CHEMICAL SENSOR AND DETECTION APPARATUS

This invention aims at providing a chemical sensor and a detection apparatus each of which can control a threshold voltage and achieve improvement in an electric-charge retention characteristic. A chemical sensor provides: a sensitive portion having a sensitive membrane sensitive to a chemical substance; a transistor having a floating gate and a gate insulating film; and a first potential controlling portion configured to control a potential of the floating gate in accordance with a voltage applied to the sensitive membrane. The first potential controlling portion has: a P-well region connected to the sensitive portion via a wiring line; a control insulating film formed to make contact with the P-well region; and a control floating portion placed at a position where the control floating portion faces the P-well region across the control insulating film, the control floating portion being conductive with the floating gate. A capacitance of the sensitive membrane is larger than a series combined capacitance of respective capacitances of the gate insulating film and the control insulating film.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a chemical sensor and a detection apparatus each configured to convert the type of a chemical substance, an ion concentration, and so on into electrical signals.

Description of the Related Art

An ion sensitive field effect transistor (ISFET) is often used for measurement of ion concentration in solution, measurement of gas concentration, sequence analysis of DNA, and so on.

The ISFET configured to measure an interface potential of a sensitive membrane interface has a problem that a threshold voltage varies when accumulated electric charges are present in a device using the ISFET. Electric charges that cause disturbance in the device are accumulated on floating gates, sensitive membranes, gate oxide films, interfaces of electrodes, and so on. These accumulated electric charges are caused mainly because of a film defect or the like at the time of etching or deposition by plasma during a semiconductor process.

PTL 1 describes that the threshold voltages of the ISFET is offset by about ±10 V due to these accumulated disturbance charges. Large variations in initial offsets of threshold voltages of the ISFET result in measurement errors and an increase in calculation throughput.

Further, the ISFET also has a problem called drift in which its output shifts over time during measurement. Drift is a phenomenon caused such that chemical reactions are caused on an interface of a sensitive membrane provided in the ISFET by application of a voltage to the ISFET, and electric charges are trapped by the interface.

PTL 2 describes a method for controlling a threshold of the ISFET by changing the amount of electric charges in a floating gate provided in the ISFET, in order that such output variations caused in the ISFET are corrected by hardware.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

PTL 2 discloses a first structure in which a sensitive portion sensitive to a chemical substance is connected to a floating gate constituted by one layer of polysilicon and configured to accumulate electric charges. The first structure is a structure in which the floating gate configured to accumulate electric charges extends from the polysilicon to a wiring layer, and the volume of an electric-charge storage portion is larger than that before the sensitive portion is connected to the floating gate. Because of this, the first structure has a problem that a parasitic capacitance increases, and leakage current easily occurs. As a result, the first structure has a problem that it is difficult to keep an adjusted threshold voltage for a long time.

Further, in addition to the first structure, PTL 2 discloses a second structure in which an electric-charge accumulation floating gate is divided from a metal plate part by use of a capacitor such as MIM. The sensitive portion as a maximum factor for an increase in volume is removed from the second structure. However, insulating films such as MIM configured to seal electric charges in the floating gate are deposited in a low temperature process, and therefore, leakage current easily occurs in comparison with a thermal oxide film used in a front end of line. On this account, the second structure has a problem that an electric-charge retention characteristic is lower than those of normal memories to which sensitive membranes are not connected.

An object of the present invention is to provide a chemical sensor and a detection apparatus each of which can control a threshold voltage and achieve improvement in an electric-charge retention characteristic.

Means for Solving the Problem

In order to achieve the above object, a chemical sensor according to one aspect of the present invention provides: a sensitive portion placed on a semiconductor substrate and having a sensitive membrane sensitive to a chemical substance; a transistor having a floating gate and a gate insulating film formed to make contact with the floating gate; and a first potential controlling portion configured to control a potential of the floating gate in accordance with a voltage applied to the sensitive membrane. The first potential controlling portion has: a first impurity diffused region formed in the semiconductor substrate and connected to the sensitive portion via a wiring line; a control insulating film placed on a first surface side of the semiconductor substrate and formed in the semiconductor substrate to make contact with the first impurity diffused region; and a control floating portion placed on the first surface side and placed at a position where the control floating portion faces the first impurity diffused region across the control insulating film, the control floating portion being conductive with the floating gate; and a capacitance of the sensitive membrane is larger than a series combined capacitance of respective capacitances of the gate insulating film and the control insulating film.

Further, in order to achieve the above object, a detection apparatus according to one aspect of the present invention provides: two chemical sensors according to the one aspect of the invention; an electrode structure having a metal electrode made of platinum or gold as a pseudo-reference electrode; and a detecting circuit configured to detect an output difference between the two chemical sensors with respect to the pseudo-reference electrode. The sensitive portion provided in one of the two chemical sensors has a first sensibility, and the sensitive portion provided in the other one of the two chemical sensors has a second sensibility. The sensitive portion provided in the one of the two chemical sensors, the sensitive portion provided in the other of the two chemical sensors, and the pseudo-reference electrode are provided to be immersible in a test sample at the same time.

Effects of the Invention

With one aspect of the present invention, it is possible to control a threshold voltage and achieve improvement in an electric-charge retention characteristic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

The following describes a chemical sensor according to a first embodiment of the present invention with reference toFIGS. 1 to 6. First described is a schematic configuration of a chemical sensor1according to the present embodiment with reference toFIG. 1.

<Configuration of Chemical Sensor>

As illustrated inFIG. 1, the chemical sensor1according to the present embodiment provides a semiconductor substrate19constituted by a P-type silicone substrate, for example, and a sensitive portion15placed on the semiconductor substrate19and having a sensitive membrane152sensitive to a chemical substance. Further, the chemical sensor1provides a transistor12having a floating gate123and a gate insulating film162formed to make contact with the floating gate123. The floating gate123is placed on a first surface side of the semiconductor substrate19in an electrically floating state. The first surface side of the semiconductor substrate19is a side of a surface (an element forming surface) where an element is formed by laminating a predetermined insulating film, metal film, and so on and injecting impurities to form the transistor12and so on. In the present embodiment, the sensitive portion15is placed on the same surface side as the surface side where the floating gate123is formed. The transistor12functions as an ISFET in the chemical sensor1. Further, the chemical sensor1provides a first potential controlling portion11configured to control a potential of the floating gate123in accordance with a voltage applied to the sensitive membrane152. The first potential controlling portion11has at least part formed in the semiconductor substrate19and is connected to the sensitive portion15. Further, the chemical sensor1provides a first electric-charge flow portion13through which electric charges are flowable to and from the floating gate123in accordance with an applied voltage. The first electric-charge flow portion13has part formed in the semiconductor substrate19.

As illustrated inFIG. 1, the first potential controlling portion11has a P-well region111(one example of a first impurity diffused region) formed in the semiconductor substrate19and connected to the sensitive portion15via a wiring line. The P-well region111has a P-type (one example of a first conductivity type), for example. Although details will be described later, the wiring line via which connects the P-well region111and the sensitive portion15has a plug21a, a plug21b, an intermediate wiring line25a, a plug21c, an intermediate wiring line25b, and a plug21d. The first potential controlling portion11has a highly-concentrated impurity diffused region112containing impurities at a concentration higher than that in the P-well region111and formed in the P-well region111. The P-well region111and the highly-concentrated impurity diffused region112are formed in an upper part of the semiconductor substrate19, for example. Further, the p-well region111and the highly-concentrated impurity diffused region112are formed in a surface layer of the semiconductor substrate19, for example. The p-well region111and the highly-concentrated impurity diffused region112correspond to the part formed in the semiconductor substrate. Note that “PW” illustrated inFIG. 1indicates a P-well region. The P-well region111is electrically connected to the sensitive membrane152of the sensitive portion15via the highly-concentrated impurity diffused region112and the wiring line having the plug21aand so on.

In the section illustrated inFIG. 1, the highly-concentrated impurity diffused region112is placed in the P-well region111to be closer to an end part side of the chemical sensor1than to a central part side of the chemical sensor1. The highly-concentrated impurity diffused region112has a P+ region112aand an N+ region112b. An element isolation layer191bformed in the P-well region111is placed between the P+ region112aand the N+ region112b. Hereby, the P+ region112aand the N+ region112bdo not make direct contact with each other. The P+ region112ais placed closer to the end part side of the chemical sensor1than the N+ region112b.

The first potential controlling portion11has an N-well region115formed in the semiconductor substrate19. The N-well region115is formed around the P-well region111in a state where part of the N-well region115makes contact with the P-well region111. The N-well region115is formed to have generally the same depth as the P-well region111. The first potential controlling portion11has an N+ region116formed in the N-well region115. The N+ region116is formed in the N-well region115on the end part side of the chemical sensor1. An element isolation layer191cis formed between the N-well region115and the P-well region111. The element isolation layer191cis also placed between the N+ region116and the P+ region112a. Hereby, the N+ region116and the P+ region112ado not make direct contact with each other. Note that “NW” illustrated inFIG. 1indicates an N-well region.

As illustrated inFIG. 1, the chemical sensor1provides a deep N-well region114(one example of a fifth impurity diffused region) formed in the semiconductor substrate19to surround the P-well region111at a position deeper than the P-well region111and having an N-type (one example of a second conductivity type). The deep N-well region114is formed to cover the lower side of the P-well region111. Since the deep N-well region114is formed to surround the P-well region111as such, the chemical sensor1can prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to the P-well region111.

The first potential controlling portion11has a control insulating film161placed on the first surface side of the semiconductor substrate19and formed in the semiconductor substrate19to make contact with the P-well region111. The first potential controlling portion11has a control floating portion113placed on the first surface side and placed at a position where the control floating portion113faces the P-well region111across the control insulating film161. The control floating portion113is conductive with the floating gate123. The control floating portion113is insulated from the P-well region111and is placed to be connected to the floating gate123in an electrically floating state. The control floating portion113is formed on the control insulating film161. That is, the control floating portion113is formed to make contact with the control insulating film161. The control floating portion113is insulated from the P-well region111by the control insulating film161. The control floating portion113has a single layer (that is, one layer) structure, for example. The control floating portion113is made of polysilicon, for example.

The control insulating film161is a thermal oxide film, for example. The control insulating film161is formed by thermally oxidizing a surface of the semiconductor substrate19at high temperature. The control insulating film161and an after-mentioned gate insulating film162and first insulating film163are formed at the same time in the same heat treatment step, for example. The control insulating film161is made of a silicon oxide film (SiO2), a silicon oxynitride film (SiON), or the like, for example. Since the control insulating film161is made of a thermal oxide film by high-temperature treatment, it is possible to prevent the occurrence of leakage current in the first potential controlling portion11.

Since the control insulating film161is formed in the same heat treatment step as the first insulating film163, for example, the control insulating film161has a film thickness in the same film thickness range (e.g., not less than 6 nm but less than 15 nm) as the first insulating film163. Electric charges do not pass through the control insulating film161differently from the first insulating film163described below, and therefore, the control insulating film161may have a film thickness thicker than the first insulating film163.

As illustrated inFIG. 1, the floating gate123provided in the transistor12has a single layer (that is, one layer) structure, for example. The floating gate123is made of polysilicon, for example.

The transistor12has a P-well region121(one example of a fourth impurity diffused region) formed in the semiconductor substrate19and having the P-type. The P-well region121has the same impurity concentration as the P-well region111, for example. The transistor12has the gate insulating film162placed to be sandwiched between the P-well region121and the floating gate123and formed to make contact with the P-well region121and the floating gate123. The gate insulating film162is formed right under the floating gate123. The P-well region121is formed to include a region right under the gate insulating film162.

The gate insulating film162is a thermal oxide film, for example. The gate insulating film162is formed by thermally oxidizing the surface of the semiconductor substrate19at high temperature. The gate insulating film162is made of a silicon oxide film (SiO2), a silicon oxynitride film (SiON), or the like, for example. As described above, since the gate insulating film162is made of a thermal oxide film by high-temperature treatment, it is possible to prevent the occurrence of leakage current in the transistor12.

Since the gate insulating film162is formed in the same heat treatment step as the first insulating film163, for example, the gate insulating film162has a film thickness in the same film thickness range (e.g., not less than 6 nm but less than 15 nm) as the first insulating film163. Electric charges do not pass through the gate insulating film162differently from the first insulating film163described below, and therefore, the gate insulating film162may have a film thickness thicker than the first insulating film163.

The transistor12has a source S formed in the P-well region121on either one of the opposite sides of the floating gate123and having the N-type, and a drain D formed in the P-well region121on the other one of the opposite sides of the floating gate123and having the N-type. The source S and the drain D are constituted by a highly-concentrated impurity diffused region having an impurity concentration higher than that of the P-well region121.

The transistor12has a P+ region122(one example of a highly-concentrated impurity diffused region) having the P-type and containing impurities at a concentration higher than that in the P-well region121. The P+ region122is formed in the P-well region121, and a voltage is applicable to the P+ region122. An element isolation layer192bis formed between the P+ region122and the source S. The element isolation layer192bis formed in the P-well region121. Hereby, the P+ region122and the source S do not make direct contact with each other.

The transistor12has an N-well region125formed in the semiconductor substrate19. The N-well region125is formed around the P-well region121in a state where the N-well region125partially makes contact with the P-well region121. The N-well region125is formed to have generally the same depth as the P-well region121. The transistor12has an N+ region126formed in the N-well region125. An element isolation layer192cis formed between the N-well region125and the P+ region122. The element isolation layer192cis formed in the P-well region121. The element isolation layer192cis also placed between the N+ region126and the P+ region122. Hereby, the N+ region126and the P+ region122do not make direct contact with each other.

The chemical sensor1provides a deep N-well region124having the N-type and formed in the semiconductor substrate19to surround the P-well region121at a position deeper than the P-well region121. The deep N-well region124is formed to cover the lower side of the P-well region121. Since the deep N-well region124is formed to surround the P-well region121as such, the chemical sensor1can prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to the P-well region121.

As illustrated inFIG. 1, the chemical sensor1provides the first electric-charge flow portion13through which electric charges are flowable to and from the floating gate123in accordance with an applied voltage. The first electric-charge flow portion13has at least part formed in the semiconductor substrate19. Although details are described later, the chemical sensor1is configured such that electrons as electric charges are injected into the floating gate123or electrons are discharged from the floating gate123via the first electric-charge flow portion13. Hereby, the chemical sensor1can adjust a threshold voltage of the transistor12.

The first electric-charge flow portion13has a P-well region131(one example of a second impurity diffused region) formed in the semiconductor substrate19and having the P-type, and a highly-concentrated impurity diffused region132formed in the P-well region131and containing impurities at a concentration higher than that in the P-well region131and to which a voltage is applied. The P-well region131and the highly-concentrated impurity diffused region132are formed in the upper part of the semiconductor substrate19, for example. Further, the P-well region131and the highly-concentrated impurity diffused region132are formed in the surface layer of the semiconductor substrate19, for example. The p-well region131and the highly-concentrated impurity diffused region132correspond to the part formed in the semiconductor substrate.

In the section illustrated inFIG. 1, the highly-concentrated impurity diffused region132is placed on a side closer to the central part side of the chemical sensor1from a central part of the P-well region131. The highly-concentrated impurity diffused region132has a P+ region132aand an N+ region132b. An element isolation layer193bformed in the P-well region131is placed between the P+ region132aand the N+ region132b. Hereby, the P+ region132aand the N+ region132bdo not make direct contact with each other. The P+ region132ais placed closer to the central part side of the chemical sensor1than the N+ region132b.

The first electric-charge flow portion13has an N-well region135formed in the semiconductor substrate19and having the N-type. The N-well region135is formed around the P-well region131in a state where the N-well region135partially makes contact with the P-well region131. The N-well region135is formed to have generally the same depth as the P-well region131. The first electric-charge flow portion13has an N+ region136formed in the N-well region135. In the section illustrated inFIG. 1, the N+ region136is formed in the N-well region135on the central part side of the chemical sensor1. An element isolation layer193cis formed between the N-well region135and the P-well region131. The element isolation layer193cis also placed between the N+ region136and the P+ region132a. Hereby, the N+ region136and the P+ region132ado not make direct contact with each other.

As illustrated inFIG. 1, the chemical sensor1provides a deep N-well region134(one example of the fifth impurity diffused region) having the N-type and formed in the semiconductor substrate19to surround the P-well region131at a position deeper than the P-well region131. The deep N-well region134is formed to cover the lower side of the P-well region131. Since the deep N-well region134is formed to surround the P-well region131as such, the chemical sensor1can prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to the P-well region131.

The first electric-charge flow portion13has a first insulating film163formed to make contact with the P-well region131, and a first floating portion133making contact with the first insulating film163and formed on the first surface side of the semiconductor substrate19in an electrically floating state. The first floating portion133is connected to the floating gate123. The first floating portion133is formed on the first insulating film163. The first floating portion133is insulated from the P-well region131by the first insulating film163. The first floating portion133has a single layer (that is, one layer) structure, for example. The first floating portion133is made of polysilicon, for example.

