Abstract:
Tools and methods for making molded an optical integrated circuit including one or more waveguides are disclosed. In one embodiment, a molding die is provided that includes a substrate that has a topographically patterned first surface. A conformal protective film is provided over the first surface of the substrate. The substrate may be formed of silicon or gallium arsenide, and may be patterned using conventional semiconductor patterning techniques, such as plasma etching. The protective film may be metal (e.g., nickel or titanium), diamond, or some other hard material. Typically, a plurality of such molding dies are formed from a wafer of the substrate material. The die is pressed into a moldable material, such as thermal plastic, to form the wave guide(s) of the optical integrated circuit. A plurality of the dies may be mounted around the curved surface of a heated roller, and a heated tape of the waveguide material may be fed under the roller in a mass production process. Alternatively, the die may be mounted in an injection molding cavity, and the IOC may be formed by an injection molding process.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention involves integrated optical circuits (“IOC”) formed of moldable materials, such as thermal plastic, and tools for making such IOCs.  
           [0003]    2. Description of Related Art  
           [0004]    An integrated optical circuit (IOC) is a collection of one or more miniature optical waveguides on a substrate that provides optically transmitting paths for connection between optical components. Typically, such optical components include lasers, optical amplifiers, optical modulators, and optical detectors. Usually IOCs are similar in size to electronic integrated circuits, with areas ranging between 1 and 625 square millimeters.  
           [0005]    Some IOCs are formed by compression-molding a material that is optically transparent. The molded material may be a polymer, a thermoplastic, or another moldable plastic. As is typical in compression-molding, a molding die is shaped as a “negative copy” of the IOC. The molding die is pressed into the moldable material, and the die forms the moldable material into a “positive copy” of the IOC, a copy identical in shape to the desired waveguide.  
           [0006]    Demand for IOCs is increasing due to increased usage of fiber optics and optical chips. Accordingly, methods and tools capable of making IOCs in efficient and cost effective manner are need.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides tools and methods for economically making IOCs including one or more waveguides using conventional moldable materials.  
           [0008]    One embodiment of a tool within the present invention includes a molding die. The molding die includes a substrate having a topographically patterned first surface. A conformal protective film is provided over said first surface. The film has an outer second surface that forms a negative copy of the IOC to be molded using the molding die.  
           [0009]    In one method of making the molding die, a silicon or gallium arsenide wafer is provided. The wafer may be used to form a plurality of the molding dies simultaneously. The wafer is patterned to form the patterned first surface, which typically includes trenches and/or ridges. The patterning may be done using methods common to semiconductor manufacturing, such as plasma etching through a photoresist mask. The protective film may be any hard, durable material compatible with the material of the substrate. For example, the film may be metal, aluminum oxide, or diamond, among other possibilities. The film may be deposited on the wafer by plating or sputter deposition, among other possibilities. Finally, the wafer is cut into various pieces, with each piece comprising one of the molding dies or a strip of the molding dies. If desired, a backing plate may be attached to the substrate opposite the first surface to lend support to the substrate.  
           [0010]    An alternative embodiment of a tool for molding such an IOC includes a roller having the shape of a cylinder with a curved outer surface. One or more of the novel molding dies described above (or one or more conventional molding dies) are applied to the curved outer surface of the roller. The molding dies may be bent so as to conform to the curved outer surface of the roller.  
           [0011]    The present invention also includes methods of compression molding one or more IOCs. An exemplary method includes providing a molding die and a moldable first material. The molding die includes a substrate with a topographically patterned first surface, and a hard protective film over the first surface. The exposed outer surface of the film is the molding surface of the molding die. The first material is positioned on a holding substrate. One or both of the molding die and the first material are heated to a selected molding temperature. The molding surface of the molding die is pressed into the first material at a selected pressure, thereby molding a patterned IOC surface in the first material. The first material is then cured.  
           [0012]    In one embodiment, a molded IOC surface includes a plurality of channels. The channels are filled with a second moldable material that is optically transmissive, thereby forming a waveguide. The first and second materials are cured simultaneously or in separate steps.  
