Formation of silicon and gallium arsenide wafers  (Page 11/11)

Cleaning

During the processes described above, semiconductor wafers are subjected to physical handling that leads to significant contamination. Possible sources of physical contamination include:

1. airborne bacteria,
2. grease and wax from cutting oils and physical handling,
3. abrasive particulates (usually, silica, silicon carbide, alumina, or diamond dust) from lapping, grinding or sawing operations,
4. plasticizers which are derived from containers and wrapping in which the wafers are handled and shipped.

Chemical contamination may also occur as a result of improper cleaning after etch steps. Light-metal (especially sodium and potassium) species may be traced to impurities in etchant solutions and are chemisorbed on to the surface where they are particularly problematical for metal oxide semiconductor (MOS) based devices, although higher levels of such impurities are tolerable for bipolar devices. Heavy metal impurities (e.g., Cu, Au, Fe, and Ag) are usually caused by electrodeposition from etchant solutions during fabrication. While wafers are cleaned prior to shipping, contamination accumulated during shipping and storage necessitates that all wafers be subjected to scrupulous cleaning prior to fabrication. Furthermore, cleaning is required at each step during the fabrication process. Although wafer cleaning is a vital part of each fabrication step, it is convenient to discuss cleaning within the general topic of wafer fabrication.

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 (Cl ₃ CH ₃ ) or xylene (C ₆ H ₄ Me ₂ ), 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 H ₂ O-H ₂ O ₂ -NH ₄ OH 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 (H ₂ O ₂ ) produces water soluble products. Similarly, metal contaminants such as cadmium, cobalt, copper, mercury, nickel, and silver are solubilized by the NH ₄ OH through the formation of soluble amino complexes, e.g., [link] .

The second solution consists of H ₂ O-H ₂ O ₂ -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 H ₂ O-H ₂ O ₂ -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).

Measurements, inspections and packaging

Quality control measurements of the semiconductor crystal and subsequent wafer are performed throughout the process as an essential part of the fabrication of wafers. From crystal and wafer shaping through the final wafer finishing steps, quality control measurements are used to ensure that the materials meets customer specifications, and that problems can be corrected before they create scrap material and thus avoid further processing of reject material. Quality control measurements can be broadly classified into mechanical, electrical, structural, and chemical.

Mechanical measurements are concerned with the physical dimensions of the wafer, including: thickness, flatness, bow, taper and edge contour. Electrical measurements usually include: resistivity and lateral resistivity gradient, carrier type and lifetime. Measurements giving information on the perfection of the semiconductor crystal lattice are classified in the structural category and include: testing for stacking faults, and dislocations. Routine chemical measurements are limited to the measurement of dissolved oxygen and carbon by Fourier transform infrared spectroscopy (FT-IR). Finished wafers are individually marked for the purpose of identification and traceability. Packaging helps protect the finished wafers from contamination during shipping and storage.

Industry standards defining in detail how quality control measurements are to be made and determining the acceptable ranges for measured values have been developed by the American Society of Testing Materials (ASTM) and the Semiconductor Equipment and Materials Institute (SEMI).

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