Materials Science and Engineering, B2 (1989) 211 -216 211

An Approach to a New Machine Design for Implantation at Medium and High Energies*

S. BLIMAN

LA GRIPPA/DRF G, Bdtiment 10.05, Centre D 'Etudes Nuch;aires de Grenoble, 85 X 38041 Grenoble ('~:dex (l+an('e)

(Received June 2, 1988)

Abstract

7he features which a good machine for implan- tation should have are a large energy range, a high current output at all energies, stable operation over long periods of time, limited beam pollution and good vacuum conditions. With the development during the last few years" of highly reliable new electron (yclotron resonance ion sources, it is" now possible to design a machine to meet these require- ments. Some recent experiments" on highly charged ions interacting with surfaces show features rele- vant to the understanding of ion implantation.

1. Introduction

In recent years, ion implantation has become very largely accepted in integrated-circuit pro- cessing and in metallurgical applications. The kiloelectronvolt implanters currently in use are in the main restricted in their energy range and in current outputs. Moreover, one important draw- back is the ion source lifetime in addition to the complex gas-handling system to feed the source. For megaelectronvolt ion implantation, which is not yet widely used, the equipment design is limited to classical tandem and linear accelera- tors; to some extent, the problems met in the kiloelectronvolt ion beams are also encountered in megaelectronvolt ion beams.

Ways of solving these problems now seem to exist; in the last few years, research dedicated to the field of electron cyclotron resonance ion sources (ECRISs) has reached a point where the design of a new machine with unprecedented features is possible.

*Paper presented at the Symposium on Deep Implants: Fundamentals and Applications at the E-MRS Spring Meeting, Strasbourg, May 31 -June 2, 1988.

We first describe two recently developed ECRISs and present their main properties and performances; then an overview on ion-surface interactions is discussed on the basis of recent experimental results; finally the effectiveness of an accelerator concept based on the use of an ECRIS is discussed.

2. The electron cyclotron resonance ion source

A research and development programme on ECRISs was initiated in the early 1970s [1 ]. The goal was the design of an ion source delivering sizable beams of highly charged ions. An ECRIS is basically a high frequency discharge. The plasma is confined to a minimum B magnetic field configuration.

The basic principles for operating such a source are as follows.

(1) The requirement of stable plasma confine- ment is met when, in the minimum B field,

/ ~ ~1/~ ~Opc<Wc~ [1] where o)w=lnevmE,) ~ is the plasma frequency and ~o~ = eB/m electron cyclo- tron frequency.

(2) Electron cyclotron resonance heating is realized by coupling high frequency power at a frequency such that ~%f = C0cc [2].

(3) The neutral pressure in the source volume is adjusted taking into account the fact that in the hot electron plasma the ionization degree ct ( =n/(no +n))is close to 100%.

In a source operated at 10 GHz--a classical situation--the resonance magnetic field is 3.6 kG, and the base pressure is of the order of 3 x 10 ~ torr. This is at variance with all classical sources: Nielsen type, unoplasmatron, duoplasmatron or d.c. discharges operated at quite high pressures (p>~ 10 _3 torr) and where the ionization degree does not exceed 10%-20%. Moreover, ECR1Ss can be either pulsed with an adjustable duty cycle

0921-5107/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands

212

or fully d.c. Since there is no cathode, the lifetime of this source does not raise any problem. In these conditions, with a proper adjustment of the high frequency power to the source which is held at high potential, an ion beam is extracted showing a distribution of charge states.

A feature common to all ECRIS designs discussed here is that, to obtain beams of solid elements at normal temperature and pressure, no oven is required. An overall review of ECRIS uses and properties as of 1986 has been given previously [2].

We now describe two ECRISs one of which is fully operational: one is CAPRICE [3] and the other NEOMAFIOS [4] which is an advanced prototype; both of these could be part of a new machine design for implantation at medium and high energies.

2.1. C A P R I C E electron cyclotron resonance i o n s o u r c e s

This is a compact source operated at 10 GHz. The magnetic field is provided by a hexapole (SmCo 5 permanent magnets) for the radial field and a pair of solenoids for the bottle-like axial field. All these elements are contained within an iron yoke. The plasma chamber on the axis of the magnetic field has an inner diameter of 7 cm and a length of 22 cm. The overall power required to operate it is of the order of 35 kVA mostly dissi- pated in the magnetic field generation, the r.f. power reaching 1 kW at 10 GHz.

