Abstract:
An optical device ( 10 ) including a first semiconductor layer ( 12 ) on which is deposited a dielectric layer that is patterned and etched to form dielectric strips ( 14 ) as part of a diffraction grating layer. Another semiconductor layer ( 16 ) is grown on the first semiconductor layer ( 12 ) between the dielectric strips ( 14 ), resulting in alternating dielectric sections ( 14 ) and semiconductor sections. In an alternate embodiment, a dielectric layer is deposited on a first semiconductor layer ( 64 ), and is patterned and etched to define dielectric strips ( 66 ). The semiconductor layer ( 64 ) is etched to form openings ( 68 ) between the dielectric strips ( 66 ). Another semiconductor material ( 70 ) is grown within the openings ( 68 ) and then another semiconductor layer ( 72 ) is grown over the entire surface after removing the dielectric strips ( 66 ). Either embodiment may be modified to provide diffraction grating with air channels ( 20 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to semiconductor diffraction gratings and, more particularly, to a semiconductor structure that includes semiconductor epitaxial layers lattice matched to each other and including a diffraction grating therebetween, where the semiconductor layers and the diffraction grating material having a greater difference in their indexes of refraction than the difference in the indexes of refraction of the semiconductor layers.  
           [0003]    2. Discussion of the Related Art  
           [0004]    There is a need in the art for optical semiconductor diffraction gratings in certain optical semiconductor devices, such as distributed feedback (DFB) optical filters, optical couplers, etc. A conventional optical semiconductor diffraction grating will typically include a semiconductor waveguide layer positioned between outer cladding layers, where the waveguide layer has a higher index of refraction than the cladding layers so that light propagates down the waveguide layer by reflecting off of the cladding/waveguide interfaces and is trapped therein. The diffraction grating is formed at the interface between one of the cladding layers and the waveguide layer by fabricating a ripple or corrugated structure on one of either the waveguide surface or the cladding layer surface so that as the light is reflected off of the interface, it interacts with the grating. Thus, the diffraction layer is the periodic longitudinal index difference between the peaks and troughs defined between the semiconductor layers forming a grating region. As the light propagates down the waveguide layer, the wavelength of light related to the periodic index change or spatial period of the peaks in the diffraction layer is reflected backwards or transmitted through the waveguide layer in such a manner that it is separated from the other wavelengths of light to provide for example, optical filtering.  
           [0005]    Processing techniques for fabricating the conventional grating interfaces in semiconductor devices are well established. The grating can be formed on the waveguide layer or cladding layer surface by direct electron beam writing or holography patterning, both well understood to those skilled in the art. In order to make the interfaces between the waveguide layer and the cladding layers of a high optical quality with minimum defects and imperfections, it is desirable to use a semiconductor epitaxial growth process, such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), to provide crystal nucleation and growth and lattice matching between these layers. Because the waveguide layer must have a higher index of refraction than the cladding layers, the waveguide layer must be made of a different semiconductor material or material composition than the cladding layers. However, to provide the necessary crystal structure and lattice matching during the semiconductor fabrication process, the waveguide layer and the cladding layers must be compatible to allow the crystal growth process to occur.  
           [0006]    The interaction of the optical modes in the waveguide layer is a function of the differences in the indexes of refraction in the diffraction layer formed by the semiconductor layers, which is characterized by a grating coupling coefficient. The semiconductor materials that are compatible and satisfy the crystal growth requirement have nearly the same indexes of refraction, and thus, the optical wavelength separation capability provided by the diffraction grating layer in these devices is limited. In other words, because the crystal growth process requires that the waveguide and cladding materials have nearly the same index of refraction, the optical filtering capability, or other optical wavelength separation process, is limited. Typical semiconductor indexes of refraction are approximately 3, and the difference between compatible semiconductor materials is usually at most about 0.5.  
           [0007]    The small difference in the indices of refraction between the waveguide layer and the cladding layers is adequate for many applications, such as optical mode pumping in a laser, but for other applications, such as optical filtering, a larger difference between these indices of refraction is desirable. In many applications, a significant improvement in device performance would be achieved if it were possible to fabricate gratings with much larger coupling coefficients.  
