Semiconductor cleaner and method of cleaning semiconductor

Abstract

A semiconductor cleaner includes (a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, (b) an outer wet bath which is filled with liquid and in which the inner wet bath is soaked, (c) a megasonic-wave irradiator located outside the inner wet bath for irradiating megasonic waves to the wafer to clean the wafer, and (d) a unit which moves the megasonic-wave irradiator towards or away from the inner wet bath.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a semiconductor wafer cleaner by means of megasonic waves, and a method of cleaning a semiconductor wafer by means of megasonic waves.

[0003] 2. Description of the Related Art

[0004] Supersonic waves such as megasonic waves are used in a lot of fields for cleaning an object. For instance, megasonic waves are used in a fabrication process of a semiconductor device for cleaning a silicon wafer.

[0005] An example of a conventional silicon wafer cleaner for cleaning a silicon wafer by means of megasonic waves is illustrated in FIG. 1.

[0006] The illustrated silicon wafer cleaner 50 is comprised of a cleaning bath 52 filled with cleaning solution 51 such as water, an oscillation plate 53 adhered to an outer surface of a bottom of the cleaning bath 52, and an oscillator 54 which oscillates the oscillation plate 53.

[0007] A silicon wafer 55 to be cleaned is kept soaked in the cleaning solution 51 in the cleaning bath 52. By activating the oscillator 54, the oscillation plate 53 is oscillated by the oscillator 54, resulting in that megasonic waves 56 are irradiated towards the cleaning bath 52 from the oscillation plate 53. The silicon wafer 55 is cleaned by the thus irradiated megasonic waves 56.

[0008] As illustrated in FIG. 1, the megasonic waves 56 are irradiated from the oscillation plate 53 in all directions. Hence, the megasonic waves 56 includes first megasonic waves 56a irradiated upwardly from the oscillation plate 53, and second megasonic waves 56b irradiated obliquely upwardly from the oscillation plate 53.

[0009] A part of the second megasonic waves 56b is reflected at a sidewall 52a of the cleaning bath 52, and is directed towards inside of the cleaning bath 52 as reflected megasonic waves 56c. As a result, the first megasonic waves 56a and the reflected megasonic waves 56c interfere with each other at locations indicated with solid circles 57 (&Circlesolid;). This means that the silicon wafer 55 locally receives the megasonic waves 56 having high intensity.

[0010] As mentioned above, since the silicon wafer 55 receives the megasonic waves 56 too much at the locations 57 at which the first megasonic waves 56a and the reflected megasonic waves 56c interfere with each other, there is caused a problem that wirings formed on the silicon wafer 55 are partially broken.

[0011] Many attempts have been made so far in order to solve the above-mentioned problem.

[0012] As an example of the solution to the above-mentioned problem, Japanese Unexamined Patent Publication No. 11-221534 has suggested a supersonic wave cleaner. FIG. 2 illustrates the suggested supersonic wave cleaner.

[0013] The illustrated supersonic wave cleaner 60 is comprised of a cleaning bath 62 filled with a cleaning solution 61, a first supersonic wave oscillator 63 fixed onto an outer surface 62a of a bottom of the cleaning bath 62, a first variable oscillator 64 which controls a frequency of supersonic waves emitted from the first supersonic wave oscillator 63, a second supersonic wave oscillator 65 designed to be movable upwardly and downwardly relative to the cleaning bath 62, and a second variable oscillator 66 which controls a frequency of supersonic waves emitted from the second supersonic wave oscillator 65.

[0014] A silicon wafer 67 to be cleaned is kept soaked in the cleaning solution 61 with being fixed to the second supersonic wave oscillator 65. The silicon wafer 67 moves upwardly and downwardly in the cleaning solution 61 together with the second supersonic wave oscillator 65. While the silicon wafer 67 moves upwardly and downwardly in the cleaning solution 61, the first supersonic wave oscillator 63 irradiates supersonic waves to the silicon wafer 67, and thus, the silicon wafer 67 is cleaned.

[0015] It is said in the Publication that since the silicon wafer 67 is cleaned by supersonic waves propagated from both of the first and second supersonic wave oscillators 63 and 65 in different manners, the above-mentioned problem caused by applying only supersonic waves irradiated from the first supersonic wave oscillator 63 to the silicon wafer 67 can be solved.

[0016] Japanese Unexamined Patent Publication No. 61-194727 has suggested a supersonic wave cleaner. FIG. 3 illustrates the suggested supersonic wave cleaner 70.

[0017] The supersonic wave cleaner 70 includes a reflector 72 inclined by a predetermined angle &thgr; in a cleaning bath 71. The reflector 72 is formed at a surface thereof with slight irregularity. Supersonic waves irradiated from a supersonic wave oscillator 73 arranged on a sidewall of the cleaning bath 71 are reflected at the reflector 72, and then, irradiated to a silicon wafer 75 being kept soaked in cleaning solution 74 filling the cleaning bath 71 therewith.

