AuO+ and AuO+ 2 gaseous ions formed during the sputter deposition of Au films in ArO2 dischargesCarolyn Rubin Aita

Citation: Journal of Applied Physics 61, 5182 (1987); doi: 10.1063/1.338295 View online: http://dx.doi.org/10.1063/1.338295 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/61/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigating the plasma parameters of an Ar/O2 discharge during the sputtering of Al targets in an invertedcylindrical magnetron Phys. Plasmas 21, 093510 (2014); 10.1063/1.4896062 Alumina films by sputter deposition with Ar/O2: Preparation and characterization J. Vac. Sci. Technol. A 7, 1298 (1989); 10.1116/1.576273 (ArO)+ and (ArO2)+ ions in rf sputter deposition discharges J. Appl. Phys. 60, 837 (1986); 10.1063/1.337384 Discharge characteristics for magnetron sputtering of Al in Ar and Ar/O2 mixtures J. Vac. Sci. Technol. 17, 743 (1980); 10.1116/1.570553 Mechanisms of the reactive and chemicalsputter deposition of TiO2 from Ti and TiC targets in mixed Ar+O2discharges J. Appl. Phys. 50, 4966 (1979); 10.1063/1.325573

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lD. L. Flamm, V, M, Donnelly, and D. E. Ibbotson, in VLSI Electronics Microstructure Science, VoL 8, edited by N. G. Einspruch and D. M, Brown (Academic, Orlando, 1984), p. 230.

25. J. Fonash, Solid State Techno!. 28,150, (1985). 'J. E. Bjorkolm, J. Eicher, J. C. White, and R. E. Howard, J. App!. Phys. 58, 2098 (1985).

<Y, C. Du, Z. Lu,Z. Q, Yu, D. C. Sun, F. M. Li, andG. J. Collins, ChineseJ. Semicond. 6, 159 (1985) (in Chinese); or see: Chinese Phys. 6, 208

(1986). 's. J. Fooash, Solid State Techno!. 28, 201 (1985). 6C. A. Moore, J. J. Rocca, T. Johnson, G. J. Collins, and P. E. Russell, App!. Phys. Lett. 43, 777 (1983).

'c. A. Moore, J. 1. Rocca, and G. 1. Collins, Appl. Phys. Lett. "5, 169 (1984).

'Y. C. Du, H. Wang, D. C. Sun, and F. M. Li (unpublished).

AuO+ and AuOt gaseous ions formed during the sputter deposition of Au fUms in Ar ... 02 discharges

Carolyn Rubin Aita Materials Department and the Laboratory for Surface Studies, University of Wisconsin-Milwaukee, P.O. Box 784, Milwaukee, Wisconsin 53706

(Received 26 November 1986; accepted for publication 2 February 1987)

AuO+ and AuO/ gaseous ions formed during the sputtering of an Au target in 1.0 X 10-2

Torr, rf-excited Ar-02 discharges were studied by glow-discharge mass spectrometry. These ions are created from neutral species in the negative glow and are incident on the substrate during a sputter deposition. The relative flux of Au02+ I Au + , AuO + I Au + , and AuO/ I AuO+ was determined for gas compositions from 100% Ar to 100% 02' The results show that the arrival of Au-oxide species at the substrate, in addition to Au atoms, must be taken into account when modeling the growth of Au films sputter deposited in Oz-bearing discharges.

The effect of oxygen on Au film growth on oxide glass substrates has been of continuing interest for the past two decades. 1--4 In the case of sputter-deposited Au, eady work by Mattox I has shown that film adhesion is enhanced when O2 is added to the At sputtering discharge. The reason for this behavior is is not yet understood.

