Investigations on the Influence of Surface Textures on Optical Reflectance of Multi-crystalline Silicone (MC-Si) Crystal Surfaces-Simulations and Experiments

MC-Si is the most widely used material for making solar PV cells. In spite of the considerable research on improving the conversion efficiency of MC-Si solar PV cells still it remains well within the range of 15-20%. Optical reflectance being the major loss of incident solar energy, efforts are being made to reduce the optical reflectance of solar cell surfaces. Among the several methods proposed, creation of well-defined surface topography on the cell surface remains a promising option. Micro/nano level features with various dimensions and distributions have been created on MC-Si crystal surfaces using a femto-second pulsed laser and the influence of surface topography on optical reflectance in the incident light wave length of 350 – 1000 nm have been studied and compared with the simulation results obtained using OPAL2 software. Experimental results indicate that surface textures on the wafer surface lead to the reduction of optical reflectance in the range of 20-35% in comparison with plain surface. Width of micro grooves have less significant effect on the optical reflectance in comparison with pitch between the micro grooves. Best reduction in reflectance is exhibited by the texture having a groove width of 30 mm and a pitch of 100 mm. A post texturing etching operation is found to have detrimental effect on the ability of micro/nano level features in decreasing the optical reflectance in the preferred wavelength of solar spectrum due to the flattening of nano level features created within the micro grooves due to laser texturing.

Prediction And Verification of Wafer Surface Morphology in Ultrasonic Vibration Assisted Wire Saw (UAWS) Slicing Single Crystal Silicon Based On Mixed Material Removal Mode

Monocrystalline silicon is one of the most important semiconductor materials, widely used in chip manufacturing, solar panels. Slicing is the first step in making chips and the surface quality of silicon wafers directly affects the quality of later processing and accounts for a large proportion in the chip manufacturing cost. Ultrasonic vibration assisted wire saw (UAWS) is an effective sawing process for cutting hard and brittle materials such as monocrystalline Si, which can significantly improve the surface quality of silicon wafers. In order to further study the formation mechanism of the surface morphology of single crystal silicon sliced by UAWS, a new model for prediction of wafer surface morphology in UAWS slicing single crystal silicon based on mixed material removal mode is presented and verified in this paper. Firstly, the surface model of diamond wire saw tool is established by equal probability method. Then according to the equation of transverse vibration dynamics about the wire saw with ultrasonic excitation, the trajectory equation of arbitrary abrasive particles on the surface of wire saw is derived and analyzed. Thirdly, a new model for prediction of the wafer surface morphology based on mixed material removal mode is presented, which can be used to predict the wafer surface morphology of single crystal silicon sliced by UAWS. Finally, the prediction model is verified by UAWS slicing experiment, and the effects of slicing parameters such as wire saw speed, feed speed and workpiece rotate speed on the surface quality of silicon wafer were studied. It shows that the predicted wafer surface morphology and the experimental wafer surface morphology are similar in some characteristics, and the average error between the experimental and the theoretical values of the wafer surface roughness is 11.9%, which verifies the validity of the prediction model.

Role of Slurry Additives on Chemical Mechanical Planarization of Silicon Dioxide Film in Colloidal Silica Based Slurry

Chemical mechanical planarization (CMP) is a critical process for smoothing and polishing the surfaces of various material layers in semiconductor device fabrication. The applications of silicon dioxide films are shallow trench isolation, an inter-layer dielectric, and emerging technologies such as CMOS Image Sensor. In this study, the effect of various chemical additives on the removal rate of silicon dioxide film using colloidal silica abrasive during CMP was investigated. The polishing results show that the removal rate of silicon dioxide film first increased and then decreased with an increasing concentration of K+, pH, and abrasive size. The removal rate of silicon dioxide film increased linearly as the abrasive concentration increased. The influence mechanisms of various additives on the removal rate of silicon dioxide film were investigated by constructing simple models and scanning electron microscopy. Further, the stable performance of the slurry was achieved due to the COO- chains generated by poly(acrylamide) hydrolysis weaken the attraction between abrasives. High-quality wafer surfaces with low surface roughness were also thus achieved. The desirable and simple ingredient slurry investigated in this study can effectively enhance the planarization performance, for example, material removal rates and wafer surface roughness.

Obtainment of Residual Stress Distribution from Surface Deformation under Continuity Constraints for Thinned Silicon Wafers

Precision machining (e.g., fine grinding, polishing) induced residual stress is very small and often not constant across the wafer and it is difficult to be directly obtained by stress testing equipment or Stoney equation. The residual stress could be obtained theoretically based on the principle of superposition in which the entire wafer deformation is taken as the sum of all deformations induced by the residual stresses of different positions on the wafer surface. However, the solved residual stress is affected greatly by deformation measurement errors and fluctuates greatly across the wafer surface. To solve the problem, a regularization method with continuity constraints was proposed in this study. The mechanisms for the discontinuity of the residual stress distribution and the sensitivity of calculation results to the measurement errors were studied. The influences of the number of subareas of the silicon wafer were investigated and the continuity constraint term was constructed based on the positional relationship of different subareas. Stable and continuous residual stress distribution was successfully obtained after using the proposed regularization method. The method may also be applied to estimate the residual stress from surface deformation for thin substrate plates of other materials.

