Many modern technological processes require clean high vacuum environments. Diffusion pumps (coupled with mechanical pumps) can achieve high vacuum pressures (3×10-8 mbar) and are relatively inexpensive and easy to maintain. Their greatest drawback is the backstreaming of oil vapors that could potentially contaminate the chamber where the process of interest is taking place.
Likewise, some of these processes produce, as byproduct, contaminants that must be kept from entering into the vacuum system: the pipes and, especially, the pumps. One example is the pulsed laser deposition of thin films of alloys containing elements with high vapor pressure, such as sulfur, selenium, or tellurium. These contaminants travel down the vacuum pipes and may not only be hazardous to the operator´s health but could also degrade the properties of the oils used in the pumps.
We propose a new baffle technology that overcomes the disadvantages associated with liquid nitrogen or compressor cooling systems. Our baffle is aimed at trapping both oil vapors going up the vacuum pipes (in the direction of the process chamber) and chalcogenide contaminant vapors (i.e. sulfur, selenium, or tellurium) going down the vacuum pipes (in the direction of the diffusion pump), while avoiding the undesirable partial cryopumping of water. This application does not require temperatures in the range of liquid nitrogen, but in the -50°C to -30°C range.
Our baffle is cooled down with thermoelectric coolers (TECs). Compared to compressor cooling systems, TECs have useful advantages for this application: they are compact, silent, vibration-free, almost maintenance-free, inexpensive, widely commercially available, and capable of providing a precise temperature control. The baffle is partly built inside a tee vacuum fitting. A refrigerated cooper helix provides the trapping surfaces for contaminants flowing between the process chamber and the vacuum pumps. All the components of this TEC-based baffle are either commercially available or simple to manufacture. The cost of the baffle is below USD 250 (manufacturing costs not included). The most expensive components are the tee (USD 80) and the four water blocks (USD 40).
Our TEC-based baffle may be used in small or medium sized high vacuum systems (up to NW50 flanges and pressures above 10-8 mbar). The baffle can be easily adapted for larger systems using other sizes or types of flanges. The vacuum seal may be improved for systems requiring ultra-high vacuum using a ceramic-to-metal seal, instead of the series of two silicone O-rings and PTFE spacers.
A virtual prototype, analyzed by the finite element method, proved that the trapping surfaces can reach -50°C when the baffle is cooled down with TEC1-12706 modules fed with 5A. Even lower temperatures could be achieved with higher power rating TECs and more efficient water blocks. However, -50°C is enough for trapping oil vapors and chalcogenide contaminants while avoiding the undesirable partial cryopumping of water. With a feed current of 4A, the helix of the baffle reach -48°C and consumes 215W, which is roughly half of the power requirement of a compressor used in baffles cooled down with pumped liquids.