           [0008]    To provide semiconductor diffraction gratings of the type discussed above that have a much greater difference between the indexes of refraction within the grating layer, it has heretofore been known to use a wafer-to-wafer bonding technique to bond the waveguide layer and cladding layers together that eliminates some of the restrictions imposed on the semiconductor growth fabrication processes. Different wafer-to-wafer bonding techniques are known in the art, where separate semiconductor structures are adhered together in a non-crystal growth process. By bonding a semiconductor structure to another semiconductor structure that includes the diffraction ripples, an interface is created where air gaps are defined between the peaks in the ripple structure. Therefore, as the optical beam propagates down the waveguide and interacts with the diffraction grating layer, the optical beam sees alternating regions of air and semiconductor material. Because the index of refraction of air is one, there is a significant difference between the materials that define the grating, providing increased filtering capabilities. Wafer-to-wafer bonding, however, has a number of drawbacks making this technique somewhat undesirable for fabricating optical diffraction gratings. Particularly, the wafer-to-wafer bonding process introduces strain between the crystalline structure of the two semiconductor layers that affects the optical interaction in the diffraction layer. Additionally, defects and impurities present at the interface can affect the optical integrity as a result of the bonding process that would not be present during a crystalline growth process. Additionally, the wafer-to-wafer bonding process is relatively expensive to implement, and thus adds a significant level of cost above the typical diffraction grating fabrication process.  
           [0009]    What is needed is a process for making an optical diffraction grating that employs semiconductor crystal growth processes, and provides a relatively significant difference between the indices of refraction between the waveguide layer and the cladding layers in a diffraction grating layer for increased optical filtering. It is therefore an object of the present invention to provide such a process.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the teachings of the present invention, an optical device including an optical diffraction grating is disclosed. In one embodiment, the optical device includes a first semiconductor layer on which is patterned and etched a dielectric layer to form dielectric strips. A second semiconductor layer is grown by an epitaxial growth process on the first semiconductor layer between the dielectric strips to enclose the dielectric strips, so that the spaced apart dielectric strips, together with the second semiconductor layer between the strips and up to the height of the strips, define the diffraction grating layer. The dielectric strips can then be etched away to form the diffraction grating with air channels. Subsequent material growth steps determine the location of the waveguide layer. The second semiconductor layer can be made of the same material as the first semiconductor layer and a waveguide layer remotely located, or the second semiconductor layer can be the waveguide itself, as long as it is compatible with the material of the first dielectric layer for the crystal growth process.  
           [0011]    In another embodiment, a first semiconductor layer is provided and a dielectric layer is deposited on the first semiconductor layer, and then patterned and etched to define dielectric strips. The semiconductor layer is then exposed to a chemical etch that forms openings in the semiconductor layer between the dielectric strips. Another semiconductor material is grown within the openings in the first semiconductor layer by an epitaxial growth process, and then the dielectric strips are etched away. Another semiconductor layer is grown on the first layer and the diffraction regions. The semiconductor material in the diffraction regions can be a sacrificial material that is etched away to form the diffraction grating with air channels. As above, additional semiconductor layers can be provided to define a waveguide layer between cladding layers where light propagating down the wave guide layer interacts with the diffraction layer, or the waveguide layer can be remotely located.  
           [0012]    Additional objects, advantages, and features of the present invention will become apparent in the following description independent claims, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    FIGS.  1 - 3  show cross-sectional views of several fabrication steps of a semiconductor optical diffraction grating structure, according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 4 is a top view of a fabrication step of the semiconductor optical diffraction grating structure shown in FIG. 3;  
         [0015]    [0015]FIG. 5 is a cross-sectional view of an optical device including a semiconductor optical diffraction grating structure according to the present invention;  
         [0016]    [0016]FIG. 6 is a cross-sectional view of an optical device including another semiconductor optical diffraction grating structure according to the present invention; and  
         [0017]    FIGS.  7 - 10  show cross-sectional views of a fabrication process of an optical semiconductor diffraction grating structure, according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The following discussion of the preferred embodiments directed to a semiconductor optical diffraction device and method of making same is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses.  
