Plasma processing apparatus with an electrically conductive wall

[0002] The present invention pertains to a plasma processing apparatus useful in plasma-assisted processes such as etching, deposition or sputtering processes. More particularly, the invention relates to an Inductive Coupled Plasma source.

[0003] Inductive Coupled Plasma sources are conventionally used for etching substrates or depositing material thereon. They are generally composed of a plasma chamber having a lateral dielectric wall surrounded by an external solenoid inductor. The plasma chamber contains a process gas. The external inductor is fed by a Radio Frequency generator and produces electromagnetic energy which is provided to the interior of the plasma chamber through the dielectric wall. A plasma is thus generated inside the chamber, by ionization of the process gas molecules.

[0004] The composition of the process gas is selected as a function of the treatment to be applied to the substrate. Thus, for example, if the plasma apparatus is used for material deposition on a substrate, the process gas will contain material elements to be deposited on this substrate. If the plasma apparatus is used as an etching apparatus, the process gas will contain reactive species that will react with the substrate and remove material therefrom.

[0005] An interesting characteristic of the plasma-assisted processes, in particular with respect to the Chemical Vapour Deposition technique, is that, by virtue of the use of the plasma, a lower substrate temperature is required in order to activate the reaction or deposition. Plasma-assisted processes may therefore be advantageously applied notably when the substrate to be treated cannot withstand high temperatures.

[0006] However, Inductive Coupled plasma sources remain expensive, in particular when they are shaped so as to have curved portions. The material from which their external wall is made, i.e. a dielectric material, is indeed not appropriate for the manufacture of complex pieces, such as curved or cylindrical pieces. Furthermore, drilling a hole in the dielectric material, for example in order to provide an access to the interior of the chamber for the process gas, is a difficult task.

[0007] WO-A-96/41897 describes a plasma treatment apparatus comprising a parallelepiped plasma chamber surrounded by electrical conductors. The side walls of the plasma chamber are composed of parallel metallic bars extending longitudinally over the length of the chamber. Gaps or slots are defined between the metallic bars in order to allow the magnetic field produced by the electrical conductors to penetrate into the interior of the plasma chamber. The plasma chamber is sealed by dielectric plates, each covering the external side of a corresponding chamber side wall.

[0008] The use of a dielectric plate for each side wall of the chamber may pose a problem when it is desired to have a chamber with a more complex shape, such as a cylindrical shape. Curved dielectric plates would indeed have to be constructed, which would significantly increase the manufacturing costs. Furthermore, the plasma chamber according to WO-A-96/41897 is not appropriate for providing a lateral access for the process gas, since such lateral access would require drilling a hole through a dielectric plate.

[0009] The present invention aims at remedying the above-mentioned drawbacks.

[0010] To this end, there is provided a plasma processing apparatus comprising a plasma chamber comprising an electrically conductive wall, wherein said electrically conductive wall has at least one aperture for interrupting a current path in said wall when electromagnetic energy is provided into the chamber from the outside of the chamber, external electromagnetic means for supplying electromagnetic energy into said plasma chamber through said electrically conductive wall in order to create a plasma inside the chamber, and sealing means for sealing said at least one aperture, characterised in that said sealing means comprises at least one electrically conductive enclosure element which is electrically insulated from said electrically conductive wall.

[0011] Thus, according to the present invention, one or more electrically conductive enclosure elements are used to seal the aperture(s) defined on the conductive wall of the plasma chamber. The plasma chamber is therefore essentially made from an electrically conductive material, such as metal, which renders the construction of the chamber more flexible. In particular, the chamber may have a complex shape, without the manufacturing costs significantly increasing, since conductive materials may be machined and deformed rather easily.

[0012] Furthermore, as the conductive enclosure elements are electrically insulated from the conductive wall, a current path through the conductive wall is interrupted at the level of the aperture(s), thus preventing a current loop to be formed all around the conductive wall when electromagnetic energy is supplied to the interior of the chamber. By virtue of the provision of the aperture(s) and of the fact that the conductive enclosure element(s) is insulated from the conductive wall, the electromagnetic energy produced by the external electromagnetic means is not or little screened out by the conductive wall. An important portion of the electromagnetic energy produced by the external electromagnetic means is therefore effectively supplied to the inside of the chamber.

[0013] Preferably, said at least one aperture extends in a first direction which is substantially parallel to the magnetic field induced by the external electromagnetic means, so as to interrupt a current path through the electrically conductive wall in a direction orthogonal to said first direction.

