Vinay Agarwal

CMOS Microelectrode Array for Electrochemical Lab-on-a-Chip Applications

Microelectrode arrays (MEAs) offer numerous benefits over macroelectrodes due to their smaller sample size requirement, small form factor, low-power consumption, and higher sensitivity due to increased rates of mass transport. These features make MEAs well suited for microfluidic lab-on-a-chip applications. This paper presents two implementations of MEAs with and without an on chip potentiostat. We first describe an 8times8 array of 6 mum circular microelectrodes with center to center 37 mum spacing fabricated on silicon using conventional microfabrication techniques. Pads are provided for external connections to a potentiostat for electrochemical analysis. The second implementation is an individually addressable 32times32 array of 7 mum square microelectrodes with 37 mum center to center spacing on a CMOS chip with built-in very-large-scale integration potentiostat for electrochemical analysis. The integrated CMOS MEA is post processed at the die level to coat the exposed Al layers with Au. To verify microelectrode array behavior with individual addressability, cyclic voltammetry was performed using a potassium ferricyanide (K3Fe(CN)6) solution.

DNA-decorated carbon-nanotube-based chemical sensors on complementary metal oxide semiconductor circuitry.

We present integration of single-stranded DNA (ss-DNA)-decorated single-walled carbon nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry as nanoscale chemical sensors. SWNTs were assembled onto CMOS circuitry via a low voltage dielectrophoretic (DEP) process. Besides, bare SWNTs are reported to be sensitive to various chemicals, and functionalization of SWNTs with biomolecular complexes further enhances the sensing specificity and sensitivity. After decorating ss-DNA on SWNTs, we have found that the sensing response of the gas sensor was enhanced (up to approximately 300% and approximately 250% for methanol vapor and isopropanol alcohol vapor, respectively) compared with bare SWNTs. The SWNTs coupled with ss-DNA and their integration on CMOS circuitry demonstrates a step towards realizing ultra-sensitive electronic nose applications.

The heterogeneous integration of single-walled carbon nanotubes onto complementary metal oxide semiconductor circuitry for sensing applications

A simple methodology for integrating single-walled carbon nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry is presented. The SWNTs were incorporated onto the CMOS chip as the feedback resistor of a two-stage Miller compensated operational amplifier utilizing dielectrophoretic assembly. The measured electrical properties from the integrated SWNTs yield ohmic behavior with a two-terminal resistance of approximately 37.5 kOmega and the measured small signal ac gain (-2) from the inverting amplifier confirmed successful integration of carbon nanotubes onto the CMOS circuitry. Furthermore, the temperature response of the SWNTs integrated onto CMOS circuitry has been measured and had a thermal coefficient of resistance (TCR) of -0.4% degrees C(-1). This methodology, demonstrated for the integration of SWNTs onto CMOS technology, is versatile, high yield and paves the way for the realization of novel miniature carbon-nanotube-based sensor systems.

A PVT independent subthreshold constant-G m stage for very low frequency applications

This paper presents the design of an ultra low power sub-threshold Gm stage for Gm-C filters with very low cut-off frequency. The sub-threshold region of operation provides a robust trans-conductance performance against temperature variations. We propose a new topology for a PVT (process, voltage & temperature) independent gm stage with constant gm biasing circuit operating in sub-threshold region. The significant variation to temperature due to channel length modulation is corrected using a parallel combination of two gm blocks of different value. The circuit is made immune to process variations by using replica-biasing circuit to track the changes in process parameters. The circuit performance also remains nearly constant for a supply voltage range of 1.1V to 1.7V. A general biquad filter has been implemented and simulations show about 1% variation in the 3 dB bandwidth (in the range of few hertz) for a temperature range of -30degC to 110degC. The circuit has been implemented using 0.5mum CMOS technology with a nominal supply voltage of 1.2 V and power consumption for each gm block is 55.3 nW.

A CMOS integrated thermal sensor based on Single-Walled Carbon Nanotubes

This paper presents a fully functional thermal sensor based on Single-Walled Carbon Nanotubes (SWNTs) integrated with CMOS interface circuitry utilizing die-level post-CMOS processing. The SWNTs are incorporated on the CMOS circuitry by utilizing a low temperature Dielectrophoretic (DEP) assembly process, which includes a pretreatment of an electroless zincation to prepare the top metal layer of the CMOS chip for assembly. The entire sensor system is next encapsulated with a parylene-C layer for improving the contacts between the SWNTs and the electrodes. The SWNTs were assembled as the gain element of an integrated inverting amplifier circuit. I-V measurements indicate that the temperature coefficient of resistance for the SWNT-based thermal sensor is -0.40% over a temperature range from 25degC to 105degC. The indirect measurement of the TC from the AC gain of the amplifier displayed a temperature coefficient of -0.33% over the same temperature range. This is the first successful demonstration of a fully functional SWNT-based thermal sensor on CMOS and the entire concept can be easily extended to other nanostructures for numerous other applications.

