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Etching, Molding and LIGA
Etching
• the objective is to selectively remove material using imaged photoresist as a masking template
• The pattern can be etched directly into the silicon substrate or into a thin film, which may in turn be used as a mask for subsequent etches
Electrochemical Etching
• The relatively large etch rates of anisotropic wet etchants (>0.5 μm/min) make it difficult to achieve uniform and controlled etch depths.
• Some applications, such as bulk-micromachinedpressure sensors, demand a thin (5- to 20-μm) silicon membrane with dimensional thickness control and uniformity of better than 0.2 μm, which is very difficult to achieve using timed etching.
• This method is commonly referred as electrochemical etching (ECE)
Plasma Etching• Plasma (or dry) etching is a key process in the
semiconductor industry• Conventional plasma-phase etch processes are commonly
used for etching polysilicon in surface micromachining and for the formation of shallow cavities in bulk micromachining.
• deep reactive ion etching (DRIE) tool for the etching of very deep trenches (over 500 μm) with nearly vertical sidewalls.
• Plasma etching involves the generation of chemically reactive neutrals (e.g., F, Cl), and ions (e.g., SFx+) that are accelerated under the effect of an electric field toward a target substrate.
• A fully suspended n-type crystalline silicon island electrochemically etched in TMAH after the completion of the CMOS processing.
• (Courtesy of: R. Reay, Linear Technology, Inc., of Milpitas, California, and E. Klaassen, Intel Corp. of Santa Clara, California.)
DRIE• Profile of a DRIE trench
using the Bosch process.
• The process cycles between an etch step using SF6 gas and a polymer deposition step using C4F8.
• The polymer protects the sidewalls from etching by the reactive fluorine radicals. The scalloping effect of the etch is exaggerated.
• A limitation of DRIE is the dependence of the etch rates on the aspect ratio (ratio of height to width) of the trench (see Figures 3.13 and 3.14).
• The effect is known as lag or aspect-ratio-dependent etching (ARDE).• The etch rate decreases with increasing trench aspect ratio. (Courtesy of:
GE NovaSensor of Fremont, California.)
Reactive Ion Etching
• an ion-assisted reactive etching method used in the semiconductor fabrication process.
• excellent process control (homogeneity, etch-rate, etch-profile, selectivity), which is critical for high-fidelity pattern-transfer
• used in the manufacturing of topographical structures in micro- and nano-system technologies
• RIE is based on a combination of chemical and physical etching which allows isotropic and anisotropic (uni-directional) material removal.
• The etching process is carried out in a chemically reactive plasma containing positively and negatively charged ions generated from the gas that is pumped into the reaction chamber.
• A mask on top of the substrate is used to protect certain areas from etching, exposing only the areas to be etched.
• The ions are accelerated into the etching region, where they attack the substrate surface and react with it
• RIE primarily employs ion-assisted processes, where heavy ion bombardment damages chemical bonds and the radicals chemically react with exposed surface atoms producing a volatile product.
• RIE provides excellent pattern-transfer results even for very fine structures significantly under 100nm.
*) Resolution testpattern in Silicon *)Teststpattern etched 500 nm deep into silicon( 193 nm DUV-Litho. in 110 nm thick PMMA)
*) Detail of a high-resolution neurochip structure *)„μ-Cocktail Glass Set” fabricated in Silicon(e-Beam Lithography & RIE)
*) 100 μm μ-gears *) Etched silicon templates for micro gears
*) 420 μm deep etched Si-micro-“needles” *)200 μm deep etched Si-trench pattern
Deep Reactive Ion Etching
• a RIE modification for silicon deep etching using alternating etch and passivation cycles (gas chopping, time-multiplexed etching, ASE -Advanced Silicon Etching)
• It is used for high anisotropy and etch rates to obtain patterns with maximal aspect ratios and resolution.
• Selectivity, anisotropy and etch rates can be controlled through the process chemistry (gases) and process parameters (RF-power, pressure, gas flow, substrate cooling etc.)
• High anisotropy of etched structures is guaranteed by applying side-wall polymer passivation.