           [0013]    An alternative method within the invention for compression-molding an IOC includes providing a molding tool having one or more molding dies mounted on a roller. A tape of a moldable first material also is provided. The molding tool and/or the tape are heated. The tape is fed under the rolling molding tool, which presses its molding die(s) into the tape of the first material, thereby molding a patterned IOC surface. The tape is then cured, and individual IOCs are singulated from the tape using a saw or some other severing device.  
           [0014]    Another method within the present invention for molding an IOC includes providing a mold having a cavity defined by an interior surface. A molding die is mounted on the interior surface of the cavity. The molding die includes a substrate with a topographically patterned first surface, and a hard protective film over the first surface. The exposed outer surface of the film is the molding surface of the molding die. A moldable first material is injected into the cavity so that the first material contacts and conforms to the molding surface of the die, thereby molding a patterned IOC surface. The first material is cured, and removed from the mold.  
           [0015]    These and other aspects of the present invention may be better appreciated in view of the attached drawings and the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 depicts a cross-sectional side view of a molding die  1  with a patterned substrate  10  and protective film  20 .  
         [0017]    [0017]FIG. 2 depicts a cross-sectional side view of a molding die  2  with a patterned substrate  10 , protective film  20 , and curved molding surface  35 .  
         [0018]    [0018]FIG. 3 depicts a cross-sectional side view of a molding die  3  with a patterned substrate  10 , protective film  20 , and curved molding surface  35 .  
         [0019]    [0019]FIG. 4 a  depicts a molding tool  4  including a roller  40  and multiple molding dies  44  applied to an outer surface  42  of roller  40 .  
         [0020]    [0020]FIG. 4 b  is a cross-sectional side view of molding tool  4  of FIG. 4 a.    
         [0021]    [0021]FIG. 5 a  depicts a molding tool  5  including a roller  40  and multiple molding dies  44  that are applied to and bent around outer surface  42  of roller  40 .  
         [0022]    [0022]FIG. 5 b  is a cross-sectional side view of molding tool  5  of FIG. 5 a.    
         [0023]    [0023]FIG. 6 a  is a flow chart of an exemplary method  50  of compression-molding an IOC.  
         [0024]    [0024]FIG. 6 b  is a cross-sectional side view of molding die  1  of FIG. 1 being used according to method  50  of FIG. 6 a.    
         [0025]    [0025]FIG. 6 c  is a cross-sectional side view of an alternative IOC that was molded according to method  50  of FIG. 6 a.    
         [0026]    [0026]FIG. 6 d  is a cross-sectional side view of a further alternative IOC that was molded according to method  50  of FIG. 6 a.    
         [0027]    [0027]FIG. 7 depicts molding tool  5  of FIG. 5 a  being used in accordance with an exemplary molding method  70  of compression-molding an IOC.  
         [0028]    [0028]FIG. 8 is a cross-sectional side view of a molding die  44  being used according to an exemplary method  80  of injection-molding an IOC. 
     
    
       [0029]    In the drawings, where the different embodiments have similar structures, the same reference numbers are usually used.  
       DETAILED DESCRIPTION  
       [0030]    [0030]FIG. 1 depicts a cross-sectional side view of a molding die  1  in accordance with one embodiment of the present invention. Molding die  1  includes a substrate  10  that has a patterned first surface  12  and an opposite back surface  14 . A hard protective film  20  is superimposed over first surface  12 . Film  20  has a contact surface  25  that is applied over first surface  12  and an opposite, exposed second surface  30 .  
         [0031]    Substrate  10  may be made of silicon, gallium arsenide, silicon nitride, or silicon carbide or any other material compatible with the very fine manufacturing techniques that are commonly used in the semiconductor industry, such as plasma etching, sputter etching, sputter deposition, and plating. For the sake of example, assume that substrate  10  is silicon unless otherwise specified.  