         [0019]    [0019]FIG. 1 is a cross-sectional view of a fabrication step for an optical diffraction grating structure  10 , according to an embodiment of the present invention. The grating structure  10  includes a semiconductor layer  12  that has been grown by an epitaxial growth process, such as MOCVD or MBE, on a compatible semiconductor substrate. After the semiconductor layer  12  has been grown to the desired thickness for a particular optical application, the wafer is removed from the crystal growth apparatus, and a dielectric layer, for example silicon dioxide or silicon nitride, is deposited on a top surface of the semiconductor layer  12 . The dielectric layer is patterned by a suitable semiconductor patterning process, such as a holography or electron beam writing exposure technique, to define a pattern of dielectric diffraction grating strips  14 . An etching process, such as any suitable chemical or plasma based etch, is used to etch away the unwanted portions of the dielectric layer between the strips  14 . The etch is selectively controlled so that the unwanted dielectric portions of the dielectric layer are etched far enough down to expose the layer  12  between the strips  14 , as shown in FIG. 1.  
         [0020]    Once the dielectric grating strips  14  have been formed on the layer  12 , the structure  10  is returned to the semiconductor growth apparatus. FIG. 2 shows a subsequent fabrication step of the structure  10  where a second semiconductor layer  16  has been grown on the layer  12  by an epitaxial re-growth process. The layer  16  is lattice matched or nearly lattice matched to the layer  12  that is exposed between the strips  14  to define the layer  16 . The growth process is continued until the strips  14  are completely enclosed. For purposes of the present discussion, the layer  12  and the layer  16  are made of the same semiconductor material, but as will become apparent form the discussion below, the layer  16  can be a different semiconductor material than the layer  12 , as long as it is compatible for the growth process. The dielectric strips  14  define a diffraction grating for an optical device, such as a filter. The dielectric material of the strips  14  has an index of refraction from about 1.5 to 2, making the difference in the index of refraction between the strips  14  and the semiconductor material greater than the difference between the standard epitaxial growth semiconductor diffraction gratings.  
         [0021]    The difference in the index of refraction between the diffraction strips  14  and the semiconductor material can be further increased by removing the dielectric strips  14  after the re-growth step to define air channels  20 , as shown in FIG. 3. FIG. 4 shows a top view of the structure  10  depicting a fabrication process for selectively removing the strips  14  once the layer  16  has been fabricated. An optical device region  22  is defined in the structure  10  that represents the usable optical area for the final optical device. The optical beam will propagate down the region  22  transverse to the strips  14 . Via etching channels  24  and  26  are patterned and etched down through a top surface of the structure  10  by a suitable masking and etching process until the ends of the strips  14  are exposed to air. A suitable etchant is then introduced into the via channels  24  and  26  that selectively removes the strips  14  to form the air channels  20 . The etching process is complete when the entire length of the strips  14  have been removed to define the air channels  20 . If necessary, the channels  20  can be sealed by depositing a passivating and dielectric film into the via channels  24  and  26  that is sufficient to close the channel openings.  
         [0022]    The structure  10  as fabricated by the process described above can be used for different optical devices, in accordance with the teachings of the present invention. FIG. 5 shows a cross-sectional view of an optical device  30  including a bottom semiconductor cladding layer  32  representing the layer  12  and a top semiconductor cladding layer  34 . For the device  30 , the semiconductor layer  16  is a semiconductor waveguide layer  36  having a higher index of refraction than the cladding layers  32  and  34  such that an optical wave propagating down the waveguide layer  36  is substantially confined therein. In this embodiment, a diffraction grating layer  38  is defined at the interface between the waveguide layer  36  and the cladding layer  32 , and includes periodically spaced dielectric strips  40 , representing the strips  14 , and semiconductor regions  42  grown between the strips  40  that are part of the crystal re-growth making up the waveguide layer  36 . When the structure  10  is reintroduced back into the crystal growth apparatus after the strips  14  have been patterned, the waveguide layer  36  is regrown as a compatible semiconductor material that is different than the material making up the cladding layer  32 . The cladding layer  34  is then grown on the waveguide layer  36 , and is the same material as the cladding layer  32 . Therefore, all of the layers  32 ,  34  and  36  are compatible for the crystal growth process. The strips  40  can be removed to define the air channels  20  in the manner as discussed above.  