[0018] In accordance with the supersonic wave cleaner 70, since supersonic waves are irregularly reflected by the slight irregularity formed at a surface of the reflector 72, it would be possible produce supersonic waves having no directivity, but having a uniform density. Hence, it is said in the Publication that the above-mentioned interference problem can be solved.

[0019] However, the supersonic wave cleaner 60 illustrated in FIG. 2 and the supersonic wave cleaner 70 illustrated in FIG. 3 are accompanied with the following problems.

[0020] In the supersonic wave cleaner 60 illustrated in FIG. 2, since it is necessary to attach the silicon wafer 67 to the second supersonic wave oscillator 65, a cleanness of the silicon wafer 67 might be deteriorated in a step of attaching the silicon wafer 67 to the second supersonic wave oscillator 65, or by the second supersonic wave oscillator 65 making contact with the silicon wafer 67.

[0021] In addition, since steps of attaching the silicon wafer 67 to the second supersonic wave oscillator 65 and separating the second supersonic wave oscillator 65 from the silicon wafer 67 have to be additionally carried out, the number of steps for fabricating the supersonic wave cleaner 60 would be unavoidably increased.

[0022] Furthermore, since the supersonic wave cleaner 60 has to include two oscillators, specifically, the first and second supersonic wave oscillators 63 and 65, the number of parts for constructing the supersonic wave cleaner 60 is unavoidably increased, and hence, costs for fabricating the supersonic wave cleaner 60 is also increased.

[0023] In the supersonic wave cleaner 70 illustrated in FIG. 3, the reflector 72 arranged in the cleaning bath 71 produces a useless space 76 therebelow, resulting in an unnecessary increase in a size of the cleaning bath 71 and hence the supersonic wave cleaner 70.

[0024] In addition, since the reflector 72 is inclined, supersonic waves reflected at an upper portion of the reflector 72 and supersonic waves reflected at a lower portion of the reflector 72 reach the silicon wafer 75 in different distances. Accordingly, the supersonic waves reflected at an upper portion of the reflector 72 would have a greater attenuation factor than the supersonic waves reflected at a lower portion of the reflector 72. Thus, it is not always possible to produce supersonic waves having a uniform density.

[0025] Japanese Unexamined Patent Publication No. 2001-120918 has suggested a method of cleaning a filtering plane in a solid-liquid separator, including the steps of producing supersonic waves by means of a supersonic wave oscillator, applying the supersonic waves to a movable oscillator, moving the oscillator relative to the filtering plane, and applying the supersonic waves to the filtering plane to thereby clean the filtering plane.

[0026] However, the above-mentioned problems remain unsolved even in the suggested method.

SUMMARY OF THE INVENTION

[0027] In view of the above-mentioned problems in the conventional supersonic wave cleaner, it is an object of the present invention to provide a semiconductor cleaner which is capable of avoiding megasonic waves from interfering with each other to thereby prevent local concentration of megasonic waves to a semiconductor wafer to be cleaned.

[0028] It is also an object of the present invention to provide a method of doing the same.

[0029] In one aspect of the present invention, there is provided a semiconductor cleaner including (a) a wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, (b) a megasonic-wave irradiator located outside the wet bath for irradiating megasonic waves to the wafer to clean the wafer, and (c) a unit which moves the megasonic-wave irradiator towards or away from the wet bath.

[0030] In the semiconductor cleaner in accordance with the present invention, the megasonic-wave irradiator is moved upwardly and downwardly relative to the wet bath with megasonic waves being irradiated to a wafer from the megasonic-wave irradiator. As a result, megasonic waves are deviated in phases from each other, and thus, megasonic waves interfere with each other in broader areas. Hence, it would be possible to prevent local concentration of megasonic waves to a wafer.

[0031] It is preferable that the unit moves the megasonic-wave irradiator by a distance in the range of 2 mm to 20 mm both inclusive.

[0032] It is preferable that the megasonic-wave irradiator irradiates megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

[0033] The wet bath may be formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of the megasonic waves.

[0034] It is preferable that the unit moves the megasomicwave irradiator at a rate which is not equal to multiples of a phase velocity of the megasonic waves and multiples of a half of a phase velocity of the megasonic waves, and the unit moves the megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of the megasonic waves and multiples of a half of a wavelength of the megasonic waves.

[0035] There is further provided a semiconductor cleaner including (a) a wet bath which is filled with liquid and in which first to N-th wafers to be cleaned are soaked wherein N is an integer equal to or greater than two, (b) first to N-th megasonic-wave irradiators located outside the wet bath and associated with the first to N-th wafers, respectively, for irradiating megasonic waves to the first to N-th wafers to clean the first to N-th wafers, and (c) first to N-th units which move the first to N-th megasonic-wave irradiators towards or away from the wet bath, respectively.

[0036] It is preferable that the first to N-th units move the first to N-th megasonic-wave irradiators, respectively, by a distance in the range of 2 mm to 20 mm both inclusive.

