A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.     

金刚石自支撑膜的高温红外透过性能

由于金刚石具有低吸收和优异的力学与导热性能使其成为长波(8~12μm)红外光学窗口材料的重要选择。对于许多极端条件的应用,化学气相沉积(CVD)金刚石自支撑膜的高温光学性质至关重要。应用直流电弧等离子喷射法制备光学级金刚石自支撑膜进行变化温度的红外光学透过性能研究,采用光学显微镜、X射线衍射、激光拉曼和傅里叶变换红外-拉曼光谱仪检测CVD金刚石膜的表面形貌、结构特征和红外光学性能。结果表明:在27℃时金刚石膜长波红外8~12μm之间的平均透过率达到65.95%,在500℃时8~12μm处的平均透过率为52.5%。透过率下降可分为3个阶段。对应于透过率随温度的下降,金刚石膜的吸收系数随温度的升高而增加。金刚石自支撑膜表面状态的变化,对金刚石膜光学性能的影响显著大于内部结构的影响。     

Porous silicon oxide sacrificial layers deposited by pulsed-direct current magnetron sputtering for microelectromechanical systems

The development of silicon oxide layers with high etch rates to be used as sacrificial layers in surface micromachining for microsystems fabrication poses a great technological challenge. In this work, we have investigated the possibility of obtaining easily removable silicon oxide layers by pulsed-direct current (DC) magnetron reactive sputtering. We have carried out a comprehensive study of the influence of the deposition parameters (total pressure and gas composition) on the composition, residual stress and lateral etch rate in fluoride wet solutions of the films. This study has allowed to determine the sputtering conditions to deposit, at high rates (up to 0.1 μm/min), silicon oxide films with excellent characteristics for their use as sacrificial layers. Films with roughness around 5 nm rms, residual stress below 100 MPa and very high lateral etch rate (up to 5 μm/min), around 70 times higher than for thermal silicon oxide, have been achieved. The structural characteristics of these easily removable silicon oxide layers have been assessed by infrared spectroscopy and atomic force microscopy, which have revealed that the films exhibit a porous structure, related to very specific sputter conditions. Finally, the viability of these films has been demonstrated by using them as sacrificial layer in the fabrication process of AlN-based microresonators.     

Enhanced light absorption of Ag films deposited onto femtosecond laser microstructured silicon

Absorptive properties of silver (Ag) films with the thickness varied from 160 nm to 340 nm deposited onto the surface of femtosecond laser microstructured silicon by vacuum thermal evaporation were measured in a wavelength range of 0.3-16.7 μm. Greatly enhanced light absorption of Ag films has been observed in the whole measured wavelength range. For the same Ag film thickness (268 nm), the light absorption was strongly depended on the height and spacing of the spikes, especially in the region of 1-16.7 μm. The relation between light absorption and thickness of Ag films has also been investigated, it was shown that the light absorption decreases with the increasing thickness of Ag films. The strongly enhanced light absorption in such a wide wavelength range is mainly ascribed to the multiple reflection of light between spikes and surface plasmon excitation of noble metal nano-particles on the spikes surface.     

Production of orientation-patterned GaP templates using wafer fusion techniques

Nonlinear optical frequency conversion is an effective technique for generating infrared (IR) and terahertz (THz) wavelengths not readily available from existing laser sources. Birefringent materials such as LiNbO₃ are often used to generate wavelengths where gaps exist, but are unsuitable in the mid-IR, far-IR, and THz regions as these materials are often opaque in these regions. As an alternative, GaAs has been employed for frequency conversion in these regions using quasi-phase-matching (QPM) to overcome the material’s lack of birefringence. QPM has been successfully demonstrated in GaAs using fused stacks of thin alternately oriented layers or inverted orientation patterned (OP) grating templates overgrown with thick columnar GaAs layers. Although GaAs has a high nonlinear coefficient d₁₄ = 170 pm/V at 1.064 μm and good thermal conductivity (52 W/m K), it suffers from strong two-photon absorption below 1.7 μm making it inefficient when pumped with a source less than or equal to this wavelength. GaP also has a high nonlinear coefficient d₁₄ = 71 pm/V at 1.064 μm, better thermal conductivity (110 W/m K) and much lower two-photon absorption in the 1 μm region. Therefore, OPGaP is desirable for NLO applications in the mid-IR and THz that use commercially available pump lasers in the 1.06-1.55 μm wavelength range. In this work the fabrication of OPGaP templates suitable for thick columnar hydride vapor phase epitaxial growth of GaP is reported using a commercially viable wafer fusion technique.     

