ELECTRON BEAM-EXCITED ION BEAM SOURCE

Abstract: An electron beam-excited ion beam source having a plasma region, an accelerating cathode, an electron beam accelerating region, an accelerating anode, an ion produc­ ing region and a target cathode in this order, and further comprising means for applying a negative electric potential to the target cathode as against the accelerating cathode and an ion extracting electrode for extracting positive ions or negative ions produced in the ion producing region whereby a high current ion beam can be obtained in a low input power.

Description: This invention relates to an electron beam-excited ion beam source in which ions are produced by electron beam irradiation. Ion beam sources have been used to etch semiconductor substrates. In order to efficiently etch the semiconductor substrates without causing large damage to the substrates, a high current and low energy ion beam is needed. However, the high current and low energy ion beam could not be obtained so far. In the known ion beam sources, the ion beam currents for low energy are below 10mA/cm². One of the known ion beam sources is disclosed by J. Uramoto in Research Report of Institute of Plasma Physics, Nagoya University, Nagoya, Japan, IPPJ-395 (1979). From "Electron Temperature Effects for an Ion Beam Source" by J. Uramoto, Research Report of Institute of Plasma Physics, NAGOYA UNIVERSITY/Japan, IPPJ-395, May 1979, an electron beam-excited ion beam source is known. This electron beam-excited ion beam source has a discharge cathode, a plasma region, an accelerating cathode, an electron beam accelerating region, an accelerating anode, an ion producing region and a target cathode in this order. It further comprises an ion extracting electrode for extracting ions generated in the ion producing region. The portion between the ion beam extracting electrode and the target cathode is an energy analyser for ions. The ion extracting electrode is located between the accelerating anode and the target cathode. The ion extracting electrode is further biassed by a power supply. Therefore, electron beam accelerated by the accelerating anode does not pass through the ion extracting electrode and does not reach to the target cathode. It is therefore not possible to obtain a high current and low energy ion beam. An object of the invention is to provide an ion beam source which can produce a high current and low energy ion beam. Another object of the invention is to provide an ion beam source which can produce an ion beam in high efficiency. The above objects are attained by an electron beam-excited ion beam source according to claim 1 In this ion beam source, electrons in the plasma region are entered into the ion producing region by the accelerating anode. The entered electrons excite and ionize atoms of an inert gas (or a metal vapor) charged in the ion generating region. A part of produced ions flow to the plasma region and prevent generation of negative space charge near an outlet of the accelerating cathode. Therefore, an electron beam of high current proportional to a plasma density of the plasma region enter into the ion producing region. In the ion producing region, ions, the number of which is proportional to the current value of the introduced electron beam, are generated. In the ion beam source according to this invention, as shown in Fig. 2, a potential well is formed by the accelerating cathode, the accelerating anode and the target cathode. The electrons entered from the plasma region go and return in the potential well and plasma is effectively produced. Ions in the plasma are extracted by the ion extracting electrode and emitted from the ion producing region. Energy (velocity) of the ions are determined by the potential difference between the ion extracting electrode and the accelerating anode. In the case that a multipole magnetic field, a mirror magnetic field or an electric potential well for preventing lateral diffusion of charged particles is provided at the ion producing region, plasma is more effectively produced. The ion beam source according to this invention can be used as that for ion implant apparatus. When the inside of this ion beam source is filled with an inert gas such as Argon or a reactive gas such as CF₄, this ion beam source can be used as an ion beam source for etching. Additionally, if this ion beam source is filled with a gas for producing films such as SiF₄, this ion beam source can be used as that for producing films. Moreover, this ion beam source can be used to effectively heat plasma in nuclear fusion because a ratio of an ion beam current to an input power is large. According to this invention, the high current and low energy ion beam can be obtained, in other words, the ratio of an ion beam current to an input power is large. Therefore, it becomes possible to effectively process workpieces such as semiconductor substrates without causing defects in crystal. In the field of nuclear fusion, it becomes possible to heat plasma in high efficiency. The foregoing and other objects of this invention will be seen by reference to the description, taken in connection with the drawings in which: Fig. 1 is a schematic of the first embodiment according to this invention, Fig. 2 is a drawing for illustrating the operation of this invention, Fig. 3 is a graph showing a relation between an ion beam current and an initial discharge current obtained in the first embodiment, Fig. 4 is a schematic of the second embodiment according to this invention, Fig. 5 is a graph showing a relation between an ion beam current and an initial discharge current obtained in the second embodiment, Figs. 6 to 9 are schematics of the third, the fourth, the fifth and the sixth embodiments, respectively, Figs. 10a and 10b are a longitudinal section and a transverse section respectively which show a multiple magnetic field formed at the ion producing region, Figs. 11 and 12 are schematics of the eighth and the ninth embodiments which are used as sputter apparatus, Fig. 13 is