POSITIONING AND WIDTH ANALYSIS OF IDT WITH MICROHEATER FOR

BETTER GAS SENSITIVITY

OUTLINEINTRODUCTION TO MEMSMEMS BASED GAS SENSOR NEED OF MICRO HEATER IN GAS SENSORJOULE HEATINGIDTLITERATURE REVIEWGAPS IN PRESENT STUDYOBJECTIVESREFERENCES

MEMS and Micro SensorMicro Sensor: It is a sensor that has at least one

physical dimension at the sub millimeter (micron) level.

MEMS –Micro Electro Mechanical System.

MEMS is a device where micro sensor and mechanical parts along with signal processing circuits are integrated on a small piece of silicon.

INPUT SENSOR PROCESSOR ACTUATOR

Why MEMS ?

IC Compatible Higher Performance

Miniaturization Smaller Ruggedness Lower Power

Consumption

Batch Fabrication Low Cost

Applications Automobiles Machines Robotics Medicine Aerospace etc.

GAS SENSOR Gas sensor is a subclass of chemical

sensors. Gas sensor determines which gas is present

around and measures the concentration of gas on its surface. Gas sensor interacts with a gas to measure its concentration.

Applications: Process control industries Environmental monitoring Alcohol breath tests Detection of hazardous gases in

environment Home safety

Chemiresistive semiconducting metal oxides are widely used as gas sensor because of certain advantages like, very low cost, high sensitivity, fast response, simple electronic interface, ease of use, low maintenance and ability to detect large number of gases.

Mainly gas sensor are used to detect gases like, Nitrogen dioxide (NO2), Nitric oxide (NO), Nitrous oxide (N2O), Carbon monoxide (CO), Ammonia (NH3), Methane (CH4), Sulfur dioxide (SO2), Carbon dioxide (CO2).

NEED OF MICROHEATERMicro-Heaters have been the subject of

great interest owing to their extensive applications in gas sensors, humidity sensors and other micro-systems.

A micro-heater has low power consumption and better temperature uniformity.

As we increase the heating area of micro heater, we can achieve proper detection of selective gases in terms of ppm or ppb.

Joule HeatingIn micro-heaters the basic principle is base

on joule heating. Joule heating is also known as ohmic heating and resistive heating.

The electric field equals the negative of gradient of the potential V. All The generated resistive heat Q is proportional to the square of the magnitude of the electric current density J. Also Current density J is proportional to the electric field E:

Q α │J│2

Cont…

The proportionality constant is the electric resistivity ρ or the reciprocal of the temperature dependent electric conductivity σ. Combining these facts we have,

ρ = 1/σ σ = σ (T) Q = ρ *│J│2 = (1/σ) *│σ E│2 = σ

│∆V│2

IDT The term interdigited, selected for use throughout this, refers to a digitlike or

fingerlike periodic pattern of parallel in-plane electrodes used to build up the resistance when a gas is present on the material sample or sensitive coating.

A typical chemical interdigited sensor design is to deposit interdigited electrodes on an insulating substrate. The electrodes are coated with a thin layer of material that is sensitive to the concentration of chemicals present in the ambient atmosphere. The most common outputs of measurement are changes in resistance and capacitance between electrodes. The sensing mechanism is that when the sensor is exposed to ambient chemicals, the interaction of the chemicals with the sensitive material coating changes the material’s conductivity , dielectric constant ,and/or the effective thickness of the sensitive layer. The change in conductivity and effective thickness result in a resistance change, and the change of dielectric constant and effective thickness of the sensitive layer changes the capacitance.

Interdigital chemical sensors are inexpensive to manufacture and can be integrated on a chip consisting of the sensor element and signal processing electronics.

LITERATURE REVIEWIn [1], four different

geometries single meander, double

meander, fan type and square geometries are analyzed.

For same material, same surface area and same applied voltage, square type geometry gives better temperature distribution among all.

In [2], it describes the fabrication and characteristics of a NO sensor using ZnO thin film integrated SiC micro heater based on SiC thin film of operation in harsh environments.

By using the Pt added ZnO thin film was showed higher sensitivity, lower working temperature, and faster adsorption characteristics to NO gas than the pure ZnO thin film.

In [3], it gives simulation results of micro-heaters by using six different patterns like, Plane plate with central square hole, Double spiral, Honey comb, S- shape, fan shape and meander type with their Electro thermal simulated temperature profile.

For the same supply voltage applied, the uniform temperature profile and same power consumption, high temperature uniformity is achieved in fan type patterns of micro-heater

In [4], this paper presents simulation results of Polyimide (PI) based micro-heater operating in the temperature range of 200- 250⁰C which could be operated at low voltage. Three different micro heater geometries have been simulated.

The resistive track width and the gap between the consecutive tracks are varied and the area over which uniform temperature is obtained.

