Patent Publication Number: US-2009223924-A1

Title: Method of fabricating reflective mirror by wet-etch using improved mask pattern and reflective mirror fabricated using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on Japanese Patent Applications No. 2004-166511 filed Jun. 4, 2004 and No. 2004-321632 filed Nov. 5, 2004, and International Application No. PCT/JP2005/010027 filed Jun. 1, 2005, the contents of which are incorporated hereinto by reference. 
     This application is a continuation application of International Application No. PCT/JP2005/010027 filed Jun. 1, 2005, now pending, which was published in Japanese under PCT Article 21(2). 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to techniques of fabricating by etch a plate-shaped reflective mirror having a reflective surface on which light is incident, and more particularly to techniques of fabricating such a reflective mirror by wet-etch. 
     2. Description of the Related Art 
     For example, in the applications where images are formed optically, some situations exist where a plate-shaped reflective mirror is employed which has a reflective surface on which light is incident. 
     An exemplary type of such a reflective mirror is used for optical scan, in a manner that the reflective mirror is excited to vibrate about its oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     There exists an example of a conventional reflective mirror of the aforementioned type (see, for example, Japanese Patent Application Publication No. 2003-57586). 
     This conventional reflective mirror constitutes an oscillating body in cooperation with a plate-shaped spring which extends from the reflective mirror along the oscillation axis and which is excited to cause at least a torsional vibration about the oscillation axis. This conventional reflective mirror is used for optical scan, in a manner that at least a portion of the oscillating body is excited to vibrate, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     As disclosed in the aforementioned Japanese Patent Application Publication No. 2003-57586, the above-described conventional reflective mirror is conventionally fabricated so as to have a quadrangular planar-shape having a center line of symmetry coincident with the oscillation axis of the reflective mirror. Further, this conventional reflective mirror is fabricated by etch. In addition, etch is categorized into dry-etch and wet-etch. 
     BRIEF SUMMARY OF THE INVENTION 
     In the applications where such a reflective mirror is used, it is highly desired to increase a maximum oscillating speed at which the reflective mirror can be angularly oscillated about its oscillation axis in some situations. 
     More specifically, for example, in the case of an image forming apparatus including a scanner for use in optical scan using such a reflective mirror, there exist cases where, for an image resolution to increase, increase in an optical scan rate of the scanner is highly desired, and for the scan rate to increase, increase in a maximum oscillating speed of the reflective mirror is desired. 
     On the other hand, for the oscillating speed of such a reflective mirror to increase, it is effective to reduce the moment of inertia of the reflective mirror about its oscillation axis. 
     However, a conventional reflective mirror, when fabricated by wet-etch, is conventionally formed so as to have a quadrangular planar-shape having a center line of symmetry coincident with the oscillation axis of the reflective mirror, as described above. 
     For this reason, when this conventional (quadrangular) reflective mirror is unavoidably used, it is more difficult to reduce the moment of inertia of the reflective mirror than when a circular reflective mirror is used instead. The circular reflective mirror, for ensuring its reflective surface to have the same size or area as that in the conventional (quadrangular) reflective mirror, has the same width dimension as that of the conventional reflective mirror. 
     When a reflective mirror is fabricated by dry-etch, it is easier to downsize or micromachine an associated mask pattern than when the reflective mirror is fabricated by wet-etch, and is also easier to fabricate the reflective mirror precisely into a desired shape than when the reflective mirror is fabricated by wet-etch. 
     The fabricating process of reflective mirrors by dry-etch is not adequately suitable to a butch process in which a large number of etchable materials are etched at a time for fabricating a large number of reflective mirrors. For this reason, the fabricating process of reflective mirrors by dry-etch is not adequately suitable to improvements in manufacturing efficiency and reductions in manufacturing costs of reflective mirrors. 
     It is therefore an object of the present invention to provide techniques of fabricating by etch a plate-shaped reflective mirror having a reflective surface on which light is incident, more particularly, techniques of fabricating such a reflective mirror by wet-etch. 
     According to a first aspect of the present invention, there is provided a method of fabricating by an etching technique a plate-shaped reflective mirror having a reflective surface on which light is incident. 
     The method according to the first aspect of the present invention comprises: 
     a coating step of coating at least one of opposite faces of a plate-shaped etchable material made of a single crystal material, with a film-like etching mask; 
     a mask-pattern forming step of forming a mask pattern on at least one of opposite faces of the etching mask which has been deposited on the etchable material, the mask pattern having a planar shape to which a circle is more similar than a quadrangle; and 
     a wet-etching step of wet-etching the etchable material on which the etching mask has been deposited, by immersing the etchable material in an etchant having a predetermined temperature and a predetermined concentration, 
     whereby the reflective mirror is fabricated so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     According to a second aspect of the present invention, there is provided a plate-shaped reflective mirror which has a reflective surface on which light is incident. 
     The reflective mirror according to the second aspect of the present invention is shaped to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     Further, this reflective mirror is fabricated by implementing: 
     a coating step of coating at least one of opposite faces of a plate-shaped etchable material made of a single crystal material, with a film-like etching mask; 
     a mask-pattern forming step of forming a mask pattern on at least one of opposite faces of the etching mask which has been deposited on the etchable material, the mask pattern having a planar shape to which a circle is more similar than a quadrangle; and 
     a wet-etching step of wet-etching the etchable material on which the etching mask has been deposited, by immersing the etchable material in an etchant having a predetermined temperature and a predetermined concentration. 
     According to a third aspect of the present invention, there is provided a process of integrally fabricating by an etching technique an oscillating body having a unitary configuration including both a reflective mirror having a reflective surface, and a plate-shaped spring. 
     In this regard, the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     The aforementioned spring is shaped to have a beam structure which extends from the reflective mirror along the oscillation axis and which has a stepped portion. 
     Further, this spring is excited by a vibration occurring in at least a portion of the oscillating body, to cause at least a torsional vibration about the oscillation axis, to thereby angularly oscillate the reflective mirror about the oscillation axis by at least the caused torsional vibration. 
     The process according to the third aspect of the present invention comprises: 
     a coating step of coating opposite faces of a to-be-processed portion of a plate-shaped etchable material made of a single crystal material, with two film-like etching masks, respectively, wherein the to-be-processed portion is to be processed into the beam structure; 
     a mask-pattern forming step of forming a pair of mask patterns on the two etching masks which have been deposited on the opposite faces of the to-be-processed portion, respectively, wherein the pair of mask patterns have respective shapes for forming the beam structure; and 
     a wet-etching step of, after formation of the pair of mask patterns, wet-etching the etchable material by immersing the etchable material in an etchant. 
     According to a fourth aspect of the present invention, there is provided an oscillating body having a unitary configuration including both a reflective mirror having a reflective surface, and a plate-shaped spring. 
     In this regard, the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     The aforementioned spring is shaped to have a beam structure which extends from the reflective mirror along the oscillation axis and which has a stepped portion. 
     Further, this spring is excited by a vibration occurring in at least a portion of the oscillating body, to cause at least a torsional vibration about the oscillation axis, to thereby angularly oscillate the reflective mirror about the oscillation axis by at least the caused torsional vibration. 
     The aforementioned reflective mirror is fabricated so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface, by implementing the steps of: 
     coating at least one of opposite faces of a plate-shaped etchable material made of a single crystal material, with a film-like etching mask; 
     forming a mask pattern on at least one of opposite faces of the etching mask which has been deposited on the etchable material, the mask pattern having a planar shape to which a circle is more similar than a quadrangle; and 
     wet-etching the etchable material on which the etching mask has been deposited, by immersing the etchable material in an etchant having a predetermined temperature and a predetermined concentration. 
     The aforementioned stepped portion is fabricated by wet-etching the etchable material such that the stepped portion is ultimately shaped to include: 
     (a) a higher sub-portion having the same height as a basic surface of the beam structure; 
     (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and 
     (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1  is a schematic diagram illustrating a retinal scanning display including a reflective mirror for optical scan which has been fabricated by a reflective-mirror fabricating method according to a first embodiment of the present invention; 
         FIG. 2  is an exploded perspective view illustrating an optical scanner  104  depicted in  FIG. 1 ; 
         FIGS. 3(   a ) and  3 ( b ) are a sectional side view and a perspective view illustrating an actuator  154  depicted in  FIG. 2  and its neighborhood, respectively; 
         FIG. 4  is a perspective view illustrating an oscillating body  124  depicted in  FIG. 2 ; 
         FIG. 5  is a block diagram illustrating a horizontal scanning drive circuit  180  depicted in  FIG. 1 ; 
         FIG. 6  is a perspective view illustrating a specific shape of the oscillating body  124  depicted in  FIG. 2 ; 
         FIG. 7  is a perspective view for explaining how light enters a circle-shaped reflective mirror  122  of the oscillating body  124  depicted in  FIG. 2 ; 
         FIG. 8  is a perspective view for explaining how light enters a quadrangle-shaped reflective mirror  302  of an oscillating body which is a comparative example of the oscillating body  124  depicted in  FIG. 2 ; 
         FIG. 9  is a process chart illustrating the aforementioned reflective-mirror fabricating method; 
         FIG. 10  is a top plan view illustrating a mask pattern formed in a step S 3  depicted in  FIG. 9 ; 
         FIGS. 11(   a ) and  11 ( b ) are perspective views for explaining step by step the progression of wet-etching performed in a step S 4  depicted in  FIG. 9 ; 
         FIGS. 12(   a ) and  12 ( b ) are additional perspective views for explaining step by step the progression of the wet-etching performed in the step S 4  depicted in  FIG. 9 ; 
         FIG. 13  is a perspective view illustrating in enlargement an etchable material  400  depicted in  FIG. 12(   b ); 
         FIGS. 14(   a ) and  14 ( b ) are vertical sectional-views illustrating the reflective mirror  122  ultimately fabricated the reflective-mirror fabricating method illustrated in  FIG. 9 , and  FIG. 14(   c ) is a vertical sectional-view illustrating a comparative example of the reflective mirror  122 : 
         FIG. 15  is a top plan view illustrating an example of a modified version of the mask pattern depicted in  FIG. 10 ; 
         FIG. 16  is a top plan view illustrating a mask pattern formed for fabricating a reflective mirror  122  by a reflective-mirror fabricating method according to a second embodiment of the present invention; 
         FIGS. 17(   a ) and  17 ( b ) are perspective views for explaining step by step the progression of wet-etching performed in the second embodiment; 
         FIGS. 18(   a ) and  18 ( b ) are additional perspective views for explaining step by step the progression of the wet-etching performed in the second embodiment; 
         FIG. 19  is a perspective view illustrating in enlargement an etchable material  480  depicted in  FIG. 18  ( b ); 
         FIGS. 20(   a ) and  20 ( b ) are a perspective view and a top plan view illustrating the reflective mirror  122  ultimately fabricated in the second embodiment, respectively; 
         FIG. 21  is a top plan view illustrating an example of a modified version of the mask pattern depicted in  FIG. 16 ; 
         FIG. 22  is a process chart illustrating an oscillating-body fabricating process according to a third embodiment of the present invention; 
         FIGS. 23(   a ) and  23 ( b ) are sectional views taken on lines A-A and B-B in  FIG. 2  for explaining steps S 12  and S 13  depicted in  FIG. 22 , respectively; 
         FIGS. 24(   a ) and  24 ( b ) are top plan views illustrating an upper mask pattern  630  and a lower mask pattern  632  formed in the step S 13  depicted in  FIG. 22 , respectively; 
         FIGS. 25(   a ),  25 ( b ), and  25 ( c ) are perspective views illustrating the upper mask pattern  630 , an etchable material  600 , and a basic desired-shape of a representative frame-side leaf spring  144  which represents a plurality of frame-side leaf springs  144  depicted in  FIG. 2 , respectively; 
         FIG. 26  illustrates in top plan view a basic desired-shape of a stepped portion  160  of the representative frame-side leaf spring  144  in  FIG. 25 , and a stepped-portion-oriented mask pattern  650 ; 
         FIGS. 27(   a ) and  27 ( b ) are perspective views for explaining step by step the progression of wet-etching performed in a step S 14  depicted in  FIG. 22 ; 
         FIGS. 28(   a ) and  28 ( b ) are additional perspective views for explaining step by step the progression of the wet-etching performed in the step S 14  depicted in  FIG. 22 ; 
         FIGS. 29(   a ) and  29 ( b ) are a top plan view and a perspective view illustrating in enlargement the stepped portion  160  of the etchable material  600  depicted in  FIG. 28  ( b ), respectively; 
         FIGS. 30(   a ) and  30 ( b ) are perspective views for explaining step by step the progression of wet-etching performed in a comparative example of the third embodiment; 
         FIGS. 31(   a ) and  31 ( b ) are additional perspective views for explaining step by step the progression of the wet-etching performed in the comparative example of the third embodiment; 
         FIGS. 32(   a ) and  32 ( b ) are top plan views illustrating an upper mask pattern  740  and a lower mask pattern  742  formed for fabricating a stepped portion  160  by an oscillating-body fabricating process according to a fourth embodiment of the present invention, respectively; 
         FIG. 33  illustrates in top plan view a stepped-portion-oriented mask pattern  760  in  FIG. 32 , and a basic desired-shape of a stepped portion  160  to be formed using the stepped-portion-oriented mask pattern  760 , respectively; 
         FIGS. 34(   a ) and  34 ( b ) are perspective views for explaining step by step the progression of wet-etching performed in the oscillating-body fabricating process according to the fourth embodiment; and 
         FIGS. 35(   a ) and  35 ( b ) are additional perspective views for explaining step by step the progression of the wet-etching performed in the oscillating-body fabricating process according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The object mentioned above may be achieved according to any one of the following modes of this invention. 
     These modes will be stated below so as to be sectioned and numbered, and so as to depend upon the other mode or modes, where appropriate. This is for a better understanding of some of a plurality of technological features and a plurality of combinations thereof disclosed in this description, and does not mean that the scope of these features and combinations is interpreted to be limited to the scope of the following modes of this invention. 
     That is to say, it should be interpreted that it is allowable to select the technological features which are stated in this description but which are not stated in the following modes, as the technological features of this invention. 
     Furthermore, stating each one of the modes of the invention in such a dependent form as to depend from the other mode or modes does not exclude the possibility that the technological features set forth in a dependent-form mode become independent of those set forth in the corresponding depended mode or modes and to be removed therefrom. It should be interpreted that the technological features set forth in a dependent-form mode is allowed to become independent, where appropriate. 
     (1) A method of fabricating by an etching technique a plate-shaped reflective mirror having a reflective surface on which light is incident: the method comprising: 
     a coating step of coating at least one of opposite faces of a plate-shaped etchable material made of a single crystal material, with a film-like etching mask; 
     a mask-pattern forming step of forming a mask pattern on at least one of opposite faces of the etching mask which has been deposited on the etchable material, the mask pattern having a planar shape to which a circle is more similar than a quadrangle; and 
     a wet-etching step of wet-etching the etchable material on which the etching mask has been deposited, by immersing the etchable material in an etchant having a predetermined temperature and a predetermined concentration, 
     whereby the reflective mirror is fabricated so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     Upon this method being practiced, a reflective mirror is fabricated so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     Therefore, upon this method being practiced for fabricating a reflective mirror required to have at least a circular reflective area with a desired size, such a reflective mirror is provided that is easier in removing a wasteful reflective area from the reflective mirror than when the reflective mirror has a silhouette of a quadrangle. 
