WO2012005566A1

WO2012005566A1 - Apparatus for isfet gate characterization

Info

Publication number: WO2012005566A1
Application number: PCT/MY2011/000062
Authority: WO
Inventors: Zaini Abdullah Ali; Abdul Rani Rozina; Ismahadi Syono Mohd
Original assignee: Mimos Berhad
Priority date: 2010-07-06
Filing date: 2011-06-02
Publication date: 2012-01-12

Abstract

The present invention provides an apparatus [18] capable of characterizing the ISFET membrane at an earlier stage of ISFET manufacturing process flow and thereby providing an improved ion concentration sensitivity of ISFET. A solution storage part [20] with an opening [21], said opening is to be in contact with ISFET membrane. Two electrodes [22, 24] are dipped into the solution storage part [20]. A capacitor is connected in series with one of the electrode. A capacitance-voltage characteristic between the electrodes in contact with the solution of known pH and another electrode at the back surface of the semiconductor is monitored. The shift of the capacitance-voltage characteristic is represented in voltage-pH graph and thereby sensitivity of ISFET is determined.

Description

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APPARATUS FOR ISFET GATE CHARACTERIZATION

The present invention relates to ISFET gate and ISFET characterization, and in particular apparatus involved in characterization of ISFET gate at wafer level.

BACKGROUND ART

ISFETs (Ion Sensitive Field Effect Transistor) are constructed by substituing an ion sensitive film for the metal gate on the gate oxide of a traditional MOSFET (Metal-Oxide Semiconductor Field Effect Transistor). It is commonly used to measure the ion concentration such as pH in a solution. The sensing method is based on potentiometric detection. When the ISFET is in contact with a solution, the interfacial potential of the ion sensitive film and the solution will influence the charge density in the inversion layer of the semiconductor surface and thereby the ion concentration such as pH of the solution can be measured from the source/drain current and gate voltage of ISFET thereof.

Good desirable ISFET has sensitivity above 50 mV/pH. It is usually tested in package form, wherein die is attached to board, wire bond and encapsulation of the surroundings. There are problems associated with the characterization of the ISFET; in particular, it is time consuming for using conventional method of ISFET characterization in which it is characterized in its packaged form (after encapsulation). For commercial purpose, it is important to characterize the ISFET at an earlier stage of the ISFET manufacturing process flow, which is less time consuming.

Stand-alone study of ISFET gate layer has to be conducted to improve the process parameter in which to produce higher sensitivity ISFET in shorter time.

SUMMARY

The aim of the present invention is to provide an apparatus to perform stand-alone study of ISFET gate membrane, in particular at wafer level.

Within this aim, an object of the present invention is to provide an apparatus for measuring the sensitivity of ISFET gate at wafer level. In the present invention, the ISFET wafer is provided comprising a semiconductor substrate, an insulating film provided on the semiconductor substrate, and an ion sensitive layer provided on the insulating film in which to be in contact with a solution. In the present invention, the apparatus for the measurement of the sensitivity of the ISFET comprising a solution storage part provided to hold a solution in which holes are provided at the bottom of the storage part which can be open to allow contact of the solution with said ion sensitive layer or closed to prevent spillage of the solution, two metal electrodes in which a reference electrode and a working electrode provided to be in contact with the solution, a capacitor provided in series with the working electrode, a lever provided on said solution storage part in which its fulcrum at one end of the lever and a screw pair mechanism at the opposite end of the lever.

In the present invention, a pH measurement method is provided in which a solution of predetermined pH is introduced into the solution storage part and is in contact with the ion sensitive layer as describe above, and then a capacitance-voltage characteristic is determined between the electrodes in contact with the solution and another electrode at the surface of said semiconductor substrate which is opposite to the surface having insulating film provided thereon. A solution of different pH is replaced into the invention and the process repeated. The C-V curves of different pH are compared and the sensitivity of the ISFET ion sensitive layer is determined.

The present invention provides the capability of stand-alone study of ISFET gate membrane at wafer level which is less time-consuming for improvement of ISFET gate sensitivity and lower cost of manufacturing ISFET.

BRIEF DESCRIPTION OF DRAWINGS

Further characteristics and advantages of the present invention will become better apparent from the following detail description of a preferred but not exclusive embodiment thereof, illustrated by way of non-limiting example in the accompanying drawings, wherein:

FIG. 1 is a schematic cross section of an ISFET at wafer level. FIG. 2 is a schematic cross section of an apparatus for ion concentration measurement according to the present invention.

