RLE Currents | December 1988 (8.9Mb PDF)

Volume 2, Number 1 " December 1988 The Research Laboratory of Electronics at the Massachusetts Institute of Technology MATERIALS RESEARCH: Meeting the Challenge of Microelectronics Technology Since the discovery of the transis- tor at Bell Laboratories in 1947, scientists have been challenged by the need to produce materials for highly minia- turized and increasingly fast microelectronic components. Driven by the technology of the information age, scientists have examined the unique properties of semiconductors, funda- mentally exploited new combinations of these novel materials, and devel- oped innovative design techniques to miniaturize electronic components. At the heart of this research is the ever- shrinking and exceedingly complex integrated circuit microchip. In addition, the development of microscaled transistors and other electronic components has resulted in systems with larg- er and faster capabilities, particularly for information processing and high- speed communication. I Every year since 1960, the number of circuit components on the most ad- Professor Henry I. Smith e.p loins the development of an alignment ststemJbr x-ray vanced microchip has nearly doubled. nanolithography that should be capable of 100-angstrom precision. Under his direction. Today, over 4 million transistors can be RLE's Submicron Structures Laboratory has pioneered new technologies in submicron onto one packed neatly computer structuresfabrication and explored deep -submicron MOSPETs and the exciting new memory or DRAM random (dynamic field of quantum-effect electronics (see related article on page 7.). Working with Prqfes- access memory) chip. The dramatic sor Dimitri Antoniadis and graduate student Ghavam Shahidi, dx. have discovered that miniaturization of electronic compo- the deleterious hot- electron effects seen in silicon MOSFETs with channel lengths below nents over the last forty years has raised 0.25 microns actually decrease at linewidths below 0.15 microns. This is apparent/)) questions about the physical limits of because the source-drain transit timesare shorter than electron energy-relaxation time. the materials, devices, and systems in- The quantum-effect electronicsgroup (Professors Dimi/ri Antoniadis,Jesus del Alamo, volved. Such limitations in materials Clifton Ponstad, Marc Kastnei; Leslie Kolodziejski, Patrick Lee, Terry Orlando, and science are fundamental Henry Smith.) has demonstrated several novel quantum transportphenomena which imposed by can occur in sub-0.1-micrometerstructures. It is hoped that their basic research into such quantum phenomena may eventual!)' lead to more advanced computation and (continued on pg. 2) electronic systems. MATERIALS RESEARCI I (continued) Director's Message physical science and the composition, structure, and behavior of the materials used. In the 1960s, the smallest feature size of an electronic circuit was 30 mi- Electronic and optical materi crons. In contrast, today's scientists als form the basis upon which work with electronic struc- structures, devices, circuits, and typically tures one micron and below systems are built. They are the measuring (submicron). By the mid-1990s, scien- starting point for much of RLE's tists anticipate that the smallest features search in electronics and optics, will be approximately 0.1 micron. and increasingly they are engi- Materials research seeks to ad- neered products that are designed dress these limitations through differ- to meet stringent and innovative ent fields of requirements of the smallest and investigation: processing and fabrication of silicon and com- fastest systems. In earlier days, nat- fundamental ural elements like silicon were pound semiconductors; materials pro- doped in hulk to form the starting principles underlying cessing effects; the growth and material for electronic integrated characterization of and me- circuits. But now, much of RLE re- crystalline tallic thin films for microelectronic ap- search focuses on synthetic materi- plications; experiments with novel sili- als, using novel techniques of hand con heterostructure formation gap engineering to provide en- epitaxy; in compound semiconductors; and the hanced carrier mobility and desir- control of thin-film crystalline struc- able optical properties. tures on amorphous substrates. The achievement of high-quality, defect-free electronic materials (continued on pg. 3) is the focus of several projects aimed at the controlled micro- structural evolution of thin films. In this way, device-quality material can be formed on a substrate, and Proj ssor Jonat/)an Allen, Director, grain boundaries in conducting SHORT CIRCUITS Researcb Electronics. wires can be oriented to drastically Laboratory of reduce the effects of electromigra- The staff of currenLc