[Bill Kennedy]

Editor's Note: The following article first appeared as the cover story in the Fall 2008 issue of MICROmanufacturing magazine, our print counterpart.

From their invention to their obsolescence, manufacturing techniques evolve and improve along with the items they produce.

This nickel-cobalt part measures 5mm × 2.5mm × 0.8mm, weighs 5mg and features a moving jaw. It was produced by the EFAB process. Photo courtesy Microfabrica Inc.

Today,the shrinking size and growing need for sophisticated components used in all types of devices are fueling demand for fabrication techniques that can mass-produce parts ranging from a micron to a few millimeters. To meet demand, processes previously limited to laboratory production of nanoscale parts are being adapted to mass-produce microscale parts. Hybrid processes are available in which
nano- and macro-scale technologies are combined, and established fabrication methods once used for just producing macroscale parts are coming online. An example of the latter is stereolithography
(STL), the first rapid prototyping (RP) technique developed.

With STL, a 3-D model of a part is drawn in
a CAD program and digitally segregated into
layers that are output as an STL file. The part
is built from thin layers (usually about 0.10mm thick) of a liquid polymer resin that cures when exposed to ultraviolet light. Guided by the STL file, a UV laser traces a cross section of the part’s features on the resin, solidifying and bonding it to the previous layer. After a layer is cured, a new
layer of polymer is distributed across the part. The laser tracing and curing cycle repeats until the desired 3-D object is formed.

A recent advance in the STL process is two photon photopolymerization, which can produce 0.1μm-sized features. In this method, a focused, near-infrared laser traces the desired shape in a block of gel. The gel solidifies only where the laser is focused.

Another RP technology that fabricates an object from a succession of layers is 3-D printing. The process creates a 3-D form by depositing and connecting successive cross sections of material. The hotosensitive polymer material is deposited in minute drops by an inkjet-style printhead. Alternately, the head can distribute adhesive that bonds the layers.

RP is excellent for producing models for evaluation. Production, though, is usually limited to single-digit part runs and parts consisting of one or two materials.

Improving traditional subtractive methods

TRADITIONAL SUBTRACTIVE machining technologies are being downsized and made more precise, allowing them to better process millimeter- and micronscale parts. One such technology is CNC Swiss-type automatic lathes. (See article on page 39.) The machines can handle bar stock down to 1mm in diameter and turn 0.076mm-dia. parts having proportionately smaller features. Milling machines designed specifi cally for microscale work can be fi tted with 0.0004"-dia. (and smaller) endmills and provide 2μm positioning accuracy, 50nm resolution and 200nm repeatability. In the area of thermal machining, EDMs run 0.0027"- to 0.0008"-dia. wire and produce surface fi nishes as fi ne as 0.002μin. These and other traditional subtractive methods, including electrochemical, electron beam and laser machining, produce individual parts that generally require assembly to create functional products. Injection molding is an additive technique increasingly used to create microscale parts from plastic, metal or ceramic. (See article on page 29.) Improvements in process steps, such as work-material fl ow and debinding, are providing increased accuracy while enabling production of ever-smaller parts. CoorsTek Inc. is a manufacturer of advanced ceramic, plastic and metal products. The Golden, Colo., company reports that it can mold ceramic parts down to 1mm3, with walls as thin as 0.2mm, while holding tolerances on certain products to within ±0.25 percent. —B. Kennedy

Lithography and MEMS The most widely applied mass-production methods are those used to microfabricate millions of silicon-based microprocessors that store and manipulate data in computers and other devices.

A microprocessor can contain hundreds of millions of transistors. Intel Corp.’s 45nm processors, for example, incorporate 410 million transistors in each dual-core chip and 820 million in each quad-core chip. The chips are miniscule; hundreds are made via batch processing on a single 300mm-dia. silicon wafer.

Traditional fabricating processes cannot supply the precise detail such small devices require. The chips are made by lithographic techniques that exploit the precision of focused light. The method is a development of the manufacturing methods originated to produce integrated-circuit (IC) boards. The process begins by outlining features representing one layer of the chip on a photomask that functions like a stencil or a photographic negative.

A single mask can hold multiple images, permitting simultaneous batch processing of hundreds of chips. After an insulating layer of silicon oxide is generated on a silicon wafer’s surface, it is coated with a photosensitive substance called a “resist.”

The mask is placed over the wafer and then exposed to a light source. As light passes through the mask, the outline of the layer’s features transfer to the substrate surface.

Chemical treatments remove those areas not exposed to light, and the process is repeated until the desired chip geometry is built. The layers’ electrical conductivity are
altered via ion implantation, also known as “doping.” During the layering process, atoms of metals are deposited on some areas of the chip to form electrical connections.

