Abstract:
The present invention relates generally to micromachining. More particularly, the present invention relates to a method for combining directional ion etching and anisotropic wet etching and devices and structures fabricated thereby. The present invention is particularly applicable to silicon micromachining and provides architectures that combine crystallographic surfaces and vertical dry etched surfaces together in the same structure.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 60/472,910 filed on May 23, 2003 and U.S. Provisional Application No. 60/472,909 filed on May 23, 2003, the entire contents of which applications are incorporated herein by reference. This application is related to the subject matter of U.S. application Ser. No. 10/860,811 filed on May 24, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to micromachining. More particularly, the present invention relates to a method for combining directional ion etching and anisotropic wet etching and devices and structures fabricated thereby. The present invention is particularly applicable to silicon micromachining. 
     BACKGROUND OF THE INVENTION 
     The art of bulk micromachining in silicon has changed since the invention of practical means to etch vertical sidewalls in silicon using dry etching technology, as has largely resulted from the invention of the BOSCH process. Deep dry etching technology has allowed new architectures in single crystal silicon to be created, especially in released structures such as those based on SOI silicon like accelerometers and electrostatic actuators. While such deep dry etch processes have allowed the realization of many new architectures, there are still advantages in traditional crystallographic (anisotropic) etching in silicon. Wet anisotropic etching is often based on opening a hard mask deposited on silicon, with features oriented on the surface to create v-grooves, u-grooves, precision inverted pyramidal pits, and other shapes, which are well known in the art. The exact shapes depending on the crystal orientation, mask opening shape, and particular wet etch used. Advantages of wet etching, with proper alignment and etch conditions, include the ability to batch process many wafers at one time, the ability to achieve very smooth surfaces, and the ability to achieve non perpendicular surfaces such as v-grooves, and the ability to produce highly accurate mechanical dimensions. Alternatively, the use of deep plasma etching has the advantage of being independent of crystallographic axis limitations and allows vertical surfaces to be created with high aspect ratio. What is lacking in the art is a method to combine these two etching formats so that the benefits of both techniques can be brought together allowing new freedom in possible resulting shapes and structures. Combining these to etching formats would be particularly useful for the art of silicon optical bench, where elements such as micro-optics, semiconductor lasers, photodetectors, optical fibers, and other elements can be hybridly integrated on the silicon wafer surface using mechanical features etched into the silicon, along with integrated patterned metals, solders, resistors, and MEMS that can be fabricated directly into or onto the silicon wafer. Thus microsystems can be achieved with assembly economy difficult to otherwise achieve. It should be clear that other such wafer level micro-systems will clearly benefit from this improvement in micromachining technology such as sensors, actuators, micro-fluidics, RF microdevices, and so on. For example, can apply the methods of the present invention to the fabrication of known bulk micromachined products such as accelerometers to create architectures leveraging dry and wet etching. By way of example and not limitation, the instant disclosure describes mechanical structures which can be realized and that are useful in hybrid micro-optical electrical systems, also known as silicon optical bench, SiOB, silicon wafer board, or simply silicon bench. 
     The ability to precisely locate optical elements relative to one another is of critical importance in the fabrication of micro-optical devices, since the alignment tolerances between elements are often specified in submicron dimensions. Typically, such elements may include an optical signal source, such as a laser, a detector, and an integrated or discrete waveguide, such as a fiber-optic, integrated optics, or GRIN rod lens. Additionally, such elements may include a fiber amplifier, optical filter, modulator, grating, ball lens, or other components for conveying or modifying or splitting an optical beam. Micro-optical devices containing such components are crucial in existing applications such as optical communication and consumer opto-electronics, as well as applications currently being developed, such as optical computing. 
     Maintaining precise alignment among the optical elements may be conveniently provided by an optical microbench, such as a silicon optical bench. An optical microbench comprises three-dimensional structures having precisely defined surfaces onto which optical elements may be precisely positioned. One material well-suited for use as an optical microbench is single crystal silicon, because single crystal silicon may be etched anisotropically to yield three-dimensional structures having planar sidewalls formed by the precisely defined crystallographic planes of the silicon. For example, the {111} silicon plane is known to etch more slowly than the {100} or {110} planes with proper choice of etchant. Thus, structures may be formed comprising walls that are primarily {111} planes by anisotropic etching. 
     Since the many optical elements sit within the three-dimensional structures at a position at least partially below a top surface of the silicon substrate, a portion of the optical path often lies below the top surface of the substrate, within the volume of the substrate. Accordingly, passageways must be provided in the optical microbench between three-dimensional structures so that light may travel between the elements disposed in the associated three-dimensional structures. Hence, an optical microbench should contain three-dimensional structures that communicate with one another through structures such as a passageway. 
     While discrete, non-communicating, three-dimensional structures may be conveniently formed by an anisotropic etching, etched structures which communicate with one another at particular geometries, such as a convex corner, pose significant problems for applications in which it is desirable to maintain the precise geometry defined by the crystallographic planes. For example, where two {111} planes intersect at a convex (or exposed) corner, the convex corner does not take the form of a straight line intersection between two planes, but is rather rapidly attacked by the etchant to create a rounded or complex intersection between the two {111} planes. As etching continues to reach desired depth of the structure containing the {111} planes, the rounding or attack of the corners can grow to such an extent that a substantial portion of the intersection between the two {111} planes is obliterated. Since the {111} planes are provided in the three-dimensional structures to form a planar surfaces against which optical elements may be precisely positioned, absence of a substantial portion of the {111} planes at the intersection can introduce a great deal of variability of the positioning of the elements at the intersection. Thus, the benefits provided by the crystallographic planes can be unacceptably diminished. 
     Traditionally, to avoid etching intersecting features, dicing saw cuts may be used. However, dicing saw cuts can be undesirable, because such cuts typically must extend across the entire substrate, or consume an undesirably large portion of it, and may not conveniently be located at discrete locations within the substrate. Moreover, dicing saw cuts create debris which may be deposited across the substrate surface and lodge within the three-dimensional structures, which may interfere with the precise positioning of optical elements within such a structure. 
     Therefore, there remains a need in the art for optical microbench technology which permits three-dimensional structures having crystallographic planar surfaces to intersect with other surfaces, without degrading the crystallographic orientation of the intersected planar surfaces. Further, there remains a need in the art to combine crystallographic surfaces and vertical dry etched surfaces together in the same structure. 
     SUMMARY 
     The present invention provides a substrate having an etch-stop pit and an etched feature, such as an anisotropically etched feature, disposed adjacent the etch-stop pit. The anisotropically etched feature may comprise a V-groove. The etch-stop pit may have a shape suited to supporting an etch-stop layer on the surfaces of the etch-stop pit. The etch-stop pit may desirably be created prior to creating the etched feature. The etch-stop layer may comprise a material resistant to the etchant which is used to create the etched feature. After the etch-stop layer is provided, the anisotropically etched feature is etched in the substrate. The etch-stop layer prevents the feature etch from extending into the region where the etch-stop pit is located. The prevention of such etching by the etch-stop layer provides that the crystallographic planar walls of the anisotropically etched feature maintain their crystallographic orientation adjacent the stop-etch pit. For example, the present invention provides a micromachined crystalline substrate, comprising: an anisotropically etched groove disposed in a substrate surface; an anisotropically etched pit disposed in the substrate surface, the pit comprising a sidewall to provide a reflection surface for optical communication with the groove; and a directionally-etched etch-stop pit comprising a first sidewall portion that intersects the groove at a non-orthogonal angle relative to a longitudinal axis of the groove so that at least a portion of a wedge-shaped end portion of the groove is absent and comprising a second sidewall portion that intersects the pit so that at least a portion of a pit sidewall adjacent the etch-stop pit is absent. 
     The present invention also provides a method for micromachining a crystalline substrate, comprising: providing a crystalline substrate having a surface; directionally etching a first etch-stop pit in the substrate; coating the first etch-stop pit with a mask material; anisotropically wet etching an area adjacent to the first etch-stop pit to provide a groove disposed in the substrate surface; and anisotropically wet etching a first wet-etched pit after formation of the first etch-stop pit, the first wet-etched pit comprising a sidewall to provide a reflection surface for optical communication with the groove, wherein the first etch-stop pit is disposed between the groove and the first wet-etched pit. In addition, the present invention provides a micromachined crystalline substrate fabricated by the method recited in the preceding sentence. 
