# Patent

## Title
ISFETs fabrication method

## Abstract

Methods for fabricating ion sensitive field effect transistors (ISFETs) with SnO2 extended gates. A SnO2 detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO2 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 SnO2 detection film and the conductive wire are left exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.

## Background
BACKGROUND
The invention relates to ion sensitive field effect transistors (ISFETs), and more particularly, to methods for fabricating SnO2 extended gate ISFETs.
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.
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.
A variety of materials are known to be capable of serving as ISFET detection film, such as, Al2O3, Si3N4, a-WO3, 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.
Thus, an easily fabricated, low cost ISFET and the detection film thereof, eliminating packaging problems, are desirable.
SUMMARY
ISFET fabrication methods are provided. In an exemplary embodiment of a fabrication method for ISFETs with SnO2 extended gates, wherein a SnO2 detection film is formed on a substrate by sol-gel technology to serve as an extended gate. The SnO2 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 SnO2 detection film and part of the conductive wire exposed. The exposed conductive wire is electrically connected to a gate terminal of a MOS transistor.
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.
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
The invention can be more fully understood by the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:
FIGS. 1A ̃1D are flowcharts illustrating an embodiment of an ISFET fabrication method; and
FIGS. 2A and 2B are an exemplary embodiment of a read circuit.



DETAILED DESCRIPTION
FIGS. 1A ̃1D are flowcharts illustrating an embodiment of a SnO2 extended gate ISFET fabrication method.
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 SnO2 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.
In the step of forming SnO2detection film 12, SnCl2 powder (SnCl2·2H2O) 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 SnO2 detection film 12, and then cooled to room temperature.
The substrate 10 with the SnO2 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 SnO2 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.
The aluminum conductive wire 108 is installed through a capillary 16, and the SnO2 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 SnO2 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.
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 SnO2 detection film 12 is completed.
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.
The detection circuit 120 detects the drain-source voltage VDS and drain-source current IDS of an extended gate ISFET (called EGFET hereinafter) 100. The detection circuit 120 includes a current mirror 121, a first operational amplifier A1, a resistor R2, a second operational amplifier A2, a drain-source voltage detection module 123 and a drain-source current detection module 125.
The current mirror 121 includes three bipolar junction transistors Q1 ̃Q3 and a resistor R2, and provides a reference current IREF for the detection circuit 120. The transistor Q1 includes a first terminal coupled to a power voltage VDD, a second terminal coupled to a non-inversion input terminal of the operational amplifier A1, and a control terminal coupled to a control terminal of the transistor Q2. The transistor Q2 includes a first terminal coupled to the power voltage VDD, a second terminal coupled to the resistor R1, and a control terminal coupled to the control terminal of the transistor Q1.
The transistor Q3 includes a first terminal coupled to the control terminals of the transistors Q1 and Q2, a second terminal coupled to a ground voltage, and a control terminal coupled to the resistor R2 and the second terminal of the transistor Q2. The resistor R1 includes a first end coupled to the ground voltage and a second end coupled to a second terminal of the transistor Q1 and the control terminal of the transistor Q2. For example, the resistor R2 can be a variable resistor to adjust the drain-source voltage VDS within 0.0 ̃1.0V, and the current mirror 121 is a constant current source. Further, the current mirror 121 and the resistor R2 combine a constant voltage source to generate a required voltage VDS to control the EGFET 100. The first and second operational amplifiers A1 and A2 constitute a source follower to prevent a loading effect, and the desired voltage VD at the drain terminal of the EGFET 100 is obtained by the source voltage VS and the loop constituted by the operational amplifiers A1 and A2 and the resistor R2.
The operational amplifier A1 includes a non-inversion input terminal coupled to second terminal of the operational amplifier A2 and the resistor R2, 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 A2 includes a non-inversion input terminal coupled to the drain terminal of the transistor 20, an output terminal coupled to the resistor R2, and an inversion terminal coupled to the output terminal thereof.
The drain-source voltage detection module 123 is coupled to the source terminal of the EGFET 100 to detect the drain-source voltage VDS of the EGFET 100. As shown, the drain-source voltage detection module 123 includes an operational amplifier A3 and resistors R6 ̃R9. For example, the resistors R6 ̃R9 can be the same, such that the output voltage of the operational amplifier is equal to VD ̃VS, namely, the drain-source voltage VDS. The resistor R6 includes a first end coupled to the non-inversion input terminal of the operational amplifier A3 and the resistor R7, and a second end coupled to the output terminal of the operational amplifier A1 and the drain terminal of the transistor 20. The resistor R8 includes a first end coupled to the non-inversion input terminal of the operational amplifier A2 and the source terminal of the transistor 20, and a second end coupled to the inversion input terminal of the operational amplifier A3 and the resistor R9. The resistor R9 is coupled between the second end of the resistor R8 and the output terminal of the operational amplifier A3.
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 A4 and a resistor R5. The Wildar current source includes four transistors Q4 ̃Q7 and two resistors R3 and R4 to control and limit the drain current ID. The resistor R4 can be a variable resistor to adjust the current IDS, and the transistor Q7, the resistor R5 and the operational amplifier A4 constitute a sub-detection unit to detect the current IDS.
The transistor Q4 includes a first terminal coupled to the ground voltage, a second terminal coupled to the control terminal of the transistor Q2 and a control terminal coupled to a first end of the resistor R3, and a second end of the resistor R3 is coupled to the ground voltage. Transistor Q5 includes a first terminal coupled to the first end of the resistor R3 and the control terminal of the transistor Q4, a control terminal coupled to the control terminal of the transistor Q6, and a second terminal coupled to the power voltage Vss. The transistor Q6 includes a first terminal coupled source terminal of the transistor 20, a control terminal coupled to control terminal of the transistor Q5, and a second terminal coupled to a first end of the fourth resistor R4, and a second end of the fourth resistor R4 is coupled to the power voltage Vss. The transistor Q7 includes a first terminal coupled to the first end of the resistor R5, a control terminal coupled to the control terminal of the transistor Q4, and a second terminal coupled to the first end of the resistor R4. The operational amplifier A4 includes a non-inversion input terminal coupled to the first end of the resistor R5 and the first terminal of the transistor Q1, and an inversion input terminal coupled to an output terminal thereof. The output terminals of the operational amplifiers A3 and A4 and the source terminal of the transistor 20 are coupled to the input terminals of the processing unit 130 respectively.
The reference voltage generation module 150 includes two operational amplifiers A5 and A6, two zener diodes Z1 and Z2 and resistors R10 and R11. The zener diodes Z1 and Z2 provide stable voltages to the processing unit 130 through the operational amplifiers A5 and A6, such that the processing unit 130 can potentially prevent source noise and reduce fluctuation errors.
The oscillation signal generation module 160 includes an oscillator OCS and two capacitors C1 and C2 to provide oscillation signals to the processing unit 130.
The processing unit 130 determines the pH value of unknown solutions according to the detected drain-source voltage VDS and the detected drain-source current IDS 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.
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.

