Shape deposition manufacturing of microscopic ceramic and metallic parts using silicon molds

Micro-Mold Shape Deposition Manufacturing (.mu.-Mold SDM) is a method for fabricating complex, three-dimensional microstructures from layered silicon molds. Silicon wafers are etched using conventional silicon-processing techniques to produce wafers with surface patterns, some of which contain through-etched regions. The wafers are then stacked and bonded together to form a mold, which is filled with part material. In one embodiment, the part material is a ceramic or metallic gelcasting slurry that is poured into the mold and solidified to form a part precursor. The mold is removed, and the precursor is sintered to form the final part. The gelcasting material may also be a polymer or magnetic slurry, in which case sintering is not needed. The mold can also be filled by electroplating a metal into it; if necessary, each layer is filled with metal after being bonded to a previously filled layer. Patterned silicon wafer layers may also be combined with macroscopic wax layers formed by Mold SDM to create macroscopic parts with some microscopic parts or features.

FIELD OF THE INVENTION
 This invention relates generally to micromachining methods. More
 particularly, it relates to a method for fabricating complex millimeter
 and sub-millimeter parts out of ceramic and metallic materials using
 layered molds.
 BACKGROUND ART
 A variety of methods are available for fabricating complex microscopic and
 mesoscopic parts out of ceramics and metals, in which feature or part
 sizes are below 1 millimeter. These sizes are not accessible by
 conventional machining techniques, including milling and turning.
 Micromachining techniques can be divided into a few main groups: thin film
 techniques, micro-stereolithography (.mu.-SLA), photo-chemical etching,
 and LIGA.
 Thin film techniques include etching, sputtering, and chemical vapor
 deposition. Material is deposited onto a silicon substrate, which is
 etched away after the part is formed. Complex multi-material assemblies
 can be produced with high accuracy. However, only certain materials can be
 deposited in sufficient quality with these techniques. Feature height is
 limited to about 10 .mu.m, above which internal stresses built up during
 thin-film deposition cause delamination. In addition, microscopic parts
 formed by this technique cannot be combined with conventionally machined
 parts to form an assembly. A related technique involves electroplating
 into patterned silicon substrates. Generally, a thin seed layer is
 deposited onto the wafer as an electroplating base before electroplating.
 This technique also has height limitations, and complex structures with
 overhangs cannot be electroplated.
 SLA is widely used for manufacturing macroscopic prototypes out of
 polymeric materials, and has been adapted for micropart fabrication, as
 described in T. Nakamoto et al., "Manufacturing of three-dimensional
 micro-parts by UV laser induced polymerisation," Journal of Micromechanics
 and Microengineering, Vol. 7, pp. 89-92 (1997). Parts are made by scanning
 a UV laser over a photo-curable resin. In regions where the laser
 penetrates the liquid surface, the resin solidifies and forms the part.
 SLA achieves complex three-dimensional structures, but accuracy is limited
 by the spot size of the laser and its penetration depth. It is also
 difficult to remove the uncured resin from microscopic cavities.
 In photo-chemical etching, a focused laser beam removes material by
 photo-chemical ablation. This process has been used to shape silicon with
 an argon-ion laser in a chlorine atmosphere, and is described in T. M.
 Bloomstein and D. J. Ehrlich, "Laser deposition and etching of
 three-dimensional microstructures," IEEE, Transducers '91, pp. 507-511.
 Laser ablation allows the creation of real three-dimensional structures.
 Its main disadvantage is the slow speed of the process. Since surfaces
 must be scanned in a serial fashion, photo-chemical etching cannot compete
 with the highly parallel lithographic processes, especially when a large
 number of devices are manufactured or if large volumes of material must be
 removed.
 LIGA (Lithographie, Galvanik, Abformung) involves the use of a bright X-ray
 beam to irradiate polymers such as polymethyl methacrylate (PMMA). The
 irradiated PMMA degenerates into shorter polymer chains that are soluble
 in certain solvents. By covering some regions Qf the polymer with an X-ray
 mask, microstructured polymer parts can be manufactured. LIGA can produce
 polymer microstructures with very high precision and high aspect ratios.
 By electroplating into the polymer mictrostructures, metal parts can be
 formed. However, standard LIGA cannot be used to produce complex,
 multi-layered parts.
