Cover Story
Wafer temperature control
in batch processing
As consolidation provides less custom for tool and materials manufacturers in the
semiconductor industry, companies seek alternative markets to sell advanced tools
and materials. There are opportunities for companies outside the semiconductor
Issue II 2009
square4
industry. One of these is the growing LED industry which faces its own manufacturing
challenges in developing new and more powerful LEDs every year. Mark Dineen is a
.com
Principal Applications Engineer and Mike Cooke is the New Product Introduction
Manager at Oxford Instruments Plasma Technology and here they discuss meeting
the challenge of controlling temperature in batch wafer processing.
oasiasemiconductor
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he throughput of a plasma etcher used for
texturing sapphires depends not only on
having an efficient high density plasma
source, but also on the ability of the tool to
remove heat from the substrates. In many
plasma etch processes, there is more plasma density
available, but the etch rate is limited by the survival of the
photoresist mask in the face of heat fluxes of up to 1 W cm-2
from the high power ICP380 source.
The silicon wafer industry is accustomed to clamping
single wafers to a temperature-controlled table, and
introducing a heat transfer medium, normally helium gas,
between the table and the wafer. ‘Helium backside cooling’
has become the standard method for single wafer
temperature control. However, the HBLED manufacturing Figure 1: Electrode surface and sample temperature
route currently uses batches of smaller substrates, passed into
the etch tool on a carrier plate.
This significantly affects heat transfer, by introducing a conditions without helium backside cooling. A fibre-optic
second low pressure gas interface in series with the normal probe was used to measure temperatures in vacuum, in the
wafer-table interface. Heat must now pass from wafer to presence of rf excitation.
carrier plate, then from carrier plate to table. The main The probe was attached to the front surface with
temperature drops are at these interfaces – the temperature polyimide tape, and a little Fomblin grease to measure the
drops across the wafer thickness or carrier plate thickness are substrate temperature within about 5 degrees. It was also
very small by comparison. helpful to create a small ‘umbrella’ of metal foil over the
sensing tip, to avoid direct heating of the tip by the plasma.
Results count Without the He cooling, the temperature of the sample
Figure 1 shows the temperatures of the electrode table placed on the electrode continues to rise over time,
surface (lower line) and of a single sapphire wafer (upper line) eventually reaching over 200
0
C! An even higher temperature
placed directly on the table, under high power etch rise would be expected if the wafer sat on a carrier plate.
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