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UCRL- 10558

UNIVERSIT Y O F CALIFORNIA

Lawrence. Radiation Laboratory Berkeley, California

Contract NO.. W-7405-eng-48

FILM DEPOSITS AND WHISKER GROWTH IN THE ELECTRON MICROSCOPE

P. W.. Osborne and A. E. Bayce I - - - - . - -- - - . . - . . - - . . I August 1963

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F i l m Deposits and Whisker Growth - in the Electron Microscope

P, W. Osborne* and A. E. Bayce+ I norganic Materials Research ~ i k s i o n , Lawrence Radiation Laboratory

University of California, B e rkeley, California

August 1963

ABSTRACT

Whisker grnwth has bcsn observed on a soda-lime-silica

.. . glass during observation in an electron microscope under a

high intensity beam. A s imi lar growth was a lso noted on a n

electron microscope copper support grid which had been

dipped .in diffusion pump oil. Both the whiskers on the g lass

and the grid had identical electron diffraction patterns. It

m a y be "concluded that the whiskers are a decomposition

product of the diffusion pump oil.

. . r

INTRODUCTION 1 That films deposit on certain materials when these materials ark

subjected to electron and ion bombardment in vacuo was well enough i established by 1934 that R. L. Stewart devoted a paper to the phenome-

nonl. He concluded that the films were carbon compounds, that they j

resulted from the polymerization of organic vapors, and that they cou$l

be an inoidiouo and prtv alenL source of experimental e r ro rs . Later ,

J. H. L. Watson reported on the deposit of such films with specific-

reference to the e r r o r s introduced in the measurement of particle size

with the electron microscope2. Watson suggested that nonconducting

materials may be relatively f ree of film deposits. The following i s a

report on film deposits encountered while investigating the structure

of glass with the electron microscope. The d e p ~ s i t s a r e of interest

- not only because they a r e a potential source of trouble in electron

microscope studies of glasses but also because they can, under ce r -

tain conditions, manife st themselves a s whiskers . This whisker

growth phe.nomena occurs under almost ideal conditions for observa-

tion.

ENVIRONMENTAL CONDITIONS FOR WHLSKER GROWTH

The whisker growth phenomenon was encountered on both a

Siemans Elmiskop-I and an Hitachi HU - 11 electron microscope.

Of several glasses examined, whisker growth was. noted only ,on the

soda-lime- silica glass previously described by Levingood and Vong

in connection with i t s interesting spiral defects3. Whisker growth

was also noted to occur, in the absence of any glass, on a grid that

had been dipped in diffusion pump oil.

..'I

The electron beam current was critical in encouraging whisker : ':.

F growth. On the Siemans microscope, with the beam sharply focused, ' 2

- . I

a current of two to six microamperes was most favorable for whisker :,$'

d growth, F o r the Hitachi microscope the beam current ranged f rom *:,

K 50 to 70 rnicroarnperes with the beam moderately sharply focused. $+

?

h his discrepancy in beam current i s not a s great as it seems if the

over-all performanee of the microscopes i s considered.) F o r both

microscopes 100 KV electrons. were used and the larges t condenser

aperture was in place.

OBSERVATIONS

Pertinent observations were as follows :

la) Whisker growth was most pronounced near protuberances.

Spikes of glass (see Pig. 1) a r e not necessary, but they do

intensify and dramatize the effect.

(b) The whisker growth was influenced by the electron beam

current and the beam focus, Extension of the whiskers

could be halted by either lowering o r raising the beam

current.

(c) The maximum growth ra te observed was about 50 g/sec.

(on the HU - 11).

(d) The whiskers, a t their tips during growth, were a s smal l

0 a s 20 A across. This i s a limiting and possibly a typical

dimension, The conditions of observation did not always

permit this low limit of resolution. )

(e) The whiskers were more resistant to high beam currents

than the glass itself. Spikes of the glass softened a t . ,

beam currents (on t6e Siemens) of about 10 microampere s j

while matured whiakers were stable under beam currents .

of 25 microamperes.

(f) Anexampleofwhisker growthis showninFig. 1, a n d a 1

more detailed sequence of a single whisker is shown in

Fig. 2. As shown, the growth normally occurs in stages. :1

The whiskers grow out up to a micron in length, tapering

to a point. ' Extension then ceases while the whiskers

thicken and the tips round off.' Then new growth is nucle-

ated on the old whisker and the cycle i s repeated. As

many as four such cycles of growth have been observed. \

(g) Whiskers avoid each other by ceasing to extend o r !by

curving (Fig. 3).

