Parylene is a polymer that can be deposited from the gas phase at room temperature. It gives conformal coatings that are pinhole-free, show excellent moisture, chemical and dielectric barrier properties and dry-film lubricity.
On planar surfaces Parylene can be structured using photoresist and anisotropic plasma etching in oxygen plasma. For this purpose we have created an anisotropic etch recipe in ALOES.
Coating procedure:
Parylene is provided as a dimer powder with a consistency resembling common salt. A weighed amount of Parylene is loaded into the vaporization chamber of the coater, before loading the substrates into the coating chamber and pumping down the machine. The pressure reached upon pumping down, with the furnace at 690°C and liquid nitrogen loaded into the cold trap, is called the "base pressure". The base pressure should be ~ 8mTorr, it can be slightly higher but should not exceed 20 mTorr.
The Parylene dimer itself has no liquid phase so it sublimizes upon heating, going directly from the solid to the gas phase in the vaporization chamber. The gas then gets introduced to the coaters furnace. Inside of this furnace the Parylene dimers are broken up into monomers, the building blocks of a polymer, at 690°C.
The monomerized gas is then introduced into the coating chamber at room temperature, where it polymerizes. It is going to conformly coat not just the sample but also the chamber walls, really any kind of exposed surface.
Process control
As monomerized Parylene gas enters the evacuated coating chamber, the pressure inside the chamber will rise proportionally to the amount of parylene present in the gas. The chamber pressure is therefore used as a control parameter for the coating process in the PDS 2010 machine and is coupled to the vaporization temperature by a PID controlling unit.
On the machine we set the "operating pressure" in the vacuum tab before coating:
A good rule of thumb is that the operating pressure should be 15 mTorr higher than the base pressure of the machine for Parylene C.The quality of the deposition depends on the deposition rate. If you enter a higher operating pressure, the coating process will happen faster and the parylene quality will be lower.
Coating thickness calculation
The deposited thickness on your sample can be tuned by varying the amount of Parylene that is introduced into the vaporization chamber. But it also depends on the exposed surface area inside of the chamber. If more surface area is exposed, the overall thickness of the coating will be reduced.
The coated area consists of the baffle and rotating plate (~ 1213 cm2), the chamber surface area (~4374 cm2), (and the surface area of the used fixture, if one is used) for a total of 5587 cm^2.
If the substrate is doubled from one wafer (~78,54 cm2) to 2 wafers (157,08 cm2) the total area is therefore changed from 5665,54cm2 to 5744,08cm2 or by 1,014%. To achieve the same coating thickness the weighed parylene sample should be increased by 1,014%.