The sawing of silicon wafers with diamond coated wires still requires further development for a widespread application in the photovoltaic industry. The technique has the potential for a cost reduction due to higher cutting rates and the use of water as a low-cost cooling fluid, but it is also necessary to integrate the technique into the established processing chains particularly for sawing multicrystalline silicon (mc-Si). One of the requirements is an increasing industrial demand on the wafer surface quality, such as the optical appearance, the total thickness variations (TTV), the etching behavior and the sub-surface and surface damage, which determines the mechanical wafer stability. The goal of this work is to analyze the impact of different wire velocities on the surface damage of multicrystalline silicon wafers. First, the distribution of amorphous regions was measured using Raman microscopy. The results reveal slightly higher local fractions of the amorphous phase with increasing wire velocity. This also correlates with more scattering and higher inhomogeneity in the surface roughness values. Furthermore, the microcrack depths were analyzed on polished and etched bevel cut samples of wafers using confocal laser scanning microscopy (CLSM). Additionally, the present study investigates the impact of cleaning procedures and different grain orientations on the sawing damage characteristics
Mechanical preparation of mono-and multicrystalline silicon wafers by sawing, lapping, or grinding damages the surface. Depending on the mechanical impact of the treatment different degrees of damage structures occur. It is necessary for further processing of wafers to have damage free and contactless methods to characterize structures such as changes of the lattice structure by phase transformations or plastic deformations, and microcracks below the surface. In this paper we present results of investigations on the sub-surface damage by two contactless optical methods: Scanning infrared reflection polarimetry with SIREX and Raman spectroscopy. The investigations were carried out on silicon wafers sawn with the multi-wire technique using diamond coated wires. The sub-surface damage mainly consists of microcracks of different length, which penetrate several micrometers into the bulk. The results were compared with confocal microscopy and scanning electron microscopy analyses of the same damage structures.
The fracture strength of silicon wafers used for photovoltaic and microelectronic applications mainly depends on the damage structure, which is introduced on the surface during processing of the wafers. The present paper investigates the formation and development of the damage structure by scratching, which occurs during grinding processes or when sawing with diamond coated wires. The basic scratching process has been studied with a newly developed scratch technique, where test parameters comparable to a real process could be used. Single scratch tests have been performed with diamond particles on monocrystalline silicon wafers with a defined surface orientation and under different applied forces. The resulting microcrack structure, which develops under the surface, has been investigated by confocal laser scanning microscopy, scanning electron microscope and Raman spectroscopy. Details of the shape, depth and distance of the cracks have been obtained by high quality sample cross sections. The orientations of the main microcrack planes are determined
The purpose of this work is to verify the possibility to process highly doped Si supporting substrates using a 2-step process: (i) sintering of a low-cost Si powder based ingot using hot pressing and a ELKEM Silgrain material as a feedstock; and (ii) wafering of such ingots using multi-wire sawing technique similar to that, which is used for cast multi-Si or Cz-Si grown ingots. Moreover, the possibility to dope Si powder ingot at sintering temperatures below melting point of Si, using a mixture of Si and boron powders to fabricate highly conductive Si wafers is verified as well. The slurry technique has been chosen for multi-wire sawing of sintered Si powder based ingots. Surface properties of Si powder based substrates as well as their chemical composition have been studied by optical microscopy imaging and energy-dispersive X-ray spectroscopy (EDX). Although the overall concentrations of oxygen, carbon and possibly also metal impurities, which are initially present in a low-cost Si feedstock, are too high to achieve acceptable semiconducting properties, it is concluded, that sintered Si powder based wafers have high enough conductivity (resistivity similar to 0.001 Omega cm) to serve as supporting substrates for low cost Si wafer equivalent structures
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