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 system, in which etchant modifiers such as acetic acid also commonly used. In the case of GaAs HCl-HNO₃ 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 (HNO₃) and acetic acid (MeCO₂H) 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 (HNO₃) in water or acetic acid (MeCO₂H). The etching involves a reduction-oxidation (redox) reaction, followed by dissolution of the reaction products. In the HF-HNO₃ system the HNO₃ oxidizes the silicon and the HF removes the reaction products from the surface. The overall reaction is:

(1)

The oxidation reaction involves the oxidation of Si⁰ to Si⁴⁺, and it is auto-catalytic in that the reaction product promotes the reaction itself. The initial step involves trace impurities of HNO₂ in the HNO₃ solution (Equation 2), which react to liberate nitrogen dioxide (NO₂), Equation 3.

(2)

(3)

The nitrogen dioxide oxidizes the silicon surface in the presence of water, resulting in the formation of Si(OH)₂ and the reformation of HNO₂ (Equation 4). The Si(OH)₂ decomposes to give SiO₂ (Equation 5). Since the reaction between HNO₂ and HNO₃ (Equation 2) is rate limiting, an induction period is observed. However, this is overcome by the addition of NO₂^- ions in the form of [NH₄][NO₂].

(4)

(5)

The final step of the etch process is the dissolution of the SiO₂ by HF (Equation 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 Equation 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.

(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 (Equation 4) and the etching is anisotropic, since the oxidation reaction is sensitive to doping, crystal orientation, and defects. In contrast, the use of a HNO₃-rich solution produces isotropic etching since the dissolution process is rate limiting (Table 3). The reaction of HNO₃-rich solutions has been found to be diffusion-controlled over the temperature range 20 - 50 °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 HNO₃ (79%), HF (49%), and MeCO₂H (99%). There are some etchant formulations that are based on alternative (or additional) oxidizing agents, such as: Br₂, I₂, and KMnO₄.

Alkaline etching (KOH/H₂O or NaOH/H₂O) 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 H₂O at 90 °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- that is oxidized by a suitable oxidizing agent (e.g., H₂O₂) to the soluble oxide, As₂O₃ (Equation 7). The gallium ions form the oxide Ga₂O₃ via the hydroxide (Equation 8). Both oxides are soluble in acid solutions, resulting in their removal from the surface.

(7)

(8)

The peroxide based oxidative etches for GaAs are divided into acidic and basic etches. The composition and application of some of these systems are summarized in Table 5. The most widely used of these is H₂SO₄/H₂O₂/H₂O and is referred to as Caro's acid. The high viscosity of H₂SO₄ results in diffusion-limited etching with high acid concentrations. Etches with low acid concentrations tend to be anisotropic. Phosphoric acid (H₃PO₄) or citric acid (Figure 9) may be exchanged for sulfuric acid (H₂SO₄). Replacement of the acid component with bases such as NH₄OH or NaOH can result in near to truly isotropic etchants, although certain combinations can result in strong anisotropy.

Figure 9: Structure of citric acid.

One of the earliest etching systems for GaAs is based on the use of a dilute (ca. 0.05 vol.%) solution of bromine (Br₂) in ethanol. The Br₂ acts as the oxidant, resulting in the formation of soluble bromides. The etch rate of this system is different for different crystallographic planes, i.e., the etch rates for the (111) As, (100), and (111) Ga faces are in the ratio 6:5:1, although more uniform etch rates are observed with high Br₂ concentrations (ca. 10 vol.%). These higher concentration solutions are used for the removal of damage due to cutting with the saw.

Differences between polishing and lapping

If the surface of a wafer that has undergone lapping (or grinding) is examined with an electron microscope, cracks, ridges and valleys are observed. The top "relief layer" consists of peaks and valleys. Below this layer is a damaged layer characterized by microcracks, dislocations, slip and stress. Figure 10 shows a schematic representation of the abraded surface. Both of these layers must be removed completely prior to further fabrication. Decreasing the particle size of the abrasive during lapping only decreases the scale of the damage, but does not eliminate it entirely. In fact this surface damage is a characteristic of the brittle fracture of single crystal Si and GaAs, and occurs because during lapping the abrasive grains are moved across the surface under a pressure beyond that of the fracture strength of the wafer materials (Si or GaAs). In contrast to the mechanical abrasion employed in lapping, polishing is a mechano-chemical process during which brittle fracture does not occur. A polished wafer does not display any evidence of a relief surface such as that produced by lapping, even at highest resolution electron microscope.

Figure 10: Schematic representation of a cross sectional view of an abraded wafer surface prior to polishing.

