Strainless precision after-treatment process by radical reaction

Abstract: The invention relates to a precision process, wherein a radical reaction of a radical produced from a gas on discharge or laser beam excitation in a gaseous atmosphere with an atom or molecule con­ stituting a workpiece is utilized, a compound pro­ duced according to the reaction is vaporized and removed, and thus a workpiece such as silicon single crystal, ceramics material or the like can be smoothed, cut, bored, grooved and so forth without leaving a residual defect such as residual crack, thermally affected layer or the like on the surface.

Description: This invention relates to a strainless precision process on radical (free radical) reaction and, more particularly, to a precision process ensuring strainless cutting, boring and grinding of brittle materials hard to work on such as silicon and germanium single crystal for manufacturing semiconductors, gallium-arsenic compound, various ceramics materials or the like. A prior art precision process, or cutting for example, of brittle materials hard to work on such as silicon and germanium single crystal for manufacturing semiconductors, gallium-arsenic compound, various ceramics materials or the like depends only on dicing on a diamond wheel, and since its working principle comprises a brittle fracture by fine cracking, a serviceable surface will not be secured due to residual cracks unless the cut face is removed about 100 µm in thickness on the average. Further, the brittle fracture due to cracks is rather probable and a considerably large crack may remain according to circumstances, which is capable of deteriorating the reliability of an obtained surface. Thus, the cut face of a workpiece must be removed at about 100 µm in thickness, for which lapping is employed, however, since its working principle also comprises a brittle fracture, a residual affected layer is rather deep while not so heavy as compared with the aforementioned dicing, and thus an after-treatment such as etching, polishing or the like is further required. Then, lapping is also carried out for grinding the surface of a workpiece, however, the lapping is realized by transferring the flatness of a lapping tool to the workpiece, and since it depends in this case on a brittle fracture likewise according to a conventional process using a grade, both surface roughness and surface property are not to withstand the final purpose. Further, the prior art process is that of utilizing a thermal fusion phenomenon on laser for boring, grooving and so forth, therefore a thermal affected layer or a residual crack arsing on thermal stress is present on a finished surface. Thus in the prior art process, not only a residual crack and a thermal affected layer are present on the finished surface, but also a finished portion is removed so much, therefore yield of a workpiece is high, and further a production process increases in number, thus preventing a cut in cost of semiconductors and others using expensive silicon and germanium single crystal, or gallium-arsenic compound. US-A-4,668,366 relates to a method for figuring a surface plasma assisted chemical transport. In this method, a plasma of a reactive gas is produced between a first electrode and a workpiece being contacted at the rear side of a second electrode wherein the area of the second electrode is smaller than the area of the workpiece surface to be etched. In this manner, by varying the time of contact and the location of the second electrode a figuration (etching) of the workpiece surface is performed. US-A-4,624,736 discloses a process for etching a substrate surface by generating radical species in a plasma as a high frequency discharge which diffuses to said surface and by irradiating a part of said surface with a beam of light. Said part is etched by simulated radical reaction with atoms or molecules of the substrate. US-A-4,297,162 relates to a method of plasma etching by a reactive gas, wherein an electrode is arranged opposite to a substrate surface and has, for instance, a convex shape in order to achieve a uniform etching of a substrate also in its central area. On the electrode, a high frequency voltage is impressed. US-A-4,461,954 relates to an ion-processing method for material removal of a substrate by energetic ions or an additional chemical reaction of a gas forming the ions. In this process, accelerated ions fed from outside are directed to the substrate in a tubular member, wherein the substrate surface is processed. The tubular member and the substrate can be shifted relative to each other. US-A-2,902,584 relates to a method of detaching material by electric erosion. A tool electrode of variable form adapted to the processing mode is used and the electrode and the workpiece are shifted