AU632269B2 - Plasma ion implantation - Google Patents

Description

i;r~ -i COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION Form (ORIGINAL) 6 2 FOR OFFICE USE 3 i 26 Short Title: Int. Cl: Application Number: Lodged: PI 6457 28 January 1988 Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE CO: PLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION New Illawarra Road, Lucas Heights, New South Wales 2234, Australia IAN JAMES DONNELLY and JOHN TENDYS GRIFFITH HACK CO.

71 YORK STREET SYDNEY NSW 2000

AUSTRALIA

Complete Specification for tlat invention entitled: PLASMA ION IMPLANTATION The following statement is a full description of this invention, including the best method of performing it known to us:- 6316A/HM

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The present invention relates to ion implantation in materials by use of a plasma.

Ion implantation is useful in many fields and techniques for ion implantation have been published in the literature. For example it is known to treat steels by implanting nitrogen ions to achieve specific metallurgical surface characteristics. Ion implantation has also been proposed in the field of doping semi-conductors and implanting ceramics. In these processes a beam of ions is generated and is incident on the target material where implantation occurs.

It has also been proposed in the literature to use a plasma, rather than a beam; ions in the plasma are implanted into a target immersed in the plasma. For example a plasma is formed at a temperature of 50,000 0 K, and a negative potential is applied to the target whereby an electric field normal to the surface of the target is provided, thereby establishing a sheath across which ions accelerate onto the target.

Improved and alternative methods of ion implantation would be desirable as limitations exist with previous proposals.

According to the present invention, in one aspect, there is provided a method of ion implantation comprising locating a target in a working chamber, generating a plasma in the working chamber for providina the ions required for implantation into the target, applying a negative potential to the target to establish an electric field substantially normal to the surface of the target and for accelerating the ions into the target, the method being characterised by generating the plasma with electromagnetic energy.

Preferably, the invention is implemented using high frequency electromagnetic energy, for example radio frequency energy) or microwaves.

Typically the plasma will have a temperature of around 50,000°K and a pulsed negative potential of the order of kV will be applied to the target.

-2- 5809S/JM/12.4.91 In a second aspect, the invention consists in an apparatus for ion implantation in a target, the apparatus having a plasma chamber, means for applying high frequency electromagnetic radiation in the chamber to form a plasma therein, means for mount:ing a target in the plasma, and means for applying a high negative voltage to tne target whereby ion implantation into the target occurs.

It has now been found that plasmas generated with electromagnetic energy have characteristics such that, in co-operation with the other method steps, an implantation technique having significant advantages is achieved. For example, large and complex shaped targets should be capable of being treated effectively. The implantation of the ions should be substantially perpendicular to the surface of the target and the implantation should be substantially uniform.

Sad Furthermore, use of the present invention advantageously includes controlling the negative voltage applied to the target whereby a desired depth profile of the implanted species of ion can be achieved and the effects of any arc formation in the sheath can be minimised.

Furthermore, use of the present invention advantageously includes controlling the plasma density by adjusting the level of the high frequency electromagnetic energy used to generate the plasma. This allows control of the implantation rate.

Compared with prior art proposals in which a beam of ions is used, it appears that the use of the present invention offers potential advantages in terms of method control, enhanced rate of implantation and uniformity of implantation, particularly where complex shaped targets are used.

h Previously, the use of plasmas for ion implantation has been suggested, but the plasmas have been generated by a filament discharge; these plasmas have relatively low density and low ionisation. This results in limits on the energy per unit ion mass obtained during acceleration of ions across the sheath which surrounds the target and which results from the electric field created by the negative potential applied to the target. Furthermore, with such -3- 5809S/JM/12.4.91 -ir: i "1 ::i plasmas the sheaths have large widths and the ionisation of gas in the sheath region can lead to the formation of arcs.

The present invention should be applicable to ion implantation of a wide range of materials which can be placed into plasma form. Furthermore, the method may be useful for recoil ion implantation of any material deposited as a thin layer on the surface of the target, for example by a sputtering technique. With this technique, ions froii the plasma impact on the thin layer and cause atoms (or molecules) from the layer to be implanted into the substrate.

By way of example only, the invention will be further described with reference to the accompanying drawings, of which:- 25 Fig. 1 is a schematic sectional view illustrative of implementation of the invention using r.f. energy applied in a device known as a rotamak, and Fig. 2 is a schematic illustration of alternative modes of implementing the invention.

Refering first to Fig. i, a known rotamak device is illustrated, this device having been developed for laboratory research work on the characteristics of plasmas.

The device comprises a generally spherical glass housing having, on diametrically opposed sides, first and second pumping ports 11 and 12 and two Helmholtz coils one of which is arranged about a vertical axis and has coil port,.ons 13A and 13B surrounding the upper aird lower portions of the vessel. The other Helmholtz coil (not shown in the drawing), is arranged about a horizontal axis perpendicular to the drawing. A radio frequency signal (typically 1 MHz) is applied to the Helmholtz coils, thereby establishing a rotating magnetic field which is schematically illustrated and which has the effect of ionising the filling gas and establishing a plasma. Vertically orientated field coils 14 are disposed around the necks of the housing which lead to the pumping ports 11 and 12 for applying a steady magnetic field which helps control the plasma density and establishes a magnetic bottle effect.