Typical feature sizes are around 70nm. Efforts are under way to develop a technology that will allow fabricating
20nm features. Roughly 20 layers may be connected to form microprocessor circuitry. The actual number depends on
the chip’s intended function.

Including the mask exposures, etching and deposition processes, more than 300 steps may be involved in making a chip — a process that consumes weeks. Lithographic techniques are proven ways to produce chips. But they have
limitations with regard to workpiece materials (usually only silicon can be used), structure height, and the time and expense of the process.

While computer chips are produced in runs of millions, the technologies used to produce them also are applied in the manufacture of smaller, more specialized runs of micro-electrical mechanical systems.

MEMS devices incorporate micron-sized components, including microprocessors, sensors and actuators. Common uses for MEMS include motion and pressure sensors, inkjet printer cartridges and drug-delivery devices. Accelerometers that actuate automotive airbags and the Wii video game are wellknown MEMS applications.

The lithographic manufacturing technique known as surface micromachining is used to produce Analog Devices’ ADXL202 dual-axis iMEMS accelerometers. Photo courtesy Analog Devices.

The Wii incorporates
a 3-axis ADXL330 MEMS
accelerometer from Analog Devices Inc. that senses and replicates the player’s hand motions.

Norwood, Mass.-based
Analog Devices’ marketing
program manager for MEMS accelerometers and gyroscopes in automotive and consumer products, Howard Wisniowski, said the company’s ADXL32x family of multiaxis iMEMS accelerometers come in 4mm × 4mm × 1.45mm packages. Analog has packages under development that are smaller than 2mm × 2mm × 0.9mm.

Wisniowski cited a recent advancement in silicon-based lithographic manufacturing techniques. An early method,
bulk micromachining, involves coating and etching a silicon wafer that serves as the chip’s structure. The newer technique, called surface micromachining, involves deposition of silicon layers on a substrate’s surface to create the chip’s structure. It offers numerous advantages
compared to bulk machining.

“Bulk micromachining sculpts the moving pieces by removing material from a relatively thick substrate,” Wisniowski said, “which is not complementary to the demands of high-volume IC processes.”

Surface micromachining, on the other hand, involves depositing thin films on the substrate and then etching them. It is compatible with on-chip signal conditioning circuitry. (Signal conditioning is amplification, or other conversion, of a sensor’s output to make it suitable for digital representation.)

Putting the amplifier or converter directly on the chip with the sensor requires extremely compact
elements.

The EFAB process produces micrometer- and millimeter-scale devices by forming and stacking thin metal layers. Deposition and, later, elimination of sacrificial material in addition to structural metal permits fabrication of 3-D, monolithic devices.

Therefore, the smaller, more
precisely patterned structures possible with surface micromachining facilitate
production of on-chip signal conditioning arrangements.
Surface micromachining typically allows production of structures 20 times smaller than the bulk method. It “enables the most functionality and the highest performance in the smallest and most cost-effective package,” Wisniowski said.

‘Handshake’ technologies
Although both traditional manufacturing techniques and lithography-based methods can be used to fabricate microscale
parts and assemblies, the conceptual
and physical differences between
the processes historically have been
significant.

However, combining traditional and
lithographic techniques is key to progress in the area of microfabrication, according
to Dr. Marc Madou, chancellor’s professor at the University of California’s Department of Mechanical and Aerospace Engineering and Department of Biomedical Engineering, Irvine.

“Traditional manufacturers did very much go their own way, and lithographybased people did, too,” Madou said. “It’s only when they communicate that we get breakthrough manufacturing methods.”
An example of blending traditional and lithographic techniques—a “handshake” technology, as Madou described it—is the microfabrication method LIGA.

LIGA is a German acronym for X-ray lithography (lithographie), electrodeposition (galvanoformung) and molding (abformtechnik). Developed in the 1980s, it combines the lithographic technology of chip manufacturing with electroplating and molding. With LIGA, a thick photolithographic resist is exposed to X-ray or UV light to create a 3-D shape. The shape is electroplated, and when the resist is removed, a
3-D metal structure remains. The metal form can be an end product or be used to make molds.

Large depth-to-width ratios (exceeding 100:1) are possible, and the process produces straight, smooth walls with no burrs. Features as small as tens of microns and part heights up to several centimeters can be produced. Among the products made via LIGA are electronic testing probe pins, EDM electrodes and microgears. LIGA-built molds are able to mold more precise parts than those made via stamping and
EDMing. And when used to create multiple molds, LIGA technology is well-suited for mass fabrication of parts, particularly polymer ones.

The multistep LIGA process is relatively slow, however, and the complexity of the parts produced and the number of materials that can be employed are limited.

