Abstract:
A polymer micro-ring resonator and a method of manufacturing the same that is capable of providing reduced surface roughness and improved submicron gap separation between a waveguide and a micro-ring. Nanoimprinting is employed to achieve these advantages without the need for a final lithography and etching step. According to a first method, a hard mold is used to directly imprint a polymer film to form optical waveguides in micro-ring devices. A second method employs a template filling approach, which allows a thicker waveguide to be fabricated, as well as polymers that are difficult to directly imprint. Later buffering of the substrate is used to form pedestal structures under the waveguide and micro-ring for improved performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Ser. No. 60/383,010, filed May 24, 2002. The disclosure of the above application is incorporated herein by reference. 
     
    
     GOVERNMENT RIGHTS  
       [0002] This invention was made with government support under Contract No. F49620-01-0-0135 awarded by the Air Force Office of Scientific Research. The Government has certain rights in the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates to the fabrication of a polymer waveguide devices and, more particularly, relates to a polymer micro-ring or micro-disk resonator waveguide device.  
         BACKGROUND AND SUMMARY OF THE INVENTION  
         [0004]    Micro-ring resonator-based photonic devices have been researched extensively in recent years due to their important applications in integrated photonic circuits. These devices are typically in the form of a micro-ring closely coupled to a waveguide, which offers unique properties such as narrow bandwidth filtering, high quality factor, and compactness. A wide range of functionality has been exploited using micro-ring resonator-based devices for future optical communications, including channel add/drop filters, WDM demultiplexers, true ON-OFF switches, dispersion compensators, lasers, and enhanced nonlinear effects. To date, most of the micro-ring resonator devices have been fabricated in semiconductor materials by using a combination of electron-beam lithography and dry etching of semiconductor materials.  
           [0005]    However, these prior art fabrication techniques may suffer for a number of disadvantages. For example, it is known that dry etching often leads to increased surface roughness, which results in large scattering loss. It is important to note that scattering loss is believed to be the main loss mechanism associated with fabricated micro-ring devices. Such a high loss places a significant limitation on the practical use of micro-resonator devices. That is, since scattering loss from surface roughness is proportional to(n WG   2 -n C   2 ), where n WG  and n C  are the refractive indices of the waveguide and the cladding, respectively, the use of low refractive index polymers as used in the present invention will significantly reduce such loss. In addition, using the disclosed thermal reflow process could further reduce the surface roughness of the polymer waveguide, resulting in micro-ring resonators with extremely high quality-factor. Furthermore, polymer waveguides provide better coupling efficiency to optical fibers than prior art semiconductor waveguides due to the low index and the large cross section of the polymer waveguide. Still further, use of polymer materials also allows one to easily explore nonlinear optical effect for active devices by using many existing Nonlinear Optical (NLO) polymers. Devices such as tunable filters, optical switches, optical modulators can be made by using NLO or EO polymer materials.  
           [0006]    Electron-beam lithography is known to be a slow serial patterning technique, which includes several limitations preventing efficient high volume manufacturing of micro-ring resonator based photonic integrated circuits.  
           [0007]    Accordingly, there exists a need in the relevant art to provide a polymer micro-ring resonator that is capable of overcoming the disadvantages of the prior art.  