Moore and Thornton5 studied the reaction of molten Au droplets with a silica substrate in an oxygen atmosphere. They concluded that Au reacted with oxygen from the atmo~ sphere to form a stable Au~oxide which alloyed with the silica. There was strong bonding between the Au-doped sili­ca and subsequent Au droplets placed upon it. Mattox sug­gested a similar process was occurring during sputter depo­sition, with Au-oxide formation at the substrate assisted by active oxygen species created in the glow discharge. The analysis assumed that the only target species incident on the substrate were Au. However, Aita and Tran6 have shown that Pt-O bond formation in sputter-deposited Pt-O alloys is controlled by oxidation of Pt metal at the target surface. Hecq et aU and Aita8 have shown that Pt-oxide species ar­rive at the substrate along with metallic Pt.

In the present paper, the occurrence of AuO+ and Au02+ gaseous ions formed during the sputtering of an Au target in Ar-02 discharges is monitored by glow-discharge mass spectrometry as a function of gas O2 content. Glow­discharge mass spectrometry (GDMS) is a method of deter­mining the relative fiux and energy of positive ions incident on the substrate plane. 8,9 Ions bearing target species detected by GDMS originate from sputtered neutrals which have been ionized in the negative glow. A positive ion sputtered

from the target will not be able to escape the cathode field and therefore will not be detected by GDMS.

Possible reactions in the plasma volume which lead to the formation of Au-oxide gaseous ions can be divided into two categories: (1) Ionization reactions involving a Au-ox­ide molecule sputtered from an oxidized layer at the target surface. (2) Oxidation + ionization reactions involving a Au atom sputtered from the target surface. With respect to the second category, two-body collisions to form AuO+ and AuOz+ directly are highly unlikely because of the momen­tum conservation constraint. Associative ionization reac­tions between Au and various oxygen species in which one or both reactants are in excited electronic states are energetical~ ly possible, However, in genera! an associative ionization reaction is unlikely unless one of the reactants is in a metasta­ble state. l!) The same argument holds for the occurrence of ea) dissociative ionization involving Au and O2 in which the product AuOii- dissociates to form AuO+ and 0, and (b) dissociative ionization involving Au and higher order oxy­gen molecules, 0 3 and 0 4 , It is therefore assumed that Au­oxide ions detected by GDMS are sputtered as some form of neutral Au-oxide molecule from the oxidized surface of the Au target. The relationship of ionic to neutral flux cannot be determined for the Au-O system because this requires l

!

knowledge of the ionization potentials of AuO and Au02,

which are unknown. References 8 and 12 describe the rf diode sputter depo­

sition apparatus with attached spectrometer used in this ex­periment. Positive ions from the negative glow were sampled through an orifice in the substrate table covering the anode.

5182 J. Appl. Phys. 61 (11), 1 viune 1987 0021-8979/87/115182-03$02.40 © 1987 American Institute of PhysiCS 5182 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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A 20-cm-diam Au target was bonded to the cathode. The chamber was pumped to 5 X 10-7 Torr base pressure and backfilled with the Ar-Oz mixture under investigation. 99.999% pure At and 99.97% pure O2 were admitted sepa­rately and mixed in the chamber. Sputtering gas composi­tions from 100% Ar to 100% O2 were investigated. The majority of data were taken at a total discharge pressure of 1 X 10-2 Torr, although a limited amount of data were taken at higher pressure. An anode-cathode separation of9 cm and a cathode voltage of - 1400 V (p-p) were used throughout the experiment. GDMS signals in the 1-300 amu range were collected.

Figure 1 shows the intensity ofionic signals of 12 species as a function of sputtering gas O2 content for a total dis-

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UO

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{I.10

[Il

Au02+/Aif ! 0

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AuOt/AuO+

cl O~-----50~----~10~O

%02 FIG. 2. The relative flux to the substrate of (a) AuO+ I Au" ions, (bj AuO,+ I Au + ions, and (c) AuO," I AuO + ions as a function of sputtering gas O2 content. Total pressure is: 0-1.0, e-:2,O, 0-2.5, D-3.0X 10-- 2

Torr.

