OIPT Systems
Oxford Instruments Plasma Technology
System Manual
6.1.2
Systems using Helium purge
The specifications for the Helium gas supply are as for those of the process gases given
in sub-sections 6 and 6.1 with the exception that the maximum pressure at the inlet to
the pressure controller must not exceed 3.5 bar (43 psig). The design of the Helium
pressure controller is such that it can be destroyed by higher pressures.
6.1.3
Installation of low vapour pressure gases (e.g. SiCl
4
, BCl
3
, C
4
F
8
)
The low vapour pressure can lead to condensation in the gas supply lines, particularly at
cold points or when the gas passes into a cooler region. This condensation can result in a
build up of liquid in the gas pipe, usually at the low points or u-bends in the gas line,
often leading to instability of gas flow, especially if liquid condenses or flows into the
MFC.
The low vapour pressure can also result in very low gas pressure if the gas cylinder is very
cold, e.g. if it is kept outdoors in the winter.
Therefore, it is important to adhere to the following guidelines:
(A) It is necessary to keep the gas cylinder indoors (in an extracted gas cabinet) to
avoid loss of line pressure when the outside temperature is cold.
However, do NOT heat the gas cylinder with a heated jacket as this can cause
condensation problems when the gas passes into the cooler gas lines. Room
temperature is warm enough to provide sufficient vapour pressure.
(B) It is important to maintain a positive temperature gradient from the cylinder to
the MFC, or at least keep them at the same temperature. The simplest method is
to position the gas cabinet close to the gas pod, minimising the chances of
temperature differences, reducing the length of the gas pipe, and hence
minimising the chances of condensation. If this is not possible, then it is
necessary to heat the gas lines by the use of heater tape.
The MFC will also need to be heated. OIPT offers a heated MFC kit for these
gases. Alternatively, heater tape can be wrapped around the MFC. However, in
this case, it may also be necessary to detach the MFC from the backing plate to
avoid heat loss through the plate, and to cover the MFC in insulation material to
avoid cooling from air flow within the gas pod (from the gas pod exhaust).
It will then be necessary to set the MFC temperature hotter than the gas line
temperature, which in turn is hotter than the gas cylinder temperature. A typical
set-up might be MFC 40 °C or above, gas line 30-40 °C, and gas cylinder at room
temperature.
(C) If condensation problems are suspected, it will be necessary to pump out the gas
lines completely, and optimise the heater tape arrangement and temperature
setpoints before refilling the gas line.
(D) For
SiCl
4
it is important to use a dedicated SiCl
4
MFC as this is designed
specifically for low-pressure condensable SiCl
4
operation.
Services Specifications
Issue 21: January 10
Page 16 of 22
Printed: 6-Jan-10, 8:53
Summary of Contents for OpAL
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