LIGA moves into the mainstream
07/01/2007
Newly robust electroforming processes enable rugged, affordable microsystems
BY JOHN RASMUSSEN AND JOHN KRAFCIK, AXSUN TECHNOLOGIES
The LIGA electroforming process (see “What is LIGA?” next page) has been a tantalizing yet largely inaccessible option for commercial MEMS development. It offers several important advantages, including superior feature definition and radii, plus smooth, straight sidewalls without burrs (see figure, below). LIGA-produced devices withstand high pressures and temperatures and are rugged and reliable. They can transmit useful loads, forces, and torques. In addition, LIGA-produced moving parts are free from stiction and operate without lubricants.
But LIGA has traditionally presented a number of challenges for designers wishing to use it, and thus has been relegated to research labs. Supported in part by funding from the U.S. Army ARDEC, recent advances have overcome these drawbacks and have made LIGA a very real option for MEMS production. Today, LIGA processes are producing completely assembled and rugged parts made from metal, ceramic, and plastic (see figures on p. 25) at newly affordable prices without sacrificing reliability. Axsun Technologies alone is shipping as many as 10,000 parts per month from its contract LIGA foundry.
LIGA is not just a research process any more. To the contrary, it is increasingly attracting the attention of product designers.
Advancements enable application
Led by the development of ultraviolet lithography (UV-LIGA), manufacturing improvements have enabled a greater-than-two order of magnitude cost reduction, making rugged, precise microsystems practical for an increasing variety of commercial and military applications. Axsun expects that costs will continue to drop; its stated goal is “pennies to $0.50 per part, and a few dollars per finished assembly” in large quantities by 2009. UV-LIGA is reducing reliance on synchrotrons, and product yields have increased dramatically.
UV-LIGA and other manufacturing processes-in combination with low-cost masks-are making prototyping affordable, and sample parts can be shipped in only a few weeks.
Micro-scale parts fabricators have traditionally delivered unassembled parts in waffle packs or loosely jumbled in vials, leaving customers with the tedious tasks of inspection, micro assembly, and packaging-performed by hand under microscopes. But while most customers prefer completely assembled devices ready for insertion into end products, until recently inspection and assembly have not been readily available on a contract basis. Now, for the first time, the U.S. micro-scale electroforming industry is providing high-speed automated kitting, metrology, inspection, and assembly services for their own parts and those produced by others. State-of-the-art equipment provides precise soldering, liquid dispensing, and adhesive bonding, and can assemble parts with clearances of a few microns.
 Examples of the fine feature definition and smooth sidewalls produced LIGA processes. |
Injection molders and die casters seeking tools and mold inserts with precise micron-scale features and smooth sidewalls are frequently frustrated with products made by conventional processes such as stamping, micro machining, or EDM. LIGA-produced nickel alloy tools and inserts have proven to be rugged, and they exhibit fine micro feature fidelity with smooth, straight sidewalls. They are being used for applications such as EDM sinkers, and pick-and-place tools. They are also being used as mold inserts for micro die casting, hot embossing, and micro injection molding of powdered metals and metal alloys such as stainless steel, ceramics such as alumina, and a wide variety of plastics.
LIGA fabricators have accumulated extensive data on micro-scale material properties, and they are increasingly using statistical process control, tensile testing, and process-parameter monitoring to produce components with consistent and predictable material properties. For example, a user of micro-scale metal springs for high-reliability safety devices is consistently obtaining components with tightly specified lot-to-lot and device-to-device spring performance.
LIGA to the rescue
Three examples will demonstrate how LIGA processes are helping designers overcome challenges posed by other approaches to microsystems development.
A designer of micro-optical instruments and control devices was challenged to reduce the size and cost of existing electro-optical products to create new markets. The company needed production quantities of rugged structures for mounting micro-optical components and required precise, six-axis component alignment to 0.1 micron and stable 25-year field life.
 Precise micro-scale electroformed metal springs (left) and cups (center, and right next to the head of a pin) are now highly affordable. |
The company’s early microsystem-based products were assembled by hand using solders and adhesives, and over time moved because of residual stresses, and outgassing contaminated optical surfaces. Its second-generation devices were welded and soldered together from stampings and extrusions and produced better results, but relaxation of highly stressed components caused gradual misalignment.
Micro-scale electroforming met the challenge, producing stress-free alignment structures with precise surfaces that enable easy optical device mounting, highly accurate alignment, and stable long-term field performance that meets or exceeds standards.
A manufacturer of smart munitions needed to enhance the performance of, and add features to, its products without increasing their size. The company needed large quantities of fully assembled, rugged safety and arming devices 60% smaller than devices made with watchmakers’ tools-and at a fraction of the cost.
 This safety and arming device for smart munitions contains multiple moving parts among tens of total parts. |
Previously, the company tried using silicon devices, but they were reduced to rubble during operation. Micro-scale electroforming allowed the company to consider other materials, and today it is using microsystems with multiple, high-precision moving parts and frames. These, along with a variety of miniature parts from other vendors, are automatically measured, inspected, and assembled. The LIGA-produced, low-cost safety and arming devices (see figure, below) have demonstrated ruggedness and reliability, even when they are subjected to 40-ft drops, extreme vibration, and supersonic launch forces exceeding 70,000 Gs.
A producer of high-end wristwatches needed to improve the quality and life of existing products and reduce their power needs. Precise LIGA-manufactured metal mold inserts enable injection molding of parts that are far more precise than those made by stamping and EDM. The result: smoothly operating mechanisms that last longer and require only a fraction of the battery power.
Recent advancements have overcome barriers to widespread adoption of LIGA-produced Microsystems, and today complete, fully inspected, operational microsystems are being delivered ready for use in all kinds of applications. Product designers are increasingly taking advantage of these capabilities to significantly reduce the size, cost, and weight of mechanical devices without reducing reliability.
John Rasmussen is director of business development, and John Krafcik is director of technologies, both at the Axsun-West LIGA Foundry of Axsun Technologies Inc. (www.ligafoundry.com). For more information, contact [email protected].
What is LIGA?
LIGA-an acronym representing the German words for lithography, electroforming, and molding-is a method for lithographically producing microcavities (mandrels) and using them to electroform parts having precise micron-scale features. LIGA is inherently a mass-production process, using wafer-scale technologies similar to those used in the semiconductor industry.
In its simplest form, the process begins with CAD drawings. Images of the parts are laid out in a pattern (often as many as thousands of parts per wafer) and a photomask is created. In parallel, a sacrificial substrate is coated with a layer of photoresist. The mask is positioned over the photoresist and exposed to collimated ultraviolet light or X-rays. The resist is then developed, leaving precise mandrels in the shape of desired parts, and the cavities are filled with metal using an electroplating process. Finally, the resist is dissolved away leaving behind loose parts ready for inspection and assembly.