     As a result, this method makes it easier to fabricate a reflective mirror reduced in weight and moment of inertia than when the reflective mirror has a silhouette of a quadrangle. 
     Additionally, upon this method being practiced, a reflective mirror is fabricated not by dry-etch but by wet-etch. Therefore, this method makes it easier to fabricate reflective mirrors in an efficiency-improving and cost-saving manner than when the reflective mirrors are fabricated by dry-etch. 
     In addition, dry-etch allows a reflective mirror to be fabricated so as to have the same shape as that of a mask pattern formed on an etching mask deposited on the surface of an etchable material. In contrast, wet-etch allows a reflective mirror to be fabricated so as to have a different shape from the mask pattern. This is caused by differences in etch rate between crystallographic planes in the etchable material. 
     Therefore, it is required, when the method according to the present mode is practiced, to prepare a mask pattern, based on a desired shape of a resulting reflective mirror, by allowing for differences in etch rate between crystallographic planes in an etchable material used. 
     Upon this method being practiced, the reflective mirror is fabricated so as to ultimately have a silhouette of a planar shape to which a circle is more similar than a quadrangle. Based on this fact, a mask pattern is prepared so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, and which is different from a desired shape of the reflective mirror. 
     As will be evident from the above, even when a reflective mirror is fabricated by wet-etch, if a mask pattern is defined by allowing for differences in etch rate between crystallographic planes in an etchable material used, the reflective mirror can be fabricated to achieve its desired shape. 
     This method according to the present mode may be practiced in an arrangement in which the opposite faces of the etchable material are coated with respective etching masks, or an arrangement in which one of the opposite faces of the etchable material is coated with an etching mask. 
     For this method to be practiced in the former arrangement, its mask-pattern forming step may be implemented to form mask patterns on two etching masks which have been deposited on the opposite faces of the etchable material, respectively, or may be implemented to form a mask pattern on a selected one of those two etching masks. 
     That is to say, for this method according to the present mode to be practiced, the forming at least one mask pattern on at least one of opposite faces of an etchable material is sufficient. 
     (2) The method according to mode (1), wherein the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     This method allows a reflective mirror for use in optical scan to be fabricated by wet-etch so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle. 
     (3) The method according to mode (2), wherein the reflective mirror constitutes an oscillating body in cooperation with a plate-shaped spring which extends from the reflective mirror along the oscillation axis and which is excited to cause at least a torsional vibration about the oscillation axis, and 
     wherein the reflective mirror is used for optical scan, in a manner that at least a portion of the oscillating body is excited to vibrate, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     (4) The method according to any one of modes (1)-(3), wherein the planar shape of the mask pattern is generally in the shape of a convex octagon. 
     This method allows a reflective mirror generally having a silhouette of a convex octagon to be fabricated in the form of a reflective mirror which is easily reduced in moment of inertia than when the reflective mirror has a silhouette of a quadrangle. 
     (5) The method according to mode (4), wherein the planar shape of the mask pattern has a basic shape of a convex octagon with protrusions at eight corners of the octagon, and wherein the protrusions protrude outwardly from the octagon at the eight corners, as viewed in a direction perpendicular to a surface of the mask pattern. 
     This method allows portions of the etchable material which face the corners of the mask pattern during the wet-etching step, to be etched with a start-of-etching timing delayed with respect to that when the mask pattern does not have the protrusions at the respective corners. The reduction in etch rate affects the fabrication of the reflective mirror. 
     The “protrusions” set forth in the present mode may be defined to exhibit, for example, a shape allowing each protrusion extends from a corresponding one of the corners of a regular octagon toward a region in which an external angle for the corresponding corner is formed. 
     (6) The method according to mode (4) or (5), wherein the mask pattern is shaped to have first sides parallel to a reference line, and second sides perpendicular to the reference line, wherein the mask pattern is located relative to the etchable material, such that the first and second sides are each perpendicular to at least one of &lt;110&gt; crystallographic directions of the etchable material on at least one of {100} crystallographic planes of the etchable material, and 
     wherein the mask pattern has an outline including a first portion corresponding to at least a separate one of the {100} crystallographic planes, and a second portion corresponding to at least one of {111} crystallographic planes of the etchable material, per each one of four regions into which a surface of the etchable material is separated by two center lines of symmetry orthogonally intersecting at a center point of the mask pattern. 
     Throughout the description and drawings, crystallographic planes are denoted as {abc}. The designation of planes {abc} includes not only a specific plane (abc) but also all of its equivalent planes. Therefore, {abc} crystallographic planes should be interpreted by generic definition to identify a family of equivalent planes. This holds true for crystallographic directions denoted as &lt;abc&gt;. 
     The method according to the present mode allows the fabrication of a reflective mirror having a silhouette of an m-sided polygon (m: four times an integer equal to or greater than three) having at least one of {100} crystallographic planes and at least one of { 111 } crystallographic planes, both as exposed surfaces. 
     (7) The method according to mode (6), wherein the outline of the mask pattern further includes a third portion corresponding to at least one of {n11} crystallographic planes of the etchable material (n: an integer equal to or greater than two), disposed between the first and second portions, per each region of the etchable material. 
     This method allows the fabrication of a reflective mirror having a silhouette of an m-sided polygon (m: multiples of eight, which is greater than eight) which is obtained by partially cutting away a presupposed octagon (defined below) at its eight corners. 
     The presupposed octagon is shaped to have a silhouette having an outline defined only by at least one of {100} crystallographic planes and at least one of {111} crystallographic planes. 
     The m-sided polygon is obtained by partially cutting away the presupposed octagon at its eight corners such that each corner is exposed at least one of different crystallographic planes from the above planes. That is to say, the thus-obtained m-sided polygon can refer to a chamfered octagon. 
     Therefore, this method allows a reflective mirror to be fabricated so as to have a silhouette of a planar shape to which a circle is more similar than an octagon. 
     Further, this method makes it easier to allow the silhouette of a resulting reflective mirror to approximate to a circle rather than a quadrangle. 
     Therefore, this method makes it easier to remove a wasteful reflective area from a reflective mirror required to achieve a desired reflective area (a desired width dimension), than in the case of a quadrangle-shaped reflective mirror, resulting in facilitated reduction in weight and moment of inertia of the reflective mirror. 
     The degree of circularity or roundness of the circumference of a reflective mirror (i.e., how much a reflective mirror approximates to a complete circle) becomes better when crystallographic surfaces at which each corner of the aforementioned presupposed octagon is exposed are different in type, than when the exposed crystallographic surfaces are identical in type. 
     For example, when the exposed crystallographic surfaces are identical in type, the silhouette of a corresponding reflective mirror is a 16-sided polygon. On the other hand, when the exposed crystallographic surfaces have two different types, the silhouette of a corresponding reflective mirror is a 24-sided polygon. 
     There exits the tendency that, as the types of the exposed crystallographic surfaces increase in number, the degree of circularity of the reflective mirror improves. The tendency is amplified as the types of the exposed crystallographic surfaces increase in number. 
     The method according to the present mode is advantageous in increasing the number of types of the exposed crystallographic surfaces. 
     (8) The method according to mode (6), wherein the outline of the mask pattern further includes a fourth portion corresponding to at least one of {520} crystallographic planes of the etchable material, disposed between the first and second portions, per each region of the etchable material. 
     This method allows the fabrication of a reflective mirror having a silhouette of a specific shape which is obtained by partially cutting away a presupposed octagon (defined below) at its eight corners. 
     The presupposed octagon is shaped to have a silhouette having an outline defined only by at least one of {100} crystallographic planes and at least one of {111} crystallographic planes. 
     The specific shape is obtained by partially cutting away the presupposed octagon at its eight corners such that each corner is exposed principally at least one of {520} crystallographic planes. A reflective mirror which can be fabricated by this method has a planar shape having a silhouette approximate to a 16-sided polygon. 
     Therefore, this method allows a reflective mirror to be fabricated so as to have a silhouette of a planar shape to which a circle is more similar than an octagon. 
     Further, this method allows a reflective mirror to be fabricated so as to have a shape permitting each corner of the octagon to be exposed, as a result of each corner being partially cut away, principally at a plurality of crystallographic surfaces identical in type, with the exposed crystallographic surfaces being kept unchanged in type. 
     Therefore, this method allows a plurality of reflective mirrors to be fabricated so as not to be variable in ultimate shape, resulting in improved stability of the circumferential profiles of the reflective mirrors. 
     (9) The method according to any one of modes (1)-(8), wherein the etchant includes KOH or TMAH. 
     (10) The method according to mode (9), wherein the predetermined concentration is in the range from about 0.35 wt. % to about 45 wt. %. 
     (11) The method according to mode (9) or (10), wherein the predetermined temperature is in the range from about 60 degrees Celsius (.degree. C.) to about 80.degree. C. 
     (12) The method according to any one of modes (1)-(11), wherein the mask-pattern forming step includes a step of forming the mask pattern on each of two etching masks which have been deposited on the opposite faces of the etchable material, respectively. 
     Upon the method according to any one of above modes (1)-(11) being practiced, an instance exists where a reflective mirror is fabricated such that a lateral side or circumference of the reflective mirror has an inclined region in a vertical sectional-view, as illustrated in  FIGS. 14(   b ) and  14 ( c ), for example. 
     In this instance, upon an etchable material being wet-etched only from one of opposite faces of the etchable material, as illustrated in  FIG. 14(   c ) in vertical sectional-view, for example, the inclined region is formed at a lateral side of an etchable material  400 ′ in the shape of a continuous inclined surface. The inclined surface is shaped to be asymmetrical with respect to a line extending parallel to the etchable material  400 ′ and passing through the center of the thickness of the etchable material  400 ′. 
     In contrast, upon an etchable material being wet-etched from opposite faces of the etchable material, as illustrated in  FIG. 14(   b ) in vertical sectional-view, for example, the inclined region is formed at a lateral side of an etchable material  400  in the shape of discontinuous inclined surfaces. The inclined surfaces are shaped to be symmetrical with respect to a line extending parallel to the etchable material  400  and passing through the center of the thickness of the etchable material  400 . 
     In comparison of the former type of wet-etch (i.e., single-sided wet-etch) with the latter type of wet-etch (i.e., double-sided wet-etch), in terms of the weight of a resulting reflective mirror, the double-sided wet-etch can reduce the weight of a resulting reflective mirror more easily than the single-sided wet-etch. The easier the reduction in weight of a reflective mirror becomes, the easier the reduction in moment of inertia of the reflective mirror becomes. 
     Based on the findings described above, in the method according to the present mode, mask patterns are formed on two etching masks which have been deposited on opposite faces of an etchable material, respectively. That is to say, opposite faces of an etchable material are formed to have respective mask patterns, and eventually, the etchable material is wet-etched from the opposite faces of the etchable material, respectively. 
     (13) The method according to any one of modes (1)-(12), further comprising a removing step of, upon completion of the wet-etching, removing the etching mask from the etchable material. 
     (14) The method according to mode (13), further comprising a reflective-layer forming step of, after removal of the etching mask from the etchable material, forming a reflective layer on at least one of the opposite faces of the etchable material. 
     (15) A plate-shaped reflective mirror which has a reflective surface on which light is incident, 
     wherein the reflective mirror is shaped to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface, and 
     wherein the reflective mirror is fabricated by implementing: 
     a coating step of coating at least one of opposite faces of a plate-shaped etchable material made of a single crystal material, with a film-like etching mask; 
     a mask-pattern forming step of forming a mask pattern on at least one of opposite faces of the etching mask which has been deposited on the etchable material, the mask pattern having a planar shape to which a circle is more similar than a quadrangle; and 
     a wet-etching step of wet-etching the etchable material on which the etching mask has been deposited, by immersing the etchable material in an etchant having a predetermined temperature and a predetermined concentration. 
     This reflective mirror has a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     Therefore, this reflective mirror, when used to satisfy the requirements to have its reflective surface having at least a circular reflective area with a desired size, makes it easier to remove a wasteful reflective area from the reflective mirror than when a reflective mirror has a silhouette of a quadrangle. 
     As a result, this reflective mirror allows reduction in weight and moment of inertia more easily than when the reflective mirror has a silhouette of a quadrangle. 
     (16) The reflective mirror according to mode (15), wherein the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     (17) The reflective mirror according to mode (16), wherein the reflective mirror constitutes an oscillating body in cooperation with a plate-shaped spring which extends from the reflective mirror along the oscillation axis and which is excited to cause at least a torsional vibration about the oscillation axis, and 
     wherein the reflective mirror is used for optical scan, in a manner that at least a portion of the oscillating body is excited to vibrate, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto. 
     (18) A process of integrally fabricating by an etching technique an oscillating body having a unitary configuration including both a reflective mirror having a reflective surface, and a plate-shaped spring, 
     wherein the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto, 
     wherein the spring is shaped to have a beam structure which extends from the reflective mirror along the oscillation axis and which has a stepped portion, 
     wherein the spring is excited by a vibration occurring in at least a portion of the oscillating body, to cause at least a torsional vibration about the oscillation axis, to thereby angularly oscillate the reflective mirror about the oscillation axis by at least the caused torsional vibration, and 
     wherein the process comprises: 
     a coating step of coating opposite faces of a to-be-processed portion of a plate-shaped etchable material made of a single crystal material, with two film-like etching masks, respectively, wherein the to-be-processed portion is to be processed into the beam structure; 
     a mask-pattern forming step of forming a pair of mask patterns on the two etching masks which have been deposited on the opposite faces of the to-be-processed portion, respectively, wherein the pair of mask patterns have respective shapes for forming the beam structure; and 
     a wet-etching step of, after formation of the pair of mask patterns, wet-etching the etchable material by immersing the etchable material in an etchant. 
     This process is directed to techniques of fabricating a one-dimensionally extending beam structure by an etch method, and more particularly to techniques of integrally fabricating the beam structure shaped to have a stepped portion disposed at a local position on a path extending in a length-wise direction of the beam structure. 
     For example, in the applications where images are optically formed, instances exist where a beam structure is used for optical scan, or depth control of a virtual image perceived by a viewer through the viewer&#39;s eye, by varying a direction in which light travels, or varying the curvature of wavefront of light entering a viewer&#39;s eye, with high accuracy. 
     In operation, the beam structure is excited to cause a torsional vibration about a straight line in parallel to the beam structure for optical scan, or is excited to cause a vertical vibration along a direction perpendicular to the surface of the beam structure for modulation of the curvature of wavefront of light, for example. 
     Such a beam structure, when used for optical scan or depth control of a virtual image, is configured to include a plate-shaped reflective mirror having a reflective surface on which light is incident, and an elastically deformable portion extending coplanar with the reflective mirror, in a unitary configuration, for example. 
     Japanese Patent No. 2981600 discloses an example of a conventional thus-configured beam structure. 
     (19) The process according to mode (18), wherein the stepped portion is ultimately shaped to include: 
     (a) a higher sub-portion having the same height as a basic surface of the beam structure; 
     (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and 
     (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure. 
     By this process, an etchable material patterned to have a mask pattern is wet-etched, to thereby fabricate a beam structure having a stepped portion. The stepped portion, as described above, is shaped to include the higher sub-portion, the lower sub-portion, and the shoulder sub-portion. 
