Sputtering process and an apparatus for carrying out the same

Abstract: A sputtering process and an apparatus for carrying out the same for forming a film of a film forming material over the surface of a substrate (25). A target (5) formed of the film forming material is held on a sputtering electrode (4) receiving a voltage, and the substrate (25) is disposed at a predetermined distance and opposite to the target (5). A high-density plasma is produced by producing a cusp field between the target (5) and the substrate (25) and a bias voltage is applied to the surface of the substrate (25) to make ions of the high-density plasma fall on the surface of the substrate (25) in order to make the particles of the film forming material sputtered from the target deposit in a thin film over the surface of the substrate (25).

Description: Background of the Invention The present invention relates to a sputtering process for forming a thin film and, more particularly, to a sputtering process capable of satisfactorily coating minute stepped parts, minute grooves and minute holes in the surface of a substrate for a highly integrated multilayer semiconductor device having minute wirings, and an apparatus for carrying out the same. Japanese Patent Provisional Publication No. 60-221563 discloses a bias sputtering apparatus designed to apply a strong magnetic field to a plasma to increase current flowing into the substrate on which a film is to be formed. That is, in this bias sputtering apparatus, an objective substrate is disposed opposite to a magnetron sputtering electrode, and part of ions in a plasma generated on the magnetron sputtering electrode is caused to flow into the surface of the substrate by applying a negative bias voltage to the surface of the substrate. U.S. Pat. No. 3,325,394 discloses a sputtering technique employing a cusp field. According to this technique, an objective substrate is disposed opposite to a sputtering electrode, and a cusp field is produced between the objective substrate and the sputtering electrode by two sets of electromagnets to increase the density of the plasma and to increase the film forming speed. Since a magnetron sputtering process, which was disclosed after the sputtering process employing a cusp field, is simpler in the construction of the apparatus for carrying out the same and more effective than the sputtering process employing a cusp field for increasing the film forming speed, no attention has been paid to the use of a cusp field. EP-A-0 173 164 discloses a microwave-assisted sputtering method and apparatus for forming a thin film on a substrate at a high rate. This is achieved by combining microwave excitation with a cusp field generated by a pair of magnets, located in the target side and the substrate side of the apparatus, respectively. By these means, a plasma density, and accordingly, the deposition rate is increased. In the bias sputtering process, a negative bias voltage is applied to the surface of a substrate while a film forming material is deposited over the surface of the substrate in order that ions of a plasma fall on the surface of the substrate to give the energy of the ions to the particles of the film forming material in order to enhance the movement of the particles of the film forming material over the surface of the substrate so that the spread of the particles of the film forming material over the surfaces of grooves and holes formed in the substrate is improved. Accordingly, it is essential to increase the quantity of ions which fall on the substrate, which can be attained by increasing the plasma density on the substrate to increase an ionic current flowing into the substrate or by increasing the bias voltage. Since the conventional magnetron sputtering electrode retains a plasma on the surface thereof facing the substrate, plasma density on the surface of the substrate cannot be increased to a sufficiently high degree simply by applying a bias voltage to the surface of the substrate. Our preliminary experiments showed that the intensity of ionic current flowing into the substrate was on the order of 0.5 A/125 mm diam., plasma density on the substrate was on the order of 2 x 10¹⁰/cm⁻³ and the spread of the film forming material (aluminum) over the surface of a square hole of 1.0 µm square and 1.0 µm in depth. As generally known, the quantity of Ar gas absorbed by the film formed by sputtering increases with bias voltage. It was also found through our preliminary experiments that voids and blisters appear in a film formed by sputtering using a high bias voltage on the order of 140V or above, when the film is subjected to annealing after sputtering. Summary of the Invention It is the object of the present invention to prevent excessive temperature rise in the central portion of the substrate due to the concentration of high-energy electrons on the central portion of the substrate, to make ions fall uniformly on the substrate, to form a