WO2015178755A1

WO2015178755A1 - Ion-sensitive field-effect transistor (isfet) with nanostructures and fabrication method thereof

Info

Publication number: WO2015178755A1
Application number: PCT/MY2015/000033
Authority: WO
Inventors: Daniel Chia Sheng Bien; Khairul Anuar ABD WAHID; Muhammad Aniq Shazni MOHAMMAD HANIFF; Mai Woon LEE
Original assignee: Mimos Bhd
Current assignee: Mimos Bhd
Priority date: 2014-05-21
Filing date: 2015-05-13
Publication date: 2015-11-26
Anticipated expiration: 2016-11-21
Also published as: MY197896A

Abstract

Disclosed herein is an ion-sensitive field-effect transistor (ISFET) (100) having nanostructures (109) for sensing ions and measuring ion concentration in solutions. In general, the base layer (101) at the sensing region of the ISFET (100) is etched to form the nanostructures (109). Each of the nanostructures (109) has a diameter of less than 100nm, and the nanostructures (109) have a distance of less than 100nm from each other. The nanostructures (109) are nanopillars with cylindrical shape, needle-like shape, or a combination thereof. Due to all these particular features, the surface area of the ISFET (100) that is exposed to ions is increased, and therefore the sensitivity and efficiency of the ISFET (100) are improved. Also disclosed herein is a fabrication method thereof.

Description

ION-SENSITIVE FIELD-EFFECT TRANSISTOR (ISFET) WITH

NANOSTRUCTURES AND FABRICATION METHOD THEREOF

TECHNICAL FIELD OF THE INVENTION

This invention relates to an ion-sensitive field-effect transistor (ISFET), and more particularly to an ISFET with nanostructures.

BACKGROUND OF THE INVENTION

An ion-sensitive field-effect transistor (ISFET) is a device that is widely used for biosensor applications. One of the most common biosensor applications of ISFET is to determine the pH value of a sample solution. The ISFET works so by sensing ions in the sample and reading the potential difference caused by ion concentration. There are many characteristics of ISFET, such as fast response time, only requires small quantity of sample, small in size, and on-chip circuit integration, that make it suitable for biosensor applications.

A conventional ISFET mainly consists of a base layer, a source and a drain that are implanted in the base layer, an oxide layer, and a sensing membrane layer. The area between the source and the drain is known as the sensing or gate region, where exposure to ions occurred.

Generally, the conventional ISFET is fabricated with a planar sensing region, which is the main reason why the ISFET has low sensitivity and low efficiency. The planar sensing region serves a limited total surface area for the ions to be attached to.

Therefore, it has become an aim of the present invention to solve the above- mentioned technical issues and disadvantages by presenting an ISFET for sensing ions and measuring ion concentrations with improved sensitivity and efficiency by increasing the surface area of the sensing region of the ISFET. This is achieved by introducing nanostructures at the sensing region. SUMMARY OF THE INVENTION

It is an objective of the present invention to introduce an ion-sensitive field-effect transistor (ISFET) having nanostructures for sensing ions and measuring ion concentration in solution with improved sensitivity and efficiency.

In an embodiment of the present invention, the ISFET comprises a base layer; a source at one end of the base layer and a drain at the other end of the base layer, forming a sensing region in between; a layer of oxide covering the source, the drain, and the sensing region; a layer of sensing membrane covering the layer of oxide, and optionally a layer of chemical membrane on top of the layer of sensing membrane; characterized in that the base layer at the sensing region comprises the nanostructures.

In a further embodiment of the present invention, each nanostructure has a diameter of less than 10Onm.

In another embodiment of the present invention, the nanostructures have a distance of less than 10Onm from each other. In an additional embodiment of the present invention, the nanostructures are nanopillars having cylindrical shape, needle-like shape, or a combination thereof.

In a preferred embodiment of the present invention, the base layer is a silicon layer.

In a preferred embodiment of the present invention, the layer of oxide is a layer of silicon dioxide.

In a preferred embodiment of the present invention, the layer of sensing membrane is a layer of silicon nitride. It is another objective of the present invention to introduce a method to fabricate the above discussed ISFET having nanostructures for sensing ions and measuring ion concentration in solutions with improved sensitivity and efficiency. In an embodiment of the present invention, the method is characterized by a first step of doping the base layer to form the source and the drain, and therefore forming the sensing region in between; a second step of depositing a layer of thin metallic film on top of the base layer, covering the sensing region; a third step of patterning the layer of thin metallic film; a fourth step of nucleating the layer of thin metallic film at a temperature above 400°C to form nanoparticles, wherein the nanoparticles act as masks in a subsequent etching process; a fifth step of depositing a layer of resist material on top of the source and the drain; a sixth step of etching the base layer to form the nanostructures; a seventh step of removing the nanoparticles and the layer of resist material; an eight step of growing the layer of oxide; a ninth step of depositing the layer of sensing membrane; and optionally a tenth step of depositing a layer of chemical membrane on top of the layer of sensing membrane.

