GB2311164A

GB2311164A - Large area plasma generator

Info

Publication number: GB2311164A
Application number: GB9605337A
Authority: GB
Inventors: Peter Charles Johnson
Original assignee: AEA Technology PLC; UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority; Ricardo AEA Ltd
Priority date: 1996-03-13
Filing date: 1996-03-13
Publication date: 1997-09-17
Also published as: GB9605337D0

Abstract

An inductively-coupled radio-frequency plasma generator includes a plurality of radio-frequency antennae 14 symmetrically disposed at one end of the chamber in the end and/or side walls. A number of embodiments are described adapted to operate as plasma processing equipment e.g. for ion implantation, ion beam and neutral particle beam generators.

Description

Larae Area Plasma Generator The present invention relates to plasma generators and, more specifically to plasma generators of the type in which a plasma is generated in an enclosure by means of a radio-frequency electromagnetic field which is generated within the enclosure by means of an antenna.

In UK Patent GB 2,231,197B there is disclosed a plasma processing apparatus which includes a plasma generator incorporating an electrode assembly which includes an antenna in the form of a flat, or nearly so, circular coil situated at one end of a cylindrical chamber into which a gaseous medium in which a plasma is to be generated is admitted. The end of the cylindrical chamber adjacent to the antenna is made of a dielectric material so that radio frequency electromagnetic energy can be coupled into the chamber via the antenna. As explained in the above mentioned specification, because electrons respond to the high frequency electromagnetic field far more rapidly than do molecules, because of the greater inertia of the latter, they are stripped off the molecules which then become ionised, forming a plasma.

Subsequently the electrons are removed from the plasma leaving a positively ionised gaseous medium.

The antenna arrangement described in GB 2,231,197B is satisfactory for use with plasma generating apparatus having a diameter up to about eight inches. Above this size, however, the thickness of the dielectric window required to withstand the pressure differential between the interior of the plasma generator, which is at a vacuum of about 10 millibar, and its surroundings, becomes excessive, as do the power/voltage requirements of the antenna it an adequate magnetic field is to be generated in the plasma generation chamber. Also, it becomes more difficult to provide for tuning of the antenna to maximise the ionisation of particular gases.

Similar arrengements are shown in US patent specification 4,948,458 and EPO 379 828.

In US patent 4,948,458 there is disclosed an apparatus for the plasma processing of semiconductor wafers in which a wafer to be treated is placed in a cylindrical chamber in which the end of the chamber opposite the wafer includes a window made of a dielectric material, quartz being disclosed, specifically. Adjacent the outer surface of the window is a planar spiral antenna which is powered by an rf generator via a matching circlt. as in the arrangement disclosed in GB 2,231,197. Adjacent the inner surface of the window is a process gas inlet manifold which has orifices which direct the process gas into the chamber in a plane parallel to the inner surface of the window and with a tangential cm.lponent of velocity.

The apparatus shown and described in US patent 4,948,458 is very conceptual, but it will suffer the same electrical/mechanical weaknesses as that shown in GB 2,231,197. In addition, its efficiency will be reduced because there appears to be no attempt to prevent electron loss from the plasma by drift to the wall of the chamber.

In EPO 3?9 828 again there is disclosed a cylindrical plasma generation/work chamber with a quartz end adjacent: the outer surface of which there is a planar spiral rf anterira adjacent the inner surface of which there is an inlet for a process gas. In this case, the side wall of the chamber also is made of quartz. A plurality of magnets are so disposed about the cylindrical side wall and the end of the chamber as to reduce drift of the plasma to the wall and end of the chamber.

This apparatus again will suffer from the same problems as that of GB 2,231,197 if attempts are made to scale it beyond some eight inches in diameter. The use of quartz for the whole of the chamber, moreover, means that the problem of mechanical weakness would be exacerbated.

Thus, none of the prior art devices would be capable of processing satisfactorily the larger ( > 12 inches) semiconductor wafers which are coming into use, and in particular, flat panel devices of A4 size and above.

The above described plasma generators also have another common restricting feature and that is that the geometry of the magnetic field produced by their circular antenna, and hence the plasma profile, is fixed.

It is an cbject of the present invention to provide an improved rf powered plasma generator which is capable of being scaled to dimensions not hitherto practicable.

According to the present invention there is provided a plasma generator including a chamber having an axis of symmetry, a plurality of radio-frequency inductive plasma exciter antennae disposed symmetrically with respect to the axis of symmetry of the chamber at one end thereof and means for admitting into the chamber in the proximity of the antennae a gaseous medium to be converted into a plasma.

