Dr.G.Velmathi, Prof. S.Mohan and N.Ramshanker Importance ... 5/Issue2/8. Importance...Abstract: This paper presents the design, simulation of an S ... By controlling the voltage .

Dr.G.Velmathi, Prof. S.Mohan and N.Ramshanker 33

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 5, Issue. 2, July-2013.

Importance of Temperature in Gas Sensors and Design, Fabrication, Testing of S shaped Low

Power Platinum Microheater for Gas Sensor Applications

1Dr.G.Velmathi, 2Prof. S.Mohan , 2N.Ramshanker

1Dept. Of ECE. Velammal College of Engineering and Technology, Madurai, Tamilnadu 2 CeNSe, Indian Institute of Science, Bangalore.

Abstract: This paper presents the design, simulation of an S shaped microheater for high temperature operation, low power consumption and good thermal uniformity required for various gas sensor applications. The S shaped platinum (Pt) heater pattern embedded in between the Si3N4 layers with a very good heater/membrane area ratio offers good stress compensation, thermal isolation and thermal uniformity and has low power consumption. Results show that the simulated thermal profile is in good agreement with the electrical characteristics results of the fabricated heater. Keywords: Microheater, S type, Electro-thermal, COMSOL TM, FLIR, Thermal Image

I. INTRODUCTION 1.1 General Design considerations of Gas Sensors:

A large demand exists for small Gas Sensors for measuring the various gas constituents that are present in automotive exhaust. Gas sensors also play a major role in measurement of gases for environmental monitoring, industrial safety and home land security. High performance and portable gas sensing devices are employed to avoid and reduce human exposure to dangerous gases in industrial plants and other possible exposed environments that are hazardous in nature. The efficient use of a gas sensor is mainly related with its characteristics of performance and its design. When selectivity of gas sensor is considered as an important aspect, there are four popular methods to obtain selectivity in semiconductor gas sensors namely use of (i) catalysts and promoter (ii) temperature control (iii) specific surface additives and (iv) use of filters. Of the various approaches, the use of filters is by far the most unambiguous. The use of dopants and promoters, temperature control are common [1-3]. This work mainly focuses on the design and implementation of microheater and its optimization towards the temperature control aspect of the gas sensors.

1.2 Specific Needs of Microheater design in Gas Sensing:

Semiconductor gas sensors utilize the chemical sensitivity of their surfaces for gas sensing applications. Metal-oxide n-type semiconductors like SnO2, ZnO, and TiO2 are the most commonly used sensing materials. The gas detection technique is primarily based on a change in the electrical resistance of the semi conducting metal oxide films. The reaction model in fig.1 on the SnO2 surface mentioned will explain the sensing mechanism of the metal oxide based sensors. The principal detection process is the change of the oxygen concentration at the surface of the metal oxides, caused by the adsorption and heterogeneous catalytic reaction of oxidizing and reducing gaseous species.

.

Fig 1. Sensing reaction model of the metal oxide based gas sensor

The electrical conductivity depends on the gas

atmosphere and on the temperature of the sensing materials exposed to the test gas. The signal generated from the sensing element strongly depends on the temperature of the element. The adsorption and desorption are temperature activated processes, and hence dynamic properties like



response and recovery time of the sensor depend mainly on the operating temperature. And also, temperature has an effect on the physical properties of the semiconductor sensor material (charge carrier concentration, Debye length, work function etc.,). At very high temperature the charge carrier concentration increases and Debye length decreases which results in a reduction in sensitivity of the film.[4-7] At low temperature most of the gas sensor materials especially metal oxides are insulators rather than semiconductors. So the signal generated from the sensing element strongly depends on the temperature of the element. Hence it becomes necessary to keep the temperature of the sensing film with an external heater, which is usually attached on the back side of the sensing film in a précised value as shown in fig.2

Fig 2. Integrated structure of gas sensor

And also the heating device should be capable of producing optimum temperature which enables the sensing action of the film and regeneration of the film. Moreover in the portable applications of gas sensors the heating elements are the major power consuming devices apart from the electronics which is used in the readout circuit of the sensing signals. So in this work it is mainly focused on the geometrical aspect of the heater which will lead to a considerable power reduction, keeping uniform temperature also into considerations. Towards this aim several heater geometries are designed and analyzed iteratively and the result of one type of heater named S shaped pattern is been reported in this paper.

2 Microheater geometries

The geometries presented in this work are (i) Fan

Shape (ii) S-Shape (iii) Double Spiral (iv) Honeycomb (v)

Meander (vi) Plane plate with central hole. However the

performance of basic meander and double spiral shaped

structures [8-9] are explained by some researcher, the

homogeneous temperature profile at the sensing area has not

been achieved fully because the outer bound of the heater

experiences more heat loss through conduction, while heat

losses of the inner part are mainly caused by radiation and

heat transfer through the passivation layer to the sensing

film.

