Parylene Deposition: Process Parameters
Parylene deposition uses a room-temperature CVD process to create ultra-conformal, pinhole-free films. This guide summarises the critical process parameters, how to control rate and thickness, and the preparation and inspection steps that drive reliable outcomes.
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Process Flow (CVD Steps)
- Sublimation: Solid dimer is heated to form vapour.
- Pyrolysis: Vapour cracks to reactive monomer.
- Deposition/Polymerisation: Monomer enters the cool vacuum chamber and polymerises on all exposed surfaces.
- Vacuum Hold/Purge & Cool: Complete polymerisation, evacuate by-products, then unload.
Background reading: Parylene overview
Key Process Parameters
- Dimer Mass & Feed Rate: Primary driver of film build; match mass to target thickness and total surface area.
- Pyrolyser Temperature: Must fully crack dimer without side-products; stable temperature prevents under-cracking (poor films) or over-cracking (losses).
- Chamber Pressure: Controls mean free path and penetration into features; typical low-pressure vacuum for uniformity.
- Monomer Flow Distribution: Manifold/nozzle placement affects uniformity across complex 3D loads.
- Substrate Temperature: Keep parts cool to favour surface polymerisation and adhesion control.
- Loading Density & Fixturing: Line-of-sight and diffusion paths matter; rotate or rack to avoid shadowing.
- Cycle Timing: Stabilise soak/hold and cool periods for repeatable results.
Thickness & Rate Control
- Setpoint Strategy: Convert target thickness (e.g., 10–25 µm) to dimer mass using your chamber’s calibration curve.
- Deposition Rate: Tune via dimer feed, pyrolyser temp, and chamber pressure; slower rates can improve uniformity on dense assemblies.
- Load Mapping: Place witness coupons at corners/centre to verify uniformity across the load.
- Verification: Measure with optical thickness meters or eddy-current gauges; manage plans via AQL, coupons & SPC.
Adhesion & Surface Prep
- Cleanliness: Pre-clean to remove oils/ionics; verify no residues that can cause de-wetting.
- Adhesion Promoters: Apply silanes (e.g., A-174) where compatible; allow proper hydrolysis/flash-off.
- Surface Activation: Use plasma cleaning for low-energy plastics, ceramics, or oxidised metals.
Masking & Keep-outs
- Select robust masks for vacuum: reusable boots, tapes , dots, or custom shapes.
- Verify edge quality under magnification to avoid leakage or feathering.
- Plan demask and local repair workflow before loading.
Inspection & Verification
- Coverage: Check under magnification; confirm edges/keep-outs.
- Thickness: Verify per plan (AQL/coupons/SPC) using optical or eddy-current methods.
- Defects: Record and address common defects such as edge bleed.
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