WO2009064166A2

WO2009064166A2 - An integrated ion sensitive field effect transistor sensor

Info

Publication number: WO2009064166A2
Application number: PCT/MY2008/000140
Authority: WO
Inventors: Rozina Abdul Rani; Hing Wah Lee; Ali Zaini Abdullah; Mohd Rofei Mat Hussin; Azlan Zakaria; Azlina Mohd Zain; Siti Aishah Mohamd Badaruddin; Nor Azhadi Ngah
Original assignee: Mimos Berhad
Priority date: 2007-11-15
Filing date: 2008-11-14
Publication date: 2009-05-22
Also published as: MY142676A; WO2009064166A3

Abstract

A chemical sensor having an ion sensitive field effect transistor (ISFET) comprising a substrate (10) situated with a source (4) and a drain (3); an ion sensing gate (5) disposed between the source and the drain; an ion-sensitive film (1) formed on the surface of the substrate and the ion sensing gate; an electrode domain (6) formed on the ion-sensitive film surrounding the periphery of the ion sensing gate (5) characterized in that the electrode domain (6) is made of tungsten, titanium or tungsten suicide.

Description

AN INTEGRATED ION SENSITIVE FIELD EFFECT TRANSISTOR

SENSOR

FIELD OF THE INVENTION The present invention relates to an improved ion-sensitive field effect transistor (ISFET) sensor that gives good sensitivity to detect ion concentration in an aqueous solution. In more particular, a noble material is introduced into the fabrication of the ISFET of the present invention as the reference electrode thus renders the fabricated ISFET possessing such properties.

BACKGROUND OF THE INVENTION

Chemical sensors for detecting chemical compounds are widely used in different field such as medical diagnosis, water treatment system, food processing and the like. Conventional chemical sensor such as glass electrode and spectrophotometer are relatively large in size, difficult to be handled, and expensive as required lots of building parts for construction. In view of this, miniaturized semiconductor sensors were devised and now replacing the use of conventional chemical sensor.

Ion sensitive filed effect transistor (ISFET) is one of the most widely employed semiconductor at the present which is incorporated into chemical sensor for chemical compounds detection. Since the fabrication of the chemical sensor with incorporated ISFET is conducted by integrated circuit process technology, the chemical sensor produced are much smaller in size, ease to the be standardized with high reproducibility, and ready for mass production thus cheaper in manufacturing cost. Nevertheless, attempts to reduce the manufacturing cost and further improve the sensitivity performance of the ISFET have never end. Substituting the existing components within an ISFET with new materials or coating is known to be one of the effective ways to attain the aforesaid purposes. United State patent no. 5182005 discloses an ISFET which it contains an reference electrode consisted of an insulator or semiconductor substrate coated with polyglutamate with an ion sensitive layer made of phthalocyninato-polysiloxane polymers. With such features, the disclosed invention is said to be advantageous in long term stability and sensitivity.

Another United State patent no. 5385659 claims a planar reference electrode for chemical sensors that contains a structured polymer layer defining a trench which serves as a diffusion channel to slow the ion diffusion thus avoiding negative effect due to the high electrical resistance between the sensor and the reference electrode.

Further United State patent no. 5387328 discloses a bio-sensor using ISFET for detecting chemical compounds while platinum is proposed to be the reference electrode in this invention. From the disclosure, the disclosed invention claims be able to generate three fold hydrogen ions as compared to conventional sensor thus increase the sensitivity performance of the fabricated bio-sensor.

SUMMARY OF THE INVENTION

The present invention aims to provide a chemical sensor with an improved ISFET structure which is easy and convenient to be fabricated in opposed to the conventional chemical sensor as noble reference electrode is introduced into the present invention. In more specific, the noble reference electrode employed can be deposited onto the substrate of the ISFET without additional processing steps as compared to some of the chemical sensors available at the present.

