Abstract-We report two integrated thermoelectric infrared sensors on thin silicon oxide / nitride microstructures realized by industrial CMOS IC technology, ...
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IEEE ELECTRON DEVICE LETTERS, VOL. 13, NO. 9, SEPTEMBER 1992
Thermoelectric Infrared Sensors by CMOS Technology R e n t Lenggenhager, Student Member, IEEE, Henry Baltes, Member, IEEE, Jon Peer, and Martin Forster
Abstract-We report two integrated thermoelectric infrared sensors on thin silicon oxide / nitride microstructures realized by industrial CMOS IC technology, followed by one compatible single maskless anisotropic etching step. No additional material is needed to enhance infrared absorption since the passivation layer, as provided by the CMOS process, is sufficient for certain spectral bands. The responsivities are between 12 and 28 V / W.
11. DESIGNAND FABRICATION
We designed different microbeam or suspended-bridge membrane structures in order to investigate the optimal geometry for high responsivity, short etching time, and good mechanical stability. Excellent thermal insulation [9] is obtained by removing the bulk silicon under the absorbing area and hot thermoelectric contacts by a maskless post-processing anisotropic etching step. The thickness of I. INTRODUCTION the resulting silicon oxide/nitride membranes is about ERMOPILES based on thin-film thermocouples 3 pm. The multiple silicon oxide layers (thermal oxide and supported by membranes are currently studied as CVD oxides) of the CMOS process serve as "natural" radiation [1]-[31, gas flow [4], and low vacuum [5] sensors. etching masks. Etch windows are defined by superimposBi/Sb thermocouples are used for highly sensitive thinfilm thermopiles [61, but these materials are not available ing a device well, contact cut, and pad opening in the in standard IC technology. We report thermopile infrared CMOS mask layout. This prevents the formation of thermal oxide, CVD oxide, and passivation at the desired sensors fabricated by an industrial CMOS process (prolocations during the CMOS process and thus allows the vided by Austria Mikro Systeme (AMs),Unterpremstatten, post-processing underetching of free oxide structures. The Austria) followed by minimal compatible post-processing. anisotropic etching is performed typically with an EDP We use aluminum/polysilicon contacts [2], [ l l ] on a silisolution containing 1000-ml ethylenediamine, 160-g pyrocon oxide/nitride layer as provided by the CMOS process. catechol, 133-ml water, and 6-g pyrazine [lo], which leads Our approach allows the cointegration of pertinent cirto an etching rate of about 30 p m / h in the (100) cuitry [7], [8] and inexpensive batch fabrication. We recall direction. Using this etchant we remove bulk silicon where the thermopile voltage L& = N . a * AT with N denoting desired, but preserve the metal pads and passivation. The the number of thermocouples, a the Seebeck coefficient, etch rate of aluminum is about 180 times lower than that and AT the temperature difference between the hot and of silicon; the rate of silicon dioxide is about 2000 times cold contacts. The radiation is absorbed and converted to lower than that of silicon. This provides an etch window of a temperature increase near the hot junctions of the about 4 h for retaining bondable aluminum pads. Judithermopile located on the thin membrane, whereas the cious design for this etch window is crucial for infrared cold junctions are located on the bulk silicon acting as sensors which usually require large areas. Thus our design heat sink. The Seebeck coefficient for aluminum/polyincludes appropriate alignment of the structures in relasilicon is 65 pV/K for the CMOS process of AMS. We tion to the crystal orientation and additional windows in found similar values for other commercially available the large areas to be underetched. The highest etch rates CMOS technologies [41. We determine the responsivity for (100) wafers are found for an alignment of 27" or 45" R = K p / P with the incident radiation power P and the of the etch window in respect to the wafer flat [lo]. normalized detectivity D* = NEP-'( A . A f ) ' I 2 with NEP Moreover, the design has to be optimized with respect denoting the noise equivalent power, A the detector area, to mechanical stability of the multilayer oxide in view of and A f the bandwidth, considering thermal noise only. mechanical stress induced by the different materials and deposition methods, which may lead to buckling and fissures. Thus, freestanding cantilever beams are not approManuscript received May 29, 1992. strutR. Lenggenhager and H. Baltes are with the Physical Electronics priate because their bending leads to very Laboratory, Institute of Quantum Electronics, CH-8093 Zurich, Switzer- tures. The structures have to be suspended by additional land. supports. We found a suspended bridge fixed at the four J. Peer and M. Forster are with Cerberus AG, CH-8708 Maennedorf, edges (Fig. 1) or a suspended structure with two supports Switzerland. (Fig. 2) at each end to be the most appropriate structures. IEEE Log Number 9202587.
