Uncooled Micro Research Articles

Study of boron doped amorphous silicon lightly hydrogenated prepared by DC magnetron sputtering for infrared detectors applications

The objective of this study is to investigate the effect of boron doping concentration on the bolometric properties of lightly hydrogenated amorphous silicon doped with boron (a-Si: H(B)) films. Thin film a-Si: H(B) samples with different boron concentrations are prepared by co-sputtering of boron and silicon at relatively low hydrogen pressure. FTIR analyses show that the intensity of the characteristic peak of the substitutional boron gradually increases with the addition of boron. Increasing in boron concentration affects the bolometric properties of the lightly hydrogenated a-Si: H (B) films, including conductivity at room temperature (σRT) and thermal resistance coefficient (TCR). Indeed, when the boron concentration increases from 1.5 to 43%, σRT increases from 1.4 10−6 to 2 10−3 Ω−1 cm−1 while the absolute value of TCR decreases from 3% to 8% K−1, respectively. In addition, lightly hydrogenated a-Si: H (B) films exhibit good thermal stability. We have showed in this study that lightly hydrogenated a-Si: H(B) can be considered as a potential candidate for low-cost, high-performance uncooled micro bolometers.

Multilayer Thin Film Getter for Sustainable Vacuum in MEMS Packaging

Some applications require MEMS devices to be packaged in a high vacuum environment, for example, high-quality factor mechanical micro-resonators or low heat loss microbolometers can only be obtained in a vacuum better than 10-3 mbar [1-2]. For this purpose, many Non-Evaporable Getter (NEG) multilayers or alloys films based on sputtered highly reactive transition metals have been investigated. For example, R.K. Sharma [3] found the Ti21Zr24V55 getter and O.B. Malyshev [4] found the Ti-Zr-Hf-V getter can be activated at 160 °C for 2 hours and 150 °C for 24 hours respectively. The getter function after its thermal activation is to absorb gases generated inside the packaging cavity by outgassing, permeation and by real and virtual leaks after packaging cap sealing by a highly hermetic bonding process. In this work we investigate Au/Zr/V/Zr getter multilayers in order to get a sustainable low pressure in a packaging cavity. All films were initially deposited by e-beam evaporation on 100 mm in diameter Si（100）wafers. The samples of Au/Zr/V/Zr were deposited in an ultra-high vacuum set-up with a background pressure of 2 10-10 mbar. To investigate their activation and oxidation by gas sorption at typical pressures found in packaging cavities, the films were annealed for 1 hour at various annealing temperatures in a rapid thermal annealing (RTA) chamber (Jipelec Jetfirst 100) at a vacuum level 10-2 mbar. Then, these films were analyzed by using four probe resistivity measurement, SEM and TEM cross section observations, EDX measurement and SIMS. All results are consistent and support that the activation temperature of zirconium under vanadium depends on vanadium layer thickness. For example, a 25nm zirconium oxide layer is found under the vanadium film after annealing at 300°C for 1h when the vanadium thickness is less than 200nm. However, for a 450nm thick vanadium layer, the 1-hour annealing temperature must reach 350°C to obtain the same thickness of zirconium oxide layer. Next, these annealed Au/Zr/V/Zr getters were exposed to air at room temperature for 7 months. After this exposure, the sheet resistance was found to increase by 6 % and 15 % for samples annealed at 250°C-1h and 300°C-1h respectively. TEM cross section observations of these samples showed that the thickness of the zirconium oxide layer under vanadium increases by about 3 nm. Finally, to evaluate its sorption ability in a real application case, the Au/Zr/V/Zr getter film was integrated in the 1.3 cmx1.3 cmx2.5 mm packaging cavity of a resonant gyroscope and activated at 300°C for 30 min before cavity sealing by Au/Sn bonding at 285°C for 30min. The time dependence of the pressure inside the packaging was evaluated from the variation of the quality factor Q of a selected mechanical resonance of the gyroscope. Quantitative values of the internal pressure were obtained owing to a previous calibration of the Q versus pressure curve that was in agreement with a squeeze film damping model. The evaluated pressure inside the cavity was 4.5 x10-3 mbar after annealing, 3x10-3 mbar after 2 weeks, and then this vacuum level still maintained after 6 months. In conclusion, this work demonstrates that Au/Zr/V/Zr getter films have a low activation temperature and suitable sorption properties to get a sustainable low pressure in a medium size vacuum package. [1] M. Elßner, Vacuum quality evaluation for uncooled micro bolometer thermal imager sensors, Microelectronics Reliability 54 (2014), 1758. [2] G.-D. Anna, Miniature and MEMS-type vacuum sensors and pumps, Vacuum 82 (2009), 1419. [3] R.K. Sharma, Jagannath, S. Bhattacharya, R.B. Tokas, K.G. Bhushan, S. Sen, S.C. Gadkari, S.K. Gupta, Journal of Alloys and Compounds 651 (2015) 375-381. [4] O.B. Malyshev, R. Valizadeh, R.M.A. Jones, A. Hannah, Vacuum 86 (2012) 2035-2039.

Vacuum quality evaluation for uncooled micro bolometer thermal imager sensors

This paper presents an innovative and effective method of measuring the internal vacuum quality of un-cooled micro bolometer thermal imager sensors where the bolometer sensor elements themselves are used for vacuum measurement. A feasible thermal calculation model using an extended Fourier’s Law is presented which is integrated in thermal FEM simulations. Experimental results correlating with FEM simulations prove the feasibility of this method. A measuring range to pressures as low as 5×10−3mbar was achieved that fully covers the needed range where the internal package pressure is leading to performance losses of the IRFPA. The vacuum quality evaluation method supported by a developed temperature compensation method is showing a high repetitive accuracy with a remaining mean failure of 0.2%. Without the need to integrate additional pressure sensors this method reduces costs and chip area and it is fast and highly accurate. Therefore, it can be used for stationary test systems as well as in mobile infrared camera systems.

IR optical system design of uncooled thermal imaging camera in long band (8—12µm)

For 384×288 pixel uncooled micro bolometer (a-Si) focal plane array (FPA) detector an infrared optical system (lenses) has been designed in this paper for work in long wavelength infrared region (LWIR) 8— 12 µm. the design results show that the optimum optical performance can be obtained by our proposed approach, so the modulation transfer function (MTF) at Nyquist frequency above 0.5 and closed to diffraction limit, and diameter of geometrical blur image less than detector unit cell (pixel) and concentrated on it. The main advantages of this design beside quality of it optically, it's simple in structure (consists of one conic surface - ellipse; and five spherical surfaces), lightweight (about 77.59g), small volume (entrance pupil diameter=40mm), and short length (total track=58.5mm). These mechanical parameters in addition to chosen tolerance level for manufacture of lenses have a positive affect on the cost of production for this designed system. So the designed optical system should good enough to meets the technical performance requirements for: high image quality, small size, light weight, and low cost thermal camera. This system can be applied in

Micro-Scale Thermal Imaging of Organic and Polymeric Materials with Cooled and Uncooled Infrared Cameras

The emissivity corrected thermal imaging combined with a real-time direct imposed-signal system on the freezing of biological cells is presented, which makes it possible to visualize the exothermic latent heat at a minus temperature. The applicability of the uncooled micro bolometer (thermal detector) to the micro-scale thermal analysis on the phase transitions of organic and polymeric materials is discussed in comparison with the photon detector, equipped with the optics originally designed.