Vacuum pumping systems

Details Vacuum pumping systems Vacuum pumping systems

1998-08-12

2001-03-13

Thorpe, Timothy S. (Department: 3746)

Pumps

Successive stages

With interstage discharge or additional discharge from...

C417S307000, C417S520000

Reexamination Certificate

active

06200107

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to vacuum pumping systems and, more particularly, to such systems for use in controlling the pressure in a semiconductor processing chamber.
The requirements for a vacuum pumping system for use in the semiconductor industry are many and varied. In addition to evacuating the semiconductor processing chamber down to the required level of vacuum and exhausting the reaction gases used in the chamber in the manufacture of semiconductor devices from the chamber to atmosphere or to one many types of collection or scrubbing means, the pumping system is increasingly being used to control the pressures associated with the processing chamber by varying the rate at which the reaction gases are exhausted from the chamber.
In particular, there is a need in the semiconductor industry to provide a control on the pressure in the processing chamber independently of the process reaction gas flow quantity in, and from, the chamber. In addition, there may also be a need to provide a control on the reaction or other gas species present in the processing chamber in order to vary partial pressures of the reactive gases and reactive gas by-products, for example to exhaust the reactive by-products from the chamber at a rate faster than that of the reactive gases themselves or to promote means to reduce the time between cleaning operations in a chamber and the normal processing operations.
In a typical simple vacuum pumping system for use in the semiconductor industry, the processing chamber is connected to a system comprising a first vacuum pump (or pumps)—commonly a turbo-molecular pump—which is backed by a forepump (or pumps) connected to the first pump by a foreline and which can exhaust the gases from the semiconductor chamber to atmosphere.
In such a simple system, in an attempt to provide a means to exercise control on the pressure in the processing chamber to which it is attached, it has previously been prepared to provide a variable throttle valve either between the process chamber and the first pump(s) or in the foreline between the first pump(s) and the fore pump(s).
However, it has been found that the presence of a throttle valve can cause certain disadvantages. For example, if the throttle valve is at the inlet to the first pump, there is necessarily a restriction in to that pump even when the throttle valve is fully open, so that a larger and therefore more costly first pump (or pumps) is required.
If the throttle valve is in the foreline, the effect of it on the pumping rate of the first pump is to render it highly non-linear so that it becomes effective only over a narrow range of pressure. As such, the system as a whole is difficult to regulate in a stable manner if the process gas flow rate varies by a large amount.
In such a simple pumping system, it has also been proposed to introduce a variable flow of a ballast gas (or a spoiling gas) in to the foreline. However, this has generally not proved effective in allowing a control of the pressure in the processing chamber. In addition, the introduction of a ballast gas of a different composition to that used in the semiconductor processing may contaminate or dilute the process gases. If it is of the same composition, the flow rate may be large and therefore costly.
Furthermore, for the same general reasons, it has also been proposed to provide means to regulate the rotational speed of the first pump(s) or the forepump(s) or both. However, regulation of the rotational speed of the first pump, for example a turbo-molecular pump, cannot normally be achieved rapidly without requiring a large amount of extra power, due to the large moment of inertia of the pump rotor. This leads to the need for more expensive motor and drive electronics. Alternatively, the time required to regulate pressure in the processing chamber is long which in itself reduces the effectiveness of the pumping system as a whole.
Regulation of the rotational speed of the forepump suffers from the same disadvantages and, additionally, can make the pumping rate of the first pump non-linear and effective only over a narrow region of pressure.
Attempts to provide means to regulate the rotational speed of both the first pump and the forepump result in further expense and complexity and, in any event, do not fully overcome the disadvantages.
There is therefore a need for alternative means in such pumping systems for controlling the pressure as necessary in the processing chamber.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a vacuum pumping system for use with a process chamber, comprising a first vacuum pump whose inlet is adapted for communication via a first line with a chamber outlet and a further vacuum pump whose inlet is adapted for communication via a second line with a first pump outlet, wherein a third line containing a throttle valve is linked to the first and to the second lines in parallel to the first vacuum pump to enable variable amounts of gas to flow through the valve from the second line to the first line depending on the position of the valve member.
The system of the invention therefore includes a recirculating loop for exhaust gases that have passed through the first pump back to the inlet of the first pump in amounts (including zero) dependent on the degree of opening of the throttle valve.
When the throttle valve is at least partially open, the gas will flow from the second line to the first line by means of the pressure differential across the first pump.
It has been found that the restriction caused by the throttle valve is greater for the reactive gases (typically having lighter molecular mass) than for the reaction by-product gases (typically having heavier molecular mass). This causes the pumping system to remove reactive gases from the chamber more quickly than the reaction by-product gases.
As such in the invention, because the restriction presented by the recirculation loop is greater for reactive gases than for reaction by-product gases, the loop modifies the pumping characteristic of the system so as to improve the pumping of by-products in relation to reactive gases.
The conductance of the throttle valve can be regarded as being inversely proportional to the square of the molecular mass of the gas passing through. This has been found to be a key reason why a throttle valve positioned at the inlet to the pump (as described above) causes light gases to be pumped more quickly than heavy ones and therefore why it is advantageous to eliminate the inlet throttle.
Similarly, it has been found that the throttle loop recirculates light gases more readily than heavy ones and therefore the addition of the throttle loop can suppress the pumping of lighter gases.
Additionally, the reactant gases are generally lighter than the reaction by-products and therefore it has been found that the combined effect of removing the inlet throttle and adding the throttle loop causes the by-products to be pumped preferentially in relation to the reactant gases.
The first pump preferably comprises a turbo-molecular pump having a stator and a rotor with both having a number of arrays of angled blades to effect a pumping action in a manner known per se. The first pump may have additional stages of the same or different type or may comprise two or more separate pumps collectively referred to as the “first pump”.
In preferred embodiments, the first pump comprises a turbo-molecular pump and one or more molecular drag or regenerative stages contained in the same pump body.
In certain embodiments, when the first vacuum pump comprises one or more stages in the same pump or two or more separate pumps, the third line containing the throttle valve should link the first line at the first inlet to the first pump but may be linked at its other end to the outlet of any of the first pump stages.
The second pump may comprise any type of vacuum pump normally used for backing a turbo molecular pump and cable of delivering the gases exhausted from the system to atmospheric pressure. The second pump may therefore

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