s Nuclear Instruments and Methods in Physics Research B 94 (1994) 597-600

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ELSEVIER Letter to the Editor

A magnetically confined hollow cathode duoplasmatron for the PINSTECH ion implanter

Abdul Qayyum, Shoaib Ahmad *

Pakistan Institute of Nuclear Science and Technology (PINSTECH), P.O. Nilore, Islamabad, Pakistan

Received 19 July 1994; revised form received 16 September 1994

Abstract A duoplasmatron ion source is being reported which operates at low gas pressures, uses small discharge power, has a

long life time and is of small dimensions and weight. The source incorporates a magnetically confined hollow cathode with the main plasma constriction between the intermediate electrode and anode. Ring shaped permanent magnets produce the special magnetic field profile in the source that is responsible for the enhanced performance. The source can deliver up to 5 mA of singly charged ion beam at moderate operating conditions.

At PINSTECH we have experimented with various designs of the cold and hollow cathode duoplasmatrons with and without magnetic field between the cathode and intermediate electrode [l]. In this communication we dis- tinguish between two distinct regions i.e., the cathode-to- intermediate electrode and the intermediate electrode-to- anode, while reporting and discussing the role of the axial magnetic field. The paper is concerned with the design and results of a compact and highly efficient duoplasmatron ion source based on a magnetically confined hollow cath- ode. The source was designed to meet the requirements of our recently designed PINSTECH accelerator, a 250 keV ion implanter. Due to limited space and power available in the high voltage terminal we required an ion source of small dimensions and weight. Typical dimensions were 70-80 mm diameter X 90-100 mm length and the total weight not to exceed 1.5 kg. The source has to operate at low power (I 100 W) and the source gas pressure Ps to remain I 10e3 mbar and yet provide stable and sizable ion beams.

The source operates in the glow discharge regime at fairly low operating temperatures as opposed to high tem- peratures usually expected in the arc discharge mode of the hollow cathode operation [2,3]. Cold and hollow cathode duoplasmatrons were developed by Batalin et al. [4] and Kerkow et al. [5] based on electromagnets as well as permanent magnets. However, these authors have either not measured or reported the complete B, profile inside the source which in our view has immense significance vis

* Corresponding author.

a vis the overall source performance. The schematic dia- gram as well as the electric circuit of the source is shown in Fig. la. Four commercially available ring shaped per- manent magnets of ID = 40 mm, OD = 80 mm and 11.5 mm thickness are used to produce the axial magnetic field B, profile shown in Fig. lb. The profile is measured with an axial probe. Plates K and K’ of the hollow cathode are of mild steel whereas the central ring C is of aluminum. The magnetic field’s +ve maximum lies in the center of the hollow cathode while the - ve maximum is in between the aperture K’ and the intermediate electrode IE. The intermediate electrode’s 50 mm long and 20 mm diameter inner cylinder is essentially a magnetic field free region, This is followed by a conventional IE-A region where constriction of B, takes place. IE is a magnetic field free region but the electric field between K’ and IE ensures electron flow up to the IE-A region. The beam current is extracted and measured with a Faraday cage as shown in the figure.

Initiation of the discharge requires a higher gas pres- sure P. = 10-l mbar and a higher discharge voltage V, -

600-700 V, but once the discharge has been established, Pg can be reduced to any desired value in the range 10-1-10-4 mbar. After the discharge initiation and set- ting up of the required Pg, a lower value of V, = 400-450 V can be maintained even at fairly low power Pd = i, X V,.

Fig. 2 shows the output from the source operated at 5 X 10e4 mbar with argon where the extracted beam current It, versus Pd is plotted for various extraction voltages V,,. Even for a power as low as 7.5 W at V,,, is 4 kV, a 500 pA beam current is obtained. This nicely meets one of the main requirements of our PINSTECH accelerator that the initial beam energy should be in the

0168-583X/94/$07.00 0 1994 Elsevier Science B.V. AI1 rights reserved

SSDI 0168-583X(94)00340-8

598 A. Quyyum, S. Ahmad/Nucl. In&. andkfeth. in Phys. Res. B 94 (1994) 597-600

ALUMINIUM

V ext - 30kV. 16mA

(al

I I I

200 I

f I

t I

P 150- I I

5 t I

(b) -loo0 0 10 20 30 40 50 60 70 80 90 100

DISTANCE (mm)

Fig. 1. (a) Sketch of the ion source, the electric circuit, extraction geometry and its components including the ring type permanent magnets

and the three regions i.e., hollow cathode (K-C-K’), the intermediate electrode IE and the anode A. (b) The measured axial magnetic field B, profile as a function of 2 along the source.

