Formation of silicon and gallium arsenide wafers  (Page 19/11)

Introduction

Integrated circuits (ICs) and discrete solid state devices are manufactured on semiconductor wafers. Silicon based devices are made on silicon wafers, while III-V (13-15) semiconductor devices are generally fabricated on GaAs wafers, however, for certain optoelectronic applications InP wafers are also used. The electrical and chemical properties of the wafer surface must be well controlled and therefore the preparation of starting wafers is a crucial portion of IC and device manufacturing. In order to obtain high fabrication yields and good device performance, it is very important that the starting wafers be of reproducibly high quality. For example, the front surface must be smooth and flat on both a macro- and microscale, because high-resolution patterns (lithography) are optically formed on the wafer. In principle, cutting a crystal into thin slices and polishing one side until all saw marks are removed and the surface appears smooth and glossy could produce a suitable wafer. However, due in part to the brittleness of Si and GaAs crystals, as well as the increasing requirements of wafer cleanliness and surface defect reduction with ever decreasing device geometries, a very complex series of processing steps are required to produce analytically clean, flat and damage-free wafer surfaces.

The following focuses on the general principles and methods with regard to wafer formation. Detailed formulas, recipes, and specific process parameters are not given as they vary considerably among different wafer producers. However, in general, techniques for fabrication of Si wafers have generally become standardized within the semiconductor industry. In contrast, GaAs wafer technology is less standardized, possibly due to either (a) the similarity to silicon practices or (b) the lower production volume of GaAs wafers. There are two general classes of processes in the methodology of making wafers: mechanical and chemical. As both Si and GaAs are brittle materials, the mechanical processes for their wafer fabrication are similar. However, the different chemistry of Si and GaAs require that the chemical processes be dealt with separately.

Wafer formation procedures

Each of the processing steps in the conversion of a semiconductor ingot (formed by Czochralski or Bridgeman growth) into a polished wafer ready for device fabrication, results in the removal of material from the original ingot; between ¹ / ₃ and ¹ / ₂ of the original ingot is sacrificed during processing. Methods for the removal of material from a crystal ingot are classified depending on the size of the particles being removed during the process. If the removed particles are much larger than atomic or molecular dimensions the process is described as being macro-scale. Conversely, if the material is removed atom-by-atom or molecule-by-molecule then the process is termed micro-scale. A further distinction between various types of processes is whether the removal occurs as a result of mechanical or chemical processes. The formation of a finished wafer from a semiconductor ingot normally requires six machining (mechanical) operations, two chemical operations, and at least one polishing (chemical-mechanical) operation. Additionally, multiple inspection and evaluation steps are included in the overall process. A summary of the individual steps, and their functions, involved in wafer production is shown in [link] .