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0741-3106/92$03.00 0 1992 IEEE
LENGGENHAGER et al.: THERMOELECTRIC INFRARED SENSORS BY CMOS TECHNOLOGY
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Wavelength h [pml Fig. 3. Measured reflectance of CMOS passivation layers (1, 2, and 3 p m thickness) on aluminum. B
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(b) Fig. 1. (a) SEM picture of thermopile infrared sensor on a suspended silicon oxide/nitride membrane with four thermopiles of 10 thermocouples each. The size of the absorbing area is 500 p m x 250 pm. (b) Schematic cross section A-B through the hot contacts on the membrane.
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Fig. 2. (a) SEM picture of microbridge with two thermopiles of 24 thermocouples each. The size of the absorbing area is 500 p m X 250 p m ; the length of the suspension beams is about 450 pm. (b) Schematic cross section A-B through the hot contacts parallel to the metal and poly lines.
The problem of stress is eased by modern CMOS technology aimed at minimal stress on the fine aluminum lines and achieving this goal by oxide/nitride sandwich passivation, with oxide and nitride having opposite signs of stress (tensile or compressive).
111. DEVICECHARACTERISTICS Responsivity measurements were carried out in air using a black-body source at 353 K with 8.2-pm peak wavelength. With this radiation source we obtain a variation of the irradiance on the sensor surface of 500 pW/cm2 caused by the action of the chopper. Fig. 1 shows an SEM picture of a suspended-membrane infrared sensor structure. The absorbing area is 0.12 mm2. Four thermopiles, each consisting of ten thermocouples, are located on the suspension beams and are connected in ser-ies with a total internal resistance of 38 k R . For radiation chopped at 1 Hz we measured a responsivity of about 12 V/W. Assuming only Johnson noise, this leads The time to a detectivity D* = 1.74. lo7 cm =/W. constant of this sensor is about 10 ms. Fig. 2 shows a bridge structure with two thermopiles, each consisting of 24 thermocouples. The absorbing area is 0.116 mm2 and the total internal resistance is 178 k R . We measured a responsivity of 28 V/W at 1 Hz, a detectivity D* = 1.78 lo7 cm . m / W , and a time constant of about 20 ms. The difference in internal resistance of these two sensors is due to the different length and width of the polysilicon thermocouple legs. For certain spectral bands no additional material for enhancing infrared absorption is needed, since the passivation layer as provided by the CMOS process shows absorption bands between 8- and 14-pm wavelengths as shown in Fig. 3. For better absorption and thermal equalization, we deposited an aluminum layer on the absorbing area underneath the passivation layer. The incoming radiation will thus be reflected at the aluminum mirror and pass the absorbing layer twice. The measured absorption bands are due to Si-0 bonds (8-10 pm), Si-N bonds (11-13 pm), and interference effects. Our values for responsivity and D* are 2 to 5 times smaller than comparable sensors reported in [31, which have a responsivity of 64 V/W and a D* of 7.7 * lo7 cm &/W for a blacked absorbing area. I v . CONCLUSION AND OUTLOOK We described two infrared sensor structures fabricated with an industrial CMOS process, combined with one
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post-processing etching step. The passivation layer as it comes with the CMOS process provides sufficient infrared absorption for certain spectral bands of interest. We achieved sensitivities between 12 and 28 V/W with detectivities of about 1.7. lo7 cm . &/W. The next task is to develop a very low-noise CMOS preamplifier with high impedance for signal conditioning to be cointegrated with the thermopile.
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