Bake Plate Process Theory

Hotplate Bake Process Theory

Hotplate bake processing has increased in popularity since the early 1980s. Previously the most common technique for film drying and curing as the convection oven. 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 a peak temperature slightly lower than the hotplate surface temperature. Drying and curing steps generally take about 1 minute. This is in contrast to traditional oven processes taking 30 minutes or more.

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 Skin Effect 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.

Another reason for using higher temperatures is to increase process throughput. In oven processes there is a problem commonly known as the "skin effect". This is a result of the outer exposed layer of the film drying and forming a skin before all of the solvents in lower layers have evaporated. Most oven processes are adjusted to use lower temperatures and bake times measured in minutes and hours to prevent this. During a hotplate bake process the film is baked from the bottom up thus preventing the formation of a skin over the surface. Because of this it is possible to increase temperatures and adjust bake times to be measured in seconds without danger of blistering or cracking in the film.

Bake Time

Good Process The selection of the bake time parameter plays an important role in the reproducibility of the bake process. Substrate thermal properties and the choice of bake method greatly affect the amount of time necessary for the substrate and therefore film temperature to stabilize during the bake. Thicker substrates and the use of proximity bake methods will increase the time necessary for the film to reach its final temperature. It is important that most of the baking action in the film takes place after this temperature is reached. A silicon wafer will reach a stable temperature within a few seconds and so it is traditional to adjust a photoresist bake processes to be completed in 60-90 seconds with an appropriate bake temperature.

For thicker substrates such as photomasks and ceramic modules the increased time necessary to heat the larger mass of the substrate results in bakes times Unstable Processapproaching five minutes. It should be noted that these substrates can be processed with higher temperature and much shorter bake times but reproducibility may suffer. If the bake time is too short then a significant amount of the actual bake process will take place during the loading and unloading steps as well as while the substrate is cooling after removal from the hotplate. This is an unstable condition since it is very difficult to exactly reproduce conditions during these steps.

In general the temperature-time relationship in a bake process can be taken as a "dose" of the (temperature) x (time) product. Increasing the bake temperature results in a need for decreasing bake time. The limits for both of these parameters can be considered to be reached when the process is no longer reproducible or when the physical temperature limitations of the resin or substrate have been reached.

Bake Method

Another important factor is the method of bake. Cee® hotplates allow for three distinct bake methods. These are Proximity, Soft contact and Hard contact. The choice of bake method is programmable and up to three consecutive bake steps may be programmed into the onboard microcomputer. Hard ContactIn a hard contact bake the substrate is clamped 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 ContactSoft 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 as in hard contact baking and can be somewhat random in variation. Proximity BakeProximity baking is accomplished by forcing heated gas (usually nitrogen) through the same ports in the hotplate surface that are used for vacuum in the hard contact method. 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. This is a significant advantage not found in "pin lift" type systems.

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.

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 exhaust lid

Unstable ambient conditions

Protect against major fluctuations

Bake time too short

Increase bake time

Hotplate surface contaminated

Clean surface of hot plate