(h) Selected a r e a diffraction patterns were made of the

whiskers and of the glass. The whiskers yielded two

diffuse rings typical of noncrystalline materials. These

rings, of approximately equal intensity, indicated dis- ' 0

tances of 1.'17 and 2.03 A. The glass yielded a three

ring pattern (strong, weak, very weak) indicative of 0

distances of 2.90, 1.16, and 1.55 A, respectively, A

reasonable estimate of the accuracy of these measure-

ments may be & 5%.

(i) Vacuum pump oil, when diffracted, yielded a diffraction

pattern identical to that of the whiskers.

DLSCUSSION .

Composition of the whiskers: F rom the diffraction data i t can

be concluded that the whiskers a r e a decomposition product of the '..

. . diffusion pump oil.

. . . .

Mechanism of growth: The whisker growth sequence of Fig. 2 i,

shows that growth is not caused by extension a t the base, since the

fork of a whisker does not move out f rom the base with growth. Two . other possibilities a r e that the whiskers grow by vapor condensation

o r by surface diffusion. Growth of whiskers by condensation of

vapors is frequently associated 'with dislocation mechanisms in

crystalline materials. As the whiskers a r e as small as 20 2 ac ros s

at the tip it is difficult to clearly distinguish between the glassy and

crystalline state. However, the gradual curvature of some whiskers,

their frequent random forking a t the tips, the general lack of angu-

larity, and the diffuse nature of. t3ie diffraction pattern all suggest a

noncrystalline growth mechanism.

A stalactitic type of growth would appear to be compatible with

the observed whisker behavior, ' When the film i s f i r s t deposited on

the specimen i t should retain some of the fluid properties of the

diffusion pump oil. The driving force for whisker growth; hh ich is

presumably the mutual repulsion of positive ions of the film and their

attraction to the electron beam, would force the film to the edges of

the specimen. At the edge, local charge concentrations could nucleate

whiskers. Growth of whiskers would then be dependent on the volati-

lization of one of the constituents of the viscous film or the poly-

merization of the constitueuts so as to leave a strong skeletal structure.

The growth of -whiskers in stages indicates that a certain charge

concentration i s necessary a t the tip in order to string out molecules.

If this charge concentration i s not maintained, the surface tension of

the film becomes dominant, the tip rounds off, and'extension ceases.

With'inevitable fluctuations in the beam current of the microscope '.

such behavior would explain growth in stages. , . . ,. ,, .

General remarks: The beam current a t which spikes of the ,

glass softened (about 10 .microamperes) and the softening tempera- 3:

ture of the glass in a furnace (roughly 8 0 0 ~ ~ ) place the temperature

of the specimen in the 200 to 6 0 0 ~ ~ range during whisker growth.

Whisker growth was not noted on thin films of glass (obtained by

blowing a bubble) but this may merely indicate that thin films a r e

at a different temperature than the thicker chips 'when exposed to

the same beam currents. That whisker growth was noted on only

one type of ,glass chip may indicate an affinity of the film deposits . --

for this glass. Such an affinity of the film deposits for carbon

black was noted i n footnote 2 .

All glasses examined (fused silica, Pyrex, sodium disilicate

glass, ruby glass a r e a few) yielded a diffusion ring indicative of a

distance of about 1.2 R. While this i s undoubtedly a result of a

, common &m,ension. .in the structure of these glasses, it is a lso a

distance characteristic of the decomposition product of the diffusion,

pump oil.

CONCLUSIONS

(1) The material of the whiskers was identified as a decomposition

product of the diffusion pump oil.

. ( 2 ) A stalactitic mechanism of whisker growth i s consistent with

the observed behavior.

ACKNOWLEDGMENTS 4

This work was supported by the Inorganic Materials Research .

Division of the Lawrence Radiation Laboratory, U, S. Atomic

Energy Commission. The authors wish to express their apprecia-

tion to Professor E a r l R. Parker of the Department of Mineral

Technology, University of California, for his encouragement in

thio worlc,

FOOTNOTES

*Present Address : N. S, F, Fellow, Max-Planck-Institut fuer MetaU-

forschung, Stuttgart, Germany.

+Present Address: Metallurgy Section, Stanford Research Institute,

, Menlo Park, California,

1; K. L.' Steward, phys: Rev. - 4,488 (1934).

'.

2 J, H.: L.' Watson, J.' Appl. Phys. - 18, 153 (1947).

3W. C . Levengood and T. S. Vong, J, Appl. Phys. - 31,1416 (1960)~'

Fig. 1. Spike of glass (A) with subsequent whisker growth (B-D). (A) 10,000 X, (33 and C ) 12,000 X, (D) 50,000 X.

Fig. 2. Growth sequence of a single whisker. The growth rate was relatively slow, about two hours being required for the sequence. (A-F) 20,000 X, (G and H) 40,000 X,

UCRL- 10558

Fig. 3. Detail showing how whiskers curve to avoid contact. 120,000 X.

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