Slobodan Rajic

Performance of uncooled microcantilever thermal detectors

It has recently been shown that bimaterial microcantilevers can be used as uncooled infrared detectors. Bimaterial microcantilevers deform as their temperature changes due to the absorption of infrared photons. Infrared imaging using uncooled cantilever arrays has already been achieved by a number of groups. In this paper, we examined the performance of microcantilevers as uncooled infrared detectors with optical readout. As in the case of other kinds of uncooled thermal infrared detectors, temperature fluctuation noise and background fluctuation noise are fundamental limits to the performance of microcantilever thermal detectors. Since microcantilevers are mechanical devices, thermo-mechanical noise will also influence their performance. We fabricated a SiNx microcantilever thermal detector with an Al layer in the bimaterial region. For the microcantilever geometry and materials used, the background fluctuation noise equivalent temperature difference, NETDBF, calculated for f/1 optics and a 30 Hz frame r...

Photoinduced and thermal stress in silicon microcantilevers

The photogeneration of free charge carriers in a semiconductor gives rise to mechanical strain. We measured the deflection of silicon microcantilevers resulting from photoinduced stress. The excess charge carriers responsible for the photoinduced stress, were produced via photon irradiation from a diode laser with wavelength λ=780 nm. For Si microcantilevers, the photoinduced stress is of opposite direction and about four times larger than the stress resulting from only thermal excitation. In this letter we report on our study of the photoinduced stress in silicon microcantilevers and discuss their temporal and photometric response.

“Self-leveling” uncooled microcantilever thermal detector

Bimaterial microcantilevers can be used as highly sensitive uncooled infrared (IR) detectors. Since the transduction efficiency of microcantilevers depends on their deflection, it is important to minimize cantilever deflections caused by factors other than IR radiation (e.g., intrinsic mechanical stresses and ambient temperature fluctuations). In this letter we report on a design of a microcantilever IR detector that is immune to ambient temperature changes and other sources of interfering mechanical stresses. We modeled and experimentally measured responses of such devices to IR radiation as well as to ambient temperature changes. Both modeling and experimental results indicate that the implemented innovative designs can combine excellent IR sensitivity with negligible sensitivity to the ambient temperature changes.

Detection and differentiation of biological species using microcalorimetric spectroscopy.

We report on the application of infrared (IR) microcalorimetric spectroscopy ( micro -CalSpec) to the identification and detection of trace amounts of biological species. Our approach combines principles of photothermal IR spectroscopy with ultrasensitive microcantilever (MC) thermal detectors. We have obtained photothermal IR spectra for DNA and RNA bases and for Bacillus Cereus (an anthrax simulant) in the wavelength range of 2.5-14.5 micro m (4000-690 cm(-1)). The measurements are accomplished by absorbing biological materials directly on a MC thermal detector. The main advantage of the developed micro -CalSpec is its unprecedented sensitivity as compared to any of the previously explored IR techniques, including FTIR and photothermal FTIR methods. Our results demonstrate that <10(-9)g of a biological sample is sufficient to obtain its characteristic micro -CalSpec spectrum that contains information-rich chemical (vibrational) signatures. This opens up a new opportunity to create inexpensive high-throughput analytical systems for biochemical detection.

Detection of infrared photons using the electronic stress in metal–semiconductor cantilever interfaces

We report on a new method for detecting photons using the stress caused by photoelectrons emitted from a metal film surface in contact with a semiconductor microstructure which forms a Schottky barrier. The detection of photons results from measuring the photo-induced bending of the Schottky barrier microstructure due to electronic stress produced by photoelectrons diffusing into the microstructure. Internal photoemission has been used in the past to detect photons, however, in those cases the detection was accomplished by measuring the current due to photoelectrons and not due to electronic stress. In this work we studied the photon response of 500 nm thick Si microcantilevers coated with a 30 nm layer of Pt. Photons with sufficient energies produce electrons from the platinum-silicon interface which diffuse into the Si and produce an electronic stress. Since the excess charge carriers cause the Si microcantilever to contract in length but not the Pt layer, the bimaterial microcantilever bends. The charge carriers responsible for the photo-induced stress in Si, were produced via internal photoemission using a diode laser with wavelength lambda = 1550 nm.

Fabrication of quantum well microcantilever photon detectors

We have developed a new method for fabricating quantum well microcantilever arrays that can be used in a variety of sensing applications. Microcantilevers with quantum wells allow real-time manipulation of energy states using external stress thus providing photon wavelength tunability. For example, this can result in an effective and rapid change in electron energy levels in photon detection devices. We applied this microfabrication technique to develop InSb microcantilevers and small arrays of GaAs/GaA1As microcantilever quantum wells. Such arrays can be useful in the detection of infrared (IR) radiation at room temperature.

