Lapping and Polishing

Lapping and polishing are loose abrasive finishing processes, which are among the commonly used methods for fabrication of extremely smooth surfaces, e.g., a critical requirement of optical lenses, bearings, dies and molds, silicon wafers, and other precision components (Nakazawa 2004; Venkatesh and Izman 2008). Figure 1 shows the schematic diagram of lapping and polishing.
Lapping is applied to produce a smooth surface. The lapping uses a tool, called a lap, which has a reverse shape of the desired form. The lap rubs the loose abrasive which is generally suspended in fluid, against the workpiece surface, and replicates the tool form to the workpiece while smoothening the surface. In general, surface with Ra = 0.04 to 0.01 μm are obtained as a result of lapping.
Laps are generally made of cast iron, soft steel, brass, hardened steels, as well as glass. Copper and steel laps enable the lapping process to be accelerated, but cast iron laps retain their shape better and produce a smoother surface at the same time. Glass laps are capable to remove high levels of metal when used with fine-grain abrasives and produce an even better surface than cast iron laps.
Lapping media that are commonly used are aluminum oxide, silicon carbide, emery, boron carbide, and pastes or compounds mixed with oil or bonding vehicle (gasoline, kerosene, vegetable oil, etc.)
Polishing compared to lapping is applied to produce extremely smooth surface. The polishing process uses a tool, relatively soft tools, such as pads made of soft cloth or resin which is used to rub loose abrasive slurry against the workpiece surface thus improve the smoothness of workpiece.

Chemical Mechanical Polishing (CMP)

Chemical mechanical polishing (CMP) was introduced by IBM for the production of the 64 Mbit DRAM chip (Zantyea et al. 2004). This process is frequently used for the planarization of semiconductor wafers; CMP takes advantage of the synergetic effect of both mechanical and chemical forces in the polishing of wafers (Patrick et al. 1991; Zantyea et al. 2004; Matijevic and Babu 2008; Venkatesh et al. 1995). This is performed by applying a load force to the back of a wafer while it rests on a pad. Both the pad and wafer are then counter rotated while introducing slurry containing both abrasives and reactive chemicals underneath.
A schematic diagram of CMP is shown in Fig. 2. A wafer containing the films, which is a workpiece, is mounted in a carrier and pressed facedown at a known pressure and rotated against a porous polyurethane pad mounted on a rotating table. Both the carrier and the platen are normally rotated in the same direction. When the rotational speed of the carrier and the platen are the same, the relative velocity of each point on the wafer with respect to the pad is the same, facilitating a uniform material removal from across the entire wafer surface. After determining that the proper amount of material has been removed by using one of a variety of end point determination techniques, the wafer is removed from the carrier and washed.
To obtain the better quality surface, this set of CMP process and cleaning processes are repeated for each level of metallization. It is useful to remember that in a typical CMP process, the thickness of the material removed is limited to about a micrometer, with a removal uniformity of perhaps a few tenths of a nanometer across a heterogeneous surface on a 300 mm diameter silicon wafer – a daunting and inherently complex task.
A large number of parameters influence the outcome of the CMP process. These can be divided into two groups. The first group, which depends on the polishing tool configuration, includes the design of the wafer carrier and retainer ring, applied pressure and its distribution across the wafer, rotational speeds of the carrier and the platen, etc. The second group consists of those dictated by the consumables used during the process, which include the characteristics of the pad like its hardness, modulus, porosity, surface roughness, grove design, conditioning, etc., and the large number of properties associated with the slurry. The latter contains abrasives (e.g., silica, alumina, ceria, zirconia, etc.) as well as several different chemical additives. The components of the slurry, i.e., the abrasive size, shape, method of preparation and concentration, as well as the chemical additives, and more importantly the pH strongly influence the polishing process in terms of both removal rates and defects caused.

Elastic Emission Machining (EEM)

EEM is one of the atomic size machining methods (Mori et al. 1987; Mori et al. 1988). When two solid phase materials composed of different chemical elements come into contact with each other, different kinds of interactions are being generated at the interface. Therefore, if these solids are separated mechanically, chances that the atoms from one surface moving onto the other surface might occur. This kind of phenomenon is applied in machining and is known as EEM.
Ultrafine powder particles consisting of diameters that are much smaller than 1.0 μm are homogeneously mixed with water. Making use of the flow of this mixture, powder particles are accelerated and transported onto the work surface with minimal load (Fig. 3). When in contact with the work surface, surface atoms will be removed through the process mentioned above. Limitation of working area is within the contacting area which needs to be smaller than 10 nm2, and removal is possible only where mutual surface atoms are ideally binding.
Furthermore, to remove the target atom, the interface has to possess the characteristics to decrease the binding energy between the atoms in the surface and second layers. As a result, to obtain a geometrically perfect surface, both machines area and the depth need to be of approximate atomic order. The interface characteristics have shown that the removal of atoms from the work surface is semi-spontaneous, thus finished surfaces from the point of view of physical properties can be perfect.