WO2013175503A2

WO2013175503A2 - Nanostructure based mems preconcentrator

Info

Publication number: WO2013175503A2
Application number: PCT/IN2013/000322
Authority: WO
Inventors: Sheetal PATIL; V. Ramgopal RAO; Soumyo Mukherji; Nehul GULLAIYA
Original assignee: NANOSNIFF TECHNOLOGIES PVT Ltd
Current assignee: NANOSNIFF TECHNOLOGIES PVT Ltd
Priority date: 2012-05-22
Filing date: 2013-05-20
Publication date: 2013-11-28
Anticipated expiration: 2014-11-22
Also published as: WO2013175503A3

Description

NANOSTRUCTURE BASED MEMS PRECONCENTRATOR

Technical Field

The present invention relates to a preconcentrator and more specifically, a nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases.

Background

A vapor/gas preconcentrator is a device whose main function is to improve sensitivity and selectivity of detection systems towards target vapor analytes. This is done by collecting and concentrating analytes in vapor phase for a predetermined period of time and then subsequently facilitating a controlled release of a particular analyte as a concentrated wave into the sensor/detector chamber. The preconcentrator operates on the principle of thermal heating with a short-time constant for facilitating the controlled release of various analytes of interest at different temperatures. It is a generic front-end of vapor phase sensing systems and can also be used with a variety of analytical and detection systems, such as gas chromatographs, mass spectrometers, ion mobility spectrometers (IMS), and microelectromechanical systems (MEMS)-based chemical sensors.

Conventional preconcentrators which are known in the art, have one or the other drawback such as low thermal stability, high power consumption, low precision and selectivity, bigger dimensions, difficult and expensive to manufacture.

WO2008021275 discloses an Insulated preconcentrator for use in e.g. chemical detector. The preconcentrator has a hollow enclosure such as capillary tube, and a sorbent material such as polymers, tenax, Carbosieve, maintained within a hollow enclosure. A heating current source provides an electrical current for heating the hollow enclosure, where the sorbent material absorbs a target chemical at a temperature and desorbs the target chemical at another temperature. The hollow enclosure is formed from an electrically conductive material such as stainless steel.

WO2008/144076 discloses a thermal preconcentrator unit and a method for concentrating chemical species. The thermal preconcentrator unit comprises a thermoelectric device having a temperature controlled surface; and a sorbent material disposed on and in thermal contact with the surface and configured to concentrate a chemical species. The thermoelectric device is configured to cool and heat the temperature controlled surface to promote sorption and desorption of chemical species onto and from the sorbent material. The sorbent material comprises a carbon nanofibre, nanoparticles or fibres.

Carbon based nanostructures suffer from high cost of fabrication, complex manufacturing process with the requirement of high temperatures ~900 °C for growth, incompatibility with semiconductor manufacturing technologies and the use of hazardous processes materials.

Therefore, there is a need of a low-cost, small size preconcentrator device having additional features such as high precision and selectivity, high thermal stability, low response time, low power consumption.

Summary

An embodiment of the present invention describes a nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases. The preconcentrator comprises a sorbent layer formed using semi-conducting oxide nanostructures. The sorbent layer sorbs or desorbs analytes on varying temperature in the preconcentrator.

Another embodiment of the present invention describes a nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases comprising a semiconductor substrate, a micro heater formed on the substrate for heating the preconcentrator, a temperature sensor formed on the substrate for controlling the temperature of the preconcentrator, and a sorbent layer formed over the micro heater and the temperature sensor. The sorbent layer is formed using semi-conducting oxide nanostructures.

The present invention also describes a process for fabricating a nanostructure based MEMS preconcentrator. The process comprises cleaning a silicon substrate layer, forming a first insulating layer over the substrate layer, forming a micro heater and a temperature sensor, forming a second insulating layer which is followed by a nanostructure seed layer over the micro heater and the temperature sensor, removing excess silicon from under the micro heater and the temperature sensor to reduce the thermal mass, and forming a sorbent layer on the second insulating layer using semi-conducting oxide nanostructures.

The nanostructures play a dual role, as a sorbent layer during the collection phase and that of a desorbing surface during the release phase of operation. The release is done by heating the sorbent layer and directing the desorbed analytes towards the detection chamber of the system.

Brief Description of the Drawings

The aforementioned aspects and other features of the present invention will be explained in the following description, taken in conjunction with the accompanying drawings, wherein:

Figure 1 illustrates a nanostructure based MEMS preconcentrator according to an embodiment of the present invention.

Figure 2 illustrates a process for fabricating a Nanostructure based MEMS preconcentrator according to an embodiment of the present invention.

Figure 3 illustrates a schematic of microheater and resistive temperature detector (RTD) according to an embodiment of the present invention. Figure 4 illustrates a response of the micro cantilever sensor based detection system.