         [0032]    During fabrication, molding die  1  typically will be is fabricated on a wafer, e.g., a silicon wafer, using conventional semiconductor manufacturing processes. Typically, a plurality of molding dies  1  will be made in a matrix form on a single wafer, similar to how integrated circuit chips are made on a wafer. Subsequently, the individual molding dies may be singulated from the wafer by a conventional wafer sawing method. Alternatively, the wafer may be sawn so that an array of molding dies are in a single, monolithic strip (e.g., a one by five array of molding dies).  
         [0033]    Protective film  20  may be made of any hard and durable material that is compatible with being applied on a wafer of material of the types listed above, e.g., silicon, and is compatible with molding processes, such as are provided below. Exemplary materials include nickel, titanium, aluminum oxide, and diamond, among other possibilities.  
         [0034]    First surface  12  of substrate  10  is topographically patterned to include one or more trenches, ridges or both, which are subsequently coated by film  20 . For example, in FIG. 1, substrate  10  includes a plurality of film-coated trenches  16 . Each trench  16  has vertical sidewalls and a bottom. A film-coated ridge  18  is between two trenches  16 . Each ridge  18  has vertical sidewalls and a top. A trench  16  in molding die  1  may be used to mold a ridge in an optically transmissive material so as to form an optical waveguide of an IOC. Alternatively, a ridge  18  on molding die  1  may be used to mold a trench in a first material that may be filled in with a second material to form an optical waveguide of an IOC. Typically, the first material would have a higher index of refraction that the second material, as discussed below.  
         [0035]    First surface  12  may be patterned by any fine patterning method compatible with the materials used for substrate  10 . For example, where substrate  10  is silicon or gallium arsenide, techniques such as plasma etching, chemical etching, or e-beam milling may be used, typically in conjunction with a photoresist mask or other type of mask, to pattern first surface  12 . Film  20  may be applied over first surface  12  by any method compatible with substrate  10  and the materials of film  20 . For example, where substrate  10  is silicon and film  20  is metal, such as nickel, then sputtering, electroplating, or electrodeless plating may be used to apply film  20  over first surface  12  of substrate  10 .  
         [0036]    In an exemplary embodiment of the present invention, molding die  1  includes substrate  10  that is made of silicon and is 1-2 mm thick. First surface  12  is patterned by plasma etching to include a plurality of trenches  16 . Film  20  is nickel, is 0.4-0.8 mm thick, and is applied by electrodeless plating. A metal backing plate may applied to opposing second surface  14  of substrate  10  to lend support to substrate  10 .  
         [0037]    When film  20  is applied over first surface  12  of substrate  10 , lower contact surface  25  of film  20  conforms to the shape of first surface  12 . However, the thickness of film  20  causes the shape of the opposing upper second surface  30  of film  20  to be slightly different from the shape of first surface  12 . For example, the thickness of film  20  on opposing sidewalls of a trench in first surface  12  causes the film-coated trench  16  to be narrower than the underlying trench in first surface  12  of substrate  10 . Applying film  20  over first surface  12  may also cause the shape of second surface  30  to differ from the shape etched into first surface  12  of substrate  10 . Accordingly, first surface  12  of substrate  10  is patterned so that, after film  20  is applied over first surface  12 , second surface  30  is a negative copy of the desired IOC, i.e., a surface that can be used to mold the IOC.  
         [0038]    [0038]FIG. 2 depicts a cross-sectional side view of an alternative embodiment of a molding die within the present invention. Molding die  2  includes a substrate  10 , first surface  12 , and film  20  similar to those of molding die  1  of FIG. 1. However, in molding die  2 , first surface  12  of substrate  10  is patterned such that the vertical sidewalls of the trenches include intermediate steps  17  between the top and the bottom of the trench. Accordingly, second surface  30  of film  20  will have a vertically curved surface  35  within trench  16 . During molding of the IOC, curved surface  35  will act to produce a molded optical waveguide with a matching curved outer surface.  