5183 J. Appl. Phys., Vol. 61, No. 11, 1 June 1987

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FIG. 1. The GDMS ionic signal in­tensity as a function of sputtering gas O 2 content for: (a) Ar+ (40 amu), O,+- (32 amu), 0+ (16 amu), Ar+ 2

(20 amu) , and Ar/ (80 amu); (b) Au+ (l96amu), AuO+ (212amu), AuO l " (228 amu), and AuAr+ (236 amu); (e) HP+ (18 amu), 03' (48 amu), 0.+ (64 amu),

ArO+ (56 amu), and ArO'" (72 amn). Total discharge pressure is 1 X 10--2 Torr.

charge pressure of 1 X 10--2 Torr. Ions bearing target species are:Au+ (l96amu), AuO+ (212amu),Au0::t (228amu), and AuAr+ (236 amu). Unusual ions containing Ar, such as AuAr+ , ArO+ , and ArOt are the products of associ­ative ionization reactions involving neutral Ar atoms in long-lived metastable states and neutral ground state Au or O 10,12-14

2'

Figures 2(a) and 2(b) show the relative flux of AuO+ / Au + and Au02+ / Au+ species. In both cases, there is a rapid increase in the flux of Au-oxide relative to Au species as the sputtering gas O2 content is increased above 50%. Comparing the Ar+ and O;\- fluxes shown in Fig, 1(a}, it can be seen that for gas containing < 50% O2 the majority particle population capable of sputtering the target changes from Ar+ to 02+

A limited amount of data taken at discharge pressure > 1 X 10-2 Torr is presented in Fig. 2(a). It can be seen that increasing the discharge pressure from 1 to 3 X 10-2 Torr does not affect the relative AuO+ / Au + flux. This result sug­gests that both ions are fonned from their corresponding neutral molecules by a single-step ionization process. It

The relative AuO/ /AuO+ flux, shown in Fig. 2(c), increases linearly with increasing sputtering gas O2 content. The 02+ flux is also a linear function of sputtering gas O2

content. Simple correlation of these results indicates that the sputtered target species is different depending upon whether the bombarding ion is Ar+ or O2+, However, characteriza­tion of the target surface is essential to further understand the sputtering dynamics which result in the ejection of Au­oxide species.

In summary, we have shown that AuO+ and at gase­ous ions are formed when a Au target is sputtered in Ar-02 discharges. The arrival of Au-oxide molecules at the sub­strate, in addition to Au atoms, must be taken in account when modeling the growth of Au films sputter deposited in 02-bearing discharges.

The author wishes to thank Professor M. E. Marhie and Professor G_ S. Baker for helpful comments. Preparation of this manuscript was supported under U.S. Army Research Office Grant No. DAAG29-84-K-0126.

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ID. M. Mattox, J. Appl. Phys. 37, 3613 (1966). 2L. Holland, The Properties o/Glass Substrates (Wiley, New York, 1964). 3J. Salem and F. Sequeda, J. Vae. Sei. Technol. 18, 149 (1981). 4R. J. Martin, W. G. Sainty. and R. P. Netterneld, Vacuum 35, 621 (1985). sD. C. Moore and H. P. Thornton, J. Res. Nat!. Bur. Stand. 62,127 (1959). 6C. R. Aitaand N. C. Tran, J. Appl. Phys. 55,6051 (1983); 56, 985 (1984). 7M. Hecq, A. Hecq, and T. Robert, J. Vac. Sci. Technol. 16,960 (1979). 8C. R Aita, J. Vac. Sci. Techno!. A 3,630 (1985). 9J. W. Coburn, Rev. Sci. lnstrum. 41,1219 (1970).

IOE. E. Muschlitz, Jr. and M. J. Weiss, in Atomic Collision Processes, edited by M. R. C. McDowell (North-Holland, Amsterdam, 19(4), Pl'. 1073-1079.

11J. W. Coburn, E. W. Eckstein, and Eric Kay, J. Vac. Sci. Techno!. 12, 151 (1975).

1ZC, R. Aita and R. J. Lad, J. Appl. Phys. 60,837 (1986). 13Z. Herman and V. Cermek, Nature 199, 558 (1963). ;4J. W. Coburn and Eric Kay, J. Chern. Phys. 64, 907 (1976).

