Removable semiconductor wafer susceptor

Details Removable semiconductor wafer susceptor Removable semiconductor wafer susceptor

2002-12-05

2004-10-05

Tran, Khoi H. (Department: 3651)

Material or article handling

Associated with semiconductor wafer handling

Including means for charging or discharging wafer cassette

C414S935000, C901S030000

Reexamination Certificate

active

06799940

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to semiconductor wafer processing, and more particularly to a removable semiconductor wafer susceptor which can be used in batch processing of semiconductor substrates.
For common semiconductor films such as silicon nitride, polysilicon, and thermal oxides, substrate processing usually proceeds by elevating the substrate to some process temperature, conducting the process, and finally cooling the substrate. Generally, most processes are conducted in a 200 mm batch furnace where substrates (hereafter referred to as wafers) are placed in a vertically stacked arrangement. Because of process and throughput requirements, the wafer stack often undergoes rapid heating and cooling at the beginning and end of the process. However, some thermal ramping limits exist at higher processing temperatures. It is now known that for 300 mm wafer, serious limitations exist on wafer heating/cooling rates and maximum process temperatures, well below the operational limits of the processing equipment.
The gravitational force and elevated process temperature (typically above 850° C.) cause considerable stress on the silica on wafer, leading to situations where slip and plastic deformation may occur. Fast thermal ramping can further degrade the situation because within-wafer (WinW) thermal gradients from uneven heating of wafers in a vertical stacked arrangement may cause slip to occur even before the process temperature is reached. Of course, fast thermal ramping is employed to increase productivity by decreasing the overall cycle time or reduce thermal budget by decreasing the ramping cycles. Therefore, a serious situation arises for high temperature processing of 300 mm substrate, especially in batch processing environments. Additionally, even if slip does not occur, the induced thermal gradient on the wafer may be of sufficient magnitude as to cause significant differences in the thermal histories of the die spread across the wafer. This will result in an unexpected die performance variation between the wafer center and edge locations.
Two approaches can be taken to solve this slip problem. One approach is to improve the wafer's chemical and mechanical characteristics, such as decreasing the oxygen precipitate concentration within the silicon wafer. This approach is an area of responsibility for the wafer manufacturers. The other approach is to improve the substrate susceptor(support) design.
The current industry standard for vertical batch wafer processing is the ladder boat and its variations (FIG.
1
). The wafer boat is a holding device shaped like a hollow cylinder and is made of, for example quartz which has high heat resistance and high chemical stability. It comprises, for example, four vertical support rods, each having grooves. The wafer boat holds wafers, each set at its circumferential portion in the four grooves made in the four support rods. Hence, the wafers are held parallel to one another and one above another. This is the simplest design for vertical batch processing. However, it does not provide the most optimum mechanical support possible with respect to gravitational forces. Also, the standard ladder boat provides little reduction in thermal gradients. The ladder boat's greatest advantages are its low cost and compatibility with standard automation.
Two previously developed innovations have addressed the WinW wafer thermal issue for batch processing. The first wafer support method, shown in
FIG. 2
, was developed and patented by Tokyo Electron Ltd. (TEL). This “ring” support method uses a ring of material (typically quartz) designed to come into physical contact with the edge of the wafer. The addition of mass near or at the wafer's edge reduces the WinW thermal gradient because of the increase in heat capacity and change in radiation view factors. The method also provides a larger area of mechanical support than a ladder boat. The method gives good performance on 200 mm wafers, as thermal WinW gradients are controlled to under 10° C. for fast thermal ramps (above 75° C./min). However, this support method is complex and such designs are more expensive to manufacture and purchase. Additionally, this method requires more complex automation to load and unload wafers from the support appliance, leading to added cost for the associated support automation.
Another approach found in the prior art (previously patented by SVG, Thermco Systems) is the “band” method as shown in FIG.
3
. Here, a thin band of material, typically quartz, is placed around the edge of the wafer, but not in intimate contact. The quartz material is either opaque or mechanically modified to be translucent. This method, like the ring support, reduces or screens incident radiation onto the wafer's edge, while permitting radiation through the unblocked areas and onto the wafer's center. Although not as effective as the ring support method shown in
FIG. 2
, the “band” method does reduce WinW thermal gradients and can be manufactured at a lower cost.
Other approaches to wafer support methodologies have been previously explored by others and are well known within the industry. In
FIG. 4A
, the best theoretical point contact support at a single radius value is shown. This method places point supports at 70% of the radial distance from the center to the wafer's edge, to balance the weight of the wafer on either side of the support and reduce gravitational stress effects. This approach when implemented in a ladder boat configuration will provide better support, but the cost will be greater due to the additional manufacturing complexity of very long support tabs. Also, this method does not address the WinW thermal gradient problem. A corresponding analogy exists for the ring support (point contact) where the location for a single ring would be also at 70% of the radial distance from the center to the wafer's edge. In this case, the ring support's axial symmetry greatly improves the control of the gravitational stress magnitude and symmetry compared to the ladder boat method.
FIG. 4C
shows the absolute best theoretical support design possible, as all pints on the wafer are mechanically supported. Clean, simple, and efficient mechanical wafer loading and unloading for this design becomes a serious problem, if not impossible, with current automation technology.
The vast majority of single wafer processing equipment currently use supports shown in
FIGS. 4B and 4C
. Here either a ring of material or a flat plate or susceptor composed of quartz, SiC or similar material supports the wafer. These design are preferred for reasons of simplicity or reduction of thermal mass to permit rapid wafer heating and cooing (up to 100° C./sec). The supports in
FIGS. 4B and 4C
are not necessarily employed for thermal WinW control in single wafer processing equipment because they rely on heating element design to accomplish WinW thermal uniformity. In some cases, there may be some benefit based on material selection with reducing thermal non-uniformity. As an added benefit, gravitational forces are reduced and in the case of
FIG. 4C
, are completely eliminated if the right support material is used. However, these designs do add complexity to the method of wafer handling and are best suited for single wafer environments where the automation comprises a larger percentage of the overall equipment set and cost.
WinW Thermal Gradients
The primary issue with batch processing and rapid heating of large substrates is the resultant thermal gradients, as demonstrated in FIG.
5
. During the heating phase of the process cycle (see FIG.
5
A), the edges of the wafer receive the majority of the incident radiation and as a result heat up at a faster rate. Heating of the interior regions of the wafer is chiefly accomplished by thermal conduction through the substrate itself. As a result, a “bowl”-shaped thermal profile forms across the wafer. This thermal gradient can add to the gravitational stress and—if large enough—cause warping, bowing, plastic deformati

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