         [0039]    [0039]FIG. 3 depicts a cross-sectional side view of a further alternative embodiment of a molding die  3  within the present invention. Molding die  3  of FIG. 3 is similar to molding die  2  of FIG. 2, except that substrate  10  is patterned with steps  17  that extend further up the sidewall of the trench toward first surface  12 . Accordingly, second surface  30  forms curved surfaces  35  at both the top and the bottom of trench  16  with no sharp vertical corners. A ridge formed in a moldable material by curved surface  35  of FIG. 3 will have a matching curved surface.  
         [0040]    The elimination of sharp vertical corners in molding dies  2  and  3  reduces the stress on the molding die during molding. Therefore, the durability of the molding die is increased and the tendency for the molding die to crack during molding is decreased over that of a similar molding die that has sharper vertical corners. The embodiment of FIG. 3 has fewer sharp corners than that of FIG. 2.  
         [0041]    [0041]FIG. 4 a  depicts a molding tool  4  in accordance with another embodiment of the present invention. Molding tool  4  includes a roller  40  that is cylindrically shaped. Roller  40  may be made of any material, such as steel, that is durable and consistent with rolling and applying pressure to a moldable material. FIG. 4 b  depicts a cross-sectional side view of molding tool  4 .  
         [0042]    One or more molding dies  44  each having a topographically patterned surface  30  is applied to the curved outer surface  42  of roller  40 . Patterned surface  32  faces outwards from roller  40 . Dies  44  may be any of dies  1 ,  2 , or  3  of FIGS. 1, 2 or  3 . Alternatively, another die (e.g., a conventional die) may be used as die  44 . Typically, a plurality of molding dies  44  are applied fully around outer surface  42  of roller  40 , as depicted in FIG. 4 a . A narrow space may be allowed between the molding dies  44 . Molding dies  44  may be attached to outer surface  42  by any method compatible with the material of substrate  10 , such as soldering, brazing, or bonding with an epoxy or other adhesive.  
         [0043]    [0043]FIG. 5 a  depicts a further alternative embodiment of a molding tool  5  within the present invention. Roller  40  and outer surface  42  of molding tool  5  are similar to those of molding tool  4  of FIGS. 4 a ,  4   b . However, in molding tool  5 , molding dies  44  are bent around the curvature of surface  42  as they are applied to surface  42 . FIG. 5 b  depicts a cross-sectional side view of molding tool  5 . Molding dies  44  may be thinned to increase its flexibility for bending around the curvature of surface  42 . With such an embodiment, excess spacing between the dies  44  may be eliminated, and the molded IOCs may have a straighter alignment.  
         [0044]    Molding die  44  may be thinned using any method compatible with the material of the die. For example, where die  44  is one of dies  1 ,  2  or  3  of FIGS. 1, 2, and  3 , respectively, and substrate  10  is made from a silicon wafer, then substrate  10  may be thinned using conventional polishing or etching techniques. The amount of thinning done will typically be the amount necessary to obtain the flexibility necessary to allow substrate  10  to be bent around curved outer surface  42  of roller  40 .  
         [0045]    For example, a conventional silicon wafer is 0.5 mm thick. After patterning, the unpatterned back surface  14  of the wafer may be polished to reduce the thickness of the wafer to 50 microns or less, which allows silicon substrate  10  to be flexibly bent around a roller  40  that has a radius of curvature of 10 mm. Subsequently, strips of molding dies  1 ,  2 , or  3  may be sawed from the wafer, with each strip including a plurality of molding dies  1 ,  2 , or  3 . The strips are then applied over outer surface  42  and attached thereto, so as to form a continuous ring of molding dies  1 ,  2 , or  3 .  
         [0046]    [0046]FIG. 6 a  is a flow chart of an exemplary method  50  of compression-molding an IOC that includes at least one optical waveguide. The order of the steps may vary.  
         [0047]    In step  52 , a molding die  44  is provided, such as molding dies  1 ,  2 , and  3  of FIGS. 1, 2, or  3  respectively. As mentioned above, each of molding dies  1 ,  2 , and  3  includes a substrate  10 , a patterned first surface  12 , a film  20  over first surface  12 , and an exposed second surface  30  of film  20 .  