Figure 5 illustrates the response of the micro cantilever sensor along with the preconcentrator to different nitro compounds and moisture.

Figure 6 illustrates a flow diagram of a process for fabricating a nanostructure based MEMS preconcentrator according to an embodiment of the present invention.

While the invention will be described in conjunction with the illustrated embodiment, it will be understood that it is not intended to limit the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined.

Detailed Description

The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments. The present invention can be modified in various forms. Thus, the embodiments of the present invention are only provided to explain more clearly the present invention to the ordinarily skilled in the art of the present invention. In the accompanying drawings, like reference numerals are used to indicate like components.

Figure 1 illustrates a nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases according to an embodiment of the present invention. The preconcentrator comprises a semiconductor substrate, a micro heater, a temperature sensor, and a sorbent layer. The micro heater is formed on the substrate for heating the preconcentrator to various temperatures. The sensor is a micro-resistance based temperature sensor and is formed on the substrate for sensing and controlling the temperature of the preconcentrator. The sorbent layer is formed on the micro heater and the temperature sensor. The sorbent layer sorbs or desorbs analytes based on the temperature. The sorbent layer is formed using semiconducting oxide nanostructures (such as nano-particles, nano-belts, nano-rods, ' nano-tubes and nano-wires (NWs)). Semiconducting Oxide Nano-structures include but not limited to following materials such as ZnO, SnO₂, TiO₂, WO₃ and other material, which act as a sorbent layer and provide very high sorption capacity due to their high surface area to volume ratio. In one embodiment, the semiconductor substrate is a silicon substrate.

Nanostructures play the dual role, as a sorbent layer during the collection phase and that of a desorbing surface during the release phase of operation. The release is done by heating the sorbent layer and directing the desorbed analytes towards the detection chamber of the system. In the present invention, due to the small size of the device and low power requirements, it can also be easily incorporated and accommodated within small form factor systems such as the handheld detection systems.

The micro heater is integrated with a RTD (resistive temperature detector) sensor. Its main role is to quickly heat up the preconcentrator surface and desorb the adsorbed analytes as per the requirement. The dimension of the micro heater and the amount of adsorbent mass are determined based upon the critical bed mass, length, resident time, breakthrough volume, and adsorbent properties. According to an embodiment of the present invention the individual Pt/Ti micro heater and micro resistive temperature sensor have the following dimensions:

• Width of 150 ym,

• Length of 800 μηι and

• Thickness of 110 nm.

STEP 1: RCA cleaning and thermal oxidation: In the first step of the fabrication, a silicon substrate of size 2" is cleaned by a Radio Corporation of America (RCA) process followed by a formation of an insulating layer of a silicon dioxide of thickness 1 μιτι.

STEP 2: Transfer of Microheater, RDT and insulating layer pattern:

In the second step, the patterns are transferred using conventional liftoff photolithography process. For this positive photo resist, the photolithography process with the corresponding masks is used. The samples are exposed to UV light using a single sided mask aligner and then subjected to development followed by rinsing with water. The Ti/Pt (10 nm/100 nm) thin film as heater and RTD shown in Figure 2b as Layer-1 , the contacts pads Ti/Au (10 nm/200 nm) thin film shown in Figure 2c as Layer-2 and Si0₂ (~150 nm) thin film shown in Figure 2d as an insulating layer (Layer3) are deposited using metal and dielectric sputtering techniques. Deposition is followed by liftoff in acetone, rinsing with water and subsequent N₂ drying. Schematic of microheater and RTD is shown in Figure 3. Excess silicon is etched from the backside of the silicon substrate using TMAH process to reduce thermal mass. Hydrothermal growth of nanostructures viz, nanowires, nanorods, nanotubes etc. is selectively carried out on patterned Si0₂ layer. The nanostructures of different semiconducting oxide materials like ZnO, Sn02, T1O2, WO3 etc. can be used as a sorbent layer. These nanostructures can be used as it is or coated by forming SAMs of sorbent polymer which has affinity towards a particular chemical, gas or explosive.

STEP 3: Hydrothermal growth of nanowires:

In the third step, the sorbent layer of ZnO nanowires / nanorods is grown by a hydrothermal method. The nanowires are grown on a substrate coated with a ZnO nanoparticles synthesized using a chemical route acting as a seed. The seed layer of ZnO can also be deposited using dielectric sputtering. For ZnO nanoparticles a 30 mM NaOH solution in methanol is added drop wise in a zinc acetate di-hydrate solution in methanol under continuous stirring at 60°C. The stirring is continued for additional 2 hours. The synthesis results in ZnO nanoparticles having dimensions of the order of ~50 nm. The nanoparticle solution is stable for a period of 15 days. These nanoparticles are drop casted / spin coated onto the substrate. The growth of nanowires is achieved by suspending the nanoparticle coated wafer upside down over an aqueous equimolar (25 mM) solution of zinc nitrate and hexamine kept at 90°C. The aspect ratio of resulting nanowires can easily be varied by controlling the process parameters namely concentration of solution, temperature and reaction duration.

