ADVANCED APPLICATIONS: MICROFABRICATION - Lasers carve precision microscale features

The manufacture of micromachines is rapidly becoming the next frontier in miniaturization technology. Machines with micron-scale moving parts have been prototyped for applications such as chemical processing, biological assaying, and optical information processing. Devices with millions of miniscule movable mirrors may soon replace conventional film projection in movie theaters, eliminating the scratches and spots that film watchers have put up with for decades. Microscopic pressure meters and accelerometers have been on the market for several years.

In some cases, the build process relies on the same lithographic techniques used to produce integrated circuits. Increasingly, though, laser-based devices are becoming the tools of choice for machining microparts. Using femtosecond pulses, excimer lasers, and near-field optics, researchers have demonstrated precision machining on scales approaching 10 nm.

System components

Laser-micromachining systems require a laser source, optics for conditioning and focusing the beam, and a way to precisely control and point the beam. Typical laser sources are either excimer lasers or Ti:sapphire femtosecond-pulsed devices, but some researchers also use Nd:YAG systems.

The advantage provided by excimer lasers is that most work materials strongly absorb the ultraviolet (UV) radiation. While system optics are more complex, the laser energy can be absorbed in a very small region. Benefits of Ti:sapphire systems include compactness and the capability to be all solid state. Femtosecond pulses also can create very-fine ablation patterns because electrons absorb the laser energy before adjacent atoms heat up, which reduces damage to the neighboring workpiece areas. In some cases, technologies are combined to deliver excimer radiation in less than a picosecond.

Typical micromachining lasers provide an average output power of between 50 mW and 2 W, with pulse energies from tens of microjoules to several millijoules. Processing can use very small laser pulses, with a few hundred microjoules of energy being sufficient to ablate a 100-µm focal spot. For many applications, though, nanosecond-length pulses from Nd:YAG lasers are adequate.

There are two basic approaches to control irradiation of the workpiece. One illuminates a mask uniformly. The other involves direct-writing a pattern with a point-source beam. With the mask technique, optics demagnify the resulting image, focusing it on an area typically 5-15 times smaller in linear dimensions. During direct-writing, either a mirror controls the beam direction or the workpiece is moved on a stage.

An example of the mask-based system is apparatus developed at the Fachbereich Physikalische Technik (Steinfurt, Germany).¹ The system feeds a UV beam at 248 nm to a homogenizer consisting of two crossed arrays of cylindrical lenses. This illuminates a mask homogenously that is up to 1.5 cm across. The beam is then fed through the mask to a demagnification lens, which compresses the image to as small as 1 mm across. With an energy density of 4 J/cm² at the image plane and 150 pulses, researchers were able to ablate a test pattern with a resolution of 10 µm into a Kapton substrate, producing sharp-edged contours over the whole pattern.

The direct-writing process ablates a pattern in a surface with a moving laser beam. For example, researchers at BNFL Springfields (Preston, England) ablated channels into fused silica and Pyrex using 170-fs pulses produced by a Ti:sapphire laser at 790 nm.² The pulses, focused to 50 µm in diameter, were directed at a workpiece moving on a translation stage. Initial experiments ablated tracks with 500 µJ /pulse, a 1-kHz pulse rate, and a 1-mm/s scan rate but produced severe cracking at the edges of the track. Reducing the energy to 50 µJ/pulse and increasing the scan rate to 7 mm/s allowed production of cleaner and shallower tracks. Scientists could then laser-mill deeper trenches-repeatedly ablating the same trench, but reducing the width milled at each pass to form a triangular-shaped trench. This allowed achieving a surface roughness below 1 µm.

Beyond micromachining

Researchers are also exploring other techniques that will expand their ability to create extremely small devices. For example, scientists at the Institute of Electronic Structure and Lasers (Athens, Greece) used lasers with femtosecond UV radiation to precisely transfer material to a workpiece through ablation.3 Here, lasers were focused onto chromium films layered on transparent quartz wafers. A glass target surface was placed near the film, and a 500-fs pulse at 248 nm ablated the chromium from it for deposition onto the glass. This technique could place 4-µm-diameter dots onto the glass at a rate of 10 pulses/s.

Another research project reduced the size of laser-generated trenches from micron to nanometer scale using near-field optics. This relied on the Flolant (focusing laser radiation in the near field of a tip) method to enhance the field in the near field of a scanning tunneling tip, a process common in scanning tunneling microscopes capable of observing individual atoms.⁴

Scientists at the Physikalische Technik Laserlabor (Steinfurt, Germany) focused a frequency-doubled Nd:YAG laser at 532 nm onto the gap between the tip of the scanning tunneling microscope and the work region, covering an area about 500 µm across that was far larger than the scan region. In the immediate vicinity of the tip, the field was enhanced by a combination of Rayleigh scattering and surface plasma excitation.

For metal tips, the field enhancement can approach a millionfold. One experiment carved lines in a gold or palladium surface only 15 nm across using tungsten tips with a 30-nm radius of curvature. Such narrow cuts could lead to the production of devices several orders of magnitude smaller than current technology. The trench production mechanism, however, will likely involve some combination of field enhancement and physical contact between the tip and working material and will require more study.

Chemical competition

Lasers are not the only way to micromachine. For a decade, commercial products have been micromachined with lithographic techniques used to produce microcircuits. A pattern is embodied in a lithographic mask and the workpiece overlaid with a resist. The mask exposes this to produce regions that are then chemically etched away. To make undercut regions essential to most micromachining, a sacrificial layer between the substrate and the overlying cantilever is etched away from the side.

Chemical micromachining is a maturing, commercialized technology with total annual sales exceeding a billion dollars, mainly in the form of pressure sensors and accelerometers. With many laser micromachining techniques still laboratory-based, how will they fit into the existing market?

Compared with chemical techniques, laser micromachining has some definite advantages. First, lasers can machine metals, while lithographic techniques are used almost exclusively for silicon wafers. In many applications, metals are essential work materials, even at small scales. Second, lasers are capable of much smaller dimensions-in the range of a few microns-than current silicon-based techniques, which typically produce lateral device dimensions of 100-500 µm. More-exotic techniques like Flolant can offer potentially nanometer-scale features.

Working devices

Laser-micromachining researchers have a diverse range of end uses in mind. One involves the assembly of extremely small trenches into microfactories for chemical and biological processing. With this technique, scientists at the University of Wales (Bangor, Wales) built a prototype biofactory on a chip that detects and determines the viability of microorganisms in water supplies.⁵ The chip channels water samples through the trenches and labels harmful microorganisms with latex beads coated with antibodies for those microorganisms. Measurements of electrorotation of the samples can then identify the number of organisms present. The device, with its 10-µm feature sizes, was fabricated with a krypton excimer laser.

Another emerging application involves micromachining of diffractive optical elements. These patterns or gratings can focus or bend light. Because they are planar, they can be made exceedingly small and then integrated with electro-optical devices such as diode-laser arrays, detector arrays, and optical interconnects. While such devices can be produced with e-beams, the process is time-consuming and costly.

In contrast, excimer lasers can create the optical elements quickly and at a relatively low cost. A device projects a UV laser beam though a pattern mask that can contain a variety of patterns for different diffractive optical elements. Researchers could select the required mask with high-speed motion of the pattern. One pulse produces the optical element, with the UV beam ablating the material to form the diffraction grating.

Researchers used micromachined holes and bubbles to demonstrate a three-dimensional optical-data storage device. The ultimate storage density of such devices may exceed 10 trillion bits per cubic centimeter.