     In addition, the present invention provides method for micromachining a crystalline substrate, comprising: providing a crystalline substrate having a surface; directionally etching in the substrate two or more open ended, ring-shaped etch-stop pits with at least two of the etch-stop pits oriented so that their respective open ends are not in facing relation; coating the etch-stop pits with a mask material; and anisotropically wet etching an area surrounding the etch-stop pits to provide a wet-etched pit having a base disposed in the substrate and to provide a respective wedge-shaped protrusion interior to each etch-stop pit, the protrusions extending upward from the wet-etched base. Further the present invention provides a method for micromachining a crystalline substrate, comprising: providing a crystalline substrate having a surface; directionally etching an etch-stop pit in the substrate in the surface; coating the etch-stop pit with a mask material; and anisotropically wet etching an area surrounding the etch-stop pit to provide a wet-etched pit having a base disposed in the substrate and to provide two or more wedge-shaped protrusions adjacent the etch-stop pit, the protrusions extending upward from the wet-etched base. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of the preferred embodiments of the present invention will be best understood when read in conjunction with the appended drawings, in which: 
         FIG. 1  schematically illustrates a top view of a V-groove provided in an upper surface of a substrate, where the V-groove includes two ends which include wedge-shaped end portions; 
         FIGS. 2A–2D  schematically illustrate top views of a substrate showing the changes to the substrate as features are added to the substrate to create a structure having a V-groove and an adjoining etch-stop pit in accordance with a first embodiment of the present invention; 
         FIGS. 3A–3F  and  4 A– 4 D schematically illustrate top views of alternative etch-stop pit configurations with the adjoining V-grooves in accordance with the present invention; 
         FIG. 5  schematically illustrates a top view of an alternative etch-stop pit configuration in accordance with the present invention for preventing formation of a wedge-shaped end wall in a V-groove; 
         FIG. 6  schematically illustrates a top view of an alternative etch-stop pit configuration in accordance with the present invention for providing a partial, wedge-shaped end wall in a V-groove; 
         FIG. 7  schematically illustrates a top view of an alternative etch-stop pit configuration in accordance with the present invention for preventing formation of a wedge-shaped end walls in three V-grooves which adjoin the etch-stop pit; 
         FIGS. 8–10  schematically illustrate top views of further configurations of etch-stop pits and V-grooves along with a device mount for providing optical subassemblies in accordance with the present invention; 
         FIGS. 11–16  schematically illustrate top views of alternative configurations etch-stop pits with two or more V-grooves adjoining the etch-stop pits; 
         FIGS. 17–20  schematically illustrate top views of substrates having etch-stop pits, V-grooves, and an optional V-pit, for providing optical subassemblies in accordance with the present invention; 
         FIGS. 21–26  schematically illustrate top views of substrates having two V-grooves oriented at 90 degrees with respect to one another and having an etch-stop pit disposed at the location of the intersection of the two V-grooves; 
         FIGS. 27 and 28  schematically illustrate top views of substrates having an etch-stop pits disposed at locations where an inside, convex corner of two intersecting V-grooves would be located; 
         FIGS. 29A–29D ,  30 ,  31 ,  32 A– 32 B, and  33  schematically illustrate top and cross-sectional views ( 29 B,  29 D,  30 ,  32 B) of substrates having an etch-stop pit which circumscribes a selected area of the substrate in which an anisotropically etched feature is formed; 
         FIGS. 34 ,  35 A,  36 , and  37  schematically illustrate top views of substrates having a U-shaped etch-stop pit adjacent to a V-pit to provide a location on the substrate for a laser mount and to provide a location for retaining a spherical optical element; 
         FIG. 35B  schematically illustrates a cross-sectional view of the substrate illustrated in  FIG. 35A ; 
         FIGS. 38A and 39A  schematically illustrate top views of substrates having a V-groove with an etch-stop pit and fiber stop disposed internally to the groove; 
         FIGS. 38B and 39B  schematically illustrate cross-sectional views of the substrates illustrated in  FIGS. 38A and 39A , respectively; 
         FIG. 40  illustrates a flowchart representing a process in accordance with the present invention for creating an etch-stop pit and an adjacent anisotropically etched feature; 
         FIG. 41  illustrates a flowchart representing another process of the present invention for creating an etch-stop pit and adjacent an anisotropically etched feature; 
         FIG. 42  illustrates a flowchart representing yet another process of the present invention for creating an etch-stop pit and an adjacent anisotropic feature; 
         FIGS. 43 and 44  schematically illustrate a top view and a cross-sectional view, respectively, of a substrate comprising a V-groove and an adjoining etch-stop pit; 
         FIGS. 45–51  schematically illustrates cross-sectional views of a substrate at selected steps of processing in accordance with the method illustrated in the flowchart of  FIG. 41 ; 
         FIGS. 52–58  schematically illustrates cross-sectional views of a substrate at selected steps of processing in accordance with the method illustrated in the flowchart of  FIG. 42 ; 
         FIGS. 59–64  schematically illustrates cross-sectional views of a substrate at selected steps of processing in accordance with the method illustrated in the flowchart of  FIG. 43 ; 
         FIG. 65A  schematically illustrates a top view of a substrate having a V-groove and a truncated V-pit with an etch-stop pit therebetween, where the V-pit provides a reflector surface for reflecting light out of the plane of the substrate and the etch-stop pit provides a fiber stop; 
         FIGS. 65B and 65C  schematically illustrate a top view and a cross-sectional side view taken along the line A—A in  FIG. 65B , respectively, of the substrate of  FIG. 65A  but with an optical fiber disposed in the V-groove; 
         FIG. 66  schematically illustrates a top view of an alternative configuration of a substrate having a V-groove and a V-pit with an etch-stop pit therebetween, where the V-pit provides a reflector surface for reflecting light out of the plane of the substrate and the etch-stop pit provides a fiber stop; 
         FIG. 67A  schematically illustrates a top view of a substrate having two V-grooves with an etch-stop pit therebetween, where the one of the V-grooves provides a reflector surface for reflecting light out of the plane of the substrate and the etch-stop pit provides a fiber stop; 
         FIG. 67B  schematically illustrates a top view of the substrate of  FIG. 67A  but with an optical fiber disposed in one of the V-grooves; 
         FIG. 68A  schematically illustrates a top view of a substrate having two V-grooves with two etch-stop pits and a V-pit disposed therebetween, where the one of the V-grooves provides a reflector surface for reflecting light out of the plane of the substrate and one the etch-stop pits provides a mechanical stop which can be used to align a fiber along the surface; 
         FIG. 68B  schematically illustrates a top view of the substrate of  FIG. 68A  but with an optical fiber disposed in one of the V-grooves; 
         FIG. 69A  schematically illustrates a top view of an alternative configuration of a substrate having two V-grooves with two etch-stop pits and a V-pit disposed therebetween, where the one of the V-grooves provides a reflector surface for reflecting light out of the plane of the substrate and one the etch-stop pits provides a fiber stop; 
         FIG. 69B  schematically illustrates a side cross-sectional view taken along the line A—A in  FIG. 69A  but with an optical fiber disposed in one of the V-grooves and a ball lens disposed in the V-pit; 
         FIG. 69C  schematically illustrates a side cross-sectional view similar to that of  FIG. 69B  but having the optical fiber and ball lens disposed below and upper surface of the substrate; 
         FIG. 70  schematically illustrates a side view of an optical fiber disposed in a V-groove; 
         FIGS. 71A and 711B  schematically illustrate a top view and a side cross-sectional view taken along the line A—A in  FIG. 71A , respectively, of a substrate having a U-shaped etch-stop pit disposed within a wet-etched pit to provide a wedge-shaped protrusion in the wet-etched pit; 
         FIG. 72  schematically illustrates a top view, respectively, of a substrate having four U-shaped etch-stop pits disposed within a wet-etched pit to provide four wedge-shaped protrusions in the wet-etched pit 
         FIGS. 73A and 73B  schematically illustrate a top view and a side cross-sectional view taken along the line A—A in  FIG. 73A , respectively, of a substrate having four generally triangular etch-stop pits disposed within a wet-etched pit to provide four wedge-shaped protrusions in the wet-etched pit; 
         FIG. 73C  schematically illustrates the substrate of  FIG. 73B  but having an additional V-pit for holding a ball lens; 
         FIGS. 74A and 74B  schematically illustrate a top view and a side cross-sectional view taken along the line A—A in  FIG. 74A , respectively, of a substrate having an “I”-shaped etch stop pit disposed within a wet-etched pit to provide two wedge-shaped protrusions in the wet-etched pit; 
         FIG. 75  schematically illustrates a top view of a substrate having an “X”-shaped etch stop pit disposed within a wet-etched pit to provide four wedge-shaped protrusions in the wet-etched pit; and 
         FIGS. 76–80  schematically illustrate additional, optional steps for use with the method illustrated in  FIGS. 52–58 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures, wherein like elements are numbered alike throughout, several different embodiments of devices in accordance with the present invention are illustrated. The different embodiments include a substrate having at least two common features, an etch-stop pit and an anisotropically etched feature adjacent the etch-stop pit. The etch-stop pit has a shape suited to supporting an etch-stop layer on the surfaces of the etch-stop pit. The etch-stop pit is created prior to creating the anisotropically etched feature. The etch-stop layer comprises a material resistant to the etchant which is used to create the anisotropically etched feature. After the etch-stop layer is provided, the anisotropically etched feature is etched in the substrate. The etch-stop layer prevents the anisotropic etching from extending into the region where the etch-stop pit is located. The prevention of such anisotropic etching by the etch-stop layer provides that the crystallographic planar walls of the anisotropically etched feature maintain their crystallographic orientation adjacent the stop-etch pit. The advantages of preventing such etching are illustrated in the accompanying figures depicting several desirable embodiments of the present invention. 