The antennae may be in the form of loops, helical coils or flat spiral or involute coils.

The word 'flat' is not used herein in its geometrical sense, but to denote an antenna in which the radiating element conforms to a surface as opposed to forming a helix.

In one form of the invention, the exciter antennae are helical coils and are mounted so that they have their longitudinal axes parallel to that of the chamber.

Alternatively, the exciter antennae are mounted around the periphery of the chamber so that their longitudinal axes are perpendicular to that of the chamber.

If so desired, a combination of antennae of both orientations can be used.

The helical antennae can be simple coils or so formed that the bottom of the coil is closed by a spiral coil.

In a second form of the invention, the antennae are flat in the above sense and are situated outside an end wall of the chamber substantially at right angles to the axis of symmetry of the chamber and there is a region of dielectric medium in the end wall of the chamber opposite each antenna, the antennae being disposed in an array such as to generate a substantially uniform source of plasma across the end of the chamber.

Antennae also can be positioned at the side of the chamber parallel to the walls of the chamber, with or without antennae at the end wall of the chamber.

If the walls of the chamber have appreciable curvature, then the antennae can be shaped to conform to the walls of the chamber.

With both types of antennae, if the chamber has a circular cross-section then preferably the antennae are positioned at the end wall of the chamber in a closepacked array of at least three antennae.

If the chamber has a square or rectangular crosssect ion, then the minimum number of antennae in the array is two.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows schematically, a longitudinal section of a plasma generator embodying the invention, Figure 2 shows schematically, a longitudinal section of a plasma processing apparatus, incorporating the plasma generator of Figure 1, Figure 3 shows schematically, a longitudinal section of an ion beam generator, incorporating the plasma generator of Figure 1, Figure 4 shows schematically, a longitudinal section of a plasma generator embodying the invention in which antennae are disposed about the side of the plasma generator, Figure 5 snows schematically, a longitudinal section of another form of helical exciter antenna which can be used with the embodiments of the invention shown in Figures 1 to 4, Figure 6 shows schematically, a longitudinal section of a second form of plasma generator embodying the invention, Figure 7 shows schematically, a longitudinal section of a plasma processing apparatus, incorporating the plasma generator of Figure 6, Figure 8 shows schematically, a longitudinal section of an ion beam generator incorporating the plasma generator of Figure 6, Figure 9 shows schematically, a plasma generator embodying the invention in which flat antennae are disposed about the side of the plasma generator as well as the end wall of the plasma generator, Figure 11 shows schematically a plan of an array of antennae suitable for use with a plasma generator of square cross-section, and Figure i.2 ows schematically a plan of an array of antennae suitable for use with a plasma generator of rectangular cross-section.

Referring to Figure 1 of the drawings, a plasma generation chamber embodying the invention consists of a chamber 1 which is made of a non-magnetic metal such as stainless steel or aluminium. The chamber 1 consists of a cylindrical portion 2 which is closed at one end by an end wall 3. The ,unction between the end wall 3 and the cylindrical portion 2 of the chamber 1 is rendered vacuum tight by an O-ring seal 4. Symmetrically disposed in the end wall 3 of the chamber 1 are three windows 5 only one of which is shown in Figure 1. Each window 5 has a mounting flange 6 and an O-ring seal 7. Associated with each window 5 is an inlet 8 to allow a gaseous medium to be turned into a plasma to be admitted into the chamber 1. The inlets 8 are arranged to direct the gaseous medium across the associated windows 5. The inlets 8 are fed from a plenum chamber 8' which is incorporated within the end wall 3 of the chamber 1. Also incorporated in the end wall 3 of the chamber 1 are cooling passages 9.

Similar cooling passages 10 are included in top and bottom flanges, 11 and 12, respectively, which form part of the cylindrical portion 2 of the chamber 1. As is common practice in the plasma generator art, a plurality of linear arrays of magnets 13 are disposed regularly around the cylindrical portion 2 of the chamber 1. These magnets produce a cusped magnetic field around the periphery of the cylindrical portion 2 of the chamber 1 which acts to confine the plasma to the central region of the chamber 1.

Mounted in each window 5 is an inductive exciter antenna 14. Each antenna 14 consists of a mounting flange 17 to which there is attached a closed cylinder 18 made of a dielectric ceramic such as alumina. Inside the cylinder 18 is a helical coil 19 so dimensioned as to radiate electromagnetic radiation efficiently at a frequency which is known to be effective for the creation of gaseous plasmas. The coil 19 is hollow so that a coolant can be circulated through it if required.

