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An ion-source of ceramic construction suitable for ion-implantation
Vacuum 64 (2002) 37–40
An ion-source of ceramic construction suitable for ion-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 ion currents B100 mA at 25 kV, extraction voltage. The source is of straightforward construction and is characterised by a ceramic 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 of particular value for high sample throughputs and/or situations where time is at a premium (e.g. high dose requirements in 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 ionimplantation machine is a versatile ion-source. With most high current density ion-sources currently in use, ions are produced by a high intensity plasma discharge in a gas or vapour at a pressure typically of the order of 10 3 mbar. The ion beam is extracted with a high electric field through an aperture or canal in the source chamber using an external electrode held at a high negative potential. The plasma boundary plays a critical role in focusing the extracted ion beam similar to that of the cathode surface of an electron gun. The present study describes a simple, low*Corresponding author. Tel.: +91-1596-42230; fax: +911596-42294. E-mail address: [email protected] (G.S. Virdi).
voltage d.c. arc-discharge type ion-source which has been indigenously fabricated and tested satisfactorily in a 30 KeV ion-implanter. The theory and practical aspects of this type of ionsource are now fairly well-known [1–4]. The main new features of the ion source described in this paper are its low cost, straightforward fabrication employing a ceramic casting and improved degassing features when compared with similar imported boron nitride arc discharge ion-sources.
2. Construction A schematic diagram of the ion-source is shown in Fig. 1. A discharge chamber which contains a heated filament or an indirectly heated cathode and an anode with an exit aperture has to satisfy the requirements of a good thermal insulating
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
38
G.S. Virdi / Vacuum 64 (2002) 37–40
Fig. 1. Schematic diagram of the ion-source.
material capable of with standing high temperatures of 10001C and above in vacuum with resistance to a wide range of possible vapours. Materials satisfying the above conditions include BN, pyrophyllite and SiN which are not always readily available. A possible alternative is to use a silica-alumina based ceramic material for the source and investigation suggested that a material of suitable composition could be found that would satisfy the above requirements. Tests were therefore made of silica-alumina compositions in the range 60 : 40% to 70 : 30%. We found that an optimum proportion of 65 : 35 silica-alumina mixture provided the best surface finish, the least brittleness, and lowest degassing time in vacuum for the finalised design of the ceramic discharge chamber. A ceramic discharge chamber of the above mentioned composition was fabricated by a local company,1 the design allowed the housing of a filament, anode and, when required, a small oven. The geometry of the source was similar to that used by Menzinger and Wahlin [5] and is shown in Fig. 1. To confine the discharge within a small spatial region, resulting in high source efficiencies, a ceramic ring was introduced between the filament and the anode. The source charge could 1
Pennar. Ceramic and General Industries, Panipat, India.
be admitted externally as a gas/vapour or for solid material, placed in an internal oven (not shown in the diagram). The ion-source is cylindrically symmetric and consists of the following parts: (1) Anode, which is a 25 mm stainless steel disc, placed in the ceramic chamber having a central circular hole of 0.5 mm through which the ions can be extracted. (2) A filament in the form of a spiral wound tungsten wire, the filament to anode distance being of the order of 5 mm. (3) Filament and anode contact leads, which are fed through 1 mm holes at one end of the chamber. (4) A gas line in the form of a tube with a tapered end, made of stainless steel and fitted at the back of the ceramic cup to ensure gas tight connections.
3. Experimental procedures The source was initially thoroughly outgassed for about an hour in high vacuum using a filament current of 5 A. To strike an arc, the filament current was increased to B18 A to produce sufficient electron emission and the anode voltage toB200 V. Gas was slowly admitted to the source through a needle value until the arc struck when the arc current would increase suddenly to a high value of about 800 mA. The electrical circuit diagram for operation of the ion-source is shown
G.S. Virdi / Vacuum 64 (2002) 37–40
in Fig. 2. Once equilibrium was established, anode voltage, source pressure and filament current were adjusted to give the desired maximum ion current.