Review of pyroelectric thermal energy harvesting and new MEMs based resonant energy conversion techniques

Harvesting electrical energy from thermal energy sources using pyroelectric conversion techniques has been under investigation for over 50 years, but it has not received the attention that thermoelectric energy harvesting techniques have during this time period. This lack of interest stems from early studies which found that the energy conversion efficiencies achievable using pyroelectric materials were several times less than those potentially achievable with thermoelectrics. More recent modeling and experimental studies have shown that pyroelectric techniques can be cost competitive with thermoelectrics and, using new temperature cycling techniques, has the potential to be several times as efficient as thermoelectrics under comparable operating conditions. This paper will review the recent history in this field and describe the techniques that are being developed to increase the opportunities for pyroelectric energy harvesting. The development of a new thermal energy harvester concept, based on temperature cycled pyroelectric thermal-to-electrical energy conversion, are also outlined. The approach uses a resonantly driven, pyroelectric capacitive bimorph cantilever structure that can be used to rapidly cycle the temperature in the energy harvester. The device has been modeled using a finite element multi-physics based method, where the effect of the structure material properties and system parameters on the frequency and magnitude of temperature cycling, and the efficiency of energy recycling using the proposed structure, have been modeled. Results show that thermal contact conductance and heat source temperature differences play key roles in dominating the cantilever resonant frequency and efficiency of the energy conversion technique. This paper outlines the modeling, fabrication and testing of cantilever and pyroelectric structures and single element devices that demonstrate the potential of this technology for the development of high efficiency thermal-toelectrical energy conversion devices.

Novel photon detection based on electronically-induced stress in silicon

The feasibility of microcantilever-based optical detection is demonstrated. Specifically, we report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers. In this work, optical transduction techniques were used to measure microcantilever response to photons and study the electronic stress in silicon microcantilevers, and their temporal and photometric response. The photo-generation of free charge carriers (electrons, holes) in a semiconductor gives rise to photo-induced (electronic) mechanical strain. The excess charge carriers responsible for the photo-induced stress, were produced via photon irradiation from a diode laser with wavelength (lambda) equals 780 nm. We found that for silicon, the photo-induced stress results in a contraction of the crystal lattice due to the presence of excess electron-hole-pairs. In addition, the photo-induced stress is of opposite direction and about four times larger than the stress resulting from direct thermal excitation. When charge carriers are generated in a short time, a very rapid deflection of the microcantilever is observed (response time approximately microseconds).

Infrared imaging using arrays of SiO2 micromechanical detectors

In this Letter, we describe the fabrication of an array of bimaterial detectors for infrared (IR) imaging that utilize SiO(2) as a structural material. All the substrate material underneath the active area of each detector element was removed. Each detector element incorporates an optical resonant cavity layer in the IR-absorbing region of the sensing element. The simplified microfabrication process requires only four photolithographic steps with no wet etching or sacrificial layers. The thermomechanical deflection sensitivity was 7.9×10(-3) rad/K, which corresponds to a noise equivalent temperature difference (NETD) of 2.9 mK. In the present work, the array was used to capture IR images while operating at room temperature and atmospheric pressure without the need for vacuum packaging. The average measured NETD of our IR detector system was approximately 200 mK, but some sensing elements exhibited an NETD of 50 mK.

Ultra-responsive thermal sensors for the detection of explosives using calorimetric spectroscopy (CalSpec)

We have developed a novel chemical detection technique based on IR micro-calorimetric spectroscopy that can be used to identify the presence of trace amounts of very low vapor pressure target compounds. Unlike numerous recently developed low-cost sensor approaches, the selectivity is derived from the unique differential temperature spectrum and does not require the questionable reliability of highly selective coatings to achieve the required specificity. This is accomplished by obtaining the IR micro-calorimetric absorption spectrum of a small number of molecules absorbed on the surface of a thermal detector after illumination through a scanning monochromator. We have obtained IR micro- calorimetric spectra for explosives such as TNT over the wavelength region 2.5 to 14.5 micrometers . Thus both sophisticated and relatively crude explosives compounds and components are detectable with this technique due to the recent development of ultra sensitive thermal-mechanical micro-structures. In addition to the above mentioned spectroscopy technique and associated data, the development of these advanced thermal detectors is also presented in detail.