Lapping method and lapping machine

Abstract

An object such as an electronic device can be favorably lapped by a method composed of preparing a lapping machine having a longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board and a supporting means arranged adjacently to the lapping board; placing the object on the lapping board via abrasive grains and further on the supporting means; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping board whereby the object is caused to rotate on an axis thereof and lapped.

Description

FIELD OF THE INVENTION

[0001] This invention relates to a lapping method and a lapping device. In particular, the invention relates to a method and machine favorably employable for lapping objects having a relative large size.

BACKGROUND OF THE INVENTION

[0002] The lapping machine is generally employed for lapping accurately a surface of an object such as a silicon substrate to be employed for manufacturing an integrated circuit or an alumina-titanium carbide substrate to be employed for manufacturing a magnetic head.

[0003] FIG. 1 is a front view of a typical structure of the conventionally employed lapping machine, and FIG. 2 is a top view of the lapping machine of FIG. 1. The lapping machine of FIGS. 1 and 2 comprises a lapping board 13 which is fixed to an axis of rotation 12 of a motor 11, a abrasive grain-supplying means 15 which supplies a slurry 14 containing abrasive grains on the surface of the lapping board 13, and disc means 17 for rotatably supporting an object 16 to be lapped.

[0004] The object 16 is temporarily attached to the supporting means 17 via wax or the like. The supporting means 17 is supported on its circumferential side with a pair of rollers 20. Each roller 20 is rotatably supported by a roller-supporting means 23 which is arranged on the top of a pole 22 standing on a base board 21.

[0005] The lapping board 13 rotates in the direction indicated by the arrow 19 (see FIG. 2) by activating the motor 11. With the rotation of the lapping board 13, the supporting means 17 holding the object 16 rotates under guidance with the pair of rollers 20.

[0006] On the surface of the lapping board 13, the abrasive grain slurry 14 is dropwise supplied from the abrasive grain-supplying means 15. Abrasive grains generally are diamond grains, alumina grains, or silica grains. The abrasive grain slurry 14 is moved toward the object 16 and supplied between the lapping board 13 and the object 16 by the rotation of the lapping board 13.

[0007] The lapping board 13 temporarily holds abrasive grains between the object 16 and the board 13. Thus, the lapping board 13 is generally made of relatively soft material such as tin, as compared with the abrasive grains. For instance, a lapping board made of tin is slightly deformed on its surface to temporarily hold the abrasive grains.

[0008] When the lapping board 13 and the object to be lapped 16 are independently rotated with the intervening abrasive grains, the under-surface of the object 16 is polished. The polishing using abrasive grains is generally named “lapping”.

[0009] Japanese Patent Provisional Publication No. 11-320376 describes an improvement of the lapping machine which has means for adjusting conditions of contact between the lapping board and the object to be lapped.

SUMMARY OF THE INVENTION

[0010] The conventional lapping machine is favorably employed for lapping an object having a relatively small size. If an object having a large size such as silicon substrate of 12 inches is to be lapped in a lapping machine or a number of objects are lapped simultaneously in one lapping machine, it is required that the lapping machine employs a large sized lapping board and a high-powered motor. It is known that the horizontal accuracy on the surface of the lapping board should be less than 100 &mgr;m, if the object is lapped with a high accuracy. Apparently, it is not easy to manufacture a large sized lapping board with a high horizontal accuracy. Accordingly, it is not easy to lap a large-sized object with a high accuracy using the conventional lapping machine. If a large-sized lapping board can be manufacture, its manufacturing cost is very high.

[0011] Accordingly, it is an object of the invention to provide a method particularly favorable for lapping a large-sized object with a high accuracy.

[0012] The present invention resides in a method for lapping an object which comprises preparing a lapping machine comprising at least one longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board and a supporting means arranged adjacently to the lapping board; placing the object on the lapping board via abrasive grains and further on the supporting means; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping board whereby the object is caused to rotate on an axis thereof and lapped. This lapping method is named a first lapping method in the specification.

[0013] In the first lapping method, the following embodiments are preferred.

[0014] (1) The rotation of the object is guided by a rotation-guiding means attached to the lapping machine.

[0015] (2) The supporting means is a longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies single-mode vibration to the lapping board and the object is placed on the supporting means via abrasive grains.

[0016] (3) The elliptical vibration is a vibration of progressive wave travelling along the longitudinal or circular lapping board.

[0017] (4) The elliptical vibration is a vibration of standing wave occurring along the longitudinal or circular lapping board.

