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Microelectronics Journal 40 (2009) 1042–1047

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Thermal transient characterisation of the etching quality of microelectro mechanical systems

Peter Szaboa,b,�, Balazs Nemetha, Marta Rencza,b

aDepartment of Electron Devices, Budapest University of Technology and Economics, XI. Goldmann Gy. ter 3, H-1521 Budapest, HungarybMicReD Ltd., XI. Etele u. 59-61, H-1119 Budapest, Hungary

Received 11 October 2006; received in revised form 26 June 2007; accepted 13 July 2007

Available online 7 September 2007

Abstract

The paper presents a non-destructive thermal transient measurement methodology that can reveal micron-sized differences among

etched layers of MEMS structures. MEMS resonator devices and bridge structures made of polycrystalline silicon differing in etching

times were investigated by simulations and measurements as well. Simulations showed that tiny differences in etching times of the

sacrificial layers can cause significant changes in heat distribution. In the measurement process the voltage transients of the devices were

captured. The results were transformed into temperature transients. Utilising temperature transients, small differences could be detected

among the structures. The paper demonstrated simulation and measurement experiments by the applicability of thermal transient

methodology for non-destructive testing of the etching quality in MEMS structures.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Thermal transient; Etching quality; MEMS

1. Introduction: thermal transient testing of the etched layers

of micro electro mechanical systems (MEMS)

The proper operation of MEMS devices stronglydepends on the qualities of the manufacturing processes.The etching process is one of those that might beresponsible for the malfunctioning of MEMS devices. Inthe manufacturing process parts of different depositedsacrificial layers are removed either by fluids in case of wetetching, or by ionised gas in case of dry etching. Thequality of the etching in the wet etching process stronglydepends on the material to be etched, on the concentrationof the etching chemicals and on the etching time as well.Regarding the dry etching procedure the materials, thepressure, the etching gas and the etching time play the mostimportant role in the settings of parameters. If parts remainin the etched layers then the movable parts in the non-

e front matter r 2007 Elsevier Ltd. All rights reserved.

ejo.2007.07.119

ing author. Department of Electron Devices, Budapest

echnology and Economics, XI. Goldmann Gy. ter 3,

st, Hungary. Tel.: +361 463 3072; fax: +36 1 463 2973.

esses: szabo@eet.bme.hu, szabop@micred.com (P. Szabo),

me.hu (B. Nemeth), rencz@micred.com (M. Rencz).

sacrificial layers might be immobilised. On the other hand,the over-etching of the remaining parts can cause fracturesor sticking.To check the quality of the etching process is a crucial

point of the fabrication. There are several methods that canbe used to investigate the state of the etched layers, e.g.,optical microscopy, scanning electron microscopy (SEM)and AFM inspections. The discussed techniques are onlyapplicable either for localising problems on the surfaces ofthe chips or for identifying elements that are not shroudedby other ones. Unfortunately, to get information aboutdeeper levels, the above-mentioned methods are barelyusable.The goal of our investigations was to check if the

thermal transient method [1] was applicable to characterisethe etching quality of MEMS structures. In the experi-ments method, the T3Ster Thermal Transient Testerequipment [2] was used. The applied methodology wasdeveloped originally for the measurement of thermalresistances of semiconductor devices. In the thermaltransient methodology, a power step is switched on thesystem and the resulted temperature change, the so-calledheating or cooling curve is observed. This depends on the

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Fig. 1. The circuit arrangement for the calibration and transient

measurement.

P. Szabo et al. / Microelectronics Journal 40 (2009) 1042–1047 1043

geometrical and material parameters of the structure. Anyirregularity in the structure results in modification in theheating curve, offering a way to detect the source of thechange. As differences in the etching time or quality resultin changing the structure itself, the thermal transient curvesare expected to show the potential etching problems.

The temperature in MEMS structures is measured byobserving some temperature dependent quantity that is inmost of the cases the electrical resistance. MEMS usuallyhave elements with temperature dependent electricalparameters, e.g., electrical resistances of bridges andthermopiles. In transient measurement processes, theelectrical resistances of the appropriate elements of MEMSstructures are usually heated by electrical current. After thetemperature has been stabilised, the current is switched offand the temperature change induced voltage change ismeasured on the electrical resistance. From the tempera-ture elevation and the duration of the transient in time, theetching ratio between different samples can be determined.In the subsequent sections, we present the method throughsimulations and measurements of MEMS elements. Inorder to carry out experiments on samples having differentetching times we needed special MEMS samples. Thesamples were designed in the TIMA laboratory [3] andwere fabricated in the same industrial manufacturingprocess. The devices went through the same manufacturingsteps, except for the etching. Different etching times wereapplied for the different dies enabling us to detect theetching time dependence of various physical properties ofthe otherwise similar MEMS devices.

In the rest of the paper, first, a physical realisation of thethermal transient measurement method is presented. InSection 3, the simulation and measurement experiments onvarious MEMS elements are presented. The paper isfinished with the discussion of the obtained results anddraws some conclusions.

