Ion source

Abstract: An ion source for producing an ion beam utilized for fabrication and processing of semiconductors, thin films or the like includes a plasma producing chamber (1) equipped with first magnetic means (10a) for limiting a plasma region (20a) and a plasma expansion chamber (2) provided in combination with the plasma producing chamber (1) on the side where a beam extracting electrode (4) is disposed. The plasma expansion chamber (2) is provided with second magnet array (10b) for confining and holding a plasma region (20b) therein which is of a larger area than that of the plasma region (20a) formed in the plasma producing chamber (1).

Description: The present invention generally relates to an ion source as defined in the first part of claim 1 and more particularly to an ion source for to an ion source for producing an ion beam to be utilized in the processing as well as fabrication of semiconductors, thin films and the like. In recent years, tendency of an ion beam being utilized in etching apparatus and sputtering apparatus for processing and manufacturing functional thin films, semiconductors and the like becomes more and more flourish in accompaniment to an increasing demand for the fineness in the implementation of these devices. Many of the ion sources for producing the ion beam destined for the applications mentioned above are so designed that an arc discharge is sustained by emission of thermal electrons from a filament to thereby produce plasma of a desired gas, see Rev. Sci. Instrum., Vol. 53, No. 7, Jul. 1982, p. 1038-1048, S. Tanaka et al.: "Reduction of gas flow into a hollow cathode ion source for a neutral beam injector". However the conventional ion source of this type suffers a serious disadvantage that the use life of the filament is so short that frequent replacement thereof is required. This is because the plasma has to be produced from a gas which exhibits a high reactivity in such applications as etching or the like. The problem mentioned above may be solved by resorting to adoption of a high-frequency (radio frequency or rf) discharge and a microwave discharge for producing the plasma. The ion source of this type can be operated over an extended use life because the filament is no more employed. The ion source operating based on the high-frequency discharge has long been studied and is reported, for example, in C.J. Cook et al's article "Energy Anomalies Observed in Ion Beams Produced by RF Sources" in Rev. Sci. Instrum. Vol.33, No.6, p.649-652, (1962). As discussed also in this literature, it is known that the ion beam extracted from a plasma produced through the high-frequency discharge has an energy level which is higher than the extracting potential by several hundred eV on an average (referred to as excess energy) and exhibits a considerably broad energy spread. On the other hand, the inventors of the present application have experimentally discovered that the number of ions contained in the ion beam of an energy level not higher than 1 keV extracted from the plasma produced through the high-frequency (rf) discharge is small when compared with that of the ion beam extracted from the plasma produced through the arc discharge or microwave discharge and having a density equivalent to that of the plasma produced by the high-frequency discharge mentioned above. The ion sources in which the microwave is utilized are disclosed in detail, for example, in N. Sakudo et al's article "Microwave Ion Source" of "Rev. Sci. Instrum.", Vol. 48, No. 7, (July, 1977) p.762-766. The ion source of this type has a narrow energy spread and produces no excess energy, differing from the case of the high-frequency discharge type ion source. By virtue of these features, the microwave ion source can be advantageously used as the ion source for the ion implantation process involving mass separation. However, density distribution of the plasma produced by the microwave ion source does not always have a desired uniformity. When the ion beam of an energy level on the order of several hundred eV is extracted from the plasma produced by the microwave ion source for an etching process, the ion beam has an intensity distribution substantially proportional to the distribution of the plasma density. Accordingly, with the etching apparatus employing the ion source of the microwave discharge type, difficulty is encountered in realizing a uniform etching rate over a large area. For realizing a large-diameter beam (i.e. an ion beam having a large area), the plasma chamber may be constructed in a large size. In that case, however, it is necessary to establish the impedance matching for injection of microwave power into the plasma chamber, imposing thus restriction on the selection of dimensions of a waveguide as well