US20060027290A1 - Microstructure and manufacturing process thereof - Google Patents

Abstract

It is an object of the present invention to attain a microstructure having a miniature continuous structure which has high throughput and has been processed with high accuracy. To achieve this, provided is a microstructure having a column-shaped structure and a slit-forming portion which extends in a side-face direction from a side face of the column-shaped structure, wherein the slit-forming portion has a plurality of slits aligned in parallel at an interval from 20 to 1,000 nm in a direction along a center axis of the column-shaped structure.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to a microstructure, and manufacturing process thereof, which is minute and has excellent molding precision. The present invention particularly relates to a process for manufacturing a microstructure which can be employed as an optical element.
• 2. Description of the Related Art
• In recent years, microminiaturization, increasing precision and increasing super-high-end performance for a variety of structures has been progressing across a wide-range of technical fields, wherein for example, a structure miniaturized to a dimension in the order of nanometers (hereinafter referred to as a “microstructure”) has been sought after. Microstructures have been manufactured using various processes in the past. Specific examples include the following.
• (1) Utilization of a self-organized structure
• (2) An optical shaping method using a laser or a light confocus
• (3) A method for fabricating a three-dimensional structure using an electron beam or ion beam.
• (4) Utilizing a semiconductor process
• (5) A nanoimprinting process
• Here, (1) a self-organized structure, as disclosed in Japanese Patent Laid-Open No. 2002-023356 is a method which places a molecule capable of self-organization at a specific site of an underlying layer, such as a substrate, to form a highly oriented compound onto the substrate in an oriented manner through interaction with a molecule having an associated functional group which can react with the molecule capable of self-organization. (2) the optical shaping method, as disclosed in Japanese Patent Laid-Open No. 1995-329188, is a method for manufacturing a microstructure by irradiating ultraviolet rays or similar laser beam onto a liquid photosetting resin to thereby form a thin film, and then successively laminating this thin film. (3) a method for fabricating a three-dimensional structure using an electron beam or ion beam, as disclosed in Japanese Patent Laid-Open No. 1989-261601, is a method for manufacturing a microstructure by irradiating an intensity-modulated electron beam onto a resist film coated onto a substrate. (4) a semiconductor process is a method for forming a structure by repeatedly carrying out the steps of forming a mask pattern by photolithography and removing an exposed portion by etching. (5) nanoimprinting is a method for transcribing a template pattern onto a substrate by pressing the substrate with a template having a nano-size pattern.
• However, in (1) a self-organized structure, the position of the portion which undergoes shape-processing and self-organization is restricted, thus making it difficult to attain a structure having a desired shape or position. For (2) optical shaping method, since light is employed for the resin curing, shape-processing of a structure in the order of nanometers is difficult. Furthermore, when performing complete curing by a full-cure step after molding of the photosetting resin, the entire structure shrinks from one to several percent, whereby molding of a structure with a high degree of precision is difficult. For (3) a method for fabricating a three-dimensional structure, the thickness that can be processed is restricted, whereby the degree of freedom for the shape in a thickness direction is small, and throughput is also small. For (4) a semiconductor process and (5) nanoimprinting process, since a three-dimensional structure is made by fabricating a planer structure and then building these planar structures up, a long time is required for structure fabrication. Furthermore, since these techniques undergo a number of steps, a high precision processing of structure is difficult.
• Meanwhile, at pages 304 to 306 of Micromachine/MEMS Technology Outlook, a Bosch process is disclosed. The Bosch process is a type of processing method for silicon substrates, which etches a silicon substrate layer in its thickness direction by alternating between etching with SF₆gas and forming a passivation film from C₄F₈gas, whereby a minute and continuous structure can be attained.
SUMMARY OF THE INVENTION
• The present invention was created with the above-described problems in mind, wherein it aims at obtaining a microstructure comprising a minute structure which has a high throughput and in which shape-processing is possible with high precision.
• The invention also aims at providing an optical element having excellent optical processing characteristics comprising a microstructure.
• To resolve the above-described problems, the present invention is characterized by having the following structure. That is, the present invention relates to a microstructure comprising a column-shaped structure and a slit-forming portion which extends in a side-face direction from a side face of the column-shaped structure, wherein the slit-forming portion has a plurality of slits aligned in parallel at intervals from 20 to 1,000 nm in a direction along a center axis of the column-shaped structure.
• The present invention also relates to a process for manufacturing a microstructure which comprises a column-shaped structure and a slit-forming portion which extends in a side-face direction from a side face of the column-shaped structure, wherein the slit-forming portion has a plurality of slits aligned in parallel in a direction along a center axis of the column-shaped structure, the process comprising the steps of:
• • (1) preparing a substrate which has a thickness greater than a height of the column-shaped structure;
  • (2) providing a mask extending in a prescribed direction of an upper face of the substrate which comprises a narrow-width portion in a direction which intersects with the extending direction for defining a portion to serve as the slit-forming portion and a broad-width portion in a direction which intersects with the extending direction for defining a portion to serve as the column-shaped portion;
  • (3) forming two facing grooves by carrying out isotropic etching on an upper face of the substrate by a reactive ion etching method using SF₆gas using the mask as a etching mask, and excavating in a thickness direction at least a portion of both sides opposing the extending direction of the mask of the upper face of the substrate;
  • (4) covering the upper face of the substrate forming the grooves with a passivation film formed by plasma reaction using C₄F₈gas;
  • (5) providing apertures for connecting between grooves which are faced sandwiching the narrow-width portion of the mask at least below the narrow-width portion of the mask, by carrying out isotropic etching on the upper face of the substrate covered with the passivation film by a reactive ion etching method using SF₆gas; and
  • (6) repeating the steps (3) to (5) for aligning in parallel the apertures in a thickness direction below the narrow-width portion of the mask, to thereby attain the microstructure as well as extending the grooves in a thickness direction of the substrate.
• According to the manufacturing process of the present invention, a microstructure can be attained which has a high throughput and in which the shape has been processed with high precision. Furthermore, according to the manufacturing process of the present invention, the shape, size and intervals, etc of the connecting portions and the aperture can be controlled easily. Therefore, a microstructure according to the present invention can be used in a wide variety of applications by utilizing such characteristic. The microstructure according to the present invention can be, in particular, used as an excellent optical element by utilizing its minuteness and the high precision of its processed shape.
