Because wet-type and dry-type mechanochemical polishing are dependent on the chemical properties of workpieces, they can only be applied to a limited range of materials. However, this polishing method can process almost all kinds of materials to a strain-free, mirror-like surface. The final result depends, however, on the hardness of workpieces, which sometimes causes degradation of processing efficiency. Other polishing methods include Si wafer processing, which combines the polishing mechanism of wet-type mechanochemical polishing with the present polishing mechanism. This polishing method will be discussed later in this chapter.
5.
Noncontact Polishing. This method corresponds to the micro-minimized processing reaction of mechanical polishing to the order of 1/10–1/100. The particles of approximately 100 Å diameter act on the surface atoms of workpieces thereby removing several to several tens of atoms. This representative example is based on what was proposed as elastic emission machining (EEM) [38] as seen in Figure 6.28 and is applied to noncontact processing such as float polishing [39].
Figure 6.28.
A processing principle of elastic emission machining (EEM)
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When the processing unit diminishes to the atomic or molecular order, particle surface atoms join with the workpiece surface atoms without inducing plastic behavior, followed by destruction of the joints. The polishing progresses this way. Consequently, almost no mechanical damage is left after the atoms are removed. In mechanical and mechanochemical polishing methods, abrasives retained by a polisher, in some cases, in a solution, act upon workpieces. On the other hand, EEM and EEM-applied noncontacting polishings by the collision of abrasives with workpieces accomplish high-quality and high-precision polishing.
In general, processing efficiency, processing precision, and depth of damaged layers are reciprocally related. For those chemical reactions included in the processing mechanism such as mechanochemical polishing, the higher the processing efficiency, the fewer the damaged layers. However, because the processing surface precision degrades as chemical actions become large, chemical reactivity needs to be controlled in order to obtain better precision.
Figure 6.29 shows the general characteristics chemical compound polishing. With lapping, the efficiency increases approximately in proportion to the diameter of abrasives used; however, such a relation is not greatly noticeable when polishing with particles below 1 μm. For polishing with particles below 0.1 μm, the magnitude of the number of active particles has a large effect on efficiency. In other words, high efficiency can be achieved by increasing the processing pressure with the polisher uniformly working on the surfaces of workpieces. Because transcribing the polisher surface is a basic principle, processing precision is largely dependent on the precision of the polisher or the plate on which workpieces are mounted. However, in the case of processing methods such as EEM, which scans the area to be processed, the efficiency depends primarily upon its scanning accuracy.
Figure 6.29.
General characteristics of chemical compound polishing
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Table 6.5.
Main Factors for Polishing
Processing mechanism Efficiency Accuracy Quality (low damaged layers)
1. Mechanical polishing particle dia., hardness: large (diamond, alumina, cellium, oxide, etc) pressure, relative speed: high plate shape accuracy, system motion accuracy, particle dia. and distribution, polishier materials opposite to what listed in “efficiency”
2. Wet-type MCP/CMP reagent (chemicals) concentration, particle density: high atmosphere temp.: high plate shape accuracy, system motion accuracy, plate/chemicals temp. control reagent concentration: high atmosphere temp.: high particle density: low soft abrasives/soft polisher
3. Dry type mechano-chemical polishing particle material: reactivity against workpiece (solid-phase reaction), soft abrasives plate shape accuracy, system motion accuracy particle dia: small particle material: reactivity against workpiece, soft abrasives
4. Colloidal Silica Polishing particle density: high gelling speed, high pressure particle/workpiece: correlation, colloid chemical action plate shape accuracy, system motion accuracy particle density: high particle dia./ distribution: small particle/workpiece: correlation, soft particle, colloid chemical action
5. Non-contact Polishing particle dia., kinetic energy: large, number of particles: large correlation particle/workpiece: correlation particle kinetic energy control, system motion accuracy, particle dia. and distribution particle dia, kinetic energy: small, number of particles: large particle/workpiece: correlation
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Wet-Type Mechanochemical Polishing and Chemical Mechanical Polishing
This chapter deals with chemical compound polish, namely mechanochemical polishing (MCP) or chemical mechanical polishing (CMP), that combines mechanical removal action (mechanical polishing) of abrasives with chemical action generated by the processing reagent or abrasives.
