Deposition Technique Targets Middle Ground in Electronics Manufacturing Direct-write technology precisely draws midsize electronic features without masks and resists. Major microelectronic fabrication techniques do a good job of depositing very small electronic features and relatively large ones. But what about medium-size features such as resistors, capacitors, and inductors? No deposition technology on the market is really aimed at these crucial midsize structures. This paper looks at a new technology developed specifically to produce midsize electronic features. Direct-write technology can deposit electronic materials onto low-temperature, non-conformal substrates without masks or other photolithographic accessories. The technology can write lines of organic and inorganic materials less than 10 microns wide onto polymer, glass, or ceramic substrates. Soon, a direct-write tool will be ready for high-volume manufacturing of circuit boards, batteries, fuel cells, antennas, and even biological products. Today, a single microelectronic substrate can hold semiconductor devices, external wiring, and passive electronic components such as resistors, inductors, and capacitors. Integration of these discrete components minimizes device size and cost, while improving thermal management and overall reliability. Two very different technologies dominate microelectronic fabrication. In thick-film processes, screen printing is used to apply paste or ink patterns to a substrate. Capacitors, inductors, resistors, and conductive lines can be created by successive applications of materials, followed by firing at temperatures of 800ºC or higher. Thick-film technology is simple and relatively inexpensive. But it has limitations. For one thing, the high temperatures it requires would damage flexible polymer substrates. Moreover, screen printing usually can’t produce lines and spaces smaller than 100 microns. To produce tiny features on densely packed devices, manufacturers normally use thin-film processes such as sputtering or chemical vapor deposition. In thin-film processes, materials are patterned using masks and photoresists. Though it’s complicated and costly, thin-film technology can produce complex devices with sub-micron features. But what about features between 1 and 100 microns wide? As electronic devices continue to shrink, thick-film fabricators are approaching the physical limits of screen printing. Thin-film technology can deposit midsize features, but it requires a highly skilled workforce and a major investment in new manufacturing facilities. What’s needed is a moderately priced deposition technology that produces features in the intermediate size range. Recognizing this, the Defense Advanced Research Projects Agency (DARPA) is spending $9 million to create technology that will produce “meso-scale” electronic features. Launched in 1998, the four-year Meso-scale Integrated Conformal Electronics program aims to develop a high-volume conformal manufacturing process and tool for building micro-scale passive and active electronic components on high- and low-temperature substrates. DARPA’s prime contractor for the job is Optomec Inc. (Albuquerque, NM), a leading developer of laser-directed material deposition processes. Optomec is working on a deposition process and tool based on so-called “direct write” technology, which deposits electronic materials onto low-temperature, non-conformal substrates without masks or other thin-film equipment. Simpler and less expensive than thin-film techniques, direct-write technology can lay down 10-micron lines of organic and inorganic materials on polymer, glass, or ceramic substrates. In the direct-write process, metal and ceramic precursor particles as small as 20 nanometers in diameter are part of an aerosol mixture that flows into the path of a focused laser beam. From there, the process can employ two different deposition techniques. Laser particle guidance (LPG) uses laser radiation pressure to move particles to the substrate via a hollow fiber. LPG deposits about 10,000 particles per second with an accuracy that can reach sub-micron levels. The other technique, flow guidance, combines laser pressure and co-flow air to deposit about a billion particles per second. But the technique is much less precise than LPG, with accuracies in the 10-25 micron range. Once the precursor materials are deposited, they must be densified and annealed to produce the best electrical and mechanical properties. While this can be done by firing the materials in an oven, direct-write technology relies on laser sintering, which concentrates high temperatures on the deposited materials rather than the substrate. This lets manufacturers use low-cost polymer substrates that can’t stand up to high-temperature oven firing. Direct writing is done by a compact tool developed for rapid manufacturing of both electronic and biological devices. An in-line conveyor system moves substrates into the tool’s 12 x 12-inch work area. There an auto vision camera looks at positional reference points around the substrates. If a substrate is misaligned, system software can compensate for the error. Managing the process is QNX, a Unix operating system developed specifically for precise machine control. QNX is true multi-tasking software that provides process verification via closed-loop feedback. The tool features a continuous-wave Nd:YAG laser that deposits and sinters materials through a deposition head. To maintain an optimal dispensing height, a contact surface sensor calibrates the deposition head to the substrate. Deposition control is handled by an X, Y, Z overhead gantry, which is driven by three-axis