Patent Application
20220276197
The present invention relates generally to sensors, and more particularly to olfactory sensors using gas-sensitive polymers materials to detect analytes.
Sensors are an important part of the present day electronic systems. Availability of wide variety of sensors to detect different physical responses is assisting in the design of better solutions to improve the living environment. These sensors are an essential part of many handheld devices, earning them a tag of being ‘smart’. Humans have five basic senses: vision, hearing, touch, taste and olfaction. The first three of these senses are responsive to physical interaction whereas the taste and olfaction abilities are based on chemical responses to different analytes. To develop an artificial intelligence system, capable of replicating human olfaction abilities, sensors capable of detecting chemical stimulants need to be developed.
The sense of smell provides very useful information to mammals by helping to analyse, distinguish or identify numerous odorants. Research on developing the means to extract information from odorants has grown tremendously[1][2]. The approach towards development of artificial olfactory systems generally resembles their biological counterparts where the olfactory receptors react to chemical stimuli of the odorant and generate signals for the information to be perceived by the brain. The broad and diverse range of smells mammals can process are a result of at the very least, thousands of years of evolution. Research in chemistry has shown promise and potential to develop advanced vapour sensitive materials, taking these devices closer to a truly artificial olfactory sensor platform that closely mimics its biological equivalent. The recent advancements in chemistry has given new potential materials and multiple chemical derivatives thereof, with the potential to deliver an effective olfactory sensing platform. One such class of materials is conducting polymers (CP) which exhibit a change in their electrical properties with exposure to different odorant vapours[3][4][5]. The objective of this research is to integrate these gas sensitive conducting polymers with an electronic platform for development of a small and inexpensive olfactory sensor chip.
Over the years, a number of gas sensing systems have been developed using many different sensing mechanisms[2][6][7]. The first reported olfaction system was introduced as a Mechanical nose by Moncrieff in early 19605[8]. This research was followed by development of a number of different sensing mechanisms which can be broadly categorised as metal oxide sensors, conducting polymers, bioelectronics noses, optical and/or piezoelectric sensors [2][6][7]. Most commercially available electronic nose systems are based on metal oxide sensors technology[7][9]. The metal oxide sensors have a strong sensitivity, a relatively fast response time for analyte detection and are compatible with standard silicon processing which makes them cost effective [7][9]. The operation of metal oxide gas sensors is based on principle of a change in conductance of an oxide layer when it is exposed to a gas analyte. This change in conductance is (normally) proportional to concentration of the exposed analyte[6]. The selectivity of these sensors are typically modified by doping the oxide layer with different noble metals[9]. The metal oxide sensors require high operating temperatures which is a major limiting factor. For an integrated design application, they would require an on-chip microheater which is linked to higher power consumption, making it difficult to be used in handheld or mobile devices[6][9]. However, the metal oxide sensor technology still remains the most common olfactory sensor platform and different research efforts have been reported that show an improvement in their performance. The recent work in the in implementation of such systems, use nanostructured materials such as nanowires/nanotubes as well as other new materials some of which show promise for the future of metal oxide sensor technology[10].
A bio-electronic nose is a relatively new but promising class of olfaction system based on the use of biological olfactory receptors as sensing elements for detecting different odorant molecules[2][11]. The biological sensing elements in these system are either olfactory receptor proteins or olfactory receptor cells[12]. The sensing mechanism of a bio-electronic nose is a two layer structure where the primary layer of biological olfactory receptor cells or receptor proteins, interacts with the exposed analyte vapour to generate a biochemical signal and the second layer of transducer converts it to an electrical signal[2]. Different mechanisms, such as the use of microelectrodes, resonance detection, piezoelectric layers and optical detectors have already been used as a secondary layer electrical transducer[2][12]. The bio-electronic nose has a compatibility with traditional silicon systems, which makes them economical for fabrication on a mass production scale[2][13]. The selectivity of the bioelectronic nose is high as its receptor layer is developed using biological olfactory receptor proteins/cells which are able to detect most of the odors to which a human nose can respond [14]. The sensitivity of these systems is dependent on the properties of transducer layer and its integration with biological receptor cells[14]. Recent advancements in biotechnology are helping researchers find new methods of binding the olfactory bio-cells of the bio-electronic noses to the transducer layer. New nanomaterials, like graphene and carbon nanotubes, have also been reported for their possible application in bioelectronic nose system for improving its sensitivity[2][14]. The bioelectronic nose has shown potential to be a promising olfactory sensor platform. However, there are still some limitations that include stability, repeatability of measurements and the ease of integration as a single chip olfactory sensor platform [2]. With continued research in this area, improvements in performance of bioelectronic noses can be expected in the future.
Piezoelectric sensors are very popular for wide range of sensing applications. They are also reported to be used as acoustic wave sensors in different gas sensing applications[6][7][15]. These sensors employ different piezoelectric materials to generate an acoustic wave which travels through or along their surface[15]. The nature travel for acoustic wave is used to classified sensors as surface acoustic wave sensors (SAW) or bulk acoustic wave sensor (BAW) also known as Quartz crystal microbalance (QCM) [7][15][16]. When used in gas sensing applications, these acoustic wave sensors use a thin coating of different gas sensitive materials on piezoelectric structures. Upon exposure to a vapour analyte, the gas sensitive layer interacts with vapour molecules of the analyte to produce a change in its physical properties which is reflected as a resultant change in resonant frequency of the sensor[6][15]. These sensors are designed in a silicon compatible environment which gives them advantages of small size, low power operation and lower cost because of mass production facilities. For olfactory applications, they are reported to have advantages of high sensitivity and low response time. Reproducibility of results and higher dependency on environment variables like temperature or humidity are primary causes of concern for these systems[6][7][15].