Associated with each antenna 14 is an electrical matching turning circuit 15 and a common high frequency power generator 16, although separate power generation circuits can be used if desired. The matching/tuning circuits 15 enable the antennae to be tuned to a frequency which is most efficient for exciting any given gaseous medium within the chamber 1. Typically, the power generator 16 is capable of operating over the range 1 MHz to 100MHz and at a power of about 1 kW and the matching/tuning circuits 15 are capable of causing the antennae 14 to resonate at a chosen frequency within this range. A frequency commonly used to produce plasma is 13.56 MHz and this frequency is suitable for the purposes of this invention.

The central section of the end wall 3 is depressed so that the plane of the antennae 14 is lower than the uppermost of the magnets 13. By this means, drift of electrons to the top of the cylindrical portion 2 of the chamber 1 and the periphery of the end wall 3 of the chamber 1 is greatly reduced.

Figure 2 shows the plasma generator of Figure 1 incorporated in an apparatus for the plasma processing of semiconductor wafers or larger planar semiconductor substrates for use in the production of flat-panel displays. The apparatus also can be used for the plasma processing of other artefacts.

Referring to Figure 2, in which those components which are common with those of the plasma generator described with reference to Figure 1 have the same reference numerals, the bottom of the cylindrical portion 2 of the chamber 1 is closed by a baseplate 21 which includes a port 22 through which the closed chamber 1 can be connected to a vacuum system (not shown). The junction between the base plate 21 and the bottom flange 12 of the cylindrical portion 2 of the chamber 1 is sealed with an O-ring 23.

Mounted centrally in the base-plate 21 is an insulated workpiece support 24 which is connected via a capacitor 25 to a second rf power generator 26. The purpose of the capacitor 25 and the power generator 26 is to enable a rf potential to be applied to the workpiece support 24 to attract ions from the plasma generated, in the chamber 1 to a workpiece 27 on the workpiece support 24 so that they bombard the workpiece 27 with an appreciable, controlled, energy. In practice, rf potentials of between -10 volts to -2,000 volts can be used. Preferably the power generator 26 is operated at a different frequency to that of the power generator 15 and at a power level less than ten per cent that of the power generator 15.

In use, the chamber 1 is evacuated to a pressure typically of about 10-3 millibar to 10-7 millibar and then a mixture of a plasma forming gas and a reactive gas is admitted to the chamber 1 until a pressure of about 10-3 millibar is reached. The power generator 15 is then energised to create a plasma in the chamber 1 and the power generator 26 is energised to initiate whatever treatment is to be carried out on the workpiece 27.

Examples of suitable gases are: Q; Cl2; SF6; CF4; C2F; or a C2F6/CHF3- In the apparatus shown, workpieces are placed in, or removed from, the chamber 1 by raising the end wall 3 of the chamber 1. If preferred, however, a cylindrical addition with access ports can be provided between the cylindrical portion 2 of the chamber 1 and the base plate 21 to form a processing chamber. In this case, one or more evacuation ports can be provided in the side wall of the processing chamber rather than in the base plate 21.

Alternatively, the cylindrical portion 2 of the chamber 1 can include a lower region in which there are no plasma confining magnets 13 and in which access and/or evacuation ports are situated.

The capacitor 25 and the r.f. generator 26 can be replaced by a dc or pulsed power supply so as to enable other forms of plasma processing operations to be carried out.

For example, in a plasma immersed ion implantation process the workpiece support 24 is so arranged as to support a workpiece, which need not be planar but can be of complex shape for example, a component of a mechanical device, in such a position within the chamber 1 that, in use, the workpiece will be immersed in the plasma in the chamber 1. A gaseous medium including appropriate species is admitted to the chamber and ionised in the plasma, a dc or pulsed potential is applied to the workpiece and the workpiece is subjected to overall bombardment by ions extracted from the plasma by the dc or pulsed potential applied to the workpiece.

Figure 3 shows the plasma generator of Figure 1 incorporated into an ion beam source. Referring to Figure 3 in which as before, those components which are common with those of the plasma generator described with reference to Figure 1 have the same reference numerals, the bottom end of the cylindrical portion 2 of the chamber 1 is closed by a metal grid 31 which is mounted on and insulated from the lower flange 12 of the cylindrical portion 2 of the chamber 1, by a support ring 32. A series of ion accelerating and beam-forming electrodes 33, only one of which is shown, is positioned outside the grid 31 and insulated from it by a support ring 34.