Plane plate with central hole (fig.3): This is a rectangular

plate design with a square hole in its

center which causes natural

convection. The problem with this

structure is its undistributed hot spots at the center and high

power consumption.

Meander (fig.4) : The basic

meandering structure shown in figure

which is widely used by some

researchers show some undistributed hot spot at very high

temperature.

Double spiral shape (fig.5) : To avoid the radial temperature

gradient of the conventional meander type

design, the design of double spiral was

investigated. In this thesis work, a novel

structure of double spiral shape which has

varied gap size and spiral widths in the pattern is explained

in detail in next section.

Fan shape (fig.6) : A modified double

spiral shape results in a fan shape geometry.

The structure is later proved to be one of the

best to obtain uniform temperature profile and less power

consumption.

Honey comb shape(fig.7): the honeycomb

design employs a strategy that redistributes

the thermal energy.

S-Shape (fig.8): One of the suitable

structures for uniform temperature profile

and lower power. But will be best for

sensing film of small dimensions.

3 Design optimization

The aim of this section is to point out a strategy to

optimize the geometrical structure designs. Resistive

microheaters generate heat by the inherent resistance of

metal conductors to electron flow. By controlling the voltage



supplied to a resistor, a predictable amount of power in the

form of heat energy per unit time can be emitted from the

resistor. This heat can be then used to increase the

temperature of the nearby environment, in our case the

Silicon Nitride which is a thermally conducting passivation

layer. The equation which relates conductor is2VP

R . The

resistance of material properties and its geometry LA

R

where - resistivity / , L-Length, A-Cross sectional area

of the conductor. Thermoresistive effects on thin metal films

can be taken advantage to measure temperature to a high

degree of precision and linearity. The equation which relates

the resistance and temperature for thin metal films is given

by

RT = Ro[1+αR(T-To)] (1)

RT - Resistance measured for different Temperature Ro - Resistance at Room Temperature To αR – Temperature Co-efficient of Resistance (TCR)

of the heater material T – Measured Temperature in oC

RT = L

WT

[1+αR(T-To)] (2)

For a conductive line made with material resistivity

, length L, width W and thickness T, the heater pattern can

be tailored within limits of the processing capabilities by

variably reducing the line width of the pattern.

4. Electro-thermal Simulation of the heater geometries

The heater geometry simulations were carried out

by means of COMSOLTM. In this tool the Electro-Thermal

analysis [10-11] in heat transfer module for MEMS materials

have been used. The heat transfer is assumed to be

transmitted from the hot plate to the surrounding air at an

ambient temperature of 30oC through conduction only. The

concept of Joule heating plays a major role in resistive type

microheaters. In Joule heating, the temperature increases due

to the resistive heating from the electric current.

Over a range of temperature the electric conductivity is a function of temperature T as per equation

1 ( )

o

oT T

where o is the conductivity at the reference temperature To,

α is the TCR of Pt material which typical value for platinum

is 0.003/ oc. The steady state temperature analysis has been

performed to determine the temperature distributions and

thermal resistance of the heater device. The related heat

equations chosen is COMSOL electro thermal module

which have been solved under Dirichlet, Neumann and

mixed boundary conditions numerically using the finite

element method (FEM). In this analysis, according to the

application requirement, the fixed thermal boundary is

defined for all side walls of the 3D model and the walls were

kept at room temperature of 300K while other sides were

adiabatic. Fixed temperature boundary and potentials are

applied at the ends of the heater. Several material properties

which are used to solve the heat equation are given in the

table1.

Material (Si) (SixNy) (SiO2)

Ti (Pt)

Thermal Conductivity (W/mK)

157 22 1.4 21.9 73

Young’s Modulus (GPa)

190 290 73 104 170

Poisson’s Ratio

0.17 0.24 0.20 0.342

0.39

Thermal expansion (10-6 K-1)

2.33 2.33 0.55 8.5 8.9

Density (kg/m3)

2.32e3

3.1e3 2.2e3 4506 2.145e4

Heat Capacity (J/ kg oC)

700 600-800

730 522 130

The optimized meshing for the simulation is determined by

performing an independent grid study to minimize modeling

error.



Fig.9 Temperature profile of S-shaped Microheater

The thermograph shown in fig.9 gives good overall

visualization of the temperature distribution in the S-shape

geometry of area 500µm x 500µm. From this study it is

realized that the coupled Electro-thermal analysis provides

an estimation of thermal losses and temperature distribution

on the heater plane for realistic geometrical and material

parameters pertinent to the fabrication technology. The

power consumption details of the heater geometry have also

been calculated from the resistive heating power analysis

graphs of COMSOL is given in Fig 10.