tion. One of RLE's major new em- would like to phases is the epitaxial growth of dope and deposit materials at the note the follow- novel materials and devices involv- submicron scale in the planar di- corrections to ing Ill-V and II-VI heterostructures, mensions, leading to new device ing the issue: using both molecular beam and configurations and techniques for June 1988 chemical beam techniques. These surface defect repair. RLE is also "Faculty Profile," page 11: Dr. Charles expanded experimental programs heavily involved in the study of su- V. "Chuck" Shank of AT&T Bell Labo- support the synthesis of materials perconducting materials, which ratories in Holmdel, NewJersev, was that involve direct control of have much sci- single recently generated mistakenly referred to as Chuck atomic layers as the material is entific excitement. Chang. grown, yielding unat- In all of these areas of experi- previously of at RLE," 16: In tainable structures and dramatical- mental materials, structures, and 'Ilistory Optics page the 1945 photograph depicting Al- ly improved devices. devices, RLE leads the develop- bert G. Hill andJerrold R. Zacharias While epitaxy builds up a ma- ment of new fabrication tech- examining a microwave waveguide, terial in one dimension, high-reso- niques, driven by the needs of Albert McCavour Clogston (left in lution lithography permits submi- electronic and optical systems. En- photograph) was misidentified as cron control of structure and gineered control at atomic dimen- George Briggs Collins. device planar dimensions, provid- sions provides the means to syn- 'UPDA'I'E: ing the world's smallest FET tran- thesize exciting performance that Communications," page sistor. Focused ion beam tech- is breathing new life into high- 20: Author Donald George Baltus niques also furnish the ability to speed miniature systems. was misnamed [)avid. Thanks to our eagle-eyed readers who called our attention to these errors. Principal Research Scientist Dr. John Alelngailis uses focused ion beamsfor patterned depositionfrom ad- sorbed gas molecules andfor patterned implantation or lithography. The unique capabilities ffo- casedion beam implantation have been usedatMIT tofabricate tunable (iunn diodes and to expose re- sistfeatures down to 0.05 micron linewidths. With the apparatus shown in thephotograph, goldfilms have been deposited in linewidths down to 0.1 micron. The focused ion beam column is mounted on an ultrahigh vacuum chamber, and will be usedfor microdeposition ofreactive metals such as tungsten or aluminum. It will also he used to develop tech- Professor G4fton C. Fonstad examines a heterostructure specimen grown by niquesfor x-ray lithography mask repair andfor in- molecular beam epitaxy for use in lacer diode fabrication. He has successfully tegrated circuit restructuring and repair. In addi- grown state-of-the-art multiple quantum well heterostructures in two materi- tion to deposition, both of these applications require als systems; gallium-aluminum-arsenide/gallium arsenide and indium-gal- material removal, or micromilling, with submicro- lium-aluminum-arsentde/indium phosphide. In collaboration with institute meter resolution. Grooves 0. / micron wide and 0.2 Professor Jlerinann A. Haus, he has incorporated heterostructures in wave- micron deep have been milled with ivell-defined, guide devicesfor high-speed, guided wave optical circuits. Professor Fonstad steep sidewalls. The deposition andremoval ofhigh has also demonstrated the growth of strained-layer heterostructures on (iii) atomic number materials; such as or arsenide, and the in gold tungsten gallium was first to observe the internal electricfields these with submicrometer resolution andhigh aspect ra- layers. The growth ofstrained layers on (111)-oriented substrates willfacilitate tio, is essential to x-ray lithography mask repair, highly efficient optical modulators for integrated optical circuits; and will en- while good electrical conductivity is requiredfor cir- hance laser diode performance. cuit repair. An integrated circuit microchip contains the integrated-circuit pattern tronic properties than the bulk silicon. primarily consists of transistors, resis- exposed by electromagnetic radiation Thus, a semiconductor's electrical tors, capacitors, and various intercon- or charged particles. Selected areas on properties can be changed and closely nections finely mapped out and fabri- the photoresist are etched away, leav- controlled using doping techniques. cated on a single semiconductor ing a circuit pattern on the remaining Because silicon's few valence electrons crystal. Microchips are made by slicing photo res ist. are entirely consumed by covalent a cylinder of single-crystal silicon into Next, the silicon is

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