LIGA Etching

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