[0037] It is preferable that the first to N-th megasonic-wave irradiators irradiate megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

[0038] It is preferable that each of the first to N-th units moves the megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of the megasonic waves and multiples of a half of a phase velocity of the megasonic waves, and each of the first to N-th units moves the megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of the megasonic waves and multiples of a half of a wavelength of the megasonic waves.

[0039] There is still further provided a semiconductor cleaner including (a) a wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, and (b) a megasonic-wave irradiator located outside the wet bath for irradiating megasonic waves to the wafer to clean the wafer, the wet bath being formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of the megasonic waves.

[0040] By forming irregularity having a surface roughness greater than a wavelength of the megasonic waves, on an inner surface of a sidewall of the wet bath, the megasonic waves are irregularly reflected at the inner surface of a sidewall of the wet bath. As a result, megasonic waves are deviated in phases from each other, and thus, megasonic waves interfere with each other in broader areas. Hence, it would be possible to prevent local concentration of megasonic waves to a wafer.

[0041] There is yet further provided a semiconductor cleaner including (a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, (b) an outer wet bath which is filled with liquid and in which the inner wet bath is soaked, (c) a megasonic-wave irradiator located outside the inner wet bath for irradiating megasonic waves to the wafer to clean the wafer, and (d) a unit which moves the megasonic-wave irradiator towards or away from the inner wet bath.

[0042] For instance, the megasonic-wave irradiator may be comprised of (c1) a megasonic-wave oscillation plate located in the outer wet bath and below a bottom of the inner wet bath in parallel with the bottom of the inner wet bath, and (c2) a megasonic-wave oscillator connected at one end to the megasonic-wave oscillation plate and at the other end to the. unit, the megasonic-wave oscillator being fit into an opening formed at a bottom of the outer wet bath for slidable movement.

[0043] It is preferable that the inner wet bath is composed of quartz.

[0044] There is further provided a semiconductor cleaner including (a) an inner wet bath which is filled with liquid and in which first to N-th wafers to be cleaned are soaked wherein N is an integer equal to or greater than two, (b) an outer wet bath which is filled with liquid and in which the inner wet bath is soaked, (c) first to N-th megasonic-wave irradiators located outside the inner wet bath and associated with the first to N-th wafers, respectively, for irradiating megasonic, waves to the first to N-th wafers to clean the first to N-th wafers, and (d) first to N-th units which move the first to N-th megasonic-wave irradiators towards or away from the inner wet bath, respectively.

[0045] There is further provided a semiconductor cleaner including (a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, (b) an outer wet bath which is filled with liquid and in which the inner wet bath is soaked, and (c) a megasonic-wave irradiator located outside the inner wet bath for irradiating megasonic waves to the wafer to clean the wafer, the inner wet bath being formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of the megasonic waves.

[0046] In another aspect of the present invention, there is provided a method of cleaning a wafer soaked in liquid filled in a wet bath, including the step of irradiating megasonic waves to the wafer with a relative distance between the wafer and a megasonic-wave irradiator being varied.

[0047] It is preferable that the relative distance is varied by reciprocating the megasonic-wave irradiator relative to the wafer.

[0048] It is preferable that the relative distance is varied in the range of 2 mm to 20 mm both inclusive.

[0049] It is preferable that the megasonic waves have a frequency in the range of 750 KHz to 1 MHz both inclusive.

[0050] It is preferable that relative distance is varied by moving the megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of the megasonic waves and multiples of a half of a phase velocity of the megasonic waves, and by a distance which is not equal to multiples of a wavelength of the megasonic waves and multiples of a half of a wavelength of the megasonic waves.

[0051] The advantages obtained by the aforementioned present invention will be described hereinbelow.

[0052] In accordance with the present invention, it would be possible to increase the number of locations at which megasonic waves interfere with one another, and scatter the locations in a broader area. Hence, it would be possible to reduce an intensity of megasonic waves locally concentrating to a wafer due to interference of megasonic waves with one another. Thus, it would be possible to prevent a wafer from being damaged due to interference of megasonic waves with one another.

[0053] The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 is a schematic view illustrating a conventional semiconductor cleaner.

[0055] FIG. 2 is a schematic view illustrating another conventional semiconductor cleaner.

[0056] FIG. 3 is a schematic view illustrating still another conventional semiconductor cleaner.

[0057] FIG. 4 is a schematic view of a semiconductor cleaner in accordance with the first embodiment.

[0058] FIG. 5 illustrates irradiation of megasonic waves in the semiconductor cleaner in accordance with the first embodiment.

[0059] FIG. 6 is a schematic view of a semiconductor cleaner in accordance with the second embodiment.

[0060] FIG. 7 is a schematic view of a semiconductor cleaner in accordance with the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings.

[0062] [First Embodiment]

[0063] FIG. 4 illustrates a semiconductor cleaner 10 in accordance with the first embodiment.