 A method for producing complex structures using PMMA sheets patterned by
 LIGA has been disclosed in U.S. Pat. Nos. 5,378,583 and 5,496,668, issued
 to Guckel et al. Multiple layers can be stacked to form complex
 microstructures of up to 1 mm in height. This method has some significant
 drawbacks, however. Both X-ray masks and access to the synchrotron light
 source needed for X-ray generation are extremely expensive. In fact, while
 the LIGA method is, in theory, capable of highly parallel manufacturing of
 numerous identical or different parts, incorporating an X-ray source into
 mass production is unfeasible. Furthermore, part material that can be cast
 or plated into polymer microstructures is quite limited. The mold must be
 able to withstand the filling process without heat- or pressure-induced
 deformation, and the part material must be unaffected by the method
 (usually chemical) used to remove the polymer. The thin, flexible polymer
 is also difficult to handle and align, with both the X-ray mask and
 subsequent layers. Polymer layers also cannot be combined with macroscopic
 layers to form macroscopic parts incorporating microscopic features.
 There is still a need for a process for accurately fabricating complex,
 three-dimensional microstructures that can be used for mass production.
 OBJECTS AND ADVANTAGES
 Accordingly, it is a primary object of the present invention to provide a
 highly accurate method for fabricating complex, three-dimensional
 microscopic and mesoscopic parts. The parts can include overhangs and
 curved surfaces.
 It is a further object of the invention to provide a method for fabricating
 parts up to 1 mm high.
 It is an additional object of the invention to provide a method in which
 the finished part can easily be removed from its mold, without retaining
 any unwanted material.
 It is another object of the present invention to provide a method that can
 produce large quantities of parts in parallel, in an economically viable
 fashion.
 It is an additional object to provide a method for fabricating parts from a
 wide range of ceramic and metallic materials.
 It is a further object to provide a method that uses a mold that is easy to
 form, handle, and align, and is made of rigid, strong material.
 Finally, it is an object of the invention to provide a technique for
 fabricating macroscopic parts that that have regions or features of
 microscopic or mesoscopic dimensions.
 SUMMARY
 These objects and advantages are attained by Micro-Mold Shape Deposition
 Manufacturing, a method for shaping microscopic parts and assemblies out
 of ceramic and metallic materials using layered silicon molds. The molds
 are made using standard silicon-processing technologies, and then filled
 with the desired material. The mold is removed to obtain the final part.
 In one embodiment, a method of fabricating a precursor to a part, "surface
 patterns are etched in a plurality of silicon wafer layers, " at least one
 of which contains a through-etched region. The wafers are stacked and
 bonded together to create a mold, which is then filled with a gelcasting
 material, which may be a ceramic, metallic, or other gelcasting slurry.
 The gelcasting material is then solidified to produce the precursor. The
 mold may also contain patterned wax layers, which have pattern features
 wider than 1 mm, so that the resulting precursor has both microscopic and
 macroscopic regions. The method may also include a first additional step
 of removing the precursor from the mold, preferably by chemically etching
 the silicon mold, and a second additional step of sintering the precursor
 to form the part.
 This embodiment also includes the use of a mold containing only one layer.
 The layer is made by creating a surface pattern in a mold material; the
 surface pattern includes a feature of width between 1 .mu.m and 1 mm.
 Preferably, the mold material is a silicon wafer, and the pattern is
 created by wet or dry etching; features may be up to 10 mm wide. The mold
 is then used as described above to create a precursor and a micropart.
 Also provided is a method for fabricating parts made of a material that
 does not require sintering to form the finished part. A layered mold is
 formed as described above. In this case, the layered mold is filled with
 part material to produce the part. The mold can either be created from the
 wafer layers and then filled, or each wafer layer can be filled after it
 is stacked and bonded to the previously filled wafer layer. The mold may
 also be attached to a support surface before being filled. Part material
 may be a polymer, metal, metal powder, magnetic slurry, or other material.
 For a metal powder, the filling step also includes liquid-phase sintering.
 Preferably, the part material is a metal, in which case the mold is
 attached to a metal support surface and the metal is electroplated into
 the mold to create the part.

DETAILED DESCRIPTION
 Although the following detailed description contains many specifics for the
 purposes of illustration, anyone of ordinary skill in the art will
 appreciate that many variations and alterations to the following details
 are within the scope of the invention. Accordingly, the following
 embodiments of the invention are set forth without any loss of generality
 to, and without imposing limitations upon, the claimed invention.
