IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 1459

Effect of Thin Film Thicknesses and Materials on theResponse of RTDs and Microthermocouples

Muhammad Imran and Abhijit Bhattacharyya

Abstract—Fabrication and thermal characterization of a resis-tance temperature detector (RTD) heater and microthermocou-ples (MTs) on silicon substrates have been reported in this paper.The influence of film thickness and nickel–gold (Au) electroplatingon RTD on its steady-state temperature with respect to its steady-state electrical power input and resistance is studied. Further, thethermal effects of multiple thermocouples in a thermopile as wellas the effects of Au layers in the contact pads of the thermopileson their open-circuit Seebeck voltage are studied. Therein lies thenovelty of this paper. The in situ operating relationships for theRTD heater and the MT are provided.

Index Terms—Microthermocouple (MT), relative Seebeck coef-ficient (RSC), resistance temperature detector (RTD).

I. INTRODUCTION

THIN-FILM resistance temperature detectors (RTDs) andmicrothermocouples (MTs) are the most frequently used

temperature sensors to measure surface temperatures in mi-crosystems [1]–[8]. The RTDs are usually characterized withrespect to their resistance versus temperature response [2], [3],whereas a typical approach to characterize an MT is to measureits open-circuit Seebeck voltage, hot junction, and cold junc-tion temperatures [9]–[11]. Gregory and You [2] have devel-oped and experimentally characterized indium-tin-oxide (ITO)RTDs and ITO-based ceramic MTs for the surface temperaturemeasurements of gas turbine engines employed in power andpropulsion systems. The RTDs were characterized with respectto resistance versus temperature curves at low and high oxygenpartial pressures. The Seebeck voltage versus hot junctiontemperature characteristics were studied for the ceramic MTs.Shi and Majumder [3] have experimentally investigated differ-ent heat transfer mechanisms of gold (Au) samples on silicon(Si) substrates using Au-RTD heaters and thin-film platinum(Pt)–chromium (Cr) thermocouples. Shi et al. [4] have alsoused Pt RTD heaters to determine the one-dimensional (1-D)thermoelectric properties of single-wall carbon nanotubes. The

Manuscript received October 25, 2005; revised February 26, 2006 and April5, 2006. This work was supported by the Defense Advanced Research ProjectsAgency under Contract DAAD19-02-1-0270 administered by the Army Re-search Office and in part by the National Science Foundation-EPSCOR Pro-gram under Award EPS-70236967 to the University of Arkansas at Fayetteville(subcontract SA0402138 from the University of Arkansas at Fayetteville to theUniversity of Arkansas at Little Rock). The associate editor coordinating thereview of this paper and approving it for publication was Dr. Kailash Thakur.

The authors are with the Smart Materials and MEMS Laboratory, Depart-ment of Applied Science, University of Arkansas at Little Rock, Little Rock,AR 72204-1099 USA (e-mail: axbhattachar@ualr.edu).

Color versions of Figs. 1, 2, and 4 are available at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JSEN.2006.884167

Fig. 1. Optomicrographic images of (a) thin-film Cu-RTD heater, (b) thin-film Cu-RTD heater and 1-MT thermopile, (c) thin-film Cu-RTD heaterand 3-MT thermopile. The dimensions of RTDs in (b) and (c) are the sameas shown in (a).

Pt and titanium (Ti) RTD heaters have been characterized byLourenço et al. [5] for high-temperature applications. In [6],the physical and the thermoelectric characteristics of ger-manium (Ge)–aluminum (Al) thermocouples on polyethyleneterephthalate plastic substrates have been compared with nickel(Ni)–Cr thermocouples on glass substrates for gas flow mea-surements. The characterization and the use of poly-Si heatersto measure the Seebeck coefficient of complementary metaloxide semiconductor have been demonstrated in [7]. Choi andWise [8] have developed the poly-Si–Au thermocouples forautomated manufacturing and process monitoring. They usedthe thermocouples as infrared sensing detectors and studied theresponse time, responsivity, and detectivity characteristics ofthe thermopile with respect to the number of thermocouples,the width of the thermocouple legs, and the radius of the blackbody absorption window at the hot junctions of the thermopile.Völklein et al. [12] have also developed n-doped poly-Si andAl thin-film thermocouples on Si wafers for infrared radi-ation detection. Bhatt et al. [13] calibrated the Ti carbide(TiC)–tantalum carbide (TaC) thin-film thermocouples on alu-mina substrates against temperature under vacuum (pressure <10−6 torr) for different sensor applications in inert atmosphere.

