GENERAL VACUUM TECHNIQUES

                       
                          GENERAL VACUUM TECHNIQUES

INTRODUCTION

In this unit we discuss the procedures and equipment used to maintain a vacuum system at pressures in the range from 10-3 to 10-11 torr. Total and partial pressure gauges used in this range are also described.
Because there is a wide variety of equipment, we describe each of the various components, including details
of their principles and technique of operation, as well as their recommended uses. SI units are not used in this unit. The American Vacuum Society attempted their introduction many years ago, but the more traditional units continue to dominate inthis field in North America. Our usage will be consistent with that generally found in the current literature. The following units will be used.

Pressure is given in torr. 1 torr is equivalent to 133.32 pascal (Pa).
Volume is given in liters (L), and time in seconds (s). The flow of gas through a system, i.e., the ‘‘throughput’’
(Q), is given in torr-L/s. Pumping speed (S) and conductance (C) are given in L/s.

PRINCIPLES OF VACUUM TECHNOLOGY

The most difficult step in designing and building a vacuum system is defining precisely the conditions required to fulfill the purpose at hand. Important factors to consider include:

1. The required system operating pressure and the gaseous impurities that must be avoided;
2. The frequency with which the system must be vented to the atmosphere, and the required recycling time;
3. The kind of access to the vacuum system needed for the insertion or removal of samples.

For systems operating at pressures of 10-6 to 10-7 torr,venting the system is the simplest way to gain access, but for ultrahigh vacuum (UHV), e.g., below 10-8 torr, the pumpdown time can be very long, and system bakeout would usually be required. A vacuum load-lock antechamber for the introduction and removal of samples may be essential in such applications. Because it is difficult to address all of the above questions,
a viable specification of system performance is often neglected, and it is all too easy to assemble a more sophisticated and expensive system than necessary, or, if budgets are low, to compromise on an inadequate system that cannot easily be upgraded. Before any discussion of the specific components of a vacuum system, it is instructive to consider the factors that govern the ultimate, or base, pressure. The pressure can be calculated from
P =Q/S  ------(1)

where P is the pressure in torr, Q is the total flow, or throughput of gas, in torr-L/s, and S is the pumping speed in L/s.
The influx of gas, Q, can be a combination of a deliberate influx of process gas from an exterior source and gas originating in the system itself. With no external source, the base pressure achieved is frequently used as the principle indicator of system performance. The most important internal sources of gas are outgassing from the walls and permeation from the atmosphere, most frequently through elastomer O-rings. There may also be leaks, but these can readily be reduced to negligible levels by proper system design and construction. Vacuum pumps also contribute to background pressure, and here again careful selection  and operation will minimize such problems.

The Problem of Outgassing Of the sources of gas described above, outgassing is often the most important. With a new system, the origin of outgassing may be in the manufacture of the materials used in construction, in handling during construction, and in exposure of the system to the atmosphere. In general these
sources scale with the area of the system walls, so that it is wise to minimize the surface area and to avoid porous materials in construction. For example, aluminum is an excellent choice for use in vacuum systems, but anodized aluminum has a porous oxide layer that provides an internal surface for gas adsorption many times greater than the apparent surface, making it much less suitable for use in vacuum.

The rate of outgassing in a new, unbaked system, fabricated from materials such as aluminum and stainless
steel, is initially very high, on the order of 10-6 to 10-7 torr-L/s - cm2 of surface area after one hour of exposure to vacuum (O’Hanlon, 1989). With continued pumping, the rate falls by one or two orders of magnitude during the first 24 hr, but thereafter drops very slowly over many months. Typically the main residual gas is water vapor. In a clean vacuum system, operating at ambient temperature and containing only a moderate number of O-rings, the lowest achievable pressure is usually 10-7 to mid-10-8 torr. The limiting factor is generally residual outgassing, not the capability of the high-vacuum pump.

The outgassing load is highest when a new system is put into service, but with steady use the sins of construction are slowly erased, and on each subsequent evacuation, the system will reach its typical base pressure more rapidly. However, water will persist as the major outgassing load. Every time a system is vented to air, the walls are exposed to moisture and one or more layers of water will adsorb virtually instantaneously. The amount adsorbed will be greatest when the relative humidity is high, increasing the time needed to reach base pressure.
Water is bound by physical adsorption, a reversible process,but the binding energy of adsorption is so great
that the rate of desorption is slow at ambient temperature.Physical adsorption involves van der Waal’s forces, which are relatively weak. Physical adsorption should be distinguished from chemisorption, which typically involves the formation of chemical-type bonding of a gas to an atomically clean surface—for example, oxygen on a stainless steel surface. Chemisorption of gas is irreversible under all conditions normally encountered in a vacuum system.
After the first few minutes of pumping, pressures are almost always in the free molecular flow regime, and
when a water molecule is desorbed, it experiences only collisions with the walls, rather than with other molecules.
Consequently, as it leaves the system, it is readsorbed many times, and on each occasion desorption is a slow process.One way of accelerating the removal of adsorbed water is by purging at a pressure in the viscous flow region, using a dry gas such as nitrogen or argon. Under viscous flow conditions, the desorbed water molecules rarely reach the system walls, and readsorption is greatly reduced. A second method is to heat the system above its normal operating temperature.

Any process that reduces the adsorption of water in a vacuum system will improve the rate of pumpdown. The
simplest procedure is to vent a vacuum system with a dry gas rather than with atmospheric air, and to minimize the time the system remains open following such a procedure.

Dry air will work well, but it is usually more convenient to substitute nitrogen or argon. From Equation 1, it is evident that there are two approaches to achieving a lower ultimate pressure, and hence a low impurity level, in a system. The first is to increase the effective pumping speed, and the second is to reduce the outgassing rate. There are severe limitations to the first approach. In a typical system, most of one wall of the chamber will be occupied by the connection to the high-vacuum pump; this limits the size of pump that can be used, imposing an upper limit on the achievable pumping speed. As already noted, the ultimate pressure achieved in an unbaked system having this configuration will rarely reach the mid-10-8 torr range. Even if one could
mount a similar-sized pump on every side, the best to be expected would be a 6-fold improvement, achieving a base pressure barely into the 10-9 torr range, even after very long exhaust times.

It is evident that, to routinely reach pressures in the 10-10 torr range in a realistic period of time, a reduction
in the rate of outgassing is necessary—e.g., by heating the vacuum system. Baking an entire system to 400C
for 16 hr can produce outgassing rates of 10-15 torr-L/ s-cm2 (Alpert, 1959), a reduction of 108 from those found after 1 hr of pumping at ambient temperature. The magnitude of this reduction shows that as large a portion as possible of a system should be heated to obtain maximum advantage