The first insulating film163at least partially has a region with a film thickness of not less than 6 nm but less than 15 nm, for example. In the present embodiment, the first insulating film163has the region with a film thickness of not less than 6 nm but less than 15 nm as a whole. The first insulating film163may have a film thickness within a range of not less than 6 nm but less than 15 nm as a whole, and an irregular shape may be formed on a surface of the first insulating film163. Alternatively, the first insulating film163may have a uniform film thickness (e.g., a uniform thickness of 6 nm) within a range of not less than 6 nm but less than 15 nm, for example, and the surface of the first insulating film163may have a flat shape. When the first insulating film163has a film thickness thinner than 6 nm, direct tunneling easily occurs in the first insulating film163, and the electric-charge retention characteristic (retention characteristic) of the first floating portion133worsens. In the meantime, when the first insulating film163has a film thickness thicker than 15 nm, injection of electric charges into the first floating portion133and discharge of electric charges from the first floating portion133become slow. In view of this, when the first insulating film163at least partially has the region with a film thickness of not less than 6 nm but less than 15 nm, the chemical sensor1can achieve improvement in the electric-charge retention characteristic and improvement in injection speed and discharge speed of electric charges between the first floating portion133and the P-well region131.

The first insulating film163is a thermal oxide film, for example. The first insulating film163is formed by thermally oxidizing the surface of the semiconductor substrate19at high temperature. The control insulating film161, the gate insulating film162, and the first insulating film163are formed at the same time in the same heat treatment step, for example. The first insulating film163is made of a silicon oxide film (SiO2), a silicon oxynitride film (SiON), or the like, for example. Since the first insulating film163is made of a thermal oxide film by high-temperature treatment, it is possible to prevent the occurrence of leakage current in the first electric-charge flow portion13.

The chemical sensor1provides a first connecting portion17avia which the floating gate123is connected to the control floating portion113, and a second connecting portion17bvia which the floating gate123is connected to the first floating portion133. The first connecting portion17aand the second connecting portion17beach have a single layer (that is, one layer) structure, for example. The first connecting portion17aand the second connecting portion17bare each made of polysilicon, for example. The first connecting portion17ais formed on the element isolation layers191a,192b,192cand other element isolation layers (not illustrated). The second connecting portion17bis formed on the element isolation layers192a,193b,193cand other element isolation layers (not illustrated).

The control floating portion113, the first connecting portion17a, and the floating gate123are formed integrally. Further, the floating gate123, the second connecting portion17b, and the first floating portion133are formed integrally. Accordingly, the floating gate123, the control floating portion113, and the first floating portion133are formed integrally. An overall shape of a combination of the control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, and the first floating portion133has an E-shape, for example, when viewed in a direction perpendicular to the surface of the semiconductor substrate19on which the gate insulating film162and so on are laminated. The overall shape of the combination of the control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, and the first floating portion133is not limited to the E-shape and may be other shapes.

As such, the first potential controlling portion11, the transistor12, and the first electric-charge flow portion13are electrically connected to each other via the floating gate123, the control floating portion113, and the first floating portion133, in the upper part of the semiconductor substrate19. In the meantime, the first potential controlling portion11, the transistor12, and the first electric-charge flow portion13are electrically isolated from each other in the semiconductor substrate19by the element isolation layer191aand the element isolation layer192a. More specifically, the element isolation layer191ais formed in the semiconductor substrate19between the P-well region111provided in the first potential controlling portion11and the N-well region125provided in the transistor12. Hereby, the element isolation layer191aelectrically isolates the first potential controlling portion11from the transistor12in the semiconductor substrate19. The element isolation layer192ais formed in the semiconductor substrate19between the P-well region121provided in the transistor12and the N-well region135provided in the first electric-charge flow portion13. Hereby, the element isolation layer192aelectrically isolates the transistor12from the first electric-charge flow portion13in the semiconductor substrate19.

Further, the chemical sensor1has a P-well region195formed in the semiconductor substrate19between the N-well region115and the N-well region125. Hereby, the N-well region115and the N-well region125do not make direct contact with each other. As a result, even when different voltages are applied to the N-well region115and the N-well region125, respective potentials of the N-well region115and the N-well region125do not interfere with each other.

Further, the chemical sensor1has a P-well region196formed in the semiconductor substrate19between the N-well region125and the N-well region135. Hereby, the N-well region125and the N-well region135do not make direct contact with each other. As a result, even when different voltages are applied to the N-well region125and the N-well region135, respective potentials of the N-well region125and the N-well region135do not interfere with each other.

Further, the chemical sensor1provides an element isolation layer191dformed in a peripheral edge of the chemical sensor1. The element isolation layer191dis formed in the semiconductor substrate19. the element isolation layer191dis provided such that, when a plurality of chemical sensors1is formed in an array form, the element isolation layer191delectrically isolates adjacent chemical sensors1from each other.

As illustrated inFIG. 1, the chemical sensor1provides an interlayer insulating film18formed on the semiconductor substrate19. The interlayer insulating film18is formed at least in a region where the control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, the first floating portion133, the source S, the drain D, the highly-concentrated impurity diffused regions112,132, the N+ regions116,126,136, and the element isolation layers191a,191b,191c,192a,192b,192c,193b,193c,191dare provided. The interlayer insulating film18has a function as a protective film that protects the control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, the first floating portion133, the source S, the drain D, the highly-concentrated impurity diffused regions112,132, the N+ regions116,126,136, and so on.

As illustrated inFIG. 1, the chemical sensor1provides the plugs21a,21bembedded in openings that are formed in the interlayer insulating film18and that expose part of the highly-concentrated impurity diffused region112to respective bottom faces of the openings, and the intermediate wiring line25aelectrically connected to the plugs21a,21band formed in the interlayer insulating film18. A first end of the plug21ais formed to make contact with the P+ region112aof the highly-concentrated impurity diffused region112. A first end of the plug21bis formed to make contact with the N+ region112bof the highly-concentrated impurity diffused region112. Silicides are formed on respective surfaces of the P+ region112aand the N+ region112b, for example. The plug21ais formed on the silicide formed on the surface of the P+ region112a. This is to reduce contact resistance between the plug21aand the P+ region112a. The plug21bis formed on the silicide formed on the surface of the N+ region112b. This is to reduce contact resistance between the plug21band the N+ region112b. The intermediate wiring line25ais formed to make contact with a second end of the plug21aand a second end of the plug21b. Hereby, the plug21aand the plug21bare connected to each other via the intermediate wiring line25a.

The chemical sensor1provides the plug21cembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line25ato a bottom face of the opening, and the intermediate wiring line25belectrically connected to the plug21cand formed in the interlayer insulating film18. A first end of the plug21cis formed to make contact with the intermediate wiring line25a. The intermediate wiring line25bis formed to make contact with a second end of the plug21c.

The chemical sensor1provides the plug21dembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line25bto a bottom face of the opening. A first end of the plug21dis formed to make contact with the intermediate wiring line25b. A second end of the plug21dis connected to the sensitive portion15.

The chemical sensor1provides a plug21eembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the N+ region116to a bottom face of the opening, and an intermediate wiring line25celectrically connected to the plug21eand formed in the interlayer insulating film18. A first end of the plug21eis formed to make contact with the N+ region116. A silicide is formed on a surface of the N+ region116, for example. The plug21eis formed on the silicide formed on the surface of the N+ region116. This is to reduce contact resistance between the plug21eand the N+ region116. The intermediate wiring line25cis formed to make contact with a second end of the plug21e.

The chemical sensor1provides a plug21fembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line25con a bottom face of the opening, and an intermediate wiring line25delectrically connected to the plug21fand formed in the interlayer insulating film18. A first end of the plug21fis formed to make contact with the intermediate wiring line25c.

The intermediate wiring line25dis formed to make contact with a second end of the plug21f. The intermediate wiring line25dis connected to an external terminal Tb1(not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the N+ region116via the external terminal Tb1, the intermediate wiring line25d, the plug21f, and so on.

As illustrated inFIG. 1, the chemical sensor1provides a plug22aembedded in an opening that is formed in the interlayer insulating film18and exposes part of the P+ region122on a bottom face of the opening. A first end of the plug22ais formed to make contact with the P+ region122. A silicide is formed on a surface of the P+ region122, for example. The plug22ais formed on the silicide formed on the surface of the P+ region122. This is to reduce contact resistance between the plug22aand the P+ region122.

The chemical sensor1provides a plug22bembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the source S to a bottom face of the opening. A first end of the plug22bis formed to make contact with the source S. A silicide is formed on a surface of the source S, for example. The plug22bis formed on the silicide formed on the surface of the source S. This is to reduce contact resistance between the plug22band the source S.

The chemical sensor1provides an intermediate wiring line26aelectrically connected to the plugs22a,22band formed in the interlayer insulating film18. The intermediate wiring line26ais formed to make contact with a second end of the plug22aand a second end of the plug22b. Hereby, the plug22aand the plug22bare connected to each other via the intermediate wiring line26a.

The chemical sensor1provides a plug22cembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26ato a bottom face of the opening, and an intermediate wiring line26belectrically connected to the plug22cand formed in the interlayer insulating film18. A first end of the plug22cis formed to make contact with the intermediate wiring line26a. The intermediate wiring line26bis formed to make contact with a second end of the plug22c. The intermediate wiring line26bis connected to an external terminal Ts (not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the source S and the P+ region122via the external terminal Ts, the intermediate wiring line26b, the plug22c, the intermediate wiring line26a, the plugs22a,22b, and so on.

The chemical sensor1provides a plug22gembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the drain D to a bottom face of the opening, and an intermediate wiring line26eelectrically connected to the plug22gand formed in the interlayer insulating film18. A first end of the plug22gis formed to make contact with the drain D. A silicide is formed on a surface of the drain D, for example. The plug22gis formed on the silicide formed on the surface of the drain D. This is to reduce contact resistance between the plug22gand the drain D. The intermediate wiring line26eis formed to make contact with a second end of the plug22g.

The chemical sensor1provides a plug22hembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26eto a bottom face of the opening, and an intermediate wiring line26felectrically connected to the plug22hand formed in the interlayer insulating film18. A first end of the plug22his formed to make contact with the intermediate wiring line26e.

The intermediate wiring line26fis formed to make contact with a second end of the plug22h. The intermediate wiring line26fis connected to an external terminal Td (not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the drain D via the external terminal Td, the intermediate wiring line26c, the plug22e, and so on. Further, currents flowing through the external terminal Td and the external terminal Ts can be detected.

The chemical sensor1provides a plug22eembedded in an opening that is formed in the interlayer insulating film18that exposes part of the N+ region126on a bottom face of the opening, and an intermediate wiring line26celectrically connected to the plug22eand formed in the interlayer insulating film18. A first end of the plug22eis formed to make contact with the N+ region126. A silicide is formed on a surface of the N+ region126, for example. The plug22eis formed on the silicide formed on the surface of the N+ region126. This is to reduce contact resistance between the plug22eand the N+ region126. The intermediate wiring line26cis formed to make contact with a second end of the plug22e.

The chemical sensor1provides a plug22fembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26cto a bottom face of the opening, and an intermediate wiring line26delectrically connected to the plug22fand formed in the interlayer insulating film18. A first end of the plug22fis formed to make contact with the intermediate wiring line26c.

The intermediate wiring line26dis formed to make contact with a second end of the plug22f. The intermediate wiring line26dis connected to an external terminal Tb2(not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the N+ region126via the external terminal Tb2, the intermediate wiring line26d, the plug22f, and so on.

As illustrated inFIG. 1, the chemical sensor1provides plugs23a,23bembedded in openings that are formed in the interlayer insulating film18and that expose part of the highly-concentrated impurity diffused region132to respective bottom faces of the openings, and an intermediate wiring line27aelectrically connected to the plugs23a,23band formed in the interlayer insulating film18. A first end of the plug23ais formed to make contact with the P+ region132aof the highly-concentrated impurity diffused region132. A first end of the plug23bis formed to make contact with the N+ region132bof the highly-concentrated impurity diffused region132. Silicides are formed on respective surfaces of the P+ region132aand the N+ region132b, for example. The plug23ais formed on the silicide formed on the surface of the P+ region132a. This is to reduce contact resistance between the plug23aand the P+ region132a. The plug23bis formed on the silicide formed on the surface of the N+ region132b. This is to reduce contact resistance between the plug23band the N+ region132b. The intermediate wiring line27ais formed to make contact with a second end of the plug23aand a second end of the plug23b. Hereby, the plug23aand the plug23bare connected to each other via the intermediate wiring line27a.

The chemical sensor1provides a plug23cembedded in an opening that is formed in the interlayer insulating film18that is exposes part of the intermediate wiring line27ato a bottom face of the opening, and an intermediate wiring line27belectrically connected to the plug23cand formed in the interlayer insulating film18. A first end of the plug23cis formed to make contact with the intermediate wiring line27a. The intermediate wiring line27bis formed to make contact with a second end of the plug23c. The intermediate wiring line27bis connected to an external terminal Tc2(not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the highly-concentrated impurity diffused region132via the external terminal Tc2, the intermediate wiring line27b, the plug23c, the intermediate wiring line27a, the plugs23a,23b, and so on.

The chemical sensor1provides a plug23eembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the N+ region136to a bottom face of the opening, and an intermediate wiring line27celectrically connected to the plug23eand formed in the interlayer insulating film18. A first end of the plug23eis formed to make contact with the N+ region136. A silicide is formed on a surface of the N+ region136, for example. The plug23eis formed on the silicide formed on the surface of the N+ region136. This is to reduce contact resistance between the plug23eand the N+ region136. The intermediate wiring line27cis formed to make contact with a second end of the plug23e.

The chemical sensor1provides a plug23fembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line27con a bottom face of the opening, and an intermediate wiring line27delectrically connected to the plug23fand formed in the interlayer insulating film18. A first end of the plug23fis formed to make contact with the intermediate wiring line27d.

The intermediate wiring line27dis formed to make contact with a second end of the plug23f. The intermediate wiring line27dis connected to an external terminal Tb3(not illustrated inFIG. 1) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the N+ region136via the external terminal Tb3, the intermediate wiring line27d, the plug23f, and so on.

As illustrated inFIG. 1, the sensitive portion15provided in the chemical sensor1is placed above the floating gate123and the control floating portion113. The sensitive portion15has a sensitive membrane152and a conductive portion151connected to the first potential controlling portion11, and the sensitive membrane152is formed on a first surface side of the conductive portion151. The conductive portion151has a flat-plate shape, for example. Here, the first surface out of the both surfaces of the conductive portion151is a surface where the sensitive membrane152is formed, and a second surface out of the both surfaces is a surface facing the semiconductor substrate19. That is, the sensitive membrane152is provided to make contact with the surface, out of the opposite surfaces of the conductive portion151, that does not face the semiconductor substrate19. The conductive portion151is made of metal, for example. The sensitive portion15has a flat-plate shape as a whole. A second end of the plug21dmakes contact with a back surface (a surface where the sensitive membrane152is not formed) of the conductive portion151. Hereby, the sensitive portion15is electrically connected to the first potential controlling portion11.

The sensitive membrane152has a film thickness of not less than 1 nm but not more than 1000 nm, for example. Further, the sensitive membrane152may be any film sensitive to a specific chemical substance, e.g., an insulating sensitive membrane, an organic sensitive membrane, an antibody membrane, or the like. Accordingly, an analysis target to be analyzed by the chemical sensor1is not limited. For example, in a case where the chemical sensor1measures hydrogen ions, the sensitive membrane152may be made of silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide, tin oxide, silicon dioxide, or the like. The sensitive membrane152is deposited on an uppermost surface of the conductive portion151and placed in an uppermost part of the sensitive portion15. The interlayer insulating film18above the sensitive portion15is opened, and therefore, the sensitive membrane152is exposed to outside. Hereby, a test sample91can be directly placed on the sensitive membrane152. Thus, the chemical sensor1has a structure that allows the sensitive membrane152to make direct contact with the test sample91.

As illustrated inFIG. 1, in the chemical sensor1, a reference electrode81is placed to make contact with the test sample91. An input terminal Tc1into which a predetermined reference voltage is input is connected to the reference electrode81. The predetermined reference voltage is input into the input terminal Tc1in accordance with a detection operation to detect an ion concentration or the like of the test sample91, an adjustment operation to adjust the threshold voltage of the transistor12, and so on. Although details are described later, the reference voltage input from the reference electrode81is applied to the floating gate123of the transistor12via the control floating portion113of the first potential controlling portion11. On this account, at the time of the detection operation of the chemical sensor1, the reference voltage input from the reference electrode81is applied to a circuit in which a capacitance in the sensitive portion15, a capacitance in the first potential controlling portion11, and a capacitance in the transistor12are connected in series to each other.

