A simple ion source for target preparation via ion beam sputtering

N U C L E A R INSTRUMENTS AND METHODS 150 ( 1 9 7 8 )

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N O R T H - H O L L A N D PUBLISHING CO

LETTERS TO THE EDITOR A SIMPLE ION SOURCE FOR TARGET PREPARATION VIA ION BEAM SPUTTERING + J A NOLEN J R , M S CURT1N and T E. DYSON

Cyclotron Laboratory and Department o/Phyaws, Michigan State Umverstty, East Lansmg, MI 48824 U S A Received 1 December 1977 A simple glow discharge ~on source' has been developed and used to prepare thin-film nuclear targets of W, Pt, Mo, Zn, TI and Sl wa ion beam sputtering with good efficiency lor separated isotopes

The utility of the ion beam sputtering method for the preparation of thin-film targets for nuclear physics has been discussed by Sletten and Knudsen~). They have used a duo-plasmatron ion source to generate an Ar + beam with an energy of about 10 keV and an intensity of about 1 mA to prepare numerous isotopic targets, primarily of refractory materials such as W and Hf. Further development of the sputtering technique for target making has been done by Scarfe, Hanley and Purser2), and by Wirth and Baumann 3) who utilize duo-plasmatron and Penning ion sources, respectively. The present communication describes a highly simplified ion source which has also proved very useful in the preparation of a wide variety of targets. Emphasis has also been placed on the use of a very efficient sputtering geometry and the application of the sputtering method to higher vapor pressure elements as well as the refractories. + Research supported in part by the U S National Science Foundation

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An assembly drawing indicating the components of the simple glow discharge ion source is given in fig. 1 This is a modified version of the source manufactured by Edwards High Vacuum 4) and intended primarily for surface cleaning rather than target making. Modification of the Edwards system was necessary to achieve stable operation for long times (many hours) at power levels higher than originally intended for their source. An argon glow discharge is established in the confined region between the anode and cathode by applying a positwe potential of 10-15 kV to the anode. The argon gas flow is adjusted via a needle valve to give a discharge current of about 1 mA between the anode and cathode. An Ar + beam of about 100 ~a emerges from the aperture in the cathode. The beam has a large energy spread with an average energy corresponding to about half of the voltage applied to the glow discharge. The divergence of the beam from the aperture is relatively small so that a spot size about 3 mm in diameter is obtained at a distance of 5 cm from the ion gun. The present system differs from the original Ed-

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Fig 1 SchemaUc assembly drawing indicating the components of the glow d~scharge ~onsource.

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wards system in the following aspects: a) all teflon parts were replaced with boron nitride, b) a larger power supply is used, 0 - 2 0 kV at up to 3 m A instead of 0 - 8 kV at 2 m A , c) a finer needle valve is used to achieve more stable argon flow rates, and d) the needle valve is operated at the high voltage to prevent glow discharge on the low pressure side of the argon feed line. A block diagram of the external c o m p o n e n t s is given in fig. 2. The diffusion-pump v a c u u m system maintains a v a c u u m of 3 × 10-6 torr when the argon flow is off and typically 1 × 10 -5 torr w h e n the ion source is in operation. T h e argon line and high voltage leads enter an insulating box on the side of the v a c u u m c h a m b e r where the needle valve is located The ion source shaft passes through a v a c u u m feedthrough at this location, Fig 3 is a schematic drawing of the sputtering geometry used for high efficiency with separated isotopes and to reduce sputtering time as m u c h as possible. The angular distribution of sputtered atores as a function of the angle of incidence of the ion b e a m is discussed by RiSdelsperger et al.5). With an ~on b e a m angle near grazing incidence, as indicated in fig. 3, a t o m s are sputtered into a lobe with the m a x i m u m yield nearly perpendicular to the surface of the sample In the present work the sample material, which was typically available as powder, was formed into a flat disc about 8 m m in d m m e t e r by pressing with a hydraulic press with a force of 10 tons. Although about 50 mg of matenal was normally used to form these source

discs, less could be used for particularly expensive isotopes. The efficiency of this geometry is such that it is possible to m a k e a target with a thickness of 1 5 0 / l g / c m 2 by sputtering less than 1 mg from the source material. The resulting targets appear to be quite uniform over a central area about 104 -

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Ftg. 4 Spectrum of alpha pamcles scattered from a thin Pt target prepared via ton beam sputtering (lower) compared w~th one obtained from a thicker rolled fod target (upper) The carbon and oxygen peaks in the lower spectrum are due to the formvar backing The peaks just to the left of the Pt elastic near channel 1200 m the lower spectrum are due to excttation of low lying collectwe states of the various isotopes m the natural Pt targets

SIMPLE ION SOURCE FOR TARGET PREPARATION

5 mm in diameter, but become rapidly thinner at large radii. The substrate is mounted level with and is shielded from the aperture of the ion source so that atoms sputtered from the aperture cannot contaminate the target. The source disc rests on the end of a 5 mm diameter vertical rod of graphite so that the source disc is the only material struck by the ion beam in the vicinity of the substrate. The beam spot becomes larger as the aperture increases in diameter due to sputtering of the aperture material, which necessitates replacement of the aperture after about 24 h of ion source operation. This is not a serious problem, however, because the apertures are simple stainless steel discs. They are easy to change and several typical targets can be made with each aperture. The graphite pedestal is electrically insulated and connected to a current integrater to provide a measure of target thickness Targets of separated isotopes of Pt and Mo have been prepared and used in nuclear physics experiments. The Pt isotopes were sputtered directly onto thin formvar backings in approximately three hours each and required about 800 #g of each isotope for 150/~g/cm 2 target thicknesses. Fig. 4 shows a test spectrum obtained with a target sputtered from natural Pt and evaluated for impurities by elastic scattering of alpha particles. The only impurities noted are C and O which are present in the formvar backing. Test targets of natural isotopic abundance have also been successfully prepared from W, Zn, Ti and Si. Self supporting metallic foils in the 200 ~g/cm 2 thickness range have been prepared with standard water soluble salt release agents. Tests with metallic samples of

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U, Th and Hf did not produce good metalhc targets. These elements all have very low sputtering rates 6) and they seemed to oxidize about as fast as they sputtered in the present system, although such materials have been successfully sputtered with more elaborate ion sources at higher rates~,2,3). The ability to make targets of refractory materials, such as W, via ion beam sputtering at low temperatures has been previously emphasized t,2,3). However, the present success with Zn indicates another valuable application of the sputtering method, i.e. to higher vapor pressure metals which have erratic sticking probabilities in conventional thermal evaporation. Sputtered films are tenacious, presumably because of the higher energies of the sputtered atoms relative to those from thermal evaporations, and this seems to eliminate the normal sticking problems with elements such as Zn and Cd. These elements also have very high sputtering rates 6) which makes them even more suitable for this method. References l) G. Sletten and P. Knudsen, Nucl Instr and Meth 102 (1972) 459 2) W Scarfe, P Hanley and K Purser, Proc 4th Ann. Conf of Nuclear Target Development Society, Argonne National Lab, Argonne Rep no 1, Section B (1975) 75 3) H L W~rth and H Baumann, contribution to the Ann. Conf of the Nuclear Target Development Society, Umverslty of Cahforma, Berkeley, CA, October 1977. 4) Edwards High Vacuum Inc, 3279 Grand Island Blvd., Grand Island, NY 14072. 5) K Rodelsperger, W Kruger and A Scharmann, Z Physlk A272 ~1975) 127 6) G Carter and J S. Colhgon, Ion bombardment of sohds (American Elsevier, New York, 1968) p. 310