Microscope Technology & News1

The most sophisticated members of the integratedcircuit family are microprocessors that serve as thecentral processing units of modern calculators andcomputers. Microprocessors have found their wayinto a myriad of everyday appliances such as radio,television, refrigerators, stoves, microwaves ovens,automobiles, motorcycles, electronic watches,cameras, bicycle, odometers, and home securitysystems. In the scientific arena, microprocessors arenow found in such simple instruments as pH meters,thermometers, balances, pipettes, and even the simplelight microscope.

Michael W. Davidson Institute of Molecular Biophysics and

Center for Materials Research and Technology, The Florida State University, Tallahassee, Florida 32306

Keywords: Photomicrography, reflected differential interference contrast microscopy, integrated circuits, and microprocessors.

The complex electronics involved with transmis-sion and scanning electron microscopy led, early on,to the widespread use of microprocessors in theseinstruments. Only recently, however, has the micro-processor been utilized as an aid to light microscopy.Many modern light microscopes are equipped within-camera light detection systems that feed illumina-tion intensity information to a microprocessor that, inturn, calculates exposure times. As time goes on, themicroprocessor will find increasingly more complextasks to perform in light microscopy. Nikon hasrecently introduced the Microphot-FXA/L light

erhaps no single technology has produced such a widespread impact on

science as the current revolutionary developments occurring in integrated circuit

design and fabrication. Integrated circuits are now found in virtually every piece of

scientific equipment on the market and new upgrades of existing circuitry are being

released at an amazing rate.

P

INTEGRATED CIRCUITS IN AND UNDER THE MICROSCOPE

Microscope Technology & News2

microscope that is almost entirely controlled by a 16-bit microprocessor. Variables such as light sourcevoltage, film speed, number of exposures, andautomatic bracketing are computer controlled in theFXA/L. The user communicates with the microscopethrough a miniature keyboard complete with aneasily discernable liquid crystal display. Themicroscope can even be interfaced to a printer via aparallel port on the back of the instrument or to amodem or external computer with an optional RS-232C connector. With this setup the microscopist canimport a specific set of values to control any or allfour of the microscope’s optical ports. With theappropriate software, the microscope can be totallymanaged by the external computer. The expectedintroduction of an autofocus system coupled to anautomated objective rotation nosepiece for the FXA/L should result in a microscope capable of makingcorrectly exposed photomicrographs without directhuman intervention. Even so, this is not the ultimatein computer-assisted light microscopy. Compleximage analysis methods can digitize optical informa-tion for analysis with sophisticated computer systemsand the recent escalation in scanning confocalmicroscopy research will help to permanently embedcomputers into light microscopy systems.

FABRICATION OF INTEGRATED CIRCUITS

Visible light microscopy plays a paramount roleduring the manufacturing process of integratedcircuits. Wafers of pure silicon by a process thatresembles dipping candles. These thin (one-tenth ofa millimeter in thickness) wafers are polished andsterilized at high temperature before undergoing thecomplex optical lithographic process of masking,doping, and etching that can exceed 30 individualsteps and take more than two months to complete.All processes are conducted in “clean rooms” that arescrupulously sterilized to remove all foreign particlesand have filtered air to insure against contamination.A single contaminating dust speck during any stageof the complicated manufacturing process can ruinan integrated circuit.

Hundreds of circuits can be fabricated on a single6-inch diameter silicon wafer. The smaller the circuit

design, the more circuits can be packed onto a wafer.A single wafer, for instance, can be used to fabricateseveral hundred tiny binary counter circuits whereasthe same wafer can only accommodate 50 or so80286 microprocessors that are about 1 cm2. Circuittesting is conducted throughout the manufacturingprocess. Wafers are examined with reflected lightmicroscopes for analysis of gross morphologicalfeatures and various portions of the wafer are probedwith detectors to determine overall circuit integrity.Often, test circuits are fabricated directly on thewafer to aid in component examination. Typically asmany as 70% of the circuits on a particular waferbecome defective before completion of the manufac-turing process.

