Abstract:
An athermal optical modulator includes a waveguide, a ring resonator configured to receive light input from the waveguide and output modulated light to the waveguide, the ring resonator including a ridge unit located at a center of the ring resonator in a vertical section, a first contact connected to one side of the ridge unit and a second contact connected to the other side of the ridge unit, the first contact and the second contact forming paths for applying electricity to the ring resonator to form an electric field in the ring resonator, and a polymer layer covering the ridge unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2013-0040028, filed on Apr. 11, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to athermal optical modulators and methods of manufacturing the same, and more particularly, to optical modulators that have a relatively small change in emitted wavelength with respect to temperature change and methods of manufacturing the same. 
     2. Description of the Related Art 
     An opto-photonic device uses an optical device for transmitting signals and uses electricity for inputting and outputting the signals. An optical modulator used in an opto-photonic device is manufactured by using mainly a semiconductor substrate, and a device that transmits optical signals may be formed of Si that is formed in a process compatible with a complementary metal-oxide-semiconductor (CMOS) process. 
     Optical modulators based on a silicon substrate include a mach-zehnder type modulator and a ring resonator type modulator. The ring resonator type modulator has a smaller size when compared with the mach-zehnder type modulator, is operated at a higher speed and has lower power consumption. 
     However, the optical modulator that includes Si has a problem of changing refractive index as temperature increases. Accordingly, due to the relatively high thermo-optic coefficient of Si, a propagation wavelength is changed, and as a result, a desired optical signal may not be transmitted. For example, an optical modulator formed of a silicon substrate may have a wavelength variation with respect to temperature of 0.11 nm/K. 
     SUMMARY 
     Example embodiments provide athermal optical modulators that have a minimum wavelength change according to temperature change and may be manufactured in a process compatible with a CMOS process, and methods of manufacturing the same. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of example embodiments. 
     According to example embodiments, an athermal optical modulator includes a waveguide, a ring resonator configured to receive light input from the waveguide and output modulated light to the waveguide, the ring resonator including a ridge unit located at a center of the ring resonator in a vertical section, a first contact connected to one side of the ridge unit and a second contact connected to the other side of the ridge unit, the first contact and the second contact forming paths for applying electricity to the ring resonator to form an electric field in the ring resonator, and a polymer layer covering the ridge unit. 
     The ring resonator may include doping regions sequentially stacked on an insulating layer, the doping regions including a p+ doping region doped with a higher concentration of p type impurity, a p doping region doped with a lower concentration of p type impurity, an n doping region doped with a lower concentration of n type impurity, and an n+ doping region doped with a high concentration of n type impurity. 
     The first contact may be connected to the p+ doping region, and the second contact may be connected to the n+ doping region. A protection layer may cover the ring resonator on the insulating layer, the first contact and the second contact may fill vias in the protection layer that expose the p+ doping region and the n+ doping region, and the first contact and the second contact may include a metal. 
     The first contact and the second contact may have a ring shape when viewed from a plan view, and one of the first contact and the second contact formed on an outer side of the ring resonator may include an opening on a portion thereof facing the waveguide. The polymer layer may fill a trench between the first contact and the second contact and exposing the ridge unit. The trench may have a ring shape when viewed from a plan view. The trench may have a width greater than a width of the ridge unit. The ridge unit may include the p doping region and the n doping region. The waveguide and the ring resonator may include single crystal silicon. 
     According to example embodiments, a method of manufacturing an athermal optical modulator includes forming a waveguide and a ring resonator by etching an upper silicon layer of a silicon-on-insulation (SOI) substrate, wherein forming the ring resonator includes sequentially stacking a p+ doping region doped with a higher concentration of p type impurity, a p doping region doped with a lower concentration of p type impurity, an n doping region doped with a lower concentration of n type impurity, and an n+ doping region doped with a higher concentration of n type impurity on an insulating layer, forming a protection layer on the ring resonator, forming a first contact and a second contact by forming vias in the protection layer exposing the p+ doping region and the n+ doping region and filling a metal in the vias, etching the protection layer to form a trench exposing the p+ doping region and the n+ doping region between the first contact and the second contact, and forming a polymer layer filling the trench. 