relative to each other. It is preferred to detach material by means of spark erosion. However, it is also suggested to use said electrode in combination with ions or electrons. US-A-4,387,287 discloses a method of shaping a poly-crystalline synthetic diamond by electric spark erosion. In this method, an electrode is provided which comprises a shaped surface according to a desired shape of the diamond. US-A-3,046,206 relates to a method of electrochemical removal of metal. In this method, a gear is provided as one electrode and a lap element is provided as the other electrode, wherein an electrolyte is applied between the contact areas of the gear and the lap element, and the gear is lapped during relative movement thereof by electrolytic material removal. In view of the situation mentioned above, an object of the invention is to provide a strainless precision process on radical reaction, wherein defects such as residual crack, thermal affected layer and the like which are capable of deteriorating a reliability of products are not left on the surface of a workpiece, still further lapping, or a work for removing the workpiece surface at about 100 µm or over in thickness is not required as an after-treatment unlike the prior art process, thus not only the post-process is considerably simplified, but also the workpiece can be utilized effectively. In order to attain the aforementioned object, the invention provides a strainless precision process on radical reaction according to claim 1. Other object, features and advantages of this invention will become apparent from the following description, the appended claims and the accompanying drawings. Fig. 1 is a sectional view illustrating a principle of a strainless precision process on radical reaction according to the invention. Fig. 2 is a simplified sectional view showing a cutting on discharge of a wire electrode. Fig. 3 is a simplified sectional view showing a smoothing on discharge of a lapped electrode. Fig. 4 is a simplified sectional view showing a cutting on excitation of radiated laser beams not according to the invention. Fig. 5 is a simplified sectional view showing a smoothing on excitation of laser beams radiated evenly not according to the invention. Fig. 6 is a simplified perspective view showing cutting and grooving by means of a wire electrode as in the case of Fig. 2. Fig. 7 is a simplified perspective view showing a boring by means of a needle electrode. Fig. 8 is a simplified perspective view showing a transfer for transferring sectional form to a workpiece by means of a prismatic electrode. Fig. 9 is a simplified side view showing a smoothing of the outer peripheral surface of a columnar workpiece by means of a tool-like electrode. Fig. 10 is a simplified side view showing a smoothing for shaping the outer peripheral surface of a workpiece symmetrically in rotation by means of a tool-like electrode. Fig. 11 is a simplified side view showing a threading of a cylindrical workpiece by means of a sawtooth electrode. Fig. 12 is a simplified perspective view showing a smoothing of the inner peripheral surface of a cylindrical workpiece by means of a tool-like electrode. Fig. 13 is a simplified perspective view showing a smoothing for shaping the inner peripheral surface of a cylindrical workpiece symmetrically in rotation arbitrarily by means of a tool-like electrode. Fig. 14 is a simplified side view showing a gear cutting by means of a gear-toothed electrode. Fig. 15 is a simplified perspective view showing a transfer by means of a ring electrode as in the case of Fig. 8. Fig. 16 is a simplified perspective view showing a reshaping by means of a correction electrode. Fig. 17 is a simplified perspective view showing an arbitrary shaping by means of a rod electrode with a hemispherical surface on the nose. The invention comprises carrying out a strainless precision process through a radical reaction (free radical reaction) of a reactive radical (free radical) having an unpaired electron with atom or molecule constituting a workpiece. Here, as a method for generating a radical, it is conceivable that a plasma which can easily be produced by discharge at a degree of vacuum 1,3 mbar (1 Torr) or below [1,3.10 -3 to 1,3 mbar (10 -3 to 1 Torr)] will be utilized, however, since a density of the plasma thus produced is low and the working rate is slow, a high density plasma condition is generated for process by vibrating a charged corpuscle in a high frequency field under high pressure 1 bar. or over in the invention. More specifically, the strainless precision process is carried out such that a workpiece of a brittle material hard to work such as silicon and germanium single crystal for manufacturing semiconductors, gallium-arsenic compound, various ceramics materials or the like is