MEMS: a nickel for your thoughts: a novel approach to building microstructures based on nickel has paved the way to lower cost MEM/MOEM applications.

Electroforming of microstructures in nickel is not itself novel. How often in the electronics world have you heard the phrase a revolutionary concept attached to new products or technologies? The answer is probably too often, as most developments turn out to be the victim of an overactive marketing department.

So, when something is described as a breakthrough technology allowing unparalleled synergy between previously unrelated fields, skepticism comes easily. However, micro-electromechanical systems (MEMS) and their optical counterparts, micro-opto-electromechanical systems (MOEMS), are truly revolutionary.

Millions of dollars are being invested around the world into developing MEMS and MOEMS technology, and applications as diverse as medical, military, aerospace and home office are already benefiting from it. Even the normally sober-speaking U.S. Defense Advanced Research Projects Agency (DARPA) has described MEMS technology as "an important development that will lead to an unprecedented merging of sensing, actuation and computing."

A Technology of the Future

Three main characteristics making microstructures an important technological development are that systems can be made in a batch process, individual features can he made very small and accurate, and optical elements can be fully integrated. When cost and real estate savings are factored in, many more applications are most likely undiscovered, representing a significantly untapped source for such solutions.

To date, microstructures have been predominantly manufactured in silicon using expensive technology borrowed from the semiconductor industry. The silicon approach has dominated because of its familiarity with designers and the availability of suppliers. However, it is not necessarily the best approach, particularly with regards to cost, application and time to market.

An alternative manufacturing process has been developed that offers advantages such as lower cost and quick turnaround from design to finished product. This new approach is based on using nickel instead of silicon and has the potential to be used with other conductors such as copper and gold.

This microstructure technology has been described as a hybrid application building on ideas from the semiconductor, high-volume audio CD and micro-embossing industries. Using this approach, traditional manufacturing obstacles have been overcome to deliver low cost micro-metal parts and larger parts with ultra-fine features.

The Benefits of Nickel

A main advantage of using nickel instead of silicon is that nickel is a much cheaper raw material. Physically, it is less brittle, more flexible and offers a higher electrical conductivity. Nickel also offers good optical properties and can be made into very smooth mirrors ideally suited for use in optical applications (Table 1).

In the silicon industry, a frequently asked question of component manufacturers is: With what size wafers are you working? In most cases, companies are working with 3 in. or 6 in., wafers, with some doing 8 in. or 12 in. With nickel-based systems, component manufacturers can work with substrates measuring 12 in. by 12 in. (300 x 300 ram). The benefit of being able to work on this scale is that large parts with fine features and/or a large number of parts to tight tolerances can be accurately manufactured.

Besides the cost and mechanical benefits, nickel-based systems can be produced on a much shorter timescale, typically taking less than a month from the initial design to delivered products. This short lead time is because nickel-based systems are manufactured in a much simpler way than silicon products.

A typical process begins with a substrate such as copper or glass. This substrate is then covered in a layer of photoresist and exposed using a tool like photomask containing the pattern. After developing the image and rinsing, the part is made by filling nickel into the gaps using high-end photo-electroforming techniques.

Multilayered parts are possible with this technique, because it allows layer alignment down to less than one micron over the entire 12 in.2 substrate. Microstructure parts can be built in nickel to silicon tolerances.

Typical Applications

The applications for nickel-based technology are limitless. So far, the technique has been used to make products as diverse as sensors, actuators, hearing aids, medical devices, optical instruments, micro-lenses, meshes, masks, displays and micro-fluidic devices.

The three most important trends in the electronics industry are to make (1) smaller devices that can run (2) faster, which as a consequence run (3) hotter. The nickel microstructure technology naturally accommodates all three.

Nickel microstructures can be manufactured to extremely small scale, with features such as apertures, fluid channels or raised lands, down to one or two microns and tolerances at sub-micron levels. As they run hotter, the ability to build in fluidic channels raises the possibility of building devices with integrating cooling systems.