     Throughout the figures, the substrate material is selected to be &lt;100&gt;-oriented silicon. However, other orientations of silicon, such as &lt;110&gt;-oriented silicon, may also be used in accordance with the present invention. In addition, other anisotropic crystalline materials, such as III–V semiconductor materials, e.g., InP, GaAs, InAs, or GaP, may be used in accordance with the present invention. The substrate material is chosen with regard to the nature of the particular optical device and the features to be fabricated. The crystal orientation of the substrate may be chosen with respect to the desired orientation of the sidewalls of the fabricated features. For example, &lt;100&gt;-oriented silicon may be selected to create features having sidewalls that are sloped with respect to the upper surface of the substrate. Alternatively, &lt;110&gt;-oriented silicon may be selected to create features having sidewalls that are perpendicular to the upper surface of the substrate. 
     An example of a typical feature which may be formed by anisotropic etching in an &lt;100&gt;-oriented silicon substrate  6  is a V-groove  2 , as illustrated in  FIG. 1 . In a first aspect of the present invention a modified V-groove  2  is provided, V-groove  12 , having a configuration particularly well-suited to retaining a cylindrical element, such as an optical fiber or GRIN rod lens, as illustrated in  FIGS. 2A–2D . 
     Turning first to the V-groove  2  illustrated in  FIG. 1 , each surface of the V-groove  2  is a {111}-plane of the silicon substrate  6 . The V-groove  2  may be made by known methods such as etching through a rectangular aperture mask using an aqueous solution of KOH. A V-groove  2  which does not extend to the edges of the substrate  6  includes two wedge-shaped end walls  4 . The end walls  4  slope upwardly towards the upper surface  1  of the substrate  6  from an apex  5 . A wedge-shaped end wall  4  is often undesirable in optical subassemblies, because a wedge-shaped end wall  4  can partially or completely occlude the optical path to block light transmitted to or from an optical element disposed in the V-groove  2 . In addition, the wedge-shaped end wall  4  functions poorly as an optical fiber stop, since the wedge-shaped end wall  4  is sloped with respect to the endface of the fiber which is usually perpendicular to the longitudinal axis of the fiber. Thus, it is desirable to create a V-groove without one or more of the wedge-shaped end walls  4 . 
     In particular, referring to  FIG. 2D , a V-groove structure in accordance with the present invention is illustrated where one of the wedge-shaped end walls  14   a , shown in phantom, is eliminated from the V-groove  12 . The device includes a substrate  10  having an upper surface  11  in which a V-groove  12  and adjacent etch-stop pit  16  are provided. The edges  13  where the V-groove  12  and the stop-etch pit  16  intersect are straight line segments that lie within the {111}-plane of the V-groove sidewalls  15 . The ability to remove the right end wall  14   a  while maintaining the {111}-orientation of the sidewalls  15  in the vicinity of the removed end wall  14   a  is provided by the etch-stop pit  16  and etch-stop layer  18 . 
     The sequence in which the etch-stop pit  16  and V-groove  12  are formed in the surface of the substrate  10  is illustrated in  FIGS. 2A–2D . Turning to  FIG. 2A , a top elevational view of the substrate  10  is shown in which an etch-stop pit  16  is formed. As depicted, the etch-stop pit  16  has a triangular cross-section in the plane of the upper surface  11  of the substrate  10 . Other shapes than triangular cross-section may be used so long as such shapes are suited to preventing the formation of a wedge-shaped end wall  14   a  of the V-groove  12 . The types of shapes which may be used are discussed below with reference to  FIGS. 3 and 4 . 
     The walls of the etch-stop pit  16  may desirably extend into the substrate  10  at a 90 degree angle, i.e. vertical, relative to the upper surface  11  of the substrate  10 , and the etch-stop pit  16  may contain a flat bottom parallel to the plane of the upper surface  11 . Such a configuration of the etch-stop pit  16  may be fabricated by high-aspect ratio dry etching, such as reactive ion etching such as the BOSCH etching process. Alternatively, the etch-stop pit  16  may include sidewalls that are sloped with respect to the plane of the upper surface  11 . Regardless of the sidewall slope that is utilized, the portions of the sidewalls  16   a  located proximate the region at which the V-groove  12  is to be formed, i.e. at intersecting segments  13 , should extend into the substrate  10  a greater depth than the depth intended for the adjoining portion of the V-groove  12 . Providing such a deeper sidewall portion ensures that a subsequently applied etch-stop layer  18  provides a barrier between an etchant in the V-groove  12  and the etch-stop pit  16 . 
     After formation of the etch-stop pit  16 , an etch-stop layer  18  is conformally provided on the sidewalls  16   a  and the bottom of the etch-stop pit  16 . The etch-stop layer  18  comprises a material that is resistant to the etchant that will be used to create the V-groove  12 . For example, the etch-stop layer  18  may comprise silicon dioxide, which may be provided by CVD or by thermally oxidizing surfaces  16   a  of the etch-stop pit  16 , or silicon nitride, which may be provided by CVD. Optionally, the upper surface  11  of the substrate  10  may be provided with a layer of the same material used for the etch-stop layer  18 . 
     The V-groove  12  is formed in the surface  11  of the substrate  10  by a suitable process, such as anisotropic wet etching with KOH or EDP through a rectangular aperture mask. The rectangular aperture mask is oriented such that the perimeter of the rectangular aperture is registered to the perimeter of the V-groove  12  located in the upper surface  11  of the substrate  10 . The rectangular aperture mask is oriented such that a portion of an end of the rectangular aperture overlies the etch-stop pit  16 . Further details regarding how the masks are provided is discussed below in connection with the method of the present invention. 
     As an optional additional step, the etch-stop layer  18  may be removed from the etch-stop pit  16 . Removal of the etch-stop layer allows the V-groove  12  to communicate with the etch-stop pit  16 . Such communication permits an element, such as a fiber, disposed within the V-groove  12  to extend into the region over the etch-stop pit  16  and abut the sidewall  16   a  of the etch-stop pit  16  that is disposed perpendicular to the longitudinal axis of the V-groove  12 , to provide a fiber stop  17 . 
     The process described above with respect to  FIG. 2D  is suited to forming all of the structures illustrated herein. For example, each of the following structures described below includes at least one etch-stop pit which is formed before an adjacent anisotropically etched feature, such as a V-groove is formed adjacent to the pit. In addition, an etch-stop layer is provided in the etch-stop pit prior to forming a anisotropically etched feature. While the etch-stop layer may not be illustrated in the figures, because it has been removed after the formation of the anisotropically etched feature, it is understood that the etch-stop layer is present within the etch-stop pit while the anisotropically etched feature is being formed. 
     In addition to the triangular cross-sectional shape of the etch-stop pit  16  illustrated in  FIGS. 2A–2D , other cross-sectional shapes may be used in accordance with the method of the present invention to completely or partially prevent the formation of a wedge-shaped end wall of a V-groove, as shown in  FIGS. 3C–3F  and  4 A– 4 D. For example, a first type of etch-stop pit configuration for completely preventing the formation of a wedge-shaped end wall  44  is illustrated in  FIGS. 4A–4D .  FIGS. 4A–4D  illustrate top elevational views of a V-groove  42  adjacent to etch-stop pits  46 ,  47 ,  48 ,  49  of differing cross-sectional areas. In each configuration, the etch-stop pit  46 ,  47 ,  48 ,  49  completely overlays a region of the substrate in which the wedge-shaped end wall  44  of the V-groove  42  would otherwise be formed. The etch-stop pit  46 ,  47 ,  48 ,  49  and V-groove  42  may be formed in the substrate by the procedure described above with respect to the device illustrated in  FIG. 2D . 
     One desirable configuration of an etch-stop pit  49  comprises two sidewalls joined at an apex that lies along the longitudinal axis of the V-groove  42  such that the apex angle, a, is bisected by the longitudinal axis. Such a configuration of an etch-stop pit  49  can prevent the formation of a wedge-shaped end wall  44  when the apex angle is less than or equal to 90 degrees. 
