High density plasma forming device

Abstract: The invention relates to a device, simple in construction, adapted to produce high density plasma. The device comprises a process chamber (3) in which a target (54) is mounted and at least one source chamber (2) which further comprises a RF helical coil antenna (10) . Plasma is formed by the ionisation of gas within the chamber under a vacuum, such that plasma formed in the source chamber may be focused mounted within a source chamber induces the formation of a high density plasma which is focused and directed towards a target by magnetic means (12, 13, 56, 57) .

Description: This invention relates to methods and apparatus for the production of high density plasmas. A variety of different processes are operated in a plasma environment including thin film coating, for example sputtering, evaporation, chemical vapour disposition, cleaning and etching processes. For example, it has been known for many years that thin film deposition processes and the properties of the deposited films themselves can be improved by allowing energetic ions to impinge on the growing surface of the film being deposited; this is due to an energy transfer between the energetic ions and the so-called "adatoms" of the thin film being deposited. This increases the surface mobility of the adatoms and allows them to migrate more readily to the preferred lattice sites. Sputtering processes are widely used for the deposition of thin coatings or films of materials on to substrates. Such processes take place in an evacuated chamber containing a small quantity of an ionisable gas, for example argon. Electrons emitted from a source held within the chamber ionise the gas to form a plasma; a (cathode) target comprising the material to be sputtered is bombarded by the ions causing atoms of the target material to be dislodges and subsequently deposited on to the substrate being coated. It is also well known that the rate of deposition in sputtering process may be increased by the use of magnetic means, for example an array of permanent magnets positioned in a predetermined manner (commonly as a closed loop) associated with the cathode target to create in use a plasma which is localised and concentrated along a sputtering zone of the target and thereby defined the area or region from which sputtering, or erosion of the target occurs. In vapour deposition processes, especially plasma enhanced chemical vapour deposition (PECVD), a chemical gad contained within a vacuum chamber is dissociated and activated by the plasma at a rate generally proportional to the density of the plasma. In plasma etching or cleaning processes, energetic ions and/or chemically active ions and/or chemically active ions produced by a plasma present in a vacuum chamber are employed to remove material from a substrate, for example in the production of semi conductor integrated circuits. In all such processes, an important parameter is the ability to produce high density plasmas at the lowest possible power consumption. Benefits of high density plasmas include the production of higher grade coatings in general, commonly allow for a greater homogeneity in the coating microstructure. It has previously been proposed to produce high density plasmas by means of electron cyclotron resonance (ECR) by use of microwaves at a specific frequency of 2.45 GHz and a magnetic field of 875 Gauss in vacuum. Although this produces a useful high density plasma, the size of the required magnetic field is such that it consumes high power levels and is therefore expensive. A more recent proposal for the production of high density plasmas is the use of high density plasma waves. These can be produced in a chamber at high vacuum, for example 10 -2 to 10 -4 mbar, by interaction between a uniform magnetic field and an electric field profile of an external antenna operating at a radio frequency (RF). Wave energy from the antenna emissions is transferred to the electrons produced in a plasma discharge, for example argon, present in the chamber by the well known mechanism known as Landau damping; with these waves, energy exchange is thought to occur in a much more efficient manner than with other types of discharge. This effect can generally be regarded as a collisionless damping of waves in a plasma due to particles in the plasma which possesses a velocity almost equal to the phase velocity of the wave. Such particles tend to travel with the wave without "seeing" a rapidly fluctuating electric field and can therefore exchange energy with the wave. A plasma containing electrons some of which are faster and some of which are slower than the wave itself. However, in a Maxwellian distribution, there are more slower electrons than faster ones and therefore there are more particles receiving energy from the plasma wave than those imparting energy to the wave. The 13.56 MHz frequency discharge is the most widely used to produce high density plasma waves, although related frequencies of 6.78 MHz and 27.12 MHz could also be employed. High density plasma wave induced plasma generation can usefully be performed in a chamber in which, or about which, field coils are present to effect a uniform magnetic field (commonly cylindrical) in a pre-determined area of the chamber. Three such field coils in a linear array spaced along the length of the relevant past of the chamber are generally sufficient. At one end of the array is positioned the RF antenna to cause, in use of the process, the intense plasma wave produced by interaction between the magnetic field and the RF power supply, which in turn accelerates plasma electrons by the Landau damping mechanism. The design of the RF antenna required to produce helicon waves and the subsequent generation of a high density plasma is, important. The design has been the subject of considerable discussion in original papers by R.W. Boswell and D. Henry published in "Applied Physics Letters" 47 (1985)1095, and subsequently by F.F. Chen published in "Journal Of Vacuum Science and Technology" A10 (1992) 1389, and thereafter by various