PLASMA CHEMICAL VAPOUR REACTION APPARATUS

Abstract: An improved plasma chemical vapour reaction apparatus is described. The apparatus comprises a reaction chamber (1,8), a gas feeding system (10,11) for introducing a reactive gas into the reaction chamber, Helmholtz coils (5) for inducing a magnetic field in the reaction chamber, a microwave generator (4) for inputting microwaves into the reaction chamber, and a substrate holder (3) for supporting a substrate (2) to be treated in the reaction chamber. The substrate holder is so located with respect to said magnetic field inducing means that the field strength gradient vector of the magnetic field induced by the Helmholtz coils is directed towards the substrate in the vicinity of the surface of the substrate to be treated. By virtue of the magnetic field characterized by such a gradient vector, the plasma gas in the reaction chamber is caused to drift towards the substrate.

Description: BACKGROUND OF THE INVENTION The present invention relates to a plasma chemical vapour reaction apparatus, and particularly, though not exclusively, relates to a plasma ECR (Electron Cyclotron Resonance) CVD (Chemical Vapour Deposition) apparatus. Plasma ECR CVD apparatuses are disclosed, for example, in the Matsuo et al article "Low Temperature Chemical Vapour Deposition Method Utilizing an Electron Cyclotron Resonance Plasma" published in the Japanese Journal of Applied Physics, Vol 22, No. 4, April 1983, pp. L210-L212, and in the Kawarada et al article "Large Area Chemical Vapour Deposition of Diamond Particles and Films Using Magneto-Microwave Plasma" published in the Japanese Journal of Applied Physics, Vol 26, No.6 June 1987, pp. L1032-L1034. Both of these Matsuo et al and Kawarada et al articles disclose a plasma chemical vapour reaction apparatus comprising a reaction chamber, a gas supply system for introducing a reactive gas into the reaction chamber, Helmholtz coils for developing an axial magnetic field within the reaction chamber, a microwave generator for inputting microwave energy into the reaction chamber in an axial direction, the magnetic field and microwave energy producing conditions for sustaining electron cyclotron resonance within the reactive gas, and a substrate holder for supporting a substrate at a position in the reaction chember such as to have a surface of the substrate exposed to deposition of material from the plasma. The ECR CVD technique has attracted the interest of researchers due to its high rate excitation ability. As a deposition method it makes use of ECR (Electron Cyclotron Resonance) which takes place between charged particles and microwaves in a magnetic field. Since the energy induced by ECR is very high, the disadvantage arises that a substrate to be treated tends to suffer from sputtering damage. As shown in the abovementioned Matsuo et al article, substrates have therefore been located spaced apart from the position in which the magnetic field and microwave energy conditions required to establish ECR are satisfied and the plasma products are extracted onto the substrate by a divergent magnetic field. This has given rise to the disadvantage of the prior art ECR CVD technique that the plasma density in the substrate region is relatively small and the pressure of the reactive gas in the apparatus has to be of the order of 10⁻⁴ Torr (1 Torr = 133.3 N/m²) in order to enable the excited gas to reach the substrate to be processed, and highly crystallized depositions, such as of diamond, could not be formed. OBJECTS AND SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a plasma chemical vapour reaction apparatus capable of effective treatment of a substrate. It is another object of the present invention to provide a plasma chemical vapour reaction apparatus which is capable of depositing highly crystallized films on substrates. According to the present invention there is provided a plasma chemical vapour reaction apparatus comprising a reaction chamber; a gas supply system for introducing a reactive gas into said reaction chamber; magnetic field inducing means for inducing a magnetic field in said reaction chamber; a microwave generator for inputting microwave energy into said reaction chamber; said magnetic field inducing means and said microwave generator producing conditions in operation of said apparatus for sustaining electron cyclotron resonance within a reactive gas plasma in said reaction chamber; and a substrate holder for supporting a substrate in said reaction chamber at a position to have a surface thereof exposed to particulate material from said reactive gas plasma; all as known from the Matsuo and Kawarada articles abovementioned; the apparatus being characterized in accordance with the invention in that second magnetic field inducing means are provided for generating centripetal forces such as to pinch the plasma; and said substrate holder and the first-mentioned and second magnetic field inducing means are arranged so that the substrate holder is located where the magnetic fields of the first-mentioned and second magnetic field inducing means interact so as to produce a resultant magnetic field having a field strength gradient directed to concentrate the plasma towards the substrate that is to be treated. The invention also provides a plasma chemical vapour reaction method comprising: disposing a substrate in a reaction chamber; introducing a reactive gas into said reaction chamber; establishing a magnetic field within said reaction chamber and inputting to said reaction chamber microwave energy propagated approximately parallel to said magnetic field so as to produce a reactive gas plasma by the development of electron cyclotron resonance within the reaction chamber; and carrying out plasma processing with said plasma gas on said substrate; as known from the Matsuo and Kawarada articles abovementioned; the method being characterized in accordance with the invention in that the reactive gas plasma is additionally subjected to a further magnetic field developing centripetal forces such as to pinch