In situ compression tests on micron-sized silicon pillars by Raman microscopy—Stress measurements and deformation analysis

Mechanical properties of silicon are of high interest to the microelectromechanical systems community as it is the most frequently used structural material. Compression tests on 8 μm diameter silicon pillars were performed under a micro-Raman setup. The uniaxial stress in the micropillars was derived from a load cell mounted on a microindenter and from the Raman peak shift. Stress measurements from the load cell and from the micro-Raman spectrum are in excellent agreement. The average compressive failure strength measured in the middle of the micropillars is 5.1 GPa. Transmission electron microscopy investigation of compressed micropillars showed cracks at the pillar surface or in the core. A correlation between crack formation and dislocation activity was observed. The authors strongly believe that the combination of nanoindentation and micro-Raman spectroscopy allowed detection of cracks prior to failure of the micropillar, which also allowed an estimation of the in-plane stress in the vicinity of the crack ti

Diamond wire-sawn silicon wafers - from the lab to the cell production

Wafers for the PV industry are mainly sawn with a multi-wire slurry saw. This process is slow (it takes almost half a day to complete a cut) and generates a lot of waste: around half the silicon is sawn away and contaminating the slurry, and the wire is worn and has lost strength. After each cut, the slurry has to be cleaned from the silicon debris and the wire has to be exchanged. In contrast, sawing the wafers with a diamond-plated wire is faster, requires only a cooling liquid that is easy to filter from silicon debris and uses a wire that can be kept for several cut. But this new sawing technique only has a chance to develop if the solar cell production lines developed for slurry sawn wafers is capable of processing these diamond-plated wire sawn wafers efficiently. This study focused on the differences of surface properties of wafers cut via a slurry wire-saw and via a diamond-plated wire-saw. From these surface differences, it is possible to explain the differences in cell processing behaviour and to update the cell production line. Finally, it is shown that wafers sawn with a diamond-plated wire can give cells that are as efficient as the slurry sawn wafers, which validates this new diamond-plated wire wafering method for the production of solar cells

Effect of debris on the silicon wafering for solar cells

The wafers used by the photovoltaic industry are mostly produced by multi-wire slurry sawing. One of the key factors determining the wafer quality (presence of saw marks and chips, increased roughness, wafer thickness variations and wafer strength) is the abrasive slurry. For cost reduction, the slurry is regularly exchanged and the debris it contains is removed in a recycling operation. To optimise the slurry usage, it is of utmost importance to understand the effects of the silicon debris concentration in the slurry. This was studied by sawing several bricks one after the other with the same slurry. It was found that when the amount of debris is too high (more than 4% of the slurry volume), saw marks appear on the wafers and they become more fragile. Finally, a first qualitative model explaining the apparition of the saw marks and the reduction of wafer strength is proposed. (C) 2011 Elsevier B.V. All rights reserved

Wire-sawing processes: parametrical study and modeling

Reducing the wafer breakage rate without changing the wafer thickness and sawing thinner wafers while maintaining constant breakage rate are two possibilities to decrease the costs of solar cells. They are similar in the sense that both require stronger wafers. To achieve this goal, it is important to gain insight into the wire-sawing process, its underlying defect creation mechanisms and the impact of sawing parameters on wafer strength. Consequently, a series of bricks were sawn with different slurry densities, wire tensions and feed rates, and the results were analyzed in terms of the wafer strength measured by bending tests. Roughness and wafer thickness were also measured. It is found that the strongest wafers were obtained by using a low abrasive volume fraction in the slurry, a low wire tension and a slow feed rate. From the analyses, we provide a qualitative interpretation of the effects of the processes at work in slurry-based wafering that explains, for instance, the wafer thickness and roughness variations. Based on physical arguments about the interaction between the wire, the silicon carbide particles and the silicon wafer, a semi-empirical model relating defect creation to the sawing parameters is developed. With this model, the wafer strength distribution can be predicted, thus simplifying optimization of the sawing process. (C) 2014 Elsevier B.V. All rights reserved

In situ compression tests on micron-sized silicon pillars by Raman microscopy: Stress measurements and deformation analysis

ISSN:0884-2914ISSN:2044-532

Influence of abrasive concentration on the qualiy of wire-sawn silicon wafers

The sawing parameters have an impact on the depth of the defects in the wafers, and hence on their mechanical strength. However, as sawing is a highly complex system, the wafering industry is still relying on a “trial and error” approach to improve the sawing parameters. In this contribution, the effects of the abrasive concentration are studied with the help of the “rolling-indenting model”, the model most commonly used to describe the sawing process. From roughness and cracks depth measurement correlated with flexure tests, we show that using a lower silicon carbide concentration in the slurry decreases the depth of the defects as well as the roughness and results in a higher breakage strength of the wafers

Mechanisms of wafer sawing and impact on wafer properties

Silicon wafer wire-sawing experiments were realized with different sets of sawing parameters, and the thickness, roughness, and cracks depth of the wafers were measured. The results are discussed in relation to assumptions underlying the rolling-indenting model, which describes the process. It was also found that the silicon surface at the bottom of the sawing groove is different from the wafer surface, implying different sawing conditions in the two positions. Furthermore, the measured parameters were found to vary along the wire direction, between the entrance of the wire in the ingot and its exit. Based on these observations, some improvements for the wire-sawing model are discussed. Copyright (C) 2010 John Wiley & Sons, Ltd