other authors. Previous designs have centred on complex configurations in which the various operating modes are embodied to provide, in particular, a double loop system made from a continuous length of conductive material in which: a first loop having a main axis which is orientated parallel to the direction of the magnetic field present in the chamber; a second loop also having a main axis which is orientated parallel to the direction of the magnetic field present in the chamber; and an electrical cross linkage between the first and second loops of varying design. An example of apparatus including such an antenna is disclosed in US 4,990,229. The disclosed apparatus has a cylindrical plasma generation chamber about which is located the antenna and magnetic field coils. The magnetic field coils provide a magnetic field which transports the plasma to a separate process chamber. Gas is injected into both the source chamber and the process chamber. The size of the plasma in the process chamber can be controlled by varying the current in a further magnetic field coil. A substrate to be coated or etched is located within the process chamber, and is bombarded by the plasma which may have a density of up to 1x10 13 /cm 3 . Thus, US 4,990,229 discloses a high density plasma forming device having a vacuum chamber comprising a process chamber, a source chamber completely open at one end to the process chamber and an ionisable gas inlet; a target mounted within the process chamber; a radio frequency antenna for the formation of a plasma within the source chamber and a magnetic means by which the plasma may be directed and focused onto the target. Although it has been shown that the use of helicon wave systems can produce high density plasmas to good effect, the complexity of the RF antenna and their spatial configuration with the magnetic array required to interact with the RF power can provide difficulties in the implementation of helicon wave production of helicon wave plasmas. The invention is concerned with the provision of high density plasma systems embodying an antenna of simpler, easier to use design and having further benefits as described below. EP 0 489 407 A2 discloses a plasma reactor comprising a vacuum processing chamber into which etching gas and/or deposition gas is supplied. A section of the chamber is defined by a dielectric dome and an RF antenna is positioned about said dome for developing an etching and/or deposition plasma. The preferred RF antenna is a single turn configuration but an alternative multiple turn spiral arrangement is mentioned. One or more electromagnets are mounted adjacent the chamber which may be used to provide a magnetic field that enhances the density of the plasma at a semiconductor wafer positioned in the chamber. In accordance with the invention, there is provided a high density plasma forming device having a vacuum chamber comprising a process chamber, at least one source chamber completely open at one end to the process chamber and an ionisable gas inlet; a target mounted within the process chamber; a radio frequency antenna for the formation of a plasma within the source chamber; and a magnetic means by which the plasma may be directed and focused onto the target, wherein said antenna is a helical coil associated with the source chamber characterised in that the device comprises further magnetic means configured such that the plasma flow is deflected from the longitudinal axis of the source chamber onto the target by said further magnetic means. In accordance with a further aspect of the invention there is provided a process for the preparation of a high density plasma comprising the steps of: (i) supplying a helical coil radio frequency antenna associated with a source chamber with a discharge power supply, (ii) inducing the ionisation of an ionisable gas within the source chamber completely open at one end to a process chamber to form a plasma, and (iii) directing and focusing of the plasma onto the target within the process chamber by magnetic means, characterised in that said process comprises the further step of (iv) deflecting the plasma flow from the longitudinal axis of the source chamber onto the target by further magnetic means It has surprisingly been found that the use of a helically wound coil antenna, as opposed to the complex antennae used previously, can be used to good effect and to provide additional technical benefits leading to a factor of three enhancement in the plasma ion density. The helically wound coil antenna in the invention generally comprises an electrically conducting strip or wire with one end connected to the RF live supply and the other to earth (ground) and hence returns to an RF matching unit. As such, the RF power passes through each turn of the helically wound coil in turn between the two ends. The RF supply should generally be within the range of 1 to 200 MHz, however, the preferred frequencies would be within the range 1 to 30 MHz with frequencies of 6.78 MHz, 13.56 MHz and 27.12 MHz the most preferred. In general the RF frequency is surprisingly not crucial as long as the RF source is matched with the antenna in a manner know per se. The process of the invention should preferably be performed in a chamber made of an insulating material for example a generally cylindrical chamber, characterised in that quartz and glass are preferred materials. In preferred embodiments, the helically wound coil antenna can conveniently be wound around the outside of the relevant part of the chamber. This is especially applicable in the case of a metallic, for example brass, strip antenna around an insulating, for example quartz, chamber. The antenna is preferably cooled, for example water cooled. The helically wound coil antenna preferably comprises at least three turns and advantageously comprises from three to eight turns, for example three, four or five. The RF power applied to the helically wound coil antenna must be matched in a manner known per se to ensure the formation of a high density plasma wave in to the magnetic field present in