Method and apparatus for treating material by using plasma

Abstract: A sputter etching apparatus comprises a vacuum chamber (1) provided with a gas supply system (4) and an evacuator (3), a sputter etching electrode (6) disposed within the vacuum chamber (1) on which a substrate (5) is disposed, a plasma generator (2) for generating plasma (15; 20) by applying microwave energy and disposed in opposition to the sputter etching electrode (6), a voltage applying stud (9) provided in association with the sputter etching electrode (6) for causing ions in the plasma to impact against the substrate (5), a first power supply source (10) provided in association with the plasma generator (2) for generating the plasma, and a second power supply source (8) provided independent of the first power supply source (10) for supplying the voltage for causing the ions to impact against the substrate (5).

Description: The present invention is generally concerned with the technique of treating a material such as a substrate, wafer or the like by utilizing a plasma and it more particularly relates to an apparatus for performing such technique for instance for etching a substrate by sputtering at high speed using a plasma without damaging or injuring the substrate or for depositing a film by a plasma CVD process at high speed. In the prior art a sputter etching apparatus is known in which an etching electrode having a substrate disposed thereon is mounted within a vacuum chamber. By applying high-frequency power to the etching electrode plasma is produced over the substrate and ions from the plasma are caused to impact against the substrate under acceleration by the electric field generated under the potential appearing on the substrate, whereby the latter is etched by the sputtering. With this structure of the sputter etching apparatus, however, the plasma produced over or above the substrate is of a low density, as the result of which the sputter etching can proceed only at a low speed. When the high-frequency power applied to the etching electrode is increased in an effort to speeding up the sputter etching, the ions will then impact against the substrate with increased energy because of a high voltage appearing on the electrode and substrate, giving rise to the possibility of the substrate being undesirably damaged or injured. To increase the sputter etching rate or speed in JP-A-60-74436 a plasma produced by applying a high-frequency power to the etching electrode is confined within a delimited space over the substrate by using a floating electrode in combination with a magnet for the purpose of preventing the plasma from diffusing radially outwardly while increasing the number of ions in the plasma and hence the density thereof. With this arrangement, the voltage making appearance on the etching electrode can certainly be prevented from being increased relative to the power applied to the electrode. It is however noted that the increase in the density of plasma is not proportional to the increase in the power applied to the etching electrode at a definite one-to-one ratio. Consequently, when the applied high-frequency power is increased in order to further speed up the etching, the voltage appearing on the electrode is increased to such an extent that damage or injury can occur in the substrate to a problem. Another plasma treatment apparatus is disclosed in JP-A-57-26441. With that known apparatus there is provided in combination with a reaction chamber a preliminary excitation chamber into which only a gas difficult to activate is supplied for preliminary excitation, the gas thus activated being then introduced into the reaction chamber together with a gas easy to activate. In this way, concentration of the activated gases which partake straightforwardly in the formation of a film can be increased without need for applying a high power to the electrode within the reaction chamber, whereby the material to be treated such as substrate can be protected against damage or injury. Besides, by regulating the electric power supplied to the preliminary excitation chamber and the reaction chamber, the ratio of composition of the film formed by deposition can be controlled for ensuring a high quality of the film. However, this known apparatus suffers from a shortcoming that the activation of the gas difficult to activate is inadequate because excitation of the gas of this species relies on the high-frequency discharge. A further plasma treatment method in the prior art is disclosed in JP-A-57-167631. According to this known technique, the preliminary excitation of a reactant gas is realized by utilizing microwave energy. By using a microwave (usually having a frequency of 2.45 GHz) in place of a high-frequency electrical power supply (normally of 13.56 MHz) as proposed, the plasma density is increased by one to two orders of magnitude (approximately up to 10¹¹/cm³) because the ratio at which the reactant gas is excited due to impact of electrons in the plasma is increased, whereby concentration of the activated gas is enhanced. However, a long distance intervenes between the activating chamber and the reaction chamber. Moreover, a passage for introducing the activated gas into the reaction chamber is provided at an offset position. Consequently, difficulty is encountered in introducing the gas activated through the preliminary excitation onto a substrate under treatment uniformly without lowering concentration of the activated gas. Such being the circumstances, it is difficult to carry out the treatment uniformly at a high speed, although it is admitted that a film of improved quality can be formed on the substrate. Finally, in 'Applications of Plasma Processes to VLSI Technology' of T. Sugano, 1985, J. Wiley & Sons, Inc., on pages 150/151 there is disclosed a plasma processor of the electron cyclotron resonance (ECR) type in which a magnetic line of force is formed in a vacuum chamber in parallel with a travelling of microwaves. When the intensity of the magnetic field is set to be coincident with a cyclotron frequency of electrons (0.875 T at a microwave frequency of 2.45 GHz), the ECR condition is satisfied and the microwave is travelled in the plasma irrespectively of the plasma density. The plasma thus obtained has a density which is greater than 10¹¹/cm³. Under those circumstances, in order to uniformly form the magnetic field in parallel