There are olfactory systems based on optical sensors for vapour detection which work on interaction of gas molecules with electromagnetic light waves. Optical sensors for olfactory systems offer multiple possibilities for extraction of information, like measurement of reflection, refraction, luminance, fluorescence, wavelength or absorbance[7][17]. This can be very helpful in designing a higher sensitivity system with a lesser number of sensors in an array. A general design of optical olfactory sensor array is incorporated with a group of multimode optical fibers with their tip coated with different gas sensitive materials, generally polymers[7][18]. The optical olfactory systems have fast response time and good sensitivity for many analytes but are complex and expensive. Packaging of these systems is an important limiting factor that needs to be addressed well in order to overcome the noise generated because of optical interference[18].
Conducting polymers, after their evolution in late 1970's, became a well-researched class of materials in the field of olfactory sensors[19][20]. Since the year 2000, when the joint Nobel Prize in chemistry was awarded to Heeger, MacDiarmid and Shirakawa “for the discovery and development of conductive polymers”, the research in this domain has intensified[20]. The conducting polymers operate at room temperature and can be easily deposited using electrochemical deposition techniques. The electrochemical process using three-electrode setup for electrodeposition of conducting polymers provides better control over the polymerization process and is a preferred method for polymer synthesis for different sensor applications [19]. The conducting polymers offer fast response time and high sensitivity towards number of analytes[20]. The sensitivity of polymers is based on a number of possible mechanisms such as oxidation or reduction of polymer, mobility variation of charge carriers in polymer chains, change in energy band structure of polymer or possible physical change such as swelling or shrinking of polymer on interaction with analyte particles[5][21]. The high sensitivity of the conducting polymers results in their lower selectivity for different analytes[22]. Techniques to improve selectivity and synthesise multiple chemically diverse conducting polymers have been reported, including the use of different monomer units for polymer synthesis, co-deposition of different monomer units to create a co-polymer, polymerisation at different oxidation potentials and the use of different dopants for polymer depositions[4][5][23][24]. Eighty-one chemically diverse conducting polymer derivatives[24] have been reported. These polymers were used for chemical identification of twelve different analytes by analysing the change in their resistivity upon exposure to these analyte vapours and then using principal component analysis techniques for processing the measurement results[24]. These findings indicate a modification in electrical properties of the conducting polymer on exposure to different analytes.
In past work, a floating gate metal oxide semiconductor (FGMOS) transistor with Polypyrrole (PPy) as the sensing polymer was successfully tested for sensitivity to different analytes[25][26]. The FGMOS is a dual gate transistor (control gate and floating gate) in which a change in the charge density on floating gate causes a shift in its normal electrical characteristics.
Further research and development was undertaken by the inventive entity of the present application to build and improve upon the forgoing groundwork laid in the field of olfactory sensing technology.
According to a first aspect of the invention, there is provided a floating gate metal oxide semiconductor (FGMOS) transistor comprising:
a substrate having a source region, a drain region, and a channel region residing therebetween;
gate stack layers deposited on said substrate, among which there is defined a stacked gate structure that resides in overlying relation to the channel region, and comprises, in sequential order starting from said substrate, a first dielectric layer, a floating gate, a second dielectric layer and a control gate;
an extension pad that resides in exposed condition outside said stacked gate structure, comprises a constituent material of an outermost conductive layer of said gate stack layers situated furthest from the substrate, and is conductively linked to the floating gate; and
a floating gate terminal by which an electrical bias is applicable to the floating gate and the extension pad conductively linked thereto for use in electrodeposition of a conducting polymer onto said extension pad.
According to a second aspect of the invention, there is provided a sensing device comprising an array of sensors each comprising a respective transistor of the forgoing type, wherein the extension pads of the transistors of at least some of the sensors comprise outer surfaces composed of polymer material of varying chemical composition to one another.
According to a third aspect of the invention, there is provided a method of producing the sensing device of the forging type recited in the second aspect of the invention, said method comprising performing electrodeposition of chemically diverse polymeric films onto the extension pads of different subsets of said sensors basis by, for each subset of said sensors, applying an electrical bias to the extension pad(s) of said subset while said subset is submerged in a polymer precursor solution in order to deposit a respective polymer film onto the extension pad(s) of said subset.
One embodiment of the invention will now be described in conjunction with the accompanying drawings in which:
One objective of the research behind the present invention was to develop a small, inexpensive programmable olfactory sensor platform using a commercially available silicon technology. The Complementary Metal Oxide Semiconductor (CMOS) devices on the silicon substrate are used as the fundamental building blocks for many integrated circuits in present day electronics. The CMOS technology has many advantages that include high speed of operation, low power consumption and a well-established mass production technology.
The Floating Gate Metal Oxide Semiconductor (FGMOS) transistor is well-known device that had been used extensively in flash semiconductor memories. It also has been used as a sensor in many electronic systems[27][28]. The structure of FGMOS transistor is different from that of conventional CMOS transistors in terms of number of gate terminals. The FGMOS transistor has two gate terminals, referred as the control gate and the floating gate. These transistor gate structures are designed with polysilicon layers and a silicon technology with two distinct polysilicon layers is required. In building and testing prototypes of the present invention, a 0.35 μm silicon technology available through Taiwan Semiconductor Manufacturing Company (TSMC) was used for the fabrication of the integrated circuit design. This 0.35 μm silicon technology is one of the few available that offer two polysilicon layers.