Comparison of shortwave and longwave measuring thermal-imaging systems

A comparison study of shortwave (3-5-μm) and longwave (8-12-μm) measuring thermal imaging systems has been conducted. The study was limited to systems working in indoor conditions, as is typical in many industrial and scientific applications. A theory of the influence of measurement conditions and system parameters on the accuracy of temperature measurements has been developed. On the basis of the developed formulas an analysis of the influence of signal disturbances (because of incorrectly assumed emissivity, radiation reflected by the object, radiation emitted by system optics, limited transmittance of the atmosphere, and limited temperature resolution of the system) on the accuracy of temperature measurement has been made. It has been found that the shortwave systems in typical measurement conditions offer generally better accuracy in temperature measurement than the longwave ones do.     

Optical and mechanical characteristics of zinc sulphide-thorium fluoride mixed composition thin films for use in the near infrared region (1–10µm)

Mixed composition thin films of zinc sulphide-thorium fluoride have been deposited on glass and silicon substrates by thermal evaporation of mixtures of these materials in different proportions, from a single resistively heated source. The films are characterized for their optical properties (refractive index and extinction coefficient), mechanical properties (intrinsic stress), surface morphology and chemical composition. It is found that these films have tailorable refractive indices and low losses, and that films with certain compositions have low intrinsic stress and smooth surface morphology, making them suitable for incorporation in thin film multilayers for use in the near infrared region up to at least 10μm.     

Boron induced recrystallization of amorphous silicon film by a rapid thermal process

Rapid thermal process (RTP) is to induce boron-doped amorphous silicon into a high degree of crystallization of polycrystalline silicon in 5 min. In addition to the short time characteristic, it also provides a relatively lower temperature route to prepare high percentage of polycrystalline silicon in comparison with solid phase crystallization method. Before RTP, boron is homogeneously doped into the amorphous silicon film by ion implantation technology. After rapid thermal processing, the grain size of the polycrystalline silicon was found about at 0.1-0.5 μm. The degree crystallization of silicon is reached up to 99.1% with a good hole mobility of 138.6 cm²/V s.     

Optical Temperature Measurement Method for Glowing Microcomponents

A measurement method and measurement results for the temperature of miniature microbridge emitters integrated on silicon are presented. First, the extinction coefficient of highly doped silicon was measured at high temperatures: a piece of a silicon-on-insulator wafer was heated to several temperatures in a high-temperature furnace, and the emitted spectra were measured using a spectroradiometer with focusing optics. The optical behavior of the sample was modeled with Fresnel equations. The extinction coefficient of silicon was obtained from the model, because other optical properties, the dimensions, and the temperature of the structure were known. An emissivity model was then developed and adapted for the microbridge with the known extinction coefficient values, which allows the temperature to be determined from the measured spectrum. We can now measure optically the temperatures of the microbridges of dimensions 400 × 25 × 4 μm³ in the temperature range 600 °C to 1200 °C with an uncertainty of 100 °C.     

