Cropping

The as-grown ingots have conical shaped seed (top) and tang (bottom) ends that are removed using a circular diamond saw for ease of further manipulation of the ingot (Figure 1a). The cuttings are sufficiently pure that they are cleaned and the recycled in the crystal growth operation. Portions of the ingot that fail to meet specifications of resistivity are also removed. In the case of silicon ingots these sections may be sold as metallurgical-grade silicon (MGS). Conversely, portions of the crystal that meet desired resistivity specifications may be preferentially selected. A sample slice is also cut to enable oxygen and carbon content to be determined; usually this is accomplished by Fourier transform infrared spectroscopic measurements (FT-IR). Finally, cropping is used to cut crystals to a suitable length to fit the saw capacity.

Grinding

The primary purpose of crystal grinding is to obtain wafers of precise diameter because the automatic diameter control systems on crystal growth equipment are not capable of meeting the tight wafer diameter specifications. In addition, crystals are seldom grown perfectly round in cross section. Thus, ingots are usually grown with a 1-2 mm allowance and reduced to the proper diameter by grinding Figure 1b.

Crystal grinding is a straightforward process using an abrasive grinding wheel, however, it must be well controlled in order to avoid problems in subsequent operations. Exit chipping in wafering and lattice slip in thermal processing are problems often resulting from improper crystal grinding. Two methods are used for crystal grinding: (a) grinding on center and (b) centerless grinding.

Figure 2 shows a schematic of the general set-up for grinding a crystal ingot on center. The crystal is supported at each end in a lathe-like machine. The rotating cutting tool, employing a water-based coolant, makes multiple passes down the rotating ingot until the requisite diameter is obtained. The center grinder can also be used for grinding the identification flats as well as providing a uniform ingot diameter. However, grinding the crystal on centers requires that the operator locate the crystal axis in order to obtain the best yield.

Figure 2: Schematic representation of grinding on center.

Centerless grinding eliminates the problems associated with locating the crystal center. The centerless method is superior for long crystals; however, a centerless grinder is much larger than a center grinder of the same diameter capacity. In centerless grinding the ingot is supported between two wheels, a grinding wheel and a drive wheel. A schematic of the centerless grinder is shown in Figure 3. The axis of the drive wheel is canted with respect to that of the crystal ingot and the grinding wheel pushing the crystal ingot past the stationary (but rotating) grinding wheel, see Figure 3b.

Figure 3: Schematic representation of centerless grinding viewed (a) along and (b) perpendicular to the crystal axis.

Orientation/Identification Flats

Following grinding of the ingot to the desired diameter, one or two flats are ground along the length of the ingot. The identification flats (one or two) are ground lengthwise along the crystal according to the orientation and the dopant type. After grinding the crystal on centers the crystal is rotated to the proper orientation, then the wheel is positioned with its axis of rotation perpendicular to the crystal axis and moved along the crystal from end to end until the appropriate flat size is obtained. An optical or X-ray orientation fixture may be used in conjunction with the crystal mounting to facilitate the proper orientation of the crystal on the grinder.

The largest flat is called the primary flat (Figure 1c) and is parallel to one of the crystal planes, as determined by X-ray diffraction. The primary flat is used for automated positioning of the wafer during subsequent processing steps, e.g., lithographic patterning and dicing. Other smaller flats are called "secondary flats" and are used to identify the crystal orientation (<111> versus <100>) and the material (n-type versus p-type). Secondary flats provide a quick and easy manner by which unknown wafers can be sorted. The flats shown schematically in Figure 4 are located according to a Semiconductor Equipment and Materials Institute (SEMI®) standard and are ground to specific widths, depending upon crystals diameter. Notches are also used in place of the secondary flat; however, the relative orientations of the notch and primary flat with regard to crystal orientation and dopant are maintained.

Figure 4: SEMI locations for orientation/identification flats.

Etching

The cropping and grinding processes are performed with relatively coarse abrasive and consequently a great deal of subsurface damage results. Pits, chips, and cracks all contribute to stress in the cut wafer and provide nuclei for crack propagation at the edges of the finished wafer. If regions of stress are removed then cracks will no longer propagate, reducing exit chipping and wafer breakage during subsequent fabrication steps.

The general method for removing surface damage is to etch the crystal in a hot solution. The most common etchants for Si are based on the HNO3-HF HNO3-HF size 12{"HNO" rSub { size 8{3} } "-HF "} {} system, in which etchant modifiers such as acetic acid also commonly used. In the case of GaAs HCl- HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} is the appropriate system. These etchants selectively attack the crystal at the damaged regions. After etching, the crystal is transferred to the slicing preparation area.

Wet Chemical Etching

The wet chemical etching of any material can be considered to involve three steps: (a) transportation of the reactants to the surface, (b) reaction at the surface, and (c) movement of the reaction products into the etchant solution (Figure 7). Each of these may be the rate limiting step and thus control the etch rate and uniformity. This effect is summarized in Table 3.