By using different combinations of grids and acceleration electrodes, beams of ions having energies ranging from less than 100 eV to greater than 100 KeV can be formed.

The ion source may be converted into a neutral particle beam generator by including an ion neutraliser, such as a charge exchange cell, downstream of the ion acceleration electrode or electrodes 32.

Figure 4 shows an embodiment of the present invention in which the helical exciter antennae are incorporated in the upper end of the side wall of the cylindrical portion 2 of the chamber 1. Each of these antennae is of the same form as the antenna 14 shown in Figure 1 and has the same gas feed and matching/tuning circuit associated with it, hence the same reference numerals are used as before. As shown, all the antennae 14 are driven from a common power source, but this is not necessarily so.

Magnets similar to the magnets 13 in the side wall of the chamber 1 can be incorporated in the end wall 3 of the chamber 1 to provide a similar cusped magnetic field to stop drift of the plasma to the end wall 3 of the chamber 1.

As before, the plasma generator of Figure 4 can be incorporated into a plasma processing apparatus, used as a high-current ion source or as a neutral particle beam source.

In another variation of the present invention which is not illustrated, exciter antennae 14 are incorporated in the end wall 3 of the chamber 1, and the upper end of the cylindrical portion 2 of the chamber 1. The use of the additional antennae 7 enables a larger volume of plasma to be generated in the chamber 1, so increasing the number of ions which can be extracted from a given size of chamber 1. Again, the plasma generator can be incorporated into a plasma processing apparatus, used as a high-current ion source or as a neutral particle beam source.

Although in Figures 1 to 4 the end wall 3 of the chamber 1 is shown as flat, it can be dished, so increasing its ability to withstand the pressure loading upon it.

Furthermore, the cross-section of the chamber 1 can be other than circular. Figures 10 and 11 show plan views of the arrangements of antennae in the end wall 3 of the chamber 1 which can be used with a square or rectangular chamber 1, respectively. These arrangements of antennae are not limiting. For example, the antennae can be made square in plan to improve the packing of them.

Referring to Figure 5, another form of helical plasma exciter antenna is generally similar to that previously described and the same reference numerals are used to describe similar features. In this form of antenna, however, the base of the helical coil 19 is closed by a flat spiral or involute portion 51.

Referring to Figure 6, there is shown a longitudinal section of a second form of plasma generator embodying the invention. Many components are the same as are used in the embodiment of the invention described with reference to Figure 1 and these components have the same reference numerals. In this embodiment of the invention, the window 5 is closed by a membrane 61 of dielectric medium sufficiently robust to be capable of withstanding the pressure differential between the inside and the outside of the chamber 1. Again, a suitable material is alumina, although silica also is suitable. An exciter antenna 62 in the form of a flat spiral or involute is positioned adjacent the membrane 61 outside the chamber 1. As before, the dimensions of the antenna 63 are chosen to be such that it radiates efficiently at the desired frequency. Also, again the antenna 62 is hollow so that a coolant can be circulated through it if so desired.

Figure 7 shows a plasma processor corresponding to that described with reference to Figure 2 and incorporating flat antennae as described with reference to Figure 6. Again, corresponding features have corresponding reference numerals.

Figure 8 shows an ion source corresponding to that described with reference to Figure 3 and incorporating flat antennae as described with reference to Figure 6.

Again similar features have the same reference numerals.

Figure 9 shows another embodiment of the second form of the invention in which there is included a number of additional flat antennae 14 disposed regularly around the perimeter of the upper end of the chamber 1. The remainder is the same as the plasma generator described with reference to Figure 6, and the same reference numerals are used for the same components.

Also, as before this form of plasma generator can be incorporated into plasma processing apparatus, an ion beam generator or a source of energetic neutral particles. It is not thought necessary to illustrate these embodiments of the invention.

If the chamber 1 is of a size such that, for mechanical reasons, the end wall 3 has to be curved, then the antennae can be dished to conform with the shape of the end wall 3. The same is true of any antennae in the side wall of the chamber 1.

It can be seen readily from the above embodiments of the present invention that plasma generators made according to the present invention can be scaled to sizes far beyond those achievable with the prior art systems.

Also, compared with the prior art, the present invention gives far more flexibility in the choice of cross-section of the chamber 1, which can be circular, square, rectangular or polygonal, as required.

