Small excimer lasers create tiny pieces

Micromanufacturing workstations with UV light source, video output, and motion control perform cutting, scribing, and drilling in various materials.

Small excimer lasers create tiny pieces

Micromanufacturing workstations with UV light source, video output, and motion control perform cutting, scribing, and drilling in various materials.

Sarah Cohn Christensen and C. Paul Christensen

The world as we know it is shrinking. We are starting to carry our telephones in our pockets and our computers under our arms. Automobiles are sprouting tiny sensors and actuators. Eyes, knees, and hearts are being repaired with miniature tools and implants. This miniaturization revolution is fueling an expanding demand for new equipment for micromanufacturing.

Fabrication of microcomponents and miniature systems requires tools capable of shaping, measuring, and marking materials of interest on very small spatial scales. Conventional mechanical operations carried out by milling machines, lathes, calipers, engravers, and similar equipment can address most requirements when feature sizes are larger than a few hundred micrometers. However, when hole diameter, slot width, character height, or similar feature sizes fall below this range, optical tools are one of the few viable options.

Lasers for micromanufacturing

The spatial resolution associated with lasers, high-resolution imaging systems, and precision motion-control devices allows relatively easy access to feature sizes in the 1-100-µm range. Most optically controlled material processing uses either optical exposure of photoresist or laser ablation as the primary process mechanism. No one would argue that any single light source is optimum for all applications, but ultraviolet-emitting excimer lasers play important roles in both of these processes.

Excimer-laser ablation has several features that are desirable for effective micromanufacturing. The deep-UV wavelengths (351, 308, 248, or 193 nm) at which excimer lasers operate are strongly absorbed by a wide range of materials and also can produce very small spot sizes. Additionally, because short-wavelength UV photons can directly break chemical bonds, material can be literally vaporized with minimal heat transfer to the surrounding substrate.

In general, very little laser energy is needed to ablate small areas on absorbing substrates. Typically, only a few hundred microjoules of optical energy are required for ablation in a 100-µm focal spot. The material-removal rate is proportional to the average laser power delivered to the substrate, and the laser energy should be delivered in short pulses to minimize heat conduction to unilluminated areas.

Micromachining-system configurations

The miniaturization of excimer-laser technology has made available a light source that is ideal for work on small spatial scales. These small excimer lasers produce pulse characteristics and average powers that are adequate to meet the production rate requirements of many applications, while keeping operating costs relatively low and facility and installation requirements minimal. Using small excimer lasers, workstations can be configured that are compact, flexible, and cost-effective (see Fig. 1).

The laser. Small excimer lasers used in micromachining typically have average power outputs between 50 mW and 2 W and pulse energies ranging from tens of microjoules to several millijoules. The table on p. 85 lists operating parameters of two systems from Potomac Photonics (Lanham, MD) designed for micromachining and marking at different wavelengths. One of these is a high-repetition-rate, low-energy device that is well suited to sheet cutting, patterning, and text writing; the other is a low-repetition-rate, higher-pulse-energy device well adapted to cutting larger holes and slots and dot-matrix marking. Experience with a wide range of applications has shown that for the vast majority of applications krypton fluoride (KrF; 248 nm) and argon fluoride (ArF; 193 nm) wavelengths are most desirable.

Beam shaping and focusing. The relative incoherence of UV excimer-laser output allows beam shaping and image projection using masks backlit by the laser source. High-energy laser systems allow larger work surfaces to be processed, but the associated optical system also has increased complexity. Small excimer machining systems, with their modest pulse energy, normally use only simple beam-shaping masks to hel¥define the size and shape of the focal spot at the work surface. This design reduces the cost and complexity of the optical train and simplifies setu¥and maintenance. Machining is usually done with a single focal spot that is moved over the work surface to produce effects analogous to milling, drilling, or lathe operations.

Motion. Because these applications are normally single-point machining processes, precise motion of the work surface with respect to the laser focal spot is usually an essential feature of the tool. Resolution and accuracy of the motion system must be significantly better than the desired feature size. Precision stages with dc servomotor drive and high-accuracy encoders can provide submicron resolution, accuracy, and repeatability. Synchronization of laser pulsing with stage motion is useful in avoiding substrate exposure variations associated with varying stage velocity.

Fixturing of components and feedstock in micromanufacturing can be a challenging problem. This requirement is an area in which the greatest degree of differentiation among tools is found. Microcomponents can be delicate to handle and are often susceptible to electrostatic, capillary, and kinetic forces that are negligible on larger scales. During processing, the work surface must be positioned within the depth of focus of the optical system, and process economics often dictates that all parts-handling be carried out in a short amount of time.

Viewing and measurement. Quality micromachining is almost impossible without a good system for viewing the work surface. Video imaging systems can be useful in setu¥and focusing of the laser beam on the work surface, feature measurement, and pattern registration. These processes are adaptable to machine vision and automated control. Through-the-lens imaging is desirable for accurate measurement and registration and is essential in high-magnification systems used in very-small-scale work. Dimensional measurement can be accomplished by direct calibration of the video image scale or through use of the motion system to precisely translate the part under the video microscope.

Operator interface. The operator interface connects the microworld to the macroworld. It can be as simple as a button that starts an automated process to a sophisticated CAD/CAM package integrated with a machine-vision system by a complex graphical user interface. The general goal must be linkage of laser, viewing, and motion system to allow the operator to easily set the system operating parameters and carry out the necessary sequences of illumination and motion required for part registration, processing, and inspection.