• Deep etching down to 1 mm• Aspect ratios (width/height) from 1:10 to
1:100 (material dependent)• Resolution below 100 nm (mask dependent)
• http://www.fhv.at/fhv-science/microtechnology/dry-etching-rie-and-drie
Fields of application
• Semiconductor technology• Micro-reactors, micro-flow sensors, micro-
switches, optocouplers, micro-motors, biosensors
• Small-sized components for movable and unmovable microstructures
MICRO-ELECTROPLATING• Micro-electroplating involves the electro-chemical
deposition of metallic layers on objects• In combination with optical lithography, various metallic
microstructures can be produced (so-called LIGA ("Lithographisch-Galvanische Abformung") process using lithography, electroplating and moulding)
• In this approach a negative form of the desired metal structure is generated by means of lithography in a photo-sensitive film (photoresist).
• In the next technological step electroplating is used to fill metal into the empty spaces in this form, which then form a secondary structure (the desired metal structure).
• To achieve this, different electrolytes are used according to specific requirements: nickel, hard nickel, nickel alloys and copper
• for the electrodeposition of copper layers and nickel layers with thicknesses up to several millimetres
• Undercoating layers and start (seed) layers for electrodeposition are generated by means of a sputter (vacum deposition) process.
*) 80 μm thick electroplated μ-coils realized by electroplating of Cu on a 4-inch wafer *)Detail of a 87 μm thick μ-coil of nickel
*) Details of 70 μm thick Cu-electroplated μ-coil for RFID deposited on Pyrex wafer
*)3 μm wide and 10 μm thick Cu-reliefpattern *)Cu-electroplated resolution-testpattern
*) Resolution test evaluation in 3D *)50 μm Lines/Spaces gridpattern filled by electroplated copper
Electroplating and Molding
• Electroplating is a well-established industrial method that has been adapted in micromachining technology to the patterned deposition of metal films.
• A variety of metals including gold, copper, nickel, and nickel-iron (Permalloy™) have been electroplated on silicon substrates coated with a suitable thin metal plating base.
• a conducting seed layer (e.g., of gold or nickel) is deposited on the substrate.
• A thick (5- to 100-μm) resist is then deposited• The largest aspect ratio achievable with optical lithography
is approximately three, limited by resolution and depth of focus.
• In LIGA, optical lithography is replaced with x-ray lithography to define very high aspect ratio features (>100) in very thick (up to 1,000 μm) poly(methylmethacrylate) (PMMA) (plexiglass based).
• The desired metal is then plated• Finally, the resist and possibly the seed• layer outside the plated areas are stripped off.
Illustration of mold formation using either optical or x-ray lithography and electroplating (LIGA).
• The process may be stopped at this point with a metal microstructure suitable for some purposes.
• Alternatively, the metal can be used as a mold for plastic parts (the “A” in LIGA).
• Precision gears and other microstructures have been fabricated using LIGA, but the method is considered expensive because of the requirement to use collimated x-ray irradiation available only from synchrotrons.
• Mold formation using optical lithography is often called “poor man’s LIGA.”
Fields of application
• Form and mint applications• Electrodes for micro-erosion• Masks for laser ablation• Metallic micro-prefabricated parts
Ultraprecision Mechanical Machining
• Cutting tools such as mills, lathes, and drills using a specially hardened cutting edge
• Using modern computer-numerical-controlled (CNC) machines with sharply tipped diamond-cutting tools
• many metals and even silicon have been milled to a desired shape, with some features smaller than 10 μm.
• Resolution of about 0.5 μm can be achieved, with surface roughnesses on the order of 10 nm
• Example applications include optical mirrors and computer hard drive disks.
Laser Machining
• Focused pulses of radiation, typically 0.1–100 ns in duration, from a high-power laser can ablate material (explosively remove it as fine particles and vapor) from a substrate.
• Incorporating such a laser in a CNC system enables precision laser machining.
• Holes as small as tens of microns in diameter, with aspect ratios greater than 10:1, can be produced.
