COVER STORY
Right:Figure 1a: temperature uniformity is caused by the
Uniform heat source introduction and removal of the target. By
introducing a relatively cold target to a pre-heated
Far right: Figure 1b: source the potential for non-uniformity increases
Uniform heat source with the direction and speed of the introduction.
utilizing edge margins Ideally a target would be introduced to a planar
source in a direction perpendicular to the emitting
plane of the heat source. In many processing
scenarios this is impractical. In which case the
source to target temperature delta and speed of
introduction must be closely managed to
minimize non-uniformity.
If uniformity is also required throughout the entire
Issue II 2009
thickness of the substrate, (typically required
when the heat source is opposite the side being
processed) an appropriate soak or dwell time
must also be considered since the lower thermal
Right: Figure 1c: conductivity of non-metallic substrate materials
Zoned heat source does not allow for rapid heat transfer through the
utilizing edge margins substrate.
-pv-management.com
Far right: Figure 1d: The use of pre-heat chambers or inclusion of a
.solar
Zoned heat source heat source on both faces of the target can
www
utilizing edge margins minimize dwell times.
and reflector shields
14
Improper orientation of heat source
(continuously moving target - web/foil)
In some thin film processing lines the target takes
the form of a moving web. In this scenario the
orientation of the heat source is critical to avoid
“thermal striping” of the moving web (Figure 3a).
In applications utilizing exposed element, tubular,
leads as well as chamber walls, doors, lift pins, cable or cartridge style heating elements
ports, etc. (Figure 2a). Thermal leakage can be uniformity can be enhanced by orienting the long
minimized by utilizing the smallest geometries as axis of the heating element perpendicular to the
Below left: Figure 2a: mechanically possible to support both the heat movement of material (Figure 3b).
Standard mechanical source and the target (Figure 2b). Further
supports reduction in thermal leakage can be achieved by It is also common to embed the heating elements
increasing the path length between hot and cold in a metallic plate. Aluminum is preferred due to
Below : Figure 2b: design features or by utilizing thermal breaks its favorable thermal conductivity; however,
Thinner mechanical (lengths of non-thermally conductive materials) stainless steel and other alloys may be used in
supports within the leak path (Figure 2c). higher temperature applications. The metallic
plate serves as a heat spreader that when
Below right: Figure 2c: Not accounting for the introduction and properly designed can provide a thermally
Long path mechanical removal of the target (single piece target homogeneous source. Orientation in relation to
supports with thermal such as a glass plate) the moving web becomes less important when
breaks A much more subtle, yet quantifiable effect on employing this technique. It is important however
to address edge effects using the techniques
discussed above.
Not accounting for gas flows within
a chamber
A final factor that must be considered during any
thermal system design are gas flows. Although
the majority of photovoltaic cell processing
occurs in an evacuated chamber there are some
that involve gas flows into and out of the process
chambers. Examples include load locks where
the chamber cycles between vacuum and
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44