Infrared ray sensing element and method of producing the same

An infrared ray sensing element, includes: 1) a semiconductor substrate; 2) an infrared ray receiver disposed above the semiconductor substrate in such a manner as to be isolated from the semiconductor substrate, the infrared ray receiver being configured to receive an infrared ray; and 3) a beam configured to support the infrared ray receiver to the semiconductor substrate and include a thermopile configured to sense a temperature increase of the infrared ray receiver, wherein one of the following has a cross sectional shape that includes at least one protruding part: i) the beam, and ii) the thermopile.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an infrared ray sensing element provided with a thermopile, and a method of producing the infrared ray sensing element.

2. Description of the Related Art

Japanese Patent Application Laid-Open No. 2000-111396 (=JP2000111396) discloses a conventional infrared sensing element and its manufacturing method. Specifically, JP2000111396 discloses a thermopile-type infrared ray sensing element. There is provided an infrared ray absorber on a center upper face of a substrate and a beam is used for supporting the infrared ray absorber to the substrate, thereby the infrared ray absorber is heat-isolated from the substrate.

FIG. 1AandFIG. 1Bshow a structure of a beam of the conventional infrared ray sensing element like that according to JP2000111396, specifically,FIG. 1Ashows a perspective view of a beam120andFIG. 1Bshows an upper face of the beam120. InFIG. 1, the beam120has two polysilicons121,122each serving as thermopile, specifically, an N-type polysilicon121and a P-type polysilicon122are formed in parallel to each other in a longitudinal direction of the beam120. The N-type polysilicon121and P-type polysilicon122are covered with a protective film123made of silicon oxidized film and the like, with the beam120shaped rectangular.

When an external bending force is applied around a first center line b1inFIG. 1B, the beam120is deformed as shown inFIG. 2Ataken in the direction II-A inFIG. 1B. In this case, most of the above bending is caused to the protective film123since the N-type polysilicon and P-type polysilicon121,122(thermopiles) are harder than the silicon oxidized film of the protective film123. Meanwhile, when an external torsional force is applied around a second center line b2inFIG. 1B, a sheer stress caused by the torsional force deforms the beam120as shown inFIG. 2Btaken in the direction II-B inFIG. 1B. When the beam120has a bent portion between the substrate and the infrared ray absorber, displacement of the beam120in a height direction (Z-direction) is increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an infrared ray sensing element having an increased mechanical strength against bending stress or torsional stress, and a method of producing the infrared ray sensing element.

According to a first aspect of the present invention, there is provided an infrared ray sensing element, comprising: 1) a semiconductor substrate; 2) an infrared ray receiver disposed above the semiconductor substrate in such a manner as to be isolated from the semiconductor substrate, the infrared ray receiver being configured to receive an infrared ray; and 3) a beam configured to support the infrared ray receiver to the semiconductor substrate and include a thermopile configured to sense a temperature increase of the infrared ray receiver, wherein one of the following has a cross sectional shape that includes at least one protruding part: i) the beam, and ii) the thermopile.

According to a second aspect of the present invention, there is provided a method of producing an infrared ray sensing element, comprising: 1) a first step for forming a sacrificial layer on a semiconductor substrate; 2) a second step for forming on the sacrificial layer an infrared ray receiver configured to receive an infrared ray; 3) a third step for forming a thermopile configured to sense a temperature increase of the infrared ray receiver; 4) a fourth step for forming an insulative film configured to cover the thermopile and to serve as a protective film of the thermopile; 5) a fifth step for forming, by selectively removing the insulative film, a beam configured to support the infrared ray receiver to the semiconductor substrate and include the thermopile, the beam having a cross sectional shape that includes at least one protruding part; 6) a sixth step for forming an etching hole through the insulative film; and 7) a seventh step including the following sub-steps: i) a first sub-step for selectively removing the semiconductor substrate and the sacrificial layer via the etching hole, ii) a second sub-step for forming a void between the semiconductor substrate and the infrared ray receiver, and iii) a third sub-step for isolating the semiconductor substrate from the infrared ray receiver.

