Low energy electron diffraction method of identifying atomically clean surface after laser surface interactio

Smith, H. P., Jr.

English

NASA Technical Reports Server

Space Sciences Labora tory  
University of California 
Berkeley, California 94720 
Semi-Annual Report  on 
LASER SURFACE INTERACTIONS: 
CREATION AND DETECTION O F  ATOMICALLY CLEAN SURFACES 
Support e d b y 
NASA Grant  
NGR-05-003 -161 
f o r  the period 
October 1, 1966 through March 31, 1967 
Principal  Investigator: P ro fes so r  Harold P. Smith,  Jr. 
Space Sciences Laboratory Se r i e s  No.  8, I ssue  No.  20 
March 31, 1967 
1 
https://ntrs.nasa.gov/search.jsp?R=19670017002 2020-03-12T11:24:02+00:00Z
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LASER SURFACE INTERACTIONS: 
CREATION AND DETECTION OF ATOMICALLY CLEAN SURFACES 
NASA Grant 
NGR 05-003-161 
INTRODUCTION 
Advancing technology and increased in te res t s  in a r e a s  of surface 
physics, such as the behavior of surfaces  exposed to the hard vacuum 
of the space environment, thin film technology, and surface catalysis 
have created a need for  the development of a convenient and reliable 
technique f o r  producing atomically clean surfaces.  
have included high-temperature bulk annealing, sputter cleaning, 
vacuum cleaving,. and chemical cleaning, to mention the most  common. 
Each  technique has  shortcomings o r  disadvantages of inconvenience o r  
inapplicability to  the surface being cleaned. 
Previous techniques 
As a resul t ,  the NASA Lewis Research  Center,  through the 
University of California Space Sciences Laboratory,  has sponsored a 
r e s e a r c h  program beginning in Fall, 1966, t o  investigate the applicability 
of l a s e r  -surface bombardment for the production of atomically clean 
surfaces.  - 
localized portion of the mater ia l  surface, essentially resulting in ' 
the thermal  evaporation of only surface l aye r s  of mater ia l .  
nes s  of these surface layers can of course be controlled by the intensity 
of the l a s e r  pulse. 
simplicity of the equipment (a l a se r  external to  the vacuum sys tem that 
has, at most ,  an additional port for  the l a s e r )  and universal  applica- 
bility to  all mater ia l s .  
The l a s e r  cleaning mechanism is intense heating of a 
The thick- 
The advantages of this  cleaning technique a r e  the 
This document is  submitted as a repor t  of progress  in this  
investigation for  the period f r o m  October 1966 through March  1967. 
SURFACE CLEANING AND DETECTION 
In the above discussion of the potential of l a s e r  cleaning, no 
mention has  been made  of the method of identifying a clean surface.  
Two methods will be used fo r  this purpose. One is the use of LEED 
I .  
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\ 
1 
(Low Energy Electron Diffraction) by which the diffraction of low- 
energy electrons can identify an  atomically-ordered surface;  the other 
method is by high-energy (- 100  keV) proton-induced character is t ic  
X-rays of the surface contaminants. 
The Laser -Leed  experimental arrangement is shown schemati-  
cally in Figure 1 .  The target  to be cleaned is aligned perpendicular 
to the axis of the l a s e r  and then pulsed by the Q-switched l a s e r  with 
pulses that range in peak power density f r o m  20 to 200 megawatts /cm2 
and have total energy densities of 2 j ou le s / cm2 .  
ment  the ta rge t  is annealed and is rotated back into alignment with the 
electron gun, and the diffraction pattern is observed. 
and intensity of the diffraction pattern it is possible to identify the type 
of surface s t ructure  o r  i t s  contamination. 
After l a s e r  bombard- 
F r o m  the type 
During the previous six-month period, the LEED device was 
received f r o m  the manufacturer and installed in our laboratory.  
Following the initial checkout a n  aluminum target  was prepared and 
installed in  the LEED. 
the surface to  give the expected aluminum diffraction pattern. 
observe the effect of a l a se r  pulse in disordering the surface,  
1 . 2  joules/cm” , 30 megawatts /cma pulses caused no degradation of 
the pattern, but did produce sizable burs t s  (from 10-l’ T o r r  to l o - *  
T o r r ) .  
f r o m  the target  and the vacuum chamber by reflected l a s e r  light. 
Annealing at 600’ C for 40 hours  conditioned 
To 
These pressure  bursts  presumably come f r o m  gases  desorbed 
Focusing the l a s e r  pulse to  higher energy densit ies caused a de-  
gradation of the diffraction pattern that was readily removed by annealing 
at 600’ C for  10 hours. 
50 megawatts /cm2 there  is visual damage caused to  the surface. 
at this  extreme the diffraction pattern was partially recovered by 
annealing. 
For energy densities above approximately 
Even 
Having thus established the capability of the l a s e r  to disorder  
the surface by heating and evaporation, and also having established the 
energy and power densities neccessary  to achieve this ,  it is expected that 
during the next six months attempts will be made  to  c rea te  atomically 
clean surfaces  for  LEED. This will entail the preparation of a nickel 
surface by conventional methods to  identify i t s  s t ructure  and then 
the intentional contamination of this surface by oxygen. The 
cleaning potential of the l a se r  w i l l  then be investigated by attempting 
to  desorb  the contaminating oxygen. Following these experiments ,  an 
unprepared nickel su r f ace  will be  laser-bombarded to c rea t e  an 
atomically clean surface.  
PARTICIPATING PERSONNEL: 
P ro fes so r  Harold P. Smith, J r .  - Principal  Investigator; 
P ro fes so r  N .  Thomas Olson; Dr .  Jhan M. Khan; Dr.  Ferdinand 
Hofmann; Salah Bedair and Giulio V a r s i  - Research  Assistants.  
. 
FLUORESCENT SCREEN 
Figure  1 .  Schematic showing arrangement by which 
LEED targets are bombarded by laser 
p u l s e s .  


Laser surface interactions - Creation and detection of atomically clean surfaces Semiannual report, Oct. 1, 1966 - Mar. 31, 1967

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