Abstract:
Methods for fabricating ion sensitive field effect transistors (ISFETs) with SnO 2  extended gates. A SnO 2  detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO 2  detection film is electrically connected to a conductive wire, and an insulating layer is formed on the surface of the ISFET but part of the SnO 2  detection film and the conductive wire are left exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.

Description:
BACKGROUND  
       [0001]     The invention relates to ion sensitive field effect transistors (ISFETs), and more particularly, to methods for fabricating SnO 2  extended gate ISFETs.  
         [0002]     The ion sensitive field effect transistor (ISFET) was presented by Piet Bergveld in 1970. An ISFET with reference electrode is similar to Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), except that the ISFET has exposed the gate insulator to measure a selected ion concentration in electrolyte. When the pH-ISFET is immersed in an aqueous solution, a surface potential is induced at the surface of the detection membrane of the pH-ISFET. However, the surface potential at the sensing membrane will affect the carrier concentration within the inversion layer of the semiconductor, due to the gate dielectric layer being extremely thin. Thus, the current, which flows through the channel, is adjusted. Furthermore, the surface potential is related to the hydrogen ion activity within the aqueous solution. As the pH values change, different surface potentials are induced at the detection membrane, leading to different channel currents. Thus, the pH-ISFET can be used to detect the pH values of solution.  
         [0003]     Further, the extended gate field effect transistor (EGFET) structure was presented by J. V. D. Spiegel et al, in which the detection film is extended from the gate terminal of the field effect transistor by a conductive line. Thus, only the detection film requires immersion in a testing solution, without the field effect transistor.  
         [0004]     A variety of materials are known to be capable of serving as ISFET detection film, such as, Al 2 O 3 , Si 3 N 4 , a-WO 3 , a-C:H, and a-Si:H, etc. The manufacture of detection films is typically accomplished by deposition, such as, sputtering or plasma enhanced chemical vapor deposition (PECVD). Thus, the cost is relatively high and the time required for thin film fabrication is excessive.  
         [0005]     Thus, an easily fabricated, low cost ISFET and the detection film thereof, eliminating packaging problems, are desirable.  
       SUMMARY  
       [0006]     ISFET fabrication methods are provided. In an exemplary embodiment of a fabrication method for ISFETs with SnO 2  extended gates, wherein a SnO 2  detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO 2  detection film is then electrically connected with a conductive wire. An insulating layer is then formed on the surface of the ISFET but leaving part of the SnO 2  detection film and part of the conductive wire exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.  
         [0007]     In some embodiments of a detection circuit, a current mirror provides a reference current, and a first operational amplifier comprises a non-inversion input terminal coupled to the reference current and an inversion input terminal coupled to a drain terminal of the ISFET. A first resistor comprises a first end coupled to the non-inversion input terminal of the first operational amplifier, and a second end. A second operational amplifier comprises a non-inversion input terminal coupled to a source terminal of the ISFET, and an inversion input terminal coupled to the second end of the first resistor. A drain-source voltage detection module is coupled to the source terminal of the ISFET, and a drain-source current detection module is coupled to the source terminal of the ISFET.  
         [0008]     In some embodiments of a read circuit, a detection circuit detects the drain-source voltage and a drain-source current of the ISFET. A processing unit is coupled to the detection circuit to determine a pH value of an unknown solution according to the detected drain-source voltage and the detected drain-source current. A liquid crystal display is coupled to the processing unit to display the determined pH value. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention can be more fully understood by the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:  
         [0010]     FIGS.  1 A˜ 1 D are flowcharts illustrating an embodiment of an ISFET fabrication method; and  
         [0011]      FIGS. 2A and 2B  are an exemplary embodiment of a read circuit. 
     
    
     DETAILED DESCRIPTION  
       [0012]     FIGS.  1 A˜ 1 D are flowcharts illustrating an embodiment of a SnO 2  extended gate ISFET fabrication method.  
         [0013]     A substrate is cut into squares of 8 cm×8 cm and rinsed with propyl alcohol, isopropanol and deionized water (DI water) respectively for 20 minutes. A SnO 2  detection film  12  is then formed on the substrate  10  by sol-gel technology to serve as an extended gate, as shown in  FIG. 1A .  
