[i] A tubular heater utilizes a dielectric material, e.g. mica, inside a tube with a small coiled wire running through it. Due to the high resistivity of the wire, when a current passes through it heat is generated according to the relationship I2R, Joule heating. The thermal energy then transfers from the wire to the ceramic filler and then to the outer tube. These tubes typically form loops inside the chuck heater, with a similar pattern to an electric kitchen stove. Some of the manufacturers of routine heaters make the high tech versions used in semiconductor processing. Some tubular heating elements can reach temperatures of 870o C (1600o F). An etched foil heating system consists of a labyrinth of highly resistive foil. Each sheet of foil is surrounded by an adhesive and dielectric material, such as polyimide or mica. A polyimide insulator can operate at 260o C with up to 75% plate coverage while mica heaters can operate at up 600o C but with only about 45% of the heating surface covered.[ii]
Engineering Constraints Of A Chuck Heater
Following the heating element itself, the size, shape, and material of the chuck significantly impacts the temperature distribution, as does the of the metallic plate, if utilized. If the heating element does not have a uniform heat output or not profiled well, then the thickness of the plate can be altered or increased to improve the uniformity. A thicker plate will increase the cross-sectional area for the heat to conduct laterally before it reaches the surface of the plate. The thickness of the plate is constrained because increasing the cross-sectional area causes the amount of surface on the plate to increase, thus requiring more energy input. In a plate with a tubular heating system the area near the heating element will be hotter than those that are in between the heating element. Increasing the thickness in the plate makes these hot and cold areas less evident. Other factors that can affect the thermal uniformity in the plate and therefore must also be taken into account are the mounting of the chuck heater, how to monitor the temperature, and how to mate the wafer handling mechanisms. Many of these constraints arise because a relatively cold spot forms wherever the plate is in contact with another surface. Even a temperature sensor, which needs to be in an area of high thermal conductivity, can have an effect on the heat transfer capabilities of the plate.[iii]
The Designing Of The Chuck Heaters
As an experiment with variation in design of a chuck heater, Glew Engineering has designed three possible layouts of the heating element. The first is a standard kitchen stove design, starting in the middle and spiraling outward. While this design is the simplest and has the smallest gaps between heating portions, it is not optimized for a chuck heater because it does not begin and end in the middle of the plate, as is a design requirement to ensure thermal uniformity. The second design is a series of towers that cover the area of the plate while the third is a series of circles. Each of these heating elements have been modeled utilizing 3D CAD software. Before the next blog is released each will undergo thermal finite element analysis (FEA) with the primary goal of understanding which creates the most uniform thermal output. These results will then be discussed in a future blog.
[i] Strehlow, Russell Minco “Designing Heated Chucks for Semiconductor Processing Equipment” 2008
[ii] Strehlow, Russell Minco “Designing Heated Chucks for Semiconductor Processing Equipment” 2008
[iii] Durex Industries “Cast In Heaters for Semiconductor Processing” 2013