• Arbitrary shapes of varying depths are laser machined by scanning the beam
• to remove a shallow layer of material, then scanning again until the desired depth has be reached
• Laser machining can be used to create perforations• in silicon wafers for subsequent cleaving to form individual chips, as
well as simply cutting though the full wafer thickness.
• Laser machining examples: • (a) microlenses in polycarbonate; and • (b) fluid-flow device in plastic. Multiple depths of material
can be removed.• (Courtesy of: Exitech Ltd., of Oxford, United Kingdom.)
Electrodischarge Machining
• Electrodischarge machining, also called electrical-discharge machining or sparkerosion machining (EDM) uses a series of electrical discharges (sparks) to erode material from a conductive workpiece.
• High-voltage pulses, repeated at 50 kHz to 500 kHz, are applied to a conductive electrode, typically made of graphite, brass, copper, or tungsten
• Electrodes as small as 40 μm in diameter have been used, limiting features to about the same size
• Features with aspect ratios of over 10 can be fabricated, with a surface roughness on the order of 100 nm
• Each discharge removes a small volume of material, typically in the range of 103 to 105 μm3, from the workpiece
• EDM has been used to create the tooling for molds and stamping tools, as well as final products such as nozzles and holes in microneedles.
Screen Printing
• silk screening• In electronics, it has long been used in the
production of ceramic packages and more recently for large flat-panel displays
• A wide variety of materials, including metals and ceramics, can be applied using screen printing.
• It does not have same resolution as photolithography, but is cost effective and is readily applied to large substrates
Screen Printing
• Screen printing begins with the production of a stencil, which is a flat, flexible plate with solid and open areas
• The stencil often has a fine-mesh screen as a bottom layer to provide mechanical rigidity
• Separately, a paste is made of fine particles of the material of interest, along with an organic binder and a solvent.
• A mass of paste is applied to the stencil, then smeared along with a squeegee.
• A thin layer of paste is forced though the openings in the stencil, leaving a pattern on the underlying substrate.
• Drying evaporates the solvent• Firing burns off the organic binder and sinters the remaining metal
or ceramic into a solid, resulting in a known amount of shrinkage.
• Metal lines with 125-μmlines and spaces are made in the production of ceramic packaging , with 30-μm features demonstrated .
• Film thicknesses after firing range from roughly 10 to 200 μm. • Multiple layers of different materials can be stacked
Microcontact Printing/Soft Lithography
• Microcontact printing, a microscale form of ink printing also called softlithography
• It enables low-cost production of submicrometer patterns and has been studied as an alternative to conventional photolithography, but is not presently a product fabrication method.
• the production of the original, hard, three-dimensional master pattern which can involve conventional photolithography and etching, electron-beam lithography, laser scribing, diamond scribing, or any other suitable method.
• A mold of an elastomer, usually poly(dimethylsiloxane) (PDMS), is made against the master, then peeled off to create a stamp with raised patterns.
• An “ink,” a liquid solution typically of an alkanethiol (a hydrocarbon chain ending in a thiol, an –SH group) such as hexadecanethiol, is poured onto the PDMS stamp and dried.
• The inked stamp is then held against a substrate coated with gold, silver, or copper, then removed
• The thiol end of each “ink” molecule bonds to the metal, forming a densely packed, single-molecule-thick coating of hexadecanethiolwhere the raised areas of the stamp were
• Microcontact printing: (a) create master; (b) form PDMS stamp and peel off; (c) coat with “ink”; (d) press inked stamp against metal and remove, leaving ink monolayer; (e) use selfassembled monolayer as an etch mask; or (f) as a plating mask
Hot Embossing
• In the hot embossing process, a pattern in a master is transferred to a thermoplastic material.
• If the dimensions are relatively large (>100 μm), the master can be made with conventional machining
• Smaller dimensions can be produced using nickel electroplated through patterned photoresist
• The master is pressed into the thermoplastic (e.g., PMMA, polycarbonate, polypropylene) just above the material’s glass transition temperature
• The master and plastic are cooled while in contact, then separated, leaving a pattern in the plastic.
Nanoimprint Litography