2. The applied transient measurement method

As it was mentioned before electrical resistances of thestructures were used for driving and measuring the thermalresponse. For temperature measurements, we examined thetemperature-induced change of the electrical resistances ofthe structures. The change in the electrical resistance isdescribed with the following linearised equation:

DRel ¼ aRel0DT , (1)

where a is the coefficient, Rel0 is the electrical resistance at agiven T0 temperature, T is the actual temperature, DT is thetemperature difference, DT ¼ T�T0, and DRel is thechange of the electrical resistance. The value of a canbe determined by calibration. In the calibration process,the device was put onto a cold-plate and was excited withthe so-called sensor current, Isense. The temperature of thecold-plate was changed and the voltage change wascaptured along the temperature change. The measurements

have been done by the thermal transient tester. Thearrangement is shown in Fig. 1.After the calibration process, the thermal transient

measurement can be carried out. In our case, the electricalresistance was excited, heated, with the so-called drivingcurrent, Idrive. After the temperature was stabilised in thesystem, the driving current was switched off and thevoltage transient was captured by the sensor channel ofthe tester with 1ms sampling rate in logarithmic samplingmode using 200 sample pro decade resolution. The voltageresolution of the channel was 10 mV. Applying thecalibration results the temperature change was calculated.The main advantage of our method is that both heating

and measurement are feasible at the same device at thesame time. Different structures are not necessary to senseor to drive in the measurement process. Another advantageis that the self-impedance of the driven element of thedevice can be measured [4]. The thermal transient methodgives the possibility to separate the electrical cross effectsfrom the thermal ones as the response is captured in time.

3. Thermal simulations and transient measurements of the

structures

The investigated test MEMS devices were fabricated bythe PolyMUMPs technology [5]. In this process, the PSGlayers existing between the polysilicon layers are etched off.The sample chips were etched for different length of times:30, 60 and 90 s. In some cases and figures, we will refer tothe samples by using only the etching times. All the chipswere packaged similarly. The properties of the etched PSGlayers play the most important role in the operation ofthese devices as the ability of moving is strongly dependenton the potentially remained parts of the PSG layers [6].Four different types of MEMS devices, bridge struc-

tures, presented in Figs. 2 and 10 and resonators, shown inFig. 12, were selected to demonstrate the effects of thedifferent etching times in the thermal transients.

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3.1. Simulations and measurements of the thermal behaviour

of bridge1

At first a bridge, called bridge1, was examined by opticalmicroscopy. Its top view is shown in Fig. 2. The cross-sectional view of the structure is shown in Fig. 3. Thebridge is 300 mm long, 10 mm wide and the average electricalresistance of the bridge is about 500O.

Firstly, a calibration was carried out applying 2mAsensor current. The voltage change results of the bridgesare shown in Fig. 4. The calibration curves show goodlinearity, consequently linear approximation is feasible tobe used in the measurement. The slope of the curves can be

Fig. 2. The investigated bridge1.

Fig. 3. The cross-sectional view of the bridge1.

Fig. 4. Calibration curves of the bridges of samples etched at different

time.

regarded as sensitivity factors in the temperature calcula-tion (see in Fig. 4).After the calibration, thermal transients were measured

driving the devices with 5mA driving currents. Theobtained transient curves of the different samples areshown in Fig. 5. It is observable that the measuredtransient of the 90-s etched sample shows larger tempera-ture elevation in consequence of the larger thermalresistance coming from the longer etching time. As it isnoticeable in Fig. 5, the first part of the transients untilabout 20 ms is different for the simulated and measuredcurves. The reason is the so-called parasitic electricaltransient. The electrical transients are the consequences ofthe presence of electrical capacitances. The separation ofthe parasitic electrical and thermal parts of the transientsby measurement methods was presented in details inRef. [6].In this paper, another method is used to distinguish

between the thermal and electric part of the measuredcurves. Thermal transient simulations of the structureswere accomplished using the SUNRED thermal simulator[7]. Two simulation models of the bridges have beencreated. In the model referring to the 90-s etching time allthe PSG are removed between the substrate and the bridge.In the case of the model of the 60-s etching time the 1/20 ofthe whole PSG was left as shown in Fig. 6. The thermalcapacitances of the bridges were changed in the models toget the best fit between the simulated and the measuredtransients. From the comparison of the simulated and themeasured curves, it can be asserted that the thermal part ofthe measured transients begins at about 20 ms. This is thepoint after which the simulated and the measured curvesrun together. The results also suggest that the thermalparameters of the polysilicon change slightly during theetching process, as the simulated and measured curves runparallel in case of the 90-s curves if we calculate with the60 s parameters. The effects of the etching time weresimulated on the temperature distribution in completely(see Fig. 7) and in partially etched case (see Fig. 8).

Fig. 5. Measured and simulated thermal transient curves of the 60 and

90 s etched samples.

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Fig. 6. The model of the partially etched structure; the bridge1 is

transparent for the better visibility of the considered PSG layer.

Fig. 7. 3D-simulated temperature distribution of the completely etched

bridge by equidistant mesh.

