Precision Machining of Silicon Substrates

Abstract

Brittle nonmetallic structural materials, such as quartz, glass, silicon, ceramics, ferrites and glass ceramics, are widely used in mechanical engineering, instrument making and radio electronics, as well as in optical, clock and jewelry industries. These materials have high hardness, strength, wear resistance and brittleness; therefore, their machining is a complicated problem. For production of piezoelectric resonators and filters, semiconductor devices, solid-state lasers, optoelectronic devices and other parts of electronics, plates made from monocrystals of quartz, lithium tantalate and niobate, silicon, germanium, sapphire, etc. are used. Semiconductor plates, which were obtained after cutting of a boule, have a number of defects, such as presence of a mechanically affected layer, nonflateness of sides, twisting and large variation of thickness. Therefore, after cutting it is necessary to carry out grinding and polishing. Grinding (abrasive finishing) is machining of semiconductor plates by means of hard finishing grinding discs (lapping tools) using abrasive flour grains. Finishing discs (lapping tools) are usually made from cast iron, glass, steel, copper or tin. The grain size of flour grains for grinding of semiconductor plates is selected from M14 to M5. Finishing machining of silicon substrates is complicated by a number of factors, such as provision of stable sizes, and, which is more important, the required microrelief of a machined surface, and unacceptability of machining marks and micro fissure on a surface. In Perm National Research Polytechnic University, the method of abrasive finishing and polishing of surfaces of silicon substrates by means of “Rast-350” flat-finishing machine was developed. The feature of the machines is in sliding movement of a tool (lap) along a nonrecurring trajectory, which has a form of a grid of complicated configuration. At that, the speeds of movement of all points of the working face of the lapping tool are the same. The studied and proposed technological recommendations for operations of finishing and polishing of silicon substrates allowed determining the main technological regimes (time, specific pressure, and density of the grid of tool marks), as well as the required abrasive material of corresponding grain size, which allowed providing the required microrelief of substrates (Rz 25 nm, Ra 5 nm) and eliminating machining marks and micro fissures on a polished surface.