Figure 1 is a schematic diagram of the CAPRICE ECRIS. The operation with gases is simple and, to increase the currents of the higher charge states of a heavy gas, it is mixed with a light gas.

Figure 2 shows the currents measured at constant extraction voltage (15 kV) vs. charge states for some gaseous elements. For solid ele- ments at normal temperature and pressure, the source operation does not require a furnace. This is easily understood on the basis of plasma and collision considerations; the source plasma contains a high energy electron population dif- fusing slowly radially and axially. These electrons are responsible for both solid element sublima- tion and ionization. In fact, it is sufficient for many elements with a low melting temperature to introduce a small piece of the element directly into the source body. In operation, coating and recycling on the wall occur. For refractory ele- ments, a small rod of the element has to be slowly

Fig. 1. Schematic diagram of the C 'APRICE ECRIS: 1, magnets ihexapolar field); 2, coils axia l field); 3, closed electron cyclotron resonance surface; 4, 5, cooling water inlet and outlet; 6. r.f. power coupling; 7. gas feed; 8 , tu rbo- molecular pump; 9, ion extraction: 10, gas inlet; II r.f. window; 12, vacuum chamber,

~A

10 2

101

10 0

10-1

I i I i z ! ! i i

~ i l i i

Fig. 2. Current vs. charge state for differcnt gases as obtained with C A P R I C E (r.f. power, 500, cont inuous wave: 1/~ = 15 kV).

pushed into the r.f. discharge close to the c~. resonant surface; the discharge is performed using a buffer gas (generally argon).

Figure 3 shows the current vs. charge for various elements; the extraction voltage is t 5 kV.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

mo ~A

~Ae

lo HA

I ,HA

0.1 HA

0.01 /JA

Fig. 3. Current vs. chargc state for the same solid elements with CA PRI CE (r.f. power (10 GHz), 6(I0 W, continuous wave: V.,, = 15 kV ).

2.2. 7he N E O M A F I O S electron cyclotron resonance ion source

This source is basically a prototype in which the magnetic field is provided by permanent magnets (Fe-Nd 'B) . The power required for operation is limited (about 1.5 kW). A schematic diagram of this device is given in Fig. 4.

2.3. Long-tirne runs It is now well established that in the field of

cyclotrons, where these sources are currently coupled to upgraded machines, long-time runs are a necessity [5]. For this purpose a 100 h run was made to obtain molybdenum ions in view of its application to the Ganil. Figure 5 shows that, with 220 W r.f. power, it has been possible to obtain a stable Mo 9 + beam of current 5 HA___ 1(l% over 100 h.

3. A possible machine design

The basis of the design for a versatile machine is the use of an ECRIS coupled to an analysing magnet for selection of the proper ion charge. In the usual operation of ECRIS, ion beam extrac-

213

Fe Nb B AXIAL MAGNEI

IRON

F e N b B HEXAPO[ E

/ ~ IRON

] ~ UHF FEED F

J ] ~ GAS

Fig. 4. Schematic diagram of the N E O M A F I O S ECRIS.

O : displacement of M piston (Vs) t

I uA

D

5

4

3

?

I

• I , I I t

~0 ~s ~025 30 35 40 45 so 55 600~ ~ 0 ~ s 8 0 ~ 5 90 95~00

Fig. 5. Mo '~+ current vs. time (opcrating conditions: r.f. power, 200 W, continuous wave: molybdenum feed, about 3 m g h i).

tion is performed through a hole 6 mm in diam- eter. With a source operated at + 15 kV with respect to a reference potential the classical normalized emittance was measured to be of the order of less than 150 mm grad . Operat ion at + 30 kV with respect to the reference is easy and this would ensure higher currents (according to the Child-Langmuir law) but allows a better emittance. It should be noted that, for coupling to

214

cyclotrons, this extraction condition is normally employed. Under these conditions, the design for a machine could be as follows.

A high voltage platform on which the source and analysing magnet would sit; this sample could be tested up to 450 kV. From a technical point of view, the construction of high voltage isolation power transformers gives no specific problems (with a CAPRICE- type ECRIS, 3 5 k W is required; with the N E O M A F I O S a level of less than 2 kW is needed). The optics which will with-

R~ LT IF I F R

Fig. 6. Single-loop induction high voltage supply.

stand 450 kV is of a classical design long known with SAMES electrostatic machines. The high voltage generator of the multiplier type exists and characteristic performances reach 5 mA. With a high voltage transformer, 450 kV and 100 mA are within reach of the ISU-type single-loop inductor [6J (Fig. 6 .