Fig. 8. 3D-simulated temperature distribution of the bridge, the 1/20 of

the PSG is left back between the bridge and the substrate, equidistant

mesh.

Fig. 9. The comparison of the measured curves of the three bridges etched

for different times.

P. Szabo et al. / Microelectronics Journal 40 (2009) 1042–1047 1045

In Fig. 8, it is shown that the temperature distribution isnot homogeneous in the bridge, as a result of theinhomogeneous material distribution of the parts of thePSG left. The simulation has demonstrated that already asmall amount of material, e.g. 1/20 of PSG, in the etchedlayer if it is in connection with the bridge and the siliconsubstrate may cause large differences in the temperatureelevation and distribution. It is important that theinhomogeneity of the temperature distribution may causeinhomogeneous stresses in the material and this isresponsible eventually for cracking during the applicationof the MEMS. The comparison of the measured results onthe samples with 30, 60 and 90 s is shown in Fig. 9.

Note that there is practically no difference between the30-s and the 60-s etching cases. It means that the thermalproperties of the devices show no significant difference inthe first part of the etching process even the etching time istwice as long. In the 90-s etching case, the elevation of the

curve is about two times higher at 15 ms than that of thecurves of the 30 s and the 60 s demonstrating that longeretching time is needed to form the structure.

3.2. Transient measurements on bridge2

Further transients were also measured on the samesample dies but on a different (bridge2) structure,presented in the black frame in Fig. 10. The voltagetransients were converted into temperature transients withthe sensitivities derived from the calibration; the transientresults are shown in Fig. 11. The temperature elevation ofthe sample etched for 90 s shows the highest temperaturevalues compared to the transients of the samples etched for60 and 30 s. The sample etched for 30 s has the lowesttemperature elevation values. Note that the transientsof the 60 and 90 s etched case run almost together

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Fig. 10. The examined bridge2 is in the black frame; destroyed devices can

be observed on the left-hand side of the figure.

Fig. 11. Thermal transients of the bridge2 in case of 30, 60 and 90 s etched

samples.

Fig. 12. The 1st electro mechanical resonator is shown in the black and

the 2nd in the white frame.

Fig. 13. Cross-sectional view of the resonators.

Fig. 14. SEM picture of the 1st electro mechanical resonator.

P. Szabo et al. / Microelectronics Journal 40 (2009) 1042–10471046

demonstrating that the two etching times result in a fairlysimilar etching quality in case of the present structure.

3.3. Investigations of two electromechanical resonators

In the last series of investigations two electromechanicalresonators were measured, as shown in Fig. 12. Theresonator at the top of the figure is called 1st resonator andthe other at the bottom of the figure is called 2ndresonator. The cross-sectional view of the resonators ispresented in Fig. 13. The SEM picture of the 1stelectromechanical resonator is shown in Fig. 14. Theresonator has two beams crossing each other; each beamhas bonds at the ends connected to pins. Transient resultswere obtainable for the 1st resonator of the 60 s and the90 s samples only as the beam of the 30 s has been cracked.Fig. 15 represents the transients. The effect of the differentetching times is well observable in Fig. 15. The temperaturetransient measured at the sample etched for 90 s showshigher temperature elevation. The SEM picture of the 2ndelectromechanical resonator is shown in Fig. 16. The

transients have been measured at the top, narrower beams.Transient results of the three sample etched at differenttimes are presented Fig. 17.

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Fig. 15. Thermal transient results of the top beam of the first resonators.

Fig. 16. SEM picture of the 2nd electro mechanical.

Fig. 17. The thermal transient results of the top beams of the 2nd

resonators.

P. Szabo et al. / Microelectronics Journal 40 (2009) 1042–1047 1047

In the measurements, the expected effects were observed;the samples etched for 90 s have the highest temperatureelevation, its transient is the longest in time. The transient

of the sample with 60 s etching time shows lower elevationand its length is shorter than the previous one. The sampleetched for 30 s has the smallest temperature elevation andends as the earliest. The change in the length of thetransients in time is explained in Ref. [1].

4. Conclusions

With the measurements, we have demonstrated thatthermal transient measurements result in significantlydifferent curves for samples of the same structure withdifferent etching times. We have presented a non-destruc-tive thermal transient measurement method also insimulations and measurements. Simulations have shownthat the amount of the material in layers of MEMS candrastically change the temperature.The suggested method is able to reveal micron-sized

differences in the etched layer of MEMS structures. Wetested the method to the etching quality of appropriateMEMS structures. We showed in our measurementprocedure that the captured voltage-change at a givenstructure can be transformed into temperature change thatcan reveal even micron-sized differences. The advantage ofthe thermal transient method is that we can drive andmeasure at the same device that can be any resistor-typeelement in the MEMS structure. No special test structuresare needed either for sensing or driving in the suggestedmeasurement method.

Acknowledgements

This work was supported by the PATENT IST-2002-507255 Project of the EU and by the OTKA-TS049893Project of the Hungarian Government. Special thanks forBernard Courtois (TIMA labs) and Benoit Charlot fortheir help in preparing and providing us the samples.

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