film of uniform quality, and to make the migration of the particles of the film forming material over the entire surface of the substrate uniform. This object is achieved by the method of claim 1 and the apparatus of claim 5. Brief Description of the Drawings Figure 1 is a schematic longitudinal sectional view of a sputtering apparatus, in a first embodiment, according to the present invention; Figure 2 is a diagrammatic illustration showing the distribution of magnetic lines of force between a substrate and a target in the sputtering apparatus of Fig. 1; Figure 3 is a graph showing inflow ionic current density on a substrate and inflow electron energy density on a substrate with the center magnetic flux density of a substrate coil; Figure 4 is a schematic longitudinal sectional view of a sputtering apparatus, in a second embodiment, according to the present invention. Detailed Description of the Preferred Embodiment Referring to Fig. 1 showing a sputtering apparatus, in a first embodiment, according to the present invention, a sputtering electrode 4 is attached to an insulating member 3 so as to be positioned in an opening 2 formed in the upper wall of a vacuum chamber 1. A target 5 formed of a film forming material is attached to the lower surface, namely, a surface facing the interior of the vacuum chamber 1, of the sputtering electrode 4, and a target coil 6 and a yoke 7 enclosing the coil 6 are attached to the upper surface, namely, a surface facing the exterior of the vacuum chamber 1, of the sputtering electrode 4. The target coil 6 is connected to a target coil power source 23. The sputtering electrode 4 is connected to a sputtering power source 20. An anode 28 is disposed around the target 5 with a gap which will prevent discharge from the target 5. The anode 28 is secured through an insulator 8 to the vacuum chamber 1. The potential of the anode 28 is set selectively for floating potential, ground potential, an optional positive voltage or an optional negative voltage. A substrate electrode 10 for supporting a substrate 25 is disposed opposite to the target electrode 4. The substrate electrode 10 can be shifted between a substrate transfer position indicated by full lines in Fig. 1 and a processing position indicated by alternate long and short dash lines in Fig. 1 by an electrode shifting cylinder 10B. The substrate electrode 10 has a substrate elevating member 39 for vertically moving the substrate 25. A temperature control system for cooling the substrate 25 comprises a gas supply pipe 29 inserted through a through hole formed coaxially through the stem 10B of the substrate electrode 10 and through the central portion of the substrate holding table 10A of the substrate electrode 10, a gas supply device, not shown, for supplying a cooling gas the same as that filling the vacuum chamber 1, namely, an inert gas such as Ar gas, through the gas supply pipe 29 into the vacuum chamber 1, and a temperature regulator, not shown, for regulating the temperature of the cooling gas. The cooling gas supply pipe 29 is extended through the substrate elevating member 39 to supply a cooling fluid for cooling the substrate 25 between the substrate 25 and the substrate supporting table 10A. The substrate electrode 10 is provided also with a built-in inner substrate coil 30 in the substrate supporting table 10A. An outer substrate coil 17 is disposed at a position corresponding to the processing position of the substrate electrode 10. When the substrate electrode 10 is at the processing position, the inner substrate coil 30, the outer substrate coil 17 and the target coil 6 produce a cusp field. A bias voltage applying unit 38 for applying a bias voltage to the substrate 25 is disposed on the right-hand side of the processing position of the substrate supporting table 10A of the substrate electrode 10 to apply a bias voltage to the substrate 25 positioned at the processing position. The bias voltage applying unit 38 comprises a contact member 40 with which the substrate 25 is brought into contact, a base member 41 supporting the contact member 40, rollers 42 supported on the base member 41, a guide plate 44 having guide grooves 43 respectively for guiding the rollers 42, a spring 45 biasing the base plate 41 to the right as viewed in Fig. 1, and a power cylinder 46 for pushing the base plate 41 to the left as viewed in Fig. 1 so that the base plate 41 is moved obliquely downward. The substrate 25 is conveyed into the vacuum chamber 1 through an entrance gate 47 provided on one side wall of the vacuum chamber 1 by a conveying device 49 and is conveyed outside the vacuum chamber through an exit gate 48 provided on another side wall of the vacuum chamber 1 by a conveying device 50. When only a DC bias voltage is applied to the substrate 25, the high-frequency power source 21 shown in Fig. 1 may be omitted. The DC power source 22 shown in Fig. 1 may be omitted when only a high-frequency bias voltage is applied