• According to the manufacturing process of the present invention, a microstructure can be manufactured which is minute, has high throughput and which has a shape processed with high precision. Furthermore, since control of the manufacturing conditions is easy and the manufacturing steps are simple, a microstructure can be manufactured in short time using a simple apparatus. Furthermore, according to the manufacturing process of the present invention, the mask formed on the manufacturing substrate may comprise at least one or more width-varying portions, and the mask having a variety of shapes can be employed. Therefore, the manufacturing process of the present invention can have a high degree of design freedom.
• According to the manufacturing process of the present invention, by forming a plurality of width-varying portions in the mask, a structure of a slit-forming portion can be easily controlled. In addition, a microstructure can be attained wherein an intended characteristic varies depending on the position in the microstructure. According to the manufacturing process of the present invention, by forming a width-varying portion at both ends of the mask, a structure can be formed wherein a slit-forming portion is sandwiched between column-shaped structures, whereby the slit-forming portion can be protected from damage during manufacture.
• Furthermore, if the microstructure according to the present invention is employed as an optical element, the desired characteristics which are required to be an optical element can be exhibited by utilizing the minuteness and high precision of the shape. In addition, a microstructure according to the present invention can exhibit even more excellent desired optical element characteristics by arranging the connecting portions and the apertures (slits) in equal intervals in an axial direction.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention;
• FIG. 2 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention;
• FIG. 3 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention;
• FIG. 4 is a schematic view illustrating one example of a microstructure according to the present invention;
• FIG. 5 is an electron microscope photograph illustrating one example of a microstructure according to the present invention;
• FIG. 6 is a schematic view illustrating one example of a microstructure according to the present invention;
• FIG. 7 is an electron microscope photograph illustrating one example of a microstructure according to the present invention;
• FIG. 8 is a schematic view illustrating one example of a microstructure according to the present invention;
• FIG. 9 is a schematic view illustrating one example of a microstructure according to the present invention;
• FIG. 10 is a schematic view illustrating one example of a microstructure according to the present invention;
• FIG. 11 is a view illustrating one example of a mask pattern used in the process for manufacturing a microstructure according to the present invention;
• FIG. 12 is a schematic view illustrating one example of a branching filter according to the present invention;
• FIG. 13 is a schematic view illustrating one example of a wire grid according to the present invention;
• FIG. 14 is a diagram illustrating a mask pattern used and a microstructure manufactured in examples;
• FIG. 15 is a view illustrating one example of a mask pattern used in the process for manufacturing a microstructure according to the present invention.
• FIG. 16 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention;
• FIG. 17 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention; and
• FIG. 18 is a schematic view illustrating one example of a process for manufacturing a microstructure according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• (Process for Manufacturing a Microstructure)
• A microstructure according to the present invention can be manufactured by alternating between isotropic etching and process fabricating a protective film on the entire etching surface. For example, when fabricating a microstructure by using a silicon substrate, a Bosch process (also called ASE: Advanced Silicon Etching) can be employed. A Bosch process is process silicon etching which alternates between etching using SF₆and fluorocarbon deposition using C₄F₈, thus enabling etching with high selectivity and a high aspect ratio to be realized.
• One example of a process for manufacturing a microstructure according to the present invention will now be described in detail with reference toFIG. 1. First, asubstrate11 having a thickness greater than the height of the column-shaped structures is prepared. As the substrate, a silicon substrate or an SOI substrate is used, and a thermal oxidation film SiO₂is formed on a surface of the substrate. Narrow-width portion33 is connected with broad-width portion32 via width-varyingportion31 in thismask12.
• Then, substrate exposedportion13 which is not covered by the mask is formed on the substrate by photolithography.FIG. 1 (a) and (b) are schematic views which illustrate this state, whereinFIG. 1 (a) is a view showing the substrate from an upper face in a thickness direction andFIG. 1 (b) is a cross-sectional view in the A-A′ direction ofFIG. 1 (a). (InFIG. 1, as one example, a mask is used having two width-varyingportions31.) Although themask12 is not especially restricted, a resist mask or a SiO₂film, for example, can be employed.
• After this, a Bosch process is carried out. That is, using the mask on the substrate as the etching mask, isotropic etching of the substrate by a reactive ion etching process using SF₆gas, deposition of a passivation film by a plasma reaction using C₄F₈gas and isotropic etching using SF₆gas in the same manner as the initial isotropic etching are carried out a number of times. This will now be explained in more detail.
• First, in the initial isotopic etching, at least a mask-side portion of exposedportion13 of the substrate (an aperture formed on a portion having both sides sandwiching the mask in direction intersecting withdirection14 along which the mask extends) is excavated in thickness direction of a substrate to form a pair of grooves16 (FIG. 1 (d)). Viewed along a cross-section parallel to the substrate, thesegroves16 are formed in a shape which stretches overoutline13 of the mask.FIG. 1 (c) to (e) are schematic views which illustrate this state, whereinFIG. 1 (c) is a view showing the substrate from an upper face in a thickness direction,FIG. 1 (d) is a cross-sectional view in the A-A′ direction ofFIG. 1 (c) andFIG. 1 (e) is a cross-sectional view in the B-B′ direction ofFIG. 1 (c).
• As the isotropic etching proceeds, the pair ofgrooves16 connect partially and abut onto a width-varying portion within the mask, wherebyaperture17 connecting (connecting indirection15 intersecting with extendingdirection14 of the mask) the opposing grooves and sandwiching the narrow-width portion33 below narrow-width portion33, is formed. The portion where aperture17 (slit) is formed is not limited to the portion below narrow-width portion33. Depending on the shape of the mask and etching conditions, the aperture may be formed in a form stretching from the lower part of narrow-width portion33 to the lower part of width-varyingportion31. Furthermore, since the isotropic etching finishes before the pair ofgrooves16 completely connect, connectingportion18 which is not removed by the isotropic etching is formed directly below narrow-width portion33.
• Next, afluorocarbon passivation film19 using C₄F₈is formed on the substrate by a CVD chemical vapor growth process.FIG. 2(a) to (c) are schematic views which illustrate this state, whereinFIG. 2(a) is a view showing the substrate from an upper face in a thickness direction,FIG. 2(b) is a cross-sectional view in the A-A′ direction ofFIG. 2(a) andFIG. 2(c) is a cross-sectional view in the B-B′ direction ofFIG. 2(a).