Recently, we have experimentally characterized an on-chipassembly of an RTD heater, sputtered sample, and a thermopile(see Fig. 1 for the optical micrographs of some of the devicesthat have been characterized [14]). In particular, the RTD with a0.25-µm-thick copper (Cu) layer was characterized with respectto its temperature versus time and resistance versus temperaturecharacteristics. The 5-MT thermopile with Au contact pads atits two cold junctions was characterized based on open-circuitSeebeck voltage measurements. We studied the contact effectsof external thermocouples on the response of the 5-MT ther-mopile; those effects were found to be small but measurable, as

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1460 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006

reported in an earlier paper [14]. The temperature sensors werethen used to determine the heat transfer coefficients of thin-film Cr samples of different dimensions. In another paper [15],we have reported a detailed steady-state analytical thermalmodel for the in-plane 1-D temperature profile of an assemblyof RTD—sputtered sample—thin-film thermopile in which thelength effects of the thermopile on the cold junctions were stud-ied. While open-circuit measurements of thermopiles have beentraditionally used as a sensory parameter, we have investigatedthe closed-circuit response of thermopiles where the Seebeckcurrent is a sensory parameter. This work—both experimentsand thermal modeling—has been communicated recently [16].During the course of this research, we have been curious aboutthe effects of thin film geometry on the response of the RTDs aswell as the effects of Au pads on the thermoelectric propertiesof MTs. We have not been able to locate any such work in theopen literature.

In this paper, we study the effects of film thickness andsputtered materials on the thermal response of the RTD. Inparticular, two different RTDs with identical architecture butdifferent film thicknesses are studied. In reality, the response ofan RTD as well as an MT is not only dictated by the materialsinvolved but also their architecture as well as their geometry.The operating relationships of the RTDs are then given. TheCr–Ti MTs are fabricated with and without Au pads at the twocold junctions. The Seebeck voltage, hot junction temperature,and cold junction temperature characteristics are studied. Therelative Seebeck coefficients (RSCs) are found. The MTs withAu pads at its cold junctions were found to have higher RSCsthan the MTs without Au pads. The effects of thermal mass onthe response of MTs are also given. Further, while multiple ther-mocouples in a thermopile are instrumental in increasing thesensitivity of the thermopile (higher Seebeck voltage for a givenchange in temperature), the measured temperature itself isaffected due to a higher thermal mass. This issue is alsoinvestigated in this paper.

We have carried out an extensive literature survey, and whilethere is a significant body of literature on MTs, we have notbeen able to find any systematic studies that focus on the geo-metric effects on the RTD resistivity as well as the thermal masseffect of multiple thermocouples. Thus, the focus and the nov-elty of this paper are more on the methods used to characterizeon-chip MTs rather than the specific materials that have beenused here for the RTDs (Cu) and MTs (Ti and Cr). It is hopedthat these methods will apply to RTDs and MTs made fromother materials as well.

II. DESIGN, FABRICATION, AND EXPERIMENTAL

CHARACTERIZATION OF RTD HEATERS

AND THIN-FILM MTS

The fabricated thin-film RTD heater source and a 1-MT anda 3-MT thermopile are shown in Fig. 1(a)–(c), respectively.We have used Cu for the RTD, Cr, and Ti for the MTs inthe thermopile. The choice of these materials was driven bytheir availability, ease of fabrication, and the excellent adhesivestrength of Cr and Ti to oxidized Si wafers. Further, goodadhesive property of Cr with Ti ensured better contact at the

Fig. 2. Schematic diagram of thin-film RTD heater and an MT.

hot junctions of the MT. The arms of the thin-film MTs weredesigned long enough to ensure that the cold junction tem-perature stayed much lower than the hot junction temperature.With reference to Fig. 2, the width of the RTD heater wRTD =30 µm (1 µm = 10−6 m), the length of the RTD heater LRTD =3280 µm, the width of the MT legs wMT = 50 µm, the lengthof the MT legs LMT = 10 100 µm, and the length of the MThot junction tip LMT-tip = 200 µm whereas ws and Ls arethe sample width and length, respectively. The contact padsfor the MT and the Cu-RTD are square in cross section, withdimensions of 2000 × 2000 µm and 1000 × 1000 µm, respec-tively. The schematic of the process flow is shown in Fig. 3,whereas the details of the fabrication process are given inthe Appendix.

