Parylene Coating Measurement Methods

Used as moisture and dielectric barriers, Parylene (p-xylylene/XY) coatings are conformal and pinhole free. Applied by a unique vapor deposition polymerization (VDP) method, Parylene penetrates beneath substrate facades, simultaneously attaching above surfaces at the molecular level. VDP generated films cover crevices, exposed internal regions, points and sharp edges uniformly, without gaps or breaches. Compared to liquid coating materials – acrylic, epoxy, silicone and urethane – XY film layers are micron-thin, enhancing their utility for micro-electrical-mechanical systems (MEMS) and nanotechnology. 

At the same time, XY application is never flawless and needs to be carefully monitored. With VDP, deposition thickness is typically regulated by total quantity of solid-phase dimer loaded in the sublimation chamber at process commencement; process end-point occurs when dimer is exhausted. Thickness measurements are desirable because this VDP end point can hinder precise, repeatable control of film thickness. The inaccuracy of predicting and controlling the deposition thickness is especially problematic when the deposition thickness is small (e.g., less than 1 µm). Film thickness appropriate to requirements of the coating assignment is key to reliable Parylene performance.

Applied too thin, the coated device will be vulnerable to environmental risks – caustic fluids, chemical incursion, salt air, etc. – and malfunction. Too little Parylene can mean incomplete coverage, pinholes and reduced wear-ratings. In contrast, overly-thick film application can generate:

• Disproportionate stress on components and solder joints
• Waste of precursor materials required to initiate VDP
• Reduction in device performance after film application

XY polymers exist in a variety of types, including the Parylenes C, N, ParyFree^® and Parylene HT^®. Each has its own range of chemical and performance properties, appropriate to precise coating functions. Measurement of coating thickness is essential to assure the specific XY type is applied at proper thickness for optimal operation. Measurement helps maximize the benefits of Parylene coatings, including biocompatibility, high reliability, light weight, stress-free impact and transparency. Typically, coating thickness is specified according to the material used for the film and the function of the particular assembly.

Laser micrometers can be used to measure the coated device, recording its dimensions across various areas of its topography prior to VDP and re-measuring the same regions after coating, allowing for the calculation of coating thickness, defined as the difference from the original reading. The film’s uniformity of application is assessed by standard deviation analysis. Coating both sides of a device is common with Parylene; division of the total by two provides reliable measure of the film’s thickness on all sides.

Test coupons economically and accurately measure coating thickness which are positioned near substrates and measured for Parylene thickness after the run. Ideally, they are composed either of the same material as the coated device; otherwise, non-porous materials like glass or metal, are used. A test probe set flat on the XY film’s surface produces measures accurate to ±1 µm, for films as thin as 25-50 µm. Micrometers or similarly dependable instruments compare finished coating to assure quality specifications; non-contact techniques use eddy currents.

With different chemical and optical properties, refractive indexes (RI — dispersion of XY material) of each Parylene varies, often significantly. Algorithmic estimates of coating thickness are convenient in the production environment, but if RI is unknown, the thickness reading will be inaccurate. Highly variable, this method is not suited for applications requiring critical thicknesses. Precise thin film depositions require commensurately precise measurement tools in real-time, according to reliably calculated Parylene-to-substrate thickness ratios.

Optical reflectometry offers accurate and verifiable measurement and control of film thickness(t) for XY coating systems.

• If a plane monochromatic wave of wavelength λ exists on a substrate, some of the light is reflected. λ = v f , {displaystyle lambda ={frac {v}{f}},,,}
• A substrate coated with Parylene containing an RI of n will display sinusoidally modulated reflectivity as the depth of coating increases.
• A functional relationship between the substrate’s reflectivity and the coating’s optical thickness allows film thickness measurement.
• One complete cycle of reflectivity modulation occurs as each thickness increment is applied to the substrate, according to the formula, Δt = λ/2n.

Monitoring thickness levels during application requires balancing the total number of cycles during XY application with the fraction of cycles measured, based on a pre-determined standard for film thickness for that coating run.

Accurate and precise control of coating thickness is essential to successful target fabrication. Recently devised in-situ sensors working from thermal transfer principles end VDP when a targeted thickness is achieved. The device consists of a temperature sensor and a heater split by a well-defined gap, defined as d. When the process reaches one half d’s value, d/2, the heat conducts through both the gap and the substrate. The thermal signature determines the gap-closing event, stopping the process. The sensor measures both the quantity of Parylene dispensed and its film thickness as deposited on the substrate, signaling VDP completion at specified thickness. This method both manages and measures XY deposition thickness, with a functional efficiency improving the +/- 10% of desired thickness standard often used, making it far more desirable for MEMS and nano technology applications. Downsides to the use of this technology is the need for extra steps to prepare the system for subsequent use, often consuming all savings gained by its use.

Optical interferometry is the most reliable method of Parylene thickness measurement. Filmetrics offers a number of systems capable of this task, one of the earliest being their F3-CS system, an exceptional instrument for measuring Parylene thickness. It is capable of measuring XY films applied on flat surfaces, to measure small witness or coupon samples. USB-powered F3-CS acquires reflectance data via a contact stage that maintains the same working distance of the measurement sample throughout the process. Placed face-down on the device’s contact stage, the sample’s spectral data is collected and analyzed automatically by FILMeasure thickness measurement software to determine the coating thickness.

The optical constants (n and k) for Parylenes C and N layers are fundamental components of F3-CS, powered by any computers running Windows XP^® through 64-bit Windows 8^®. Presented in real-time, thickness results can be upgraded to measure refractive index.

Other Filmetrics products calibrate Parylene thickness for curved surfaces and coatings exceeding the 0.25-75 µm standard.