 The present invention, Micro-Mold Shape Deposition Manufacturing (.mu.-Mold
 SDM), is a process for fabricating complex, three-dimensional parts out of
 ceramic and metallic materials using layered molds. FIG. 1A shows an
 example of a layered silicon mold 10, made from layers 12, 14, and 16. The
 individual layers are etched silicon wafers; in this example, all of the
 wafers are through-etched. FIG. 1B is a cross-sectional view of a part 20
 that is formed in mold 10 of FIG. 1A. Part 20 contains three prismatic
 sections, 22, 24, and 26, shown separated by dotted lines. Prismatic
 sections 22, 24, and 26 correspond to mold layers 12, 14, and 16,
 respectively. A prismatic section has coplanar faces bound by sides
 perpendicular to the faces. For example, prismatic section 22 contains
 coplanar faces 28 and 30 bound by sides 32 and 34.
 Multiple prismatic sections can be constructed in a single silicon wafer by
 repeated etchings using different photographic masks, as will be described
 below. However, it is not possible to create an overhang 18 without using
 the layered mold of the present invention. Micro-Mold SDM allows for
 fabrication of much more complex parts than can be produced by standard
 silicon-processing techniques.
 Adaptive process-planning software can be used to determine an appropriate
 set of prismatic mold layers from a computer-aided design (CAD) part
 model. For each prismatic section, a photographic mask for silicon
 processing is produced using the bottom face geometry of the layer. FIG. 2
 shows a portion of a CAD model 36 containing a slanted surface 38 and a
 curved surface 40. If the model cannot be oriented in a way that allows
 division into prismatic sections, a stair-step approximation 42 to the
 required accuracy is used to obtain the prismatic sections.
 Dividing a mold into layers of equal height is well known in the art.
 Algorithms describing stair-step approximations with varying layer heights
 have also recently been developed. These techniques minimize the number of
 steps or layers necessary to approximate a given geometry while satisfying
 particular accuracy specifications. Mathematical tools such as Fourier or
 Wavelet transformations are used to estimate how rapidly the part geometry
 changes in a given area, which determines how thick the layer must be.
 After the layer thicknesses and patterns have been determined, silicon
 wafers are etched using conventional processes to create the surface
 patterns in the layers. The following description illustrates a preferred
 process; of course, any method that creates anisotropically through-etched
 wafers may be used.
 Commercially-available wafers have thicknesses between 10 .mu.m and 1 mm.
 If a wafer with the predetermined thickness is not available, a slightly
 thicker wafer can be chosen, and the thickness can be reduced by
 dry-etching in a deep reactive ion-etching machine, wet-etching with
 chemical agents, mechanically polishing, or any other suitable technique.
 FIG. 3 illustrates the silicon-processing step. A wafer 44 is coated with a
 layer of photoresist 46, preferably by spin coating. Photoresist 46 is
 preferably between 1 and 15 .mu.m thick. Wafer 44 is mounted in a
 commercial mask aligner (not shown) and exposed to ultraviolet (UV) light
 through a photographic mask 48 containing the necessary patterns for the
 layer being produced. UV light changes the molecular structure of
 photoresist 46 in the exposed regions. Negative photoresist hardens after
 exposure and is resistant to solvents capable of dissolving unexposed
 photoresist. Positive photoresist breaks down when exposed to UV light and
 is easily removed with solvents that cannot dissolve unexposed positive
 photoresist; regions are chemically dissolved using a solvent known as a
 "developer", leaving a pattern of hardened photoresist 50 on wafer 44.
 This pattern resists subsequent wet or dry etching, whereas the exposed
 surface of wafer 44 does not. Photoresist 46 is a positive photoresist,
 but negative photoresist may also be used with appropriate photographic
 masks.
 Wafer 44 with photoresist layer 50 is then etched to create a patterned
 wafer 52 containing features in the areas not covered by layer 50. A
 feature is any discrete section of a pattern, such as a trough or edge.
