Vacuum 64 (2002) 37–40

An ion-source of ceramic construction suitable forion-implantation

G.S. Virdi*

Semiconductor Devices Area, Central Electronics Engineering Research Institute, Pilani-333031, India

Received 10 December 2000; received in revised form 24 April 2001; accepted 25 April 2001

Abstract

A simple low voltage arc-discharge type ion-source for gases and solids is described which is capable of delivering ioncurrents B100mA at 25 kV, extraction voltage. The source is of straightforward construction and is characterised by aceramic housing which has superior degassing characteristics for commercially available boron nitride (BN) sources,

thus reducing impurity content, pump down time and time to achieve full output of the required ion species. This is ofparticular value for high sample throughputs and/or situations where time is at a premium (e.g. high dose requirementsin volume production). r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Ion-source; Ion; Outgassing; Ion-implantation; Ceramic; Plasma; Ion beam; Arc discharge

1. Introduction

A key requirement for accelerators and ion-implantation machine is a versatile ion-source.With most high current density ion-sourcescurrently in use, ions are produced by a highintensity plasma discharge in a gas or vapour at apressure typically of the order of 10�3mbar. Theion beam is extracted with a high electric fieldthrough an aperture or canal in the sourcechamber using an external electrode held at a highnegative potential. The plasma boundary plays acritical role in focusing the extracted ion beamsimilar to that of the cathode surface of an electrongun. The present study describes a simple, low-

voltage d.c. arc-discharge type ion-source whichhas been indigenously fabricated and testedsatisfactorily in a 30KeV ion-implanter. Thetheory and practical aspects of this type of ion-source are now fairly well-known [1–4]. The mainnew features of the ion source described in thispaper are its low cost, straightforward fabricationemploying a ceramic casting and improved degas-sing features when compared with similar im-ported boron nitride arc discharge ion-sources.

2. Construction

A schematic diagram of the ion-source is shownin Fig. 1. A discharge chamber which contains aheated filament or an indirectly heated cathodeand an anode with an exit aperture has to satisfythe requirements of a good thermal insulating

*Corresponding author. Tel.: +91-1596-42230; fax: +91-

1596-42294.

E-mail address: virdi@ceeri.ernet.in (G.S. Virdi).

0042-207X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 7 1 - 2

material capable of with standing high tempera-tures of 10001C and above in vacuum withresistance to a wide range of possible vapours.Materials satisfying the above conditions includeBN, pyrophyllite and SiN which are not alwaysreadily available. A possible alternative is to use asilica-alumina based ceramic material for thesource and investigation suggested that a materialof suitable composition could be found that wouldsatisfy the above requirements. Tests were there-fore made of silica-alumina compositions in therange 60 : 40% to 70 : 30%. We found that anoptimum proportion of 65 : 35 silica-aluminamixture provided the best surface finish, the leastbrittleness, and lowest degassing time in vacuumfor the finalised design of the ceramic dischargechamber.

A ceramic discharge chamber of the abovementioned composition was fabricated by a localcompany,1 the design allowed the housing of afilament, anode and, when required, a small oven.The geometry of the source was similar to thatused by Menzinger and Wahlin [5] and is shown inFig. 1. To confine the discharge within a smallspatial region, resulting in high source efficiencies,a ceramic ring was introduced between thefilament and the anode. The source charge could

be admitted externally as a gas/vapour or for solidmaterial, placed in an internal oven (not shown inthe diagram). The ion-source is cylindricallysymmetric and consists of the following parts: (1)Anode, which is a 25mm stainless steel disc, placedin the ceramic chamber having a central circularhole of 0.5mm through which the ions can beextracted. (2) A filament in the form of a spiralwound tungsten wire, the filament to anodedistance being of the order of 5mm. (3) Filamentand anode contact leads, which are fed through1mm holes at one end of the chamber. (4) A gasline in the form of a tube with a tapered end, madeof stainless steel and fitted at the back of theceramic cup to ensure gas tight connections.

3. Experimental procedures

The source was initially thoroughly outgassedfor about an hour in high vacuum using a filamentcurrent of 5A. To strike an arc, the filamentcurrent was increased to B18A to producesufficient electron emission and the anode voltagetoB200V. Gas was slowly admitted to the sourcethrough a needle value until the arc struck whenthe arc current would increase suddenly to a highvalue of about 800mA. The electrical circuitdiagram for operation of the ion-source is shown

Fig. 1. Schematic diagram of the ion-source.

1Pennar. Ceramic and General Industries, Panipat, India.

G.S. Virdi / Vacuum 64 (2002) 37–4038

in Fig. 2. Once equilibrium was established, anodevoltage, source pressure and filament currentwere adjusted to give the desired maximum ioncurrent.

4. Results

The degassing characteristic of the ceramicsource was compared with a commercially avail-able BN source, as shown in Fig. 3. The resultsshow that the time to achieve equilibrium condi-tions for a typical set of an operating conditions isreduced from about 140min for the BN source toless than 50min with a new ceramic source. It maybe mentioned that after setting the ion-sourceparameters during operation, it takes only abouthalf an hour to restore the system vacuum to10�5mbar with the new ceramic source (for a 15Afilament current) compared with a minimum of 2 hwith the BN ion source operating under similarconditions. Fig. 4 shows the variation of extractedion beam current as a function of extractionvoltage. The measurements were taken using aFaraday cup placed 250mm from the ion sourceexit aperture. A maximum ion current of 100mAFig. 2. Electrical circuit diagram for operating the ion-source.

Fig. 3. Degassing characteristics of ceramic and BN ion-source.

G.S. Virdi / Vacuum 64 (2002) 37–40 39

was obtained for an extraction voltage of 25 kVwith a filament current of 17A, a discharge currentof 350mA, an anode voltage of 200V, and a gaspressure of B10�2mbar.

5. Conclusions

An ion-source using a silica-alumina ceramicconstruction has been tested and shown to yield

significantly improved outgassing characteristicswhen compared to a BN source. This means lessimpurity contamination in the beam, a threefoldimprovement in the time to reach equilibriumconditions and maximum output of the desired ionspecies. This is of particular value for high samplethroughputs and/or situations where time is apremium (e.g. production situations where highdoses are required). The source may also be usefulwhere it is not possible to use mass separation toeliminate impurity content.

Acknowledgements

The author is thankful to C.S.I.R., New Delhifor the partial financial support during the courseof the present investigations and also to Mr. M.Bawaja of Pennar Ceramic Industries, Panipat,India, for his help in fabrication of the ceramiccup.

References

[1] Dearnaley G, Freeman JH, Nelson RS, Stephen J. Ion-

implantation. Amsterdam: North Holland, 1973.

[2] Wilson RG, Brewer GR. Ion beams with applications

to ion-implantation. New York: Wiley InterScience,

1973.

[3] Livingston MS, Blewett JP. Particle accelerators. New

York: McGraw Hill, 1962.

[4] Townsend PD, Kelly JC, Hartley NEW. Ion implantation

sputtering and their applications. New York: Academic

Press, 1976.

[5] Menzinger M, Wahlin L. Rev Sci Instr 1969;40:102.

Fig. 4. Variation of extraction current at Faraday cup with

extraction voltage.

G.S. Virdi / Vacuum 64 (2002) 37–4040