Process for manufacturing microstructured bodies

A method for manufacturing a microstructured body, comprises irradiating a material with a pattern of X-rays; and dissolving selectively irradiated or non-irradiated regions of the material, where the material comprises a light-hardening and/or UV-hardening epoxy coating. The epoxy coatings need shorter irrdiation times than known plastics. The resulting microstructures can have high aspect ratios, and the developed structures can be produced with greater structure depths, free of defects and faults. Structural precision is in the submicron range.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a process for manufacturing microstructured bodies
 with structural depths ranging from several microns to several
 millimeters, with lateral dimensions in the micron range, by irradiation
 of polymers with X-rays and subsequent development with suitable
 developing media
 2. Description of the Background
 In microelectronics, the technological advances of increasing
 miniaturization and integration have led to a myriad new products.
 Compared to other sectors, the advances in miniaturization in
 microelectronics in just a few recent years have been remarkable.
 Indications are that in the future other microtechniques as well will
 become important. Areas which might be mentioned are micromechanics,
 integrated optics, and microfluidics. Such technologies, in combination
 with microelectronics, offer the possibility of numerous novel electronic,
 optical, biological, and mechanical functional elements of revolutionary
 types. The highly productive manufacturing methods of semiconductor
 technology can be exploited in a wide range of applications in the mass
 production of miniaturized non-electronic components, modules, and
 subsystems. Classical methods of fine fabrication for micromechanics have
 been strengthened and combined with appropriately modified methods from
 semiconductor fabrication, to go beyond the constraints of planar silicon
 technology and make available novel options in the forning of materials,
 applicable for numerous shapes and configurations, and numerous materials.
 An advance of this nature is the LIGA method
 (Lithographie-Galvanoformung-Abformung (i.e.,
 lithography-galvanoforming-molding)), which combines fabrication steps
 involving lithography, galvanoforming (electrochemical forming), and
 molding, and is described in German Patent Application No. 41 42 001,
 incorporated herein by reference. The method was developed at
 Kernforschungszentrum Karlsruhe (the Karlsruhe, Germany Nuclear Research
 Center). The essential fabrication step of the original LIGA method is the
 structurally accurate irradiation of a polymer material. The basic
 practicability of the LIGA method can be demonstrated by microstructures
 produced in a specially manufactured polymethyl methacrylate (PMMA). A
 number of other plastics have been developed with the aim of compatibility
 with forming techniques employing X-ray irradiation. Among these one might
 mention polyoxymethylene (POM), and polyesters, particularly
 polyglycolides and polylactides, as described in German Patent Application
 No. 41 41 352 A1.
 The use of "SU8" epoxy photoresist for microstructuring using UV
 lithography has been described in a number of publications: Despont, M.,
 Lorenz, H., Fahrin, N., Brugger, J., Renaud, P., and Vettiger, P., 1997,
 "High-aspect-ratio, ultrathick, negative-tone near-UV photoresist for
 micro electro mechanical systems (MEMS) applications", in "Proc. of the
 10th IREE International Workshop on Micro Electro Mechanical Systems (MEMS
 '97)", Jan. 26-30, 1997, Nagoya, Japan; and Lee, La Bianca, et al.,
 "Micromachining applications of a high resolution ultrathick photoresist",
 J.Vac.Sci.Technol., B13(6), November/December 1995, all of which are
 hereby incorporated by reference. In addition, epoxide mixtures are used
 for the encapsulation of, e.g., microelectronic, electronic, or optical
 components (see PROTAVIC brochure).
 In the fabrication of complex three-dimensional structures with structural
 depths ranging from several microns to several millimeters, using the LIGA
 method, it has been found that the existing plastics require a substantial
 amount of irradiation. It has also been found that in the development of
 irradiated polymer articles with a suitable developing medium the
 un-irradiated polymer regions tend to swell, wherewith the fine structures
 formed may be distorted or otherwise faulty. Further, the swollen polymer
 regions can result in stress cracking when the material is dried, thereby
 rendering the microstructure bodies unsuitable for electrochemical
 processing. Another problem is that for some of the plastics used, the
 processing is complex and expensive. This is true of polylactides and
 polyglycolides, wherewith costly stamping steps must be employed to apply
 the materials to substrates prior to irradiation.
 UV-hardening coating systems may be used for fabricating microstructures.
 With these it has been impossible with the means at hand to achieve the
 level of submicron precision and accuracy which is often needed for
 optical and fluidic components; the difficulties are attributed to
 diffraction, scattering, and interference effects on the irradiating light
 (wavelengths 300 mn to 460 mn), if the coating thicknesses are in the
 micron and millimeter range.
 SUMMARY OF ThE INVENTION
 An object of the invention is to provide a polymer which requires
 relatively little radiation when irradiated with synchrotron radiation.