In order to improve detectivity to detect the ion concentration or the like of the test sample91, the chemical sensor1should be configured such that the capacitance in the sensitive portion15is larger than the capacitance in the transistor12. This configuration has an effect to efficiently transmit the reference voltage input from the reference electrode81to the floating gate123, in comparison with a configuration in which the capacitance in the sensitive portion15is smaller than the capacitance in the transistor12. Further, in order to efficiently apply the reference voltage input from the reference electrode81to the transistor12, the chemical sensor1should be configured such that the capacitance in the first potential controlling portion11is larger than the capacitance in the transistor12. This configuration has an effect to efficiently transmit the reference voltage input from the reference electrode81to the floating gate123, in comparison with a configuration in which the capacitance in the first potential controlling portion11is smaller than the capacitance in the transistor12. Further, in order to efficiently apply the reference voltage input from the reference electrode81to the transistor12, the chemical sensor1should be configured such that the capacitance in the sensitive portion15is larger than the series combined capacitance of the capacitance in the first potential controlling portion11and the capacitance in the transistor12. This configuration has an effect to efficiently transmit the reference voltage input from the reference electrode81to the floating gate123, in comparison with a configuration in which the capacitance in the sensitive portion15is smaller than the series combined capacitance of the capacitance in the first potential controlling portion11and the capacitance in the transistor12. When the reference voltage input from the reference electrode81is efficiently transmitted to the floating gate123as described above, it is possible to improve the detectivity.

Here, when the capacitance in the transistor12is referred to as C12, the capacitance in the first potential controlling portion11is referred to as C11, the series combined capacitance of the capacitance in the first potential controlling portion11and the capacitance in the transistor12is referred to as Cs, and the capacitance in the sensitive portion15is referred to as Cm, the chemical sensor1should satisfy relationships expressed by Formula (1) and Formula (2).

In the meantime, the capacitance in the sensitive portion15is dominated by a capacitance formed by the sensitive membrane152. On this account, the capacitance formed by the sensitive membrane152can be regarded as the capacitance in the sensitive portion15. Further, a capacitance in the control floating portion113can be regarded as the capacitance in the first potential controlling portion11. The capacitance in the control floating portion113is dominated by a capacitance formed by the control insulating film161. On this account, the capacitance formed by the control insulating film161can be regarded as the capacitance in the control floating portion113. Further, a capacitance in the floating gate123can be regarded as the capacitance in the transistor12. The capacitance in the floating gate123is dominated by a capacitance formed by the gate insulating film162. On this account, the capacitance formed by the gate insulating film162can be regarded as the capacitance in the floating gate123. Accordingly, from Formula (1), the capacitance of the sensitive membrane152should be larger than a series combined capacitance of the capacitances in the gate insulating film162and in the control insulating film161.

More specifically, the chemical sensor1is configured such that the ratio of the capacitance in the sensitive membrane152to the sum of the capacitance in the sensitive membrane152and a series combined capacitance of the capacitance in the floating gate123and the capacitance in the control floating portion113is not less than 0.7 but not more than 1.0. On this account, a voltage of 70% or more of the reference voltage input from the reference electrode81is applied to the series combined capacitance of the control floating portion113and the floating gate123. Hereby, in the chemical sensor1, the reference voltage can be applied to the floating gate123of the transistor12more efficiently, and thus, it is possible to improve the detectivity.

The chemical sensor1is configured such that the ratio of the capacitance in the control floating portion113to the sum of the capacitance in the floating gate123and the capacitance in the control floating portion113is not less than 0.7 but not more than 1.0. On this account, a voltage of 49% or more of the reference voltage input from the reference electrode81is applied to the floating gate123. Hereby, in the chemical sensor1, the reference voltage can be applied to the floating gate123of the transistor12further more efficiently, and thus, it is possible to improve the detectivity.

<Operation of Chemical Sensor>

Next will be described the operation of the chemical sensor1according to the present embodiment by taking, as an example, a hydrogen-ion concentration sensor, with reference toFIGS. 2 to 5as well asFIG. 1. First described is the detection principle of the chemical sensor1to detect a test sample, with reference toFIG. 2. In order to facilitate understanding, the first potential controlling portion11and the first electric-charge flow portion13are not illustrated inFIG. 2.

(Detection Principle of Chemical Sensor)

As illustrated inFIG. 2, in the chemical sensor1, when the test sample91is placed on the sensitive portion15, and a reference voltage is input into the sensitive portion15from the reference electrode81via the test sample91, a sensitive group on an interface of the sensitive membrane152, e.g., a hydroxy group, dissociates, and an electrochemically balanced state is established. At this time, a potential corresponding to the concentration of the test sample91occurs on the interface of the sensitive portion15.

An interface voltage changes in accordance with the concentration of the test sample91. Accordingly, even when the voltage value of the reference voltage input from the reference electrode81is the same, the voltage value of a gate voltage applied to the floating gate123of the transistor12changes in accordance with the concentration of the test sample91. By directly reading a drain current Id flowing through the transistor12or an interface potential at this time, the concentration of the test sample91can be detected.

The characteristic of the drain current to the gate voltage changes in accordance with the threshold voltage of the transistor. The chemical sensor1according to the present embodiment can adjust the threshold voltage of the transistor12by adjusting the amount of electric charges (electrons) present in the floating gate123. On this account, the chemical sensor1can detect a hydrogen-ion concentration or the like of the test sample91without being affected by manufacture variations, variations with time, and so on in the threshold voltage of the transistor12. Hereby, the chemical sensor1can improve the detection accuracy of the test sample91.

Next will be described the detection operation of the chemical sensor1according to the present embodiment with reference toFIGS. 3 to 5. Upon describing the detection operation of the chemical sensor1, an equivalent circuit of the chemical sensor1will be described first with reference toFIG. 3.

(Equivalent Circuit of Chemical Sensor)

The first potential controlling portion11has the highly-concentrated impurity diffused region112provided on one of the both sides of the control floating portion113. However, the element isolation layer191a(not illustrated inFIG. 3, seeFIG. 1) is placed on the other of the both sides of the control floating portion113. On this account, as illustrated inFIG. 3, the first potential controlling portion11can be expressed by a circuit mark indicative of a field effect transistor (FET) that one of two terminals that corresponds to a source or a drain is the highly-concentrated impurity diffused region112and the other of the two terminals is an opened state.

The input terminal Tc1for the reference voltage is connected to the highly-concentrated impurity diffused region112of the first potential controlling portion11via the plugs21a,21b(not illustrated inFIG. 3, seeFIG. 1) and so on, the sensitive portion15, and the reference electrode81(not illustrated inFIG. 3, seeFIG. 1). The highly-concentrated impurity diffused region112is connected to the P-well region111via the P+ region112a. Between the P-well region111and the N+ region116, a PN junction pn11is formed by the P-well region111, the deep N-well region114(not illustrated inFIG. 3, seeFIG. 1), the N-well region115(not illustrated inFIG. 3, seeFIG. 1), and the N+ region116.

The external terminal Tb1is connected to the N+ region116via the plug21f(not illustrated inFIG. 3, seeFIG. 1) and so on. Between the N+ region116and a P-type region (a region indicated by “Psub” inFIG. 1) where well regions of the semiconductor substrate19are not formed, a PN junction pn12is formed by the N+ region116, the N-well region115, the deep N-well region114, and the P-type region. In the chemical sensor1according to the present embodiment, the P-type region is connected to the ground, for example.

As illustrated inFIG. 3, the control floating portion113of the first potential controlling portion11is connected to the floating gate123of the transistor12via the first connecting portion17a(not illustrated inFIG. 3, seeFIG. 1). The external terminal Ts is connected to the source S of the transistor12via the plug22band so on. The external terminal Td is connected to the drain D of the transistor12via the plug22eand so on.

Further, the P+ region122is connected to the source S of the transistor12. The P-well region121of the transistor12is connected to the P+ region122. Between the P-well region121and the N+ region126, a PN junction pn21is formed by the P-well region121, the deep N-well region124(not illustrated inFIG. 3, seeFIG. 1), the N-well region125(not illustrated inFIG. 3, seeFIG. 1), and the N+ region126.

The external terminal Tb2is connected to the N+ region126via the plug22f(not illustrated inFIG. 3, seeFIG. 1) and so on. Between the N+ region126and the P-type region of the semiconductor substrate19where well regions are not formed, a PN junction pn22is formed by the N+ region126, the N-well region125, the deep N-well region124, and the P-type region.

As illustrated inFIG. 3, the floating gate123of the transistor12is connected to the first floating portion133of the first electric-charge flow portion13via the second connecting portion17b(not illustrated inFIG. 3, seeFIG. 1). The first electric-charge flow portion13has the highly-concentrated impurity diffused region132provided on one of the both sides of the first floating portion133. However, the element isolation layer191d(not illustrated inFIG. 3, seeFIG. 1) is placed on the other of the both sides of the first floating portion133. On this account, as illustrated inFIG. 3, in the first electric-charge flow portion13can be expressed by a circuit mark indicative of a field effect transistor (FET) that one of two terminals that corresponds to a source or a drain is the highly-concentrated impurity diffused region132, and the other of the two terminals is an opened state.

The external terminal Tb3is connected to the N+ region136via the plug23f(not illustrated inFIG. 3, seeFIG. 1) and so on. Between the N+ region136and the P-type region of the semiconductor substrate19where well regions are not formed, a PN junction pn32is formed by the N+ region136, the N-well region135, the deep N-well region134, and the P-type region.

(Detection Operation of Chemical Sensor)

As illustrated inFIG. 3, in the chemical sensor1, in a case where the hydrogen-ion concentration or the like of the test sample91is to be detected, a positive direct voltage Vdc is input into the input terminal Tc1as the reference voltage. Further, in this case, in the chemical sensor1, a voltage of zero volts (V) is input into the source S of the transistor12, and a voltage as a drain voltage Vd (e.g., 0.1 V) is input into the drain D of the transistor12. Further, in this case, in the chemical sensor1, the external terminals Tb1, Tb2, Tb3, Tc2are brought into an opened state.

Hereby, a gate voltage obtained by adding an interface voltage on the sensitive membrane152to the direct voltage Vdc is applied to the floating gate123of the transistor12via the sensitive portion15and the first potential controlling portion11. Further, respective voltages described above are input into the source S and the drain D of the transistor12. Hereby, the transistor12operates. Since the interface voltage of the sensitive membrane152changes in accordance with the hydrogen-ion concentration of the test sample91, a gate voltage on which the hydrogen-ion concentration of the test sample91is reflected is applied to the floating gate123of the transistor12. As a result, a drain current on which the hydrogen-ion concentration of the test sample91is reflected flows through the transistor12. Hereby, the chemical sensor1can detect the hydrogen-ion concentration of the test sample91.

(Adjustment Operation on Threshold Voltage of Transistor in Chemical Sensor)

With reference toFIGS. 4A and 4B, the following describes an operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor1into an enhancement state by injecting electric charges into the floating gate123. First described is a first operation to adjust the threshold voltage to bring the transistor12into the enhancement state, with reference toFIG. 4A.

As illustrated inFIG. 4A, in the first operation, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. The reference sample is a sample the hydrogen-ion concentration of which is known. In the first operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the first floating portion133via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, the first connecting portion17a, the floating gate123of the transistor12, and the second connecting portion17b. Further, in the case of the first operation, in the chemical sensor1, the same pulse voltage as the input terminal Tc1is applied to the external terminal Tb1. Further, in the case of the first operation, in the chemical sensor1, the external terminal Ts connected to the source S of the transistor12and the external terminal Td connected to the drain D of the transistor12are brought into an opened state. Furthermore, in the case of the first operation, in the chemical sensor1, a pulse voltage of which is the voltage value inverted from 0 V to −Vpp is input into the external terminal Tc2of the first electric-charge flow portion13, and a voltage of 0 V is input into the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tb3is connected to the ground, for example).

Hereby, Fowler-Nordheim tunneling conduction (FN tunneling) occurs in the first insulating film163, and as indicated by straight arrows inFIG. 4A, electrons e− are injected into the floating gate123from the P-well region131of the first electric-charge flow portion13through the first insulating film163, the first floating portion133, and the second connecting portion17b. Hereby, the threshold voltage of the transistor12increases. After electric charges are injected, the threshold voltage of the transistor12in a state where the reference sample is placed is checked by a method similar to the detection operation of the chemical sensor1as described with reference toFIG. 3. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are injected into the floating gate123by the first operation, the capacitance in the control floating portion113should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the control floating portion113to the first floating portion133, in comparison with a configuration in which the capacitance in the control floating portion113is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

Next described is a second operation to adjust the threshold voltage to bring the transistor12into the enhancement state, with reference toFIG. 4B.

As illustrated inFIG. 4B, in the second operation, in the chemical sensor1, the input terminal Tc1and the external terminal Tb1are brought into an opened state. In the second operation, the reference sample is not placed on the sensitive portion15. Further, in the case of the second operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the source S and the drain D of the transistor12and the external terminal Tb2. Furthermore, in the case of the second operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc2of the first electric-charge flow portion13, and a voltage of 0 V is input into the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tb3is connected to the ground, for example).

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 4B, electrons e− are injected into the floating gate123from the P-well region131through the first insulating film163, the first floating portion133, and the second connecting portion17b. Hereby, the threshold voltage of the transistor12increases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor1as described with reference toFIG. 3. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are injected into the floating gate123by the second operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

With reference toFIGS. 5A and 5B, the following describes an operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor1into a depression state by discharging electric charges from the floating gate123. First described is a first operation to adjust the threshold voltage to bring the transistor12to into the depression state, with reference toFIG. 5A.

As illustrated inFIG. 5A, in the first operation, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. In the first operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the first floating portion133via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, the first connecting portion17a, the floating gate123of the transistor12, and the second connecting portion17b. Further, in the case of the first operation, in the chemical sensor1, a voltage of 0 V is input into the external terminal Tb1(the external terminal Tb1is connected to the ground, for example). Further, in the case of the first operation, in the chemical sensor1, the source S and the drain D of the transistor12are brought into an opened state. Furthermore, in the case of the first operation, in the chemical sensor1, a pulse voltage the of which voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13.

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 5A, electrons e− are discharged from the floating gate123to the P-well region131through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor12decreases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor1as described with reference toFIG. 3. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are discharged from the floating gate123by the first operation, the capacitance in the control floating portion113should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the control floating portion113to the first floating portion133, in comparison with a configuration in which the capacitance in the control floating portion113is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

Next described is a second operation to adjust the threshold voltage to bring the transistor12into the depression state, with reference toFIG. 5B.

As illustrated inFIG. 5B, in the second operation, in the chemical sensor1, the input terminal Tc1and the external terminal Tb1are brought into an opened state. In the second operation, the reference sample is not placed on the sensitive portion15. Further, in the case of the second operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the source S and the drain D of the transistor12and the external terminal Tb2. Furthermore, in the case of the second operation, in the chemical sensor1, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13.

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 5B, electrons e− are discharged from the floating gate123to the P-well region131through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor12decreases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor1as described with reference toFIG. 3. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are discharged from the floating gate123by the second operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

(Effect of Chemical Sensor)

Next will be described an effect of the chemical sensor1according to the present embodiment with reference toFIG. 6.FIG. 6is a graph illustrating measurement results of threshold voltages before and after adjustment regarding transistors provided in samples having a configuration similar to the chemical sensor1. Triangular marks illustrated inFIG. 6indicate measured values of threshold voltages of the transistors before adjustment, and circular marks illustrated inFIG. 6indicate measured values of threshold voltages of the transistors, the threshold voltages being adjusted to a range of 1.5 V to ±0.01 V by use of phosphate buffer of pH 6.9. Each of the measured values of the threshold voltages before and after the adjustment, illustrated inFIG. 6, is the average value of 10 measured values of the threshold voltage of each sample. The horizontal axis in the graph illustrated inFIG. 6indicates sample. The vertical axis in the graph illustrated inFIG. 6indicates threshold voltage [V]. InFIG. 6, a gate voltage at which a drain current of 3 nA flows is defined as the threshold voltage.

As illustrated inFIG. 6, regarding 10 samples from sample1to sample10, before the threshold voltages are adjusted, the threshold voltages of the transistors vary within a range from −1 V to 3.5 V. On the other hand, after the threshold voltages are adjusted, the threshold voltages fall within the range of 1.5 V to ±0.01 V in all the 10 samples from sample1to sample10. As such, the chemical sensor1according to the present embodiment can adjust the threshold voltage of the transistor12to a desired value, and thus, it is possible to improve the detection accuracy of a test sample.

The chemical sensor1according to the present embodiment includes the control insulating film161, the gate insulating film162, and the first insulating film163having an excellent membrane quality and formed by thermal oxidation. The floating gate123is capacitively coupled to the gate insulating film162, the control floating portion113is capacitively coupled to the control insulating film161, and the first floating portion133is capacitively coupled to the first insulating film163. Hereby, the chemical sensor1has a structure in which electric charges are trapped in the control floating portion113, the floating gate123, and the first floating portion133by the control insulating film161, the gate insulating film162, and the first insulating film163. On this account, the chemical sensor1can achieve improvement in the electric-charge retention characteristic.