     (20) The process according to mode (19), wherein the etchable material is originally shaped to include: 
     (d) a to-be-fully-etched portion of the etchable material which is to be etched through a thickness of the etchable material when wet-etched, to thereby produce the beam structure from the plate-shaped etchable material; 
     (e) a to-be-unetched portion of the etchable material which is to remain unetched when wet-etched, to thereby form the higher sub-portion, and 
     (f) a to-be-half-etched portion of the etchable material which is to be etched in half-way of the thickness of the etchable material when wet-etched, to thereby form the lower sub-portion. 
     For this process to be implemented, an etchable material is configured to include the to-be-fully-etched portion to produce the beam structure from the plate-shaped etchable material; the to-be-unetched portion to produce the higher sub-portion of the stepped portion, from the plate-shaped etchable material; and the to-be-half-etched portion to produce the lower sub-portion of the stepped portion, from the plate-shaped etchable material. 
     (21) The process according to claim 20, wherein the to-be-fully-etched portion includes opposite sub-portions which are opposed to each other in a width-wise direction of the beam structure and between which the to-be-half-etched portion is interposed, and 
     wherein the pair of mask patterns are originally shaped to include; 
     (g) a basic pattern shaped to cover a surface of the to-be-unetched portion; and 
     (h) a compensating pattern shaped to cover a surface of at least opposite sub-portions of the opposite sub-portions and the to-be-half-etched portion. 
     The present inventors conducted researches into a technique of integrally fabricating a beam structure having a stepped portion disposed at a position on a path extending in a length-wise direction of the beam structure. 
     The stepped portion is shaped to include: (a) a higher sub-portion having the same height as a basic surface of the beam structure; (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure. 
     The research has made the inventors recognize that dry-etching of a wafer makes it easier to fabricate a beam structure so that a shoulder sub-portion of a stepped portion of the fabricated beam structure is formed at a less-variable position on a path extending in a length-wise direction of the beam structure, while wet-etching of a wafer in a conventional fashion makes it more difficult to fabricate a beam structure so that a shoulder sub-portion of a stepped portion of the fabricated beam structure is formed at a less-variable position on a path extending in a length-wise direction of the beam structure. That is to say, a conventional wet-etch forms the shoulder sub-portion at a position sensitive to a variable etch time. 
     The research has further made the inventors recognize that a conventional wet-etching technique makes it difficult to fabricate beam structures so that their shoulder sub-portions are formed at respective positions on length-wise directions of the beam structures, with these positions being less variable irrespective of individual differences between the beam structures. That is to say, a conventional wet-etch forms the shoulder sub-portion at a position sensitive to individual-differences between the beam structures. 
     Describing in greater detail, the shoulder sub-portion of the stepped portion is formed by etching a flat plane portion of an etchable material. During such etch, at the etchable material, an intermediate is produced which has a similar shape to the resulting shoulder sub-portion. 
     The intermediate extends along a plane intersecting a basic surface of the etchable material. As a result, at the intermediate, the etch progresses not only in a direction allowing the thickness of the etchable material to be reduced, but also in directions allowing the width of the etchable material to be reduced and allowing the position of the intermediate to go back in the length-wise direction of the beam structure. 
     For these reasons, at the intermediate, etch progresses at a faster rate than at the basic surface of the same etchable material, and the composition of exposed crystallographic surfaces is prone to be complicated. 
     Therefore, if a mask pattern which is pre-deposited on an etchable material for etching a flat surface portion of the etchable a material to form a solid stepped-portion, is formed to have the same shape as that of a basic pattern reflecting faithfully only the shape of the aforementioned to-be-unetched portion, then the stepped portion of the beam structure is fabricated such that the shoulder sub-portion of the stepped portion is formed at a variable position on a path extending in the length-wise direction of the beam structure. 
     In other words, the position of the shoulder sub-portion is very sensitive to unintended variations in actual etching conditions (including, e.g., an etch time), with the result that shoulder sub-portions of beam structures (intended to be identical in shape) which have been fabricated by etch are prone to be variable in position. 
     With this in mind, the process according to the present mode has been invented to provide a technique of integrally fabricating a beam structure having a stepped portion at a local position on a path extending in a length-wise direction of the beam structure, such that a shoulder sub-portion of the stepped portion is positioned at a controlled position on a path extending in a length-wise direction of the beam structure. 
     This process is practiced using a mask pattern configured to include a basic pattern covering the surface of the to-be-unetched portion. The mask pattern is configured to further include a compensating pattern shaped to cover opposite sub-portions of the to-be-fully-etched portion which are opposed to each other in a width-wise direction of the beam structure and between which the to-be-half-etched portion is interposed, and to optionally further cover the to-be-half-etched portion. 
     (i) opposite sub-portions which are opposed to each other in a width-wise direction of the beam structure and between which the to-be-half-etched portion is interposed; and (ii) a compensating pattern shaped to cover a surface of at least opposite sub-portions of the opposite sub-portions and the to-be-half-etched portion. 
     The compensating pattern has a function to reduce an etch rate at a portion of the etchable material which has been coated with the basic pattern. 
     Further, this compensating pattern makes it easier to control the composition of exposed surfaces of an ultimately formed shoulder sub-portion, depending on one of various shapes of this compensating pattern, in a manner that crystallographic surfaces resistant to etch are exposed at appropriate positions. The exposure of the shoulder sub-portion at crystallographic surfaces resistant to etch contributes to the formation of the shoulder sub-portion at a less-variable position (i.e., with enhanced position stability) even under a varying actual-etching-condition. 
     In view of the above findings, the process according to the present mode is implemented using a mask pattern configured to include a compensating pattern in addition to a basic pattern, allowing a beam structure having a solid stepped portion to be integrally fabricated from a plate-shaped etchable material with greater ease, so that a shoulder sub-portion of the stepped portion is improved in position accuracy. 
     (22) The process according to any one of modes (18)-(21), wherein the to-be-processed portion includes a portion of the etchable material which is to be processed into the reflective mirror, and wherein each of the pair of mask patterns has a planar shape for forming the reflective mirror, to which a circle is more similar than a quadrangle, 
     whereby the reflective mirror is fabricated so as to have a silhouette of a planar shape to which a circle is more similar than a quadrangle, when viewed in a direction normal to the reflective surface. 
     (23) The process according to any one of modes (18)-(22), wherein each of the pair of mask patterns has a uniform thickness throughout each mask pattern. 
     This process allows a mask pattern to be formed with greater ease and higher stability than when the mask pattern is not uniform in thickness. 
     (24) The process according to any one of modes (18)-(23), wherein the etchable material is made of a single crystal silicon, and at least one of a plurality of { 100 } crystallographic planes of the single crystal silicon is assigned an initial exposed surface. 
     (25) The process according to mode (21), wherein the compensating pattern includes a first etch compensator disposed to cover surfaces of the opposite sub-portions, to thereby reduce a rate of the wet-etching performed for the to-be-half-etched portion, for preventing the etchant from reaching the to-be-unetched portion. 
     By this process, the surfaces of the opposite sub-portions of the etchable material are coated with the first etch compensator, which is a portion of the mask pattern. 
     In this regard, the opposite sub-portions are opposed to each other in a width-wise direction of the beam structure, and the to-be-half-etched portion is interposed between the opposite sub-portions. 
     The coating of the opposite sub-portions results in a reduced etch rate at which wet-etch progresses at the to-be-half-etched portion of the etchable material. 
     Therefore, this process, because of the first etch compensator, prevents wet-etch from reaching the to-be-unetched-portion, in the presence of the possibility that the to-be-unetched-portion of the etchable material is wet-etched at its shoulder sub-portion from its forward-facing side. 
     As a result, this process allows the shoulder sub-portion to be ultimately formed at a less variable position (i.e., with enhanced position stability) even under a varying actual-etching-condition. 
     (26) The process according to mode (25), wherein the first etch compensator includes a pair of wings which coextend in a length-wise direction of the beam structure, and which are disposed on respective opposite sides with respect to the to-be-half-etched portion, and 
     wherein the wings are associated with the basic pattern, such that the wings are partially coupled at one end side of the wings to the basic pattern, and such that the wings are partially open at opposite end side of the wings, whereby the wings and a portion of the basic pattern which is coupled to the wings cooperate to form a substantial U-shape. 
     (27) The process according to mode (25) or (26), wherein the first etch compensator includes a rectilinear portion which is parallel to a width-wise direction of the beam structure and which is perpendicular to at least one of a plurality of &lt;110&gt; crystallographic directions of the etchable material. 
     Portions of an etchable material which are oriented perpendicular to &lt;110&gt; crystallographic directions of the etchable material are resistant to etch. Therefore, if a shoulder sub-portion is wet-etched so that such portions are exposed, then the shoulder sub-portion is formed with enhanced position stability. 
     In view of the above findings, this process according to the present mode is implemented in a manner that the first etch compensator set forth in the above mode (25) or (26) is configured to include a rectilinear portion which is oriented parallel to the width-wise direction of the beam structure, and perpendicular to at least one of a plurality of &lt;110&gt; crystallographic directions of the etchable material. 
     (28) The process according to mode (21) or any one of modes (25)-(27), wherein the compensating pattern includes a second etch compensator disposed to cover a surface of the to-be-half-etched portion, to thereby reduce a rate of the wet-etching performed for the to-be-half-etched portion, for preventing the etchant from reaching the to-be-unetched portion. 
     By this process, the surface of the to-be-half-etched portion of the etchable material is coated with the second etch compensator, which is a portion of the mask pattern. 
     The coating of the to-be-half-etched portion results in a reduced etch rate at which wet-etch progresses at the to-be-half-etched portion of the etchable material. 
     Therefore, this process, because of the second etch compensator, prevents wet-etch from reaching the to-be-unetched-portion, in the presence of the possibility that the to-be-unetched-portion of the etchable material is wet-etched at its shoulder sub-portion from its forward-facing side. 
     As a result, this process allows the shoulder sub-portion to be ultimately formed at a less variable position (i.e., with enhanced position stability) even under a varying actual-etching-condition. 
     (29) The process according to mode (28), wherein the compensating pattern includes the first etch compensator defined in any one of modes (25)-(29) and the second etch compensator, 
     wherein the compensating pattern and a portion of the basic pattern which is coupled to the compensating pattern cooperate to form a substantial rhombus-shape having four corners and four sides, 
     wherein each side of the compensating pattern is perpendicular to at least one of a plurality of &lt;100&gt; crystallographic directions of the etchable material, and 
     wherein the compensating pattern is shaped such that the compensating pattern is coupled at one of two opposite corners of the four corners to the basic pattern, and such that the compensating pattern is cut-away at the other of the opposite corners. 
     (30) The process according to any one of modes (18)-(29), wherein the wet-etching step is implemented such that the etchable material is immersed in the etchant once, to thereby fabricate the oscillating body at a time. 
     The process according to any one of the above modes (18)-(29) may be practiced such that, in the wet-etching step, the same etchable material is immersed in an etchant several times, to thereby fabricate a beam structure in several steps. 
     In contrast, this process according to the present mode is practiced such that, in the wet-etching step, an etchable material is immersed in an etchant once, to thereby fabricate a beam structure in one step. 
     (31) A beam structure having a stepped portion at least one position on a path extending in a length-wise direction of the beam structure, 
     wherein the stepped portion is fabricated by wet-etching a plate-shaped etchable material made of a single crystal material such that the stepped portion is ultimately shaped to include: 
     (a) a higher sub-portion having the same height as a basic surface of the beam structure; 
     (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and 
     (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure. 
     This beam structure, which is configured to have a stepped portion at least one position on a path extending in a length-wise direction of the beam structure, allows this beam structure to be integrally fabricated by wet-etch. 
     This beam structure can be fabricated by the practice of the process according to any one of the above modes (18)-(30) in a suitable manner. 
     (32) The beam structure according to mode (31), fabricated by implementing the process according to any one of modes (18)-(30). 
     (33) The beam structure according to mode (31) or (32), configured to have a unitary configuration of a plate-shaped reflective mirror on which light is incident and an elastically-deformable portion extending coplanar with the reflective mirror, the elastically-deformable portion having the stepped portion formed therein. 
     (34) The beam structure according to mode (33), for use in varying an optical characteristic of light incident on the reflective surface, by vibration of the oscillating body. 
     The “vibration” set forth in the present mode and the following modes may be achieved as, for example, an angular oscillation of the beam structure about an oscillation axis oriented parallel to the reflective surface, a reciprocal motion of the beam structure along a rectilinear line oriented perpendicular to the reflective surface, etc. 
     The “optical characteristic” set forth in the present mode may be interpreted to mean, for example, an angle at which outgoing light from the reflective surface is deflected from incoming light by use of the reflective surface, the curvature of wavefront of outgoing light from the reflective surface, etc. 
     (35) The beam structure according to any one of modes (31)-(34), further comprising a laminate formed on the lower sub-portion, such that an upper face of the laminate is not above an upper face of the higher sub-portion. 
     This beam structure allows its laminate (e.g., a vibrator, an actuator) to be formed at this beam structure while preventing the laminate from protruding from the basic surface of the beam structure in the thickness-wise direction. 
     Therefore, this beam structure, when used in an environment, for example, where there is a need to thin the beam structure, allows the laminate to be formed within the beam structure, while satisfying the need. 
     (36) A process of integrally fabricating by an etching technique an oscillating body having a unitary configuration including both a reflective mirror having a reflective surface, and a plate-shaped spring, 
     wherein the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation-axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto, 
     wherein the spring is shaped to have a beam structure which extends from the reflective mirror along the oscillation axis and which has a stepped portion disposed at a position on a path extending in a length-wise direction of the beam structure, 
     wherein the spring is excited by a vibration occurring in at least a portion of the oscillating body, to cause at least a torsional vibration about the oscillation axis, to thereby angularly oscillate the reflective mirror about the oscillation axis by at least the caused torsional vibration, 
     wherein the process comprises the method according to any one of modes (1)-(14), 
     wherein the coating step includes a step of coating opposite faces of a to-be-processed portion of the etchable material with two film-like etching masks, respectively, for fabricating the beam structure, 
     wherein the mask-pattern forming step includes a step of forming a pair of mask patterns on the two etching masks which have been deposited on the opposite faces of the to-be-processed portion, respectively, for fabricating the beam structure, 
     wherein the wet-etching step includes a step of wet-etching the to-be-processed portion on which the pair of mask patterns have been deposited at the mask-pattern forming step, such that the etchable material is immersed in the etchant, for fabricating the beam structure, 
     wherein the stepped portion is shaped to include: 
     (a) a higher sub-portion having the same height as a basic surface of the beam structure; 
     (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and 
     (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure, 
     wherein the etchable material is originally shaped to include: 
     (d) a to-be-fully-etched portion of the etchable material which is to be etched through a thickness of the etchable material when wet-etched, to thereby produce the beam structure from the plate-shaped etchable material; 
     (e) a to-be-unetched portion of the etchable material which is to remain unetched when wet-etched, to thereby form the higher sub-portion; and 
     (f) a to-be-half-etched portion of the etchable material which is to be etched in half-way of the thickness of the etchable material when wet-etched, to thereby form the lower sub-portion, wherein the to-be-fully-etched portion includes opposite sub-portions which are opposed to each other in a width-wise direction of the beam structure and between which the to-be-half-etched portion is interposed, and 
     wherein the pair of mask patterns are shaped to include: 
     (g) a basic pattern shaped to cover a surface of the to-be-unetched portion; and 
     (h) a compensating pattern shaped to cover a surface of at least opposite sub-portions of the opposite sub-portions and the to-be-half-etched portion. 
