Cutting force modeling and control of single crystal silicon using wire saw velocity reciprocation

Abstract

Single crystal silicon wafers are often used as substrate material for integrated circuits. Often the wafer is cut by a wire with fixed abrasive diamond owning to a narrow kerf and a low cutting force. The cutting force changes during the process as the direction of wire movement continuously reverses (i.e., reciprocates), which may cause the wire saw to break, the wafer to collapse, and the wafer surface roughness to decrease even if the wire saw tension and the contact length between the wire and the wafer are fixed. In this work, a cutting force model including both normal and tangential forces was established to determine the relationship between the normal and tangential forces and the commanded wire speed. Separate controllers were developed to regulate both the normal force and the tangential force by adjusting the wire velocity. Experimental studies were conducted for wire saw processing of single crystal silicon wafers. Compared with a process using constant wire velocity, regulating the normal force can significantly reduce the processing time and the wafer surface roughness. The average improvements in processing time and wafer surface roughness are approximately 11% and 56%, respectively, when using normal force control, and approximately 29% and 30%, respectively, when using tangential force control. The results show that the normal and tangential forces can be well regulated during the machining process. In addition, the wafer surface roughness and machining time were lower in both experiments where the forces were regulated than in the experiment where a constant wire velocity was used. This paper demonstrates that the novel concept of regulating process forces in wire saw machining by adjusting the wire velocity can be used to optimize the cutting process of single crystal silicon, making the process more productive while decreasing the part roughness.