Further object of the present invention is to offer a more cost-effective chemical sensor as the building materials of the disclosed chemical sensor are less expensive in view of the conventional chemical sensor. In contrast to the ISFET using platinum or gold as the reference electrode, the cost o the noble reference electrode employed in the present invention is much cheaper thus greatly reduce the manufacturing cost of the chemical sensor.

Still another object of the disclosed invention to provide a chemical sensor with improved long term stability and sensitivity as the reference electrode used in the chemical sensor is resistant against both acidic and basic corrosion. The reference electrode employs tungsten or its derivatives as the making material shall possess an inert oxide surface which is enduring to different chemical solution tested.

At least one of the preceding objects is met, in whole or in part, by the present invention, in which one of the embodiment of the present invention includes a chemical sensor having an ion sensitive field effect transistor (ISFET) comprising a substrate situated with a source and a drain; an ion sensing gate disposed between the source and the drain; an ion-sensitive film formed on the surface of the substrate and the ion sensing gate; an electrode domain formed on the ion-sensitive film surrounding the periphery of the ion sensing gate characterized in that the electrode domain is made of tungsten, titanium or tungsten suicide.

In accordance with another preferred embodiment of the present invention, the ISFET of the chemical sensor further comprises one or more p-type ion implantation deposited around the source and the drain on the substrate.

Further embodiment of the disclosed invention further comprises a layer of titanium nitrate deposited in between the electrode and substrate to reinforce the attachment of the reference electrode onto the substrate.

Still another embodiment of the present invention, the p-type ion implantation is preferably derives from the element of boron. Accordingly, the ion sensitive film used in the present invention is preferably made of silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows one of the layout design of an integrated ISFET sensor;

Figure 2 is a cross-sectional view for the integrated ISFET as shown in Figure l;

Figure 3 shows another embodiment of the integrated sensor which adapts a round shape design;

Figure 4 shows further embodiment of the sensor which employs multi-finger gate design;

Figure 5 is a graph showing the output voltage of the fabricated ISFET using tungsten as the reference electrode in response to solution with different pH;

Figure 6 is a graph showing the output voltage of the fabricated ISFET using titanium as the reference electrode in response to solution with different pH; and

Figure 7 is a graph showing the output voltage of the fabricated ISFET using tungsten suicide as the reference electrode in response to solution with different pH. DETAILED DESCRIPTION OF THE INVENTION

The most preferred embodiment of the invention is now described with reference to the figures, where like reference numbers indicate identical or functionally similar elements. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the art will recognize that the other configurations and arrangements can be used without departing from the scope of the invention.

The present invention relates to a chemical sensor having an ion sensitive field effect transistor (ISFET) as shown in figure 1 comprises a substrate (12) situated with a source (4) and a drain (3) ; an ion sensing gate (5) disposed between the source (4) and the drain (3); an ion-sensitive film (1) formed on the surface of the substrate (12) and the ion sensing gate (5); an electrode domain (6) formed on the ion-sensitive film surrounding the periphery of the ion sensing gate (5) characterized in that the electrode domain (6) is made of tungsten, titanium or tungsten suicide.

According to the preferred embodiment as presented in the figure 2 of the present invention, the substrate (10) in the present invention can be constructed from different types of semiconductor materials such as glass, polyimide, gallium arsenide, silicon, germanium, indium or any derivatives thereof. By far silicon-based material, like silicon oxide, are most preferred, the applicability of other materials in the present invention is not limited. Moreover, it is preferred that single crystal of the semiconductor wafer is used in the fabrication as substrate for the ISFET, yet utilization of other forms like amorphous silicon or polycrystalline silicon is not ruled out. The substrate (12) in the ISFET functions as a platform for the deposition or situation of source (4) and drain (3) which electrons will flow from the source (4) to the drain (3) under an influential applied voltage. In order to carry out the chemical compound sensing property, the substrate (12) used herein is preferably doped with p-type ion implantation around the source (4) and drain (3) to have the ISFET behaves as a voltage amplifier thus enhances the sensitivity of the chemical sensor disclosed. Different materials can be implanted at variable dose with different methods. It is preferable to use boron, phosphorus or arsenic ion as the ion implant. Dose of the ion implant can be in varied amount depending on the properties of the ISFET to be fabricated. Conductive material (9) such as metal may be disposed into the substrate (12), preferably below the p-type ion implant (11) to reinforce the conductivity of the substrate (12) caused by the p-type ion implant (11). However, the source (4) and the drain (3) are preferably to be n- type, or be doped of opposite type to the substrate for the ISFET to perform. Preferably, phosphorus material is used for the making of the source (4) and drain (3).

