Bake plate enhancements for optimal thick-film curing results
High-uniformity bake plates have been displacing convection ovens for well over two decades in the microelectronics industry. The disadvantages of variable temperature zones, lengthened cure times, and considerable particle contamination have been thoroughly analyzed and confirmed. Although temperature uniformity remains the primary advantage for precision hot plates, virtually eliminating skinning effects for thin-film (< 1-5 µm) applications has been equally beneficial. The skin effect is well understood when baking “from the outside in” and is typical of baking in a conventional oven. This phenomenon is more pronounced with thick-film materials such as SU-8 photoresist, KMPR photoresist, BPR-100 photoresist, and WaferBOND® HT-10.10 material, which are very prone to the skin effect. This effect occurs when the outer exposed layer of the film dries and forms a skin before all of the solvents in the lower layers have evaporated. These process anomalies are often observed with today’s bake plates if the heating accelerations are too aggressive and/or the bake temperature or time is below recommended levels. Often the result is post-bake blistering and/or cracking, which renders the film unusable for downstream processing.
To combat this issue, customers have deployed the use of multiple high-uniformity bake plates with multiple set points (dual stage) for performing a soft bake, post-exposure bake, and post-develop hard bake for final curing. However, the multi-plate option fails to provide a repeatable method for ramping up from ambient temperature to the set point and over-utilizes precious clean room or laboratory working surfaces. One potential solution to this problem is instituting programmable plates with multiple bake method options (proximity, gravity, and hard contact).
Proximity baking essentially floats the substrates on a “pillow” of inert gas that is blown through orifices in the chuck surface. A combination of heated gas, radiant heat from the chuck, and reflective radiant heat from the hood baffle uniformly heats the substrate. This slower heating of the substrate reduces blistering and cracking of films made from materials containing fast-drying solvents. Proximity baking can eliminate the need for two independent bake plates if the set points are relatively close (± 10°C). Proximity baking with an N2 pillow mitigates the risk of physical contact between the substrate and the hot plate and has been adopted for many photomask and display processing applications.
In a “soft-contact” bake, gravity alone holds the substrate to the surface of the chuck. This method is typically utilized as an intermediate option between a “hard contact” bake and a proximity bake, for multiple step warm-up.
In a “hard contact” bake, the substrate is held to the surface by applying a vacuum to the underside of the substrate. This method ensures baking uniformity and minimizes bowing and warping of the substrate.
The latest process enhancement combines the existing bake methods (proximity, soft contact, or hard contact) with programmable lift pins. The electronic lift pins are used for automated loading and unloading of the process wafers and provide enhanced repeatability. Furthermore, they have been designed for maximum flexibility and will accommodate all standard and semistandard sizes from 2 inches (with proper placement of the flat) through 200 mm. The electronic lift pins provide the user with 100 specific proximity process steps above the surface in any sequence or combination. The heights are programmed in 0.001-inch increments with an overall operating window from 0.001 inch through 0.750 inch (± 0.002 inch). In many scenarios, this feature allows for faster ramping acceleration/deceleration and emulates several bake plates temperatures simultaneously. This feature is also extremely valuable for safe handling of thermomechanically sensitive materials such as GaAs, LiNbO3, InP, GaN, SiC, and sapphire substrates. The Brewer Science® Cee® line of high-uniformity bake plates are the only ones to bring together all referenced capabilities in a compact footprint designed for a lab-scale environment. See the Cee® bake plate web page for additional information and detailed tool specifications.