     If the apex angle were greater than 90 degrees, as illustrated in  FIGS. 3C and 3D , a partial wedge-shaped end wall  34  would be formed in the V-groove  32 . In the configuration where the “apex angle” is equal to 180 degrees, i.e. a straight line, the typical wedge-shaped end wall  24  would be formed in the V-groove  22 , as illustrated in  FIGS. 3A and 3B . That is, an etch-stop pit  26  having a straight sidewall  23  adjacent to the area in which the V-groove  22 , is to be formed, and oriented perpendicular to the longitudinal axis of the V-groove  22 , allows for the formation of a wedge-shaped end wall  24 . Other cross-sectional shapes of an etch-stop pit are contemplated in accordance with the present invention, such as the “W” cross-sectional shape depicted in  FIGS. 3E and 3F . 
     Yet another configuration of an etch-stop pit  386  in accordance with the present invention may be provided so that a fiber stop  387  is created within a V-groove  384 , as illustrated in  FIGS. 38A–38B  and  39 A– 39 B.  FIGS. 38A and 39A  illustrated top views of a substrate  380  in which a V-groove  384  is formed.  FIGS. 38B and 39B  illustrate cross-sectional views taken along the lines B—B in  FIGS. 38A and 39A , respectively. The etch-stop pit  386  has a shape that promotes the formation of a wedge-shaped fiber stop  387  along a {111} crystallographic plane adjacent a first sidewall  383  of the etch-stop pit  386 . In particular, the straight sidewall  383  oriented perpendicular to the longitudinal axis of the V-groove  384  promotes the formation of the wedge-shaped fiber stop  387  in an analogous fashion to the formation of the wedge-shaped end wall  24  in  FIG. 3B . The etch-stop pit  386  also comprises a pair of angled sidewalls  385  across the dark and  386  from the first end wall  383 . The angled sidewalls  385  intersect at a selected apex angle which has a magnitude and orientation suitable for preventing the formation of wedge-shaped surfaces, i.e. {111} surfaces, in the V-groove  384  adjacent to the angled sidewalls  385 . The angled sidewalls  385  may have a similar configuration to corresponding sidewalls depicted in  FIG. 2D . As illustrated in the cross-sectional views of  FIGS. 38B and 39B , the wedge-shaped fiber stop  387  extends above the deepest portion of the V-groove  384  so that a fiber  381  disposed within the V-groove  384  may abut the wedge-shaped fiber stop  387 . 
     A second type of etch-stop pit configuration that prevents a wedge-shaped end wall from forming has a parallelogram cross-sectional shape oriented at an angle, β, of 45 degrees or less, with the longitudinal axis of the V-groove  52 , as illustrated in  FIG. 5 . If, the angle, β, is larger than 45 degrees, as depicted in the configuration of  FIG. 6 , then a partial wedge-shaped end wall  64  is formed in the V-groove  62 . In a case where β is 90 degrees, the configuration of the etch-stop pit becomes functionally equivalent to that of the etch-stop pit depicted in  FIG. 3A . In addition, V-grooves  52 ,  53 ,  55  may be provided on opposing sides of the etch-stop pit  56  as illustrated in  FIG. 7 . So long as the longitudinal axis of each V-grooves  52 ,  53 ,  55  is oriented at an angle less than 45 degrees relative to an adjacent surface of the etch-stop pit  56 , the etching process in accordance with the present invention will not produce wedge-shaped end walls in the V-grooves  52 ,  53 ,  55  in the region adjacent the etch-stop pit  56 . Any number of V-grooves may be so provided, and such grooves need not have the same size. 
     Returning now to the configuration illustrated in  FIG. 2D , where the combined V-groove  14  and etch-stop pit  16  provide a cavity having a fiber stop  17  for retaining a fiber optic, further optical subassemblies may be fabricated by providing additional features in or on the substrate  10 . Such subassemblies may provide for optical communication with the fiber. In particular, the structure of  FIG. 2D  is well-suited for use with other optical elements, because the fiber stop  17  provides a fiducial reference point to precisely identify where the end of the fiber is located. 
     For example,  FIGS. 8–10  illustrate top elevational views of additional configurations in accordance with the present invention that provide optical subassemblies comprising a fiber  81 ,  91 ,  101 , a V-groove  84 ,  94   104 , and a laser mount  85 ,  95 ,  105 . Alternatively, detector or VCSEL mounts could be provided in place of the edge emitting laser mounts  85 ,  95 ,  105 . In particular, with reference to  FIG. 8 , a V-groove  84  and adjoining etch-stop pit  86  with fiber stop  83  are provided in a configuration similar to that depicted in  FIG. 2D  described above. Advantages of the design in  FIG. 8  include allowing a fiber, lensed tipped fiber, or cylindrical lens to be placed arbitrarily close to the edge emitting facet of the laser without interference from the otherwise adjoining &lt;111&gt; facet and without requiring a dicing cut to remove the facet. The etch-stop pit  86 , however, is not precisely triangular in cross-section, but rather includes an etched area  87  that protrudes, in cross-section, from the fiber-stop edge of the etch-stop pit  86 , so that the cross-sectional shape of the etch-stop pit  86  is similar to that of an arrowhead. The optional etched area  87  allows for beam expansion. In addition, a laser mount  85  is provided proximate the etched area  87  and is disposed along the longitudinal axis of the V-groove  84 . It may be desirable to provide an optical device between the end of the fiber optic  81  and the laser mount  85 . Accordingly, the configurations illustrated in  FIGS. 9 and 10  provide slots  99 ,  109  for receiving optical elements. The slots  99 ,  109  communicate with the respective etch-stop pits  96 ,  106  and may be formed at the same time as the etch-stop pits  96 ,  106 . The slots  99 ,  109  comprise vertical sidewalls, however, sloped sidewalls may also be provided. The slot  109  of  FIG. 10  conveniently has a cross-sectional shape of a plano-convex lens, whereas the slots  99  is well-suited to receiving flat optics or lenses other than ball lenses. 
     In yet another etch-stop pit configuration in accordance with the present invention, the etch-stop pit may have a diamond-like cross-sectional shape which is suited to device configurations that include two or more V-grooves disposed on opposing sides of the etch-stop pit, as illustrated in  FIGS. 11–16 . Referring to  FIG. 11 , a substrate  110  is shown which includes a diamond cross-sectional shaped etch-stop pit  116  with two V-grooves  114 ,  115  disposed on opposing sides of the etch-stop pit  116 . The V-grooves  114 ,  115  have longitudinal axes are collinear and intercept at a respective vertex of the etch-stop pit  116 . The region of intersection between each V-groove  114 ,  115  with the respective portion of the etch-stop pit  116 , has a similar geometry to the intersection between the V-groove  14  and etch-stop pit  16  depicted in  FIG. 2D . Thus, for the same reasons given above, no wedge-shaped end wall is formed in the V-grooves  114 ,  115  at the locations adjacent the etch-stop pit  116 . To allow the end faces of respective fibers disposed in two V-grooves  164 ,  165  to be space more closely together, the etch-stop pit  166  may comprise a diamond-like shape that is compressed, as illustrated in  FIG. 16   
     In a similar manner, the etch-stop pit  136  may have a cross-sectional shape suited to having a single V-groove  134  on one side of the etch-stop pit  136  and having two or more V-grooves  135 ,  137 ,  139 , disposed at an opposing side of the etch-stop pit  136 . In addition, the etch-stop pit  136  may have a cross-sectional shape suited to preventing the formation of a wedge-shaped end wall in each V-groove  134 ,  135 ,  137 ,  139  at the respective positions where the V-grooves  134 ,  135 ,  137 ,  139  adjoin the etch-stop pit  136 . A suitable shape for such an etch-stop pit  136  is depicted in  FIG. 13 . The etch-stop pit  136  provides two fiber stops  137  for a fiber disposed in the V-groove  134 . Yet additional shapes of an etch-stop pit  126  may be provided for preventing the formation of wedge-shaped end walls in multiple V-grooves  124 ,  125 ,  126 ,  127 , as illustrated in  FIG. 12 . Wedge-shaped end walls do not form for the reasons given above with regard to  FIGS. 4D and 7 , for example. 
     In silicon optical bench, applications for such structures include aligning several fibers to a single grin lens or larger multi-mode fiber. Alternatively, in microfluidics, structures such as these can be use for fluidic mixing of multiple flow channels. 
     Still further, two of the ‘etch-stop pit with adjoining V-groove’-structures illustrated in  FIG. 2D  may be provided in a single substrate  140  in back-to-back coaxial relationship with a passageway  149  extending between the two triangular sections of the etch-stop pit  146 , as illustrated in  FIG. 14 . Such a configuration provides a fiber stop  147  for each of the V-grooves  144 ,  145  so that the distance, D, between the ends of two fibers, or cylindrical objects, located within the V-grooves  144 , 145  may be precisely specified. In addition, the passageway  159  may be sufficiently long so as to provide for insertion of an optical element between the two mechanical stops  157 . A slot  153 , or other suitable shape, may be provided to receive, for example, an optical element such as filter, isolator, etc. 