[0064] The semiconductor cleaner 10 in accordance with the first embodiment is comprised of an inner wet bath 18 which is filled with cleaning solution 11 and in which a wafer 12 to be cleaned is soaked, an outer wet bath 15 which is filled with water 14 and which surrounds the inner wet bath 13, and a megasonic-wave irradiator 16 arranged below a bottom of the inner wet bath 13 for irradiating megasonic waves.

[0065] The megasonic-wave irradiator 16 is comprised of a megasonic-wave oscillation plate 17 located in the outer wet bath 15 and below a bottom of the inner wet bath 13 in parallel with a bottom of the inner wet bath 13, a megasonic-wave oscillator 18 connected at one end to the megasonic-wave oscillation plate 17 for oscillating the megasonic-wave oscillation plate 17 to produce megasonic waves, and at the other end to a later mentioned motor 19, and a motor 19 which moves the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 upwardly and downwardly relative to the inner wet bath 13.

[0066] A wafer 12 is kept soaked in the cleaning solution 11 by means of an appropriate holder (not illustrated).

[0067] The outer wet bath 15 may be composed of any material, whereas the inner wet bath 13 is composed preferably of quartz.

[0068] As illustrated in FIG. 4, the megasonic-wave oscillation plate 17 is soaked in the wafer 14 filling the outer wet bath 15 therewith. The outer wet bath 15 is formed centrally at a bottom thereof with an opening through which the megasonic-wave oscillator 18 is fit for slidable movement. A waterproof seal 20 is sandwiched between the megasonic-wave oscillator 18 and the opening for preventing leakage of the water 14.

[0069] The motor 19 is mechanically connected to the megasonic-wave oscillator 18 through a converter 19a which converts rotational motion into linear motion, such as rack and pinion. Accordingly rotational motion output from the motor 19 is converted to linear motion by the converter 19a, and then, the linear motion is transferred to the megasonic-wave oscillator 18. Hence, if the motor 19 rotates in a first direction (for instance, a clockwise direction), the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 move upwardly, whereas if the motor 19 rotates in a second direction (for instance, a counterclockwise direction), the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 move downwardly.

[0070] An operation of the semiconductor cleaner 10 in accordance with the first embodiment is explained hereinbelow.

[0071] The motor 19 moves the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 upwardly or downwardly below a bottom of the inner wet bath 13 with the wafer 12 being kept soaked in the cleaning solution 11. While moving together with the megasonic-wave oscillation plate 17, the megasonic-wave oscillator 18 oscillates the megasonic-wave oscillation plate 17 to thereby emit megasonic waves from the megasonic-wave oscillation plate 17. The thus emitted megasonic waves are irradiated to the wafer 12, and resultingly, the wafer 12 is cleaned.

[0072] FIG. 5 illustrates that megasonic waves 21 emitted from the megasonic-wave oscillation plate 17 are irradiated to the wafer 12. In FIG. 5, only the wafer 12, the inner wet bath 13, the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 are illustrated, and other parts are omitted for simplification of FIG. 5.

[0073] The megasonic waves 21 emitted from the megasonic-wave oscillation plate 17 include first megasonic waves 21a emitted upwardly from the megasonic-wave oscillation plate 17, and second megasonic waves 21b emitted obliquely upwardly from the megasonic-wave oscillation plate 17.

[0074] A part of the second megasonic waves 21b is reflected at a sidewall 13a of the inner wet bath 13, and is directed towards inside of the inner wet bath 13 as reflected megasonic waves 21c. As a result, the first megasonic waves 56a and the reflected megasonic waves 56c interfere with each other at locations indicated with solid circles 57. This means that the silicon wafer 55 locally receives the megasonic waves 56 having high intensity.

[0075] In the conventional semiconductor cleaner 50 illustrated in FIG. 1, the first megasonic waves 56a irradiated upwardly from the oscillation plate 53 and the reflected megasonic waves 56c interfere with each other at the solid circles 57, and resultingly, the silicon wafer 55 is damaged at the solid circles 57.

[0076] This is because the oscillation plate 53 and the cleaning bath 52 are kept stationary relative to each other, and hence, an intensity of megasonic waves is increased at uniform locations due to interference of megasonic waves with each other.

[0077] In contrast, the megasonic-wave oscillation plate 17 from which megasonic waves are irradiated is always made to move upwardly or downwardly in the semiconductor cleaner 10 in accordance with the first embodiment. Accordingly, since a relative distance between the megasonic-wave oscillation plate 17 and the inner wet bath 13 (exactly, between the megasonic-wave oscillation plate 17 and the wafer 12 kept soaked in the inner wet bath 13) is varied, locations at which an intensity of megasonic waves is increased due to interference of megasonic waves with each other are not uniform unlike the conventional semiconductor cleaner 50. Hence, it is possible to prevent megasonic waves from locally concentrating to the wafer 12, and thus, it is possible to protect the wafer 12 from being damaged due to interference of megasonic waves.