 Another object of the invention is to provide a polymer which undergoes
 depolymerization or crosslinidng under the influence of X-ray irradiation.
 Another object of the invention is to provide a polymer which is selectably
 removable with the use of special developers.
 In addition, the polymer should be easy to fabricate into test bodies,
 should not experience stress cracking, and should not contain faults.
 Preferably, it should be maximally compatible with semiconductor
 fabrication processes.
 The objects are provided by a method of manufacturing microstructure bodies
 with structural depths ranging from several microns to several millimeters
 by pattemwise irradiation of polymers with X-rays, wherein the polymers
 employed comprise UV-hardening and/or light-hardening epoxy resin
 coatings. The X-rays used preferably are supplied by synchrotron
 radiation. In carrying out the inventive method, the epoxy coatings may be
 applied to a support by means of pressing, extruding, stamping, embossing,
 injection molding, or spin-coating. The method can be carried out in a
 plurality of stages.
 Surprisingly, it was discovered that light-hardening epoxy coatings known
 from the technologies of semiconductor fabrication and plastic adhesives
 can be microstructured using X-rays, and such coatings satisfy the
 above-indicated criteria It was surprising that X-ray hardenable epoxide
 coatings could be used for producing microstructure bodies wherein a high
 aspect ratio is an important factor, as is required, e.g., for the LIGA
 method. Further, the advantages achieved with said coatings were
 surprising and were not suggested by known publications.
 DETAILED DESCRIPTION OF THE INVENTION
 According to the inventive method, microstructure bodies with structural
 depths ranging from one micron to 10 millimeters can be fabricated, such
 that with synchrotron radiation and the action of selective developers
 with a removal depth of 1 micron to 1000 micron, bodies or features can be
 structured which have lateral dimensions in the micron to submicron range.
 Suitable selective developers are organic solvents and alkaline media,
 preferably, e.g., propylene glycol monomethyl ether acetate (PGMEA),
 hydroxide solutions with glycol components, or alcoholic alkali hydroxide
 solutions.
 The irradiation in the inventive method is carried out by high energy
 parallel radiation from X-ray sources. The wavelengths of these sources
 are in the range 0.1 nm to 10 nm, preferably 0.1-1 nm. Such irradiation
 may comprise, e.g. irradiation 1 to 300 minutes with an average ring
 current of e.g. 25 mA, in a synchrotron, with special pre-absorbers, e.g.
 comprised of beryllium or polyimide film (e.g. "Kapton", supplied by the
 firm DuPont de Nemours).
 The amount of irradiation depends on the electron energy in the electron
 storage ring branched off from the synchrotron. In general, the electron
 energy is in the range 1.0-2.7 GeV.
 For pattemwise irradiation, typically one uses special X-ray masks, e.g.
 comprised of a support foil of titanium, beryllium, or diamond, bearing
 absorber structures comprised of gold or tungsten.
 Particularly suitable for the inventive method are:
 "SU8" epoxy coating (supplied commercially by the firm Micro Resist
 Technology), or
 the liquid photoresists supplied by the firm Lackwerke Peters GmbH for the
 manufacture of printed circuits by means of etching techniques, which
 photoresists have become well known under the trade name ELPEMER.
 Examples of other materials which may be used are the protective coatings
 supplied commercially by the firm Protex under the trade names PROTAVIC PU
 and PROTAVIC UV.
 The epoxy coatings used in the production of the microstructure bodies may
 be applied to solid supports, preferably electrically conducting metal
 supports such as nickel, copper, steel, or titanium, by means of pressing,
 injection molding, extruding, or spin-coating, at temperatures in the
 range 20-100.degree. C. In this connection, adhesive layers, adhesive
 coatings, or special adhesion promoters may be employed. The thickness of
 the epoxide coatings on the supports is generally in the range 1-10,000
 micron, preferably 10-1000 micron, particularly preferably 100-800 micron.
 Following the pattemwise irradiation, development is carried out using
 suitable developing media. Developer systems which may be used may be
 based on alkylene glycol alkylether alkylmonocarboxylic acid ester
 compounds of the general formula
 ##STR1##
 where a=0 to 5, b=2 to 4, and c=0 to 5, or isomers of these; e.g.,
 propylene glycol monomethylether acetate, propylene glycol monoethylether
 acetate, ethylene glycol butylether acetate, butylene glycol
 isopropylether propanoate; or basic developers, such as: alkali hydroxide
 solutions combined with glycols, or alcoholic alkali hydroxide solutions.
 The inventive method has the following advantages:
 The polymer used is suitable for microstructuring. The polymer enables the
 production of microstructures with structure widths as small as a few
 microns, and having high aspect ratios (ratio of height to width of the
 structure) in the range 5:1 to 1000:1 (preferably in the range 10:1 to
 100:1).
