Gas gives big assist to many lasing jobs

Many of today’s laser processes are performed with an assist gas, which is a pressurized stream of gas directed either coaxially with or lateral to the laser beam. In cases where an assist gas isn’t required, it’s often applied to reduce processing time and enhance workpiece quality.

Assist gases are used in laser operations like cutting, drilling, welding, deposition and surface alteration. A common application is the production of stents: coronary, urinary, urethral/prostatic, colonic, duodenal, vascular and peripheral vascular. The stents are made from biocompatible metals that are laser-cut in ways that give them the flexibility to be snaked through the body and sufficient rigidity to hold open the orifice into which they are being inserted. Kerf widths of tens to hundreds of microns are typical, and outstanding surface finish is a must—especially for drug-eluting stents.

A typical vascular stent has a wall thickness of about 0.003" and strut-size tolerances around 0.0003". The required surface finish is about 0.00015" (mirror finish), or ±5 percent of wall thickness. Such requirements couldn’t be met without an assist gas.

Among the reasons to apply assist gases are:

• To remove molten debris from the workpiece surface. Efficient removal of slag minimizes post-process cleaning and helps protect the laser’s lens.
• Cool the workpiece.
• Inhibit oxidation. Frequently, a “cover” gas is applied that blankets the processing area with an inert gas. Doing this prevents oxidation at the high temperatures associated with laser welding and laser cutting of metal. (Note: The process sometimes is performed in an enclosed environment, like a laser glovebox.)
• Enhance oxidation, which speeds the burn process. This is often done when cutting stainless steel.

Assist gases are teamed with infrared lasers more often than ultraviolet lasers. One reason is that, generally, IR lasers are used to weld—a common application—and UV lasers are not. The short wavelength and associated short pulse length of the UV laser do not lend themselves to joining applications.

When an assist gas is delivered coaxial with the laser beam, beam centering is critical.

Another reason is that UV spot sizes are generally much smaller than IR spot sizes, meaning there is less molten material for an assist gas to remove during a UV-laser operation.

Finally, with the exception of excimer lasers, UV lasers are generally delivered through galvanometer beam-delivery systems. These do not lend themselves to coaxial gas processing.

Two UV applications where assist gas is used are pulsed-laser deposition and laser cleaning. Thin films of HtSs (high-temperature superconductors) can be deposited uniformly and rapidly with UV lasers and the appropriate gas environment. Other coatings can be applied by this method as well, including thin films of crystalline and amorphous silicon.

Laser cleaning systems for the semiconductor market remove organics from wafers. One of them, a patented cleaning technology from UV Tech System, Sudbury, Mass., utilizes “green” gases coupled with laser light to remove organic materials from the surface of silicon wafers and other substrates. The light and gas reaction creates a “gas reaction zone” that photochemically and/or photo-ablatively removes selected material from the surface. No solvents or chemicals are required—only an inert gas like oxygen or nitrogen.

The most common assist gas applied in traditional laser processing is CDA (clean, dry air). Because the slightest presence of water will kill many processes, the air needs to be extremely clean, dry and pure. Other gases usually have some purity specification.

An obvious choice to enhance oxidation is pure oxygen. Used for cutting and drilling certain metals, it generally is placed in the reactive-gas category.

In order to limit oxidation, such as when welding metals, an inert cover gas is used. These gases include nitrogen, helium, neon and argon. A light gas like He moves quickly and can easily enter very small spaces. Larger gas molecules, like Ne and Ar, are heavy and tend to make better cover gases. They have enough mass—and therefore momentum—to deflect ejected material from the processing area. Even H₂ and CO₂ are used on occasion. It has been shown that gas mixtures sometimes perform better than pure gases.

Laser cutting and drilling metals usually create a HAZ (heat-affected zone) that sometimes must be removed with a secondary process. This is undesirable and can be minimized or eliminated by applying the right gas mixture. Steel, for instance, responds well to a mixture of O₂ and N₂. Stainless steel and aluminum respond well to a mixture of N₂ and CDA, while titanium and nickel alloys respond well to Ar and He.

Welding requires an assist gas to perform three main functions: protect the HAZ from oxidation, minimize plasma effects in the weld area and expel plasma from the weld joint. Helium is the gas of choice because of its high ionizing potential and minimal metallurgical concerns, but it is expensive. Argon is less expensive, but it has a lower ionizing potential and the performance is not as good as with He. Here again, a gas blend may be preferable.

Gas pressure is vital and can vary greatly. For micromachining purposes, 80 psi probably is sufficient. Applications involving larger lasers, though, call for pressures up to 300 psi and flow rates up to 1,000 standard cu. ft./hr.

Beam centering with a coaxial delivery system is extremely important, especially when trying to achieve consistent results in multidirectional cutting. For the

oxygen-assisted cutting of steel, for example, concentricity should be within 50µm. Centering of the gas nozzle is done after focusing.

When coaxial gas-assist delivery is not possible, external nozzles can be used to direct the gas at the target area obliquely. The direction of the gas—i.e., toward, away from or perpendicular to the direction of travel—can make a big difference in on-target quality.

Finally, a key to maintaining excellence in processing over an extended time period is to consistently apply high-quality gases. This is achieved by partnering with a supplier known for providing a pure product. µ

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.