A versatile broadbeam ion sourceYusheng Rao, Duiyi Tang, Xiaozeng Liu, and Boli Shen Citation: Review of Scientific Instruments 61, 321 (1990); doi: 10.1063/1.1141282 View online: http://dx.doi.org/10.1063/1.1141282 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/61/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Broadbeam multiampere metal ion source Rev. Sci. Instrum. 61, 577 (1990); 10.1063/1.1141922 Broadbeam ion sources (invited) Rev. Sci. Instrum. 61, 230 (1990); 10.1063/1.1141883 Broadbeam electron source J. Vac. Sci. Technol. A 3, 1774 (1985); 10.1116/1.573377 Summary Abstract: Developments in broadbeam ion source technology and applications J. Vac. Sci. Technol. A 1, 337 (1983); 10.1116/1.572128 Developments in broadbeam, ionsource technology and applications J. Vac. Sci. Technol. 21, 764 (1982); 10.1116/1.571822

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A versatile broad-beam ion source Yusheng Rao,Duiyi Tang, Xiaozeng Uu, and Boli Shan

DepartmentoJElectronic Engineering, Xi 'an Jiaotong University, Xi'an, Shaanxi, 710049 People's Republic of China

(Presented on lOJuly 1989)

A broad-beam ion source has been developed for thin-film technology and ion implantation. A broad ion beam of 20-3000 e V and 100 rnA can be extracted from the source. Three shapes of beam can be produced that are suitable for appropriate thin-film processes, uniform, convergent, and spherically divergent beams. The ion optics for forming these multishaped beams is discussed. According to the various beam-extraction energy, triple, double, and single grids are employed. A single-grid system is used for an ion energy less than 200 e V. It can operate with a variety of gases, such as Nz, O2, Ar, CH4, etc. The source has been applied to ion-beam sputtering deposition, ion­beam direct deposition, ion-beam-assisted deposition, and ion implanter without mass analyzer.

INTRODUCTION

A versatile broad-beam ion source has been developed for thin-film technology and ion implantation. As is well known, a broad ion beam should possess some chosen shape. However, so far only two beam shapes have been used, 1,2 i.e., uniform and convergent or focused beam. These cannot al­ways satisfy the requirements for various thin-film technolo­gy. For instance, the uniform beam from a broad-beam ion source less than 8 cm diameter will not be suitable for a larger processed surface like an arched-type substrate holder bigger than 60 em diameter. Besides uniform and convergent beams, a spherically divergent beam can be produced by our broad-beam source that is suitable for the large processed surface mentioned above. A uniformity of ± 5% over 8 em diameter has been obtained for a uniform beam, and a mini­mum beam spot of 2 cm diameter has been measured at a distance of 11 em from the source exit for convergent beams. A broad ion beam of 20-3000 e V and more than 100 rnA can be extracted from the source. According to the beam-extrac­tion energy, triple-, double-, and single-grid extraction sys­tems are employed. A single-grid system is used for ion ener­gies less than 200 eV. Although a single-grid system has previously been described,3 a new electric connection is giv­en here, with which beam can be steadily extracted. The factors influencing discharge stability are discussed. The maximum gas consumption is 1.2 Pa m3/s, and the oper­ational pressure is 1.33 X 10-2 Pa in the discharge chamber. It can operate with a variety of gases, such as N 2' Oz, Ar, and CH4 , etc.

The source has been applied to thin-film technology and ion implantion without mass analysis. High-Tc supercon­ducting films and W films on Be substrates have been depos­ited by an ion-beam sputtering technique with the conver­gent or focused beam, diamondlike films by ion-beam direct deposition with the uniform beam, and ion-beam-assisted deposition of optical films with the spherically divergent beam, in a 700-mm-diam box coater (LEYBOLD A 700 Q). Obviously, the broad-beam source can also be used for etch­ing, thinning, and cleaning.