disposed in an atmospheric gas such as chlorine (Cℓ 2 ), fluorine (F 2 ) or the like, a high frequency voltage is impressed between a workpiece of conductor and an electrode of material relatively high in reaction temperature with a chloride gas such as tungsten (W), platinum (Pt) or the like which is disposed a little apart from the workpiece, a high frequency voltage is impressed between the aforementioned electrode and another electrode disposed in the vicinity of the work piece of insulator, a discharge is caused near the surface of the workpiece to generate a radical reactive chemically from the atmospheric gas, and in case, for example, silicon is used as the workpiece and a chloride gas is used as the gas, a radical reaction is caused on the surface of a working portion of the silicon to generate a silicon chloride, and the silicon chloride is vaporized and removed at ordinary temperature or at 200 °C or below. The invention will now be described further in detail with reference to the accompanying drawings representing preferred embodiments thereof. Fig. 1 illustrates the principle of a process of the invention, wherein a reference numeral 1 denotes a workpiece, 2 denotes a gas molecule, 3 denotes a radical (free radical), 4 denotes a constituent atom or molecule of the workpiece 1, and 5 denotes a compound. The radical 3 is produced by subjecting the gas molecule 2 to a discharge near the surface of the workpiece 1 disposed in the atmospheric gas 2, the compound 5 capable of vaporizing at relatively low temperature is produced on reaction of the radical 3 with the constituent atom or molecule 4 of the workpiece 1, the compound 5 is vaporized by warming up the workpiece 1 to a set temperature relatively low or, for example, 200 °C or below on ordinary temperature or heater or the like (not indicated), thus removed from the working surface to form successively a space 6 of atom or molecule unit on the workpiece 1 to a macro recess, thus carrying out strainless cutting, boring, grooving and so forth. Similarly in a strainless smoothing, the radical reaction may also take place most probably at a small portion projecting from the surface of the workpiece 1, consequently the projecting portion is removed selectively to smoothing on atom or molecule level. Fig. 2 is a principle drawing of a device for strainless cutting by means of the radical 3 generated on a discharge of the workpiece 1, wherein a wire electrode 7 such as tungsten, platinum or the like is wound on feeding and take-up reels 8, 8 spaced apart from each other, the wire electrode 7 is laid apart a little from the workpiece 1 through a plurality of guide rollers 9, ... and in case the workpiece 1 is a conductor, a discharge is generated on a high frequency voltage impressed by a RF (high frequency) power (not indicated) between the workpiece 1 and the wire electrode 7 to activate the atmospheric gas 2 (not indicated), the space 6 is produced only in an area where the radical 3 is present as shown in Fig. 1 according to a radical reaction of the generated radical 3 with the constituent atom or molecule 4 of the workpiece 1, and further a distance between the wire electrode 7 and the workpiece 1 is changed relatively to cutting. Then, ir case the workpiece 1 is an insulator, a discharge is generated on a high frequency voltage impressed on the wire electrode 7 to a radical reaction likewise. Here, the wire electrode 7 normally uses a fine wire several µm to several ten µm in the precision process, therefore the reels 8 are rotated to have the wire wound on one side for feeding at all times, thereby preventing the wire electrode 7 from cutting due to a discharge at the identical portion. Further, the workpiece 1 and the wire electrode 7 are shifted relatively on computer control or other means available, and thus a radical reaction area is changed successively to cutting of the workpiece 1. Fig. 3 is a principle drawing of a device for smoothing the surface of the workpiece 1 by means of the radical 3 generated on discharge likewise, wherein a plane lapped electrode 11 is disposed in parallel and also at a small distance d over the plane workpiece 1 fixed on a surface plate 10, a high frequency voltage is impressed between the lapped electrode 11 and the workpiece 1 according to the workpiece 1 being a conductor or an insulator, a discharge is thus caused to generate the radical 3, thereby removing the constituent atom or molecule 4 of the workpiece 1. Here, the discharge is generated selectively on a point effect between the lapped electrode 11 and the workpiece 1 at a salient 12 present on the surface of the workpiece 1, a removal of the constituent atom or molecule 4 on the radical reaction goes on at the portion, thus smoothing the surface of the workpiece 1. Then, the distance