According to a third aspect of the present invention, there is provided a method of producing an infrared ray sensing element, comprising: 1) a first step for forming a sacrificial layer on a semiconductor substrate; 2) a second step for forming on the sacrificial layer an infrared ray receiver configured to receive an infrared ray; 3) a third step for forming a first resistor film configured to serve as a base part of a thermopile configured to sense a temperature increase of the infrared ray receiver; 4) a fourth step for forming a first insulative film configured to cover the first resistor film and to serve as a protective film of the thermopile; 5) a fifth step for forming a second resistor film on the first resistor film by selectively removing the first insulative film on the first resistor film, the second resistor film being configured to serve as a protruding part of the thermopile; 6) a sixth step including the following sub-steps: i) a first sub-step for forming a second insulative film configured to cover the second resistor film and to serve as the protective film of the thermopile, and ii) a second sub-step for forming a beam configured to support the infrared ray receiver to the semiconductor substrate and include the thermopile, the beam having a cross sectional shape having at least one of the protruding parts; 7) a seventh step for forming an etching hole through the protective film; and 8) an eighth step including the following sub-steps: i) a first sub-step for selectively removing the semiconductor substrate and the sacrificial layer via the etching hole, ii) a second sub-step for forming a void between the semiconductor substrate and the infrared ray receiver, and iii) a third sub-step for isolating the semiconductor substrate from the infrared ray receiver.

The other object(s) and feature(s) of the present invention will become understood from the following description with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, various embodiments of the present invention will be described in detail with reference to the accompanying drawings.

For ease of understanding, the following description will contain various directional terms, such as left, right, upper, lower, forward, rearward and the like. However, such terms are to be understood with respect to only a drawing or drawings on which the corresponding part of element is illustrated.

FIG. 3is a front view of a structure of an infrared ray sensing element10, according to a first embodiment of the present invention. The infrared ray sensing element10inFIG. 3includes i) a substrate (otherwise referred to as “semiconductor substrate”)1made of silicon and the like, ii) an infrared ray absorber (otherwise referred to as “infrared ray receiver”)2formed above the substrate1and is made of a membrane for absorbing and sensing an infrared ray, and iii) an L-shaped beam3.

The substrate1and the infrared ray absorber2are heat-isolated by, for example, a pyramid void55(or a rectangular cone void, to be described afterward referring toFIG. 7E) which is a heat-isolating region formed on an upper face of the substrate1. According to the first embodiment, the two beams3are formed along an outer periphery of the infrared ray absorber2, thus connecting the substrate1with the infrared ray absorber2and supporting the infrared ray absorber2to the substrate1in such a manner that the infrared ray absorber2is isolated from the substrate1. The beam3includes a pair of polysilicons serving as a thermopile covered with a protective film which is made of silicon oxidized film and the like and serves as a main member of the beam3. The thermopile senses a temperature increase of the infrared ray absorber2.

As shown inFIG. 3, the infrared ray absorber2is far larger in area and volume than the beam3supporting the infrared ray absorber2. Therefore, the infrared ray absorber2works on the beam3like a weight. InFIG. 4which is a perspective view of structure of the infrared ray sensing element10, the infrared ray absorber2has a gravitational center x positioned in the center of the infrared ray absorber2. Therefore, with an inertia caused by an acceleration, vibration and the like which are applied to the beam3in Z-direction inFIG. 4, a bending moment and a torsional torque operate from the infrared ray absorber2to the beam3, causing a stress, thus deforming the beam3in the Z-direction. In this case, amount of the deformation depends on rigidity of the beam3. Therefore, the smaller the deformation is, the higher a resonance frequency is, thus increasing rigidity of the beam3.

FIG. 5is a perspective view of the structure of the beam3inFIG. 3. InFIG. 5, the beam3includes a pair of an N-type polysilicon30and a P-type polysilicon31each serving as a thermopile and disposed in parallel. The N-type polysilicon30and the P-type polysilicon31are covered with a protective film32made of silicon oxidized film. As shown inFIG. 5, the beam3has a structure where the protective film32has a base part32a(bottom face part) and a protruding part32b(convex part) which protrudes in the Z (height) direction. The protruding part32bhas a height h2higher than a height h1of the base part32a(h1<h2). In this case, however, the cross section of the beam3according to the first embodiment of the present invention is as large as the cross section of the beam120inFIG. 1Aaccording to the related art.

A Z-direction displacement inFIG. 4may apply the bending moment to the beam3having the cross section inFIG. 6A. For the following reason, the deformation amount of the beam3inFIG. 6Ais smaller than that according to the related art inFIG. 1Ashowing the rectangular cross sectional structure of the beam120: Reason) InFIG. 5(showing the L-shaped beam3) andFIG. 6A, the height h2of the protruding part32bis higher than the height h1of the base part32a. The above sums up that the bending rigidity according to the first embodiment of the present invention is increased.

