Fine working method of crystalline material

Abstract: A fine working method of a crystalline material comprises forming an ion injection region in a crystalline material by irradiating a focused ion beam on a crystalline material and subsequently removing a predetermined region of the ion injection region by applying a chemical etching treatment, the method having the steps of performing ion injection under the ion injecting conditions where no removal of the predetermined region occurs even if the crystalline material may be exposed to an etchant, and removing the predetermined region of the ion injection region by injecting an etchant into the ion injection region formed by ion injection under the ion injecting conditions.

Description: This invention relates to a fine working method of a crystalline material. Particularly, the present invention relates to a fine working method of a crystalline material which performs etching working by use of a focused ion beam as a part of the working steps. Related Background Art In the prior art, as the fine working method by use of etching of a crystalline material such as bulk or thin film single crystalline materials, etc., there have been generally employed the method in which a resist, etc. is coated on its worked surface, a desired resist pattern is formed by lithography and then the portion without such resist pattern is removed chemically or physically. However, in the above working method, for fine etching working with a line width of 1 micron or less, etching with high precision can be effected with difficulty, with resolution of the resist and expansion of the working region during etching being obstacles. Also, according to the method as described above, working of a crystalline material into a cross-sectional shape having a region of reverse tapered shape or hollow could be done with extremely difficulty for following reasons. In the prior art, when into a reverse tapered shape or a tunnel shape, anisotropic working has been practiced. Such anisotropic working utilizes the fact that etching speed is remarkably different depending on the crystal orientation. Accordingly, the crystal orientation of a crystalline material must be matched previously to a shape desired to be worked, and therefore this method could applied only to a single crystalline meterial with limited orientation. And, it could not be applied to with different crystal orientations within the plane such as a polycrystalline material. Further, when working in shape of a tunnel is to be performed, a bore must be opened at the side surface of sample and the bore must be excavated by etching. On the other hand, other than the fine working method according to lithography use of a resist, there is also the method by use of focused ion beam. For example, J. Vac. Sci. Tehnol. B, Vol. 6, No.3, p. 1014 - 1017, May/Jun 1988 "Fabrication of one-dimensional GaAs wires by focused ion beam implantation" shows fine working with a line width of 1 µm (micron) or less. Also, here is shown an example, in which etching is effected by use of HCI after injection of focused ion beam injection of Si ions onto a GaAs substrate. However, also nothing is shown here about application of a complicated working such as hollow shape onto a crystalline material. EP-A-0 148 448 discloses an etching method in which focused ion beams are implanted in a substrate, and a treatment method in which the etching rate varies with the ion concentration is subsequently applied thereto, whereby it is made possible to produce etching holes having various sectional patterns or even a tunnel-shaped etching in the substrate. It is also possible to produce patterns projected on the substrate. US-A-4 450 041 discloses a method of semiconductor production by transforming the structure of a material by ion and/or energy beams such that the regions impacted by these beams etch at a different rate than the non-perturbed regions. By the use of ion beams utilized to transform crystalline materials into amorphous ones such that the amorphous materials are selectively etchable. FR-A-2 334 199 discloses a method for shaping a crystalline material by removing portions thereof by injecting ions into the area to be removed and then removing said area by exposing it to an etchant. According to the invention there is provided a method of fine working material by forming an ion injection region in the material and removing the region by a chemical etchant, and characterised by focusing a beam of either Be or He ions on a surface of a crystalline workpiece and selecting the amount of ions injected and the acceleration energy thereof in accordance with the material being worked so as to change an inner region of the workpiece to an amorphous structure whilst leaving the outer surface substantially unchanged, forming first and second inlet regions at both ends of said region by focusing an ion beam on said regions and selecting the amount of ion projected and the acceleration energy thereof in accordance with the material being worked so that an amorphous region is formed in contact with said inner region in order that a chemical etchant can reach the inner region, and chemically etching away the inner region to provide an internal hollow. The present invention enables fine working method of a crystalline material to be achieved and meets the problems of the prior art. The present invention is intended to provide a fine working method which can work a crystalline material into a complicated shape having a cross-section of reverse tapered shape or hollow shape which has been worked with difficulty in the prior art. The invention may also provide a fine working method of a crystalline material capable of forming any desired three-dimensional