with the microwave as required, it is necessary to arrange a large-sized magnetic field generating coil at a periphery of the plasma generator. Moreover, a wafer to be treated must be arranged in a predetermined interval from a microwave injection window in order to allow the microwave to travel in the plasma. Generally, in the prior art technique, neither endeavour nor consideration is paid to uniform formation of a high quality film on a substrate at high speed without injuring the substrate. Therefore, with the said known techiques, degradation will occur in the quality and/or the uniformity of the filmed formed when the treating rate or speed is increased. To say it in other words, every attempt to form a uniform film of high quality will be inevitably accompanied by a lower treating speed. Therefore, it is a first object of the present invention to provide a sputter etching apparatus making available a plasma having a high and uniform density for allowing the etching of a substrate to be carried out at a high speed owing to a reliable and simple mechanism without damaging the substrate by providing power supply sources separately for generation of the plasma on one hand and for the voltage causing ions in the plasma to impact against the substrate on the other hand, which power supply sources are thus adapted to be controlled independent of each other. A second object of the present invention is to provide a plasma treatment method and apparatus capable of forming a film of high quality uniformly on a substrate by plasma CVD process at high speed while avoiding damage of the substrate given by the plasma. The said objects according to the present invention are achieved by plasma treatment apparatus as defined in the appended claim 1. Advantageous further developments of the invention are defined in the respective subclaims. More specifically, the invention first provides a plasma treatment apparatus which comprises a microwave generating source generating a microwave, a waveguide feeding said microwave, a cavity resonator resonating said microwave fed along said waveguide so that an electric field of said microwave is intensified, a gas supply source, an evacuator, a vacuum chamber connected to said gas supply source and said evacuator and containing a plasma treatment electrode supporting a substrate thereon, a microwave injection window supplying microwave energy resonated in said cavity resonator so as to generate plasma by said microwave energy, wherein the plasma is disposed against said substrate in said vacuum chamber, said microwave injection window being interposed between said cavity resonator and said vacuum chamber so that said microwave injection window is provided in opposition to said substrate and a voltage supply electrically connected to said plasma treatment electrode causing ions in the plasma to impact against said substrate. Secondly the invention provides a plasma treatment apparatus which comprises a microwave generating source generating a microwave, a waveguide feeding said microwave, a cavity resonator resonating said microwave fed along said waveguide so that an electric field of said microwave is intensified, a gas supply source, an evacuator, a vacuum chamber connected to said gas supply source and said evacuator and containing a plasma treatment electrode supporting a substrate thereon, a microwave injection window supplying microwave energy resonated in said cavity resonator so as to generate plasma by said microwave energy, wherein the plasma is disposed against said substrate in said vacuum chamber, said microwave injection window being interposed between said cavity resonator and said vacuum chamber so that the microwave injection window is provided in opposition to said substrate, a voltage supply electrically connected to said plasma treatment electrode causing ions in the plasma to impact against said substrate and a magnetic field generator located in the periphery of said vacuum chamber adjacent to said microwave injection window to prevent the plasma from diffusing radially and outwardly in a space defined by said plasma treatment electrode, said microwave injection window and magnetic lines of force generated by said magnetic field generator so as to keep a plasma density highly and uniformly in distribution in the vicinity of said substrate. With the present invention, a cavity resonator is utilized to generate a high density plasma without using a magnetic field. The high density plasma and a high frequency permit the supply of a large amount of ions to a wafer with low energy. The plasma is made higher in its density with the increase of the microwave power. When a plasma frequency is in coincidence with a microwave frequency, the microwave is prevented from travelling in the plasma (7 x 10¹⁰/cm³ at a microwave frequency of 2.45 GHz). Thus a high density plasma can be produced in a restricted area in the vicinity of a microwave injection window. A plasma thus produced, however, is widely diffused. In order to prevent the diffusion of the plasma in its radial direction, a magnetic field generator may be arranged at an outer periphery of the microwave injection window to form a magnetic line of force at the outer periphery only. In general, a mode of microwave to be injected has a symmetrical electric field distribution with respect to its axis. The electric field is weak at its periphery and the density distribution of the plasma is made higher in its center portion. However, the forming of a magnetic field prevents loss of plasma and allows an increase of plasma density as a whole and particularly an increase of the rate of the plasma density at its periphery and permits the generation of a uniform plasma. The present invention does not require a large-sized coil and hence the electrodes can be reduced in size. Further, according to the present invention the waver is arranged in the vicinity of the high density plasma at the microwave injection window and that permits a plasma processing with good efficiency. Moreover, there is no magnetic field perpendicular to a surface of the wafer and hence no electric field is produced in the wafer and the occurrence of charge-up damage is prevented. The fundamental constitution of the magnetic field according to the present invention is provided by a magnet in the vicinity of the microwave injection window. It is not absolutely necessary