1.
GaAs Crystal Wafer [44]. Considering the common points in the conventional slurries applied so far to the GaAs crystals, it has been revealed that slurries are oxidizing agents containing either Na or Br. The research has shown sodium bromate (NaBrO2), which is inexpensive and used as a desizing agent for fiber processing and as a slime control agent for industrial water, does not produce a toxic gas, and its oxidization action is mild.
When disk-type chemical polishing was carried out only with NaBrO2 solution, reaction products partially left a film, while a smooth mirror-like surface was obtained.
Furthermore, when fine particles are mixed in, the reaction products are positively wiped off, thus achieving a high efficiency. Figure 6.35 shows the mechanochemical polishing rate of GaAs crystals with the slurry of NaBrO2 solution (0.6% of NaOH included) to which the DN agent and SiO2 fine particles were added. As expected, the removal rate increases as the size of SiO2 particles become large. With this mixed slurry applied to the polishing of a GaAs wafer, a surface toughness Rz of below 10 was obtained as shown in Figure 6.36.
Figure 6.35.
Relation between SiO2 particle size, polishing rate, and surface roughness when using β-slurry (Specimen: GaAs)
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Figure 6.36.
Comparison of surface roughness by the new polishing slurry (γ-Slurry): (C4H9)4N BrO2 solution + SiO2: (a), with cloven surface, (b) specimen: GaAs
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Based on these polishing characteristics, the processing mechanism is as follows.
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Both Ga and As are oxidized by the NaBrO2 solution and produce gallium Oxide (Ga2O3) and diarsenic pentoxide (As2O5) as a reaction product, respectively.
equation(6.5)
2GaAs+4NaBrO2→Ga2O3+As2O5+4NaBr
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Immediately after the As2O5 is produced, it is converted into sodium arsenate (Na3AsO4) by NaOH.
equation(6.6)
As2O5+6NaOH→2Na3AsO4+3H2O
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Ga2O3 becomes gallium hydroxide Ga(OH)3 by alkali hydroxide.
equation(6.7)
Ga2O3+6OH→2Ga 3(OH)+3O2−
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This Ga(OH)3 easily dissolves in sodium hydroxide.
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During these reactions (6.5), (6.6) and (6.7) and while surface activity penetration function is working, the chemical actions are further accelerated by the mechanical action of the added fine particles and the frictional exoergic reaction with the processing surface.
From this processing mechanism of GaAs, a slurry with an oxidant to which a small quantity of alkali hydroxide is added is basically effective for the mechanochemical polishing of GaAs crystals. Achieving high-quality surfaces by adding a DN agent with a surface penetration function allows for removal of the reaction products (films, stains) from the surfaces.
An experimental polishing of GaAs has been made using as a slurry an oxidant H2O2 with a small quantity of NaOH. Figure 6.37 is an example of the processing characteristics with a H2O2 – NaOH type solution to which a DN agent is added. Maximum removal rate is given at about pH = 10. Thus, similar removal rate and surface quality to those of NaBrO2-type solutions have been obtained. However, although SiO2 fine particles are added, the increase of the removal rate will not be as remarkable as that of NaBrO2 solution, presumably because reaction products are not formed on the surfaces as solidly as NaBrO2-type solutions.
Figure 6.37.
Example of polishing characteristics of GaAs using a slurry containing H2O2 (oxidizing agent), NaOH (alkali), and DN agent (nonionic solvent)
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2.
CdTe Crystal Wafer [45]. CdTe crystals are compound semiconductors belonging to the II–VI group while GaAs crystals belong to the III–V group. CdTe has come into focus particularly as an infrared/radiation detector, light emitting diode, and a solar battery substrate. CdTe crystals, like GaAs crystals, have a marked cleavage and are soft (Mohs hardness of GaAs and CdTe is 4.5 and 2.8, respectively).
When establishing a selection guide of polishing slurries for CdTe crystals based on the processing mechanism of CaAs crystals, processing solutions that oxidize CdTe surfaces are required. An assumption was made to remove (with a polisher or fine particles) the oxide, like CdTeO3, formed by such a processing solution.
Figure 6.38 indicates the removal rate of the CdTe crystals when the oxidizing bleaching agent that is harmless and commonly used at home is primarily used; it clearly shows the effect of the oxidizing bleaching agent and fine particles (SiO2). Through the microscopic observation of the processed surfaces, it has been found that more uniform surfaces were produced when DN added-polishing slutty was used as in the mechanochemical polishing of GaAs wafers. However, its effect is not as obvious as that of GaAs crystals. The surface roughness Ry was below 2 nm, which means that better surface roughness has been efficiently obtained when compared to the conventional polishing slurry.
Figure 6.38.
Polishing rate by various slurries (Workpiece: CdTe)
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From the processing experiments of CdTe crystals and the results of its chemical analysis, the following will be drawn as a processing mechanism to the mechanochemical polishing of CdTe crystals.
First, by creating an atmosphere that makes the CdTe surface easily oxidized by a DN agent, and finally some kind of reaction product is deposited on the surface. Then, oxide films or reaction products deposited on the surface are mechanically removed by abrasive grains or a polisher. By repeating these processes, a high-quality surface is produces.
We have discussed the processing mechanism of GaAs and CdTe crystals for compound semiconductors and drew a conclusion that its processing mechanism is basically the same as that of the mechanochemical polishing of Si crystals.
Colloidal Silica Polishing
Basic Processing Characteristics
Table 6.7 shows an example of the composition and properties of colloidal silica. In this alkaline solution (pH 9.5–10.5), SiO2 fine particles (of ϕ100∼200 Å) are suspended by 50 wt%. In general, gelling of colloidal silica is controlled by the addition of alcohol. By the addition of salt, like sodium sulfate (Na2SO4), gelling is accelerated.
Table 6.7.
Composition of Colloidal Silica and its Characteristics (for the First Polishing)
SiO2 content 50 wt% (ϕ 100∼200 Å)
NaO2 content below 0.4
Hydrogen ion exponent (pH) 10.8∼11.2
Particle diameter (at 20°C) below 25 CP
Specific gravity (at 20°C) 1.36∼1.40
Appearance transparent milky colloidal solution
Freezing point 0°C
Stability semipermanent
TEM photo of colloidal silica
(transmission electron microscope)
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The relationship between the processing pressures and the removal rates of polishing are shown inFigure 6.39, using colloidal silica on a variety of single crystals such as GGG, LiTaO3, Si, and sapphire, which are chemically and mechanically stable. Removal rates of all crystals increase in proportion to the increase of processing pressures; particularly high pressures were needed for the hard sapphire single crystals.
Figure 6.39.
Relation between polishing pressure and polishing rate of various single crystals using colloidal silica
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Referring to the removal rate shown in Figure 6.35 as a parameter, the relation between Vickers hardness of several crystal and processing pressures necessary to obtain a given constant removal rate is illustrated inFigure 6.40. In the case of Si single crystal recognized clearly as affected by the chemical action of the alkaline solution, the stock of removal in the figure is considerably off the straight line, achieving as a sequence a high removal rate at low polishing pressure. For the rest of the crystals, the polishing pressure P (kg/mm2) necessary to obtain the removal rate V (μm/h) is expressed in the following equation.
equation(6.8)
P=104exp (K1•HV)
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where K1 is a constant determined by the processing conditions and HV is Vickers hardness (kg/mm2). The constant K1 can be obtained with the equation, K1 = 2 × 10-4V + 1.4 × 10−3, and when its relation is substituted for the equation (6.8), it will be expressed as:
equation(6.9)
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Figure 6.40.
Relation between necessary processing pressure and Vickers hardness of various kinds of crystals