 Features are preferably between 1 .mu.m and 1 mm in width, but may be up
 to 10 mm. In order to attain prismatic sections and high accuracy of part
 geometry, the etching must be anisotropic: wafer material is etched
 vertically, but not laterally. No significant underetching of photoresist
 50 can occur. Because through-etched regions of several hundred
 micrometers are produced, high etch rates are also required. Both
 requirements can be met by a commercially-available deep reactive
 ion-etching machine, which can etch at a rate of up to 4 .mu.m/min.
 When {100} silicon wafers are used, anisotropic etching can also by
 performed by wet etching using chemical etchants. These wafers have a
 particular, unidirectional crystal orientation. Possible etchants include,
 but are not limited to, potassium hydroxide (KOH) and tetra-methyl
 ammonium hydroxide (TMAH). Of course, any other method for producing
 anisotropically etched wafers may be used.
 Etching also removes some amount of photoresist. For example, dry etching
 of silicon wafers to a depth of 70 .mu.m etches about 1 .mu.m of
 photoresist, and the photoresist thickness must be chosen accordingly.
 Photoresist remaining after etching may be removed by, for example,
 acetone or a 4:1 volume mixture of sulfuric acid and hydrogen peroxide, to
 reveal patterned wafer 52.
 The entire etching process is repeated as many times as necessary. If the
 wafer has only one prismatic section, no further silicon processing steps
 are required. The process must be performed for each required layer.
 "Closed" molds may have a bottom layer that does not require etching.
 The wafers are then aligned with each other for stacking. FIGS. 4A-4C show
 wafer 52 of FIG. 3 being aligned with subsequent wafer layers 54 and 56.
 Alignment may be done using mechanical alignment marks that were etched
 into the silicon or using an optical microscope. Using these methods,
 wafers may be aligned to a precision of 1 .mu.m; that is, edges or
 alignment marks are at most 1 .mu.m apart. Of course, any method may be
 used to align the wafers. It is relatively easy to align the wafers
 because of their high strength and rigidity.
 Wafers 52, 54, and 56 are laminated together to produce a mold 58,
 preferably by applying an adhesive on the side surfaces of the layers or
 by bonding the layers in a commercial wafer bonder. If one photographic
 mask contains patterns for several layers, the wafer is cut into pieces
 after being etched, and the patterned pieces are stacked to form the mold.
 A single mold can also be used to form many of the same parts or many
 different parts. This allows the method to be used in highly parallel
 manufacturing processes.
 Next, the mold is filled with part material. One method of filling the mold
 is gelcasting, a relatively new method for fabricating macroscopic ceramic
 parts. Compared with traditional ceramics, gelcast ceramics have a lower
 binder content, and can be machined before "sintering". They also shrink
 isotropically when the binder is removed, allowing for more accurate mold
 construction. Gelcasting is described in U.S. Pat. No. 4,894,194, issued
 to Janney, and U.S. Pat. No. 5,028,362, issued to Janney et al. Briefly,
 the process is as follows. A ceramic powder is mixed with water or a
 nonaqueous solvent, a dispersant, and a monomer to form a slurry. A
 partial vacuum is applied to the slurry to remove air bubbles. Next, an
 initiator is added to the slurry, and then the slurry is cast into a mold.
 The mold and slurry are heated, which causes the initiator to begin
 polymerization of the monomer into a solvent-filled gel that supports the
 ceramic particles in the desired shape. This precursor stage of the
 finished part is called the "green" part. The green part is unmolded and
 dried to remove the solvent, which causes the part to shrink
 isotropically. A slurry that is about half solids produces a green part
 that will shrink by about 3%. Finally, the green part is sintered to burn
 off polymer and fuse the ceramic particles into a dense material.
 The present invention creates molds that allow gelcasting to be used on
 microscopic and mesoscopic scales. Any type of gelcasting material may be
 used: ceramics, for example, alumina and silicon nitride; metals, such as
 stainless steels, tool steels, and superalloys; and castable polymers,
 including polyesters, epoxies, polyurethanes, and polyurethane foams.
 Castable polymers are not sintered. Gelcasting can also be used to form
 weak magnetic parts, in which the particles supported by the gel are
 magnetic. In this case, however, the green part is not sintered, and the
 polymer remains in the part to support the magnetic particles. For all
 gelcasting materials, solvents may be both aqueous and nonaqueous.
