- Microsprings Connect Dense Laser Array
Daniel S. Burgess
Researchers at Palo Alto Research Center in California have demonstrated a micromachined spring structure that enables optoelectronic packaging densities five times greater than is possible with current techniques. Initially developed for laser printing, the electrical interconnect technology may find a place in other applications, such as LED print bars, VLSI packaging and optoelectronic modules for free-space communications.
A microspring-based electrical interconnect offers manufacturers the ability to package optoelectronic devices at densities five times greater than is now possible. Courtesy of Palo Alto Research Center.
"Most laser printers today use a multifaceted, rotating mirror that sweeps one or two laser beams across a photoreceptor drum," explained Christopher L. Chua, a research scientist on the project. "The moving mechanical part presents a bottleneck to high print speeds. We are working on an imaging architecture that does away with the mechanical part."
Specifically, the team's all-solid-state approach uses an array of more than 14,000 GaAs vertical-cavity surface-emitting lasers (VCSELs) spaced on a 3-µm pitch to write the information directly on the drum, one laser for each pixel. "The photoreceptor drum can then be exposed by simply turning each laser in the array on or off," Chua said.
Although it is possible to pack the lasers onto a 4-cm-long chip, conventional interconnect technologies such as wire bonding and solder bump bonding cannot interconnect individual VCSELs at such tight pitch.
Wire bonding, for example, would short several contiguous emitters at the connection site. The researchers therefore turned to micromachined spring cantilevers.
To fabricate the interconnect, they sputtered a 300-nm-thick film of MoCr at two pressures onto a layer of SiN on a quartz substrate. They subsequently coated the film with gold to improve its electrical conductivity. Patterning through a mask defined a series of 4-µm-wide lines on a 6-µm pitch in the Au/MoCr film. Because they sputtered the film at two pressures, it displayed a 3-GPa stress differential between the top and bottom. This ensured that the ends of the lines would curve upward from the substrate to a height of 14 µm when freed from the SiN by selective undercut etching through another mask.
To assemble the laser imager, they bonded the contact pads on the VCSEL array and on an ASIC driver chip to their respective sides of the interconnect using a UV-curable adhesive. Alignment was accurate to within 500 nm, and initial thermal shock experiments on daisy-chained test packages suggest that the process yields devices that are MIL-STD-883-compliant.
Although the laser printing imager still is in the laboratory research phase, the interconnect technique can be easily adopted for applications that require a large number of interconnects, off-chip interconnects with large mechanical compliance or interconnects with densely packed arrays of contact pads. The team, led by David K. Fork, is working with chip manufacturers to incorporate the microsprings in their products and welcomes the interest of potential joint development commercial partners in the electronics and optoelectronics industries.
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