[0018] The present invention further resides in a method for lapping an object which comprises preparing a lapping machine comprising a pair of longitudinal lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged in parallel; placing the object on the lapping boards via abrasive grains; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping boards whereby the object is caused to rotate on an axis thereof and lapped. This lapping method is named a second lapping method in the specification.

[0019] In the second lapping method, the following embodiments are preferred.

[0020] (1) The rotation of the object is guided by a rotation-guiding means attached to the lapping machine.

[0021] (2) An auxiliary lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical or single-mode vibration to the auxiliary lapping board is arranged between the pair of the lapping boards, and the object is further lapped on the auxiliary lapping board.

[0022] (3) The elliptical vibration is a vibration of progressive wave travelling along the longitudinal lapping board.

[0023] (4) The elliptical vibration is a vibration of standing wave occurring along the longitudinal lapping board.

[0024] The present invention furthermore resides in a method for lapping an object which comprises preparing a lapping machine comprising a pair of circular lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged coaxially; placing the object on the lapping boards via abrasive grains; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping boards whereby the object is caused to rotate on an axis thereof and lapped. This lapping method is named a third lapping method in the specification.

[0025] In the third lapping method, the following embodiments are preferred.

[0026] (1) The object is further moved along the circular lapping boards.

[0027] (2) The elliptical vibration is a vibration of progressive wave travelling along the circular lapping board.

[0028] (3) The elliptical vibration is a vibration of standing wave occurring along the circular lapping board.

[0029] The present invention furthermore resides in a lapping machine comprising at least one longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board and a supporting means arranged adjacently to the lapping board.

[0030] This lapping machine is named a first lapping machine in the specification.

[0031] In the first lapping machine, the following embodiments are preferred.

[0032] (1) Means for guiding an object to be lapped is further comprised.

[0033] (2) The supporting means is a longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies single-mode vibration to the lapping board.

[0034] The present invention furthermore resides in a lapping machine comprising a pair of longitudinal lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged in parallel. This lapping machine is named a second lapping machine in the specification.

[0035] In the second lapping machine, the following embodiments are preferred.

[0036] (1) Means for guiding an object to be lapped is further comprised.

[0037] (2) An auxiliary lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical or single-mode vibration to the auxiliary lapping board is arranged between the pair of the lapping boards.

[0038] The present invention furthermore resides in a lapping machine comprising a pair of circular lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged coaxially. This lapping machine is named a third lapping machine in the specification.

[0039] In the third lapping machine, the following embodiments are preferred.

[0040] (1) Means for guiding an object to be lapped is further comprised.

BRIEF DESCRIPTION OF THE DRAWING

[0041] FIG. 1 is a front view of a conventional lapping machine.

[0042] FIG. 2 is a top view of the machine of FIG. 1.

[0043] FIG. 3 is schematic view of an example of the lapping machine according to the invention.

[0044] FIG. 4 is a section view taken along the line I-I of FIG. 3 which illustrates the structure of the rotation-guiding means and the object to be lapped.

[0045] FIG. 5 is a view illustrating the progressive wave of elliptical vibration travelling on the longitudinal lapping board.

[0046] FIG. 6 is a schematic view of another example of the lapping machine according to the invention.

[0047] FIG. 7 is a section view taken along the line II-II of FIG. 6 which illustrates the structure of the rotation-guiding means and the object to be lapped.

[0048] FIGS. 8 to 11 are top views showing various lapping machines according to the invention.

[0049] FIG. 12 is a schematic view showing a lapping machine according to the invention.

[0050] FIG. 13 is a partially cutaway schematic view showing assembly of a lapping machine of the invention.

[0051] FIG. 14 is a view explaining a mechanism producing an elliptical vibration of standing wave occurring along the longitudinal lapping board.

DETAILED DESCRIPTION OF THE INVENTION

[0052] The present invention is further described by referring to the figures given in the attached drawings.

[0053] In FIG. 3, a representative construction of the second lapping machine of the invention is schematically illustrated.

[0054] The lapping machine of FIG. 3 comprises a pair of longitudinal lapping boards 31, 32 which are equipped with a ultrasonic oscillation-applying device and arranged in parallel. The ultrasonic oscillation-applying device is capable of applying elliptical vibration to the lapping board. The lapping boards 31, 32 can be made of tin or tin-antimony alloy.

[0055] In FIG. 3, the ultrasonic oscillation-applying device capable of applying elliptical vibration to the lapping board 31 comprises a pair of Langevin vibrators 41a, 41b, while the ultrasonic oscillation-applying device capable of applying elliptical vibration to the lapping board 32 comprises a pair of Langevin vibrators 42a, 42b.