a schematic of the tenth embodiment which is obtained by modifying the second embodiment, Fig. 14 is a schematic of the eleventh embodiment which is a sputtering type ion source, Figs. 15 and 16 are schematics of the twelfth and the thirteenth embodiments in which negative ions can be extracted. Fig. 1 is a schematic of the first embodiment according to this invention. A discharge cathode 1, an accelerating cathode 2 also operating as a discharge anode, an accelerating anode 3, and a workpiece disposed conductive portion 4 operating as a target cathode and an ion extracting cathode are disposed in this order. The first space between the cathode 1 and the accelerating cathode 2 is a plasma region 5 filled with plasma. The second space between the accelerating cathode 2 and the accelerating anode 3 is an electron accelerating region 6 for accelerating the electrons which are extracted from the plasma region 5. And the third space between the acceleration anode 3 and the workpiece disposed portion 4a,4b is an ion producing region 7. A power source 8 applies a voltage between the discharge cathode 1 and the acceleration cathode 2 and then discharge for producing plasma is caused. An accelerating voltage source 9 produces a potential difference between the accelerating cathode 2 and the accelerating anode 3. The electrons in the plasma region 5 and ions in the ion producing region 7 are accelerated to the accelerating anode 3 and the accelerating cathode 2 respectively by the potential difference. The accelerating cathode 2 is directly grounded. The workpiece disposed portion 4a,4b is grounded through a voltage source for controlling ion energy 10 so that a voltage between the workpiece disposed portion 4a,4b and the accelerating anode 3 is controlled. The inert gas, usually used, such as Argon enters into the ion beam source from inlets 11a, 11b and exits from outlets 12a, 12b, 12c. In this embodiment, a magnetic field is applied along the propagating direction of ions and electrons, so that lateral expansion of electron beam and ion beam is prohibited. Partitions 13a, 13b which are disposed at the plasma region 5 is to make a pressure difference for the gas charged in the plasma region. The ion beam source as constructed above operates as follows. Plasma is produced in the plasma region 5 by means of the discharge which is caused between the cathode 1 and the accelerating cathode 2. The electrons, which make up the plasma, are attracted by the accelerating anode 3, enter into the ion producing region 7, and produce ions. A part of these ions is attracted by the accelerating cathode 2 and neutralizes the negative space charge which is produced near an aperture of the accelerating cathode 2. Therefore, an electron beam having a current proportional to the plasma density in the plasma region grows into the ion producing region 7. In the ion producing region 7, ions, the number of which is proportional to the current value or the number of the entered electrons, are produced. A workpiece 18 on the workpiece disposed portion 4a,4b is irradiated with the produced ions after their energy is controlled by the potential of the workpiece disposed portion 4a,4b. Fig. 3 shows a relation between an ion beam current and an initial discharge current (almost equal to an electron beam current) in the above embodiment having the condition that the gas charged in the ion beam source is Argon gas, the voltage between the accelerating cathode 2 and the accelerating anode 3 is 110V, the voltage between the workpiece disposed portion 4a,4b and the ground is -34V, the pressure at the plasma region 5 is 93 Pa (0.7 Torr), the pressure at the electron accelerating region 6 is 0,015 Pa (1.1 x 10⁻⁴ Torr), the pressure at the ion producing region 7 is 0,27 Pa (2.0 x 10⁻³ Torr), and the distance between the accelerating anode 3 and the workpiece disposed portion 4a,4b is 10cm. From Fig. 2, it can be found that a large ion beam current, which is larger than that of conventional ion beam source by a factor of 100, was obtained. The ion beam current was measured by reading the voltmeter 16 measuring a voltage across a resistor 14. An alternate (high-frequency) voltage source can be used as the voltage source for controlling ion energy 10. The voltage of the workpiece 18 can be changed by controlling the voltage of the alternate voltage source. Fig. 4 is a schematic of the second embodiment according to this invention. Compared with the first embodiment, a target cathode 4a and an ion extracting anode 4b are disposed instead of the workpiece disposed conductive portion 4a,4b. The first space between the accelerating anode 3 and the target cathode 4a correpsonds to the ion producing region 7. The ion accelerating electrode 4b connects to the accelerating anode 3 through the variable voltage source for controlling ion energy 10. A voltage between the target cathode 4a and the accelerating anode 3 can be controlled by a voltage source 10'. The ions produced in the ion producing region 7 are extracted by the ion extracting electrode 4b and emitted from the ion beam source. Fig. 5 shows a relation between the extracted ion beam current Ii and the discharge current Id (almost equal to an electron beam current) in the above embodiment having the condition that the gas charged in the ion beam source is Argon gas, the voltage between the accelerating cathode 2 and the accelerating anode 3 is 100V, the voltage between the ion extracting electrode 4b and the accelerating anode 3 is -400V, the pressure at the plasma region 5 is 107 Pa (0.8 Torr), the pressure at the electron accelerating region 6 is 0,011 Pa (0.8 x 10⁻⁴ Torr), the pressure at the ion producing is 0,27 Pa (2.0 x 10⁻³ Torr), and the diameter of an aperture of the ion producing region 7 is 1mm. The curve A corresponds to the case that the potential of the target cathode 4a is the same as that of the accelerating anode 3, the curve B corresponds to the case that the potential of the target cathode 4a to the accelerating anode 3 is -100V. From