Anand Gawarikar received his Bachelors in Technology (with honours) in Electronics Engineering from the Institute of Technology, Banaras Hindu University, India. He completed his doctoral degree in electrical engineering in 2013, centering on research on MEMS infrared detectors and semiconductor fabrication.
Anand now holds a position in semiconductor fabrication at Intel in Portland, OR.
We have derived an expression for the effective absorbing area for thermal infrared detectors having non-zero absorption in the support legs, which is different from the geometric areas of the constituent detector elements. This technique is particularly applicable to devices where sensitivity is more important than fill-factor, as opposed to standard imaging arrays. The effective area can simply be substituted in standard equations to obtain a good estimate of the detector performance under uniform flood illumination conditions. The formalism can also be used for estimating the contributions of the individual signal generating elements to the total measured signal. This approximation has been tested for MEMS infrared detectors with thermoelectric readout operating under vacuum. The responsivity of the same device calculated using the effective area approximation and measured using a tightly constrained absorbing area are found to match very closely, within 5% over the most wavelengths and within 15% at the shortest thermal infrared wavelengths.
Incorporation of bismuth telluride/antimony telluride co-sputtered thermoelectric junctions into MEMS devices requires process developments for patterning and encapsulation as well as characterization of properties such as film stress and contact resistance. Test structures are presented for measuring important thermoelectric properties, resistivity, thermal conductivity, carrier concentration, and Seebeck coefficient. A fabrication process is presented that allows the junctions to be deposited, patterned, encapsulated, and etch released. Measurement of the thermoelectric junctions reveals a room temperature figure of merit, Z_T, of 0.43 with a total Seebeck coefficient difference of 150 μV/K, resistivities of 17.4 and 7.6 μΩ-m, and thermal conductivity of 0.34 and 0.30 W/mK for antimony telluride and bismuth telluride, respectively. The junctions have been incorporated into state of the art uncooled thermopile infrared detectors with a peak detectivity of 3 \texttimes 10^9 cm*Hz^(1/2)/W.
We describe uncooled thermal detectors with a peak detectivity of at least 3 \texttimes109 cm \textsurdHz/W with spectrally selective absorption in the long-wave infrared. The spectral selectivity in absorption is achieved through resonant cavity coupling of a thin metal film with a low-order air-gap optical cavity. The electrical readout uses thermoelectric thin films with a Johnson noise limited performance. The detectors are of multiple sizes but those with 100- μm2 area have time constants of 58 ms and thermal conductances of 2.3 \texttimes10-7 W/K.
Long wave infrared is an important region of the electromagnetic spectrum due to strong thermal emission in this region by room temperature blackbodies and good atmospheric trans- parency which enables transmission of electromagnetic energy over large distances. Detectors for this spectral region, especially ones that can operate at room temperature, have been an active area of research due to applications in surveillance, remote sensing and chemical detection. Of particular interest is the integration of spectral and spatial filtering directly with the detector to incorporate multispectral capabilities with reduced hardware complexity. This thesis explores several aspects of spectral selectivity in infrared detectors operating at room temperature. The effects of spectral selectivity on the fundamental photon noise limit are first explored using the formalism of an ideal resonant optical cavity. It is shown that the photon noise limit of such a detector is higher than that of a broadband detector. The theoretical performance of this detector architecture is investigated for the specific application of passive standoff detection of gases. Some practical aspects and trade-offs involved in optical and electrical design of such detec- tors is discussed in detail. A process for fabrication of these detectors using standard silicon micromachining techniques is described. Various optical and electrical characterization tech- niques are used to demonstrate spectrally selective high sensitivity detectors operating at room temperature. These detectors have amongst the highest sensitivities reported in the literature. Finally, a thermal model for detector responsivity is developed for the particular case of spatially non-uniform absorption. An approximate expression for detector absorbing area is derived from this model, which can be directly substituted in standard equations to estimate responsivity to good accuracy. Detailed derivation and experimental verification of this model is described.
On extremely small scales, traditional microcooler performance estimates must be corrected to include losses due to radiation. We present a method for analysis of microcoolers having a significant radiative contribution to their thermal conductance. We have fabricated ultrasmall microcoolers from sputtered Bi2Te3/Sb2Te3 thermoelectric junctions with cooling volumes of 200 μm \texttimes 200 μm \texttimes 65 nm, which we believe to be the smallest microcoolers ever made. The devices are highly thermally isolated with total thermal conductance under 5 \texttimes 10-7 W/K in vacuum at room temperature. By fitting the temperature response to input power of the devices in vacuum, we have quantified the nonlinearity of the response to calculate the radiative and film contributions to the total thermal conductance of the device. Three device geometries are presented, with radiative contributions to thermal conductance of 15%, 26%, and 100% depending on their emissive area and support structure. The cooling capabilities of these devices are also measured with maximum cooling of 3.1 K for the 15% radiation-limited device and 2.6 K for the 26% radiation-limited device, with power consumptions below 5 μW.