For all the experiments reported in this paper, the followingcharacterization devices were used: a microscope probe station(model no. Meiji EMZ5RTD from Emcal Scientific Inc.), a dig-ital multimeter (model no. HHM290 from Omega Engineering,Inc.) for ac source voltage measurement, a digital multimeter(model no. CDM 250 from Tektronix Enabling Innovation) forac current measurement, a nanovoltmeter (model no. 2182Afrom Keithley Instruments, Inc.) for Seebeck voltage mea-surements, external K-type thermocouples (model no. SE-GG-K-30-36 from Omega Engineering, Inc.), and an ac voltagesource (model no. BP124201 from Superior Electric Company)for heating the RTD heater. The K-type thermocouples had atip diameter of about 250 µm and were used in the temperaturerange of 23.7 C to about 270 C. In a previous paper [14], wereported on the contact effects of the external thermocouple andfound those to be small. The schematic of the device placed on

IMRAN AND BHATTACHARYYA: EFFECT OF THIN FILM THICKNESSES AND MATERIALS ON RTD RESPONSE 1461

Fig. 3. Fabrication process sequence of the device. (a) Thin-film Cu-RTD heaters. (b) Thin-film Cr–Ti MTs. tCu is the thickness of Cu layer. Some of thestandard photoresist application and strip processes are not shown.

the chuck (diameter 100 mm and made of 360 brass1) is shownin Fig. 4(a), and a photograph of the setup with microscope isshown in Fig. 4(b). The device was diced from the wafer andplaced horizontally on the chuck, as shown in Fig. 3(b). Allthe experiments were performed in air (under free convectionconditions), and each experiment was repeated at least threetimes. We typically give averages of the multiple runs in theplots to follow, except where explicitly stated otherwise.

We have taken a cautious approach in estimating the errors inthe measurements of different quantities. The cumulative errorξC in a measured quantity is defined as ξC = (ξR2

+ ξS2)1/2,

where ξR is the random measurement error, and ξS is the sys-tematic error of the specific device used to make the measure-ment [17]. The random error is defined as ξR = (

∑Ni=1(xi −

1A 360 brass is composed of 60–63 wt.% Cu, 35.5% zinc (Zn), 2.5%–3.7%lead, 0.35% iron.

x)2)1/2/(N − 1), where xi is the ith repetition among a total ofN repetitions of a given measurement, and x = (

∑Ni=1 xi)/N

is the mean of N repetitions. The systematic errors in thehardware were obtained from the technical specifications of themanufacturers. We are reporting errors at the maximum valueof the measured quantity seen in this work. The systematicerrors are given as follows: 1) K-type thermocouple: ξs

TC =±1.26 C at 269.03 C; 2) voltage source: ξs

V = ±2.13 Vat 282.26 V; 3) ammeter: ξs

I = ±1.7 mA at 169.5 mA; and4) nanovoltmeter: ξS

Vs= ±4.12 nV at 1.4 mV. The cumulative

errors are mentioned subsequently in the text whenever wereport the corresponding experimental measurements.

A. Thin-Film RTD Heaters

The thin-film Cu-RTD heater is shown in Fig. 1(a). TheRTD heater with a 0.15-µm-thick Cu layer and without Ni–Au

1462 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006

Fig. 4. (a) Schematic of the experimental setup of the device placed on thechuck for characterization with external thermocouples, voltage source, andSeebeck voltage measurement. (b) Photograph of the experimental setup of thedevice with microscope.

pads will be referred to as RTD1. Its chip size was 9.55 ×7.11 mm. The other RTD heater with a 0.25-µm-thick Cu layerand with Ni–Au pads will be called RTD2. Its chip size was7 × 6 mm. The room temperature To was at 23.7 C.

