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ION BEAM PROCESSING OF OPTICAL MATERIALS

F. L. Williams, L. L. Boyer, 1I. W. Reicher, J. J. McNallyt, G. A. Al-JumailytT, and J. R. McNeil University of New Mexico Department of Electrical and Computer Engineering Center for High Technology Materials Albuquerque, NM 87131

ABSTRACT We have deposited thin films of optical materials using ion beam sputtering and ion assisted deposition techniques.

It is possible to obtain good quality film material

deposited on substrates at temperatures lower than normally required.

Ion assisted

deposition influences film stoichiometry and packing density, which in turn determine optical and mechanical properties of the film material. We discuss two general indicators which appear helpful in predicting the degree to which these occur.

1.0

Introduction The technique of processing optical and electronic materials using low energy ion

beams (Ei1 implies that oxygen atoms are

more readily sputtered from the surface than are metal atoms, so that ion bombardment results in a reduced metal oxide material. This process is known as preferential sputtering,18 and is most likely responsible for the increase in extinction coefficients observed from thin films of Ta 2 0 5 . Malherbe et al. (1986) have developed a method for predicting the composition of metal oxides reduced by ion bombardment.9

Their argument gives the fractional

sputtering yield of a compound target as YO YM

_

AM )113( Ao

UM )2/3 Up

where A. and U. are the atomic weight and surface binding energy, respectively, of the metal (M) and oxygen (0). While the expression for (Yo/YM) was derived for bulk target material, the preferential sputtering for thin films is expected to be approximately the same.

Values of (Yo/YM) calculated for the metal oxides investigated in this study are The values in Table I show that the theory of Malherbe et al. is

listed in Table 1.

reasonably accurate in predicting the experimental result that use of IAD to deposit Ta 2 0 5 produces films having significantly greater optical absorption compared to that of films of A12 0 3 , Si0

2

and TiO 2. Table 1 Fractional Sputtering Yields

Target

AM/Ao

UM/UO

Yo/YM

A12 0 3

1.69

0.77

1.00

Si0

2

1.76

0.81

1.00

TiO 2

2.99

1.09

1.50

Ta 20 5

11.31

1.23

2.50

The model for preferential sputtering does not include the effects of bombarding with reactive ions.

Reactive ions may engage in specific chemical interactions with the

target to synthesize compound thin films. 20 Also, oxygen IAD of metal oxides occurs in a reactive atmosphere, where ion bombardment may induce chemical reactions between

487

neutral gas species and the surface.

Such chemically reactive mechanisms are probably

responsible for the low values of extinction coefficient observed in TiO 2 thin films deposited with oxygen lAD, even though the theory of preferential sputtering predicts that lAD of TiO 2 should result in reduced thin film material. Much work has been done in recent years to model the effects of ion bombardment during film growth. The most succe