[0056] The Langevin vibrator 42a comprises a piezoelectric transducer 43a, a supporting frame 44, and another piezoelectric transducer 43b which are combined between a pair of metallic blocks 45a, 45b and bolted on each other. The Langevin vibrators 41a, 41b, and 42b have the same structure as that of the Langevin vibrator 42a. Each of the metallic blocks 45a, 45b is, for instance, made of alminum metal.

[0057] The lapping machine of FIG. 3 has a rotation-guiding means 33 which assists a rotation of an object to be lapped on its axis. The rotation-guiding means 33 comprises an object-holding means 33a and an arm 33c which are connected via a bearing 33b. An object to be lapped is held on the object-holding means 33a on the lapping board side.

[0058] FIG. 4 of a section view of a combination of the rotation-guiding means 33 and the object 40 to be lapped taken along the line I-I in FIG. 3. The object 40 is attached to the object-holding means 33a in such manner that the object 40 is rotatable on its axis by the aid of a bearing 33 of the rotation-guiding means 33. The object 40 is attached temporarily to the object-holding means 33a, for instance, via wax.

[0059] The lapping machine of FIG. 3 is equipped with a pair of abrasive grain-supplying means 36 which supply a slurry of abrasive grains between the object 40 and the lapping boards 31, 32. The abrasive grains are generally supplied onto the lapping boards in the form of a slurry. Representative examples of the abrasive grains include grains of diamond, alumina, and silica. In FIG. 3, pipe lines and pump system for supplying the abrasive grains to the abrasive grain-supplying means 36 and a frame for fixing the abrasive grain-supplying means 36 to the base board 30 are not shown.

[0060] The abrasive grains can be placed on the lapping boards prior to start of the lapping procedure. A lubricant such as olive oil is preferably supplied onto the lapping board on which the abrasive grains are placed.

[0061] The lapping procedure corresponding to the aforementioned second lapping method are described below using the lapping machined illustrated in FIGS. 3 and 4. In the second lapping method, the object 40 is placed on the longitudinal lapping boards 31, 32 via abrasive grains; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping boards 31, 32, whereby the object is caused to rotate on an axis thereof and lapped.

[0062] In the lapping machine illustrated in FIGS. 3 and 4, each of the lapping boards 31, 32 is provided with a progressive wave of elliptical vibration which is generated the ultrasonic oscillation-applying device. Each of the arrows 31a, 32b indicates the direction of progressive wave travelling on each lapping board. As is shown in FIG. 3, the ultrasonic oscillation-applying devices attached to lapping boards are preferably set to apply progressive waves of an elliptical vibration of opposite directions to the lapping boards. The object 40 on the lapping boards 31, 32 on which the progressive waves of elliptical vibration are travelled receives moments directed reversely to the progressing waves. Accordingly, the object 40 rotates on its axis in the direction of allow 34. The object 40 is lapped in the course of the rotation on the lapping boards 31, 32. Details of mechanism to apply the progressive wave of elliptical vibration to the longitudinal lapping board are described hereinafter. The longitudinal lapping board preferably has a rounded edge 39 so as to smoothly rotate the object 40 on the lapping boards 31, 32.

[0063] As is apparent from the above-given explanations, the lapping machine of the invention can be employed for lapping an object of large size, with no use of a large sized lapping disc board.

[0064] A plurality of objects can be lapped simultaneously on the lapping boards of the invention, by placing the objects along the longitudinal lapping boards.

[0065] The lapping machine of the invention preferably comprises a linear driving means for moving the object to be lapped along the longitudinal lapping boards, whereby obviate occurrence of uneven wear on the surface of the lapping boards. The lapping machined of FIG. 3 has a voice coil motor 37 as the linear driving means. The voice coil motor 37 is fixed to a supporting means 35 standing on the base board 30 and has a driving axis connected to the arm 33c of the rotation-guiding means 33. In the course of lapping, the object 40 under rotation is alternately moved forward and backward on the longitudinal lapping boards 31, 32 so that the lapping boards are kept from occurrence of uneven wear on their surfaces.

[0066] The mechanism for applying a progressive wave of elliptical vibration to the longitudinal lapping board is explained by referring to FIG. 5. The longitudinal lapping board 51 of FIG. 5 is equipped with Langevin vibrator 52a, 52b which generates an elliptical vibration. The progressive wave of elliptical vibration can be applied to the lapping board 51 in the following manner.

[0067] To the Langevin vibrator 52a is electrically connected an alternating current source 53, while to the Langevin vibrator 52b are electrically connected a resistance 54 and a coil 55.