As aforementioned, a gate structure (5) is positioned in between the drain (3) and the source (4) to permit electrons flow from the source (4) to the drain (3). The passage for the electron flow is literally known as ion channel or conductive channel. It is known in the art that the flow of the electrons from the source to the drain is affected by the applied voltage across the gate and the source. Preferably, the conductive channel in the present invention is doped of the opposite type to the substrate doping which the conductive channel is preferred to be n-type. In accordance with the preferred embodiment, application of negative voltage upon gate-to-source at the ISFET of the chemical sensor cause depletion of the conductive channel, while positive voltage enhances the width of the conductive channel. Further embodiment in the present invention possesses a layer of polysilicon (8) on top of the gate structure (5) to structure (5) to protect the gate structure (5) from possible corrosion comes from the etching solvent during the fabrication process. Besides, the polysilicon layer (8) also stabilizes the performance of the ISFET by shielding the gate structure (5) from any light that may possibly change the conductivity property. In another embodiment of the present invention, an oxide layer (10) is deposited in between the surface of ion-sensitive film and the substrate (12) as well as the source and drain. Nevertheless, a thin oxide layer (2) may be formed in between the polysilicon layer (8) and the ion-sensitive film (1) too. This oxide layer (10 or 2) is situated at the bottom surface of the ion-sensitive film (1) to prevent direct contact of the ion sensitive film (1) with the other components below, it also serves as an insulator between 2 different layers.

Further embodiment of the present invention involves applying an ion sensitive film (1) or electroactive membrane onto the surface of the substrate (12). This ion sensitive film is silicon nitride (SisN₄) in the preferable embodiment which is used as a charge blocking interface thus improves the pH response. Yet, other types of ion sensitive membrane can be incorporated into the present invention also, for example polyvinyl chloride containing valinomycin, tin dioxide, tantalum pentoxide, and alumina. Embodiments with such features are conferred with improved ion selectivity to be applied in task that requires detecting specific types of chemical compounds in an aqueous solution.

Another embodiment of the present invention is related to the utilization of a noble material as the electrode domain (6) or reference electrode in the ISFET of the present invention. It is more preferred that the electrode domain is fabricated from tungsten (W), titanium (Ti) or tungsten suicide (WSix), and most preferred is tungsten. It was found by the inventors of the present invention that tungsten is suitable to be used for the fabrication of the disclosed chemical sensor. Owing to its inert oxide surface, electrode domain (6) made of tungsten is resistant upon the acidic condition of the sample to be tested. Therefore, the disclosed invention is more sustainable and offer long-term stability in terms of performance. Derivatives of tungsten such as tungsten suicide are found to be useful as the building material of the electrode domain. Nevertheless, titanium can be integrated as the electrode domain too in the present invention.

Attention is now drawn to the fact that an additional layer of titanium nitrate is required to be deposited in between the electrode domain (6) and substrate (12). The titanium nitrate layer serves as a reinforcing layer to provide a good adhesion of the electrode domain (6) onto the substrate (12).

It is to be understood that the present invention may be embodied in other specific forms and is not limited to the sole embodiment described above. However modification and equivalents of the disclosed concepts such as those which readily occur to one skilled in the art are intended to be included within the scope of the claims which are appended thereto.