The steady-state temperature at the center of the RTD withrespect to the electrical power input (Pin) and the resistanceof the RTD is plotted in Fig. 5(a) and (b), respectively. Theseexperiments were repeated three times, and the average valuesof three runs are given in Fig. 5. The cumulative errors arelisted as follows: 1) ±1.92 C (RTD1) and ±1.8 C (RTD2)for temperature measurement; 2) ±0.75 W for electrical powerinput (both RTDs); and 3) ±42 Ω (RTD1) and ±19.8 Ω (RTD2)for resistance. The electrical power input to the RTD heater wascalculated from the product of applied input voltage and thecorresponding steady-state current flow through the respectiveRTD heater and was plotted with respect to the center RTD tem-perature as shown in Fig. 5(a). For a given input power to theRTDs, RTD1 heats up by about 9 C higher than RTD2. Whenthe input power is increased beyond 25 W, the temperature ofRTD1 is about 5 C higher than that of RTD2. The higher RTD1temperature is due to its less thermal mass compared to that ofRTD2 since both the RTDs have the same dimensions but dif-ferent Cu layer thickness. The temperature at the center of eachRTD heater T center

RTD versus the steady-state resistance of theRTD RRTD is shown in Fig. 5(b). The steady-state resistancewas computed by taking the ratio of the applied voltage andthe corresponding steady-state current. At room temperature ofTo = 23.7 C, the resistances of the RTD1 and RTD2 heaterswere calculated as RRTD1(To) = 961 Ω and RRTD2(To) =522 Ω, respectively. We note that, as expected, RTD1 has ahigher resistance than RTD2 in the temperature range reported

in Fig. 5(b). Interestingly, the change in the resistance of RTD1is about 1000 Ω as compared to a much lower change of about400 Ω over a temperature range of about 210 C. This indicatesthat RTD1 has a higher sensitivity than RTD2 to a change in itsresistance with respect to a change in temperature.

In order to measure the sensitivity of the resistance of an RTDto a change in temperature T , a widely used measure is thetemperature coefficient of resistance (TCR) β2 defined as [18]

β =RRTD(100) − RRTD(0)

100RRTD(0). (1)

We determined RRTD1(100) = 1226.5 Ω and RRTD1(0) =739.4 Ω, leading to β1 = 6587 ± 676 ppm/C,3 whereasthe TCR for RTD2 was determined to be β2 = 4279 ±372 ppm/C [14]. Note that β for commercially available bulkCu-RTDs is 4274 ppm/C [19].

Note also that the TCR for RTD1 is significantly higherthan that of RTD2. Our hypothesis is described next. RTD2has Ni/Au plating on its Cu contact pads, whereas RTD1 doesnot. Furthermore, the Cu trace of RTD2 is 66% thicker thanRTD1. Since the Ni/Au plating will add to the room temperatureresistance of RTD2 and a higher thickness of the Cu trace willreduce it, clearly, the latter effect is more dominant as the roomtemperature resistance of RTD2 is significantly lower than thatof RTD1 [see Fig.5(b)]. On the other hand, the Ni–Au platingand higher thickness of the Cu trace significantly add to thethermal mass of RTD2 as compared to RTD1. Therefore, for agiven electrical power input, the entire Cu trace of RTD2 is notexpected to heat up as much, and the resistance is not expectedto increase as much, as that of RTD1. These are the probableeffects that could have gone into a significantly higher TCR forRTD1 as compared to RTD2.

With a view toward providing an operating relationship foruse in applications, a third-order fit (using Matlab) of RRTD

versus TRTD is done. The corresponding trendlines are shownin Fig. 5(b). The equation is given as follows:

T centerRTD = a

RRTD

RRTD(To)

3

+ b

RRTD

RRTD(To)

2

+ c

RRTD

RRTD(To)

+ d (2)

where the parameters in (2) for RTD1 and RTD2 are givenin Table I.

B. Thin-Film RTD Heater and Cr–Ti Thermopile

In this section, we shall describe the experimentalcharacterization of a Cr–Ti thin-film MT thermopiles in contact

2We have used β to represent the TCR instead of the more commonly usedsymbol α, as we have used α subsequently to represent Seebeck coefficient.

3The resistance of the RTD at 0 C was measured using a digital voltmeterwhile freezing the RTD in ice obtained from double-distilled water.

IMRAN AND BHATTACHARYYA: EFFECT OF THIN FILM THICKNESSES AND MATERIALS ON RTD RESPONSE 1463

Fig. 5. (a) RTD temperature versus electrical input power. (b) Temperature of the RTD heaters with respect to their resistance values. tCu is the Cu layerthickness, and RRTD(To) is the room temperature resistance of the RTD.

