Figure 7.

LIST OF ILLUSTRATIONS — (Cont'd)

Three-dimensional surface potential map of capacitors

Figure 8. ASLEEP micrograph showing dislocation pipes in silicon wafer as denoted by arrows

Figure 9.

X-ray topograph showing Lomer-Cattrell dislocation networks

Figure 11.

Energy band diagram for silicon showing the position of the Fermi level,
the intrinsic Fermi level, the band gap, electron affinity, and surface
potential

(A) Expected work function values from Fermi level shifts
(B) Expected work function values after Allen and Gobeli [5].
(C) Measured work function values

Figure 13. Optical photomicrograph of integrated circuit utilized in the
ASLEEP experiment

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13

15

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Figure 14. Micrographs of integrated circuit showing specific area analyzed (A) Optical photomicrograph of test circuit

(B) ASLEEP image of test circuit

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Figure 15.

ASLEEP image and Auger spectra of indium phosphide surface after an
acetone rinse

ASLEEP image and Auger electron spectra of indium phosphide
annealed at 330°C for 15 min.

25

Figure 18.

ASLEEP image of indium phosphide prior to oxidation treatment ...... 26

Figure 19.

ASLEEP image of indium phosphide after oxidation

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Figure 20.

A comparison of ASLEEP image, SEM image, and scanning Auger
images of gallium arsenide contamination ..

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Figure 21.

ASLEEP image of gallium arsenide wafer showing the effect of two
different etch treatments . . .

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PREFACE

This study was carried out at the Naval Research Laboratory as a part of the Semiconductor Technology Program in the Electronic Technology Division at the National Bureau of Standards. The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability, and reliability of discrete semiconductor devices and integrated circuits through improvements in measurement technology for use in specifying materials and devices in national and international commerce and for use by industry in controlling device fabrication processes. The work was supported by the Defense Advanced Research Projects Agency* through the National Bureau of Standards' Semiconductor Technology Program, NBS Order Nos. 501718 and 711782. The contract was monitored by R. L. Raybold as the Contracting Officer's Technical Representative (COTR). Drs. W. M. Bullis and K. F. Galloway provided technical review of this report for the National Bureau of Standards.

Certain commercial equipment, instruments, or materials are identified in this report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

Larger scale drawings of the mechanical parts and detailed listings of the computer programs and subroutines are available on request from the COTR, TECH-A-361, National Bureau of Standards, Washington, DC 20234.

*Through ARPA Order 2397.

AUTOMATED SCANNING LOW-ENERGY ELECTRON PROBE
(ASLEEP) FOR SEMICONDUCTOR WAFER DIAGNOSTICS

A. Christou

Naval Research Laboratory
Washington, D.C. 20375

This report summarizes the results of a three-year effort in the develop-
ment of a computer Automated Scanning Low-Energy Electron Probe
(ASLEEP) for semiconductor wafer diagnostics. Experiments designed
to explore the measurement capabilities of ASLEEP for measurements
on silicon, gallium arsenide, and indium phosphide are described. Four
areas were emphasized: 1) semiconductor resistivity, 2) semiconductor
defect density, 3) lateral inhomogeneities, and 4) oxide uniformity.
Although oxide charging problems limit the utility of ASLEEP in its
present form for silicon, defects and lateral inhomogeneities could be
readily detected in gallium arsenide and indium phosphide.

Key words: Automated scanning low-energy electron probe; lateral
inhomogeneity; oxide uniformity; scanning low-energy electron probe;
semiconductor defect density; semiconductor resistivity; semiconductor
wafer diagnostics.

The scanning low-energy electron probe (SLEEP) is an electron beam probing technique whereby an electron beam is first accelerated (to provide beam definition) and then decelerated by a grid placed in front of the sample to be probed [1]. The sample under investigation is scanned by the electron beam in the retarding field region. Only those electrons whose energies are sufficient to overcome the local sample potential barriers are collected. This collected current is measured in the sample-cathode circuit. Similarly, electrons with insufficient energy to overcome sample surface potential are reflected from the sample and may be collected to form the mirror mode operation. Thus, either the directly collected current or reflected current provides a surface potential map of the sample. In the work reported here, only the collected current mode was utilized.

The SLEEP technique is inherently simple. The basic apparatus consists of a low-energy gun structure (800 to 900 V) and a standard vidicon electromagnetic beam focusing and deflection system. Depending on the type of data, precision, accuracy and detail required, control and collection can be carried out by manual techniques or by minicomputer. Similarly, depending on the sophistication of the measurement required, vacuum equipment may range from the rudimentary (10-6 torr) to the more advanced oil-free ultra-high vacuum (10-11 torr).

In the present investigation a Data General Nova 800 computer has been interfaced to the scanning low-energy electron probe to control data collection and signal processing, thus forming an Automated Scanning Low-Energy Electron Probe (ASLEEP) system. An ultrahigh vacuum system was also utilized in order to provide for a contamination free system. Figure 1 shows the vacuum system for the ASLEEP experiments while figure 2 shows the computer and interface electronics. The ASLEEP experiments have been designed to explore

oil-free pump station.