IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 12, DECEMBER 2011 1761

Robust Array-Composite MicromachinedThermopile IR Detector by CMOS Technology

Dehui Xu, Student Member, IEEE, Bin Xiong, Yuelin Wang, Senior Member, IEEE, and Tie Li

Abstract—An improvement in mechanical stability and a re-duction in time constant have been obtained over a traditionalCMOS-compatible micromachined thermopile infrared detec-tor. The single big thermopile (SBT) is replaced with a series-connected small thermopile array (STA). The small thermopilein array-composite structure is scaled down compared with thetraditional SBT structure. Compared with SBT, STA shows abetter compromise among mechanical stability, time constant, andsensitivity. Because of the small floating structure design, the STAdetector only shows a one-fourth deflection as the SBT detector.In addition, the time constant of the STA detector is also muchreduced due to the size effect, while sensitivity and detectivity forSBT and STA detectors are nearly the same.

Index Terms—Array composite, CMOS, infrared detector, me-chanical stability, micromachined thermopile.

I. INTRODUCTION

IN RECENT years, micromachined thermopiles have foundmany military and commercial applications, such as remote

temperature sensing, infrared gas sensing, terahertz detection,etc. [1], [2]. Because a thermopile sensor utilizes Seebeck effectto transduce temperature difference, the output voltage of athermopile is self-generated. Thus, a thermopile detector hasthe merits of ease of operation and no 1/f noise. Towardhigh cost efficiency and miniaturization, much attention hasbeen focused on CMOS-compatible front-side-etched bridge-type thermopile IR detectors [2]–[5]. However, compared withtraditional closed-membrane thermopiles, the front-etched ther-mopile is mechanically unstable [5]. Moreover, due to theresidual stress in CMOS thin film [6], mechanical stabilityis of particular importance for front-etched micromachinedthermopiles by CMOS technology [5].

Sensitivity (Rs) and time constant (τ) are two of the mostimportant figures of merit for thermopile IR detectors. Highsensitivity is desired for a large output voltage, while smalltime constant is also desired for fast response and large work-

Manuscript received July 11, 2011; revised September 4, 2011; acceptedSeptember 13, 2011. Date of publication October 21, 2011; date of current ver-sion November 23, 2011. This work was supported in part by the National High-Tech R&D Program of China (863 Program) under Contract 2009AA04Z324,by the National S&T Major Project of China under Grant 2011ZX02507-003,and by the Major State Basic Research Development Program of China (973Program) under Grant 2011CB309501. The review of this letter was arrangedby Editor W. T. Ng.

The authors are with the State Key Laboratory of Transducer Technol-ogy and the Science and Technology on Micro-system Laboratory, ShanghaiInstitute of Microsystem and Information Technology, Chinese Academyof Sciences, Shanghai 200050, China (e-mail: dehuixu@mail.sim.ac.cn;bxiong@mail.sim.ac.cn; ylwang@mail.sim.ac.cn; tli@mail.sim.ac.cn).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2011.2169037

Fig. 1. Optical micrographs of (a) SBT structure by traditional design,(b) STA with 16 series-connected small thermopiles, (c) absorber area of SBTdetector, and (d) single small thermopile in the STA detector. The device areasare 1.65 × 1.65 mm2 (2.723 mm2) for SBT and 1.93 × 1.41 mm2 (2.721 mm2)for STA.

ing bandwidth. For CMOS-compatible thermopile IR detec-tors with polysilicon/aluminum as thermocouple, Rs and τ asfunctions of thermopile geometric parameters can be foundin [7] and [8]. In order to obtain high sensitivity, a largethermocouple leg length (Ltp) is usually desired to reducethermal conductance. However, a small time constant requiresa small Ltp, i.e., a small thermal resistance. Thus, Rs and τ ofa thermopile detector seem to be a paradox.

In this letter, we report on an array-composite thermopile de-sign for uncooled IR detection, which provides high mechanicalstability and small time constant. By scaling down the tradi-tional big thermopile structure [Fig. 1(a)], the array-compositethermopile structure is adopted for IR sensing [Fig. 1(b)]. Thenumber of thermocouples (N), Ltp, and absorber width (La)of a thermopile in a small thermopile array (STA) are one-fourth of those of the single big thermopile (SBT), while thethermocouple widths for SBT and STA are the same.

II. EXPERIMENTAL

The thermopile detector was fabricated by 2-μm CMOSbaseline process and one maskless post-CMOS XeF2 etching

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1762 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 12, DECEMBER 2011

TABLE ISTRUCTURAL PARAMETERS OF TWO DESIGNED

THERMOPILE IR DETECTORS

step [9], [10]. n-type polysilicon and aluminum were used torealize the thermocouple. The dielectric film in CMOS processwas adopted for IR absorber to fully exploit the potential ofCMOS process and simplify the fabrication [9], [10]. After0.5-μm Si3N4 and 0.9-μm SiO2 were deposited for passivation,bond pads and etching windows were opened by reactive ionetching. Finally, thermopile structure was released by XeF2

front-side dry etching.The fabricated SBT and STA detectors are shown in Fig. 1.

Fig. 1(c) shows the absorber area of the SBT detector. Etchingwindows in the absorber are designed to avoid cutting off theheat transfer path from the absorber to hot junction. Further-more, aluminum strips are patterned in the absorber to uniformthe temperature in the absorber area. Fig. 1(d) shows the smallthermopile structure in the STA detector after XeF2 release. Thestructural parameters of SBT and STA detectors are presentedin Table I. Since the thermopile in STA is one quarter of thesize of the SBT, STA and SBT detectors are placed in the samefootprint (2.72 mm2) and show nearly the same fill factor.