Fig.10 Power consumption analysis of the S shaped

Microheater

5. Experimental Characterization

In order to validate the above strategy and

simulations, the microheater geometry is fabricated and their

thermal characterization results were compared.

5.1 Micro-Heater fabrication

A typical structure of the heater is on a thin membrane

composed of multiple thermal isolated layers to provide

better performance. The resistance of the passivation layer

(SiO2/ Si3N4) must be high enough to ensure that leakage

currents are not affecting the measurement of the sensible

film at working temperature of 400oC.

Fig.11 shows the fabrication process flow of the microheater chip developed.

Microheater fabrication process - (1) Thermally grown SiO2 on Si Substrate

(2) Si3N4 deposition on top side

(3) Pt heater deposition

(4) Si3N4 deposition over Pt heater (5) contact pad open

(6) back side window open

(7) Backside etching using TMAH

In this study, the sensor is fabricated on a 2 inch diameter,

275µm thick, double-sided polished Silicon wafer <100>

using standard Micro-Electro-Mechanical System (MEMS)

process. The thin film Pt heater (200nm) was embedded in a

SiO2/Si3N4 membrane (which offers a high electrical

insulation). The SiO2 layer (1.2µm thick) oxide was

thermally grown by wet oxidation at 1223K, which serves as

the etch stop for TMAH-etching, as well as electrical

insulation. Then a Si3N4 layer (250 nm) was deposited using

RF sputtering over SiO2 and annealed at 600oC for 45

minutes. The membrane package of SiO2 and Si3N4 layers

were used because of their low combined intrinsic stress

level. After the nitride deposition, the Pt layer (200 nm) with

a Ti adhesion layer (20 nm) was deposited by DC sputtering.

The thickness of platinum films is optimized as 200 nm as it

is more suitable for the crystallization of ceramic thin films

due to the fact that higher heating rates can be achieved using

a low power supply. The Pt/Ti film was patterned using a lift

off process. fig.12 shows the photograph of the heater devices

on the wafer during liftoff process.



Fig.12 The photograph of the heater devices on the wafer during liftoff process.

In this process a sacrificial lift-off resist layer was patterned

using a standard S1823 photo-resist on top. All the platinum

films were heat treated at 600oC in order to minimize the

subsequent micro-structural changes during the experiments.

Finally the Si substrate underneath the heater plane was

anisotropically etched by TMAH solution at 353K in order to

produce the membrane.

Images of Fabricated Heater patterns, etched membrane and

packaged heater are shown in following figures.

Fig. 13 Fabricated Microheater

Fig. 14 TMAH etched

Backside Membrane

Fig.15 Gold wire bonded Microheater Chip

5.2 Thermal characterization

5.2.1Electrical Characterization The fabricated micro-heater was placed inside a furnace and

electrically connected to measure the Pt heater’s resistance as

a function of temperature. The temperature of the Pt heater

was set externally by heating the furnace from room

temperature (RT) to 673K using different heating rating

(from 0.5 K/s to 18.9 K/s). The temperature inside the

furnace was measured with a thermocouple located right

above the micro-heater. The resistance was measured using a

digital Multi-Source-Meter (Keithley 2410C).

With reference to the eqn1., the calculation of the

temperature coefficients can be obtained from the polynomial

fit through of the Resistance versus Temperature curves.

From the above experiment, it was found that the TCR value

was 2.2e-3/oC for a Pt heater of thickness 200 nm as shown in

fig 16.



0 100 200 300 400 500 600 700 8003.64

3.66

3.68

3.70

3.72

3.74

3.76

3.78

3.80

Shee

t Res

ista

nce

(ohm

/Sq)

Temperature (deg C )

Fig. 16 Temperature Vs Sheet

Resistance to find TCR

Using these coefficients and continuously measuring the

resistance by applying a defined range of voltage to the

heater, the temperature of the heater plane is calculated

precisely

and the SEM of the Fan shape heater at 400oC is shown in

Fig 17. The power analysis of the heater for different

temperature is been shown in fig.18.

Fig. 17 SEM of the Fan shape heater at 400oC

0 50 100 150 200 250 300 350 400 450 500 5500

500

1000

1500

2000

2500

3000

Mea

sure

d po

wer

in m

W

Temperature in Deg C

Bulk membrane

Power Analysis of 500µm x 500µm S shaped Microheater

Fig. 18 Power analysis results of the Fabricated

Microheater

5.2.2 Thermal Imaging of Microheaters

With refer to fig.9, the electro-thermal simulation

analysis of the heater geometry provides very high

temperature uniformity in some specified areas, which is to

be proved practically by Thermal Images captured by FLIR

SC 5200.