the plasma; and the substrate is located within the reaction chamber at a position whereat the magnetic fields applied to the plasma interact so as to produce a resultant magnetic field having a field strength gradient which is directed to concentrate the plasma towards the substrate that is to be treated. Further features of the invention are set forth in the appended claims and, together with the abovementioned features, will be well understood by consideration of the following detailed description given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing an exemplary plasma CVD apparatus in accordance with the present invention; Figs. 2(A) and 2(B) are cross-sectional and plan views respectively showing Ioffe bars comprising electromagnets; Figs. 3(A) and 3(B) are cross-sectional and plan views respectively showing Ioffe bars comprising permanent magnets; Fig. 4 is a cross-sectional view showing a modification of the Ioffe bars illustrated in Figs. 3(A) and 3(B); Fig. 5 is a schematic illustration showing the magnetic field strength profile in the plasma generating space of an ECR CVD apparatus which does not embody the present invention; Fig. 6 is a schematic illustration showing the magnetic field strength profile in the plasma generating space of an apparatus in accordance with the present invention; and Fig. 7 is an explanatory view showing the drift forces exerted on plasma in an apparatus in accordance with the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring now to Fig. 1, an exemplary microwave enhanced plasma CVD apparatus in accordance with the present invention is illustrated. The apparatus comprises a reaction chamber in which a plasma generating space 1 and an auxiliary space 8 are defined which can be maintained at an appropriate negative pressure by means of an evacuating system 9. Gas introduction systems 11 and 10 are connected to the plasma generating space 1 and the auxiliary space 8 respectively. Microwave energy is introduced into the plasma generating space 1 from a microwave generator 4 through a waveguide 7. Helmholtz coils 5 and Ioffe bars 6 are located around the plasma generating space 1. Ioffe bars are known per se, for example from Patent Abstracts of Japan, Vol 11, No. 317(C-452)(2764) dated 15 October 1987. The Ioffe bars 6 are arranged parallel to the microwave propagation direction and as depicted in Figs. 2(A) and 2(B) can be constructed with electromagnets or as depicted in Figs. 3(A) and 3(B) with permanent magnets. In Fig. 3(A) the arrows indicate the magnetic moments of the permanent magnets. Fig. 4 shows a modification of the Ioffe bars illustrated in Figs. 3(A) and 3(B). A substrate 2 to be treated is mounted on a substrate holder 3 which is adapted to heat the substrate 2. The substrate holder 3 is made of a ceramic, e.g. aluminium nitride, which causes little disturbance of the magnetic field induced by the magnets 5 and 6. The relative location of the substrate 2 with respect to the Helmholtz coils 5 is very important. Fig. 5 shows the configuration of an ECR CVD apparatus which does not embody the present invention and shows the location of the coils 5 relative to the substrate 2, the substrate being placed between the two coils. A number of curves have been drawn within the plasma generating space 1 to trace equi-strength surfaces of the magnetic field induced by the magnets 5 and 6. The magnetic field strength increases as a function of the distance from the centre of the substrate 2. In this apparatus configuration the plasma experiences drift forces towards the weaker magnetic field strength positions, i.e. in directions away from the substrate 2. Consequently, there is a tendency to gather plasma in the generating space 1 rather than in the region of the substrate surface to be processed. Generally speaking, the drift force exerted on plasma in a magnetic field is in proportion to -grad B, where B is the absolute strength of the magnetic field. The equi-strength surfaces in the magnetic field that is established in the case of the present invention are shown in Fig. 6. The magnetic field as shown becomes weaker at a nearer position to the substrate 2. The drift forces exerted on the plasma are shown in Fig. 7 by the arrows. The arrow 12 represents a drift force which serves for drifting plasma towards the substrate 2 and is mainly induced by the Helmholtz coils 5. The arrows 13 represents a centripetal force which is induced mainly by the Ioffe bars 6. Next, an exemplary method for depositing carbon films on a substrate by use of the apparatus according to the invention will be described. After evacuating the chamber by means of the evacuating system 9, hydrogen is admitted to the plasma generating space 1 at 10SCCM from the gas feeding system 11. The Helmholtz coils 5 are energized in order to induce a 2k Gauss magnetic field in the plasma generating space 1. The Ioffe bars 6 modify the magnetic field to produce a centripetal drift force. 2.45 GHz microwave energy is introduced from the microwave generator 4 at 500 W. By virtue of resonance between the microwaves and hydrogen atoms and electrons, excited hydrogen ions and electrons are produced. The resonance may take place in an ECR mode, a MCR (Mixed Cyclotron Resonance) mode or the whistler mode. The pressure of hydrogen is maintained at 0.1 Pa. Hence, high energy hydrogen atoms or electrons collide with the surface of the substrate 2 and perform plasma cleaning. After cleaning the substrate surface, the introduction of hydrogen is stopped and a carbon compound gas such as CH₄, C₂H₄, C₂H₂, CH₃OH or C₂H₅OH for example is introduced instead. The carbon compound gas is excited in the same manner as described above for the hydrogen cleaning process and deposits a carbon thin film of diamond or an i-carbon. In accordance with the results of experiments that we have conducted, diamond films can be formed when the substrate temperature is not lower than 450°C. When the deposition was carried out in the same manner but without a magnetic field, only graphite films were