• Subsequently, in the same manner as the initial isotropic etching, isotropic etching is conducted by a reactive ion etching process.FIG. 3(a) to (c) are schematic views which illustrate this state, whereinFIG. 3(a) is a view showing the substrate from an upper face in a thickness direction,FIG. 3(b) is a cross-sectional view in the A-A′ direction ofFIG. 3(a) andFIG. 3(c) is a cross-sectional view in the B-B′ direction ofFIG. 3(a). During this etching, since a voltage bias is being applied,passivation film21 of the side wall of the initial groove (side etching portion) is not removed by the etching, wherein the passivation film in the horizontal direction of substrate is preferentially removed. Etching then further proceeds in a thickness direction lower portion of the substrate to formgrooves20. Thesegrooves20 connect partially belowaperture17, whereby aperture (slit)22 connecting the opposing grooves and sandwiching a narrow-width portion of the mask, is formed. Because the isotropic etching finishes beforegrooves20 completely connect, connectingportion23 which was not removed by the etching is formed in the space between initially formedaperture17 andaperture22.
• Next, the slit-forming portion can be fabricated by forming in parallel a plurality of slits in a thickness direction (a direction along a center axis of the column-shaped structure) of the substrate by carrying out isotropic etching and the passivation film deposition as illustrated in FIGS.1 to3. The number of times that such isotropic etching and passivation film deposition are carried out is preferably at least two times or more, but the number of times is not especially restricted.
• Furthermore, the other example of a process for manufacturing a microstructure according to the present invention will now be described in detail with reference to FIGS.16 to18. First,mask12 is formed on asubstrate11 in the same manner asFIG. 1. Then, substrate exposed portion which is not covered by the mask is formed on the substrate by photolithography. After this, a Bosch process is carried out. First, in the initial isotopic etching using SF₆gas, at least a mask-side portion of exposed portion of the substrate is excavated in thickness direction of a substrate (FIG. 16(c)).FIG. 16(a) to (c) are schematic views which illustrate this state, whereinFIG. 16(a) is a view showing the substrate from an upper face in a thickness direction,FIG. 16(b) is a cross-sectional view in the A-A′ direction ofFIG. 16(a) andFIG. 16(c) is a cross-sectional view in the B-B′ direction ofFIG. 16(a).
• Next, afluorocarbon passivation film19 using C₄F₈gas is formed on the substrate by a CVD chemical vapor growth process.FIG. 17(a) to (c) are schematic views which illustrate this state, whereinFIG. 17(a) is a view showing the substrate from an upper face in a thickness direction,FIG. 17(b) is a cross-sectional view in the A-A′ direction ofFIG. 17(a) andFIG. 17(c) is a cross-sectional view in the B-B′ direction ofFIG. 17(a).
• Subsequently, isotropic etching is conducted by a reactive ion etching process. As the isotropic etching proceeds, the pair ofgrooves16 connect partially and abut onto a width-varying portion within the mask, wherebyaperture17 connecting (connecting indirection15 intersecting with extendingdirection14 of the mask) the opposing grooves and sandwiching the narrow-width portion33 below the narrow-width portion33, is formed. FIGS.18(a) and (b) are schematic views which illustrate this state, whereinFIG. 18(a) corresponds to a cross-sectional view in the A-A′ direction of microstructure ofFIG. 17(a) andFIG. 18(b) corresponds to a cross-sectional view in the B-B′ direction of microstructure ofFIG. 17(a). During this etching, since a voltage bias is being applied, passivation film of the side wall of the groove (side etching portion) is not removed by the etching, wherein the passivation film in the horizontal direction of substrate is preferentially removed to thereby form grooves. These grooves connect partially, whereby aperture (slit) connecting the opposing grooves and sandwiching a narrow-width portion of the mask, is formed. The isotropic etching finishes before grooves completely connect.
• Thus, one aperture is formed by process of FIGS.16 to18. Next, the slit-forming portion can be fabricated by repeating process as illustrated in FIGS.16 to18.
• The conditions for each isotropic etching and passivation film deposition may be the same or different. If these conditions are the same, each of the connecting portions and the apertures (slits) have the same shape and size, thus enabling the intervals in thickness direction of the substrate to be formed with high precision at equal intervals. Therefore, by forming the aperture intervals with high precision at equal intervals, a microstructure having intended characteristics depending on the purpose can be manufactured. On the other hand, if the conditions for each isotropic etching and passivation film deposition are changed, each of the connecting portions and the apertures (slits) have a different shape and size, whereby these intervals in thickness direction of the substrate are also different.
• Next, once the isotropic etching is conducted to a desired depth in the substrate, etching is finished. This etching may be stopped before the grooves penetrate the substrate, or may be conducted until penetrating through the substrate. If etching is stopped before the grooves penetrate the substrate, a microstructure formed on the substrate can be attained, while if etching is conducted until the grooves penetrate through the substrate, only a microstructure can be attained. The substrate thickness is preferably from 5 to 100 μm, and more preferably from 10 to 50 μm.
• Subsequently, a microstructure according to the present invention is formed by removing the remainingmask12 andpassivation film19. This microstructure is illustrated inFIG. 4(a).FIG. 4(b) is a perspective view illustrating only the connecting portions of the microstructure of theFIG. 4(a).
• FIG. 4(c) is a cross-sectional view in the A-A′ direction of the connecting portions ofFIG. 4(b). The uppermost connecting portion59 (the connecting portion is uppermost in thickness direction of the substrate; corresponding to uppermost connectingportion44 inFIG. 4(a)) is composed offace53 andface54. Connecting portion60 (corresponding to the connecting portion second from the top among the connectingportions45 inFIG. 4(a)) is composed offace54 andface55. Connecting portion58 (corresponding to the connecting portion third from the top among the connectingportions45 inFIG. 4(a)) is composed offace56 andface57. While in the microstructure ofFIG. 4(a), two connecting portions are formed below the connectingportion58, the connecting portions formed below connectingportion58 of these connecting portions have the same shape as connectingportion58, and the connecting portion formed at the lowermost part has the reverse shape to the uppermost connectingportion59. Therefore, the two connecting portions formed below connectingportion58 have been omitted from FIGS.4(b) and (c).
• An electron microscope photograph of an actually fabricated microstructure is illustrated inFIG. 5.FIG. 5(a) is a photograph of the microstructure viewed from an oblique direction.FIG. 5(b) is enlarged view of slit-forming portions of the microstructure ofFIG. 5(a). From the electron microscope photograph ofFIG. 5, it can be understood that a periodic structure is formed with excellent precision.
• In the manufacturing process according to the present invention, the mask to be used is not restricted to the above-described masks. By changing the extending direction of mask, the shape and size of the width-varying portion, the position of the width-varying portion is formed in the mask, or the width of the narrow-width portion and the broad-width portion, or adjusting the etching conditions, the shape, size and interval of connecting portions and slits of the microstructure can be processed into a desired shape with excellent precision. In addition, a minute three-dimensional microstructure can be fabricated at a high throughput with good reproducibility.