I. DESIGN AND PERFORMANCE

The source design is shown schematically in Fig. 1. The discharge chamber consists of anode, cathode, and screen grid. Usually, the cathode and screen grid are electrically connected and at a negative potential with respect to the anode to form an electron-obstructing barrier, with elec­trons oscillating backward and forward. An axial magnetic field confines the electrons and prolongs their effective path in the discharge chamber. A discharge in a magnetic field is a complex process. V-I characteristics of the discharge are shown in Fig. 2, in which the dashed line indicated uncon­trolled voltage-falling processes, and the solid line represents controlled self-sustaining ones.

The magnetic field configuration and magnitude influ­ence the discharge losses and the beam current extracted. They are optimized by means of orthogonal test methods,4

based on optimum operational efficiency. It should be pointed out that the discharge stability of

the source is a vital problem. The factors which influence the stability are mainly as follows: (1) The match between the

FIG. L Schematic diagram of the source (1) Ground grid, (2) screen grid, (3) accelerating grid. (4) anode pole piece, (5) anode, (6) permanent mag­nets, (7) cathode, (8) cathode pole piece. (9) gas inlet, (10) anode insula­tor, (11) intergrid insulator, and (12) neutralizer.

321 Rev. Sci.lnstrum. 61 (1), January 1990 0034-6748/90/010321-03$02.00 @ 1990 American institute of Physics 321

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<t: ~

'" r--

+-' 4

" C! ... ~ '3

u g, 2 H

'" .c u "' i:5

o

System Pressure P

O~5.~9X10~3 Fa

O~5.19XlO~3 Pa

t. ~4. 66X ltr3 Pa

60 80 100 12() 140 160 180 200 220

Discharge Voltage Ua(V)

FIG. 2. V-J characteristics of discharge for various system pressure.

electron emission capability of the cathode and the gas pres­sure in the discharge chamber. (2) The insulation of the electrode and feedthrough. Labyrinth-type structures and sputter shields are adopted for minimizing contamination so as to stably operate for long duration without cleaning. (3) Magnetic scale sedimentation should be cleared away as nec­essary.

With these measures, the source operates steadily, con­tinuously, and reliably. Its continuous and stable operation can last longer than 13 h.

When the substrate or target processed is an insulator, a neutralizer should be used which can spray electrons toward the substrate or target to avoid charge accumulation on it.

II. EXTRACTION SYSTEM

As is well known, a broad ion beam can be formed by a muItiaperture extraction system; in other words, a broad ion beam consists of a number of beamlets.

The extraction system often consists ofthree electrodes, i.e., screen grid, accelerating grid, and ground grid. For de­sign simplification and beam current enhancement, a dia­phragm may be used instead of the ground grid. A two-grid system (the screen and ground grid) can also be used. But the advantage ofthe three-grid system over the two-grid one is the ability to operate at low ion energies without extreme penalties in current density.

A single-grid system may be used when the beam energy is below 200 eV? In the single-grid system, the acceleration distance is the plasma sheath thickness between the dis-

- -----

FIG. 3. Electrical connection of single-grid extraction system.

322 Rev. SCi.lnstrum., Vol. 61, No.1, January 1990

_.,.......-0--"" 2A

lA

')20 400

Target Voltage, Vt(V)

FIG. 4. Extraction characteristics of single-grid system.

charge plasma and the single grid. If n = 1010 cm< 3, Te = 5 eV, the sheath thickness will be of the order of magnitude of 10-2 cm. The connection diagram for the single-grid system is shown in Fig. 3, in which a steady beam can be extracted. The relation between extracted ion current 1; and target vol­tage V, for the single-grid system is shown in Fig. 4. The range of beam energy adjustment is 20-200 eV with the sin­gle-grid system.

As mentioned above, the broad beam source can provide three beam shapes, uniform, convergent, and spherically di­vergent beams. Its design principle and method have been explained in a previous article.5 They are simply described as follows.

