TECHNOLOGY NEWS
Injection Moulding Enables Mass Production of Microstructure Components
Microstructure technology has encountered a number of obstacles in seeking to make the transition from research laboratory to mass industrial applications. 
While there have been a handful
of successful applications using silicon to mass-
produce pressure and acceleration sensors, the relative immaturity of the technology and the reluctance of industry to devote sufficient resources to R&D have impeded its development in other areas. According to Nezih Ünal, marketing director of MicroParts Gesellschaft für Microstructurtechnik mbH (Dortmund, Germany), the introduction of LIGA technology may accelerate its acceptance by industry.
Microstructure technology in electronics essentially consists of repeating a highly complicated and sensitive production process each time the final part is produced, says Ünal. "The great idea of Forschungszentrum Karlsruhe GmbH (Karlsruhe, Germany), which developed the technology and licensed it to MicroParts, is to limit this process to the fabrication of a metal matrix. This model is then used to mass-produce the product by injection moulding."
LIGA is an acronym for the German words that designate lithography, electroforming, and moulding, the three principal processes used in the fabrication of microstructure components. Initially, explains MicroParts managing director Reiner Wechsung, a mask is produced by optical or E-beam lithography. This mask is typically composed of a thin titanium or beryllium membrane and a layer of x-ray-absorbent material. The structure is exposed to synchrotron radiation, which transfers the shape of the final product from the mask onto the receiving material. The radiated material is removed with a solvent to reveal the primary microstructure model. In the electroforming phase, metal is deposited on the ground plate until it fills and slightly overflows the cavities formed in the x-ray-resistant material. The mould insert is then fabricated by removing the ground plate and the x-ray-resistant structure.
Microspectrometers, dosing mechanisms for drug-delivery systems, channel plates, and optical-fiber couplers are among MicroParts' first industrial applications of LIGA technology.
A spectrometer's demanding optical and mechanical construction and its overall fragility have largely precluded mass production of the devices in the past, according to Wechsung. "LIGA technology has eliminated these obstacles," he says. "All the critical components of the spectrometer--radiation entrance slit, planar waveguide, grating, and connectors--are integrated into the moulded plastic part." Measuring only 25 * 30 * 5 mm, the device incorporates lead-selenide detectors with a permanent self-adjustment feature, thereby eliminating long-term drift. The spectrometer has a spectral range of 34.3 µm; custom designs with spectral ranges between 2 and 10 µm can be supplied. The IR-Micro Spectrometer is available as an OEM component for the detection of CO2 in diagnostic equipment and CH in process control applications. It can be custom configured to monitor the presence of other gases in medical equipment as well as in other applications.
LIGA technology also enabled the design and manufacture of a nebulizer for Boehringer Ingelheim KG (Ingelheim, Germany) that did not require the use of fluorocarbons and substantially increased drop-size distribution. "Wet nebulizers routinely use fluorocarbons," says Ünal, "and it is rather difficult to find the fluorocarbon that is compatible with a given drug. And traditional nebulizers tend to have a greatly undefined drop-size distribution," he adds. Droplets must be in the 5-µm range to reach the lung, according to Ünal. Anything larger than that remains in the mouth and consequently is ineffective. "With LIGA technology, we were able to produce extremely minute channels. Whereas in traditional nebulizers only 30% of the drug is small enough to reach the lung, the product we developed for Boehringer attains a ratio of more than 80%," says Ünal.
In addition, the company has developed microstructure channel plates with precise combinations of different geometries to optimize flow and optical-fiber couplers (see photo above). "The couplers' optical wave-guide structure has a surface roughness of 2030 nm, which results in low scattering losses," notes Wechsung. "The ability to mould polymers with well-suited optical properties, such as polycarbonate and PMMA, is yet another advantage of LIGA technology."
Indeed, polymers are destined to play a defining role in the future of microstructure components, according to Wechsung. "The ongoing development of polymeric materials that enhance such properties as conductivity, piezoelectricity, and temperature resistance most certainly will enlarge the field of potential applications."