In another embodiment of the present invention, the nanostructures have a distance of less than 10Onm from each other. In an additional further embodiment of the present invention the nanostructures are nanopillars having cylindrical shape, needle-like shape, or a combination thereof.

In a preferred embodiment of the present invention, the layer of thin metallic film is of cobalt, iron, nickel, gold, or a combination thereof. In a preferred embodiment of the present invention, the base layer is etched via chemical etching, or plasma etching with hydrogen bromide or sulfur hexafluoride.

In a preferred embodiment of the present invention, the layer of oxide is grown by thermal oxidation.

In a preferred embodiment of the present invention, the layer of sensing membrane is a layer of silicon nitride. In a preferred embodiment of the present invention, the layer of sensing membrane is deposited by using plasma enhanced chemical vapour deposition (PECVD).

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will be more readily understood and appreciated from the following detailed description when read in conjunction with the accompanying drawings of the preferred embodiment of the present invention, in which:

Figure 1 shows the cross-sectional view of the ISFET with nanostructures of the present invention;

Figure 2 shows the top view of the ISFET with nanostructures of the present invention;

Figure 3A to 3H shows the ISFET with nanostructures of the present invention in different fabrication phases; and Figure 4 shows the nanoparticles formed from the step of nucleating the layer of thin metallic film.

DETAILED DESCRIPTION OF THE INVENTION

The above mentioned and other features and objects of this invention will become more apparent and better understood by reference to the following detailed description. It should be understood that the detailed description made known below is not intended to be exhaustive or to limit the invention to the precise disclosed form as the invention may assume various alternative forms. On the contrary, the detailed description covers all the relevant modifications and alterations made to the present invention, unless the claims expressly state otherwise. The present invention will now be described with reference to Figure 1 , Figure 2, Figures 3A - 3H, and Figure 4.

Two main aspects will be discussed herein.

The first aspect relates to an ion-sensitive field-effect transistor (ISFET) (100) having nanostructures (109) for sensing ions and measuring ion concentration in solution with improved sensitivity and efficiency.

The first aspect of the present invention will now be elaborated with the aid of Figure 1 and Figure 2.

In essence, the ISFET (100) of the present invention comprises a base layer (101), for example a silicon layer; a source (102) at one end of the base layer (101), and a drain (103) at the other end of the base layer (101), forming a sensing region in between; a layer of oxide (104), for example a layer of silicon oxide, covering the source (102), the drain (103), and the sensing region; and a layer of sensing membrane (105), for example a layer of silicon nitride, covering the layer of oxide (104). Optionally, the ISFET (100) may further comprise an additional layer of chemical membrane on top of the layer of sensing membrane (105). This further expands the use of the ISFET (100) as the additional layer renders it to be used for chemical sensing applications.

In order to increase the sensitivity and efficiency of the present ISFET (100), the surface area that is exposed to ions, which is the sensing region, must be increased. Therefore, the sensing region of the present ISFET (100) is introduced with nanostructures (109), as illustrated in Figure 2. Note that the nanostructures (109) are located at the base layer (101 ) of the sensing region. The layer of oxide (104) and the layer of sensing membrane (105) are deposited thereafter, taking over the shape of the nanostructures (109).

Each of the nanostructures (109) has a diameter of less than 100nm. Further, the nanostructures (109) have a distance of less than 100nm from each other.

The nanostructures (109) mentioned herein can be of any shape. For example, the nanostructures (109) can be of nanopillars having cylindrical shape, needlelike shape, or a combination thereof.

The second aspect relates to a method to fabricate the aforementioned ISFET (100) having nanostructures (109) for sensing ions and measuring ion concentration in solutions with improved sensitivity and efficiency. The second aspect of the present invention will now be elaborated with the aid of Figure 3A to Figure 3H, and Figure 4

In essence, the method comprises a first step of doping the base layer (101) to form the source (102) and the drain (103), and therefore forming the sensing region in between (Figure 3A). As mentioned above, the base layer (101) can be of the silicon layer. The next step is to deposit and pattern a layer of thin metallic film (106) on top of the base layer (101), covering the sensing region (Figure 3B). Preferably, the layer of thin metallic film (106) is of cobalt, iron, nickel, gold, or a combination thereof.