• The mask can be formed so as to extend in a prescribed direction, and may comprise a single closed curve or have both ends open. The mask may also be a linear shape extending in a fixed direction, or a curved shape in which the extending direction changes. In addition, a plurality of linear masks, curved masks or combination of these masks linked together are also preferable. Still further, the mask may branch into a plurality of masks midway, or the plurality of masks branched out midway may be linked together. The branched mask and linked mask may each be closed or open, and may also be linear or curved.
• The width-varying portion is acceptable as long as its width varies. In the present specification, regardless of whether the width variation is continuous or discontinuous. If the width variation is continuous, viewed from extending direction of the mask, a portion where the width increases or decreases continuously is taken to be a width-varying portion. Thus, if there are a portion where the width increases continuously and a portion where the width decreases continuously, each of these width-varying portions independently compose the different width-varying portions. For example, when the width changes in a discontinuous step-like manner (e.g.,FIG. 1 (a)31), the width-varying portion is linear in the direction perpendicular to the extending direction of the mask, and without any length in the extending direction of the mask (e.g.FIG. 1 (a)14). The ratio by which the width of the width-varying portion varies is not especially restricted, and neither is such shape especially restricted. A side portion of a width-varying portion where the width continuously varies is constituted from a curve or a straight line. In such case, the curve may be concave or convex or an uneven shape. The width-varying portion may be constituted from a single straight line having different slopes, or from a plurality of straight lines. Furthermore, these shapes may be plural or a combined shape.
• The narrow-width portion and the broad-width portion are connected via a width-varying portion. That is, a portion having a narrow width is termed “narrow-width portion”, and a portion having a broad width is termed a “broad-width portion” of the mask formed on both sides sandwiching a width-varying portion. Thus, “narrow-width” and “broad-width” are relative terms, so that even the same portion of a mask can be termed narrow-width or broad-width. For example,FIG. 15 is a diagram viewed from an upper part of a mask formed on a substrate in a step-like manner. The mask inFIG. 15 has width-varying portions1501 to1503. Taking width-varyingportion1503 as a reference, masks1506 and1507 exist on either side of the portion. Becausemask1506 is the broader of the twomasks1506 and1507,mask1506 is the “broad-width portion” andmask1507 is the “narrow-width portion”. However, if width-varyingportion1502 is taken as the reference, masks1505 and1506 exist on either side of the portion. Becausemask1505 is the broader of the twomasks1505 and1506,mask1505 is the “broad-width portion” andmask1506 is the “narrow-width portion”. That is,mask1506 is a “broad-width portion” if it takes width-varyingportion1503 as a reference, and is a “narrow-width portion” if it takes width-varyingportion1502 as a reference. Thus, “narrow-width portion” or “broad-width portion” are depends on the width-varying portion taken as a reference, so that even the same portion in a mask can be a “narrow-width portion” or a “broad-width portion”. Furthermore, the “narrow-width portion” and “broad-width portion” may have no length in the extending direction (e.g.FIG. 1(a)14) and be a straight line in direction intersecting with extending direction of mask (e.g.FIG. 9(a)904,FIG. 10(a)1024), or may have a prescribed length in the extending direction of the mask.
• For example,FIG. 8(a) is a drawing which illustrates one example of a mask used in the present invention. InFIG. 8(a),reference numerals81 to84 denote width-varying portions, andreference numerals810,820,830 and840 denote narrow-width portions.Reference numerals815,825,835,845 and855 denote broad-width portions. When such mask is used, a microstructure having a structure as shown in FIGS.8(b) and (c) can be manufactured.FIG. 8(b) is a cross-sectional view in the A-A′ direction ofFIG. 8(a) andFIG. 8(c) is a perspective view illustrating only the connecting portions of the microstructure ofFIG. 8(a). Uppermost connectingportion811 and connectingportion812 are formed below narrow-width portion810, uppermost connectingportion821 and connectingportion822 are formed below narrow-width portion820, uppermost connectingportion831 and connectingportion832 are formed below narrow-width portion830 and uppermost connectingportion841 and connectingportion842 are formed below narrow-width portion840.
• As can be understood fromFIG. 8, if the width ratio of the narrow-width portion to the broad-width portion becomes increasing, the groove formed in the region including the mask aperture portion becomes bigger, and the height H and width W′ of the uppermost connecting portion and the connecting portions becomes smaller. If the width ratio of the narrow-width portion to the broad-width portion becomes decreasing, the groove formed in the region including the mask aperture portion becomes smaller and the height H and width W′ of the uppermost connecting portion and the connecting portions becomes larger. Furthermore, if the length L of the narrow-width portion becomes longer, the length L′ of the uppermost connecting portion and the connecting portions also becomes longer, and if the length L of the narrow-width portion becomes shorter, the length L′ of the uppermost connecting portion and the connecting portions also becomes shorter. (Thus, by adjusting the height H of the uppermost connecting portion and the connecting portions, the intervals between the parallel slits can also be adjusted.)
• A mask as illustrated inFIG. 9 can also be used.FIG. 9(a) is a drawing which illustrates one example of a mask used in the present invention.FIG. 9(b) is a cross-sectional view in the A-A′ direction of the microstructure ofFIG. 9(a) andFIG. 9(c) is a perspective view illustrating only the connecting portions of the microstructure ofFIG. 9(a). InFIG. 9(a),reference numerals91 and94 to96 denote width-varying portions. The sides of the width-varyingportions91,94 and96 are constituted from a straight line, and a side portion of width-varyingportion95 is partially constituted from a curve. Furthermore,reference numerals901 to904 denote narrow-width portions andreference numerals905 to909 denote broad-width portions. Uppermost connectingportion92 and connectingportions93 are mainly formed below width-varyingportion91, uppermost connectingportion97 and connectingportions98 are mainly formed below two width-varyingportions94, uppermost connectingportion99 and connectingportions100 are mainly formed below two width-varyingportions95 andstructure101 is mainly formed below two width-varyingportions96.
• When a microstructure is manufactured using the width-varyingportion91 fromFIG. 9, the height and width of connectingportions92 and93 are at a minimum at oneend901. Furthermore, when a microstructure is manufactured using width-varyingportions94 and95, the height and width of connectingportions97 to100 are at a minimum atcenters902 and903 in extendingdirection14. When a width-varyingportion96 is used, the height and width of connectingportion101 is at a minimum atcenter904 in extendingdirection14. (In the case ofreference numeral101 inFIG. 9(c), a portion of the column-shaped structure is also included in the drawing.)
• Thus, height H and width W′ of the connecting portions corresponding to the broad-width portions of the mask become larger, and the height H and width W′ of the connecting portions corresponding to the narrow-width portions of the mask become smaller. Furthermore, if the width W of the width-varying portions is dramatically varied, the portions corresponding to the connecting portions also dramatically vary.