 The described epoxy coatings allow microstructures to be produced which
 have substantially greater structure depths, for a given irradiation cost,
 than do the plastic materials known in the art. The sensitivity of said
 coatings is greater than that of PMMA by a factor of 100 to 1000.
 The method enables fabrication of structures with structure heights of,
 e.g., 500 micron and lateral dimensions in the submicron range, without
 defects.
 Structural precision in the submicron range can be achieved.
 The microstructures have sharp, steep edges, and smooth walls.
 The polymer is mechanically stable up to 80.degree. C., and has adequate
 thermomechanical strength.
 The polymer can withstand galvanic processes without problems, and is
 resistant to the action of, e.g., copper and nickel acid baths, sulfiric
 acid, sulfamic acid, and complex-forming agents. At temperatures in the
 range 20-80.degree. C., the microstructures are unaffected after 24 hr in
 galvanic baths.
 The surface of the plastic is smooth. The plastic can be applied to the
 support in uniform coating thicknesses.
 The described developers have very good selectivity.
 The described epoxy coatings are very well suited for the LIGA process, and
 are very amenable to processing in equipment used in semiconductor
 technology.

Having generally described this invention, a further understanding can be
 obtained by reference to certain specific examples which are provided
 herein for purposes of illustration only and are not intended to be
 limiting unless otherwise specified.
 EXAMPLE 1
 A coating of "SU8" epoxy coating material of thickness 505.+-.5 micron was
 applied to a support comprised of a silicon wafer, having dimensions 100
 mm diameter.times.0.5 mm thick. This resist was dried on a hot plate at
 90.degree. C., after which it was subjected 35 minutes to patternwise
 synchrotron irradiation with electron energy 2.3 GeV and average ring
 current 20 mA; the scanner stroke was 20 mm and the scanner speed 1
 mm/sec. The X-ray mask used was configured for test structures comprising
 fields comprised of hexagonal prisms of diameters 1-500 micron. Developing
 was carried out with PGMEA at 25.degree. C., 25 min. The structure was
 developed in completely free form. The typical structures produced in the
 structured epoxy coating were completely free of residues from the
 developing, and displayed excellent developability down to a diameter of 5
 micron (side to side cross dimension of the hexagon).
 COMITIVE EXAMPLE 1
 A test body comprised of PMMA was irradiated 60 min under conditions
 corresponding to those of Example 1, and was developed with GG-developer,
 which is the optimum developer for PMMA. The depth of material removed was
 only 100 micron. Using the same prismatic test structure, the
 smallest-diameter prism which could be effectively produced was 50 micron
 in diameter; prisms with smaller diameters had distortion or other serious
 defects.
 As seen from the comparison of Example 1 and Comparitive Example 1, a
 structure height of c. 500 micron in PMMA requires irradiation for at
 least 10 hr under the same conditions; aspect ratios up to 40 are
 achievable. In contrast, "SU8" can yield deeper structures with shorter
 irradiation times and higher aspect ratios, at a lower cost, and with
 higher quality.
 EXAMPLE 2
 A coating of "ELPEMER SD 2054" epoxy coating material of a thickness of 300
 microns was applied by spin-coating, to a support comprised of copper, and
 was subjected for 15 minutes to pattemnwise irradiation in a synchrotron
 with 2.3 GeV electron energy and average ring current 20 mA. The scanner
 stroke was 20 mm. The X-ray mask used had a spectrometer configuration.
 The subsequent developing was carried out using a 5% solution of sodium
 hydroxide in 1:1 water/ethylene glycol at 25.degree. C. for 75 min. The
 structure was developed in completely free form. The structured epoxy
 displayed outstanding definition of the functional tooth edges of the
 typical grid teeth of the spectrometer structure. The spectrometer was
 free of individual and statistical fault points.
 COMITIVE EXAMPLE 2
 A test body comprised of PMMA having a thickness of 300 microns, on copper,
 was irradiated under the same conditions as in Example 2, until a dose of
 irradiation typical for 300 microns of structuring in PMMA had been
 received; the irradiation time was 6 hr. Developing with GG-developer
 followed. The teeth showed indistinct etching, and rounding of the
 structures over the entire height of the teeth. The resulting structure
 had individual and statistical fault points.
 As seen from the comparison of Example 2 and Comparitive Example 2, the
 prescribed grid structure for a microspectrometer with typical grid teeth
 can be realized using PMMA only if the microstructure is less than 150
 micron thick. In contrast, "SD 2054" allows a thicker irradiated layer.
 Despite the thicker layer, the irradiation time required is substantially
 less and the toothed structure of the grid is free of defects.
 Obviously, numerous modifications and variations of the present invention
 are possible in light of the above teachings. It is therefore to be
 understood that within the scope of the appended claims, the invention may
 be practiced otherwise than as specifically described herein.
 The priority document of the present application, German Patent Application
 No. 197 41 492.3, filed Sep. 19, 1997, is hereby incorporated by
 reference.