brushless servo motors with attached encoders. With its precision stages, the tool can direct write electronic features at high speeds. Deposition rates are already approaching 1 meter per second, allowing the tool to meet the requirements of both rapid prototyping and high-volume electronic manufacturing. So far, the direct-write tool has successfully deposited a number of materials, including metals such as gold, silver, platinum, copper, and rhodium. Conductive metal deposits exhibit less than twice the resistivity of bulk material. The tool has also deposited resistor materials such as ruthenates and silver/glass. Other deposits include barium titanate, ZST, BNT, and several polyimides, all of which turned into electronic features with good material properties and dielectric performance. Why use direct-write technology? In the design phase, the process gives engineers great flexibility. No longer must their designs be constrained by the mask sets and other accessories required by conventional deposition techniques. Other electronic manufacturing processes include many steps between the creation of CAD drawings and the start of production. But the mask-less direct-write process lets manufacturers move right from “art to part.” Though simple, direct write technology can be tailored to meet the needs of many different manufacturing scenarios. By eliminating masks and resists, the process permits on-the-fly changes and rapid refinement cycles. Direct-write technology can produce the ultra-fine features and spacing envisioned by proponents of the National Electronics Manufacturing Initiative, which calls for 15-micron line widths on printed circuit boards by 2009. Though some development work remains, the direct-write tool is already producing 10-micron electronic features. In the process, the tool wastes less material than conventional deposition technologies. Rather than coating an entire masked surface, manufacturers can deposit exact amounts of material exactly where they need to be. Once the material has been deposited, conventional approaches require high-temperature treatment that can take minutes or even hours. By contrast, laser treatment occurs at temperatures of around 200ºC and takes less than 10 milliseconds. The end result is a high-quality thin film with excellent edge definition and near-bulk electronic properties. A flexible technology, direct-write can deposit a wide variety of materials. These include metals, conductors, insulators, ferrites, and polymers, as well as organic materials such as cells, enzymes, and glucose oxidase. Deposits can be made on virtually any surface material — silicon, glass, plastics, metals, and ceramics — and on both high- and low-temperature substrates. Since it can accommodate a wide range of materials, direct-write technology can be used to fabricate components for many industries. Soon, the direct-write tool will be ready for high-volume manufacturing of resistors, capacitors, inductors, batteries, fuel cells, and antennas. In the near future, tiny direct-write antennas could be deposited on flex substrates. The volume of the thin copper spirals would be about 1000 times less than that of antennas produced by conventional methods. After deposition, the material would be processed in ambient conditions and at relatively low temperatures. With the direct-write tool, manufacturers will be able to integrate many active and passive components into one compact, lightweight, and conformal electronic system. On a typical PC board, most of the space is taken up by surface-mount resistors, capacitors, and inductors. But direct-write can embed these components into the board. By embedding components and reducing interconnect pitch and line widths, direct write can clear up to 70 percent of the space on today’s crowded PCBs. Among other things, this will allow PCB designers to pack boards with all the components required by next-generation wireless devices offering e-mail, Internet access, cell phone functions, digital music, and color video in one package. In flex circuit manufacturing, the technology can precisely deposit metal on non-conformal substrates. In this process and others, the direct-write tool can place very small amounts of metal exactly where they need to be. Other possible applications for the technology include: · Bond-pad redistribution · Custom bump fabrication for flip-chip interconnects · Custom deposition for under-bump metallization · Creation of RF filter metallization patterns · Prototyping and fabrication of micro electromechanical systems · Rework and repair of electronic circuitry · Printing electrodes and active materials in displays Direct-write technology will also move into the biomedical field. Biomedical applications include organic cell deposition and the manufacture of closed-loop biosensors. Capable of femtoliter-range dispensing, the technology could also save money in drug testing by depositing very small amounts of expensive reagents. Farther down the road, technologies that direct-write electronic circuits may combine with tissue engineering to produce living machines. Summary Poised to claim the neglected middle ground in microelectronic fabrication, direct-write technology produces “meso- scale” features between 1 and 100 microns wide. Simpler and cheaper than thin-film techniques, the technology can deposit a variety of materials onto high- and low-temperature substrates without masks or resists. A direct-write tool is being developed for rapid prototyping and high-volume manufacturing of many electronic and biological products. I wrote this white paper for a public relations firm working for Optomec Inc.