The temperature of the heater element was

measured using a Thermal Camera from FLIR P65 and FLIR

SC5600 system [11]. The thermal imaging camera can

convert the IR radiant energy received by the detector into an

electrical signal as point by point in the view area, the analog

signals are then amplified, modified and converted into

digital signal and displaced in the monitor as a thermal

image. This system is capable of recording 50 images per

second allowing the dynamics of the system to be analyzed.

The heater positioned on the released membrane reached a

temperature of about 400oC in 3 seconds.

The radiation E received by the detector is given

by[9]:

Where o is the Stefan Bohzman constant with the

value5.67x10-8 w/m2k-4

- emissivity of the heater materials

a – emissivity of the ambient approximately equal to 1

Ta – Ambient temperature.

E= o[T4 + (1+ )aTa4]



To capture the thermal image, the heater is placed

on the test bench and the electrodes are connected to the

Keithley source meter 2410C and current is given as input in

regular step. When the heater temperature is raised, the

thermal image is captured by the FLIR camera. The detector

used in this camera is InSb and the lens used is G5. The

scanned spectrum range is 1.5-5.1µm and the temperature

range is from -20 oc to 3000 oc [12]. During this process the

emissivity of the Platinum is kept between 0.105 to 0.255 [9]

form IR range of 1.8µm to 3.1µm.The inbuilt software

program [ALTAIR] of the camera enabled us to directly read

the true temperature from the images.

Fig.9. FLIR SC5200 Thermal Imaging camera.

Fig.19 Infra Red camera with source-meter and computer control and probe facility

The thermal image corresponds to S type heater is given in following figures.

Fig.20.FLIR thermal image of S Shape microheater with simulated result as a inset.

The area numbered ad 4,5,6 ( 500µm X 100 µm) shows the

uniform temperature area

( the area of interest for the gas sensing film). From the

Thermal Imaging analysis and also from the electrical

characterization it is clearly explained that the heater

geometry designs and their simulation results are in well

agreement with that of the Fabricated heaters. The power

analysis graph shown below is another proof for the

coherence of the Thermal imaging and electrical

characterization results.

220 240 260 280 300 320 340 360 380 400

1.0

1.2

1.4

1.6

1.8

2.0

Pow

er c

onsu

med

in W

Temperature in Deg C

electrical Mst. FLIR Mst.S Shape heater

Fig. 21 Power analysis Comparison of S type

Conclusion

The Microheaters are fabricated in Center for

Excellence in Nanoelectronics (CEN) IISc with the available

3’’ mask facilities.

From the results it is very clear that the temperature

profile of the heater is in well agreement with the simulated

results. The power consumption analysis data presented

indicates that the power consumed for raising the

temperature of the heater to 400oC is nearly 1.8 W which is

considerably lower than the results obtained on similar

heaters reported by other researchers .Yaowu Mo, Yuzo

Okawa, ed. all [13] designed a microheater of meander type

with Ti/Pt thickness 20/100 nm and the heater pattern of size

40µm X 40µm. The power consumed for 400oC for the

heater is reported as 10mW. Dae Sung Yoon ed. all [14]



have used a microheater of spiral groove of size 280µm X

280µm for polymerase chain reaction (PCR) application.

Their results indicate that the power consumed is 1.34 W for

a temperature rise to 90oC Li Liu ed.all [15] reported a

power consumption of 422mW to heat the La0.7Sr0.3FeO3

based gas sensing film to 340oC,. From the analysis it is

explained S shape heater pattern presented in this thesis

consumes less power on comparison with the other reports.

The fabricated structure also shows high stability even at very

high temperature (700oC).

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[3] G. Gaggiotti, A. Galdikas, S. Kacˇiulis, G. Mattogno,

A. Sˇetkus “Temperature dependencies of sensitivity and surface chemical composition of SnOx gas sensors Sensors and Actuators B 25, 1-3, 1995, 516-519

[4] Peigang Deng, Yi-Kuen Lee, Ping Cheng “An experimental study of heater size effect on micro bubble generation” International Journal of Heat and Mass Transfer, Volume 49, Issues 15-16, July 2006, Pages 2535-2544

[5] Per Johander, Igor Goenaga, et al.Design and manufacturing of micro heaters for gas sensors 4M 2006 - Second International Conference on Multi-Material Micro Manufacture, 2006, Pages 117-121

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[10] www.comsol.com

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Solid-State Electronics 51(2007) 1029-1033

Dr.G.Velmathi, Prof. S.Mohan and N.Ramshanker Importance ... 5/Issue2/8. Importance...Abstract: This paper presents the design, simulation of an S ... By controlling the voltage .

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