structure by applying an etching treatment on the region where ion injection is effected overlappingly. In the accompanying drawings:- Figs. 1A and 1B are schematic sectional views showing an example when an etching treatment is applied after one line ion injection by use of a focused ion beam scanning device; Fig. 2 is a schematic constitutional illustration showing an example of the focused ion beam scanning device to be used for practising the present invention; Fig. 3 and Fig. 5 are illustrations showing respectively the etching depths when the ion injecting conditions are varied; Figs. 4A - 4D are a schematic sectional views of the crystal obtained when etching treatment is applied after the acceleration energy of injected ions is varied; Figs. 6A and 6B are schematic sectional views of an example when a tunnel-shaped etching groove is formed; Fig. 7 is a perspective view thereof; Figs. 8A and 8B are schematic sectional views showing an example when etching treatment is performed by effecting ion injection overlappingly. DESCRIPTION OF THE INVENTION The fine working method of a crystalline material of the present invention comprises forming an ion injection region in a crystalline material by irradiating a focused ion beam at a predetermined region of crystalline material and subsequently removing a predetermined region of said ion injection region by applying a chemical etching treatment, said method having the steps of: performing ion injection under the ion injecting conditions where no removal of said predetermined conditions occurs if the crystalline material may be exposed to an etchant, and removing the predetermined region of said ion injection region by injecting etchant into the ion injection region formed by ion injection under said ion injecting conditions. Also preferable is the fine working method of a crystalline material by forming an ion injection region in the crystalline material by irradiating a focused ion beam at a predetermined position of the crystalline material, and subsequently applying a chemical treatment to remove the predetermined region of said ion injection region, wherein ions are injected overlappingly by changing the ion injecting conditions during irradiation of said focused ion beam. As the result, it has become possible to increase the integration degree of electronic device, whereby a fine micromechanics device utilizing mechanical action or a quantum effect device having an extremely fine structure can be obtained. As the working of micromechanics device for which the present invention can used, for example, there may be included working of cantilever of accelerating sensor, working of pressure sensor utilizing internal and external pressure difference of hollow groove, etc. As the working of quantum effect device, for example, by use of an electroconductive crystalline material, an electroconductive fine wire structure can be obtained by applying the working of forming a hollow portion in said electroconductive crystalline material and the etching removal working of the portion adjacent to said hollow portion. And, by making the fine wire about 100 or less, it is applicable to a quantum effect device capable of observing the interference effect by the progressive electron wave effect or the fluctuation phenomenon. The present invention is described in detail by referring to the drawings. Fig. 2 is a schematic illustration showing schematically the focused ion beam scanning device in embodying the fine working method of a crystalline material of the present invention. The ion beam released by electrical field from an ion source 201 (e.g. Au-Si-Be liquid metal) is focused by an electronic condenser lens 202, and only necessary ion species are separated by an ExB mass separator 203. Then, the ion beam is again focused by an objective lens and the ion beam is deflected by computer control to be irradiated on the target 207. The target 207 is moved freely within the XY plane by a moving means having a stage 206 equipped at a freely movable staging device 205 to be set at a desired position. In Fig. 2, 208 is SEI (secondary electron imager) and 209 is Faraday Cup for measuring the current value of the ion beam. As the ion injecting conditions when the device of Fig. 2 is used, Be and He can be employed as the injected ions by changing the ion source 201. As the acceleration voltage of ions, 20 KV to 200 KV may be employed, and as the beam diameter of the ion beam, preferably 0.05 µm to 1 µm may be suitably employed. As the crystalline material which can be used as the target, in addition to Si crystal, Ge crystal, for example, any crystalline material such as yttriumaluminum-garnet (YAG), yttrium-iron-garnet (YIG), etc. epitaxially grown on a substrate can be used, and other than single crystal materials, polycrystalline materials are also available. This is because, in the present invention, the crystalline structure in the injection region is destroyed by injection of focused ion beam to become an amorphous structure and selective etching is effected by utilizing the difference in etching speed between the crystalline region and the amorphous region. Here, specific examples of the working conditions are to be described. By use of a target sample having yttrium-iron-garnet (YIG) epitaxially grown in liquid phase to a thickness of 4 µm on a gadlinium-gallium-garnet (GGG) (111) substrate, and by use of Au-Si-Be liquid metal as the ion source 201, Be ions are irradiated vertically on the sample surface with an acceleration voltage of 40 to 80 KV and a beam diameter of 0.1 µm. The thin layer YIG used here is a magnetic garnet which is useful for Bloch line memory, optical eye relator, etc. Etching can be performed in the case of the