[0014] In practice, the electrically conductive wall extends in a direction parallel to a longitudinal axis of the plasma chamber, and said at least one aperture extends in a direction which is substantially parallel to the longitudinal axis of the plasma chamber, so as to interrupt a current path through said electrically conductive wall in a direction orthogonal to said longitudinal axis.

[0015] The plasma chamber may further comprise end plates at opposite ends of the electrically conductive wall, which end plates extend perpendicularly to the conductive wall.

[0016] The plasma chamber may be cylindrical, with the electrically conductive wall forming the cylindrical face of the plasma chamber, or parallelepiped, with the electrically conductive wall being composed of plane adjacent faces of the chamber. The plasma chamber may also be of any other suitable shape, and in particular of a shape that is adapted to the shape of the substrate to be plasma treated.

[0017] The external electromagnetic means may comprise an inductor surrounding the electrically conductive wall and connected to an AC power supply.

[0018] According to the present invention, at least one insulating means or electrically non-conductive gasket is provided for electrically insulating each conductive enclosure element from the conductive wall.

[0019] Preferably, each non-conductive gasket is disposed on the conductive wall in such a manner as to be in indirect line of sight from the interior of the chamber, i.e. from the plasma generated inside the chamber. This feature is particularly advantageous when the plasma processing apparatus according to the invention is used for depositing or sputtering electrically conductive material, such as metal. Indeed, in such a case, there would be a risk that a continuous layer of such conductive material be coated on the non-conductive gasket when the plasma deposition or sputtering process is being carried out. This could result in the creation of a current loop all around the conductive wall of the chamber and, accordingly, in the electromagnetic energy supplied by the external electromagnetic means being screened out by this continuous layer in the manner as explained above.

[0020] By avoiding any direct line of sight between the interior of the chamber and the insulating gasket, the risk of energy screening out due to conductive material deposition on the chamber wall is prevented or at least greatly reduced. This result is attained without the present invention requiring any Faraday shield within the chamber, nor any additional complex labyrinth structure, as conventionally used in the prior art. Furthermore, as the enclosure element(s) is/are made of a conductive material, such as metal, the deposition of conductive material thereon is not detrimental. The present invention makes it thus possible to use wide apertures or slots on the conductive wall in order to simplify the construction of the apparatus and to reduce its cost.

[0021] Typically, said at least one aperture consists of a plurality of apertures, said at least one electrically conductive enclosure element consists of a plurality of electrically conductive enclosure elements, and each one of the plurality of conductive enclosure elements is mounted on the electrically conductive wall in registration with a corresponding one of the plurality of apertures.

[0022] According to an embodiment of the present invention, said at least one gasket consists of a plurality of gaskets, each one of the plurality of gaskets surrounding, on an external surface of the electrically conductive wall, a corresponding one of the plurality of apertures, and each one of the plurality of electrically conductive enclosure elements is applied on a corresponding one of the plurality of gaskets.

[0023] According to another embodiment of the present invention, each one of the plurality of apertures comprises a narrow portion emerging into the plasma chamber and a wider portion communicating with the narrow portion and forming a recess on an external surface of the electrically conductive wall. Each one of a plurality of gaskets surrounds, in the recess of a corresponding aperture, the narrow portion of the corresponding aperture, and each one of the plurality of conductive elements is applied on a corresponding gasket in the recess of a corresponding aperture.

[0024] Advantageously, the at least one gasket and the at least one conductive enclosure element are removably mounted on the conductive wall. Preferably, a gap between the at least one conductive enclosure element and the conductive wall is determined according to the Paschen's law, such that no plasma is generated between the conductive enclosure element(s) and the conductive wall. Typically, the gap may be less than one millimeter.

[0025] The plasma chamber may further comprise one or several sputtering targets. These sputtering targets may be disposed on an inner surface of the conductive wall. An insulating plate may be interposed between the sputtering targets and the inner surface of the conductive wall. The sputtering source may be connected to a power source by a lead crossing the conductive wall and electrically insulated from the conductive wall.

[0026] According to another advantageous feature of the present invention, the plasma chamber comprises at least one gas inlet formed on the electrically conductive wall. This gas inlet serves to supply the inside of the chamber with a process gas. Preferably, the at least one gas inlet consists of a plurality of gas inlets provided at regular intervals on the electrically conductive wall. The process gas can thus be introduced into the chamber with a uniform distribution.

