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BEEM takes STM imaging below the surface.

Scanning tunneling microscopy (STM) started a revolution in surface characterization a decade ago that has begun expanding into the subsurface realm with the invention of ballistic electron emission microscopy (BEEM) at the Jet Propulsion Laboratory's (JPL) Center for Space Microelectronics Technology, Pasasena, CA, in 1988. Because of the significance of the technology, BEEM received an R&D 100 Award in 1990.

BEEM was conceived with semiconductor devices in mind. Much of the action here takes place at buried interfaces, all but the shallowest of which lie beyond the reach of STM and other surface microscopies.

The BEEM technique helps overcome this limitation by injecting a highly localized electron tunnel current into a semiconductor device structure with an STM. The injected electrons propagate ballistically (without undergoing scattering or energy loss) for a depht of 100 to 1000 [angstroms].

As the STM tip scans the top of the device, the ballistic electron tunnel current is detected penetrating subsurface interfaces. The transmission and reflection of these electrons provides information about material quality, interface integrity, and device performance.

At least three terminals are required for a BEEM measurement: one third at the tip, a second at the metal layer contact, and a third at the semiconductor substrate. Electrons that successfully pass through the interface are detected as a current between the substrate and the metal layer. In the process of scanning the tunneling tip across the area of interest, a surface topograph and an interface BEEM image are acquired simultaneously.

Although BEEM can be performed in air, ultrahigh vacuum is still often preferred, especially when the objective is to correlate surface and interface features.

How BEEM is Being Used

BEEM is in a stage of development comparable to STM seven or eight years ago. A small research community around the world oversees the refinement of the technique, and its capabilities are still unfolding. Applications to date have ranged from studies of epitaxial multilayer structures to interface lithography.

Wherever BEEM has been used, it has had considerable impact.

The JPL group, for example, has used BEEM as part of a program to improve the quality of AU/GaAs interfaces. BEEM demonstrated the existence of nonuniformities at these interfaces, caused by interdiffusion (see figure). BEEM then showed that as few as two atomic layers of A1As make for a sufficient diffusion barrier.

A group led by Robin Williams at the Univ. of Wales, Cardiff, UK, has similarly mapped nonuniformities at interfaces between metals and II-IV semiconductor compounds, the systems upon which much of our infrared imaging technology is based.

Young Kuk at AT&T Bell Laboratories, Murray Hill, NJ, and a group led by Robert Burhman and John Silcox at Cornell Univ., Ithaca, NY, used BEEM to resolve a debate about whether regions of different crystal structure have different Schottky barrier heights in the [Si/NiSi.sub.2] system.

Such silicide systems are important because of the possibility of growing complicated device structures epitaxially. These structures would have exceptional thermal stability.

Future BEEM Applications

One future application being actively pursued concerns memory devices. BEEM could be used to write/read patterns placed at subsurface interfaces. There certainly are advantages to placing information here, because then it is protected.

A step in that direction has been taken by the Buhrman and Silcox group at Cornell. Without modifying surface topography, they burned the logo of Cornell's nanofabrication facility into a Au/Si interface in 80-nm tall letters using BEEM. They then read the lithographic image (see figure) with the same instrument.

Similar experiments could be performed, such as reading patterns with BEEM that were placed at an interface using other tools like electron-beam lithography.

Another future use could involve SiGe heterostructures. These epitaxial layers are proving crucial to the development of silicon-based ultrafast circuits. One of the most important issues concerning them is elecrical transport, which is precisely the type of information BEEM provides.

Though the applications emphasized here all pertain to the metal/semiconductor interface, it should be recognized that BEEM is a general-purpose technique for injecting low energy beams of electrons or holes into layered structures with high spatial resolution.

So BEEM can as readily be applied to quantum structures (e.g., resonant tunneling devices), metal multilayers, or metal-insulator-metal junctions. With modification, BEEM could probe magnetic or superconducting structures. It could also measure the effect of external perturbation by light, strain, or temperature. Alternative methods of detection such as luminescence are being investigated.

Many new uses will undoubtedly be found when commercial instruments become readilly available. The first such instrument was recently offered by Surface/Interface INC., Mountain View, CA. Their BEEMBox performs BEEM and other STM measurements at temperatures between 77 and 300 K. Surface/Interface now markets a BEEM system that incorporates data handling capabilities from Atomis Inc. (see photo).

Other manufacturers, notably Omnicron Inc., are marketing ultrahigh vacuum STMs with modified software and electronics to help users implement BEEM.

Michael H. Hecht, L. Douglas Bell, and William J. Kaiser developed the BEEM Technique at the Center for Space Microelectronics Technology of the Jet Propulsion Laboratory, Pasadena, CA. Their work was sponsored by ONR and SDIO/IST through an agreeement with NASA.
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Title Annotation:ballistic electron emission microscopy; scanning tunneling microscopy
Author:Hecht, Michael H.; Bell, L. Douglas; Kaiser, William J.
Publication:R & D
Date:Aug 1, 1991
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