标签: cmos vlsi

Synchronous and asynchronous MOS VLSI logic circuit methodologies are now in the mainstream of electronics design. But the end is near for this approach: five or ten years at most, by most knowledgeable projections. And while further integration is possible, the cost of doing so is escalating: it now takes a company with the net worth of a small country to afford the cost of fabricating today's nanometre circuits. Intel Corp., long an advocate of the synchronous VLSI circuit design, recently faced the fact that it is not possible to achieve higher levels of integration with the traditional planar MOS approach that has worked so well for 30 or 40 years. So it is now pouring billions of dollars into going vertical with 3-D FinFETs to achieve higher density, and more importantly, lower power. It also recently acquired Fulcrum Microsystems, whose main claim to fame was the low power and blazingly fast networking devices it built using a proprietary asynchronous logic based on technology originally developed at Caltech. So, what next? There are quite a few alternatives being proposed: nanoscale logic built from carbon-based Fullerene structures, nanowires, DNA nanoscale structures structured to operate in an on/off form similar to silicon-based logic – and, of course MEMS logic. This last is the alternative that I favour, partly because I have always been attracted to the elegant "no nonsense" simplicity of all of Dr. Feynman's ways of thinking as reflected in his ideas and theories. Working in its favour is that MEMS are already used as a complement to the CMOS logic in many consumer and embedded devices to replace mechanical functions such as gyroscopes and motion sensing in a range of automotive, industrial and mobile communications devices not amendable to scaling. Already in existence are a number of design tools from the likes of Mathworks, Cadence and Coventor to help in the fabrication of MEMS sensors and other miniaturized mechanical components. Using this as a starting point – and possibly attracted as I was by the elegant simplicity of the approach – there are a growing number of researchers looking at the feasibility of VLSI scale MEMS logic. Since the year 2000 virtually every Integrated Solid Circuits Conference (ISSCC) has had one or more papers on ways to build MEMS logic devices. A search of papers on ACM, IEEE and Google Scholar on the concept has turned up some interesting results. In a paper presented at the 2010 ISSCC (pdf) , a team of researchers from MIT, UC Berkeley, and UCLA demonstrated the feasibility of building a wide range of integrated MEM logic circuits, including switches, inverters, carry-generation circuits, flip-flops and latches. They also created a digital-to-analogue converter (DAC) from MEMS logic structures, as well as a 10bit DRAM column composed of MEMS switches. And this year, in a paper in IEEE Transactions on Electron Devices (pdf) , a team of researchers from UC Berkeley analysed the feasibility of building and scaling MEMS switches and relays for use in ultra-low power digital logic. One of their conclusions was that scalable MEMS logic technology could make it possible to achieve a 10-fold increase in energy efficiency as compared to equivalent CMOS VLSI technology for circuits operating at clock frequencies of about 100MHz. While not up to the bleeding edge performance necessary in many PC and mobile designs, this is more than enough to interest many embedded developers. Despite all of the barriers to MEMs logic ever replacing MOS VLSI, think about some of the advantages, even if only as a niche or complementary technology to mainstream semiconductor logic. For one thing, compared with semiconductor-based circuits, MEMS-based logic devices would be virtually immune to most harsh environmental conditions, including the radiation environments in outer space. Second, a MEMS logic structure is absolutely nonvolatile. If the power goes out, there is no need to quickly store valuable information in nonvolatile storage. Power outages would have no impact: the logic state it was in before would be the logic state it would be in after the power is restored. Because of its nonvolatile nature a MEMS-logic based computer would require very little power when compared to existing systems where there is always power required to maintain the logical state of the system. Realistically, I expect that even if VLSI level MEMS-based logic devices were possible, institutional and educational momentum would work against their widespread use. But darn it, I have liked the elegant simplicity of the approach ever since I heard Dr. Feynman talk about it (but that is another story). And nothing I have learned about semiconductor electronics since then has changed that. What do you think?  