The pumping ports are used for connection to valves and pumping equipment for initially evacuating the housing and 4 5809S/HM then bleeding into the housing a very small amount of gaseous substance which, under excitation by the magnetic field, establishes a plasma.

The invention is implemented by mounting a target on a target holder 16 so as to be located within the plasma and connecting the target to a negative pulsed voltage supply 17, an anode 18 connected to the voltage supply also protruding into the housing. The target holder may be connected to a cooling/heating system.

Referring now to Fig. 2, a modified configuration is sllown including two options. A different shape of glass housing 20 is utilised with a magnetic coil 21 at one end over an extension portion 23 of the housing 20, the magnetic coil. being connected to a very high frequency r.f. power source, typically operating at 10-30 MHz. An alternative means of energising the plasma is through a microwave source for example in the form of a microwave system having a horn antenna 24 located near one end of the housing.

S 20 The configuration has optional coils, not shown, similar to the magnetic field coils 14 in Fig. 1, and placed so as to generate a steady magnetic field which enhances the plasma production by micro-wave energy.

Similarly to Figure 1, the housing 20 has a target 25 mounted there.n on a holder 26; the target is connected to a negative pulsed voltage source 27, and an anode extends into the housing spaced from the target.

Furthermore, the apparatus embodying the invention may, partly or wholly, be of metal construction. In the latter case, access of the electromagnetic energy to the plasma is obtained by placing the r.f. magnetic field coils or the microwave horn antenna inside the metal housing.

In the above ex~nples, typically the voltage supply is at 20 kV with a. 50 microsecond negative voltage pulse applied for about 200 times at intervals of 500 microseconds in order to cause ion implantation. For example a mild steel sample inserted in a nitrogen plasma has been given a dose of about 3 x 1016 nitrogen ions per square centimetre. Implantation is found to be to a depth consistent with ion energies of 20 keV.

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Because electro-magnetic radiation is used to generate a plasma the sheath which forms around the target 15 or reaches a state of equilibrium and reaches this state in a relatively short time for example, 5 micro seconds for an 8 mm spherical electrode with a voltage power supply of 18 kV.

Increasing the filling pressure of the housing 20 slightly decreases the ion density but increases the ionisation rate (implying a decrease in particle confinement time).

In contrast to the prior art the preferred embodiments described above not only result in a steady state sheath but also an implantation current which itself reaches an equilibrium value so that the ions entering the sheath are accelerated by the full implantation voltage as previously mentioned because the plasma density can be controlled by adjusting the level of high frequency electro-magnetic energy used to generate the plasma. This also means that the ionisation rate can be increased by simply raising the electro-magnetic energy level.

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Claims (16)

1. A method of ion implantation comprising locating a target in a working chamber, generating a plasma in the working chamber for providing the ions required for implantation into the target, applying a negative potential to the target to establish an electric field substantially normal to the surface of the target and which is arranged to accelerate the ions into the target, the method being characterised by generating the plasma with electromagnetic energy.

2. A method as claimed in Claim 1, wherein the electromagnetic energy is in the form of radio frequency fields or microwaves.

3. A method as claimed in Claim 1 or Claim 2 wherein the plasma is generated to have a temperature of 50,000°K.

4. A method as claimed in any one of the preceding claims and wherein a pulsed negative potential of the order of 50 kV is applied to the target.

5. A method as claimed in any one of the preceding claims and including controlling the negative potential applied to the target such that the depth profile of the implanted species of ion is controlled.

6. A method as claimed in any one of the preceding claims wherein the electromagnetic energy is in the form of high frequency energy and the method includes adjusting the level of the high frequency energy for controlling the plasma density whereby a controlled implantation rate is achieved.

7. A method as claimed in any one of the preceding claims and wherein the target has a thin surface layer and the method is operated to cause recoil ion implantation.

8. A method as claimed in any one of the preceding claims wherein the negative potential applied to the targe is pulsed with a pulse duration of substantially microseconds and an interval between pulses of substantially 500 microseconds and of the order of 200 pulses are applied. 6 5809S/JM/12.4.91 A

9. A method as claimed in any one of the preceding claims and wherein the method utilises an apparatus substantially as described herein with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

10. An apparatus for ion implantation in a target, the apparatus having a plasma chamber, means for applying high frequency electromagnetic radiation in the chamber to form a plasma therein, means for mounting a target in the plasma, and means for applying a high negative voltage to the target whereby ion implantation into the target occurs.

11. An apparatus as claimed in Claim 10, and wherein the electromagnetic radiation means generates high frequency radio fields or microwaves.

12. An apparatus as claimed in Claim 10 or 11, wherein said voltage means is adapted to apply a pulsed negative voltage of the order of 50 kV.

13. An apparatus as claimed in Claim 12 and wherein the voltage means is adapted to provide pulses of abot microsecond duration with about 500 microsecond intervals between pulses.

14. An apparatus as claimed in any one of Claims !0 to 13, and including means for controlling the negative voltage applied to the target.

15. An apparatus as claimed in any one of Claims 10 to 14, and including means for adjusting the level of the electromagnetic energy whereby plasma density is controlled. i

16. An apparatus substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings. DATED this 12th day of April, 1991 AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION By their Patent Attorneys GRIFFITH HACK CO. -7- 5809S/JM/12.4.91