Abstract:
The present disclosure discloses silicon waveguides with embedded active circuitry fabricated from silicon wafers utilizing photolithographic microfabrication techniques to define waveguide structures and embedded circuit recesses for receiving integrated circuitry. The method of fabricating the waveguides utilizes a double masking layer, one layer of which at least partially defines at least one waveguide and the other layer of which at least partially defines the at least one waveguide and at least one embedded circuit recess. The photolithographic microfabrication techniques are sufficiently precise for the required small structural features of high frequency waveguides and the double masking layer allows the method to be completed more efficiently. The basic fabrication method may be extended to provide batch arrays to mass produce silicon waveguide devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of, and claims the benefit of, U.S. patent application Ser. No. 13/542,996 to Philip A. Stupar, et al., entitled METHOD OF FABRICATING SILICON WAVEGUIDES WITH EMBEDDED ACTIVE CIRCUITRY, filed on Jul. 6, 2012, and published on Jul. 24, 2014 as United States Patent Publication 2014/0205231 A1, which is hereby incorporated herein in its entirety by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002]    This invention was made with government support under Contract Number HR0011-09-C-0060 under the THETA Program &amp; GO 71325 Defense Advanced Research Projects Agency (DARPA). The government has certain rights to this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]    This invention relates generally to silicon-based waveguides, and more particularly to fabrication methods utilizing embedded active circuitry with said waveguides and the resulting structures. 
       BACKGROUND OF THE INVENTION  
       [0004]    High frequency waveguides are commonly made from brass split blocks using conventional machining. This gives rise to several problems. For example, waveguides for higher frequencies require extremely small features and precise dimensional control. For example, a 670 Ghz waveguide (WR 1.5) requires a 191 μm waveguide width and corresponding depth. Conventional machining processes are typically not precise enough for such high frequency waveguides. Additionally, production of these waveguides is a slow and expensive process, and the resulting waveguides cannot be easily integrated into arrays. Furthermore, as waveguides get smaller and begin operating at frequencies approaching the terahertz (THz) range, the placement of integrated circuitry such as control circuits with E-Plane probes must become increasingly precise; up to micron accuracy may be required. 
         [0005]    The Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA) has made waveguides with chip recesses out of brass using conventional machining. These waveguides suffer from problems similar to those mentioned above. The precision with which an integrated circuit can be placed within such a waveguide is largely dependent on the precision of the machine used. Such commonly used machines are generally not capable of the necessary precision discussed above. 
         [0006]    The above processes used to fabricate waveguides require multiple steps, which translates into increased cost, lost time and decreased productivity. These waveguide fabrication methods also do not address the issue of providing the precision needed to install integrated circuitry into a waveguide with exceedingly small features within micron accuracy. 
       SUMMARY OF THE INVENTION  
       [0007]    A silicon (Si) waveguide fabrication method is presented which overcomes the problems noted above, providing high frequency Si waveguides with integrated circuitry, precise features and a simplified fabrication method. 
         [0008]    The present method relies on using a silicon-on-insulator (SOI) wafer with precisely defined layers. A double masking layer is formed on the SOI wafer, leaving at least one portion of the surface of the SOI wafer unmasked. The first layer of the double masking layer is arranged to define the dimensions of the first half of a waveguide and an embedded circuit recess to receive a circuit having an associated embedded circuit thickness and embedded circuit width. The second masking layer is arranged to define the dimensions of the first half of the waveguide only. 
         [0009]    The unmasked portions of the SOI wafer are then etched, defining a portion of the first half of the waveguide, by etching through the buried insulator to a depth equal to the desired waveguide depth minus the embedded circuit thickness. The second layer of the double masking layer is then removed, unmasking at least one additional portion of the SOI wafer. Another etch is then conducted to define the circuit recess and to complete the etching of the remaining portion of the desired waveguide depth. The resulting waveguide structure is one half of a complete waveguide. The photolithographic etching process allows for structure formation within sub-micron accuracy. 