Measurements of transverse .. gain profiles in rf .. and dc"'excited CO2 gain cavities

Richard C. Sharp Raytheon Research Division, 131 Spring Street, Lexington, Massachusetts 02173

(Received 13 November 1986; accepted for publication 2 February 1987)

Spatially resolved transveTse~gain profile measurements have been made in rf- and dc-excited waveguidelike cavities for CO2 laser gas mixtures. Both current density and gas temperature impact the CO2 gain profile. As measured., transversely rf-excited cavities show higher CO2

gain near the wall boundaries when compared to longitudinally dc-excited cavities. We propose that transverse-rf excitation may be inherently better than dc-longitudinal excitation in producing a higher average spatial gain, owing to more uniform current density pumping.

In recent years the use of rf excitation for waveguide COzlasers has become quite common. While widely applied, there are a number of poorly understood aspects associated with the discharge physics. For example, it is easily observed in the laboratory that the use of transverse-rf excitation pro~ duces macroscopic inhomogeneities in the visible lumines­cence emitted from the discharge (see Fig. 1). Electrostatic double-probe measurements have indicated that the regions of bright visible luminescence near the electrode walls are regions of higher electron temperature. 1 The impact of such inhomogeneities on laser performance was unknown, a!­though it was believed such brightly luminous areas would be regions of low CO2 gain, I and there was speculation that power extraction per unit volume and mode quality would be significantly affected.2

Spatially resolving the gain profile requires focusing a low-intensity probe beam into a cavity much shorter than a typical laser. The probe beam doubling distance is kept long with respect to the cavity length. In our experiments, the cavity length was reduced to 1.5 em, improving spatial reso­lution at the sacrifice of gain and signal-to-noise ratio. The spot radius (lie point of the E field) for the probe was 110 pm at the focal point and the beam doubling distance was one half the cavity length. Alignment of the cavity with re­spect to the probe beam was accomplished by, first, using a helium-neon laser beam to overlap the path of the CO2

( 10.59 pm) laser beam and, second, using the visible beam in conjunction with an autocollimator to align the test cavity with respect to the laser beam. A micrometer driven stage moved the test cavity through the fixed probe beam in 100-f-tID steps, with the probe beam situated at mid~height in the cavity. The traversal took place from one IT-electrode waH to the other, thereby cutting across the striations in the visible

luminescence emitted by such a discharge. After passing through the cavity the entire probe beam was collected by a HgCdTe PV detector, whose signal was collected by a two­channel, gated data acquisition system. The gating was syn­chronous with the plasma, which was modulated on and off at about 1 Hz (to provide probe beam transmission with and without gain),

The conditions for Fig. 1 include a 112-MHz excitation frequency, 38-W input power, at 34-Torr gas pressure. The actual discharge cavity dimensions are 1.5 cm long by 5 mm wide by 5 mm high, producing a nominal energy loading of about 100 W Icc. The pressure is reduced from the more typical 100 Torr pressures found in the narrower waveguide laser in order to increase the gain and provide improved sig­nal-to-noise ratios. It also facilitates comparisons to subse­quent measurements in dc capillaries at similar pressures. The gas was a 1:1:5 CO2:N2:He mix, and flowed to prevent any effects due to gas decomposition.

Figure 2 (upper trace) shows the result of such gain measurements for the metal walled cavity of Fig. 1, while the lower trace shows similar data for an all ceramic (A120 3 )

cavity (42 Torr; the visible luminescence is identical to Fig. 1 ) . Relative error bars ( 1 standard deviation) were calculat­ed for several typical points in each measured gain profile. The error in the absolute gain measurement (a shifting up or down of the entire profile) could be as high as 20%, a result of the small signals and the possible base line difference in the electronics for the two independent channels (gain and no gain), from not keeping the probe laser beam stabilized precisely at gain line center from one experiment to the next, and from blooming of the plasma beyond the ends of the cavity. Except for a slight difference in features directly adja­cent to the waH, the gain appears to follow the same general

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