IBM Scientists Build New Atom Imaging Microscope---_----_ _Tom Murphy, International Business Machine Corporation

IBM scientists in Zurich, Switzer­land, have made a new ScanningTunneling Microscope with a scanningassembly small enough to be held inone hand. Because it is so small, thisinnovative new instrument can beused with other microscopes to "zoomin" on atomic surface structures andmake images of them.

This is the latest development in ascientific technique-scanning tunnel­ing microscopy-invented and firstused in 1981 by IBM at its ResearchLaboratory in Zurich. A tunneling mi­croscope can show how individualatoms are arranged on a wide varietyof surfaces.

In future technologies, objects mayhave dimensions only a few hundredor dozens of atoms wide. As struc­tures approach such dimensions, theirsurfaces become increasingly criticalbecause the surface area becomesproportionally larger compared to thebulk. This means that surface struc­ture could determine conductivity orother properties of these structures.

A solid's surface differs from thebulk because atoms under the sur­face-where they are surrounded byother atoms on all sides-arrange

Figure 1. The new small scanning assem­bly of the Scanning Tunneling Microscopeis able to fit in a person's hand, yet so­phisticated enough to enable scientists tosee individual atoms on the surface ofmater ials .

themselves in the most stable posi­tion with regard to one another. Onthe other hand, when a surface forms,the "exposed" atoms must rearrangethemselves in a new configuration thatmaximizes their stability.

Such atomic arrangements and howthey form is what surface scientistsstudy. This understanding is valuableto many technologically important areassuch as catalysis, chemical reactions,crystal growth, and the performanceof many electronic components.

The scanning assembly of the origi­nal Scanning Tunneling Microscopewas much larger. However, both in­struments are based on the samephysical principle and use similarelectronics.

Tunneling, which involves the pas­sage of electrons between two materi­als that are narrowly separated by anonconducting area such as an insula­tor or vacuum, is an aspect of thetheory of quantum mechanics. Inquantum mechanics, electrons behavelike "clouds" that spill out slightlybeyond the surfaces of the materialsin which they originate. If two materi­als are brought so close that theirelectron clouds Intermingle, and if avoltage is the applied across them, acurrent-the movement of electrons­will occur between the two separatedmaterials. This "tunneling" phenome­non is the key to the operation of theScanning Tunneling Microscope.

MICROSCOPE OPERATION

In operation, the Scanning Tunnel­ing Microscope takes advantage of thefact that this tunneling current changesdrastically as the distance between thetwo materials increases or decreases.

Isolated from all vibration by aunique double-damping method, theScanning Tunneling Microscope movesa finely pointed probe to within tenangstroms of the surface. (A typicalatom is about three angstroms in

diameter.) At these small distances, a. change in tip-to-surface distance bythe diameter of a single atom pro­duces a tunnel-current change of onethousand times.

If the tip's vertical position were notchanged as it is moved laterallyacross the surface, the current-wouldfluctuate in direct correspondence tothe ups and downs of the surfaceatoms. Instead, the needle is movedup and down so that the current re­mains constant.

The probe scans a two-dimensionalcross-section of the surface atoms.Many parallel scans are combined toproduce a three-dimensional picture ofthe surface. The atomic images ap­pear as "bumps" or "hills" that pro­trude from the surface.

These vertical and lateral move­ments are accomplished using threepiezoelectric elements that can posi­tion the tip with an accuracy less thanthe diameter of a single atom. (A piezo­electric substance contracts or ex­pands when electricity is applied.)

The Scanning Tunneling Microscopecan achieve a maximum vertical reso­lution or better than 0.1 angstrom (1angstrom = 2.5 billionths of an inch)and a horizontal resolution better thantwo anqstroms-cqood enough to re­solve the electron cloud that formsthe outer boundary of each atom. Noother microscope has ever achievedthis combination of vertical and hori­zontal resolution.

For the first time, the actual surfaceatomic structure has been glimpsed,directly and in "real" space-as it ac­tually exists at the surface.

IBM scientists believe that the newsmaller device will permit very ad­vanced research into the nature ofthin films, the atomic surface struc­ture of materials such as silicon andgallium arsenide and of junctures be­tween materials that make up semi­conductor circuits.

replacing many primary and second­ary metal structures, including nosecones, wing components, engine partsand interior construction.

The BCC survey found that the ma­jor aircrafUaerospace markets for ad­vanced composites are fighters wherecomposite usage will double in fiveyears, rocket motor cases for missiles,pressure vess~ls, secondary structuresin civilian aircraft and in helicopters.All these applications should increasethe use of composites by one-third to

and resins which are processed tofinished product by methodology.

Of the 11.5 million pounds con­sumed worldwide (80% in ' the U.S.),80% is accounted for by the aircraft!aerospace industry, while the remain­ing 20% goes to sporting goods, in­dustrial equipment, marine and printedcircuit boards.

Thanks to a high strength-to-weightratio, the current center of advancedcomposite activity is the aircraft/aero­space sector where the materials are

15% Growth Rate Projected for Advanced Composites _According to "Advanced Compo­

sites: An Evaluation of CommercialProspects," a recent report by theBusiness Communications Company(BCC) of Stamford, Connecticut, themarket value of finished advancedcomposite parts has reached $1.1 bil­lion worldwide and should continue togrow at an annual rate of 15% overthe next ten years. Often used to re­place metals in certain applications,advanced composites consist of care­fully engineered laminates of fibers

56 JOURNAL OF METALS· December 1985