• FIG. 10(a) is a drawing which illustrates the other example of a mask used in the present invention.Reference numerals1010 to1013 denote width-varying portions,reference numerals1021 to1024 denote narrow-width portions, andreference numerals1031 to1035 denote broad-width portions.FIG. 10(b) is a cross-sectional view in the A-A′ direction of the microstructure ofFIG. 10(a) andFIG. 10(c) is a perspective view illustrating only the connecting portions of the microstructure ofFIG. 10(a). The width-varying portions ofFIG. 10(a) differ from the width-varying portions ofFIG. 8(a) andFIG. 9(a) in that the broad-width portions and narrow-width portions of the width-varying portions are aligned alongface1014. Uppermost connectingportion1001 and connectingportions1002 are formed below narrow-width portion1021, uppermost connectingportion1003 and connectingportions1004 are formed below narrow-width portion1022, uppermost connectingportion1005 and connectingportions1006 are mainly formed below width-varyingportion1012 and uppermost connectingportion1007 and connectingportions1008 are mainly formed below width-varyingportion1013.
• Furthermore, a mask having the same shape as that shown inFIG. 11 can be used as the mask. In the masks ofFIG. 11 (a) and (c), the mask is closed, while the mask ofFIG. 11 (b) is open. Furthermore, in the masks ofFIG. 11 (a) and (b), a width-varying portion is formed at their respective apex.
• Thus, even if a microstructure is manufactured using masks having a variety of shapes, a narrow-width portion is formed in at least one location, so that a connecting portion and an aperture is formed at least below this portion by isotropic etching.
• While the number of width-varying portions, narrow-width portions and broad-width portions formed in the mask is not especially restricted, at least one or more need to be formed. In addition, while the position of these width-varying portions, narrow-width portions and broad-width portions in the mask is not especially restricted, for an open mask, these portions are preferably formed so that the broad-width portion is at both ends. More preferably, the broad-width portion is formed at both ends of the mask and at a portion sandwiched by both ends of the mask. By forming a broad-width portion at both ends of the mask, the slit-forming portion can be sandwiched by a column-shaped structure, to thereby prevent damage to the slit-forming portion. Furthermore, by also forming a broad-width portion at portions other than both ends, a microstructure can be attained having apertures with high precision and high surface area density in the slit-forming portion.
• When forming a plurality of width-varying portions, narrow-width portions and broad-width portions, their length in extending direction of the mask is not especially restricted, and may be set freely. Preferably, all these portions are made to have the same length. For example,FIG. 6(b) illustrates a microstructure manufactured using a mask having a plurality of the width-varyingportions31, narrow-width portions33 and broad-width portions32 shown inFIG. 6(a). Using such a mask, by carrying out isotropic etching and passivation film deposition by plasma reaction using C₄F₈gas in the same manner as that described above, a microstructure is formed having four column-shapedstructures61 aligned in a prescribed direction, and slit-formingportions62 in the space adjacent to column-shapedstructures61. On each slit-formingstructure62 are formed an uppermost bridge structure (an uppermost connecting portion)64, threeapertures63 and three bridge-shaped structures (connecting portions)65.
• Furthermore, an electron microscope photograph of a microstructure actually fabricated using such a mask is shown inFIG. 7. (InFIG. 7, as one example, two microstructures are illustrated, wherein the closer microstructure has been fabricated as far as a middle portion in the photograph.) FromFIG. 7 it can be understood that the apertures and the connecting portions in the slit-forming portion are aligned with uniform intervals.
• The isotropic etching and passivation film deposition of steps (3) and (5) can be carried out using conventionally-known Bosch process operation conditions.
• The substrate is not restricted to being a silicon substrate or a SOI substrate. Substrates made from a variety of materials can be employed. For example, when using a substrate made from SiO₂, as the isotropic etching conditions, it is preferable to carry out isotropic etching using anhydrous HF vapor, and as the passivation film deposition conditions, it is preferable to set conditions in the same manner as those described above.
• (Microstructure)
• The microstructure according to the present invention has at least one column-shaped structure and a slit-forming portion which extends in a side face direction (direction intersecting with an axis direction of the column-shaped structure) from a side face of the column-shaped structure. The slit-forming portions has a plurality of slits which are aligned in intervals from 20 to 1,000 nm in a direction along the center axis of the column-shaped structure, and are minute and excellent in shape uniformity. By utilizing these characteristics of being minute and excellent in shape uniformity, the microstructure according to the present invention can be used across a wide range of fields, and can be used, for instance, as a filter for optical telecommunications. In such a case, by setting the intervals between slits (slit period) to be 20 nm or more, filtering in the visible light region becomes possible. Furthermore, by setting to 1,000 nm or less, transmission loss is lessened and filtering is possible as far as the telecommunications waveband (wavelength of 2 μm or less). Although the interval between slits corresponds to the height H of the connecting portions or the uppermost connecting portion in the direction along the center axis of the column-shaped structure, if the height H of the connecting portions or the uppermost connecting portion is varied in the length direction of the connecting portions or the uppermost connecting portion, the height of any portion can be taken as the slit interval.
• The slits may be aligned with a fixed interval, or with two kinds of interval therebetween (The slits can have a first interval and a second interval). In such case, the intervals between the slits may be aligned so as to alternate between the first interval and the second interval, or may be aligned with the first intervals in a prescribed region, and with the second intervals in the other prescribed region. The slits may also be aligned with three or more different intervals. For example, a structure is also acceptable in which the intervals between the slits gradually decrease in the direction along the center axis of the column-shaped structures.
• The interval between slits is preferably from 120 to 750 nm. In addition, slit thickness (width of a slit in the direction along the center axis of the column-shaped structures) is also preferably about the same width as the interval between slits.
• FIG. 4 illustrates one example of a microstructure according to the present invention. The microstructure comprises two column-shapedstructures41 and a slit-formingportion42 formed between adjacent column-shaped structures. As seen from anaxial direction43 of the column-shaped structures, slit-formingportions42 alternately have connecting portions (44 and45) and a slit46 (wherein the slits are aligned at a prescribed interval).
• Here, “column-shaped structure” is defined as a portion which does not manifest any slits in its cross-section when viewed from face parallel in an axial direction of the microstructure and perpendicular to the aligneddirection14 of the column-shaped structures. That is, inFIG. 4(a), the portions ofcross-section48 and49 does not manifest any slits in its cross-section, and is thus a column-shaped structure when viewed from face parallel in an axial direction of the microstructure and perpendicular to the aligneddirection14 of the column-shaped structures. A “connecting portion” is defined as a structure which connects a portion of a side face of adjacent column-shapedstructures41, a portion which manifests any slits in its cross-section when viewed from face parallel in an axial direction of the microstructure and perpendicular to the aligneddirection14 of the column-shaped structures. Seen from an axial direction, the connecting portion at the uppermost location is termed the uppermost connecting portion. The uppermost connecting portion connects the uppermost face of a column-shaped structure (corresponding to the uppermost face of the substrate) with a part of a side face of a column-shaped structure. The shape of the cross-section intersecting with the axial direction of the column-shaped structure is mainly prescribed by the broad-width portion of the mask formed on an upper portion thereof.