Thermal lensing in silver gallium selenide parametric oscillator crystals

We performed an experimental investigation of thermal lensing in silver gallium selenide (AgGaSe(2)) optical parametric oscillator crystals pumped by a 2-μm laser at ambient temperature. We determined an empirical expression for the effective thermal focusing power in terms of the pump power, beam diameter, crystal length, and absorption coefficient. This relation may be used to estimate average power limitations in designing AgGaSe(2) optical parametric oscillators. We also demonstrated an 18% slope efficiency from a 2-μm pumped AgGaSe(2) optical parametric oscillator operated at 77 K, at which temperature thermal lensing is substantially reduced because of an increase in the thermal conductivity and a decrease in the thermal index gradient dn/dT. Cryogenic cooling may provide an additional option for scaling up the average power capability of a 2-μm pumped AgGaSe(2) optical parametric oscillator.     

Optical constants and Drude analysis of sputtered zirconium nitride films

Opaque and semitransparent dc magnetron-sputtered ZrN films on glass and silicon have been optically characterized with spectral reflectance measurements and ellipsometry. High rate sputtered ZrN has good optical selectivity, i.e., higher than 90% infrared reflectance and a pronounced reflectance step in the visible to a reflectance minimum of less than 10% at 350 nm. The results are comparable with those obtained for single crystalline samples and those prepared by chemical vapor deposition. The complex optical constant (N = n v ik) for opaque films has been determined in the 0.23-25-μm wavelength range with Kramers-Kronig integration of bulk reflectance combined with oblique incidence reflectance for p-polarized light. A variable angle of incidence spectroscopic ellipsometer has been used for determination of the optical constants in the 0.28-1.0-μm wavelength region. The results of the two methods show excellent agreement. The results indicate that ZrN is free electronlike and the Drude model can be applied. The best opaque films had Drude plasma energies (?ω(p) between 6.6 and 7.5 eV and relaxation energies (?/τ) between 0.29 and 0.36 eV. Ellipsometer data for the semitransparent films show that the refractive index (n) in the visible increases with decreasing film thickness whereas the extinction coefficient (k) is essentially unchanged. The optical properties are improved by deposition upon a heated substrate.     

单晶硅太阳电池发射区结构设计与工艺优化

首先利用TCAD半导体器件仿真软件全面系统地分析了不同发射区表面浓度和结深对P型单晶硅太阳电池短路电流、开路电压、填充因子及转换效率的影响。然后以获得最优的发射区结构参数为目标,对热扩散工艺和离子注入工艺进行了仿真研究。仿真结果表明,发射区表面浓度和结深的变化对单晶硅太阳电池输出特性产生显著影响。当发射区表面浓度为5×1020 cm-3,结深为0.1μm时,太阳电池转换效率最高,可达20.39%。若采用热扩散工艺制备发射区,扩散温度范围为825~850℃,扩散时间范围为10~20min;若采用离子注入工艺制备发射区,当注入剂量为1×1017 cm-2,注入能量为5keV时,退火温度范围为850~875℃,退火时间范围为5~15min。     

A Microbolometer Asynchronous Dynamic Vision Sensor for LWIR

In this paper, a novel event-based dynamic IR vision sensor is presented. The device combines an uncooled microbolometer array with biology-inspired (“neuromorphic”) readout circuitry to implement an asynchronous, “spiking” vision sensor for the 8–15 $mu{rm m}$ thermal infrared spectral range. The sensor's autonomous pixels independently respond to changes in thermal IR radiation and communicate detected variations in the form of asynchronous “address-events.” The 64 $times$ 64 pixel ROIC chip has been fabricated in a 0.35 $mu{rm m}$ 2P4M standard CMOS process, covers about $4times 4 {rm mm}^{2}$ of silicon area and consumes 8 mW of power. An amorphous silicon (a-Si) microbolometer array has been processed on top of the ROIC and contacted to the pixel circuits. We discuss the bolometer detector properties, describe the pixel circuits and the implemented sensor architecture, and show measurement results of the readout circuits. Subsequently, a DFT-based approach to the characterization of asynchronous, spiking sensor arrays is discussed and applied. Test results and analysis of sensitivity, bandwidth, and noise of the fabricated IR sensor prototype are presented.      

