Bake Plate Process Theory
Hotplate bake processing has increased in popularity since the early 1980s. Previously the most common technique for film drying and curing was the convection oven. Bake plates (also known as hotplates) offer several advantages in the form of increased throughput, increased uniformity and reproducibility, and decreased particle contamination. In a typical bake process the substrate is placed into contact with a heated surface of known temperature. The substrate quickly rises to temperature. Drying and curing steps generally take about 1 minute. This is in contrast to traditional oven processes taking 30 minutes or more.
Bake Plates vs Conventional Ovens
Hotplates have several advantages over convection type ovens:
- Decreased bake time
- Increased reproducibility
- Better film quality.
Stratification, the formation of different temperature zones, is a problem associated with convection ovens and can severely affect film quality and reproducibility.
The heating rate of a substrate in an oven depends not only on the heated air flow past a substrate but also on its proximity to other cold substrates. Thus the heating rate for each substrate in a cassette of substrates that are being baked, will be less than if each substrate is baked alone.
In addition, substrates near the ends of a cassette heat faster than the substrates in the middle, thus producing a non-uniform heating
Particle generation also occurs within a standard oven. In a forced-air, convection oven, substrates are commonly exposed to a flow of particle laden air for at least 30 minutes.
During resin film cures, the substrates will be exposed to considerable particulate contamination. The substrates are vulnerable since the film may still contain solvents and during this ‘soft’ state, the film is very susceptible to having particles adhere to it.
The Skin Effect
Another disadvantage in normal oven baking results from baking substrates from the “outside in”. Since heat is applied to the outer surface of the film first, a skin forms on the surface of the film thus trapping solvents. Upon vaporizing, these solvents form blisters or bubbles which results in adhesion loss or even bulk film failure. This problem prevails in processes involving thick film resins, e.g. polyimides.
No skin effect occurs on a hotplate since hotplate baking heats the substrate from the bottom up. This “inside out” approach offers advantages for thick films since solvents in the film nearest the substrate are baked off before the fim surface seals over.
Hotplate Bake Variables and Methods
A typical bake process consists of preheating the surface to a known temperature, loading the substrate onto the surface for a specific length of time and removing it promptly at the end of the cycle. The selection of the temperature and time values used as well as the bake method employed all affect the overall performance of the process.
Bake Temperature
The bake temperature used is dependent on several factors. The material and substrate being baked as well as the results desired are key factors to be considered in developing a bake process.
In general, hotplate baking will be performed at temperatures slightly higher than those used in oven bake processes. The film being baked will reach a temperature somewhere between the temperature of the hotplate and the ambient air above the film. As an example, with a hotplate surface temperature of 115°C, a layer of photoresist on a silicon wafer will reach a final temperature of about 105°C after a few seconds. Thicker substrates and/or substrates with lower coefficients of thermal conductivity will require even higher temperatures to compensate for this phenomenon.
Bake Methods
Another important factor is the method of bake. Cee® hotplates allow for three distinct bake methods; proximity, soft contact, and hard contact.
In a hard contact bake the substrate is held onto the hotplate surface by the application of vacuum to the underside of the substrate. Small holes are machined into the hotplate surface in a pattern which optimizes vacuum distribution without the formation of cold spots or warping of the substrate. This method is usually preferred for silicon and other flat substrates where back side contact is not a problem.
Soft contact baking uses gravity alone to hold the substrate onto the hotplate. This method generally offers less uniformity since the substrate-hotplate thermal interface is not as efficient.
Proximity baking is accomplished by forcing nitrogen through ports in the hotplate surface. This forces the substrate to float at a distance of one to four mils (25-100µm) above the hotplate surface. Proximity baking allows a slower warm-up than contact bake methods and can be advantageous when baking thick films where blistering would otherwise be a problem.
Another advantage of proximity baking in this manner is that in many cases cambered or warped substrates can be baked with a high degree of uniformity. This is usually not possible with the contact methods since it is not possible to achieve a vacuum under a substrate that is not flat to start with. Processing cambered substrates with the soft contact method creates hot spots where the substrate touches the hotplate and cold spots where it does not. It should be noted as well that this type of proximity process is “self-leveling” in that the substrate will tend to form a uniform gap to the hotplate surface.
Proximity baking also offers the unique advantage of allowing hotplate processing without touching the bottom side of the substrate. An example of this application is photomask processing. In processing these relatively thick glass plates it is important that the back side of the glass not directly touch the hotplate since this causes micro-fractures in the glass itself from rapid heating. By performing the entire bake process in the proximity mode the integrity of the substrate is not endangered and the uniformity is excellent.
Exhaust Cover
The design of the Cee®exhaust cover promotes the dissipation of vapors removed from a substrate placed on the chuck, without actually drawing air across the chuck surface.
Oven vs. Bake Plate Examples
The chart below presents process examples for a commonly used resins. These figures should not be use as a rigid guideline, since the best method with a particular baking application can only be achieved through experimentation.
- APPLICATION: Positive Photo Resist
- OVEN BAKE
- 90°C – 30 minutes, Polyimide beta (partial imidization)
- 135°C – 30 minutes, Polyimide alpha (solvent removal)
- HOTPLATE BAKE
- 115°C – 30 secs w/Hard Contact bake, Polyimide beta (partial imidization)
- 150°C – 15 secs w/Proximity bake, 150°C – 90 secs w/Hard Contact bake, Polyimide alpha (solvent removal)
Hotplate Process Troubleshooting
As with the spin coating process there are no absolute rules for hotplate baking, only general guidelines. Following is a list of issues to consider for specific hotplate process problems.
Film Overbaked
Bake temperature too high
Select lower temperature
Bake time too long
Decrease bake time
Film Underbaked
Bake temperature too low
Select higher temperature
Bake time too short
Increase bake time
Film Blistering or Cracking
Unstable balance in temp. / time parameters
Decrease temp. / increase time
Warm-up time too fast
Use proximity bake to preheat substrate
Non-Uniform Bake
Unstable balance in temp. / time parameters
Decrease temp. / increase time
Operating with exhaust lid raised
Lower the lid exhaust
Unstable ambient conditions
protect against major fluctuations
Bake time too short
Increase bake time
hotplate surface contaminated
Clean surface of hot plate