Medical Device Link.

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Thin films in medical technology Layers of material deposited as a coating onto a surface are used to enhance the physical and optical properties of otherwise inappropriate materials in terms of passivation, hygiene, insulation or hardness. In health-related industries, both optical and non-optical coatings are being applied in a range of applications. These include passivation coatings on dental and orthopaedic prostheses, drug or passivation coatings on implants, and coatings as enhancement treatments on fabrics and as moisture or hygiene barriers on the inside of packaging. In medical devices such as catheters, guide wires and stents, the use of non-optical coatings can preclude issues with trauma or irritation without altering the device’s bulk material properties. In addition, thin films, or more exactly thin foils, are used to form catheter balloons that are employed to expand stents against blood vessel walls; the proper mechanical integrity of these balloons is obviously critical.

Market growth

Figure 1: The inset shows a close-up of the surface of a heart catheter balloon with a material thickness of approximately 20 µm.

Modest growth of almost 4% is expected in the well-established optical coatings market over the next few years. However, the coatings market for medical applications is predicted to experience sales growth of several times that rate. The increasing use of coatings in this area raises the issue of adequate quality control (QC). Probably more than in any other industry, safety and reliability are paramount.

To guarantee correct function, coatings must fulfil requirements for proper thickness and layer integrity, have correct material composition and exhibit proper surface smoothness/roughness for the application in question. Formed by processes such as spin coating, vacuum evaporation, sputtering, vapour deposition and dip coating, and ranging in thickness typically from sub-nm up to sub-mm, these coatings can be accurately characterised using optical metrology techniques that were originally developed for semiconductor wafer processing.

Optical characterisation

Optical techniques are used to assess thin film thickness because they are accurate, non-destructive and require little or no sample preparation. One technique, spectroscopic reflectometry, employs a broad spectrum of light that is directed straight down onto a thin film or coating. The reflections from the twin surfaces mutually interfere to produce a modulation of the light intensity as a function of wavelength seen in the reflected beam. Systems employing this technique can measure films of approximately 10 nm up to 400 µm with an accuracy of 1 nm, the only requirement being that they are partially transparent. Three or four multilayer stacks, or metallisation layers of thicknesses above 1 nm can also be measured provided some degree of transparency remains.

Applications

Today’s optical measuring systems can be used to measure coatings applied as passivation layers to

• implant components, including stents and bone replacement parts
• prosthetics and items used in the dental industry
• urological, arterial and venous catheters that are designed to ease insertion, avoid irritation and/or prevent encrustation
• the balloon material used in catheterisation to distend the blood vessel and thus dislodge any obstruction (Figure 1 shows a heart catheter and a microscope image, inset, of the film itself, here the balloon film has a thickness of just under 20 µm)
• packaging material such as Tyvek (DuPont), which is a spunbonded olefin, to enhance its flame-retardance, anti-static properties, performance in electronics packaging (aluminium lamination) and offset lithographic printing (aqueous coating).
• underlying “surface preparation” layers such as the initial primer layer that is often needed to improve hard coat adhesion of the optical coatings for scratch resistance and to enhance the optical performance (reflectance, transmittance) of transparent materials; given proper initial estimates for material parameters, the thickness of these layers can be measured individually or simultaneously with the upper layer.

Figure 2: Measurement (screen shot) of a 25-nm thick metallic coating on Tyvek. (click image to enlarge)

The coating process needs to be monitored for QC purposes and to ensure the desired function. Any of the coatings described above can be assessed using optical metrology, provided of course the layer retains some degree of transparency. Figure 2 shows measurement of a metallic coating of 25 nm thickness on Tyvek and Figure 3 shows a close up of its surface.

Measurement versatility

In addition to aids for measurements on curved surfaces such as those found on aspherical lenses, specific adaptors also exist for accessing enclosed surfaces. Accessories for two-dimensional topographic mapping over intermediate distances are also available. When used in conjunction with a microscope, the spatial resolution can be reduced from approximately 400 µm down to 1 µm.

Figure 3: A close up of Tyvek’s surface texture and the overlying metal coating.

The accurate measurement of thin films created for optical applications is usually straightforward. These coatings are optically flat and homogeneous, have a high degree of transparency and usually exhibit strong reflections due to large changes in refractive index at smooth interfaces. This is not the case for non-optical coatings, which often exhibit the opposite characteristics. Accurate measurement is then hampered by aspects such as weak bottom surface reflection, which is worsened by poor layer transparency and interface roughness. In addition to rough coatings, even the substrates destined for use with thin films may not be smooth. Optically rough metal, ceramic and plastic substrates can be coated with a hard wearing diamond-like carbon layer and the net effect is reduced contrast in the interference signal, which in turn complicates the measurement process. The use of robust algorithms is employed for modelling interface roughness, based on expected and actual reflectance amplitudes of a known material interface. Together with the large spectral range that is utilised (from 250 nm in the ultraviolet to 1100 nm in the near infrared), measurement of critical layers for non-optical, medical applications of hardness coatings, and of transparent foils are all easily performed.

The short measurement time and the flexible positioning of the fibre head, together with its simplicity, robustness, cost-effectiveness and ease of integration, has meant that this type of measurement tool has found favour in numerous application fields.

Gerald Nitsch is Managing Director, Mikropack GmbH, Maybachstrasse 11, D-73760 Ostfildern, Germany, tel. +49 711 341 6960, e-mail: info@mikropack.de www.mikropack.de

Gregory Flinn Putting Photonics into Context, Munich, Germany, e-mail: gregory.flinn@gmx.net