Applications of micromachining and parts

For most polymers, laser energy densities between 0.2 and 2 J/cm2 are optimum for efficient precision machining. At the 248- and 193-nm excimer wavelengths, these energy densities will typically result in removal of 0.1-0.5 µm of material from the work surface with each laser pulse. Materials such as glass, ceramics, and silicon require energy densities in the 2-20-J/cm2 range for good machining, and material-removal rates are in the somewhat lower 0.05-0.2-µm per pulse range.

These basic material parameters together with laser operating characteristics such as those in the table determine the production rate and cost associated with fabrication of small components with small UV lasers. It is useful to consider some specific examples to better understand the capabilities of small excimer-laser micromachining systems. Three examples of processes that require between one second and one minute of machine time are described.

Polyimide sheet. Sheets of polymer materials such as polyimide and Mylar can be used very effectively as feedstock for production of microcomponents (see photo, p. 83). These materials are easily cut and shaped by pulsed UV sources to make components used in microsurgery, fluid control, and sensing. The component shown in the photo was fabricated with a waveguide excimer laser operating at 248 nm at a pulse-repetition rate of 1500 H¥and an average power at the work surface of 50 mW.

A 75-µm-thick polyimide sheet was mounted on a vacuum chuck and translated under the focused beam using a motion-control system with CAD/CAM interface and 0.25-µm resolution. Process time for the cutting operation is about 40 s. Using step-and-repeat processing, as many as 10,000 parts can be fabricated from a single polymer sheet, allowing the machine to operate unattended for several shifts.

Catheter holes. Cylindrical components such as tubes, wires, and rods can be cut, drilled, and marked with appropriate fixturing. Hole patterns can be made in tubing by configuring the laser and beam-delivery system to produce a focal spot of suitable size and shape and moving from location to location along the tube with translation and rotation operations (see Fig. 2).

A waveguide excimer laser such as the one described producing 50 µJ of energy at a pulse-repetition rate of 500 H¥allows the hole pattern to be cut in a few seconds. Overall process time of 60 s is influenced by the 55-s parts-handling time, providing a good example of the impact of parts-handling and fixturing on machine throughput.

Glass marking. Most common glasses can be marked and machined with either the 248- or 193-nm excimer wavelengths. For example, alphanumeric text can be inscribed on borosilicate glass using 30-µJ pulses from a 248-nm waveguide excimer laser (see Fig. 3). Depth of the mark can range from about 1 µm to as much as 15 µm and is controlled by pulse overlap. This mark was approximately 2 mm high and 10 µm deep, and each character was inscribed in about 3 s. Marking time is determined by character size and parts-handling. Characters with heights as small as 35 µm can be produced with small lasers of this type, and character-writing rates tend to increase with decreasing character size.

Two-dimensional (2-D) dot-matrix patterns have been shown to be an effective marking format for storage of relatively large amounts of information in small areas. The 2-D matrix test pattern in Fig. 4 was written sequentially with a 193-nm excimer laser delivering approximately 1-mJ pulses of energy to the work surface. With a 100-H¥pulse-repetition rate, a 10 ¥ 10 pattern can be written in approximately 1 s.

Production costs and throughput

Because the viability of any production process is determined by cost and rate, it is important to consider these factors as they apply to microfabrication with small excimer lasers. The processing costs of parts similar to the previous examples are driven by depreciation costs, labor, laser maintenance and consumables, process duty factor, and yield. On the one hand, for single-point machining processes such as those described, uniform laser exposure of each area of the work surface is easily achieved by fixing the laser energy and optical focus. As a consequence, process yield can be high. On the other hand, the process duty factor, or laser on/off ratio, can vary widely and is influenced by part geometry, fixturing, and registration requirements as well as other factors. For the sheet-cutting operation, the duty factor can exceed 95%. A short laser process that requires substantial operator intervention, as in the tube-porting example, can have a duty factor as low as a few percent.

Hourly costs for the ownershi¥and operation of the machine are usually driven by equipment depreciation and labor costs because the cost of consumables, utilities, and scheduled maintenance is relatively low. A typical machining system amortized over a 10,000-hour period would have a depreciation cost in the $10-$15/hour range. Burdened labor cost for the operator will often lie in the same range. Consumables and scheduled maintenance will be roughly proportional to the duty factor of the laser process and should not exceed $5-$8/hour. Total hourly costs will vary with the production scenario but will normally range from $20 to $40/hour. Fabrication processes such as those in these examples can be carried out in less than one minute with associated costs measured in cents.

With full, three-shift utilization, a machine producing one part in 20 seconds can generate one million parts per year. Therefore, we expect machining systems using small excimer lasers to be capable of production volumes in the million-per-year range for processes similar to some of the examples described. This technology addresses a need for laser-based equipment that is reliable, economical, and user-friendly in a micromanufacturing production environment. o

Click here to enlarge image

Micromachining of polyimide sheet stock can produce small "comb-like" components (shown with pin for size comparison) that can be used in microsurgery, sensing, and controls. Component dimensions are 0.75 ¥ 1.1 ¥ 0.075 mm.

Click here to enlarge image

FIGURE 1. Operator controls small excimer-laser micromachining system, consisting of laser, beam-delivery equipment, and video monitor, through control interface. Precision stage allows beam position to remain stable while part being machining is moved

Click here to enlarge image

Click here to enlarge image

FIGURE 2. Excimer laser machines two holes--ranging between 75 and 1150 µm--in the side wall of a 1-mm- diameter polyester tube.

Click here to enlarge image

FIGURE 3. A high-repetition-rate waveguide excimer laser performs alphanumeric text marking on borosilicate glass; this text is 2 mm high, with a stroke width of 25 µm.

Click here to enlarge image

FIGURE 4. In two-dimensional matrix marking on borosilicate glass, small excimer laser creates complex pattern.

More in Lasers & Sources