above conditions by, for example, dipping a sample injected with ions in phosphoric acid of room temperature. Specifically, it may be also dipped in, for example, 85 % phosphoric acid solution for about 5 hours, but the etching time can be shortened by dipping in phosphoric acid solution by giving ultrasonic by means of an ultrasonic cleaner. The etchant is related to the substrate material, injected ions, the ion injecting conditions, and may be suitably selected from those with great selectivity in etching between the ion injected region which has been changed to amorphous and the crystalline region injected with no ion. Figs. 1A and 1B show cross-section of an example of sample which is by dipping in a 85 % phosphoric acid solution in combination with ultrasonic treatment after one line irradiation of YIG layer 101 with an ion beam 102 of Be²⁺ at an injection amount of 2 x 10¹⁶ ions/cm² with accelerated energy of 160 keV focused to 0.1 µm. Fig. 1A shows the ion injected state, and Fig. 1B the state after etching. According to the examples in Figs. 1A and 1B, only the ion injected portion 103 is subjected to etching, and its cross-sectional shape 104 took the water droplet form by reflecting the concentration distribution within the YIG layer 101 of Be ions. The cross-sectional dimensions of the etching shape were 0.6 µm of depth and 0.5 µm of the maximum width. Similarly, for the sake of comparison when one line scanning was performed with Si²⁺ ion beam of an acceleration voltage of 120 keV at an injection amount of 5 x 10¹⁵ ions/cm², and etching was effected, the cross-sectional dimensions of the etching shape of an example were found to be 0.2 µm of depth and 0.2 µm of the maximum width. Fig. 3 shows a plot of Be ion injected amount as the abscissa versus etching depth as the ordinate. This drawing shows the change in etching depth when ion injection was effected by varying the amount of Be ions injected and the acceleration energy of ions. Here, etching was conducted under the same etching conditions as employed for obtaining Fig. 1, and etching depth was measured by use of a pointer type surface roughness meter. In Fig. 3, line (A), line (B) and line (C) show ion injections when the accleration energy is 40 keV, 80 keV and 160keV, respectively. In line (A), etching occurs at a Be injected amount of 2 x 10¹⁵ ions/cm² or more, and here the amount of Be injected amount 2 x 10¹⁵ ions/cm² is the threshold for causing etching to occur. In line (B), etching occurs at a Be injected amount of 1 x 10¹⁶ ions/cm² or more, and here this value is the threshold. Further, in line (C), 1 x 10¹⁶ ions/cm² is also the threshold. In the range of the ion injected amount from about 1 x 10¹⁵ to about 6 x 10¹⁵ ions/cm², as compared with line (B), line (C) when ion injection is effected at acceleration voltages of 80 keV and 160 keV, the etching depth of line (A) when ion injection is effected at a lower acceleration voltage of 40 keV is deeper. The reason is explained by use of Figs. 4A-4D. Figs. 4A and 4B show the injection states when Be ions are injected at an injected amount of 2 x 10¹⁵ ions/cm². Fig. 4A shows the case of acceleration voltage of 40 keV and Fig. 4 (B) the case of acceleration voltage of 160 keV, respectively. In Fig. 4A, the ion beam 402a accelerated at 40 keV is low in acceleration energy. For this reason, the ion injected portion 403a is formed in the vicinity of the surface of the YIG layer 401. In contrast, in Fig. 4B, the ion beam 402b accelerated at 160 keV is high in acceleration energy. For this reason, the ion injected portion 403b is formed internally of the YIG layer 401. And, when etching is effected, the crystalline structure of the YIG layer 401 surface is destroyed and the ion injected portion 403a converted to amorphous structure is removed. Accordingly, the etching groove 404 shown in Fig. 4C is formed. In contrast, the ion injected portion 403b injected with ions at high acceleration energy is formed internally of the YIG layer 401. For this reason, it remains as the ion injected portion 403b shown in Fig. 4 D without being'removed. By referring here again to Fig. 3, if the ion injected amount exceeds 6 x 10¹⁵ ions/cm², then the etching depth of the line (C) where ion injection is effected at the acceleration voltage energy of 160 keV becomes deeper than the line (A) when ion injection is effected at the acceleration voltage of 40 keV. This is because the 160 keV ion injected portion 403b has become more dense by increase of ion injected amount, whereby said injected portion 403b is expanded to the surface of the YIG layer 401. Next, also for the sake of comparison Fig. 5 is to be explained. Fig. 5 is obtained by replacing Be ions in Fig. 3 with Si ions, with Si ion injected amount being plotted on the abscissa versus etching depth on ordinate. Here, line (A) and line (B) show respectively the cases when ion injection is effected at acceleration voltages of 120 keV and 160 keV. Also, in this Figure, it is shown that threshold exists for whether etching is possible or not. And, similarly as in Fig. 3, the etching depth is reversed (line (A), line (b)) according to the difference in acceleration energy depending on the ion injected amount. This is because of the same reason as explained for Fig. 3. Figs. 6A and 6B are schematic illustrations of the steps (sectional views) when applying working of a hollow shape in a crystalline material. The steps are shown below. First, as shown in Fig. 3 and Fig. 5, an ion beam 602 is irradiated in YIG layer 601 under appropriate ion injecting conditions at a value lower than the threshold where etching occurs to form an ion injected portion 603. Then, etching treatment is performed by use of phosphoric acid. In this way, a hollow