 Microscopic gelcast parts can be made using either a layered silicon mold
 or a single patterned silicon wafer, acting as a silicon mold. FIGS. 5A-5D
 illustrate the gelcasting process using an "open" layered mold 60 made as
 described above. A gelcasting slurry 62 is poured into mold 60, which is
 supported on a support surface 64, for example, a flat aluminum plate.
 Alternately, the bottom layer of the mold may be an unetched wafer, or a
 patterned wafer that has not been through-etched. A partial vacuum may be
 imposed on the cast slurry to remove air bubbles; the rigid silicon mold
 resists deformation under vacuum. The slurry is cured, preferably by
 heating at 50-100.degree. C. for approximately 1-2 hours, to form a hard
 gel or green part 66, shown in FIG. 5B. Any excess green part above the
 mold surface may be removed, in this case by grinding with a grinder 68.
 FIG. 5C shows the finished green part 70, a precursor to the final
 micropart. After the mold is removed (described below), the green part is
 sintered to produce the final micropart 72, shown in FIG. 5D. Suitable
 temperature profiles for sintering are well known in the art.
 Gelcast parts can also be formed in a "closed" mold 74, shown in FIG. 6. In
 this case, slurry 76 must be poured through sprue opening 78. After mold
 removal, the green part will have a sprue 80, which can then be removed by
 grinding.
 Metal parts can also be formed by electroplating into the silicon mold. Any
 desirable metal that can be electroplated to sufficient quality may be
 used; common examples are copper and nickel. Preferably, the plated metal
 is copper. In most cases, the metal itself will be the desired part.
 However, the metal may also be used as an injection mold for plastic
 parts. As opposed to silicon molds, the metal mold can be reused.
 In the case of mold 82 of FIG. 7A, the entire part can be electroplated at
 once. For mold 84 of FIG. 7B, regions 86 and 88 will not fill properly if
 the entire mold is created before electroplating; in this case, each layer
 is filled separately as described below. When the entire mold is filled at
 once, the resulting part is more likely to be free of defects. However,
 suitable parts may also be formed by sequentially filling layers.
 FIG. 8 illustrates the process of building a mold and electroplated part
 together. A first patterned silicon wafer layer 90 is mounted on a support
 92 using a suitable adhesive. Support 92 may be made of a solid copper or
 aluminum plate with a thin metallic seed layer 94, which acts as an
 electroplating base. The electroplating base can be any metal suitable for
 electroplating the metal to be plated. Preferably, layer 94 has been
 sputtered to a thickness of less than 1 .mu.m and is made of copper.
 The assembly is immersed in an electroplating solution, which is preferably
 an acidic copper-sulfate solution. The solution may also contain a leveler
 and brightener, which are used to make the plating more uniform and to
 improve the surface finish of the copper being plated. Suitable quality
 parts may also be formed without the leveler and brightener.
 Electroplating is preferably done by periodic pulse reverse plating, the
 most efficient way of providing dense, nearly void-free copper having a
 smooth surface finish. Optimal current density is 30 mA/cm.sup.2, which
 achieves a deposition rate of 25 .mu.m/hr. The electroplating current is
 periodically reversed, with the reverse current being three times stronger
 than the forward current, but having a duration twenty times shorter. This
 technique is described in G. Milad, "Periodic Pulse Reverse," Printed
 Circuit Fabrication, Vol. 20, No. 7, July, 1997.
 Layer 90 is plated with copper until a plated part 96 has the same height
 as or is slightly higher than layer 90. If necessary, the excess copper of
 part 96 can be ground off to reveal a level part 98. A next layer 100 is
 aligned with layer 90, using an optical microscope, mechanical alignment
 pins, or any other method. Layers 90 and 100 may be held together with any
 suitable wafer adhesive. Layer 100 is then plated with a metal 102, which
 is ground to form a level metal 104. The steps are repeated as necessary
 until all layers have been plated to form a part 106. If support 92 is
 aluminum, it may be removed after plating by a chemical etchant, for
 example, hydrochloric acid or potassium hydroxide.
 Depending on the geometry of the mold, the silicon wafer layers may need to
 be coated with an electroplating base, preferably by sputtering. A thin
 layer of titanium or chromium, preferably 0.2 .mu.m thick, forms an
 adhesion layer on the wafer. A thin copper layer is then sputtered,
 preferably to about 0.1 .mu.m, on the adhesion layer.
