晶圆清洗：湿法仍然领先

There are two principal methods of  cleaning wafers: gas-phase - dry cleaning - (Figs. 1 & 2) and liquid phase - wet cleaning - (Figs. 3 & 4.) Although the respected “standard clean,”  the RCA wet clean, continues its foothold, other technologies, including dry  methods, are beckoning to users.

Dr. Werner Kern, then at RCA Laboratories, devised the RCA clean for  RCA’s use in 1965. He detailed the  process publicly five years later. (Table  2). This two-step clean is an oxidizing  and complexing treatment. It employs  aqueous H202 - NHIOH and Hz02 - HC1  mixtures at 75-80°C for 10 min.

The use of advanced wetchemical cleaning for making  ultrapure silicon wafers will  persist for several more years, ‘  says Kern. The trend, however, 7  is from liquid to gaseous reac- 7  tants.

“Eventually and ideally,” Kern adds,  the entire cleaning sequence will be conducted in situ by a sequence of gas  phase reactions at low or reduced pressure with elevated temperature. This  will take place in a cluster tool integrated with other cluster modules for  film deposition, annealing, dry etching  and other fabrication steps.

He doesn’t expect dry cleaning to replace wet cleaning, except in applications where wet cleans cannot be used. tinue to be important, other factors - surface roughness, surface termination  and oxide and growth characteristics - need to be completely understood, Deal  says. Not only that, but cleaning success will increasingly be “based on electrical test as opposed to particles and  metals.’’

In fact, Deal, whose firm is one of  the largest suppliers of cleaning equipment, calls for a “holistic approach’’ to  wafer cleaning needs. This means that  the relationship between cleaning and  the process steps prior and after must  be understood and development completed together.

Tomorrow’s problem, being generally  recognized today, is the difficulty of removing smaller particles in the 0.1 pm  (and smaller) range, adds Robert Donovan of Research Triangle Institute.  Most of today’s particle removal technology becomes less effective, Donovan  adds, as particle size decreases.

The future of cleaning development  will rely on application-specific uses,  says Dr. Donald Deal, chief technologist  for FSI International. Each clear., Deal  adds, will have specific requirements  dictated by the subsequent processing  step. The variation in these requirements will cause a variation in the clean  technology, sequencing and tool that  the process engineer selects. This, Deal  contends, will lead to variations in  chemical sequence, chemical concentrations and changes to new chemistries.  The bottom line: “All basic cleaning  technologies, wet, vapor or dry, will be  required and used either separately or  in combination.”

Fig1

Wet cleans, based on the RCA clean,  will not soon disappear, despite some  hardy band pushing for their elimination. Dry cleaning techniques, as Motorola’s DePinto notes, “are pretty application-specific.”

Typically,” says Micron’s Paduano,  “alternative cleaning methods have not  been implemented where high throughput is needed.” Look for the use of both  wet and dry cleans to become quite application-specific and to work side-byside.