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ISSN: 2321-2667 Volume 2, Issue 4, July 2013 6

Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE)

FABRICATION OF MICROHEATER AND INTER - DIGITED

ELECTRODES ON SILICON PLATFORM FOR LOW POWER IN-

TEGRATED GAS SENSOR APPLICATIONS

Mr. Santanu Maity1 , Assistant Professor, ECE, NIT,AP, e-mailID: [email protected]; Mr. Abhishek Kumar2, Assistant Professor, EEE, NIT,AP

e-mailID: [email protected], Mr. Sahadev Roy3, Assistant Professor, ECE, NIT,AP, e-mailID: [email protected], Mr. Vinay Kumar Dwive-

di4, MNNIT, e-mailID: [email protected], BalajiTripathi5,Student, ECE, NIT,AP, Ashmit Sharma6, Student, ECE, NIT,AP, Dayasagar Agra-

hari7 Student, ECE, NIT,AP

Abstract

Power consumption and sensing material selection is the most

important issue for gas sensor realization. MEMS based MetalOxide Sensors using micro-machining structure can be used forlow power consumption. In this paper we are proposing a novel

coplanar micro heater structure along with two Inter Digited

Electrodes (IDE) for different gas sensing purpose on thin Sili-

con dioxide membrane for an operational tem perature of ≤200oC

with <50mW power consumption.

Introduction

Battery power consumption for a hand held electronic instrument

is the primary requirement of gas sensor application. Micro elec-

tro mechanical system (MEMS) based technology is very much

applicable for reduction of consumed power. A study on piezore-

sistive Micro-Electro-Mechanical Systems (MEMS) cantileverfor a chemical sensitive - mass based sensor has been carried out

to enhance sensor sensitivity by Subhashini et al.[1]. The sensi-

tive region attracts the CO2 molecules.Huge work reported so far

deal with the design of either platinum [1] or polysilicon [2] mi-

croheater particularly applicable in the higher temperature range

(400-700ºC) [3]. MEMS based devices those produced through a

combination of standard CMOS technology and MEMS post-

processing as described in the several literature [4]-[10]. Varietyof heater based devices has been demonstrated such as resistive

temperature sensors [11], resistive heaters [12], thermal actuators

[13], thermoelectric sensors [14] and also conductometric gassensors have also been designed and fabricated by several groups[15]. Micromachining technique is best and established technique

for getting different MEMS structure. Wang et al. [16] con-

structed an array of sensor elements fabricated on silicon mem-

branes, which provided improved thermal isolation of the active

area Using micromachining techniques. Advances in thin film

processing, and micromachining techniques, have made this an

extremely attractive approach which are Ikegami et al. [17] fabri-

cated a uniformly heated array of six elements on an alumina

substrate, and used this array to identify gases such as ammonia

methanol, and hydrogen sulfide. Individual heater element with

sensor fabricated using silicon integrated structure and use of

thin film techniques have also been described by Chang et al

[18]. Inter digitated electrodes (IDEs) for sensing application areimplemented in various sensing devices including surface acous

tic wave (SAW) sensors, chemical sensors as well as curren

MEMS biosensors [19]. Different researchers show that increas

ing electrode thickness yielded an increase in current and overalsignal with small increases in signal noise. This increase in cur

rent is thought to be due to increased surface area [20]. Radke eal. in [18] also optimize another type of IDE for better perfor

mance. Gas sensors based on solid-state property have been used

by the semiconductor industry for both life safety and proces

applications [21].

In our study we have taken two identical electrodes with differ-

ent sensing layer for identify different gasses by using singleheater as a result it consume low power. And the heater can pro-

duce more than 350oc at low voltage (1.5v).

Fabrication and result analysis:

Device structure:

The device structure is shown in figure 1 is the single cantilev

er based structure. Where within one die there are one heater and

one electrode. It is considered the electrode as the Inter DigitedElectrode from which it can measure the gas sensing property

Silicon substrate of <100> orientation and resistivity is 1-5 ohm

cm is taken as the platform for making micro machined structure

The die size of the micro heater is 3mm X 3mm because it re

duces the power consumption. In figure 2 it is showing fabricated

gas sensing device where yellow portion is electroplated gold

and black portion is Ti/Pt.

mailto:[email protected]
















(a)

(b)Figure 1:(a)Top view of micro heater, (b) colour code

Fabrication and discussions:

Sensing layer is the most important part of gas sensor. By chang-

ing sensing different parameters like material properties, thick-

ness of sensing materials, crystal orientation, and also deposition

procedure it is possible to get different sensing result. Here it is

shown the alignment of sensing material as it is needed to depositon the electrode. Activation of sensing layer is also one of theimportant parts of gas sensor fabrication.

Figure 2: Top view of micro heater

Figure 3: Top view of micro heater (bad alignment)

It shown in figure 3 is that the zinc oxide sensing layer max

imally placed on the top of the micro heater but partially on theIDE. So there is an alignment problem. The deposition was done

by using tensile mask. The alignment problem can be overcome

by using lithography technique where pattern can make on the

top of the electrode by using positive photo resist.