PHOTOMICROGRAPHY OFINTEGRATED CIRCUITS

One need not be employed by the semiconductorindustry to be able to photograph integrated circuitsthrough the microscope. Complete wafers aredifficult to obtain, however discarded computer partsare an excellent source of integrated circuits. Thesesilicon “chips” are cut from wafers with a diamond-tipped saw and are generally packaged by eithermolding into an epoxy resin case or cemented into aceramic case. It is virtually impossible to recover anintegrated circuit from an epoxy resin case. Duringthe manufacturing process, the resin flows onto thesurface of the chip and penetrates into the etchedmicrostructure. When the epoxy cases are broken orcracked open, the silicon fractures through the centerof the integrated circuit and all surface detail is lost.The cement in a ceramic case can, however, be scoredwith a hacksaw and split with a fine chisel to revealthe internal chip. Most programmable read-onlymemory (PROM), microprocessors, mathcoprocessors, and some random access memory(RAM) integrated circuits are protected with ceramiccases. In addition, many analog and binary circuitsthat perform simple functions are packaged inceramic cases. In addition, many analog and binarycircuits that perform simple functions are packagedin ceramic cases. To identify an integrated circuit,take note of the markings on the case before opening.

Microscope Technology & News3

These markings usually consist of a manufacturedate, the manufacture, and a symbol or number codethat identifies the circuit. In most instances, thisinformation is also etched onto the surface of thechip itself and can be revealed by examination with areflected light microscope.

Examination of integrated circuits with reflectedlight can serve many purposes. For instance, detailsof a particular circuit structure such as register areas,data busses, memory storage, logic units, and evenindividual transistors can be identified and exam-ined. By observing differences in the architecture ofvarious integrated circuits, one can begin to get ahandle on the complex electronics involved inmodern devices. The figure illustrates a NipponElectric Company (NEC) copy of the famous Intel8080 microprocessor introduced in 1974 as the firststand-alone microprocessor. The introduction of thisrevolutionary microprocessor was a key factor in theearly growth of a fledgling personal computerindustry and it’s application in the MITS Altair 8800computer was introduced on the July 1975 cover ofPopular Electronics.

Reflected light microscopy is the method ofchoice for examining integrated circuits. Circuitscan be visualized using brightfield, darkfield, anddifferential interference contrast, and fluorescencemicroscopy. Usually, more detail is apparent whenthe microscope is operated in the differential inter-ference contrast (DIC) mode. By inserting a retarda-tion plate between the polarizer and sample, a colorspectrum is generated in DIC that allows for a greaterdifferentiation of the various details on the surface ofintegrated circuits. This method also produces themost colorful photomicrographs.

A stereo microscope also can be very useful inthe examination of surface detail on integratedcircuits. This is especially true of the larger micro-processors that exceed 1 cm2 in area. By addingcolor filtration to the light sources for stereo re-flected light microscopy, one may acquire coloredhighlighting effects that enhance the appearance ofintegrated circuits in photomicrographs. It isinteresting to experiment with several light sourceseach with a different color filter. Quite a number ofspecial highlighting effects can be obtained thismanner.

HISTORY AND FUTURE OFTHE MICROPROCESSOR

As I have described above, the microprocessorhas had a major impact on science and industry andtouches the lives of almost every individual indeveloped nations. The most prominent micropro-cessors in use are probably the 8000 series firstintroduced by Intel in 1972. The development ofmicroprocessors has been a blend of increasing buswidth and register size coupled to a steady reductionin component size. The apparent limit is dictated bythe minimum allowable channel length of the metal-oxide-silicon field-effect transistor (MOSFET) andalso limitations on transistor packing efficiency. Ascomponents grow smaller and more tightly packed,total overall chip size increases to accommodatelarger registers and additional busses. Intel hasspeculated that by the year 2000 they will havedeveloped a microprocessor with 100 milliontransistors, operating at 250 MHz with an executionrate of 2000 million instructions per second (MIPS).This superchip is expected to operate 80 times fasterthan the current highest-speed i486.

