Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 lm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases.New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase.
Fabrication of high aspect ratio structures requires the use of a photoresist able to form a mold with vertical sidewalls. Thus the photoresist should have a high selectivity between the exposed and the unexposed area in the developer. It should be relatively free from stress when applied in thick layers necessary to make high aspect ratio structures. PMMA (Poly Methyl Methacrylate) is the photoresist of choice in the LIGA process, mainly for its ability to hold vertical sidewalls for tall structures. It is applied to the substrate by a glue-down process in which a pre-cast, high molecular weight, sheet of PMMA is attached to the plating base on a substrate. The applied photoresist is then milled down to the precise height by a fly-cutter prior to pattern transfer by x-ray exposure. The requirement that the applied layer be relatively free from stress dictates the choice of glue-down over casting. The substrate preparation steps, as well as the conditioning of the PMMA sheet prior to the glue-down, are done, in part, to reduce the stress in the glued down sheet of photoresist. The cutting of the PMMA sheet in the fly-cutter requires specific operating conditions as well as particular cutting tools to avoid introducing any stress and the resultant crazing of the photoresist.
The cost effectiveness of the deep X-ray lithography and electrodeposition process, LIGA, depends directly on the throughput of the process. The use of high energy photons allows the exposure of stacked photoresist and results in high throughput. High energy X-ray exposures require a different mask than low energy X-ray exposures. The high energy mask allows a large area exposure but requires a thicker X-ray absorber. The cost of generating high energy X-ray masks can be drastically reduced by using a thick optical photoresist process rather than an X-ray exposure process. The cost can be further reduced by using alternatives to the typical X-ray absorber, gold. High atomic weight (high Z) materials are ideal absorbers. Lead has been demonstrated as being a useable alternative as an X-ray absorber. IntroductionThe basic deep X-ray lithography and electrodeposition process creates submicron tolerance, prismatic patterned, thick photoresist. Photoresist recesses are electroplated to produce metal structures. High throughput may be obtained using one of three methods. The original process uses the resulting metal pattern as a mold in an injection molding process [1]. Electro-discharge machining [2] using precision LIGA discharge tips has recently been shown to be an excellent, cost effective method of producing precision die molds. An alternative to the injection molding process uses high energy photon exposures and stacked photoresist [3,4] to obtain high throughput without requiring the use of injection molding. Thicknesses of 10 cm of photoresist have been exposed [5] using high energy photons.Alternative methods of exposing thick resist layers have been introduced. Successive exposure and development processes using conformal masks make it possible to fabricate thick microstructures [6]. However, the process is not cost effective when compared to high energy photon exposures which produce the same result in one exposure. High energy exposures produce a higher degree of precision and cost effectiveness from sample to sample. This is due to the fact that the same mask is used on successive samples. The large expense of mask making is a one time overhead cost. Using the same mask guarantees identical patterns for all printed samples. Deep X-ray lithographyDeep X-ray lithography requires complete exposure of the unshielded area while keeping X-ray doses in the masked area low enough so that the photoresist which is shielded is not attacked in the subsequent developing process.X-ray power decays through all substances and this attenuation is given by an exponential decay law, px p o e ÀxaL W/cm 2 1where p is the X-ray power per unit area and L is called the absorption length. The absorption length is dependent on the photon energy and type of material. The absorption length for high energy photons (at 20,000 eV: L Si = 1000 lm, L PMMA = 2 cm) allows for the exposure of extremely thick photoresist (to a practical maximum of $3L) and this enables highly parallel exposures. A computer program [7] written for exposure de...
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