Cleaning Examples

Proof of Process

We present two simple cleaning examples that demonstrate the effectiveness of carbon dioxide snow cleaning.

Sherman{1994} has qualitatively demonstrated particle removal. The pair of micrographs at 1000x magnifications demonstrates particle removal effectiveness by comparing the exact same areas before and after cleaning. A silicon wafer was scribed with a carbide tip, and many micron and submicron particles were generated near the scratch. After CO2 snow cleaning, the second micrograph, also at 1000x magnification, shows no particles. This example demonstrates the cleaning of silicon particles from a substrate and this data would be typical of particle removal from many different substrates including other wafers, glass, ceramics, metals, etc.

 

Sherman{1994} provided further microscopic evidence of organic removal by comparing the exact same areas before and after cleaning a facial grease residue. A pair of micrographs is shown below at 1000x magnification of the same area of a scribed silicon wafer before and after cleaning. The initial wafer condition is shown below with extensive contamination:

and after CO2 snow cleaning, no contamination is observed.

This visual evidence of removing organic contamination is typical for many surfaces and materials.

The above two older examples are simple optical examples showing the power of CO2 Snow Cleaning. Many other examples have been shown, two older examples follow:

• Whitlock {1989} performed the first set of laboratory measurements that quantified the effectiveness of removing micron and submicron particles from a surface. The approach was to disperse an aerosol of micron and submicron particles on a wafer, count and size them, then clean the wafer with CO2 snow and count the remaining particles.
• Zinc orthosilicate powder was suspended in ethyl alcohol and sprayed on to a 2 inch silicon wafer with an airbrush. Spraying continued until a noticeable deposit was observed. An automated microprobe equipped with an energy dispersive spectrometer (EDS) set to detect only zinc-rich particles was used for particle counting and sizing. A total of 295 zinc-rich particles was identified after counting 100 frames, each frame being a 27.4 by 20.0 micron area. Next, the wafer was CO2 snow cleaned. After cleaning, the wafer was analyzed again in the microprobe and only 3 zinc rich particles were found after scanning 1600 frames. Normalizing the initial measurements to 1600 frames gives an initial particle population of 4720 particles. This gives a removal ratio of over 99.9% under laboratory cleanroom conditions. Another comparison would be to normalize the particle populations to a square centimeter. In this situation, an initial particle population of 538,00 particles per square centimeter was reduced to 342 particle per square centimeter for particles larger than 0.1 micron.
• Sherman and Whitlock{1991} demonstrated the effectiveness of carbon dioxide snow cleaning in removing organic residues from surfaces. Here, X-ray Photoelectron Spectroscopy (XPS) was used to investigate the surface chemistry of new silicon wafers, contaminated wafers, and then CO2 snow cleaned wafers. A fingerprint or facial grease was applied to the wafer and the extent of the contamination was measured by XPS. Next, each stain was removed using CO2 snow cleaning and these regions were analyzed by XPS. The typical background level for the surface hydrocarbons on new wafers ranged from 27 to 29 atomic percentage. The fingerprint and facial grease increased the contamination levels by a factor of 2 to 3. Cleaning with CO2 snow removed all visible signs of the contamination, and yielded hydrocarbon levels that were actually lower than the "new" wafer values (between 19 - 23 atomic percent). These results not only demonstrated that CO2 snow cleaning can remove contamination but it can also reduce the native hydrocarbon contamination found on many surfaces.