Radical generator and molecular beam epitaxy apparatus

Abstract

A molecular beam epitaxy apparatus includes a radical generator for generating a radical species, a molecular beam cell for generating a molecular beam or an atomic beam, and a vacuum chamber for accommodating a substrate therein, in use, the substrate being irradiated with the radical species and the molecular beam or the atomic beam in vacuum, to thereby form, on the substrate, a crystal of a compound derived from the element of the radical species and the element of the molecular beam or the atomic beam.

Claims

1. A molecular beam epitaxy apparatus comprising: a radical generator for generating a radical species, a molecular beam cell for generating a molecular beam or an atomic beam, and a vacuum chamber for accommodating a substrate therein, in use, the substrate being irradiated with the radical species and the molecular beam or the atomic beam in vacuum, to thereby form, on the substrate, a crystal of a compound derived from an element of the radical species and an element of the molecular beam or the atomic beam, the radical generator comprising: a supply tube comprising a conductive material for supplying a gas; a plasma-generating tube which is cylindrical and comprises a dielectric material, the plasma-generating tube being connected to a supply tube at a downstream end thereof; a coil winding about an outer circumference of the plasma-generating tube, for generating an inductively coupled plasma in the plasma-generating tube; an electrode comprising a double-pipe cylinder having an inner space in which cooling material can be refluxed and covering an outer circumference wall of the plasma-generating tube and which is disposed between the coil and the supply tube, for generating a capacitively coupled plasma in the plasma-generating tube and adding the capacitively coupled plasma to the inductively coupled plasma; and a parasitic-plasma-preventing tube comprising a dielectric material which extends from a bottom of the plasma-generating tube to an opening of the supply tube in a space between the bottom and the opening, and a tip part thereof is inserted into the supply tube to cover an inner wall of the supply tube for preventing a generation of a parasitic plasma between the electrode and the inner wall of the supply tube.

2. The molecular beam epitaxy apparatus according to claim 1, wherein the radical generator further comprises: a plurality of permanent magnets which are disposed along an outer side circumference of a zone of the plasma-generating tube where the capacitively coupled plasma is generated and which localize the capacitively coupled plasma to a center of the plasma-generating tube, wherein the respective permanent magnets have been magnetized in a direction orthogonal to a cylinder center axis of the plasma-generating tube, and a plane proximal to the plasma-generating tube has been magnetized as an N pole or an S pole, and two permanent magnets adjacent to each other have inner planes of different magnetic poles such that an N pole and an S pole are alternatingly repeated along a circumferential direction of the plasma-generating tube.

3. The molecular beam epitaxy apparatus according to claim 2, wherein the permanent magnets are disposed so as to be exposed to the inner space of the electrode.

4. The molecular beam epitaxy apparatus according to claim 1, wherein the radical generator comprises a nitrogen radical generator in which nitrogen is supplied through the supply tube, to thereby generate nitrogen radicals, and a crystal of a nitride compound is grown.

5. The molecular beam epitaxy apparatus according to claim 1, wherein the molecular beam cell generates a molecular beam of a Group III metal, and a crystal of a Group III nitride semiconductor compound is grown.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 A schematic view of the configuration of the radical generator of Embodiment 1.

(2) FIG. 2 A cross-section of the radical generator of Embodiment 1, cut along A-A shown in FIG. 1.

(3) FIG. 3 A graph showing the relationship between the gas flow rate in the radical generator of Embodiment 1 and the nitrogen radical density.

(4) FIG. 4 A schematic view of the configuration of the MBE apparatus of Embodiment 2.

(5) FIG. 5 A graph showing the relationship between the Ga flux in the MBE apparatus of Embodiment 2 and the GaN film formation rate.

MODES FOR CARRYING OUT THE INVENTION

(6) Specific embodiments of the present invention will next be described in detail with reference to the drawings. However, these embodiments should not be construed as limiting the present invention thereto.