The vacuum problems are solved very casdy: this is at variance with classical accelerators.

As seen in Fig. 7. for the source itself a 50 I ~ lurbomolecular pump is used (Fig. IL on the extraction side a 350 1 s ~ turbomolecular pump and on the analysing magnet exit side a 150 1 ~ ' lurbomolecular pump on the platform. On the ground side, a 500 1 s ~- ~ turbomolecular pump will ensure in operation a pressure lower than 10 ~ torr for a classical vacuum mount ]'his is because the ECRIS has a very high ionizauon efficiency (about 100% and releases no gas to the extraction and accelerator parts. It is e v e n pos- sible, depending on the requirements, to realize a high vacuum machine.

The operation of such a machine, based on the possibility of setting the high voltage platform from 50 to 450 kV, would allow an energy range from 50q to 450q keV with the same species (q is the ion charge state selected on the platform).

E.C.R. SOURCE F

SOURCE SUPPLIES

J

HIGH VOLTAGE ELECIRODE

< "~ALTERNATOR MOTOR

Fig, 7. Schematic outline of a flexible versatile implanter.

DAMPING RESISTOR

DEFLECIlNG MAGNE]

CONSIANI FIELD TUBE , • . . . n L . , • ,

* i i i i | i i i

I ; ; ; ~ ; ; '~ ; !

II.V. GENERATOR I SLI

FOCUSING SELECTION MAGNET

We now consider some problems connected with the ion-surface interaction.

4. Interaction of highly charged ions with metal surfaces

The studies of ion interaction with solids have been timely; they have mostly been performed utilizing singly charged ions: protons or heavier projectiles [7, 8]. Recently, with the introduction of the new ECRISs, a renewed interest in these studies has been observed, mostly because of the possibility of covering a very large energy range ]9] and a very extended range of ion charges ]I0-1 2].

In these new situations, the studies were oriented towards the analysis of electron emis- sion by surfaces and of their energy distribution. Among the main features, it can be stated that two types of interaction occur: at low velocities, the electron yield y (the number of electrons emitted per incident ion) is mostly related to the ion potential energy and as such increases with increasing ion charge. With an increase in the ion velocity, y decreases to a minimum and increases again, but at a smaller rate. This increase is usually associated with the kinetic interaction. The yield associated with the turning point increases with increasing ion charge [9]. The experimental device uses an ECRIS and the characteristic of the X-ray emission when the incident ion captures electrons is measured.

4.1. Experimental method An ion beam delivered by the ECRIS is mass

and charge analysed to select Ne ~÷ and Ar 17+. The energy range was 45-120 keV for neon and 60-350 keV for argon. The ion beam is incident on a metal surface at an incidence angle of 45°; X-rays are detected utilizing an Si(Li) detector looking at the target at 45 ° . The background pressure is less than 10 7 torr.

4.2. Resuhs and discussion Two series of measurements were performed

for each ion. First a charge exchange collision with an atom

is observed. It is well known that in an electron transfer the capture is associated with a large cross-section and the most populated level to which transfer [ 13] occurs is defined as

?3,,4 Hp ~ 11!2

215

where Q is the incident ion charge and I the atom ionization potential in atomic units. With hydro- gen-like projectiles, the decay gives a helium-like spectrum. For the excited levels of these ions, the lifetime and cascades are well known [14]. A typi- cal decay time is of the order of 10 ~-L10 I~ s. The energy of the most intense transition (basi- cally Lyman ct) end term of the cascade is at 0.92 keV for Ne s+ and at 3.1 keV for Ar 1~+.

Then, in a second series of experiments, the interaction of the hydrogen-like ions with a solid surface is observed. A common feature should be noted. The main peak is displaced to lower energies and is at energies intermediate between the Lyman ct unperturbed value and the charac- teristic K X-ray values a,,2K, I~j, I~H (0.85 keV) for neon and Ctl,_~K, I~l, L m (2.95 keV) for argon. This can be understood as arising because the ion

(a)

k/3 350 300 250 2[]O 15Q IOO

5Q

350 300 250 20Q 150 100

50

lOO 150 200 25Q 300 Ly

Ly,~: ! : : .. "~ B

";'..:...".'.. ~ ~ : : ~ "7 , .V g :

JJ. / .