Process of Polishing

id13362530 shows a schematic of the polishing process. Polishing may be conducted on single wafers or as a batch process depending on the equipment employed. Single wafer polishing is preferred for larger wafers and allows for better surface flatness. In both processes, wafers are mounted onto a fixture, by either wax or a composite Felx-Mount™, and pressed against the polishing pad. The polishing pad is usually made from an artificial fabric such as polyester felt-polyurethane laminate. Polishing is accomplished by a mechano-chemical process in which aqueous polishing slurry is dripped onto the polishing pad, see id13362530. The polishing slurry performs both a chemical and mechanical process, and consists of fine silica (SiO₂) particles (100 Å diameter) and an oxidizing agent. Aqueous sodium hydroxide (NaOH) is used for Si, while aqueous sodium chlorate (NaOCl) is preferred for GaAs. Suspending agents are usually added to prevent settling of the silica particles. Under the heat caused by the friction of the wafer on the polishing pad the wafer surface is oxidized, which is the chemical step, while in the mechanical step the silica particles in the slurry abrade the oxidized surface away.

Figure 11: Schematic representation of the wafer polishing process.

In order to achieve a reasonable rate of removal of the relief and damaged layers and still obtain the highest quality surface, the polishing is done in two steps, stock removal and haze removal. The former is carried out with a higher concentration slurry and may proceed for about 30 minutes at a removal rate of 1 μm/min. Haze removal is performed with a very dilute slurry, a softer pad with a reaction time of about 5 to 10 minutes, during which the total amount of material removed is only about 1 μm. Due to the active chemical reaction between the wafer and the polishing agent, the wafers must be rinsed in deionized water immediately after polishing to prevent haze or stains from reforming.

There are many variables that will influence the rate and quality of polishing. High pressure results in a higher polishing rate, but excessive pressure may cause non-uniform polishing, excessive heat generation and fast pad wear. The rate of polishing is increased with higher temperatures but this may also lead to haze formation. High wheel speeds accelerate the polishing rate but can raise the temperature and also results in problems in maintaining a uniform flow of slurry across the pad. Dense slurry concentrations increase the polishing rate but are more costly. The pH of the slurry solution can also affect the polishing rate, for example the polishing rate of Si gradually increases with increased pH (higher basicity) until a pH of about 12 where a dramatic decrease is observed. In general, the optimum polishing process for a given facility depends largely upon the interplay of product specification, yields, cost, and quality considerations and must be developed uniquely. The wafer polishing process does not improve the wafer flatness and, at best, polishing will not degrade the wafer flatness achieved in the lapping operation.

Cleaning silicon

The first step in cleaning a Si wafer is removal of all physical contaminants. These contaminates are removed by rinsing the wafer in hot organic solvents such as 1,1,1-trichloroethane (Cl₃CH₃) or xylene (C₆H₄Me₂), accompanied by mechanical scrubbing, ultrasonic agitation, or compressed gas jets. Removal of the majority of light metal contaminants is accomplished by rinsing in hot deionized water, however, complete removal requires a further more aggressive cleaning process. The most widely used cleaning method in the Si semiconductor industry is based on a two step, two solution sequence known as the “RCA Cleaning Method”.

The first solution consists of H₂O-H₂O₂-NH₄OH in a volume ratio of 5:1:1 to 7:2:1, which is used to remove organic contaminants and heavy metals. The oxidation of the remaining organic contaminants by the hydrogen peroxide (H₂O₂) produces water soluble products. Similarly, metal contaminants such as cadmium, cobalt, copper, mercury, nickel, and silver are solubilized by the NH₄OH through the formation of soluble amino complexes, e.g., Equation 9.

(9)

The second solution consists of H₂O-H₂O₂-HCl in a 6:1:1 to 8:2:1 volume ratio and removes the Group I(1), II(2) and III(13) metals. In addition, the second solution prevents re-deposition of the metal contaminants. Each of the washing steps is carried out for 10 - 20 min. at 75 - 85 °C with rapid agitation. Finally, the wafers are blown dry under a stream of nitrogen gas.

Cleaning GaAs

In principle GaAs wafers may be cleaned in a similar manner to silicon wafers. The first step involves successive cleaning with hot organic solvents such as 1,1,1-trichloroethane, acetone, and methanol, each for 5-10 minutes. GaAs wafers cleaned in this manner may be stored under methanol for short periods of time.

Most cleaning solutions for GaAs are actually etches. A typical solution is similar to the second RCA solution and consists of an 80:10:1 ratio of H₂O-H₂O₂-HCl. This solution is generally used at elevated temperatures (70 °C) with short dip times since it has a very fast etch rate (4.0 μm/min).