Further, the chemical sensor1does not have a structure in which a conductive portion of a sensitive portion is directly connected to a floating gate made of polysilicon through a wiring via like conventional chemical sensors. When the conductive portion is directly connected to the floating gate like the conventional chemical sensors, an electric-charge accumulation part expands to the wiring via and the conductive portion. The wiring via and the conductive portion are not designed to retain electric charges. Because of this, when a floating portion extends to the wiring via and the conductive portion such that an electric-charge accumulation layer expands, a parasitic capacitance increases, and leakage current easily occurs. As a result of diligent studies by the inventors of the present invention, it was found that, when the electric-charge accumulation layer expands to the wiring via and the conductive portion, the amount of leakage current increases, and an adjusted threshold voltage of a transistor cannot be retained for a long time.

The chemical sensor1according to the present embodiment has a structure in which the conductive portion151of the sensitive portion15is connected not to the floating gate123but to the highly-concentrated impurity diffused region112of the first potential controlling portion11. Hereby, electric charges can be accumulated only in the floating gate123, the control floating portion113, and the first floating portion133that are designed to be used as memories. As a result, the chemical sensor1can retain the electric charges accumulated in the floating gate123, the control floating portion113, and the first floating portion133for a long time. Further, since the chemical sensor1is configured such that the conductive portion151is connected to the highly-concentrated impurity diffused region112that is part of the semiconductor substrate19, plasma damage caused when plugs and intermediate wiring lines are formed can be released to the substrate. As a result, the chemical sensor1can achieve improvement in reliability of the control insulating film161, the gate insulating film162, and the first insulating film163.

The following describes chemical sensors according to modifications of the present embodiment with reference toFIGS. 7 and 8. Upon describing the chemical sensors according to the modifications, the same reference sign as a constituent in the chemical sensor1according to the present embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor1, and descriptions of the constituent are omitted.

As illustrated inFIG. 7, a chemical sensor1aaccording to Modification 1 of the present embodiment does not have the N+ regions126,136, the plugs22e,22f,23e,23f, and the intermediate wiring lines26c,26d,27c,27d, as compared with the chemical sensor1according to the above embodiment. Further, the chemical sensor1ahas an N-well region198ainstead of the N-well regions115,125,135and the P-well region195,196, as compared with the chemical sensor1according to the above embodiment. The N-well region198ais placed right under the element isolation layers191a,192a.

The chemical sensor1aprovides a deep N-well region199ahaving the N-type and formed in the semiconductor substrate19to surround the P-well region111, the P-well region121, and the P-well region131at a position deeper than the P-well region111, the P-well region121, and the P-well region131. Further, the chemical sensor1ahas the N-well region198aformed around the P-well region111, the P-well region121, and the P-well region131. The N-well region198ais formed in the deep N-well region199a. Hereby, the chemical sensor1acan prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to at least one of the P-well region111, the P-well region121, and the P-well region131, similarly to the chemical sensor1.

As illustrated inFIG. 8, a chemical sensor1baccording to Modification 2 of the present embodiment does not have the N+ region136, the plugs23e,23f, and the intermediate wiring lines27c,27d, as compared with the chemical sensor1according to the above embodiment. Further, the chemical sensor1bhas a P-well region121ainstead of the P-well regions121,131, as compared with the chemical sensor1according to the above embodiment. The P-well region121ais formed continuously over the transistor12and the first electric-charge flow portion13.

The chemical sensor1bprovides a deep N-well region124ahaving the N-type and formed in the semiconductor substrate19right under the P-well region121aover the arrangement region for the transistor12and the arrangement region for the first electric-charge flow portion13at a position deeper than the P-well region121a. Hereby, the chemical sensor1bcan prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to at least one of the P-well region111and the P-well region121a, similarly to the chemical sensor1.

As described above, the chemical sensors according to the present embodiment and the modifications can control the threshold voltage of the transistor and achieve improvement in the electric-charge retention characteristic.

Second Embodiment

The following describes a chemical sensor according to a second embodiment of the present invention with reference toFIGS. 9 to 13. First described is a schematic configuration of a chemical sensor2according to the present embodiment with reference toFIG. 9. Upon describing the chemical sensor according to the present embodiment, the same reference sign as a constituent in the chemical sensor according to the first embodiment or the modifications is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor according to the first embodiment or the modifications, and descriptions of the constituent are omitted.

<Configuration of Chemical Sensor>

As illustrated inFIG. 9, the chemical sensor2according to the present embodiment does not include an electric-charge flow portion, differently from the chemical sensor1according to the first embodiment. The chemical sensor2has a feature in that electric charges are injected into the floating gate123or electric charges are discharged from the floating gate123by use of the transistor12. The first potential controlling portion11and the sensitive portion15included in the chemical sensor2have the same configurations as the first potential controlling portion11and the sensitive portion15included in the chemical sensor1according to the first embodiment.

The transistor12includes the P-well region121(one example of the fourth impurity diffused region) formed in the semiconductor substrate19and having the P-type. The P-well region121has the same configuration as the P-well region111in the first embodiment. The transistor12includes a gate insulating film16placed to be sandwiched between the P-well region121and the floating gate123and formed to make contact with the P-well region121and the floating gate123. The gate insulating film16is formed right under the floating gate123. The P-well region121is formed to include a region right under the gate insulating film16.

The gate insulating film16at least partially has a region with a film thickness of not less than 6 nm but less than 15 nm, for example. In the present embodiment, the gate insulating film16has the region with a film thickness of not less than 6 nm but less than 15 nm as a whole. The gate insulating film16may have a film thickness within a range of not less than 6 nm but less than 15 nm as a whole, and an irregular shape may be formed on a surface of the gate insulating film16. Alternatively, the gate insulating film16may have a uniform film thickness (e.g., a given thickness of 6 nm) within a range of not less than 6 nm but less than 15 nm, for example, and the surface of the gate insulating film16may have a flat shape. When the gate insulating film16has a film thickness thinner than 6 nm, direct tunneling easily occurs in the gate insulating film16, and the electric-charge retention characteristic (retention characteristic) of the floating gate123worsens. In the meantime, when the gate insulating film16has a film thickness thicker than 15 nm, injection of electric charges into the floating gate123and discharge of electric charges from the floating gate123become slow. In view of this, when the gate insulating film16at least partially has the region with a film thickness of not less than 6 nm but less than 15 nm, the chemical sensor2can achieve improvement in the electric-charge retention characteristic and improvement in injection speed and discharge speed of electric charges between the floating gate123and the P-well region121.

The gate insulating film16is a thermal oxide film, for example. The gate insulating film16is formed by thermally oxidizing the surface of the semiconductor substrate19at high temperature. The control insulating film161and the gate insulating film16are formed at the same time in the same heat treatment step, for example. The gate insulating film16is made of a silicon oxide film (SiO2), a silicon oxynitride film (SiON), or the like, for example. Since the gate insulating film16is made of a thermal oxide film by high-temperature treatment, it is possible to prevent the occurrence of leakage current in the transistor12.

The transistor12has the source S formed in the P-well region121on one of the both sides of the floating gate123and having the N-type, and the drain D formed in the P-well region121on the other of the both sides of the floating gate123and having the N-type. The source S and the drain D are constituted by a highly-concentrated impurity diffused region having an impurity concentration higher than that of the P-well region121.

The transistor12has the P+ region122(one example of the highly-concentrated impurity diffused region) having the P-type and containing impurities at a concentration higher than that in the P-well region121. The P+ region122is formed in the P-well region121, and a voltage is applicable to the P+ region122. The element isolation layer192bis formed between the P+ region122and the source S. The element isolation layer192bis formed in the P-well region121. Hereby, the P+ region122and the source S do not make direct contact with each other.

The transistor12has the N-well region125formed in the semiconductor substrate19. The N-well region125is formed around the P-well region121in a state where the N-well region125partially makes contact with the P-well region121. The N-well region125is formed to have generally the same depth as the P-well region121. The transistor12has the N+ region126formed in the N-well region125. The element isolation layer192cis formed between the N-well region125and the P+ region122. The element isolation layer192cis formed in the P-well region121. The element isolation layer192cis also placed between the N+ region126and the P+ region122. Hereby, the N+ region126and the P+ region122do not make direct contact with each other.

<Operation of Chemical Sensor>

Next will be described the operation of the chemical sensor2according to the present embodiment by taking, as an example, a hydrogen-ion concentration sensor, with reference toFIGS. 10 to 12as well asFIG. 9.

(Equivalent Circuit of Chemical Sensor)

As illustrated inFIG. 9, the chemical sensor2according to the present embodiment can be expressed by an equivalent circuit similar to that of the chemical sensor1according to the first embodiment except that the chemical sensor2does not include the first electric-charge flow portion.

(Detection Operation of Chemical Sensor)

As illustrated inFIG. 10, in the chemical sensor2according to the present embodiment, in a case where the hydrogen-ion concentration or the like of the test sample91is to be detected, a positive direct voltage Vdc is input into the input terminal Tc1as a reference voltage. Further, in this case, in the chemical sensor2, a voltage of zero volts (V) is input into the source S of the transistor12, a voltage as a drain voltage Vd (e.g., 0.1 V) is input into the drain D of the transistor12, and the external terminals Tb1, Tb2are brought into an opened state.

Hereby, a gate voltage obtained by adding an interface voltage on the sensitive membrane152to a pulse voltage Vpp is applied to the floating gate123of the transistor12via the sensitive portion15and the first potential controlling portion11. Further, respective voltages described above are input into the source S and the drain D of the transistor12. Hereby, the transistor12operates. Since the interface voltage of the sensitive membrane152changes in accordance with the hydrogen-ion concentration of the test sample91, a gate voltage on which the hydrogen-ion concentration of the test sample91is reflected is applied to the floating gate123of the transistor12. As a result, a drain current on which the hydrogen-ion concentration of the test sample91is reflected flows through the transistor12. Hereby, the chemical sensor2can detect the hydrogen-ion concentration of the test sample91. Thus, the chemical sensor2can detect the hydrogen-ion concentration or the like of the test sample91by an operation similar to that of the chemical sensor1according to the first embodiment.

(Adjustment Operation of Threshold Voltage of Transistor in Chemical Sensor)

With reference toFIG. 11, the following describes an operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor2into the enhancement state by injecting electric charges into the floating gate123.

As illustrated inFIG. 11, in the adjustment operation on the threshold voltage to bring the transistor12into the enhancement state, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. The reference sample is a sample of which the hydrogen-ion concentration is known. In the adjustment operation on the threshold voltage, in the chemical sensor2, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the control floating portion113via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, and the first connecting portion17a. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, the same pulse voltage as the input terminal Tc1is applied to the external terminal Tb1. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Ts connected to the source S of the transistor12and the external terminal Td connected to the drain D of the transistor12. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, a voltage of 0 V is input into the external terminal Tb2(the external terminal Tb2is connected to the ground, for example).

Hereby, FN tunneling occurs in the gate insulating film16, and as indicated by straight arrows inFIG. 11, electrons e− are injected into the floating gate123from the P-well region121through the gate insulating film16. The threshold voltage of the transistor12increases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor2as described with reference toFIG. 10. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are injected into the floating gate123, the capacitance in the control floating portion113should be larger than the capacitance in the floating gate123. This configuration has an effect to efficiently transmit a voltage input into the control floating portion113to the floating gate123, in comparison with a configuration in which the capacitance in the control floating portion113is smaller than the capacitance in the floating gate123. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

With reference toFIG. 12, the following describes an operation to adjust the threshold voltage to bring the transistor12included in the chemical sensor2into the depression state by discharging electric charges from the floating gate123.

As illustrated inFIG. 12, in the adjustment operation on the threshold voltage, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. In the adjustment operation on the threshold voltage, in the chemical sensor2, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the floating gate123via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, and the first connecting portion17a. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, a voltage of 0 V is input into the external terminal Tb1(the external terminal Tb1is connected to the ground, for example). Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Ts connected to the source S of the transistor12and the external terminal Td connected to the drain D. Furthermore, in the case of the adjustment operation on the threshold voltage, in the chemical sensor2, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tb2.

Hereby, FN tunneling occurs in the gate insulating film16, and as indicated by straight arrows inFIG. 12, electrons e− are discharged from the floating gate123to the P-well region121through the gate insulating film16. Hereby, the threshold voltage of the transistor12decreases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor2as described with reference toFIG. 10. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished. Note that, in a case where electric charges are discharged from the floating gate123, the capacitance in the control floating portion113should be larger than the capacitance in the floating gate123. This configuration has an effect to efficiently transmit a voltage input into the control floating portion113to the floating gate123, in comparison with a configuration in which the capacitance in the control floating portion113is smaller than the capacitance in the floating gate123. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

Thus, the chemical sensor2according to the present embodiment can adjust the threshold voltage of the transistor12by controlling the amount of electric charge of the floating gate123via the P-well region121and the gate insulating film16of the transistor12.

The following describes a chemical sensor according to a modification of the present embodiment with reference toFIG. 13. Upon describing the chemical sensor according to the present modification, the same reference sign as a constituent in the chemical sensor2according to the second embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor2according to the second embodiment, and descriptions of the constituent are omitted.

As illustrated inFIG. 13, a chemical sensor2aaccording to the present modification provides a deep N-well region199bhaving the N-type and formed in the semiconductor substrate19to surround the P-well region111and the P-well region121at a position deeper than the P-well region111and the P-well region121, as compared with the chemical sensor2according to the above embodiment. Further, the chemical sensor2ahas an N-well region198bformed around the P-well region111and the P-well region121, as compared with the chemical sensor2according to the above embodiment. The N-well region198bis formed in the deep N-well region199b. Hereby, the chemical sensor2acan prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to at least one of the P-well region111and the P-well region121, similarly to the chemical sensor2.

As described above, the chemical sensors according to the present embodiment and the modification can control the threshold voltage of the transistor and achieve improvement in the electric-charge retention characteristic. Further, since the transistor functions as an injection portion and a discharge portion of electric charge, the chemical sensor according to the present embodiment does not provide an electric-charge flow portion. Hereby, the chemical sensor according to the present embodiment can be reduced in size as compared with the chemical sensor1according to the first embodiment.

Third Embodiment

The following describes a chemical sensor according to a third embodiment of the present invention with reference toFIGS. 14 to 19. First described is a schematic configuration of a chemical sensor3according to the present embodiment with reference toFIG. 14. Upon describing the chemical sensor according to the present embodiment, the same reference sign as a constituent in the chemical sensor according to the first embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor according to the first embodiment, and descriptions of the constituent are omitted.

<Configuration of Chemical Sensor>

As illustrated inFIG. 14, the chemical sensor3according to the present embodiment has a feature in that the chemical sensor3provides a second electric-charge flow portion14in addition to the configuration of the chemical sensor1according to the first embodiment.

As illustrated inFIG. 14, the chemical sensor3provides the second electric-charge flow portion14through which electric charges are flowable to and from the floating gate123in accordance with an applied voltage, and that has at least part formed in the semiconductor substrate19. Although details are described later, the chemical sensor3is configured such that electric charges are injected into the floating gate123by use of either of the first electric-charge flow portion13and the second electric-charge flow portion14, and electric charges are discharged from the floating gate123by use of the other one of the first electric-charge flow portion13and the second electric-charge flow portion14. Hereby, the chemical sensor3can adjust the threshold voltage of the transistor12.

The second electric-charge flow portion14has a P-well region141(one example of a third impurity diffused region) formed in the semiconductor substrate19, and a highly-concentrated impurity diffused region142formed in the P-well region141and containing impurities at a concentration higher than that in the P-well region141, and to which a voltage is applied. The P-well region141and the highly-concentrated impurity diffused region142are formed in the upper part of the semiconductor substrate19, for example. Further, the P-well region141and the highly-concentrated impurity diffused region142are formed in the surface layer of the semiconductor substrate19, for example. The p-well region141and the highly-concentrated impurity diffused region142correspond to the part formed in the semiconductor substrate. Here, the second impurity diffused region in the present embodiment may have the P-type (one example of the first conductivity type) or the N-type (one example of the second conductivity type) different from the P-type. In the present embodiment, the second impurity diffused region has the P-type.

In the section illustrated inFIG. 14, the highly-concentrated impurity diffused region142is placed closer to a central part side of the chemical sensor3from a central part of the P-well region141. The highly-concentrated impurity diffused region142has a P+ region142aand an N+ region142b. An element isolation layer194bformed in the P-well region141is placed between the P+ region142aand the N+ region142b. Hereby, the P+ region142aand the N+ region142bdo not make direct contact with each other. In the section illustrated inFIG. 14, the P+ region142ais placed closer to the central part side of the chemical sensor3than the N+ region142b.

The second electric-charge flow portion14has an N-well region145formed in the semiconductor substrate19and having the N-type. The N-well region145is formed around the P-well region141in a state where the N-well region145partially makes contact with the P-well region141. The N-well region145is formed to have generally the same depth as the P-well region141. The second electric-charge flow portion14has an N+ region146formed in the N-well region145. In the section illustrated inFIG. 14, the N+ region146is formed closer to the central part side of the chemical sensor3in the N-well region145. An element isolation layer194cis formed between the N-well region145and the P-well region141. The element isolation layer194cis also placed between the N+ region146and the P+ region142a. Hereby, the N+ region146and the P+ region142ado not make direct contact with each other.