     Each of the first contact and the second contact may have a ring shape when viewed from a plan view, and one of the first contact and the second contact on an outer side of the polymer layer may include an opening on a portion thereof facing the waveguide. The protection layer may be etched to form the trench having a ring shape when viewed from a plan view. The protection layer may be etched to form the trench having a width greater than a width of the ridge unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic plan view of a structure of an athermal optical modulator according to example embodiments; 
         FIG. 2  is a cross-sectional view taken along the line II-II′ of  FIG. 1 ; and 
         FIGS. 3A through 3D  are cross-sectional views showing a method of manufacturing an athermal optical modulator according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Hereafter, example embodiments will be described more fully with reference to the accompanying drawings. In the drawings, thicknesses of layers and regions may be exaggerated for clarity of the specification. The embodiments described below are only examples, and thus, it should be understood that various changes may be made from the example embodiments set forth herein. When an element is referred to as being “on” or “above” another element, it may include an element directly on the element and elements that are not in contact with the element. In the drawings, like reference numerals are used for elements that are substantially identical to each other, and the descriptions thereof will not be repeated. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used to distinguish one element from another. Thus, a first element discussed below could be termed a second element without departing from the teachings of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present inventive concept. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a schematic plan view of a structure of an athermal optical modulator  100  according to example embodiments.  FIG. 2  is a cross-sectional view taken along the line II-II′ of  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , an insulating layer  112  may be formed on a substrate  110 , and a waveguide  120  and a ring resonator  130  may be formed on the insulating layer  112 . The waveguide  120  and the ring resonator  130  may be formed of single crystal silicon. The waveguide  120  and the ring resonator  130  may be formed in an upper silicon layer  114  of a silicon-on-insulation (SOI) substrate. The ring resonator  130  may have a circular ring shape. Also, the ring resonator  130  may have various annular shapes including a ring shape and an oval shaped ring. 
     The ring resonator  130  includes a p+ doping region  131  doped with a relatively high concentration of p type impurity, a p doping region  132  doped with a relatively low concentration of p type impurity, an n doping region  133  doped with a relatively low concentration of n type impurity, and an n+ doping region  134  doped with a relatively high concentration of n type impurity sequentially formed in the stated order from the outer side of the ring resonator  130 . However, the configuration of the ring resonator  130  is not limited thereto, for example, the ring resonator  130  may include the n+ doping region doped with a relatively high concentration, an n doping region doped with a relatively low concentration, a p doping region doped with a relatively low concentration, and a p+ doping region  131  doped with a relatively high concentration sequentially formed in the stated order from the outer side of the ring resonator  130 . 
     The p+ doping region  131  and the n+ doping region  134  may be regions respectively doped with an impurity at a concentration of 10 18 ˜10 19 /cm 3 , and the p doping region  132  and the n doping region  133  may be regions respectively doped with an impurity at a concentration of 10 15 ˜10 17 /cm 3 . 
     In the ring resonator  130 , the n doping region  133  and the p doping region  132 , which are formed in the center and may be consecutively formed, constitute a ridge unit R that protrudes higher than the p+ doping region  131  and the n+ doping region  134 . In the ring resonator  130 , light moves mainly through the ridge unit R. The ring resonator  130  may have a diameter in a range from about 1 μm to about 50 μm. The ridge unit R may have a height in a range from about 90 nm to about 150 nm. The height of the ridge unit R may be substantially the same as that of the waveguide  120 . The ridge unit R may have a cross-section width in a range from about 200 nm to about 1,000 nm. A width between the waveguide  120  and the ridge unit R may be in a range from about 200 nm to about 1,000 nm. 
     The waveguide  120  and the ring resonator  130  may be covered by a protection layer  140  on the insulating layer  112 . The protection layer  140  may be formed of silicon oxide or silicon nitride. The protection layer  140  may function as a cladding layer of a light propagation path together with the insulating layer  112 . 
