Planarization of hard and brittle materials normally requires a multi-level process control: grinding processes serve to be the pre-forming processes, while lapping and polishing processes take care of a good end-result. At the IMPT, the following processes are developed and applied:
Plane Grinding Process (“surface grinding” also commonly used)
Plane grinding processes are used to process level functioning surfaces with high material removal rates. Plane grinding is based on the track-bound principle because the rigidly-bound particle penetrates the workpiece along a prescribed path. Due to the extreme hardness and wear resistance of brittle-hard materials (ceramics, monocrystalline), diamond is predominately used as cutting material. Machining-related component damage such as bursts, cracks, and internal stresses at the border zone are critical and often require a reworking of the components. Research is conducted at the IMPT on plane grinding for the processing of metals and coarse thickness reduction of ceramic substrates.
Lapping is defined as the machining with mainly loose, in a liquid or paste dispersed cutting grain which performs a rolling motion due to the relative motion between the lapping wheel and the workpiece. The lapping grain moves in the space between the workpiece and the tool which makes lapping space-bound principle method. In addition to grinding, lapping of ceramics is another process that represents a production process by which high surface quality and dimensional accuracy can be reached economically. For the processing of ceramics, mainly diamond is used as the lapping grain (abrasive grain). For the processing of ductile materials (metals), however, SiC grains are preferred. Due to the relatively large abrasive grains and hard lapping plate, component damage such as bursts, cracks, and internal stresses at the border zone can be reduced but not completely avoided. If, instead of a typically hard lapping plate (cast iron), a lapping plate made of softer material (Cu, Al, Zn) is used, the border zone damages can be significantly reduced. In this case, fine lapping or loose abrasive grinding is preferrec. At the IMPT, fine lapping is mainly used for coarse planarization of ceramic substrates (wafers).
An alternative to the above-mentioned finishing processes is the nano grinding process which is an ultraprecision process for the production of flat or slightly curved surfaces. While the grain between the workpiece and the tool performs a rolling motion when lapping, with nano grinding the cutting grain is bonded in a soft metallic working plate. In comparison to the space-bound principle with lapping, the principle with nano grinding is power bound due to the constant surface pressure acting on the workpiece. Nano grinding is also known in the literature as fixed abrasive grinding. It is a two-phase ultraprecision manufacturing process, consisting of a generation and a processing phase, to produce flat or slightly curved surfaces. In the generation phase, the nano grinding tool is produced by embedding 0.5 µm to 1 µm large diamonds in a tin working plate. This tool is then used for the machining of the workpiece in the processing phase. This method is suitable for the final step in high-strength ceramics production. Compared to the competing processes, nano grinding achieves both high surface quality and relatively high separation rates. At the IMPT, nano grinding is used for fine planarization of ceramic substrates as well as for the surface processing (angle, slope, side walls) of small chips.
Plane Polishing (solely mechanical)
Plane Polishing is a smooth fine machining process applicable to a variety of materials. The smoothing effect is usually achieved by two mechanisms: First, the protrusions of the surface structure are plastically and partially-plastically deformed and leveled. Second, depending on the nature of the polish, a very low to low material removal is carried out in addition to filling of depressions. Eventually, the surface often reaches a shiny appearance due to the smoothness. Although the roughness of the workpiece is reduced by polishing, the accuracy in shape also generally decreases. Polishing clothes (textile, felt, polymer) are mainly used as the polishing pads. Stiffer polishing pads are more suitable to preserve shape accuracy because softer polishing pads tend to round the edges of components. In addition to diamond, other nano-grained particles can be used as abrasive grains such as Al2O3, SiO2, and CeO2. At the IMPT, plane polishing is used for the surface finishing of ceramics or ductile substrates (metals).