 Any method for filling the layered mold is within the scope of the present
 invention. For example, liquid phase sintering may also be used. For this
 technique, the mold is filled with a metal powder containing two metals
 having different melting temperatures. As the mold and powder are heated,
 the metal with the lower melting point melts to form a matrix that
 supports the grains of the second metal. After the part cools and
 solidifies, the mold can be removed. Hot pressing a metal powder mixture
 also produces suitable parts. In this case, the powder-filled mold may be
 pressed in a conventional 10-ton hot press in a graphite die, preferably
 at 750.degree. C. and 30 MPa. For both methods, the metal mixture is
 preferably silver-tungsten, and particle diameters are preferably between
 1 and 5 .mu.m.
 After the part or precursor is complete, the mold is removed, preferably by
 chemical etching. Any etchant that completely removes the mold, but does
 not attack or corrode the part itself, may be used. Gelcast green pieces
 still contain polymer, and are therefore especially susceptible to attack.
 Silicon is an excellent mold material, in part because it can be etched by
 a variety of chemicals, and so it is likely that a suitable etchant can be
 found that does not affect the part. In contrast, it would be difficult to
 remove a polymer mold from a green piece without attacking the polymer
 binder within the green piece itself.
 The following four chemicals can etch silicon at a reasonable rate:
 potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH),
 catechol (1,2-dihydroxybenzene), and a mixture of nitric acid (HNO.sub.3),
 hydrofluoric acid (HF), and water (or acetic acid). The HF/HNO.sub.3
 mixture works at room temperature, and the other three at temperatures
 between 50.degree. C. and 110.degree. C. For copper parts, KOH is the most
 preferable etchant: it does not attack copper, etches {100} silicon at up
 to 100 .mu.m/hour, and can be used without complicated safety precautions.
 Potassium degrades the high temperature properties of ceramics, and so KOH
 cannot be used for ceramics. Depending on the sensitivity of the polymeric
 binder in the green part to high temperatures, either TMAH or HF/HNO.sub.3
 can be used. After the mold material is removed, the part may require
 further finishing steps before being combined with other parts into an
 assembly. As described above, the gelcast green piece is sintered after
 mold removal.
 An important advantage of the present invention is that it allows for a
 link between conventional macroscopic manufacturing techniques and
 micromachining. Macroscopic parts often have a few regions or features
 that either have microscopic dimensions or must be manufactured to an
 especially high precision. Using existing technologies, either the entire
 part must be made using micromachining techniques, or the microscopic
 feature must be made separately and then joined with the macroscopic part.
 These solutions are either too complicated or unnecessarily expensive. The
 present invention, .mu.-Mold SDM, may be combined with conventional Mold
 Shape Deposition Manufacturing (Mold SDM) to produce a single mold
 containing both microscopic and macroscopic layers.
 Mold SDM is a technique for producing complex, three-dimensional
 macroscopic parts from layered molds, usually made of wax. Sequential mold
 layers and support layers are built up, allowing for complicated undercut
 mold structures. The support material is eventually removed, and the
 resulting mold can be filled with part material.
 As shown in FIG. 9, .mu.-Mold SDM and Mold SDM can be combined to form a
 compound mold 108 filled with part material 110. Mold 108 contains silicon
 wafer layers 112 and wax layers 114. In general, the layers requiring high
 precision or small features are made from patterned silicon wafer layers,
 and the remaining layers from wax. Layers 112 and 114 of compound mold 108
 can be joined together using a standard commercial adhesive. Using this
 method, there is virtually no limit in part size.
 A CAD/CAM system can determine which layers to make of silicon and which of
 wax. It then sends layer information to the appropriate station within the
 system. .mu.-Mold SDM uses the layer information to generate photographic
 masks, and Mold SDM generates computer numerical control (CNC) code for
 material removal and deposition to form the wax layers.
 Compound mold 108 can be filled with any of the part materials described
 above. After the finished part or green part is formed, depending on the
 technique, the wax mold is first dissolved in a heated solvent. The
 remaining silicon mold is then removed as described above.
 It will be clear to one skilled in the art that the above embodiment may be
 altered in many ways without departing from the scope of the invention.
 For example, multiple part materials may be used within a single mold.
 Accordingly, the scope of the invention should be determined by the
 following claims and their legal equivalents.