As illustrated inFIG. 14, the chemical sensor3provides a deep N-well region144(one example of the fifth impurity diffused region) having the N-type and formed in the semiconductor substrate19to surround the P-well region141at a position deeper than the P-well region141. The deep N-well region144is formed to cover the lower side of the P-well region141. Since the deep N-well region144is formed to surround the P-well region141as such, the chemical sensor3can prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to the P-well region141.

The second electric-charge flow portion14has a second insulating film164formed to make contact with the P-well region141, and a second floating portion143making contact with the second insulating film164and formed on the first surface side of the semiconductor substrate19in an electrically floating state. The second floating portion143is connected to the floating gate123. The second floating portion143is formed on the second insulating film164. The second floating portion143is insulated from the P-well region141by the second insulating film164. The second floating portion143has a single layer (that is, one layer) structure, for example. The second floating portion143is made of polysilicon, for example.

The second insulating film164at least partially has a region with a film thickness of not less than 6 nm but less than 15 nm, for example. In the present embodiment, the second insulating film164has the region with a film thickness of not less than 6 nm but less than 15 nm as a whole. The second insulating film164may have a film thickness within a range of not less than 6 nm but less than 15 nm as a whole, and an irregular shape may be formed on a surface of the second insulating film164. Alternatively, the second insulating film164may have a uniform film thickness (e.g., a uniform thickness of 6 nm) within a range of not less than 6 nm but less than 15 nm, for example, and the surface of the second insulating film164may have a flat shape. When the second insulating film164has a film thickness thinner than 6 nm, direct tunneling easily occurs in the second insulating film164, and the electric-charge retention characteristic (retention characteristic) of the second floating portion143worsens. In the meantime, when the second insulating film164has a film thickness thicker than 15 nm, injection of electric charges into the second floating portion143and discharge of electric charges from the second floating portion143become slow. In view of this, when the second insulating film164at least partially has the region with a film thickness of not less than 6 nm but less than 15 nm, the chemical sensor3can achieve improvement in the electric-charge retention characteristic and improvement in injection speed and discharge speed of electric charges between the second floating portion143and the P-well region141.

The second insulating film164is a thermal oxide film, for example. The second insulating film164is formed by thermally oxidizing the surface of the semiconductor substrate19at high temperature. The control insulating film161, the gate insulating film162, the first insulating film163, and the second insulating film164are formed at the same time in the same heat treatment step, for example. The second insulating film164is made of a silicon oxide film (SiO2), a silicon oxynitride film (SiON), or the like, for example. Since the second insulating film164is made of a thermal oxide film by high-temperature treatment, it is possible to prevent the occurrence of leakage current in the second electric-charge flow portion14.

The chemical sensor3provides a third connecting portion17cvia which the first floating portion133is connected to the second floating portion143. The third connecting portion17chas a single layer (that is, one layer) structure, for example. The third connecting portion17cis made of polysilicon, for example. The third connecting portion17cis formed on the element isolation layers193a,194band other element isolation layers (not illustrated).

The control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, the first floating portion133, the third connecting portion17c, and the second floating portion143are formed integrally. Accordingly, the floating gate123, the control floating portion113, the first floating portion133, and the second floating portion143are formed integrally. The control floating portion113, the first connecting portion17a, the floating gate123, the second connecting portion17b, the first floating portion133, the third connecting portion17c, and the second floating portion143are formed on insulating films such as the gate insulating film162and element isolation layers such as the element isolation layer191aand have a comb shape, for example.

As such, the first potential controlling portion11, the transistor12, the first electric-charge flow portion13, and the second electric-charge flow portion14are electrically connected to each other via the floating gate123, the control floating portion113, the first floating portion133, and the second floating portion143, in the upper part of the semiconductor substrate19. In the meantime, the first potential controlling portion11, the transistor12, the first electric-charge flow portion13, and the second electric-charge flow portion14are electrically isolated from each other in the semiconductor substrate19by the element isolation layer191a, the element isolation layer192a, and the element isolation layer193a. More specifically, the element isolation layer193ais formed in the semiconductor substrate19between the P-well region131provided in the first floating portion133and the N-well region145provided in the second floating portion143. Hereby, the element isolation layer191celectrically isolates the first floating portion133from the second floating portion143in the semiconductor substrate19.

Further, the chemical sensor3has a P-well region197formed in the semiconductor substrate19between the N-well region135and the N-well region145. Hereby, the N-well region135and the N-well region145do not make direct contact with each other. As a result, even when different voltages are applied to the N-well region135and the N-well region145, respective potentials of the N-well region135and the N-well region145do not interfere with each other.

As illustrated inFIG. 14, the chemical sensor3provides plugs24a,24bembedded in openings that are formed in the interlayer insulating film18and that expose part of the highly-concentrated impurity diffused region142to respective bottom faces of the openings, and an intermediate wiring line28aelectrically connected to the plugs24a,24band formed in the interlayer insulating film18. A first end of the plug24ais formed to make contact with the P+ region142aof the highly-concentrated impurity diffused region142. A first end of the plug24bis formed to make contact with the N+ region142bof the highly-concentrated impurity diffused region142. Silicides are formed on respective surfaces of the P+ region142aand the N+ region142b, for example. The plug24ais formed on the silicide formed on the surface of the P+ region142a. This is to reduce contact resistance between the plug24aand the P+ region142a. The plug24bis formed on the silicide formed on the surface of the N+ region142b. This is to reduce contact resistance between the plug24band the N+ region142b. The intermediate wiring line28ais formed to make contact with a second end of the plug24aand a second end of the plug24b. Hereby, the plug24aand the plug24bare connected to each other via the intermediate wiring line28a.

The chemical sensor3provides a plug24cembedded in an opening that are formed in the interlayer insulating film18and that exposes part of the intermediate wiring line28ato a bottom face of the opening, and an intermediate wiring line28belectrically connected to the plug24cand formed in the interlayer insulating film18. A first end of the plug24cis formed to make contact with the intermediate wiring line28a. The intermediate wiring line28bis formed to make contact with a second end of the plug24c. The intermediate wiring line28bis connected to an external terminal Tc3(not illustrated inFIG. 14) via plugs (not illustrated) or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the highly-concentrated impurity diffused region142via the external terminal Tc3, the intermediate wiring line28b, the plug24c, the intermediate wiring line28a, the plugs24a,24b, and so on.

The chemical sensor3provides a plug24eembedded in an opening that are formed in the interlayer insulating film18and that exposes part of the N+ region146to a bottom face of the opening, and an intermediate wiring line28delectrically connected to the plug24eand formed in the interlayer insulating film18. A first end of the plug24eis formed to make contact with the N+ region146. A silicide is formed on a surface of the N+ region146, for example. The plug24eis formed on the silicide formed on the surface of the N+ region146. This is to reduce contact resistance between the plug24eand the N+ region146.

The chemical sensor3provides a plug24fembedded in an opening that are formed in the interlayer insulating film18and that exposes part of the intermediate wiring line28cto a bottom face of the opening, and an intermediate wiring line28delectrically connected to the plug24fand formed in the interlayer insulating film18. The intermediate wiring line28dis formed to make contact with a second end of the plug24f. The intermediate wiring line28dis connected to an external terminal Tb4(not illustrated inFIG. 14) via plugs (not illustrated) or intermediate wiring lines. Hereby, a voltage can be applied to the N+ region146via the external terminal Tb4, the intermediate wiring line28d, the plug24f, and so on.

<Operation of Chemical Sensor>

Next will be described the operation of the chemical sensor3according to the present embodiment by taking, as an example, a hydrogen-ion concentration sensor, with reference toFIGS. 15 to 17as well asFIG. 14.

(Equivalent Circuit of Chemical Sensor)

As illustrated inFIG. 15, the first floating portion133of the first electric-charge flow portion13is connected to the second floating portion143of the second electric-charge flow portion14via the third connecting portion17c(not illustrated inFIG. 15, seeFIG. 14). The second electric-charge flow portion14has the highly-concentrated impurity diffused region142placed on one of the both sides of the second floating portion143. However, the element isolation layer191d(not illustrated inFIG. 15, seeFIG. 14) is placed on the other of the both sides of the second floating portion143. In view of this, as illustrated inFIG. 15, the second electric-charge flow portion14can be expressed by a circuit mark indicative of a field effect transistor (FET) that one of two terminals that corresponds to a source or a drain is the highly-concentrated impurity diffused region142and the other of the two terminals is an opened state.

The external terminal Tb4is connected to the N+ region146via the plug24f(not illustrated inFIG. 15, seeFIG. 14) and so on. Between the N+ region146and the P-type region of the semiconductor substrate19where well regions are not formed, a PN junction pn42is formed by the N+ region146, the N-well region145, the deep N-well region144, and the P-type region.

(Detection Operation of Chemical Sensor)

As illustrated inFIG. 15, in a case where the chemical sensor3detects the hydrogen-ion concentration or the like of the test sample91, a positive direct voltage Vdc is input into the input terminal Tc1as a reference voltage. Further, in this case, in the chemical sensor3, a voltage of zero volts (V) is input into the source S of the transistor12, and a voltage as a drain voltage Vd (e.g., 0.1 V) is input into the drain D of the transistor12. Further, in this case, in the chemical sensor3, the external terminals Tb1, Tb2, Tb3, Tb4, Tc2, Tc3are brought into an opened state.

Hereby, a gate voltage obtained by adding an interface voltage on the sensitive membrane152to the direct voltage Vdc is applied to the floating gate123of the transistor12via the sensitive portion15and the first potential controlling portion11. Further, respective voltages described above are input into the source S and the drain D of the transistor12. Hereby, the transistor12operates. Since the interface voltage of the sensitive membrane152changes in accordance with the hydrogen-ion concentration of the test sample91, a gate voltage on which the hydrogen-ion concentration of the test sample91is reflected is applied to the floating gate123of the transistor12. As a result, a drain current on which the hydrogen-ion concentration of the test sample91is reflected flows through the transistor12. Hereby, the chemical sensor3can detect the hydrogen-ion concentration of the test sample91.

(Adjustment Operation of Threshold Voltage of Transistor in Chemical Sensor)

With reference toFIG. 16, the following describes an operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor3into the enhancement state by injecting electric charges into the floating gate123.

As illustrated inFIG. 16, in the operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor3into the enhancement state, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. In the adjustment operation on the threshold voltage, in the chemical sensor3, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the second floating portion143via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, the first connecting portion17a, the floating gate123of the transistor12, the second connecting portion17b, the first floating portion133, and the third connecting portion17c. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor3, the same pulse voltage as the input terminal Tc1is applied to the external terminal Tb1. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor3, the external terminal Ts connected to the source S of the transistor12and the external terminal Td connected to the drain D are brought into an opened state. Further, in the case of the adjustment operation of the threshold voltage, in the chemical sensor3, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc3of the second electric-charge flow portion14, and a voltage of 0 V is input into the external terminal Tb4of the second electric-charge flow portion14(the external terminal Tb4is connected to the ground, for example). Furthermore, in the case of the adjustment operation on the threshold voltage, in the chemical sensor3, a voltage of 0 V is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tc2and the external terminal Tb3are connected to the ground, for example).

Hereby, FN tunneling occurs in the second insulating film164, and as indicated by straight arrows inFIG. 16, electrons e− are injected into the floating gate123from the P-well region141of the second electric-charge flow portion14through the second insulating film164, the second floating portion143, the third connecting portion17c, the first floating portion133, and the second connecting portion17b. Hereby, the threshold voltage of the transistor12increases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor3as described with reference toFIG. 15. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished.

Although not illustrated herein, in the chemical sensor3, the threshold voltage of the transistor12can be adjusted by an operation similar to the second operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor1according to the first embodiment into the enhancement state. In this case, the first potential controlling portion11and the transistor12in the present embodiment are operated in respective states similar to those of the first potential controlling portion11and the transistor12in the second operation in the first embodiment. Further, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc3of the second electric-charge flow portion14in the present embodiment, and a voltage of 0 V is input into the external terminal Tb4of the second electric-charge flow portion14(the external terminal Tb4is connected to the ground, for example). Further, the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13in the present embodiment are brought into an opened state. Hereby, the chemical sensor3can inject electrons e− into the floating gate123via the second electric-charge flow portion14. Note that, in a case where electric charges are injected into the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the second floating portion143. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the second floating portion143, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the second floating portion143. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

With reference toFIG. 17, the following describes an operation to adjust the threshold voltage to bring the transistor12included in the chemical sensor3into the depression state by discharging electric charges from the floating gate123.

As illustrated inFIG. 17, in the operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor3into the depression state, a reference sample used for adjustment of the threshold voltage of the transistor12is placed on the sensitive portion15. In the adjustment operation on the threshold voltage, in the chemical sensor3, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the first floating portion133via the reference sample, the sensitive portion15, the control floating portion113of the first potential controlling portion11, and the first connecting portion17a. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor3, a voltage of 0 V is input into the external terminal Tb1(the external terminal Tb1is connected to the ground, for example). Further, in the case of the adjustment operation of the threshold voltage, in the chemical sensor3, the source S and the drain D of the transistor12are brought into an opened state. Furthermore, in the case of the adjustment operation of the threshold voltage, in the chemical sensor3, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13. Furthermore, in the case of the adjustment operation on the threshold voltage, in the chemical sensor3, the external terminal Tc3and the external terminal Tb4of the second electric-charge flow portion14are brought into an opened state.

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 17, electrons e− are discharged from the floating gate123to the P-well region131through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor12decreases. The threshold voltage of the transistor12is checked by a method similar to the detection operation of the chemical sensor3as described with reference toFIG. 15. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor12are repeatedly executed until the transistor12reaches its desired threshold voltage. When the transistor12reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor12is finished.

Although not illustrated herein, in the chemical sensor3, the threshold voltage of the transistor12can be adjusted by an operation similar to the second operation to adjust the threshold voltage to bring the transistor12provided in the chemical sensor1according to the first embodiment into the depression state. In this case, the first potential controlling portion11, the transistor12, and the first electric-charge flow portion13in the present embodiment are operated in respective states similar to those of the first potential controlling portion11, the transistor12, and the first electric-charge flow portion13in the second operation in the first embodiment. Further, the external terminal Tc3and the external terminal Tb4of the second electric-charge flow portion14in the present embodiment are brought into an opened state. Hereby, the chemical sensor3can discharge electrons e− from the floating gate123via the first electric-charge flow portion13. Note that, in a case where electric charges are discharged from the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the control floating portion113. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the control floating portion113, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the control floating portion113. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

As such, in the chemical sensor3according to the present embodiment, the first electric-charge flow portion13is used to discharge electric charges (electrons in the present embodiment) from the floating gate123, and the second electric-charge flow portion14is used to inject electric charges (electrons in the present embodiment) into the floating gate123. In the chemical sensor3, the second electric-charge flow portion14may be used to discharge electric charges (electrons in the present embodiment) from the floating gate123, and the first electric-charge flow portion13is used to inject electric charges (electrons in the present embodiment) into the floating gate123. Since different paths are used for a path where electric charges are discharged from the floating gate123and for a path where electric charges are injected into the floating gate123as such, the amount of electric charge passing through the first insulating film163provided in the first electric-charge flow portion13and the second insulating film164provided in the second electric-charge flow portion14can be decreased. Hereby, the chemical sensor3can restrain deterioration of the first insulating film163and the second insulating film164and achieve improvement in the electric-charge retention characteristics of the floating gate123, the control floating portion113, the first floating portion133, and the second floating portion143.

The following describes chemical sensors according to modifications of the present embodiment with reference toFIGS. 18 and 19. Upon describing the chemical sensors according to the modifications, the same reference sign as a constituent in the chemical sensor3according to the present embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor3according to the third embodiment, and descriptions of the constituent are omitted.

As illustrated inFIG. 18, a chemical sensor3aaccording to Modification 1 of the present embodiment does not have the N+ regions126,136,146, the plugs22e,22f,23e,23f,24e,24f, and the intermediate wiring lines26c,26d,27c,27d,28c,28d, as compared with the chemical sensor3according to the third embodiment. Further, the chemical sensor3ahas N-well regions198cinstead of the N-well regions115,125and the P-well region195, and N-well regions198cinstead of the N-well regions135,145and the P-well region197, as compared with the chemical sensor3according to the third embodiment. The N-well regions198care placed right under the element isolation layers191a,192a,193a.

The chemical sensor3aprovides a deep N-well region199chaving the N-type and formed in the semiconductor substrate19to surround the P-well region111, the P-well region121, the P-well region131, and the P-well region141at a position deeper than the P-well region111, the P-well region121, the P-well region131, and the P-well region141. Further, the chemical sensor3ahas the N-well region198cformed around the P-well region111, the P-well region121, the P-well region131, and the P-well region141. The N-well regions198cplaced right under the element isolation layers191a,192a,193aare formed in the deep N-well region199c. Hereby, the chemical sensor3acan prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to at least one of the P-well region111, the P-well region121, the P-well region131, and the P-well region141, similarly to the chemical sensor3.

