Growth of GaN films with controlled out-of-plane texture on Si wafers

GaN films were deposited on Si (400) wafers by a pulsed laser deposition technique, and it was shown that out-of-plane texture of the film is controllable although the film and the substrate do not have any interface epitaxy. The texture of the film can be set either in c-axis or a-axis direction, thereby achieving polar or nonpolar film surfaces as desired. The GaN film and Si substrate were found to be separated by a thin amorphous interface layer consisting of Si, Ga, and O atoms, that can enhance the bonding between GaN and Si. This study shows the possibility of depositing GaN films on Si wafers at low cost and the potential of integrating Si based electronics with GaN based optoelectronics.     

Interference effects in photoacoustic and reflectance spectroscopies on TiO2/Si structures and TiO2 band gap

Experimental results of photoacoustic (PAS) and reflectance (RS) spectroscopies of titanium dioxide thin films (TiO2), deposited on Si substrates, are compared in a wide optical range including transparent and absorbent regions of TiO2. Due to the fact that the light modulation frequency f used in the photoacoustic experiments was so low that the thermal diffusion length of the TiO2 (mu = 100 microm) is always larger than the thickness of the studied films, the PAS turns out to be complementary to RS over the entire range. The presence of multiple reflection interference effects makes difficult a direct evaluation of the TiO2 band gap from the PAS signal. However, by employing k(lambda) values, obtained from transmission experiments on equivalent TiO2 films deposited on transparent fused quartz substrates, the PAS spectra for the films deposited on silicon are reconstructed by using those theoretical models that consider multiple reflections. The reasonable agreement of the simulated and experimental PAS spectra allows one to obtain reliable Eg values for the TiO2 films deposited on opaque silicon substrates.     

Image reconstruction for photoacoustic scanning of tissue structures

Photoacoustic signal generation can be used for a new medical tomographic technique. This makes it possible to image optically different structures, such as the (micro)vascular system in tissues, by use of a transducer array for the detection of laser-generated wide-bandwidth ultrasound. A time-domain delay-and-sum focused beam-forming technique is used to locate the photoacoustic sources in the sample. To characterize the transducer response, simulations have been performed for a wide variety of parameter values and have been verified experimentally. With these data the weight factors for the spectrally and temporally filtered sensor signals are determined in order to optimize the signal-to-noise ratio of the beam former. The imaging algorithm is investigated by simulations and experiments. With this algorithm, for what is to our knowledge the first time, the three-dimensional photoacoustic imaging of complex optically absorbing structures located in a highly diffuse medium is demonstrated. When 200-mum-diameter hydrophone elements are used, the depth resolution is better than 20 mum, and the lateral resolution is better than 200 mum, independent of the depth for our range of imaging (to ~6 mm). Reduction of the transducer diameters and adaptation of the weight factors, at the cost of some increase of the noise level, will further improve the lateral resolution. The synthetic aperture algorithm used has been shown to be suitable for the new technique of photoacoustic tissue scanning.     

A large-area mesoporous array of magnetic nanostructure with perpendicular anisotropy integrated on Si wafers

Large-area, over several square centimeters, mesoporous array of magnetic nanostructure with perpendicular anisotropy is prepared by depositing Co/Pt multilayers (MLs) on a mesopore array of anodized alumina (AAO) fabricated on Si wafers. The MLs are mainly deposited on the top of AAO walls and perimeters of the pores; very small amounts of magnetic material reach the bottom due to the high aspect ratio of AAO. Consequently, ordered pores are present in the magnetic MLs. The mean pore diameter of the fabricated mesoporous array is 8.83?nm with a standard deviation of 3.16?nm and density of about 2.1 × 10(11)?cm(-2). The Co/Pt MLs deposited on AAO and Si both exhibit strong perpendicular magnetic anisotropy, but the perpendicular coercivity (H(c)) increases by 15 times on AAO compared to that on Si. On the other hand, the magnetic cluster size decreases from 1000?nm (on Si) to 100?nm due to the presence of high-density pores. The dramatic increase in H(c) and the decrease in magnetic cluster size suggest that the pores behave as effective pinning sites. The magnetization-switching characteristics of the fabricated porous structure are different from those of the continuous films or Stoner-Wohlfarth-type (S-W) particles. One of the potential applications of this mesoporous structure may be in the field of high-density magnetic data storage.     