In
There are different tunnelling mechanisms responsible for the charge transfer in the FGMOS devices. The most common mechanism is based on Fowler Nordheim (FN) tunneling[30]. The energy band diagram of the FGMOS if shown in
A 3D representation of a novel FGMOS sensor structure of the present invention is shown in
This top metal layer (M4) from this 0.35 μm silicon technology is thus used as an extension pad that is conductively connected to the floating gate poly1 layer of the sensor. To create the floating gate layer connectivity to the topmost metal layer, a stacked bridging structure is created that connects all of the intermediate metal and via layers between the poly1 layer and the top metal layer (M4). For example, the first metal layer (M1) is connected to the floating gate poly1 layer using a “CONTACT” hole though the first inter-layer-dielectric (ILD1) sandwiched between poly1 and M1. The second metal layer (M2) is then connected to M1 through a first via “VIA1” in the second inter-layer-dielectric ILD2 layer sandwiched between M1 and M2, and so on through to M4. The different stacked structures formed among the overall topology of gate stack layers of the FGMOS transistor can be seen in
The floating gate extension pad is the surface onto which the conducting polymers are electrochemically deposited, effectively functionalizing the active sensing area of the sensor. Unfortunately, all four metal layers in this 0.35 μm silicon technology are made from aluminum, which oxidizes and inhibits the electrodeposition of the polymers. To overcome this problem, in preferred embodiments of the present invention, a layer of gold is selectively electrodeposited onto the floating gate extension pad using several post processing steps. In a clean room environment, a process for the selective electroless deposition of gold onto the aluminium extensions was developed, as described in more detail below. The now gold-coated surface of the floating gate extension pad is used as the working electrode of the sensor for the electrodeposition of the desired conducting polymer to be used for olfactory sensing functionality. After successfully depositing the polymer onto the extension pad, characterization of the sensor systems is conducted in a controlled electrical and analyte environment.
The operation of the FGMOS sensor is dependent upon the applied electrical bias to the gates, source, drain and substrate terminals, as well as any charge that has been induced on the floating gate when the conducting polymer on the extension pad interacts with an analyte vapor. In
Each conducting polymer responds in a unique way to different vapour analytes. An example of the time dependence of the source-drain current (IDS) is shown in
The basic feasibility of this type of sensor functionality, though without the novel extension pad of the present invention, has been previously verified [25]. However, to provide a chemically diverse olfactory sensor system, embodiments of the present invention include a novel chip having an array of FGMOS sensors thereon, along with associated electronic control circuits need, all integrated onto a single silicon substrate.
A schematic view of such an embodiment is shown in
The sensors in the array require suitable electrical signals to bias them in a favourable operating region. Access to each extended floating gate sensor pad individually is required for the selective electrodeposition of the polymers. To provide an automated addressing scheme for system testing, a special address and control circuit was designed using counters, multiplexers, decoders and analog buffers as shown in
All the circuit schematics were designed in Cadence Virtuoso schematic composer using TSMC 0.35 μm technology parameters. The address and control circuit of the illustrated embodiment has two operational modes. In a manual mode, the sensors in the array can be addressed manually using the address lines A5-A0 to generate a 6-bit address for all 64 sensors. In the automated mode of address generation, a clock signal is used to trigger a 6-bit counter circuit which counts through all addresses automatically. An array of six multiplexer circuits is used to switch in-between these two modes. The 6-bit address generated by counter or address lines is used to control 8 rows and 8 columns buses that run through the array of sensors. The signals for these row and column buses are generated by decoding the 3-bit address signal to 8-bits using two 3:8 decoder circuits. Every row and column bus combination is used to excite an individual array cell which has one FGMOS sensor of the type disclosed above with the novel polymer coated extension pad, a set of digital gates and two specially designed analog buffer circuits. The digital AND gate uses inputs from row and column bus to generate an output which is used to enable our buffer circuits. An ON-state buffer connects the gate terminals of FGMOS sensor to external pins which are used to pass electrical signals to the sensor. In the manually-addressed programming or setup mode, these electrical signals are used for the polymer electrodeposition onto the extension pads of the arrayed sensors to create the finished sensing device ready for use. In automatically-addressed sensing mode of the finished device, electrical signals are used to perform the analyte detection process.