[0068] When the alternating current source 53 applies an alternating voltage to the Langevin vibrator 52a, the Langevin vibrator 52a generates ultrasonic wave and applies the ultrasonic wave to the longitudinal lapping board 51, whereby the lapping board 51 vibrates in the depth direction. The vibration applied to the lapping board 51 is transmitted to the Langevin vibrator 52b and converted into an electric energy in the vibrator 52b. The electric energy is then consumed in the resistance 54 and coil 55. Thus, the vibration applied to the lapping board 51 by the Langevin vibrator 52a does not produce a standing wave in the longitudinal direction of the lapping board 51, but produces a progressive wave of elliptical vibration which has a depth vibration component and a vibration component in the longitudinal direction. The arrow 51a shown in FIG. 5 indicates the direction of the progressive wave of elliptical vibration transmitting on the lapping board 51. If a progressive wave of elliptical vibration advancing in the opposite direction is desired, the alternating current source is connected to the Langevin vibrator 52b, while the resistance and coil are connected to the Langevin vibrator 52a.

[0069] The longitudinal lapping board preferably has a length not less than twice, more preferably not less than three times, the width of the board, so that the progressive wave of elliptical vibration is smoothly transmitted in the longitudinal direction.

[0070] The mechanism for applying the progressive wave of elliptical vibration to the longitudinal board is the same as that for applying an progressive wave of elliptical vibration to a stator of a surface wave type ultrasonic linear motor. In more detail, the longitudinal lapping board 51 corresponds to a stator of a surface wave type ultrasonic linear motor, while the object to be lapped corresponds to a slider of the surface wave type ultrasonic linear motor. More details for applying a progressive wave of elliptical vibration to a longitudinal board are seen in “Piezoelectric/Electrostrictive Actuators” (Morikita Publishing, 1996, pages 183-186).

[0071] FIG. 6 schematically illustrates another example of the second lapping machine of the invention employing a pair of longitudinal lapping boards. The lapping machine of FIG. 6 comprises a pair of longitudinal lapping boards 31, 32 and four Langevin vibrators, which are the same as those of lapping machined of FIG. 3.

[0072] The lapping machine of FIG. 6 further comprises a rotation-guiding means consisting of a pair of bar magnets 64a, 64b. The “S” and “N” shown in the bar magnets 64a, 64b indicate poles of respective bar magnets. The object-holding means 63 has an object to be lapped, on its lapping board side.

[0073] FIG. 7 is a section view of a combination of the object 40 to be lapped and the object-holding means 63 taken along the line II-II in FIG. 6. The object-holding means 63 comprises a stainless steel disc 63a and a ring-shaped magnet 63b placed around the disc 63a. The “S” and “N” shown in the ring-shaped magnets 63b indicate poles in the magnet 63b.

[0074] Each of the bar magnets 64a, 64b repels the ring-shaped magnet 63b of the object-holding means 63. Therefore, the rotation of the object 40 (which is attached to the object-holding means 63) on its axis is assisted by the bar magnets 64a, 64b. The bar magnets 64a, 64b further function to define the movement of the object-holding means 63 so that the object 40 attached to the object-holding means 63 can move only along the longitudinal lapping board.

[0075] The lapping machine of FIG. 6 further comprises a pair of magnetic contact switches 65a, 65b arranged on either ends of the longitudinal lapping boards 31, 32. The magnetic contact switches 65a, 65b detects the position of the object-holding means 63 when it becomes into contact with the switches 65a, 65b.

[0076] In the same manner as in the lapping machine of FIG. 3, the progressive waves of elliptical vibration indicated by the arrows 31a, 32b are applied to the lapping boards 31, 32 of FIG. 6, and the object 40 to be lapped which is attached to the object-holding means 63 rotates on its axis in the direction indicated by the arrow 34.

[0077] In the lapping machine of FIGS. 6 and 7, the object 40 can be not only rotated on its axis but also moves alternately forward and backward along the longitudinal lapping boards 31, 32 by adjusting the directions of the progressive waves of elliptical vibration applied to the lapping boards. For instance, the progressive waves of elliptical vibration advancing in the same directions are applied to both of the lapping boards 31, 32, the object 40 to be lapped linearly moves in the direction opposite to the directions of the progressive waves. If such linear movement of the object 40 is periodically caused on the lapping boards, troublesome uneven wear of the surfaces of the lapping boards can be obviated.

[0078] The lapping of the object can be done with periodical linear movement along the longitudinal lapping boards in the below-mentioned manner.

