MEMS and SAWs 'zipped' into packages
11/01/2002
Ziptronix, Research Triangle Park, NC, has adapted its proprietary materials bonding technology for dissimilar semiconductor materials to a wafer-scale method for encapsulating surface sensitive devices, including MEMS and SAW filters. The covalent bonding technology does not use adhesives, takes place at room temperature, and uses standard wafer processing equipment, materials, and process chemicals (see Technology News, July 2001, p. 34).
The Ziptronix technology has been adapted to bond wafers of various materials that provide custom cavities for specific MEMS applications (Fig. 1), including optical windows, thermal spreaders, etc. This process creates a variety of flexible, low-cost packaging options and enables the integration of MEMS and IC technologies without the complex packaging, testing, and assembly that typically accounts for up to 80% of the cost of MEMS devices. The resulting package is fully hermetic to Mil-Std-883E (Fig. 2).
Doug Milner, CEO of Ziptronix, says, "Because of their sensitivity, MEMS have traditionally required special assemblyand packaging, creating significant costdisadvantages vs. a standard IC. Our methodeliminates this special handling. Now,semiconductor foundries, MEMS manufacturers, and IDMs can cost-effectively assemble, package, and test devices in-house. This paves the way for volume commercialization of MEMS, and for streamlining their integration with electronic systems." Surface sensitive MEMS are exposed to yield loss "hits" during dicing, pick-and-place, wire bonding, and package sealing.
The new process, in which the covalent bonding is activated on both sides of a wafer, can be confined to a single side. This is sometimes desirable with surface-sensitive MEMS devices, as it helps to avoid conflicts with proprietary headspace (clearance) chemistries needed for optimum performance.
The technique's two-step sequence readily accommodates fab production cycles. Surface activation can precede bonding by several hours without deterioration of the activated surfaces. When active surfaces are bonded, the process occurs on contact without appliedpressure or an electric field, facilitating high production throughput. Bondingtypically takes only about 4 sec. The result is low-cost, wafer-scale, hermetic encapsulation of MEMS and SAW filters that eliminates costly special handling and complex encapsulation of individual devices.
 Figure 1. Helium leak rates for MEMS cavities fabricated with a silicon-to-silicon covalent bonding process.Bumps on Cu pillars tighten pitch |
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Bumps on Cu pillars tighten pitch
Calling it "breakthrough technology," wafer bumping house Advanpack Solutions (APS), Singapore, has achieved lead-free wafer bumping below the industry target of 100µm — specifically 80µm-pitch"copper pillar" bumps with a lead-freesolder tip (see figure). Engineers at APS have targeted this technology to meet the European Commission's legislativerestrictions requiring lead-free electronics packaging by January 1, 2006.
Company engineers attribute success to their proprietary copper-pillar. This is a rigid nonreflowable bump that supports a reflowable-solder wetting tip. Special manufacturing methods are used to restrict wetting of the solder to the sides of the copper pillar, thereby maintaining a constant spacing between the reflowable portion of the bumps.
Achieving equivalent results using conventional, fully reflowable solder bump technologies has been difficult. In normal fully reflowable solder bumping, bumps change shape or move during the liquid phase of reflow. During the molten or liquid stage, bridging of adjacent bumps can occur.
APS president John Briar says, "This new technology eliminates solder bridging worries by creating a nonreflowable stand or pedestal to support the reflowable portion of the bump during flip-chip attachment. Since the solder can never wet to the sides of the pillar or pedestal, an effective bridging barrier is created and shorting is prevented even at pitches of 100µm and below."
APS's copper pillar also increases bump standoff, meeting the 80–90µm height needed for good reliability and consistent underfilling of flip-chip ICs.
Tiny MEMS relay for 20GHz RF
Claiming "the world's smallest, among relays with contacts," Omron Corp., Tokyo, Japan, has developed a 20GHz RF MEMS relay (i.e., a micromachined relay or MMR). The MMR is fabricated using Omron's proprietary vertical feed-through MEMS structure, a wafer-level chip-size package process that uses no plastic, ceramic, or other type of external packaging (see figure).
The MMR is 1/100th the size of an equivalent Omron mechanical relay and achieves insertion loss of 1.3 dB or less and isolation of 20 dB or greater, specifications unmatched by any other relay. The MMR uses tiny mechanical contacts, a 2–20GHz bandwidth, and switching that minimizes power loss. Furthermore, it eliminates noise generated with conventional RF semiconductor switches and so eliminates previously necessary RF filters and other peripheral parts.
In the future, Omron engineers believe they will be able to extend this technology for extremely high frequency band (10–30GHz) millimeter-wave communication applications. Utilizing wireless communication equipment tied to a wireless LAN, home wireless or other network, use of RF signals over a wide band is essential for smoothly transmitting large volumes of information such as moving images. Eventually, applications will switch from the present 2–5GHz spectrum to RF. Currently, mainstream RF semiconductor switches used for RF signal switching are compliant to only a narrow band and have too large a signal power loss.
Targeting 20GHz information communications, Omron is aiming for commercialization during its 2004 fiscal year.— P.B.
 The Omron vertical feed through MMR is 1.8mm ¥ 1.8mm ¥ 1.0mm. |
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