etching region 604 is obtained. In this case, an inlet for etchant to permit the etchant to reach the ion injecting portion 603 is required. This is to be explained by use of Fig. 7. At the region 703b where a hollow etching region is desired to be formed, an ion beam 702 is irradiated. And, for example, at both ends of said region 703b, injection inlets 703a, 703c for etchant are formed. Said injection inlets 703a and 703c are formed by removing the crystalline surface by etching. Accordingly, prior to performing etching, ion injection is effected at the regions where injection inlets 703a and 703c are to be formed under the ion injecting conditions of the threshold where etching occurs as shown in Fig. 3 and Fig. 5. The ion injecting conditions of the threshold or higher are to effect ion injection at an ion injected at an amount of the ion injected amount in the region 703b, for example at an injected amount of 5 x 10¹⁶ ions/cm². Subsequently, by dipping the YIG layer 701 after ion injection into an etchant, the region injected with ions under the ion injecting conditions of the threshold or higher is removed to form the injection inlets 703a and 703c for the etchant. Then, by further continuing the etching treatment, the etchant injected through the injection inlets 703a, 703c reaches the region 703b, whereby the region 703b is removed to form a hollow etching region 604. Here, the etchants used for formation of the injection inlets 703a and 703c for the etchant may be either the same or different in kind. When hollow etching working is applied as shown in Fig. 6, ion species of light mass such as Be, He are used. This is because ions with greater mass have strong power to destroy crystalline structure, thus tending to destroy also the crystalline structure on the surface side of the crystal surfaces. Here, the concentration distribution of the ions injected into the target which is the material to be treated in the depth direction generally indicates a Gaussian distribution in the depth direction within the target. In this way, a region where the ion concentration becomes the maximum is formed internally in the target rather than on the target surface. Also, as can be seen from comparison between Fig. 3 and Fig. 5, the etching depth depends on the ion species (inversely proportional to mass of ions). Further, the etching depth is proportional to the acceleration energy of ions and the injected amount of ions. Next, another example of the present invention is shown. Figs. 8A and 8B show examples wherein the present invention is practiced as one approach for obtaining a rectangular etching cross-section. On the YIG layer 801, focused ion beams 802 to 160 keV Be²⁺, 80 keV Be²⁺ and 40 keV Be²⁺ were scanned at one line on the same position respectively at the injected amounts of 2 x 10¹⁶, 1 x 10¹⁸ and 5 x 10¹⁵ ions/cm², followed by chemical etching by use of phosphoric acid to form a rectangular groove 804. Thus, by effecting chemical etching after injecting ions with different acceleration voltages on the same position or overlappingly on different positions, any desired three-dimensional etching shape can be obtained. As described above, even when a crystalline material may be exposed to an etchant, by performing the fine working of a crystalline material having the ion injecting step of effecting ion injection under the ion injecting conditions where a predetermined region of the ion injection region in the crystalline material is not removed, and the step of removing the predetermined region of said ion injection region by injecting an etchant into the ion injection region formed by ion injection under the above ion injecting conditions, a crystalline material can be worked to a complicated shape having a cross-sectional shape of a reverse tapered shape or hollow region which has been worked with difficulty in the prior art. Also, by applying the etching treatment overlappingly on the region injected with ions, any desired three-dimensional structure can be formed. AS the result, an excellent electronic device, micromechanics device or a quantum effect device having ultra-fine structure can be provided.

Claims: A method of fine working material by forming an ion injection region in the material and removing the region by a chemical etchant, and characterised by focussing a beam of either Be or He ions on a surface of a crystalline workpiece and selecting the amount of ions injected and the acceleration energy thereof in accordance with the material being worked so as to change an inner region (703b) of the workpiece to an amorphous structure whilst leaving the outer surface substantially unchanged, forming first and second inlet regions (703a, 703c) at both ends of said region (703b) by focussing an ion beam on said regions and selecting the amount of ion injected and the acceleration energy thereof in accordance with the material being worked so that an amorphous region is formed in contact with said inner region (703b) in order that a chemical etchant can reach the inner region, and chemically etching away the inner region to provide an internal hollow. A method according to claim 1, characterised in that the or each inlet region is formed by injecting the region with a corresponding concentration of ions per unit area to that applied in the inner region (703b). A method according to any one of claims 1 or 2, characterised in that the same etchant is used to remove the inner region and the or each inlet region.

Inventors: OKUNUKI MASAHIKO, ONO HARUHITO, TAKEDA TOSHIHIKO, TSUKAMOTO TAKEO, WATANABE NOBUO, OKUNUKI, MASAHIKO, ONO, HARUHITO, TAKEDA, TOSHIHIKO, TSUKAMOTO, TAKEO, WATANABE, NOBUO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: C23F1
IPC codes: H01L21, C23F1