           [0008]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1( a ) is a schematic view illustrating a micro-ring resonator;  
         [0011]    [0011]FIG. 1( b ) is a graph illustrating narrow bandwidth filter behavior;  
         [0012]    [0012]FIG. 2( a ) is a flowchart illustrating the process steps of a first embodiment of the present invention;  
         [0013]    [0013]FIG. 2( b ) is a flowchart illustrating the process steps of a second embodiment of the present invention;  
         [0014]    [0014]FIG. 3( a ) is a SEM photograph illustrating a waveguide and micro-ring trench formed in a mold;  
         [0015]    [0015]FIG. 3( b ) is a SEM photograph illustrating a waveguide and micro-ring;  
         [0016]    [0016]FIG. 4( a ) is a SEM photograph illustrating a waveguide disposed atop of a pedestal structure;  
         [0017]    [0017]FIG. 4( b ) is a SEM photograph illustrating a waveguide and micro-ring disposed atop of a pedestal structure;  
         [0018]    [0018]FIG. 5( a ) is a SEM photograph of a waveguide and micro-ring in a racetrack configuration;  
         [0019]    [0019]FIG. 5( b ) is a SEM photograph of a waveguide and micro-ring in a microdisk configuration  
         [0020]    [0020]FIG. 6( a ) is a SEM photograph of a waveguide and micro-ring before annealing;  
         [0021]    [0021]FIG. 6( b ) is a SEM photograph of a waveguide and micro-ring annealed at 85° C. for 120 seconds;  
         [0022]    [0022]FIG. 6( c ) is a SEM photograph of a waveguide and micro-ring annealed at 95° C. for 60 seconds;  
         [0023]    [0023]FIG. 7( a ) is a graph illustrating the transmission spectrum through the micro-ring resonator device of the present invention; and  
         [0024]    [0024]FIG. 7( b ) is a graph illustrating the transmission spectrum through a micro-ring resonator device having a pair of waveguides on opposing sides of the micro-ring of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0026]    By way of background, it is believed that a brief discussion of the principles of micro-ring resonators is useful. With particular reference to FIG. 1( a ), a waveguide  10  is illustrated coupled with a micro-ring  12 . An input (E 1 ), an output (E 3 ), and circulating field inside micro-ring  12  (E 2  and E 4 ) can be described by the following coupled-mode equations  
           E   3 =α i (τ E   1   +jκE   2  )  
           E   4 =α i ( jκE   1   +τE   2 )   (1)  
         [0027]    where υ and κ is the amplitude transmission and coupling coefficient, respectively, and α i  is the insertion loss due to waveguide  10  mode mismatch in coupling region  14 . By introducing a single-pass amplitude attenuation factor a, it is appropriate to state E 2 =ae jφ E 4 , where φ is the single-pass phase experienced by light traveling inside micro-ring  12 , which is equal to 2πn eff L/λ. Here, n eff  is the effective refractive index of the propagation mode, L is the circumference of micro-ring  12 , and λ is the vacuum wavelength. Together with Eq. (1), the transmission through waveguide 10, when coupled to micro-ring  12 , is as follows:  
             T   =         I   3       I   1       =                (       α   i        τ     )     -       α   i   2        a                        j                 φ             1   -       a        (       α   i        τ     )                 j                 φ                  2               (   2   )                               
 
         [0028]    Accordingly, as set forth in Eq. (2), resonance occurs as φ=2mπ (m is an integer), and the transmission through waveguide  10  shows a periodic dip behavior as a function of input wavelength (schematically illustrated in FIG. 1( b )). It is this narrow bandwidth filter behavior that makes micro-ring devices very attractive for integrated WDM add/drop filter applications.  
         [0029]    In micro-ring resonators, the coupling coefficient plays an important role in determining the device characteristics. Generally, the coupling coefficient depends exponentially on the gap distance between the micro-ring and the straight waveguide. In order to have sufficient coupling between the micro-ring and the straight waveguide, the gap between the micro-ring and waveguide should preferably be small; alternatively, “racetrack” geometry can be used where the overall length of the coupling region is increased to enhance the coupling. According to the present invention, it has been determined that for a typical polymer with refractive index of 1.55, the polymer channel separating waveguide  10  and micro-ring  12  should be at least 1.5 μm high in order to support single mode propagation with low loss and good confinement with a gap width at the coupling region of about 100 to 200 nm. However, to fabricate polymer waveguide and micro-ring devices, especially closely coupled waveguides and micro-rings with gap distance of 100 to 200 nm and height of at least 1.5 μm, conventional patterning and RIE processes are very difficult.  