to arrange a magnet on a substrate. Such an arrangement is provided only in order to additionally improve the efficiency. In view of the first object mentioned above, it is proposed according to a first aspect of the invention that a plasma generating means is provided in opposition to an etching electrode which is mounted within a vacuum chamber and on which a substrate is disposed. By utilizing microwave energy (e.g. a microwave having a frequency of 2.45 GHz), a plasma of a high density is produced over the substrate, wherein ions in the plasma are caused to impact against the substrate under action of a voltage appearing on the substrate by applying a high frequency power to the etching electrode. More specifically, the plasma generating means for producing the plasma through excitation by using microwave energy is constituted by a microwave generating source, a cavity and a microwave injection window, wherein a matching means is provided within the cavity for operating it as a cavity resonator for the microwave. The cavity may be implemented in a coaxial structure and so dimensioned that the microwave can assume an axis-symmetrical mode within the cavity. The microwave injection window serves for two functions: first, it serves as a partition wall constituting a part of a vacuum chamber for partitioning the latter from the cavity, and second, it serves as a window for radiating or injecting the microwave into the vacuum chamber from the cavity. To this end, the microwave injection window is formed of an electrically insulating material capable of scarcely absorbing microwave energy and impermeable to gases. By way of example, the microwave injection window may be made of quartz, alumina or other ceramics. The cavity resonator may be either of coaxial or rectangular configuration, not to speak of a circular configuration, as is well known in the art. It is however preferred to implement the cavity resonator in a circular configuration of coaxial type in view of ease in obtaining a plasma of uniform density distribution over the etching electrode. When the substrate to be etched is an electrically insulating material, it is indispensable to apply a high-frequency (radio-frequency) power to the etching electrode. However, in the case where the substrate to be treated is made of an electrically conductive material such as a metal, a DC power supply source may be used in place of the high-frequency power supply source. With the structure of the plasma generating means composed of the microwave generating source, the cavity, the matching means for allowing the microwave to enter the cavity effectively and the microwave injection window through which microwave energy is introduced into the vacuum chamber as described above, the microwave generated by the microwave generating source and guided to the cavity through a suitable wave guide can be effectively introduced into the cavity by operating the cavity as the cavity resonator for the microwave of concern with the aid of the matching means. Subsequently, the microwave is introduced into the vacuum chamber through the microwave-transmissive injection window to ionize an atmospheric gas (e.g. Ar gas) within the vacuum chamber for thereby producing a plasma. In this conjunction, it should be noted that the microwave transfers energy thereof to the plasma in the course of propagation therethrough, as the result of which the plasma of increased density is made available. By way of example, with the microwave having a frequency of 2.45 GHz, a plasma density on the order of 7.4 x 10¹⁰/cm³ can be achieved. Further, by dimensioning the cavity in such a configuration that the axis-symmetrical mode of the microwave prevails within the cavity or adopting the coaxial conduit configuration, there can be produced a plasma exhibiting a high uniformity of density. A substrate to be treated is disposed on the etching electrode over which the high-density plasma produced by the plasma generating means in the manner described above prevails, wherein a high-frequency power (usually at a frequency of 100 kHz to 100 MHz) is applied to the etching electrode from the high-frequency power supply source connected thereto. A high voltage thus making appearance on the surface of the substrate disposed on the etching electrode causes ions in the plasma to impact against the substrate, whereby etching is carried out by the sputtering. By providing the power supply sources separately for producing the plasma on one hand and for generating the voltage for causing the ions to impact against the substrate, respectively, as described above, the density of the plasma as produced and energy with which the ions in the plasma bombard the substrate can be separately controlled independent of each other. Since the plasma can be produced with a high density by microwave energy in this manner, the sputter etching can be carried out at an increased rate without injuring the substrate. As mentioned hereinbefore, although the high-frequency power supply source is indispensably required in the application where the material to be etched by sputtering is of an electrically insulating material, it should be understood that a DC power supply source may be employed when the substrate is made of an electrically conductive material such as a metal. In view of the first object mentioned above, it is proposed according to another aspect of the present invention that in a sputter etching apparatus which comprises the vacuum chamber equipped with a gas supply system and an evacuator, the sputter etching electrode disposed within the vacuum chamber for supporting thereon a substrate to be treated, the plasma generating means for producing a plasma through excitation by microwave energy, the cavity of the plasma generating cavity being disposed on the vacuum chamber in opposition to the substrate with the microwave injection window being interposed between the plasma generating cavity and the vacuum chamber, the voltage applying means provided in association with the sputter etching electrode for causing ions in the plasma to impact against the substrate, the power supply source provided in association with the plasma generating means, and the second power supply source provided independent of the first power supply source for supplying the voltage for causing the ions in the plasma to impact against the substrate, there is