         [0014]     In the step of forming SnO 2 detection film  12 , SnCl 2  powder (SnCl 2 .2H 2 O) is dissolved by ethanol to prepare a mixed solution with a concentration of 0.37M. The mixed solution is then placed at room temperature for a first predetermined duration, for example 3 days, to form a light yellow color mixed solution. The light yellow color mixed solution is coated on the rinsed substrate  10 , and the substrate  10  is dried at 350° C. in an oven for 1 hour to form the SnO 2  detection film  12 , and then cooled to room temperature.  
         [0015]     The substrate  10  with the SnO 2  detection film  12  is cut into squares of 1.5 cm×1.5 cm and washed in deionized water of an ultrasonic oscillator. One end of an aluminum conductive wire  14  is bonded to the SnO 2  detection film  12  by silver paste and dried at 120° C. in an oven for 10 minutes, and then cooled to room temperature, as shown in  FIG. 1B .  
         [0016]     The aluminum conductive wire  108  is installed through a capillary  16 , and the SnO 2  detection film  12 , the substrate  10  and the capillary  16  are fixed by an insulating layer  18  comprising epoxy resin and dried at 120° C. in an oven for 20 minutes. The SnO 2  detection film  12  and the substrate  10  are then packaged with epoxy resin but an area of 2 mm×2 mm is kept to serve as a sensing window. The sensing portion  19  is obtained as shown in  FIG. 1C .  
         [0017]     Finally, the remaining end of the aluminum conductive wire  14  is electrically connected to a gate of a MOSFET  20 , and the ISFET  100  with a SnO 2  detection film  12  is completed.  
         [0018]     In order to read out pH values of unknown solutions a read circuit is also provided to detect a drain-source voltage and a drain-source current of the ISFET  100  to thereby determine pH values of unknown solutions accordingly. During pH value detection, only the detection film is required to be immersed into the unknown solutions without the MOSFET connected to the detection film.  FIGS. 2A and 2B  show an exemplary embodiment of a read circuit. Read circuit  200  includes a detection circuit  120 , a processing unit  130 , a liquid crystal display  140 , a reference voltage generation module  150  and an oscillation signal generation module  160 .  
         [0019]     The detection circuit  120  detects the drain-source voltage V DS  and drain-source current I DS  of an extended gate ISFET (called EGFET hereinafter)  100 . The detection circuit  120  includes a current mirror  121 , a first operational amplifier A 1 , a resistor R 2 , a second operational amplifier A 2 , a drain-source voltage detection module  123  and a drain-source current detection module  125 .  
         [0020]     The current mirror  121  includes three bipolar junction transistors Q 1 ˜Q 3  and a resistor R 2 , and provides a reference current I REF  for the detection circuit  120 . The transistor Q 1  includes a first terminal coupled to a power voltage V DD , a second terminal coupled to a non-inversion input terminal of the operational amplifier A 1 , and a control terminal coupled to a control terminal of the transistor Q 2 . The transistor Q 2  includes a first terminal coupled to the power voltage V DD , a second terminal coupled to the resistor R 1 , and a control terminal coupled to the control terminal of the transistor Q 1 .  
         [0021]     The transistor Q 3  includes a first terminal coupled to the control terminals of the transistors Q 1  and Q 2 , a second terminal coupled to a ground voltage, and a control terminal coupled to the resistor R 2  and the second terminal of the transistor Q 2 . The resistor R 1  includes a first end coupled to the ground voltage and a second end coupled to a second terminal of the transistor Q 1  and the control terminal of the transistor Q 2 . For example, the resistor R 2  can be a variable resistor to adjust the drain-source voltage V DS  within 0.0˜1.0V, and the current mirror  121  is a constant current source. Further, the current mirror  121  and the resistor R 2  combine a constant voltage source to generate a required voltage V DS  to control the EGFET  100 . The first and second operational amplifiers A 1  and A 2  constitute a source follower to prevent a loading effect, and the desired voltage V D  at the drain terminal of the EGFET  100  is obtained by the source voltage V S  and the loop constituted by the operational amplifiers A 1  and A 2  and the resistor R 2 .  