(7) Embodiment 1

(8) FIG. 1 is a schematic view of the configuration of the radical generator of Embodiment 1. FIG. 2 is a cross-section of the radical generator of Embodiment 1, cut along A-A shown in FIG. 1.

(9) As shown in FIGS. 1 and 2, the radical generator of Embodiment 1 has a metallic casing 18, a metallic end plate 21 disposed at an end of the casing 18, a cylindrical supply tube 10 made of SUS, and a cylindrical plasma-generating tube 11 which is connected to the supply tube 10 and which is made of insulating pyrolytic boron nitride (PBN). At the center of the end plate 21 there is provided with an opening 22 through which the generated plasma is output. The plasma-generating tube 11 has an inner diameter of 24 mm and a length in the axial direction of 90 mm. An orifice plate 19 is disposed at the opening of the plasma-generating tube 11 which opening is disposed opposite the supply tube 10. The orifice plate 19 has a pore 20 with a diameter of 5 mm at the center of the opening of the plasma-generating tube 11. The pore 20 is co-axially provided with the opening 22 of the end plate 21.

(10) A double-pipe cylindrical CCP electrode 13 is disposed in the vicinity of the connection portion between the supply tube 10 and the plasma-generating tube 11 and outside the plasma-generating tube 11. The CCP electrode 13 has an inner space 13a defined by the double-pipe cylinder. To the CCP electrode 13, a water-supplying tube 16 and a water-discharging tube 17 are connected. Thus, the inner space 13a of the CCP electrode 13 is communicated with the water-supplying tube 16 and the water-discharging tube 17. Through this configuration, cooling water can be supplied via the water-supplying tube 16 to the inner space 13a of the CCP electrode 13, and the water can be discharged through water-discharging tube 17, whereby the CCP electrode 13 can be cooled through refluxing cooling water.

(11) The inner wall of the CCP electrode 13 (i.e., the inner space 13a) is provided with six permanent magnets 14 which are disposed along the outer circumference of the zone of the plasma-generating tube 11 at the equal intervals. The permanent magnets 14 are made of SmCo. Each permanent magnet 14 has been magnetized in the direction orthogonal to the cylinder center axis (i.e., magnet thickness direction), and the plane proximal to the plasma-generating tube 11 has been magnetized as an N pole or an S pole. The two permanent magnets 14 adjacent each other have inner planes (planes proximal to the plasma-generating tube 11) of different magnetic poles. That is, the inner planes of the permanent magnets 14 are magnetized such that an N pole and an S pole are alternatingly repeated along the circumferential direction. These permanent magnets 14 are exposed to the inner space 13a of the CCP electrode 13. Thus, when cooling water is refluxed through the inner space 13a of the CCP electrode 13 so as to cool the CCP electrode 13, the cooling water comes into direct contact with the permanent magnets 14. In this case, rise in temperature of the permanent magnets 14, which would otherwise be caused by heating of the CCP electrode 13, can be effectively suppressed.

(12) A coil 12 is disposed so that the coil winds about the outer circumference of the plasma-generating tube 11. The coil 12 is located outside the plasma-generating tube 11 and at the downstream end of the CCP electrode 13 (i.e., opposite the supply tube 10 side). The coil 12 is formed of a hollow stainless steel tube which has been wound 3.5 times. The coil 12 can be cooled through passage of cooling water in the inner space of the stainless steel tube.

(13) To the CCP electrode 13 and the coil 12, a high-frequency power source (not illustrated) is connected. The supply tube 10, the casing 18, and the end plate 21 remain at the same voltage and are grounded. One end of the coil 12 is grounded. By means of the high-frequency power source, high-frequency electric power is applied between the CCP electrode 13 and the ground. Also, high-frequency electric power is applied between the other end of the coil 12 and the ground by means of the high-frequency power source. Through this configuration, an inductively coupled plasma can be generated in the zone inside the plasma-generating tube 11 where the coil 12 is disposed at the outer circumference of the tube, and a capacitively coupled plasma can be generated in the zone inside the plasma-generating tube 11 where the CCP electrode 13 is disposed at the outer circumference of the tube.