the chamber. The antenna must be adapted to generate an RF electric field, interaction between this electric fields and the magnetic field generates a modified electric field which in turn produces a plasma wave which couples energy to plasma electrons through a Landau damping mechanism which in turn causes high ionisation of the plasma gas and hence a high intensity plasma. The magnet means can comprise a single magnet, for example an annular solenoid placed coaxially about the chamber and positioned remote from the antenna to produce, in use, a magnetic field relative to the antenna axis between the magnet and the antenna. Preferably, however, the magnetic means comprises more than one magnet. In preferred embodiments, there is provided a first (source) magnet associated with the antenna and a second (chamber) magnet positioned remote from the antenna to generate a linked magnetic field. In certain embodiments, the source magnet is preferably an annular solenoid positioned co-axially about the helically wound coil antenna with a small clearance between the external diameter of the antenna and the internal diameter of the solenoid. In some cases, however, the source chamber magnet is slightly removed to the side of the antenna away from the chamber magnet, or even to the opposite side of the process chamber whilst remaining co-axial therewith. In other preferred embodiments, the process chamber magnet also comprises an annular solenoid positioned co-axially with the antenna axis but having a larger diameter than the source magnet. In order to allow the magnetic lines of flux to bridge the process chamber. As an example, the course magnet can be such that it generates a magnetic field of the order of 5 x 10 -3 Tesla parallel to the axis of the source chamber and/or antenna and the process chamber magnet can generate a stronger magnetic field of the order of 5 x 10 -2 Tesla again parallel to the central axis of the source chamber and/or antenna with the linked magnetic field being in accordance with the invention. In the case of the use of solenoid magnet(s) in particular, the current flowing through the solenoid(s) determines the magnetic field strength, the magnetic field gradient across the chamber (if any) and the direction of the overall field. This is critical to the overall operation of the process and the control of its operating parameters. In general, in the case of using more than one magnet, the source and each chamber magnet must generate respective magnetic fields in the same direction; it should be noted, however, that the process works irrespective of the selected same direction. An advantage of the invention is the use of a further magnet on one side of the chamber between the source magnet (or the antenna if no source magnet is employed) and the chamber magnet surprisingly allows the plasma beam to be attracted or "steered" in the process chamber towards or away from the further magnet depending on the polarity of the magnet. In such embodiments, the use of a solenoid is advantageous in that switching the solenoid current in one direction will deflect the plasma towards the solenoid and switching the current in the other direction would repel the plasma away from the solenoid. There may, in certain circumstances, be problems associated with the deposition of material from a target present in the plasma not only on to a substrate held within the vacuum chamber but also on to the internal surfaces of the vacuum chamber, particularly when used for coating processes. Such problems are generally associated with RF leakage via such coatings on the internal surfaces which has an effect increasingly as the coatings become thicker and/or more widespread of reducing the overall efficiently of the coating process. In further preferred embodiments of the invention, therefore, it is proposed that the apparatus includes a vacuum process chamber having a source chamber in which the antenna is deployed, with magnet means present in the vicinity of the source chamber and the process chamber to propagate the magnetic field and thereby to generate the high density plasma out of the source chamber and in to the process chamber. In such an arrangement, a target comprising material to be deposited and a substrate on to which the target material is to deposited can be situated in the process chamber. In such an arrangement, even though some deposition may occur on the process chamber surfaces, relatively low or zero amounts of deposition of target material will generally occur on the interior surfaces of the source chamber where the antenna is positioned. As a result, RF leakage from the RF antenna via chamber surface deposits should be avoided. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings in which: Figure 1 is a schematic representation of an apparatus of the invention; Figure 2 is a schematic representation of the cross-section of an embodiment of the invention; Figure 3 is a further schematic representation of an apparatus of an embodiment of the invention; Figure 4 is a schematic representation of the cross-section of a second preferred embodiment of the invention possessing magnetic means to divert the flow of plasma; Figure 5 is a further schematic representation of the cross-section of an the second preferred embodiment of Figure 4. Figure 1 shows the external appearance of an embodiment of the invention 1 having a process chamber 3 a source chamber 2 with an associated coil 10. Figure 2 shows a schematic representation of apparatus of the invention. There is shown therein a substantially cylindrical vacuum chamber 1 having a source chamber 2 of a first cross section, a process chamber 3 of larger cross section and a termination end 4 of tapering cross section. The source chamber has an inlet 5 in to which ionisable process gas can be introduced. The process chamber 3 has an outlet 6 attached to vacuum pumps at 7 for evacuating the vacuum chamber 1 and, in use of the apparatus, causing a flow of process gas therethough. The termination end 4 is water cooled by a helically arranged water pipe 