as the plasma chamber. Further, in practical applications, restriction is also imposed on the size of a cylindrical coil for generating a magnetic field. An ion source using a high frequency discharge between two electrodes and a plasma expansion chamber for obtaining an improved intensity distribution is disclosed in FR-A-2 550 681. In the light of the state of the art described above, it is an object of the present invention to provide an ion source which is able to supply an ion beam of a large area without increasing the volume of a plasma producing chamber. This object is met by the characterizing features of claim 1. An ion source according to the invention comprises a plasma producing chamber having an inner cavity supplied with a gas for producing a plasma through microwave discharge and provided with first magnetic means for restricting a plasma region, and a plasma expansion chamber provided on the side of the plasma producing chamber where ion beam extracting means is disposed, wherein the plasma expansion chamber is equipped with second magnetic means for confining and holding a plasma region of a larger area than that of the plasma region within the plasma producing chamber. The above and other objects, features and advantages of the present invention will be more apparent upon reading the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical sectional view showing an exemplary embodiment of the ion source according to the present invention. Fig. 2 is a sectional view taken along the line V - V in Fig. 1. Fig. 3 is a sectional view taken along the line VI - VI in Fig. 1. Fig. 4 is a vertical sectional view showing another embodiment of the ion source according to the present invention. Fig. 5 is a plan view of the ion source shown in Fig. 4. Figs. 6 and 7 are views for graphically illustrating plasma density distributions. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the present invention will be described in detail with reference to the drawings. Referring to Fig. 1 which shows in a vertical sectional view an exemplary embodiment of the ion source according to the invention, a plasma producing chamber generally denoted by a reference numeral 1 comprises a cylindrical side wall 15. The plasma producing chamber 1 is provided with a plasma expansion chamber 2 at the bottom side. More specifically, the plasma expansion chamber 2 has a top end coupled to the plasma producing chamber 1 and includes an enlarged cylindrical side wall 16 and a beam extracting electrode 4. A vacuum container 17 is connected to the plasma expansion chamber 2 at the bottom end thereof through an interposed insulation member 9a. There is disposed within the vacuum chamber 17 a holder 6 mounted rotatably while being cooled by water. A workpiece 5 to be processed is disposed on the holder 6. The whole assembly comprising the plasma producing chamber, the plasma expansion chamber and the vacuum container are evacuated through an evacuation outlet 18 so that vacuum prevails within the whole system. In the case of the instant embodiment, the plasma expansion chamber 2 has a volume which is five times as large as that of the plasma producing chamber 1. The plasma expansion chamber 2 is effective to expand the plasma produced in the chamber 1 and having a tendency of diffusion. A cylindrical coil 14 is disposed around the outer periphery of the side wall 15 of the plasma producing chamber 15 to apply a DC magnetic field in the longitudinal direction of the cylindrical coil 14. A reactive gas is introduced into the plasma producing or source chamber 1 after evacuation thereof through the inlet port 8, while a microwave power having a frequency of 2.45 GHz is injected through a waveguide 13. Intensity of the magnetic field as applied is so set that electron cyclotron resonance (ECR) frequency is approximately equal to that of the injected microwave power. Alternatively, the intensity of applied magnetic field may be set higher than the latter in some degree. Under the action of resonance taking place between plasma and the microwave power, the gas as introduced is ionized to form a plasma of a high density. By way of example, in the case where the oxygen gas (O₂) was employed, plasma density of 0.5 x 10¹² to 1 x 10¹² (cm⁻³) could be realized. However, the uniformity in the plasma density distribution was rather unsatisfactory, as will be seen from Fig. 6. When the ion beam is extracted directly from the plasma producing chamber 1, the intensity distribution of the extracted ion beam becomes non-uniform. To deal with this problem, the invention teaches the provision of the plasma expansion chamber 2. With this structure, the distribution of the plasma density within the plasma expansion chamber 2 which is equiped with permanent