MICROELECTRONICS MANUFACTURING

Laser-drilled vias could make multichi¥modules cheap High-density circuit boards are needed to interconnect many integrated-circuit chips or dies for multichi¥modules (MCMs) to pack lots of electronics into small areas. But one of the major limitations that has slowed MCM use is the high cost of substrates. This cost is driven largely by the time-consuming process of forming thousands or even hundreds of thousands of vias needed to interconnect various levels and signal lines in the cir

MICROELECTRONICS MANUFACTURING

Chris Chinnock

Laser-drilled vias could make multichi¥modules cheap High-density circuit boards are needed to interconnect many integrated-circuit chips or dies for multichi¥modules (MCMs) to pack lots of electronics into small areas. But one of the major limitations that has slowed MCM use is the high cost of substrates. This cost is driven largely by the time-consuming process of forming thousands or even hundreds of thousands of vias needed to interconnect various levels and signal lines in the circuit. Many new alternatives to traditional drilling techniques are now emerging, but one of the most promising is the use of lasers to form these vias. As a result, MCMs could become much more common and cost-effective in the next few years.

One way to use the laser is to essentially replace the mechanical drill by laser drilling vias one hole at a time. Bill Payne, vice president of engineering at printed-circuit-board-manufacturer Merix Corp. (Forest Grove, OR) notes, "We are evaluating a solid-state UV laser system from ESI (Portland, OR) that is faster and more economical than mechanical drilling for very small holes. It can get down to about 1-mil holes in polyimide and related materials, but, in the tests we performed, it was still not as fast as more-parallel processes such as plasma etch or photoimageable vias."

Recently, however, Litel Instruments (San Diego, CA) demonstrated a laser-based system that comes close to a true parallel-via-generation approach. Chet Farris, vice president, says, "We can make 750,000 to 1 million vias per minute in 50-µm-thick polyimide with a nominal size of 1 mil (25 µm)." Litel plans to deliver the first high-volume production machine to MCM-substrate-manufacturer Sheldahl (Longmont, CO) this fall.

Litel uses an excimer laser at a wavelength of 248 nm, which, after some beam conditioning and stabilizing, is imaged onto the workpiece or MCM substrate with Litel`s patterned holographic phase mask. The mask is used to generate a series of holographic features that alter the phase and amplitude of the light and focus it onto the substrate to produce the vias (see photo). Rapidly scanning the laser tool with a high-power beam allows the holes to be vaporized (ablated) in a nearly parallel fashion. Not having to move the laser to each via allows the Litel system to generate vias quickly. This also eliminates the undesirable effects of mechanical motion, registration, and stabilization.

Farris notes that the system can be used by manufacturers of several kinds of MCMs. For large-area MCMs, u¥to 24 ¥ 24 in., the holographic pattern is stepped across the panel to generate all of the vias. But for smaller-area MCMs, several via patterns or layers can be fabricated on the same holographic mask, reducing overall tooling costs for a new circuit. In addition, the company has developed a new tool designed to include signal routing patterns.

The via holographic patterns can also be tailored for a specific application. For example, Farris says, "Manufacturers of MCMs made by thin-film deposition deal with thin-film metalization, so they want vias that are tapered to insure good metal ste¥coverage at the subsequent metal-deposition step. Alternatively, MCMs formed on laminate materials have thicker layers and require very stee¥vias for plating." In addition, other features, such as excision holes, die cavities, alignment fiducials, and so forth, also can be made with the same holographic phase mask.

The computer-generated holograms (masks) are fabricated in transparent fused-silica quart¥plates. The end products have no dielectrics or metal coatings, so they suffer essentially no tool wear and are capable of being used at very high fluence (energy) levels. Simple binary holograms are formed with standard semiconductor-fabrication techniques that use electron-beam lithography to define the features. Repeating the process produces multilevel holographic elements for more-complex and higher-density via patterns.

However, the tools are expensive. Sheldahl microproducts grou¥director Larry Lemke notes that "these tools can cost $15K-$16K, so the circuit-board design must be fairly stable and have at least a modest volume to make it worthwhile." But lower MCM costs could lead to volume usage.

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