[0079] (1) In the first place, the object 40 is lapped under rotation on its axis on the lapping boards 31, 32, by applying progressive waves of elliptical vibration advancing opposite directions on the lapping boards. At a lapse of a predetermined period of time, progressive waves of elliptical vibrations advancing in the same directions (such as the direction indicated by the arrow 31a) are applied to the lapping boards 31, 32, to move the object 40 to advance towards the magnetic contact switch 65b. If the lapping of the object 40 under rotation and the movement of the object along the longitudinal lapping boards are periodically repeated, the object 40 reaches the ends of the longitudinal lapping boards. The position of the object 40 is then detected by the magnetic contact switch 65b.

[0080] (2) Subsequently, progressive waves of elliptical vibrations reversely advancing in the same directions (such as the direction indicated by the arrow 32b) are applied to the lapping boards 31, 32, to move the object 40 to advance towards the magnetic contact switch 65a. At a lapse of a predetermined period of time, the object 40 is again lapped under rotation on its axis on the lapping boards 31, 32, by applying progressive waves of elliptical vibration advancing opposite directions on the lapping boards. If the lapping of the object 40 under rotation and the movement of the object along the longitudinal lapping boards are periodically repeated, the object 40 reaches the ends of the longitudinal lapping boards. The position of the object 40 is then detected by the magnetic contact switch 65a.

[0081] If the rotations and linear movements of the object 40 are repeated, the object 40 moves forward and backward periodically along the lapping boards 31, 32.

[0082] Each of FIGS. 8 and 9 is a top view of the first lapping machine to be used for performing the first lapping method. In FIGS. 8 and 9, two longitudinal boards 81, 82 and the object 40 to be lapped only are shown.

[0083] In operating the lapping machine of FIG. 8 or 9, the progressive wave of elliptical vibration is applied only to the longitudinal board 82. The longitudinal board 81 may serve only to support the object 40.

[0084] As is shown in FIG. 8, when a progressive wave of elliptical vibration in the direction indicated by the arrow 82b is applied to the lapping board 82, a power is given to the object 40 by the progressive wave to move in the direction indicated by the arrow 84a. By this power, the object 40 rotates on its axis in the direction indicated by the arrow 70a and is lapped, and further moves in the direction indicated by the arrow 80a along the longitudinal lapping board 82.

[0085] In the above-mentioned procedure, the rate of rotation and rate of movement can be controlled by adjusting the amplitude of vibration of the progressive wave of elliptical vibration applied to the lapping board 82. The amplitude of vibration of the progressive wave can be adjusted by varying the amplitude of vibration and the frequency of alternating voltage applied to the Langevin vibrator.

[0086] In the lapping machine of FIG. 9, when a progressive wave of elliptical vibration in the direction indicated by the arrow 82a is applied to the lapping board 82, a power is given to the object 40 by the progressive wave to move in the direction indicated by the arrow 84b. By this power, the object 40 rotates on its axis in the direction indicated by the arrow 70b and is lapped, and further moves in the direction indicated by the arrow 80b along the longitudinal lapping board 82.

[0087] The longitudinal board 81 shown in FIGS. 8 and 9 serves to assist the rotation and movement of the object 40. It is preferred to apply a single mode vibration to the longitudinal board 81 so that the object can rotate and move on the longitudinal board 81. The vibration can be of a vertical mode, a slide mode, a flexural mode, a longitudinal mode, or a spread mode. The single mode vibration consists of a vibration according to one of these modes. For instance, the single mode vibration of vertical mode can be given to the longitudinal board 81 by applying simultaneously an alternating voltage to each of two Langevin vibrators attached to the longitudinal board 81. Since the vibrations given by the Langevin vibrators are not absorbed by any of the generators, no progressive wave of elliptical vibration cannot be produced, and a standing wave of a vertical mode is produced in the longitudinal board 81.

[0088] The single mode vibration can be applied to the longitudinal board 81 by other means than the Langevin vibrator. For instance, a piezoelectric transducer capable of emitting a single mode vibration is attached to the longitudinal board 81 on the underside to apply a single mode vibration to the longitudinal board.

[0089] A slurry containing abrasive grains can be supplied onto the longitudinal board 81 so that the longitudinal board 81 can serve as a lapping board. Thus, the object 40 can be lapped not only on the lapping board 82 but also on the longitudinal board 81.

[0090] The longitudinal board 81 can be replaced with a simple supporting means having a slidable or rotatable head.

[0091] FIG. 10 is a top view of an example of the second lapping machine according to the invention. In FIG. 10, there are shown a pair of longitudinal lapping boards 81, 82 and an object 40 to be lapped.