to the substrate 25. When a high-frequency bias voltage is applied to the substrate 25, a substrate holder 12 is formed of an insulating material to shield a high-frequency plasma. The target coil 6 and the target coil power source 23 for producing a magnetic field may be substituted by an equivalent permanent magnet. The manner of operation of the sputtering apparatus will be described hereinafter. The conveying device 49 conveys the substrate 25 through the entrance gate 47 into the vacuum chamber 1 to a position corresponding to the center of the substrate electrode 10. Then, the elevating member 39 is raised to receive the substrate 25 from the conveying device 49. Then, the conveying device 49 is retracted, the elevating member 39 is lowered to mount the substrate 25 on the substrate supporting table 10A, and then the substrate holder 12 holds the substrate 25 on the substrate supporting table 10A. An inert gas such as Ar gas is supplied into the vacuum chamber 1 by a gas supply system 19. The vacuum chamber 1 is evacuated by an evacuating system 18. Then, an electrode driving cylinder, not shown, is actuated to raise the substrate electrode 10 to the processing position and the entrance gate 47 is closed. The evacuating device 18 is operated at an evacuating rate in relation to the flow rate of gas supplied through the gas supply pipe to maintain the internal pressure of the vacuum chamber, typically, at a pressure on the order of 0.1 Pa. The target coil power source 23, the substrate coil power source 24 and the substrate inner coil power supply 31 supply coil currents to the target coil 6, the substrate coil 17 and the inner substrate coil 30, respectively, so that magnetic fields of opposite polarities are produced by the target coil 6 and the substrate coil 17, respectively, and magnetic fields of the same polarity are produced by the target coil 6 and the inner substrate coil 30. Consequently, a cusp field as represented by magnetic lines of force 26 in Fig. 1 is procuced. Then, the sputtering power source 20 applies a sputtering voltage to the sputtering electrode 4, and thereby a high-density plasma 27 is produced in a space defined by the magnetic lines of force 26 between the surface of the target 5 and the surface of the substrate 25. Then, the DC power source 22 applies a DC bias voltage through the substrate holder 12 to the surface of the substrate 25 to maintain the surface of the substrate 25 at a negative bias potential or the high-frequency power source 21 supplies a high-frequency power to the substrate electrode 10 to maintain the surface of the substrate 25 at the negative bias potential by producing a high-frequency plasma and causing the negative bias potential on the surface of the substrate 25, in order to cause ions in the plasma 27 to flow into the substrate 25. The substrate 25 is held at a predetermined temperature by the Ar gas supplied through the gas supply pipe 29. By applying sputtering power to the target electrode 7 a plasma 27 is produced between the target electrode 7 and the substrate electrode 10. Consequently, the target 5 of a film forming material sputters particles, and then the particles of the film forming material deposit over the surface of the substrate 25 in a thin film of a predetermined thickness. Since the high-density plasma is caused to prevail near the substrate 25 by the agency of the cusp field 27, an ionic current sufficient to make the particles of the film forming material migrate satisfactorily in stepped parts, grooves and holes formed in the substrate 25 is produced at a low bias voltage which will not cause defects in the film. Then, the power cylinder 46 is actuated to push the base plate 41 and thereby the base plate 41 is moved obliquely downward to the left as the rollers 42 move along the guide grooves 43 to bring the contact member 40 into contact with the substrate 25. Then, a DC bias voltage is applied to the substrate 25 for a secondary (bias) sputtering process. A high-frequency power is supplied to the substrate supporting table 10A of the substrate electrode 10 to induce a high-frequency self-bias voltage on the surface of the substrate 25. In such a case, the bias voltage applying unit 38 need not be provided. Fig. 2 illustrates calculated magnetic lines of force 26 obtained through a simulation, in which the respective central magnetic flux densities of the target coil 6, the inner coil 30 and the outer coil 17 were 338G, 280G and 250G. Magnetic lines of force (magnetic flux) a among those (a and b) extending from the fore end of the central core of the yoke 7 of the target coil 6 are curved so as to extend along the surface of the target 5 under the influence of magnetic flux c generating from between the inner coil 30 and the outer coil 17. Magnetic lines of force b extending from the free end of the central core of the yoke 7 reach the inner coil 30. Then, the target power source 20 supplies a sputtering power