Two analog buffer circuits are used in each array cell to transmit the floating gate and control gate voltages from respective floating gate and control gate signal lines Vcontrol_gate and Vfloating_gate to the floating gate and the control gate terminals, respectively, of the FGMOS sensor of the cell as shown in
To create the greatest sensitivity, the FGMOS sensors can be biased in the subthreshold or weak inversion region of operation where a slight change in floating gate voltage produces a substantial change in sensor source-drain current. For a linear change in the control gate voltage, the source-drain current changes exponentially and its magnitude can range from a few nanoamperes to many microamperes. To rescale this exponential response onto a linear voltage scale, a transimpedance amplifier circuit may be used, such as that schematically shown in
The transimpedance amplifier output voltage from a simulation is shown in
The final stage of this sensor array system is an analog to digital (A/D) converter designed to produce an 8-bit digital result from the amplifier output. The 8-bit A/D converter yields a voltage resolution of 13 mV (3.3 V/255) which means the sensor can discriminate (a 1-bit change) between voltages having a difference of more than 13 mV. The digital data from the entire array, given that each of the sensors could contain a different polymer, and react differently to given group of analytes, can collectively produce a “digital” fingerprint or 2D “image” for a given analyte. The digital information is easier to store and process. Therefore the A/D converter may be included on the chip in the interest of decreasing the complexities in development of processing algorithms by representing information in more convenient digital form. A block schematic for one embodiment of A/D converter implementation on the chip is shown in
In the illustrated example, the 8-bit counter is synchronized to the clock which is incremented on the positive edge of the clock. The output bus of the counter is connected to an R-2R ladder circuit which converts the 8-bit binary number generated by the counter to its equivalent voltage level. For a counter counting up the R-2R circuit will generate a ramp signal of voltages for every cycle of the count. The analog ramp signal generated with R-2R circuit has high frequency components from the clock superimposed on the voltage ramp. To minimize these high frequency components and its effect on circuit operation, a low pass filter is used between the R-2R ladder circuit and the comparator. The filtered voltage ramp signal is then compared with the output voltage of trans-impedance amplifier using a comparator circuit. Once the ramp signal voltage exceeds the trans-impedance amplifier output, the comparator generates a trigger signal which is used as a latch enable control signal for an 8-bit latch circuit which stores the data. Hence an 8-bit digital number, equivalent to the voltage generated by trans-impedance amplifier output is latched at the output of the A/D converter. A number of simulations were performed to evaluate the performance of the A/D converter circuit. The simulation results show good linearity between the analog input and the digital output for our A/D converter. The primary limitation of the A/D converter is the long high response time as it could require it run through all possible 256 digital states (i.e. 256 clock cycles) to find a match with the input signal. The response time of the polymers to detect presence of any vapour analyte is usually much longer that this response time (256/clock), therefore this A/D circuit is suitable for this application.
The chips from three different fabrication runs were tested for electrical performance of the inventive sensing device employing an array of sensors, each having the novel FGMOS design with the polymer coated extension pad.
Under ideal circumstances, the integration of the polymers with the extended floating gate pad of the sensor should require only one process of electrodeposition of a given polymer. However, past experiences have shown that the polymers do not deposit well on the aluminum surface of the M4 extended floating gate pads of this 0.35 μm silicon-based implementation. In fact, it was observed that instead of polymer deposition, an etching of the aluminium layer was observed[26]. An ideal solution is to coat the surface of extended floating gate pads with some non-oxidizing, non-reactive noble metal. Gold is often used in a thin from for the deposition of organic polymers using the an electrochemical process[33]. It was believed and subsequently discovered that Au would be a suitable metal to work with these chips. However, the process of selective deposition of gold onto the contact pad surface using a standard lithography technique would be difficult to perform due to the size of this silicon chips (˜3×5 mm). Coating the contact pad surface using electroplating is promising but the conventional electroplating process would require a series of electrical connections which would be very complex. An electroless deposition technique for plating has been shown to be an easy and reproducible process with compatibility of electrodeposition on a micron scale[34]. The electroless plating process simply requires only an aqueous solution of the target material and works without the need of any external electrical connections. The aqueous solution used for electroless plating contains a reducing agent for the target material which triggers a chemical reaction when the substrate electrode is immersed into the solution, resulting in reduction of target material onto the substrate, effectively coating it.
Aluminium is very reactive to presence of oxygen and forms a thin native oxide layer on its surface soon after it comes in contact with any oxygen environment. This native oxide layer prevents direct contact with the aluminium surface which makes the electroplating of gold onto the aluminium contact pads very difficult. A well-known industrial solution to this problem includes a three-stage plating process for electroplating gold onto an aluminum surface. The process requires sequential plating of zinc, nickel and then gold layers onto a clean aluminium surface. Some researchers[34] have reported that this process is compatible with microelectronics applications.
Before the plating process is begun, it is very important to clean the surface of the chips to ensure a homogenous deposition. The chips may be rinsed thoroughly in organic solvents (methanol and acetone) followed by a deionised water rinse to remove any organic contaminants. The chips are then immersed into a room temperature aqueous solution of “zincate” a zinc compound from Casewell Inc. The zincate solution first etches the thin aluminium oxide layer present on the surface and immediately follows it up with the deposition of zinc onto the surface which prevents re-oxidation of the aluminium until the next plating process is initiated. The zincate solution is alkaline in nature and can generate complex intermetallic compounds of aluminium which are found to be insoluble in the zincate solution[35]. These insoluble compounds are known as ‘smut’ which can adversely affect the uniformity of the following electroplated layers. To achieve uniformly electroplated surfaces, a process of “desmutting” with a dilute nitric acid solution followed by one more zincate baths may be used. This combined process is called as double zincate process. Desmutting after first zinc bath helps in stripping of undesired smut and nucleated zinc depositions onto the surface of aluminium which helps achieve a homogenous, thin zinc layer on the aluminium surface[35]. The presence of zincate layer on top of aluminium surface was confirmed with optical microscope images and energy dispersive x-ray spectroscopy (EDS) using a FEI Quanta 650 scanning electron microscope available through the Manitoba Institute of Materials at University of Manitoba. The zinc coated samples were processed for electroless nickel growth using another plating solution purchased from Casewell Inc.
The nickel bath requires a proportionate mixing of three nickel concentrates to prepare the final plating solution. The electroless deposition of nickel is an autocatalytic process where the product of the initial chemical reaction acts as the catalyst for the next chemical reactions. The process may employ a bath temperature of 90° C. to trigger the autocatalytic process. When the zincated samples are immersed in the heated nickel bath, a uniform deposition of a nickel film is formed onto the zinc at the plating rate of 400 nm/minute begins.
