Figure 1: Composite layered heater from patent US 9,224,626 B2
Our eponymous Dr. Glew, P.E. recently contributed to a new patent on an advanced thin-film electric heater, titled “Composite substrate for layered heaters”. Watlow Electric, based in St. Louis, hired Glew Engineering and Dr. Glew to help develop this heater technology due to his experience in the Silicon Valley’s semiconductor industry. As a semiconductor equipment expert and materials engineering consultant, Dr. Glew’s familiarity with semiconductor manufacturing meant he understood both the limitations of common semiconductor chuck heating methods and the techniques that could be used to construct a better heater. In this post, I’ll take a look at how this composite heater capitalizes on semiconductor processing methods to provide more uniform heat over a large area than standard coil heaters can.
Thermal Management of Semiconductor Processing
Semiconductor manufacturing involves the sequential deposition and removal of patterns of different materials. These layers can be less than a micrometer thick, and any temperature disruptions across the area of the wafer or through each deposition period can lead to costly mistakes in layer uniformity. With such thin layers, the timing of the process also has to be exact. Not only do gases have to be released in precise increments at precise intervals, but the semiconductor processing equipment must also be able to rapidly adjust to the temperatures required by the different precursor gases. Furthermore, the temperature of the wafers needs to be precisely controlled across the wafer, accounting for process chamber conditions. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are two common semiconductor processing techniques with demanding temperature control requirements.
Problems with Resistive Heating Coils
While some older semiconductor chambers used heat lamps, and others might use uncommon techniques like a circulating heated liquid channel in the chuck, most semiconductor chucks rely on resistive heating. Running a current through coiled wire embedded in ceramic is a reliable and easily-controllable method of heating a chuck, but it has drawbacks. If the resistive element is too close to the surface, or the pattern is too sparse or isn’t uniform, then the surface of the chuck can experience uneven heat distribution (Figure 2). In order to evenly disperse heat and minimize the formation of hot and cold spots on the surface, a rule of thumb is to keep the coil 15 mm away from the surface of the chuck. However, increasing the thickness of the chuck will cause more heat loss along the edge, which again prevents uniform heat distribution across the top surface. To counteract this, chucks with heating coils must be manufactured with a larger diameter than the wafer in production. The larger a surface gets, however, the more difficult it becomes to heat the entire surface evenly. Increasing the thickness of the chuck also increases its thermal mass, which in turn increases the time constant for the wafer temperature’s responsivity to heater adjustments. Ultimately, refining heating coil design is an increasingly difficult problem to address as wafers increase in size and complexity. The patent Dr. Glew developed provides a different method of creating a semiconductor chuck heater than addresses these issues.
Figure 2: Comparison of resistive coil heater and layered heater
Composite Substrate for Layered Heaters
The layered heater that Dr. Glew developed solves the problems of chuck thickness and heating uniformity by relying on the same semiconductor layering processes that will use the heater in the future. Just as it sounds, this device consists of stacked layers of different materials, one of which is a patterned resistive heating material. This resistive heater layer can be deposited and then etched into a precise circuit pattern, just as semiconductor and metal traces are patterned for the integrated circuits on a wafer. With this micron-level deposition accuracy, the heater can be perfectly tuned to minimize variance and account for heat loss at different locations on the chuck and provide uniform heat distribution across the whole surface (Figure 2). That same fine level of control over the heater pattern also allows for complex multi-zone or non-uniform heating distributions. Furthermore, since the heater layer is only separate from the wafer by a thin layer of dielectric material, the time constant for the wafer’s temperature change is very small. Since the heater is thin, it also minimizes the heat loss on the edges of the chuck. Overall, this heater would allow operators to more precisely control the wafer’s temperature across its whole area and throughout the entire process.
Figure 3: Cross-section of the composite layered heater
This layered heater, in one embodiment, is comprised of 5 layers, listed below in the order they are deposited and shown in Figure 3. Each of these layers is made of a different material, and as such will have a different coefficient of thermal expansion (CTE). As I have covered in a previous blog, two bonded materials with different CTEs will expand and contract at different rates when their temperature changes. This can lead to cracks or delamination over the life of the materials. Therefore, it’s essential that each layer’s material be chosen such that its thermal properties are not too dissimilar from the neighboring layers.
- Application substrate (12). This layer bonds to the top surface of the semiconductor chuck or other surface(22). The material for this layer is chosen to match the CTE of the application surface.
- Heater substrate (14). This layer is brazed or otherwise adhered to the application substrate. This material is chosen to match the CTE of the layer above.
- First dielectric layer (16). This insulating dielectric layer forms a base for the resistive heating layer above (18).
- Resistive heating layer (18). This is the conductive layer, deposited on the dielectric layer and then laser-etched into a circuit pattern.
- Second dielectric layer (20). This insulating layer is deposited last, filling the etched spaces in the resistive heating layer and creating a smooth upper surface.
Applications for a Layered Heater
This composite layered heater should prove very useful not just in the thermal management of semiconductor processing, but outside the semiconductor industry as well. Any large, thin substrates that need even heating over both area and time, like coatings or optics, could benefit. This heater film could also be applied in areas where volume is a concern, whether due to confined space or very large surface area. Sometimes, the solution to your problem, and many others, might have been right in front of your eyes the entire time.