Multi wire sawing process is the main technology that is used for cutting hard materials such as silicon, SiC and sapphire. The multi wire sawing process is important for determining the remaining wafering process time for such steps as diamond mechanical polishing(DMP) and chemical mechanical polishing(CMP), as the wire sawing process generates the maximum shape error, which should be reduced to the target value through successive wafering processes. An electroplated diamond wire is the most important component in a multi wire sawing process and it crucially affects the accuracy of the wafer shape. The greatest advantages of using an electroplated diamond wire in a wire sawing process are low kerf loss, minimum surface damage, and high productivity. During the wire sawing process of sapphire ingots, the wire motion is designed to move back and forth in order to reduce production cost by reducing the total wire consumption, which has not been applied in a normal wire sawing process with free abrasive slurry. Therefore, efforts should be made to provide a better cutting ability for higher material removal rates and effective use electroplated diamond wires. The reciprocating motion results in a gradual wear of diamond abrasives. Therefore, the wear of the wire due to repeated contact has to be optimized to maintain a balance between the cutting ability and the effectively use of the wire by appropriately controlling the wear of diamond abrasives.
The main purpose of this study is to understand the cutting process under different process conditions, the concentration of diamond on the wire, the wear of diamond abrasives, and the resulting cutting ability. To accomplish this, the cutting ability of diamond wires was tested under various cutting conditions by using different wires and sapphire ingots. In addition, a characterization method for the cutting ability of an electroplated diamond wire is proposed based on mathematical modeling and experimental results using the single wire saw and wire vision system.
Based on the experiments, the results can be summarized into four main categories.
First, in terms of the process conditions, the cutting speed is proportional to the speed of the wire and the applied load on the electroplated diamond wire.
Second, theoretical and experimental results showed that the cutting ability of the wire is greater when the concentration of abrasives is lower. This phenomenon can be explained by the fact that the total cutting cross-sectional area produced by the indentation depth of each abrasive multiplied by the number of contact points is greater when the concentration of abrasive is lower because of the increased indentation force on each abrasive. However, the lifetime of the electroplated diamond wire is lower when the concentration of abrasive is lower because of the increased wear rate of diamond abrasives. Therefore, the optimal ratio for each cutting process needs to be improved for a better cutting ability and lifetime of electroplated diamond wires.
Third, the cutting phenomenon of electroplated diamonds for the same experimental condition is classified into three steps. At the initial stage of cutting with a new wire, the cutting speed was always high, and then a consistent increase in cutting depth was observed with a stable cutting speed. At the final stage, the cutting speed of the electroplated diamond wire was reduced as the wire reached its lifetime. A higher cutting speed at the initial stage is caused by the contact condition between the abrasives and the workpiece. Because of the statistical distribution of abrasives, only higher abrasives can contact the workpiece; therefore, the initial amount of contact is much lower than the given concentration. Moreover, in the next stage, the cutting speed rapidly decreased because a fast wear of high spots in the wire resulted in a rapid increase of contact points. The increased number of contact points generates a reduction in the actual contact force between abrasives and workpiece. Furthermore, in addition to the increase in the number of active abrasives, the nickel layer surrounding the abrasives also plays an important role in reducing the contact force because the surrounding nickel layer also contacts the workpiece, resulting in a fast reduction of contact force. Therefore, the concentration of diamond and the thickness of Ni plating are dominant factors that determine the lifetime of electroplated diamond wires.
Finally, the methods used to verify the cutting ability of electroplated diamond wires were the defined cutting ability, the suggested cutting ability curve and an experimentally evaluated model of cutting ability. The measurement and evaluation of the cutting ability of electroplated diamond wires were feasible by measuring cutting load. The reduced cutting ability when the feed speed is higher was revealed by comparing the comparison with the area of the cutting ability curve for the same electroplated diamond wire. In addition, the increased cutting ability when the speed of the wire is higher was revealed by the comparison with the area of the cutting ability curve for the same electroplated diamond wire. From the results of the two experiments, it is observed that applying a high wire speed is required to improve the lifetime and precise cutting of electroplated diamond wires, and the characteristics of the initial cutting ability and lifetime can be evaluated by an analysis of the cutting ability curve.
From this study, one can evaluate the phenomena of cutting mechanism more consistently, and the experimental and theoretical results investigated in this thesis could help in understanding the wire sawing process.