         [0010]    The first layer of the double masking layer is then removed, as are the exposed portions of the buried insulator layer. A conductive material is then deposited on the surface of the SOI wafer. An integrated circuit is then installed into the embedded circuit recess. 
         [0011]    The second half of the waveguide structure is produced as above. The two halves are then bonded together to form a complete waveguide with embedded active circuitry. 
         [0012]    These and other further features and advantages of the invention would be apparent to those skilled in the art from the following description, taken together with the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The detailed description of embodiments of the invention will be made with reference to the accompanying drawings, wherein like numerals designate corresponding parts in the figures. 
           [0014]      FIGS. 1   a - 1   e  are sectional views of a fabrication method for forming a Si waveguide structure that constitutes one-half of a complete Si waveguide. 
           [0015]      FIGS. 2   a - 2   g  are sectional views of a fabrication method for forming a double masking layer for use in the fabrication method for forming a Si waveguide structure that constitutes one-half of a complete Si waveguide. 
           [0016]      FIGS. 3   a - 3   d  are sectional views of a fabrication method for forming a complete waveguide with integrated embedded circuitry. 
           [0017]      FIG. 4  is a sectional view of a complete waveguide with integrated embedded circuitry with a top chip-probe access port. 
           [0018]      FIG. 5  is a sectional view of a complete waveguide with integrated embedded circuitry with a bottom chip-probe access port. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present method is directed to a means for fabricating Si based waveguides with embedded active circuitry. The basic method steps are illustrated in the series of sectional views shown in  FIGS. 1   a - 1   e,  which show a fabrication method for forming a Si waveguide structure that constitutes one-half of a complete Si waveguide. In the step shown in  FIG. 1   a,  a SOI wafer  100  is provided. SOI wafer  100  comprises a first Si layer  102 , a buried insulator layer  104 , which is typically an oxide layer, on first Si layer  102 , and a second Si layer  106 , which can serve as the device layer, on buried insulator layer  104 . Preferably, buried insulator layer  104  comprises silicon dioxide (SiO 2 ). By using a SOI wafer, wherein the various layers can have dimensions precisely specified during fabrication of the wafer, the dimensions of the post-etch structures can also be precisely defined. For example, pre-calculating the thickness of second Si layer  106  to be equal to a desired thickness  108  for an embedded circuit, the manufacturing method can be simplified (as will be discussed below). 
         [0020]    It is understood that when an element or layer is referred to as being “on”, “connected to”, “coupled to” or “in contact with” another element or layer, it can be directly on, connected or coupled to, or in contact with the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to”, “directly coupled to” or “directly in contact with” another element or layer, there are no intervening elements or layers present. 
         [0021]    In one embodiment of the present method, an optional raised Si mesa  110  can be formed on second Si layer  106 . While it is not necessary to form raised Si mesa  110 , the mesa can be advantageous as it serves to minimize the bonding area of the completed waveguide device, thereby increasing the bonding pressure and facilitating a bond with no voids extending into the waveguide area. Raised Si mesa  110  should be wide enough to accommodate the formation of the desired waveguide trench(s) and circuit recess(es) (which will be discussed further below). Raised Si mesa  110  can be formed using conventional etching and/or other micro-fabrication techniques, with the preferred method being a wet or dry photolithographic etch. In one embodiment of the present method, the thickness of raised Si mesa  110  is equal to desired embedded circuit thickness  108 , while the thickness of the portions of second Si layer  106  not constituting raised Si mesa  110  is less than desired embedded circuit thickness  108 . 