     This process can achieve the effects in common to those of the process according to the above mode (21), by the principle in common to that of the process according to the above mode (21). In other words, this process allows the beam structure of the oscillating to be fabricated so that a shoulder sub-portion of the stepped portion is improved in position accuracy. 
     (37) The process according to mode (36), wherein each of the pair of mask patterns has a uniform thickness throughout each mask pattern. 
     This process can achieve the effects in common to those of the process according to the above mode (23). 
     (38) The process according to mode (36) or (37), wherein the etchable material is made of a single crystal silicon, and is exposed at least one of a plurality of {100} crystallographic planes. 
     (39) The process according to any one of modes (36)-(38), wherein the compensating pattern includes a first etch compensator disposed to cover surfaces of the opposite sub-portions, to thereby reduce a rate of the wet-etching performed for the to-be-half-etched portion, for preventing the etchant from reaching the to-be-unetched portion. 
     This process can achieve the effects in common to those of the process according to the above mode (25), by the principle in common to that of the process according to the above mode (25). 
     (40) The process according to mode (39), wherein the first etch compensator includes a pair of wings which coextend in a length-wise direction of the beam structure, and which are disposed on respective opposite sides with respect to the to-be-half-etched portion, and 
     wherein the wings are associated with the basic pattern, and that the wings are partially coupled at one end side of the wings to opposite end side of the wings, whereby the wings and a portion of the basic pattern which is coupled to the wings cooperate to form a substantial U-shape. 
     (41) The process according to mode (39) or (40), wherein the first etch compensator includes a rectilinear portion which is parallel to a width-wise direction of the beam structure and which is perpendicular to at least one of a plurality of &lt;110&gt; crystallographic directions of the etchable material. 
     This process can achieve the effects in common to those of the process according to the above mode (27), by the principle in common to that of the process according to the above mode (27). 
     (42) The process according to any one of modes (36)-(41), wherein the compensating pattern includes a second etch compensator disposed to cover a surface of the to-be-half-etched portion, to thereby reduce a rate of the wet-etching performed for the to-be-half-etched portion, for preventing the etchant from reaching the to-be-unetched portion. 
     This process can achieve the effects in common to those of the process according to the above mode (28), by the principle in common to that of the process according to the above mode (28). 
     (43) The process according to mode (42), wherein the compensating pattern includes the first etch compensator defined in any one of modes (39)-(41), and the second etch compensator, wherein the compensating pattern and a portion of the basic pattern which is coupled to the compensating pattern cooperate to forms a substantial rhombus-shape having four corners and four sides, 
     wherein each side of the compensating pattern is perpendicular to at least one of a plurality of &lt;100&gt; crystallographic directions  6  of the etchable material, and 
     wherein the compensating pattern is shaped such that the compensating pattern is coupled at one of two opposite corners of the four corners to the basic pattern, and such that the compensating pattern is cut-away at the other of the opposite corners. 
     (44) The process according to any one of modes (36)-(43), wherein the wet-etching step is implemented such that the etchable material is immersed in the etchant once, to thereby fabricate the oscillating body at a time. 
     This process can achieve the effects in common to those of the process according to the above mode (30), by the principle in common to that of the process according to the above mode (30). 
     (45) An oscillating body having a unitary configuration including both a reflective mirror according to any one of modes (15)-(17), and a plate-shaped spring, 
     wherein the reflective mirror is used for optical scan, in a manner that the reflective mirror is angularly oscillated about an oscillation axis parallel to the reflective surface, to thereby vary a direction in which light is reflected from the reflective surface upon entry thereinto, 
     wherein the spring is shaped to have a beam structure which extends from the reflective mirror along the oscillation axis and which has a stepped portion, 
     wherein the spring is excited by a vibration occurring in at least a portion of the oscillating body, to cause at least a torsional vibration about the oscillation axis, to thereby angularly oscillate the reflective mirror about the oscillation axis by at least the caused torsional vibration, and 
     wherein the stepped portion is ultimately shaped to include: 
     (a) a higher sub-portion having the same height as a basic surface of the beam structure; 
     (b) a lower sub-portion lower than the basic surface, and lower than the higher sub-portion in a thickness-wise direction of the beam structure; and 
     (c) a shoulder sub-portion which is located at a border between the higher and lower sub-portions and which traverses the beam structure. 
     (46) The oscillating body according to mode (45), fabricated by implementing the process defined in any one of modes (36)-(44). 
     (47) The oscillating body according to mode (45) or (46), having a unitary configuration of the reflective mirror and an elastically-deformable portion extending coplanar with the reflective mirror, the elastically-deformable portion having the stepped portion formed therein. 
     (48) The oscillating body according to any one of modes 
     (45)-(47), for use in varying an optical characteristic of light incident on the reflective surface, by vibration of the oscillating body. 
     (49) The oscillating body according to any one of modes (45)-(48), further comprising a laminate formed on the lower sub-portion, such that an upper face of the laminate is not above an upper face of the higher sub-portion. 
     Several presently preferred embodiments of the invention will be described in detail by reference to the drawings in which like numerals are used to indicate like elements throughout. 
     First Embodiment 
     Referring first to  FIG. 1 , there is schematically illustrated a retinal scanning display including a reflective mirror for optical scan which has been fabricated by a reflective-mirror fabricating method according to a first embodiment of the present invention. 
     This retinal scanning display (hereinafter, abbreviated to “RSD”) is an apparatus adapted to allow a laser beam, with wavefront curvature and light intensity being properly modulated, to impinge onto a retina  14  through a pupil  12  of a viewer&#39;s eye  10 . This RSD allows the laser beam to be two-dimensionally scanned on the retina  14 , to thereby directly project a desired image onto the retina  14 . 
     This RSD includes a light source unit  20 , and a wavefront-curvature modulating optical system  22  and a scanning unit  24  both of which are disposed between the light source unit  20  and the viewer&#39;s eye  10  arrayed in the description order. 
     For generating a beam of laser of any color by combining three beams of laser of three primary colors (RGB) into a single beam of laser, the light source unit  20  includes an R laser  30  emitting a red-colored beam of laser, a G laser  32  emitting a green-colored beam of laser, and a B laser  34  emitting a blue-colored beam of laser. These lasers  30 ,  32 , and  34  each may be constructed as a semiconductor laser, for example. 
     For beams of laser emitted from the respective lasers  30 ,  32 , and  34  to be eventually combined, these beams of laser are collimated by collimating optical systems  40 ,  42 , and  44 , respectively, and thereafter, these beams of laser are caused to enter respective dichroic mirrors  50 ,  52 , and  54  all of which are wavelength-selective. As a result, these beams of laser are selectively reflected from or transmitted through the corresponding respective dichroic mirrors  50 ,  52 , and  54 , depending on the wave length of each beam of laser. 
     More specifically, a red-colored beam of laser emitted from the R laser  30  is caused to enter the dichroic mirror  50  after collimated by the collimating optical system  40 . A green-colored beam of laser emitted from the G laser  32  is caused to enter the dichroic mirror  52  through the collimating optical system  42 . A blue-colored beam of laser emitted from the B laser  34  is caused to enter the dichroic mirror  54  through the collimating optical system  44 . 
     Upon entry into the respective three dichroic mirrors  50 ,  52 , and  54 , the beams of laser of three primary colors eventually enter the dichroic mirror  50 , which is a representative one of the dichroic mirrors  50 ,  52 , and  54 , resulting in the beams of Laser being combined thereat. The combined beam of laser is subsequently focused at a combining optical system  56 . 
     Although the optical section of the light source unit  20  has been described above, then there will be described the electrical section of the light source unit  20 . 
     The light source unit  20  includes a signal processing circuit  60  constructed principally with a computer. The signal processing circuit  60  is configured to perform, in response to an externally-supplied video signal, signal processing for driving the respective lasers  30 ,  32 , and  34 ; and signal processing for implementing a scanning operation of a laser beam. 
     In operation, the signal processing circuit  60  supplies drive signals for driving the respective lasers  30 ,  32 , and  34 , in response to the externally-supplied video signal, per each pixel of a desired image to be projected onto the retina  14 . These drive signals, which are required for obtaining desired color and intensity of a beam of laser, are routed to the corresponding respective lasers  30 ,  32 , and  34  via corresponding respective laser drivers  70 ,  72 , and  74 . The signal processing for scanning a laser beam will be described below. 
     The light source unit  20  described above causes a laser beam to be focused at the combining optical system  56  and to be entered into an optical fiber  82 . Upon entry into the optical fiber  82 , the laser beam passes through the optical fiber  82  functioning as a light transmissive medium, and enters the wavefront-curvature modulating optical system  22  via a collimating optical system  84  which collimates the laser beam exiting the optical fiber  82  at its rearward end divergently. 
     This wavefront-curvature modulating optical system  22  is an optical system for modulating a curvature of wavefront of a laser beam emitted from the light source unit  20 . This wavefront-curvature modulating optical system  22  may be of a type, although it is inessential to practice the present invention, that performs the wavefront curvature modulation per each pixel of an image to be projected onto the retina  14 , or alternatively, may be of a type that performs the wavefront curvature modulation per each frame of an image. 
     Modulating a wavefront results in the adjustment of depth perception of a displayed image, or the adjustment of a in-focus-position of a displayed image. 
     In any case, this wavefront-curvature modulating optical system  22  modulates the curvature of wavefront of an incoming laser beam, based on a depth signal inputted from the signal processing circuit  60 . In this wavefront-curvature modulating optical system  22 , a laser beam incoming from the collimating optical system  84  in the form of parallel light is transformed into converging light by means of a converging lens  90 . 
     The converging light into which parallel light has been transformed is transformed into diverging light due to reflection of a movable mirror  92 . The diverging light into which the converging light has been transformed passes through the converging lens  90 , and leaves the wavefront-curvature modulating optical system  22  in the form of a laser beam having a desired curvature of wavefront. 
     As illustrated in  FIG. 1 , this wavefront-curvature modulating optical system  22  includes: a beam splitter  94  causing a laser beam entered from the outside to be reflected from or passed through the wavefront-curvature modulating optical system  22 ; the converging lens  90  to converge the laser beam entered thereinto through the beam splitter  94 ; and the movable mirror  92  to reflect the laser beam converged by the converging lens  90 . 
     This wavefront-curvature modulating optical system  22  further includes an actuator  96  for causing the movable mirror  92  to be displaced in a direction allowing the movable mirror  92  to move toward or away from the converging lens  90 . An example of the actuator  96  is a piezoelectric element. The actuator  96  moves the movable mirror  92  in response to a depth signal entered from the signal processing circuit  60 , to thereby modulate the wavefront curvature of a laser beam emerging from the wavefront-curvature modulating optical system  22 . 
     In the wavefront-curvature modulating optical system  22  constructed as described above, a laser beam entered from the collimating optical system  84  is reflected from the beam splitter  94  into the converging lens  90  and is then reflected from the movable mirror  92 . Thereafter, the laser beam passes through the converging lens  90  again, and then passes through the beam splitter  94  to be directed to the scanning unit  24 . 
     The scanning unit  24  includes a horizontal scanning system  100  and a vertical scanning system  102 . 
     The horizontal scanning system  100  is an optical system for performing a horizontal scan in which a laser beam is scanned horizontally, per each frame of an image to be displayed. On the other hand, the vertical scanning system  102  is an optical system for performing a vertical scan in which a laser beam is scanned vertically, per each frame of an image to be displayed. 
     The horizontal scanning system  100  is configured to scan a laser beam at a higher scan rate, namely, a higher frequency than the vertical scanning system  102 . 
     More specifically, in the present embodiment, the horizontal scanning system  100  includes an optical scanner  104  in which a resilient material incorporating a mirror for performing mechanical deflection is vibrated, to thereby angularly oscillate the mirror. The optical scanner  104  is controlled in response to a horizontal sync signal supplied from the signal processing circuit  60 . 
     Referring next to  FIG. 2 , the optical scanner  104  is illustrated in exploded perspective view. As illustrated in  FIG. 2 , the optical scanner  104  is constructed by mounting a body  110  onto a base  112 . 
     The body  110  is formed with an elastic material such as silicon. As illustrated at the top of  FIG. 2 , the body  110  is generally in the shape of rectangle-shaped thin-plate with a light-transmissive through-hole  114 . The body  110  includes at its outside a stationary frame  116 , while the body  110  includes at its inside an oscillating body  124  having a reflective mirror  122  on which a reflective surface  120  is formed. 
     In comply with the construction of the body  110 , as illustrated at the bottom of  FIG. 2 , the base  112  is constructed to include a support  130  on which the stationary frame  116  is to be mounted when the body  110  is mounted on the base  112 , and a cavity  132  opposing to the oscillating body  124 . The cavity  132  is formed to have a shape avoiding interference with the base  112  even when the oscillating body  124  is displaced due to vibration thereof, with the body  110  being mounted on the base  112 . 
     As illustrated in  FIG. 2 , the reflective surface  120  of the reflective mirror  122  is oscillated about an oscillation axis  134 , which is also a symmetry line of the reflective mirror  122 . The oscillating body  124  further includes beams  140 ,  140  extending from the reflective mirror  122  in coplanar relation, for coupling the reflective mirror  122  to the stationary frame  116 . In the present embodiment, a pair of beams  140 ,  140  oppositely extend from opposite ends of the reflective mirror  122 , respectively. 
     Each of the beams  140  is so constructed as to include a single mirror-side leaf spring  142 , a pair of frame-side leaf springs  144 ,  144 , and a connection  146  for connecting the mirror-side leaf spring  142  to the pair of frame-side leaf springs  144 ,  144 . 
     The mirror-side leaf springs  142  extend on and along the oscillation axis  134 , from both ends of the reflective mirror  122  opposing to each other in a direction of the oscillation axis  134 , respectively, up to the corresponding respective connections  146 ,  146 . 
     For each beam  140 , the corresponding pair of frame-side leaf springs  144  extend from the corresponding connection  146  along the oscillation axis  134  so as to be offset from the oscillation axis  134  in opposite directions. 
     As illustrated in  FIG. 2 , for the respective beams  140 ,  140 , actuators  150 ,  152 ,  154 , and  156  are secured to the pairs of frame-side leaf springs  144  and  144 , with the actuators  150 ,  152 ,  154 , and  156  extending to the stationary frame  116 . 
     As illustrated in  FIG. 3 , each frame-side leaf spring  144  is locally thinned on its proximal side to the stationary frame  116 , resulting in formation of a recess  158 . A recess  159  is formed at the stationary frame  116  to achieve surface continuity between the recesses  158  and  159 . Utilization of these recesses  158  and  159  allows each actuator  150 ,  152 ,  154 ,  156  to be disposed such that each actuator  150 ,  152 ,  154 ,  156  extends between the corresponding frame-side leaf spring  144  and the stationary frame  116 . 
