Thermal Properties Measurement of Micro-electromechanical System Sensors by Digital Moire Method

Abstract:  The thermal properties of a micro-electromechanical system sensor were analysed by a novel digital moiré method. A double-layer micro-cantilever sensor (60  μ m long, 10  μ m width and 2  μ m thick) was prepared by focused ion beam milling. A grating with frequency of 5000 lines mm⁻¹ was etched on the cantilever. The sensor was placed into a scanning electron microscope system with a high temperature device. The observation and recording of the thermal deformation of the grating were realised in real-time as the temperature rose from room temperature to 300 °C at intervals of 50 °C. Digital moiré was generated by interference of the deformed grating and a digital virtual grating. The thermal properties including strain distribution of the sensor and the linear expansion coefficient of polysilicon were accurately measured by the phase-shifted moiré patterns.     

Optimization of diffraction grating profiles in fabrication by electron-beam lithography

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 microm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.     

衍射频谱法测光栅膜层厚度

根据傅立叶光学理论从光栅的衍射频谱中可以反推光栅本身的多种信息,包括其形貌特征。 利用矩形相位光栅的傅立叶变换,推导出零级和一级衍射光强和矩形相位光栅膜层厚度之间的函数关系,据此可在测得零级和一级次光强后,推算出光栅膜层厚度。以台阶仪作为标准,该方法的测量误差在4%以内。     

High-efficiency diffractive micromachined chopper for infrared wavelength and its application to a pyroelectric infrared sensor

We have developed a diffractive micromachined chopper (DMC) for an IR wavelength of ~10 mum. This device operates mechanically by movable reflection grating beams. It modulates the diffraction efficiency by controlling the displacement of grating beams by an electrostatic force. For a CO(2) laser beam, a high modulation efficiency of 84% with an -0.8-dB small insertion loss was obtained by detecting 0th-order diffracted light. A novel pyroelectric IR microsensor with a DMC and a diffractive multilevel Si microlens was proposed and it demonstrated the detection of human existence.     

Wideband two-port beam splitter of a binary fused-silica phase grating

The usual beam splitter of multilayer-coated film with a wideband spectrum is not easy to achieve. We describe the realization of a wideband transmission two-port beam splitter based on a binary fused-silica phase grating. To achieve high efficiency and equality in the diffracted 0th and -1st orders, the grating profile parameters are optimized using rigorous coupled-wave analysis at a wavelength of 1550 nm. Holographic recording and the inductively coupled plasma dry etching technique are used to fabricate the fused-silica beam splitter grating. The measured efficiency of (45% x 2) = 90% diffracted into the both orders can be obtained with the fabricated grating under Littrow mounting. The physical mechanism of such a wideband two-port beam splitter grating can be well explained by the modal method based on two-beam interference of the modes excited by the incident wave. With the high damage threshold, low coefficient of thermal expansion, and wideband high efficiency, the presented beam splitter etched in fused silica should be a useful optical element for a variety of practical applications.     

带有力反馈控制的三明治式微机械干涉加速度计

设计了一种静电力反馈控制的三明治式微机械干涉加速度计,加速度计由敏感芯片、半导体激光器、光电二极管以及相应的驱动电路和反馈控制电路组成.敏感芯片为玻璃-硅-玻璃3层结构,通过硅-玻璃键合体硅工艺制成.硅质量块由铝梁支撑,底部玻璃基片上有金属光栅和电极,通过在质量块和底部玻璃基片上的电极之间施加电压可以调节质量块与玻璃基片间的间隙.入射激光照射到敏感芯片上的光栅上,产生衍射光束,其光强随质量块与下玻璃的间距而变化.反馈控制电路通过测量衍射光强的变化来改变质量块与底电极之间的电压,使得质量块与底部玻璃基片的距离保持为入射光波长1/8的奇数倍,从而提高输出线性度,改善灵敏度,增大量程.     