8; the end of the termination end 4 contains a glass view port 9 for observing the plasma 'P' generated in the chamber. A helically wound coil annular antenna 10 having four turns and being in the form of a brass strip whose ends are electrically insulated from each other with one end connected to an RF live supply 11 and the other end connected to earth. The four turns were each spaced apart for adjacent turns by about one or two centimetres. The overall length of the antenna was of the order of six to eight centimetres. The RF supply was at a frequency of 13.56MHz. About the antenna and co-axial therewith is a source magnet 12 in the form of an annular solenoid having an internal diameter slightly greater than the external diameter of the antenna and electrically isolated therefrom. Activation of the solenoid magnet 12 is by connection to a power source (not shown). About the termination end 4 of the chamber 1 is a chamber magnet 13 in the form of another annular solenoid having a larger diameter than the source magnet 12. The exemplified apparatus of Figure 2 has a source chamber 2 made of quartz and an internal diameter of one-hundred-and-fifty millimetres, a wider process chamber 3 and a termination end 4 of initial diameter the same as that of the source chamber 2 but tapering in a direction away from process chamber 3. In use of the apparatus with the vacuum pumps at 7 turned on, the source and chamber solenoids 12 , 13 respectively are both turned on with the windings of both solenoids generating magnetic fields parallel, to the axis of the process chamber in the same direction and the chamber magnet 13 generating the larger individual magnetic field effect (5 x 10-2 Tesla) than that of the source magnet (5 x 10-3 Tesla) but with the fluxes of the respective magnetic fields being linked to produce a non-axial magnetic field overall relative to the main (longitudinal) axis of the helically wound coil antenna 10. Typical flux lines of such a non-axial magnetic field are shown in Figure 3 showing the same type of apparatus to that of Figure 2. It should be noted, however, that the use of a uniform magnet field is not essential as generally there will be some non-uniformity in respect of the magnetic field. The magnetic field so formed means that in use there is a magnetic gradient increasing in a direction away from the antenna 10 and the generated RF electric field must interact with lines of magnet flux in the vacuum chamber. In addition, cooling water was passed through the helically arrayed pipe 8 and argon (ionisable) gas was present in the evacuated vacuum chamber 1 the chamber pressure should preferably range between 7 x 10 -5 mbar and 2 x 10 -2 mbar. The operation of the apparatus shown in Figure 2 showed that a high density plasma beam P was formed at low pressures. In particular, the argon ion efficiency was calculated to be in excess of thirty percent at 5kW power and 8 x 10 -4 mbar pressure. Figure 4 shows the use of a steering magnet 40 as an addition to the apparatus shown in Figure 2. This steering magnet is in the form of an annular solenoid positioned on one side of the process chamber 3 (the top as shown). In use, the polarity of the solenoid magnet determines whether the plasma beam P is deflected or steered towards the steering magnet 40 (as shown) or alternatively away from the magnet. The ability to steer the plasma beam may be of considerable benefit in certain coating methods in which the control of the direction of the plasma in relation to the substrate is important, for example a substrate positioned in the upper or lower part of the process chamber 3. In this respect, Figure 5 shows the use of a source chamber having a central longitudinal axis parallel with, but not co-linear with the plane of the target. The target being positioned such that plasma entering the process chamber from the source chamber needs to be substantially deflected towards the target. The apparatus of Figure 5 includes a cylindrical vacuum chamber 1 having a process chamber 51 and a source chamber 52 in which a RF helically wound coil antenna 53 is deployed. Also present in the chamber 1 is a target 54 having a surface of material to the sputtered and a substrate 55 in to which the target material is to be deposited. Electromagnetic means 56 at the top (as shown) of the process chamber 51 opposite the source chamber 52, or to the right of the antenna and further electromagnetic means 57 positioned about the process chamber 51 provide, in use of the apparatus, magnetic means to generate a magnetic field which, by interaction with the electric field profile of the RF antenna 53 in use of the apparatus, produces a high density plasma wave. In use of the apparatus of Figure 5 , introduction of gas, for example again, in to the chamber in the direction of the arrow G will be chamber 1 evacuated by means of vacuum pumps (not shown) acting in the direction of the arrow V again allows a very high intensity plasma to the generated by means of the high density plasma wave propagated from the helically wound coil antenna 53 and to be present by virtue of the electromagnetic 56, 57 in the general area with dotted lines indicated by the reference letter P between the target 54 and a substrate 55 . The ability to produce a high intensity plasma P in Figure 5 between the target 54 and the substrate 55 from an antenna 53 present in the source chamber and therefore remote from the coating area of the chamber clearly affords the possibility of avoiding or at least minimising RF leakage from the antenna in that substantially no coating of the internal wall of the chamber should occur in the source chamber in the vicinity of the antenna 53. It is possible for the source chamber 52 to have a different orientation to that shown in Figure 5 by making the angle α more than the ninety degrees shown in Figure 5 for example one hundred and thirty-five degrees. This would afford the greater possibility of avoiding RF leakage from the antenna 53 in that even less coating should be effected on the wall of the source chamber 52.

