Advanced Materials Research Vols. 76-78

Abstract: Current polishing pads cannot polish a workpiece without using slurry with free abrasive. The new slurry is required to be continually poured into the working area, so more than half of the slurry may be lost from the table without contacting the wafer surface; this leads to economic and environmental problems. In the current work, the fixed abrasive pad was used, where nano-sized diamond abrasives were embedded in the polishing pad; distilled water, rather than slurry, was used. The effect of various fixed abrasive pad designs on polishing characteristics during silicon wafer polishing was investigated. Moreover, the primary function of fixed abrasive was to remove the rough parts of silicon wafer as they were being polished. Consequently, it needed to disperse the nano-sized abrasives into the pad material with high hardness value; this way, working abrasives are not pressed into the pad material. Furthermore, with the use of a pad conditioner, the interior working abrasives were exposed to the pad surface. As a result, the best outcome of using the fixed abrasive pad with a nano-sized diamond was a surface roughness of Ra 0.47 nm.

Abstract: Laser micromachining has been widely used for micro-component fabrication of various materials, such as silicon substrates where silicon wafer is ablated accurately and precisely through marking, scribing, drilling or dicing. Thermal damages can occur on the substrates when improper process parameters and methods are used. This paper presents a review on the micromachining of silicon substrates using conventional and novel lasers as well as water-assisted laser micromachining technologies. The basic concepts and approaches of the technologies are discussed along with the challenges to damage-free laser micromachining at commercially acceptable cutting rates.

Abstract: Multi-wire sawing process with slurry has been popularly adopted for wafer slicing of silicon substrates for solar cells. This paper is to investigate the chip size estimation for effective blending ratio of mixing slurry of wire sawing. Different combination of slurry has been studied with microscopic pictures of abrasive grits by SEM and distribution of particle size of slurry. The chip size can be estimated by the developed method and then the TTV of sliced substrates is used to evaluate the blending ratio of slurry for cost efficiency of wire sawing. Experimental results indicate that the slurry with certain ratio of reborn grits increases the TTV value of sliced substrates. As the weight percentage of silicon chips is above 6 % for the blending ratio of 50% reborn abrasive grits after sawing run 2, the slurry is suggested to be changed to new slurry for wire sawing. Under this developed rule, the average TTV can be maintained under 0.016 mm for nominal thickness of 200 m silicon substrates. Results of this paper can be used to estimate the feasible blending ratio for maintaining the TTV under the desired specifications of silicon substrates for cost efficiency.

Abstract: Chemo-mechanical grinding (CMG) process is a promising process for large-sized Si substrate fabrication at low cost. An encountered issue in current CMG process of Silicon (Si) wafers is metallic contaminations on ground Si wafer surface, which is attributed to the existence of sodium carbonate in wheel compounds. In this paper, four different CMG wheels were developed and grinding experiments were conducted to study the effects of exclusion of sodium carbonate and concentration of ceria abrasive on grinding performance. The grinding characteristics of the four wheels were analysized and discussed to reveal the effects of different compositions.

Abstract: The demand for extremely-thin Si wafers is expanding. Current manufacturing technologies are meeting great challenges with the continuous decrease in Si wafer thickness. In this study, a novel single step thinning process for extremely thin Si wafers was put forward by use of an integrated cup grinding wheel (ICGW) in which diamond segments and chemo-mechanical grinding (CMG) segments are alternately allocated along the wheel periphery. The basic machining principle and key technologies were introduced in detail. Grinding experiments were performed on 8-in. Si wafers with a developed ICGW to explore the minimal wafer thickness and grinding performance. The experimental results indicate that the proposed grinding process with the ICGW is an available thinning approach for extremely thin Si wafer down to 15μm

Abstract: Polycrystalline ingot slicing by wire electric discharge machining (W-EDM) has been investigated to reduce kerf loss and wafer thickness. In order to use the sliced wafers for semiconductor devices, the modified surface layer induced by W-EDM must be removed. In this paper, we have demonstrated the elimination of the layer by abrasive blasting. Three types of abrasives were blasted at a speed of 100 m/s. The surfaces blasted with WA #1000 and GC #1000 were smoother than that sliced with a wire saw. The modified layer induced by W-EDM slicing could be removed by blasting with WA #1000 while scanning the surface three times. Solar cells were fabricated using wafers with the blasted surface with an efficiency of 15.2%, which was almost the same as that of cells fabricated from the wire-sliced wafers.

Abstract: Many models [1-3] have been proposed to study the infeed grinding of Si wafers and to understand its effects on the surface generation. However, most exiting models are based on 2D/3D kinematical analysis of the wheel or its cutting edges, thus unable to suggest the behavior at the interface of wheel/substrate including effects of the elastic deformation and the cutting path density. In this paper, a new grinding model of capable to incorporate the machine tool stiffness and the substrate contact rigidity has been developed. By taking the effect of cutting path density into account, this model is able to give a better explanation of interference between the cutting edge and substrate, and the geometrical profile generated in an infeed grinding scheme.

Abstract: Ultraprecision diamond-ground silicon wafers were irradiated by a high-frequency nanosecond pulsed Nd:YAG laser equipped on a four-axis numerically controlled stage. The resulting specimens were characterized using a white-light interferometer, a micro-Raman spectroscope and a transmission electron microscope. The results indicate that around the laser beam center where the laser energy density is sufficiently high, the grinding-induced amorphous silicon was completely transformed into the single-crystal structure. The optimum conditions for one- and two-dimensional overlapping irradiation were experimentally obtained for processing large-diameter silicon wafers. It was found that the energy density level required for completely removing the dislocations is higher than that for recrystallizing the amorphous silicon. After laser irradiation, the surface unevenness has been remarkably flattened.

Abstract: Chemical mechanical polishing (CMP) is a technique used in semiconductor fabrication for planarizing the top surface of an in-process semiconductor wafer. Especially, Post-CMP thickness variations are known to have a severe impact on the stability of downstream processes and ultimately on device yield. Hence understanding how to quantify and characterize this non-uniformity is significant step towards statistical process control to achieve higher quality and enhanced productivity. The main reason is that the non-uniformed interface between the wafer and the machine-pad adversely affects the polishing performance and ultimate surface uniformity. The purpose of this paper is to suggest a new measure that estimates the uniformity of wafer surface considering the difference of the amount of abrasion between the center and the edge. This new measure which is called the Coefficient of Uniformity is defined as the following ratio: Geometric Mean (GM) / Arithmetic Mean (AM). This metric can be evaluated regionally to quantify the non-uniformity on the wafer surface from the center to the edge. Further simulations show that this new measure is insensitive to shift of the wafer center and sensitive to shift of the wafer edge. This trend indicates that this new measure is a very useful to test the non-uniformity of wafer after CMP polishing.

Abstract: The principle of stereopsis involves measuring an object’s geometry from a pair of images taken at slightly different viewing positions. This technique is frequently used for geographical mapping in satellite-based reconnaissance, however, the same practice has not been reliably applied at the other end of the scale spectrum: i.e. optical microscope imaging. The impediments have been identified and addressed in this work, concluding that optical stereopsis can be applied to microscopical surface examinations, and that the resulting digital elevation models can be of particular use in tribological investigations for performance and failure analysis.