Chemical Gas Sensors and Biosensors Developments

At The George Washington University

Institute for MEMS and VLSI Technologies

Department of Electrical and Computer Engineering

Professor M.E.Zaghloul zaghloul@gwu.edu

www.seas.gwu.edu/~mems/

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Three different types of Chemical Gas sensors were developed at GWU. The three types were fabricated and tested. Test results are excellent and the devices are sensitive to the small chemical gas concentration. All devices were fabricated using commercial CMOS foundries with simple post processing steps. Electronics circuits are integrated on the same substrate to control the performance of the sensor. The types of sensors and the chemical Gas detected are described in details the following sections.

1. Impedance Measurement Chemical Gas Sensor in CMOS Technology:

The system utilizes a thin film of tin-oxide (SnO2) as a sensing material. The conductance of this tin-oxide film changes in the presence of gas molecules. The chip has on board a digital decoder to select each element of the sensing array, and an operational amplifier to monitor the change in conductance of the tin-oxide thin film.. The chip was fabricated using CMOS technology through MOSIS. The chip is post-processed to create micro hotplates using bulk micro-machining techniques. Tin-oxide-sensing films are grown over post-patterned gold sensing electrodes using a low-pressure chemical vapor deposition (CVD) technique. Figure 1 shows array of sixteen gas sensors.

Figure 1- Array of 16 Tin Oxide Gas Sensors

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The metal oxide in this case Tin Oxide is heated and the gas to be measured interacts with it changing its resistance. Additives to the metal oxide can catalyze the reaction and improve sensitivity and selectivity. The heater and the electronics to control the temperature are integrated on the CMOS chip. Figure 2 shows gas sensor integrated with electronics.

Figure 2- Gas sensor Chip Integrated with Electronics

The following results are for Methanol.

Figure3: Conductance versus time for 50ppm Methanol

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For further reading please refer to the publication

M. Afridi, J.S. Suehle, M.E.Zaghloul, D.W. Berning, A.R.Hefner, R.E.Cavicchi, S.Semacik, C.B. Montgomery, C. J. Taylor, “ A Monolithic CMOS Microhotplate-Based Gas Sensor System”, IEEE SENSORS Journal, Vol. 2, No.6, December 2002, PP644-655.

2. Cantilever Resonance Frequency Measurement for Chemical Gas Sensors

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FFiigguurree 44-- CCaannttiilleevveerr iinn CCMMOOSS tteecchhnnoollooggyy ffoorr ddeetteeccttiinngg ggaass mmoolleeccuulleess

before exposure to the analyte ∆f1

∆f2

-36.6

-36.4

-36.2

-36

-35.8

-35.6

-35.4

-35.2

88050 88250 88450 88650 88850 89050 89250Frequency [Hz]

Am

plitu

de [d

Bm

]

DMMP concentration 3.6mg/m³

DMMP concentration 22mg/m³

Fig.5. Cantilever response upon exposure to various DMMP concentrations. Frequency shifts are marked ∆f1 and ∆f2

The above principle can be used for preliminary tests in the areas of a) Medical practice, b) Detection of explosives as bombs and unexploded land-mines, c) Detection of toxic or dangerous gases that may come from spills and leaks, d) Process of monitoring food, e) Perfume, and beverage industries.

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For further reading please refer to

I. Voiculoescu, M.E.Zaghloul, A. McGill, G. Fedder, “ Electrically Actuated Resonant Microcantilever in CMOS Technology for Detection of Chemical Weapons” Accepted to appear in the IEEE Sensors Journal, Special Issue on Sensors for prevention of Terrorist Acts. I. Voiculescu, M. Zaghloul, A. McGill, “ Cantilever Gas Sensor”, The Proceedings of the IEEE International Symposium on Circuits and Systems, Bangkok, Thailand, May 25-28, 2003.

3. Surface Acoustic Wave (SAW) Chemical Gas Sensors In this type of gas sensor SAW devices are built on a CMOS chip. The surface acoustic waves are loaded by absorption of gas molecules polymer coating. This causes delay in the loaded wave and the measurement of the delay is function of the number of gas molecules absorbed by the polymer. At this time we are measuring the fabricated chips, and we will have the results by the end of the summer. This device could be used as biosensors and it is possible to use it with microfluidic channels. In addition this device may be used for RF Communications systems as part of Filters, VCO, and other parts of transmitters/receivers.

SAW resonators die with electronics circuits

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Further references are S. Ahamadi, C. Korman, M.E.Zaghloul, “ ZnO based CMOS Surface Acoustic Wave gas Sensor”, the proceedings of the IEEE Sensors Conference , Tornto, Canada, October 2003. S. Ahmadi, F. Hassani, C. Korman,M. Rahman, and M. Zaghloul, “ Characterization of Multi and Single layer Structure SAW Sensor”, in IEEE Proceedings of IEEE-Sensors 2004, Vienna, October 2004. The above devices descriptions are example of using commercial CMOS technology to implement chemical gas sensors. Those sensors can be used as biosensors for several bio sensing applications in addition to communications systems applications.

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