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CO2 Snow Jet Sample Cleaning

Content:


CO2 Snow Jet Cleaning uses a carbon dioxide snow plume to remove micron and sub
micron particles and hydrocarbon based contamination. The CO2 Snow Jet Cleaning is
  • Non-destructive
  • Non-abrasive
  • Residue-Free
  • Environmentally Safe

Equipment

Snow cleaning systems are for the most part straightforward. They consist of a CO2 source, a nozzle with an internal orifice, and the means to transport the CO2 from the source to the nozzle.A typical system, such as shown below, consists of a CGA320 fitting, tubing, an on/off gun or valve and a nozzle. Most units are made with a PTFE lined flexible stainless steel hosing (we do offerall stainless steel units). Available on/off controls include solenoid, pneumatic, or manual valves, and hand guns. The standard unit from Applied Surface Technologies shown below is supplied with two nozzles - one polymer and one stainless steel, an on/off gun, a 10 foot flexible stainless steel PTFE lined hose, a CGA320 cylinder fitting, an optional 0.5 micron sintered stainless steel filter, and a 0 - 2000 psi pressure gauge.
 CO2 Snow Jet Cleaning

The high purity unit from Applied Surface Technologies shown below has a packless electropolished stainless steel diaphram valve to control carbon dioxide flow. Also shown is a 0.01 micron filter, a polymer nozzle, and a 10 foot flexible stainless steel PTFE lined hose. The CGA320 fitting can be electropolished if requested. All fittings for this unit are compression fittings as opposedto NPT fittings for the standard unit shown above. A stainless steel nozzle also comes with this unit. This unit can be made cleanroom ready.

 High Purity Unit

Two nozzle come with the standard and high purity units - one polymer (a fluorpolymer) and the other stainless steel or brass.The metal nozzles are stable for either gas or liquid carbon dioxide feeds while the polymer nozzle is stable for gaseous carbon dioxide feed. Further, the polymer nozzle always has a lower flow rate than the metal nozzles though all have the same internal dimensions.The standard nozzles have a 16 mil orifice though other diameters ranging from 8 to 39.5 mils have been made and are available.

The nozzle design is by far the most important factor in performing CO2 snow cleaning. Nozzles involve either a single or double expansion arrangement. The simplest nozzles are the single expansion nozzles with one orifice and are usually variations of a Venturi orifice design. The exit side is more typically a nosecone and this chamber is for dry ice nucleation. Its angle and length play an important role in determining the velocity of the stream and the snow size. Other nozzle designs are available including small diameter orifice tubes, metering leak valves or others. Different cleaning abilities can result from different designs.

The nozzles from Applied Surface Technologies can operate with gas or liquid CO2 while other nozzle geometries do not. This difference in operation is related to the fact that other commecial nozzles have a sudden expansion internal to the nozzle. This sudden expansion violates the need for a constant enthalpy expansion and causes the enthalpy to decrease enough to exit the mixed gas-liquid phase region shown earlier. When these nozzles are used with a liquid source, the same inefficiency is present but does not interfere with snow formation or cleaning - just wastes carbon dioxide.

The nozzles described above are point sources and clean about a 1/4 inch diameter. Layden in 1990 introduced large area double expansion nozzles and made units ranging from 1/2 inch to over 28 inches in length. The picture below shows a two inch wide large area nozzle along with the standard nozzle and an inner tube cleaning nozzle.


Nozzle
Automated Cleaning Systems

The CO2 Snow Jet Clening System can be used in a new fully automated cabinet. The sample is inserted on a X, Phi table for scanning linear and rotational patterns. The CO2 Snow Jet Cleaning nozzle is positioned on a X, Y motion device. Both moving mechanismes (sample nad nozzle) can be programmed with an easy teach-in mode for reproducible and optimised sample cleaning.
The cabinet has a venting system to enable the cleaning process under gas, i.e. N2. In the bottom part of the cabinet is the gas supply. The gas is inserted in the cleaning chamber through bottom slits and vented through the top part of the cabinet. Two temperature PID controller control the gas temperature.


 Automated Cleaning System   Cleaning Cabinet



Cleaning Examples

-- Proof of Process --

We present several cleaning examples that demonstrates the effectiveness of carbon dioxide snow cleaning.
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 air brush. 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 countingand sizing. A total of 295 zinc-rich particles was identified after counting 100 frames, each frame being a 27.4 by 20.0 micronarea. 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 (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 ofsilicon dust 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 belowat 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. 
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 27to 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. Further tests on cleaning new, uncontaminated wafers indicated the potential for reductions in hydrocarbons on the order of 50%.


Link to: Snow Jet Cleaning


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  contact : Dipl.-Ing. Andreas Gati
tectra GmbH
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D-60323 Frankfurt
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fax: Germany (0) 69 - 72 04 00
email: info@tectra.de
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  last update: 27.5.06
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