[0078] The reason is explained in detail hereinbelow.

[0079] In FIG. 5, phases of first megasonic waves 22a irradiated from the megasonic-wave oscillation plate 17 when the megasonic-wave oscillation plate 17 reaches its uppermost point are illustrated in broken lines, and phases of second megasonic waves 22b irradiated from the megasonic-wave oscillation plate 17 when the megasonic-wave oscillation plate 17 reaches its lowermost point are illustrated in solid lines.

[0080] In FIG. 5, locations at which the reflected megasonic waves 21a and the first megasonic waves 22a interfere with each other are indicated with hollow circles (◯), and locations at which the reflected megasonic waves 21a and the second megasonic waves 22b interfere with each other are indicated with star marks (★). It is understood that locations at which megasonic waves interfere with each other are scattered relative to the solid circles 57 illustrated in FIG. 1.

[0081] As mentioned above, the semiconductor cleaner 10 in accordance with the first embodiment makes it possible to scatter locations at which megasonic waves interfere with each other, and hence, reduce an intensity of megasonic waves locally concentrating to the wafer 12 due to interference of megasonic waves with each other. As a result, it is possible to prevent the wafer 12 from being damaged due to interference of megasonic waves with each other.

[0082] A distance by which the megasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18 move upwardly and downwardly is dependent on a size of the wafer and/or a size of the inner wet bath 13. According to the experiments having been conducted by the inventor, the above-mentioned advantageous effects can be ensured regardless of sizes of the wafer 12 and the inner wet bath 13, if the distance is set equal to or greater than 2 mm, and equal to or smaller than 20 mm.

[0083] The megasonic waves 21 irradiated from the megasonic-wave oscillation plate 17 may have any frequency. However, according to the experiments having been conducted by the inventor, the megasonic waves 21 having a frequency in the range of 750 KHz to 1 MHz both inclusive could clean the wafer 12 to the best degree. Accordingly, it would be preferable that the megasonic-wave oscillator 18 oscillates the megasonic-wave oscillation plate 17 such that the megasonic waves 21 having a frequency in the range of 750 KHz to 1 MHz both inclusive are irradiated from the megasonic-wave oscillation plate 17.

[0084] [Second Embodiment]

[0085] FIG. 6 is a top view of a semiconductor cleaner 30 in accordance with the second embodiment, when viewed from upward.

[0086] In the semiconductor cleaner 30 in accordance with the second embodiment, eight wafers (not illustrated) are arranged in the inner wet bath 13 in parallel with one another. The semiconductor cleaner 30 is designed to include first to eighth megasonic-wave oscillation plates 17a to 17h and first. to eighth megasonic-wave oscillators 18a to 18h in association with eight wafers.

[0087] Each of the first to eighth megasonic-wave oscillation plates 17a to 17h is identical in structure with the megasonic-wave oscillation plate 17 in the first embodiment, and each of the first to eighth megasonic-wave oscillators 18a to 18h is identical in structure with the megasonic-wave oscillator 18 in the first embodiment.

[0088] Accordingly, a distance by which each of the eight wafers moves upwardly and downwardly can be controlled for each of the eight wafers.

[0089] A single megasonic-wave oscillation plate may be used in place of the first to eighth megasonic-wave oscillation plates 17a to 17h. However, by using the first to eighth megasonic-wave oscillation plates 17a to 17h in place of a single megasonic-wave oscillation plate, it would be easier to control each of the megasonic-wave oscillation plates.

[0090] [Third Embodiment]

[0091] FIG. 7 illustrates a semiconductor cleaner 40 in accordance with the third embodiment.

[0092] The semiconductor cleaner 40 in accordance with the third embodiment is comprised of an inner wet bath 43 which is filled with cleaning solution 41 and in which a wafer 42 to be cleaned is soaked, an outer wet bath 45 which is filled with water 44 and which surrounds the inner wet bath 43, and a megasonic-wave irradiator 46 arranged below a bottom of the inner wet bath 43 for irradiating megasonic waves.

[0093] The megasonic-wave irradiator 46 is comprised of a megasonic-wave oscillation plate 47 located in the outer wet bath 45 and below a bottom of the inner wet bath 43 in parallel with a bottom of the inner wet bath 43, and a megasonic-wave oscillator 48 connected to the megasonic-wave oscillation plate 47 for oscillating the megasonic-wave oscillation plate 47 to produce megasonic waves.

[0094] The wafer 42 is kept soaked in the cleaning solution 41 by means of an appropriate holder (not illustrated).

[0095] The inner wet bath 43 is formed at an inner surface 43a of a sidewall thereof with irregularity having a surface roughness greater than a wavelength of megasonic waves irradiated from the megasonic-wave oscillation plate 47.