         [0022]    In the step shown in  FIG. 1   b,  a double masking layer  112  is provided on second Si layer  106  of SOI wafer  100 . Double masking layer  112  comprises a first masking layer  114  on second Si layer  106 , and a second masking layer  116  on first masking layer  114 . In addition to the portions of second Si layer  106  that are masked by both layers ( 114 ,  116 ) of double masking layer  112 , there is at least one unmasked portion  118  of second Si layer  106  (two portions shown), and at least one partially masked portion  120  of second Si layer  106  (one shown); partially masked portion  120  is only masked by second masking layer  116 . These unmasked and/or partially masked portions will at least partially define the relevant dimensions of the waveguide trench and circuit recess structures. For example, unmasked portion  118  of second Si layer  106  can correspond to a desired waveguide trench width  122 . Likewise, partially masked portion  120  of second Si layer  106  can correspond to a desired embedded circuit width  124 . It is important to note that at least one partially masked portion  120  of second Si layer  106  should be near enough to an unmasked portion  118  of second Si layer  106  to enable an embedded integrated circuit to be near enough to a waveguide trench structure to interact with the completed waveguide (as will be discussed in further detail below). 
         [0023]    First masking layer  114  can comprise any suitable masking material that is known in the art, with the preferred material being a material that will not be damaged or etched by the process chosen to remove second masking layer  116 . In one embodiment of the present method, first masking layer  114  and buried insulator layer  104  comprise the same material. For example, in embodiments of the present method wherein buried insulator layer  104  comprises SiO 2 , first masking layer  114  would also comprise SiO 2 . This particular embodiment is advantageous as first masking layer  114  and exposed portions of buried insulator layer  104  can be removed in a single step (which will be discussed in further detail below), thus simplifying the fabrication method and conserving resources. 
         [0024]    Second masking layer  116  can comprise any suitable masking material that is known in the art, with the preferred material being a material that can be selectively removed by a process that will not damage or etch underlying first masking layer  114 . A preferred material for second masking layer  114  is a photoresist material. The use of photoresist materials is known in the art. Such materials can be easily applied, patterned and removed without negatively affecting underlying masking layers. 
         [0025]    In  FIG. 1   c,  unmasked portions  118  of second Si layer  106  are subjected to a first etch and are etched through second Si layer  106 , through buried insulator layer  104 , and partially into first Si layer  102 , forming a first portion of at least one waveguide trench structure  126  (two shown). These waveguide trench structures are an active portion of the completed waveguide. It is understood that the term “waveguide” as used in this disclosure can refer to both the waveguide trench structures and the completed waveguide itself. The waveguide depth can be further controlled by using a timed etch or by using an SOI wafer with multiple layers. In one embodiment, unmasked portions  118  are etched to a depth corresponding to a desired waveguide depth  128  minus desired embedded circuit thickness  108 . During this etch, double masking layer  112 , including first masking layer  114  and second masking layer  116 , is arranged to at least partially define the dimensions of this first portion of waveguide trench structure  126 , by masking portions of the surface of SOI wafer  100  that are not to be etched. 
         [0026]    In addition to at least partially defining the first portion of the dimensions of trench structure  126 , second masking layer  116  is also arranged to cover partially masked portion  120 . Accordingly, second masking layer  116  protects underlying second Si layer  106  from the etch. Additionally, aside from at least partially defining the first portion of the dimensions of trench structure  126 , first masking layer  114  at least partially defines partially masked portion  120 , and therefore at least partially defines the relevant dimensions of a circuit recess structure. Such dimensions can include, for example, embedded circuit width  124  and embedded circuit thickness  108 . 
         [0027]    Any wet or dry photolithographic process can be used to carry out the above described etch. Resulting structures defined by photolithographic processes will have precisely defined features with sub-micron accuracy. The preferred etching method is deep reactive ion etching (DRIE). A preferred DRIE process known as the Bosch process utilizes time-sequenced alternating etch and passivation steps. An etchant such as sulfur hexafluoride SF 6  is used to etch a portion of waveguide trench structure  126 . To passivate the side wall of waveguide trench structure  126  and prevent further lateral etching, an insulating layer is subsequently deposited using a separate gas composition which includes a species such as octafluorocyclobutane C 4 F 8 . This process is repeated until the desired depth is achieved. Etching via this process allows for high selectivity and achieves substantially vertical side walls, with aspect ratios as high as 40:1 or more. This high aspect ratio facilitates the production of smaller diameter cavities and smaller via-to-via spacings, as it reduces the amount of lateral blooming during etching. 