     Each actuator  150 ,  152 ,  154 ,  156  is constructed principally by a piezoelectric material  170  (referred to also as “piezoelectric vibrator” or “piezoelectric element”). The actuator  154  is illustrated representatively in  FIG. 3 . The piezoelectric material  170 , which is thin-plate-shaped, is attached to the oscillating body  124  at its one side face and is interposed between an upper electrode  172  and a lower electrode  174  in a direction perpendicular to the one side face. 
     As illustrated in  FIG. 3 , the upper and lower electrodes  172  and  174  are electrically connected to a pair of input terminals  178  and  178 , respectively, which are mounted on the stationary frame  116 , via respective lead wires (not shown). 
     Alternatively, the present invention may be carried out in a mode in which the upper and lower electrodes  172  and  174  are electrically connected to an external terminal (not shown), respectively, via respective lead wires (not shown). 
     Application of a voltage to these upper and lower electrodes  172  and  174  causes the piezoelectric material  170  to be displaced in a direction perpendicular to a direction in which the voltage has been applied. The displacement causes the beams  140  to bend or curve, as illustrated in  FIG. 4 . The bending occurs in a manner that a portion of the beam  140  which is coupled to the stationary frame  116  acts as a fixed end, while a portion of the beam  140  which is coupled to the reflective mirror  122  acts as a free end. 
     As a result, whether the free end is displaced upwardly or downwardly depends on whether the beams  140 ,  140  bend upwardly or downwardly. 
     As will be evident from  FIG. 4 , among the four actuators  150 ,  152 ,  154 , and  156  attached onto the respective four frame-side leaf springs  144 , a pair of actuators  150  and  152  are positioned on one of sides opposite to each other with respect to the oscillation axis  134 , with the reflective mirror  122  being interposed between the actuators  150  and  152 . 
     A pair of actuators  154  and  156  which are positioned on the other side, with the reflective mirror  122  being interposed between the actuators  154  and  156 , individually bend in a manner that two of the piezoelectric materials  170  which belong to each of the pair of actuators  150  and  152  and the pair of actuators  154  and  156  are displaced in the same direction at their free ends. 
     On the other hand, a pair of actuators  150  and  154  are positioned on one of sides opposite to each other with respect to the reflective mirror  122 , with the oscillation axis  134  being interposed between the actuators  150  and  154 . 
     A pair of actuators  152  and  156  which are positioned on the other side, with the oscillation axis  134  being interposed between the actuators  152  and  156 , individually bend in a manner that two of the piezoelectric materials  170  which belong to each of the pair of actuators  150  and  154  and the pair of actuators  152  and  156  are displaced in opposite directions at their free ends. 
     As a result, as illustrated in  FIG. 4 , unidirectional rotational displacement of the reflective mirror  122  is excited by both the displacement in one direction of the pair of actuators  150  and  152  positioned on one of sides opposite to each other with respect to the oscillation axis  134 , and the displacement of the pair of actuators  154  and  156  positioned on the other side in a reverse direction of the displacement of the actuators  150  and  152 . 
     To summarize the above, each frame-side leaf spring  144  has the function of transforming the rectilinear displacement (lateral displacement) of the piezoelectric material  170  attached onto each frame-side leaf spring  144  into a bending movement (vertical displacement). Each connection  146  has the function of transforming the bending movement of the corresponding frame-side leaf springs  144  into a rotary movement of the corresponding mirror-side leaf spring  142 . The rotary movement of the mirror-side leaf spring  142  causes a rotation of the reflective mirror  122 . 
     Therefore, in the present embodiment, for the control of the four actuators  150 ,  152 ,  154 , and  156 , two actuators  150  and  15 S which are positioned on one of sides opposite to each other with respect to the oscillation axis  134 , which is to say, the actuator  150  positioned at the upper right of  FIG. 2  and the actuator  152  positioned at the upper left of  FIG. 2 , constitute a first pair, while two actuators  154  and  156  which are positioned on the other side, which is to say, the actuator  154  positioned at the lower right of  FIG. 2  and the actuator  156  positioned at the lower left of  FIG. 2 , constitute a second pair. 
     In the present embodiment, for displacing the two actuators  150  and  152  forming the first pair and the two actuators  154  and  156  forming the second pair in opposite directions to thereby excite reciprocal rotation or angular oscillation of the reflective mirror  122  about its oscillation axis  134 , alternating voltages identical in phase to each other are applied to the two actuators  150  and  152  forming the first pair, while alternating voltages identical in phase to each other but opposite in phase to the alternating voltages for the first pair, are applied to the two actuators  154  and  156  forming the second pair. 
     As a result, where both of the two actuators  150  and  152  forming the first pair bend downwardly in  FIG. 4 , both of the two actuators  154  and  156  forming the second pair bend upwardly in  FIG. 4 . 
     For achieving the control described above, the horizontal scanning system  100  includes a horizontal scanning drive circuit  180  depicted in  FIG. 1 . As illustrated in  FIG. 5 , in this horizontal scanning drive circuit  180 , an oscillator  182  generates an alternating voltage signal in response to a horizontal sync signal entered from the signal processing circuit  60 . 
     The oscillator  182  is electrically coupled to the two actuators  150  and  152  forming the first pair via a first path through a phase shifter  184  and an amplifier  186 , while the oscillator  182  is electrically coupled to two actuators  154  and  156  forming the second pair via a second path through a phase inverter circuit  188 , a phase shifter  190 , and an amplifier  192 . 
     The phase inverter circuit  188  inverts the phase of an alternating voltage signal entered from the oscillator  182  and supplies to the phase shifter  190  the alternating voltage signal which has been inverted in phase. 
     This phase inverter circuit  188  is provided only to the second path. Therefore, the alternating voltage signals supplied from the corresponding respective amplifiers  186  and  192  become opposite in phase to each other between the two actuators  150  and  152  forming the first pair and the two actuators  154  and  156  forming the second pair. 
     With both the first and second paths, the respective phase shifters  184  and  190  are provided for alternating the phase of an alternating voltage signal to be supplied to the actuators  150 ,  152 ,  154 , and  156  to establish synchronization between the aforementioned video signal and the oscillation of the reflective mirror  122 . 
     In  FIG. 6 , there is illustrated in perspective view a specific shape of the oscillating body  124 . In this oscillating body  124 , the reflective mirror  122  is fabricated so as to have a silhouette generally in the shape of a circle when viewed in a direction normal to the reflective surface  120 . This reflective mirror  122  constitutes an example of the “reflective mirror” set forth in the above mode (15). 
     In the present embodiment, the reflective mirror  122  has a diameter of 1 mm and a thickness of 100 μm, while each beam  140  has a length of 2 mm. The method of fabricating the reflective mirror  122  will be described below in more detail. 
     In  FIG. 7 , there is illustrated in perspective view how light emerged from the optical fiber  82  enters this optical scanner  104  through the collimating optical system  84 . 
     The light emerged from the collimating optical system  84  enters the reflective surface  120  of the reflective mirror  122 , wherein the reflective surface  120  is generally in the shape of circle. The reflective mirror  122  is angularly oscillated about the oscillation axis  134 , resulting in the reflected light from the reflective surface  120  being horizontally scanned. 
     In  FIG. 8 , there is illustrated in perspective view how light emerged from the optical fiber  82  enters a conventional optical scanner  300  through the collimating optical system  84 . In this optical scanner  300 , a reflective mirror  302  is in the shape of a quadrangle. 
     When comparing the weight of the reflective mirror  302  and the weight of the reflective mirror  122  of the present embodiment with each other, assuming that these mirrors  302  and  122  are identical to each other in material density, thickness, and maximum width dimension, the reflective mirror  122  of the present embodiment is smaller in weight than the counterpart. 
     Therefore, when comparing the moment of inertia of the reflective mirror  122  of the present embodiment about the oscillation axis  134  and the moment of inertia of the reflective mirror  302  about its oscillation axis  304  with each other, the reflective mirror  122  of the present embodiment is smaller in moment of inertia than the counterpart. 
     As a result, where the reflective mirror  122  of the present embodiment and the conventional reflective mirror  302  are each adapted to scan light using its own resonance, the reflective mirror  122  of the present embodiment is more suitable than the conventional reflective mirror  302  in an attempt to increase a resonant frequency for increasing a scan frequency. 
     As illustrated in  FIG. 1 , a laser beam scanned horizontally by the optical scanner  104  described above is relayed to the vertical scanning system  102  through a relay optical system  194 . 
     This RSD is provided with a beam detector  200  at a fixed position for detecting a laser beam which has been deflected by the optical scanner  104 , to thereby detect the position of the scanned laserbeam in a main scan direction (i.e., horizontal scan direction). An example of the beam detector  200  may be a photodiode. 
     The beam detector  200  outputs a BD signal indicating that a laser beam has reached a predetermined position. The outputted BD signal is delivered to the signal processing circuit  60 . 
     In response to the delivery of the BD signal from the beam detector  200 , the signal processing circuit  60  applies needed drive signals to the respective laser drivers  70 ,  72 , and  74 , upon elapse of a predetermined length of time since the beam detector  200  detected the laser beam. 
     This allows a start-of-image-displaying timing to be determined per each scan line, and with the determined timing, displaying of an image starts. This ensures synchronization between an image signal and a laser beam scan position. 
     While the horizontal scanning system  100  has been described above, the vertical scanning system  102  includes a galvanometer mirror  210  as an example of an angularly-oscillating mirror performing mechanical deflection, as illustrated in  FIG. 1 . 
     The galvanometer mirror  210  is adapted such that a laser beam emerged from the horizontal scanning system  100 , upon focused by the relay optical system  194 , enters the galvanometer mirror  210 . The galvanometer mirror  210  is oscillated about an axis of rotation crossing the optical axis of the laser beam entering the galvanometer mirror  210 . The start-up timing and the rotational speed of the galvanometer mirror  210  are controlled in response to a vertical sync signal supplied from the signal processing circuit  60 . 
     It is added that this vertical scanning system  102 , although is constructed principally using the galvanometer mirror  210 , may be constructed in an alternative arrangement. 
     The horizontal scanning system  100  and the vertical scanning system  102  both described above cooperate to scan a laser beam two-dimensionally. An image represented by the scanned laser beam impinges on the viewer&#39;s eye  10  via a relay optical system  214 . 
     Now, there will be described the fabricating method of the reflective mirror  122  in more detail with reference to a process chart of  FIG. 9 . 
     This fabrication method begins with a step S 1  to prepare a plate-shaped material (a silicon wafer) which is made of a single crystal silicon and which has a thickness of 100 μm, for the material to be used as an etchable material  400  (see  FIG. 11 ). The single crystal silicon is initially exposed at some of {100} crystallographic planes of the single crystal silicon. That is to say, the single crystal silicon has an initial exposed {100} surface. 
     The step S 1  is followed by a step S 2  to coat opposite faces of the etchable material  400  with etching masks  410  (see  FIG. 11 ). The etching masks  410  are, for example, silicon oxide coatings formed on the opposite faces of the etchable material  400  as a result of heating the etchable material  400 . That is to say, the step S 2  constitutes a coating step. 
     The step S 2  is followed by a step S 3  to form predetermined mask patterns by a lithographic technique on the respective etching masks  410  which have been deposited on the opposite faces of the etchable material  400 . 
     The shape of each mask pattern formed on each etching mask  410  determines the shape of a portion of the etchable material  400  which will start being contacted with and then being etched by an etchant held in an etching bath (not shown) when the etchable material  400  is immersed in the etching bath. That is to say, the step S 3  constitutes a mask-pattern forming-step. 
     In  FIG. 10 , an example of each mask pattern is illustrated in top plan view. 
     In the present embodiment, for convenience of explanation of ultimately-exposed crystallographic planes of the etchable material  400 , a surface of the etchable material  400  is conceptually divided into four equal regions I, II, III, and IV around a center point PC (Point of Center) of the surface of the etchable material  400 , by relying on symmetric ultimate-shape of the etchable material  400 , as illustrated in  FIG. 10 . In  FIG. 13 , one of the four regions I, II, III, and IV is representatively illustrated. 
     In the present embodiment, as illustrated in  FIG. 10 , each mask pattern includes first sides each parallel to the oscillation axis  134 , and second sides each perpendicular to the oscillation axis  134 . The oscillation axis  134  constitutes an example of the “reference line” set forth in the aforementioned mode (6). 
     Each mask pattern is located relative to the etchable material  400  depicted in  FIG. 11 , such that each of the first and second sides is perpendicular to a selected one of &lt;110&gt; crystallographic directions of the etchable material  400  on a selected one of {100} crystallographic planes of the etchable material  400 . 
     In  FIG. 10 , the selected {100} crystallographic plane, which is the aforementioned initial exposed {100} surface, is parallel to the sheet of  FIG. 10 . 
     In the present embodiment, as illustrated in  FIG. 10 , the first sides correspond to ones of a plurality of second portions  422  described below which extend parallel to the oscillation axis  134 , wherein the ones extend horizontally in  FIG. 10 . On the other hand, the second sides correspond to ones of the plurality of second portions  422  which extend perpendicular to the oscillation axis  134 , wherein the ones extend vertically in  FIG. 10 . 
     For defining the shape of this mask pattern, for convenience of explanation, the etchable material  400  is divided into the four regions I, II, III, and IV by two center lines of symmetry orthogonally intersecting each other at the center point of the mask pattern. One of the center lines is the oscillation axis  134 , and the other is a straight line orthogonally intersecting the oscillation axis  134  at the center point PC. 
     In the present embodiment, as illustrated in  FIG. 10 , each mask pattern further includes, at its both ends opposite to each other in the direction of the oscillation axis  134 , extensions  431  and  431  which each extend outwardly of each mask pattern along the oscillation axis  134 . The extensions  431  and  431  are provided to the mask pattern for allowing the beams  140  and  140  illustrated in  FIG. 4  to be fabricated together with the reflective mirror  122  by a wet-etching technique. 
     Referring now to  FIG. 13 , there will be described the relationship between the orientation of each of the four regions (i.e., quarter regions) I, II, III, and IV, and the orientation of an x-y-z coordinate system assigned to the etchable material  400  for defining a plurality of crystallographic planes of the etchable material  400  made of a single crystal material. 
     The orientation of each quarter region and the orientation of the x-y-z coordinate system are defined, such that a bisecting line of a central angle of the quarter region of interest coincides with an x-axis of the x-y-z coordinate system. For the example illustrated in  FIG. 13 , the central angle is one of four corners of a quadrangle which is located uppermost in  FIG. 13 , wherein the quadrangle is the shape of the quarter region of interest. 
     That is to say, the etchable material  400  is quartered such that a normal to a selected one of the {100} crystallographic planes of each region coincides with the bisecting line of the central angle of each quarter region. For the example illustrated in  FIG. 13 , the selected {100} crystallographic plane is denoted by “{ 100 }” in  FIG. 13 . 
     In  FIG. 10 , the aforementioned x-y-z coordinate system is illustrated not with the etching mask  410  but with the etchable material  400 . In  FIG. 10 , the mask pattern is divided into four equal regions I, II, III, and IV which are located on the surface of the etchable material  400  and around the center point PC of the surface of the etchable material  400 . Although the x-y-z coordinate system is assigned to each region,  FIG. 10  illustrates the assignment only for a representative region I. 
