2.2.3.2 Pumping high gas loads with 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. At temperatures above this level, irreversible plastic deformation of the rotors will occur with an unpredictable timescale.

By measuring the rotor temperature and restricting the maximum temperature, pumps in the HiPace series with pumping speeds of > 1,000 l s^-1 can be prevented from overheating. Precise characterization of the process allows the rotor temperature to be estimated for a large number of pumps and defines a process window for safe operation and long-term stability.

The suitability of a turbopump for pumping high gas loads can be influenced by the rotor and stator design as well as precise control of the temperature profile in the pump. ATH-M series pumps, for instance, are explicitly designed for high gas throughputs and comparatively high process pressures. These turbopumps were specially developed for coating and dry etching processes in the semiconductor industry. The specific challenges which they face here are pumping corrosive media, heated use of the pumps to prevent the condensation of process chemicals or secondary products and in particular high process gas throughputs for heavy gases. These developments can also be used in applications in the solar and LED lighting industries. The design of these turbopumps also allows them to be used in load locks with a high transfer pressure between the backing pumps and the turbopump as well as under industrial operating conditions with high cooling water temperatures.

Pumps designed specially for generating low pressures which are suitable for light gases due to their high compression ratios, can also within limits be used for vacuum processes with high gas throughputs. Because the friction power 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 measure can extend the process window for pumps.

Pumping heavy noble gases such as krypton or xenon is particularly critical. Due to their high atomic weight, 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 very little heat to the stator or to the housing, which results in high rotor temperatures. The maximum gas throughputs for these gases are therefore relatively low compared to gas molecules or monatomic gases with a lower mass, i. e. higher mobility and collision frequency.

When operating with process gases, the turbopump performs two important functions:

• Fast evacuation of the process chamber to a low pressure (clean initial conditions through degassing the surfaces and substrates)
• Maintaining the desired pressure at a constant level during the vacuum process (coating, etching, etc.)

Gas throughput $Q$ and working pressure $p_{process}$ during a process are typically specified, and thus the volume flow rate at the process chamber as well.

$S=\frac{Q}{p_{process}}$

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.

Figure 2.7: Gas throughput of different turbopumps at high process pressures

In Figure 2.7 the gas throughput graphs for different turbopumps with a NW 250 flange are given. The backing pump for the ATH 2303 is from a typical process-capable Roots pumping station as used in the semiconductor industry. The throughput must be the same for both pumps, because the same gas flow will pass through both pumps successively:

$S_{fore-vacuum}=\frac{Q}{p_{fore-vacuum}}$

The choice of backing pump affects the temperature balance of the turbopump. If the pumping speed of the backing pump is designed precisely so as to attain the turbopump's maximum fore-vacuum compatibility with its gas throughput, then the turbopump rotor will be thermally loaded. A backing pump with a higher pumping speed should be selected to reduce gas friction and the thermal load on the turbopump.

The pumping speed at the process chamber is restricted to the required level either through the RPM or a regulating valve. Pressure regulation using the speed of rotation of the turbopump is hampered by the high inertia of the rotor which prevents a faster variation of the rotation speed. In some process windows it is possible to control the pressure by regulating the speed of rotation of the backing pump.

Figure 2.8: Vacuum system with pressure and throughput regulation

Let us take as an example a vacuum process system as shown in Figure 2.8 with the parameters

Where $S=\frac{Q}{p_{process}}$

we obtain a nominal pumping speed for the turbopump of 600 l s^-1. At this high process pressure it is not possible to attain the maximum pumping speed for turbopumps. We therefore select a turbopump (2) of type ATH 2303 M which still attains a pumping speed of more than 800 l/s with a splinter shield at this pressure and a backing pump of type A 603 P. With this process pump we reach fore-vacuum pressure of 3.0 Pa with a gas throughput of 0.24 hPa m³ s^-1. With a maximum turbopump fore- vacuum pressure of 3.3 hPa, this configuration is conservative despite the thermally demanding process gas argon.

The process gas is admitted to the chamber (1) via a mass flow controller (5). The butterfly valve (4) that is controlled by pressure $p_{process}$ 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 evacuate the chamber until the final pressure is reached. As this is happening, a new workpiece is loaded into the process chamber. Further information relating to pumping high gas loads as well as corrosive and abrasive substances is provided in Chapter 4.8.3.