Research on Wafer Surface Defect Pattern Detection Method Based on Incremental Learning

Wafer manufacturing is an important step in quality control and analysis in the semiconductor industry. The defect pattern classification algorithm of wafer maps has received extensive attention from academia and industry. At present, most methods for detecting wafer surface defect patterns focus on static data model classification and analysis. However, in the production process, static data models cannot satisfy the dynamic analysis of wafer defect patterns in the form of streaming data. In this regard, this paper proposes a wafer surface defect pattern detection method based on incremental learning. Our experiment uses Resnet as the backbone network, and the data set uses the WM811K wafer data set. Experiments have proved that our method can achieve better classification accuracy in the field of wafer defect detection, which provides the possibility for continuous learning of wafer defects in the future.

Research on The Bonding Properties of Vitrified Bonds With Porous Diamonds and The Grinding Performance of Porous Diamond Abrasive Tools

Ordinary diamond presents the disadvantages of poor self-sharpening and concentrated grinding stress when it is used as an abrasive. Moreover, this kind of diamond cannot be well wetted by the vitrified bond, resulting in a lower holding force of the binder to the abrasives (i.e., the diamond is easy to detach from the binder matrix during grinding). These comprehensive factors not only reduce the surface quality of the processed workpiece, but also hinder the processing efficiency. In order to solve these problems, a new type of porous diamond with high self-sharpening properties was prepared using a thermochemical corrosion method in this study. Our results showed a great improvement in pore volume and specific surface area of the porous diamond compared with ordinary diamond abrasive particles, and the holding force and wettability of vitrified bond to the porous diamond abrasive particles were also improved. Compared with ordinary diamond abrasive tools, porous diamond abrasive tools showed a 29.6% increase in grinding efficiency, a 15.5% decreased in grinding ratio, a 27.5% reduction in workpiece surface roughness, and the scratches on the silicon wafer surface were reduced and refined.

Variation of the silicon optical parameters after rapid heat treatment

The results of the effect of rapid heat treatment on the optical characteristics of a silicon wafer surface in the region of the G-point in the Brillouin zone are presented for different types of silicon wafers conductivity, their doping level, the covalent radii of dopants and the crystallographic orientation of the wafer surface. The absorption coefficient and refractive index of the initial 100 mm diameter samples KDB-12 <100>, KDB-10 <111>, KDB-0.005 <100> and KES-0.015 <100>, underwent standard chemical-mechanical polishing, was measured on a Uvisel 2 ellipsometer (Horiba Scientific, France) in the spectral range 0.6–6.0 eV (200–2100 nm) before and after rapid heat treatment. The incidence angle of the light beam was 70° relative to the sample plane. It is shown that the changes in the optical characteristics of the silicon surface in the spectral region of the location of the G-point in the Brillouin zone after rapid heat treatment is due to a decrease in the surface deformation potential due to solid-phase recrystallisation of the mechanically damaged layer. It has been established that carrying out the rapid heat treatment of silicon samples with a high boron concentration leads to a more significant decrease in the refractive index and absorption compared with silicon with a low boron concentration, due to an increase in the depletion of the silicon surface with boron as a result of diffusion processes at the silicon – silicon dioxide interface.

DEPOSITION OF Al/SnAl OHMIC CONTACT ON SILICON FROM A LIQUID PHASE

Background. Single- and multi-layer metal films are widely utilized in modern electronics and optoelectronics as ohmic contacts. As a rule, the films are deposited by thermal evaporation, ion sputtering and chemical vapour deposition. However the methods of deposition from a liquid phase are the most simple and cost-effective. Thus the ohmic contact deposition by these methods is still an actual problem. Objective. The purpose of the paper is to study the possibility of deposition of multi-layer ohmic metal films over a semiconductor wafer surface from a liquid phase, particularly by scanning liquid phase epitaxy technique. Methods. In this work we considered the influence of a long-term temperature gradient at the interface metallic solution-melt – semiconductor wafer on the possibility of deposition of multi-layer ohmic metal films on the semiconductor wafer surface during segmental contact between the solution-melt and the wafer. For this purpose we carried out the simulation of heat transport process, wafer wetting process as well as the process of wafer cleansing off the solution-melt taking into account capillary phenomena in the mask openings using the method of scanning liquid phase epitaxy. For experimental confirmation of adequacy of the model proposed we carried out the deposition of Al/SnAl layer on silicon wafer in the above mentioned conditions. Results. We have deposited the contact layer Al/SnAl on the surface of silicon wafer from Al-Sn solution-melt by scanning liquid phase epitaxy technique using supplementary heater for the wafer and mask installed in the apparatus. The contact layer is made as three identical pads located at different distance one from each other. By the analysis of current-voltage characteristic we determined that the metallic film contact with the semiconductor is a non-rectifying, i.e. ohmic contact. The specific contact resistance was determined by the Transmission Line Method using linear configuration of the contact pads (LTLM). Its value was 7.2∙10-4 Ohm·cm2. Conclusions. The principal possibility of obtaining of multi-layer ohmic contacts to the semiconductor by scanning liquid phase epitaxy technique in conditions of segmental contact between the solution-melt and the wafer as well as long-term gradient at the contact interface was shown. The conditions were realized by using extra heating of the wafer back side and the high-temperature mask through which the solution-melt contacted the wafer.