     As illustrated in  FIG. 10 , the mask pattern is in the shape of a convex octagon. More specifically, an outline of the mask pattern includes, per each region, a first portion  420  corresponding to one of {100} crystallographic planes of the etchable material  400  which is different from the aforementioned {100} crystallographic plane (i.e., the initial exposed {100} surface). 
     The outline of the mask pattern further includes second portions  422  each corresponding to one of {111} crystallographic planes of the etchable material  400 . The {111} crystallographic planes includes a specific (111) crystallographic plane and its equivalent crystallographic planes. The second portions  422  are located at opposite ends of the first portion  420 , respectively. 
     The mask patterns, each of which has the shape described above, are formed on the opposite faces of the etchable material  400  without misregistration. Thereafter, as illustrated in  FIG. 9 , in a step S 4 , a laminate of the etchable material  400  and the etching masks  410  is immersed in the etching bath containing the etchant. 
     In the present embodiment, the etchant is prepared to include a Potassium Hydroxide (KOH) solution with a preset concentration of 40 wt. % and a preset temperature of 70.degree. C. Under this condition, the etchable material  400  is wet-etched. That is to say, the step S 4  constitutes a wet-etching step. 
     It is added that the etchant may be alternatively prepared to include a Tetramethyl Ammonium Hydroxide (TMAR) solution, although in this alternative the shape of a corresponding mask pattern will be modified from that illustrated in  FIG. 10 . 
     In  FIGS. 11(   a ),  11 ( b ),  12 ( a ), and  12 ( b ), how the etchable material  400  is wet-etched is illustrated step by step. In each figure, however, only one of the aforementioned four regions I, II, III, and IV is representatively illustrated by virtue of the symmetry of the etchable material  400 . 
     In  FIG. 11(   a ), the wet-etching of the etchable material  400  starts at portions of the etchable material  400  which have been un-coated with the etching masks  410 . In this stage, the etchable material  400  is wet-etched to expose only some of the {100} crystallographic planes (which are different from the aforementioned initial exposed {100} surface) and some of {111} crystallographic planes. 
     As illustrated in  FIG. 11(   b ), when the wet-etching of the etchable material  400  progresses slightly from the stage illustrated in  FIG. 11(   a ), the etchable material  400  starts exposing additional crystallographic planes at portions between the firstly-exposed {100} and {111} planes, which is to say, the corners of an octagon exhibited by the etching mask  410  in the stage illustrated in  FIG. 11(   a ). As a result, the corners of the etchable material  400  are rounded relative to their original corners. 
     As illustrated in  FIG. 12(   a ), when the wet-etching of the etchable material  400  progresses slightly from the stage illustrated in  FIG. 11(   b ), the crystallographic planes previously created between the {100} and {111} planes in the stage illustrated in  FIG. 11(   b ) grow, and the etchable material  400  starts exposing still additional crystallographic planes. As a result, the corners of the etchable material  400  are rounded relative to their original corners. 
     As illustrated in  FIG. 12(   b ), when the wet-etching of the etchable material  400  progresses slightly from the stage illustrated in  FIG. 12(   a ), the crystallographic planes previously created between the {100} and {111} planes in the stage illustrated in  FIG. 12(   a ) grow, and the etchable material  400  starts exposing yet additional crystallographic planes. As a result, the corners of the etchable material  400  are further rounded relative to their original corners. 
     The stage illustrated in  FIG. 12(   b ) is an ultimate stage of the wet-etching in which the etchable material  400  has been partially penetrated in its thickness-wise direction because of the etchant. In  FIG. 12(   b ), the ultimate shape of the etchable material  400  is illustrated with the mask pattern. 
     In  FIG. 13 , the etchable material  400  depicted in  FIG. 12(   b ) is illustrated in enlargement. Upon completion of the wet-etching of the etchable material  400 , there are exposed between the {100} and {111} crystallographic planes a plurality of additional crystallographic planes including {211}, {311}, {411}, and {511} crystallographic planes. 
     These {211}, {311}, {411}, and {511} crystallographic planes are defined to include specific crystallographic planes specified by a string of figures in parenthesis and their equivalent crystallographic planes, as with the {111} crystallographic planes. 
     That is to say, in the present embodiment, as illustrated in  FIG. 10 , the etching mask  410  includes third portions  424  corresponding to {n11} (n: an integer equal to or greater than two) crystallographic planes disposed between the first portions  420  and the second portions  422 . 
     The reflective mirror  122  fabricated by the wet-etching technique described above has a silhouette generally in the shape of a convex octagon as viewed in a direction normal to the reflective surface  120  of the reflective mirror  122 . More precisely, the silhouette is in the shape of an m-sided polygon (m: an integer greater than sixteen). 
     In  FIG. 14(   a ), a completed reflective mirror  122  is illustrated in a vertical sectional-view taken on a plane passing through some of the {100} crystallographic planes. 
     In  FIG. 14(   b ), the completed reflective mirror  122  is illustrated in a vertical sectional-view taken on a plane passing through some of the {111} crystallographic planes. 
     In  FIG. 14(   c ), a comparative example of the reflective mirror  122  is illustrated in a vertical sectional-view. The comparative example is a reflective mirror  122 ′ which has been fabricated by wet-etching an etchable material  400 ′ only from one of opposite faces of the etchable material  400 ′. The vertical sectional-view is taken on a plane passing through some of the {111} crystallographic planes of the etchable material  400 ′. 
     As illustrated in  FIG. 14(   b ), in the present embodiment, the etchable material  400  is wet-etched from its opposite faces, to thereby form at lateral faces of the etchable material  400 , discontinuous inclined surfaces which are symmetrical to each other with respect to a line extending parallel to the etchable material  400  and passing through the center of the thickness of the etchable material  400 . 
     Therefore, the present embodiment makes it easier to fabricate the reflective mirror  122  which is reduced in weight and moment of inertia, than when the aforementioned comparative example is fabricated. 
     In the present embodiment, between the {100} and {111} crystallographic surfaces, additional crystallographic surfaces (hereinafter, referred to as “interposed crystallographic surfaces”) are exposed in the form of selected ones of families of crystallographic planes. 
     Therefore, the present embodiment enables the shape of the reflective mirror  122  to be rounded relative to the shape of a sixteen-sided polygon which is exhibited by the reflective mirror  122  when alternatively fabricated so as to expose the interposed crystallographic surfaces in the form of a selected one of families of crystallographic planes. 
     This is because a tendency exists that the larger the number of line segments (corresponding to the crystallographic surfaces) constituting an outline of a silhouette of the reflective mirror  122  when viewed in a direction normal to the reflective mirror  122 , the more exactly the outline approximates a circle. 
     Further, in the present embodiment, the etchable material  400  is etched at both the {100} and {111} crystallographic planes by a wet-etching technique, with the result that the {100} and {111} crystallographic surfaces of the etchable material  400  are reduced in length from their original dimensions. The length reductions are filled by replacement with newly-created additional crystallographic surfaces in an oblique orientation with respect to both the {100} and {111} crystallographic surfaces. 
     Therefore, the present embodiment allows a reduction in length of the exposed crystallographic surfaces of the reflective mirror  122  which together form its outer circumferential surface, resulting in the promotion of equalization in length among the exposed crystallographic surfaces of the reflective mirror  122 . This enables the shape of the reflective mirror  122  to be rounded, as with the creation of the interposed crystallographic surfaces of plural types. 
     This is because the tendency exists that, assuming that the number of the line segments (corresponding to the crystallographic surfaces) constituting the outline of the silhouette of the reflective mirror  122  is held constant, the shorter the longest one of the line segments, the more exactly the outline approximates a circle. 
     It is added that, in  FIG. 15 , an example of a modified version of the mask pattern is illustrated. In this example, similarly with the aforementioned example, an outline of the mask pattern includes first portions  430  corresponding to {100} crystallographic planes, second portions  432  corresponding to {111} crystallographic planes, and third portions  434  corresponding to {n11} (n: an integer equal to or greater than two) crystallographic planes. 
     This example depicted in  FIG. 15  is different from the previous example depicted in  FIG. 10  in terms of the positions of the first portions  420 ,  430  relative to the positions of the second portions  422 ,  432 . 
     Once the wet-etching step is terminated in a manner as described above, as illustrated in  FIG. 9 , a step S 5  follows to eject the etchable material  400  from the etching bath, whose opposite faces have been coated with the etching masks  410 . A step S 6  follows to remove the etching masks  410  from the opposite faces of the etchable material  400 . A step S 7  follows to form a reflective layer made of aluminum or silver on at least one of the opposite faces of the etchable material  400 . 
     Then, a succession of implementations in the reflective-mirror fabricating method is completed. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. 
     The present embodiment is different from the first embodiment only with respect to the shape of each mask pattern and the specific shape of the reflective mirror, and is common to the first embodiment with respect to other elements. 
     Therefore, the common elements of the present embodiment will be omitted in detailed description by reference using the identical reference numerals or names, while only the different elements of the present embodiment will be described in greater detail below. 
     As illustrated in  FIG. 16 , in the present embodiment, each mask pattern includes, similarly with the first embodiment, first sides each parallel to the oscillation axis  134 , and second sides each perpendicular to the oscillation axis  134 . The oscillation axis  134  constitutes an example of the “reference line” set forth in the above mode (6). 
     Each mask pattern is located relative to an etchable material  480  depicted in  FIG. 17 , such that each of the first and second sides is perpendicular to a selected one of &lt;110&gt; crystallographic directions of the etchable material  480  on a selected one of {100} crystallographic planes of the etchable material  480 . 
     In the present embodiment, the first sides correspond to ones of a plurality of second portions  472  described below which extend parallel to the oscillation axis  134 , wherein the ones extend horizontally in  FIG. 16 . On the other hand, the second sides correspond to ones of the plurality of second portions  472  which extend perpendicular to the oscillation axis  134 , wherein the ones extend vertically in  FIG. 16 . 
     For defining the shape of this mask pattern, for convenience of explanation, the etchable material  480  is divided into four regions I, II, III, and IV by two center lines of symmetry orthogonally intersecting each other at a center point of the mask pattern. One of the center lines is the oscillation axis  134 , and the other is a straight line orthogonally intersecting the oscillation axis  134  at the center point PC. 
     In the present embodiment, as illustrated in  FIG. 16 , each mask pattern further includes, similarly with the first embodiment, at its both ends opposite to each other in the direction of the oscillation axis  134 , the extensions  431  and  431  which each extend outwardly of each mask pattern along the oscillation axis  134 . 
     In the present embodiment, similarly with the first embodiment, each mask pattern is generally in the shape of a convex octagon and an outline of the mask pattern includes first portions  470  each corresponding to one of {100} crystallographic planes of the etchable material  480 , and second portions  472  each corresponding to one of {111} crystallographic planes of the etchable material  480 . 
     Further, in the present embodiment, fourth portions  474  are located symmetrically relative to each other at opposite ends of the first portion  470 , respectively, as with the second portions  472 . 
     As will be apparent from the above, in the present embodiment, the mask pattern has a planar shape having a basic shape of a convex octagon, and fourth portions  474  in the form of protrusions which protrude outwardly from the octagon at its eight corners. Each fourth portion  474  is shaped so as to extend from a corresponding one of the corners of a regular octagon, toward a region in which an external angle for the corresponding corner is formed. 
     The fourth portions  474  provide the function of delaying a timing at which wet-etch starts at corners which, would be formed by the first portions  470  and the second portions  472  without interposition of the fourth portions  474 . This function allows the reflective mirror  122  to be formed with its outer circumferential surfaces being simplified in composition (i.e., surface structure) than that in the case of the first embodiment. 
     In the present embodiment, similarly with the first embodiment, opposite faces of the etchable material  480  are coated with etching masks  490 , and on the thus-coated two etching masks  490 , respective mask patterns each having the shape as described above are formed. The etchable material  480  on which the mask patterns have been assigned in a manner described above is wet-etched. 
     In  FIGS. 17(   a ),  17 ( b ),  18 ( a ), and  18 ( b ), how the etchable material  480  is wet-etched is illustrated step by step. In  FIG. 17(   a ), the wet-etching of the etchable material  480  starts at portions of the etchable material  480  which have been un-coated with the etching masks  490 . 
     In this stage, the etchable material  480  exposes principally some of {100} crystallographic surfaces and some of {111} crystallographic surfaces. Further, the etchable material  480  exposes additional crystallographic surfaces between these {100} and {111} crystallographic surfaces. 
     Portions of the etchable material  480  which exposes the aforementioned additional crystallographic surfaces constitute protrusions  492  located between the {100} and {111} crystallographic surfaces (see  FIG. 18(   b )). The protrusions  492  are created as a result of the start-of-etch timing at the protrusions  492  being delayed owing to the fourth portions  474  of the mask pattern, with respect to the start-of-etch timings in the neighborhood of the fourth portions  474 . 
     As illustrated in  FIG. 17(   b ), when the wet-etching of the etchable material  480  progresses slightly from the stage illustrated in  FIG. 17(   a ), the protrusions  492  located between the {100} and {111} crystallographic surfaces are wet-etched, resulting in reduction in protrusion amount of each protrusion  492 . In this stage, the etchable material  480  starts exposing ones of {520} crystallographic surfaces at the tips of the protrusions  492 . 
     As illustrated in  FIG. 18(   a ), when the wet-etching of the etchable material  480  progresses slightly from the stage illustrated in  FIG. 17(   b ), the protrusions  492  located between the {100} and {111} crystallographic surfaces are further wet-etched, resulting in further reduction in protrusion amount of each protrusion  492 . In this stage, ones of {520} crystallographic surfaces which have been previously exposed at the tips of the protrusions  492  grow. 
     As illustrated in  FIG. 18(   b ), when the wet-etching of the etchable material  480  progresses slightly from the stage illustrated in  FIG. 18(   a ), ones of {520} crystallographic surfaces further grow between the {100} and {111} crystallographic surfaces, with the result that the ones of {520} crystallographic surfaces act as intervening inclined surfaces for joining these {100} and {111} crystallographic surfaces to each other. 
     The stage illustrated in  FIG. 18(   b ) is an ultimate stage of the wet-etching in which the etchable material  480  has been partially penetrated in its thickness-wise direction because of the etchant. In  FIG. 18(   b ), the ultimate shape of the etchable material  480  is illustrated together with a representative one of the first portions  470  of the mask pattern. 
     In  FIG. 19 , the etchable material  480  depicted in  FIG. 18(   b ) is illustrated in enlargement. Upon completion of the wet-etching of the etchable material  480 , only ones of the {520} crystallographic planes are exposed between the {100} and {111} crystallographic surfaces. As a result, according to the present embodiment, the {100} and {111} crystallographic surfaces are joined to each other via surfaces which are simplified in composition (i.e., surface structure) than in the case of the first embodiment. 
     In  FIG. 20(   a ), a completed reflective mirror  122  is illustrated in perspective view, while the same is illustrated in top plan view in  FIG. 20(   b ). The reflective mirror  122  has a silhouette generally in the shape of a convex octagon as viewed in a direction normal to the reflective surface  120  of the reflective mirror  122 . More precisely, the silhouette is in the shape of a sixteen-sided polygon, and still more precisely, the silhouette has a shape approximate to that of a sixteen-sided polygon. 