(14) Into the opening of the supply tube 10 disposed at the connection portion between the supply tube 10 and the plasma-generating tube 11, a parasitic-plasma-preventing tube 15 made of an insulating ceramic material is inserted toward the supply tube 10. The parasitic-plasma-preventing tube 15 is built in a holder 23, which covers the tube 15 and extends to the supply tube 10. The holder 23 and the supply tube 10 connecting to the holder 23 are connected to the plasma-generating tube 11 such that the end face of plasma-generating tube 11 is sandwiched by the end face of the holder 23 and the stopper 24. Each of the parasitic-plasma-preventing tube 15, the holder 23, and the stopper 24 is made of an insulating ceramic material of boron nitride. The parasitic-plasma-preventing tube 15 has an inner diameter of 1 mm and an outer diameter which is almost equivalent to the inner diameter of the supply tube 10. Through insertion of the parasitic-plasma-preventing tube 15 into the supply tube 10, the inner wall of the supply tube 10 is covered with the parasitic-plasma-preventing tube 15, whereby generation of a parasitic plasma between the CCP electrode 13 and the inner wall of the supply tube 10 is prevented.

(15) In order to effectively prevent generation of a parasitic plasma, the insertion length of the parasitic-plasma-preventing tube 15 is preferably 10 times or more the inner diameter of the supply tube 10, more preferably 20 to 50 times the inner diameter of the supply tube 10.

(16) The plasma-generating tube 11, the coil 12, and the CCP electrode 13 are built in the cylindrical casing 18. The radical-radiating end of the casing 18 is connected to the end plate 21 having the opening 22 at the center thereof. An additional electrode for removing ions (not illustrated) or an additional magnet (not illustrated) may be disposed in the vicinity of the opening 22.

(17) In use of the radical generator of Embodiment 1, a gas is supplied to the plasma-generating tube 11 through the supply tube 10. Then, high-frequency electric power is applied to the coil 12 and the CCP electrode 13, to thereby generate an inductively coupled plasma and a capacitively coupled plasma in the plasma-generating tube 11. High-density radicals are generated through injection of the capacitively coupled plasma to the inductively coupled plasma.

(18) In the radical generator of Embodiment 1, the parasitic-plasma-preventing tube 15 is inserted into the supply tube 10, to thereby prevent generation of a parasitic plasma in the supply tube 10, which would otherwise be caused by electric discharge between the CCP electrode 13 and the inner wall of the supply tube 10. Through insertion of the parasitic-plasma-preventing tube 15, the capacitively coupled plasma is generated only in the plasma-generating tube 11, and the plasma density of the capacitively coupled plasma is enhanced. Thus, the density of the generated radicals is also enhanced.

(19) The capacitively coupled plasma is localized at high density in the center of the plasma-generating tube 11 by the mediation of a cusp field provided by the six permanent magnets 14. Specifically, a magnetic flux is formed from the N-polar inner plate of one permanent magnet 14 to the S-polar inner plane of one permanent magnet 14 adjacent thereto, whereby arc-shape magnetic flux units are formed at intervals of 60 degrees. The plasma is expelled from the magnetic flux units, whereby the capacitively coupled plasma is localized at high density in the center of the plasma-generating tube 11. In the case where high gas pressure is employed in order to enhance molecule decomposition performance, the inductively coupled plasma is generally in the low-bright mode, rather than in the high-bright mode. The high-bright mode is a state in which a plasma is generated in the center of the plasma-generating tube 11, with the radical density being higher in the vicinity of the center. In contrast, the low-bright mode is a state in which a plasma is generated along the inner wall of the plasma-generating tube 11, with the plasma density being lower in the vicinity of the center. In this case, the entire radical density is low, and the output radicals have a low density. However, through adding a capacitively coupled plasma localized to the center to the inductively coupled plasma, the low-bright-mode plasma is modified, to thereby compensate lowering in plasma density at the center. As a result, even when high gas pressure is employed, the plasma density at the center is enhanced, and considerably high radical density can be attained, as compared with the case in which only an inductively coupled plasma is generated. In addition, by virtue of a large number of high-energy electrons present in the capacitively coupled plasma, gas molecules are effectively decomposed to the corresponding gas atoms, and the thus-generated atomic radicals come to have an enhanced internal energy. When such atomic radicals having high internal energy are employed in, for example, a source element of crystal growth, growth temperature can be lowered, which is very advantageous.