[0096] In general, if a wave had a wavelength smaller than a surface roughness of a wall at which the wave is reflected, the wave would be irregularly reflected. In the semiconductor cleaner 40 in accordance with the third embodiment, megasonic waves irradiated obliquely upwardly from the megasonic-wave oscillation plate 47 are reflected at the inner surface 43a of the sidewall of the inner wet bath 43, and then, interfere with megasonic waves irradiated upwardly from the megasonic-wave oscillation plate 47. Since the megasonic waves irradiated obliquely upwardly from the megasonic-wave oscillation plate 47 are irregularly reflected at the inner surface 43a, locations at which those megasonic waves interfere with each other are in a broader area than the conventional semiconductor cleaner 50 illustrated in FIG. 1.

[0097] Hence, it is possible to reduce an intensity of the megasonic waves from locally concentrating to the wafer 42 due to interference of megasonic waves with each other, and thus, it is possible to protect the wafer 42 from being damaged.

[0098] It should be noted that the inner wet bath 13 in the semiconductor cleaner 10 in accordance with the first embodiment may be designed to have slight irregularity at an inner surface of a sidewall thereof.

[0099] Hereinbelow is explained an example of the semiconductor cleaners 10 and 40 in accordance with the first and third embodiments.

[0100] In theory, if the cleaning solution 11 and 41 is comprised of water, supersonic waves such as megasonic waves are reflected at a surface of the water 11 or 41 by 99.9%, and the thus reflected supersonic waves are directed to inside of the inner wet bath 13 or 43. The supersonic waves directed to inside of the inner wet bath 13 or 43 interferes with supersonic waves directed to a surface of the water 11 or 41, resulting in that sound pressure is locally strengthened every pitch of a standing wave. This is explained in detail hereinbelow.

[0101] It is assumed that megasonic waves irradiated from the megasonic-wave oscillation plate 17 or 47 have a frequency of 950 KHz.

[0102] A phase velocity of supersonic waves in pure water is equal to 1483 m/s at 20 degrees centigrade. A wavelength &lgr; is calculated as follows. 1 Wavelength ⁢   ⁢ λ = Phase ⁢   ⁢ velocity ⁢   ⁢ C ⁢   [ m / s ] / Frequency ⁢   ⁢ F ⁢   [ Hz ] = 1483 / 950 = 1.56 ⁢   ⁢ mm

[0103] Accordingly, a pitch of a standing wave is calculated as follows.

1.56/2=0.78 mm

[0104] This is small enough to be canceled by a wave generated at a surface of the water 11 or 41 by supersonic waves. Accordingly, it is possible to ignore non-uniformity in cleaning the wafer 12 or 42, caused by a pitch of a standing wave, that is, damages of the wafer 12 or 42 caused by megasonic waves locally having a high intensity due to interference of megasonic waves with each other.

[0105] Accordingly, reflected megasonic waves which may interference with non-reflected megasonic waves are limited to megasonic waves reflected from either the inner wet bath 13, 43 or the wafer 12, 42.

[0106] In order to prevent cancellation of cleaning effects of megasonic waves and/or interference of megasonic waves with each other while the megasonic-wave oscillation plate 17 moves upwardly or downwardly, it would be necessary for the megasonic-wave oscillation plate 17 to move at a rate which is not equal to multiples of a phase velocity of megasonic waves and multiples of a half of a phase velocity of megasonic waves, and by a distance which is not equal to multiples of a wavelength of megasonic waves and multiples of a half of a wavelength of megasonic waves.

[0107] Herein, there is considered a specific example having the following specification.

[0108] Frequency of megasonic waves: 950 KHz

[0109] Cleaning solution: Water

[0110] Temperature of cleaning solution: 20 degrees centigrade

[0111] Revolution speed of the motor 19: 10,000 per second

[0112] Gear ratio of the motor 19: 1:4

[0113] Material of which the inner wet bath 13 and 43 is composed: Quartz

[0114] In order to avoid a rate Ca of the megasonic-wave oscillation plate 17 from being equal to multiples of a phase velocity of megasonic waves and multiples of a half of a phase velocity of megasonic waves, it is assumed that the rate Ca is equal to one-third ({fraction (1/3)}) of a phase velocity C.

Ca=C/3=1483/3=494.33 [m/s]=4943.3 [cm/s]

[0115] A distance L which ensures the rate Ca of 4943.3 [cm/s] under the above-mentioned conditions is calculated as follows.

L=Ca[cm/s]/the number of reciprocation=4943.3/2500=1.977 cm

[0116] Herein, the number of reciprocation is calculated as 2500, because the motor 19 rotates at a revolution speed of 10000/s and has a gear ratio of 1/4.

[0117] Accordingly, a distance La by which the megasonic-wave oscillation plate 17 moves upwardly or downwardly is calculated as a half of the distance L.

La=L/2=1.977/2=0.989 [cm]

[0118] Since the distance La of 0.989 cm is not equal to multiples of a wavelength of megasonic waves and multiples of a half of a wavelength of megasonic waves, the megasonic waves do not interfere with each other.

