MEMS薄膜中的残余应力问题

在制造微电子机械系统(MEMS)的器件的过程中,通常要进行高温的薄膜淀积或生长,因此薄膜中存在的残余应力很多情况下影响着器件结构的特性,有时甚至严重劣化器件的性能。本文以实例具体分析了薄膜残余应力的影响,并介绍了残余应力的起源、产生机制以及控制。     

Crystallographic evaluation of ZnSe crystals by x-ray diffraction

Crystallographic quality and the lattice constant of ZnSe crystals grown from Te/ Se solutions by the temperature gradient solution growth method were evaluated by using a high resolution x-ray diffractometer. The full width at half maximum of the x-ray rocking curve was 5.7 sec, a value almost equivalent to that of GaAs. The distribution of crystallographic properties along the growth direction was nearly the same excepting just on the heat-sink. The accurate lattice constant of the ZnSe crystal measured by this system was 5.6700 ± 0.000025 A.     

High-energy X-ray diffraction and topography investigation of CdZnTe

High-energy transmission x-ray diffraction techniques have been applied to investigate the crystal quality of CdZnTe (CZT). CdZnTe has shown excellent performance in hard x-ray and gamma detection; unfortunately, bulk nonuniformities still limit spectroscopic properties of CZT detectors. Collimated high-energy x-rays, produced by a superconducting wiggler at the National Synchrotron Light Source’s X17B1 beamline, allow for a nondestructive characterization of thick CZT samples (2–3 mm). In order to have complete information about the defect distribution and strains in the crystals, two series of experiments have been performed. First, a monochromatic 67 keV x-ray beam with the size of 300×300 μm² was used to measure the rocking curves of CZT crystals supplied by different material growers. A raster scan of a few square centimeter area allowed us to measure the full-width at half-maximum (FWHM) and shift in the peak position across the crystal. The rocking curve peak position and its FWHM can be correlated with local stoichiometry variations and other local defects. Typically, the FWHM values ranging from 8.3 arcsec to 14.7 arcsec were measured with the best crystal used in these measurements. Second, transmission white beam x-ray topography (WBXT) was performed by using a 22 mm×200 μm beam in the energy range of 50 keV to 200 keV. These types of measurements allowed for large area, high-resolution (50 μm) scans of the samples. Usually, this technique is used to visualize growth and process-induced defects, such as dislocations, twins, domains, inclusions, etc. the difference in contrast shows different parts of the crystal that could not be shown otherwise. In topography, good contrast is indicative of a high quality of the sample, while blurred gray shows the presence of defects. Correlation with other techniques (e.g., infrared (IR) mapping and gamma mapping) was also attempted. Our characterization techniques, which use highly penetrating x-rays, are valid for in-situ measurements, even after electrical contacts have been formed on the crystal in a working device. Thus, these studies may lead to understanding the effects of the defects on the device performance and ultimately to improving the quality of CZT material required for device fabrication. It is important to study crystals from different ingot positions (bottom, center, and top); consequently, more systematic studies involving scans from center to border are planned.     

Measuring stiffnesses and residual stresses of silicon nitride thin films

The mechanical deflection of circular membranes of SiN _x is presented as a technique for measuring the stiffness and residual stress of very thin, single-layer films. The dimensions of the membranes are controlled precisely using standard photolithography, dry etching and wet etching techniques. Thicknesses vary between 0.09 μm and 0.27 μm and average diameters range between 1100 μm and 4100 μm. A Nanoindenter is used to deflect the membranes with a point force at their centers, and to continuously record the applied forces and the resulting deflections. The analysis of the force-deflection data yields the values of Young’s moduli and residual stresses for the films.     

X射线衍射形貌术在碲锌镉晶体中的应用

碲锌镉晶体中存在着各种典型晶体缺陷,X射线衍射形貌术是一种非破坏性地整体研究晶体材料结构完整性、均匀性的有效方法。本文将反射式X射线衍射形貌术应用于碲锌镉晶体质量的评价,研究了入射线狭缝宽度、积分时间、扫描步长等测试参数以及样品表面加工状态对X射线衍射形貌的影响。结果表明入射线狭缝宽度对碲锌镉晶体的X射线衍射成像及晶体质量筛选应用影响很大,积分时间、样品扫描步长等测试参数的选择与入射线狭缝宽度密切相关。     

Composition and thickness control of thin LPE HgCdTe layers using x-ray diffraction

Double-axis x-ray rocking curve measurements have been used to nondestructively characterize the composition profile of HgCdTe heterojunction photodiode structures grown by liquid phase epitaxy (LPE). In particular, the thickness and composition profile of the thin graded-composition cap layer are determined through an empirical correlation between rocking curve parameters and composition profiles measured by SIMS. Spatial maps of cap layer thickness and composition are generated from automated measurements of x-ray rocking curves across a wafer. X-ray mapping has been instrumental in improving the spatial uniformity of cap layers and in maintaining control of the growth process in Hg-rich LPE dipping reactors.     