Temperature-insensitive fiber Bragg grating tilt sensor

A temperature-insensitive optical fiber tilt sensor is presented. The sensor scheme uses a prestrained fiber Bragg grating to sense the strain, which depends on the tilt angle. To compensate for the temperature effect, materials that have different linear thermal expansion behaviors are used for implementation of the sensor body. The differentiation in the linear thermal expansion would then cause a counter effect to the original temperature effect. Experimental tests show an accuracy of +/-0.167 degrees in tilt angle measurement. A temperature stability better than +/-0.33 degrees over the temperature range from 27 degrees C to 75 degrees C is demonstrated. The resolution 0.0067 degrees in tilt angle measurement is achieved by using our preliminary sensor with a dimension of 1 6 x 5 x 5 cm(3).     

Temperature sensor based on the power reflected by a Bragg grating in a tapered fiber

We present a temperature sensor based on two chirped gratings made in optical fibers tapered by fusion. One of the gratings has a metallic shielding and acts as sensor element, whereas the second grating provides a reference signal. The sensor is interrogated by measuring the power reflected by the two gratings, and the system has an accuracy of 0.05 degrees C over a linear operation range of more than 10 degrees C that can be adjusted in the fabrication process.     

CO2 laser-grooved long period fiber grating temperature sensor system based on intensity modulation

A long period fiber grating (LPFG) temperature sensor system based on intensity modulation is developed. The LPFG employed is fabricated by the use of a focused CO2 laser beam to carve periodic grooves on the fiber. The temperature measurement resolution of up to 0.1 degrees C has been obtained within the temperature range between 20 degrees C and 100 degrees C. The system uses a simple intensity measurement method and exhibits the advantages of convenient intensity measurement, double temperature sensitivity, high resolution, simple configuration, and low cost.     

Metal oxide-based monolithic complementary metal oxide semiconductor gas sensor microsystem

A fully integrated gas sensor microsystem is presented, which comprises for the first time a micro hot plate as well as advanced analog and digital circuitry on a single chip. The micro hot plate is coated with a nanocrystalline SnO2 thick film. The sensor chip is produced in an industrial 0.8-microm CMOS process with subsequent micromachining steps. A novel circular micro hot plate, which is 500 x 500 microm(2) in size, features an excellent temperature homogeneity of +/-2% over the heated area (300-microm diameter) and a high thermal efficiency of 6.0 degrees C/mW. A robust prototype package was developed, which relies on standard microelectronic packaging methods. Apart from a microcontroller board for managing chip communication and providing power supply and reference signals, no additional measurement equipment is needed. The on-chip digital temperature controller can accurately adjust the membrane temperature between 170 and 300 degrees C with an error of +/-2 degrees C. The on-chip logarithmic converter covers a wide measurement range between 1 kOmega and 10 MOmega. CO concentrations in the sub-parts-per-million range are detectable, and a resolution of +/-0.1 ppm CO was achieved, which renders the sensor capable of measuring CO concentrations at threshold levels.     

Optical grating diffraction method: from strain microscope to strain gauge

The grating diffraction method for direct strain measurement is reviewed. Two systems which use this method are presented. The first system is a compact strain microscope. A Leitz optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. Gratings with median density of between 40 and 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed with high image quality on the CCD sensor plane. Software is developed to precisely, quickly and automatically determine the diffraction spot centroids. The second system is a new strain sensor based on a high-frequency grating and two Position Sensor Detectors (PSDs). The grating, attached to the surface of the specimen, is illuminated by a focused laser beam, generally with a frequency of 1,200 lines/mm. The centroids of diffracted beam spots from the grating are automatically determined using two PSD sensors connected to a personal computer. The shift of diffracted beam spots due to specimen deformation is detected. Strain sensitivity of one micro-strain can be obtained, as can a 0.4 mm spatial resolution for strain measurement. The system can be used for both static and dynamic tests.     