Claims

1. A plasma generator including a chamber having an axis of symmetry, a plurality of radio-frequency inductive plasma exciter antennae disposed symmetrically with respect to the axis of symmetry of the chamber at one end thereof and means for admitting into the chamber in the proximity of the antennae a gaseous medium to be ? converted into a plasma.

2. A plasma generator according to Claim 1 including a plurality of magnets so disposed in a side wall of the chamber as to produce a magnetic field in the region of the side wall of the chamber adapted to confine a plasma in the central region of the chamber and also to increase its uniformity.

3. A plasma generator according to Claim 1 or Claim 2 wherein the plasma exciter antennae are in the form of helical coils or loops.

4. A plasma generator according to Claim 3 wherein the plasma exciter antennae are in the form of helical coils, the bottoms of which are closed by a flat spiral or involute portion.

5. A plasma generator according to Claim 3 or Claim 4 wherein the plasma exciter antennae loops or coils have a dielectric medium interposed between them and the plasma.

6. A plasma generator according to Claim 5 wherein the plasma exciter antennae are inside a closed cylinder of dielectric material.

7. A plasma generator according to any of Claims 3 to 6 wherein the plasma exciter antennae are mounted in an end wall of the chamber with their longitudinal axes parallel to the axis of symmetry of the chamber.

8. A plasma generator according to any one of Claims 3 to 6 wherein the plasma exciter antennae are mounted regularly around the periphery of the side wall of the chamber with their longitudinal axes perpendicular to the longitudinal axis of the chamber.

9. A plasma generator according to any of Claims 3 to 6 wherein the plasma exciter antennae are mounted in an end wall of the chamber with their longitudinal axes parallel to that of the chamber and in the side wall of the chamber adjacent the end wall with their longitudinal axes perpendicular to that of the chamber.

10. A plasma generator according to Claim 1 or Claim 2 wherein the antennae are flat and situated outside an end wall of the chamber substantially at right angles to the axis of symmetry of the chamber and there is a region of dielectric medium in the end wall of the chamber opposite each antenna, the antennae being disposed in an array such as to generate a substantially uniform magnetic field across the end of the chamber.

11. A plasma generator according to Claim 10 wherein there is included a plurality of flat antennae regularly disposed about the side wall of the chamber there being a region of the sice wall of the chamber opposite each antenna which is made of a dielectric medium.

12. A plasma generator according to Claim 9 or Claim 10 wherein the antennae are shaped to conform with part of the chamber wall where they are situated.

13. A plasma generator according to any of Claims 10 to 12 wherein the dielectric medium is alumina.

14. A plasma generator according to any of Claims 1 to 8 or 9 to 13 wherein the cross-section of the chamber is circular and the antennae are positioned in a closepacked array of at least three antennae.

15. A plasma generator according to any of Claims 1 to 14 wherein the cross-section of the chamber is a square or rectangle and the number of antennae is two or more.

16. A plasma generator according to any of Claims 1 to 8 or 9 to 13 wherein the cross-section of chamber is a polygon having at least six sides.

17. A plasma processing apparatus including a plasma generator according to any preceding claim wherein the end of the chamber remote from the antennae is closed by a base plate having a workpiece support electrically isolated from the baseplate, and means for applying to the workpiece support a radio frequency potential, the frequency of which differs from that applied to the antennae thereby to generate a negative bias on the workpiece support with respect to ions in the plasma formed in the chamber.

18. A plasma processing apparatus according to Claim 17 wherein there is included a region of the chamber below that in which the plasma is generated and the baseplate including one or more vacuum tight access ports.

19. An ion source including a plasma generator according to any of Claims 1 to 16 together with at least one electrode to which a potential can be applied to extract ions from a plasma formed in the plasma chamber.

20. An ion source according to Claim 19 wherein there is included further ion accelerating and/or beam-forming electrodes.

21. A neutral particle beam source including an ion source according to Claim 19 or Claim 20 and means for neutralising the ions after they emerge from the final accelerating electrode.

22. A plasma generator substantially as hereinbefore described and with reference to Figures 1, 4, 5, 6 and 9 of the accompanying drawings.

23. A plasma processing apparatus substantially as hereinbefore described and with reference to Figures 2 and 7 of the accompanying drawings.

24. An ion source substantially as hereinbefore described and with reference to Figures 3 and 8 of the accompanying drawings.

25. A neutral particle beam generator substantially as hereinbefore described.