to the target electrode 4 to produce a high-density plasma confined by the magnetic lines of force a and c extending perpendicularly to the direction of the electric field between the target 5 and the surface of the substrate supporting table 10A of the substrate electrode 10. It was found through observation that the density of the plasma is higher in a region in which the magnetic lines of force a extend than in a region in which the magnetic lines of force b extend. The target 5 is caused to produce a large number of secondary electrons mainly by ions supplied from the plasma in the region of the magnetic lines of force a, and then the secondary electrons are accelerated at a high rate and discharged into the plasma by the agency of the potential difference of several hundreds volts between the target 5 and the plasma. The electrons having a high energy move helically along the magnetic lines of force a and c. Since the magnetic lines of force a are convexly curved with respect to the surface of the target 5, the electrons other than those lost by collision are confined by the magnetic lines of force a. Electrons discharged into the magnetic lines of force c travel along the magnetic lines of force c toward the substrate 25. However, since the magnetic lines of force c are deviated from the substrate 25 and pass across the circumference of the substrate 25, electrons having a high energy never fall on the substrate 25. On the other hand, most of the electrons discharged into the magnetic lines of force b are high-energy electrons diffused by the collision with ions and neutral particles and hence the speed of those electrons are reduced to a sufficiently low speed. The low-speed electrons travel along the magnetic lines of force b toward the substrate 25. Since the magnetic lines of force b extending from a sufficiently small area in the target 5 are distributed uniformly over the surface of the substrate 25 in a uniform magnetic flux density, the electrons are distributed uniformly over the surface of the substrate 25. Furthermore, since the distribution of magnetic flux density over the surface of the substrate 25 is uniform, ions contained in the plasma 27 and traveling along the magnetic lines of force b fall on the surface of the substrate 25 in a uniform distribution. Still further, since the magnetic lines of force a are caused to collect near the surface of the target 5 by the magnetic lines of force c, a high-density plasma is formed in a wide region. Fig. 3 shows the variation of ionic current flowing into the substrate 25 and electron energy density on the substrate 25 with the central magnetic flux density of the inner coil 30, in which the bias voltage applied to the substrate 25 is -100V and the respective central magnetic flux densities of the target coil 6 and the outer coil 17 are 338G and 248G. When the magnetic flux density of the inner coil 30 is zero, a magnetic field the same as a cusp field is produced. As the central magnetic flux density of the inner coil 30 is increased, the concentration of ions on the central portion of the substrate 25 is reduced, ions fall in a uniform distribution on the substrate 25 and no high-energy electron falls on the substrate 25 when the magnetic flux density of the inner coil 30 exceeds 260G. A temperature regulating gas is introduced between the substrate supporting table 10A of the substrate electrode 10 and the substrate 25 held thereon to regulate the temperature of the substrate 25 at an appropriate temperature for forming a thin film of satisfactory quality. Thus, abnormal temperature rise in the central portion of the substrate and irregular migration of the particles of the film forming material over the surfaces of minute stepped parts and minute holes attributable to the maldistribution of ions on the surface of the substrate, which are liable to occur in the conventional cusp field bias sputtering process, are obviated, a film is formed at a high film forming speed, a film having a uniform quality can be formed and the particles of the film forming material are caused to migrate satisfactorily over the surface of the substrate. Fig. 4 shows a sputtering apparatus, in a second embodiment, according to the present invention. The second embodiment is similar in construction to the first embodiment except for the means for producing a magnetic field. In the second embodiment an annular permanent magnet 33 is employed instead of the inner coil 30 and the outer coil 17 of the first embodiment. The permanent magnet 33 has an inside diameter greater than that of the substrate and is disposed at the back of the substrate 25. The external magnetic lines of force of the permanent magnet 33 produce a cusp field in cooperation with the magnetic lines of force of a target coil 6. The internal magnetic lines of force of the permanent magnet 33 are distributed similarly to and provide the same effect as that provided by those shown in Fig. 2.