[0027] In addition to the above-mentioned gas inlets, the plasma chamber may further comprise one or more gas inlets formed on a cover plate disposed at the top or the bottom of the chamber. The gas inlets on the conductive wall may then serve to supply the interior of the chamber with a reactive gas mixture, while the gas inlets provided on the cover plates may be used for introducing inert gases whose role notably consists in facilitating the mixing of the various elements contained in the gas mixture. The gas inlets provided on the conductive wall, which forms a lateral wall of the chamber, enable the introduction of the reactive gas mixture into the chamber in an area close to the substrate to be treated. In this way, the present invention avoids supplying the reactive gas mixture to areas which are remote from the substrate and which therefore need not receive the gas mixture.

[0028] In practice, the at least one electrically conductive enclosure element preferably consists of one or several plates, and both the conductive wall and the plates are made of metal. Furthermore, the at least one aperture may take the form of one or several slots.

[0029] FIG. 1 is a partial sectional front view of a plasma apparatus according to a first embodiment of the present invention;

[0030] FIGS. 2 and 3 are perspective views respectively showing a plasma apparatus according to the invention with a cylindrical shape and a rectangular parallelepiped shape;

[0031] FIG. 4 is a diagrammatic front view of a part of a plasma chamber wall included in the plasma apparatus shown in FIG. 1;

[0032] FIG. 5 is a diagrammatic top view of a part of a plasma chamber wall included in the plasma apparatus shown in FIG. 1;

[0033] FIG. 6 is a diagrammatic top view of a part of a plasma chamber wall included in the plasma apparatus shown in FIG. 1, illustrating current paths;

[0034] FIG. 7 is a diagrammatic top view of a part of a plasma chamber wall included in a plasma apparatus according to a second embodiment of the present invention;

[0035] FIG. 8 is a diagrammatic top view of a part of a plasma chamber wall included in a plasma apparatus according to a third embodiment of the present invention;

[0036] FIG. 9 is a sectional front view showing a plasma apparatus according to a fourth embodiment of the present invention;

[0037] FIG. 10 is a top sectional view of the plasma apparatus according to the fourth embodiment of the present invention;

[0038] FIG. 11 is a partial sectional front view showing a plasma apparatus according to a fifth embodiment of the present invention; and

[0039] FIG. 12 is a diagrammatic top view of a part of a plasma chamber wall included in a plasma apparatus according to a sixth embodiment of the present invention.

[0040] FIG. 1 shows an Inductively Coupled Plasma apparatus according to a first embodiment of the present invention. The plasma apparatus according to this first embodiment is used as a Plasma Enhanced Chemical Vapour Deposition apparatus, in order to deposit material layers on substrates.

[0041] It generally comprises a plasma, or process, chamber 1 containing a process gas, an external inductor 2 for creating a plasma inside the plasma chamber 1, a holder 3, disposed within the plasma chamber, for supporting a substrate 4, a pump device 5 for evacuating the plasma chamber 1, and a gas controller 6 for supplying the plasma chamber with the said process gas through a gas inlet 6′.

[0042] The holder 3 is connected to a power source 7 thereby enabling the substrate 4 to be biased. The holder 3 may also be connected to a heating or cooling system (not shown) for temperature regulation of the substrate.

[0043] The pump device 5 serves to evacuate the plasma chamber 1 before the latter is used. The gas controller 6 supplies a process gas composed of a reactive gas, comprising species that are to be deposited on the upper surface of the substrate, and an inert gas, enabling an efficient mixing of the various compounds contained in the process gas.

[0044] The inductor 2 is connected to a Radio Frequency (RF) power source 20 and provides RF magnetic energy, the frequency of which preferably ranges between 100 kHz and 100 MHz. Such RF magnetic energy permits the transformation of the process gas into a plasma, whose reactive species will deposit on the substrate 4.

[0045] The general characteristics and operation of a plasma apparatus are known to the skilled person, and will not therefore be explained in detail.

[0046] The plasma chamber 1 according to the present invention may have a cylindrical shape, as shown in FIG. 2, or any other suitable shape adapted to the shape of the substrate 4. FIG. 3 shows, as an example, a plasma chamber according to the invention having a parallelepiped shape.

