Plasma Enhanced Chemical Vapor Deposition (PECVD)

PECVD Principle

Depositing SiO₂ by conventional CVD takes substrate temperatures of several hundred degrees, which rules out electronics, solder joints and polymers. PECVD replaces thermal energy with electrical energy. A radio-frequency field at 13.56 MHz sustains a plasma in the reactor, a partially ionized gas containing free electrons, ions and radicals. The electrons, being light, pick up the RF energy and reach effective temperatures of tens of thousands of kelvin, while the heavy molecules and the substrate stay near room temperature. Electron impact does the chemistry that heat would otherwise have to do. In practice this allows film growth below 100 °C.

PECVD Chemistry

Precursor delivery. The silicon source is an organosilane liquid, usually hexamethyldisiloxane (HMDSO) or hexamethyldisilazane (HMDS), evaporated and carried into the reactor with oxygen or N₂O. These liquids are much safer to handle than silane gas.

Dissociation in the plasma. Electron collisions break the precursor and oxygen molecules into reactive fragments: silicon-bearing radicals, atomic oxygen, hydroxyl radicals. None of these species would form at the substrate temperature; the plasma creates them in the gas phase.

Film growth and densification. The radicals adsorb and react on the substrate to build a silicon oxide network. Atomic oxygen oxidizes the methyl groups of the precursor into CO₂ and water, which are pumped away. At the same time, ions accelerated across the plasma sheath strike the growing film with energies of a few tens of electronvolts. This is enough to displace weakly bound atoms and compact the network, but not enough to damage the substrate underneath. The ion bombardment gives the film a density that thermal deposition could not reach at this temperature, and density is what determines barrier performance.

The oxygen-to-precursor ratio is the main process lever. With plenty of oxygen, almost all the carbon burns off and the film approaches stoichiometric SiO₂, hard and dense. With less oxygen, residual carbon stays in the network and the film becomes softer and more polymer-like (SiOₓCᵧ). The same precursor and the same reactor therefore cover a whole range of compositions, from ceramic to nearly organic, which is put to use in multilayer stack design.

PECVD Multilayer

A defect-free SiO₂ film would be an almost perfect barrier; permeation through the dense oxide itself is negligible. The problem is stress. As the plasma-deposited film grows thicker, intrinsic stress accumulates, and beyond roughly 500 nm it relaxes by forming nanocracks and pinholes. Water and gas then flow through these defects, not through the bulk. Permeation is governed by defect density, so past a certain point a thicker film actually performs worse.

The solution is architectural rather than chemical: alternate thin oxide layers with polymer interlayers. Parylene, deposited by the Gorham process above, is the natural partner because it is conformal and pinhole-free. Two mechanisms are at work in such a stack. First, defect decoupling: a pinhole in one oxide layer rarely sits directly above a pinhole in the next, so a water molecule that gets through one defect has to diffuse laterally through the polymer before it finds the next opening. The effective diffusion path becomes very long. Second, stress relief: the compliant polymer absorbs the mechanical stress of the rigid oxide layers, keeping each of them below its cracking threshold. Measurements confirm the approach. A parylene/SiOₓ multilayer of 5.9 µm total thickness reached the same helium leak-tightness as 40 µm of pure parylene and passed the MIL-STD-883 hermeticity criterion.

Based on: Choi et al., Surface and Coatings Technology (2000); Hogg, PhD Thesis, University of Bern (2014); Buchwalder, PhD Thesis, University of Bern / COAT-X SA (2023).