Patterning thin-film coatings with lasers

Thin-film-coating technology has played a huge role in the miniaturization of many products and devices. Among them are circuits, semiconductor wafers and MEMS devices, photovoltaic cells and optical devices—to name a few.

Lasers sometimes are used to apply thin-film coatings to surfaces. But the more common methods are chemical vapor deposition, physical vapor deposition and spray coating. All the processes uniformly coat the target surface.

If some areas should not be coated, then they need to be masked before the deposition process or the coating must be patterned post-deposition. Masks have some inherent problems, including limitations on the resolution that can be achieved, potential damage to the part from physical contact, higher costs and the need to inventory a mask for each application.

Thin-film patterning of dielectric material on a printed circuit board. All images courtesy PhotoMachining.

Close-up view of a gobo used with a projector.

A 355nm laser, with a 30µm beam width on target, removed gold from this Mylar diabetes-test strip.

Post-deposition patterning methods include mechanical scribing, sand blasting and lasing. The first two offer limited resolution and can potentially damage the underlying substrate, and, with respect to sand blasting, it’s a messy process. Lasers cleanly and precisely remove films at a competitive cost. And the achievable resolution is much better, for instance, than with lithography or mechanical processes—down to a few microns when using an ultraviolet laser and a lens with a focal ratio (f-number) near 1.

Often, by choosing the appropriate laser-beam wavelength and energy density on target, material can be selectively removed with little or no damage to the substrate.

Materials include optical films, for dielectric masks and “gobos” (colored films used with light projectors), and conductive films, for miniature circuits, resistive heaters and cell isolation. Conductive films can be metal or metal oxides, like ITO (indium-tin oxide).

Types of film

Before discussing specifics of laser patterning thin films, let me define “thin”: a film with a total thickness of less than 1µm. Most films range from hundreds of angstroms to thousands of angstroms thick, which usually allows removal of the target material with one laser pulse.

Some films, metal ones in particular, react quite differently in the bulk state than in the film state. For instance, clean removal of most bulk metals usually requires an energy density of 10 J/cm² or higher. A thin film, conversely, can be removed at almost the raw fluence of the laser—a couple hundred millijoules per centimeter squared. (It’s worth noting that many films do not exist in the bulk state.)

Common substrates include metals, glass, ceramics and plastics. These materials can be either opaque or transparent to some wavelengths. Glass, for example, is transparent to visible-light wavelengths. Because of this, potential damage caused by overexposure is minimized or eliminated. Also, it is frequently desirable to position the film facing downward and lase through the substrate. This promotes clean liftoff of films and minimizes the amount of debris that collects on the substrate surface.

Patterning examples

One example of patterning an optical film is the gobos found in projectors used for light shows. Gobos are substrates approximately 1" in diameter and transparent in visible light. They are first coated with light-reflecting films. Then they are patterned by lasers so that when the projector’s light passes through them, highly detailed and unique patterns are displayed. Edge quality is crucial because the projected image is many times larger than the gobo.

A disposable diabetes-test strip is an example of a laser-patterned conductive film. A strip typically consists of several layers of material laminated together and incorporates a window where the blood sample collects. One layer is a gold-coated plastic a few hundred angstroms thick. The layer can be removed at the low fluence of a 355nm UV laser with little or no damage to the plastic. The patterned gold forms the electrical circuit necessary for digital readout of blood sugar levels.

Test strips represent a huge market for the laser industry, as do photovoltaic cells, the technology underlying solar panels. Lasers are used to pattern rigid and flexible thin-film photovoltaic cells.

Flexible substrates coated with conductive inks are easy to pattern with near-infrared lasers on roll-to-roll processing lines. While this technology has a lower solar-energy-to-electricity conversion efficiency than rigid films, it provides a great way to make low-power, portable products. (Once the initial setup is paid for and in place, it is almost like printing money!)

For rigid products, primarily those made with silicon-based thin films, lasers are used to scribe their three main layers—designated P1, P2 and P3—and perform an operation called “edge isolation.”

The P1 layer is scribed with a 1,064nm laser. Its beam, directed through the glass, removes the TCO (transparent conductive oxide) and defines cells. The P2 scribe is performed with a 532nm laser after deposition of the silicon. The P3 scribe is then made after deposition of the metal electrode with a 532nm laser; the laser’s light is transparent to the glass substrate and the TCO, but ablates the silicon and metal.

Edge isolation involves removal of all layers of conducting and absorbing material from the edges of a panel. The process ensures that a water- and oxygen-tight bond exists between the glass sheets. Failure to perform this operation would cause short circuiting of the finished product. µ

(Editor’s note: To learn more about processing photovoltaic cells, read the article “Solar Flair.”)

About the author: Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. E-mail: rschaeffer@photomachining.com.