Experimental details:

The MEMS pre-concentrator has been manufactured and tested for preconcentrating nitro based compounds and increasing their detection, sensitivity and selectivity. The invented preconcentrator has been tested with MEMS devices such as micro- cantilever based detection systems. These micro cantilevers are coated with coating material which is sensitive to nitro based compounds. This micro-cantilever sensor used in the experiment also responds to moisture/humidity and thus suffers selectivity issues. The pre-concentrator of the present invention has eliminated the above mentioned problem.

The experiments were conducted using micro-cantilever based detection system with and without the use of the invented preconcentrator device.

Example 1: Experiments without preconcentrator:

Figure 4 illustrates a response of the sensor when the nitro based compounds and water vapor/moisture are exposed one by one to the micro cantilever sensor based detection system. It is observed that the sensor responds to both the nitro based compounds and moisture equally.

Example 2: Experiment with preconcentrator:

The preconcentrator was then added in series before the sensor chamber in the detection system. The same experiment was repeated with the use of the preconcentrator device. The nitro based compounds and water vapor / moisture were exposed one by one to the preconcentrator device mounted in a flow cell for a particular time intervals. The preconcentrator device was then heated to temperatures in the range of 40 - 70°C. The nitro based compounds were released at their respective temperatures and were detected by the micro cantilever sensor based detection system. Figure 5 shows two ^distinct responses to the nitro compounds and the moisture when the preconcentrator is used along with the micro cantilever sensor. The detection system show peaks of higher sensitivity for nitro compounds and no response to moisture when used with a preconcentrator.

The above exemplary experiments demonstrate and validate the intended purpose of the invention which is to increase sensitivity and improve selectivity of gas / chemical and vapor sensing and detection systems.

Figure 6 illustrates a flow diagram of a process for fabricating a nanostructure based MEMS preconcentrator according to an embodiment of the present invention. A silicon substrate is cleaned in step 601. A first insulating layer is formed over the silicon substrate in step 602. A micro heater and a temperature sensor are formed on the insulated silicon substrate in step 603. A second insulating layer is formed which is followed by a nanostructure seed layer over the micro heater and the temperature sensor in step 604. Excess silicon is removed from under the micro heater and the temperature sensor to reduce the thermal mass in step 605. A sorbent layer is formed/ grown on the second insulating layer using semi-conducting oxide nanostructures in step 606.

The nano-structure based MEMS preconcentrator of the present invention has overcome the limitations of prior arts and has following advantages: a) Semiconducting oxide nano-structures of materials such as ZnO, SnO2, TiO2, WO3 etc. acting as a sorbent layer give a very high sorption capacity due to their high surface area to volume ratio. However, the conventional preconcentrator systems use carbon based nanostructures such as Carbon nano-tubes, Carbon nano-fibers etc. as the sorbent layer. Carbon based nanostructures suffer from the drawbacks of high cost of fabrication, complex manufacturing processes with the requirement of high temperatures ~900°C for growth, incompatibility with semiconductor manufacturing technologies and the use of hazardous process materials. b) Semi-conducting oxide nanostructures grown by hydro-thermal processes offer various advantages that include low growth temperatures ~90°C, non hazardous and simple fabrication processes, requirement of low cost, simple equipment for structural growth making the current invention easy to manufacture and cost effective. c) The preconcentrator devices fabricated as part of the present invention support and are compatible with Semiconductor Manufacturing Technologies. Besides, the fabrication process has mass manufacturing capability at low costs. The chemicals used in the present invention for the growth of nanostructures are non hazardous, eco-safe and safe to handle. d) Semi-conducting oxide nano-structures based preconcentrator sorbent surfaces are thermally stable, chemically inert during release and do not undergo a permanent reaction with analytes. This is critical as the device requires very weak bonding between the analyte and the surface so that the analyte can be released at desired temperatures. Earlier materials used for this purpose such as Carbon nano-tubes etc., disclosed in prior art are reactive and generate toxic gases during release which is highly undesirable for preconcentrator action. e) Small device size (in micrometers): Conventionally disclosed pre- concentrators are large in size and designed using stainless steel tubes with heating coils. The invented devices are very small in size - of the order of microns, which can be integrated in small form factor applications such as handheld systems, miniaturize sensor modules etc. The net flow cell volume is approximately 200-300 micro-liters. The small size advantage also brings down the fabrication cost of the device and the cost of raw materials required. It can easily be used in applications where devices have to be mass deployed. f) The present invention is a single chip solution that requires low power consumption i.e. maximum of a few Watts of power to achieve higher temperature applications. This additionally provides the flexibility of employing them in small battery operated applications that provide systems only a few voltage for operation. The conventional preconcentrators were large, instrument like designs of stainless steel tubes with heating coils which consumes very high power to operate. g) Low response time: The preconcentrator device has very low thermal mass which allows instant heating and cooling of the sorption layer (in the order of milliseconds). For accurate preconcentrator action, fast heating and release of the adsorbed analyte is the key. The preconcentrator device of the present invention has this advantage and is very durable over hundreds, possibly thousands of heating and cooling cycles. h) The invented device acts as a moisture adsorbent layer as semi conducting oxide nano-structures tend to adsorb moisture even in as high as 95% RH in the environment and the same can be completely released at high temperature such as ~200°C. This allows the use of the preconcentrator as a very effective moisture removal device for sensors susceptible to moisture. i) The use of metal such as Platinum as a resistive heating material implies that the device has the capability of very high precision temperature control. This comes from the fact that the resistance of Platinum is highly sensitive towards change in temperatures. Hence, it can be used to improve selectivity of sensors to certain adsorbed analytes to release on known temperatures. Rest of the analyte remain trapped on the sorbent layer and are purged later. j) The invented device and the materials used in it have the uniqueness of being thermally stable in normal operation and upto the order of several hundred degrees Celsius. This opens up the possibility of using this preconcentrator in various gas/ chemical detection applications with varying desorption temperatures.