provided magnetic field generating means for generating a magnetic field in such a manner in which the plasma produced within a space defined between the substrate and the microwave injection window is peripherally surrounded by magnetic lines of force. The magnetic field generating means may be implemented in various structures mentioned below according to further features of the present invention. (1) A ring-like array of juxtaposed magnets which are magnetized in the radial direction with polarities thereof being alternately inverted from one to another along the circumferential direction. (2) An array of paired ring-like magnets one of which is disposed on the side of the microwave injection window with the other being disposed on the side of the etching electrode. (3) An array of the paired ring-like magnets mentioned above in which the magnets are however constituted by coils, respectively. (4) An array of the paired ring-like coils mentioned above which are arranged to conduct respective electric currents in the directions opposite to each other. (5) An array of the paired ring-like coils mentioned above which are arranged to conduct respective electric currents in a same direction. (6) An array of the paired ring-like magnets mentioned above in which the magnets are constituted by permanent magnets, respectively. (7) An array of the paired ring-like permanent magnets mentioned above in which the permanent magnets are magnetized circumferentially in the directions opposite to each other. (8) An array of the paired ring-like permanent magnets mentioned above in which the permanent magnets are magnetized thicknesswise in the directions opposite to each other. (9) An array of the paired ring-like permanent magnets mentioned above in which the permanent magnets are radially magnetized in the directions opposite to each other. When the magnetic field generating means is constituted by coil(s), the latter may be realized by using a superconductor so that a current can flow therethrough permanently for generating a highly stable magnetic field. The magnetic field for confining stably the plasma should preferably be of a cusp-like profile. The power applied to the etching electrode is usually of a high-frequency power having a frequency in the range of 100 kHz to 100 MHz. However, when the substrate for treatment is made of an electrically conductive material, the power applied to the etching electrode may be a DC power. It goes without saying that the application of high-frequency power is indispensable when the substrate is of an electrically insulating material. By virtue of disposition of the magnetic field generating means in such a manner as to enclose the plasma around the periphery thereof, the plasma is confined within a space above the substrate by the magnetic field produced by the magnetic field generating means, whereby the plasma is positively prevented from diffusing radially outwardly. Thus, the plasma is maintained stably at a high density substantially independent of the pressure within the vacuum chamber. In this way, the etching electrode having the substrate disposed thereon is positioned below the high-density plasma produced by the above mentioned plasma generating means, wherein the high-frequency power (generally of a frequency in the range of 100 kHz to 100 MHz) is applied to the etching electrode by connecting thereto the high-frequency power supply source. Under the voltage thus making appearance on the surface of the substrate disposed on the etching electrode, ions in the plasma are caused to impact against the substrate, as the result of which the substrate is etched by sputtering. Because of provision of separate power supply sources for the plasma generation and the ion acceleration for the sputter etching, respectively, the plasma density and energy at which ions bombard the substrate can be separately controlled independent of each other. Further due to high density of the plasma generated through excitation by the microwave, the sputter etching can be achieved at a high etching speed without damaging the substrate. For further explaining the invention and its advantages reference is now made to the accompanying drawings in which: Fig. 1 shows in a partially sectioned side elevational view a sputter etching apparatus according to a first embodiment of the present invention; Fig. 2 shows in a partially sectioned side elevational view a sputter etching apparatus according to a second embodiment of the invention; Fig. 3 is a view for graphically illustrating relationship between the etching rate and the high-frequency power in the sputter etching apparatus according to the invention comparatively with that of the prior art apparatus; Fig. 4 shows in a partially sectioned side elevational view a sputter etching apparatus according to a third embodiment of the present invention; Fig. 5 is a plan view of a ring-like magnet array employed as the magnetic field generating means for confining a plasma within a defined space in the sputter etching apparatus shown in Fig. 4; Fig. 6 shows in a partially sectioned side elevational view a sputter etching apparatus according to a fourth embodiment of the invention; Fig. 7 shows in a partially sectioned side elevational view a sputter etching apparatus according to a fifth embodiment of the invention; Fig. 8 shows in a partially sectioned side elevational view a sputter etching apparatus according to a sixth embodiment of the invention; Fig. 9 shows in a partially sectioned side elevational view a sputter etching apparatus according to a seventh embodiment of the invention; Fig. 10 shows in a partially sectioned side elevational view a sputter etching apparatus according to an eighth embodiment of the invention; Fig. 11 is a view for graphically illustrating relations between the etching rate and the distance from the center of a substrate in the sputter etching apparatus equipped with a plasma confining magnet system comparatively with the apparatus without such magnet system; Fig. 12 is a view for graphically illustrating relations between the etching rate and the high-frequency power in the sputter etching apparatus according to the present invention comparatively with that of the prior art apparatus; Now, the invention will be described in detail in conjunction with preferred and exemplary embodiments thereof by reference to the drawings. Fig. 1 shows