     A first contact  161  may be formed to contact the p+ doping region  131  of the ring resonator  130 , and a second contact  162  may be formed to contact the n+ doping region  134  of the ring resonator  130 . Via holes  142  may be formed in the protection layer  140  to expose the p+ doping region  131  and the n+ doping region  134 . Each of the via holes  142  may be filled with a via metal. The via metals may be the first contact  161  and the second contact  162 . As shown in  FIG. 1 , the first contact  161  and the second contact  162  may have the shape of the ring resonator  130 . When the first contact  161  has a ring shape, as shown in  FIG. 1 , an opening  161   a  may be formed in a region of the first contact  161  adjacent to the waveguide  120 . The opening  161   a  may be formed to prevent or inhibit interferences in light propagation between the waveguide  120  and the ring resonator  130  by the first contact  161 . 
     A trench T may be formed between the first contact  161  and the second contact  162  on the upper silicon layer  114 . The trench T may be formed to include the ridge unit R. The trench T may have a width W1 greater than the width W2 of the ridge unit R. The trench T may be formed to have the same shape as the ring resonator  130 . 
     The trench T may be filled with a polymer. A polymer layer  150  that is formed by filling a polymer may be formed of, for example, poly (methyl methacrylate) (PMMA). The polymer layer  150  may be formed to have a thickness in a range from about 100 nm to about 5 μm. In  FIG. 2 , an upper surface of the polymer layer  150  may be exposed, but the present disclosure is not limited thereto. For example, the protection layer  140  may extend on the polymer layer  150 . A detailed structure will be described below with reference to  FIG. 3D . 
     A first electrode pad  171  and a second electrode pad  172  respectively connected to the first contact  161  and the second contact  162  may be formed on the protection layer  140 . 
     Light that enters the waveguide  120  passes through the waveguide  120  and is transmitted to the ring resonator  130  through the opening  161   a . After circulating in the ring resonator  130 , the light is re-transmitted to the waveguide  120  through the opening  161   a , and afterwards, is output. At this point, when a predetermined or given voltage is applied to the ring resonator  130  through the first and second electrode pads  171  and  172 , a frequency of the light propagated to the waveguide  120  through the ring resonator  130  is modulated. When detecting light output from the waveguide  120 , an optical signal is determined according to the detected frequency. 
     The refractive index of silicon varies as the temperature increases. Accordingly, a wavelength of propagation light may vary due to a relatively high thermo-optic coefficient of silicon, and as a result, desired optical signals may not be transmitted. However, the polymer layer  150  has a thermo-optic coefficient lower than that of silicon. The athermal optical modulator  100  that includes the polymer layer  150  may have a wavelength change with respect to temperature of 0.5 pm/K. Accordingly, the athermal optical modulator  100  may stably transmit almost uniform optical signals. 
     Also, because the first contact  161  and the second contact  162  are formed to have the same shape as the ring resonator  130 , a power input from the first and second electrode pads  171  and  172  is stably supplied to the ring resonator  130 , and forms a uniform electric field in the ring resonator  130 , and thus, the athermal optical modulator  100  may generate stable optical signals. 
       FIGS. 3A through 3D  are cross-sectional views showing a method of manufacturing an athermal optical modulator  200  according to example embodiments. 
     Referring to  FIG. 3A , a SOI substrate is prepared. The SOI substrate includes a silicon substrate  210  and an insulating layer  212  and an upper silicon layer  214  which are sequentially formed on the silicon substrate  210 . As depicted in  FIGS. 1 and 3A , a waveguide (refer to  120  of  FIG. 1 ) and a ring resonator  230  may be formed by patterning the upper silicon layer  214 . In  FIGS. 3A through 3D , the waveguide is omitted for convenience of explanation. 
     The ring resonator  230  includes a protruded ridge unit R. A p+ doping region  231  doped with a relatively high concentration, a p doping region  232  doped with a relatively low concentration, an n doping region  233  doped with a relatively low concentration of n, and an n+ doping region  234  doped with a relatively high concentration may be sequentially formed in the stated order from an outer side of the ring resonator  230  by sequentially doping them using an implant process well known in the semiconductor process. The p doping region  232  and the n doping region  233  correspond to the ridge unit R. For convenience, it is depicted that the p+ doping region  231  is formed on an outer side of the ring resonator  230  facing the waveguide (not shown). 