Tracks and channels can be fabricated as narrow as two microns wide on a four-micron pitch, with smooth walls and sub-micron tolerances. Resultant surface smoothness is also high precision, being sub-wavelength at 600 nm across the surface.

Raised areas in the design can be produced with a currently unprecedented aspect ratio of 5:1--and potentially much higher.

All of these tolerances and accuracy figures are valid across the full 12 in. x 12 in. (300 x 300 mm) surface area of the substrate, representing a significant alter native over existing silicon techniques.

HDI Technology

One of the most interesting developments to emerge from this nickel-based manufacturing technology is in the field of high-density interconnect (HDI) PCBs. An approach called imprint patterning has been developed for high density circuit fabrication.

Imprint patterning is an application of the established micro-replication technology that is used to make products such as CDs, reflective roadside signs and plastic Fresnel lenses. Micro-replication allows ultra-precise three-dimensional surfaces to be produced on a variety of substrates, usually in high-volume. Micro-surface structures such as dimples, grooves and blind holes are formed by a hot stamping process.

The technique involves the production of a high-density fine-pitch metal stamp, or tool-loll, which is then used in an ordinary laminating press to imprint its image directly onto a substrate to produce circuit traces and vias. The imprinted substrate can then be metallized to produce accurate high-density circuitry.

The tool-foil is imprinted onto an uncured, non-tacky, imprintable film and then removed. Etch resist is squeegeed into the resulting grooves and microvias, before etch is applied to the remaining exposed copper surfaces. When the resist is finally stripped away, copper is left only ill the desired grooves and microvias.

This new technology may revolutionize the manufacture of high-density and microvia-based printed circuit boards (PCBs), as it is significantly faster and less expensive and has the capability of providing finer pitches and vias than current technology. The process involves no drilling, no photo tools and no registration issues--and the plating procedures are reduced.

Imprint patterning is also a padless approach, unlike laser drilling; savings in board real estate can be significant, particularly in the interconnect and I/O areas. Full vias can be produced across the whole surface of a single imprinting tool foil, allowing all of the vias to be made in a single operation rather than individually. The same tool can also be reused many times.

The technology eliminates much of the front-end processing required by conventional HDI. At the same time, new equipment is not required because imprinting is performed in an ordinary laminating press.

Summary

Microstructures, MEMS and MOEMS are all exciting area of technological activity and are being used in an increasingly wide range of new applications and industries. The established manufacturing approach is based on silicon, using techniques borrowed from the semiconductor industry. However, nickel represents a far more cost-effective alternative that delivers very fine features over large areas of substrate.

Offering feature sizes down to the micron-level, the nickel-based approach has been used to make devices ranging from sensors and actuators through optical instruments and displays to medical and micro-fluidic instruments. It has also been successfully used in a new approach to high-density circuit fabrication, which is far simpler than the standard approach based on photo imaging and laser drilling.

TABLE 1: The benefits of nickel-based microstructures as compared to
silicon.

Feature                              Benefits

Resist thickness from sub-micron
  and up                             * Ability to provide very accurate
                                       and strong parts in different
                                       thicknesses
Resolution to sub-micron             * Accuracy of product
                                     * Repeatable process
Aspect ratio 5:1                     * Very fine tolerances and fea-
                                       tures with rigidity
                                     * Flexibility in product design
                                       and thickness
Surface area 12 [in..sup.2]          * Productivity
                                     * Economics
                                     * Ability to make large components
Rapid turnaround                     * Time to market
                                     * Flexibility
Use of nickel                        * Lower cost than silicon
                                     * Metal is better heat conductor
                                     * Metal is conductive
                                     * Metal is more flexible
                                     * Metal parts can be made thinner
                                       and thicker
Low stress                           * Flat, not bowed, surfaces
                                     * Good optical properties
Controllable surface finish
  (mirror/optical)                   * Good release for embossing tools
                                     * Perfect reproduction of masters
                                     * Excellent optoelectronics
                                       characteristics

Noel Cherowbrier is vice president of sales and marketing for Tecan Components, Dorset, UK, [phone]; e-mail: [e-mail].