In [5], the geometric optimization for the micro-heater has been performed on platinum micro-heater. For applying different conditions like,

(1) A spiral micro-heater (2) A spiral heater with a

cavity in the silicon at the center of micro-heater

(3) A suspended spiral micro-heater on four bridges

(4) A spiral micro-heater with sensing layer of ZnO,

And results are analyzed.

In [6], they present the fabrication of a carbon monoxide (CO) micro gas sensor integrated with an inverting amplifier circuit and a micro heater on chip using CMOS process.

The structure is made up such that Polysilicon resistor is on to sensing film (SnO2) and micro-heater is attached with it.

The inverting amplifier circuit is utilized to convert the resistance of the gas sensor into the voltage output.

In [7], this paper presents design, simulation, fabrication and experimental characterization of the platinum micro-heater elements built on alumina substrate. Micro-heaters are designed for application where the device has to be heated up to 700 °C in less than a 200 ms.

Choosing the proper combination of electrical, thermal and mechanical characteristics of thick film heating resistor, glass layer and substrate are taken and analyzed.

In [8], presented for the first time is the design and fabrication of high temperature, low power consumption, and good thermal uniformity micro-heaters for environmental gas sensors.

A Wheatstone-bridge type resistive Pt heating structure and a multi-ringed heat spreading structure was applied to enhance the thermal uniformity.

Figure:

[9] This paper describes the structural design and electrothermal analysis of a microheater array to find out the thermal characteristic with different bias voltage.The array consists of four unit cell, each of 3mm x 3mm of dimension with a membrane size of 1.5mm x 1.5 mm.This device has the unique advantage of making the microheater and interdigitated electrode co-planner using a single lithography process.

GAPS IN PRESENT STUDYMicro-heater geometries generally used for proper detection

of gases on MEMS based gas sensor, there is more preference given to high temperature. But oxidation and reduction processes occur in MOS (Metal Oxide Semiconductor) based gas sensor around 380K to 600K. And some micro-heater materials do not support high temperature as high temperature changes its parameters. Although at less temperature some sensing material like, WO3

(Tungsten trioxide), SnO2 (Stannic oxide), ZnO (Zinc Oxide) are widely used and give very good gas concentration value on surface.

There are different geometries analyzed and compared to one another. But very less work is done regarding the positioning and width analysis of IDT in a coplanar micro-heater design.

OBJECTIVESTo simulate co-planar micro-heater

geometries and to make analysis which geometry is better.

To simulate a fixed geometry by varying IDT position and dimensions to improve the sensitivity for gas detection.

To achieve a geometry optimization in fixed surface area for given structure for temperature uniformity.

References [1] L. Sujatha, V. S. Selvakumar, S. Aravind, R. Padamapriya, B. Preethi, “Design and

Analysis of Micro-Heaters using COMSOL Multiphysics for MEMS Based Gas Sensor”, Proceedings COMSOL Conference, India, 2012.

[2] Jae-Cheol Shim, Gwiy-Sang Chung, “Fabrication and Characteristics of Pt/ZnO NO Sensor Integrated SiC Micro Heater”, IEEE SENSORS Conference, India, 2010.

[3] Velmathi G., Ramshanker N., Mohan S., “2D Simulations and Electro-Thermal Analysis of Micro-Heater Designs Using COMSOLTM for Gas Sensor Applications” COMSOL Conference, India, 2010.

[4] M. Gayake, D. Bokdas, S. Gangal, “Simulations of Polymer based Microheater Operated at Low Voltage”, COMSOL Conference, India, 2011.

[5] Vineet Bansal , Anil Gurjar, Dinesh Kumar, B. Prasad, “3-D Design, Electro-Thermal Simulation and Geometrical Optimization of spiral Platinum Micro-heaters for Low Power Gas sensing applications using COMSOLTM”, COMSOL Conference, India, 2011.

[6] Ching-Liang Dai, Mao-Chen Liu, “Nano particle SnO2 Gas Sensor with Circuit and Micro Heater on Chip Fabricated Using CMOS-MEMS Technique”, IEEE International Conference on Nano/Micro Engineered and Molecular Systems pp. 16 - 19, Bangkok, Thailand, 2007.

[7] Sasa Toskov, Ronald Glatz, Goran Miskovic, Goran Radosavljevic, “Modeling and Fabrication of Pt Micro–Heaters Built on Alumina Substrate”, 36th Int. Spring Seminar on Electronics Technology, 2013.

[8] H. Y. Lee, S. Moon, S. J. Park, J. Lee, K.-H. Park, J. Kim, “Micro- Machined resistive micro-heaters for high temperature gas sensing applications”, Electronics Lett. Vol. 44 No. 25, 2008.

[9] S.S.Mondal, S.Roy, C.K.Sarkar, “Design and Electrothermal analysis of MEMS based Microheater Array for Gas Sensor using INVAR alloy”, International Conference on Communications, Devices and Intelligent Systems (CODIS), 2012.

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