Simulation of photoacoustic imaging of microcracks in silicon wafers using a structure-changeable multilayered thermal diffusion model

The detection characteristics for photoacoustic imaging of microcracks in silicon wafers were theoretically and quantitatively investigated using a numerical simulation. The simulation is based on a one-dimensional multilayered thermal diffusion model coupled with the thermal-wave impedance of each layer, the layer structures of which are constructed along the wafer surface and are variable according to the scanning position of the point heat source. As the modulation frequency was reduced, the spatial resolution of the temperature amplitude profile at the cracks decreased, showing good agreement with the experimentally obtained photoacoustic amplitude images. At a modulation frequency of 200 kHz, for cracks with narrow air gaps of up to 20 nm, which is much smaller than both the beam spot size of 1.5 microm and the thermal diffusion length of 12 microm, the temperature amplitude is twice that of regions without cracks, and the temperature contrast increased with an increase in the modulation frequency. These calculation results suggest the effectiveness of using a high modulation frequency, making it possible to detect microcracks of the order of 10 nm.     

High-frequency differential piezoelectric photoacoustic investigation of ion-implanted (100) silicon wafers via laser beam position modulation

An exploratory application of position-modulation photoacoustic imaging of ion-implanted (100)-oriented Si wafers was undertaken to assess its potential as a diagnostic probe in semiconductor processing. Wafer scans were performed using acoustooptic modulation of a 1.06-mum Nd(3+):YAG laser beam up to 0.2 MHz with piezoelectric photoacoustic detection. Sensitivity ranges to ion-implanted parameters (ionic species and fluences) were studied and the capability of the technique to monitor processing-induced damage was established. Results indicate that position-modulated photoacoustic detection offers higher sensitivity than single-beam photothermal imaging and has distinct advantages over other analytical techniques.     

Dislocation structure in low-angle interfaces between bonded Si(001) wafers

Dislocation structures of interfaces between bonded (001) Si wafers with co-existing low-angle twist and tilt misorientations were studied by transmission electron microscopy. At dominating twist, a square screw dislocation network accommodates the twist, and interacts with steps at the interface, forming 60-degree dislocations. As the step density, i.e., the tilt angle, increases relative to the twist angle, the density of so-called zigzag reactions increases. Finally, hexagonal dislocation meshes dominate the dislocation configuration. It was found that the plan-view observations give the crystallographic relations accurately. The structures of the dislocation configurations were analyzed using Bollmann's dualistic representation. The rotation axes and angles were determined.     

A study of Si epitaxial layer growth on SOI wafers prepared by SIMOX

The epitaxial Si layers were deposited onto silicon on insulator (SOI) substrates by chemical vapor deposition technology, and SOI substrates were manufactured with separation by implantation of oxygen technology. The dislocations and stacking faults of epitaxial Si layer and substrate were examined and their densities were calculated, respectively. The surfaces of epitaxial Si layer and SOI substrate were studied by atomic force microscopy. The SOI substrates and the epitaxial Si layers were characterized by Rutherford backscattering and channeling spectroscopy. Transmission electron microscopy was used to observe the defect in epitaxial layer. The result shows that the defects in the epitaxial Si layer on low dose substrate are less than those in the epitaxial Si layer on standard dose substrate, and also that the defects in low dose substrate are less than those in standard dose substrate. The crystallinity of epitaxial Si layer on low dose substrate is better than that of epitaxial Si layer on standard dose substrate.     

