The usual reason for the constricted line from mechanical pump to the chamber is to slow down the initial part of the pumpdown to avoid contamination effects due to adiabatic cooling. Basically, when you quickly evacuate a chamber, if there is any water vapor present, the rapid expansion of the gas will cause cooling (to 10s of degrees below zero) and the condensable vapor will freeze and settle on the substrates causing defect points. This is especially important in semiconductor and critical thin film applications where any such defects will lower yield. This slow, or soft, pump takes place from atm pressure to about 10 Torr where the gas starts to become less viscous and this effect disappears.
A great tutorial on diff pump system operation is on Roy Schmaus' page at http://www.ee.ualberta.ca/~schmaus/vacf/vacsys.html
Roy is at the Univ. Alberta and his main page is at http://www.ee.ualberta.ca/~schmaus/vacf/index.html
Steve
Created on Sunday, May 06, 2001 8:32 AM EDT by Steve Hansen