TABLE IROOM TEMPERATURE, ROOM TEMPERATURE RESISTANCE, AND THE COEFFICIENTS OF (2) FOR RTD1 AND RTD2

with a Cu-RTD heater [see Fig. 1(b) and (c)]. The experimentalcharacterization of RTD2 in contact with a 5-MT thermopilehas already been reported in our previous paper [14]. Here, wehave characterized RTD1 in contact with a 1-MT and a 3-MTthermopile. The chip sizes of the Cu-RTD1 heater with a 1-MTand a 3-MT thermopile were 20 × 7.5 mm and 20 × 7.20 mm,respectively; and for a Cu-RTD2 heater with 5-MT, the chipsize was 18.5 × 10 mm. Recall that the width of both the RTD1and RTD2 heaters is 3280 µm, and the width of the hot junctionof each MT is 200 µm. For a given voltage input to RTD1or RTD2, an external thermocouple is used to measure thetemperature at the center of their respective RTDs. Likewise, anexternal thermocouple is also used to measure the temperaturesof the hot junction of each MT. The temperatures at the hotjunctions of 1-MT, 3-MT, and 5-MT THJ are plotted in Fig. 6with respect to their corresponding T center

RTD . The cumulativeerrors in the measurement of THJ of the 1-MT, 3-MT, and5-MT thermopiles were estimated to be ±1.86 C, ±1.83 C,and ±2.80 C, respectively. The measured temperatures for all3-MT and 5-MT hot junction locations were close. Therefore,in Fig. 6, the 3-MT and 5-MT data correspond to the averageof the three and five hot junctions, respectively. Let us compare

Fig. 6. Hot junction temperatures versus temperature of the RTD at its centerfor the 1-MT, 3-MT, and 5-MT assemblies. The reported data are the average ofthree runs at each MT hot junction location. The schematic of RTD with 3-MTis shown in the inset.

the data for 1-MT (shown by “•”) and 3-MT (shown by “”),both of which have an identical RTD1 architecture. The higher

1464 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006

thermal mass4 of 3-MT as compared to 1-MT results in a lowerhot junction of the former as opposed to the latter for a giventemperature at the center of the RTD. The 5-MT data have thelowest hot junction temperature. This is the result of two addi-tive effects, namely 1) higher thermal mass of RTD2 (to which5-MT is connected) as compared to RTD1 and 2) higherthermal mass of a 5-MT thermopile as compared to a 1-MT ora 3-MT thermopile.

The Seebeck voltage and the hot junction temperatureof a thermopile is usually represented by the followingequation [20]:

Vs = N(αCr − αTi)(THJ − TCJ) (3)

where “N” is the number of MTs connected in series,(αCr − αTi) is the RSC between Cr and Ti, THJ is the hotjunction temperature, and TCJ is the cold junction temperatureof the MT. When the thermopile is integrated on a chip, itis quite possible that the two cold junctions may not just beat an ambient temperature but may also be at two differenttemperatures. Regardless, as long as the change in the coldjunction temperature can be accurately characterized and theSeebeck voltage is easily measurable, the on-chip thermopilecan be used as a temperature sensor. These issues have beenaddressed in the characterization of the 1-MT and 3-MTthermopiles, and the results are summarized in Fig. 7.

In Fig. 7(a) and (b), the hot junction temperature THJ forthe 1-MT and 3-MT thermopiles as well as the two coldjunction temperatures TCJ are plotted with respect to theircorresponding Seebeck voltage Vs. The left cold junction andthe right cold junction have been denoted as LCJ and RCJ,respectively. The cumulative errors for 1-MT in TCJ and Vs areestimated to be ±1.48 C and ±4.08 µV, whereas for 3-MT,the errors are determined to be ±1.45 C and ±10.89 µV,respectively. Notice that the cold junction temperatures do notremain cold; they heat up significantly. Furthermore, the right(Cr leg) and the left (Ti leg) contact pads of the cold junctionfor 1-MT demonstrate practically identical temperatures.However, the right and left cold junction temperatures of 3-MTare somewhat different. Thus, for example, when T center

RTD =200 C, the right cold junction temperature is 109.7 C,and the left cold junction temperature is 105.2 C. The highertemperatures at the right cold junction pad are expected, as thethermal conductivity of its leg (Cr) is higher than the thermalconductivity of the other leg (Ti). The characterization of the5-MT thermopile was reported in our earlier paper [14].

