Ž .Thin Solid Films 394 2001 284�291

Measurements of residual stress in the thin film micro-gassensors containing metallic layers

Youngman Kim�, Sung-Ho Choo

Department of Metallurgical Engineering, Chonnam National Uni�ersity, Kwangju 500-757, South Korea

Received 23 September 2000; received in revised form 14 February 2001; accepted 27 April 2001

Abstract

The mechanical properties of thin film materials are known to be different from those of bulk materials, which are generallyoverlooked in practice. The difference in mechanical properties can be misleading in the estimation of residual stress states inmicro-gas sensors with multi-layer structures during manufacturing and in service. In this study the residual stress of each filmlayer in a micro-gas sensor was measured according to the five different sets of film stacking structure used for the sensor. The Ptthin film layer was found to have the highest tensile residual stress, which may affect the reliability of the micro-gas sensor. Forthe Pt layer the changes in residual stress were measured as a function of processing variables and thermal cycling. In addition,the residual stresses in the Pt layers caused by microstructural changes were also investigated. � 2001 Elsevier Science B.V. Allrights reserved.

Keywords: SnO gas sensor; Residual stress; Thermal stress; Thermal cycling; Laser scanning method; Pt thin film2

1. Introduction

Gas sensors are widely used to detect and control avariety of harmful gas species due to high concern ofatmospheric environments and human welfare. Semi-conductor gas sensors are being manufactured in theforms of bulk, thick and thin films. Bulk type sensorsare generally used in practice with the sensing materialof SnO containing noble catalysis such as Pd.2

A SnO micro-gas sensor processed in combination2with thin film and micro-machining technologies hasthe advantages in terms of manufacturing such as massproduction oriented stabilized process, low cost, lowpower consumption and precise temperature control.Since the relevant circuits for the sensor can be manu-factured on one chip, the miniaturization of the systemis achievable and the reliability can be improved simul-

� Corresponding author. Tel.: �82-62-530-1698; fax: �82-62-530-1699.

Ž .E-mail address: kimy@chonnam.ac.kr Y. Kim .

taneously. Thus, it is a general tendency that thin filmtype sensors gradually substitute for bulk and thick filmtypes. The standardization for the sensor manufactur-ing is a challenging task and the mechanical strength of

� �the sensors is known to be a problem 1�3 .Thin film micro-gas sensors are composed of gas-

sensing material, electrodes, heaters and insulating lay-ers. The sensing mechanism for the micro-gas sensor isdetecting the difference in conductivity of sensingmaterial before and after the adsorption of gases. Sincethe current passing through the gas-sensing material isapproximately several pAs, the sensitivity of sensorhighly depends on the interfaces of gas-sensing materi-als, electrodes and electrode materials. Micro-gas sen-sors require heaters to increase the operation tempera-ture of gas-sensing material to a specific temperaturerange for the highest sensitivity of specific gases. Lowpower consumption and thermal stability are essentialfeatures for the heaters of the sensors in the widetemperature range. For low power consumption themicro-gas sensors have diaphragm structures with

0040-6090�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 1 1 3 7 - 3

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291 285

multi-layers of metals and ceramics, such as the layersof insulator, heater, electrode and sensing material

� �layers 2 .Any type of multi-layer structure results in the dif-

ference in thermal expansion coefficient and elasticmodulus between layers due to the artificial bonding ofdissimilar materials. Inevitably, thermal and residualstresses between layers arise from the bonding. In turnthe stresses may cause dimensional instability of parts,such as twisting and bending, which can trouble theengineering products during fabrication and even in

� �service 4�7 .The current provided to the heaters of prototype gas

sensors is imposed at the frequency of approximately30 Hz; the transient thermal stress occurring during theheating cycles may affect the lifetime and stability of

� �the sensor 1 . While the excessive tensile stresses maycause peeling of the thin film from the substrate, theexcessive compressive stresses may buckle the thin filmfrom the substrate. Also, it is known that these residualstresses may affect the electric, magnetic and optical

� �properties of thin film 4 . The origin of residual stressin thin film structures should be understood to improvethe reliability of thin film type micro-gas sensors.

