The LIGA Process

As stated above, the development of the LIGA process began at the Research Center in Karlsruhe (FZK), Germany, in the 70s as a rather inexpensive method of producing very small slotted nozzles of any lateral shape for uranium-isotope separation by the nozzle process. Its usage is now wide spread globally as well as to a much 

In the first of five steps (see Fig. 1) a pattern of an X-ray absorber mask is transferred onto a resist layer hundred or so micrometers in thickness by X-ray shadow projection. In practice often synchrotron radiation is used due to its very high degree of collimation and short wavelength (0.2-1 nm). 

The X-ray masks consist of a thin membrane (e.g., 3-pm titanium or 30-pm beryllium) together with absorbers consisting of X-ray opaque gold layers that are more than 10 pm in thickness in order to achieve the required mask contrast. Within the irradiated sections of the resist layer, the polymer chains are destroyed reducing their molecular weight. In most experiments polymethyl methacrylate (PMMA) is used as the X-ray resist. 

As a consequence of the second, development step, the exposed resist is selectively dissolved while at the same time the masked and un-irradiated parts remain unchanged. 

Filling tfie voids of the primary structure by clectrofprming 

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Fig. 2.73a-d The LIGA process. Reprinted from Kan-dlikar and Grande (2002) with permission... 

Chemical activities in the field of mass screening are often related to combinatorial chemistry [51,52]. One major goal, especially in the field of solid phase chemistry involving polymers like DNA or peptides, aims at the increase in the number of compounds per reactor volume and time. Commercially available microtiter plates are established as reactors in this case whereby robotic feed systems fit perfectly to their dimensions. A drastic reduction of reaction volume and increase in number of reaction vessels ( wells ) leads to the so-called nanotiter plates (e.g. with 3456 wells). Microfabrication methods such as the LIGA process are ideal means for the cost effective fabrication of nano-titer plates in polymeric materials by embossing or injection molding techniques so that inexpensive one-way tools are realized. 

The fabrication of microelectromechanical systems (MEMS), e.g. actuators and sensors, is also one of the promising applications for nickel films. Nickel is currently electroplated into preform molds. One typical process is the LIGA process, where pure or alloyed nickel films are... 

A combined fabrication technology made possible through a sacrificial layer technique was developed to obtain partly or totally movable microstmctures together with fixed structures on a single substrate. Many practitioners consider this ability to be the hall mark of the LIGA process. 

The substrate is first coated, for instance, via electroless or physical vapor deposition with a thin (< 1 pm) metallic layer, which in turn is patterned by photolithography and wet etching. This layer serves two roles, as a plating base and as an electrically conducting layer for the finished structures. In the subsequent step a sacrificial layer, of about 5 pm in tliickness, is deposited on the substrate and also patterned by photolithography and wet etching. Titanium is used most often as the sacrificial material because it adheres well to the resist and to the electrodeposited layer and can be etched with hydrofluoric acid that does not attack other materials such as chromium, silver, nickel, copper, and which are usually used in the LIGA process. 

Fig. 13.2 The LIGA process consists of first exposing a thick PMMA resist coUimated, synchrotron x-rays.

Fig. 13.3 A prototype scroll pump for a miniature GCMS system fabricated using the LIGA process, (a) 3 mm PMMA mold, (b) schematic of scroU pump, (c) electroformed scroll pump...

Fig. 2 X-ray absorption spectra calculated for the polymers PMMA and PTFE. PMMA is mainly used in deep x-ray lithography, namely the LIGA process, while PTFE is mainly used in SR direct photoetching, namely the TIEGA process...

Fig. 3 SEM picture of the high aspect ratio structure made of PMMA by the LIGA process. Typically, the structural height (depth) is 100-500 jizm and the feature size is 2-20 //m thus, the aspect ratio is between 5 and 100...

Fig. 4 Synchrotron radiation spectrum of AURORA-2S between 0.1 and 100 nm as well as the typical spectra used for x-ray lithography and the LIGA process...

The PMMA polymer is the best known resist, due to the fact that it has the best resolution down to at least 5 nm [23]. It has been used exclusively in the LIGA process since the thick resist layer of the order of a few 100 jum could be made readily [14]. Many previous studies, including those on the self-development phenomena [2, 3] and laser ablation [5, 10, 12, 17], have been carried out on this polymer. Therefore, there are relatively more data... 

A considerably different approach employs a mini-grid electrode. This type of electrode can be made of a variety of metals (gold, platinum, etc.) and is basically a very fine mesh. Expanded metal-like structures, finely woven wires [28] or structures prepared via the LIGA-process [29, 30] are currently in use [31-33]. They are inserted into electrochemical cells of basically the same type as that used with OTE. The light beam passes the grid or the honeycomb structure of the LIGA element. Because the wires or the structural LIGA elements are extremely fine, only... 

Cohen [50] showed that Ee +-ions dissolved in the electrolyte can be deposited as FeOOH on top of the usual passive film. Another example is given by Ti in alkaline solution of H2O2. Ti-ions corrode very fast and become redeposited as a sub-stoichiometric Ti02-x- The oxide film is conducting and thicker than a usual Ti02-passive film. Such substrates are used in microtechnology, for example, in the LiGA-process for Ni-deposition which is much faster than on a usual passive film. 

The original resist used for the LIGA process was thick films of poly(methyl methacrylate) (i.e., PMMA). Because spin coating the very thick... 

Fig. 1.28. An example of a flowchart illustrating the fabrication steps involved in the LIGA process. Adapted from Spearing (2000).

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