The thickness of the plated nickel layer may be controlled using the immersion time of the samples in the heated nickel bath. An immersion time of 75 seconds with constant agitation of 100 rpm may be used to achieve approximately a 500 nm thick nickel layer. To confirm the successful deposition of a uniform nickel films on the extended floating gate pad surface, optical microscope images were taken. A uniform metallic appearance of the surface as seen in
A cyanide free immersion gold solution was ordered from Transene Company Inc., Canada. The process may employ a bath temperature of 75° C. to initiate electroless gold depositions, which was found to have a typical deposition rate of −25 nm/minute. This solution was agitated at 100 rpm to ensure uniform depositions. The nickel coated samples were immersed in the heated gold bath for 2 minutes. A bright gold appearance of extended floating gate pads surface was easily visible using the microscope and was again verified using EDS analysis. The electroless plating technique gave an easy and efficient process of producing a gold coated surface for the extended floating gate pads. The next stage was the electrodeposition of the polymers onto these gold-coated surfaces. For simplicity, the initial polymer employed in the tests was limited to polypyrrole, though it will be appreciated that other polymers (conductive or otherwise), may be employed.
For the process of electrodeposition of the conducting polymers, a three-electrode electrochemical cell was used with a platinum electrode as the counter electrode, a silver-silver chloride (Ag/AgCl) electrode as the reference electrode, and the extension pad surface to be electroplated acting as the working electrode. Every electrode the potential was measured with respect to the standard potential of the Ag/AgCl reference electrode. The process of electromigration occurs in-between working and counter electrode where the working electrode acts as a site for the Oxidation-Reduction (Redox) reactions for polymer deposition. The counter electrode acts as source or sink of the charge carriers[36]. Redox potentials for polymer depositions are selected from the analysis of the cyclic voltammetry experiments where working electrode potential is ramped linearly in time while the current is measured.
The chips were packaged in CPGA 69 ceramic packages where the Au bond wires connect electrical terminals from the chip to external pins of the package. The bond wires used are very delicate (˜25 μm diameter) and require very careful handling. In the process of electrodepositing the conducting polymers, whenever electrical potential is formed on the bond wirebonds, polymer deposition can occur. This creates a very undesirable scenario resulting in polymer depositions on undesired places and can in some cases create an electrical short between terminals. To protect the wirebonds from physical forces while processing and have them electrically isolated from the electroplating solution, SU-8 photoresist was used as an insulating layer for the encapsulation of the wirebonds. The SU-8 was carefully injected onto desired wirebonds areas using a medical syringe to achieve the selective encapsulation. The SU-8 coating successfully provided the required physical support to wirebonds but also kept them electrically isolated from electrodeposition solution. A chip processed with this selective encapsulation of wirebonds is shown in
An aqueous polymer precursor solution used for the electrodeposition of polypyrrole was prepared with a 0.1 M pyrrole solution with a 0.1 M H2SO4 in 20 ml of deionised water. A CH Instruments® model 760C potentiostat was used to generate the required electric potentials for the three-electrode deposition setup. Cyclic voltammetry was conducted to observe the electroactivity of the pyrrole monomer in the solution and to find out the available redox potentials suitable for deposition of conducting polymer.
Some sensors of the array were used to test the deposition rate of polymer film and to decide upon the time constraints for a uniform deposition. It was observed that 30 seconds was a suitable time for electrodeposition of a uniform thin film of polypyrrole. Using the automated address generation Verilog code, the Altera DE2-115 development board was used to address each sensor in the array and coat the extended floating gate pad of that addressed sensor at one of two different redox potentials of 0.7 V and 1.2V. The polymer films deposited at 0.7 V had a brownish appearance while the polymer films at 1.2 V were gray in color.
To test electrical properties of polymer-functionalized sensors in an analyte environment, a gas flow apparatus was designed using mass flow controllers. The concentration of analyte vapour in the gas flow was controlled using the ratio of a direct flow of nitrogen in the test chamber to the nitrogen flow through a glass bubbler filled with liquid analyte. A schematic diagram of the gas flow apparatus is shown in
Before the polymer deposition and system characterization, the individual FGMOS sensors were tested for their electrical performance in absence of polymers on their floating gate extension pads. The operation of the FGMOS sensor with any one of its gates used to control the channel in the substrate is expected to resemble a normal MOS transistor. The effective dielectric thickness for control gate is around 5 times that of the gate oxide thickness between floating gate layer and the substrate. The thickness of dielectric layer between gate and substrate has an inverse relationship with the magnitude of field produced in the dielectric. Therefore, it is expected that the control gate terminal requires higher voltages compared to floating gate, for the same equivalent source-drain current in the channel. In
The magnitude of source-drain current was observed to be less than 10−6 A for small control gate voltages. To have better insight into the gate control (VCG) over the source-drain current (IDS), the sensor was biased with a constant drain voltage, VDS=1 V for which the control gate voltage (VCG) was swept from 0-8V and the resultant source-drain current was measured. This data is shown in
The floating gate extension pads of the sensors were coated with polypyrrole from a solution of a 0.1 M solution of a pyrrole monomer in a 0.1 M solution of H2SO4 at a redox potential of 0.7V. This polymer-coated sensing device was kept in nitrogen environment at a constant source-drain current IDS (>10−5A) using a constant electrical biasing conditions. An Agilent 34401A digital multimeter and an Agilent 33220a function generator were programmed using LabVIEW to automate the measurement processes.