Fig. 5, it is clear that according to this invention high current ion beam can be obtained. If the voltage between the ion extracting electrode 4b and the accelerating anode 3 is not so large, the ion extracting electrode 4b and the target cathode 4a can be combined. Figs. 6 to 9 are schematics of the third, the fourth, the fifth and the sixth embodiment, respectively. The third and the fifth embodiments are obtained by modifying the first embodiment shown in Fig. 1, and the fourth and the sixth embodiments are obtained by modifying the second embodiment shown in Fig. 4. In these embodiments, the voltage between the workpiece disposed portion 4a,4b or the ion extracting electrode 4b and the accelerating anode 3 can be changed by not controlling the voltage between the workpiece disposed portion 4 or the ion extracting electrode 4b and the accelerating anode 3 but controlling the voltage of the accelerating voltage source 9 which is disposed between the accelerating cathode 2 and the accelerating anode 3. In each embodiment shown in Figs. 6 to 9, the voltage source for controlling ion energy 10 is not adopted, and the workpiece disposed portion 4 or the ion extracting electrode 4b is directly grounded. The ion energy can be controlled by changing the voltage of the accelerating voltage source 9. In either embodiment shown in Figs. 8 and 9, the cathode 1 is grounded, so that the voltage between the workpiece disposed portion 4 or the ion extracting electrode 4b and the accelerating anode 3 can also be changed by varying the voltage of the dishcarge power source 8. Even if a uniform magnetic field is used as the magnetic field for guiding the electron beam as in the embodiments shown in Figs. 1 to 6, it was recognized that an ion beam of 15mm in diameter can be obtained. When permanent magnets 20 are disposed on a periphery of the ion producing region 7 so that a multipole magnetic field is formed as shown in Figs. 10a and 10b, the diameter of the ion beam is further expanded. In the present invention many modified embodiments can be obtained. For example, the accelerating anode 3 can be divided into two anodes and the ion producing region 7 can be divided into two ion producing regions. Fig. 11 shows a schematic of the eighth embodiment which is obtained by modifying the first embodiment. This embodiment is used as a sputter apparatus. The ions produced at the ion producing region 7 collide with a target 25 on the workpiece disposed conductive portion 4a,4b. Target material which is dispersed by the ion collision deposits on a substrate 26. If an electric potential is applied to the substrate as shown in Fig. 11, atoms of the target material which are ionized by the electron beam irradiation, are attracted by the substrate 26. Fig. 12 shows a schematic of ninth embodiment which is also used as a sputter apparatus. In this embodiment, the target 25 (4a,4b) itself operates as the target cathode and the ion extracting electrode. The target material is gasified by the irradiation of the ion beam, the gasified target material is irradiated with the electron beam and ions of the target material are produced. The ions are extracted by the accelerating cathode 2. The ions on the way to the plasma region 5 are encountered with the substrate 26 and a film is produced thereon. In the embodiment, it is preferable to make the aperture of the accelerating anode 3 large and to form a magnetic field near the target 25 so as to concentrate the ion beam on the target. The table for supporting the substrate 26 may be the accelerating cathode 2 itself. Fig. 13 is a schematic of the tenth embodiment which is obtained by modifying the second embodiment. In this embodiment, the voltage of the ion extracting electrode 4b to the target cathode 4a is controlled by the voltage source 10. If an extra electrode is disposed between the target cathode 4a and the ion extracting electrode 4b, a large extracting voltage can be applied to the extra electrode. In this case, since the energy of the ion beam is also controlled by the ion extracting electrode, a higher current and low energy ion beam can be obtained. Fig. 14 shows a schematic of the eleventh embodiment which is a sputter type ion source. The ions produced at the ion producing region 7 collide with the target 25 on the workpiece disposed portion 17. The target material is dispersed by the ion collision, atoms of target material are ionized by the irradiation of electron beam, and the ionized atoms of target material are extracted to the outside of the ion source by the electrode 4a, 4b. Figs. 15 and 16 are schematics of the twelfth and the thirteenth embodiments in which negative ions can be extracted. In the twelfth embodiment, the ion extracting electrode 4b operates as the workpiece disposed portion. Larmor radius of ion is largely different from that of electron. In the case that an external magnetic field is applied, there are only ions at space away from the Larmor radius of electron in the direction perpendicular to that of the magnetic field. These embodiments are based on the fact the ion extracting electrode 4b is disposed away from an electron passage 28. When the ion extracting electrode 4b is in a negative potential as against a plasma potential (almost equal to an electric potential of the accelerating anode) positive ions are extracted. On the other hand, when the ion extracting electrode 4b has a positive electric potential as against the plasma potential, negative ions are extracted. In each embodiment described as above, it is effective to dispose an electrode for regulating a plasma potential between the discharge cathode 1 and the accelerating cathode 2 as disclosed in a specification of Japanese Patent Application No.59(1984)-276738. As this invention may be embodied in several forms without departing from the scope of the claims, the above embodiments are therefore illustrative and not restrictive, since the invention is defined by the scope of the appended claims rather than by the description proceeding them.