In this study the residual stress of each materiallayer for the thin film type micro-gas sensor was inves-tigated. Among all the thin film layers in the sensor,the most influential thin film layer to the residual stressalso was found to improve the reliability and the stabil-ity of the sensor during thermal cycling. The changes inresidual stress during thermal cycling were measured asa function of processing variables. The residual stressesin Pt layers caused by microstructural changes, whichwere used for heaters and electrodes of the sensors,were also investigated after heat treatment.

2. Techniques for measuring thin film stresses

The experimental techniques for measuring stressesin the thin films on substrates fall into three generalclasses.

1. Those based on direct measurements of the elasticstrains in the films using X-ray and neutrondiffraction testing.

2. Those based on measurements of deflection usingoptical interferometry.

3. Those based on the associated curvature or deflec-tion of the substrate.

One of the most popular techniques is a laser scan-� �ning technique 4 for measuring curvature changes

associated with thin film stresses as the third classmentioned above. A laser scanning device made forthis experiment is shown in Fig. 1. A beam of laser light

Ž .scans the specimen as a mirror rotates Fig. 1 . A lensis used for the rotating laser beam to be perpendicularto the substrate at all points during the scanning, andthe same lens causes the reflected beams to convergeinto a single point on the position-sensitive detectorŽ .PSD when the substrate is perfectly flat. When thesubstrate is curved, the reflected beam moves to dif-ferent positions on the PSD as the laser beam isscanned across the substrate. The linear position changedetected by the PSD can be used to find the curvatureof the substrate. The laser-scanning device is very sen-sitive and is capable of detecting the bending of a flatwafer to a radius of approximately 10 Km. For typicalfilm dimensions, this corresponds to a film stress ofapproximately 0.2 MPa. This sensitivity is required be-cause the film, being so much thinner than the sub-strate, bends the substrate extremely small in amount� �4 .

3. Experiment procedure

² :The n-type 111 Si wafer ground to 384 �m thick-ness was cut into the dimensions of 6�30 mm using adicing saw. The specimens were cleaned with acetoneand alcohol for 5 min each, and with deionized waterfor 10 min before blow-drying using N gas. Dried2specimens were thermally oxidized to form SiO layers2

Fig. 1. Laser scanning device for measuring radius of curvature.

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291286

Table 1Standard processing conditions of sensor materials using a RF magnetron sputter

Working Gas flow Bias power Sputtering Deposition Pre-sputterŽ . Ž .Pressure sccm W time temperature time

Ž .mtorr

�3SiO 2�10 Ar 20 175 40 min Room temp 5 min2�3Pt 8�10 Ar 5 80 10 min Room temp 5 min�3Ti 2�10 Ar 20 175 1 min Room temp 10 min

10 s�3SnO 4�10 Ar 20� 50 52 min Room temp 5 min2

O 20 30 s2

of approximately 100-nm thickness in the furnace at900�C using water vapor. Each material layer accordingto the stacking structure of the sensor was deposited byRF magnetron sputtering after measuring the curva-ture of the thermally oxidized specimens. In this exper-iment the sensor has the multi-layer structure, includ-ing a sensing material SnO layer, Pt layers for heater2and electrodes, SiO insulating layers that are de-2posited by thermal oxidization and sputtering, and a Ti

Ž .layer to improve adhesion of Pt Fig. 2 . A standardcondition of RF magnetron sputtering for sensor pro-cessing is given in Table 1.

The curvature of the specimen was measured wheneach layer was deposited using a laser scanning deviceŽ .Fig. 1 . From the change in the curvature of a certainlayer before and after the deposition, the residualstress in the layer was obtained using a modified

� �Stoney’s equation 8 with the knowledge of Young’smodulus and Poisson’s ratio of substrate, thicknesses ofsubstrate and film layer.

E t 2 1 1s s Ž .� � � 1f ž /t r rŽ .6 1�� f 0s

where E is Young’s modulus of the substrate, t , t ares s fthe thicknesses of substrate and film, respectively. Also,

Fig. 2. Schematic diagram of general micro-gas sensors and five setsof specimen micro-gas sensors.