The nitrogen flow conditions were maintained for several hours during which no noticeable change in the sensor source-drain current was observed. The first analyte vapour that was used to test these sensors was methanol. The glass bubbler was filled with 20 ml methanol through which and 100 sccm of nitrogen was bubbled through the liquid while the direct flow of nitrogen was turned off thus generating a 100% methanol environment. The constant electrical bias was applied for a 5-minute interval and the sensor source-drain current was measured many times during this interval. This experiment was repeated three times with measurements taken every 20 minutes. In
These results from the methanol vapour experiment confirmed the feasibility of the sensors to detect at least methanol vapour. To ensure the operations with other vapour analytes, similar experiments were conducted using other analytes including, but not limited to, ethanol, acetone and ammonium hydroxide. The polymer coated sensing device was exposed to each of these vapours for an interval of 65 minutes. The sensor source-drain currents were measured for the last 5 minutes of the exposure and compared. A comparative analysis for these measurements is shown in
In another experiment, the sensor transfer characteristic was measured by sweeping the control gate voltage from 0-5V while maintaining a constant source-drain potential of 3.3V. This was repeated after exposing the sensor to each of the four different analytes for a period of one hour. In
In addition to the aforementioned experiments performed on individual sensors of the array under analyte influence, a next phase of experiments were performed in which the core electrical system on the chip was tested under different analyte environments. One chip was designed in a way that every circuit block could be tested individually. This also produced some flexibility to allow externally coupled separate electrodes pads that were coated with a conducting polymer to the circuitry on the chip. The externally coupling of a polymer coated electrode to the chip's circuitry had the advantage of being an easy test setup, and allowed for the ability to try different polymer options onto a single sensor setup, thus giving the option to reuse one chip without getting it involved in multiple chemical processes, saving time in post processing of the chip. Therefore, in these experiments, instead of depositing the polymer on the surface of floating gate extension pads on the chip, an external interdigitated electrode (IDE) which was coated with the conducting polymer of interest. Initially these devices were characterized via analyte exposure with the sensor, current mirror and the transimpedance amplifier only. The polypyrrole film was coated on an IDE which was then externally coupled to the floating gate terminal on the chip. A schematic for this test system is shown in
All the aforementioned exposure experiments were repeated for this subsystem and it was observed to function well and produce differentiable voltages at the output of transimpedance amplifier for 100% flow of different analyte vapours. To determine if smaller analyte concentrations could be detected, experiments were conducted in 20% analyte flow by maintaining 120 sccm of direct nitrogen flow and 30 sccm of nitrogen bubbled through the analyte. In the plot in
For detection of broad range of analytes, it is preferable to have many chemically diverse polymers with the ability to produce unique responses for many different analytes. Chemical diversity in the conducting polymers, and therefore the uniqueness of analyte response, can be achieved by using different monomer units (Pyrrole, Aniline etc.). This may also be achieved by using different dopants (sulfuric acid, nitric acid or sodium dodecyl sulfate) in the polymeric precursor solution for the electropolymerization process. This may also be achieved by changing the oxidation state of polymer during the electrodeposition, realized by varying the deposition potential[24] applied to the individual floating gate extension pad of different subsets of the sensor array when immersed in the same polymer precursor solution. Several of these methods may be used to develop the required chemical diversity of the conducting polymers. The size of subset selected to share the same extension pad polymer composition may be varied. For example, in one embodiment, each and every sensor in the array may be given a unique polymer composition, in which case only one individual sensor is addressed during a given energization of the float gate signal line in a given immersion of the sensing device in a particular polymer precursor solution. Alternatively, it may be beneficial to have multiple sensors within the array that share the same composition, in which case one or more of the subsets may each features a plurality of sensors that are all addressed during a given energization of the float gate signal line in a given immersion of the sensing device in a particular polymer precursor solution. The electropolymerization step for each different subset can be varied from another in the selected electric deposition potential (e.g. 0.7V vs. 1.2V) applied to the floating gate extension pads of the addressed subset via the floating gate signal line, or in the particular makeup of the polymer precursor solution, whether by variation in the selected monomer units, and/or dopants used therein. The inclusion of multiple sensors within each subset may be advantageous over other embodiments in which each individual sensor has a unique polymer composition from all other sensors, as a shared composition by multiple sensors in the array may be useful, for example, to direct directional movement of an analyte using measurements from spaced apart sensors in the array, or to benefit statistical accuracy.
In a particular experiment, now described, the response of the aforementioned subsystem was measured with two different polypyrrole films electrodeposited at different electropolymerization potentials. A solution of 0.1 M pyrrole monomer solution in a 0.2M H2SO4 solution was used to deposit polypyrrole films on two different IDEs at 0.7V and 1.2V. These IDEs were externally coupled to a common sensor setup, one at a time. Each was then exposed to a 30% methanol environment while the output voltage of the transimpedance amplifier was measured after 1-hour of analyte exposure. The measurement results are compared in
As was observed in the aforementioned experiments, the change in output voltage of the transimpedance amplifier for lower concentrations of any analyte was very small. The minimum resolution of the 8-bit A/D converter is little less than 13 mV. Therefore the resolution of the A/D converter is large with respect to the observed change in output voltage of the transimpedance amplifier. Therefore an on-chip high gain amplifier may be employed to rescale the output voltage of transimpedance amplifier, suitable for the A/D converter operation. To demonstrate the working of the proposed system when such a high gain amplifier is added, the output terminal of transimpedance amplifier was connected to an off-chip high gain differential mode amplifier. The amplifier circuit was designed using LM 741 OPAMP chip. A differential amplification mode was used designed to produce a gain of 10 using suitable values of resistors R1 and R2 (see
A schematic of this test system is shown in
The tested off-chip amplifier was very useful in amplifying and rescaling the small voltage shifts from transimpedance amplifier. The 8-bit digital output generated for each analyte is different and unique. The system can be refreshed to its original digital state by flushing the system with nitrogen. This experiment was a demonstration of desired electrical operation of the full proposed system.