Claims: An electron-beam excited ion beam source comprising a discharge cathode (1), a plate-shaped accelerating cathode (2) disposed opposite said discharge cathode (1) and having an aperture at the center thereof, a plasma region (5) defined between said discharge cathode (1) and said accelarating cathode (2) for producing a plasma therein from an introduced gas by means of a discharge between said discharge cathode (1) and said accelerating cathode (2), a first variable voltage supply means (8) for supplying a voltage between said discharge cathode (1) and said accelerating cathode (2) to cause a discharge in said plasma region (5), a discharge current which flows between said discharge cathode (1) and said accelerating cathode (2) being changed by varying the voltage of said voltage supply means (8), a plate-shaped accelerating anode (3) disposed opposite said accelerating cathode (2) on a side away from said discharge cathode (1) for accelerating electrons of said plasma produced in said plasma region (5) and having an aperture at the center thereof, an electron-beam accelerating region (6) defined between said accelerating cathode (2) and said accelerating anode (3) for allowing said accelerated electrons to pass into an ion producing region (7) through said aperture, a second voltage supply means (9) for supplying a voltage between said accelerating cathode (2) and said accelerating anode (3) and accelerating electrons of said plasma produced in said plasma region (5), a target cathode (4a) disposed opposite said accelerating anode (3) on a side away from said accelerating cathode (2), an ion producing region (7) defined between said accelerating anode (3) and said target cathode (4a) for producing ions therein from an introduced gas by said accelerated electrons, a portion of said ions being attracted by said accelerating cathode (2) and neutralizing a negative space charge produced near said aperture, whereby an electron beam which has a current proportional to the density of the plasma region grows into the ion producing region (7), an ion extracting electrode (4b) for extracting ions produced in said ion producing region (7), and means (8, 9, 10) for regulating the potential difference between said ion extracting electrode (4b) and said acceleration anode (3) to control the energy of said extracted ions. An electron-beam excited ion beam source according to claim 1, further comprising a magnetic field applied along the acceleration direction of said electrons and said ions for prohibiting lateral expansion of the electron-beam and ion beam. An electron-beam excited ion beam source according to claim 1 or 2, characterized in that said target cathode (4a) and said ion extracting electrode (4b) are formed integrally with each other. An electron-beam excited ion beam source according to anyone of claims 1 - 3, characterized in that said ion extracting electrode (4b) is a workpiece disposed conductive portion. An electron-beam excited ion beam source according to anyone of claims 1 - 4, said means (8, 9, 10) for regulating the potential difference between said ion extracting electrode (4b) and said acceleration anode (3) comprising a third variable voltage supply means (10) for biassing the ion extracting electrode (4b) with regard to said accelerating cathode (2) or said accelerating anode (3). An electron-beam excited ion beam source according to anyone of claims 1 - 5, said means (8, 9, 10) for regulating the potential difference between said ion extracting electrode (4b) and said acceleration anode (3) being said voltage supply means (9) for supplying a voltage between said accelerating cathode (2) and said accelerating anode (3), said ion extracting electrode (4b) being grounded and said accelerating cathode (2) being grounded, whereby the potential between the ion extracting electrode (4b) and the accelerating cathode (3) is regulated by changing the voltage of the voltage supply means (9). An electron-beam excited ion beam source according to anyone of claims 1 - 6, said means (8, 9, 10) for regulating the potential difference between said ion extracting electrode (4b) and said accelerating anode (3) being said variable voltage supply means (8) for supplying a voltage between said discharge cathode (1) and said accelerating cathode (2), said ion extracting electrode (4b) being grounded and said discharge cathode (1) being also grounded, whereby the potential between the ion extracting electrode (4b) and the accelerating anode (3) is regulated by changing the voltage of the voltage supply means (8).

Inventors: AOYAGI, YOSHINOBU FUJIMI HEIGHTS 10-1, HAMAGAKI, MANABU, HARA, TAMIO, NAMBA, SUSUMU
Applicants: RIKAGAKU KENKYUSHO
Application year: 1986
Main classification: H01J37
IPC codes: H01J27, H01J37