Jack Ganssle recently wrote a column that reminded me that this year is the 40th anniversary of the Intel 4004, the four-bit microprocessor that transformed the electronics industry. His column also reminded me of that old saying: "what goes around, comes around"—issues that are facing embedded systems developers and electronics engineers today are more or less the same as the ones embroiling the electronics industry in the early 1970s when I first became aware of the industry's first fully integrated microcomputer on a single integrated circuit. Then, as now, the industry was trying to find ways to take planar metal oxide semiconductor (MOS) technology used to build microprocessors and microcontrollers to the next step, either by finding ways to scale the underlying MOS logic structures to get more density, more performance, and lower power, or shifting to another approach that would allow them to do so. In 1971, about the time the 4004 was introduced, I was a recent graduate of Columbia University and about a year into a job at the California Institute of Technology as associate managing editor of the in-house tech publication, Engineering and Science Magazine. At that time I was working with Caltech Professor Carver Mead (co-author with Lynn Conway of the seminal book " Introduction to VLSI System Design "), helping him with an article for the magazine titled " Computers that put the power where it belongs ."(pdf) The article was about how microprocessors like the 4004 were going to change computing and put computing power in the hands of ordinary people, either directly through what Dr. Mead called 'personal computing devices' or embedded in the existing machines, replacing the bulky inner electromechanical workings. I still have a copy of one of the photographs (below) taken as a possible illustration for the article. In the photo, the 4004 chip that Dr. Mead passed over to me rests in my hand. As I held that tiny (by the standards of that time) chip, I was struck by the potential of the microprocessor revolution. I wanted to learn more, and signed up for every electronics class I could at the institute.   Bernard Cole gives the Intel 4004 a hand. At the same time, I was helping some of the Caltech faculty and grad students in the beginning stages of proposal writing for funding from either the National Aeronautics and Space Administration (NASA) or the Defense Advanced Research Projects Agency (DARPA) on the three technologies that are still front and centre in the electronics industry: globally clocked synchronous logic, clockless asynchronous logic design, and microelectromechanical systems (MEMS). The first two originated in the EE/CS department, while the MEMS effort was the inspiration of some grad students in the physics department. The aim of the first two groups was to find ways to move beyond the medium scale (MSI) and small scale (SSI) integration planar MOS circuits used at that time in most mainframes and minicomputers (as well as many missile and space craft electronics systems) to the next steps: Large Scale (LSI) and Very Large Scale (VLSI) Integration. The two groups were split between the use of either synchronous MOS logic or asynchronous MOS logic. In simplistic terms, synchronous logic is globally clocked such that all transistors on a chip march in lock step and are always on and always consuming power. Asynchronous logic is clockless in the global sense. Instead, it is clocked locally such that transistor logic is turned on – and consuming power—only when needed. It is then turned off locally when it is not needed for switching functions. Alternatively, the MEMS group wanted to take an end run around all of the scaling problems inherent in MOS circuits extended to very large scale integration (VLSI) densities. Inspired by the lectures by Caltech physicist Richard Feynman in the late 1960s, they wanted to build devices for storing information and performing logic functions by means of MEMS relays and switches that could be etched directly in silicon or any of a number of semiconductor substrates using the same fabrication techniques used to build MOS circuits. The argument made by the micromachine proponents was that while MOS VLSI circuit techniques were the fastest way to get to the higher levels of integration that the DoD and NASA needed, both approaches were prone to a host of potential problems as transistor sizes were scaled to smaller dimensions. Synchronous logic would hit the wall first and while the use of asynchronous logic would extend the usefulness of MOS VLSI, it also would run into problems. Unlike a purely mechanical/electromechanical system based on MEMS, their arguments went, software-programmable semiconductor devices are all prone to errors and to problems introduced by the environment: heat, temperature, vibration, shock, dust, humidity, x-rays, and radioactive particles. And as size of transistors are scaled down all sorts of secondary and tertiary problems relating to reliability, power, voltage and noise issues get worse. I left Caltech shortly thereafter to take a job as Silicon Valley Bureau Chief for the now defunct Electronics Magazine . So I lost track of how successful the proposals submitted were or even if they even got to the final submission stage, but in retrospect it is obvious that for the electronics industry in general, the fast track argument won the day.