[0092] In FIG. 10, when progressive waves of elliptical vibration 81a, 82b are applied to the lapping boards 81, 82, respectively, the object 40 is rotated in the direction indicated by the arrow 70a and lapped because the object 40 receives moments of power supplied in the directions indicated by the arrows 83b, 84a. If the amplitude of the progressive wave 81a is set to a value larger than the amplitude of the progressive wave 82b, the object 40 moves in the direction indicated by the arrow 83b along the longitudinal lapping board while rotating on its axis.

[0093] On the other hand, if progressive waves of elliptical vibration advancing in the same direction (same as the progressive wave 81a) are applied to both of the longitudinal lapping boards 81, 82, the object can be moved in the direction indicated by the arrow 83b along the longitudinal lapping boards in the same manner as described for the lapping machine of FIG. 6. If the amplitude of the progressive wave 81a is set to a value larger than the amplitude of the progressive wave 82b, the object 40 moves in the direction indicated by the arrow 83b along the longitudinal lapping board while rotating on its axis.

[0094] FIG. 11 illustrates a top view of a variation of the second lapping machine according to the invention. The lapping machine of FIG. 11 is the same as that of FIG. 10 except that an auxiliary longitudinal lapping board 83 is arranged between the lapping boards 81, 82. It is preferred to apply a single mode vibration to the auxiliary lapping board 83, so that the object 40 can be more evenly lapped.

[0095] If a progressive wave of elliptical vibration to the auxiliary lapping board 83 in the direction indicated by 81a, the object 40 moves in the direction 83b, while rotating on its axis. Thus, the lapping machine of FIG. 11 can rotate the object 40 on its axis, while moving the object 40 in an optionally determined direction.

[0096] FIG. 12 is a schematic view of the third lapping machine according to the invention. The lapping machine of FIG. 12 comprises two coaxially arranged-circular lapping boards 91, 92, each of which is equipped with a circular ultrasonic transducer 95, 96 capable of generating an elliptical vibration. The circular piezoelectric transducers 95, 96 are fixed on the base board 90 via a supporting pad of elastic material.

[0097] The lapping machine of FIG. 12 further comprises an object-guiding means composed of a object-holding means 33a, a bearing 33b, and an arm 44c. The object to be lapped is attached to the object-holding means on the underside. The arm 33 is connected to a motor 97 which moves the object-holding means along the circular lapping boards 91, 92.

[0098] The third lapping method of the invention is described below, with reference to the lapping machine of FIG. 12. According to the third lapping method, the object is placed on the circular lapping boards 91, 92 via abrasive grains, and lapped while rotating on its axis by activating the piezoelectric transducers to apply elliptical vibration to the lapping boards.

[0099] In the lapping machine of FIG. 12, the piezoelectric transducers 95, 96 are activated to apply a progressive wave of elliptical vibration to each of the circular lapping boards 91, 92. The arrows 91a, 92a indicate the direction of progressive waves of elliptical vibration travelling on the circular lapping boards. The object then receives moments indicated by the arrows 93b, 94b by the travelling of the progressive waves. Apparently, the contact surface area between the object and the lapping board 92 is larger than the contact surface area between the object and the lapping board 91. Accordingly, the object receives a moment from the progressive wave travelling on the outer lapping board 92 which is larger than a moment from the progressive wave travelling on the inner lapping board 91. Thus, the object rotates on its axis in the direction indicated by the arrow 34 and is lapped on both of the inner and outer lapping boards 91, 92. It is preferred that the object to be lapped is moved along the circular lapping boards 91, 92 by means of the object-guiding means.

[0100] The object-guiding means of FIG. 12 can be replaced with a set of coaxially arranged circular magnets, which can function in the same manner as the magnetic bars arranged in parallel in FIG. 6.

[0101] The mechanism for applying the progressive wave of elliptical vibration to the circular lapping boards is described below.

[0102] Each of the circular piezoelectric transducers 95, 96 attached respectively to the circular lapping boards 91, 92 comprises a circular piezoelectric ceramic element and a pair of electrode each of which is attached to each surface of the piezoelectric ceramic element. The electrode attached on one surface of the electrode is divided into two electrode areas which are separated from each other. Thus, each circular piezoelectric transducer can generate ultrasonic vibrations from the two areas along the circular lapping board by applying an alternating voltage between each of the two separated electrodes and the electrode attached to other side. By delaying generation of ultrasonic vibration in one area from generation of ultrasonic vibration in another area, for instance, by 90°, a progressive wave of elliptical vibration is generated and travels along the circular lapping board.

[0103] The circular lapping board preferably has a length not less than twice, more preferably not less than three timer, the width of the board, so that the progressive wave of elliptical vibration is smoothly transmitted in the circular direction.