Meanwhile, when the Z-direction displacement relative to the infrared ray absorber2applies the torsional torque to the beam3, the protruding part32bhaving the height h2higher than the height h1of the base part32ais so deformed as to incline, as shown in a cross sectional view of the beam3inFIG. 6B. With this, torsion of the entirety of the beam3can be suppressed, thus preventing torsion of the base part32aof the beam3, resulting in smaller Z-direction displacement amount than the related art. Moreover, the higher the height h2of the protruding part32bis, the smaller the beam3's deformation amount caused by the torsion, thus increasing rigidity of the beam3.

Furthermore, disposing the protruding part32bon a side facing (or near to) the gravitational center x of the infrared ray absorber2can prevent a possible torsional stress or bending stress to the base part32aof the beam3.

As set forth above, the beam3having the structure according to the first embodiment of the present invention has higher rigidity of the beam3due to the protruding part32b, than the beam120according to the related art having the rectangular cross sectional structure as shown inFIG. 1A. As a result, without varying the cross section of the beam3, in other words, by keeping sensitivity of the infrared ray sensing element10, the beam3can be increased in rigidity. With this, disposing the infrared ray sensing element10according to the first embodiment in a vibrating object, for example, a vehicle and the like can decrease a possible damage to the infrared ray sensing element10, resulting in increased reliability.

The higher the height h2of the protruding part32b, the better for increased reliability of the beam3. Specifically, with a width w of the beam3, and a height h (=h1+h2) from a bottom face of the base part32ato a summit of the protruding part32b, meeting h>w can further increase the mechanical strengthen the beam3.

With the infrared ray sensing element10according to the first embodiment, the two bended beams3are shown inFIG. 3. However, the number of beams3is not specifically limited. Moreover, the two thermopiles are shown according to the first embodiment, that is, the N-type polysilicon30and the P-type polysilicon31. However, the number of thermopiles is not specifically limited.

Then, referring to cross sectional views showing production steps inFIG. 7AtoFIG. 7E, a method of producing the infrared ray sensing element10having the beam3inFIG. 5is to be set forth. Each of the cross sections of the elements inFIG. 7AtoFIG. 7Eis taken along the lines VII-VII inFIG. 3.

First, a sacrificial layer50for polysilicon etching is formed on a main surface of the substrate1made of silicon and the like, and an etching stopper51for determining a range of the void55for heat-isolating the infrared ray absorber2from the substrate1is formed.

Then, on the sacrificial layer50and the etching stopper51, a nitride film52for serving afterward as the infrared ray absorber2is formed.

On the nitride film52, the N-type polysilicon30and P-type polysilicon31for serving afterward as a resistor film of the thermopile are formed by patterning.

Then, a silicon oxidized film53made of insulative film and serving afterward as the protective film32is deposited in such a manner as to cover the thermopiles, that is, the N-type polysilicon30and P-type polysilicon31. Then, an aluminum wiring54is formed for connecting the N-type polysilicon30and the P-type polysilicon31in series.

Furthermore, the silicon oxidized film53corresponding to a part serving afterward as the beam3on the N-type polysilicon30and P-type polysilicon31is selectively removed by etching, to thereby form the protective film32corresponding to the base part32aof the beam3.

Then, the silicon oxidized film53is selectively removed by etching, to thereby form a slit56(otherwise referred to as “etching hole”) reaching the nitride film52. From the nitride film52as the infrared ray absorber2, the slit56isolates the silicon oxidized film53as the protective film32of the beam3. With this, the beam3is thus formed having the cross sectional structure where the protruding part32bis disposed on the base part32a.

Finally, via the slit56formed by the sixth step inFIG. 7D, a crystal anisotropy etching is implemented using an etching solution such as hydrazine, Through the crystal anisotropy etching, the nitride film52, the sacrificial layer50and the substrate1are removed, to thereby form the void55, thus completing the infrared ray sensing element10inFIG. 3.

Through the above production steps, the L-shaped beam3including the thermopiles (the N-type polysilicon30and the P-type polysilicon31) can be easily formed on the infrared ray sensing element10.