[0047] The plasma chamber 1 comprises a lateral wall 10 extending along and about a longitudinal axis 11 of the plasma chamber and facing the inductor 2. The lateral wall 10 is cylindrical in the case of a cylindrical plasma chamber, or constituted by four plane wall portions if the plasma chamber has a parallelepiped shape. The plasma chamber 1 further comprises upper and lower end plates 12, 13 which are perpendicular to the longitudinal axis 11. The upper and lower end plates 12, 13 are respectively sealed to end surfaces 101, 102 of the lateral wall 10 by vacuum seals 14, 15.

[0048] According to the present invention, the lateral wall 10 of the plasma chamber 1 is made of a conductive material, such as metal. Furthermore, a plurality of apertures 100 are defined through the whole thickness t of the lateral wall 10 (see FIG. 5). The apertures 100 preferably extend in a direction parallel to the longitudinal axis 11, in the form of slots. The apertures 100 may extend over the whole length of the plasma chamber, i.e. between the end surfaces 101, 102 of the lateral wall 10, to contribute to the formation of a plasma in a large area within the chamber, or over a substantial portion of this length, as shown in FIG. 1.

[0049] The inductor 2 surrounds the lateral wall 2 at the outside of the plasma chamber in order to generate, within the chamber 10, an inductive field whose lines are parallel to the longitudinal axis 11 and to the slots 100.

[0050] FIGS. 4 and 5 show in more detail the apertures 100 formed on the conductive wall 10, in the case where the plasma chamber 1 is parallelepiped. Each aperture 100 is composed of a narrow portion 101 emerging into the inside of the plasma chamber 1, and a wider portion 102 defined on the external surface 103 of the conductive wall 10 and forming a recess which communicates with the narrow portion 101. The recess 102 has a larger width w (orthogonal to the longitudinal axis 11) and preferably a larger length I (parallel to the longitudinal axis 11) than those of the narrow portion 101, as shown in FIG. 4.

[0051] According to the present invention, the apertures, or slots, 100 are sealed, i.e. hermetically closed, by conductive enclosure elements 16 which close the apertures and by vacuum seals or gaskets 17 which electrically insulate the conductive enclosure elements 16 from the conductive wall 10. The conductive enclosure elements 16 are made of a metal, such as aluminium. They preferably have the same shape as that of the recesses 102 (rectangular on FIG. 5), but a slightly smaller size. The conductive enclosure elements 16 may each take the form of a plate, which may be plane if the chamber 1 is parallelepiped or curved if the chamber 1 is cylindrical. The gaskets 17 are made of an insulating material, such as a dielectric material.

[0052] The conductive elements 16 are mounted within the recesses 102 by appropriate electrically non-conductive mounting members 18. Two mounting members 18, disposed over opposite lateral sides of the recess 102, are provided in order to mechanically fit a given conductive element 16 to the wall 10. Each mounting member 18 has a surface portion 180 which is fixed on the external surface 103 of the conductive wall 10 and another surface portion 181 on which a peripheral portion of the upper surface of the conductive element 16 is fixed. Fixing of the mounting members 18 to the wall 10 and the conductive element 16 may be carried out by bonding or screwing.

[0053] The gaskets 17 preferably have a shape corresponding to that of the recesses 102 and the conductive elements 16, i.e. rectangular in the embodiment shown in FIGS. 4 and 5. Each gasket 17 is disposed between a lower surface of the recess 102 and a peripheral portion of the lower surface of the conductive element 16, thereby sealing the aperture 100.

[0054] The apertures 100 serve to interrupt a current path through the conductive wall 10 or, in other words, to break down circulating currents induced in the conductive wall 10 into plural current loops. Indeed, the current induced by the RF inductor 2 in the conductive wall 10 would, in the absence of such apertures, define a continuous loop around the wall 10, which would screen out the electromagnetic energy delivered by the RF inductor 2. As illustrated in FIG. 6, according to the present invention, currents 19 are induced in each electrically conductive parts, namely the wall portions on both sides of an aperture 100, and each conductive element 16, but there is no current path that surrounds the conductive wall 10.

[0055] Preferably, the gaskets 17 are each disposed in such a manner as to be in indirect line of sight from the interior of the chamber 1. As shown in FIG. 5, there is thus no direct line of sight 170 between each gasket 17 and the interior of the chamber 1. This is achieved, for example, by the conductive enclosure element 16 being much wider than the narrow portion 101 and the gasket 17 being disposed close to the periphery of the conductive element 16. Such an arrangement makes it possible to avoid the possible formation of conductive layers on the gasket 17 when the plasma chamber 1 is used for depositing conductive material, such as metal, on the substrate 4.

