Microelectronic Processes: An Introduction to the Manufacture of Integrated Circuits / Edition 1 available in Hardcover
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From Chapter 6: Etching
...6-5 Photoresist Removal
Upon completion of etching, the Photoresist pattern has served its purpose. if not now completely removed, it may contaminate diffusion furnaces or deposition furnaces with drastic results. Resist removal may be considered a form of etching and can be carried out by wet chemical or plasma methods. Resist removal is sometimes called stripping or cleaning.
Wet chemical removal of photoresist can be performed by any compound that decomposes organic molecules. If there is no metal on the substrate, strong oxidizing agents can be used. One possibility is "chromic acid," a solution of chromium oxide in concentrated sulfuric acid. Perhaps more popular are mixtures of hydrogen peroxide in sulfuric acid. These "peroxide sulfuric" solutions, formed by adding a few percent of hydrogen peroxide to concentrated sulfuric acid, destroy double bonds in the resist with great enthusiasm. The operating temperatures are usually 100-135 C. Peroxide is lost due to decomposition and evaporation, and must be periodically replenished. Sulfuric-peroxide stripping, though usually trouble-free, can leave a residue on substrates unless vigorous agitation is used during rinsing.
Sulfuric acid attacks metals and thus cannot be used to clean substrates containing metal films. A number of proprietary strippers for metallized substrates are on the market. Most of them contain phenols or sulfuric acids, and operate as strong detergents. These compounds are frequently not totally inert with metals, so thorough rinsing after cleaning is vital to prevent metal corrosion. Many of these compositions also emit toxic fumes, andare subject to environmental restrictions as to disposal.
Resist removal can also be performed in a plasma. An oxygen (O2) plasma will oxidize resist to carbon dioxide and water. This process is often called plasma ashing. It is performed in a simple barrel reactor, or asher, either with an end-point detector or simply by time. Oxygen plasmas are fairly unreactive with common microelectronic materials and can thus be used on metallized wafers. Ashers are also successful in removing the extremely tough resist layer often generated during ion implantation, which can resist even sulfuric peroxide stripping. Because of its convenience and safety, ashing is fast replacing most other stripping methods. However, cheap and dependable sulfuric-peroxide stripping remains competitive for nonmetallized substrates.
In some cases, resist patterning and stripping can be used to form a physical pattern without recourse to etching, by the means of lift-off processing. While not normally used in silicon integrated circuit manufacture, lift-off methods are common in some other applications, as we shall see in Chapter 12.
Figure 6-15 shows how lift-off patterning is accomplished. Figure 6-15a shows a resist pattern on a substrate. Particular attention has been given to making the walls of the resist pattern as steep as possible. In Fig. 6-15b, the film to be patterned has been deposited over the resist. Clearly, this method is not possible with thermal oxides, but can be done by some of the deposition techniques presented in Chapters 7 and 8 if the deposition temperature is low enough. Notice that, due to the steep resist walls, the film does not completely cover the edges at the steps. This failure of step coverage makes the lift-off possible.
The resist is now stripped using some wet chemical method. Figure 6-15c shows the result. Where the film was deposited directly on the substrate, film remains. Where the film rested on the resist, stripper has entered the gaps in the film and dissolved the resist, lifting away the film. The result is a pattern, the negative of the resist pattern, formed without etching.
Lift-off methods work best with metal films, which can often be deposited at low temperature. In this case, a solvent-type stripper is required to prevent attack on the film. Where a proper combination of film, resist, and stripper can be found, lift-off patterning offers the advantages of steep walls, no undercutting, no need for etching equipment, and applicability to extremely small dimensions.
Chemically, photoresist is not unlike human tissue, and resist strippers can do great damage to the careless user. All of the precautions mentioned before for wet etching should be observed. Sulfuric acid liberates an astounding amount of heat when mixed with water or water-containing solutions such as hydrogen peroxide. The general rule is to add acid to water rather than the reverse. When water must be added to acid, as in maintaining a sulfuric-peroxide bath, the addition should be made slowly and carefully, with full precautions against splattering or breakage of the acid container. Sulfuric acid particularly should not be aspirated while hot. When using proprietary strippers, obtain the Chemical Safety Data Sheet on the product and follow it. A qualified safety professional should be consulted for additional information.
6-6 Etch Process Control
As always, process control in etching is a matter of measuring desired output, detecting deviations, and relating them to variations in input. The output variable in etching is film removal, and etching problems thus consist of overetching and underetching.
Underetching, or incomplete film removal, is usually detectable by visual inspection. Observation under a microscope will show whether some of the film still remains in the patterned areas. If oxide or nitride is being removed from silicon, the cut will be colored, rather than "silicon-colored" (variously perceived as gray, tan, or white). if metal or polysilicon is incompletely etched, shiny areas or grains will be present. Use of a dark field attachment on a microscope, which emphasizes edges rather than surfaces, is particularly useful for spotting small aluminum or polysilicon grains. Incomplete resist removal appears as a brownish color of the substrate, usually visible to the naked eye. Incomplete ashing has a particularly characteristic appearance: a brown or yellow "bull's-eye" in the center of the wafer, caused by the nonuniform etch rate of a barrel reactor. (White light must be used to inspect for residual resist: the brownish hue is invisible under yellow or green light.) If one of several transparent layers is being etched, it may be necessary to measure the film thickness to ensure complete etching. The film thickness analyzers described in Chapter 2 are useful for this purpose.
For some plasma processes with poor selectivity, overetching may be shown by attack on underlying layers. In this case, visual inspection or film thickness measurement will again suffice. But when selectivity is good, downward etching will terminate when the film is completely etched. In this case, inspection of the bottom of the patterned area will reveal nothing. Overetching must then be measured by its effect in the lateral direction...
Table of Contents
A Perspective on Microelectronic Processing
The Physics and Chemistry of Semiconductors
Dopant Diffusion and Related Operations
Thin Films: Mainly Metals
Thin Films: Mainly Non-Metals
Ion Beams and Their Applications
The Fabrication of Bipolar Devices
Fabrication of MOS Devices
Fabrication of Non-Silicon Devices