Page 90 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH07.qxd 04/04/2005 19:40 Page 7.11

VACUUM TECHNOLOGY

VACUUM TECHNOLOGY 7.11

7.6.3 Delivered Pumping Speed
Conductance elements impede the flow of gas from a vessel to a pump and therefore reduce the
effective (delivered) pumping speed. The delivered pumping speed is a function of the speed at the
pump inlet and the total conductance of the conductance elements connecting the vacuum pump to
the vessel.

1 = 1 + 1
S t S p C tot

where S = delivered pumping speed, 1/s
t
S = speed at vacuum pump inlet, 1/s
p
7.6.4 Time Required for Primary Evacuation of a Vacuum Vessel
The elapsed time required to evacuate a vacuum vessel from an initial pressure (generally 740 torr) to
a lower pressure (usually near the base pressure of a primary vacuum pump, approximately 10 mtorr)
is given by

t = V ln   P  
1
S t  P 
2
where t = time, s
V = vessel volume, 1
P = initial pressure, torr
1
P = final pressure, torr
2
7.6.5 Calculation of Base Pressure for a Vacuum System
Following the primary evacuation of a vacuum vessel, the primary pump is isolated from the vessel
and the secondary pump is used to further reduce pressure in the vessel. The time required to reach
the base pressure of the vessel by the secondary vacuum pump is a strong function of the internal
environment of the vessel. Outgassing from components internal to the vacuum vessel, permeation
of gases through the vessel walls, and leaks all contribute significantly to the gas load the secondary
pump works to remove from the vacuum vessel. At the base pressure an equilibrium is established
between the delivered pumping speed of the secondary vacuum pump and the total gas loads as
described earlier.
Q = S × P
t
where Q = total gas load, torr⋅l/s
Outgassing. Outgassing is the spontaneous vaporization of materials, which is commonly
observed in a vacuum vessel. The rate at which materials outgas tends to be a strong function of the
material composition and the temperature of the material. Tables of outgassing rates for a wide vari-
ety of materials used in vacuum are available in the books listed in the references. Materials to avoid
−5
placing inside vacuum vessels intended to achieve pressures below 10 torr include high vapor pres-
sure fluids (water, common hydrocarbon oils) and any solid materials that have a detectable smell
(you can smell these materials because they have a high vapor pressure!). To achieve pressures below
−7
10 torr, the following additional materials should be excluded from the internal volume of a vacu-
um vessel, if possible—polymers such as polyvinyl chloride (PVC) and high vapor pressure metals
such as zinc, cadmium, mercury, and lead.

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