         [0030]    According to the teachings of the present invention, a direct imprinting techniques and a template filling technique are used to fabricate micro-ring resonators. A variety of optical quality polymers may be used to form the micro-ring waveguide structures uses these techniques.  
         [0031]    A first preferred embodiment includes direct imprinting to create polymer waveguides and micro-rings, which is schematically illustrated in FIG. 2( a ), and begins with first preparing a separate imprinting mold. This mold  20  includes a silicon substrate having a 200 to 400 nm thick layer of thermally grown silicon dioxide thereon. A subsequent layer of spin-coated 4% 950 k polymethylmethacrylate (PMMA) is applied thereto. The PMMA layer is preferably about 200 to 250 nm thick. This assembly is then baked at about 180° C. for about 30 minutes. Following baking, the assembly is patterned using electron beam lithography to create features in the PMMA layer. These features are transferred into silicon dioxide underneath by CHF 3 /CF 4  reactive ion etch (RIE) and the remaining PMMA is removed via acetone. The assembly is then coated with surfactant to form a shallow mold  20  used in the succeeding nanoimprinting step.  
         [0032]    After fabricating shallow mold  20 , it may be used to create a subsequent mold having deep features through a nanoimprint technique according to the present invention. A silicon substrate  22  is first grown with a 2 μm thick silicon dioxide layer  24 , which is later spin-coated with 4% 15 k PMMA to form a PMMA layer  26 , which together define an assembly  28 . Assembly  28  is closely contacted with shallow mold  20 . Assembly  28  and shallow mold  20  are brought together under high pressure of about 900 psi and high temperature of about 150° C. for about 10 minutes in order to transfer the pattern of shallow mold  20  to PMMA layer  26 . Following cooling, assembly  28  is separated from mold  20  and the residual PMMA layer is removed via O 2  RIE.  
         [0033]    To create features in assembly  28 , hard mask  30  is used, preferably a metal material such as Ti/Ni. Metal mask  30  is evaporated on silicon dioxide layer  24  and then lifted off using PRS 2000 (photo resist stripper) solution. Consequently, the pattern in metal mask  30  is transferred into silicon dioxide layer  24  via CHF 3 /CF 4  RIE. The remaining metal mask  30  is then removed via NH 4 OH:H 2 O 2 :.H 2 O (1:1:5) solution. This arrangement is then coated with surfactant as a deep mold  32  to create 2 μm high polymer waveguides in the following step. As best seen in FIG. 3( a ), a scanning electron microscopy (SEM) picture of a fabricated deep mold  32  is provided having a micro-racetrack shape.  
         [0034]    Referring again to FIG. 2( a ), deep mold  32  is then used to imprint directly a polymer spin coating on a thermally grown oxide layer to create the desired waveguide and micro-ring structure. To this end, a silicon member  40  is grown with a 2 μm thick silicon dioxide layer  42  and spin-coated with a polymer layer  44  of polymethylmethacrylate (PMMA), polystyrene (PS), or polycarbonate (PC), which forms the core of waveguide  10  and micro-ring  12 . Preferably, the polymer spin coating is a PMMA polymer because of its high optical quality. In order to minimize the thickness of any residual polymer layer after imprinting so as to facilitate further device processing, it is preferable that the initial PMMA thickness is about 200 nm, which is thinner than the final desired waveguide and micro-ring thickness of 1.5 μm. This implies that a large amount of polymer needs to be displaced in order to fill in the mold trough region during imprinting. The residual polymer layer is removed by O 2  RIE. To provide better light confinement, the sample is immersed in buffered HF to isotropically etch part of silicon dioxide layer  24  beneath waveguide  10  and micro-ring  12  for creating the pedestal structures seen in the figures.  