III. RESULTS AND DISCUSSION

To evaluate the mechanical stability of the thermopile IRdetector, morphologies of SBT and STA are measured by an op-tical profiler (WYKO NT-2000). The measured interferometricdata are automatically computed to render 3-D imaging of thetopography. As the thermopile structure exhibits symmetricalaxes, it is possible to study the morphology of SBT by means ofa single quadrant. The measured 3-D morphologies of SBT andSTA are shown in Fig. 2. The bending caused by residual stresscan be clearly seen in the measured 3-D morphologies. Thebending variations are about 28 and 6.8 μm for SBT and STAstructures, respectively. The greatest contribution to the curl inSBT structure comes from its large floating membrane, and thesmall thermopile in STA with smaller floating membrane curls75% less. This result implies that STA is more mechanicallystable than SBT.

Fig. 3 shows the measured I–V characteristics of SBT andSTA detectors. Although the small thermopile in STA is scaleddown to one quarter of the size of the SBT, N of STA isfour times larger than N of SBT. Hence, the SBT and STAhave the same number of unit square. Therefore, they exhibitnearly the same I–V characteristics. The extracted electricalresistances (Relec) for SBT and STA detectors are 173 and176 kΩ, respectively. The sheet resistance of the polysilicon

Fig. 2. Measured 3-D morphologies of (a) SBT structure and (b) singlesmall thermocouple in the STA after thermopile structure release. The bendingvariations are about 28 and 6.8 μm for SBT and small thermopile in STA,respectively. STA is more robust than SBT.

Fig. 3. Measured I–V curves for SBT and STA IR detectors. Because ofthe same number of unit square, SBT and STA detectors show the same I–Vcharacteristics.

layer is about 35.5 Ω. The overall contact resistances (Rcont)of SBT and STA are 2 and 5 kΩ, respectively. The little largerRcont of STA is due to its large N . The number of contactvias of STA is about three times larger than that of SBT.The percentages of Rcont versus Relec for SBT and STA are1.2% and 2.8%, respectively, which shows that array-compositestructure will slightly increase Relec.

Fabricated SBT and STA detectors were tested in air atroom temperature by using a setup consisting of a blackbodyradiation source (500 ◦C), a mechanical chopper, and an ampli-fication circuit. The signal output waveform of the detector wasrecorded on an oscilloscope. By changing the distance betweenthe detector and blackbody radiation source, the signal outputwas recorded as a function of input radiation power. Fig. 4(a)shows the measured outputs in terms of linearity response at4-Hz chopping. The radiation power is assumed to be com-pletely absorbed by the absorber in power response measure-ment. The sensitivities of SBT and STA detectors are 38.66and 36.37 V/W, respectively. Sensitivity results imply that thetemperature difference between hot and cold junctions (ΔTHC)of SBT is 4.25 times larger than ΔTHC of STA. Because of

XU et al.: ROBUST ARRAY-COMPOSITE MICROMACHINED THERMOPILE IR DETECTOR 1763

Fig. 4. Measured (a) power responses and (b) frequency responses of the SBTand STA IR detectors. The SBT detector shows a 1.5 times larger time constantthan the STA detector, while sensitivities for two different detectors are close.

TABLE IIFIGURES OF MERIT OF FABRICATED THERMOPILE IR DETECTORS

the scaled-down thermopile size, the small thermopile in STApresents about four times larger thermal conductance than SBT.

The frequency responses of SBT and STA were also mea-sured by changing the chopping frequency, and the resultsare shown in Fig. 4(b). The 3-dB cutoff frequencies of SBTand STA detectors are about 30 and 48 Hz, respectively. Thedetermined time constants for SBT and STA are 5.3 and 3.3 ms,respectively. The time constant of SBT is 1.6 times larger thanthat of STA. The STA detector exhibits a better compromisebetween sensitivity and time constant than the SBT detectorby conventional design. Due to the size effect, the small ther-mopile in STA presents about 2.5 times larger thermal capacitythan SBT.

The IR performances of SBT and STA detectors are sum-marized in Table II. Because SBT and STA show nearly thesame internal resistance and absorber area, the noise equivalentpower and specific detectivity values for SBT and STA are alsovery close. Compared with SBT structure, the small Ltp and La

of STA structure result in a large thermal conductance. Thus,the output voltage and time constant of the small thermopile in

STA will be smaller than those of the SBT detector. However,the series connection of 16 thermopiles enlarges the outputvoltage of STA. Therefore, the sensitivities of STA and SBTare close. On the other hand, the series connection of array-composite structure does not increase the time constant. Thetime constant of STA is equal to the time constant of the smallthermopile in the array-composite structure. Hence, the STAdetector shows a smaller time constant.

IV. CONCLUSION

In summary, we have demonstrated a micromachined array-composite thermopile for uncooled IR detection. The SBTstructure in conventional design is replaced with series-connected STA structure. Because the small thermopile in STAis scaled down compared with the SBT, the fill factors ofSBT and STA are the same. Nevertheless, the thermopile inSTA has a larger thermal conductance and a smaller floatingarea. Compared with the SBT detector, the STA detector hasadvantages of better mechanical stability and smaller timeconstant. The array-composite series connection of the STAdetector leads to a little larger electrical resistance. However,the series connection also improves the STA output voltage,and other figures of merit of STA are nearly the same asthose of SBT. These facts open perspectives to process robustthermopile sensors with small time constant and high sensitivityfor IR detection systems with room constraints.

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