The far field radiation efficiency achievable in narrowband thermal emitters is investigated, taking into account the full spatial and spectral variation of the emissivity. A coupled Fabry-Perot cavity model is used to develop an insight into the efficiency variation with cavity coherence and device temperature. It is found that the spatial variation of emissivity has to be explicitly included in the radiation power calculations to accurately estimate the achievable power efficiencies. The calculated radiation efficiencies of an ideal coherent cavity coupled emitter were found to vary from 0.1% to 9%, with a corresponding increase in the emission linewidth from 6.3 nm to 930 nm, and were much lower than that estimated without accounting for effects of spatial coherence. The analysis presented here can be used to determine the optimal operating temperature of a coherent thermal emitter once its emission characteristics and conduction losses are known and it is demonstrated that this optimum temperature is different from the temperature of peak blackbody emission at the resonant absorption wavelength.
A review is made of the physics and technology of spectrally selective thermal detectors, especially those operating at non-cryogenic temperatures. The background radiation noise fluctuations are rederived for arbitrary spectral characteristics. Infrared absorption due to phonons and free carriers is discussed followed by a review of published works on artificial infrared absorption materials such as patterned grids, nanoparticles, plasmonic structures, metamaterials and others. Subsequently, the literature of the spectral characteristics of broadband thermal detectors and spectrally selective thermal detectors is reviewed. Finally, the authors speculate on the directions that future research and development in the area will take regarding architectures, sensitivity and spectral characteristics.
The blackbody radiation limit has traditionally been set forth as the ultimate performance limit of thermal detectors. However, this fundamental limit assumes that the detector absorbs uniformly throughout the thermal spectrum. In much the same way as photon detectors can achieve very high D* because they do not absorb photon energies below their bandgap, so too can thermal detectors except that thermal detectors are not limited to cryogenic operation. In both cases, the enhanced theoretical D* is achieved because the radiation noise is reduced in a device that does not absorb at a uniform high level throughout the thermal emission band. There are multiple ways to achieve high D* in thermal detectors. One is to use materials that absorb only in a certain spectral range, just as in photon detectors. For example a detector made from PbSe, with proper optical coupling, absorbs only photons with wavelengths shorter than 4.9μm. The radiation limited detectivity of such a device can theoretically exceed 9 x 1010cmHz1/2/W in the MWIR. Even with Johnson and 1/f noise estimates included, it can still exceed 2.5x1010cmHz1/2/W in the MWIR. Another technique, applicable for narrowband thermal detectors, is probably even more powerful. Consider a thermal detector that is almost completely transparent. Here, the radiation noise has been reduced but the signal has been reduced even more. However, if the device is now placed inside an optical cavity, then at one wavelength and in one direction, the nearly transparent detector couples to the cavity resonance to absorb at 100%. Radiation from all other wavelengths and directions are rejected by the cavity or are absorbed only weakly by the detector. It is shown that theoretically, the D* of these devices are roughly proportional to the inverse square root of the spectral resonant width under certain conditions. It is also shown that even including Johnson noise and 1/f noise, the practically achievable D* approaches or exceeds 1011 cmHz^1/2/W.
The adhesion energy is measured between InGaAsquantum wells that have collapsed across a 125 nm air gap in an InP/InGaAs heterostructure. The method relies on measuring the unadhered length and shape of collapsed microcantilevers with optical interferometry. The adhesion energy is found to be 72 \textpm 16 mJ m^-2 . Since the air gap is much smaller than has been measured previously, the influence of van der Waals forces across the gap was included in theoretical modeling. It was found that the forces should not cause significant deviation from the standard adhesion models unless the adhesion energy drops below 25 mJ m^-2 .
The adhesion energy is measured between InGaAs quantum wells that have collapsed across a 120 nm airgap in a InP/InGaAs heterostructure. The method relies on measuring the unadhered length and shape of collapsed microcantilevers with optical interferometry. The adhesion energy is found to be 72 plusmn 16 mJ m^-2. Since the airgap is much smaller than has been measured previously, the influence of van-der-Waals forces across the gap was included in theoretical modeling. It was found that the forces should not cause significant deviation from the standard adhesion models unless the adhesion energy drops below 25 mJ m^-2.