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Thermal Laser Separation (TLS) for separating multi-crystalline silicon wafers: A comparison with state-of-the-art methods
Poster at 26th European Photovoltaic Solar Energy Conference 2011, Hamburg, Germany
urn:nbn:de:0011-n-1959569 (935 KByte PDF)
MD5 Fingerprint: 983f348facde01aeef2c2f3221a1551f
Created on: 8.3.2012
Motivation During research and development regarding next generation solar cells, many experiments are performed on multi-crystalline silicon (Si) wafers. One regular task is the separation of different areas of interest on a Si wafer. Separating multi-crystalline Si wafers implies one major challenge: Breaking the samples manually leads most often to a heap of Si crumbs which are only useful with limitations afterwards. This paper investigates the applicability of a novel wafer separation method, the thermal laser separation (TLS), on the separation of multi-crystalline Si wafers. TLS has already proven its applicability in the dicing of mono-crystalline Si wafers . For the dicing of multi-crystalline Si, TLS will be compared to manual scribe and break and the mechanical wafer saw. E ach method is applied to wire saw cut multi-crystalline Si wafers. The methods are briefly described, experimental results are compared with each other and the separation methods are evaluated.Approach Samples of wire saw cut Si wafers (156 mm x 156 mm) are used to evaluate three separation methods. Each sample should be separated into squares of 20 mm edge length. For the sample preparation an accuracy of ap-prox. 100 μm in straightness is sufficient. The following separation methods are evaluated: Scribe and break: An initial scribe is applied manually at the edge of the samples with a diamond tip. The sam-ples are then broken manually by bending both sides of the sample around this initial scribe with tweezers. Mechanical wafer saw: Fast rotating (30.000 rpm) diamond blades ar e used to separate the Si wafers by remov-ing material of the size of the blade width. Thermal Laser Separation : The TLS process is a cleaving process that uses thermal-induced mechanical stress and which comprises two steps. To initiate the process, a small initial scribe is required, which is done with a diamond tool tip or with a special scribing laser (step 1). In this case, a diamondtool tip is used. The initial scribe gives the cleaving process a well-defined starting point. During the cleaving step (step 2), the material is heated up by a laser and directly after the laser spot the material is cooled down by a water aerosol. This combi-nation of heating up and cooling down the material results in a large temperature gradient inducing tensile stress inside the material. The ten sile stress opens a crack starting from the initial scribe and the crack is guided through the material by the movement of the laser-cooling combination. Sample preparation For the scribe and break no special preparation is necessary. As the mechanical wafer saw is a typical dicing technology, it is necessary to mount the wafers on dicing tape which has a sticky layer that holds the Si wafer on a thin metal dicing frame. In this case a UV tape is used. This tapes adhesiveness can be reduced by UV irradiating the tape after the dicing process. Separated samples can then be demountedmanually. Even though the TLS-technology works also normally with dicing tape, it is not necessary to use it due to the fact that no external mechanical stress is applied to the samples compared to the mechan ical wafer saw. That is why the samples are just put on a vacuum chuck without special additional preparation. Results Results of separating multi-crystalline Si samples are shown in the figures below: Scribe and break (Figure 1): The small arrows denote the manually applied diamond scribes. As can be seen from Figure 1 it is impossible to separate the samples into rectangular pieces. This process result does not fulfill the above specifications. Mechanical wafer saw (Figure 2): With the mechanical saw it is possible to separate the samples into the speci-fied rectangular pieces with good straightness. Major drawback is the time-consuming sample preparation be-cause of the need for the mounting process prior to the separation and the demounting process after the separa-tion.Chipping a t the sample edges (indicated by the arrows) might reduce the breaking strength of the samples (currently under investigation). Thermal Laser Separation (Figure 3): The TLS-process results also in rectangular pieces, but of lower straight-ness as the saw-result; compare arrow in Figure 3 (quantitative analysis ongoing). The TLS-process is less time-consuming, because no special preparation is necessary and the separation process works at high velocities up to 300 mm/s (wafer saw 10 mm/s in this case). Conclusion It was shown that for the preparation of multi-crystalline Si wafers as envisaged here, the TLS-technology pro-vides the fastest results with low preparation effort and, compared to the mechanical wafer saw, with a sufficient straightness. Investigations on the resulting bending strength and a quantitative analysis of the achieved straightness are being continued and will be presented in the final paper.