Integrated Sensors, MEMS, and Microsystems: Reflections on a Fantastic Voyage

Sensors and Actuators A 136 (2007) 39–50 Review Integrated sensors, MEMS, and microsystems: Reflections on a fantastic voyage Kensall D. Wise ∗ Engineering Research Center for Wireless Integrated MicroSystems, Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109, USA Received 19 September 2006; accepted 5 February 2007 Available online 20 February 2007 Abstract The past 40 years have seen integrated sensors move from the first micromachined silicon devices to wireless integrated microsystems that combine high-performance batch-fabricated transducers with embedded signal processing and wireless interfaces. This paper reflects on sensor activities during this period, using three of the earliest devices as examples. Neural probes for precision mapping of activity in the central nervous system have evolved from simple acute structures to complex three-dimensional electrode arrays capable of both stimulation and recording. Integrated with circuitry for amplification, multiplexing, spike detection, and the wireless transmission of power and bidirectional data, they are sparking a revolution in neuroscience and are facilitating prosthetic devices for many debilitating neurological disorders. Pressure sensors have moved from low-yield piezoresistive bridges to self-testing wireless capacitive devices hermetically sealed at wafer level. Finally, effortsto miniaturize a gas chromatograph have now realized prototype microsystems the size of a small calculator containing pressure- and temperature- programmed microcolumns able to separate and identify complex gaseous mixtures in seconds. These microsystems will be key in addressing many of the key problems of the 21st century. © 2007 Elsevier B.V. All rights reserved. Keywords: MEMS history; Microsystems; Neural probes; Pressure sensors; Gas chromatography Contents 1. Introduction ............................................................................................................. 39 2. Integrated sensors are born ................................................................................................ 41 3. New technology and new applications ...................................................................................... 42 4. Device proliferation and commercialization ................................................................................. 43 5. Proliferating into sub-fields................................................................................................ 44 6. Wireless integrated microsystems (WIMS) .................................................................................. 44 7. Conclusions ............................................................................................................. 48 Acknowledgments........................................................................................................ 48 References .............................................................................................................. 48 1. Introduction development of integrated circuits. Memory has moved from magnetic core to microelectronics, and the cost per bit has During the past 50 years, we have seen real miracles in com- dropped more than a million-fold. The number of bits per chip puting, communications, and signal processing fueled by the has moved from 1 to 1 billion, and the number of transistors on processor chips is approaching one billion as well. Disk stor- age is now driving toward densities above 100 Gb/in.2, and data ∗ Tel.: +1 734 764 3346; fax: +1 734 763 9324. converters long ago passed the 16 b level. This progress has E-mail address: [email protected]. been driven by a steady stream of products such as calculators, 0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.02.013 40 K.D. Wise / Sensors and Actuators A 136 (2007) 39–50 electronic watches, the automobile, personal computers, cellu- it attempted to meet fuel economy and emissions requirements lar phones, and music systems, along with a diverse array of using electronic engine control. New applications for the tech- communications and control equipment. Since the entry of the nology were explored, and even though most work remained at first microprocessor in 1971, computers have become common- the component level, the first real microsystems were also devel- place in all kinds of products, and the call issued in 1980 [1] for oped. Multi-sensor chips, partitioning of the electronics, and the the electronics industry to develop pervasive products has been use of embedded processors for digital data compensation were broadly fulfilled. It has been a wild ride, perhaps unduplicated being debated. During the 1980s, the technology continued to in any age in any field of human endeavor. expand. Surface micromachining emerged to complement bulk Since the earliest days of microelectronics, there has been etching, and dedicated sensor conferences and journals were cre- strong interest in using silicon technology to transduce mechan- ated to focus and accelerate the field. By the end of the 1980s, ical, chemical, optical, and thermal events into electronic signals the U.S. National Science Foundation (NSF) was funding this in addition to simply processing and interpreting the signals area, which was becoming known as “microelectromechanical themselves. Fig. 1 summarizes one view of this evolution in systems” (MEMS), at a substantial level. The 1990s saw an the United States; the reader is also referred to the excellent increasing emphasis on systems and a proliferation of MEMS history of sensor evolution written some years ago by Mid- into many different sub-disciplines, including bioMEMS, RF- delhoek [2]. The groundwork needed to understand silicon as MEMS, optical-MEMS, inertial-MEMS, and microfluidics. In a material, especially its piezoresistive and etching properties, the 2000s, we have seen a continuation of this progress, and an was done at Westinghouse, Honeywell, and Bell Telephone Lab- increasing convergence of sensing with embedded computing oratories during the 1950s and early 1960s. The 1960s then saw and wireless technology. This is likely to continue as wireless the emergence of planar integrated circuits along with the first integrated microsystems (WIMS) provide increasingly complete selectively etched silicon sensors. The term “micromachining” solutions to application needs. They will form the front-ends would not be applied to such structures for nearly a decade, of all sorts of information-gathering networks and will be key but by the end of the 1960s, diaphragms, cantilevers, valves, in tackling many of the most important problems of the 21st heaters, fluidic channels, and other structures were being devel- century, including those in health care, the environment, and oped in silicon. The 1970s saw sensor-specific technologies such homeland security. as diffused etch-stops and anodic silicon–glass wafer bonding On the occasion of this 25th anniversary of Sensors and Actu- emerge to help realize an expanding array of devices, and the first ators, I appreciate this opportunity to reflect on the progress attempts at using these technologies in high-volume applications that has been made in taking technology largely developed for began. The automotive industry was a leader in these efforts as integrated circuits and extending it to devices able to gather Fig. 1. The evolution of integrated sensors, MEMS, and microsystems in the U.S.A. K.D. Wise / Sensors and Actuators A 136 (2007) 39–50 41 information from the non-electronic world. The views expressed and was planning to spend the summer of 1966 there, I was are my own and do not necessarily reflect those of the entire com- invited to use this topic as the basis of my doctoral dissertation. munity, but it has, indeed, been a fantastic voyage, extending I spent many lunch hours that summer talking with the peo- back now over 40 years. ple doing selective silicon etching at Murray Hill and reading about the electrodes then used in neurophysiology. The proposal 2. Integrated sensors are born for funding the work was rejected by the NSF (one reviewer called it “science fiction”), but the work went forward anyway My own entry into the world of integrated sensors came in led by Professors James B. Angell (Electrical Engineering) and May 1966 in a seminar given by Dr. Frank Morrell at Stanford Arnold Starr (Neurology) and supported by the Joint Services University. (It was the same year the movie Fantastic Voy- Electronics Program. age was released.) Meetings had been held previously between The development of the neural probe [5–7] required selec- personnel from the School of Medicine and individuals from tively etched silicon microstructures that were small enough to the Department of Electrical Engineering to explore possible approach neurons in vivo with a minimum of tissue damage and areas for joint work, and from those discussions a proposal record their electrical activity (Fig. 2). Only isotropic etching had emerged [3] for two devices applicable to neuroscience. was then available so 50 ␮m-thick silicon wafers and front-back The first of these was a high-density non-invasive EEG record- etching were used to realize tapered structures a few tens of ing system, while the second focused on the development of a microns wide. Wafer diameters then were 25–32 mm, and these silicon-based multi-site neural probe. The probe was based on were typically quartered for processing. Exposed gold record- using beam-lead

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