碲锌镉晶体的X射线衍射形貌与腐蚀形貌

碲锌镉晶体中存在着各种典型晶体缺陷,其缺陷研究一直倍受关注,X射线衍射形貌术是一种非破坏性地研究晶体材料结构完整性、均匀性的有效方法.采用反射式X射线衍射形貌术对碲锌镉衬底的质量进行了研究,并将衬底的X射线衍射形貌与Everson腐蚀形貌进行了对比分析,碲锌镉衬底的X射线衍射形貌主要有六种特征类型,分别对应不同的晶体结构或缺陷,包括均匀结构、镶嵌结构、孪晶、小角晶界、夹杂、表面划伤,对上述特征类型进行了详细的分析.目前,衬底的X射线衍射形貌主要以均匀结构类型为主,划伤和镶嵌结构缺陷基本已消除,存在的晶体缺陷主要以小角晶界为主.通过对比分析碲锌镉衬底和液相外延碲镉汞薄膜的X射线衍射形貌,发现小角晶界等晶体结构缺陷会延伸到外延层上,碲锌镉衬底质量会直接影响碲镉汞外延层的质量,晶体结构完整的衬底是制备高质量碲镉汞外延材料的基础.     

Thermal stresses developed in thin films of germanium

Deposition at an elevated surface temperature introduces a stress through differential expansion of the film and the substrate. In the present paper the origin, cause and sources of such stresses are briefly described. Crystalline films of germanium were grown by vacuum evaporation on cleaned microscope glass slides and also on the cleavage faces of rock salt. From the study of thermal stresses developed it is concluded that the stresses developed would be compressive so long as the temperature of measurement is less than the temperature of deposition, when the tensile stress would take over.     

Texture and microstructure of thin copper films

Microstructure is an important factor influencing the reliability of thin film interconnects. The microstructure of copper films is of particular interest because of its use in numerous electronic applications. Pole figure x-ray diffraction and transmission electron microcopy were conducted on copper films deposited by several techniques: sputtering, partially ionized beam deposition, chemical vapor deposition, evaporation, and electroplating. Quantitative texture data are determined from fiber texture plots. A typical copper film consists of three texture components: (111), (200), and random. (220) and (511) texture components are possible under some deposition conditions. Compared to aluminum films, the fraction of the random texture component and the distribution of the (hkl) components in copper films are relatively large. Bimodal grain size distributions are observed in some films.     

The structural changes in CdS-CdTe thin films due to annealing

Thin film solar cells based upon CdS-CdTe heterojunctions have become an important alternative to silicon based devices. The film structures formed during fabrication are critical to cell efficiency and thus their study is fundamental to improving device performance. We have used synchrotron x-ray diffraction to investigate the effect of a post deposition anneal upon the film structures and, in particular, have examined the dynamic formation of intermixed regions adjacent to the original, metallurgical interface. Our results have enabled us to produce a dynamic model for the structural changes which includes the extent of interdiffusion. We show that, for a 400 nm CdTe film in the presence of chlorine, the original CdS and CdTe layers are completely transformed into layers with average compositions CdS_0.93Te_0.07 and CdTe_0.94S_0.06, respectively. We present evidence that the interdiffusion occurs during or following a recrystallization and that, to a limited extent, these changes also occur without chlorine.     

Study of indium droplets formation on the InxGa1−xN films by single crystal x-ray diffraction

Indium droplet formation during the epitaxial growth of In_xGa_1−xN films is a serious problem for achieving high quality films with high indium mole fraction. In this paper, we studied the formation of indium droplets on the In_xGa_1−xN films grown by metalorganic chemical vapor deposition (MOCVD) using single crystal x-ray diffraction. It is found that the indium (101) peak in the x-ray diffraction spectra can be utilized as a quantitative measure to determine the amounts of indium droplets on the film. It is shown by monitoring the indium diffraction peak that the density of indium droplets increases at lower growth temperature. To suppress these indium droplets, a modulation growth technique is used. Indium droplet formation in the modulation growth is investigated and it is revealed in our study that the indium droplets problem has been partially relieved by the modulation growth technique.     