Figure 7: Schematic representation of the three steps involved in wet chemical etching: (i) diffusion of the chemical etch reagents through the boundary layer, (ii) chemical reaction at the surface, and (iii) diffusion of the reaction products into the etch solution through the boundary layer.

An etchant that is limited by the rate of reaction at the surface will tend to enhance any surface features and promote surface roughness due to preferential etching at defects (anisotropic). In contrast, if the etch rate is limited by the diffusion of the etchant reagent through a stagnant (dead) boundary layer near the surface, then the etch will result in uniform polishing and the surface will become smooth (isotropic). If removal of the reaction products is rate limiting then the etch rate will be slow because the etch equilibrium will be shifted towards the reactants. In the case of an individual etchant reaction, the rate determining step may be changed by rapid stirring to aid removal of reaction products, or by increasing the temperature of the etch solution, see Figure 8. The exact etching conditions are chosen depending on the application. For example, dilute high temperature etches are often employed where the etch damage must be minimized, while cooled etches can be used where precise etch control is required.

Figure 8: Typical etch rate versus temperature plot for a mixture of HF (20%), (45%) and (35%).

Traditionally mixtures of hydrofluoric acid (HF), nitric acid ( HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {}) and acetic acid ( MeCO2HMeCO2H size 12{"MeCO" rSub { size 8{2} } H} {}) have been used for silicon, but alkaline etches using potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions are increasingly common. Similarly, gallium arsenide etches may be either acidic or basic, however, in both cases the etches are oxidative due to the use of hydrogen peroxide. A wide range of chemical reagents are commercially available in "transistor grade" purity and these are employed to minimize contamination of the semiconductor. Deionized water is commonly used as a diluent for each of these reagents and the concentration of commonly used aqueous reagents is given in Table 4.

The equipment used for a typical etchant process includes an acid (or alkaline) resistant tank, which contains the etchant solution and one or more positions for rinsing the wafers with deionized water. The process is batch in nature involving tens of wafers and the best equipment provides a means of rotating the wafers during the etch step to maintain uniformity. In order to assure the removal of all surface damage, substantial over-etching is performed. Thus, the removal of 20 μm from each side of the wafer is typical. Etch times are usually several minutes per batch.

Etching Silicon

The most commonly used etchants for silicon are mixtures of hydrofluoric acid (HF) and nitric acid ( HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {}) in water or acetic acid ( MeCO2HMeCO2H size 12{"MeCO" rSub { size 8{2} } H} {}). The etching involves a reduction-oxidation (redox) reaction, followed by dissolution of the reaction products. In the HF- HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} system the HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} oxidizes the silicon and the HF removes the reaction products from the surface. The overall reaction is shown in Eq. 1.

Si + HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} + 6 HF → H2SiF6H2SiF6 size 12{H rSub { size 8{2} } "SiF" rSub { size 8{6} } } {} + HNO2HNO2 size 12{"HNO" rSub { size 8{2} } } {} + H2OH2O size 12{H rSub { size 8{2} } O} {} (1)

The oxidation reaction involves the oxidation of Si 00 size 12{ {} rSup { size 8{0} } } {} to Si 4+4+ size 12{ {} rSup { size 8{"4+"} } } {}, and it is auto-catalytic in that the reaction product promotes the reaction itself. The initial step involves trace impurities of HNO2HNO2 size 12{"HNO" rSub { size 8{2} } } {} in the HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} solution, which react to liberate nitrogen dioxide ( NO2NO2 size 12{"NO" rSub { size 8{2} } } {}), Eq. 2 and 3.

HNO2HNO2 size 12{"HNO" rSub { size 8{2} } } {} + HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} → N2O4N2O4 size 12{N rSub { size 8{2} } O rSub { size 8{4} } } {} + H2OH2O size 12{H rSub { size 8{2} } O} {} (2)

N2O4N2O4 size 12{N rSub { size 8{2} } O rSub { size 8{4} } } {} → 2 NO2NO2 size 12{"NO" rSub { size 8{2} } } {} (3)

The nitrogen dioxide oxidizes the silicon surface in the presence of water, resulting in the formation of Si(OH)2 Si(OH)2 size 12{"Si" \( "OH" \) rSub { size 8{"2 "} } } {}and the reformation of HNO2HNO2 size 12{"HNO" rSub { size 8{2} } } {} (Eq. 4). The latter re-enters the reaction process in Eq. 2. The Si(OH)2Si(OH)2 size 12{"Si" \( "OH" \) rSub { size 8{2} } size 8{ }} {}decomposes to give SiO2SiO2 size 12{"SiO" rSub { size 8{2} } } {}, Eq. 5. Since the reaction shown in Eq. 2 is rate limiting, an induction period is observed. However, this is overcome by the addition of NO2−NO2− size 12{"NO" rSub { size 8{2} rSup { size 8{ - {}} } } } {} ions in the form of [ NH4NH4 size 12{"NH" rSub { size 8{4} } } {}][ NO2NO2 size 12{"NO" rSub { size 8{2} } } {}].