Claims: A sputtering process for forming a thin film on a substrate (25) facing a target (5) in a vacuum chamber, said substrate (25) being supported on a substrate electrode (10) and being exposed to a magnetic flux density uniformly distributed over the surface of the substrate (25), and said target (5) being formed of a film forming material and mounted on a target electrode (4) receiving a voltage comprising: producing a cusp shaped magnetic field between said target (5) and said substrate (25) using a target magnetic field produced by a target magnetic device (6, 7, 23) associated with the target electrode (4) and a substrate magnetic field produced by a substrate magnetic device associated with the substrate electrode (10), said magnetic fields having opposite polarities, whereby the magnetic lines of force of the magnetic field produced by a first magnetic means (17) of the substrate magnetic device are warped toward the periphery of the substrate (25) so that the magnetic lines of force will not extend across the central portion of the substrate (25) but extend to its periphery; introducing an inert gas and producing a plasma in the space between the target (5) and substrate (25) to thereby sputter material from the target (5) to deposit the film forming material in a thin film over the surface of the substrate (25), characterized by producing, inside said cusp shaped magnetic field, a magnetic field of opposite polarity to that produced by said first magnetic means (17) so that magnetic field lines extend from the target (5) and cross the substrate (25), and producing said plasma by applying electric power to the target electrode (4). A sputtering process according to claim 1, characterized in that a bias voltage is applied to the surface of the substrate. A sputtering process according to claim 1 or 2, characterized in that the substrate is processed without incidence of high-energy electrons by controlling the magnetic flux density in the vicinity of the surface of the substrate. A sputtering process according to any of claims 1 to 3, wherein the substrate is processed with a uniform distribution of incident charged particles and with a controlled temperature. A sputtering apparatus for forming a thin film on a substrate comprising: a vacuum chamber (1) capable of being filled with an inert gas; a substrate electrode (10) for supporting a substrate (25), disposed within the vacuum chamber (1) and exposed to a magnetic flux density uniformly distributed over the surface of the substrate; a target electrode (4) for holding a target (5) formed of a film forming material, disposed opposite to the substrate electrode (10) within the vacuum chamber (1); a target magnetic device (6, 7, 23) provided on the side of the target electrode (4) to produce a magnetic field; and a substrate magnetic device provided to the side of the substrate electrode (10) and having a first magnetic means (17, 24) to produce a cusp field in co-operation with the magnetic field produced by the target magnetic device (6, 23) so that magnetic lines of force will not extend across the central portion of a substrate (25) but extend to its periphery; a power source (20) for applying a voltage to the sputtering electrode; characterized in that the substrate magnetic device comprises an additional means (30) disposed inside of the substrate magnetic device and behind said substrate electrode (10) for providing a magnetic field having an opposite polarity to the magnetic field produced by said first magnetic means (17, 24) so that magnetic field lines extend from the target and cross the substrate. A sputtering apparatus according to claim 5, characterized in that both the first magnetic means (17) and the additional means (30) comprise each an electromagnet. A sputtering apparatus according to claim 5, characterized in that the substrate magnetic device comprises a set of permanent magnets (33) which are disposed beneath and at the periphery of the substrate. A sputtering apparatus according to any of claims 5 to 7, further characterized by: voltage applying means (12, 21, 22) for applying a voltage to a surface of the substrate (25). A sputtering apparatus according to any of claims 5 to 8, further characterized by: temperature control means for controlling the temperature of the substrate at a predetermined temperature.

Inventors: HORIUCHI MITSUAKI, SAITO HIROSHI, SASAKI SHINJI, TATEISHI HIDEKI, HORIUCHI, MITSUAKI, SAITO, HIROSHI, SASAKI, SHINJI, TATEISHI, HIDEKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: C23C14
IPC codes: C23C14, H01J37