• through the bypass line using the mech pump alone, and
• using the turbo system, preferably with both turbo and mech pumps powered.

Each of these brings a different advantage to the pumpdown approach.  The bypass line can be chosen with a limited conductance to achieve a fairly slow pumpdown in the higher pressure regime, thereby minimizing particle generation. Indeed, only the mech pump is effective at these pressures. Furthermore, any slow initial pumpdown will require some means to limit conductance, which is fundamentally incompatible with the high conductance desired once pressures are sufficiently low that particle generation is not a concern.

The turbo system offers not only a higher pumping speed than does a limited-conductance connection, but also the capability to maintain high pumping speed down to very low base pressures (to the 10^-9 torr range), so that reactive impurities which would degrade process and material quality are removed as much as possible.

Manufacturing practice
The pumpdown and venting of a reaction chamber is very crucial for manufacturing throughput, since increasingly common single-wafer process chambers - even in cluster tools - require some pumpdown and venting operations for each wafer.  The time spent in these operations is considerable; and if the turbo rotors had to be cycled from off to full speed and back, this overhead time would be increased still further.

The preferred approach is to combine continuous operation of  the turbo pump system with pumpdown and venting cycles carried out through a bypass line.  Suppose a second mech pump is available which is dedicated to pumping the bypass line to the reaction chamber. Then the operational sequence is as follows.  First the turbo pump system is put into continuous operation, reducing pressure in the turbo pump chamber, with the valve V2 closed between the turbo pump and the reaction chamber; the turbo pump system is then left running continously. Then the bypass pumping system is employed for slow pumpdown of the reaction chamber, so that particle generation is avoided.  When the chamber pressure is low enough (about 10 torr), the bypass line valve VB is closed, and the turbo pump system is opened to the reaction chamber to rapidly complete the pumpdown to very low pressure.  The venting operation is just the reverse.  First the turbo valve V2 is closed to isolate the reaction chamber.  Then the venting system valve VV is opened to bring the reaction chamber pressure back to atmospheric pressure, at which point a wafer exchange could be accomplished to remove the processed wafer and take in the next wafer.

In reality, pumpdown and venting to atmosphere constitute only a portion of the process cycle carried out in a reaction chamber.  Additional reactive gases, heating, and possible plasma discharge initiation are used at various intermediate pressures to accomplish low pressure deposition or etching.  And in cluster tools, wafer insertion and removal at the reaction chamber typically requires pressures well below atmospheric.  These concepts and practices are described in detail in other modules.

In practice, a separate, dedicated mech pump for the bypass line may not be required.  The cutoff valve V1 between the turbo and its backing mechanical pump can be closed for some time once the turbo pump system has achieved very low pressures, since the turbo exhaust pressure can only rise very slowly if the turbo chamber is isolated from a gas flow point of view.  Then for slow initial pumpdown, valve V1 can be closed while leaving the turbo powered, valve VB opened to accomplish the bypass line pumpdown of the chamber, VB then closed, V1 reopened, and finally the gate valve V2 reopened to complete chamber pumpdown to very low pressures by the turbo pump system.

Exercises

Exercise 1: Demonstrate this approach through the following steps. [Run Example] First, initiate continuous turbo pump system operation:  Start/restart the simulator (stop if already running).  Turn on the mechanical pump , open valve V1, and turn on the turbo pump. Observation: The turbo pump stack pumps down to very low pressure, while the reaction chamber remains isolated at atmospheric pressure.    Now pump down the reaction chamber using the bypass line and mechanical pump:  Close valve V1, isolating the turbo pump from the mech pump.  Open valve VB to slowly pump down the reaction chamber through the bypass line.  When about 10 torr is attained in the reaction chamber, close VB.  Open valve V1 to re-establish turbo pump backing.  The open valve V2 to use the turbo pump system to evacuate the reaction chamber down to very low pressure. Observation: Pumpdown is slow during bypass pumping at higher pressures, then rapid through the turbo pump system to reach very low pressures.    Now vent the reaction chamber to complete the cycle:  Close valve V2, isolating the turbo pump system from the reaction chamber.  Open VV to vent the reaction chamber back to atmospheric pressure.    Manufacturing requires high process throughput, of order 2 minutes per wafer.  Questions  How much time per cycle is required for slow initial pumpdown followed by fast completion to 10-7 torr given that the turbo pump system is already at base pressure?  Answers

 

Summary
A turbo pump system in combination with slow bypass pumping using a mechanical pump is an effective combination for minimizing particle generation associated with rapid pumpdown at high pressure, together with rapid final pumpdown to very low pressures.

Significant time is required for the overhead operations of pumpdown and venting in vacuum processing systems.

Cluster tools have potential advantages in reducing the time budget associated with pumpdown and venting operations per process step.