Carbon Dioxide Snow Cleaning
by Applied Surface Technologies
Five typical examples:
1 - Online Demonstration it's right below!
2 - Cleaning Si wafers and two inch slot nozzle demo demos
3 - Particle removal seen in electron microscopy
4 - Optical microscopy showing particle and hydrocarbon removal
5 - AFM image showing particle removal
6 - AFM image improvement
7 - Sharpie Removal
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Example 2 - Cleaning a Si Wafer Demo - Video on left - We contaminated a Si wafer with facial grease at a trade show. Cleaning was quick and easy as seen in the video below. The sample is held by a vacuum chuck that was on a hot plate at about 40 C. Video on right - Another Si wafer on a stainless steel plate heated to about 50 C. Video courtesy of Tectra GmbH.
A K1-10-A cleaning a mirror
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Example 1 - Particle Removal At the 2004 Fine particle Society, Jacques C.J. van der Donck et al., at TNO in the Netherlands, showed SEM images of similar areas at 20,000X magnification. Silica particles, with sizes in range from 30 to 100 nanometers were dispersed on a Si wafer and particles were counted. After cleaning, no particles were found in many test image areas. These images and their counting statistics clearly show that the CO2 Snow Jet can remove particles down to 30 nanometers or 0.03 microns with excellent efficiency. Later work by another group lowered the limit to 3 to 5 nanometers. Images can be magnified
Example 4 - Optical Microscopy example - 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.
Before Cleaning, 1000X After Cleaning, 1000X
Sherman {1994} also 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. XPS data also has shown hydrocarbon and organice removal.
Before Cleaning, 1000X After Cleaning, 1000X
Example 5- AFM and Particle Removal These images are courtesy of the research group of Prof. Stephan Herminghaus now at Univ of Ulm and Karen Jacobs, now at Saarland. The researchers were studying polymer film dewetting from silica surfaces and the first step in their process was to cut and clean the quartz substrates. They ultrasonically cleaned the quartz samples in acetone, ethanol, and toluene. Initial AFM examination of the quartz films revealed extensive contamination all over the samples. In this image, many particles ranging in size as small as 15-20 nanometers are seen in the 2µ by 2µ image.
They cleaned the surface with the CO2 snow jet to acceptable levels as seen in the second image. We observe that particles smaller than 0.02µ (20 nanometers) are removed. This is direct evidence that snow cleaning can remove particles smaller than 0.1µ, the previous limit quoted in the literature. More examples are found on the AFM page. Recent work by Chernoff and Sherman has lowered the measured particle removal limit to 3 to 5 nanometers as discussed on the AFM page.
Example 6- AFM Image Improvement - Chernoff and Sherman in 2009 showed effective cleaning of heavily contaminated AFM step height standards. Particle removal was seen down to the range of 3 to 5 nanometers without any damage to the step height value. This data is discussed on the AFM page. Here, we show another example from the JVST paper in which surface cleaning made AFM imaging much easier. The image on the left is typical of many samples. Cleaning this sample with CO2 Snow for 2 seconds led to a clear image.
Example 7 - Sharpie Removal - Four smiling faces were drawn on a Si wafer and using liquid CO2, we removed two of them