magnets 10b for confining and holding plasmas can be significantly uniformed through diffusion of the plasma, as will be seen in Fig. 7, as the result of which the intensity distribution of the ion beam extracted from the plasma expansion chamber 2 becomes more uniform over a large area. Parenthetically, loss of plasma in the plasma expansion chamber 2 is as low as negligible. As will now be appreciated, the structure of the ion source shown in Figs. 1, 2 and 3 also allows an ion beam to be extracted with a uniform intensity over an increased area by virtue of the such arrangement that the density distribution of plasma produced in the plasma source chamber 1 is uniformed by providing the plasma expansion chamber 2. Referring to Fig. 3, the magnetic field 19 of multicusp geometry is formed by the permanent magnets 10b as in the case of the preceding embodiment. It should however be mentioned that a cylindrical coil may be employed when the diameter of the plasma expansion chamber 2 is small in either embodiment. Accordingly, it is safe to say that the provision of magnetic means capable of keeping plasma off from the side wall 16 of the plasma expansion chamber 2 is sufficient. Figs. 4 and 5 show another embodiment of the ion source according to the invention which is designed to extract the ion beam having a further enlarged area produced through microwave discharge. As will be seen in Fig. 5, the cylindrical plasma expansion chamber 2 of doughnut-like configuration having a larger plasma region than that of the plasma producing chamber 1 includes a number of permanent magnets 10b disposed around the outer and inner peripheries of the side wall 16, respectively, for producing a magnetic field of multicusp geometry. A pair of plasma producing chambers 1 which have cylindrical coils 14 and waveguides 13 for producing and holding plasmas are disposed on the plasma expansion chamber 2 at locations substantially symmetrical to each other. Plasma of doughnut-like geometry is produced by the pair of plasma producing chambers 1 through the mechanism described hereinbefore in conjunction with the embodiment shown in Fig. 1. By applying a predetermined potential on the ion beam extracting electrode 4, a hollow ion beam 7 can be extracted from the plasma. In the case of this embodiment, the plasma expansion chamber 2 is realized in doughnut-like cylindrical configuration for increasing the area of the plasma region. To this end, a number of permanent magnets 10b are fixed on both the side walls and the top plate 21 of the plasma expansion chamber 2, as is illustrated. A pair of plasma producing chambers 1 are mounted on the plasma expansion chamber 2 at locations substantially symmetrical to each other. With the structure of the ion source shown in Figs. 4 and 5, an ion beam of an increased area can be extracted with a uniform distribution of intensity. Although it has been assumed that a reactive gas is employed in the foregoing description of the various embodiments, it should be understood that inactive gases may be used, to similar effects. As will now be appreciated, the invention which teaches that a plasma expansion chamber equipped with magnetic means for keeping off plasma from the expansion chamber wall be provided in combination with the plasma producing chamber on the side where the ion beam is extracted allows an ion beam of a large area and having a uniformed intensity to be extracted in the case of a microwave discharge type ion source.

Claims: An ion source comprising at least one plasma producing chamber (1) supplied with a gas (8) for producing a plasma through electric discharge and provided with first magnetic means (14) for limiting a pillar-shaped plasma region (20a) defined within said plasma producing chamber, a plasma expansion chamber (2) provided with second magnetic means (10b) for confining and holding an expanded plasma region (20b) having a cross-sectional area larger than that of the plasma region (20a) formed within said plasma producing chamber (1), beam extracting means (4) provided at a side of the plasma expansion chamber (2) remote from said plasma producing chamber (1) for extracting an ion beam (7) from the plasma, characterized in that the electric discharge is caused by a microwave supplied to the plasma producing chamber (1) by a waveguide (13). The ion source according to claim 1, characterized by a plurality of plasma producing chambers (1), each of which is provided with said plasma expansion chamber (2).

Inventors: HAKAMATA YOSHIMI, KUROSAWA TOMOE, KUROSAWA YUKIO, OHNO YASUNORI, SATO TADASHI, HAKAMATA, YOSHIMI, KUROSAWA, TOMOE, KUROSAWA, YUKIO, OHNO, YASUNORI, SATO, TADASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1986
Main classification: H01J37
IPC codes: H01J27, H01J37