Laser drilling really small holes

Laser-drilled holes with diameters less than 100µm usually are considered “small.” However, lasers can produce holes down to fractions of a micron across.

When we talk about holes this small, the only real equipment option is a laser that operates in the ultraviolet spectral range. It can be either a 355nm or 266nm DPSS (diode-pumped solid-state) or excimer laser. Also, for UV photons to produce a sufficiently small spot size on target, the correct optical system must be used, which usually means a lens with a short focal length.

Figure 1: Profiles of laser-drilled holes. All images courtesy PhotoMachining.

Figure 1 shows five different cross sections of laser-drilled holes. The straight-walled (or minimal-taper) hole is very difficult to produce, but fortunately this shape is not needed except for a few special applications. The idealized taper is one in which the entrance and exit holes can be measured and the taper is a simple linear function. What we normally see when laser drilling is the trumpet shape. There are also wine-glass and champagne-flute profiles.

Taper is not bad. In fact, most small-hole applications benefit from some structure in the hole. For example, wine-glass and champagne-flute shapes improve liquid or gas flow.

When laser drilling holes in thicker materials (i.e., high-aspect-ratio holes), we use the naturally occurring taper to produce a very small hole diameter (D), which is defined as the smallest D of the cone. This is almost always on the exit side of the workpiece (Figure 2).

Figure 2: The exit side of a laser-drilled hole (D) is usually the specified diameter, not the entry side (D_E).

Accounting for natural hole taper permits drilling Ds much smaller than what would be predicted mathematically. But, it is very difficult to achieve good hole-to-hole uniformity. The common practice of applying a fixed number of pulses on a material may not work. Slight variations in thickness or the material not being sufficiently flat or inhomogeneities in the material can cause problems unique to micron-range holes.

Instead of counting pulses, an endpoint-detection method can be used. My shop employs sensitive UV detectors, sometimes coupled with a light-gathering device like an integrating sphere, that detect UV-light breakthrough at the bottom of a workpiece. We have used this technique with a 355nm laser to drill holes less than 0.5µm in D, with hole-to-hole variability of about 0.1µm.

This technique can be performed at very high speeds with the appropriate optics. For instance, we have laser-drilled over 150 parts per minute, each of which had 500 holes smaller than 0.5µm in D. All holes were uniform to within a few percentage points. The total time required included web motion, settling and camera alignment, and drill time. We applied a roll-to-roll laser tool with two galvanometers to increase throughput, and integrating spheres for endpoint detection on each of the two beam lines. The resulting profile resembled the wine-glass taper, because the first burst of pulses were at a higher energy and the final pulses were at a much-reduced energy level.

The endpoint technique can be used with an excimer laser, too, whether drilling a single hole or multiple holes simultaneously. With the latter operation, endpoint detection will consist of an average over the total number of holes drilled. Since an excimer laser uses mask imaging, many holes can be drilled simultaneously if the mask array fits into the homogeneous portion of the laser beam. This works well if the holes are very small and tightly packed in a uniform array. We have drilled thin, plastic material, using a mask, where over 7,000 holes have been drilled simultaneously in one mask image.

Wine-glass and champagne-flute profiles can be obtained by shuttling different masks into and out of the beam and/or by controlling the laser output energy. The down side to drilling many holes simultaneously is that endpoint detection cannot be used for individual holes, but is, instead, averaged over the number of holes being drilled.

More small holes being lased

A growing number of parts and devices are being designed with very small holes. They include:

• Interconnects for satellites, medical devices, missiles and UAVs (unmanned aerial vehicles), and cell phones. Interesting tidbit: More people, worldwide, use cell phones on a daily basis than wear shoes. There are thousands of lasers throughout the world drilling vias for electrical current conduction in cell phones. Technology is available to make cell phones much smaller than they currently are, but the phones would be too difficult to hold.
• Probe cards. Holes drilled in these substrates hold an electrical conductor that is used to test wafers—normally a spring or wire. In this application, minimal hole taper is required to ensure the probe is held firmly. Current probe-wire diameters are large by our definition—around 4 mils. But research is focusing on much-smaller vias.
• Filters for cells and organisms. Submicron holes are used to filter many different biological species, and these filters are applied in cancer research, in manufacturing, and for drug delivery.
• Local drug delivery through plastic tubing and angioplasty balloons. When drilling into a balloon, the holes must be large enough to deliver the drug but small enough so that the balloon can still be inflated. This can be done by drilling a matrix of holes that has an effective overall volume or open area.
• Spray-mist nozzles for analytical devices. Many analytical devices, like ICAP (inductively coupled argon plasma) and AA (atomic absorption) spectrometers, require very small holes to form a fine mist that is then analyzed. Sample preparation and presentation to the light are keys to achieving accurate and consistent results.

The growing demand for ever-smaller devices presents great opportunities for companies able to produce the even-smaller holes that will be required. 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.