         [0048]    In step  54 , moldable first material  64  is positioned on a top surface of a holding substrate  62 , as depicted in FIG. 6 b . First material  64  may be any material suitable for compression molding, such as thermosetting polymer, thermoplastic, photopolymer, or polycarbonate.  
         [0049]    In step  56 , first material  64 , and molding die  1  are heated to selected temperatures. In step  57 , second surface  30  of molding die  1  is pressed into first material  64  with a selected pressure. FIG. 6 b  depicts second surface  30  of molding die  1  being pressed into first material  64  in accordance with step  57  of FIG. 5. In one embodiment, first material  64  and molding die  1  are heated to a temperature that is near, but below, the glass transition temperature (Tg) of first material  64 . Typically, the viscosity of first material  64  is high enough to require considerable pressure to force first material  64  into the contours of molding die  1 .  
         [0050]    In step  58  of FIG. 5, molding die  1  is removed from first material  64 . In step  59 , first material  64  is cured according to methods specific to first material  64 , e.g., by exposing first material  64  to a change in temperature, exposing first material  64  to ultraviolet light, or simply waiting for the passage of a selected time period. For example, if first material  64  is a thermoosetting polymer, curing may be accomplished by further increasing the temperature of first material  64  to a selected curing temperature (e.g., 150-170° C.) and waiting for a selected curing time (e.g., 5 to 60 minutes). As another example, if first material  64  is a thermoplastic, curing may be accomplished by reducing the temperature of first material  64 . As a further example, if first material  64  is a photopolymer, curing may be accomplished by exposing first material  64  to ultraviolet light to the gel point and then heating as above.  
         [0051]    According to one embodiment of the present invention, first material  64  of FIG. 6 b  is optically transparent and is molded to form at least one ridge-channel  66  in the IOC that is an optical waveguide. In this instance, a plurality of ridge channels  66  are formed, with each being a separate waveguide of the IOC. First material  64  also includes intervening areas  65  of the IOC where there is no ridge-channel  66  (e.g., between the ridge channels  66 ). In this embodiment, first material  64  is molded to be sufficiently thin in the intervening areas  65  (e.g. two microns thick) that optical signals traveling in a ridge-channel  66  are confined to the ridge-channel  66  and do not leak into intervening areas  65  or into other ridge-channels  66 .  
         [0052]    Alternatively, as is shown in FIG. 6 c , first material  64  may include a plurality of layers of different materials, e.g., a topmost, exposed surface layer  63  and lower, optical confinement layer  67 . Optical confinement layer  67  is beneath surface layer  63  and has an optical index of refraction that is lower than that of surface layer  63 . In this embodiment, confinement layer  67  may be of any thickness as long as surface layer  63  is sufficiently thin in the intervening areas  65  (e.g. two microns thick) that an optical signal traveling in a ridge-channel  66  is confined to the ridge-channel  66 .  
         [0053]    [0053]FIG. 6 d  depicts a cross-sectional side view of an alternative IOC produced by an alternative method within the present invention. As above, first material  64  is compression molded to form one or more channels  68 . A moldable second material  69  is inserted in channels  68  so as to substantially fill channels  68 . Second material  69  is cured, forming an optical waveguide. In this embodiment, second material  69  must be optically transparent and must have an optical index of refraction that is higher than that of first material  64 . Neither an optical confinement layer  63  nor thin intervening areas  65  are required, as in FIGS. 6 b ,  6   c . In FIG. 6 d , the topographical pattern of second surface  30  of dies  1 ,  2 , or  3  is not a negative copy of the IOC but rather is a positive copy of the IOC.  
         [0054]    [0054]FIG. 7 depicts a method  70  of using a roller to compression-mold an IOC in accordance with the present invention. Molding tool  5  of FIGS. 5 a ,  5   b  is provided and includes roller  40  and one or more molding dies  44 . Molding dies  44  have a topographically patterned surface  32  and are applied to roller  40  with patterned surface  32  facing outwards from roller  40 . Again, molding die  44  may be dies  1 ,  2 , or  3  of FIGS. 1-3, respectively, or some other type of molding die.  