Fig. 2. Electrical circuit diagram for operating the ion-source.
39
4. Results The degassing characteristic of the ceramic source was compared with a commercially available BN source, as shown in Fig. 3. The results show that the time to achieve equilibrium conditions for a typical set of an operating conditions is reduced from about 140 min for the BN source to less than 50 min with a new ceramic source. It may be mentioned that after setting the ion-source parameters during operation, it takes only about half an hour to restore the system vacuum to 10 5 mbar with the new ceramic source (for a 15 A filament current) compared with a minimum of 2 h with the BN ion source operating under similar conditions. Fig. 4 shows the variation of extracted ion beam current as a function of extraction voltage. The measurements were taken using a Faraday cup placed 250 mm from the ion source exit aperture. A maximum ion current of 100 mA
Fig. 3. Degassing characteristics of ceramic and BN ion-source.
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G.S. Virdi / Vacuum 64 (2002) 37–40
significantly improved outgassing characteristics when compared to a BN source. This means less impurity contamination in the beam, a threefold improvement in the time to reach equilibrium conditions and maximum output of the desired ion species. This is of particular value for high sample throughputs and/or situations where time is a premium (e.g. production situations where high doses are required). The source may also be useful where it is not possible to use mass separation to eliminate impurity content.
Acknowledgements The author is thankful to C.S.I.R., New Delhi for the partial financial support during the course of the present investigations and also to Mr. M. Bawaja of Pennar Ceramic Industries, Panipat, India, for his help in fabrication of the ceramic cup.
Fig. 4. Variation of extraction current at Faraday cup with extraction voltage.
was obtained for an extraction voltage of 25 kV with a filament current of 17 A, a discharge current of 350 mA, an anode voltage of 200 V, and a gas pressure of B10 2 mbar.
5. Conclusions An ion-source using a silica-alumina ceramic construction has been tested and shown to yield
References [1] Dearnaley G, Freeman JH, Nelson RS, Stephen J. Ionimplantation. 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.
An ion-source of ceramic construction suitable for ion-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 ion currents B100 mA at 25 kV, extraction voltage. The source is of straightforward construction and is characterised by a ceramic 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 of particular value for high sample throughputs and/or situations where time is at a premium (e.g. high dose requirements in 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 ionimplantation machine is a versatile ion-source. With most high current density ion-sources currently in use, ions are produced by a high intensity plasma discharge in a gas or vapour at a pressure typically of the order of 10 3 mbar. The ion beam is extracted with a high electric field through an aperture or canal in the source chamber using an external electrode held at a high negative potential. The plasma boundary plays a critical role in focusing the extracted ion beam similar to that of the cathode surface of an electron gun. The present study describes a simple, low*Corresponding author. Tel.: +91-1596-42230; fax: +911596-42294. E-mail address: [email protected] (G.S. Virdi).
voltage d.c. arc-discharge type ion-source which has been indigenously fabricated and tested satisfactorily in a 30 KeV ion-implanter. The theory and practical aspects of this type of ionsource are now fairly well-known [1–4]. The main new features of the ion source described in this paper are its low cost, straightforward fabrication employing a ceramic casting and improved degassing features when compared with similar imported boron nitride arc discharge ion-sources.
2. Construction A schematic diagram of the ion-source is shown in Fig. 1. A discharge chamber which contains a heated filament or an indirectly heated cathode and an anode with an exit aperture has to satisfy the requirements of a good thermal insulating
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
38
G.S. Virdi / Vacuum 64 (2002) 37–40
Fig. 1. Schematic diagram of the ion-source.