         [0035]    Consequently, it has been found that the conditions for imprinting need to be modified accordingly to ensure that the patterns are properly transferred from deep mold  32  to polymer layer  44 . For example, it was determined that high pressure (i.e. about 75 kg/cm 2 ) serves to assist the polymer flow. Additionally, an imprinting temperature of about 175° C. was selected. Polymer temperatures greater than about 190° C. have been found to reduce adversely the viscosity of the polymer, which may lead to non-uniform pattern thickness after imprinting due to the non-flatness of the wafer surface. In the present embodiment, it was found that by extending the imprinting time to about 10 minutes, the polymer has sufficient time to move so as to achieve a uniform pattern thickness. With these optimized imprinting conditions, it is possible to now successfully imprint polymer micro-ring resonator structures. A fabricated micro-ring device according to the principles of the present invention is illustrated in FIG. 3( b ), which consists of PMMA waveguides and micro-rings of 1.5 μm in height with a coupling gap distance of 200 nm between micro-ring  12  and waveguide  10 .  
         [0036]    In order to improve field (light) confinement in waveguide  10  and micro-ring  12 , it is preferable to optionally employ buffered HF to isotropically etch the SiO 2  beneath waveguide  10  and micro-ring  12  to create pedestal structures there below (see FIGS.  2 ( a ) and  4 ( a )-( b )).  
         [0037]    During separation of the mold from the imprinted polymer waveguide and micro-ring, it is important to avoid breakage of the curved sections of waveguide  10 . This breakage may be avoided by ensuring the surface of the mold and the substrate remain parallel to each other during separation.  
         [0038]    Polymers that are suitable for forming micro-ring and micro-disk resonators are not limited to PMMA. That is, similar processing conditions can be used to fabricate polystyrene (PS) microresonator devices. Alternatively, polymers that possess tough mechanical property may be used, such as polycarbonate (PC). As best seen in FIG. 5, imprinted PC micro-racetrack (FIG. 5( a )) and micro-disk (FIG. 5( b )) structures with a waveguide and micro-ring height of 2 μm are illustrated. By way of non-limiting example, the polycarbonate used in the present embodiment included a molecular weight of 18,000 and a glass transition temperature of 150° C. Accordingly, it was necessary to raise the imprinting temperature to about 220° C. During fabrication, polycarbonate micro-ring and micro-disk remained intact during mold separation. The increased refractive index of 1.6 of polycarbonate relative to PMMA provides improved optical field confinement, while the higher glass transition temperature of polycarbonate is more thermally stable than that of PMMA. However, it should be noted that the toughness of polycarbonate might make it difficult to cleave the polycarbonate waveguide for input and output coupling.  
         [0039]    A second preferred embodiment is illustrated in FIG. 2( b ) and includes a template filling method that facilitates the fabrication of thicker polymer waveguides and micro-rings, as well as for polymers that are not easily imprinted directly. This second preferred embodiment begins with first preparing a separate imprinting mold. This mold  20  includes a silicon substrate having a 200 to 400 nm thick layer of thermally grown silicon dioxide thereon. A subsequent layer of spin-coated 4% 950 k polymethylmethacrylate (PMMA) is applied thereto. The PMMA layer is preferably about 200 to 250 nm thick. This assembly is then baked at about 180° C. for about 30 minutes. Following baking, the assembly is patterned using electron beam lithography to create features in the PMMA layer. These features are transferred into silicon dioxide underneath by CHF 3 /CF 4  reactive ion etch (RIE) and the remaining PMMA is removed via acetone. The assembly is then coated with surfactant to form a shallow mold  20  used in the succeeding nanoimprinting step.  