We determined αCr − αTi using (3) and the following data:For a given Seebeck voltage,5 TCJ is taken as the average of

4Note that the “thermal mass” implies not only the effect of the higher contactarea of a 3-MT as compared to a 1-MT thermopile with RTD1 but also the effectof the larger number of legs of 3-MT as compared to that of 1-MT. These legsact as thermal shunts that will draw heat away from the contact zone.

5Note that the overall Seebeck voltage between the contact pads may includea certain component due to the interface between the Ti and tungsten (W)probe tip on the one hand and the Cr and W probe tip on the other hand.However, since the temperatures of both pads in case of 1-MT are almostidentical [recall Fig. 7(a)], the Seebeck voltage effects at both pads are expectedto almost nullify each other. However, in the case of 3-MT [Fig. 7(b)], wherethe temperatures are somewhat different, the absence of these thermoelectriceffects cannot be conclusively ruled out.

the left and right cold junction contact pads and was fittedwith a trendline, i.e., fourth order in Vs, using Matlab. THJ istaken as the average hot junction temperatures of the respectivethermopile. Using (3) and numerical trial and error, we deter-mined the RSC for 1-MT to be αCr − αTi = 10.56 ±0.19 µV/C. The simulated trendline has been shown inFig. 7(a). Similarly, the RSC for 3-MT was determined to beαCr − αTi = 7.72 ± 0.14 µV/C. The simulated trendline hasbeen shown in Fig. 7(b). The higher RSC for 1-MT as comparedto that of 3-MT is a direct consequence of the higher hot junc-tion temperatures of the former (with its lower thermal mass) ascompared to the latter (with its higher thermal mass). Note thatthese values are not the RSC of a free-standing Cr–Ti thin-filmpair, but that of a thin-film pair which is in electrical and thermalcontact with the underlying microstructure. The in situ RSCsfor the 1-MT and 3-MT thermopiles turn out to be somewhatlower (but not too different) than the RSC of 12.7µV/Cfor bulk Ti and Cr near room temperature (i.e., 27 C)6

[21], [22] and 16.5 ± 0.81 µV/C [14] of the 5-MT thermopilethat we characterized previously. The higher RSC of the 5-MTthermopile may be due to the thermoelectric effects of anadditional Au layer used in its pads. The temperatures THJ forthe 1-MT, 3-MT, and 5-MT thermopiles are shown in (4) atthe bottom of the next page, where THJ is in degrees Celsius,αCr − αTi is in microvolts per degree Celsius, and Vs is inmillivolts. The entire fourth-order expressions for the 1-MTand 3-MT thermopiles and the quadratic expression for the5-MT thermopile within the curved brackets on the right-handside of (4) represent the cold junction compensation. Equation(4), without the cold junction compensation, is represented asthe lower solid line in Fig. 6(a) and (b) for 1-MT and 3-MT,respectively.

In closing, we note that external thermocouples have beenused extensively for the characterization of the RTDs andMTs. In an earlier paper [14], we have reported an extensivestudy on the errors involved in using external thermocouples[14]; these errors are typically quite small. This prior studygives us confidence that the operating relationships forRTDs and MTs reported in this paper can be used with asignificant degree of confidence without having to use externalthermocouples.

III. CONCLUSION AND FUTURE WORK

In this paper, we have reported the fabrication and the exper-imental methods to characterize a thin-film Cu-RTD heater andthin-film Cr–Ti MTs. The RTD heaters were fabricated withtwo different thicknesses of Cu layer. The RTD with higherthickness had Ni–Au plating at its pads, whereas the other RTD

6We have not accounted for the temperature dependence of the relativeSeebeck coefficient. In the temperature range considered here (i.e., 25 C to230 C), the Seebeck coefficient is not expected to change significantly. Webase this observation on the small change in the relative Seebeck coefficientof the bulk Cr–Ti pair from 12.7 µV/C at 27 C to 11.1 µV/C at 227 C[21], [22].

IMRAN AND BHATTACHARYYA: EFFECT OF THIN FILM THICKNESSES AND MATERIALS ON RTD RESPONSE 1465

Fig. 7. Hot junction and cold junction temperature characteristics versus Seebeck voltage for the (a) 1-MT and (b) 3-MT thermopiles.

with less Cu layer thickness had Cu pads. The RTDs wereheated with an ac voltage source, and for a given electrical inputpower, the RTD temperature–power and RTD temperature–resistance characteristics were studied, and corresponding re-lationships useful for applications were provided. The Cr–TiMTs were fabricated with and without Au pads at the twocold junctions. The Seebeck voltage, hot junction temperature,and cold junction temperature characteristics were studied.The RSCs were found. The MTs with Au pads at its coldjunctions were found to have higher RSCs than the MTs withoutAu pads.