100 150 200 250 300

350

35Q

z

111 . . . . . . .

(b) ~ ~I AR17+ ÷ N2

fill

2559 3101 3643 4135

Fig. 8. X-ray spectra: (a Nc '~+ interacting with a carbon target (90 keV); (b Ar 17+ interacting with a copper target (170 keV ).

216

interacts with the surface, lowering the potential barrier. Electrons from the Fermi level are thus flowing in a vacuum towards the ion. Some of them are captured and a very complex competi- tion for decay takes place. Considering the dis- placement in energy E = 80 eV of the main peak, it is very likely that the L shell in argon is not completely filled and there are four L electrons as "spectators" to the filling of K shell vacancy (Fig. 8). This could probably be intepreted as taking place in vacuum before the ion reaches the surface. The broad structure at higher energies is not yet fully understood.

5. Conclusion

We have shown that the features currently desired [15] for an implanter--larger energy range, high current at all energies, stable opera- tion over long periods of time and good vacuum conditions--are within reach. Moreover, with mass-to-charge selection and good vacuum con- ditions, beam pollution is easily avoided.

The following key problems in the use of such a machine deserve research: better understanding of the kiloelectronvolt and megaelectronvolt ion range-energy curves and damage when use is made of multiply charged ions and comparison with singly charged ions at same velocity. Some aspects of the interaction of highly charged ions with metallic surfaces with energies from 50 to 350 keV indicate a possible difference.

References

1 S. Bliman et aL, IEEL ])'ans. ,\'mL 5;ci., 19 (2)(1972~ 2(10.

2 M. Delaunay, S. Dousson, R. Geller, B. Jacquot, D. Hitz, P. Ludwig, P. Sortais and S. Bliman, NucL lnstrum, ,~lethods B, 23 (1987) 177.

3 B. Jacquot and R. Geller, CAPRICE 10 GHz: new 2 , ) radial B field, l¥oe. tnt. ( '(m/ (311 I;'lectron (~lclotron Resonance loll Sourees, East l.ansing, ML No~!embec 19h'7, in NucL lnstrum. Method~' B, (1988), in the press.

4 R. Geller et al., The Grenoble ECRIS status 1987 and proposals for ECRIS scalings, Proc. Ira. Conf on t-lectron (}'clotron Resonance loft Sourees, East Lansing, MI, November 1987. in NueL lnstrum. Metttod~ B,

1988), in the press. 5 Proc. ll th Int. (ot([i ott ()clotrons and lheir Applica-

tions, 7okyo, October 13 lZ I~),~0. tonics Publishing Company, Tokyo, 1986, p. 69q i.

6 Generateur HT type ISU, Data .Sheet, 1 R E t . E ( , chemin de Malacher 38243 Meylan.

7 N. Bohr and J. Lindhard, K. Dan. l/idensk. Sel~k. Mat.- Phys. Medd., 26' (7) ( 1954).

8 W. E. Meyerhof and K. Taulbjerg, Annu. Rev. NucL Sci., 2711977) 279.

9 M. Delaunay, M. Fehringher, R. Geller, D. Hitz, P. 'v'arga and H. Winter, Phys. Rev. B, 35 i 1987) 4232.

10 M. Fehringer, M. I)elaunay, R. Geller, P. ¥arga and H. Winter. Nuel. lnstrum. Methods B, 23 (1987) 245.

11 M. Delaunay, S. Dousson, R. Geller, B. Jacquot, D. Hitz. P. l.udwig, P. Sortais and S. Bliman. NIte/. ln.wrttm. Methods B, 23 (1987) 177.

12 F. W. Meyer, C. (_. Havener, S. H. Overbury. K. J. Snowdon, D. M. Zehner, W. Heitand and H: Heroine, A'ucL lnstrum. Methods B, 23 (1987) 234.

13 S. Blimam J. P. Desclaux, D. Hilz, R lndelicato and P. Marseille, NucL lnstrum. Methods B, 31 i 1988) 33(1.

14 tt. A. Bethe and E. E. Salpelcr, Quantum Meehanic.s o! One- amt l wo-Electron Atoms, Springer, Berlin. 1957.

15 l). Pramanik and A. N. Saxena, ;Vt4cl. lnstrum. Methods B, 1¢)-11 (1985~ 493.