As illustrated inFIG. 19, a chemical sensor3baccording to Modification 2 of the present embodiment does not have the N+ region136, the plugs23e,23f, and the intermediate wiring lines27c,27d, as compared with the chemical sensor3according to the third embodiment. Further, the chemical sensor3bhas the P-well region121ainstead of the P-well regions121,131, as compared with the chemical sensor3according to the third embodiment. The P-well region121ais formed continuously over the transistor12and the first electric-charge flow portion13.

Further, the chemical sensor3bhas a deep N-well region190ainstead of the deep N-well regions124,134, as compared with the chemical sensor3according to the third embodiment. The deep N-well region190ais formed continuously over the transistor12and the first electric-charge flow portion13. The P-well region121ais formed in the deep N-well region190a. Further, the chemical sensor3bhas an N-well region141ainstead of the N-well regions135,145and the P-well region197,141in the second electric-charge flow portion14, as compared with the chemical sensor3according to the third embodiment. Further, the chemical sensor3bhas the deep N-well region190a, as compared with the chemical sensor3according to the third embodiment. Thus, the second electric-charge flow portion14in the present modification has the N-well region141a(one example of the third impurity diffused region) formed in the semiconductor substrate19. The N-well region141ain the present modification may have the P-type (one example of the first conductivity type) or the N-type (one example of the second conductivity type) different from the P-type. In the present modification, the N-well region141ahas the N-type.

In the chemical sensor3baccording to the present modification, the first electric-charge flow portion13is used to inject electric charges (electrons in the present embodiment) into the floating gate123, and the second electric-charge flow portion14is used to discharge electric charges (electrons in the present embodiment) from the floating gate123. On this account, when electric charges are discharged from the floating gate123, a positive pulse voltage is applied to the N-well region141aof the second electric-charge flow portion14, and the P-type region (the region indicated by “Psub” inFIG. 19) of the semiconductor substrate19where well regions are not formed is connected to the ground, for example. Hereby, when electric charges are discharged from the floating gate123, a reverse bias is applied to the N-well region141aand the P-type region. Accordingly, leakage current can be hardly caused. Further, the N-well region141aof the second electric-charge flow portion14is brought into an opened state except for a state where electric charges are discharged from the floating gate123. Further, the P-well region111is surrounded by the deep N-well region114, and the P-well region121ais surrounded by the deep N-well region190a.

Hereby, the chemical sensor3bof the present modification can prevent leakage current from flowing into the semiconductor substrate19in response to a voltage being applied to at least one of the P-well region111, the P-well region121, the P-well region131, and the N-well region141a, similarly to the chemical sensor3.

As described above, the chemical sensors according to the present embodiment and the modifications can control the threshold voltage of the transistor and achieve improvement in the electric-charge retention characteristic. Further, the chemical sensor according to the present embodiment provides an electric-charge flow portion for exclusive use of discharging electrons from the floating gate, and an electric-charge flow portion for exclusive use of injecting electrons into the floating gate. Hereby, the chemical sensors according to the present embodiment and the modifications can restrain deterioration of the insulating films and achieve improvement in the electric-charge retention characteristics of the floating gate and the floating portions.

Fourth Embodiment

The following describes a chemical sensor according to a fourth embodiment of the present invention with reference toFIGS. 20 to 23. First described is a schematic configuration of a chemical sensor4according to the present embodiment with reference toFIG. 20. Upon describing the chemical sensor according to the present embodiment, the same reference sign as a constituent in the chemical sensor1according to the first embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor1according to the first embodiment, and descriptions of the constituent are omitted.

<Configuration of Chemical Sensor>

As illustrated inFIG. 20, the chemical sensor4according to the present embodiment provides the semiconductor substrate19constituted by a P-type silicone substrate, for example, and the sensitive portion15placed on the semiconductor substrate19and having the sensitive membrane152sensitive to a chemical substance. Further, the sensitive portion15in the present embodiment has the same configuration as the sensitive portion15in the first embodiment.

Further, the chemical sensor4provides a transistor32having the floating gate123and the gate insulating film162formed to make contact with the floating gate123. The floating gate123is placed on the first surface side of the semiconductor substrate19in an electrically floating state. The transistor32functions as an ISFET in the chemical sensor4. Further, the chemical sensor4provides a first potential controlling portion41configured to control the potential of the floating gate123in accordance with a voltage applied to the sensitive membrane152. The first potential controlling portion41has at least part formed in the semiconductor substrate19and is connected to the sensitive portion15. Further, the chemical sensor4provides a second potential controlling portion31having at least part formed in the semiconductor substrate19and configured to control the potential of the floating gate123. The second potential controlling portion31is configured to control the potential of the floating gate123to adjust the threshold voltage of the transistor32, for example. Further, the chemical sensor4provides the first electric-charge flow portion13through which electric charges are flowable to and from the floating gate123in accordance with an applied voltage and that has at least part formed in the semiconductor substrate19.

As illustrated inFIG. 20, the second potential controlling portion31has a configuration similar to that of the first potential controlling portion11in the first embodiment except that the second potential controlling portion31is not connected to the sensitive portion15. On this account, the same reference sign as a constituent of the first potential controlling portion11is assigned to a constituent of the second potential controlling portion31that has a similar operation and function to those of the constituent of the first potential controlling portion11, and detailed descriptions are omitted.

The second potential controlling portion31has the P-well region111(one example of a sixth impurity diffused region) formed in the semiconductor substrate19and having the P-type, and the highly-concentrated impurity diffused region112formed in the P-well region111and containing impurities at a concentration higher than that in the P-well region111. Further, the second potential controlling portion31has the control floating portion113insulated from the P-well region111and formed on the first surface side of the semiconductor substrate19in an electrically floating state. The control floating portion113is connected to the floating gate123.

The second potential controlling portion31is used to apply a pulse voltage to the floating gate123at the time when the threshold voltage of the transistor32is to be adjusted. Furthermore, the second potential controlling portion31is used to apply a direct voltage to the floating gate123to operate the transistor32at the time when the ion concentration and so on of the test sample are to be detected.

An external terminal Tc6(not illustrated inFIG. 20) is connected to the highly-concentrated impurity diffused region112provided in the second potential controlling portion31via the plugs21a,21b, the intermediate wiring line25a, the plug21c, and the intermediate wiring line25b, but the sensitive portion15is not connected to the highly-concentrated impurity diffused region112. The direct voltage and the pulse voltage to be applied to the floating gate123of the transistor32by the second potential controlling portion31are input into the external terminal Tc6.

As illustrated inFIG. 20, the transistor32in the present embodiment has a configuration similar to that of the transistor12in the first embodiment except that the transistor32is connected to the sensitive portion15. On this account, the same reference sign as a constituent of the transistor12is assigned to a constituent of the transistor32that has a similar operation and function to those of the constituent of the transistor12, and detailed descriptions of the constituent are omitted.

The chemical sensor4provides the plug22bembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the source S to the bottom face of the opening, and an intermediate wiring line26gelectrically connected to the plug22band formed in the interlayer insulating film18. A first end of the plug22bis formed to make contact with the source S. A silicide is formed on a surface of the source S, for example. The plug22bis formed on the silicide formed on the surface of the source S. This is to reduce contact resistance between the plug22band the source S. The intermediate wiring line26gis formed to make contact with a second end of the plug22b.

The chemical sensor4provides a plug22iembedded in an opening is that formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26gto a bottom face of the opening, and an intermediate wiring line26helectrically connected to the plug22iand formed in the interlayer insulating film18. A first end of the plug22iis formed to make contact with the intermediate wiring line26g. The intermediate wiring line26his formed to make contact with a second end of the plug22i. The intermediate wiring line26his connected to the external terminal Ts (not illustrated inFIG. 20) via plugs or intermediate wiring lines (not illustrated). Hereby, a voltage can be applied to the source S and the P-well region121via the external terminal Ts, the intermediate wiring line26h, the plug22i, the intermediate wiring line26g, the plug22b, and so on.

As illustrated inFIG. 20, the first potential controlling portion41provided in the chemical sensor4has the P-type, contains impurities at a concentration higher than that in the P-well region121, is formed in the P-well region121. The first potential controlling portion41is constituted by a P+ region. As such, the first potential controlling portion41has part formed in the semiconductor substrate19(in the present embodiment, a whole of the first potential controlling portion41is formed in the semiconductor substrate19). The first potential controlling portion41is formed in the upper part of the semiconductor substrate19, for example. Further, the first potential controlling portion41is formed in the surface layer of the semiconductor substrate19, for example.

The transistor32is connected to the sensitive portion15via the first potential controlling portion41. The first potential controlling portion41is connected to the sensitive portion15via the plug22a, the intermediate wiring line26a, the plug22c, the intermediate wiring line26b, and a plug22d.

More specifically, the chemical sensor4provides the plug22aembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the first potential controlling portion41to the bottom face of the opening, and the intermediate wiring line26aelectrically connected to the plug22aand formed in the interlayer insulating film18. The first end of the plug22ais formed to make contact with the P+ region constituting the first potential controlling portion41. A silicide is formed on a surface of the P+ region constituting the first potential controlling portion41, for example. The plug22ais formed on the silicide formed on the surface of the P+ region constituting the first potential controlling portion41. This accordingly achieves a reduction in contact resistance between the plug22aand the P+ region constituting the first potential controlling portion41. The intermediate wiring line26amakes contact with the second end of the plug22a.

The chemical sensor4provides the plug22cembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26aon the bottom face of the opening, and the intermediate wiring line26belectrically connected to the plug22cand formed in the interlayer insulating film18. The first end of the plug22cis formed to make contact with the intermediate wiring line26a. The intermediate wiring line26bis formed to make contact with the second end of the plug22c.

The chemical sensor4provides the plug22dembedded in an opening that is formed in the interlayer insulating film18and that exposes part of the intermediate wiring line26bto a bottom face of the opening. A first end of the plug22dis formed to make contact with the intermediate wiring line26b. A second end of the plug22dis formed to make contact with the conductive portion151provided in the sensitive portion15. Hereby, the plug22dis electrically connected to the conductive portion151. Accordingly, the conductive portion151is connected to the first potential controlling portion41via the plug22d, the intermediate wiring line26b, the plug22c, the intermediate wiring line26a, and the plug22a.

As such, the sensitive portion15has the conductive portion151connected to the first potential controlling portion41, and the sensitive membrane152formed on the first surface side of the conductive portion151such that a chemical substance makes contact with the sensitive membrane152. Here, the first surface out of the both surfaces of the conductive portion151is a surface where the sensitive membrane152is formed, and the second surface out of the both surfaces is a surface facing the semiconductor substrate19. That is, the sensitive membrane152is provided to make contact with a surface, out of the both surfaces of the conductive portion151, that does not face the semiconductor substrate19.

Although details are described later, the chemical sensor4can apply, to the P-well region121, a voltage to which an interface voltage on the sensitive membrane152is added to a reference voltage input into the input terminal Tc1and apply a gate voltage having a predetermined potential to the floating gate123of the transistor32via the second potential controlling portion31. On this account, the threshold voltage of the transistor32changes based on the concentration of the test sample91due to a substrate bias effect. Thus, the chemical sensor4can detect the concentration of the test sample91based on the fluctuation in the threshold voltage of the transistor12due to the substrate bias effect.

<Operation of Chemical Sensor>

Next will be described the detection operation of the chemical sensor4according to the present embodiment by taking, as an example, a hydrogen-ion concentration sensor, withFIGS. 21 to 23. Upon describing the detection operation of the chemical sensor4, an equivalent circuit of the chemical sensor4will be described first with reference toFIG. 21.

(Equivalent Circuit of Chemical Sensor)

The second potential controlling portion31has a configuration similar to that of the first potential controlling portion11except that the second potential controlling portion31is not connected to the sensitive portion15. On this account, the second potential controlling portion31can be expressed by a circuit mark indicative of a field effect transistor (FET) that one of two terminals that corresponds to a source or a drain is the highly-concentrated impurity diffused region112and the other of the two terminals is an opened state.

As illustrated inFIG. 21, the external terminal Tc6is connected to the highly-concentrated impurity diffused region112of the second potential controlling portion31via the plugs21a,21b(not illustrated inFIG. 21, seeFIG. 20) and so on. The second potential controlling portion31can be expressed by an equivalent circuit similar to that of the first potential controlling portion11in the first embodiment except that the external terminal Tc6is connected to the highly-concentrated impurity diffused region112.

As illustrated inFIG. 21, the first potential controlling portion41is connected to the P-well region121of the transistor32. The input terminal Tc1for the reference voltage is connected to the first potential controlling portion41via the plug22a(not illustrated inFIG. 21, seeFIG. 20) and so on, the sensitive portion15, and the reference electrode81(not illustrated inFIG. 21, seeFIG. 20). The transistor32can be expressed by an equivalent circuit similar to that of the transistor12in the first embodiment except that the P-well region121is connected to the sensitive portion15.

(Detection Operation of Chemical Sensor)

As illustrated inFIG. 21, in a case where the chemical sensor4detects the hydrogen-ion concentration or the like of the test sample91, a positive direct voltage Vdc is input into the input terminal Tc6. Further, in this case, in the chemical sensor4, a direct reference voltage having a voltage value of Vc is input into the input terminal Tc1. Further, in this case, in the chemical sensor4, a voltage of zero volts (V) is input into the external terminal Ts connected to the source S of the transistor32, and a voltage as a drain voltage Vd (e.g., 0.1 V) is input into the external terminal Td connected to the drain D of the transistor32. Further, in this case, in the chemical sensor4, the external terminals Tb1, Tb2, Tb3, Tc2are brought into an opened state.

Hereby, the direct voltage Vdc is applied to the floating gate123of the transistor32via the second potential controlling portion31, and a voltage obtained by adding the interface voltage on the sensitive membrane152to the reference voltage is applied to the P-well region121of the transistor32via the first potential controlling portion41. Further, respective voltages described above are input into the source S and the drain D of the transistor32. The transistor32hereby operates in a state where a back bias is applied to the transistor32. Since the interface voltage of the sensitive membrane152changes in accordance with the hydrogen-ion concentration of the test sample91, a voltage on which the hydrogen-ion concentration of the test sample91is reflected is applied to the P-well region121of the transistor32. As a result, a drain current on which the hydrogen-ion concentration of the test sample91is reflected flows through the transistor32due to the substrate bias effect. Hereby, the chemical sensor4can detect the hydrogen-ion concentration of the test sample91.

(Adjustment Operation of Threshold Voltage of Transistor in Chemical Sensor)

With reference toFIG. 22, the following describes an operation to adjust the threshold voltage to bring the transistor32provided in the chemical sensor4into the enhancement state by injecting electric charges into the floating gate123.

As illustrated inFIG. 22, in the operation to adjust the threshold voltage to bring the transistor32into the enhancement state, in the chemical sensor4, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc6. The pulse voltage is applied to the first floating portion133via the intermediate wiring lines25a,25b, the plugs21b,21c, the highly-concentrated impurity diffused region112, the P-well region111, the control floating portion113, the first connecting portion17a, the floating gate123of the transistor32, and the second connecting portion17b. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor4, the same pulse voltage as the external terminal Tc6is applied to the external terminal Tb1. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor4, the input terminal Tc1, the external terminal Ts connected to the source S of the transistor32, the external terminal Td connected to the drain D, and the external terminal Tb2are brought into an opened state. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor4, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc2of the first electric-charge flow portion13, and a voltage of 0 V is input into the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tb3is connected to the ground, for example).

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 22, electrons e− are injected into the floating gate123from the P-well region131of the first electric-charge flow portion13through the first insulating film163, the first floating portion133, and the second connecting portion17b. Hereby, the threshold voltage of the transistor32increases. The threshold voltage of the transistor32is checked by a method similar to the detection operation of the chemical sensor4as described with reference toFIG. 21. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor32are repeatedly executed until the transistor32reaches its desired threshold voltage. When the transistor32reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor32is finished. Note that, in a case where electric charges are injected into the floating gate123by the operation, the capacitance in the second potential controlling portion31should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the second potential controlling portion31to the first floating portion133, in comparison with a configuration in which the capacitance in the second potential controlling portion31is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

Although not illustrated herein, in the chemical sensor4, the threshold voltage of the transistor32can be adjusted by an operation similar to the second operation to adjust the threshold voltage to bring the transistor12in the chemical sensor1according to the first embodiment into the enhancement state. In this case, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tc1as the reference voltage. The reference voltage is applied to the P-well region121via the reference sample, the sensitive portion15, and the first potential controlling portion41. Further, the same pulse voltage as the pulse voltage input into the input terminal Tc1is applied to the source S and the drain D of the transistor32and the external terminal Tb2. Further, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc2of the first electric-charge flow portion13in the present embodiment, and a voltage of 0 V is input into the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tb3is connected to the ground, for example).