Simple three-dimensional laser angle sensor for three-dimensional small-angle measurement

A simple three-dimensional (3D) laser angle sensor for 3D measurement of small angles based on the diffraction theorem and on ray optics analysis is presented. The possibility of using position-sensitive detectors and a reflective diffraction grating to develop a 3D angle sensor was investigated and a prototype 3D laser angle sensor was designed and built. The system is composed of a laser diode, two position-sensitive detectors, and a reflective diffraction grating. The diffraction grating, mounted upon the rotational center of a 3D rotational stage, divides an incident laser beam into several diffracted rays, and two position-sensitive detectors are set up for detecting the positions of +/-1st-order diffracted rays. According to the optical path relationship between the three angular motions and the output coordinates of the two position-sensitive detectors, the 3D angles can be obtained through kinematic analysis. The experimental results show the feasibility of the proposed 3D laser angular sensor. Use of this system as an instrument for high-resolution measurement of small-angle rotation is proposed.     

“Thermal Mirror” Method for Measuring Physical Properties of Multilayered Coatings

In this study theoretical principles underlying the photothermal displacement (“thermal mirror”) method for measuring physical properties of opaque multilayered and functionally graded coatings with low thermal conductivity are analyzed. In this method, the specimen is locally heated by a power laser beam, and a two-dimensional transient temperature field is formed in a specimen. The physical basis for the photothermal displacement method is the non-stationary buckling and displacement of an irradiated surface due to a non-uniform thermal expansion. The surface is monitored by a low-power probe beam of a second laser, which is reflected from the specimen, i.e., the system operates as a convex “thermal mirror.” The photoinduced displacement varies with time, and the probe beam is reflected at a different angle depending on the slope of the displacement. The deflection angle is measured as a function of time by a position sensor, and the results of these measurements are compared with the theoretical dependence of the deflection angle on time and physical properties of a coating. This dependence was determined analytically from the solution of the two-dimensional thermal elasticity problem. It is shown that for the specimen composed of a substrate and a coating it is feasible to determine the properties of the coating, e.g., the thermal diffusivity and coefficient of linear thermal expansion provided that the analogous properties of the substrate are previously measured or otherwise known.     

High-efficiency blazed diffractive optical elements for the violet wavelength fabricated by electron-beam lithography

Blazed diffractive optical elements (DOEs) were studied for the violet wavelength by electron-beam lithography. By optimizing electron-beam writing parameters and electron-dose distributions, we fabricated eight kinds of grating (period A = 10-0.54 microgm) with excellent blazed structure. It has been demonstrated that the measured diffraction efficiency values agreed well with the rigorous theoretical ones. For the fine period of 0.54 microm, we confirmed a peak appearance of 75.6% (TE) experimentally. A wave aberration as small as approximately 0.01 lambda (rms) was obtained for the first-order diffracted wave from the fabricated DOEs. Blazed DOEs for the violet wavelength could be used as key devices in a high-density optical disk pickup of the next generation.     

Compact sensor for measuring two-dimensional tilt using a two-dimensional transmission grating and the Talbot effect

This paper proposes a tilt sensor that measures the small two-dimensional tilt of a plane reflective object using the Talbot effect. It is an extension of a previously proposed one-dimensional tilt sensor. The light beam reflected from the object impinges on a hexagonal grating, and the intensity of the diffracted wave is detected on an image sensor located at a Talbot distance from the grating. The diffraction intensity displaces due to the tilt of the object. The displacement is calculated by the Fourier transform method to obtain the two-dimensional tilt. This sensor is very simple and compact. The principle of the sensor is explained for a grating with a general pattern. An experiment using a hexagonal grating shows its validity. Discussions are given for making it more practical.     