Understanding the role of gravity in the crystallization suppression of ZBLAN glass

ZBLAN (ZrF₄–BaF₂–LaF₃–AlF₃–NaF) glass fibers are excellent materials for the use in many applications, such as fiber optics, fiber amplifiers, and lasers for cutting, drilling, and surgery. The main advantage of ZBLAN glasses over other glasses, such as silica, is its superior infrared transmissibility. The theoretical optical transmission spectrum for a ZBLAN fiber is from 0.3 μm in the UV to 7 μm in the IR region. The main obstacle with ZBLAN glass is the extrinsic losses from impurities, which includes crystallites formed during the manufacturing process. Due to ZBLAN’s narrow working range, crystallites easily form during the drawing process, which inhibits the materials transmissibility. Microgravity (μ-g) processing has been proven to suppress crystallization in ZBLAN glass, thus allowing the material to reach its theoretical loss coefficient. Past researchers have shown that this phenomenon exists, but the mechanism of crystallization suppression in a microgravity environment is not well understood. This research endeavors to understand the role that gravity plays on the crystallization suppression of ZBLAN glass. The outcome of this study will impact the production of superior mid-IR fiber optics and could lead to a more feasible manufacturing technique. The main conclusion developed from this study is that the process is heavily dependent upon mass transfer kinetics such as diffusion and buoyancy-driven convection. Thus, suppressing buoyancy-driven convection, at relevant drawing temperatures, suppresses crystallization growth in ZBLAN glass. This theory was proven through microgravity experimentation, analytical, and computational modeling.     

卤化银光纤原料的超提纯

高纯原料的制备是制造高品质光纤最重要的环节,本文采用氯气氛下水平区熔法生长单晶的提纯方法对卤化银原料进行超提纯．区域熔融提纯后的卤化银原料红外吸收光谱测量显示在800～4000cm^－1范围内吸收随波长的增加而降低．经CO₂激光量热计法测量提纯后的卤化银原料在10．6μm处的吸收系数为5×10^－4Cm^－1,较提纯前降低两个数量级,制成的光纤传输损耗03～0．5dB／m     

Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications

In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN_x) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN_x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm⁻¹ with a-SiH layers, compared with 400 cm⁻¹ loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN_x periods have been included in an InP-based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment.     

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-μm and 8-14-μm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 μm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-μm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-μm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-μm and 8-14-μm atmospheric windows.     

Germanium enhanced large-grain growth by pulse mode rapid thermal crystallization process

The large-grain crystallization for silicon film was implemented by doping germanium in silicon film and using a rapid thermal process with near-infrared illumination. Germanium atoms acted as nuclei for crystallization of the amorphous silicon film. Because the germanium embedded in silicon film could absorb infrared photonic energy and convert it into thermal energy. Due to the low thermal conductivity of germanium, the absorbed energy remained in the germanium-doped film for a long time and helped the grain growth. With the amount of germanium in silicon film increased, a large grain will be obtained readily. A grain size of 3.92 μm was achieved in germanium-doped film.     

Linear Thermal Expansion Coefficient of Silicon from 293 to 1000 K

As a part of the program to establish a thermal expansion standard, the linear thermal expansion coefficients of single-crystal silicon have been determined in the temperature range 293 to 1000 K using a dilatometer which consists of a heterodyne laser Michelson interferometer and gold versus platinum thermocouple. The relative standard deviation of the measured values from those calculated from the best least-squares fit was 0.21%. The relative expanded uncertainty in the measurement was estimated to be 1.1 to 1.5% in the temperature range, based upon an analysis of thirteen standard uncertainties. The present data are compared with the data previously obtained by similar dilatometers and the standard reference data for the thermal expansion coefficient of silicon, recommended by the Committee on Data for Science and Technology (CODATA). The present data are in good agreement with the most recently reported data but not with the earlier high-temperature data used to evaluate the standard reference data, which suggests a need for the reevaluation of the standard reference data for the thermal expansion coefficient of silicon at temperatures above 600 K.