     In yet another aspect of the present invention, two or more the above-described ‘etch-stop pit with adjoining V-groove’ structures may be provided in a substrate with a V-pit disposed therebetween, as shown in  FIGS. 17 ,  18 , and  20 . A V-pit  179 ,  189 ,  209  may be formed by anisotropic etching by the same methods used to form V-grooves but using a square aperture mask rather than a rectangular aperture mask. The V-pit  179 ,  189 ,  209  may be anisotropically etched at the same time as the grooves  174 ,  184 ,  204 . The V-pit  179 ,  189 ,  209  should be etched after the etch-stop pit  176 ,  186 ,  206  and the etch-stop layer are provided, in accordance with the process described above in reference to  FIG. 2D . The V-pit  179 ,  189 ,  209  comprises for triangular-shaped, sidewalls that lie in the {111} crystallographic planes to form a four-sided regular pyramid that extends into the substrate  170 ,  180 ,  200 . Like the V-grooves  174 , 184  the V-pits  179 , 189  should extend into the substrate a depth less than the depth of the etch-stop pits  176 ,  186  at the point of intersection between the V-pits  179 ,  189  and the etch-stop pits  176 ,  186 , as illustrated in  FIGS. 17 and 18 . In a configuration where the V-pit  209  does not intercept the etch-stop pit  206 , the V-pit  209  depth does not need to be selected with regard to the depth of the etch-stop pit  206 . The V-pits  179 ,  189 ,  209  provide a convenient shape for retaining a spherical optical element, such as a ball lens. The V-grooves  174 ,  184 ,  204  are positioned so that an optical element disposed within the V-grooves  174 ,  184 ,  204  can optically communicate with the optical element disposed within the respective V-pit  176 ,  186 ,  206 . In alternative configuration, as illustrated in  FIG. 19 , the etch-stop pit  196  may contain a central portion  195  configured to hold a spherical optical element. For example, the central portion may have a diamond-like shape. The central portion  195  of the etch-stop pit  196  may serve the same function of retaining a spherical optical element as that of the V-pit  189 , or may be configured to contain other elements such as beam splitting cubes, filters, circulators, and so on. 
     In another aspect of the present invention, an etch-stop pit may be provided at a location where two anisotropically etched features would intersect to form an inside, convex corner. A convex corner formed by the intersection of two {111} planes in the mask opening does not etch to form a straight line intersection between two planes, but rather creates a rounded or complexly eroded intersection between the two intersecting {111} planes. The rounding can propagate to remove material in the vicinity of the intersection, such that the well-defined {111} planes can be etched away in the vicinity of the intersection to yield structures that are not {111} planes. Thus, it would be desirable to prevent the formation of such rounded corners. 
       FIGS. 21–28  illustrate several configurations of etch-stop pits in accordance with the present invention which are suited to prevent undesirable etching at an inside, convex corner. Each of the structures in  FIGS. 21–28  may desirably be formed by the process described above with reference to  FIG. 2D , with an etch-stop pit and etch-stop layer provided in the substrate prior to anisotropically etching the V-grooves. For example, referring to  FIG. 21 , a top elevational view of the substrate  210  is shown in which two V-grooves  214  are disposed. The two V-grooves  214  are oriented with their respective longitudinal axes at 90 degrees relative to one another. An etch-stop pit  216  is provided at a selected location of the substrate  210  corresponding to the location at which the two V-grooves  214  would otherwise intersect. Providing the etch-stop pit  216  at the selected location prevents intersection of the V-grooves  214 . The etch-stop pit  216  may be disposed at a 45 degree angle, β, so that wedge-shaped end walls are not formed in the V-grooves  214  adjacent the etch-stop  216 . To provide for greater ease of alignment (lower alignment tolerances) between the etch-stop pit  216  and the longitudinal axis of the V-grooves  214 , an angle of less than 45 degrees may be preferable. 
     Alternative configurations of an etch-stop pit that prevent etching of an inside, convex corner and wedge-shaped V-groove end walls are illustrated in  FIGS. 22–28 . Each configuration depicted in  FIGS. 22–28  includes V-grooves  224 ,  234 ,  244 ,  254 ,  264  oriented at 90 degrees with an intermediate etch-stop pit  226 ,  236 ,  246 ,  256 ,  266  in a similar configuration to that of  FIG. 21 . The etch-stop pit  226 ,  236 ,  246 ,  256 ,  266  is located to prevent intersection of the V-grooves  224 ,  234 ,  244 ,  254 ,  264 . Each of the etch-stop pits  226 ,  236 ,  246 ,  256 ,  266  has straight wall segments disposed at an angle of 45 degrees or less with respect to the longitudinal axis of an adjacent V-groove  224 ,  234 ,  244 ,  254 ,  264 . Referring to  FIGS. 22 and 23 , the etch-stop pit  226 ,  236  may include an interior portion  225 ,  235  for retaining optical element such as filters, lenses, micromechanical switches, for example. Such elements may be formed directly in the wafer, placed and bonded into the wafer, or may be formed on another wafer and recess into the cavity when two wafers are aligned and brought together. In addition, as illustrated in  FIGS. 25 and 26 , the etch-stop pit  256 ,  266  may have a shape configured to provide a fiber stop  257 ,  267  for fibers  251 ,  261  disposed within the V-grooves  254 ,  264 . 
     In accordance with the present invention, yet additional configurations of etch-stop pits  276 ,  286  are provided which permit the intersection of two V-grooves  274 ,  284  while preventing the formation of an inside, convex corner  275 ,  285 , thus obviating the need for corner compensation, as illustrated in  FIGS. 27 and 28 . For example, a top elevational view of a substrate  270 ,  280  is shown in which pairs of V-grooves  274 ,  284  are provided in an upper surface of the substrate  270 ,  280 . Pairs of V-grooves  274 ,  284  intersect at ends of the V-grooves  274 ,  284  at an angle of 90 degrees. An etch-stop pit  276 ,  286  is provided at a selected location of the substrate  270 ,  280  corresponding to the location at which an inside corner  275 ,  285  of the intersecting V-grooves  274 ,  284  would otherwise be formed. Providing the etch-stop pit  276 ,  286  coated with an etch-stop layer at the selected location prevents formation of the inside convex corner  275 ,  285 . 
     In a further aspect of the present invention, an etch-stop pit  296  may be provided as a continuous boundary that circumscribes a region of the substrate  294  that is to be anisotropically etched. Providing such an etch-stop pit boundary permits the anisotropically etched features to be etched more deeply than otherwise possible. For example, referring to  FIG. 30 , a cross-sectional view of a substrate  300  is shown in which a recessed V-groove  304  is provided. The ability to form the V-groove  304  below the plane of the upper surface  301  is provided by the presence of the etch-stop pit  306  (coated with an etch-stop layer) which circumscribes the region in which the V-groove  304  is formed. If the etch-stop pit  306  were not provided, the surfaces of the V-groove  304  would extend upward to the upper surface  301  as indicated by the dashed line  307 , and thus would not be recessed with respect to the upper surface  301 . 
     Turning now to  FIGS. 29A–D , an L-shaped etch-stop pit  296  is provided which circumscribes an L-shaped area in which an anisotropically etched feature may be formed. Providing the L-shaped etch-stop pit  296  permits the formation of {111} sidewalls as illustrated in  FIGS. 29A and 29B . Etching the wider pit  296  as illustrated in  FIGS. 29C and 29D  permits a deeper feature to be formed. Alternatively, other shapes than L-shaped may be utilized as an etch-stop pit. For example, the etch-stop pit  316  may have a T-shaped cross-section as illustrated in the top view of  FIG. 31 . Upon anisotropically etching the region  311  bounded by the T-shaped etch-stop pit  316 , {111} sidewalls may be formed as illustrated in  FIGS. 32A and 32B . In the vicinity of the cross-sectioning plane B—B, the anisotropically etched feature  324  may have a V-shaped cross-section, as illustrated in  FIG. 32B . Yet further shapes may be utilized in accordance with the present invention as an etch-stop pit which circumscribes an area to be anisotropically etched, such as the configuration depicted in  FIG. 33 . 
     In yet another aspect of the present invention, a U-shaped etch-stop pit  346  is provided adjacent to a V-pit  345  to provide a location on a substrate  340  for mounting an optical element, such as a laser mount  355 , and to provide a location for retaining an optical element, such as a spherical optical element  350 , as illustrated in  FIGS. 34–36 .  FIGS. 34 and 35A  illustrate a top view of a substrate  340  in which a U-shaped etch-stop pit  346  is provided adjacent a V-pit  345 . The U-shaped etch-stop pit  346  includes sidewalls that extend a selected depth into the substrate  340 . Optionally, the sidewalls of the U-shaped etch-stop pit  346  may be vertical, as illustrated in  FIG. 34 . Alternatively, the sidewalls of the U-shaped etch-stop pit  346  may be inclined relative to the upper surface  301  of the substrate  340 . The sidewalls of the U-shaped etch-stop pit  346  are conformally coated with an etch-stop layer, or, optionally, the etch-stop pit  346  is filled etch-stop layer material. In addition, the portion  343  of the substrate surface  301  interior to the etch-stop pit  346  may be provided with an etch-stop layer. As explained above with reference to the process of  FIGS. 2A–2D , the etch-stop layer comprises a material that is resistant to the etching used to form an anisotropically etched feature, such as V-pit  345 . 