[0056] With reference to FIG. 7, a plasma apparatus according to a second embodiment of the present invention differs from the first embodiment as previously described essentially in that the apertures 100ado not comprise any recess. Instead of being mounted in a corresponding recess, each conductive enclosure element 16ais mounted on the external surface 103aof the conductive wall 10aby non-conductive mounting members 18aand, for this purpose, has dimensions (length, width) larger than that of the corresponding aperture 100a. A gasket 17asurrounding the aperture 100aseals the gap between the conductive element 16aand the conductive wall 10a.

[0057] With reference to FIG. 8, a plasma apparatus according to a third embodiment of the present invention differs from the first embodiment as previously described essentially in that:

[0058] seals 20bare provided between the non-conductive mounting members 18band the conductive elements 16b, and

[0059] the seal 17bwhich surrounds the narrow portion 101bof a given aperture 100bhas a flat shape thereby defining an accurate gap d between the corresponding conductive element 16band the conductive wall 10bin a direction transverse to the longitudinal axis 11 of the plasma chamber 1.

[0060] In this third embodiment, the gap d is adjusted according to the Paschen's law, such that no plasma is created therein. Typically, for an operating pressure inside the plasma chamber of 100 mTorr, the gap d is less than 1 mm. The adjustment is carried out by using several calibrated inserts with different thicknesses for the seal 17b. The seal 17b, the conductive element 16b, the seals 20band the mounting members 18bare removably mounted, thus enabling the replacement, on the plasma chamber, of the insert 17bby another insert having a different thickness.

[0061] FIGS. 9 and 10 show a plasma apparatus according to a fourth embodiment of the present invention. The plasma apparatus according to this fourth embodiment essentially has the same structure as that of the first embodiment as previously described, except that one or several gas inlets 6c′ are provided on the lateral conductive wall 10c, and one or several gas inlets 6c″ are provided on the upper end plate 12c.

[0062] As shown in FIG. 10, the gas inlets 6c′ formed on the lateral wall 10care disposed at regular intervals between each other. The gas inlets 6c′ serve to supply the interior of the plasma chamber 1cwith a gas mixture containing the reactive species. The gas inlets 6c″ formed on the upper end plate 12cserve to supply the interior of the plasma chamber 1cwith an inert gas, whose role notably consists in facilitating the mixing of the various elements contained in the reactive gas mixture. The arrangement as illustrated in FIGS. 9 and 10 has the advantage of enabling a gas injection in an area close to the substrate 4cto be treated, thus avoiding to supply process gas to areas within the chamber 1cwhich need not receive such gas. Furthermore, the fact that the gas inlets 6c′ on the lateral wall 10care placed at regular intervals permit to have to uniform distribution of the gas mixture within the chamber 1cin the area of the substrate 4c.

[0063] FIG. 11 shows a plasma apparatus according to a fifth embodiment of the present invention. The plasma apparatus according to this fifth embodiment serves to carry out a sputtering process, wherein material to be deposited on a substrate is sputtered from a target.

[0064] The construction of this plasma apparatus differs from that of the first embodiment in that a target 21d, i.e. a material source, is sealed on the upper plate 12dof the plasma chamber 1d. A DC voltage source (or DC pulsed source) 22dis connected to the target 21dto enable the sputtering of the material to be deposited. As the plasma chamber 1dis preferably grounded, the target 21dis electrically isolated from the upper plate 12dby an insulating ring 23d. A magnetron (not shown) may be added to this arrangement to enhance the sputtering efficiency. In this case, the gas inlet 6d′ connected to the gas controller 6dis placed on the top of the lateral wall 10d.

[0065] FIG. 12 shows a sixth embodiment of the present invention. The plasma apparatus according to this sixth embodiment is identical to that of the first embodiment, except that it comprises sputtering targets 24edisposed on the conductive wall 10etowards the interior of the plasma chamber. The sputtering targets 24eare made from the material to be deposited on the substrate. Preferably, the conductive wall 10eis grounded, and the targets 24eare electrically insulated from the conductive wall 10eby electrical insulators 25e. Each target 24eis connected to a DC voltage source or a DC pulsed source 26eby an electrical lead 27ein order to enable sputtering operations. Each electrical lead 27eis electrically insulated from the conductive wall 10eby seals 28e.

[0066] In each one of the six embodiments described above, cooling means may be provided in order to cool the conductive wall 10, 10a, 10b, 10c, 10d, for example in the manner as explained in EP-A-0908922.