Growth of high Ge content SiGe on (110) oriented Si wafers

Growth of high (above 40%) Ge content SiGe by applying silane and dichlorosilane as Si precursors on (110) Si is investigated. In the case of silane based processes Ge concentration is ~ 20% higher, whereas for dichlorosilane based processes it is ~ 30% lower on (110) Si compared to (100) Si. The morphology of the grown layers is found to be dependent on Ge concentration, layer thickness and process temperature. Use of optimized deposition parameters and adequate thickness results in high quality strained SiGe layers. Integration of high Ge content SiGe layers in multiple gate filed-effect transistor structures shows the expected differences in Ge content on the different Si planes forming Si fin. These differences can be avoided by adjusting the fin orientation on the Si wafer resulting in equal planes on the fin's top and sidewalls. When the investigated SiGe layers are incorporated in the buried channel field effect transistor structures on (110) Si wafers a significant thickening at the active windows edge is observed. It is speculated that this effect is connected with elastic SiGe relaxation caused by a non optimized process temperature.     

Electrical and memory properties of Si3N4 MIS structures with embedded Si nanocrystals

Memory structures with an embedded sheet of separated Si nanocrystals were prepared by low pressure chemical vapour deposition using a Si3N4 control layer and different Si2O2 or Si3N4 tunnel layers. It was obtained that Si nanocrystals improve the charging behaviour of the MNOS structures. Memory window width of 1.3 V and 2.0 V were obtained for pulse amplitudes of +/-9 V and +/-10 V, 100 ms, respectively. The extrapolated memory window after 10 years is about 15% of its initial value.     

Self-organization of porphyrin structures on Si

The system produced by deposition of aqueous solution (10^− 3–10^− 6 M) of mesotetra(4-sulfonatophenyl)porphine (TPPS₄) on Si was studied. The morphology of self-organized structures of TPPS₄ was investigated by atomic force microscopy (AFM). The rod- and fiber-like structures of 250–1000 nm width and 30–200 nm height have been found on glass substrates and crystalline Si (c-Si). The aggregation state of TPPS₄ was determined by analysis of optical absorption of aqueous solutions in a visible spectral range and spectroscopic ellipsometry of TPPS/Si samples in the region 1–5 eV. The deposition of aqueous solution of metal-free TPPS₄ and Fe-TPPS₄ was examined. The difference in the adsorption of TPPS₄ on c-Si, porous Si and glass substrates was discussed. The self-ordered ring effect caused inhomogeneous distribution of Fe-TPPS₄ on the c-Si surface. From optical studies, it was found that in TPPS/Si samples formed from aqueous solution of TPPS₄ of high acidity the optical response of J-aggregates was dominant. In contrast, the fine structure in the optical spectra of the samples Fe- TPPS₄/Si corresponded to the excitation of monomers in aqueous solution independently on the pH-value of deposited aqueous solutions. The dependence of spectra for TPPS/Si samples on substrate type, orientation of c-Si, and acidity of aqueous solution was discussed in order to clarify the mechanism of porphyrin adsorption on Si.     

Makyoh topography for the morphological study of compound semiconductor wafers and structures

The application of Makyoh topography in the assessment of flatness and overall curvature of semiconductor wafers and layer structures is reviewed, with special emphasis on compound semiconductors. Application examples are shown as well.     

Comparison of ordered structure in buried oxide layers in high-dose, low-dose, and internal-thermal-oxidation separation-by-implanted-oxygen wafers

The ordered SiO₂ in the buried oxide (BOX) layer of high-dose, low-dose, and internal-thermal-oxidation (ITOX) separation-by-implanted-oxygen (SIMOX) wafers was investigated by X-ray diffraction. From the results, it was found that the SiO₂ molecules in the low-dose and ITOX SIMOX wafers are better ordered than those in the high-dose SIMOX wafer and that the ordered structure of the ITOX layer is different from that of the originally formed BOX layer, suggesting that the ITOX layer has a structure similar to that of the ordered SiO₂ in the thermal oxide layer.     

AES/XPS Thickness measurement of the native oxide on single crystal Si wafers

The thickness determination of a native oxide film at the semi-conductor surface is the subject of this work. The study has been carried out on silicon wafers obtained by produ$\text{c}$tion plant and after different shaping operations. XPS and Auger spectroscopies have been used to evaluate and characterize the oxide layer. Five different methods of measurement, based on spectroscopic parameters proportional to the film width, are proposed. The results obtained show that Auger spectroscopy gives the simplest and the fastest method to measure the native oxide thickness routinely.     