Simultaneous measurement of temperature and pressure by a single fiber Bragg grating with a broadened reflection spectrum

Simultaneous measurement of temperature and pressure with a single fiber Bragg grating (FBG) based on a broadened reflection spectrum is proposed and experimentally demonstrated. A novel double-hole structure of a cantilever beam is designed, and a FBG is affixed on the nonuniform strain area of the cantilever beam. The Bragg reflection bandwidth is sensitive to the spatially gradient strain but is free from the spatially uniform temperature. The wavelength peak shift and the bandwidth broadening of the FBG with a change of temperature and pressure allow for simultaneous discrimination between the temperature and the pressure effects. Standard deviation errors of 1.4 degrees C and 1.8 kPa were obtained with temperature and pressure ranges of 20 degrees C-100 degrees C and 0-80 kPa, respectively. This novel and low-cost sensor approach has considerable potential applications for temperature-insensitive strain measurement.     

Diffracted wavefront measurement of a volume phase holographic grating at cryogenic temperature

Flatness of the wavefront diffracted by grating can be mandatory for some applications. At ambient temperature, the wavefront diffracted by a volume phase holographic grating (VPHG) is well mastered by the manufacturing process and can be corrected or shaped by postpolishing. However, to be used in cooled infrared spectrometers, VPHGs have to stand and work properly at low temperatures. We present the measurement of the wavefront diffracted by a typical VPHG at various temperatures down to 150 K and at several thermal inhomogeneity amplitudes. The particular grating observed was produced using a dichromated gelatine technique and encapsulated between two glass blanks. Diffracted wavefront measurements show that the wavefront is extremely stable according to the temperature as long as the latter is homogeneous over the grating stack volume. Increasing the thermal inhomogeneity increases the wavefront error that pinpoints the importance of the final instrument thermal design. This concludes the dichromated gelatine VPHG technology, used more and more in visible spectrometers, can be applied as it is to cooled IR spectrometers.     

Measurement of the thermal diffusivity of aqueous solutions of alcohols by a laser-induced thermal grating technique

The thermal diffusivity of methanol, ethanol, and their aqueous solutions was measured at atmospheric pressure and temperature. The measurements were performed with a laser-induced thermal grating technique. The aqueous solutions have weight fractions of 20, 40, 60, and 80%. Systematic errors were taken into consideration, and corrections were made to the measured values. Focused laser beams were used, which notably intensify the diffracted signal, reduce the background to zero, and justify neglecting the heterodyne term of the diffracted signal, thus simplifying the data evaluation. Hence, the accuracy of the measurements was improved significantly. The overall accuracy of the measurements is estimated to be better than 1.5%.     

Slope measurement of bent plates using double grating shearing interferometry

A grating-based shearing interferometeric setup for slope measurement of bent plates has been proposed. The specimen under test is illuminated by a collimated beam from the laser. Light reflected from the specimen passes through two identical holographic gratings placed in tandem. The grating frequency has been so chosen that the diffracted orders from each grating are separated out distinctly. Two first-order beams diffracted from each of the gratings superpose in space. In the resulting interferogram, the fringes due to slope information of the object are visualized. Mathematical formulation for experimental determination of slope values has been undertaken. Validation of the experimental results with theoretical predictions in case of cantilever beam provides good correlation. The main advantage of the technique has been the realization of very compact geometry without the need for spatial filtering arrangement commonly associated with the grating-based techniques used to date.     

Improved Al/Si ohmic contacts to p-type 4H-SiC

An AlSi-based ohmic contact with a new composition is reported in this paper. AlSi(2%)Ti(0.15%) contacts are formed by evaporation on p-type 4H-SiC grown by liquid phase epitaxy (LPE) and annealed in the temperature range from 700 to 950°C. The ohmic behaviour has been checked by I–V characteristics and the contact resistivity has been measured by the linear transmission-line-model (TLM) method. The dependence of the contact resistivity on the annealing conditions has been studied. An ohmic behaviour has been established at 700°C while the lowest contact resistivity value of 9.6×10⁻⁵ Ω cm² has been obtained after annealing at 950°C. The thermal stability of both Al/Si/SiC and AlSiTi/SiC contacts at a temperature of 600°C has been studied. It has been found that the AlSiTi/SiC contacts are stable for 100 h at this ageing temperature while the Al/Si/SiC contacts deteriorate after 24 h.