## Claims

1. A fabrication method for ion sensitive field effect transistors (ISFETs) with a SnO2 extended gate, comprising:
providing a substrate;
forming a SnO2 detection film on the substrate by sol-gel technology to serve as an extended gate;
electrically connecting the SnO2 detection film with a conductive wire;
forming an insulating layer on the surface of the ISFET but exposing part of the SnO2 detection film and part of the conductive wire; and
electrically connecting the exposed conductive wire and a gate terminal of a MOS transistor.

2. The fabrication method as claimed in claim 1, wherein the insulating layer comprises epoxy resin.

3. The fabrication method as claimed in claim 1, wherein forming the SnO2 detection film on the substrate comprises:
dissolving SnCl2 powder by ethanol to form a mixed solution;
placing the mixed solution for a first predetermined duration to form a light yellow color mixed solution;
coating the light yellow color mixed solution on the substrate; and
heating the substrate to a first predetermined temperature for a second predetermined duration.

4. The fabrication method as claimed in claim 3, wherein the concentration of the solution comprising SnCl2 and ethanol is about 0.37M.

5. The fabrication method as claimed in claim 4, wherein the first predetermined temperature is 350° C.

6. The fabrication method as claimed in claim 4, wherein the second predetermined duration is 1 hour.

7. The fabrication method as claimed in claim 1, further comprising rinsing the substrate for a third predetermined duration by propyl alcohol, isopropanol and deionization water (DI water) respectively before forming the SnO2 detection film.

8. The fabrication method as claimed in claim 7, wherein third predetermined duration is 20 minutes.

9. The fabrication method as claimed in claim 2, wherein connecting the SnO2 detection film on substrate with the conductive wire comprises:
rinsing the substrate with the SnO2 detection film by DI water;
fixing a first terminal of the conductive wire to the SnO2 detection film by silver paste; and
heating the substrate to a second predetermined temperature for a fourth predetermined duration.

10. The fabrication method as claimed in claim 9, wherein the fourth predetermined duration is 10 minutes.

11. The fabrication method as claimed in claim 9, wherein forming an insulating layer on the surface of the ISFET comprises:
installing the conductive wire through a capillary;
packaging the substrate, the SnO2 detection film, the conductive wire and capillary by the insulating layer but exposing part of the SnO2 detection film, part of the conductive wire and part of the capillary; and
heating the substrate to the second predetermined temperature for a fifth predetermined duration.

12. The fabrication method as claimed in claim 11, wherein the second predetermined temperature is 120° C.

13. The fabrication method as claimed in claim 11, wherein the fifth predetermined duration is 20 minutes.