         [0055]    A tape  46  of optically transparent and moldable material is provided. Subsequently, tape  46 , roller  40 , and a backing-roller  48  are heated, as in method  50  of FIGS. 6 a ,  6   b . As tape  46  is conveyed past roller  40 , roller  40  and backing-roller  48  cooperatively turn and apply pressure to tape  46 . This pressure forces top surface  30  of the molding dies  44  of roller  40  into tape  46  to mold the IOC. Roller  40 , backing-roller  48 , and tape  46  may be continually operated to form a continuous tape containing multiple replications of the IOC.  
         [0056]    Tape  46  of FIG. 7 is subsequently cured, such as by heating tape  46 , cooling tape  46 , exposing tape  46  to ultraviolet light, or waiting for the passage of a selected time period, as in method  50  of FIGS. 6 a ,  6   b . For example, if tape  46  is a thermosetting polymer, curing may be accomplished by further increasing the temperature of tape  46  to a selected curing temperature (e.g., 150-170° C.) and waiting for a selected curing time (e.g., 5-60 minutes). As another example, if tape  46  is a thermoplastic, curing may be accomplished by reducing the temperature of tape  46 . Tape  46  may be heated or cooled by passing tape  46  over additional rollers that are at selected temperatures, or by using heat lamps that shine on tape  46 . As a further example, if tape  46  is a photopolymer, curing may be accomplished by exposing tape  46  to ultraviolet light. After curing, individual IOCs on tape  46  may be singulated through methods common to high-precision cutting of polymers or plastics, such as sawing or scribing and breaking.  
         [0057]    In an alternative method in accordance with the present invention, each molding die  44  of tool  5  of FIG. 7 may be used to form one or more channels  68  in tape  46 , similar to method  55  of FIG. 6 c . (Again, die  44  may be one of dies  1 ,  2 , or  3 .) Subsequently, a moldable second material  69  is inserted so as to substantially fill each channel  68 , thereby forming an optical waveguide in each channel  68 . The material of tape  46  and the second material  69  may be cured in separate steps or simultaneously. In this instance, the topographical pattern of molding die  44  is not a negative copy of the IOC but rather is a positive copy of the IOC. In such an embodiment, tape  46  may or may not be optically transparent, as long as second material  69  is optically transparent and has an optical index of refraction that is higher than that of tape  46 .  
         [0058]    [0058]FIG. 8 depicts an alternative method  80  of molding an IOC in accordance with the present invention. A mold having a top half  82 , a bottom half  83 , an interior cavity therebetween, an interior surface  84 , and an injection port  86  is provided. A molding die  44  (e.g., molding die  1 ,  2 , or  3  of FIGS. 1, 2, and  3 ) with a topographically patterned surface  32  is provided. Molding die  44  is fixed to interior surface  84  of top half  82 , with patterned surface  32  facing toward the interior of the mold cavity. Molding die  44  is placed so that injection port  86  has clear access to the interior cavity. An optically transparent and moldable first material  88  is provided into the interior of cavity  82  through injection port  86  so that first material  88  contacts and conforms to the pattern of surface  32 . An injection molding technique will typically be used. Transfer molding may also be used. First material  88  is subsequently cured by methods common to injection molding or transfer molding. The mold is then opened and first material  88  is removed.  
         [0059]    As an example of an alternative method in accordance with the present invention, injection-molding may be used as in method  80  of FIG. 8 to form at least one channel  68  in first material  88 , similar to that done in method  55  of FIG. 6 c . Subsequently, a moldable second material  69  is inserted so as to substantially fill each channel  68  and thereby forming optical waveguides. In this embodiment, the topographical pattern of molding die  44  is not a negative copy of the IOC but rather is a positive copy of the IOC. Tape  46  may or may not be optically transparent, as long as second material  69  is optically transparent and has an optical index of refraction that is higher than that of first material  88 . First material  88  and second material  69  may be cured separately or simultaneously.  
         [0060]    The embodiments described above are merely examples of the present invention. Practitioners will recognize that variations of the embodiments herein are possible within the equitable scope of the appended claims.