Tina L. Lamers

Accelerating Development of a MEMS Piezoelectric Microphone

MEMS projects are well known for their lengthy development times, hindering a company’s ability to make MEMS product development profitable. This paper describes a three-pronged methodology for rapid development of a piezoelectric MEMS microphone, utilizing concurrent design and prototyping, leveraged process technology, and a modified version of Quality Function Deployment (QFD). Avago Technologies has produced more than 300 million Film Bulk Acoustic Resonator (FBAR) piezoelectric band pass filters. FBAR uses Aluminum Nitride (AlN) as the piezoelectric film. Volume production of FBAR makes Avago the world’s only high volume producer of thin-film AlN products. This high-volume FBAR production process was greatly leveraged to enable fast prototyping of piezoelectric MEMS microphones. The concurrent design concept of simultaneously iterating on technical theory, finite element modeling, and prototyping with confirmation from testing was employed as another means of enabling swift development progress. QFD helps designers utilize the ‘voice of the customer’ to determine which product specifications are the most essential, and has long been used as a successful design methodology in the heavy industrial and automotive industries [1]. QFD and most other design methodologies have rarely been applied to MEMS products [2]. The second phase of QFD was modified for better application to MEMS products. Both Phase I and Phase II of QFD were then employed to guide the development process, giving insight into which elements of the design to focus on, which design concepts had the most merit, and which potential applications were the best fit to the technology. The combined effect of these three methods was extremely rapid development, enabling prototyping of hundreds of design variations and brisk improvement of measured results during the first eight months of the program. Achieving technical results quickly while assessing potential applications can aid in identifying a fast path to market. The methods used in this case study can easily be generalized for application to other MEMS development programs, potentially enabling MEMS products to reach production more quickly and generate increased profitability through addressing applications that best fit the technology and design.Copyright

FBAR resonator figure of merit improvements

Free-standing Bulk Acoustic Resonator (FBAR), as one type of bulk acoustic wave (BAW) devices, has extremely high Q to enable excellent filter performance, and has been successfully applied to the wireless communication market. The Bode equation to calculate the unloaded Q is used to map the Avago FBAR product line resonator Rp values. The manufacturing Rp values from old and new generations of FBAR products are collected and compared to demonstrate the importance of the figure of merit (FOM) improvements.

Analysis of Micromachined Piezoelectric Transducers Operating in a Flexural Mode

Micromachined acoustic transducers have recently emerged in multiple application domains. Bulk acoustic resonators, for example, are extensively used as filters in the cellular handset industry. As another example, silicon capacitive microphones are rapidly replacing electret-condenser devices in consumer electronics platforms. Piezoelectric materials are commonly used in acoustic sensing applications. Combining micromaching with piezoelectric thin-films opens up possibilities of multiple new applications, in addition to the miniaturization of existing sensors. Applications may be as diverse as nondestructive test, acoustic imaging, vibration sensing, acceleration/force sensing, or electroacoustics. This paper presents analysis and measured behavior of micromachined devices constructed as a molybdenum, aluminum nitride, molybdenum trilayer and operating in a flexural mode. A theoretical analysis is derived using thin-plate bending theory. Finite-element simulation results of plate dynamics are examined, and sensitivities and linearity of manufactured devices are shown.Copyright

Application of a Modified Quality Function Deployment Method for MEMS

Quality Function Deployment (QFD) has long been used as a successful design methodology in the heavy industrial and automotive industries. QFD helps designers utilize the ‘voice of the customer’, or customer requirements, to determine which engineering metrics or product specifications are the most essential [1]. This prioritization helps designers know what part of the product or process is most beneficial to focus on during design, resulting in products that better meet customer requirements and generate increased commercial success. QFD and most other design methodologies have rarely been applied to MEMS products [2]. In the case of QFD, the structure of the most common format of the tool dictates that engineering metrics should be related to parts characteristics in the second step of applying QFD. This causes difficulties in using the tool for MEMS as most MEMS do not have physical ‘parts’ that are assembled into a final device. Rather, MEMS have product specifications and a manufacturing process used to create the product. Generally there is a tight link between product and process in MEMS. This link has been utilized in creating a modified version of QFD that relates engineering metrics to design concepts, including product conceptualization and manufacturing process. The modified QFD utilizes aspects of Pugh Concept Selection, and differs from typical QFD primarily in consideration of product idea and manufacturing process in the early phases of product definition. The modified QFD was applied to a MEMS project whose goal was to develop a handheld device that allows users to control the selection and release of a variety of stored scents. The technique was also applied to a microscale heat exchanger for integrated circuits. The scent dispenser and heat exchanger were designed and prototyped at Stanford University in 2005 and 2006, respectively. The modified version of QFD gave insight early in the product definition phase on which design concept to pursue to prototype. Use of this and other design methodologies in the MEMS field could shorten the time it takes to progress through product development to volume manufacturing, and increase confidence in the marketability of the chosen design and manufacturing process. A case study demonstrating the effects of using modified QFD Phase II to assist in finding a good fit between technical capabilities and market application was performed by the author on an acoustic sensor technology [3].Copyright

The MEMS Design Process

Today’s ubiquitous commercial MEMS devices, such as accelerometers, inkjet printheads, pressure sensors, and micromirror arrays, took 10–20 years from first report to commercialization. Timelines from initial development through product release for some successful MEMS commercial products have been getting shorter as MEMS technologies have matured. Companies can no longer afford such long development times. MEMS development time can be systematically shortened through the use of structured design methods. This chapter overviews the design process and design methods, illustrating structured design methods through case studies. We suggest reading this chapter prior to getting into the details of materials and process seletion, and applying the methods described to shorten development time.