Further, the external terminal Tb1and the external terminal Tc6of the second potential controlling portion31are brought into an opened state. Hereby, the chemical sensor4can inject electrons e− into the floating gate123via the first electric-charge flow portion13. Note that, in a case where electric charges are injected into the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

With reference toFIG. 23, the following describes an operation to adjust the threshold voltage to bring the transistor32provided in the chemical sensor4into the depression state by discharging electric charges from the floating gate123.

As illustrated inFIG. 23, in the operation to adjust the threshold voltage to bring the transistor32into the depression state, in the chemical sensor4, the input terminal Tc1, the external terminal Ts connected to the source S of the transistor32, the external terminal Td connected to the drain D of the transistor32, and the external terminal Tb2are brought into an opened state. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor4, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the external terminal Tc6. The pulse voltage is applied to the first floating portion133via the intermediate wiring lines25a,25b, the plugs21b,21c, the highly-concentrated impurity diffused region112, the P-well region111, the control floating portion113, the first connecting portion17a, the floating gate123of the transistor32, and the second connecting portion17b. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor4, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13.

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 23, electrons e− are discharged from the floating gate123to the P-well region131through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor32decreases. The threshold voltage of the transistor32is checked by a method similar to the detection operation of the chemical sensor4as described with reference toFIG. 21. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor32are repeatedly executed until the transistor32reaches its desired threshold voltage. When the transistor32reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor32is finished. Note that, in a case where electric charges are discharged from the floating gate123by the operation, the capacitance in the second potential controlling portion31should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the second potential controlling portion31to the first floating portion133, in comparison with a configuration in which the capacitance in the second potential controlling portion31is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

Although not illustrated herein, in the chemical sensor4, the threshold voltage of the transistor32can be adjusted by an operation similar to the second operation to adjust the threshold voltage to bring the transistor12in the chemical sensor1according to the first embodiment into the depression state. In this case, a voltage of 0 V is applied to the input terminal Tc1, the source S and the drain D of the transistor32, and the external terminal Tb2. Further, the external terminal Tc6and the external terminal Tb1of the second potential controlling portion31are brought into an opened state. Further, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2of the first electric-charge flow portion13in the present embodiment, and a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tb3of the first electric-charge flow portion13. Hereby, the chemical sensor4can discharge electrons e− from the floating gate123via the first electric-charge flow portion13. Note that, in a case where electric charges are discharged from the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

As described above, the chemical sensors according to the present embodiment can control the threshold voltage of the transistor and achieve improvement in the electric-charge retention characteristic.

Fifth Embodiment

The following describes a chemical sensor according to a fifth embodiment of the present invention with reference toFIGS. 24 to 27. First described is a schematic configuration of a chemical sensor5according to the present embodiment with reference toFIG. 24. Upon describing the chemical sensor according to the present embodiment, the same reference sign as a constituent in the chemical sensor4according to the fourth embodiment is assigned to a constituent having a similar operation and function to those of the constituent in the chemical sensor4according to the fourth embodiment, and descriptions of the constituent are omitted.

<Configuration of Chemical Sensor>

As illustrated inFIG. 24, the chemical sensor5according to the present embodiment does not include the second potential controlling portion31, differently from the chemical sensor4according to the fourth embodiment. The chemical sensor5has a feature in that the transistor32is operated by bringing the transistor32into the depression state, namely, a normally-on state of the threshold voltage. The transistor32, the sensitive portion15, and the first electric-charge flow portion13provided in the chemical sensor5have the same configurations as the transistor32, the sensitive portion15, and the first electric-charge flow portion13provided in the chemical sensor4according to the fourth embodiment.

<Operation of Chemical Sensor>

Next will be described the operation of the chemical sensor5according to the present embodiment by taking, as an example, a hydrogen-ion concentration sensor, withFIGS. 25 to 27as well asFIG. 24.

(Equivalent Circuit of Chemical Sensor)

As illustrated inFIG. 25, the chemical sensor5according to the present embodiment can be expressed by an equivalent circuit similar to that of the chemical sensor4according to the fourth embodiment except that the chemical sensor5does not provide the second potential controlling portion.

(Detection Operation of Chemical Sensor)

As illustrated inFIG. 25, in the chemical sensor5according to the present embodiment, in a case where the hydrogen-ion concentration or the like of the test sample91is to be detected, a direct reference voltage having a voltage value of Vc is input into the input terminal Tc1for the transistor32adjusted to the depression state. Further, in this case, in the chemical sensor5, a voltage of zero volts (V) is input into the external terminal Ts connected to the source S of the transistor32, and a voltage as a drain voltage Vd (e.g., 0.1 V) is input into the external terminal Td connected to the drain D of the transistor32. Further, in this case, in the chemical sensor5, the external terminals Tb2, Tb3, Tc2are brought into an opened state.

Hereby, a back bias is applied to the transistor32. Further, the floating gate123of the transistor32is maintained at a positive predetermined potential, for example. Accordingly, the transistor32operates in a state where a back bias is applied to the transistor32. Sine the interface voltage on the sensitive membrane152changes in accordance with the hydrogen-ion concentration of the test sample91, a voltage on which the hydrogen-ion concentration of the test sample91is reflected is applied to the P-well region121of the transistor32. As a result, a drain current on which the hydrogen-ion concentration of the test sample91is reflected flows through the transistor32due to the substrate bias effect. Hereby, the chemical sensor5can detect the hydrogen-ion concentration of the test sample91.

(Adjustment Operation of Threshold Voltage of Transistor in Chemical Sensor)

With reference toFIG. 26, the following describes an adjustment operation to increase the threshold voltage of the transistor32provided in the chemical sensor5by injecting electric charges into the floating gate123.

As illustrated inFIG. 26, in the adjustment operation to increase the threshold voltage of the transistor32, in the chemical sensor5, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the input terminal Tc1. The pulse voltage is applied to the first floating portion133via the intermediate wiring lines26a,26b, the plugs22a,22c,22d, the first potential controlling portion41, the P-well region121, the floating gate123, and the second connecting portion17b. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor5, the same pulse voltage as the input terminal Tc1is applied to the external terminal Ts connected to the source S of the transistor32, the external terminal Td connected to the drain D of the transistor32, and the external terminal Tb2. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor5, a pulse voltage of which the voltage value of which is inverted from 0 V to −Vpp is input into the external terminal Tc2of the first electric-charge flow portion13, and a voltage of 0 V is input into the external terminal Tb3of the first electric-charge flow portion13(the external terminal Tb3is connected to the ground, for example).

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 26, electrons e− are injected into the floating gate123from the P-well region131of the first electric-charge flow portion13through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor32increases. The threshold voltage of the transistor32is checked by a method similar to the detection operation of the chemical sensor5as described with reference toFIG. 25. The injection of the electrons e− into the floating gate123and the check of the threshold voltage of the transistor32are repeatedly executed until the transistor32reaches its desired threshold voltage. When the transistor32reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor32is finished. Note that, in a case where electric charges are injected into the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

With reference toFIG. 27, the following describes an adjustment operation to decrease the threshold voltage of the transistor32provided in the chemical sensor5by discharging electric charges from the floating gate123.

As illustrated inFIG. 27, in the adjustment operation to decrease the threshold voltage of the transistor32, in the chemical sensor5, a pulse voltage of which the voltage value is inverted from 0 V to −Vpp is input into the input terminal Tc1. The pulse voltage is applied to the first floating portion133via the intermediate wiring lines26a,26b, the plugs22a,22c,22d, first potential controlling portion41, the P-well region121, the floating gate123, and the second connecting portion17b. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor5, the same pulse voltage as the input terminal Tc1is applied to the external terminal Ts connected to the source S of the transistor32and the external terminal Td connected to the drain D. Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor5, a voltage of 0 V is input into the external terminal Tb2(the external terminal Tb2is connected to the ground, for example). Further, in the case of the adjustment operation on the threshold voltage, in the chemical sensor5, a pulse voltage of which the voltage value is inverted from 0 V to +Vpp is input into the external terminal Tc2and the external terminal Tb3of the first electric-charge flow portion13.

Hereby, FN tunneling occurs in the first insulating film163, and as indicated by straight arrows inFIG. 27, electrons e− are discharged from the floating gate123to the P-well region131through the second connecting portion17b, the first floating portion133, and the first insulating film163. Hereby, the threshold voltage of the transistor32decreases. The threshold voltage of the transistor32is checked by a method similar to the detection operation of the chemical sensor5as described with reference toFIG. 25. The discharge of the electrons e− from the floating gate123and the check of the threshold voltage of the transistor32are repeatedly executed until the transistor32reaches its desired threshold voltage. When the transistor32reaches the desired threshold voltage, the voltage application of the pulse voltage and so on is finished. Hereby, the adjustment operation on the threshold voltage of the transistor32is finished. Note that, in a case where electric charges are discharged from the floating gate123by the operation, the capacitance in the floating gate123should be larger than the capacitance in the first floating portion133. This configuration has an effect to efficiently transmit a voltage input into the floating gate123to the first floating portion133, in comparison with a configuration in which the capacitance in the floating gate123is smaller than the capacitance in the first floating portion133. As a result, the efficiency of the adjustment of the threshold voltage can be improved.

As described above, the chemical sensors according to the present embodiment can control the threshold voltage of the transistor and achieve improvement in the electric-charge retention characteristic.

Sixth Embodiment

The following describes a detection apparatus according to a sixth embodiment of the present invention with reference toFIGS. 28 and 29. First, a schematic configuration of a detection apparatus6according to the present embodiment will be described with reference toFIG. 28.

<Configuration and Equivalent Circuit of Detecting Circuit>

As illustrated inFIG. 28, the detection apparatus6according to the present embodiment provides a first chemical sensor1H, a second chemical sensor1L (examples of two chemical sensors), an electrode structure63having a metal electrode as a pseudo-reference electrode631, and a differential amplification circuit (an example of a detecting circuit)61configured to detect an output difference between the first chemical sensor1H and the second chemical sensor1L with respect to the pseudo-reference electrode631. The pseudo-reference electrode631is made of platinum or gold, for example. A sensitive portion15H provided in the first chemical sensor1H (one example of one of the two chemical sensors1) has a first sensibility. A sensitive portion15L provided in the second chemical sensor1L (one example of the other one of the two chemical sensors) has a second sensibility. The sensitive portion15H provided in the first chemical sensor1H, the sensitive portion15L provided in the second chemical sensor1L, and the pseudo-reference electrode631are provided to be immersible in a test sample (not illustrated) at the same time.

The first chemical sensor1H and the second chemical sensor1L have the same configuration. In the present embodiment, the first chemical sensor1H and the second chemical sensor1L have a configuration similar to that of the chemical sensor1according to the first embodiment, for example. However, the first chemical sensor1H and the second chemical sensor1L may have a structure similar to any of the structures of the chemical sensors according to the second embodiment to the fifth embodiment.

As illustrated inFIG. 28, the first chemical sensor1H provides the sensitive portion15H placed on a semiconductor substrate (not illustrated) and having a sensitive membrane152H sensitive to a chemical substance. The sensitive portion15H further has a conductive portion151H connected to a first potential controlling portion11H. The sensitive membrane152H is formed on a first surface side of the conductive portion151H. Here, the first surface side of the conductive portion151H has the same meaning as the first surface side of the conductive portion151in the first embodiment. The first chemical sensor1H provides a transistor12H having a floating gate123H and a gate insulating film162H formed to make contact with the floating gate123H. The first chemical sensor1H provides the first potential controlling portion11H configured to control the potential of the floating gate123H in accordance with a voltage applied to the sensitive membrane152H. Further, the first chemical sensor1H provides a first electric-charge flow portion13H through which electric charges are flowable to and from the floating gate123H in accordance with an applied voltage. The first electric-charge flow portion13H has part formed in the semiconductor substrate.

The first potential controlling portion11H has a P-well region111H (one example of the first impurity diffused region) formed in the semiconductor substrate and connected to the sensitive portion15H via a wiring line66H. Similarly to the wiring line in the first embodiment, the wiring line66H has a structure in which a plurality of plugs and a plurality of intermediate wiring lines are put together, for example. The first potential controlling portion11H has a control insulating film161H placed on the first surface side of the semiconductor substrate and formed in the semiconductor substrate to make contact with the P-well region111H. The first potential controlling portion11H has a control floating portion113H placed on the first surface side and placed at a position where the control floating portion113H faces the P-well region111H across the control insulating film161H. The control floating portion113H is conductive with the floating gate123H. A capacitance of the sensitive membrane152H is larger than a series combined capacitance of respective capacitances in the gate insulating film162H and in the control insulating film161H.

The first potential controlling portion11H has a highly-concentrated impurity diffused region112H containing impurities at a concentration higher than that in the P-well region111H and formed in the P-well region111H. The P-well region111H has a structure similar to that of the P-well region111in the first embodiment, and the highly-concentrated impurity diffused region112H has a structure similar to that of the highly-concentrated impurity diffused region112in the first embodiment. The P-well region111H is electrically connected to the sensitive membrane152H of the sensitive portion15H via the highly-concentrated impurity diffused region112H and the wiring line66H.

Although not illustrated herein, the first chemical sensor1H provides a deep N-well region having a structure similar to that of the deep N-well region114in the first embodiment, an N-well region having a structure similar to that of the N-well region115in the first embodiment, and an N+ region having a structure similar to that of the N+ region116in the first embodiment. On this account, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the P-well region111H and the N+ region, similarly to the first embodiment. Further, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the N+ region and a P-type region of the semiconductor substrate where well regions are are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the first chemical sensor1H, similarly to the first embodiment.

As illustrated inFIG. 28, the first electric-charge flow portion13H has a P-well region131H formed in the semiconductor substrate and having the P-type, and a highly-concentrated impurity diffused region132H formed in the P-well region131H and containing impurities at a concentration higher than that in the P-well region131H. A voltage is applied to the highly-concentrated impurity diffused region132H. The first electric-charge flow portion13H has a first insulating film163H formed to make contact with the P-well region131H, and a first floating portion133H making contact with the first insulating film163H and formed on the first surface side of the semiconductor substrate in an electrically floating state. The first floating portion133H is connected to the floating gate123H.

The P-well region131H has a structure similar to that of the P-well region131in the first embodiment, and the highly-concentrated impurity diffused region132H has a structure similar to that of the highly-concentrated impurity diffused region132in the first embodiment. The first insulating film163H has a structure similar to that of the first insulating film163in the first embodiment, and the first floating portion133H has a structure similar to that of the first floating portion133in the first embodiment.

Although not illustrated herein, the first chemical sensor1H provides a deep N-well region having a structure similar to that of the deep N-well region134in the first embodiment, an N-well region having a structure similar to that of the N-well region135in the first embodiment, and an N+ region having a structure similar to that of the N+ region136in the first embodiment. On this account, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the P-well region131H and the N+ region, similarly to the first embodiment. Further, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the N+ region and the P-type region of the semiconductor substrate where well regions are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the first chemical sensor1H, similarly to the first embodiment.

As illustrated inFIG. 28, the transistor12H has a source S formed in the P-well region121H on either one of the both sides of the floating gate123H and having the N-type, and a drain D formed in the P-well region121H on the other of the both sides of the floating gate123H and having the N-type. The source S and the drain D of the transistor12H are constituted by a highly-concentrated impurity diffused region having an impurity concentration higher than that of the P-well region121H. The transistor12H includes a P+ region122H having the P-type and containing impurities at a concentration higher than that in the P-well region121H. The P+ region122H is formed in the P-well region121H, and a voltage is applicable to the P+ region122H.

Although not illustrated herein, the first chemical sensor1H provides a deep N-well region having a structure similar to that of the deep N-well region124in the first embodiment, an N-well region having a structure similar to that of the N-well region125in the first embodiment, and an N+ region having a structure similar to that of the N+ region126in the first embodiment. On this account, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the P-well region121H and the N+ region, similarly to the first embodiment. Further, in the first chemical sensor1H, a PN junction (not illustrated) is formed between the N+ region and the P-type region of the semiconductor substrate where well regions are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the first chemical sensor1H, similarly to the first embodiment.

As illustrated inFIG. 28, the detection apparatus6provides a constant current source64H connected to the drain D of the transistor12H provided in the first chemical sensor1H. The constant current source64H has an input terminal to which a voltage Vdd is input, and an output terminal connected to the drain D of the transistor12H. Hereby, the constant current source64H can supply a constant current at a predetermined level to the transistor12H.

The detection apparatus6provides a resistance element65H connected to the source S of the transistor12H. The resistance element65H has a first terminal connected to the source S of the transistor12H, and a second terminal connected to a region having a reference potential Vss. The first terminal of the resistance element65H is connected to the differential amplification circuit61. The transistor12H adjusts a current supplied from the constant current source64H in accordance with the potential of the floating gate123H and outputs the current to the resistance element65H. A detection result of the test sample in the sensitive portion15H is reflected on the voltage of the floating gate123H. Accordingly, a voltage drop in the resistance element65H is a voltage on which the detection result of the test sample in the sensitive portion15H is reflected. The first chemical sensor1H outputs the voltage drop caused in the resistance element65H to the differential amplification circuit61as an output voltage Vout1. Hereby, the first chemical sensor1H can input the output voltage Vout1on which the detection result of the test sample in the sensitive portion15H is reflected to the differential amplification circuit61.

