Scalable MEMS manufacturing
03/01/2008
Traditionally, MEMS devices have been developed using a combination of custom design and manufacturing techniques in a select number of captive MEMS foundries or in one of the commercial specialty MEMS fabs that have sprung up in recent years. While such foundries typically excel at the particular type(s) of MEMS technology they work with, they often fall short in areas of increasing value to MEMS product developers including access to transistor technology, availability of 8-in. wafers and substantial high volume production experience.
Recently, adoption of MEMS technology has been experiencing rapid acceleration in the consumer electronic market-one whose characteristics are at odds with the traditional approach to MEMS product development and the capabilities of “traditional” MEMS foundries.
What is needed is the capability to manufacture MEMS devices in existing semiconductor foundries, while maximizing the use of existing transistor process technology and materials.
In general, a MEMS process that avoids the need to add new capital equipment or to utilize new materials, and that has been proven to be manufacturable to a pre-production level will be more attractive to an IC foundry than a process not exhibiting these characteristics. IC foundries are understandably unwilling to disturb or risk their ongoing production business to install a new process that is (too) radically different. Also, the size of many MEMS companies (small) adds another level of perceived business risk.
The business and technical constraints surrounding partnering with an IC foundry typically result in a requirement for the MEMS developer to make design trade-offs versus their ideal scenario.
Design targets or margins may need to be adjusted to account for such trade-offs as well as the material control and variation targets of the foundry. In summary, these trade-offs require a change in the MEMS design methodology from creating a custom process for a specific design (i.e., full custom design) to designing “into” a defined process (i.e., semi-custom or application-specific design).
Overall, the benefits received from fabricating MEMS devices in an IC foundry far outweigh the required trade-offs. In addition to access to active devices (increasingly important as MEMS technology continues to be tightly integrated into products) and cost and capacity advantages, mainstream IC foundries provide excellent statistical process control capabilities. The also provide a shorter path to product and vendor qualification as the foundry is often already qualified at customers of interest, enhanced supply security, and the possibility of leveraging access to established semiconductor packaging and assembly vendors.
IC foundry partners should be selected on the basis of maximum overlap in target markets, customers, and maximum experience with materials and unit processes that are as close to those that will be needed for the MEMS devices as possible. MEMS developers will find themselves in the role of educating their IC foundry partner about the material science aspects of MEMS.
Over the past two years, WiSpry has partnered with Jazz Semiconductor to meet the requirements and challenges described above and to implement a scalable RF-MEMS manufacturing capability in Jazz’s 8-in. wafer foundry. Early process development and prototyping were done in a MEMS foundry. Initial porting to Jazz was accomplished by mapping the existing process flow into existing unit processes at Jazz, leading to initial functional prototypes. MEMS process tuning and the development of wafer level encapsulation followed initial prototyping. CMOS circuit development was done in parallel and then, finally, the CMOS, MEMS and encapsulation processes were integrated into a single flow resulting in 8-in. wafers that can be processed through backend operations as though they are normal CMOS wafers. The development process has involved ongoing trade-offs and education between WiSpry and Jazz and has resulted in a growing library of silicon-proven, reusable MEMS building blocks for efficient scaling of product development, production and revenue.
There is growing interest, activity and necessity in developing the capability to manufacture MEMS devices in existing semiconductor foundries. Product development methodologies and models are under development, and early successes have been demonstrated. Continued adoption of MEMS technology to meet the demands of high growth markets, such as consumer electronics, is expected to further accelerate progress in the integration and availability of MEMS technology within the mainstream IC manufacturing infrastructure.
 |
Jeffrey L. Hilbert,
EVP and co-founder, WiSpry Inc.
Contact the author at WiSpry Inc., 20 Fairbanks, Suite 198, Irvine, CA 92618; ph 949/458-9477, [email protected].