a sputter etching apparatus according to a first embodiment of the present invention. As will be seen in the figure, the sputter etching apparatus comprises as primary components thereof a vacuum cell or chamber 1, a plasma generating unit 2, an evacuating system 3 and a gas supply system 4. There is disposed fixedly within the vacuum chamber 1 an etching electrode 6 through an interposed flange 7 which serves for electrically insulating the electrode 6 from the vacuum chamber 1 in a vacuum-tight manner. A substrate 5 is disposed on the top of the electrode 6. The etching electrode 6 has a supporting stud 9 having a lower end extending outwardly from the vacuum chamber 1 and connected electrically to a power supply source 8. The plasma generating unit 2 in turn is composed of a microwave generating source 10, a waveguide 11, a cavity 12, a matching element 13 and an inlet window 14 which may be formed of a quartz plate. With the structure of the sputter etching apparatus described above, a microwave is generated through oscillation of the microwave generating source 10 to be fed into the cavity 12 by way of the waveguide 11. Through adjustment by means of the matching element 13, there can be established the conditions required for the cavity 12 to function as the cavity resonator for the microwave applied thereto. Thus, the electric field of the microwave within the cavity 12 is intensified. The microwave is then introduced into the vacuum chamber 1 through the injection window 14, as the result of which the atmospheric gas within the vacuum chamber 1 supplied from the gas supply system 4 is ionized, whereby plasma 15 is produced. The plasma 15 has a density which can be controlled by the power of the microwave up to a limit value at which the microwave can no more supply energy to the plasma 15 in the course of propagation therethrough (e.g. up to the density of 7.4 x 10¹⁰/cm³ at the microwave frequency of 2.45 GHz). Ions within the plasma 15 thus produced are accelerated under the influence of a high-frequency power applied to the substrate 5 from the power supply source 8 through the etching electrode 6 connected thereto, whereby the accelerated ions are caused to impinge against the substrate 5. In this way, the sputter etching is carried out. In this connection, it should be mentioned that a uniform distribution of density of the plasma 15 located over the substrate 5 can be realized by dimensioning the cavity 12 such that the axis-symmetrical mode of the microwave may prevail within it. Fig. 2 shows a sputter etching apparatus according to a second embodiment of the present invention. This sputter etching apparatus is also composed of a vacuum chamber 1, a plasma generating unit 2, an evacuating system 3 and a gas supply system 4 in an arrangement similar to that of the first embodiment except that a bar 16 is coaxially disposed within the cavity 12 constituting a part of the plasma generating unit, whereby the cavity 12 is implemented in the form of a coaxial cavity. With the structure of the sputter etching apparatus according to the second embodiment described above, the microwave fed into the cavity 12 exhibits a coaxial mode within the cavity 12, whereby the axis-symmetrical mode of the microwave can be easily established more stably within the cavity 12, which in turn means that uniform distribution of density of the plasma 15 over the substrate 5 within the vacuum chamber 1 can be realized more stably. According to the first and second embodiments of the invention described above, the plasma of high density generated by the plasma generating unit utilizing the microwave as the excitation or ionization source can be transformed to an axis-symmetrical stable plasma of the high density by virtue of the provision of the cavity matching element 13 for adjusting the cavity dimension as well as the coaxial bar 16, wherein the density of plasma can be controlled by correspondingly controlling the power or energy of the microwave. On the other hand, the sputter etching of a substrate by using the plasma can be carried out by controlling the power supply connected to the etching electrode 6. More specifically, rate of etching and energy with which ions bombard the substrate can be separately controlled. Further, because of availability of a high-density plasma through ionization by the microwave, speeding-up of the etching treatment can be attained with the possibility of damaging the substrate being significantly reduced. The results of the sputter etching performed on a thermally grown oxide film (SiO₂) formed on a substrate made of Si by employing the sputter etching apparatus of the structures described above are graphically illustrated in Fig. 3 comparatively with the results of the etching of the oxide film performed with the prior art apparatus. More specifically, Fig. 3 illustrates relationships between the high-frequency power applied to the substrate 5 and the etching rate (nm/min.) with the microwave powers being used as parameters. As will be seen from Fig. 3, remarkably increased etching rate can be attained when compared with the prior art apparatus. The specimen as well as the etching conditions adopted in the measurements mentioned above are as follows: (1) Specimen: Si-substrate having a thermally grown oxide film formed on a surface, (2) Atmospheric gas: Ar-gas at 1.32 Pa to 0.132 Pa (10⁻² to 10⁻³ Torr), (3) Microwave power: 200 W and 400 W changed over in the two measurements, respectively, (4) High-frequency power: 1.2 kW at maximum. Next, sputter etching apparatuses according to other embodiments of the present invention will be described by referring to Figs. 4 to 10. Fig. 4 shows a sputter etching apparatus according to a third embodiment of the invention. This apparatus is composed of a vacuum chamber 1, a plasma generating unit 2, an evacuating system 3 and a gas supply system 4. Disposed fixedly within the vacuum chamber 1 is a sputter etching electrode (hereinafter called simply as etching electrode) 6 on which a substrate 5 to be etched by sputtering is disposed and which electrode is mounted in the vacuum chamber 1 through an interposed flange 7 serving to isolate electrically the vacuum chamber 1 from the etching electrode 6 in a vacuum-tight manner. The etching electrode 6 has a projection 9 extending outwardly and connected electrically to a power supply source 8. The plasma generating