Figure 4: Top view of micro heater (better alignment)

Figure 5: Deposited ZnO on SiO2 substrate

But it increases the fabrication steps. As a solution it may be

consider tensile mask of Si substrate (2” Si wafer) by using tetra

methyl ammonium hydroxide (TMAH) solution which shown infigure 4. ZnO sensing layer is also very much important for





sensing application. The FE-SEM image of deposited ZnO is

shown in figure 5.

Fig. 6: fabrication process steps

Fig. 6 shows the proposed process flow for fabricating the mi-

croheater die. As the lift off process considered for (Ti/Pt etchant

is not friendly) making heater and IDE then adhesion is the vital

issue. So different ratio of Ti/Pt can be deposit where 40nmTi100nm Pt is good adhesive composite. Selective gold plating of

the contact pads is needed for proper wire bonding shown in fig-

ure 7. Etched out of Si from the back of the Die (micro heater) by

using TMAH is the very good technique to reduce power con-

sumption. Micromachining technique (shown in figure 8) is very

much critical because in a two inch wafer it can fabricated 30devise, so for separation of all devices scribe line can be used. In

a 25% solution of TMAH at 90oc temperature is the better con-

trolled solution for making MEMS structure the etch rate de-

creases with increasing TMAH concentration.

Fig 7. : Fabrication process steps

Fig 8. : Micromachined back surface

New proposed structure:

In the present work it is proposed to fabricate a micro heater

element for low temperature integrated gas sensors. In this pro

posal a thin 1 μm Silicon dioxide cantilever is used for housing

the heater structure and the IDE.

Figure 9: Modified die for two different types’ gases (top view)

In practical the maximum temperature of around 200 oC with a

uniform distribution over the entire micro heater membrane re

gion has been achieved with 1.5V excitation.





1 2 3

0

100

200

300

T e m p a r a t u r e ( o c )

Applied voltage (Volt)

Temparature

0 1

0

1

Figure 10: Temperature distribution of a Single microheater by

applying different voltages

Conclusion

In this paper it is mainly focused on power factor so by using

single heater and also tried to reduce power consumption. A

3mm X 3mm micro heater die with coplanar IDE has been de-

signed and the fabrication process has been initiated to sense two

different types of gases by using single device. By decreasingthe electrode spacing and increasing no of electrodes the effec-

tive electrode area is increased as a result an increase in the sen-

sitivity of the device. This device may further be miniaturized

and modified to achieve even lower power consumption and bet-ter performance.

References

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Biographies

Santanu Maity, obtained his M.Tech in Radiophysics and Elec-

tronics from Calcutta University, Rajabazar Science college and

perusing Ph.D. in Solar photovoltaics from Center of Excellence

for Green Energy and Sensor Systems(CEGESS),kolkata. Heworked on RF MEMS based switch, MEMS based gas sensor,

Crystalline solar cell etc. He has three years on laboratory re-

search experience. Now he is Assistant professor of ECE de-

partment in National Institute of Technology, Arunachal Pradesh.

[email protected]

Abhishek Kumar received his B.Tech (EEE) degree from

Pondicherry university (Central University, under MHRD, GOI)in year 2009 and M.Tech degree in Power System Engineering

from NERIST Deemed University (Autonomous Institute, underMHRD, GOI), Nirjuli, India in the year 2012. He is currently

working as an Assistant Professor with department of Electrical

& Electronics Engineering, National Institute of Technology,Arunachal Pradesh, India. His areas of intrest are Power Elec-

tronics, Electrical Machine, Power Quality, Power system,

Hybrid Energy Technologies, Distributed Generation

&Renewable Energy. Mr. Kumar is a member of Institute of

Electrical & Electronics Engineer (IEEE) USA, Institute of En-

gineering Technology (IET) UK and SCIEI. [email protected]

Vinay Kumar Dwivedi is currently pursuing his M.Tech de-

gree in Power Electronics & ASIC design from Motilal Nehru

national institute of technology (MNNIT), Allahabad, U.P., In-

dia and graduated from Uttar Pradesh technical university,

Lucknow, U.P., India in year 2008. He was a Faculty member

with Dr. K. N. Modi institute of engineering & technology,Ghaziabad, U.P. for about two years and then was with the MIT,

Ghaziabad for about one year. His areas of interest are Power

electronics, Power Quality, Network and Control system. Mr.

Dwivedi is a student member of Institute of Electrical & Elec-tronics Engineer (IEEE). [email protected]

Sahadev Roy is currently working as an Assistant Professor in

ECE Department at NIT, Arunachal Pradesh, India. He has more

than 9 years Experience in research and teaching field. He re-

ceived his B.Tech degree in ECE and M.Tech, in Mechatronics

from Bengal Engineering and Science University, Shibpur, India

He perusing PhD in VLSI from NIT,AP. He has published a

number of scientific papers in National and International Jour

nals. His research interest includes Robotics, Microwave Solid

State Device and VLSI design.

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