     After the desired etch-stop layer or layers are provided, the V-pit  345  may be formed by anisotropic etching by the same methods used to form V-grooves but using a square aperture mask. Instead of using a perfectly square aperture mask, a generally square-aperture that includes a protrusion to protect substrate surface portions  343  interior to the etch-stop pit  346  may be used. The V-pit  345  may be anisotropically etched at the same time as the optional V-groove  354 . The V-pit  345  should extend into the substrate a depth less than the depth of the etch-stop pit  346  at the point of intersection  353  between the V-pit  345  and the etch-stop pit  346 , as illustrated in  FIG. 35B . In a configuration where the V-pit  345  does not intercept the etch-stop pit  346 , the V-pit  345  depth does not need to be selected with regard to the depth of the etch-stop pit  346 . 
     The V-pit  345  provides a convenient shape for retaining a spherical optical element, such as a ball lens  350 . The interior portion  343  of the substrate surface  301  provides a convenient location at which a laser  355 , or other optical device, may be located for optical communication with the ball lens  350 . Providing the U-shaped etch-stop pit  346  permits a portion of the V-pit  345  adjacent the etch-stop  346  to be recessed below the surface  341  of the substrate  340 . Such a recess permits the ball lens  350  to be positioned more closely to the laser  355 , as illustrated in  FIG. 35B . This is particularly useful for lenses with a focal point that would otherwise exist in free-space. For example, a ball lens of cubic zirconia has a refractive index near 2.0. The ideal location for a edge emitting laser for a collimating design with such a lens would be as close to the surface of the ball lens as possible. A V-pit etched by traditional means, and designed to place the circumference of the lens near the elevation of the of the silicon surface, would prevent a laser to be placed on the surface of the silicon from being sufficiently close to the lens surface. The platform  343  would prevent such a problem, and can be constructed to meet this and similar placement needs. In addition, a V-groove  354 ,  374  may also be provided for optical communication between a fiber disposed with the V-groove  354 ,  374  and the V-pit, as illustrated in  FIGS. 36 and 37 . With respect to  FIG. 36 , the V-groove  354  may be fabricated in a similar manner as the V-grooves  174  of  FIG. 17 , for example. Alternatively, as illustrated in  FIG. 37 , the etch-stop pit  376  may circumscribe the region in which the V-pit  375  is formed. The etch-stop pit  376  comprises a U-shaped segment  366  to provide an analogous function to that of the U-shaped etched pit  346  in the configuration of  FIG. 35A . The etch-stop pit  376  also comprises a triangular-shaped segment  378  to prevent formation of a wedge-shaped end wall in the V-groove  374  and to provide a fiber stop  377 . 
     In still another aspect of the present invention,  FIGS. 65–69  illustrate several configurations of etch-stop pits in combination with V-grooves and/or V-pits which are suited to permit redirection of light out of the plane of the substrate to/from an optical fiber disposed within the plane of the substrate. Such a configuration is particularly suited for optical communication with surface normal optical or optoelectronic element, such as a VCSEL. Each of the structures in  FIGS. 65–69  may desirably be formed by the process described above with reference to  FIG. 2D , with an etch-stop pit and etch-stop layer provided in the substrate prior to anisotropically etching the V-grooves/V-pits. 
     For example, referring to  FIGS. 65A–65C , a substrate  650  is shown in which a fiber V-groove  652  and a truncated V-pit  655  are disposed. An etch-stop pit  656  in the shape of a pentagon is disposed between V-groove  652  and V-pit  655 , with a triangular portion of the pentagon directed into the V-groove  652 , in a similar manner as the triangular etch-stop pit  16  intersects the V-groove  12  of  FIG. 2D , to prevent the formation of a wedge-shaped end wall in the V-groove  652  at the end adjacent the etch-stop pit  656 . As illustrated in  FIG. 65C , the etch-stop pit  656  provides a vertical surface that functions as a fiber stop  657  against which optical fiber  651  may abut, in a similar manner to the fiber stop  17  of the etch-stop pit  16  of  FIG. 2D . The etch-stop pit  656  is disposed on the substrate at a location that permits the mechanical stop  657  to be so that the end face of the fiber  651  may be accurately located relative to the opposing V-pit inclined sidewall  654  to prevent the problem illustrated in  FIG. 70 , where some of the light  704  from the fiber  701  fails to reach an optical device  702  because the light  704  is reflected back on to the fiber  701 . The situation illustrated in  FIG. 70  is particularly problematic with microscopic submounts made from anisotropically etched single crystal silicon, because the angles of the V-groove sidewalls are fixed by the crystal structure. 
     Accurate positioning of the fiber endface is important when the optical spot size needs to be accurately controlled, as is the case in photodetectors where the optical spot size is usually made to fill ˜&gt;=90% of the active diffused junction, or in the case of VCSELS being coupled to multimode fiber where a gap of ˜100 um is often used to allow the beam to expand to more fully fill the fiber core. Moreover, it is particularly important in micro-optics, laser coupling, and fiber optic component packaging, that distances between the optical lenses, fibers, and active devices are often controlled on the order of tens of microns down to several microns or smaller to produce optimal results and coupling consistency. 
     Various other configurations of V-groove, etch-stop pit, and V-pit are possible to achieve the effects illustrated in  FIGS. 65A–65C . For example, referring to  FIG. 66 , an etch-stop pit  666  of a different shape from that of the etch-stop pit  656  may be provided between a fiber V-groove  662  and truncated V-pit  664 . The etch-stop pit  666  may still comprise a triangular portion for intersection with the V-groove  662  to prevent the formation of a wedge-shaped end wall in the V-groove  662 . Like the etch-stop pit  656 , the etch-stop pit  666  provides a fiber stop  667 , and the V-pit  665  provides a reflector surface  664 . 
     Still other configurations are possible for providing out-of-plane reflection of light to and from an optical fiber, as illustrated in  FIGS. 67A and 67B . Such reflectors can be made with 54.74 degree facets when using &lt;100&gt; oriented silicon, or for example, can produce 45 degree facets when off-axis cut &lt;100&gt; wafers are used by slicing the ingot 9.74 degrees off the &lt;100&gt; axis as is known in the art. As shown and  FIGS. 67A and 67B , a fiber V-groove  672  is provided adjacent to an etch-stop pit  676  which includes a triangular portion extending into the area of the V-groove  672  to prevent the formation of a wedge-shaped end wall in the V-groove  672 . In this case, a reflector surface  674  may be provided by a reflection V-groove  675  disposed on an opposing side of the etch-stop pit  676  from the fiber V-groove  672 . The etch-stop pit  676  also includes a triangular portion extending into the area of the reflection V-groove  675  to prevent the formation of a wedge-shaped end wall in the reflection V-groove  675 . Accordingly, the etch-stop pit  676  may be diamond-shaped as shown in  FIGS. 67A and 67B . The sidewalls of the etch-stop pit  676  closest to the reflection V-groove  675  may provide a fiber stop  677  for a fiber  671 , as shown in  FIG. 67B . 
     In addition to the above configurations for providing out-of-plane reflection of light to and from an optical fiber, other configurations are possible, including ones that provide a lens between the optical fiber and the reflector surface, such as those shown in  FIGS. 68A and 68B . For example, a fiber V-groove  682  is provided adjacent to an etch-stop pit  686  which includes a triangular portion extending into the area of the V-groove  682  to prevent the formation of a wedge-shaped end wall in the V-groove  682 . The etch-stop pit  686  also extends into a V-pit  688  for holding the ball lens  683 . This configuration is useful, for example, for single mode fiber devices where beam expansion and path length without a lens would prevent sufficient coupling between fibers and active devices. A reflection V-groove  685  is disposed in the substrate to provide a reflector surface  684  for optical communication with a fiber  681  disposed in the fiber groove  682 . In addition, to prevent that portion of the light beam below the upper surface of the substrate from being occluded, a clearance etch-stop pit  689  may be provided intermediate the ball lens of the pit  688  and the reflection V-groove  685 . 