� is Poisson’s ratio of the substrate, r is the radius ofscurvature after the deposition of the thin film and r is0the radius of curvature before the deposition of thethin film. Stoney’s equation can estimate the averageresidual stress of the thin film layer using only theelastic modulus of the substrate without knowing theelastic modulus of the thin film, and it is widely useddue to the simplicity and the convenience of the calcu-lation. However, the Stoney equation has a limitationthat it can be used when the ratio of the thickness of

� �the thin film to that of the substrate is very small 4 .Pt layers are known to have problems in adhesion

� �with SiO insulation layers 9 . In this experiment a Ti2layer was used between the Pt layer and SiO insula-2tion layer to improve adhesion and to prevent thediffusion of Si atoms to the Pt layer. Also, it is gener-

² :ally known that Pt films grow with 111 orientations topossess low surface energy. Pt is held at 800�C for 1 h

² :to change the crystallographic orientation from 111² :to 200 . The heat treatment was performed under an

Ž .inert gas Ar atmosphere to minimize the oxidation² :possibility of the Pt thin film. The Pt thin film of 200

direction is the direction that has low electric resis-tance and excellent thermal stability and does notchange electric resistivity and orientation even whenthe Pt film is exposed to a high temperature of 600�C

� �for a long time 9 .After the deposition of all the thin film layers was

completed, the final heat treatment at 500�C for 6 hwas carried out for the stability of the SnO thin film.2The rates of heating and cooling for the heat treatmentwere 6�C�min. The thickness of each deposited thinfilm was measured under SEM observation, stresses inthe thin film were obtained by measured curvatures

� �and film thicknesses using Stoney’s formula 8 . Theschematic diagram of general micro-gas sensors isshown in Fig. 2 and five sets of specimens with differentlayer stacking sequences in micro-gas sensors are alsodescribed. The five sets of specimens having differentstacking structures were subject to thermal cyclingswith heating and cooling rates of 5.66�C�min fromroom temperature to 410�C. Curvatures were measuredat intervals of 20�C during heating and cooling.

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291 287

Table 2Thickness and average residual stress of each layer under standardprocessing conditions

Thickness AverageŽ .nm residual

stressŽ .MPa

Ž .SiO thermal oxidation 113 �2Ž .SiO sputtering 125 �84.52

Pt 215 200�400Ti 36 �56.1SnO 241 25.32

After the thermal cycling experiment for the speci-mens, the thin film layer potentially affecting the stabil-

ity of the sensor the most was found. The changes inresidual stress in the film layers were investigated ac-cording to various processing conditions of RF mag-netron sputtering. The residual stresses in the thin filmlayers caused by microstructural changes due to heattreatment were also investigated.

4. Results and discussion

The residual stress of each layer is obtained from themeasured radius of curvature and the thickness of eachlayer measured from the SEM image of the cross-sec-tion using Stoney’s equation as each layer was de-posited. The SEM images of five sets of specimens

Fig. 3. SEM cross-section images according to stacking structure of sensor materials. The subscripts, ‘sp’ and ‘th’ stand for ‘sputtered’ andŽ . Ž . Ž .‘thermally oxidized’, respectively. a SiO �SiO �Si specimen b SnO �SiO �SiO �Si specimen c SnO �SiO �Pt�Ti� SiO �Si2 sp 2 th 2 2 sp 2 th 2 2 sp 2 th

Ž . Ž .specimen d SnO �Pt�Ti� SiO �SiO �Si specimen e SnO �Pt�Ti� SiO �Pt�Ti� SiO �Si specimen.2 2 sp 2 th 2 2 sp 2 th

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291288

having different stacking structure of gas-sensor mate-rials are shown in Fig. 3. The residual stress measure-ments show compressive stresses under 100 MPa in theSiO and Ti layer, and a tensile stress under 50 MPa in2SnO when processed with a standard condition de-2scribed in Table 1. The Pt layers used as the heater andthe electrode showed the high tensile stresses of ap-proximately 200�400 MPa in comparison with other