         [0028]    In  FIG. 1   d,  second masking layer  116  of double masking layer  112  is removed, unmasking the previously partially masked portion  120  of second Si layer  106  that has desired relevant dimensions corresponding to a circuit recess structure. The resulting structure on the surface of SOI wafer  100  comprises first masking layer  114 , formerly partially masked portion  120 , now unmasked, and unmasked partially etched waveguide trench structure  126 . 
         [0029]    In  FIG. 1   e,  the portions of the surface of SOI wafer  100  that are unmasked by first masking layer  112  are subjected to a second etch. Waveguide trench structure  126  is etched the remaining depth to desired waveguide depth  128 . Formerly partially masked portion  120  of second Si layer  106  is etched to the buried insulator layer  104 , forming at least one embedded circuit recess  130 , for receiving an integrated circuit. An exposed insulator portion  132  of buried insulator layer  104  remains on the Si floor of embedded circuit recess  130 . 
         [0030]    In one embodiment of the step illustrated in  FIG. 1   e,  where the first etch is done to a depth corresponding to desired waveguide depth  128  minus desired embedded circuit thickness  108 , and where the second Si layer  106  has a thickness equal to a desired embedded circuit thickness  108 , formerly partially masked portion  120  of second Si layer  106  and waveguide trench structure  126  are both etched by an additional amount equal to desired embedded circuit thickness  108 . This results in waveguide trench structure  126  having a depth equal to desired waveguide depth  128  and formerly partially masked portion  120  being etched to the buried insulator layer  104 , thus forming embedded circuit recess  130 . This particular embodiment is advantageous as it eliminates the need for additional etches and allows both waveguide trench structure  126  and the embedded circuit recess  130  to be formed with more precise dimensions. 
         [0031]    The resulting structure provides a Si waveguide structure  134  that corresponds to a completed Si waveguide&#39;s first half. Repeating the above steps as depicted in  FIGS. 1   a - 1   e  will result in providing a Si waveguide structure that corresponds to the completed Si waveguide&#39;s second half. Due to the precise nature of the photolithographic etching process, waveguide halves will be identical or nearly identical. 
         [0032]      FIGS. 2   a - 2   g  illustrate one embodiment of a method according to the present disclosure by which the double masking layer can be provided. In this embodiment, the first masking layer ( 114  in  FIG. 1   b  above) comprises SiO 2  and the second masking layer ( 116 ) comprises a photoresist material. Although this embodiment refers to specific preferred materials, it is understood that mask layers of different materials can be patterned and etched or portions removed or added in a manner similar to the disclosed methods. 
         [0033]      FIG. 2   a  illustrates a SOI wafer  200  similar to the SOI wafer  100  depicted in  FIG. 1   a.  SOI wafer  200  in  FIG. 2   a  comprises a first Si layer  202 , a buried insulator layer  204  on first Si layer  202 , and a second Si layer  206  on buried insulator layer  204 . In the step illustrated in  FIG. 2   a , an oxide masking layer  214  is deposited on second Si layer  206 . According to this particular embodiment, oxide masking layer  214  will eventually become first masking layer  114 . 
         [0034]      FIG. 2   b  illustrates the next step in which a first photoresist masking layer  215  is deposited over the entire surface of oxide masking layer  214 . Either a positive or negative photoresist can be used. As shown in  FIG. 2   c , first photoresist masking layer  215  is then patterned and developed, leaving at least some of underlying oxide masking layer  214  unmasked. The unmasked portions of underlying oxide masking layer include a least one unmasked circuit recess portion  218 , corresponding to a desired waveguide trench width  220  plus a desired embedded circuit width  222 , and any other remaining additional portions  224  (one shown) corresponding to desired waveguide trench width  220 , or corresponding to the desired waveguide trench width plus the desired embedded circuit width. 