• The column-shaped structures of the microstructure ofFIG. 4(a) are constituted from a curved face. The shape of the pair of column-shaped structures connected by a connecting portion may be such that their height H in an axial direction is the same and these structures is on the same plane, and their shape is not especially restricted. For example, a column-shaped structure may include a concave face, a convex face or an uneven face on at least a part of the side face. For example, the height of the column-shaped structure is preferably from 1 to 20 μm, and more preferably from 2 to 12 μm. Maximum width of the column-shaped structure is preferably from 150 nm to 3 μm, and more preferably from 200 nm to 1 μm. Length of the column-shaped structure is preferably no greater than twice the width, and more preferably is about the same as the width. Due to the fact that the size of the column-shaped structure (height, length, width) is within these ranges, a microstructure can be manufactured having desired characteristics depending on the intended use.
• Uppermost connectingportion59 inFIG. 4(b) is constituted fromuppermost face50 viewed fromaxial direction43 and two curved faces53. Looking towards the axial direction, the change in surface area of the cross-section perpendicular toaxial direction43 of the uppermost connectingportion59 reaches a maximum atuppermost face50, and continuously decreases towards the lower side in the axial direction, becoming zero at passingbottommost portion501. Furthermore, looking towardslength direction51, the change in surface area of the cross-section perpendicular tolength direction51 of uppermost connectingportion59 reaches a maximum at oneend502 in the length direction, continuously decreases to reach a minimum atcenter503 in length direction, then continuously increases to reach a maximum atother end502 in the length direction.
• While the shape of the connecting portion is not especially restricted, the uppermost connecting portion is constituted from an uppermost face and at least two or more curved faces. How many faces the faces other than the uppermost face are constituted from is dependent on the size of width-varying portion of the mask initially formed and the width and position etc. of the broad-width portions and narrow-width portions. The curved face may be concave, convex or uneven on its inner side.
• InFIG. 4(b), connectingportion60 is constituted from fourfaces54 and55, and connectingportion58 is constituted from fourfaces56 and57.
• Looking towards theaxial direction43, the change in surface area of the cross-section perpendicular toaxial direction43 of these connectingportions60 and58 reaches zero atuppermost portion504, continuously increases towards the lower side in the axial direction to reach a maximum, then continuously decreases to reach zero at passingbottommost portion505.
• Furthermore, looking towardslength direction51, the change in the surface area of cross-section perpendicular tolength direction51 reaches a maximum at oneend506 in the length direction, continuously decreases to reach a minimum at center of a length direction, then continuously increases to reach a maximum atother end507 in the length direction.
• While the shape of the connecting portion is not especially restricted, such portion may be constituted from a plurality of faces. How many faces the connection portion is constituted from is dependent on the size of the width-varying portion of the mask initially formed, and the width and position etc. of the broad-width portions and narrow-width portions. This curved face may be concave, convex or uneven on its inner side.
• The faces other than the uppermost face of the uppermost connecting portion and each face of the connecting portions may be the same or different. Furthermore, it is also acceptable to make only a part of the connecting portions the same, while a part can be made different. Thus, to make the shape of each face the same, the etching conditions for forming each face and the deposition conditions of the passivation film should be made the same. Furthermore, to make the shape of each face different, the etching conditions for forming each face and the deposition conditions of the passivation film should be made different.
• The maximum height of the connecting portions is preferably from 25 to 200 nm, and more preferably from 25 to 150 nm. Length is preferably from 2 to 20 times the width, and more preferably from 5 to 10 times the width. Maximum width is preferably from 50 to 500 nm, and more preferably from 100 to 250 nm. Due to the fact that the size of the uppermost connecting portion and the connecting portions (maximum height, length, maximum width) is within these ranges, a microstructure can be manufactured having desired characteristics depending on the intended use.
• The maximum height of the uppermost connecting portion is preferably from 15 to 100 nm, and more preferably from 15 to 80 nm.
• The slits are formed between the connecting portions in a slit-forming portion. Here, looking from the axial direction, a slit is constituted from faces of adjacent connecting portions which face each other. For example, in the microstructure ofFIG. 4, a slit is constituted fromface53 of uppermost connectingportion59 and face54 of connectingportion60. A slit is also constituted fromface55 of connectingportion60 and face56 of connectingportion58.
• The connecting portions and slits are preferably aligned in equal intervals in an axial direction. In such case, the alignment interval is preferably from 100 nm and 1 μm, and more preferably from 120 and 750 nm. Due to the fact that the alignment interval is within these ranges, a microstructure can be manufactured in which has the shape with high precision and a high surface area density.
• WhileFIG. 4 illustrates a microstructure in which two column-shaped structures are formed on the ends, the number of column-shaped structures included in the microstructure according to the present invention is acceptable as long as it is at least one, and the number of such structures is not especially restricted. In the case of a plurality of column-shaped structures, such structures may be aligned in a straight line in a prescribed direction, or may be aligned at random. The slit-forming portions are either formed on a side face of a single column-shaped structure, or between two column-shaped structures. When a plurality of column-shaped structures are formed, the slit-forming portions may be formed between two column-shaped structures adjacent to each other, or may be formed on a side face of a single column-shaped structure among a plurality of column-shaped structures. Furthermore, on a side face of a single column-shaped structure, three or more slit-forming portions may be formed.
• Even when three or more connecting portions are formed between a pair of column-shaped structures, or even when a connecting portion are formed between three or more column-shaped structures, connecting portions can be formed having the same shape and size as the above-described connecting portion. Furthermore, even when forming a plurality of microstructures, the connecting portions of these differing microstructures can be made the same shape and size as the above-described connecting portions. Still further, among these connecting portions, it is also acceptable to make only a part of the connecting portions the same, while a part can be made different.