(20) In addition, the permanent magnets 14 can be directly cooled by refluxing cooling water through the inner space 13a of the CCP electrode 13, whereby rise in temperature of the permanent magnets 14 is suppressed, to thereby effectively prevent degaussing of the permanent magnets 14. As a result, the CCP plasma localized to the center of the plasma-generating tube 11 can be maintained for a long period of time, and high-density radical generation can be maintained for a long period of time.

(21) The radical generator of Embodiment 1 can generate any radical species through supply of a gas of interest via the supply tube 10. Examples of the gas to be supplied include nitrogen, oxygen, hydrogen, ammonia, water, fluorocarbon, hydrocarbon, silane, and germane. Any radical species of interest may be obtained from these gases. Among these gases, nitrogen, oxygen, hydrogen, and ammonia are used for generating useful radicals. When a gas is supplied through the supply tube 10, the gas may be diluted with a rare gas such as argon.

(22) FIG. 3 is a graph showing the relationship between the density of the nitrogen radicals which have been generated by means of the radical generator of Embodiment 1 and the flow rate of the nitrogen gas. The nitrogen radical density was measured through vacuum ultraviolet spectrometry. As shown in the graph, the nitrogen radical density of the product obtained by means of a conventional radical generator employing an inductively coupled plasma is about 110.sup.11 cm.sup.3 or lower. However, when the radical generator of Embodiment 1 is employed, a radical density higher than 110.sup.11 cm.sup.3 can be attained at a nitrogen gas flow rate of 5 to 35 sccm. Particularly when the nitrogen gas flow rate 15 to 25 sccm, the nitrogen radical density is 110.sup.12 cm.sup.3 or higher, which is 10 times or higher (about 20 to 30 times) the nitrogen radical density attained by a conventional radical generator.

(23) Embodiment 2

(24) An MBE apparatus according to Embodiment 2 will next be described. FIG. 4 is a schematic view of the configuration of the MBE apparatus of Embodiment 2. As shown in FIG. 4, the MBE apparatus of Embodiment 2 includes a vacuum chamber 1 whose inside can be maintained at about 10.sup.8 Pa (i.e., ultra-vacuum), a substrate stage 2 disposed in the vacuum chamber 1 which stage can hold a substrate 3 and can rotate and heat the substrate 3, molecular beam cells 4A, 4B, and 4C which can radiate a molecular beam (atomic beam) onto the surface of the substrate 3, and a radical generator 5 for supplying nitrogen radicals onto the surface of the substrate 3.

(25) In use of the MBE apparatus of Embodiment 2, the surface of the substrate 3 which has been heated and maintained in ultra-vacuum is irradiated with Group III metal atomic beams supplied by molecular beam cells 4A, 4B, and 4C, and with nitrogen radicals supplied by the radical generator 5, whereby a Group III nitride semiconductor crystal is formed on the surface of the substrate 3.

(26) Each of the molecular beam cells 4A, 4B, and 4C has a crucible for holding a Group III metal material, a heater for heating the crucible, and a shutter. The crucible is heated to thereby generate a Group III metal vapor, and an atomic beam of the metal element is formed. The dose of the atomic beam is regulated through opening/shutting the shutter. In one embodiment, the molecular beam cell 4A contains Ga, the molecular beam cell 4B contains In, and the molecular beam cell 4C contains Al, and atomic beams of respective elements are generated. An additional molecular cell 4 holding an n-type impurity (e.g., Si) or a p-type impurity (e.g., Mg) may be provided, and the substrate 3 may be irradiated with the molecular beam provided by the molecular beam cell 4.