In the future, we shall use the fabricated RTD heaters andMTs to determine the in situ thermal conductivity of sputteredsamples of different materials and dimensions. The samplewill be heated with the RTD heater, and the temperature mea-surements will be done with the MTs. The temperature mea-surements will then be used with an experimentally validatedtheoretical model to determine the thermal conductivity ofthe sample.

APPENDIX

FABRICATION OF ON-CHIP THIN-FILM RTDS AND MTS

The fabrication process is given in the following order:1) RTD heater and 2) RTD heater and MT. All the devices were

fabricated on a 125-mm-diameter (1 mm = 10−3 m) single-side polished P-type with 〈100〉 orientation Si wafers using aseven-layer mask set. The substrate thickness for RTD1 andRTD2 was 640 µm (1 µm = 10−6 m) and 400 µm, respectively,whereas the corresponding substrate resistivity values were3.41 and 1.23 Ω · cm [23], respectively.

A. Thin-Film RTD Heaters

The optical micrographic image of the fabricated thin-filmCu-RTD heater is shown in Fig. 1(a). A graphical depiction ofthe process flow for the RTD heater is given in Fig. 3(a). TheCu-RTD heater, consisting of Ti–Cu–Ti layer, was sputteredusing a dc magnetron source. The base pressure of the systemwas brought to 1 × 10−5 torr before sputtering, whereas the op-erating pressure of 1 × 10−3 torr was used throughout the sput-tering process. An in situ RF etch at a negative bias of −600 Vis applied to the wafer in a vacuum system before any sput-tering. Argon gas is supplied to the tool at a partial pressure of5 × 10−3 torr. The argon gas is ionized, and the positive ions areattracted to the wafer. The energy of the ions “sputters” or re-moves some of the material on the wafer. This “sputter” processacts like an etch that cleans as well as roughens the surface topromote adhesion of the metal to be deposited soon after. TheTi and Cu were sputtered at 2500 and 3000 W, respectively,

THJ =

103VsN(αCr−αTi)

+(−2.8(103)V 4

s + 2.6(103)V 3s − 3.4(102)V 2

s + 48Vs + 22), for 1-MT (N = 1)

103VsN(αCr−αTi)

+(24V 4

s − 74V 3s + 89V 2

s + 18Vs + 21), for 3-MT (N = 3)

103VsN(αCr−αTi)

+(2V 2

s + 6.1Vs + 24), for 5-MT (N = 5)

(4)

1466 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006

on a 5-in Si wafer. The bottom 500 Å (1 Å = 10−10 m)layer of Ti serves as an adhesion layer, the 0.25-µm-thickCu layer acts as the bulk of the RTD heater element, and thetop 500-Å Ti layer acts as an oxidation barrier for the Cu. AfterTi–Cu–Ti sputtering, photoresist AZ4330 (4 µm) was spin-coated on the wafer, and standard photolithography techniqueswere used to define the RTD heater pattern. The top and bottomTi layers were etched for 15 s in a 2% hydrofluoric acid and0.5% nitric acid solution, whereas Cu was etched in a 10%sulphuric acid and 5% hydrogen peroxide mixture for 30 s.After the Cu etching process, Ni–Au electroplating was doneto metallize the Au pads for the RTD heaters. In order toinvestigate the effects of Cu layer thickness and Ni–Au platingon RTD heater characteristics, we also fabricated RTD heaterswith a 0.15-µm-thick Cu layer and without the Ni–Au platingon the contact pads. The thickness of Ti layers at the bottomand at the top of the Cu layer was kept at 500 Å (as before withthe 0.25-µm-thick Cu layer), and standard photolithographytechniques then were used to pattern the Cu-RTD heaters withCu contact pads.

B. Thin-Film RTD Heater and MT

The optical micrographic image of the fabricated thin-filmCu-RTD heater with a 1-MT and a 3-MT thermopile is shownin Fig. 1(b) and (c), respectively. The process flow for the 1-MT,3-MT, and 5-MT thermopiles is shown in Fig. 3(b). The onlydifference between 5-MT and 1-MT (as well as 3-MT) is thepresence of an Au layer in the contact pads of 5-MT [14]. Theprocess flow for 5-MT is described first.