From the forgoing disclosure, the manufacturability and operably of the individual sensors and the collective sensor array system have been demonstrated. In design of these olfactory sensors, the floating gate terminal of each transistor is extended to a contact pad surface designed using the topmost metal layer, which is used for deposition of sensing polymer like polypyrrole. The overall chip with the array of sensors serves as a “sensing platform” where multiple sensing polymers would be used with an array of FGMOS sensors to generate a unique electrical response for many tested analytes. This type of sensing platform would be useful in a wide variety of applications such as the automobile, food, cosmetic, packaging, drug, analytical chemistry and biomedical industries. In such industries, these sensors could be used for a broad and diverse range of purposes including quality control of raw and manufactured products, process design, freshness and maturity (ripeness) monitoring, shelf-life investigations, authenticity assessments etc. A process of electroless gold deposition was developed to coat the extended floating gate extension pads of our FGMOS sensors using a three-stage electroless plating technique where zinc, nickel and then gold layers were deposited, and confirmed using energy dispersive x-ray spectroscopy (EDS) and optical microscope imaging.
The gold-coated floating gate extension pads were used for deposition of the desired conducting polymers. The wirebonds from the chip to the ceramic package were encapsulated using SU8 photoresist, though any other suitable encapsulation material may alternatively be used, to avoid electrodeposition of the polymers onto the gold wirebonds. Electrodeposition the polymers was successfully done on individual off-chip sensors, as well as on the sensors in the chip-integrated array. The sensors in the array were selectively coated for two different chemically diverse polypyrrole films using two different redox potentials during deposition. These two different polymer films were also deposited and tested on interdigitated electrodes that were externally connected to some circuitry on the chip.
In summary of the forgoing experimentation, a special gas flow setup was created to that contained a controlled test environment for exposure of the sensors to the vapour analytes. The polymer coated sensor was tested for different analytes including methanol, ethanol, isopropyl alcohol, acetone, ammonium hydroxide and water. The sensors produced unique electrical responses for each analyte and for different concentration in the gas flow. Once the sensor operation was verified, experiments were performed to test the core processing block of the chip in an analyte environment. The polymers employed in the prototypes have been tested and found to also show sensitivity towards different fuels [24]. Since these polymer coated sensors can be designed to be sensitive to many different analytes, these sensor array systems is applicable to many other industries that include food production, agriculture, cosmetic, wine and spirit production, automobiles and even defence. Given that these chips are fabricated using a relatively simple commercial silicon CMOS technology, it would be very economical to fabricate in mass production. The prototype chip has a relatively small array of only 64 (8×8) sensors. However, the number can be easily increased in other embodiments, and for example may depend only on the number of chemically distinct polymers available for a given application. Larger array systems (1000×1000 or more) would be very sensitive to many different analytes such that a combined response from a large array would enable the use of statistical (pattern recognition, signature analysis, principal component etc.) and even learning algorithms to accurately predict very complex analyte information. Such a system may be useful in many different applications.
In further support of the utility of the invention, the forgoing experimentation employing an external interdigitated electrode (IDE) as an external sensing layer were supplemented by subsequent tests of later prototypes in which the extension pads of the chips themselves were coated with different polymers, and tested in the presence of different analytes. These subsequent “on-chip” experiments were performed using the same experimental setup shown in
The floating gate extension pad of a first on-chip sensor was coated with a polypyrrole (PPy) film from a solution of 0.1M pyrrole monomer and 0.1 M sulfuric acid (H2SO4) in 20 ml deionised water (DI) at a redox potential of 1.65 V. This polymer coated sensor was initially tested for transfer characteristics in a nitrogen environment at a constant VDS of 1 V. An Agilent 4156C precision semiconductor parameter analyzer was used for this measurement processes. The nitrogen flow conditions were maintained for several hours and the measurements were repeated. During this time, no noticeable change in the sensor drain current was observed.
To observe the effect of exposure of a given analyte vapor on the sensor operation, the chip was kept in a 7.60% relative flow of the analyte for 1 hour. The vapour concentration, as mentioned previously, was generated using a mixture of 2140 ml/min of nitrogen with 176 ml/min of bubbled nitrogen through the analyte. The measurements were performed under unchanged electrical conditions. This was repeated after exposing the sensor to 6 different analyte vapors each for a period of one hour. The analytes tested were ethanol, methanol, IPA, petrol (gasoline), toluene and water. The measurement data, (IDS vs VCG) is shown in
The measurement plot shows six visibly distinct drain current traces corresponding to the exposure to each of the analyte compared after the initially calibrated nitrogen exposure. The drain current (IDS) scales with the square of the gate voltage (VCG) in subthreshold regime. The square root of the sensor current (shown in the
Further experiments and analysis were performed to develop a fuller understanding of the observed threshold voltage shift. This enabled an estimation of the equivalent charge coupled to the floating gate under an analyte influence. Five other monomer/dopant combinations were used for the synthesis of a new set of polymers. The dopants, oxalic acid (C2H2O4), potassium chloride (KCl) and p-toluenesulfonic acid (C7H7O3S) were used in a 0.1M concentration in 20 ml DI water with a 0.1M concentration of pyrrole monomer to synthesise three new polypyrrole films. The other chemical monomer unit used for the polymerization process was aniline. A 0.1M concentration of aniline monomer was used to synthesise two chemically diverse polyaniline films using dopant of 0.1 M concentrated sulfuric acid and p-toluenesulfonic acid (pTSA).