• The microstructure according to the present invention can be used as an optical element because of its minuteness and due to the fact that the microstructure can comprise connecting portions and apertures in which the intervals and shape are controlled with high precision. This will now be described in more detail.
• (Optical Element)
• The following four different mechanisms can be employed as an application for an optical element of the microstructure according to the present invention. They are:
• (1) Structural birefringence
• (2) Guided-mode resonance
• (3) Wire grid
• (4) Periodic structure
• The principles behind each mechanism will be briefly explained, and a device applying such principles will be illustrated.
• (1) Structural Birefringence
• When the structure is sufficiently smaller than the wavelength of the light to be used, the structure can be deemed to be located in a uniform electromagnetic field. The refractive index in such case greatly differs from that where light is incident in the direction perpendicular to the slit-forming portion (e.g. direction1000 inFIG. 4(a)), depending on the incident polarization direction. The dielectric constant for TE waves (transverse electric waves) having an electric field intersecting with the slit direction (e.g. direction14 inFIG. 4(a)) is expressed as:
  εTE=fε₁+(f−1)ε₂
  Here, f denotes the volume fraction of the slit structure material, ε₁denotes the dielectric constant of the slit structure material and ε₂denotes the refractive index of the medium.
• In contrast, the dielectric constant for TM waves (transverse magnetic waves) having an electric field parallel to the slit direction (e.g. direction14 inFIG. 4(a)) is expressed as:
  1/εTE=f/ε₁+(f−1)/ε₂
• If slits are made from Si, and f is set to 0.5, the refractive index of TE direction is n_TE=2.56 and the refractive index of TM is n_TM=1.36, whereby birefringence can be achieved that could not be achieved using conventional materials.
• Wave Plate
• Providing the slit thickness (e.g. thickness ofdirection1000 inFIG. 4(a)) in accordance with the wavelength to be used allows ½ wave and ¼ wave plates to be fabricated. Applying birefringence of this magnitude enables the following self-standing type optical element to be achieved.
• Polarized Beam Splitter
• By combining a slit having a period λ1 which is sufficiently smaller than the wavelength with a slit having a period λ2 which diffracts the light wavelength to be used, a polarized beam splitter can be formed. That is, portions having a large polarization dependency is formed by λ1, and a grading of λ2 is formed by these portions. Although TE waves are diffracted because of grading of λ2, a polarized beam splitter transmitting TM waves can be fabricated from a self-standing type microstructure. The period is preferably no greater than 1/10 of the intended wavelength.
• High-Efficiency Diffraction Grating
• Gradually varying the period λ1 in the polarized beam splitter enables the diffraction efficiency to be increased.
• (2) Guided-Mode Resonance
• When the slit period is about the same as the wavelength of the light to be used, a guided-mode is formed in the slit. In the microstructure according to the present invention, the interval between slits (period) is from 20 to 1,000 nm. By setting the interval between slits to be from 20 to 1,000 nm, filtering of a broad waveband is possible from the waveband used in telecommunications (wavelength of 2 μm or less) through to visible light, whereby a reflective type filter can be attained in which transmission loss is small.
• (3) Wire Grid
• By coating a metal over the surface of a slit made from Si, a wire grid structure can be attained. When forming a wire grid structure, the wire grid can be formed by depositing a metal layer by a well-known deposition method onto a microstructure manufactured in accordance with the manufacturing process according to the present invention. As a deposition method, for example, CVD method and sputtering method can be employed.
• When the slit period is sufficiently smaller than the wavelength of the light to be used (generally P (period)/λ (wavelength)<0.1), TE waves are transmitted through and TM waves are reflected. By employing such a structure, a polarized element can be realized. Further, by selecting an appropriate period, a low-pass filter of TM waves can be formed. For example,FIG. 13 illustrates one example of a wire grid according to the present invention. Of the TE waves and TM waves that are incident fromdirection1301, in the microstructure according to the present invention TM waves are reflected, and only TE waves are transmitted. Therefore, by employing a microstructure according to the present invention, specific linear polarization can be taken Out.
• (4) Periodic Structure
• Since a connecting portion and an aperture have a periodic structure toward a axial direction in the microstructure according to the present invention, when light is incident parallel to a slit-forming portion (e.g. direction14 inFIG. 4(a)), a variety of filters can be formed. The periodic structure can be designed using the same design method as that for a thin-film dielectric filter. The thickness ofmicrostructure direction14 is from 20 to 1000 nm, when the microstructure is made from Si in combination with using air for the void portion of the microstructure, such as the slits.
• As explained above, by using a microstructure according to the present invention, most kinds of optical element can be attained. The combination of such a microstructure with an optical waveguide can realize the following optical device in a compact form and at low cost.
• (Dispersion Compensator)
• The refractive index of all optical materials such as glass changes according to wavelength, which is called wavelength dispersion. In long distance optical multiplexing telecommunication, transmission time varies depending on wavelength as a consequence of this refractive index wavelength dispersion, which becomes a problem. To prevent this, a technique called dispersion compensation is used which connects in the transmission path devices having dispersion characteristics opposite to those of the wavelength dispersion Qf the optical fiber. A dispersion compensator is realized to control wavelength dispersion having an equivalent refractive index in accordance with a structure in a dielectric multilayer film. The control of equivalent refractive index is also possible in the microstructure according to the present invention in accordance with its structure, whereby a dispersion compensator can be formed.
• (Branching Filter)
• A branching filter can be formed by making a narrow-band reflection filter utilizing guided-mode resonance align in one row at an appropriate angle (preferably 45°) toward the waveguide and optimizing the filter structure in accordance with the extracted wavelength.FIG. 12 illustrates one example of a branching filter according to the present invention. Incident light consisting of wavelengths λ3 to λ6 is branched byfilters1201 to1203 into light of respective wavelengths λ3 to λ6.
EXAMPLE 1
• Thermal oxidation was performed on a p-type silicon substrate having an orientation of (100) planes, to thereby form a 50 nm silicon oxide film on the substrate surface. Subsequently, a negative-type EB resist, Calix (6)arena 3 weight % solution, was coated onto the silicon oxide film at a substrate revolution speed of 4,000 rpm using a spin coater. The coated film was baked at a temperature of 100° C. for 1 hour. Next, the pattern shown inFIG. 14(a) was formed on the baked film using an electron beam lithography system (JEOL-5FE; manufactured by JEOL).FIG. 14(b) is an enlarged SEM photograph of a part of the resist mask pattern ofFIG. 14(a). A Rainbow 4500 (manufactured by Lam Research Corporation) apparatus was then used to transcribe a resist mask pattern onto the silicon oxide film under conditions of CF₄: CHF₃: Ar=20:10:150 sccm, 150 mTorr, RF 200 W, 10° C. and 15 seconds. Next, using a MULTIPLEX-ICP (manufactured by Sumitomo Precision Products Co., Ltd.), silicon etching and passivation film formation as 1 cycle were carried out by a Bosch process at 20° C. for 20 cycles. The conditions at this time are shown in Table 1.