[0104] The mechanism for applying the progressive wave of elliptical vibration to the circular board is the same as that for applying a progressive wave of elliptical vibration to a stator of a surface wave type ultrasonic rotary motor. In more detail, the circular lapping board 51 corresponds to a stator of a surface wave type ultrasonic rotary motor, while the object to be lapped corresponds to a slider of the surface wave type ultrasonic rotary motor. More details for applying a progressive wave of elliptical vibration to a circular board are described in “Piezoelectric/Electrostrictive Actuators” (Morikita Publishing, 1996, pages 187-192).

[0105] FIG. 13 is a partially cutaway view indicating an assembly of an example of the third lapping machine according to the invention. The lapping machine of FIG. 13 comprises two coaxially arranged-circular lapping boards 91, 92, each of which is equipped with a circular ultrasonic transducer capable of generating an elliptical vibration. The lapping machined of FIG. 13 further comprises two coaxially arranged-circular lapping boards 101, 102, each of which is equipped with a circular ultrasonic transducer capable of generating an elliptical vibration on the upper side.

[0106] Between a set of the circular lapping boards 91, 92 and a set of the circular lapping boards 101, 102, three objects 40 to be lapped are provided. The three objects 40 are pressed between the lower lapping boards and the upper lapping boards by means of a pressing means such as an air cylinder 108 which is attached to a support disc 107 to which the lapping boards 101, 102 are fixed.

[0107] Each of the three objects 40 is supported by three rollers 103 which enable the object to rotate on its axis. The rollers 103 are fixed to a metal disc 104 which is connected rotatably to a rotating axis 105 of a motor (not shown). Each of the three objects 40 rotates not only on its axis but also moves along the circular lapping boards by activating the motor.

[0108] Each of the three objects 40 in FIG. 13 receives moments from the progressive wave of elliptical vibrations travelling on the circular lapping boards 91, 92 and further from the progressive wave of elliptical vibrations travelling on the circular lapping boards 101, 102, and is lapped on both of the upper surface and lower surface.

[0109] In performing the lapping method of the invention, not only the progressive wave of elliptical vibration but also a standing wave of elliptical vibration can be utilized for the desired lapping. For instance, the standing wave of elliptical vibration can be applied to the longitudinal or circular lapping board in the longitudinal or circular direction. The mechanism for applying the standing wave of elliptical vibration to the longitudinal lapping board is described below.

[0110] In FIG. 14, the longitudinal lapping board 131 is equipped a pair of piezoelectric transducers 132a, 132b which generate a depth sliding mode vibration and a piezoelectric transducer 133 which generates a vertical mode vibration on the lower surface. The piezoelectric transducer 133 is supported by a longitudinal board 134 which is fixed to the tops of supporting poles 135 standing on the base board (not shown).

[0111] Each of the arrows 136a, 136b, 137 shown in FIG. 14 indicates a polarizing direction of the piezoelectric ceramic element attached to each piezoelectric transducer. On either surface of the piezoelectric ceramic element, an electrode (not shown) for applying an alternating voltage is placed.

[0112] When the piezoelectric transducers 132a, 132b, 133 are caused to generate ultrasonic vibrations by applying an alternating voltage, the lapping board 131 is given simultaneously a depth sliding mode vibration and a vertical mode vibration. Then, a standing wave of elliptical vibration comprising a vibration in the depth direction and a vibration in the longitudinal direction is produced on the lapping board 131.

[0113] When the time phase of the sliding mode vibration is in advance of the time phase of the vertical mode vibration by 90°, a standing wave of elliptical vibration rotating along the arrow 138 is given to the lapping board 131. Accordingly, if an object to be lapped is placed on the lapping board 131 via abrasive grains, the object is moved in the direction indicated by the arrow 139 by a moment caused by the standing wave of elliptical vibration rotating in the direction of arrow 138.

[0114] When the time phase of the sliding mode vibration is behind the time phase of the vertical mode vibration by 90°, a standing wave of elliptical vibration rotating reversely to the arrow 138 is given to the lapping board 131. Accordingly, if an object to be lapped is placed on the lapping board 131 via abrasive grains, the object is moved in the direction reverse to the arrow 139 by a moment caused by the standing wave of elliptical vibration rotating reversely to the direction of arrow 138.

[0115] The combination of vibration modes of the piezoelectric transducers attached to the lower side of the longitudinal lapping board 131 is not limited to the combination of the sliding mode vibration and the vertical mode vibration. For instance, any combination of a vibration mode having a component vibrating in the depth direction of the lapping board and a vibration having a component vibrating in the longitudinal direction of the lapping board, such as a combination of a flexural mode vibration and a vertical mode vibration can be utilized.