Si 00 size 12{ {} rSup { size 8{0} } } {} + 2 NO2NO2 size 12{"NO" rSub { size 8{2} } } {} + 2 H2OH2O size 12{H rSub { size 8{2} } O} {} → Si(OH)2 Si(OH)2 size 12{"Si" \( "OH" \) rSub { size 8{"2 "} } } {}+ 2 HNO2HNO2 size 12{"HNO" rSub { size 8{2} } } {} (4)

Si(OH)2Si(OH)2 size 12{"Si" \( "OH" \) rSub { size 8{2} } size 8{ }} {}→ SiO2SiO2 size 12{"SiO" rSub { size 8{2} } } {} + H2H2 size 12{H rSub { size 8{2} } } {} (5)

The final step of the etch process is the dissolution of the SiO2SiO2 size 12{"SiO" rSub { size 8{2} } } {} by HF (Eq. 6). Stirring serves to remove the soluble products from the reaction surface. The role of the HF is to act as a complexing reagent, and thus the reaction shown in Eq. 6 is known as a complexing reaction. The formation of water as a reaction product requires that acetic acid be used as a diluent (solvent) to ensure better control.

SiO2SiO2 size 12{"SiO" rSub { size 8{2} } } {} + 6 HF → H2SiF6H2SiF6 size 12{H rSub { size 8{2} } "SiF" rSub { size 8{6} } } {} + 2 H2OH2O size 12{H rSub { size 8{2} } O} {}(6)

The etching reaction is highly dependent on the relative ratios of the etchant reagents. Thus, if an HF-rich solution is used, the reaction is limited by the oxidation step (Eq. 4) and the etching is anisotropic, since the oxidation reaction is sensitive to doping, crystal orientation, and defects. In contrast, the use of a HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {}-rich solution produces isotropic etching since the dissolution process is rate limiting (Table 3). The reaction of HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {}rich solutions has been found to be diffusion-controlled over the temperature range 20-50 °° size 12{ {} rSup { size 8{°} } } {}C (Figure 8), and is therefore commonly employed for removing work damage produced during wafer fabrication. The boundary layer thickness (Figure 7) and therefore the dimensional control over the wafer is controlled by the rotation rate of the wafers. A common etch formulation is a 4:1:3 mixture of HNO3HNO3 size 12{"HNO" rSub { size 8{3} } } {} (79%), HF (49%), and MeCO2HMeCO2H size 12{"MeCO" rSub { size 8{2} } H} {} (99%). There are some etchant formulations that are based on alternative (or additional) oxidizing agents, such as: Br2Br2 size 12{"Br" rSub { size 8{2} } } {}, I2I2 size 12{I rSub { size 8{2} } } {}, and KMnO4KMnO4 size 12{"KMnO" rSub { size 8{4} } } {}.

Alkaline etching (KOH/ H2OH2O size 12{H rSub { size 8{2} } O} {} or NaOH/ H2OH2O size 12{H rSub { size 8{2} } O} {}) is by nature anisotropic and the etch rate depends on the number of dangling bonds which in turn are dependent on the surface orientation. Since etching is reaction rate limited no rotation of the wafers is necessary and excellent uniformity over large wafers is obtained. Alkaline etchants are used with large wafers where dimensional uniformity is not maintained during lapping. A typical formulation uses KOH in a 45% weight solution in H2OH2O size 12{H size 8{2}O} {} at 90 °° size 12{ {} rSup { size 8{°} } } {}C.

Etching Gallium Arsenide

Although a wide range of etches have been investigated for GaAs, few are truly isotropic. This is because the surface activity of the (111) Ga and (111) As faces are very different. The As rich face is considerably more reactive than the Ga rich face, thus under identical conditions it will etch faster. As a result most etches give a polished surface on the As face, but the Ga face tends to appear cloudy or frosted due to the highlighting of surface features and crystallographic defects.

As with silicon the etch systems involve oxidation and complexation. However, in the case of GaAs the gallium is already fully oxidized (formally Ga3+), thus, it is the arsenic (formally the arsenide ion, As 3-3- size 12{ {} rSup { size 8{"3-"} } } {}) that is oxidized by a suitable oxidizing agent (e.g., H2O2H2O2 size 12{H rSub { size 8{2} } O rSub { size 8{2} } } {}) to the soluble oxide, As2O3As2O3 size 12{"As" rSub { size 8{2} } O rSub { size 8{3} } } {} (Eq. 7). The gallium ions form the oxide