Semiconductor wafer support with graded thermal mass

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG.
1, a rapid thermal processing (RTP) system that has been modified in accordance with the invention includes a processing chamber 10 for processing a wafer-shaped silicon substrate 12.
The substrate 12 is mounted inside the chamber 10 on a rotatable substrate support structure 14.
A heating system 16 generates radiation which enters the processing chamber 10 through a water-cooled quartz window assembly 18.
The heating system 16 includes an array of individual heat lamps (not shown) arranged in several independently controlled concentric zones to minimize thermal gradients across the substrate 12.
Each lamp in the array has a spot size of about 1.
2 inches (30 mm).
Because of reflections within chamber 10, each lamp makes a contribution to heating the entire substrate 12.
Beneath substrate 12 is a reflector 20 which is mounted on a water-cooled, stainless steel base 22.
Reflector 20 is made of aluminum and has a highly reflective surface coating 24.
The underside 26 of substrate 12 and the top of reflector 20 form a reflecting cavity for enhancing the effective thermal emissivity of the substrate.
Temperature probes 30 (only two of which are shown in FIG.
1) measure the temperatures at localized regions of substrate 12.
The temperature probes are sapphire light pipes that pass through conduits 32 that extend from the backside of base 22 through the top of reflector 20.
The probes 30 are coupled via optic fiber cables 36 to pyrometers 34 located outside the chamber 10.
Although only two measurement probes 30 are shown in FIG.
1, the embodiment illustrated in FIG.
1 actually uses seven measurement probes distributed at different radii over the reflector 20.
During thermal processing, support structure 14 is rotated at about 90 RPM or more.
Thus, each probe 30 actually samples the temperature profile of a corresponding annular ring area on the substrate 12.
The support structure 14 that rotates the substrate 12 includes an edge ring 38 that contacts the substrate 12 underneath the substrate's outer edge, thereby leaving almost all of the underside 26 of the substrate 12 exposed except for a small annular region near the substrate's edge 40