unit generally denoted by the numeral 2 is constituted by a microwave generating source 10, a waveguide 11, a cavity 12, a matching element 13, an electrically conductive bar 16 and a microwave injection or radiation window 14 through which the microwave is introduced into the vacuum chamber 1. Additionally, magnets 18 are disposed around a space defined between the injection window 14 and the etching electrode 6. With the structure of the sputter etching apparatus described above, a microwave is generated by the microwave generating source 10 to be fed into the cavity 12 by way of the waveguide 11. Through adjustment of the matching member 13, there can be established in the cavity 12 the conditions for the resonance cavity for the microwave, as the result of which the electric field of the microwave within the cavity 12 is intensified. The atmospheric gas fed into the vacuum chamber 1 from the gas supply system 4 is ionised under the action of the microwave introduced into the vacuum chamber 1 through the inlet window 14, whereby a plasma 20 is produced. A sectional view taken in the direction thicknesswise of the magnets 18 disposed around the periphery of the plasma 20 is shown in Fig. 5. Parenthetically, it should be mentioned that although the magnets 18 are shown as constituted by permanent magnets in the instant embodiment, these magnets 18 can of course be implemented as electromagnets. Referring to Fig. 5, an even number of the magnets 18 are arrayed in a ring-like form with divisions in the circumferential direction. The polarities of these magnets at inner and outer peripheral poles thereof are alternately exchanged from one to another so that magnetic lines of force run along the inwardly bulging paths (the magnetic field of such pattern is referred to as the multi-cusp magnetic field commonly employed for confining the plasma). By virtue of this arrangement of the magnets, the plasma 20 is confined within a definite space located above the substrate 5, whereby the plasma 20 can be easily produced not only with high stability even in a low vacuum range but also with a high density. Further, the density of the plasma 20 can be controlled up to a limit value at which the microwave can no more transfer energy to the plasma 20 in the course of propagation therethrough, by correspondingly controlling the microwave power as inputted. (By way of example, the plasma density can be increased up to 7.4 x 10¹⁰/cm³ at the microwave frequency of 2.45 GHz.) Ions of the plasma produced in this way are accelerated under the voltage applied to the substrate 5 by way of the etching electrode 6 from the power supply source 8 thereto (in this illustrative case, a high-frequency power is applied), whereby the ions are caused to impact against the substrate 5. Thus, the sputter etching of the substrate 5 takes place. It should be mentioned that by dimensioning the cavity 12 so that the axis-symmetrical mode of microwave prevails within the cavity 12, there can be realized a uniform distribution of density of the plasma 20 over the substrate 5. The cavity 12 is of a coaxial type in which the electrically conductive bar 16 is mounted at the center of the cavity. It should however be noted that similar effects can be obtained with the cavity of other type where no coaxial conductive bar is employed. In the foregoing description, it has been assumed that the inlet or injection window 14 is formed of a quartz plate. However, this window may also be formed of any insulation material such as ceramics exemplified by alumina or the like which has impermeability to gases and is less susceptible to absorption of the microwave, since the microwave injection window 14 must serve also as a partitioning wall between the cavity 12 and the vacuum chamber 1. Fig. 6 shows a sputter etching apparatus according to a fourth embodiment of the invention. This apparatus is composed of a vacuum chamber 1, a plasma generating unit 2, an evacuating system 3 and a gas supply system 4 in a substantially same structural configuration as the third embodiment described above in conjunction with Fig. 4 except for a difference that ring-like coils 22a and 22b are employed in place of the ring-like magnet array 18 in the third embodiment. More specifically, the ring-like coils 22a and 22b are disposed coaxially in two stages in the vertical direction and supplied with electric currents in the directions opposite to each other so that the magnetic lines of force produced by these two coils are closed to each other around a definite space over the substrate 5, whereby the plasma can be confined within the space without diffusing outwardly in the radial direction. In other words, confinement of the plasma can be realized stably, whereby the plasma of high density can be produced with high efficiency. Figs. 7 and 8 show fifth and sixth embodiments of the invention each having a structure similar to that of the fourth embodiment shown in Fig. 6 except that the two stages of coils 22a and 22b are replaced by a pair of ring-like permanent magnets 23. More specifically, in the case of the sputter etching apparatus shown in Fig. 7, the paired ring-like permanent magnets 23 arrayed vertically are magnetized in opposition to each other in the circumferential direction of the ring, as indicated by arrows 21a. With such arrangement of the paired permanent magnet rings 23, the magnetic lines of force generated by the magnet rings are closed to each other around the space over the substrate 5 to thereby confine the plasma 20 within that space. In contrast, in the case of the structure shown in Fig. 8, the paired ring-like magnets 23 are magnetized opposite to each other in the thicknesswise direction, as indicated by arrows 21b. Also with this arrangement, substantially similar effect as that of the apparatus shown in Fig. 7 can be obtained. In other words, the plasma 20 is effectively confined within the space over the substrate 5. Fig. 9 shows a seventh embodiment of the invention which is implemented in the same structure as that of the fourth embodiment shown in Fig. 6. More specifically, in the case of the seventh embodiment, a pair of ring-like coils 22a and 22b are employed as the magnetic field generating means as in the case of the fourth embodiment shown in Fig. 6. However, difference from the fourth embodiment is seen in that both coils 22a and 22b are so connected as