The following list contains a short description ofthe most prominent members of the Intel 8000microprocessor series.

8008. This was the first microprocessor with an8-bit data bus. It was an update of the original 4004,Intel’s first microprocessor developed in 1971 for aJapanese calculator firm. The 8008 was useful in theearliest attempts at building personal computers.

8080. This was the first 8-bit microprocessordesigned for general use as a stand-alone centralprocessing unit (see the figure on page 5). It wasintroduced in 1974 and contains 4,500 transistors. Atan execution rate of less than 1 MIPS, this micropro-cessor found a limited use in the development ofpersonal computers.

8086. Introduced in 1978, this 16-bit micropro-cessor was too advanced for the industry standard 8-bit technology of the day. It’s 20-bit address bus

Microscope Technology & News4

allowed the chip to address 1 megabyte of memory incontrast to the 64-kilobyte limit of the 8080.

8087. A backward step in technology, this chipwas introduced in 1979 to compete as the processorfor the emerging IBM line of personal computers.Like the 8086, the 8088 execute commands internallywith a 16-bit data bus, however it communicates withperipherals with an 8-bit external data bus. Thismicroprocessor, which operates at a clock frequencyof 4.77 MHz, became the brains behind the IBM PCand XT series of personal computers.

80286. This 16-bit microprocessor first appearedin the IBM AT series of personal computers. Itfeatures increased speed over it’s predecessors andadded a new feature called protected mode, whichallows advanced memory management techniques.The increased address bus size allows this chip toaddress 16 megabytes of memory. Initial clockfrequencies were 8 MHz, with later versions featur-ing increased frequencies of 12 and 16 MHz.

80386. First offered as the microprocessorcontrolling the Compaq 386 in 1985, this 32-bit chipcan address 4 gigabytes of physical memory and upto 16 terabytes of virtual memory. This amazingmicroprocessor is also capable of operating in avirtual 8086 mode that allows the microprocessor tosimulate an almost unlimited number of 8086processors running independently, but simulta-neously. The original version operated at 16 MHz.,but later versions are capable of operating at clockspeeds as high as 33 MHz. This microprocessormaintains downward compatibility with previousIntel microprocessors so that a program written foran 8088, for example, can easily be executed with the80386.

80386SX. Like the 8088, this microprocessorwas a backward step introduced by Intel in 1988 as alow-cost alternative to the 80386. In a sense, thismicroprocessor is related to the 80386 in the samemanner as the 8088 are to the 8086 with a reducedexternal data bus size. The 80386SX execute 32-bitcommands internally but communicate with externaldevices through a 16-bit bus. This allows the80386SX to take advantage of cheaper 16-bit AThardware while maintaining compatibility and thememory management capabilities of its more ad-vanced cousin.

80486. Intel’s latest addition to the 8000 series isthe 80486-also called the i486-introduced in 1989.This microprocessor is essentially an 80386 with

several support peripherals built on-board. The chipcontains, in addition to a complete 80386 micropro-cessor, an 80387-math coprocessor and an 8Krandom-access memory cache for almost explosivespeed.

80586 & 80686. The 80586 are currently underdevelopment by Intel while the 80686 are still on thedrawing board. Except the release of these chipswithin the next several years.

As the microprocessor signals an “informationrevolution” in science and society, one is left onlywith speculation as to the limits of integrated circuittechnology. However, it is for certain that micros-copy and advanced computer systems will walk,hand-in-hand, into the future.

ACKNOWLEDGEMENTS

The author would like to thank the FSU Centerfor Material Research and Technology (MARTECH)for financial support and the Nikon Instrument Groupfor providing equipment and technical support.

Microscope Technology & News5

THE 8080 MICROPROCESSORThis 8-bit data engine was introduced by Intel in 1974 as the first stand-alone microprocessor for per-

sonal computers. Data busses and registers can load and execute byte-sized commands in the processors’binary logic units. The 16-bit address bus allows this chip to address an unprecedented-at the time-64 K ofmemory. The photomicrograph was taken with a Nikon Prototype 2.5x DIC objective.