         [0040]    After fabricating shallow mold  20 , it may be used to create a deep features through a nanoimprint technique according to the present invention. A silicon substrate  22  is produced having a 2 μm thick thermally grown silicon dioxide layer  24  and a 2 μm thick Plasma Enhanced Chemical Vapor Deposition (PECVD) silicon dioxide layer  50 . A subsequent layer of spin-coated 4% 15 k polymethylmethacrylate (PMMA) is applied thereto to form a PMMA layer  26 . Silicon substrate  22 , silicon dioxide layer  24 , PECVD layer  50 , and PMMA layer  26  together define an assembly  52 . Assembly  52  is then patterned using the nanoimprint technique using shallow mold  20 . Specifically, assembly  52  and shallow mold  20  are brought together under high pressure of about 900 psi and high temperature of about 150° C. for about 10 minutes in order to transfer the pattern of shallow mold  20  to PMMA layer  26 . Following cooling, assembly  52  is separated from mold  20  and the residual PMMA layer is removed via O 2  RIE.  
         [0041]    To create features in assembly  52 , hard mask  30  is used, preferably a metal material such as Ti/Ni. Metal mask  30  is evaporated on PECVD layer  50  and then lifted off using PRS 2000 (photo resist stripper) solution. Those portions of PECVD layer  50  that are not protected by hard mask  30  is anisotropically etched via CHF 3 /CF4 RIE. The remaining metal mask  30  is then removed via NH 4 OH:H 2 O 2 :.H 2 O (1:1:5) solution. The resultant member  54  is spin-coated with a polymer layer  56  that can fill in trenches to form waveguide  10  and micro-ring  12 . Preferably, polymer layer  56  should be planarized by a flat silicon mold using the nanoimprint technique. After planarization, some bubbles appeared in the trenches. These bubbles can be removed by heating the sample to about 130° C. for several minutes. The residual polymer layer is removed by O 2  RIE. To provide better light confinement, the sample is immersed in buffered HF to isotropically etch part of silicon dioxide layer  24  beneath waveguide  10  and micro-ring  12  for creating the pedestal structures seen in the figures.  
         [0042]    The final polymer micro-ring resonator structure formed by the second preferred embodiment is very similar to that obtained by the first preferred embodiment. However, an advantageous unique to the present embodiment is the ability to avoid the possible defect formation during mold separation. As a result, taller structures may be fabricated. Additionally, the present embodiment is readily adaptable for use with many polymer materials that are otherwise difficult to directly imprint.  
         [0043]    According to the principles of the present invention, polymer micro-ring resonators are successfully fabricated using a nanoimprint technique. A first method employs the use of direct imprinting to fabricate PMMA and PS micro-ring devices of less than 1.5 μm in height. This first method may also be used to fabricate taller micro-ring structures through the use of mechanically stronger polymers, such as polycarbonate. Alternatively, a second method of fabrication is provided that employs a template filling method to fabricate larger micro-ring devices than could otherwise be fabricated using the aforementioned direct imprinting technique. This second method of fabrication may also be used in connection with those polymers that are traditionally difficult to directly imprint.  
         [0044]    Additionally, according to the principles of the present invention, a thermal-flow process to reduce surface roughness of polymer waveguides is provided. This process further provides an effective way to modify the submicron gap separation that controls the coupling of the optical field to the micro-ring waveguide. The polymer micro-ring devices, made from polystyrene (PS), were fabricated by using a nanoimprinting technique. After the polymer waveguide had been formed, the samples are heated to a temperature close to the glass transition temperature of PS for a predetermined amount of time. This heat treatment reduces the viscosity of PS and enhances its fluidity. SEM characterization clearly shows that the sidewall roughness can be greatly reduced, which is a result of surface tension effect of the polymer. Higher temperature tends to produce smoother surface (see FIG. 5).  
         [0045]    Lastly, as best seen in FIGS.  7 ( a )-( b ), optical results of the transmission spectrum through the micro-ring resonator device of the present invention are illustrated. FIG. 7( a ) illustrates the filter behaviour obtained from the output port E 3  of the microresonator of the present invention. FIG. 7( b ) illustrates the filter behaviour obtained from the drop port from a second waveguide, separate from waveguide  10 , disposed adjacent to micro-ring  12 . In this example, second waveguide (not shown) is spaced on an opposing side of micro-ring  12  from waveguide  10 .  
         [0046]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.