d between the lapped electrode 11 and the workpiece 1 may be optimized for gap by gap sensor or the like, thereby enhancing a working efficiency, and further the lapped electrode 11 and the workpiece 1 can be shifted relatively so as to minimize a damage due to discharge of the lapped electrode 11 and also to work the surface as wide as possible. Further, a curved surface of the workpiece 1 can also be smoothed by means of the lapped electrode 11 with a form according to the curved surface. Still further, as shown in Fig. 4 and Fig. 5 which represent embodiments not according to the invention, the radical 3 is generated from exciting optically the atmospheric gas molecule 2 by laser beams 13 induced to a working portion of the workpiece 1, thereby realizing a strainless precision process. In Fig. 4, optical fibers 15 are buried in a disk wheel 14 radially and peripherally from the center, incident ports 16 of the laser beams 13 are formed intensively at the center, ejection ports 17, ... are formed outwardly on the peripheral edge portion, the laser beams 13 are thus ready for irradiating on the gas molecule 2 present near the surface of the workpiece 1 from a peripheral edge of the wheel 14 disposed orthogonally to the surface of the workpiece 1, and the compound 5 produced on a radical reaction of the radial 3 generated by the laser beams 13 with the constituent atom or molecule 4 of the workpiece 1 is removed accordingly, thereby realizing a strainless cutting or grooving. Then, as shown in Fig. 5, the incident ports 16 of the laser beams 13 are formed intensively on one side of a plane lap plate 18, the ejection ports 17, ... are formed in dispersion uniformly at predetermined intervals on the other side opposite to the workpiece 1, the incident ports 16 and the ejection ports 17, ... are coupled optically through the optical fibers 15, ... buried internally, the lap plate 18 is disposed a little apart from and in parallel with the workpiece 1, the laser beams 13 are irradiated to the gas molecule 2 near the surface of the workpiece 1, and thus the salient 12 present on the surface of the workpiece 1 is smoothed according to the radical reaction likewise. Here, an ejection angle of the laser beams 13 from the ejection ports 17 is large, and since a power density of the laser beams 13 deteriorates according as the distance from the lap plate 18 gets large, the constituent atom or molecule 4 at the salient 12 which is nearest to the lap plate 18 is removed, and thus the salient 12 is smoothed selectively as a result. Then, a continuous process may be realized likewise by shifting the wheel 14 or the lap plate 18 and the workpiece 1 relatively, and the wheel 14 is rotated to change a position for irradiating the laser beams 13 continuously, thereby obtaining an exceedingly uniform working plane. Then, a process on radical reaction for which an optical excitation of the laser beams 13 is utilized can be applied to various properties of the workpiece 1 irrespective of conductor or insulator. Further, for boring and grooving, a strainless precision process may be realized likewise according to a home position irradiation and scanning of focused laser beams. Examples of various processes from utilizing the aforementioned working principle by discharge will further be described, next. Fig. 6 represents a cutting method as in the case of Fig. 2, wherein the wire electrode 7 disposed in parallel with the plane workpiece 1 is delivered in the direction X, the wire electrode 7 and the workpiece 1 are shifted relatively in the direction Z to cut the workpiece 1, and further shifted in the direction Y to cut or groove a new portion. Then, the cut face is flattened according to a linearity of the laid wire electrode 7. Fig. 7 represents a method for boring the workpiece 1 by shifting needle electrode 19 kept orthogonal to the workpiece 1 with its tip disposed to come near thereto in the direction Z, and a method for grooving by shifting the needle electrode 19 or the workpiece 1 in the directions X and Y is further represented therein. Fig. 8 represents a working method or transferring method for forming a recess same in sectional form as a prismatic electrode 20 on the workpiece 1 by keeping the prismatic electrode 20 (triangular prism as indicated) having a predetermined sectional form orthogonal to the workpiece 1 and shifting it in the direction Z. Fig. 9 to Fig. 11 represent a method for working the outer peripheral surface from rotating the workpiece 1 with a symmetrical form in rotation. Fig. 9 represents a method wherein a tool-like electrode 21 having a plane is disposed near the outer periphery of the cylindrical workpiece 1 in parallel therewith, the workpiece 1 is rotated, the tool-like electrode 21 is shifted axially or in the direction