A. Uniform beam

It is well known that the nonuniform distribution of plasma in the discharge chamber of an ion source results in

< non uniformity of the beam extracted from the source. Here a perveance match approach I is used to improve the unifor­mity ofthe extracted beam, in which the grid holes increase in diameter with grid radius at constant center-to-center hole spacing, and intergrid gap. The formula of the hole di­ameter variation with radius can be described as follows:

~

" 8 u -< <t: 8

c-<

'" 0 ~

'" ~ ~

5

4

3

2

1

0

x- UNCHANGED HOLE SIZE & SPACING

tJ - VARYING HOLE \~ITH RADlUS AT CONSTANT SPACING

2 3

rCc:m)

FIG. 5. Beam density distribution of uniform beam.

Ion sources 322

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-6 -4 -2 o 2 4 6

r(cm)

FIG. 6. Beam density distribution of focused beam.

ds (r) = d; [j(rb )// (rb ) ] 112'

x(1 + (d;2/41~){1- [},(rb)I},(r)]})- 112'

(r<rb ),

where d ~ is the hole diameter of the unchanged grid with the same holes, rb is the radius of the expected unifonn zone, }' (rb ) and}' (r) are the beam current density at rb and arbi­trary radius for the unchanged grid with the same holes, respectively, j(rb ) is the expected beam current density, usually j(rb ) = j' (rb ), and I" is the gap between screen and accelerating grids. For fabrication convenience the flat grids can be divided into several annular domains, in each domain the hole diameter being the same. The beam density distribu­tion with radius is shown in Fig. 5, compared with that of an unchanged grid having constant hole size. The uniformity has been improved and was ± 5% over 8 cm diameter,

Elo Convergent beam

Convergent or focused beams are produced by a spheri­cally dished extraction grid system which consists of two dished grids at constant hole size and center-to-center hole spacing. The curvature radius of the dished grids is 10 em, Its beam density distribution with radius is shown in Fig. 6, which is measured at distance of 11 cm from the source exit and the minimum beam spot of 2 cm diameter is also mea­sured at distance of 11 cm from the source exit by a movable beam-measuring target.

C, Spherically divergent beam

A spherically divergent beam seems to be beneficial for ion-beam-assisted deposition or other ion-beam thin-film

323 Rev. Sci.lnstrum., Vol. 61, No.1, January 1990

ACCEL GRID

FIG. 7, Formation of spherically divergent beam.

FQur-DENSITY

technology, when a large arch substrate holder is used so as to process the substrate uniformly. This extraction system consists of two outward convex dished grids, which are as­sembled as a segment of a sphere, as if the beamlets were diverging from a common curvature center of the dished grids (see Fig. 7). The design principle is similar to that of the dished grid used for the focused beam.

m. CONCLUSION

( 1) By reasonable selection of the structure and param­eters of the source, the current, energy, stability, and reliabil­ity are satisfactory, and the source has been successfully ap­plied to thin-film technology,

(2) The source can steadily and reliably operate with single, double, and triple grids, and provide multishaped beams, i.e., uniform, convergent, and spherically divergent beams.

(3) Efforts to provide stable source operation have been successful. The design of the source is successful.

ACKNOWLEDGMENTS

We acknowledge Professor Shirru Xu and Haitao Zheng for help in thin-film technology and helpful discus­sion.

'G. R, Brewer, Ion PropuLvion (Gordon and Branch. New York, 1970). 2H. R. Kaufman, I. M. E. Harper, and1. J, Cuomo,J. Vac. Sci. Technol.16, 899 (1979).

3J. M. E. Harper, I. I. Cuomo,P. A. Leary,G. M. Summa,H. R. Kaufman, and F. J, Bresnock, J. Electrochem. Soc. 128,1077 (1981).

4R, Yusheng, Y. S, Rao, and Z, J. Cheng (unpublished). 5R, Yusheng, Y. S. Rao, D. Y. Tang, X. Z. Liu, and B. L. Shen, in Proceed­ings afthe Intemational Symposium on Optical Coatings, 1989, Shanghai, China (International Academic Publishers, Beijing, 1989).

Ion sources 323

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