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With respect to the heat-treatment processing and its equipment, we can identify our techniques in two areas: a proprietary Boat/Holder technique and an innovative process chamber design. Three factors, single or in combination will contribute to the generation of defects such as slip during the heat treatment of the 300mm wafer. In responding to each factor, we provide the following measures.
- Thermal Stress due to the temperature gradient within the wafer in the radial direction
The flow of gas taking place within the batch-typed heat treatment furnace tends to be non-uniform, causing local temperature difference in the process chamber. The disturbance in the flow of gas results in temperature gradient inside the wafer, which, in turn, brings about the differential expansions of the volume, - hence the thermal stress. When the temperature increases above 1000°C, the ability to overcome the stress -commonly called critical yield stress - sharply drops off. The slightest increase of stress will give rise to the crystallization defect within the wafer.
Tera's counter-measures: To provide the best-available conditions for the equipment and the processing by means of the computer simulation
By taking advantage of the computer simulation over the entire equipment and the process of work, we at Tera Semicon achieves the uniformity of temperature within the wafer by controlling the gas flow including the optimization of gas injection and exhaust scheme. After making the computer simulation of the temperature and the flow of gas at each stage of the heat treatment and of various other demands from the customers, we offer them a stable set of the processing equipment of in close coordination.
- Gravitational Stress due to the weight of the wafer
The 300mm wafer has an increased diameter and thickness, compared to 200mm wafer. Therefore, if you are to load the multi wafers in a boat asymmetrically like conventional batch loading, the gravitation stress concentrates locally on the area where the wafers are supported. This increased mechanical stress results in the crystallization defects such as the slip generated from the point of contact between the wafer and the supporting medium, which is usually the rod of the boat.
An idea may be suggested that we may well make the weight of the wafer evenly distributed all across the wafer by adopting a specially designed symmetrical medium to support the wafer. However, in this case it is a big challenge to load and unload the multi-wafers using the robot in the fully automated equipments. A modified design is required to provide an appropriate positioning of each wafer to avoid the mechanical interference.
Tera's response: To realize the line-contact in the shape of perfect symmetry using the slipMaster.
The method of distributing the wafer's gravity varies according to the processing temperature. At temperatures of 1,100°C or lower, we reduces the gravitational stress to the minimum level by optimizing the boat structure of the batch type, while at temperatures of 1,150°C or higher, we adopt a special measure of support for the wafer. The advantages of the slipMaster, which we claim, have made it possible to realize the perfect line-contact, and to perform the ultra high temperature operation with significantly reduced possibility of the slip generation. As for the slipMaster technology, we've already obtained the worldwide acclaim for its being the first-ever, unique solution.
- Frictional Stress due to the difference in the coefficient of thermal expansion between the wafer and the supporting medium
Since the silicon wafer and the rod of the boat or the holder for safe-landing differ in material, frictions invariably occur between the wafer and the holder owing to the difference in the coefficient of thermal expansion at the time of the rise/fall of temperature.
Tera's response: Success in minimizing the contact area through the optimization of the design and surface treatment of the supporting medium, together with an innovative "Dynamic Control" concept.
Upon the strength of the accumulated knowledge of materials engineering and processing technologies, we at Tera offer the choice of the material of optimized surface treatment. And also we offer the processing technique of forestalling friction at the time of rise/fall of temperature through the dynamic control - to let the wafer stay on and detach from the holder even during the process cycle at high temperatures.
- Slip Control technologies - SiC boat structuring, SiC holder structuring, technique of maintaining surface roughness of contact areas, gas inject/exhaust design, Heat block structuring, wafer transfer technique, etc.
- Technique of preventing contamination -No metal exposure in the chamber, Liner purge injection, optimized SiC ware cleaning/baking, gas pre-heat, N2 load lock techniques.
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