Ž .thin film layers Table 2 .After the heat treatments of five sets of specimens

according to the stacking structure of the micro-gassensor were carried out, thermal cycling was enforcedthree times for each specimen. Hysteresis changes instress during thermal cycling were investigated by in-creasing the number of thermal cyclings, but no hys-teresis change was observed. No appreciable amount ofplastic deformation during the thermal cycling in thetemperature range from RT to 410�C occurred for theoxidation layers of SnO and SiO , and metal layers of2 2Pt and Ti in the sensor structures. The residual stressesin the specimens containing Pt and Ti thin film layers

decreased as the temperature increased as shown inFig. 4. While the stress level in the layers composed ofonly oxidation layers is relatively low at RT as shown inthe case of Fig. 4a,b, the tensile stress is relatively high,150�250 MPa at room temperature and stress devia-tions of approximately 150�200 MPa during the ther-mal cycling from room temperature to 410�C are ob-tained for the layer structures including Pt. From theabove-mentioned results, Pt layers were found to play amajor role in the stress state of the layer structuresduring thermal cycling.

To reduce the level of tensile stress in the Pt layer,the processing variables were changed when the Pt thinfilm was manufactured. The processing variables for Ptsputtering include working pressures of 6, 8 and 10mtorr, Ar gas flow rates of 3, 5 and 7 sccm and biaspowers of 60, 80, 100 W; a total of 27 different process-ing conditions were used to deposit Pt thin film layers.After the thickness of the Pt layer was measured bySEM, the curvature of each layer was measured andthe stress of each layer was obtained according to each

Fig. 4. Stress curve according to stacking structure of sensor materials during thermal cycling. The subscripts, ‘sp’ and ‘th’ stand for ‘sputtered’Ž . Ž . Ž .and ‘thermally oxidized’, respectively. a SiO �SiO th�Si specimen b SnO � SiO �SiO th�Si specimen c SnO �SiO �Pt�Ti�2 sp 2 2 2 sp 2 2 2 sp

Ž . Ž .SiO �Si specimen d SnO �Pt�Ti�SiO sp� SiO �Si specimen e SnO �Pt�Ti�SiO �Pt�Ti� SiO �Si specimen.2 th 2 2 2 th 2 2 sp 2 th

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291 289

condition of processing variable. As the working pres-sure increased regardless of bias power, the residual

Ž .stress of Pt layer decreased Fig. 5a . Also, for theresidual stress changes of the Pt layer according to gasflow and working pressure, as the working pressureincreased, the residual stress of the Pt layer deceasedwithout much influence from the variation of gas flowŽ .Fig. 5a,b . As shown in Fig. 5c, the Pt layer was in thetensile stress state of 150�250 MPa regardless of thechanges of gas flow or bias power, especially it indi-cated lower stress at the gas flow of 5 sccm. Based onthe above results the tensile stress in the Pt layer couldbe effectively reduced when the working pressure wasraised to 10 mtorr. According to these results it can besummarized that the stress in the Pt layer is the lowestat a working pressure of 10 mtorr, and also the workingpressure is the most effective in controlling the stressin the Pt layer in comparison with other processingvariables.

The stress change in the Pt layer was observedvarying the amounts of Ar gas flow of 3, 5, 7 sccm andbias powers of 60, 70, 80, 90, 100 W at the fixedworking pressure of 10 mtorr where the lowest residualstress in the Pt layer was found to occur. A low tensilestress under 100 MPa in the Pt layer was measured at10 mtorr working pressure without showing pronounced

Ž .effects of gas flow and bias power Fig. 6 .The specimens having only Pt and Ti layers on Si

wafer were heat-treated at 800�C for 1 h under an Aratmosphere and subsequent thermal cyclings were per-formed for the specimens, to find out the effects ofprocessing variables on the stress behavior of the Pt

layer during thermal cycling. As shown in Fig. 7 thehighest tensile stress of approximately 600�750 MPa inthe Pt layer was measured at a working pressure of 6mtorr; also the stress change caused by thermal cyclingwas as high as approximately 400 MPa. The tensilestress was approximately 350 MPa and the stress devia-tion during thermal cycling was approximately 200 MPafor the Pt layer processed at the working pressure of 8mtorr. At the working pressure of 10 mtorr, the resid-ual stress in the Pt layer showed the lowest tensilestress of approximately 100�250 MPa at RT; also thestress deviation during thermal cycling was under 100MPa. As the temperature increases, the residual stressdecreases but the stress deviation during thermal cy-cling increases as temperature increases. The high levelof residual stress in the Pt film at room temperaturemay greatly affect the stability of the sensor.