         [0022]     The operational amplifier A 1  includes a non-inversion input terminal coupled to second terminal of the operational amplifier A 2  and the resistor R 2 , an output terminal coupled to the drain terminal of the transistor  20 , and an inversion input terminal coupled to the output terminal thereof. The operational amplifier A 2  includes a non-inversion input terminal coupled to the drain terminal of the transistor  20 , an output terminal coupled to the resistor R 2 , and an inversion terminal coupled to the output terminal thereof.  
         [0023]     The drain-source voltage detection module  123  is coupled to the source terminal of the EGFET  100  to detect the drain-source voltage V DS  of the EGFET  100 . As shown, the drain-source voltage detection module  123  includes an operational amplifier A 3  and resistors R 6 ˜R 9 . For example, the resistors R 6 ˜R 9  can be the same, such that the output voltage of the operational amplifier is equal to V D ˜V S , namely, the drain-source voltage V DS . The resistor R 6  includes a first end coupled to the non-inversion input terminal of the operational amplifier A 3  and the resistor R 7 , and a second end coupled to the output terminal of the operational amplifier A 1  and the drain terminal of the transistor  20 . The resistor R 8  includes a first end coupled to the non-inversion input terminal of the operational amplifier A 2  and the source terminal of the transistor  20 , and a second end coupled to the inversion input terminal of the operational amplifier A 3  and the resistor R 9 . The resistor R 9  is coupled between the second end of the resistor R 8  and the output terminal of the operational amplifier A 3 .  
         [0024]     The drain-source current detection module  125  is coupled to the EGFET  100  to detect the drain-source current of the EGFET  100 . The drain-source current detection module  125  includes a Widlar current source, an operational amplifier A 4  and a resistor R 5 . The Wildar current source includes four transistors Q 4 ˜Q 7  and two resistors R 3  and R 4  to control and limit the drain current I D . The resistor R 4  can be a variable resistor to adjust the current I DS , and the transistor Q 7 , the resistor R 5  and the operational amplifier A 4  constitute a sub-detection unit to detect the current I DS .  
         [0025]     The transistor Q 4  includes a first terminal coupled to the ground voltage, a second terminal coupled to the control terminal of the transistor Q 2  and a control terminal coupled to a first end of the resistor R 3 , and a second end of the resistor R 3  is coupled to the ground voltage. Transistor Q 5  includes a first terminal coupled to the first end of the resistor R 3  and the control terminal of the transistor Q 4 , a control terminal coupled to the control terminal of the transistor Q 6 , and a second terminal coupled to the power voltage Vss. The transistor Q 6  includes a first terminal coupled source terminal of the transistor  20 , a control terminal coupled to control terminal of the transistor Q 5 , and a second terminal coupled to a first end of the fourth resistor R 4 , and a second end of the fourth resistor R 4  is coupled to the power voltage Vss. The transistor Q 7  includes a first terminal coupled to the first end of the resistor R 5 , a control terminal coupled to the control terminal of the transistor Q 4 , and a second terminal coupled to the first end of the resistor R 4 . The operational amplifier A 4  includes a non-inversion input terminal coupled to the first end of the resistor R 5  and the first terminal of the transistor Q 1 , and an inversion input terminal coupled to an output terminal thereof. The output terminals of the operational amplifiers A 3  and A 4  and the source terminal of the transistor  20  are coupled to the input terminals of the processing unit  130  respectively.  
         [0026]     The reference voltage generation module  150  includes two operational amplifiers A 5  and A 6 , two zener diodes Z 1  and Z 2  and resistors R 10  and R 11 . The zener diodes Z 1  and Z 2  provide stable voltages to the processing unit  130  through the operational amplifiers A 5  and A 6 , such that the processing unit  130  can potentially prevent source noise and reduce fluctuation errors.  
         [0027]     The oscillation signal generation module  160  includes an oscillator OCS and two capacitors C 1  and C 2  to provide oscillation signals to the processing unit  130 .  
         [0028]     The processing unit  130  determines the pH value of unknown solutions according to the detected drain-source voltage V DS  and the detected drain-source current I DS  via detection circuit  120 . The processing unit  130  can be microprocessor PIC16F873 manufactured by Microchip. The liquid crystal display  140  is coupled to the processing unit  130  to display the determined pH value of unknown solutions.  
         [0029]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.