     According to the present embodiment, the outer circumference of the reflective mirror  122  is configured such that a {520} crystallographic surface is interposed between adjacent {100} and {111} crystallographic surfaces. The wet-etching is performed so as to create crystallographic surfaces interposed between adjacent {100} and {111} crystallographic surfaces. The interposed crystallographic surfaces are selected from principally a selected one of families of crystallographic planes (i.e., a family of {520} crystallographic planes), with the total number of selected families of crystallographic planes used for forming the interposed crystallographic surfaces being unchanged. 
     Therefore, according to the present embodiment, the shape of the outer circumference of the reflective mirror  122  is less variable (stabilized), with the result that the moment of inertia of the reflective mirror  122  is less variable (stabilized) 
     It is added that, in  FIG. 21 , an example of a modified version of the mask pattern is illustrated. In this example, similarly with the aforementioned example depicted in  FIG. 16 , an outline of the mask pattern includes first portions  500  corresponding to {100} crystallographic planes, second portions  502  corresponding to {111} crystallographic planes, and fourth portions  504 . 
     This example depicted in  FIG. 21  is different from the previous example depicted in  FIG. 16  in terms of the positions of the first portions  470 ,  500  relative to the positions of the second portions  472 ,  502 . 
     Third Embodiment 
     Next, with reference to  FIGS. 22-31 , a third embodiment of the present invention will be described. 
     The present embodiment is different from the first and second embodiments only with respect to a part of the method of fabricating the body  110 , which is for forming by wet-etch the beams  140  each having the stepped portion, and is common to the first and second embodiments with respect to other elements. 
     Therefore, the common elements of the present embodiment will be omitted in detailed description by reference using the identical reference numerals or names, while only the different elements of the present embodiment will be described in greater detail below. 
     An RSD according to the present embodiment is common in construction to the RSD according to the first embodiment which is depicted in  FIG. 1 . The present embodiment, however, may be modified by removing the wavefront-curvature modulating optical system  22  from the RSD according to the present embodiment. The modification can also apply to the first embodiment. 
     The body  110  of the RSD according to the present embodiment, in common to the body  110  of the RSD according to the first embodiment which is depicted in  FIGS. 2-4 , is constructed by integrally or monolithically fabricating the oscillating body  124  and the stationary frame  116 . 
     As illustrated in  FIG. 2 , the oscillating body  124  is constructed by integrally or monolithically fabricating the reflective mirror  122  having the reflective surface  120 , and the pair of beams  140  and  140  opposing to each other with the reflective mirror  122  being interposed between the pair of beams  140  and  140 . 
     As illustrated in  FIG. 2 , each beam  140  of the oscillating body  124  is constructed, similarly with the first embodiment, to include the single mirror-side leaf spring  142 , the pair of frame-side leaf springs  144 ,  144 , and the connection  146  for coupling the mirror-side leaf spring  142  to the pair of frame-side leaf springs  144 ,  144 . 
     That is to say, in the present embodiment, each beam  140  constitutes an example of the “beam structure” set forth in the above mode (18), and an example of the “beam structure” set forth in the above modes (31), (36), and (45). 
     Now, with reference to  FIGS. 2 and 3 , there will be described portions of the body  110  of the RSD according to the present embodiment, which are required to be specially described for an easy understanding of an oscillating-body fabricating process according to the present embodiment. 
     As illustrated in  FIG. 2 , for the beams  140 ,  140 , the actuators  150 ,  152 ,  154  and  156  are secured to the pairs of frame-side leaf springs  144  and  144 , respectively, with each actuator  150 ,  152 ,  154 ,  156  extending from a corresponding one of the frame-side leaf springs  144  and  144  to the stationary frame  116 . 
     As illustrated in  FIG. 3 , each frame-side leaf spring  144  is locally thinned on its proximal side to the stationary frame  116 , resulting in formation of the recess  158 . The recess  159  is formed at the stationary frame  116  to achieve surface continuity between the recesses  158  and  159 . Utilization of these recesses  158  and  159  allows each actuator  150 ,  152 ,  154 ,  156  to be disposed such that each actuator  150 ,  152 ,  154 ,  156  extends between the corresponding frame-side leaf spring  144  and the stationary frame  116 . 
     As described above with reference to  FIG. 3 , in the present embodiment, each recess  158  is formed on one of opposite face portions, which is to say, an upper-face portion of each frame-side leaf spring  144  of the corresponding beam  140 . Each frame-side leaf spring  144  is formed to include the stepped portion  160 , thereby allowing formation of the recess  158  on the upper-face portion of each frame-side leaf spring  144 . 
     As illustrated in  FIG. 3 , the stepped portion  160  is formed at each frame-side leaf spring  144  such that the stepped portion  160  includes: 
     (a) a higher sub-portion  162  having the same height as a basic surface of the beam  140 , which is to say, an original upper face of the beam  140 ; 
     (b) a lower sub-portion  164  lower than the original upper face of the beam  140 ; and 
     (c) a shoulder sub-portion  166  which is located at a border between the higher and lower sub-portions  162  and  164  and which traverses the beam  140 . 
     Among these sub-portions  162 ,  164  and  166 , the lower and shoulder sub-portions  164  and  166  contribute to the formation of the recess  158 . 
     As illustrated in  FIGS. 2 and 3 , each actuator  150 ,  152 ,  154 ,  156  is mounted on the corresponding recess  158 , which is to say, the corresponding stepped portion  160 , with an upper face of each actuator  150 ,  152 ,  154 ,  156  being located not above a plane coplanar with a basic surface of the corresponding frame-side leaf spring  144 . 
     As illustrated in  FIG. 3 , each actuator  150 ,  152 ,  154 ,  156  is in the form of a laminate constructed by sandwiching the thin-plate-shaped piezoelectric material  170  between the upper and lower electrodes  172  and  174 . 
     As illustrated in  FIG. 3(   a ) in side view, each actuator  150 ,  152 ,  154 ,  156  is attached at its lower face to the corresponding recesses  158  and  159 , and each actuator  150 ,  152 ,  154 ,  156  has a thickness dimension allowing an upper face of each actuator  150 ,  152 ,  154 ,  156  not to be above an uppermost surface of the higher sub-portion  162 . 
     That is to say, in the present embodiment, each actuator  150 ,  152 ,  154 ,  156  constitutes an example of the “laminate” set forth in the above mode (35). 
     The horizontal scanning drive circuit  180  of the RSD according to the present embodiment is in common in configuration and operation to the horizontal scanning drive circuit  180  according to the first embodiment which is depicted in  FIG. 4 . 
     Each frame-side leaf spring  144  of the oscillating body  124  is fabricated integrally with the reflective mirror  122  such that each frame-side leaf spring  144  includes the stepped portion  160  to form the recess  158 . 
     In the present embodiment, the height of the stepped portion  160 , which is to say, a distance between the higher and lower sub-portions  162  and  164  measured in a thickness-wise direction of each frame-side leaf spring  144  is 50 μm, while the recess  158  has a dimension of 1 mm when measured in a length-wise direction of each frame-side leaf spring  144 . 
     In  FIG. 22 , there is illustrated in process chart the oscillating-body fabricating process for fabricating by a wet-etching technique the body  110  including the stationary frame  116  and the oscillating body  124 , by an integral and batch fabrication. 
     This fabrication process begins with a step S 11  to prepare an etchable material in the form of a plate-shaped material made of a single crystal silicon. 
     In  FIGS. 23(   a ) and  23 ( b ), the etchable material is denoted by reference numeral  600 . The etchable material  600  with a thickness of 100 μm is made of a silicon wafer. As illustrated in  FIGS. 29(   a ) and  29 ( b ), the etchable material  600  is initially exposed at some of {100} crystallographic planes. 
       FIGS. 23(   a ) and  23 ( b ) are sectional views taken on lines A-A and B-B in  FIG. 2  for representatively explaining a coating step and a mask-pattern forming step, both of which, although described later, belong to a method of fabricating the body  110 . 
     In this regard, the A-A line is a cutting-plane line traversing the body  110  through a center point of the reflective mirror  122 , while the B-B line is a cutting-plane line traversing the body  110  through center points of length-wise dimensions of the recesses  158  and  158 . 
     The step S 11  is followed by a step S 12  to coat opposite faces of the etchable material  600  with two etching masks  610  and  612 , respectively, as illustrated in  FIG. 23(   a ). These etching masks  610  and  612  are silicon oxide coatings formed on the opposite faces of the etchable material  600  by heating the opposite faces of the etchable material  600 . 
     That is to say, the step S 12  constitutes an example of the “coating step” set forth in the above mode (18). 
     The step S 12  is followed by a step S 13  to employ a lithographic technique for the two etching masks  610  and  612  ( 610 : upper etching mask,  612 : lower etching mask) which have been deposited on the opposite faces of the etchable material  600 . 
     More specifically, the surfaces of the etching masks  610  and  612  are coated with resists, and then the resists which have been applied to the etching masks  610  and  612  are exposed to respective optical patterns. 
     Thereafter, according to the exposure pattern, the resists are removed in a position-selective manner using appropriate chemicals. As a result, a resist  620  having an upper mask pattern is deposited on the surface of the upper etching mask  610 , while a resist  622  having a lower mask pattern is deposited on the surface of the lower etching mask  612 . 
     The step S 13  is further implemented to remove the two etching masks  610  and  612  in a position-selective manner using appropriate chemicals (e.g., hydrofluoric acid), according to the resists  620  and  622  into which the two etching masks  610  and  612  have been previously patterned. As a result, an upper mask pattern  630  illustrated in  FIG. 24(   a ) in top plan view is formed on the upper etching mask  610 , while a lower mask pattern  632  illustrated in  FIG. 24(   b ) in top plan view is formed on the lower etching mask  612 . 
     However, in  FIGS. 24(   a ) and  24 ( b ), the upper mask pattern  630  and the lower mask pattern  632  are illustrated only with respect to portions of the upper and lower mask patterns  630  and  632  which are related to fabrication of the oscillating body  124 , and are omitted with respect to portions of the upper and lower mask patterns  630  and  632  which are related to fabrication of the stationary frame  116 , because the latter portions are not necessary for understanding the present invention. 
     Actually each of the mask patterns  630  and  632  depicted in  FIGS. 24(   a ) and  24 ( b ) includes at a center point of the length-wise dimension of each mask pattern  630 ,  632 , a portion for fabrication of the reflective mirror  122 . However, the portion is omitted in  FIGS. 24(   a ) and  24 ( b ) because the portion is in common to that of the first or second embodiment described above. 
     In the upper mask pattern  630  illustrated in  FIG. 24(   a ), there are surrounded by broken circles, portions  650  and  650  of the upper mask pattern  630 , each of which is related to the fabrication of the corresponding stepped portion  160  of the upper-face portion of each frame-side leaf spring  144 . 
     On the other hand, in the lower mask pattern  632  illustrated in  FIG. 24(   b ), there are surrounded by broken circles, portions  652  and  652  of the lower mask pattern  632 , each of which is related to the fabrication of a region of the lower-face portion of each frame-side leaf spring  144 , which region is located on the side opposite to the corresponding stepped portion  160 . 
     The shape of each mask pattern  630 ,  632  determines the shape of a portion of the etchable material  600  which will be brought into contact with and etched by the etchant held in the etching bath (not shown) if the etchable material  600  is immersed in the etching bath. 
     That is to say, the step S 13  constitutes an example of the “mask-pattern forming step” set forth in the above mode (18). 
     As illustrated in  FIGS. 24(   a ) and  24 ( b ), the portions  652  and  652  are configured to extend toward portions  654  and  654  of the lower mask pattern  632  which portions correspond to the fixed frame  116 , with each portion  652  having a simple strip-like shape similar with a basic desired-shape (an ultimate-shape achieved by fabrication) of the frame-side leaf spring  144 , while the portions  650  and  650  each have a complex shape. 
     Now, with reference to  FIGS. 25(   a ),  25 ( b ),  25 ( c ), and  26 , there will be described the shape of each portion  650  in more detail below. However, in  FIGS. 25(   a ),  25 ( b ), and  25 ( c ), the etchable material  600  and a representative one of the frame-side leaf springs  144  are simplified in illustration for convenience of explanation of the shapes of the etchable material  600  and the representative frame-side leaf spring  144 . The more practical shapes of the etchable material  600  and the representative frame-side leaf spring  144  are illustrated in  FIGS. 27-29 . 
     In  FIG. 25(   a ), there is illustrated in perspective view a portion of the upper mask pattern  630  which corresponds to a single frame-side leaf spring  144  (hereinafter, referred to as “representative frame-side leaf spring”) representing the plurality of frame-side leaf springs  144  for convenience of explanation. 
     In  FIG. 25(   b ), there are illustrated in perspective view a portion of the etchable material  600  at which the representative frame-side leaf spring  144  is to be formed and the neighborhood of the portion. 
     In  FIG. 25(   c ), there is illustrated in perspective view the basic desired-shape (an ultimate-shape achieved by fabrication) of the representative frame-side leaf spring  144 . 
     Describing the etchable material  600  in more detail with reference to  FIG. 25(   b ), the etchable material  600  includes a to-be-unetched portion  700  which is to remain unetched even after the etchable material  600  is immersed in the etchant for wet-etching. The to-be-unetched portion  700  is identical in shape to the higher sub-portion  162  within the basic desired-shape of the representative frame-side leaf spring  144  depicted in  FIG. 25(   c ). The to-be-unetched portion  700  is illustrated in  FIG. 25(   b ) in broken lines. 
     As illustrated in  FIG. 25(   b ), the etchable material  600  further includes a to-be-fully-etched portion  702  which is to be wet-etched until the etchant passes through a thickness of the etchable material  600 . 
     The to-be-fully-etched portion  702  is a portion of the etchable material  600  left after conceptually excluding a silhouette of a part of the etchable material  600  which is identical in shape to the basic desired-shape of the representative frame-side leaf spring  144  from a silhouette of the etchable material  600 . These silhouettes are conceived by viewing the entire etchable material  600  and the part of the etchable material  600 . 
     As illustrated in  FIG. 25(   b ), the etchable material  600  yet further includes a to-be-half-etched portion  704  which is to be etched in half-way of the thickness of the etchable material  600  when wet-etched. 
     The to-be-half-etched portion  704  is identical in shape to the recess  158  depicted in  FIG. 3 . The to-be-half-etched portion  704  is illustrated in two-dotted lines in  FIG. 25(   b ). In the etchable material  600 , there exist opposite sub-portions  706  and  706  which are opposed to each other in a width-wise direction of the to-be-half-etched portion  704  and between which the to-be-half-etched portion  704  is interposed. 
     As illustrated in  FIG. 25(   a ), the portion  650  (hereinafter, referred to as “stepped-portion-oriented mask pattern  650 ”) of the upper mask pattern  630  includes a basic pattern  710  which covers a surface (i.e., an upper face) of the to-be-unetched portion  700  before wet-etching. The stepped-portion-oriented mask pattern  650  further includes a compensating pattern  712  which covers surfaces (i.e., upper faces) of the opposite sub-portions  706 ,  706  before wet-etching. 
     More specifically, the compensating pattern  712  is configured to include a pair of wings  714  and  714  which coextend in a length-wise direction of the to-be-half-etched portion  704 , and which are disposed on respective opposite sides with respect to the to-be-half-etched portion  704 . 