An SEM study was done on the Pt layer processed atthe working pressure of 6 mtorr, the gas flow of 7 sccm,and the bias power of 80 W in terms of microstructureto investigate the heat treatment effect. The heat treat-ment of the Pt layer at 800�C for 1 h increased thestress level up to 700 MPa; for the Pt layer without theheat treatment the stress level was approximately200�400 MPa. The as-grown Pt layer had a columnarstructure as seen from SEM images before the heattreatment, and the grain size of approximately 20�30nm was seen on the layer surface as shown in Fig. 8.However, the shape of the columnar structure wassomewhat collapsed after the heat treatment, the thick-ness of Pt layer decreased due to the densification.Thus, the grain size observed from the surface of the Pt

Ž . Ž . Ž .Fig. 5. Residual stress changes in Pt layer according to: a bias power and working pressure; b gas flow and working pressure; c bias powerand gas flow.

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291290

Fig. 6. Residual stress changes in the Pt layer according to biaspower and gas flow at a working pressure of 10 mtorr.

layer became coarser. The recrystallization and thegrain growth may have occurred in the Pt layer by heattreatment to result in coarse grain size, in turn thethickness of the Pt layer may have decreased due to thedensification of the layer through recrystallization andgrain growth. The thickness decrease of the Pt layer

due to densification may have led to an increase in theresidual stress in the Pt layer as compared to the layer

� �before the heat treatment 10 . The surface of the Ptlayer was observed to become rough after the heattreatment. The thin film type of Pt was considered tobe thermodynamically meta-stable due to the residualstress unlike the bulk form. Also we could observe theclusters of several micrometers in size at distance inter-vals of tens of micrometers for the surface of heat-treated specimens. The clusters became larger to re-duce their own surface energy through the surfacediffusion mechanism as Ostwald ripening, but someclusters of slow mobility filed up to form larger clusterswhile they were moving by the so-called ‘cluster-mobil-

� �ity coalescence mechanism’ 11 .

5. Conclusion

In this study the residual stress in each thin filmlayer was measured for the layer stacking structure of aSnO micro-gas sensor. The compressive stresses are2

Ž . Ž .Fig. 7. Stress curves according to processing conditions of the Pt thin film during thermal cycling. a 6 mtorr, 7 sccm, 60 W b 6 mtorr, 7 sccm, 80Ž . Ž . Ž . Ž .W c 8 mtorr, 5 sccm, 80 W d 8 mtorr, 7 sccm, 100 W e 10 mtorr, 7 sccm, 60 W f 10 mtorr, 7 sccm, 80 W.

( )Y. Kim, S. Choo � Thin Solid Films 394 2001 284�291 291

Ž .Fig. 8. Cross-section and surface SEM images of Pt film processed at 6 mtorr, 7 sccm, 80 W before and after heat treatment. a Before heatŽ . Ž . Ž . Ž . Ž . Ž . Ž .treatment cross-section b before heat treatment surface c after heat treatment cross-section d after heat treatment surface .

under 100 MPa in SiO and Ti, and the tensile stress is2under 50 MPa in SnO . The Pt layers used as the2heater and the electrode showed the high tensilestresses of approximately 200�400 MPa. Also, thermalcycling was carried out for five sets of specimens ac-cording to stacking structures of the micro-gas sensor.Tensile stresses of 150�250 MPa at room temperaturewith stress deviations of approximately 150�200 MPaduring the thermal cycling were obtained for the layerstructures including Pt. The Pt layer might have themost pronounced effect on the reliability and stabilityof the sensor.

The residual stress changes in the Pt layer wereobserved as a function of processing variables; such asworking pressure, the amount of gas flow and biaspower during sputtering. The working pressure wasfound to be the most effective in controlling the stressin the Pt layer in comparison with other processingvariables. The Pt layer manufactured at the workingpressure of 10 mtorr displayed the lowest tensile stressof less than 100 MPa at RT. Also, the stress of the Ptlayer increased after the heat treatment due to graincoarsening and densification of the film.

Acknowledgements

Ž .The financial supports of ERC CAPST fund fromthe Korea Science and Engineering FoundationŽ .KOSEF and Basic Study Fund for Universities fromMinistry of Information and Communication, Koreaare gratefully acknowledged.

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