to conduct the respective electrical currents in the same direction so that the magnetic lines of force generated by both coils 22a and 22b bulge inwardly, as indicated by arrows 21c, to thereby prevent the plasma 20 from diffusing radially outwardly. Fig. 10 shows an eighth embodiment of the present invention which is similar to the seventh embodiment shown in Fig. 9 except that the paired coils 22a, 22b are replaced by paired ring-like magnets generally denoted by 24. The paired ring-like magnets 24 are magnetized in the radial direction with opposite polarities to each other. The magnetic field pattern indicated by arrows 21d is substantially similar to the pattern of the magnetic field generated in the apparatus shown in Fig. 9. Thus, the eighth embodiment can exhibit the effect of confining the plasma 20 within the space over the substrate 5. The result of the sputter etching performed on the substrate 5 consisting of a silicon wafer having a thermally grown oxide (SiO₂) film formed over a surface thereof by using the sputter etching apparatus of the structure shown in Fig. 4 is illustrated in Fig. 11 comparatively with the result of the sputter etching carried out with the apparatus equipped with no magneticfield generating means. In Fig. 11, etching rate is taken along the ordinate with the distance from the center of the wafer being taken along the abscissa. Etching Conditions Substrate: silicon wafer having a thermally grown oxide (SiO₂) film, Etching gas: argon (Ar), Pressure: 0.660 Pa (5 x 10⁻³ Torr), Microwave power: 400 W, High-Frequency Power: 600 W (applied to the sputter etching electrode). As will be seen from the illustration in Fig. 11, a curve 81 representative of the etching rate realized with the apparatus equipped with the plasma confining magnet array is in a relatively flat form over the distance extending from the center of the wafer to the periphery thereof, demonstrating that the uniformity in the etching rate is improved. On the other hand, a curve 82 representative of a control case where no magnet array is employed shows that the etching rate is decreased toward the periphery of the wafer, indicating a significant difference in the etching rate at the center portion of the wafer. Fig. 12 shows relationship between the etching rate and the high-frequency power applied to the sputter etching electrode 6 on the same etching conditions as those of the preceding example. A curve 91 represents the relation obtained with the sputter etching apparatus equipped with the magnet array according to the invention, a curve 92 represents the etching rate obtained with the apparatus having no plasma confining magnet array and a curve 93 represents the etching rate attained with the prior art parallel plate type apparatus having no microwave plasma generating means. As will be seen in Fig. 11, the curve 91 shows that the etching rate or speed can be significantly increased according to the embodiments of the invention equipped with the plasma confining magnet array over the other examples for control. The etching conditions for the measurements mentioned above are as follows: Substrate: silicon wafer having a thermally grown oxide (SiO₂) film formed on a surface, Etchant gas: argon (Ar), Pressure: 0.66 Pa (5 x 10⁻³ Torr), Microwave power: 400 W (in the case of the curves 91 and 92). As will be understood from the characteristic curves mentioned above, diffusion of the plasma is positively prevented by employment of the magnet(s) or coil(s) in the manner described hereinbefore, whereby the plasma density at the peripheral portion in particular is increased, resulting in that the sputter etching rate on an average is increased. Among the others, the sputter etching rate at the peripheral portion of a substrate which has presented a problem in the prior art can be increased with the uniformity in the sputter etching rate being simultaneously enhanced. It has been found from the currently available data that deviation in the sputter etching rate between center portion and the peripheral portion is in the range of ±5% in the case of the treatment carried out with the sputter etching apparatus equipped with the magnetic field generating means according to the invention, while in the control case where no magnetic field generating means is incorporated, the deviation amounts to ±20%. As will now be appreciated from the foregoing description, it is possible according to the teaching of the present invention that the plasma can be produced at a high density by means of a plasma generating unit operative based on the microwave. Further, by implementing the cavity as a microwave cavity resonator of the axis-symmetrical mode by adjusting the dimension of the cavity by means of the matching element 13 and the coaxial electrically conductive bar 16, there can be produced a stable plasma of high density in an axis-symmetrical form, whereby the speeding-up of etching operation and reduction of damage to the substrate can be achieved. Besides, since the generation of plasma can be controlled by correspondingly controlling the microwave power while energy at which ions in the plasma impact against the substrate can be controlled independently from the microwave power control by providing separately a second power supply source 8 for applying the high-frequency power to the etching electrode 6, the etching rate and the ion impact energy can be controlled separately from each other. This implies that the optimum etching conditions can be selected for each substrate to be treated, which in turn results in an increased yield of the sputter-etched substrates. Additionally, by virtue of the previously mentioned arrangement that the plasma is produced with high density by utilizing the microwave with the cavity dimension being so adjusted by the cavity matching element 13 and the coaxial stud 16 that the microwave cavity resonator of axis-symmetrical mode is realized and that diffusion of plasma at the peripheral portion thereof is prevented by the magnetic field, there can be produced a stable plasma exhibiting the axis-symmetry with high density even at a low vacuum level, whereby the etching can be carried out on at an increased speed while decreasing the possibility of injury to the substrate, because the radially outward diffusion of the plasma at the peripheral portion thereof can be positively prevented by the magnetic field.