     The p+ doping region  231  and the n+ doping region  234  may be regions respectively doped with an impurity at a concentration of 10 18 ˜10 19 /cm 3 , and the p doping region  232  and the n doping region  233  may be regions respectively doped with an impurity at a concentration of 10 15 ˜10 17 /cm 3 . 
     A first protection layer  240  may be formed on the upper silicon layer  214 . The first protection layer  240  may be formed of silicon oxide or silicon nitride. Vias  240   a  that respectively expose the p+ doping region  231  and the n+ doping region  234  from an upper surface of the first protection layer  240  may be formed in the first protection layer  240 , and a first contact  241  and the second contact  242  may be formed by filling the vias  240   a  with a metal. The first and second contacts  241  and  242  may be formed in a ring shape, and the first contact  241  may be formed on an outer region of the ridge unit R and the second contact  242  may be formed on an inner region of the ridge unit R. 
     An opening (refer to  161   a  of  FIG. 1 ) for an optical path may be formed in a portion of the first contact  241  that faces the waveguide (refer to  120  of  FIG. 1 ). The opening may be well understood from the opening  161   a  in  FIG. 1 , and thus, a detailed description thereof will not be repeated. 
     After coating an electrode layer (not shown) on the first protection layer  240 , first and second electrode pads  251  and  252  that are respectively connected to the first and second contacts  241  and  242  may be formed by patterning the electrode layer. 
     A second protection layer  260  that covers the first and second electrode pads  251  and  252  may be formed on the first protection layer  240 . The second protection layer  260  may be formed by using the same material used to form the first protection layer  240 . The second protection layer  260  may prevent or inhibit a polymer from contacting the first and second electrode pads  251  and  252  in a polymer forming process that will be described below. 
     Referring to  FIG. 3B , a trench T that exposes the ridge unit R may be formed between the first and second contacts  241  and  242 . The trench T may have a width W1 greater than the width W2 of the ridge unit R. 
     Referring to  FIG. 3C , the trench T may be filled with a polymer by using a spin coating method. The resultant product may be referred to as a polymer layer  270 . The polymer may be poly (methyl methacrylate) (PMMA). The polymer that fills the trench T may also be formed to cover the second protection layer  260 . 
     A mask layer  280  may be formed on the polymer layer  270 . The mask layer  280  may be formed to have a thickness in a range from about 100 nm to about 1 μm. The mask layer  280  may be formed of silicon oxide or silicon nitride. 
     Referring to  FIG. 3D , holes  282  may be formed above the first and second electrode pads  251  and  252  by patterning the mask layer  280 . The first and second electrode pads  251  and  252  may be exposed by sequentially etching the polymer layer  270  and the second protection layer  260  that are exposed by the holes  282 . The second protection layer  260  and the mask layer  280  may be etched by using a dry etching method or wet etching method. The polymer layer  270  may be etched by using an oxygen plasma etching method. 
     According to example embodiments, a polymer layer may be formed after performing an electrode formation process, which is a relatively high temperature process. Thus, contamination of a device with a melted polymer of the polymer layer in the relatively high temperature electrode formation process may be prevented or inhibited. 
     Also, because a mask layer is formed on the polymer layer, an integration process with other devices may be possible on a silicon substrate. 
     In the athermal optical modulator according to example embodiments, a polymer having an athermal characteristic surrounds a ridge unit, and light output from the athermal optical modulator has relatively a stable wavelength. 
     Also, because first and second contacts are formed as a ring shape of a ring resonator and face each other with the ridge unit therebetween, an electric field of the ring resonator is more rapidly formed and relatively stable, and as a result, output light has a uniform wavelength. 
     Also, because a polymer layer is formed in a trench after performing an electrode formation process, which is a relatively high temperature process, contamination of a device with a melted polymer of the polymer layer during the relatively high temperature electrode formation process may be prevented or inhibited. 
     It should be understood that example embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. It will be understood by those of skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the appended claims.