[0119] When the inner wet bath 13 or 43 is composed of quartz, the inner wet bath 13 or 43 is frequently dealt with hydrofluoric acid at an inner surface thereof in order to reduce a surface roughness thereof. After being dealt with hydrofluoric acid, the inner wet bath 13 or 43 usually has a surface roughness of about 0.5 mm, which is smaller than a wavelength of megasonic waves. Accordingly, megasonic waves are mirror-reflected at an inner surface of the inner wet bath 13 or 43, or pass through the inner wet bath 13 or 43.

[0120] Since the megasonic-wave oscillation plate 17 or 47 is arranged below a bottom of the inner wet bath 13 or 43, it is necessary for a bottom of the inner wet bath 13 or 43 to have such a surface roughness as mentioned above.

[0121] As having been explained in the third embodiment, it is necessary for the inner surface 43a of a sidewall of the inner wet bath 13 or 43 to have a structure for preventing megasonic waves from mirror-reflecting in order to avoid interference of megasonic waves with each other. As mentioned earlier, if a wave had a wavelength smaller than a surface roughness of a wall at which the wave is reflected, the wave would be irregularly reflected. In order to utilize this phenomenon, the inner wet bath 13 or 43 is designed to have a sidewall having the inner surface 43a having a surface roughness of 3 mm. Since the inner wet bath 13 or 43 is composed of quartz, it is sufficiently possible to design the inner wet bath 13 or 43 to have a sidewall having the inner surface 43a having a surface roughness of 3 mm.

[0122] In summary, the example of the semiconductor cleaners 10 and 40 in accordance with the first and third embodiments has the following dimensions.

[0123] Frequency of megasonic waves: 950 KHz

[0124] Cleaning solution: Water

[0125] Temperature of cleaning solution: 20 degrees centigrade

[0126] Revolution speed of the motor 19: 10,000 per second

[0127] Gear ratio of the motor 19: 1:4

[0128] Material of which the inner wet bath 13 and 43 is composed: Quartz

[0129] Portion of the inner wet bath 13 and 43 to be dealt with hydrofluoric acid: only bottom

[0130] Surface processing of the inner wet bath 13 and 43: only sidewall (surface roughness of 3 mm)

[0131] Distance by which the megasonic-wave oscillation plate 17 or 47 moves: 0.989 cm

[0132] While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.

[0133] The entire disclosure of Japanese Patent Application No. 2001-394489 filed on Dec. 26, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.

Claims

1. A semiconductor cleaner comprising:

(a) a wet bath which is filled with liquid and in which a wafer to be cleaned is soaked;

(b) a megasonic-wave irradiator located outside said wet bath for irradiating megasonic waves to said wafer to clean said wafer; and

(c) a unit which moves said megasonic wave irradiator towards or away from said wet bath.

2. The semiconductor cleaner as set forth in claim 1, wherein said unit moves said megasonic-wave irradiator by a distance in the range of 2 mm to 20 mm both inclusive.

3. The semiconductor cleaner as set forth in claim 1, wherein said megasonic-wave irradiator irradiates megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

4. The semiconductor cleaner as set forth in claim 1, wherein said wet bath is formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

5. The semiconductor cleaner as set forth in claim 1, wherein said unit moves said megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of said megasonic waves and multiples of a half of a phase velocity of said megasonic waves, and said unit moves said megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of said megasonic waves and multiples of a half of a wavelength of said megasonic waves.

6. A semiconductor cleaner comprising:

(a) a wet bath which is filled with liquid and in which first to N-th wafers to be cleaned are soaked wherein N is an integer equal to or greater than two;

(b) first to N-th megasonic-wave irradiators located outside said wet bath and associated with said first to N-th wafers, respectively, for irradiating megasonic waves to said first to N-th wafers to clean said first to N-th wafers; and

(c) first to N-th units which move said first to N-th megasonic-wave irradiators towards or away from said wet bath, respectively.

7. The semiconductor cleaner as set forth in claim 6, wherein said first to N-th units move said first to N-th megasonic-wave irradiators, respectively, by a distance in the range of 2 mm to 20 mm both inclusive.

8. The semiconductor cleaner as set forth in claim 6, wherein said first to N-th megasonic-wave irradiators irradiate megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

9. The semiconductor cleaner as set forth in claim 6, wherein said wet bath is formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

10. The semiconductor cleaner as set forth in claim 6, wherein each of said first to N-th units moves said megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of said megasonic waves and multiples of a half of a phase velocity of said megasonic waves, and each of said first to N-th units moves said megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of said megasonic waves and multiples of a half of a wavelength of said megasonic waves.

11. A semiconductor cleaner comprising:

(a) a wet bath which is filled with liquid and in which a wafer to be cleaned is soaked; and

(b) a megasonic-wave irradiator located outside said wet bath for irradiating megasonic waves to said wafer to clean said wafer,

said wet bath being formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

12. A semiconductor cleaner comprising:

(a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked;

(b) an outer wet bath which is filled with liquid and in which said inner wet bath is soaked;

(c) a megasonic-wave irradiator located outside said inner wet bath for irradiating megasonic waves to said wafer to clean said wafer; and

(d) a unit which moves said megasonic-wave irradiator towards or away from said inner wet bath.