Synchrotron white beam x-ray topography analysis of MBE grown CdTe/CdTe (111)B

The structural quality of CdTe(111)B substrates and MBE grown CdTe epilayers is examined with synchrotron white beam x-ray topography (SWBXT). Reflection SWBXT indicates that CdTe substrates with comparable x-ray double crystal rocking curve full width at half maximum values can have radically different defect microstructures, i.e. dislocation densities and the presence of inclusions. Dislocation mosaic structures delineated by SWBXT are consistent with the distribution of etch pits revealed by destructive chemical etch pit analysis. Direct one-to-one correspondence between distinct features of the topographic image and individual etch pits is demonstrated. Clearly resolved images of individual dislocations are obtained by carrying out transmission SWBXT. Our investigation demonstrates how, the extent of twinning in a CdTe epilayer is strongly influenced by the quality of the defect microstructure, and how dislocations propagate from an inclusion.     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

镁掺杂对ZnO薄膜光诱导亲水性的影响

采用溶胶-凝胶法在Si(111)制备了一系列Mg掺杂的ZnO薄膜。用X射线衍射仪、原子力显微镜和接触角测试仪测量薄膜的微结构、表面形貌和表面接触角。结果表明：Mg掺杂ZnO薄膜仍为六角纤锌矿型结构,所有薄膜均具有较好的c轴择优取向。随着Mg掺杂含量的增加,薄膜的粗超度从2.14nm增大到9.56nm,薄膜表面接触角由89? 减小到 82?。通过对薄膜交替进行紫外光照和黑暗放置(或热处理),可以实现其表面疏水与超亲水性之间的可逆转化,光诱导可逆转化效率随Mg掺杂含量的增加而增大。     

Research of mechanical stresses in irradiated tin-doped silicon crystals

An optical method of registration of mechanical stresses in undoped and tin-doped silicon samples is offered. Influence of electron irradiation on energy 5 MeV and high-temperature treatment at a 723 K on residual stresses in a silicon lattice was analyzed in the paper. The proposed method is based on а modulation of polarization of laser radiation transmitted through the anisotropic area and the definition of its anisotropy parameters by means of this modulation. The modulation polarimetry technique is an express method with high detection and resolution. The method allows identifying residual stresses in samples in absolute units with a resolution of 1·10⁻⁴ MPa.     

The role of lattice mismatch in the deposition of CdTe thin films

Cadmium telluride (CdTe) is the most well-established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high-quality thin films is of utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition (PLD) technique. Common to all substrates was the strong tendency toward the preferential alignment of CdTe’s (111) planes parallel to the substrate’s surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single-crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. The film quality showed a strong correlation with lattice mismatch. Texture analysis using the [111] pole figure confirmed that improvements in the lattice mismatch led to a higher degree of in-plane alignment of the (111) grains.     

Microstructures in CoPtC magnetic thin films studied by superpositioning of micro-electron diffraction

Cross-sectional transmission electron microscopy observation of CoPtC thin films showed that 10 nm sized ultrafine particles of CoPt typically were elongated along the substrate normal. Analysis of the superposition of 40 micro-electron diffraction patterns showed that there was no preferred crystal orientation of CoPt particles. This superpositioning technique can be applied to thin films, whose X-ray diffraction analysis is difficult due to the small size of the crystals.     

Characterization of (111) Gaas and (111) InP substrates and homoepitaxial layers by divergent x-ray beam diffraction

The back-reflection diffraction of a divergent x-ray beam has been studied for the characterization of (111) GaAs and (111) InP substrates and homoepitaxial layers with different states of surface perfection. Diffraction conditions for generating back-reflection pseudo-Kossel patterns from (111) GaAs and (111) InP are presented. Mechanical polishing was observed to produce x-ray diffraction line broadening. Uneven line broadening was found to be produced by an inhomogeneous distribution of dislocations. The diffraction angles for pseudo-Kossel lines were influenced by subgrain tilting in epitaxial, LPE-grown layers. A close relationship between diffraction line profiles and surface morphology of the epitaxial layer was demonstrated with interference-contrast optical microscopy.     

A study of etch pit density and x-ray rocking curves for GaAs substrate evaluation

The relationship of structural defects to the electrical properties of semiconductor materials is discussed. Etch pit density (EPD) measurements are normally used to evaluate dislocation density. A nondestructive, quantitative method for evaluation of substrate defect populations is needed for quality assurance. In this study, double crystal x-ray diffraction rocking curves are investigated for this purpose. Rocking curve widths are determined experimentally for a set of GaAs substrated with a range of EPD. The experimentally determined values are also compared with those calculated from simulated rocking curves based on x-ray diffraction theory. Limited correlation between rocking curve widths and EPD is observed. Formerly of Spectrum Technology Inc., Holliston, MA     

Structure and property characterization of low-k dielectric porous thin films

A novel methodology is developed that uses a combination of high energy ion scattering, x-ray reflectivity, and small angle neutron scattering to characterize the structure and properties of porous thin films. Ion scattering is used to determine the elemental composition of the film for absolute intensity calibration of the x-ray and neutron scattering techniques. X-ray reflectivity is used to measure the average electron density and film thickness. Small angle neutron scattering is used to determine the pore size, structure, and connectivity. Combining information from all three techniques, the film porosity and matrax material density can be uniquely determined.