A. Qayyum, S. Ahmad/Nucl. Instr. and Meth. in Phys. Res. B 94 (1994) 597-600

Table 1

Measured values of T, for increasing values of the discharge

current I, measured with a single probe method

Z, b-41 :V]

75 1.8

loo 2.0

125 2.2

150 2.8

0 5 10 15 20 25 30 35 60 45 50 65 00

PomRiwl

Fig. 2. The extracted ion beam Zb versus the input power P4 is

plotted for various extraction voltages V,,,.

range 4 to 30 keV with at least few hundred ~.LA of beam current.

Z, is plotted as a function of Pg in Fig. 3. Two sets of data are shown in Figs. 3a and 3b for V,,, = 8 kV and 14 kV, respectively. Both sets of data show similar trends of increasing beam output for higher power inputs. A general

599

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I

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a

; :_ Hn "XM

c

0' 2- en76

f yn

m l- 7.6

(b) o/ 45 .Ol .066 301 .ooos .wol

Pplmb6rl

Fig. 3. The beam current I, is plotted as a function of the source

gas pressure Pg for (a) V,,, = 8 kV and (b) V,,, = 14 kV.

trend of maximum Zb for Pg = lo-3 mbar is evident for various power inputs and increasing V,,. The difference from the above stated observation starts at a P,, higher than 60 W of power and V,, 2 14 kV when It, tends to increase for lower gas pressures at higher power inputs. This difference is due to the higher ionization in the source and better extraction conditions in the anode region. In a later communication [l] we will present a comparative study of the performance of this source for Pg < 1O-4 mbar and Pd 2 60 W with the data presented here.

Electron temperature T, measurements presented in Table 1 were made with a Langmuir probe [6,7] inserted in the IE-A region right inside the main ion emissive plasma region. An SS cylindrical probe of 0.4 mm diameter and 2 mm length was inserted 3 mm deep into the anode side of the plasma. The distance between the IE and A is 7 mm. The probe insertion depth was chosen to coincide with the maximum Z?, in the IE-A region. Table 1 gives the tabulated values of T, from the electron retardation current on the Z-V curves at i, values greater than 75 mA (i.e., 34 Wl. The source operates equally well in the entire regime of input power from 10 to about 200 W. The improved performance of the source is due to the higher ionization by the magnetically confined electrons in both of the regions. After about 100 h of continuous operation the inside of the extraction aperture mounted on the inside of the anode A shows a well-defined beam spot of approxi- mately 4 mm diameter that provides an estimate of the plasma confinement volume’s dimensions. The peak of the axial magnetic field B, in Fig. lb also falls about 3 mm from the anode aperture and after penetrating through it falls off to zero 3 mm outside. The overall working temperature of the entire source during a working day’s continuous operation is about 50 to 60°C in the hollow cathode region.

The calculations for E/p values for the three different gas pressures for the two regions K-IE and the IE-A show rather high values u 104-lo6 (V/cm mbar) for both the regions. This seems to indicate a high mobility glow discharge plasma without the normally expected high temperatures typical of the hollow cathode arc discharge modes [8]. The values of T, show a pressure dependence with the higher 7’, corresponding to lower operating gas pressures, which is due to a reduction in collision losses.

600 A. Qayyum, S. Ahmad/Nucl. In&. and Meth. in Phys. Rex B 94 (1994) 597-600

At lower P, higher T’ as well as increasing values of E/p References 0 -

_

are associated with h&her ion velocities that are responsi- ble for the improved ionization [9].

The ion source is compact in dimensions and a further reduction of a factor of 2 can be achieved by use of smaller magnets. It is highly efficient, consumes less gas by operating in the 10m4 mbar range and can deliver substantial beam currents. Stable discharge can be obtained at a discharge power as low as 10 W with a few hundred pA beam current at an extraction voltage as low as 4 kV. The source has been in use 5 to 6 hours a day for the last year without much degradation of its performance. Due to low operating temperatures in the source, Viton “0” rings and PTFE insulators have been used which make the fabrication, assembly, maintenance and service extremely easy. At the same time it keeps the cost of materials down to about $50 for the entire source excluding the power supplies.

[ll

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131 [41

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S. Ahmad, A. Qayyum, B. Ahmad, W. Arshad and M.N. Al&tar, to be published. J.L. Delerdix and A. Roeha Trindale, Adv. Electr. and Elect.

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H. Kerkow, D. Bourbetra and K. Holldack, Nucl. Instr. and

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