(27) The radical generator 5 has the same structure as that of the radical generator of Embodiment 1 (see FIGS. 1 and 2). In Embodiment 2, nitrogen gas is supplied through the supply tube 10 to the plasma-generating tube 11, where nitrogen gas is decomposed. As described in Embodiment 1, the capacitively coupled plasma is localized at high density in the center of the plasma-generating tube 11 by the mediation of a cusp field provided by the six permanent magnets 14. In the case where high gas pressure is employed in order to enhance nitrogen molecule decomposition performance, the inductively coupled plasma is in the low-bright mode, and the radical density is low in the center of the plasma-generating tube 11. However, through adding a capacitively coupled plasma localized to the center to the inductively coupled plasma, the low-bright-mode plasma is modified, to thereby compensate lowering in plasma density at the center. As a result, even when high gas pressure is employed, the plasma density at the center is enhanced, and considerably high radical density can be attained, as compared with the case in which only an inductively coupled plasma is generated. In addition, by virtue of a large number of high-energy electrons present in the capacitively coupled plasma, nitrogen gas molecules are effectively decomposed to the corresponding gas atoms, and the thus-generated atomic radicals come to have an enhanced internal energy.

(28) The MBE apparatus of Embodiment 2 has the aforementioned radical generator 5, which can provide high nitrogen radical density as described above. Therefore, the MBE apparatus attains an enhanced Group III nitride semiconductor film formation rate, as compared with a conventional MBE apparatus. In addition, since the MBE apparatus of the invention can emit nitrogen radicals having high internal energy, migration of nitrogen on the surface of the crystal can be promoted. In other words, the nitrogen atoms satisfactorily move in the surface layer of the crystal and reach growth sites at higher possibility, whereby the crystallinity of the grown crystal as well as the sharpness of the interlayer interface can be enhanced. Furthermore, the temperature of the substrate 3 can be lowered, whereby the crystallinity can be further enhanced. Also, since the radical generator 5 enables continuous formation of nitrogen radicals for a long period of time, consistent Group III nitride semiconductor film formation is ensured for a long period of time.

(29) FIG. 5 is a graph showing variation in GaN film formation rate with respect to variation in Ga flux, under the following conditions: substrate 3 temperature of 840 C., pressure of 110.sup.4 Torr, and nitrogen gas flow rate of 15 sccm. The GaN film formation rate at a Ga flux of 210.sup.6 Torr was 700 nm/h and that at a Ga flux of 410.sup.6 Torr was 1,400 nm/h, whereas a conventional MBE apparatus attains a GaN film formation rate as low as about 500 nm/h. Since the radical generator 5 of the MBE apparatus of Embodiment 2 provides nitrogen radicals at high density, a film formation rate about thrice as that attained by a conventional MBE apparatus can be realized.

INDUSTRIAL APPLICABILITY

(30) The radical generator of the present invention can be employed as a nitrogen radical generator of a molecular beam epitaxy (MBE) apparatus or a similar apparatus, to thereby form a nitride such as a Group III nitride semiconductor. The radical generator of the present invention also finds a variety of applications such as cleaning of a substrate and substrate surface treatment based on radical radiation.

(31) The MBE apparatus of the present invention is useful as an apparatus for forming a Group III nitride semiconductor film.

DESCRIPTION OF THE REFERENCE NUMERALS

(32) 1: vacuum chamber

(33) 2: substrate stage

(34) 3: substrate

(35) 4A, 4B, 4C: molecular beam cell

(36) 10: supply tube

(37) 11: plasma-generating tube

(38) 12: coil

(39) 13: CCP electrode

(40) 14: permanent magnet

(41) 15: parasitic-plasma-preventing tube

(42) 16: water-supplying tube

(43) 17: water-discharging tube

(44) 18: casing

(45) 19: orifice plate

(46) 20: pore

(47) 21: end plate

(48) 22: opening