Z and further shifted in the direction X to have the interval adjusted accordingly, thus smoothing the outer peripheral surface of the workpiece 1, Fig. 10 represents a method wherein the outer peripheral surface of the workpiece 1 is smoothed likewise to an arbitrary symmetrical form in rotation by means of the tool-like electrode 21 with the nose almost pointed, and further Fig. 11 represents a method wherein a saw-tooth electrode 23 having projections 22, ... arranged at regular intervals linearly is shifted axially at a velocity v in tune with a rotational angle speed ω of the workpiece 1, thereby threading the outer periphery of the workpiece 1. Fig. 12 and Fig. 13 represent a method for working the inner peripheral surface from rotating the hollow workpiece 1 with a symmetrical form in rotation. Fig. 12 represents a method wherein the tool-like electrode 21 having a plane is disposed near the inner peripheral surface of the hollow cylindrical workpiece 1 in parallel therewith, the workpiece 1 is rotated, the tool-like electrode 21 is shifted axially or in the direction Z and further shifted in the direction X to have the interval adjusted accordingly, thus smoothing the inner peripheral surface of the workpiece 1, and Fig. 13 represents a method wherein the inner peripheral surface of the workpiece 1 is smoothed likewise to an arbitrary symmetrical form in rotation by means of the tool-like electrode 21 with the nose almost pointed. Fig. 14 represents a method wherein a gear-toothed electrode 25 having projections 24, ... arranged at regular intervals on the outer periphery of a disk and the disk workpiece 1 are disposed with each outer periphery coming near and each shaft kept in the same direction, the gear-toothed electrode 25 is rotated in one direction at an angular velocity ω 1 , and the workpiece 1 is rotated reversely at an angular velocity ω 2 , thereby carrying out a gear-cutting on the outer periphery of the workpiece 1. Here, a circular pitch of the gear teeth formed on the workpiece 1 can be set properly by adjusting the angular velocities ω 1 and ω 2 . Fig. 15 represents a transfer similar to the working method shown in Fig. 8, wherein the cylindrical or prismatic workpiece 1 disposed within a ring electrode 27 with a proper shape of tongued-and-grooved face 26 necessary for working formed on the inner periphery is shifted axially, and thus a shape of the tongued-and-grooved face 26 is transferred axially on the outer peripheral surface of the workpiece 1, thereby transferring a sectional form of the ring electrode 27 to the workpiece 1. Fig. 16 represents a method for reshaping a working face entirely as working a predetermined margin only at each point on the working face of the workpiece 1 by means of a correction electrode 28 having shape and size capable of correcting a local shape of the workpiece 1 on the nose. Then, the process corresponds to a process such as end milling or the like in a conventional milling. Fig. 17 represents a method for working the workpiece 1 to a complicate shape by means of a rod electrode 29 with a hemispherical nose. The process corresponds to a conventional ball milling or the like. As described, the invention comprises removing and so working the workpiece 1 on an atom or molecule level by the radical 3 generated in the atmospheric gas 2 through discharge or laser beam excitation, and an optimum condition must be set according to a material and area to work of the workpiece 1. In case, for example, a working is carried out on discharge, where pressure of the atmospheric gas 2 is low, the working may proceed slowly in a large area as the discharge covers an extensive range, which is suitable to smoothing, but where the pressure is high, the working may proceed efficiently and locally as the discharge is limited and thus the radical 3 may be generated at high probability, which is suitable to cutting by the wire electrode 7 and also to boring by the needle electrode 19. Then, the atmospheric gas 2 optimized to a material of the workpiece 1 must be selected, and further it is necessary to heat the workpiece 1 to such temperature as will not cause a thermal denaturation according to temperature easy to cause a radical reaction of the generated radical 3 with the constituent atom or molecule 4 and also to a vaporization temperature of the compound 5 produced on the radical reaction. For example, if a silicon single crystal used best for semiconductor element material is selected as the workpiece 1, then chloride gas is employed normally as the atmospheric gas 2, a radical reaction is caused at relatively low temperature, a sublimable silicon chloride is produced, and the silicon chloride can be vaporized at ordinary temperature or at 200 °C or below. According to the strainless precision