Abstract:
Electromagnetic wave oscillators each having a multi-tunnel and electromagnetic wave generating apparatuses including the electromagnetic wave oscillators are provided. The electromagnetic wave oscillator includes: a first waveguide which has a folded structure such that a path traveled by an electromagnetic wave through the first waveguide crosses an axial direction a plurality of times; an electron beam tunnel through which an electron beam passes, wherein the electron beam tunnel extends along the axial direction and crosses the first waveguide a plurality of times; and at least one auxiliary tunnel which extends parallel to the electron beam tunnel and which crosses the first waveguide a plurality of times.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2012-0012533, filed on Feb. 7, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Apparatuses consistent with exemplary embodiments relate to electromagnetic wave oscillators each having a multi-tunnel and electromagnetic wave generating apparatuses including the electromagnetic wave oscillators, and more particularly, to electromagnetic wave oscillators for oscillating an electromagnetic wave in a terahertz band and electromagnetic wave generating apparatuses including the electromagnetic wave oscillators. 
     2. Description of the Related Art 
     A frequency band of terahertz (10 12  Hz) between a microwave frequency band and an optical frequency band is expected to be a frequency band that is very significant in the fields of molecular optics, biophysics, medicine, spectroscopy, imaging, security, and the like. However, despite the significance of the terahertz band, the development of terahertz oscillators or amplifiers that generate terahertz waves is not sufficient due to physical and engineering limitations. As several new theories and a micro-processing technology have been developed, the development of terahertz oscillators or amplifiers has been recently attempted. 
     For example, several approaches, such as increasing frequencies of several oscillators in an existing micro-wave band, or changing an operational frequency into a terahertz band by using an optical device, such as a semiconductor laser or a femtosecond laser have been tried. In addition, a variety of schemes for making small-size terahertz oscillators by using a three-dimensional (3D) micro-structure manufactured using micro electro mechanical system (MEMS) technology have been recently suggested. 
     Backward wave oscillators are an example of terahertz oscillators using MEMS technology. A backward wave oscillator is a kind of interaction circuit that forms an electron beam path in a cavity for resonating an electromagnetic wave in order to oscillate a terahertz wave by interacting an electron beam emitted from an electron gun with the electromagnetic wave. A mechanism for converting energy of the electron beam to electromagnetic wave energy by interaction between the electron beam and the electromagnetic wave is significant in the terahertz interaction circuit. 
     SUMMARY 
     One or more exemplary embodiments may provide electromagnetic oscillators for oscillating electromagnetic waves in a terahertz band and electromagnetic wave generating apparatuses including the electromagnetic wave oscillators. 
     Additional aspects of exemplary embodiments 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 the presented embodiments. 
     According to an aspect of an exemplary embodiment, an electromagnetic wave oscillator includes: a first waveguide which has a folded structure such that a path traveled by an electromagnetic wave through the first waveguide crosses an axial direction a plurality of times; an electron beam tunnel through which an electron beam passes, wherein the electron beam tunnel extends along the axial direction and crosses the first waveguide a plurality of times; and at least one auxiliary tunnel which extends parallel to the electron beam tunnel and which crosses the first waveguide a plurality of times. 
     The at least one auxiliary tunnel may be disposed one of above and below the electron beam tunnel. 
     The at least one auxiliary tunnel may be disposed one of at a left side and a right side of the electron beam tunnel. 
     The at least one auxiliary tunnel may be disposed one of above and below the electron beam tunnel and one of at a left side and a right side of the electron beam tunnel. 
     The at least one auxiliary tunnel may include at least two auxiliary tunnels disposed one of above and below the electron beam tunnel and the at least two auxiliary tunnels may be adjacent to each other at the same height. 
     The at least one auxiliary tunnel may include at least two auxiliary tunnels disposed one of at a left side and a right side of the electron beam tunnel and the at least two auxiliary tunnels may be adjacent to each other at different heights. 
     The folded structure of the first waveguide may include a plurality of coupled cavities disposed at left and right sides of the electron beam tunnel. 
     The at least one auxiliary tunnel may be disposed between a left side of the electron beam tunnel and a plurality of the coupled cavities disposed to the left of the electron beam tunnel. The at least one auxiliary tunnel may be disposed between a right side of the electron beam tunnel and a plurality of the coupled cavities disposed to the right of the electron beam tunnel. 
