220.127.116.11 Pumping high gas loads by means of turbomolecular pumps
Turbopumps are subject to high stresses under high gas loads. Gas friction heats up the rotors. The maximum gas loads are limited by the permissible rotor temperature of 120 °C. Because the friction loss is proportional to the square of the peripheral speed, it is necessary to reduce the RPM of pumps that operate under high gas loads. This means that higher gas loads are attained at the expense of pumping speed, and in particular, at the expense of the compression ratio; this is not a major disadvantage for these kinds of pumps, as they are not used for generating high vacuum. Pumping heavy noble gases is particularly critical. Due to their high atomic weights, heavy noble gases generate large quantities of heat when they strike the rotor. As a result of their low specific thermal capacity, however, they can transfer only little heat to the stator or to the housing, which results in high rotor temperatures. The maximum gas loads for these gases are therefore relatively low.
When operating with process gases, the turbopump performs two important functions:
- Fast evacuation of the process chamber to a low pressure
(clean initial conditions)
- Maintaining the desired pressure at a constant level during the vacuum process
(coating, etching, etc.)
Gas throughput Q and working pressure pa during a process are typically specified, and thus the volume S = the process chamber, as well.
The turbopump will be selected on the basis of the required gas throughput. The maximum permissible gas throughputs for various gases are specified for the respective pumps in the catalog, with the throughput curves of turbopumps and backing pumps being used in this connection (Figure 7.4). The throughput must be the same for both pumps, because the same gas flow will pass through both pumps successively: Sv = .
The following rule of thumb applies for the backing pump: If the maximum gas throughput of the turbopump is attained, the pumping speed of the backing pump must be selected high enough so that only one half of the critical backing pressure will be utilized.
The volume flow rate at the process chamber is throttled to the required level by means of either RPM or a regulating valve. It is frequently not possible to employ regulation as a function of RPM, as it takes too long to set the desired pressure via RPM.
Let us consider a system in accordance with Figure 7.5.
|Q||=||20 mbar · l/s gas throughput|
|pa||=||0.05 mbar process pressure|
This results in a volume flow rate S of 400 l/s. We select a HiPace 2300 as the turbopump (2) and a Uno 120 as the backing pump (3). With this backing pump, we can attain a backing vacuum pressure of 0.8 mbar at a gas throughput of 20 mbar · 1/s, i.e. a little less than one half of the critical backing pressure of 1.8 bar.
The process gas is admitted to the chamber (1) via a gas flow regulator (5). The butterfly valve (4) that is controlled by pressure pa throttles the pumping speed of the turbopump (2). After the conclusion of the process step, the gas supply is shut off and the control valve opens completely to cleanly evacuate the chamber again. As this is happening, a new work piece is loaded into the process chamber. Further information relating to pumping high gas loads as well as corrosive and abrasive substances is provided in Section 2.7.3.
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