The RTD heater structure was insulated from the thin-filmMT by a 2-µm-thick layer of silicon dioxide deposited at250 C for 22 min using plasma-enhanced chemical vapordeposition (PECVD) technique. The Ti leg of the thermocoupleused the same Ti–Cu–Ti metallization as the underlying heat-ing element, except 2 µm of Cu was sputtered. The Au padfor the Ti leg was developed using standard photolithographytechniques. The top Ti layer and the middle Cu layer werethen etched. A new photoresist was spin-coated to define thepattern for the Ti leg, following which the Ti was etched and thephotoresist was stripped. Evaporation and subsequent patternetch of the 1000-Å-thick layer of Cr in an aqueous solution of17% hydrochloric acid for 15 s and the 1000-Å-thick layer ofAu in an opaque solution of potassium iodide and iodine for 3 hcompleted the Cr leg of the thermocouple and the Au pad for theCr leg. In case of Cr, depassivation was needed, and it consistedof a piece of Zn to initiate the Cr etch. In order to investigatethe impact on the thermoelectric effects due to Au in the twopads of an MT, we fabricated the 1-MT and 3-MT thermopileswithout the Au layers in their contact pads, as shown in Fig. 2(b)(refer to the last step).

A spin-on organic dielectric benzocyclobutene (orBCB4024) was used to protect the fabricated thin-filmRTD heaters and the MTs from the environment. In addition,the BCB served as a mask when the PECVD oxide wasetched to expose the Cu contact pads for the RTD heaters.The oxide was etched in a solution heated to 40.2 C; thesolution consisted of equal volumes of ammonium fluoride,

1 part acetic acid, and water. The etch rate was approximately700 nm/min.

ACKNOWLEDGMENT

We would like to thank E. Porter and M. Glover of theUniversity of Arkansas at Fayetteville for their help in thefabrication process of the device.

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IMRAN AND BHATTACHARYYA: EFFECT OF THIN FILM THICKNESSES AND MATERIALS ON RTD RESPONSE 1467

Muhammad Imran received the B.Sc.(Hons.) de-gree in electrical engineering from the Universityof Engineering and Technology, Lahore, Pakistan, in2000. In 2001, he joined the University of Arkansasat Little Rock to pursue to Ph.D. degree with theDepartment of Applied Science.

He was a Telecom Analyst with PakistanTelecommunication Company Limited and a SeniorDivisional Officer with the Water and Power Devel-opment Authority in Pakistan. He was also with theDepartment of Electronics, Ghulam Ishaq Khan In-

stitute of Engineering Sciences and Technology, Topi, Pakistan, as a LaboratoryLecturer for about six months. He is currently with the Smart Materials andMEMS Laboratory, Department of Applied Science, University of Arkansas atLittle Rock, working on shape-memory-alloy-based MEMS and developmentof microsystems to determine thermophysical properties of MEMS materials,under the supervision of Dr. A. Bhattacharyya. He has five conference publica-tions and four journal publications.

Mr. Imran is a member of the International Society for Optical Engineeringand the International Microelectronics and Packaging Society.

Abhijit Bhattacharyya received the B.Tech.(Hons.)degree in mechanical engineering from the IndianInstitute of Technology, Kharagpur, India, in 1985and the M.S. degree in applied mechanics and thePh.D. degree in mechanical and aerospace engineer-ing from Rutgers University, New Brunswick, NJ, in1991 and 1994, respectively.

He was an Assistant Executive Engineer (Mechan-ical) with the erstwhile Oil and Natural Gas Com-mission, New Delhi, India, from 1985 to 1987, aSenior Research Associate with the Department of

Aerospace Engineering, Texas A&M University, College Station, from 1994to 1996, and an Assistant Professor (1997–2001) and Associate Professor(2001) with the Department of Mechanical Engineering, University of Alberta,Edmonton, AB, Canada. Since 2002, he has directed the Smart Materialsand MEMS Laboratory at the Department of Applied Science, University ofArkansas at Little Rock, where he started as an Associate Professor and isnow a full Professor of applied science. His research interests are in the areaof smart materials (shape memory alloys and ferroelectric thin films), MEMS,and shape-memory-alloy-based MEMS. He has numerous publications in topinternational journals.