     The at least one auxiliary tunnel may be disposed one of above and below the electron beam tunnel, and the at least one auxiliary tunnel may be disposed one of between a left side of the electron beam tunnel and a plurality of the coupled cavities disposed to the left of the electron beam tunnel and between a right side of the electron beam tunnel and a plurality of the coupled cavities disposed to the right of the electron beam tunnel. 
     The at least one auxiliary tunnel may include at least two auxiliary tunnels disposed between a left side of the electron beam tunnel and a plurality of the coupled cavities disposed to the left of the electron bean tunnel or disposed between a right side of the electron beam tunnel and a plurality of the coupled cavities disposed to the right of the electron beam tunnel, and the at least two auxiliary tunnels may be adjacent to each other at different heights. 
     The electromagnetic wave oscillator may further include: a second waveguide including a first end connected to the first waveguide and a second end including an electromagnetic wave output port; an electron beam input port which is connected to the first end of the electron beam tunnel and on which the electron beam is incident; and an electron beam discharge port which is connected to the second end of the electron beam tunnel and from which the electron beam is discharged. 
     The second waveguide may include a bend of about 90 degrees and may have a tapered structure in which a width of the second waveguide increases from the first end to the second end thereof. 
     The electromagnetic wave oscillator may further include: a lower structure having an upper surface; and an upper structure having a lower surface bonded to the upper surface of the lower structure, wherein the first waveguide and the electron beam tunnel are formed between over the upper surface of the lower structure and the lower surface of the upper structure, and the at least one auxiliary tunnel is disposed in at least one of the lower structure and the upper structure. 
     The lower structure may include: a first substrate including a first hole which does not extend entirely through the first substrate and a second hole which does not extend entirely through the first substrate, wherein the second hole is disposed one of at a left side of the first hole and at a right side of the first hole; and a second substrate including a first hole which does not extend entirely through the second substrate and a second hole which extends entirely through the second substrate, wherein the second hole of the second substrate is formed in a position corresponding to a position of the second hole of the first substrate. 
     The upper structure may include: a third substrate including a first hole which does not extend entirely through the third substrate and a second hole which does not extend entirely through the third substrate, wherein the second hole is disposed one of at at a left side of the first hole and at a right side of the first hole; and a fourth substrate including a first hole which does not extend entirely through the fourth substrate and a second hole which extends entirely through the fourth substrate, wherein the second hole of the fourth substrate is formed in a position corresponding to a position of the second hole of the third substrate. 
     The second substrate and the fourth substrate may be bonded to each other, and the electron beam tunnel may be formed by the first hole of the second substrate and the first hole of the fourth substrate. 
     The first waveguide may be formed by the second hole of the first substrate, the second hole of the second substrate, the second hole of the fourth substrate, and the second hole of the third substrate. 
     The at least one auxiliary tunnel may include an auxiliary tunnel formed by the first hole of the first substrate and an auxiliary tunnel formed by the first hole of the third substrate. 
     The lower structure may further include a fifth substrate interposed between the first substrate and the second substrate and including a first hole which does not extend entirely through the fifth substrate and a second hole which extends entirely through the fifth substrate, wherein the second hole is disposed one of at a left side of the first hole and at a right side of the first hole. 
     The upper structure may further include a sixth substrate interposed between the third substrate and the fourth substrate and including a first hole which does not extend entirely through the sixth substrate and a second hole which extends entirely through the sixth substrate, wherein the second hole is disposed one of at a left side of the first hole and at a right side of the first hole. 
     The at least one auxiliary tunnel may include an auxiliary tunnel formed by the first hole of the fifth substrate and an auxiliary tunnel formed by the first hole of the sixth substrate. 
     The electromagnetic wave oscillator may further include a metal thin layer coated on the first substrate, the second substrate, the third substrate, the fourth substrate, the fifth substrate and the sixth substrate. 
     According to an aspect of another exemplary embodiment, an electromagnetic wave generating apparatus includes: an electromagnetic wave oscillator having the above structure; an electron gun which supplies an electron beam to the electromagnetic wave oscillator; and a collector which collects the electron beam discharged from the electromagnetic wave oscillator. 