  TABLE 1
  			C4F8Gas	SF6Gas	Gas	Substrate
  	Time	Pressure	Concentration	Concentration	Impressed	Impressed
  Treatment	(s)	(Pa)	(SCCM)	(SCCM)	Power (W)	Power (W)

  Etching	7	16	35	90	500	30
  Passivation Film	5	16	190	0	350	0
  Formation

• The silicon oxide film and passivation film were subsequently removed, to thereby manufacture a microstructure according to the present invention. The microstructure showed a shape as illustrated inFIG. 14(c) and had a connecting portion as illustrated inFIG. 14(d). The connecting portion of the microstructure washeight 20 nm, width 70 nm and length 650 nm. The aperture of the microstructure washeight 100 nm, width 70 nm and length 650 nm.

Claims (15)

1. A microstructure comprising a column-shaped structure and a slit-forming portion which extends in a side-face direction from a side face of the column-shaped structure, wherein the slit-forming portion has a plurality of slits aligned in parallel at intervals from 20 to 1,000 nm in a direction along a center axis of the column-shaped structure.

2. An optical element comprising the microstructure according toclaim 1.

3. The optical element according toclaim 2, wherein a surface is covered with a metal layer.

4. The optical element according toclaim 2 or3, wherein the intervals of the slit in a direction along a center axis of the column-shaped structure are constant.

5. An optical filter using the optical element according to any of claims2 or3.

6. A branching filter comprising the optical filter according toclaim 5.

7. The optical element according toclaim 2 or3, wherein the slit-forming portion comprises slits aligned in parallel at a first interval and slits aligned in parallel at a second interval in a direction along a center axis of the column-shaped structure.

8. The optical element according toclaim 7, wherein a ratio of the first interval to the second interval is from 1:5 to 1:20.

9. A polarized beam splitter using the optical element according toclaim 8.

10. A process for manufacturing a microstructure which comprises a column-shaped structure and a slit-forming portion which extends in a side-face direction from a side face of the column-shaped structure, wherein the slit-forming portion has a plurality of slits aligned in parallel in a direction along a center axis of the column-shaped structure, the process comprising the steps of:

(1) preparing a substrate which has a thickness greater than a height of the column-shaped structure;

(2) providing a mask extending in a prescribed direction of an upper face of the substrate which comprises a narrow-width portion in a direction which intersects with the extending direction for defining a portion to serve as the slit-forming portion and a broad-width portion in a direction which intersects with the extending direction for defining a portion to serve as the column-shaped portion;

(3) forming two facing grooves by carrying out isotropic etching on an upper face of the substrate by a reactive ion etching method using SF6 gas using the mask as a etching mask, and excavating in a thickness direction at least a portion of both sides opposing the extending direction of the mask of the upper face of the substrate;

(4) covering the upper face of the substrate forming the grooves with a passivation film formed by plasma reaction using C4F8 gas;

(5) providing apertures for connecting between grooves which are faced sandwiching the narrow-width portion of the mask at least below the narrow-width portion of the mask, by carrying out isotropic etching on the upper face of the substrate covered with the passivation film by a reactive ion etching method using SF6 gas; and

(6) repeating the steps (3) to (5) for aligning in parallel the apertures in a thickness direction below the narrow-width portion of the mask, to thereby attain the microstructure as well as extending the grooves in a thickness direction of the substrate.

11. The process for manufacturing a microstructure according toclaim 10, wherein the mask comprises a portion extending from one end to another end which is on an upper face of the substrate, and the ends are the broad-width portions.

12. The process for manufacturing a microstructure according toclaim 11, wherein the mask further comprises the broad-width portion on a portion other than on the end.

13. The process for manufacturing a microstructure according to any ofclaims 10 to12, wherein the mask extends in a branched manner in a plurality of prescribed directions.

14. The process for manufacturing a microstructure according to any ofclaims 10 to12, wherein the substrate is an SOI substrate.

15. The process for manufacturing a microstructure according to any ofclaims 10 to12, wherein the steps (3) to (5) are finished prior to the grooves penetrating as far as a lower face of the substrate.

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