A cyclic voltammetry measurement study of the new polymer recipe indicated a suitable redox potential for growth of each polymer film. For all the polymers discussed herein, the redox potential used to grow the polymer film is mentioned on the measurement data plots. Five new polymer film, integrated sensors were used to repeat the above discussed transfer characteristics experiment. All of the pyrrole-based polymers were integrated with sensors having width to length ratio of 10:1. In
The polyaniline films were integrated to sensors having width to length ratio of 20:1. The width to length ratio of a sensor is directly proportional to the sensor current. Just like the PPy integrated sensors, the polyaniline-based sensors also had sensitivity to the vapour analytes. In
To better understand the shift of threshold voltage during analyte exposure, further analysis of the experimental results was performed. The data from
The observed change in threshold voltage (ΔVTHN) was calculated as the change in the threshold voltage under the influence of an analyte relative to its magnitude under the nitrogen environment. The observed ΔVTHN value for these experiments is given in Table 3. In
The olfactory system is designed to operate in the subthreshold regime. In the subthreshold regime, a small change in the gate bias is able to produce orders of magnitude changes in the sensor current. The performance of this system in the subthreshold regime was analyzed. For the experimental data shown in
A set of experiments were designed in an effort to test the transient performance of the sensors and analyse the final steady state equilibrium response upon exposure to any given analyte. In this set of experiments, the polymer coated sensor was initially kept in nitrogen environment at a constant drain current IDS (>10−6 A) using a constant electrical biasing condition. The nitrogen flow conditions were maintained for several hours during which no noticeable change in the sensor drain current was observed. After the nitrogen measurements, the sensors were subjected to the analyte exposure at a known flow ratio. The sensor current was measured continuously throughout the nitrogen and analyte exposure cycles.
A pTSA doped PPy based sensor was tested for transient response to four different analytes. The sensor was initially kept in a saturated nitrogen environment by maintaining a constant flow of 2140 ml/min of nitrogen in the vapour chamber. The experiment began with the application of 3 V DC bias to control gate of the sensor and the sensor current was measured. A glass bubbler was prepared for analyte test by filling it with 20 ml of analyte liquid. After 30 minutes, 176 ml/min of nitrogen was bubbled through the glass bubbler while the direct flow of nitrogen was maintained at 2140 ml/min. As stated previously, flow ratio was described as 7.60% of total analyte containing flow. The bubbled nitrogen, acting as a carrier gas, carries analyte particles into the test chamber. The measurements were concluded after 90 minutes. The sequence was repeated for four different analytes; methane, petrol, toluene and water. The measured data of the experiment is plotted in
It was observed that the sensor current remained constant under the nitrogen flow while a unique response to every exposed analyte was seen. The response time of the sensor was observed to vary for different analytes. The PPy/pTSA film integrated into this sensor showed its highest sensitivity to petrol. However, this sensor also had the slowest response time for a petrol exposure. The sensor had fastest response time for water vapours. The water absorption properties of polypyrrole are already known[38]. Toluene is the only one of the four tested analytes to cause a decrease in the sensor current. The experimental results do confirm that the sensor operation is able to produce distinguishable electrical responses upon exposure to these different analytes.
In a next experiment, shown in
The same sensor was subsequently exposed to methanol vapours and the measurement data is plotted in
The PPy/H2SO4 integrated sensor was tested for continuous exposure to 4 different analytes with a nitrogen cycle between each of the different exposures. The analytes were exposed for 50 minutes of time followed by 50 minutes of pure nitrogen prior to exposure to a different analyte. For the nitrogen cycle, the bubbled flow of nitrogen through the analyte is turned off. The analyte from bubbler is removed, the bubbler is cleaned with DI water and dried with compressed dry air. The bubbler is then filled in with 20 ml of next analyte under test and is carefully refitted into the gas flow setup. In
The next dopant that was used for synthesis of a polypyrrole film was potassium chloride (KCl). The redox potential for synthesis of this conducting polymer film was 1.56V. The PPy/KCl polymer, integrated sensor was tested for sensitivity to different concentrations of petrol. As before, the sensor was initially kept under nitrogen environment for 20 minutes. It was then exposed to 167 ml/min of nitrogen bubbled through petrol. This flow was maintained for next 30 minutes. With reference to
Another experiment involved testing for sensor sensitivity to a change in analyte concentration analyte, the results of which are shown in
Polyaniline is one of the oldest known conjugated polymer which has been explored for a number of sensing applications [39]. The polyaniline (PANI) film for the following experiment was synthesized using 0.1 M aniline monomer doped with 0.1 M pTSA. The sensor was then tested with exposure to petrol and water. The measurements for both the analytes were performed individually. The data shown in
The water vapour exposure test from the previous experiment was performed once again with different concentrations of water vapour. This data for this measurement is shown in
The current response data plot for the PANI/pTSA sensor when exposed to methanol concentration changes is shown in
From the sensor transfer characteristics, it was observed that the PANI/H2SO4 films have the maximum sensitivity to water vapour when compared to the other test analytes. After gaining an understanding of the sensitivity of the different polymers to changes in analyte concentrations, a PANI/H2SO4 polymer-based sensor was tested for repeatability in a series of repeated cycles of nitrogen and water vapour, as shown in
In summary, the six different polymer based sensors all showed sensitivity for different test analytes. The PPy and PANI films synthesised using different dopants showed unique selectivity of the sensors for all of the tested analytes. The steady state response of the sensors was observed to be very stable under the influence of each vapour analyte.
Since various modifications can be made in this invention as herein above described, and many apparently widely different embodiments of same made, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.
10.1007/s12257-009-3077-1.
10.1109/TBCAS.2008.2001473.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2020/050799 | 6/11/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62862408 | Jun 2019 | US |