As illustrated inFIG. 28, the second chemical sensor1L provides the sensitive portion15L placed on a semiconductor substrate (not shown) and having a sensitive membrane152L sensitive to a chemical substance. The sensitive portion15L further has a conductive portion151L connected to a first potential controlling portion11L. The sensitive membrane152L is formed on a first surface side of the conductive portion151L. Here, the first surface side of the conductive portion151L has the same meaning as the first surface side of the conductive portion151in the first embodiment. The second chemical sensor1L provides a transistor12L including a floating gate123L and a gate having film162L formed to make contact with the floating gate123L. The second chemical sensor1L provides a first potential controlling portion11L configured to control the potential of the floating gate123L in accordance with a voltage applied to the sensitive membrane152L. Further, the second chemical sensor1L provides a first electric-charge flow portion13L through which electric charges are flowable to and from the floating gate123L in accordance with an applied voltage. The first electric-charge flow portion13L has part formed in the semiconductor substrate.

The first potential controlling portion11L has a P-well region111L (one example of the first impurity diffused region) formed in the semiconductor substrate and connected to the sensitive portion15L via a wiring line66L. Similarly to the wiring line in the first embodiment, the wiring line66L has a structure in which a plurality of plugs and a plurality of intermediate wiring lines are put together, for example. The first potential controlling portion11L has a control insulating film161L placed on the first surface side of the semiconductor substrate and formed in the semiconductor substrate to make contact with the P-well region111L. The first potential controlling portion11L has a control floating portion113L placed on the first surface side and placed at a position where the control floating portion113L faces the P-well region111L across the control insulating film161L. The control floating portion113L is conductive with the floating gate123L. A capacitance of the sensitive membrane152L is larger than a series combined capacitance of respective capacitances in the gate insulating film162L and in the control insulating film161L.

The first potential controlling portion11L has a highly-concentrated impurity diffused region112L containing impurities at a concentration higher than that in the P-well region111L and formed in the P-well region111L. The P-well region111L has a structure similar to that of the P-well region111in the first embodiment, and the highly-concentrated impurity diffused region112L has a structure similar to that of the highly-concentrated impurity diffused region112in the first embodiment. The P-well region111L is electrically connected to the sensitive membrane152L of the sensitive portion15L via the highly-concentrated impurity diffused region112L and the wiring line66L.

Although not illustrated herein, the second chemical sensor1L provides a deep N-well region having a structure similar to that of the deep N-well region114in the first embodiment, an N-well region having a structure similar to that of the N-well region115in the first embodiment, and an N+ region having a structure similar to that of the N+ region116in the first embodiment. On this account, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the P-well region111L and the N+ region, similarly to the first embodiment. Further, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the N+ region and a P-type region of the semiconductor substrate where well regions are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the second chemical sensor1L, similarly to the first embodiment.

As illustrated inFIG. 28, the first electric-charge flow portion13L has a P-well region131L formed in the semiconductor substrate and having the P-type, and a highly-concentrated impurity diffused region132L formed in the P-well region131L and containing impurities at a concentration higher than that in the P-well region131L. A voltage is applied to the highly-concentrated impurity diffused region132L. The first electric-charge flow portion13L has a first insulating film163L formed to make contact with the P-well region131L, and a first floating portion133L making contact with the first insulating film163L and formed on the first surface side of the semiconductor substrate in an electrically floating state. The first floating portion133L is connected to the floating gate123L.

The P-well region131L has a structure similar to that of the P-well region131in the first embodiment, and the highly-concentrated impurity diffused region132L has a structure similar to that of the highly-concentrated impurity diffused region132in the first embodiment. The first insulating film163L has a structure similar to that of the first insulating film163in the first embodiment, and the first floating portion133L has a structure similar to that of the first floating portion133in the first embodiment.

Although not illustrated herein, the second chemical sensor1L has a deep N-well region having a structure similar to that of the deep N-well region134in the first embodiment, an N-well region having a structure similar to that of the N-well region135in the first embodiment, and an N+ region having a structure similar to that of the N+ region136in the first embodiment. On this account, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the P-well region131L and the N+ region, similarly to the first embodiment. Further, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the N+ region and the P-type region of the semiconductor substrate where well regions are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the second chemical sensor1L, similarly to the first embodiment.

As illustrated inFIG. 28, the transistor12L has a source S formed in the P-well region121L on one of the both sides of the floating gate123L and having the N-type, and a drain D formed in the P-well region121L on the other of the both sides of the floating gate123L and having the N-type. The source S and the drain D of the transistor12L are constituted by a highly-concentrated impurity diffused region having an impurity concentration higher than that of the P-well region121L. The transistor12L has a P+ region122L having the P-type and containing impurities at a concentration higher than that in the P-well region121L. The P+ region122L is formed in the P-well region121L, and a voltage is applicable to the P+ region122L.

Although not illustrated herein, the second chemical sensor1L provides a deep N-well region having a structure similar to that of the deep N-well region124in the first embodiment, an N-well region having a structure similar to that of the N-well region125in the first embodiment, and an N+ region having a structure similar to that of the N+ region126in the first embodiment. On this account, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the P-well region121L and the N+ region, similarly to the first embodiment. Further, in the second chemical sensor1L, a PN junction (not illustrated) is formed between the N+ region and the P-type region of the semiconductor substrate where well regions are not formed, similarly to the first embodiment. Further, an external terminal (not illustrated) is connected to the N+ region of the second chemical sensor1L, similarly to the first embodiment.

As illustrated inFIG. 28, the detection apparatus6provides a constant current source64L connected to the drain D of the transistor12L provided in the second chemical sensor1L. The constant current source64L has an input terminal to which a voltage Vdd is input, and an output terminal connected to the drain D of the transistor12L. Hereby, the constant current source64L can supply a constant current at a predetermined level to the transistor12L.

The detection apparatus6provides a resistance element65L connected to the source S of the transistor12L. The resistance element65L has a first terminal connected to the source S of the transistor12L, and a second terminal connected to a region having a reference potential Vss. The first terminal of the resistance element65L is connected to the differential amplification circuit61. The transistor12L adjusts a current supplied from the constant current source64L in accordance with the potential of the floating gate123L and outputs the current to the resistance element65L. A detection result of the test sample in the sensitive portion15L is reflected on the voltage of the floating gate123L. Accordingly, a voltage drop in the resistance element65L is a voltage on which the detection result of the test sample in the sensitive portion15L is reflected. The second chemical sensor1L outputs the voltage drop caused in the resistance element65L to the differential amplification circuit61as an output voltage Vout2. Hereby, the second chemical sensor1L can input the output voltage Vout2on which the detection result of the test sample in the sensitive portion15L is reflected to the differential amplification circuit61.

The detection apparatus6provides a switch62connected to the pseudo-reference electrode631of the electrode structure63. The switch62has a first terminal into which the voltage Vdd is input, and a second terminal connected to the pseudo-reference electrode631. The detection apparatus6is configured to appropriately change the switch62between an ON state and an OFF state such that the voltage Vdd is input or not input into the pseudo-reference electrode631via the switch62.

In the meantime, generally, an Ag/AgCl electrode the potential of which is stable to liquid is used, though its size is large for the reference electrode. In this case, the reference electrode has a structure in which the Ag/AgCl electrode is immersed in an inner liquid KCl. A metal electrode made of gold, platinum, or the like can be deposited on a semiconductor substrate by a method such as vapor deposition, and therefore, the first chemical sensor1H and the second chemical sensor1L can be reduced in size. However, the potential of the metal electrode made of gold, platinum, or the like is unstable to liquid, and therefore, the metal electrode is unsuitable for a reference electrode of a chemical sensor. Although details are described later, the detection apparatus6according to the present embodiment calculates, for example, an ion concentration of a test sample by the differential amplification circuit61based on the difference between the output voltage Vout1of the first chemical sensor1H and the output voltage Vout2of the second chemical sensor1L. On this account, the detection apparatus6can be hardly affected by instability of the potential of the pseudo-reference electrode631to liquid. Accordingly, the detection apparatus6can use the pseudo-reference electrode631made of metal such as gold or platinum. Hereby, the detection apparatus6can be reduced in size and does not have a problem of elution in the inner liquid, the problem being seen in the Ag/AgCl electrode.

The sensitive portion15H is formed to have a sensitivity higher than that of the sensitive portion15L. In the detection apparatus6, for example, the sensitive membrane152H and the sensitive membrane152L are formed by use of different materials or by changing the composition ratio of a material, and therefore, the sensitive portion15H and the sensitive portion15L have different sensitivities. In the present embodiment, the sensitive membrane152H provided in the sensitive portion15H is made of Ta2O5, for example, and the sensitive membrane152L provided in the sensitive portion15L is made of Al2O3, for example.

The sensitive portion15H provided in the first chemical sensor1H, the sensitive portion15L provided in the second chemical sensor1L, and the pseudo-reference electrode631are provided to be immersible in a test sample (not shown) at the same time. Because of this, the same voltage is applied to the sensitive portion15H and the sensitive portion15L via the test sample from the pseudo-reference electrode631. Since the sensitive portion15H is formed to have a sensitivity higher than that of the sensitive portion15L, even if the same voltage is applied to the sensitive portion15H and the sensitive portion15L, a voltage higher than a voltage to the first potential controlling portion11L is applied to the first potential controlling portion11H. Hereby, a voltage higher than a voltage to the floating gate123L of the transistor12L is applied to the floating gate123H of the transistor12H. As a result, the output voltage Vout1and the output voltage Vout2having a voltage level lower than that of the output voltage Vout1are input into the differential amplification circuit61.

The output voltage Vout1includes variations by temperature characteristics in various parts such as the sensitive portion15H and the transistor12H of the first chemical sensor1H, noise such as drift, and so on, as well as information on a measurement target of the test sample. Similarly, the output voltage Vout2includes variations by temperature characteristics in various parts such as the sensitive portion15L and the transistor12L of the second chemical sensor1L, noise such as drift, and so on, as well as the information on the measurement target of the test sample. The differential amplification circuit61calculates the measurement target (e.g., the ion concentration) of the test sample by subtracting the output voltage Vout2from the output voltage Vout1. On this account, respective noises included in the output voltage Vout1and the output voltage Vout2are offset or decreased. Hereby, the detection apparatus6can improve the measurement accuracy of the test sample.

Next will be described the operation of the detection apparatus6according to the present embodiment by taking, as an example, a case where the first chemical sensor1H and the second chemical sensor1L are hydrogen-ion concentration sensors, with reference toFIG. 29as well asFIG. 28.FIG. 29is a flowchart illustrating an example of the operation of the detection apparatus6. Here, a reference electrode is used in each of steps S11to S16(details will be described later) of the operation of the detection apparatus6, and adjustment of the threshold voltage to a standard solution is performed highly precisely. As an example, an Ag/AgCl electrode is used as the reference electrode. Further, in step S17(details will be described later) of the operation of the detection apparatus6, a pseudo-reference electrode is used.

When the operation of the detection apparatus6is started, the detection apparatus6adjusts the threshold voltage of the transistor12H in step S11by injecting electric charges into the floating gate123H of the first chemical sensor1H or discharging electric charges from the floating gate123H, and the detection apparatus6shifts to the process of step S12. The adjustment operation on the threshold voltage of the transistor12H provided in the first chemical sensor1H is similar to the adjustment operation on the threshold voltage of the transistor12provided in the chemical sensor1according to the first embodiment, and therefore, descriptions of the adjustment operation on the threshold voltage of the transistor12H are omitted.

In step S12, the detection apparatus6detects an output voltage Vout1to a reference sample placed on the sensitive portion15H and shifts to the process of step S13. The detection operation to detect the output voltage Vout1to the reference sample placed on the sensitive portion15H is similar to the detection operation to detect an output in the chemical sensor1according to the first embodiment, and therefore, descriptions of the detection operation to detect the output voltage Vout1are omitted.

In step S13, the detection apparatus6determines whether the value of the output voltage Vout1is a desired output value or not. When the value of the output voltage Vout1is the desired output value (Yes), the detection apparatus6shifts to the process of step S14. In the meantime, when the value of the output voltage Vout1is not the desired output value (No), the detection apparatus6returns to the process of step S11.

In step S14, the detection apparatus6adjusts the threshold voltage of the transistor12L by injecting electric charges into the floating gate123L of the second chemical sensor1L or discharging electric charges from the floating gate123L, and the detection apparatus6shifts to the process of step S15. The adjustment operation on the threshold voltage of the transistor12L provided in the second chemical sensor1L is similar to the adjustment operation on the threshold voltage of the transistor12provided in the chemical sensor1according to the first embodiment, and therefore, descriptions of the adjustment operation on the threshold voltage of the transistor12L are omitted. In the present embodiment, the threshold voltage of the transistor12H provided in the first chemical sensor1H and the threshold voltage of the transistor12L provided in the second chemical sensor1L are adjusted to the same value. Hereby, the detection apparatus6can prevent an offset voltage from occurring in a difference voltage between the output voltage Vout1of the first chemical sensor1H and the output voltage Vout2of the second chemical sensor1L. The offset voltage is caused due to a difference between the threshold voltages of the transistor12H and the threshold voltages of the transistor12L.

In step S15, the detection apparatus6detects an output voltage Vout2to a reference sample placed on the sensitive portion15L and shifts to the process of step S16. The detection operation to detect the output voltage Vout1to the reference sample placed on the sensitive portion15L is similar to the detection operation to detect an output in the chemical sensor1according to the first embodiment, and therefore, descriptions of the detection operation to detect the output voltage Vout2are omitted.

In step S17, the sensitive portion15H, the sensitive portion15L, and the pseudo-reference electrode631are immersed in a test sample, and the detection apparatus6measures the hydrogen-ion concentration of the test sample by the differential amplification circuit61and finishes the operation. At the time when the hydrogen-ion concentration of the test sample is to be measured, the switch62is brought into an ON state, and a voltage Vdd is applied to the test sample via the switch62and the pseudo-reference electrode631. Further, at the time when the hydrogen-ion concentration of the test sample is to be measured, a constant current at a predetermined level is input into the drain D of the transistor12H from the constant current source64H, and a constant current at a predetermined level (the same level as the current output from the constant current source64H) is input into the drain D of the transistor12L from the constant current source64L.

As described above, with the detection apparatus according to the present embodiment, it is possible to control the threshold voltages of respective transistors provided in the first chemical sensor and the second chemical sensor, and it is possible to achieve improvement in the electric-charge retention characteristic. Hereby, effects similar to those of the chemical sensors according to the first embodiment to the fifth embodiment can be achieved. Further, the detection apparatus according to the present embodiment can use a pseudo-reference electrode made of metal such as gold or platinum, and thus, it is possible to achieve a reduction in size.

The present invention is not limited to the above embodiments, and various modifications can be made.

In the chemical sensors according to the first embodiment to the fifth embodiment and their modifications and the first chemical sensor and the second chemical sensor provided in the detection apparatus according to the sixth embodiment, the sensitive portion and the floating gate are placed on the first surface side, that is, the same side of the semiconductor substrate19. However, the present invention is not limited to this. The chemical sensors may be configured such that the sensitive portion is placed on a second surface of the semiconductor substrate, that is, a back-surface side that is a side opposite to the first surface of the semiconductor substrate. That is, the sensitive portion and the floating gate may be placed across the semiconductor substrate. In this case, the sensitive portion is electrically connected to the first potential controlling portion from the second surface side (the back-surface side) of the semiconductor substrate. The chemical sensors can have various connection modes to connect the sensitive portion to the first potential controlling portion. For example, the sensitive portion placed on the second surface side (the back-surface side) of the semiconductor substrate19may be electrically connected to a predetermined well region formed in the semiconductor substrate via a plug embedded in a hole formed in the semiconductor substrate or an intermediate wiring line connected to the plug. Further, for example, the sensitive portion placed on the second surface side (the back-surface side) of the semiconductor substrate19may be placed in a hole formed in the semiconductor substrate and directly electrically connected to a predetermined well region exposed in the hole.

The chemical sensors4,5according to the fourth embodiment and the fifth embodiment provide the first electric-charge flow portion13as an electric-charge flow portion. However, the present invention is not limited to this. For example, similarly to the chemical sensor3according to the third embodiment, the chemical sensors4,5may provide the second electric-charge flow portion14. The chemical sensors4,5having this configuration can achieve the same effect as the chemical sensor3.

The conductivity types of various regions such as the well regions and the semiconductor substrates in the chemical sensors according to the first embodiment to the fifth embodiment and their modifications and in the first chemical sensor and the second chemical sensor provided in the detection apparatus according to the sixth embodiment are just examples. The P-type and the N-type may be reversed, or different combinations of the P-type and the N-type can be employed appropriately.

The technical scope of the present invention is not limited to the exemplary embodiments illustrated and described herein and covers all embodiments that provide effects equivalent to those intended by the present invention. Further, the technical scope of the present invention is not limited to combinations of features of the invention defined by Claims but can be defined by any desired combination of specific features among the features disclosed herein.

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