formed. Electron beam diffraction images of thin carbon films deposited in accordance with the present invention were taken. In the image, spot patterns indicative of the existence of diamond structure were observed together with halo patterns which are peculiar to amorphous structures. This means that the carbon films were made of i-carbon. It was confirmed that i-carbon (diamond like carbon) films can be formed when the substrate is not heated. The diamond like carbon is an amorphous carbon, but has a highly abrasion-proof surface and a high thermal conductivity. When the substrate temperature was elevated, the halo patterns gradually disappeared and, at not lower than 650°C, the thin films were confirmed to be diamond films. The Raman spectrum of the film had a blunt peak about 1500 cm⁻¹ and a sharp peak about 1333 cm⁻¹ which indicates the existence of diamond. While several embodiments have been specifically described, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departure from the invention as defined by the appended claims. Examples of such modifications are described below. Improved silicon carbide films and aluminium nitride films can be formed in accordance with the present invention using methyl silane and using a gaseous aluminium compound and ammonia respectively. Also, tungsten, titanium, molybdenum or their silicates having high melting points can be formed in the same manner. The crystallinity of films manufactured in accordance with the present invention could be increased by adding water or oxygen to the reactive gas. Although the specific embodiment described concerns deposition, the present invention is applicable to general plasma processes effected in a magnetic field, such as plasma etching processes and plasma cleaning processes. Optionally, a UV light source may be provided for irradiating the reactive gas in the chamber in order to ensure that the reaction gas is in an excited state near to the substrate to be processed. Also, a DC bias voltage may be applied to the reaction gas in the plasma state. The bias can be applied by means of a voltage supply 14 connected between the substrate holder 3 and an electrode mesh 15 as illustrated in Fig. 1. The bias voltage is applied with the holder 3 being the cathode in case of Si deposition, and with the mesh 15 being the cathode in case of carbon deposition or etching using F⁻ or B⁻. In the case of the holder being made from a conductive material, the bias voltage consists only of a DC component. In the case of the holder being made from a non-conductive material, the bias voltage consists of a DC component and a superimposed AC component.

Claims: A plasma chemical vapour reaction apparatus comprising:- a reaction chamber (1); a gas supply system (10, 11) for introducing a reactive gas into said reaction chamber; magnetic field inducing means (5) for inducing a magnetic field in said reaction chamber; a microwave generator (4) for inputting microwave energy into said reaction chamber; said magnetic field inducing means (5) and said microwave generator (4) producing conditions in operation of said apparatus for sustaining electron cyclotron resonance within a reactive gas plasma in said reaction chamber (1); and a substrate holder (3) for supporting a substrate (2) in said reaction chamber at a position to have a surface thereof exposed to particulate material from said reactive gas plasma; characterized in that :- second magnetic field inducing means (6) are provided for generating centripetal forces such as to pinch the plasma; and said substrate holder (3) and the first-mentioned and second magnetic field inducing means (5, 6) are arranged so that the substrate holder (3) is located where the magnetic fields of the first-mentioned and second magnetic field inducing means (5, 6) interact so as to produce a resultant magnetic field having a field strength gradient directed to concentrate the plasma towards the substrate that is to be treated. The apparatus of claim 1 wherein the first-mentioned magnetic field inducing means (5) comprises Helmholtz coils surrounding said reaction chamber (1). The apparatus of claim 1 or 2 wherein said second magnetic field inducing means comprises a plurality of elongate permanent magnets arranged around the periphery of the reaction chamber (1), said magnets having their magnetic poles extending along transversely opposed longitudinal edges thereof and being arranged such that their magnetic moments alternate as between one magnet and the next around the periphery of the reaction chamber, or an equivalent electromagnetic arrangement. The apparatus of claims 2 and 3 wherein said elongate permanent magnets, or equivalent electromagnetic arrangement, extend within the Helmholtz coils (5) and substantially beyond one axial end thereof to a location substantially spaced from said one axial end and generally corresponding to the location of said substrate holder (3). A plasma chemical vapour reaction method comprising:- disposing a substrate in a reaction chamber; introducing a reactive gas into said reaction chamber; establishing a magnetic field within said reaction chamber and inputting to said reaction chamber microwave energy propagated approximately parallel to said magnetic field so as to produce a reactive gas plasma by the development of electron cyclotron resonance within the reaction chamber; and carrying out plasma processing with said plasma gas on said substrate; characterized in that :- the reactive gas plasma is additionally subjected to a further magnetic field developing centripetal forces such as to pinch the plasma; and the substrate is located within the reaction chamber at a position whereat the magnetic fields applied to the plasma interact so as to produce a resultant magnetic field having a field strength gradient directed to concentrate the plasma towards the substrate that is to be treated.

Inventors: HIROSE, NAOKI, INSHIMA, TAKASHI
Applicants: SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1989
Main classification: C23C16
IPC codes: C23C16, H01J37, H01L21