process on radical reaction of the invention described as above, a gas is activated selectively on discharge in the vicinity of a workpiece disposed in a gaseous atmosphere, a compound produced on a radical reaction with a constituent atom or molecule of the workpiece is vaporized, therefore a working contactless in principle may be effected, and hence a strainless process totally freee from residual defect and thermally affected layer such as crack and the like which cannot be avoided with the prior art process using a brittle fracture by fine cracks and thus deteriorate a reliability of the working face most probably may be realized, a reliability of products can sharply be enhanced, and since the process comprises removing the workpiece on a constituent atom or molecule level, an exceedingly fine portion can be processed in precision simply in principle, and further the workpiece will never be damaged, therefore an after-treatment whereby a cut face is removed by 100 µm or over in thickness hitherto is no more required, and thus not only the post-process is so simplified, but also materials can be utilized effectively. While the strainless precision process according to the invention is so efficient as described above, it is quite simple that a reactive radical is generated by activating selectively a gas in the vicinity of the working portion of a workpiece disposed in a gaseous atmosphere through discharge or laser beam excitation, a compound produced on a radical reaction of the radical with an atom or molecule constituting the workpiece is vaporized spontaneously or by heating not to denature the workpiece and then removed from the working portion, thereby realizing a precision process such as strainless cutting, smoothing, boring, grooving and so forth.

Claims: A strainless precision process on radical reaction, wherein an electrode with a predetermined form according to a processing mode is disposed a little apart to a workpiece (1) disposed in a gaseous atmosphere (2) of 1 bar or more, a high frequency voltage is impressed on the electrode, free radicals (3) are generated by activation of gas (2) in a high frequency electric field generated in the vicinity of the electrode, a compound (5) produced on a radical reaction between radicals (3) of the activated gas (2) and atoms or molecules (4) constituting said workpiece (1) is vaporized, thereby processing the workpiece (1) being shifted relatively to the electrode. The strainless precision process on radical reaction as defined in claim 1 wherein a wire electrode (7) is used as said electrode, thereby cutting the workpiece (1). The strainless precision process on radical reaction as defined in claim 1, wherein a lapped electrode (11) having a plane surface or curved surface is used as said electrode, thereby smoothing the workpiece (1). The strainless precision process on radical reaction as defined in claim 1, wherein a needle electrode (19) kept orthogonal to the workpiece (1) used as said electrode, thereby boring the workpiece (1). The strainless precision process on radical reaction as defined in claim 1, wherein a prismatic (20) or ring electrode (27) having a predetermined sectional form is used as said electrode, thereby transferring said form to the workpiece (1). The strainless precision process on radical reaction as defined in claim 1, wherein a tool-like electrode (21) is used as said electrode, the workpiece (1) is rotated, the tool-like electrode (21) is shifted axially, thereby smoothing the workpiece (1) symmetrically in form in rotation. The strainless precision process on radical reaction as defined in claim 1, wherein a saw-tooth electrode (23) with projections (22) arranged linearly thereon at regular intervals is used as said electrode, the workpiece (1) with a symmetrical form in rotation is rotated, the workpiece (1) and the electrode (23) are shifted axially relatively, thereby threading the workpiece (1). The strainless precision process on radical reaction as defined in claim 1, wherein a gear-toothed electrode (25) with projections (24) arranged on an outer periphery of the disk at regular intervals is used as said electrode, the disk workpiece (1) and the gear-toothed electrode (25) are rotated reversely each other, thereby subjecting the workpiece (1) to gearshaping. The strainless precision process on radical reaction as defined in claim 1, wherein a rod electrode(29) with the nose as hemispherical is used as said electrode, the workpiece (1) is processed to an arbitrary shape.

Inventors: MORI YUZO, YAMAUCHI KAZUTO, MORI, YUZO, YAMAUCHI, KAZUTO
Applicants: MORI YUZO, UHA MIKAKUTO PRECISION ENGINEE, MORI, YUZO, UHA MIKAKUTO PRECISION ENGINEERING RESEARCH INSTITUTE CO., LTD.
Application year: 1988
Main classification: C30B33
IPC codes: B28D5, C30B33