13. The semiconductor cleaner as set forth in claim 12, wherein said megasonic-wave irradiator is comprised of:

(c1) a megasonic-wave oscillation plate located in said outer wet bath and below a bottom of said inner wet bath in parallel with said bottom of said inner wet bath; and

(c2) a megasonic-wave oscillator connected at one end to said megasonic-wave oscillation plate and at the other end to said unit, said megasonic-wave oscillator being fit into an opening formed at a bottom of said outer wet bath for slidable movement.

14. The semiconductor cleaner as set forth in claim 12, wherein said inner wet bath is composed of quartz.

15. The semiconductor cleaner as set forth in claim 12, wherein said unit moves said megasonic-wave irradiator by a distance in the range of 2 mm to 20 mm both inclusive.

16. The semiconductor cleaner as set forth in claim 12, wherein said megasonic-wave irradiator irradiates megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

17. The semiconductor cleaner as set forth in claim 12, wherein said inner wet bath is formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

18. The semiconductor cleaner as set forth in claim 12, wherein said unit moves said megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of said megasonic waves and multiples of a half of a phase velocity of said megasonic waves, and said unit moves said megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of said megasonic waves and multiples of a half of a wavelength of said megasonic waves.

19. A semiconductor cleaner comprising:

(a) an inner wet bath which is filled with liquid and in which first to N-th wafers to be cleaned are soaked wherein N is an integer equal to or greater than two;

(b) an outer wet bath which is filled with liquid and in which said inner wet bath is soaked;

(c) first to N-th megasonic-wave irradiators located outside said inner wet bath and associated with said first to N-th wafers, respectively, for irradiating megasonic waves to said first to N-th wafers to clean said first to N-th wafers; and

(d) first to N-th units which move said first to N-th megasonic-wave irradiators towards or away from said inner wet bath, respectively.

20. The semiconductor cleaner as set forth in claim 19, wherein each of said first to N-th megasonic-wave irradiators is comprised of:

(c1) a megasonic-wave oscillation plate located in said outer wet bath and below a bottom of said inner wet bath in parallel with said bottom of said inner wet bath; and

(c2) a megasonic-wave oscillator connected at one end to said megasonic-wave oscillation plate and at the other end to each of said first to N-th units, said megasonic-wave oscillator being fit into an opening formed at a bottom of said outer wet bath for slidable movement.

21. The semiconductor cleaner as set forth in claim 19, wherein said inner wet bath is composed of quartz.

22. The semiconductor cleaner as set forth in claim 19, wherein said first to N-th units move said first to N-th megasonic-wave irradiators, respectively, by a distance in the range of 2 mm to 20 mm both inclusive.

23. The semiconductor cleaner as set forth in claim 19, wherein said first to N-th megasonic-wave irradiators irradiate megasonic waves having a frequency in the range of 750 KHz to 1 MHz both inclusive.

24. The semiconductor cleaner as set forth in claim 19, wherein said wet bath is formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

25. The semiconductor cleaner as set forth in claim 19, wherein each of said first to N-th units moves said megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of said megasonic waves and multiples of a half of a phase velocity of said megasonic waves, and each of said first to N-th units moves said megasonic-wave irradiator by a distance which is not equal to multiples of a wavelength of said megasonic waves and multiples of a half of a wavelength of said megasonic waves.

26. A semiconductor cleaner comprising:

(a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked;

(b) an outer wet bath which is filled with liquid and in which said inner wet bath is soaked; and

(c) a megasonic-wave irradiator located outside said inner wet bath for irradiating megasonic waves to said wafer to clean said wafer;

said inner wet bath being formed at an inner surface thereof with irregularity having a surface roughness greater than a wavelength of said megasonic waves.

27. A method of cleaning a wafer soaked in liquid filled in a wet bath, comprising the step of irradiating megasonic waves to said wafer with a relative distance between said wafer and a megasonic-wave irradiator being varied.

28. The method as set forth in claim 27, wherein said relative distance is varied by reciprocating said megasonic-wave irradiator relative to said wafer.

29. The method as set forth in claim 27, wherein said relative distance is varied in the range of 2 mm to 20 mm both inclusive.

30. The method as set forth in claim 27, wherein said megasonic waves have a frequency in the range of 750 KHz to 1 MHz both inclusive.

31. The method as set forth in claim 27, wherein relative distance is varied by moving said megasonic-wave irradiator at a rate which is not equal to multiples of a phase velocity of said megasonic waves and multiples of a half of a phase velocity of said megasonic waves, and by a distance which is not equal to multiples of a wavelength of said megasonic waves and multiples of a half of a wavelength of said megasonic waves.