     Various other configurations of V-grooves and etch-stop pits are also possible to achieve the effects illustrated in  FIGS. 68A and 68B . For example, referring to  FIGS. 69A and 69B , a clearance etch-stop pit  699  of a different shape from that of the etch-stop pit  689  may be provided between a reflection V-groove  695  and a lens V-pit  698 . With the exception of the shape of the clearance etch-stop pit  699 , the remaining structures illustrated in  FIGS. 69A and 69B  are analogous to those illustrated in  FIGS. 68A and 68B . The removal of substrate material in the region of etch-stop pit  699  permits light from the fiber  691  to reach the reflector surface  694 . This particular configuration of clearance etch-stop pit  699  permits the reflector surface  694  to be located closer to a ball lens  693  than is the case in the configuration of  FIGS. 68A and 68B . In addition, as illustrated in  FIG. 69C  the substrate can be configured so that the fiber  691  and ball lens  693  lie below the upper surface  690  of the substrate. This permits an optical device  693  to be cantilevered from the upper surface  690  to communicate with an optical beam  697 . The beam shown in  69 C is shown at 45 degrees. This can be achieved in practice with off-axis cut silicon wafers, however more typically a 54.74 degree facet would be found producing a beam  697  reflecting to the left of the position shown. An exemplary version of  FIG. 69C  would be in coupling a VCSEL to a single mode fiber. In this case, the VCSEL active area can overhang the reflector facet bouncing the optical beam toward the fiber. The angular deviation produced by a 54.74 degree facet can be corrected by optimizing the height of the ball lens, allowing the beam to be focused and captured within the N.A. of a single mode (or multimode) fiber. 
     Alternatively, the substrate can be configured so that the fiber and ball lens extend above the upper surface of the substrate and an optical device may be suspended above the upper surface. 
     In a further aspect of the present invention,  FIGS. 71–75  illustrate several configurations of ring-shaped etch-stop pits disposed within a wet-etched pit to provide wedge-shaped protrusions in the wet-etched pit, which protrusions may be used as a reflecting surface, mounting structure, neuroprobes, or any application in which sharp edges or points are desired. Further examples of related structures are found in U.S. patent application Ser. No. 10/076,858, the entire contents of which are incorporated herein by reference. Each of the structures in  FIGS. 71–75  may desirably be formed in a manner similar to that described above where an etch-stop pit is provided in the substrate and coated with an etch-stop layer and then an anisotropic (wet etched) feature is etched. The case of the configuration of  FIGS. 71–75 , the region of anisotropic etching surrounds and includes the location(s) of the etch-stop pit(s). 
     For example, referring to  FIGS. 71A and 71B , a substrate  7400  is shown in which a wedge  7490  is provided on the flat bottom  7415  of a wet-etched pit  7410 . In order to form the wedge  7490 , first a U-shaped etch-stop pit  7430  is dry etched or machined into the substrate  7400 . The etch-stop pit  7430  is then coated with an etch-stop layer. The substrate  7400  is then masked to provide a rectangular opening corresponding to the outer perimeter of the wet-etched pit  7410 , and the area of the substrate  7410  within the masked opening may be anisotropically etched to simultaneously form the wet-etched pit  7410  and the wedge-shaped protrusion  7490 . That is, during the wet etching, both the inclined sidewalls of the wet-etched pit  7410  and the inclined sidewall of the wedge-shaped protrusion  7490  are formed and the etch-stop pit  7430  forms a ring about the wedge-shaped protrusion  7490 . 
     In another configuration multiple U-shaped etch-stop pits  7530  may be used to provide multiple wedge-shaped protrusions  7590 , as illustrated in  FIG. 72 . Specifically, four U-shaped etch-stop pits  7530  with etch-stop layer may be oriented relative to one another to form a “+”, with the open portion of each “U” positioned adjacent the center of the “+”. Subsequent anisotropic etching in a similar manner to that described above with regard to  FIGS. 71A and 71B  produces a wet-etched pit  7510  and four wedge-shaped protrusions  7590  with each wedge-shaped protrusion  7590  inclined downward towards the center of the “+” to provide a pocket  7595  into which a ball lens or other object may be seated. Other ring shapes of etch-stop pits may also be used. For example, with reference to  FIGS. 73A and 73B , generally triangular etch-stop pits  7630  may be oriented in a “+” configuration to provide a similar structure to that illustrated in  FIG. 72 . 
     In particular, the etch-stop pits  7690  may take the form of a truncated triangle in which one apex of the triangle is not present to provide an open end of the triangle. The open end of each triangular etch-stop pit  7690  is positioned adjacent the center of the “+”. Subsequent anisotropic etching in a similar manner to that described above with regard to  FIG. 72  produces a wet-etched pit  7610  and four wedge-shaped protrusions  7690  interior to each etch-stop pit  7690 , with each wedge-shaped protrusion  7690  inclined downward towards the center of the “+” to provide a pocket  7695  into which a ball lens or other object may be seated, as seen in  FIG. 73B . In addition to providing a wet-etched pit  7610  and wedge-shaped protrusion  7690  on a substrate  7600 , other features may be provided, such as a V-pit  7620  for retaining a ball lens or  7625 , to provide an optical bench as shown in  FIG. 73C . 
     In addition to providing mounting structures, wet etching processes in the present invention may be utilized to provide structures suitable for us in various devices such as probe tips, emission sources, microfluidic nozzles, and/or out-of-plane light reflecting surfaces. For example, an “I”-shaped etch-stop pit  7730  may dry etched or machined into a substrate  7700 , as illustrated in  FIGS. 74A and 74B . The etch-stop pit  7730  is then coated with an etch-stop layer. The substrate  7700  is masked to provide rectangular opening corresponding to the outer perimeter of the wet-etched pit  7710  and the area of the substrate  7700  within the masked opening may be anisotropically etched to simultaneously form the wet-etched pit  7710  and the wedge-shaped protrusions  7790 . A knife-sharp tip  7791  is provided at the top of each wedge  7790  which can be made very close together, the distance between each tip  7791  being determined by the width of the vertical portion of the “I”. Alternatively, an etch-stop pit  7830  may be provided in the form of an “X” followed by subsequent wet etching, to provide a wet-etched pit  7810  comprising four wedge-shaped projections  7890 , as illustrated in  FIG. 70 . 
     Methods of Fabrication 
     In accordance with the present invention, there are provided methods for fabricating optical subassemblies having an etch-stop pit and an adjacent recessed area, such as an anisotropically etched area, for receiving an optical element. Three exemplary methods are illustrated in the flowcharts of  FIGS. 40–42  and the accompanying side cross-sectional views of  FIGS. 45–64 . The orientation of the side cross-sectional views of  FIGS. 45–64  is illustrated in  FIGS. 43 and 44 . 
     Referring to  FIG. 43 , a top elevational view is shown of a substrate  440  in which a V-groove  444  and adjacent etch-stop pit  446  are provided. The structure shown in  FIG. 43  is similar to that shown in  FIG. 2D , where one of the wedge-shaped end walls is eliminated from the V-groove  444 . A cross-sectional view taken along the line B—B is illustrated in  FIG. 44  to show a cross-section of the V-groove  444  at a location where the V-groove  444  intersects the etch-stop pit  446 .  FIGS. 45–64  illustrate cross-sectional views of substrates which are taken along the same view direction, B—B, as the cross-sectional view in  FIG. 44 . The exemplary part fabricated by each of the methods illustrated in the flowcharts of  FIGS. 40–42  has a final configuration similar to that of the device shown in  FIGS. 43 and 44 . 
     Referring now to  FIG. 40 , there is shown a flowchart illustrating a method in accordance with the present invention for creating the device illustrated in  FIGS. 43 and 44 . As illustrated in  FIG. 45 , a substrate  450  made from &lt;100&gt;-oriented Si is provided. The processing of the substrate  450  begins at step  4000  of  FIG. 40  by providing a protective layer  452  on a first surface of the substrate  450  to cover that portion of the substrate  450  in which the etch-stop pit  516  is not to be provided. That is, the protective layer  452  includes an etch-stop pit aperture  451  through which a portion of the substrate  450  surface is accessible for forming the etch-stop pit  516 . 
     The protective layer  452  may be deposited over the entire surface of the substrate  450 . Thereafter, portions of the protective layer  452  may be removed to expose the surface of the substrate  450  at the selected area for the etch-stop pit  516 . The material of the protective layer  452  is chosen to be resistant to the etchant that will be used to form the V-groove  512 . For example, silicon dioxide is one suitable material. The silicon dioxide may be deposited by CVD or may be provided by thermal oxidation of the substrate surface. The silicon dioxide layer should be thick enough to serve as a mask during the etch-stop pit formation. 
     Following the application of the protective layer  452 , an aperture definition layer  454  is deposited, at step  4010 , over a selected portion of the protective layer  452 , as shown in  FIG. 45 . The aperture definition layer  454  is provided so that an aperture  457  may be provided, as explained below, through which the V-groove  512  will be etched. The location of the aperture definition layer  454  is selected to cover that portion of the substrate surface at which the V-groove  512  is to be located. 