EoPlex developed the High Volume Print Forming process to produce long runs of small, complex, 3-D ceramic and metal parts with internal structures, components and connectors. Each layer can contain up to seven materials. Typical layer thickness is 40μm, and minimal thickness is 5μm. Devices may have hundreds of layers. Materials are deposited in defined patterns to build successive layers. After layering is complete, the part is sintered, binding the desired components and eliminating the sacrificial material. Photos courtesy EoPlex.

The hybrid microfabrication technique known as EFAB combines the batch-processing advantages of chip production with RP’s additive-layer principle and
traditional electroplating. Microfabrica Inc., Van Nuys, Calif., developed the proprietary process.

EFAB is a micrometer- and millimeter-scale fabrication technology that produces devices by forming and stacking a set of thin metal layers. While the layering aspect resembles RP technologies, EFAB
is a high-volume production process that yields functional metal devices with micron-scale features.

EFAB can produce part features as small as 4μm. The 3-D components and devices can be designed with independently moving parts and generally require
no assembly. Typical products include tissue-removal, drug-delivery and timing devices, actuators, semiconductor test probes and other complex metal parts.

The process starts with a 3-D model created in CAD. The model geometry is exported into an STL file format. A proprietary Microfabrica software program
then reads the STL file and generates 2-D cross sections of the device. The cross sections are used to create a set of photomask tools that define the layer geometry with submicron resolution. The photomasks define the locations in which metal is electrodeposited to form the layers of the device. Currently, EFAB employs two materials: Valloy-120, a proprietary nickel-cobalt alloy, and, for areas where increased wear resistance is required, Edura-180, a proprietary rhodium formulation.

A photomask is essentially a quartz plate with a chrome pattern having submicron resolution, said Adam Cohen, Microfabrica’s executive vice president and chief technology officer. “You get extraordinary
accuracy and repeatability when that is what is defining your geometry on a layer,” he said. “Formation of all layers produces a device that reconstructs the original CAD geometry. As long as you follow the design rules, what you see is what you get.”

Each layer of the device being fabricated contains sacrificial material (usually copper) and structural material. One material—either sacrificial or structural—is patterned using photolithography, and the other is blanket-deposited elsewhere. Each layer is planarized to ensure precise layer thickness and flatness and the desired surface finish.

Cohen said planarization is “actually one of the hardest things to accomplish,” and is a proprietary part of the process. When deposition is complete, the sacrificial material is removed via an etching process. If the end product is to be detached from the substrate, the first layer put down is made entirely of sacrificial material.

Cohen said devices can be designed using nearly any 3-D CAD platform, and design rules can be summarized in a few pages. “But there is a learning curve associated with certain things; [those] you
just can’t put on paper.”

Cohen acknowledged EFAB’s superficial commonality with RP. “You design it in CAD, and it comes out. So, in one sense, it’s like rapid prototyping. But in another sense, it is not in that it produces functional devices from engineering materials, requires tooling and is a volume production process.”

EFAB’s batch-processing nature permits both production and prototype manufacturing by altering the details of the photomask. Cohen explained: “If you are doing production, you typically are going to have one device that is replicated across the entire wafer. If you are doing prototyping, you may have 10, 20 or more different devices that are populated across the wafer. They can be variations of one design or entirely unrelated devices.”

Cohen pointed out that EFAB is not the answer for every microfabrication task. “Sometimes you need a different material than what we work with. It is a metal process. So if you really need glass or polymer, then it’s just not going to meet your needs,” he said. Filling the gap Another handshake technique is High
Volume Print Forming (HVPF), developed by EoPlex Technologies Inc., Redwood City, Calif. It involves layer-by-layer buildup of devices and utilizes multiple printing processes.

HVPF was developed to produce large volumes of small, complex ceramic and metal parts with internal structures, components and connectors at a low cost. In some cases where internal complexity is high, such as fluidic parts, HVPF may offer the only way to produce a part cost effectively.

In HVPF, micro-units of different materials are deposited in defined patterns to build successive layers and create complex 3-D shapes. The patterns are printed layer by layer, and the parts are batch-processed on panels in production-level volumes.

A design begins with a 3-D CAD model, then EoPlex uses SolidWorks software to slice the design into layers. “Printing inks” include ceramic and metallic compositions and sacrificial materials. After the layering process is complete, the device is sintered to bind the desired components and eliminate the
sacrificial material.

EoPlex Chairman, CEO and President Arthur Chait described the technique, “It’s as if we laid up all the layers and floors of a house in the different materials, then heated it up and the negative materials went away and left the space of the stairwells and rooms, and the metal sintered into wires, and the ceramics sintered into the walls.”