FIG. 8is a front view showing the structure of the infrared ray sensing element10, according to a second embodiment of the present invention. As described above according to the first embodiment, the two beams3are formed along the outer periphery of the infrared ray absorber2. Meanwhile, four beams60are respectively formed on four sides opposing orthogonally to each other on the outer periphery where the substrate1opposes the infrared ray absorber2, to thereby connect the substrate1with the infrared ray absorber2, according to the second embodiment.

The four beams60supporting the infrared ray absorber2to the substrate1can prevent the torsion from being applied to the beam60even when the infrared ray absorber2is vibrated in the Z-direction. Therefore, with the above structure according to the second embodiment, a protruding part60bcan be formed anywhere on a base part60aof the beam60. For example, as shown in the cross sectional view inFIG. 9, the protruding part60bmay be disposed in any of the following locations:

i) in the center of the base part60acovering an N-type polysilicon70and a P-type polysilicon71(thermopiles) of the beam60(FIG. 9A),

ii) at both ends in the X-direction (shorter direction) of the base part60a(FIG. 9B), and

iii) in the center on the base part60ain such a manner as to have a step (FIG. 9C).

According to the second embodiment showing the beam60having the above structure, when the bending stress or torsional stress is applied to the infrared ray sensing element10, the protruding part60bdeformed like the protruding part32baccording to the first embodiment can decrease the stress applied to the base part60aof the beam60. With this, deformation of the entirety of the beam60can be decreased according to the second embodiment, thereby bringing about substantially the same effect as that brought about according to the first embodiment.

FIG. 10is a front view showing the structure of the infrared ray sensing element10, according to a third embodiment of the present invention.FIG. 11AtoFIG. 11Cshow a structure of a beam80inFIG. 10.FIG. 11A is a front view.FIG. 11B andFIG. 11Care cross sectional views taken along the respective lines XI-B-XI-B and XI-C-XI-C inFIG. 11A. According to the third embodiment inFIG. 10andFIG. 11AtoFIG. 11C, the polysilicon of thermopile in the beam80for supporting the infrared ray absorber2to the substrate1is shaped into an alphabetical L in cross section. A protective film for covering the thermopile has a rectangular cross sectional shape, like the related art inFIG. 1.

Specifically, the thermopiles include an N-type polysilicon81(including a base part81aand a protruding part81b) and a P-type polysilicon82(including a base part82aand a protruding part82b), and a protective film83for covering the thermopiles is made of silicon oxidized film. The polysilicon of the N-type polysilicon81and P-type polysilicon82is harder than the silicon oxidized film of the protective film83, specifically, greater in Young's modulus by three times or more. Therefore, according to the third embodiment, instead of increasing the mechanical strength of the protective film83, the N-type polysilicon81and P-type polysilicon82of the hard thermopile is increased in mechanical strength against bending stress and torsional stress.

As shown inFIG. 11BandFIG. 11C, the N-type polysilicon81and P-type polysilicon82each have an L-shaped cross section. InFIG. 11B, two (left and right) of three cross sections have the protruding part81band protruding part82bformed on a side facing (or near to) the gravitational center× of the infrared ray absorber2, thus increasing rigidity of the beam80.

With this, forming the N-type polysilicon81and P-type polysilicon82(which are harder than the silicon oxidized film of the protective film83) shaped into alphabetical L so as to have the respective protruding parts81b,82bcan increase height of the N-type polysilicon81and P-type polysilicon82, thereby suppressing deformation caused by bending of the beam80, resulting in increased mechanical strengthening of the entire beam80. Moreover, most of sheer stresses applied to the beam80can be absorbed by the N-type polysilicon81and P-type polysilicon82, decreasing deformation of the beam80. Thereby, the beam80can be increased in mechanical strength against torsion.

Preferably, the higher the protruding parts81b,82b, the higher the rigidity of the N-type polysilicon81and P-type polysilicon82. Therefore, a width W of each of the N-type polysilicon81and P-type polysilicon82and a height H of each of the N-type polysilicon81and P-type polysilicon82preferably meet H>W, thus further increasing rigidity of the beam80.