Although the invention of device and method has been described in connection with the embodiment of the present invention illustrated in the accompanying drawings, it is not limited thereto. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be made thereto without departing from the scope and spirit of the invention.

Claims

We claim:

1. A nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases comprising: a sorbent layer formed using semi-conducting oxide nanostructures, said sorbent layer sorbs or desorbs analytes on varying temperature in said preconcentrator.

2. The preconcentrator as claimed in claim 1 comprising a micro heater integrated with a temperature sensor to control the temperature of said preconcentrator.

3. The preconcentrator as claimed in claim 2, wherein said temperature sensor is a RTD (resistive temperature detector) Sensor.

4. The preconcentrator as claimed in claim 1 , wherein said semi-conducting oxide nanostructures is formed by a material which is selected from a group comprises ZnO,, SnO₂, T1O₂, and WO₃.

5. The preconcentrator as claimed in claim 1 , wherein said nanostructure is selected from a group comprises nano-particles, nano-belts, nano-rods, nano- tubes and nano-wires.

6. A nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases comprising: a semiconductor substrate;

a micro heater formed on said substrate for heating said preconcentrator; a temperature sensor formed on said substrate for controlling the temperature of said preconcentrator; and

a sorbent layer formed over said micro heater and said temperature sensor, said sorbent layer is formed using semi-conducting oxide nanostructures.

7. The preconcentrator as claimed in claim 6, wherein said semiconductor substrate is a silicon substrate.

8. The preconcentrator as claimed in claim 6, wherein said temperature sensor is a RTD (resistive temperature detector) sensor.

9. The preconcentrator as claimed in claim 6, wherein said nanostructure is selected from a group comprises nano-particles, nano-belts, nano-rods, nano- tubes and nano-wires.

10. The preconcentrator as claimed in claim 6, wherein said semi-conducting oxide nanostructures is formed by a material which is selected from a group comprises ZnO,, Sn0₂, T1O2, and W0₃.

11. The preconcentrator as claimed in claims 6 and 8, wherein said micro heater is integrated with said RTD Sensor to control the temperature of said preconcentrator.

12. A process for fabricating a nanostructure based MEMS preconcentrator, the process comprising cleaning a silicon substrate;

forming a first insulating layer over said substrate;

forming a micro heater and a temperature sensor on said insulated silicon substrate;

forming a second insulating layer which is followed by a nanostructure seed layer over said micro heater and said temperature sensor;

removing excess silicon from under said micro heater and said temperature sensor to reduce the thermal mass; and

forming a sorbent layer on said second insulating layer using semiconducting oxide nanostructures.

13. The process as claimed in claim 12, wherein said sorbent layer is formed by a hydro-thermal process.

14. The process as claimed in claim 12, wherein said temperature sensor is a RTD (resistive temperature detector) sensor to heat up the preconcentrator surface and desorbs the adsorbed analytes.

15. The process as claimed in claim 12, wherein said nanostructure is selected from a group comprises nano-particles, nano-belts, nano-rods, nano-tubes and nano-wires.

16. The process as claimed in claim 12, wherein said semi-conducting oxide nanostructures is formed by a material which is selected from a group comprises ZnO,, Sn02, T1O2, and WO₃. 7. A nanostructure based MEMS preconcentrator for detecting chemicals substantially as herein described with reference to the accompanying drawings and examples.

18. A process for fabricating a nanostructure based MEMS preconcentrator substantially as herein described with reference to the accompanying drawings and examples.