HVPF design criteria specify a minimum part length of 1mm and maximum of 200mm. The process can produce features as small as 25μm, to a tolerance of ±0.5 percent or ±0.05mm, whichever is greater. Each layer can contain up to seven materials. Typical layer thickness is 40μm, and the minimal thickness is 5μm.

Structural materials include ceramics, nickel and stainless steel alloys, precious metals, oxides, glasses, and polymers with ceramic and metal fillers. The inks are proprietary formulations developed to allow for shrinkage, thermal expansion and particle-size distributions. Devices may consist of hundreds of layers.

An example is a hydrogen reformer (a device that removes hydrogen from a compound to power a fuel cell). It consists of 300 layers with 33 features, including channels, chambers and ports. Materials
include two catalysts, a metal, a ceramic and a sacrificial material. HVPF permits integration of various
geometries and components. A cell phone antenna required integration of complex metal-ceramic structures. Another device, a piezoelectric tire pressure sensor, included a cantilevered mass, piezoelectric material, conductors, internal voids and a mount. The sensor was contained in a package that was also formed via HVPF.

Chait said the process isn’t a solution to every microfabrication application, but it “fills a gap between the macro world and the miniature.” He said between 40μm and 40mm is a “realm where assembly and other tooling methods don’t work for complex multimaterial parts. You can use MEMS and all the rest to remove material, but what happens when you want to put together a structure? When you have a small-size part with a fair degree of complexity, and multiple materials, you’re really stuck. HVPF can fill that gap.”

Dr. Marvin Kilgo, EoPlex’s vice president of operations and engineering, acknowledged the similarity of HVPF to RP in that it uses layer-type construction to form a part. The key difference is batch processing. With many parts being made in parallel on panels perhaps a half-meter square, “depending on the size of the device you are making, you could have hundreds or thousands of devices being made simultaneously,” he said. “The ability to build in parallel is actually a rather significant issue. That’s a point of distinction between HVPF and rapid prototyping.”

Part of the challenge of working with a new manufacturing method is getting designers to understand the process’ capabilities and the design rules. “What we are doing is a different approach than many folks are used to,” Kilgo said. “We have to understand the customer’s application well enough to understand the implications on our design set, and we need to bring the customer along to understand our design rules. There is some back and forth so we can reach a point where we [achieve the] optimal design.”

Kilgo said the two-way learning process is common when introducing new technologies.

“Any time you expand the available toolset, it’s exciting because it provides some real opportunities. But,
it also presents a challenge to make sure designers understand what sort of technologies are really available.”

Choosing a manufacturing technique, he said, “is like picking the right tool out of the toolkit. There are certain applications where MEMS, micromachining or micromanufacturing techniques make a great deal of sense.” Silicon-based lithography methods are well understood, and manufacturing capacity is available.

“If your structure is primarily planar in nature,” Kilgo said, “and you have a modest number of materials that you want to process, there is a great deal to be said for that type of process. But if, for some reason, you need to integrate into the mechanical world so that you are looking at an application that requires manipulating fluids, or if you are trying to actually build a device in multiple dimensions, those
standard micromachining techniques don’t work particularly well.” Generating thick parts a micron at a time is expensive and time consuming. “That’s where processes similar to what we are doing or
similar to LIGA-type processes or what Microfabrica is doing tend to become more important,” he added.
UC Irvine’s Madou advised micromanufacturers to “zero-base” their approach and make the decision on what is the best route to pursue for the specific application.

“You may often come up with different answers than your first instinct. It is pleasant when people are willing to just say, ‘OK, I need to look at all of the methods,’ and they survey all of the machining
techniques that are available.”

Madou pointed out that microfabrication techniques such as LIGA offer accuracy that traditional machining processes can’t provide. However, he added, lithography-focused manufacturers often underestimate the tolerances that can be achieved by traditional machining methods. “Every time there is a breakthrough in this area, it seems to happen somewhere between 20μm and up to, maybe, a millimeter,” he said. “This is a very difficult range. When people have this handshake across the bridge, a new set of applications is possible.”

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About the author: Bill Kennedy is a contributing editor of MICROmanufacturing and Cutting Tool
Engineering magazines. Telephone: (724) 537-6182. E-mail: billk@jwr.com.

Contributors

• Analog Devices Inc.,
  (800) 262-5643,
  www.analog.com
• BioMEMS Research Group,
  Dr. Marc Madou,
  University of California, Irvine,
  (949) 824-6585,
  mmadou.eng.uci.edu/Madou_Lab_
  Website
• CoorsTek Inc.,
  (800) 821-6110,
  www.coorstek.com
• EoPlex Technologies Inc.,
  (650) 361-9070,
  www.eoplex.com
• Microfabrica Inc.,
  (818) 786-3322,
  www.microfabrica.com