Claims: A high density plasma forming device having a vacuum chamber (1) comprising a process chamber (3, 51) , at least one source chamber (2, 52) completely open at one end to the process chamber and an ionisable gas inlet (5) ; a target (54) mounted within the process chamber; a radio frequency antenna (10, 53) for the formation of a plasma within the source chamber; and a magnetic means (12, 13, 56) by which the plasma may be directed and focused onto the target, wherein said antenna (10, 53) is a helical coil associated with the source chamber (2 , 52), characterised in that the device comprises further magnetic means (40) configured such that the plasma flow is deflected from the longitudinal axis of the source chamber (2, 52) onto the target by said further magnetic means (40, 57). A high density plasma forming device as claimed in claim 1, characterised in that the magnetic means (12,13, 56) is a solenoid. A high density plasma forming device as claimed in claim 1 or 2, characterised in that the source chamber (2, 52) is substantially cylindrical. A high density plasma forming device as claimed in claim 3, characterised in that the antenna (10, 53) is a helical coil wound around the external surface of the source chamber (2, 52). A high density plasma forming device as claimed in any preceding claim, characterised in that the antenna (10, 53) is supplied with a radio frequency discharge. A high density plasma forming device as claimed in claim 5, characterised in that the radio frequency discharge is 1 to 200 MHz. A high density plasma forming device as claimed in claim 6, characterised in that the radio frequency discharge is selected from the group 6.78 MHz, 13.56 MHz and 27.2 MHz. A high density plasma forming device as claimed in any preceding claim, characterised in that said magnetic means (12,13, 56) provide a magnetic field having a magnetic gradient which increases in a direction away from the antenna (10, 53). A high density plasma forming device as claimed in claim 3 , characterised in that the target (54) is mounted parallel to the central longitudinal axis of the source chamber (2, 52) . A high density plasma forming device as claimed in any of claims 1 to 9, wherein said antenna comprises helical coil having between three and eight turns. A high density plasma forming device as claimed in any of claims 1 to 10, characterised in that the plasma flow is deflected perpendicular to the central longitudinal axis of the source chamber (2, 52) by said further magnetic means (40, 57) . A high density plasma forming device as claimed in any one of claims 1 to 11 , characterised in that said device comprises a substrate (55) mounted opposite and substantially parallel to the target (54) such that plasma generated in the source chamber (2, 52) enters the process chamber (3, 51) between the target (54) and the substrate (55) . A high density plasma forming device as claimed in any preceding claim, characterised in that the vacuum chamber (1) further comprises a cooling means (8). A high density plasma forming device as claimed in claim 12, characterised in that the cooling means (8) comprises the provision of a circulation of water to the external surface of the process chamber (3, 51) . A process for the preparation of a high density plasma comprising the steps of (i) supplying a helical coil radio frequency antenna (10, 53) associated with a source chamber (2, 52) with a discharge power supply, (ii) inducing the ionisation of an ionisable gas within the source chamber (2, 52) completely open at one end to a process chamber (3, 51) to form a plasma, and (iii) directing and focusing of the plasma onto a target (54) within the process chamber (3, 51) by magnetic means (12, 13, 56), characterised in that said process comprises the further step of (iv) deflecting the plasma flow from the longitudinal axis of the source chamber (2, 52) onto the target (54) by further magnetic means (40, 57). The use of a high density plasma forming device as claimed in claim 15 for the preparation of a thin film coating onto a substrate (55) . The use of a high density plasma forming device as claimed in claim 15 for the etching of a substrate (55) surface. The use of a high density plasma forming device as claimed in claim 15 for cleaning the surface of a substrate (55).

Inventors: THWAITES MICHAEL JOHN, THWAITES, MICHAEL JOHN
Applicants: PLASMA QUEST LTD, PLASMA QUEST LIMITED
Application year: 2000
Main classification: H01J37
IPC codes: H01J37