LIGA (manufacturing process)

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The German acronym LIGA or LIGA (represents the steps of: Li thographie, G alvanik and A bformung) refers to a method based on a combination of deep photolithography , electroplating and micromolding based. The LIGA process was developed at the then nuclear research center in Karlsruhe in the early 1980s by a team led by Erwin Willy Becker and Wolfgang Ehrfeld as part of the development of the separation nozzle process for uranium enrichment in order to be able to manufacture extremely small separation nozzles.

Waveguides produced with the LIGA process (thickness approx. 517 µm)

The process enables the production of microstructures with the smallest dimensions down to 0.2 µm, structure heights up to 3 mm, and aspect ratios up to 50 (for detailed structures up to 500) made of plastic, metal or ceramic. LIGA is used in the field of microsystem technology , not least in micro- optics , especially when structures with very high aspect ratios are to be created.

Process flow

Process steps of the LIGA process for the production of a mold insert for micro-impressions
  1. The starting material is a flat substrate, for example a silicon wafer or a polished disk made of beryllium , copper , titanium or other materials. The substrate, if not already electrically conductive, with a metal " seed layer (engl." Seed layer ) provided, usually by sputter deposition or vapor deposition . A thick photo or X-ray sensitive positive resist (often PMMA ) is applied to the start layer (Fig. A), see photolithography .
  2. The resist is exposed (picture b).
  3. After the photoresist layer has been developed, a negative form of the metal structure remains, which is to be produced in the electroplating process (Fig. C).
  4. In a galvanic process, a metal is deposited on the substrate in the areas in which the resist was removed during development (i.e. the seed layer was exposed) (Figure d).
  5. After removing the resist, the substrate, the seed layer and the electrodeposited metal are left behind (image e, top). Now there are different options for the further procedure:
    1. By etching the seed layer (which now functions as a sacrificial layer ) and possibly the substrate, (small) metallic components can be produced directly.
    2. By further electroplating ("overgrowth") and subsequent removal of the substrate and seed layer, a mold insert can be created from the photolithographically generated microstructure , which is built into a molding tool , with which the ultimately desired plastic component is molded, for example by injection molding or hot stamping (Fig e, f, g).
    3. Alternatively, the mold insert can be cut out directly from the substrate with the photolithographically produced microstructure (e.g. by spark erosion ) and built into a molding tool .

If you repeat the exposure, development and electroplating steps several times, you can design more complex structures that have to taper towards the substrate (the wafer ), otherwise the component would not come loose from its shape. This limits the complexity of the structure to be produced.

If you use the LIGA mold for injection molding, there is a special feature in contrast to the production of macroscopic parts: no holes have to be provided for the air present in the mold to escape, since the unevenness of the contact surface when manufacturing components a few hundred micrometers in size of shape and counterpart are sufficient for air to escape. Only the holes for feeding the material to be sprayed need to be created.

variants

Depending on the type of photolithography, a distinction is made between X-ray LIGA , in which X-rays (e.g. from a synchrotron ) are used, and UV-LIGA , in which ultraviolet light is used as in common semiconductor technology. UV-LIGA was made possible in the last decade of the 20th century by the development of new photoresists , especially the SU-8 . Until then, LIGA was practically synonymous with Röntgen-LIGA .

Another variant is the creation of a master from a silicon wafer by means of photolithography and silicon deep etching , with subsequent electroplating and (if necessary) molding. Based on the traditional LIGA techniques, this process, which, like the UV-LIGA, was introduced in the last decade of the 20th century, is also known as silicon-LIGA .

Both UV-LIGA and silicon-LIGA offer less precision than the X-ray LIGA , but also usually require a significantly lower capital investment and are therefore sometimes (essentially value-neutral) referred to as Poor Man's LIGA .

materials

Application examples

LIGA technology is used to manufacture gears for miniature gears (for example in the head of a dentist's drill) and micro-fine nozzles for filters. Since the high precision of the LIGA process is particularly important in micro-optics, there are many applications in this area, for example in micro- spectrometers . Miniaturized gears that were manufactured using LIGA technology cannot be lubricated due to their small size. Therefore, the art of developing such a transmission is to find combinations of materials that are self-lubricating. For example, two gears made of the same material are worse than the combination of certain different materials. A micro electric motor is made of magnetic material, i. d. R. a nickel / iron - alloy . All components of the micromotor are manufactured using micro- electroforming .

literature

  • Volker Saile (Ed.), Ulrike Wallrabe (Ed.), Osamu Tabata (Ed.), Jan G. Korvink (Ed.): LIGA and Its Applications. In: Advanced Micro & Nanosystems. Volume 7, 1st edition, Wiley-VCH, 2009, ISBN 978-3-527-31698-4 .
  • W. Ehrfeld: Microtechnology manual. Carl Hanser Verlag, Munich / Vienna 2002, ISBN 3-446-21506-9 .
  • W. Menz, J. Mohr: Microsystem technology for engineers. VCH-Verlag, Weinheim 1997, ISBN 352730536X .

Web links

  • LIGA procedure. Karlsruhe Institute of Technology (KIT), Inst. F. Microstructure Technology (IMT), accessed July 8, 2010 .
  • Arndt Last: The LIGA process. Retrieved November 20, 2019 .

Individual evidence

  1. ^ EW Becker, W. Ehrfeld, D. Münchmeyer, H. Betz, A. Heuberger, S. Pongratz, W. Glashauser, HJ Michel, R. v. Siemens: Production of separation nozzle systems for uranium enrichment by a combination of X-ray lithography and galvanoplastics . In: Natural Sciences . tape 69 , no. 11 , 1982, pp. 520-523 , doi : 10.1007 / BF00463495 .
  2. ↑ EW Becker, W. Ehrfeld, P. Hagmann, A. Maner, D. Münchmeyer: Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, galvanoforming, and plastic molding (LIGA process) . In: Microelectronic Engineering . tape 4 , no. 1 , 1986, pp. 35-56 , doi : 10.1016 / 0167-9317 (86) 90004-3 .