Mechanochemical phenomena can be defined as follows:[40], [41] and [42]
1.
The mechanical energy (e.g., mechanical shock, grinding, rolling, tension, application of pressure, plastic deformation) applied to a solid material caused the solid surface to change its physical and chemical properties.
2.
It brings about chemical changes to the gas and liquid materials around a solid material.
3.
It induces or accelerates direct reactions between the ambient gas or liquid and the solid material.
In wet-type mechanochemical polishing (MCP)/chemical mechanical polishing (CMP), sufficient information is available that suggests these phenomena occur depending on the selection of workpiece materials, abrasives, and reagent.
In accordance with the definition of the mechanochemical phenomenon, the final finishing process of workpieces can be advantageously carried out by creating an ambient atmosphere in which chemical reactions occur in the case of conventional polishing. In other words, when viewed micrographically, it is presumed that abrasives mechanically act upon the work surfaces inducing high pressure and high temperature at its contacting areas. This presumption suggests a large chance of producing changes in the physical and chemical properties of the processing surfaces. As a consequence, by placing such substances in a processing atmosphere that initiates chemical reactions, polishing accompanied by mechanochemical phenomenon, namely MCP/CMP is realized.
With regard to chemical reactions, any solid phase reaction, solid-liquid phase reaction, or solid-gas phase reaction is applicable. Dry-type MCP for sapphire with SiO2 fine particles is a typical example of the solid phase reactions. On the other hand, wet-type MCP/CMP for Si single crystals with alkaline reagents, demonstrates solid-liquid phase reactions. This wet-type MCP/CMP has a mitigating effect on some mechanical actions and will prevent the generation of polishing defects, such as scratches, if the appropriate environmental conditions are provided. However, because soft polishers (pads) are generally used to produce high-quality, strain-free, mirror-like surfaces, edge-turndown becomes predominant in comparison with dry-type MCP (see Figure 6.26). Wet-type MCP/CMP is diverted from the Si MCP of copper replacement type [43].
By utilizing replacement reactions between Si and Cu, the chemical reactions are accelerated at the same instant that the Cu ion precipitates on the processing surface, using a mixed solution of NH4F and Cu(NO3)2, and is mechanically removed by a soft polisher (pad). Currently, this method is not applied to the polishing of a final finishing process of Si wafer for LSI due to some problems arising on the residual Cu ion and surface roughness (smoothness), although the processing efficiency is high.
In general, processing efficiency and processing precision/depth of damaged layers are reciprocally related. For polishing processes where chemical reactions are incorporated into the processing mechanisms such as wet-type MCP/CMP, the greater the chemical reactions the greater the processing efficiency with only a few damaged layers. However, the processing surface precision decreases. Therefore, good precision quality has to be achieved by controlling chemical reactions to some degree. Figure 6.29 shows the general characteristics of a wet-type mechanochemical polishing (MCP)/chemical mechanical polishing (CMP).
The following section on wet-type MCP/CMP called mechanochemical polishing or chemical mechanical polishing is based on some crystalline materials as an example.
Mechanochemical Polishing (MCP)/Chemical Mechanical Polishing (CMP) of Silicon Wafer for Semiconductor
Wet-type mechanochemical polishing of Si wafer for LSI has proved effective as a high-efficiency, strain-free, mirror-like polishing and is technically recognized as the best polishing method. When referring to the DRAM of M, G and T bit order for ULSI, because the minimum pattern rule of the circuits becomes half submicrometers, a demand for higher-precision for Si wafer increases. In general, Si wafers are polished using an artificial leather-made polisher and a slurry made from ultrafine SiO2 particles (approximately ϕ100 Å) suspended in an alkaline solution of approximately pH 10. Actually, first, second, third, and even fourth polishings are carried out with a combination of slurry and polisher (pad) selected to the best of each step (see Table 6.6).
Table 6.6.
Polishing Conditions for Bare Silicon Wafers
Processing conditions
Porcess Slurry
(polishing agent, abrasives) Pad
(polisher, polishing pad) Polishing pressure Stock of removal Target
First polishing SiO2 type abrasives (colloidal silica, pH 10–11)
Particle size: 50–100 nm
Polishing agent: Alkaline solution/Amine or KOH base Polyurethane impregnated