         [0035]      FIG. 2   d  illustrates the next step, which is the removal of the unmasked oxide portions ( 218 ,  224  above).  FIG. 2   e  illustrates the next step, which is the removal of first photoresist masking layer  215 . As shown in  FIG. 2   f , a second photoresist masking layer  216  is then deposited over the surface of oxide masking layer  214  and over the exposed portions  226  of second Si layer  206 , which correspond to the oxide portions etched in the step depicted in  FIG. 2   e  ( 218 ,  224  above). According to this particular embodiment, second photoresist masking layer  216  will eventually become second masking layer  116 . 
         [0036]      FIG. 2   g  illustrates the next step, in which second photoresist masking layer  216  is patterned and developed such that an equivalent number of portions of second Si layer  206  are left unmasked as there were unmasked oxide portions in the step described above (i.e.  218 ,  224 ) and depicted in  FIG. 2   c . These unmasked second Si layer portions  228  are located in the regions where oxide masking layer  214  was etched away above. These unmasked second Si layer portions  228  have a width equal to the desired waveguide trench width  220 . There is also at least one portion now masked only by the second photoresist masking layer  216  which covers the desired embedded circuit width  222 . The resulting mask layer is double masking layer  212 , comprising oxide masking layer  214  and second photoresist masking layer  216 . According to this particular embodiment, double masking layer  212  will become double masking layer  112 . 
         [0037]    The basic method steps for installing a recessed integrated circuit into a Si waveguide structure  134  and forming a complete waveguide with embedded integrated circuitry are illustrated in the series of sectional views shown in  FIGS. 3   a - 3   d.    FIG. 3   a  shows waveguide structure  134  produced according to the steps outlined above and illustrated in  FIG. 1   a - 1   e,  in which first masking layer  114  and exposed insulator portion  132  have been removed. In one already mentioned embodiment, buried insulator layer  104  and first masking layer  114  comprise the same material and can be simultaneously removed in this step with a single process. This simplifies the method and conserves time and resources. During the previous etching processes described above, exposed insulator portion  132  acted as an etch stop layer and protected the underlying Si floor of circuit recess  130 , insuring a favorable undamaged and substantially flat surface upon which an integrated circuit could be mounted. This etch stop also facilitates precise control of the chip recess depth which will be defined solely by the thickness of second Si layer  106 . 
         [0038]    In  FIG. 3   b , a conductive material  136  is deposited over the surface of waveguide structure  134 . A preferred conductive material is metal and a preferred metal is gold. 
         [0039]    In  FIG. 3   c , an integrated circuit  138  is installed into embedded circuit recess  130 . Integrated circuit  138  can comprise a wide variety of useful functional circuits such as control circuits. Due to the precise nature of the etching process, protective exposed insulator portion  132  discussed above, and the pre-calculated dimensions of the SOI wafer layers, embedded circuit recess  130  is precisely positioned so that integrated circuit  138  can be installed onto a substantially flat surface at a precise depth and in a precise position in relation to waveguide trench structure  126 . This is important as waveguide devices begin to operate at higher frequencies and require increasingly smaller features and more precise dimensional control. Integrated circuit  138  can be placed using any acceptable conventional means known in the art. Since the area in which integrated circuit  138  is to be installed, i.e. embedded circuit recess  130 , has such precisely formed dimensions, integrated circuit  138  can even be placed using a manually operated machine. 
         [0040]    Integrated circuit  138  can further comprise an E-Plane probe  140 . E-Plane probe  140  provides a means by which integrated circuit  138  can interact with the complete waveguide. E-Plane probe  140  can function as a waveguide-to-microstrip transition for converting and directing electromagnetic wave signals to integrated circuit  138 . E-Plane probe  140  extends from integrated circuit  138  into waveguide trench structure  126 . As E-Plane probe  140  must be positioned in relation to waveguide trench structure  126  with micron accuracy, E-Plane probe  140  is usually co-fabricated with integrated circuit  138  prior to the integrated circuit being installed into embedded circuit recess  130 . 