基于光声差分衰减频谱的骨质评估仿真研究

光声层析成像术(Photoacoustic Tomography,PAT)在骨组织微结构的量化评估方面具有潜力,但在传统PAT工作模式下,松质骨的固液两相多孔结构导致骨小梁等分布式光吸收成分激发的光声信号混叠,增加了定量分析骨微结构特性的难度和复杂度。针对这一问题,文章将PAT系统改进为偏心激励-差分检测模式,获取差分衰减频谱(Differential Attenuation Spectrum,DAS);并通过数值仿真计算和验证了松质骨孔隙率与光声差分衰减频谱特征参数的相关性。研究结果表明：提取的光声差分衰减频谱特征参数与骨头孔隙率呈强线性相关,基于光声差分衰减频谱的分析方法可有效实现骨质定量评估和诊断。     

退火温度对硅基溅射银膜微结构和应力的影响

用直流溅射法在硅(111)基底上制备银膜，膜厚为380nm。用BGS6341型电子薄膜应力分布测试仪对膜应力随退火温度的变化进行了研究，结果表明：膜应力随着退火温度的升高而增大，在400℃退火温度下膜应力变化明显。用MXP18AHF型X射线衍射仪测量了膜的衍射谱，对膜微结构随退火温度的变化进行了讨论。制备的Ag膜仍为面心立方结构，呈多晶状态，平均晶粒尺寸为23．63nm，薄膜晶格常数(0．40805nm)比标准样品晶格常数(0．40862nm)稍小。     

Functional properties of microstrip Si detector structures with implanted p-n junctions

The basic properties of microstrip Si detector structures with p-n junctions and the technological procedures for their production are described. Some results of manufacturing strip detectors with 200 μm and 20 μm strips are presented.     

Formation of through-holes in Si wafers by using anodically polarized needle electrodes in HF solution

Electrochemical pore formation in Si using an anodized needle electrode was studied. In the electrochemical process, a Pt, Ir or Pd needle with a diameter of 50-200 μm was brought into contact at its tip with a Si wafer, which was not connected to an external circuit, in HF solution. By applying an anodic potential to the needle electrode against a Pt counter electrode, a pore with a diameter slightly larger than the diameter of the needle electrode was formed in both p-type and n-type Si, of which current efficiency was higher for n-type Si. Through-holes were electrochemically formed in p-type and n-type Si wafers at speeds higher than 30 μm min(-1) using a sharpened Ir needle electrode. A model was proposed to explain the results, in which the pore formation was attributed to successive dissolution of Si atoms near the 3-phase (Si/metal/HF solution) boundary by positive holes injected from the needle electrode to the surface of Si.     

Piezoelectric transducer arrangement for the inspection of large composite structures

The aim of the present work is to develop a system of smart devices that could be permanently attached on the surface of the composite structure and monitor the interaction of low-frequency Lamb waves with defects. A linear array of transmitters would generate a relatively uniform wavefront allowing the inspection of large areas with a limited number of sensors. The asymmetric A₀ Lamb mode is generated in carbon fibre reinforced plastic (CFRP) quasi-isotropic laminates using an array of thin piezoceramic transmitters operating in-phase. In this paper, the effect of damage size on the propagation of Lamb waves is presented. Experimental verification is also presented in multidirectional CFRP composite panels. Critical size impact damage is detected. Finally, the technique is applied to a stiffened panel. Damage on the skin or flange is detectable while damage on the web or cap is not detected with the current experimental set up.     

Piezoelectric actuators and sensors location for active control offlexible structures

A method for optimal positioning of piezoelectric actuators and sensors on a flexible structure is presented. First, a two-dimensional (2-D) model of a piezoelectric actuator bonded to a plate is obtained. Then, a Ritz formulation is used to find a state model of the system in view of its control. To define an optimal positioning strategy, an energy based approach is developed. This leads quite naturally to the study of controllability and observability properties of the overall dynamical model. A new criterion based on energy assessment is proposed to locate actuators and sensors