industry interview
rhombus4
Fig. 2 Comparison of the legacy pedestal heater (left) versus the Fig. 3 Comparison of alternative solutions. Leveraged by
water-cooled variant (right). New process requirements imposed computer aided engineering (CAE) resources, an iterative virtual
on an old design became highly problematic for achieving prototyping effort rapidly identified, refined and verified the
temperature uniformity goals preferred embodiment
application, however, the heat transmission 2. Thermal stress. For shafts which attach to the heater
performance of the shaft became detrimental to near the radial midpoint, there was concern about thermal
temperature uniformity. Because the stainless steel end stresses being high at the shaft weld joint locations
of the shaft was so effective at impeding heat conduction because of a macroscopic thermal strain effect known as
to the chamber floor, the excess process heat load coning, whereby the warm end of the shaft expands more
manifested itself as an unacceptable hot spot at the than the cool end.
center of the heater. The right half of Figure 2 depicts the
attempted workaround. This design consisted of 3. Constraints. In addition to uniformity remediation and
eliminating the stainless steel spool piece, extending the thermal stress mitigation, the design improvement effort
aluminum end of the shaft down to the chamber floor and would have to account for numerous constraints. These
actively cooling the chamber floor so the vacuum seal constraints included the bending limitations of sheathed
temperature would remain within safe bounds. heating elements, nichrome (NiCr) wire watt density
Unfortunately, the net heat sinking effect of these changes limitations, shaft manufacturability, heater “castability,”
caused an unacceptable cold spot. welding limitations, construction material limitations, and a
50°C (122°F) design margin for the o-ring sealing the
Challenges shaft bottom flange.
The following considerations framed the issue set for re-
optimizing the baseline, all-aluminum design: Solution Approach
1. Accommodation of multiple operating modes. The The leading design options which emerged from concept
excess process heat load scenarios ranged from 0 to ideation are illustrated in Figure 3. The inverted cone
approximately 1000 watts, depending on which condition (Option A) was preferred from a relative ease-of
was in play. For the maximum heat load condition, manufacturing perspective.
maintaining temperature uniformity proved to be highly
problematic with a center-mounted, all-aluminum shaft. To However, due to lingering concerns about thermal stress,
improve uniformity, the radial temperature distribution the radial bellows alternative known as a reentrant post
would have to be “re-balanced” by moving the shaft (Option B) was developed in parallel as a fallback. To
mounting location closer to midway between the pedestal advance a suitable design into production prototyping,
center and its outside diameter. Watlow began by detailing 20 solid model versions
Fig. 4 FEA-
driven, rapid
virtual
prototyping to
identify the
preferred shaft
design
32
www.compoundsemiconductor.net November / December 2009
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