EXAMPLE 1

As a transducer, ISFET converts the chemical reaction to an electrical signal. Thus, a measurement system is needed to detect the electrical signal, which is indicating the pH value of sample. The system consists of interface or readout circuitry, microcontroller and liquid crystal display (LCD).

The operation of ISFET interface circuit is using the constant current constant voltage method. The voltage supplied to ISFET drain was set at 1.5 V. The working point of ISFET' s is chosen to be in the saturation region to attenuate surface potential variations the most. The output of the interface circuit provides a feedback to the ISFET gate which is the reference electrode. The gate voltage measured in such a feedback circuit provides information about the electrolyte solution. In this way, besides serving the gate voltage as the output signal in response to electrolyte solutions been tested, the feedback gate voltage which is connected to the reference electrode will establish a stable electrolyte potential with respect to the surface potential of the sensing gate of ISFET. Thus, the varying of the voltage gate will restore the drain current as the electrolyte parameters are changed. To measure the voltage value resulted from ISFET reaction, the output point of feedback gate voltage is connected to the analog input pin, PORTA.2 of PIC16F877A. The PIC16F877A which is implemented with the 10-bit Analog to digital converter (ADC) will convert the analog value to the digital value. Then, a standard LCD display with 2 x 16 characters is used to display the digital current voltage of ISFET reaction. A multimeter completed with the RS232C standard interface and computer record is used for sampling the output voltage and plots the output graph.

The experiment setup for ISFET analysis is shown in Figure A. During the measurement, ISFET sensor integrated with the tungsten reference electrode was immersed in the pH buffer solution. The experiment was carried out at a room temperature of 21 °C. Reference electrode types; Tungsten, W, Titanium, Ti and Tungsten suicide WSix were used during testing the ISFET performance, respectively. The measurement data was recorded starting from dipping the integrated ISFET-TiN/W in the sample until to 15 ~ 20 minutes afterwards. The pH buffer solution; pH4, pH7 and pHIO were used as a sample solution. The ISFET response is measured in each sample for two data measurements. Then, the output results were compared to find out which electrode is more conductive to influence a better performance of ISFET. Figure 5, Figure 6 and Figure 7 show the output voltage of ISFET response by using W, Ti and WSix as a reference electrode for three samples of solution with different pH level, respectively. The result demonstrated that the W reference electrode can differentiate the signal more clearly and stable the potential of ISFET at a different level of H+- ions concentration. From the output signal in Figure. 5, the sensitivity of ISFET by using Tungsten as a reference electrode is about 39.83 mV/pH. During conducting the experiment, every time measure in a different electrolyte sample and for restoring purpose, the ISFET probe is keeping clean by rinsing the probe in deionized water before wiping dries with a soft towel or tissue.

Claims

CLAIMS:

1. A chemical sensor having an ion sensitive field effect transistor (ISFET) comprising a substrate (12) situated with a source (4) and a drain (3); an ion sensing gate (5) disposed between the source and the drain; an ion-sensitive film (1) formed on the surface of the substrate and the ion sensing gate; an electrode domain (6) formed on the ion-sensitive film surrounding the periphery of the ion sensing gate (5) characterized in that the electrode domain (6) is made of tungsten, titanium or tungsten suicide.

2. A chemical sensor according to claim 1 further comprising p-type ion implantation deposited around the source and the drain on the substrate.

3. A chemical sensor according to claim 1 or 2 further comprising a layer of titanium nitrate deposited in between the electrode and substrate.

4. A chemical sensor according to claim 1 to 3 further comprises an oxide layer (10) situated below of the ion-sensitive film (1).

5. A chemical sensor according to claims 2 to 3, wherein the p-type ion implantation is phosphorus ions, boron ions or arsenic ions.

6. A chemical sensor according to claim 1 to 3, wherein the ion sensitive film is made of silicon nitride.