Moreover, according to the third embodiment, the parallel disposition of the L-shaped N-type polysilicon81with the L-shaped P-type polysilicon82is not limited to the one shown inFIG. 11BandFIG. 11C. For example, the above parallel disposition may be those shown inFIG. 12AtoFIG. 12D. Moreover, the cross sectional shape of each of the N-type polysilicon81and P-type polysilicon82is not limited to the alphabetical L, for example, an alphabetical T as shown inFIG. 12EtoFIG. 12F, an alphabetical C as shown inFIG. 12GtoFIG. 12H, and a cross as shown inFIG. 12Ieach are allowed. Moreover, as shown inFIG. 12J, both polysilicons of the thermopiles may be rectangular like the two polysilicons121,122according to the related art inFIG. 1. In this case, however, any one of the polysilicons inFIG. 12Jshould have an aspect ratio (height/width) of 1 or over.

Then, referring to cross sectional views showing production steps inFIG. 13AtoFIG. 13F, a method of producing the infrared ray sensing element10having the beam80inFIG. 10andFIG. 11AtoFIG. 11Cis to be set forth. Each of the cross sections of the elements inFIG. 13AtoFIG. 13Fis taken along the lines XIII-XIII inFIG. 10.

First, a sacrificial layer110for polysilicon etching is formed on a main surface of the substrate1made of silicon and the like, and an etching stopper111for determining a range of an after-described void117(seeFIG. 13F) for heat-isolating the infrared ray absorber2from the substrate1is formed.

Then, on the sacrificial layer110and the etching stopper111, a nitride film112for serving afterward as the infrared ray absorber2is formed.

On the nitride film112, the base part81a(of N-type polysilicon81) and base part82a(of P-type polysilicon82) for serving afterward as a first resistor film of the thermopile are formed by patterning.

Then, a silicon oxidized film113awhich is a first insulative film serving afterward as the protective film83is deposited in such a manner as to cover the base part81a(of N-type polysilicon81) and base part82a(of P-type polysilicon82).

Then, by an anisotropy etching such as an RIE (Reactive Ion Etching) method, the silicon oxidized film113afor serving afterward as the protruding part81band protruding part82b(second resistor films) of the beam80is selectively removed, to thereby form a slit114.

Then, the polysilicon is deposited on substantially an entire face, to thereby embed the polysilicon into the slit114which was formed in the fifth step inFIG. 13C. Then, an etch back is implemented to such an extent that the silicon oxidized film113adeposited in the fifth step inFIG. 13Chas an exposed surface, to thereby remove the polysilicon. With this, the protruding part81band protruding part82bof the polysilicons are formed respectively on an upper face of the N-type polysilicon81and an upper face of the P-type polysilicon82. Then, impurities such as boron or phosphor are added to the protruding part81band protruding part82bof the polysilicons by an ion implantation. With this, the same type protruding parts81bare formed on the N-type polysilicon81, while the same type protruding parts82bare formed on the P-type polysilicon82. Then, a silicon oxidized film113bwhich is a second insulative film serving afterward as the protective film83is deposited in such a manner as to cover the protruding part81a(of N-type polysilicon81) and protruding part82b(of P-type polysilicon82). Then, an aluminum wiring115is formed for connecting the N-type polysilicon81with the P-type polysilicon82.

Then, the silicon oxidized films113a,113bare selectively removed by etching and the like, to thereby form a slit116(otherwise referred to as “etching hole”) reaching the nitride film112. From the nitride film112as the infrared ray absorber2, the slit116isolates the silicon oxidized films113a,113b(=silicon oxidized film113) as the protective film83of the beam80.

Finally, via the slit116formed by the seventh step inFIG. 13E, a crystal anisotropy etching is implemented using an etching solution such as hydrazine. Through the crystal anisotropy etching, the nitride film112, the sacrificial layer110and the substrate1are removed, to thereby form the void117, thus completing the infrared ray sensing element10inFIG. 3.

Through the above production steps, the beam80including the L-shaped thermopiles (N-type polysilicon81and P-type polysilicon82) can be easily formed on the infrared ray sensing element10.

Although the present invention has been described above by reference to three certain embodiments, the present invention is not limited to the three embodiments described above. Modifications and variations of the three embodiments described above will occur to those skilled in the art, in light of the above teachings.

This application is based on a prior Japanese Patent Application No. P2005-330371 (filed on Nov. 15, 2005 in Japan). The entire contents of the Japanese Patent Application No. P2005-330371 from which priority is claimed are incorporated herein by reference, in order to take some protection against translation errors or omitted portions.

The scope of the present invention is defined with reference to the following claims.