Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N1/00—Sampling; Preparing specimens for investigation
  • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
  • G01N1/40—Concentrating samples
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N1/00—Sampling; Preparing specimens for investigation
  • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
  • G01N1/40—Concentrating samples
  • G01N1/405—Concentrating samples by adsorption or absorption
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/60—Construction of the column
  • G01N30/6095—Micromachined or nanomachined, e.g. micro- or nanosize

Definitions

• the present invention relates to a preconcentrator and more specifically, a nanostructure based MEMS preconcentrator for detecting chemicals, explosives and gases.
• 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.
• vapor phase sensing systems 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.
• analytical and detection systems such as gas chromatographs, mass spectrometers, ion mobility spectrometers (IMS), and microelectromechanical systems (MEMS)-based chemical sensors.
• 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.
• 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.
• 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.
• 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.
• FIG 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.
• 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, SnO 2 , TiO 2 , WO 3 and other material, which act as a sorbent layer and provide very high sorption capacity due to their high surface area to volume ratio.
• 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.
• 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:
• Figure 2 illustrates a process for fabricating a Nanostructure based MEMS preconcentrator according to an embodiment of the present invention.
• 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 ⁇ .
• RCA Radio Corporation of America
• STEP 2 Transfer of Microheater, RDT and insulating layer pattern:
• the patterns are transferred using conventional liftoff photolithography process.
• 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 Si0 2 ( ⁇ 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 N 2 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 Si0 2 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• 2013
  • 2013-05-20 WO PCT/IN2013/000322 patent/WO2013175503A2/fr not_active Ceased

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