Claims: A plasma treatment apparatus, comprising a microwave generating source (10) generating a microwave, a waveguide (11) feeding said microwave, a cavity resonator (2) including a cavity (12) provided with matching means (13) allowing its adjustment to as to resonate said microwave fed along said waveguide (11) so that an electric field of said microwave is intensified, a gas supply source (4), an evacuator (3), a vacuum chamber (1) connected to said gas supply source (4) and said evacuator (3) and containing a plasma treatment electrode (6) supporting a substrate (5) thereon, a microwave injection window (14) supplying microwave energy resonated in said cavity resonator (2) so as to generate plasma (15; 20) by said microwave energy, wherein the plasma (15; 20) is disposed against said substrate (5) in said vacuum chamber (1), said microwave injection window (14) being interposed between said cavity resonator (2) and said vacuum chamber (1) so that said microwave injection window (14) is provided in opposition to said substrate (5) and a voltage supply (8) electrically connected to said plasma treatment electrode (6) causing ions in the plasma (15; 20) to impact against said substrate (5). An apparatus according to claim 1, further comprising a magnetic field generator (18; 22a, 22b; 23; 24) located at the periphery of said vacuum chamber (1) adjacent to said microwave injection window (14) and generating magnetic lines of force to contain the plasma (20) in a space defined by said plasma treatment electrode (6) and said microwave injection window (14) thereby keeping said plasma density high and uniform in distribution in the vicinity of said substrate (5). An apparatus according to claim 1 or 2, wherein said plasma treatment electrode (6) has applied thereto a high-frequency electric power. An apparatus according to any one of the claims 1 to 3, wherein said cavity (12) is limited by said microwave injection window (14). An apparatus according to any one of the claims 1 to 4, wherein said cavity (12) is implemented in a coaxial structure. An apparatus according to any one of the claims 1 to 5, wherein said cavity (12) is so dimensioned that said microwave exhibits an axis-symmetrical mode. An apparatus according to any one of the claims 1 to 6, wherein said microwave injection window (14) is formed of an electrically insulating material selected from a group consisting of quartz and ceramics. An apparatus according to any one of the claims 2 to 7, wherein said magnetic field generator is constituted by a ring-like array of magnets (18) magnetized in the radial direction with polarities thereof being inverted alternately from one to another along the circumferential direction. An apparatus according to any one of the claims 2 to 7, wherein said magnetic field generator is constituted by a pair of ring-like magnets (22a, 22b; 23; 24) one of which is disposed to the side of said microwave injection window (14) with the other one being disposed to the side of said plasma treatment electrode (6). An apparatus according to claim 9, wherein said pair of ring-like magnets is constituted by coils respectively. An apparatus according to claim 10, wherein said pair of ring-like coils is arranged to conduct electric currents in directions opposite to each other respectively. An apparatus according to claim 10, wherein said pair of ring-like coils is arranged to conduct respective electric current in a same direction. An apparatus according to claim 9, wherein said pair of ring-like magnets is constituted by permanent magnets respectively. An apparatus according to claim 13, wherein said pair of ring-like permanent magnets (22a, 22b) is magnetized circumferentially in directions opposite to each other. An apparatus according to claim 13, wherein said pair of ring-like permanent magnets (22) is magnetized thicknesswise in directions opposite to each other. An apparatus according to claim 13, wherein said pair of ring-like permanent magnets is radially magnetized in directions opposite to each other.

Inventors: NAKAJIMA KAZUHIRO, SAITO HIROSHI, SASABE SHUNJI, SUZUKI YASUMICHI, NAKAJIMA, KAZUHIRO, SAITO, HIROSHI, SASABE, SHUNJI, SUZUKI, YASUMICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: C23C14
IPC codes: H01J37, C23C16