         [0023]    [0023]FIG. 6 is a cross-sectional view of another optical device  46  including a lower cladding layer  48  having a diffraction grating layer  50  defined by air channels  52  separated by semiconductor regions  54 . In this design, the re-growth layer  16  is made of the same material as the layer  12  to define the cladding layer  48 , and the diffraction layer  50  is an embedded diffraction layer. Instead of removing the strips  14  to make the air channels  52 , the strips  14  can be retained to provide a different type of optical device, as discussed above. Once the layer  16  has been regrown to define the layer  48 , a waveguide layer  56  having a higher index of refraction than the cladding layer  48  is grown on the cladding layer  48 , and an upper cladding layer  58  is grown on the waveguide layer  56 . The wave propagating down the waveguide layer  56  has modes that penetrate into the cladding layer  48  and contact the diffraction layer  50  for optical filtering purposes, as would be appreciated by those skilled in the art. Any combination of cladding layer, diffraction grating layer and waveguide layer can be provided within the scope of the present invention, consistent with the discussion above, where compatible semiconductor layers are epitaxially grown on top of each other.  
         [0024]    FIGS.  7 - 10  show cross-sectional views depicting fabrication steps of an optical diffraction grating structure  62  to be employed in an optical diffraction device, according to another embodiment of the present invention. In this embodiment, a semiconductor layer  64  is grown on a compatible semiconductor substrate (not shown), and a dielectric layer is then deposited on the semiconductor layer  64  and is patterned and etched by a suitable patterning process to define dielectric strips  66 . The structure  62  is then exposed to an etchant that etches away the material of the layer  64  between the strips  66 , but does not etch the dielectric material to define etched holes  68 . The etch is controlled to control the depth and shape of the holes  68 . The dielectric strips  66  serve as a mask for the selective epitaxial re-growth of a sacrificial semiconductor material within the holes  68 . The structure  62  is then returned to the growth apparatus to grow sacrificial semiconductor regions  70  within the holes  68  to a level approximately equal with the top surface of the layer  64 , as shown. The semiconductor material for the regions  70  is a different material than the layer  64  so that the regions  70  can be selectively etched away, but is compatible with the growth process.  
         [0025]    The strips  66  are then removed by a suitable etching process that does not effect the semiconductor material. When the strips  66  have been removed, the structure  62  is put back into the growth apparatus, and another semiconductor layer  72  is grown on the regions  70  and the layer  64  to enclose the regions  70 , as shown in FIG. 9. The sacrificial regions  70  can then be removed by the etching process discussed above using the etchant channels  24  nd  26  to provide a structure as shown in FIG. 10 including air channels  74  defining a diffraction grating layer  76 . The semiconductor layer  72  can be made of the same semiconductor material as the semiconductor layer  64  or can be another semiconductor material, so that the structure  62  can define an embedded diffraction layer or an interface diffraction layer between a waveguide layer and a cladding layer, consistent with the discussion above.  
         [0026]    By providing the air channels  20  and  74 , the largest possible coupling coefficient consistent with semiconductor materials can be obtained by the diffraction grating. The accuracy and control of the grating tooth height and shape are significantly improved over traditional fabrication methods. The spatial period between the strips  14  or the channels  74  would depend on the particular wavelength being filtered. The thickness of the strips  14  would depend on how strongly the designer wished to diffract the light, and would be on the order of 1000A. Semiconductor materials that can be used in the optical devices discussed above include InP, InGaAs, GaAs, and other semiconductor materials, as would be appreciated by those skilled in the art. A two-dimensional grating structure for other types of optical devices can also be generated by the processes discussed above.  
         [0027]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various, changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.