[0116] The mechanism for applying the standing wave of elliptical vibration to the longitudinal board is the same as that for applying a standing wave of elliptical vibration to a stator of a complex transducer type ultrasonic linear motor. In more detail, the longitudinal lapping board 131 corresponds to a stator of a complex transducer type ultrasonic linear motor, while the object to be lapped corresponds to a slider of the complex transducer type ultrasonic linear motor. More details for applying a standing wave of elliptical vibration to a longitudinal board are described in “Ultrasonic Engineering” (Corona Co., Ltd., 1999, page 287).

[0117] Similarly, a standing wave of elliptical vibration can be applied to a circular lapping board by placing a combination of a piezoelectric transducer generating a sliding mode vibration and a piezoelectric transducer generating a vertical mode vibration on the lower surface of the circular lapping board.

[0118] The mechanism for applying the standing wave of elliptical vibration to the circular board is the same as that for applying a standing wave of elliptical vibration to a stator of a complex transducer type ultrasonic rotary motor. In more detail, the circular lapping board corresponds to a stator of a complex transducer type ultrasonic rotary motor, while the object to be lapped corresponds to a slider of the complex transducer type ultrasonic rotary motor. More details for applying a standing wave of elliptical vibration to a circular board are described in “Ultrasonic Engineering” (Corona Co., Ltd., 1999, pages 291-292).

[0119] There are no limitations of number of the longitudinal or circular lapping boards placed in one lapping machine of the invention. Thus, one, two or more lapping boards can be placed in one lapping machine.

Claims

1. A method for lapping an object which comprises preparing a lapping machine comprising at least one longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board and a supporting means arranged adjacently to the lapping board; placing the object on the lapping board via abrasive grains and further on the supporting means; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping board whereby the object is caused to rotate on an axis thereof and lapped.

2. The method of claim 1, wherein the rotation of the object is guided by a rotation-guiding means attached to the lapping machine.

3. The method of claim 1, wherein the supporting means is a longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies single-mode vibration to the lapping board and the object is placed on the supporting means via abrasive grains.

4. The method of claim 1, wherein the elliptical vibration is a vibration of progressive wave travelling along the longitudinal or circular lapping board.

5. The method of claim 1, wherein the elliptical vibration is a vibration of standing wave occurring along the longitudinal or circular lapping board.

6. A method for lapping an object which comprises preparing a lapping machine comprising a pair of longitudinal lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged in parallel; placing the object on the lapping boards via abrasive grains; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping boards whereby the object is caused to rotate on an axis thereof and lapped.

7. The method of claim 6, wherein the rotation of the object is guided by a rotation-guiding means attached to the lapping machine.

8. The method of claim 6, wherein an auxiliary lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical or single-mode vibration to the auxiliary lapping board is arranged between the pair of the lapping boards, and the object is further lapped on the auxiliary lapping board.

9. The method of claim 6, wherein the elliptical vibration is a vibration of progressive wave travelling along the longitudinal lapping board.

10. The method of claim 6, wherein the elliptical vibration is a vibration of standing wave occurring along the longitudinal lapping board.

11. A method for lapping an object which comprises preparing a lapping machine comprising a pair of circular lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged coaxially; placing the object on the lapping boards via abrasive grains; and activating the ultrasonic oscillation-applying device to apply elliptical vibration to the lapping boards whereby the object is caused to rotate on an axis thereof and lapped.

12. The method of claim 11, wherein the object is further moved along the circular lapping boards.

13. The method of claim 11, wherein the elliptical vibration is a vibration of progressive wave travelling along the circular lapping board.

14. The method of claim 11, wherein the elliptical vibration is a vibration of standing wave occurring along the circular lapping board.

15. A lapping machine comprising at least one longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board and a supporting means arranged adjacently to the lapping board.

16. The lapping machine of claim 15 which further comprises means for guiding an object to be lapped.

17. The lapping machine of claim 15, wherein the supporting means is a longitudinal or circular lapping board equipped with a ultrasonic oscillation-applying device which applies single-mode vibration to the lapping board.

18. A lapping machine comprising a pair of longitudinal lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged in parallel.

19. The lapping machine of claim 18 which further comprises means for guiding an object to be lapped.

20. The lapping machine of claim 18 which further comprises an auxiliary lapping board equipped with a ultrasonic oscillation-applying device which applies elliptical or single-mode vibration to the auxiliary lapping board arranged between the pair of the lapping boards.

21. A lapping machine comprising a pair of circular lapping boards equipped with a ultrasonic oscillation-applying device which applies elliptical vibration to the lapping board arranged coaxially.

22. The lapping machine of claim 21 which further comprises means for guiding an object to be lapped.