Silicon Shaping

As silicon is a hard, brittle material, the most suitable materials for shaping and cutting silicon are the following.

· Industrial-grade diamonds

· SiC and Al₂O₃

Shaping Operations involves the following:

· Removing the seed and tang ends from the ingots

· Surface grinding which defines the diameter of the material.

· After diameter grinding one or more flats are ground along the length of the ingot. The larger flat is called "primary" flat. Purpose of primary flat (a) it is used as a mechanical locator in automated processing equipment to position the wafer. (b) Serves to orient the IC device relative to the crystal. Other smaller flats are called "secondary" flats. Purposes of secondary flat (a) identify the orientation and conductivity type of the material. (b) Provide a means of quickly sorting and identifying wafers should mixing occur.

Fig. 9 Standard flat orientations for different semiconductor wafers [4]

After removal of seed and tang ends from the ingot, surface grinding and grounding flats the ingot is ready to be sliced into wafers.

Slicing determines four wafer parameters:

· surface orientation

· thickness

· taper

· bow

Polishing is the last step and it is done to obtain a smooth, specular surface on which device features can be photoengraved [2]. The aim is to produce a surface with a high degree of surface flatness and minimum local slope to meet the requirements of optical projection lithography. Typical values of surface flatness are between 5 and 10 microns. In the chemical mechanical polishing (CMP) process, the wafer is held on a rotating holder and pressed on a rotating polishing pad, with slurry and water in between. The slurry is a colloidal suspension of fine silica particles with diameters of about 100°A in an aqueous solution of sodium hydroxide [3]. Sodium hydroxide oxidizes the silicon surface (chemical process) with the help of heat generated by the friction between the wafer and polishing pad. Then the silica particles abrade the silicon oxide away from the surface (mechanical process). Figure [2] below gives a schematic of the CMP process.

A schematic of CMP process[2].