         [0041]    In  FIG. 3   d , the structure depicted in  FIG. 3   c  above is bonded to a like structure produced according to the steps depicted in  FIGS. 1   a - 1   e  and  3   a - 3   b  above, thus forming a complete waveguide  142  with embedded integrated circuit  138 . Any acceptable bonding means that is known in the art can be used, including temporarily clamping the two structures together in a fixture, however, the preferred bonding means being a thermocompression bond. The first and second halves of the waveguide trench structures are thus aligned to form complete waveguide  142  with complete waveguide trenches  143 . Using this method of fabricating waveguides allows for batch processing of waveguides. Waveguide structures constituting first and second halves can be fabricated simultaneously across wafers, facilitating cost effective waveguide arrays. 
         [0042]      FIGS. 4-5  show two embodiments of complete waveguides produced according to the present method.  FIG. 4  shows a complete waveguide  144  with a top chip-probe access port  146  and  FIG. 5  shows complete waveguide  148  with a bottom chip-probe access port  150 . A chip-probe access port is an opening in the Si waveguide structure (i.e.  134  in  FIG. 1   e  above) which provides outside access to structures contained within a completed waveguide; for example, an outside connection to an installed integrated circuit  138  might pass through access port  150 . 
         [0043]    In one embodiment of the present invention, a chip-probe access port can be etched into Si waveguide structure  134  prior to the deposition of conductive material  136 . In this embodiment, conductive material  136  can cover the sidewalls of the chip-probe access port. The chip-probe access port can be etched anywhere into Si waveguide structure  134 , or a corresponding half of the structure, so long as the chip-probe access port does not substantially interfere with the function of the completed waveguide. For example, a chip-probe access port can be etched in relation to embedded circuit recess  130  of either the first or second half of a completed waveguide structure, such that an integrated circuit is either partially on or under the access port. One possible method of etching a “bottom” chip probe access port in which at least part of an integrated circuit is on the access port, is by etching the port from the underside of SOI wafer  100  through a portion of embedded circuit recess  130  upon which an integrated circuit will rest. 
         [0044]    As noted above, outside connections to installed integrated circuit  138  can be routed through access port  150 . Alternatively or in addition, input/output lines (not shown) which send and/or receive signals to and from integrated circuit  138  can be provided. These input/output lines can be run across second Si layer  106 , for example, across the lower portion of embedded circuit recess  130 . The input/output lines would be made of, coated with or treated with a conductive material. In one embodiment, the input/output lines are metalized. In another embodiment, the input/output lines are formed through a lithographic process during the processing of the waveguide. If the input/output lines are formed before the deposition of conductive material  136 , a patterning process can be used to prevent conductive material  136  from depositing over the lines. These lines can be used in lieu of, or in addition to, the chip-probe access port structure discussed above. One advantage of using the input/output lines in lieu of a chip-probe access port is that it would simplify the production of an array of waveguide devices. 
         [0045]    Waveguide structures produced according to the present method could be used as individual waveguides, or a plurality of them could be formed on a common substrate and arranged into an array, such as in a receiver array. Such receiver arrays typically require a large array of many components (such as waveguides and embedded integrated circuitry), spaced apart at lengths potentially less than a wavelength. One such ideal spacing arrangement of receiver components is λ/2. Due to the precise and accurate nature of the present method, such small, precisely spaced components can be fabricated across a common substrate. 
         [0046]    Although the foregoing described the invention with preferred embodiments, this is not intended to limit the invention. While certain specific materials and dimensions were enumerated, unless noted otherwise, other materials and dimensions could be used. For example, where a masking layer was to define the thickness or width of a device layer, it could also define additional dimensions such as length or height. Likewise, while this invention refers to a “double masking layer,” it is understood that a triple or otherwise multiple masking layer, wherein the different layers define different portions of structures in an SOI wafer, would also be within this disclosure. 
         [0047]    The foregoing is intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims, wherein no portion of the disclosure is intended, expressly or implicitly, to be dedicated to the public domain if not set forth in the claims.