material capable of with standing high temperatures of 10001C and above in vacuum with resistance to a wide range of possible vapours. Materials satisfying the above conditions include BN, pyrophyllite and SiN which are not always readily available. A possible alternative is to use a silica-alumina based ceramic material for the source and investigation suggested that a material of suitable composition could be found that would satisfy the above requirements. Tests were therefore made of silica-alumina compositions in the range 60 : 40% to 70 : 30%. We found that an optimum proportion of 65 : 35 silica-alumina mixture provided the best surface finish, the least brittleness, and lowest degassing time in vacuum for the finalised design of the ceramic discharge chamber. A ceramic discharge chamber of the above mentioned composition was fabricated by a local company,1 the design allowed the housing of a filament, anode and, when required, a small oven. The geometry of the source was similar to that used by Menzinger and Wahlin [5] and is shown in Fig. 1. To confine the discharge within a small spatial region, resulting in high source efficiencies, a ceramic ring was introduced between the filament and the anode. The source charge could 1
Pennar. Ceramic and General Industries, Panipat, India.
be admitted externally as a gas/vapour or for solid material, placed in an internal oven (not shown in the diagram). The ion-source is cylindrically symmetric and consists of the following parts: (1) Anode, which is a 25 mm stainless steel disc, placed in the ceramic chamber having a central circular hole of 0.5 mm through which the ions can be extracted. (2) A filament in the form of a spiral wound tungsten wire, the filament to anode distance being of the order of 5 mm. (3) Filament and anode contact leads, which are fed through 1 mm holes at one end of the chamber. (4) A gas line in the form of a tube with a tapered end, made of stainless steel and fitted at the back of the ceramic cup to ensure gas tight connections.
3. Experimental procedures The source was initially thoroughly outgassed for about an hour in high vacuum using a filament current of 5 A. To strike an arc, the filament current was increased to B18 A to produce sufficient electron emission and the anode voltage toB200 V. Gas was slowly admitted to the source through a needle value until the arc struck when the arc current would increase suddenly to a high value of about 800 mA. The electrical circuit diagram for operation of the ion-source is shown
G.S. Virdi / Vacuum 64 (2002) 37–40
in Fig. 2. Once equilibrium was established, anode voltage, source pressure and filament current were adjusted to give the desired maximum ion current.
Fig. 2. Electrical circuit diagram for operating the ion-source.
39
4. Results The degassing characteristic of the ceramic source was compared with a commercially available BN source, as shown in Fig. 3. The results show that the time to achieve equilibrium conditions for a typical set of an operating conditions is reduced from about 140 min for the BN source to less than 50 min with a new ceramic source. It may be mentioned that after setting the ion-source parameters during operation, it takes only about half an hour to restore the system vacuum to 10 5 mbar with the new ceramic source (for a 15 A filament current) compared with a minimum of 2 h with the BN ion source operating under similar conditions. Fig. 4 shows the variation of extracted ion beam current as a function of extraction voltage. The measurements were taken using a Faraday cup placed 250 mm from the ion source exit aperture. A maximum ion current of 100 mA
Fig. 3. Degassing characteristics of ceramic and BN ion-source.
40
G.S. Virdi / Vacuum 64 (2002) 37–40
significantly improved outgassing characteristics when compared to a BN source. This means less impurity contamination in the beam, a threefold improvement in the time to reach equilibrium conditions and maximum output of the desired ion species. This is of particular value for high sample throughputs and/or situations where time is a premium (e.g. production situations where high doses are required). The source may also be useful where it is not possible to use mass separation to eliminate impurity content.
Acknowledgements The author is thankful to C.S.I.R., New Delhi for the partial financial support during the course of the present investigations and also to Mr. M. Bawaja of Pennar Ceramic Industries, Panipat, India, for his help in fabrication of the ceramic cup.
Fig. 4. Variation of extraction current at Faraday cup with extraction voltage.
was obtained for an extraction voltage of 25 kV with a filament current of 17 A, a discharge current of 350 mA, an anode voltage of 200 V, and a gas pressure of B10 2 mbar.
5. Conclusions An ion-source using a silica-alumina ceramic construction has been tested and shown to yield
References [1] Dearnaley G, Freeman JH, Nelson RS, Stephen J. Ionimplantation. 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.