Single chip Nanotube sensors for chemical agent monitoring

In this paper, we present a single chip nanosensor composed of Single-Walled Carbon Nanotubes (SWNTs) integrated on complementary metal oxide semiconductor (CMOS) circuitry with custom designed on-chip amplifiers for chemical agent detection. The SWNTs were integrated on CMOS circuitry utilizing a low temperature and low voltage Dielectrophoretic (DEP) assembly process. Furthermore, we incorporated different sequences of single-stranded DNA (ss-DNA) on to SWNTs which improved their response to two toxic and explosive gases, namely Dimethyl methylphosphonate (DMMP) (an analog of nerve agent sarin [1]) by 9 times and Dinitrotoluene (DNT) (a byproduct of TNT [2]) by 12 times. In addition, the change in resistance (ΔR/R) of SWNT sensors increased from 12% to 24% when the concentration of DMMP vapor was increased from 8 ppm to 72 ppm; While ΔR/R increased from 7% to 23% when the concentration of DNT was increased from 9 ppm to 46 ppm. The SWNTs coupled with ss-DNA integrated onto CMOS circuitry shows great promise for monitoring low concentrations of toxic and explosive gases and potentially can be used to realize ultra-sensitive nanosensors.

DNA decorated carbon nanotube sensors on CMOS circuitry for environmental monitoring

Single-walled carbon nanotubes (SWNTs) with their large surface area, high aspect ratio are one of the novel materials which have numerous attractive features amenable for high sensitivity sensors. Several nanotube based sensors including, gas, chemical and biosensors have been demonstrated. Moreover, most of these sensors require off chip components to detect the variations in the signals making them complicated and hard to commercialize. Here we present a novel complementary metal oxide semiconductor (CMOS) integrated carbon nanotube sensors for portable high sensitivity chemical sensing applications. Multiple zincation steps have been developed to ascertain proper electrical connectivity between the carbon nanotubes and the foundry made CMOS circuitry. The SWNTs have been integrated onto (CMOS) circuitry as the feedback resistor of a Miller compensated operational amplifier utilizing low temperature Dielectrophoretic (DEP) assembly process which has been tailored to be compatible with the post-CMOS integration at the die level. Building nanotube sensors directly on commercial CMOS circuitry allows single chip solutions eliminating the need for long parasitic lines and numerous wire bonds. The carbon nanotube sensors realized on CMOS circuitry show strong response to various vapors including Dimethyl methylphosphonate and Dinitrotoluene. The remarkable set of attributes of the SWNTs realized on CMOS electronic chips provides an attractive platform for high sensitivity portable nanotube based bio and chemical sensors.

Integration of Single-Walled Carbon Nanotubes on to CMOS Circuitry with Parylene-C Encapsulation

This paper presents heterogeneous integration of single-walled carbon nanotubes (SWNTs) with CMOS integrated circuits using die-level post processing. The chip was fabricated using the AMI 0.5 mum CMOS Technology. An electroless zincation process was performed over the Aluminum assembly electrodes (Metal 3 of CMOS technology) to clean and to coat the electrodes with a thin Zinc layer. Low temperature dielectrophoretic assembly was utilized for the placement of the SWNTs on to these electrodes. Encapsulating the CMOS chip with a thin (1 mum) parylene-C layer stabilized the SWNT-electrode contact resistance and also provided environmental protection. Electrical measurements from the assembled SWNTs yield ohmic behavior with a two-terminal resistance of ~44K Omega. The SWNTs were incorporated on to the CMOS chip as a feedback element of a two-stage Miller compensated high gain operational amplifier. The measured small signal ac gain (~1.95) from the inverting amplifier confirmed the successful integration of carbon nanotubes with the CMOS circuitry. This paper lays the foundation for the realization of next generation integrated nanosystems with active nanostructures on CMOS integrated circuits.

Exploring sequence dependence in DNA-decorated CNT Gas sensors on CMOS circuitry

Single-Walled Carbon Nanotubes (SWNTs) are reported to be very sensitive to numerous odors and could serve as the next generation of miniature gas sensors. Recently, we have shown that SWNTs functionalized with DNA hold even greater promise than bare SWNTs as high sensitivity gas sensors. The DNA-decorated SWNT gas sensors were integrated on CMOS circuitry with built-in amplifiers. In the paper, we report on the performance of the nanosensors as a function of the DNA sequence used for adsorption on SWNTs. A diverse sequence-dependent response was observed, with enhancements up to 300% for methanol vapor and 250% for isopropanol alcohol vapor for specific DNA sequence compared to bare SWNTs. This work demonstrates a significant step towards ultra-sensitive electronic nose type of applications based on functionalizing CNTs with different DNA sequences.

SS-DNA-decorated Single-Walled Carbon Nanotubes integrated on CMOS circuitry for high sensitivity gas sensing

In this paper, we demonstrate the integration of single-stranded-DNA (ss-DNA) decorated Single-Walled Carbon Nanotubes (SWNT) onto Complementary Metal Oxide Semiconductor (CMOS) circuitry for gas sensing applications. Utilizing Dielectrophoresis (DEP) assembly, a simple and a low temperature process, we first demonstrate integration of SWNTs onto CMOS circuitry at the die level. SWNTs are sensitive to numerous odors, and functionalization of SWNTs with biomolecular complexes further enhances their sensing specificity and sensitivity. This was demonstrated when we attached ss-DNA around SWNTs and our results indicated twice the sensitivity of the sensors compared to the plain SWNTs. The remarkable set of attributes of the SWNTs coupled with the ss-DNA molecules provides an attractive platform for ultra sensitive electronic nose type of applications.