Maybe you’ve recently brought parylene coating in-house, been put in charge of a parylene coating line or you’ve been parylene coating for years, either way you’re looking to optimize the parylene coating process. Sure, the resulting parylene film quality you have been seeing from your coating process meets your quality standards; however, at what cost of time?
As a rule of thumb, from a quality standpoint you can’t coat too slow. Slower deposition generally decreases the chance of cloudy coating caused from uncleaved dimer. Inversely, the faster your coat, the more likely it is to yield poor quality outputs. Therefore, when developing a parylene coating process it is important to find a balance between coating run-time and parylene film quality.
How fast can you parylene coat your product before the quality of the parylene film is compromised?
In order to understand what variables must be taken into consideration when developing an optimized parylene coating process, we must first have a general understanding of the parylene deposition machine. An overview of chemical vapor deposition, or parylene coating process can be seen in the illustration, below.
During a typical coating cycle, both the parts to be coated and raw material, dimer, are loaded into the sealed parylene coating system. The system is then pumped down via the use of a vacuum pump, to a steady state pressure, or base pressure. Once base pressure has been reached, the vaporizer is heated, until an acceptable coating pressure has been reached. The vaporizer heater then fluctuates, via a feedback control system, throughout the coating process in order to maintain the desired coating pressure, which dictates the coating cycle time and parylene film quality.
A lower coating pressure or vaporization temperature will result in a longer coating cycle and yield a higher quality parylene film.
On the other hand, a higher coating pressure or vaporization temperature will result in a shorter coating cycle and may yield a lower quality parylene film.
As illustrated and described above, a glaring variable that is critical to the parylene deposition machines functionalization is pressure. The parylene coating machine uses changes in pressure and temperature in order to sublimate, pyrolize, and deposit the parylene polymer film directly onto the substrate.
With that being said, the chemical vapor deposition process for parylene coating can be seen as an art just as much as it is a science; finding the optimal balance between pressure, time and parylene film quality. This article will delve into the relationship between base pressure and coating pressure, as well as the optimal way to control pressure and maintain a reliable system that consistently yields quality outputs.
What you’ll learn:
- What is parylene?
- The parylene deposition process.
- Coating benefits.
- Detailed properties of parylene.
- Design recommendations.
To understand the function of pressure within a parylene deposition machine, it’s first important to note a few basic concepts regarding vacuums. A vacuum is generally considered to be any closed system with a pressure that is less than one atmosphere of pressure. However, due to the fact that a “perfect” vacuum (zero pressure) does not exist in the real world; as every closed container contains at least some gas molecules, “vacuum” is typically divided into five levels:
As we know, parylene is deposited at room temperature, while in a vacuum chamber that functions in the medium vacuum range of 1 – 1×10-3 Torr. Although the entire deposition process is performed within the medium vacuum range, there are multiple pressure setpoints that vary throughout the process that require control and monitoring. Furthermore, different parylene types require different pressure set points and therefore, coating cycle times, in order to achieve the properties of a quality parylene film.
As stated above, there are many variables that can contribute to coating system issues and oftentimes issues with the vacuum system can be solved with a deep clean of your parylene coating system, as described in the Maintenance section, below. If your parylene coating system is adequately cleaned and is functioning as intended, there may be other variables affecting your parylene coating system.
The base pressure of a parylene coating system is defined as the lowest attainable pressure reading. It is imperative for efficient and quality deposition that you know the base pressure, and check it frequently. Ideally, the base pressure reading is repeatable; therefore, any significant deviation from previously established readings (more than 3 mTorr) can indicate vacuum leaks, vacuum pump problems, calibration errors, and other deficiencies that degrade your systems performance.
A base pressure check should be performed in a clean, dry and empty chamber. It is recommended that a base pressure check be performed:
- Before the first coating run with a new machine
- After extended periods of non-use
- After every 5 to 10 coating cycles during regular use
- As warranted by changes in coating quality
When performing a base pressure check, ensure all vacuum system o-rings and gaskets, as well as their mating surfaces are free of parylene film or various other contaminants Turn on the system and verify that the process controller set points are at their normal operating values and the cold trap is on. Initiate pump-down of the system and wait for the system to stabilize for at least 15 minutes. If you are unable to achieve a low enough base pressure, or the base pressure will not stabilize, this may indicate a problem with the vacuum system.
The coating pressure of a parylene coating system is defined as the desired chamber pressure to be maintained during a vacuum process. The coating pressure is one of the most important variables that can affect both coating cycle time and coating quality. A higher coating pressure typically results in a shorter coating cycle; however excessive coating pressures may result in unacceptable parylene film quality.
Besides parylene coating type, there are other variables that can affect coating the coating cycle time, such as the parts and/or substrate being coated, fixture design, and the integrity of the parylene coating system itself.
When developing a parylene coating process, not only does the integrity of the parylene coating system and controls need to be considered, the parts to be coated and fixturing must also be taken into consideration. During a parylene coating cycle, the system is pumped down to vacuum and any gasses that may be trapped within the parts or even the fixture can be released into the chamber causing what’s known as outgassing.
Outgassing can have a significant effect on the coating cycle and is a variable that can be compensated for by using certain materials for fixturing that have
Pressure Feedback System
To control the relation between base and coating pressure to ensure proper pump-down and that your machine maintains the optimal pressure for your parylene coating cycle. The vacuum system will fluctuate during the parylene coating cycle so to avoid having to physically monitor the coating cycle, VSi’s parylene coating equipment has a built in pressure feedback system.
A built-in pressure feedback system ensures that the temperature is automatically varied throughout the cycle to compensate for changes in pressure within the system.
Wrapping It Up
Optimizing the parylene process requires an understanding of the vacuum system and the parylene coating process. Its important to use equipment with the right controls, high-quality parylene dimer and have support from experts. VSi Parylene is here to help and make parylene more accessible. Contact us if you think we can help, we’d love to have a conversation with you about parylene.