Multi-wire sawing is used to slice silicon blocks into wafers during solar cell production. The abrasive slurry used in the slicing process can be recycled to reduce processing costs. Image analysis enables the slurry quality to be assessed.
In solar cell production for the photovoltaic, semiconductor and microelectronics industries the main technique used for slicing of silicon blocks into wafers is multi wire sawing. This process results in high throughput and an excellent surface quality1. In order to improve efficiency and reduce waste the industry is moving towards recycling of the abrasive slurry used in the wire saw cutting process and the Sysmex Flow Particle Image Analyzer FPIA-3000 is an ideal tool for monitoring the quality of the slurry during the process.
Slicing silicon wafers
Figure 1: Schematic of a wire saw
Silicon blocks are sliced into wafers using a wire saw (schematic shown in figure 1) which consists of a single wire, with a typical diameter of 80 to 200 μm, which is fed from a supply spool through a wire tensioning system, and is wound around guide rollers to form a wire web. Finally the wire is collected on a take up spool. As the wire is wound from one side to the other it passes into the sawing channel where the abrasive slurry is supplied and is then carried on the wire in order to perform the cut-grinding process. As silicon work pieces are pushed against the wire web they are sliced into thousands of wafers in a single run.
The abrasive slurry is a suspension of hard grinding particles, such as silicon carbide, in glycol or oil. The abrasive material is manufactured under strict quality control procedures so that they have high levels of precision required for the process. Traditionally after the cutting process, the exhausted slurries are discarded. The slurry becomes exhausted when there is a high proportion of fine silicon powder particles and broken abrasive particles present. Additionally the heat and chemical reactions caused during the process can generate particle aggregates. Both of these factors reduce the efficiency of the process and the quality of the wafers produced.
However, the industry has recently pushed towards recycling of the abrasive slurry in order to reduce not only production costs but also the amount of industrial waste. One method of recycling is to remove the solid material from the liquid by centrifuge. The reusable abrasive material is then separated from fine material and is reintroduced into the liquid2. Figure 2 shows a flow chart of the wire sawing process before and after the introduction of a recycling system.
Figure 2: Flow chart of the wire saw process before and after the introduction of a recycling system
Monitoring the recycling process
Figure 3: The Sysmex FPIA-3000 Flow Particle Image Analyzer
The Sysmex Flow Particle Image Analyzer FPIA-3000 (Figure 3) is being used to monitor the deterioration of abrasive slurries in order to establish when a slurry should be withdrawn from production and recycled. The suspended sample is measured in real-time and images of up to 360,000 particles are captured by a CCD camera. The standard operation procedure (SOP) driven analysis takes about 2 minutes with a one-click starting operation. The system provides statistical results in terms of particle size and shape distributions along with particle count. This allows quantitative monitoring of the percentage of unwanted fine components (worn grinder pieces, waste silicon powder etc.) in the slurry. Additionally the two-dimensional scattergram plotting size verses shape of the particles allows distinction between primary particles and aggregates on the basis of differences in size and shape.
Analysis of slurry before and after the wire saw process
Figure 4: Results of the analyses of the abrasive slurry before and after being used in the wire saw process along with example particle images
In order to demonstrate the use of the FPIA-3000 for this application, an abrasive slurry was analyzed before and after its use in the wire saw process. Figure 4 shows the results of the analyses. The number based Circular Equivalent Diameter size distributions show the large increase in the proportion of fine material present in the abrasive slurry sample after the wire saw process compared to before it. As well as showing the change in the level of fine material (region highlighted in green), the scattergrams of Circular Equivalent Diameter Verses Circularity also indicates the presence of agglomerated particles (region highlighted in blue) after the wire saw cutting process that were not present initially. Regions of the scattergram can be defined in the SOP allowing the proportions of the unwanted particles and the usable grinder particles (highlighted in red) in the samples to be quantified and therefore readily compared. Additionally images of every particle analyzed are retained by the system. Example images of the different types are particles present in the abrasive slurry samples after the wire saw cutting process are also presented in figure 4.
The analyses presented here show that the FPIA-3000 can be used to monitor the deterioration of abrasive slurries, in the wire saw cutting process to slice silicon blocks into wafers. The quantitative analysis allows the presence of unwanted particles to be monitored. Therefore, decisions can be made as to when to remove slurry from production and send it for recycling. Additionally the FPIA-3000 system can also be used to monitor the recycling process to ensure the unwanted particles are successfully removed.
Swiss Wafer Slicing technology for the Global PV Market from Meyer + Berger AG-Novel trends for the Future in Photovoltaic Wafer Manufacture. Peter Pauli et. al. 6th Symposium Photovotaique Nation SIG, Geneve November 2005