FIG. 3 represents a capacitance-voltage characteristic determined using the apparatus shown in FIG. 2.

FIG. 4 is a graph showing the relation between pH of solution with voltage obtained from the capacitance-voltage characteristic determined with the apparatus shown in FIG.2.

DESCRIPTION OF EMBODIMENTS

In an embodiment of the present invention, an ISFET membrane at wafer level is as shown in FIG. 1. The ISFET membrane [10] comprising a semiconductor substrate [12] such as silicone, an insulating film [14] such as silicone oxide formed on the substrate [12], and an ion sensitive membrane [16] such as silicone nitride formed on the insulating film [14].

The wafer [10] is placed under the apparatus [18] under the solution storage part [20] as shown in FIG. 2. A reference electrode [22] and a working electrode [24] are provided in contact with a solution with known pH [26] and are fixed to position by a holder [28] in which the holder [28] has a notch [27] to fix its position to the solution storage part [20]. A capacitor [30] is provided in series with the working electrode [24].

The solution storage part [20] in which an opening [21] is provided at the bottom of said solution storage part [20] is fixed to a lever [32] at the center thereof and the lever [32] has a fulcrum [33] at one end of the lever [32]. A screw pair mechanism [34] is attached at the lever [32] at the opposite end of the lever [32] in which the fulcrum [33] is constructed. The screw pair mechanism operates the lever to secure or release the solution storage part. The lever [32] can be opened in a direction [35] to allow substitution of different wafer [10] for characterization. A mechanical gasket [36] is coupled with the opening [21] to form a seal contact when the screw pair mechanism [34] is screwed tight. The substrate [12] has a bottom surface provided with terminal electrode [38]. When a solution [26] is introduced into the solution storage part [20] and is in contact with the ion sensitive membrane [16], a capacitance-voltage characteristic which is measured by monitor

[39] between the reference electrode [22], working electrode [24] and terminnal electrode [38] varies based on the pH of the solution [26]. Such variation is shown in FIG. 3. Two solutions of different pH values are represented in FIG. 3. Solution 2 has higher pH value than solution 1. The C-V characteristic can shift in the direction [40] based on pH.

A method of using the apparatus to obtain ISFET membrane characterization, involves several steps. First, introduce first solution with known pH into solution storage part and monitor capacitance-voltage characteristic of first solution. Then, replace the first solution with second solution of different known pH and obtain the capacitance-voltage characteristic of second solution.

The shift of the C-V characteristic at the centerline [42] is represented in FIG. 4 to show the relation of pH and voltage and thereby the sensitivity of the ISFET is determined.

In practice, it has been found that the invention thus described solves the problems noted above. This solution reduces the problems related to the time-cosuming procedures to determine the sensitivity of ISFET.

Claims

1. An apparatus for ISFET membrane characterization [18], comprising:

a solution storage part [20] with an opening [21], said opening to be in contact with ISFET membrane [16];

two electrodes, a reference electrode [22] and a working electrode [24] dipped into the solution storage part [20];

a capacitor [30] coupled in series with working electrode [24];

a terminal electrode [38] in contact with substrate [12];

wherein the apparatus is ready to be used with solution [26] to fill into solution storage part [20]; and

a monitor [39] between reference electrode [22] and capacitor, and terminal electrode.to obtain capacitance and voltage characteristics.

2. An apparatus [18] according to claim 1 , further comprising a lever [32] on the solution storage part [20] and a screw pair mechanism [34] to secure the position of solution storage part.

3. An apparatus [18] according to claim 2, further comprising a fulcrum [33] at one end of the lever [33] to release the lever when the screw pair mechanism [34] is released.

4. An apparatus [18] according to claim 1 , further comprising a mechanical gasket [36] coupled with said bottom opening [21].

5. An apparatus [18] according to claim 1 , further comprising an electrodes holder [28] to hold both electrodes [22, 24].

6. An apparatus [18] according to claim 5, wherein the electrodes holder has a notch [27] to fix to fix its position to the solution storage part [20].

7. A method of using an apparatus of any of the proceeding claims, to obtain ISFET

membrane characterization, comprising:

introducing first solution with known pH into solution storage part;

monitoring capacitance-voltage characteristic of first solution;

replacing the first solution with second solution of different known pH; obtaining capacitance-voltage characteristic of second solution;

producing a voltage-pH relationship and deduce the sensitivity of the ISFET membrane.