     Processing continues with the selective removal, at step  4020 , of a portion of the substrate  450  located within the etch-stop pit aperture  451  to form an etch-stop pit  516  in the substrate  450 , as depicted in  FIG. 46 . The etch-stop pit  516  may conveniently be formed by reactive ion etching, plasma etching, ion milling, or by any other directional process. In addition, the etch-stop pit  516  may be formed by other methods such as isotropic or anisotropic etching, so long as the etch-stop pit  516  attains the desired shape and depth. 
     Having created the etch-stop pit  516 , the surfaces of the etch-stop pit  516  are covered, preferably conformally, with an etch-stop layer  458 , at step  4030 , as illustrated in  FIG. 47 . The etch-stop layer  458  may be conveniently provided by thermally oxidizing the substrate to provide an etch-stop layer  458  comprising silicon dioxide. An appropriate choice for the etch-stop layer  458  includes any material that is resistant to the etchant which will be used to create the V-groove  512 . During the thermal oxidation step  4030 , the previously deposited silicon dioxide protective layer  452  increases in thickness and surrounds the perimeter of the aperture definition layer  454 , as illustrated in  FIG. 47 . 
     With the etch-stop layer  458  in place, processing continues by removing, at step  4040 , the aperture definition layer  454  to provide a V-groove aperture  455  in the protective layer  452 , as shown in  FIG. 48 . A sufficient thickness of the protective layer  452  is removed, at step  4050 , to expose the surface of the substrate  450  disposed below the aperture definition layer  454  so that the V-groove aperture  455  communicates with the surface of the substrate  450 . A portion of the protective layer  452  and the etch-stop layer  458  remain on the surfaces where the V-groove  512  will not be formed, as illustrated in  FIG. 49 . A suitable process for removing a thickness of the protective layer  452  is a short duration, wet or dry, oxide etch. 
     Next, as shown in  FIG. 50 , the portion substrate  450  accessible through the V-groove aperture  455  is selectively removed, at step  4060 , to form the V-groove  512 , as illustrated in  FIG. 50 . Appropriate processes for the formation of the V-groove  512  include anisotropic etching with EDP or TMAH. KOH may also be used; however, since KOH can attack the protective layer  452  and etch-stop layer  458 , KOH should only be used if the protective layer  452  and etch-stop layer  458  are sufficiently thick so as not to be completely removed by the KOH. As a final optional step, the remaining portions of the protective layer  452  and etch-stop layer  458  may be removed at step  4070 , to yield the device illustrated in  FIG. 51 . 
     Referring now to FIGS.  41  and  52 – 58 , another method in accordance with the present invention is illustrated for creating the device shown in  FIGS. 43 and 44 . As illustrated in  FIG. 52 , a substrate  520  made from &lt;100&gt;-oriented Si is provided. The processing of the substrate  520  begins at step  4100  of  FIG. 41  by providing a first protective layer  522  on a first surface of the substrate  520  to cover that portion of the substrate  520  in which neither the etch-stop pit  586  nor the V-groove  582  is to be provided. 
     The first protective layer  522  may be deposited over the entire surface of the substrate  520 . Thereafter, portions of the first protective layer  522  may be removed to expose the surface of the substrate  520  at the selected areas for the etch-stop pit  586  and the V-groove  582 . The material of the first protective layer  522  is chosen to be resistant to the etchant that will be used to form the V-groove  582 . For example, silicon nitride is one suitable material. The silicon nitride may be deposited by CVD. 
     Following the application of the first protective layer  522 , a second protection layer  524  is deposited, at step  4110 , over a selected portion of the first protective layer  522  and the substrate surface where the V-groove  582  is to be formed, as shown in  FIG. 52 . The second protection layer  524  includes an aperture  521  through which the etch-stop pit  586  may be formed. The second protection layer  524  may comprise a CVD oxide, phospho-silicate glass, or boro-phospho-silicate glass, for example. 
     Processing continues with the selective removal, at step  4120 , of a portion of the substrate  520  located within the etch-stop pit aperture  521  to form an etch-stop pit  586  in the substrate  520 , as depicted in  FIG. 53 . The etch-stop pit  586  may conveniently be formed by reactive ion etching, plasma etching, ion milling, or by any other directional process. In addition, the etch-stop pit  586  may be formed by other methods such as isotropic or anisotropic etching, so long as the etch-stop pit  586  attains the desired shape and depth. 
     Having created the etch-stop pit  586 , the surfaces of the etch-stop pit  586  and second protective layer  524  are covered, preferably conformally, with an etch-stop layer  528 , at step  4130 , as illustrated in  FIG. 54 . The etch-stop layer  528  may be conveniently provided by CVD. An appropriate choice for the etch-stop layer  528  includes any material that is resistant to the etchant which will be used to create the V-groove  582 , such as silicon nitride. 
     With the etch-stop layer  528  in place, processing continues by removing, at step  4140 , the portion of the etch-stop layer  528  disposed on the upper surface  541  of second protective layer  524 . The portion of the etch-stop layer  528  disposed within the etch-stop pit  586  is retained, as illustrated in  FIG. 55 . The removal step  4140  may be performed by any suitable method such as planarization or polishing. Subsequently, at step  4150 , a second protective layer  524  is removed, as shown in  FIG. 56 , to provide a V-groove aperture  525 . A suitable method for removing the second protective layer  524  includes etching with dilute HF. 
     Next, as shown in  FIG. 57 , the portion substrate  520  accessible through the V-groove aperture  525  is selectively removed, at step  4160 , to form the V-groove  582 , as illustrated in  FIG. 50 . Appropriate processes for the formation of the V-groove  582  include anisotropic etching with KOH. As a final optional step, the remaining portions of the first protective layer  522  and etch-stop layer  528  may be removed at step  4170 , to yield the device illustrated in  FIG. 58 . 
     Optionally, after the step of  FIG. 54 , the etch-stop pit  586  can be filled with a fugitive mask material  529  that resists nitride etches (e.g., wax, polymer or photoresist),  FIG. 76 . After filling the etch-stop pit  586 , the portion of the etch-stop layer  528  on the upper surface  541  is removed by etching,  FIG. 77 . Subsequently, the second protective layer  524  is etched away,  FIG. 78 . Then the substrate  520  is wet etched,  FIG. 76 . Finally, the fugitive mask material  529 , etch-stop layer  528 , and first protective layer  522  are removed,  FIG. 80 . 
     Referring now to FIGS.  42  and  59 – 64 , yet another method in accordance with the present invention is illustrated for creating the device shown in  FIGS. 43 and 44 . As illustrated in  FIG. 59 , a substrate  590  made from &lt;100&gt;-oriented Si is provided. The processing of the substrate  590  begins at step  4200  of  FIG. 42  by providing protective an aperture definition layer  594  deposited over a selected portion of the substrate  590 , as shown in  FIG. 59 . The location of the aperture definition layer  594  is selected to cover that portion of the substrate surface at which the V-groove  632  is to be located. A suitable material for use as the aperture definition layer  524  is silicon nitride. 
     The processing of the substrate  590  continues, at step  4210 , by providing a photoresist layer  592  over the aperture definition layer  594  and over the portions of the substrate  590  not covered by the aperture definition layer  524 . Photoresist layer  592  is patterned, using methods known in the art, to provide an etch-stop pit aperture  591 , as illustrated in  FIG. 59 . Processing continues with the selective removal, at step  4220 , of a portion of the substrate  590  located within the etch-stop pit aperture  591  to form an etch-stop pit  636  in the substrate  590 , as depicted in  FIG. 60 . The etch-stop pit  636  may conveniently be formed by a process which does not remove the aperture definition layer  594 . In addition, the etch-stop pit  636  may be formed by other methods such as isotropic or anisotropic etching, so long as the etch-stop pit  636  attains the desired shape and depth. 
     Having created the etch-stop pit  636 , the photoresist layer  592  is removed, at step  4230 . The surfaces of the etch-stop pit  636  and exposed surfaces of the substrate  590  are oxidized to form an etch-stop layer  598 , at step  4230 , as illustrated in  FIG. 61 . With the etch-stop layer  598  in place, processing continues by removing, at step  4240 , the aperture definition layer  594  to provide an un-oxidized region  597  of the substrate  590 , as shown in  FIG. 62 . 
     Next, as shown in  FIG. 63 , the un-oxidized region  597  of the substrate  590  is selectively removed, at step  4250 , to form the V-groove  632 , as illustrated in  FIG. 63 . Appropriate processes for the formation of the V-groove  632  include anisotropic etching with EDP or TMAH. KOH may also be used; however, since KOH can attack oxide etch-stop layer  598 , KOH should only be used if the etch-stop layer  598  is sufficiently thick so as not to be completely removed by the KOH. As a final optional step, the remaining portions of the etch-stop layer  598  may be removed at step  4260 , to yield the device illustrated in  FIG. 64 . 
     These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. For example, a non-anisotropically etched feature may be formed adjacent an etch-stop pit. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.