Thereafter, the layer of thin metallic film (106) is nucleated at a temperature above 400°C to form nanoparticles (107) (Figure 3C). The nanoparticles (107) act as masks in a subsequent etching process. Figure 4 illustrates the nanoparticles that are formed from this particular nucleating process under a scanning electron microscope.

Subsequently, a layer of resist material (108) is deposited on top of the source

(102) and the drain (103) to protect the same from the subsequent etching process (Figure 3D).

After that, the base layer (101) at the sensing region, which is exposed and not covered by the nanoparticles (107), is subjected to the etching process (Figure 3E). The etching process may be of any known etching process, preferably, the process is a chemical etching process, or a plasma etching process with hydrogen bromide or sulfur hexafluoride. The nanostructures (109) that are resulted from this particular step have these two particular characteristics as previously mentioned, which are a) each of the nanostructures (109) has a diameter of less than 100nm; and b) the nanostructures (109) have a distance of less than 100nm from each other. Furthermore, the nanostructures (109) can be of any shape. For example, the nanostructures (109) can be of nanopillars having cylindrical shape, needle-like shape, or a combination thereof.

Soon after that, the nanoparticles (107) and the layer of resist material (108) are removed (Figure 3F).

The layer of oxide (104) is then formed; covering the source (102), the drain

(103) , and the sensing region, which now comprises the nanostructures (109) (Figure 3G). Preferably, the layer of oxide (104) is grown by thermal oxidation. As mentioned above, the layer of oxide (104) can be of the layer of silicon oxide.

Lastly, the layer of sensing membrane (105) is deposited on top of the layer of oxide (104) (Figure 3H). This particular step can be achieved by using plasma enhanced chemical vapour deposition (PECVD), or the like. As mentioned above, the layer of sensing membrane (105) can be of the layer of silicon nitride.

Optionally, the method may further comprise a step of depositing a layer of chemical membrane on top of the layer of sensing membrane (105).

Claims

An ion-sensitive field-effect transistor (ISFET) (100) comprises:

a) a base layer (101);

b) a source (102) at one end of the base layer (101 ) and a drain (103) at the other end of the base layer (101), forming a sensing region in between;

c) a layer of oxide (104) covering the source (102), the drain (103), and the sensing region; and

d) a layer of sensing membrane (105) covering the layer of oxide; characterized in that the base layer (101) at the sensing region comprises nanostructures (109);

wherein each nanostructure (109) has a diameter of less than 100nm; wherein the nanostructures (109) have a distance of less than 100nm from each other.

An ion-sensitive field-effect transistor (ISFET) (100) in accordance with claim 1 , wherein the nanostructures (109) are nanopillars having cylindrical shape, needle-like shape, or a combination thereof.

A method for fabricating an ion-sensitive field-effect transistor (ISFET) (100) is characterized by:

a) doping a base layer (101 ) to form a source (102) and a drain (103), forming a sensing region in between;

b) depositing a layer of thin metallic film (106) on top of the base layer (101), covering the sensing region;

c) patterning the layer of thin metallic film (106);

d) nucleating the layer of thin metallic film (106) at a temperature above 400°C to form nanoparticles (107), wherein the nanoparticles (107) act as masks in a subsequent etching process; e) depositing a layer of resist material (108) on top of the source (102) and the drain (103); f) etching the base layer (101) to form nanostructures (109), wherein each nanostructure (109) has a diameter of less than 100nm, and the nanostructures (109) have a distance of less than 100nm from each other;

g) removing the nanoparticles (107) and the layer of resist material (108);

h) growing a layer of oxide (104); and

i) depositing a layer of sensing membrane (105).

A method in accordance with claim 3, wherein the nanostructures (109) are nanopillars having cylindrical shape, needle-like shape, or a combination thereof.

5) A method in accordance with claim 3 further comprises a step of depositing a layer of chemical membrane on top of the layer of sensing membrane (105).

6) A method in accordance with claim 3, wherein the base layer (101) is a silicon layer.

7) A method in accordance with claim 3, wherein the layer of thin metallic film (106) is of cobalt, iron, nickel, gold, or a combination thereof.

8) A method in accordance with claim 3, wherein the base layer (101) is etched via chemical etching, or plasma etching with hydrogen bromide or sulfur hexafluoride.

9) A method in accordance with claim 3, wherein the layer of oxide (104) is a layer of silicon dioxide.

10) A method in accordance with claim 3, wherein the layer of oxide (104) is grown by thermal oxidation. 11 ) A method in accordance with claim 3, wherein the layer of sensing membrane (105) is a layer of silicon nitride.

12) A method in accordance with claim 3, wherein the layer of sensing membrane (105) is deposited by using plasma enhanced chemical vapour deposition (PECVD).