     In  FIG. 26 , both a basic desired-shape of the higher sub-portion  162  of the stepped portion  160  of the representative frame-side leaf spring  144 , and the shape of the stepped-portion-orientated mask pattern  650  are illustrated in top plan view, side by side, for convenience of comparison. 
     As will be evident from  FIG. 26 , the stepped-portion-orientated mask pattern  650  includes a basic pattern  710  which is identical in shape to a surface shape of the higher sub-portion  162 . The stepped-portion-oriented mask pattern  650  further includes as the compensating pattern  712 , which extend laterally and outwardly of the basic pattern  710  in opposite directions. 
     As illustrated in  FIG. 26 , the pair of wings  714  and  714  are associated with the basic pattern  710 , such that the wings  714  and  714  are partially coupled at one end side of the wings  714  and  714  to the basic pattern  710 , and such that the wings  714  and  714  are partially open at opposite end side of the wings  714  and  714 , whereby the wings  714  and  714  and a portion of the basic pattern  710  which is coupled to the wings  714  and  714  cooperate to form a substantial U-shape. 
     The upper and lower mask patterns  630  and  632  having the respective shapes described above are formed on the opposite faces of the etchable material  600 , respectively. 
     Thereafter, in a step S 14  illustrated in  FIG. 22 , a laminate of the etchable material  600  and the etching masks  610  and  612  is immersed in the etching bath containing the etchant. In the present embodiment, the etchant is prepared to include a Potassium Hydroxide (KOH) solution with a preset condition of 40 wt. % and a preset temperature of 70.degree. C. Under this condition, the etchable material  600  is wet-etched. 
     That is to say, the step S 14  constitutes an example of the “wet-etching step” set forth in the above mode (18). 
     It is added that, in an alternative, the etchant may be prepared to include a Tetramethyl Ammonium Hydroxide (TMAH) solution. In this alternative, the shape of a corresponding mask pattern is obtained by partially modifying that illustrated in  FIG. 24 . 
     Further, in the step S 14 , upon the etchable material  600  being etched through the etchable material  600  at the to-be-fully-etched portion  702 , one cycle of the wet-etching process is terminated. For one cycle of the wet-etching process, the etchable material  600  is immersed in the etchant only once. 
     That is to say, in the present embodiment, for one cycle of the wet-etching process, the etchable material  600  is immersed in the etchant not several times but only once, with the result that the body  110  is fabricated at a time (by a batch process) by wet-etching the etchable material  600 . 
     In  FIGS. 27(   a ),  27 ( b ),  28 ( a ), and  28 ( b ), how the etchable material  600  is wet-etched is illustrated step by step. In each figure, however, representatively for the representative frame-side leaf spring  144 , there is illustrated how the etchable material  600  is etched from the upper face toward the lower face of the etchable material  600 . 
     In the stage illustrated in  FIG. 27(   a ), the wet-etching of the etchable material  600  starts at portions of the etchable material  600  which have been un-coated with the etching masks  610 . In this stage, because of the stepped-portion-oriented mask pattern  650 , the etchable material  600  starts exposing some of {100} crystallographic surfaces and some of {111} crystallographic surfaces. 
     As illustrated in  FIG. 27(   b ), when the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 27(   a ), with a slight increase in an etching depth of the etchable material  600 , the etching at portions of the etchable material  600  which have been coated with the pair of wings  714  and  714  progresses from both corners at the tip of each wing  714  toward the inside of each wing  714 . However, the some of {111} crystallographic surfaces remain. 
     As illustrated in  FIG. 28(   a ), when the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 27(   b ), portions of the etchable material  600  which have been coated with the pair of wings  714  and  714  are almost completely removed or eliminated Still in this stage, the some of {111} crystallographic surfaces remain. 
     When the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 28(   a ), one cycle of the wet-etching is terminated. In this stage, as illustrated in  FIG. 28(   b ), the etchable material  600  is etched through at the to-be-fully-etched portion  702  along the thickness of the etchable material  600 . Additionally, the surface of the lower sub-portion  164  is created, and the surface of the shoulder sub-portions  164  is also created. 
     In  FIG. 29(   a ), the stepped portion  160  fabricated by the wet-etching technique is illustrated in top plan view, while in  FIG. 29(   b ), the stepped portion  160  is illustrated in perspective view. The shoulder sub-portion  166  exposes some of the {111} crystallographic surfaces. {111} crystallographic surfaces are wet-etched slower than any other crystallographic surfaces. 
     Therefore, fabrication of the shoulder sub-portion  166  using some of the {111} crystallographic planes allows the shoulder sub-portion  166  to ultimately expose {111} crystallographic surfaces at a position on a path extending in a length-wise direction of the representative frame-side leaf spring  144 , wherein the position is less sensitive to a possible somewhat variation in an actual etch time. 
     As a result, the shoulder sub-portion  166  is formed at a position on a path extending in the length-wise direction of the representative frame-side leaf spring  144 , with enhanced position stability and position accuracy, irrespective of variations of actual wet-etching conditions. 
     In the present embodiment, for the shoulder sub-portion  166  to expose some of {111} crystallographic surfaces, the compensating pattern  712  is configured to include the pair of wings  714  and  714 , as illustrated in  FIG. 26 . 
     Each wing  714  has a rectangular area  716  at a tip end of each wing  714 . The rectangular area  716  extends perpendicular to the length-wise direction of each wing  714 . The rectangular area  716  has corners each having an angle approximate to an acute angle to the maximum. 
     As illustrated in  FIG. 26 , in the present embodiment, at the tip end of each wing  714 , a rectilinear portion  720  is disposed perpendicular to the length-wise direction of each wing  714 . Each rectilinear portion  720  is an example of a rectilinear portion which is perpendicular to one of the &lt;110&gt; crystallographic directions of the etchable material  600 . Each rectilinear portion  720  enables the shoulder sub-portion  166  to expose some of {111} crystallographic surfaces as a result of the wet-etching. 
     That is to say, in the present embodiment, the rectilinear portions  720  and  720  which are located at the tip ends of the pair of wings  714  and  714 , respectively, each constitute an example of the “rectilinear portion” set forth in the above mode (27). 
     In other words, in the present embodiment, the compensating pattern  712  is predefined in shape to include the pair of wings  714  and  714 , so that the shoulder sub-portion  166  may expose some of the {111} crystallographic surfaces upon completion of the wet-etching. 
     Once the wet-etching step is terminated in a manner as described above, as illustrated in  FIG. 22 , a step S 15  follows to eject the etchable material  600  from the etching bath, whose opposite faces have been coated with the etching masks  610 . A step S 16  follows to remove the etching masks  610  from the opposite faces of the etchable material  600 . 
     Then, a succession of implementations in the oscillating-body fabricating process is completed. 
     In  FIGS. 30(   a ),  30 ( b ),  31 ( a ), and  31 ( b ), how the stepped portion  160  is fabricated by a fabricating process according to a comparative example of the present embodiment is illustrated in perspective view step by step, similarly with  FIGS. 27(   a ),  27 ( b ),  28 ( a ), and  28 ( b ). In this comparative example, a stepped-portion-oriented mask pattern is prepared so as to have a shape identical to the basic pattern  170 . 
     In this comparative example, as the wet-etching of the etchable material  600  progresses, the upper-face portion of the etchable material  600  changes in shape from the stage illustrated in  FIG. 30(   a ) to the stage illustrated in  FIG. 30(   b ). 
     As illustrated in  FIG. 31(   a ), upon further progress of the wet-etching, a portion of the etchable material  600  which corresponds to the higher sub-portion  162  has a reduced length smaller than the length of its basic desired shape of the higher sub-portion  162 . 
     As illustrated in  FIG. 31(   b ), upon still further progress of the wet-etching, the corresponding portion of the etchable material  600  to the higher sub-portion  162  has a further reduced length far smaller than the length of its basic desired shape of the higher sub-portion  162 , and in addition to this, the shoulder sub-portion  166  is formed by various crystallographic surfaces. 
     For the above reasons, this comparative example makes it difficult to accurately control the position of the shoulder sub-portion  166 , and also to reduce individual differences between the shoulder sub-portion  166  and other shoulder sub-portions with respect to position accuracy. 
     In contrast, the present embodiment makes it easy to accurately control the position of the shoulder sub-portion  166 , and also to reduce individual differences between the shoulder sub-portion  166  and other shoulder sub-portions with respect to position accuracy. 
     As will be apparent from the above, in the present embodiment, the pair of wings  714  and  714  together constitute an example of the “first etch compensator” set forth in the above modes (25)-(27). 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described. 
     The present embodiment is different from the third embodiment only with respect to the shape of each mask pattern, and is common to the third embodiment with respect to other elements. 
     Therefore, the common elements of the present embodiment will be omitted in detailed description by reference using the identical reference numerals or names, while only the different elements of the present embodiment will be described in greater detail below. 
     As illustrated in  FIGS. 32(   a ) and  32 ( b ), in the present embodiment, opposite faces of the etchable material  600  are coated with an upper etching mask  730  and a lower etching mask  732 , respectively, similarly with the third embodiment. 
     In  FIG. 32(   a ), an upper mask pattern  740  to be formed on the upper etching mask  730  is illustrated in top plan view only with respect to portions of the upper mask pattern  740  which are related to the fabrication of the oscillating body  124 . 
     In  FIG. 32(   b ), a lower mask pattern  742  to be formed on the lower etching mask  732  is illustrated in top plan view only with respect to portions of the lower mask pattern  742  which are related to the fabrication of the oscillating body  124 ; and portions of the lower mask pattern  742  which are related to the fabrication of regions of the stationary frame  116  which are coupled to the oscillating body  124 . 
     In the upper mask pattern  740  illustrated in  FIG. 32(   a ), there are surrounded by broken circles, portions  760  and  760  of the upper mask pattern  740 , each of which is related to the fabrication of the corresponding stepped portion  160  of the upper-face portion of each frame-side leaf spring  144 . 
     On the other hand, in the lower mask pattern  742  illustrated in  FIG. 32(   b ), there are surrounded by broken circles, portions  762  and  762  of the lower mask pattern  742 , each of which is related to the fabrication of a region of the lower-face portion of each frame-side leaf spring  144 , which region is located on the side opposite to the corresponding stepped portion  160 . 
     As illustrated in  FIGS. 32(   a ) and  32 ( b ), the lower mask pattern  742  is common in shape to the lower mask pattern  632  in the third embodiment, while the upper mask pattern  740  is different in shape from the upper mask pattern  630  in the third embodiment. 
     Therefore, only the shape of the upper mask pattern  740  will be described in greater detail below, while the shape of the lower mask pattern  742  will be omitted in detailed description. 
     In  FIG. 33 , both a basic desired-shape of the higher sub-portion  162  of the stepped portion  160  of the representative frame-side leaf spring  144  which represents the plurality of frame-side leaf springs  144 , and the shape of one of the portions  760  and  760  (hereinafter, referred to as “stepped-portion-oriented mask pattern  760 ) corresponding to the stepped portion  160  of the upper mask pattern  740  are illustrated in top plan view, side by side, for convenience of comparison. 
     As will be evident from  FIG. 33 , the shape of the stepped-portion-oriented mask pattern  760  includes a basic pattern  770  which is identical to a surface shape of the higher sub-portion  162 . The stepped-portion-oriented mask pattern  760  further includes a compensating pattern  772  which covers the surface of the to-be-half-etched portion  704  (see  FIG. 25 ) and the surfaces of the opposite sub-portions  706  and  706  (see  FIG. 25 ) to thereby reduce a rate of the wet-etching performed especially at the to-be-half-etched portion  704 . 
     More specifically, the compensating pattern  772  forms a rhombus-shape having four corners and four sides, in cooperation with a portion of the basic pattern  770  which is coupled to the compensating pattern  772 . Each side of the compensating pattern  772  is perpendicular to one of &lt;100&gt; crystallographic directions. The compensating pattern  772  is formed such that it is coupled at one of two opposite corners of the four corners to the basic pattern  770 , and such that it is cut-away at the other of the opposite corners. 
     Still more specifically, the compensating pattern  772  includes first etch compensators  780  and  780  which cover the surfaces of the opposite sub-portions  706  and  706  (see  FIG. 25 ), respectively, and a second etch compensator  782  which covers the surface of the to-be-half-etched portion  704  (see  FIG. 25 ), which compensator is located at the front of the to-be-unetched portion  700  (see  FIG. 25 ). Because of both the first etch compensators  780  and  780  and the second etch compensator  782 , a rate of the wet-etching performed for the to-be-half-etched portion  704  is reduced, resulting in preventing the etchant from reaching the to-be-unetched portion  700 . 
     In  FIGS. 34(   a ),  34 ( b ),  35 ( a ), and  35 ( b ), there is illustrated step by step how the etchable material  600  is wet-etched is illustrated step by step. In each figure, however, representatively for the representative frame-side leaf spring  144 , how the etchable material  600  is etched from the upper face toward the lower face of the etchable material  600 . 
     In the stage illustrated in  FIG. 34(   a ), the wet-etching of the etchable material  600  starts at portions of the etchable material  600  which have been un-coated with the etching masks  610 . In this stage, because of the stepped-portion-oriented mask pattern  760 , the etchable material  600  starts exposing some of { 100 } crystallographic surfaces. 
     As illustrated in  FIG. 34(   b ), when the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 34(   a ), with a slight increase in an etching depth of the etchable material  600 , the etching at portions of the etchable material  600  which have been coated with the compensating pattern  772  progresses from both two corners at a front end of the compensating pattern  772 , and two corners opposed to each other in a width-direction of the compensating pattern  772 , toward the inside of the compensating pattern  772 . The former two corners and the latter two corners belong to a plurality of corners of the compensating pattern  772 . However, some of {100} crystallographic surfaces remain. 
     As illustrated in  FIG. 35(   a ), when the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 34(   b ), portions of the etchable material  600  which have been coated with the compensating pattern  772  are removed excepting a rear end of the compensating pattern  772 . Still in this stage, some of {100} crystallographic surfaces remain. 
     When the wet-etching of the etchable material  600  progresses slightly from the stage illustrated in  FIG. 35(   a ), one cycle of the wet-etching is terminated. In this stage, as illustrated in  FIG. 35(   b ), the etchable material  600  is etched at the to-be-fully-etched portion  702  so as to pass through the thickness of the etchable material  600 . Additionally, the surface of the lower sub-portion  164  is created, and the surface of the shoulder sub-portion  166  is also created. 
     Therefore, the present embodiment allows the shoulder sub-portion  166  to be formed at a position on a path extending in a length-wise direction of the representative frame-side leaf spring  144 , wherein the position is less sensitive to a possible somewhat variation in an actual etch time. 
     As a result, the shoulder sub-portion  166  is formed at a position on the path extending in the length-wise direction of the representative frame-side leaf spring  144 , with enhanced position stability and position accuracy, irrespective of variations of actual wet-etching conditions. 
     As will be apparent from the above, in the present embodiment, the compensating pattern  772  constitutes an example of the “compensating pattern” set forth in the above mode (29), the first etch compensators  780  and  780  of the compensating pattern  772  constitute an example of the “first etch compensator” set forth in the above modes (25), (26), (27), and (29), and the second etch compensator  782  of the compensating pattern  772  constitutes an example of the “second etch compensator” set forth in the above modes (28) and (29). 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.