     The electromagnetic wave oscillator may be configured to cause one of a millimeter wave, a sub-millimeter wave, and an electromagnetic wave in a terahertz frequency band passing through the first waveguide to obtain energy by interaction with the electron beam that passes through the electron beam tunnel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other exemplary aspects and advantages will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic perspective view of a lower structure of an electromagnetic wave oscillator according to an exemplary embodiment; 
         FIG. 2  is a schematic perspective view of an upper structure of an electromagnetic wave oscillator according to an exemplary embodiment; 
         FIG. 3  is a schematic perspective view of an electromagnetic wave oscillator manufactured by combining the lower structure of  FIG. 1  and the upper structure of  FIG. 2 , according to an exemplary embodiment, and an electromagnetic generating apparatus including the electromagnetic wave oscillator; 
         FIG. 4A  is a partial enlarged view of a portion A of the lower structure of the electromagnetic wave oscillator illustrated in  FIG. 1 , which illustrates a structure of a first waveguide and an electron beam tunnel in detail; 
         FIG. 4B  is a partial enlarged view schematically showing a modified example of the portion A of the lower structure of the electromagnetic wave oscillator of  FIG. 1 ; 
         FIG. 5A  is a schematic cross-sectional view of a B-B′ cross-section of the lower structure illustrated in  FIG. 4A ; 
         FIG. 5B  is a schematic cross-sectional view of a B-B′ cross-section of an electromagnetic wave oscillator manufactured by combining the lower structure of  FIG. 5A  and an upper structure having the same structure as that of the lower structure of  FIG. 5A ; 
         FIG. 6  is a graph showing resonant characteristics of an electromagnetic wave generator having only a single electron beam tunnel; 
         FIG. 7  is a graph showing resonant characteristics of an electromagnetic wave oscillator having a single electron beam tunnel and two auxiliary tunnels; 
         FIG. 8  is a graph showing comparison of resonant characteristics of an electromagnetic wave oscillator when the electromagnetic wave oscillator has no tunnel and when the electromagnetic wave oscillator has three tunnels, respectively; 
         FIG. 9  is a graph showing comparison of phase characteristics of output electromagnetic waves of an electromagnetic wave oscillator when the electromagnetic wave oscillator has no tunnel and when the electromagnetic wave oscillator has three tunnels, respectively; 
         FIG. 10  is a graph showing comparison of intensities of output electromagnetic waves of an electromagnetic wave oscillator when the electromagnetic wave oscillator has no tunnel and when the electromagnetic wave oscillator has three tunnels, respectively; 
         FIGS. 11A through 11G  are cross-sectional views of various arrangement examples of auxiliary tunnels formed around an electron beam tunnel. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments will be described more fully with reference to the accompanying drawings. Like reference numerals in the drawings refer to like elements throughout, and sizes of elements in the drawings may be exaggerated for clarity and convenience. 
       FIG. 1  is a schematic perspective view of a lower structure  110  of an electromagnetic wave oscillator according to an exemplary embodiment. Referring to  FIG. 1 , a first waveguide  130  functions as a path in which an electromagnetic wave, for example, in a terahertz band is generated, and in which the electromagnetic wave travels. A second waveguide  134  is connected to the first waveguide  130  and outputs an electromagnetic wave, and an electron beam tunnel  131  is arranged in an axial direction to perforate the first waveguide  130  and functions as an electron beam proceeding path. The first waveguide  130 , the second waveguide  134 , and the electron beam tunnel  131  may be formed on a surface of the lower structure  110  of the electromagnetic wave oscillator. 
     In addition, as illustrated in  FIG. 1 , an electron beam input port  132  formed to perforate one side of the lower structure  110  so as to be connected to one end of the electron beam tunnel  131 , and an electron beam discharge port  133  formed to perforate the other side of the lower structure  110  so as to be connected to the other end of the electron beam tunnel  131 , may be formed on the surface of the lower structure  110 . Thus, an electron beam is incident on the electromagnetic wave oscillator via the electron beam input port  132  and then passes through the electron beam tunnel  131  and interacts with an electromagnetic wave, and after the interaction, a remaining electron beam may be discharged from the electron beam discharge port  133 . An electromagnetic wave output port  135  is formed to perforate the side of the lower structure  110  so that the generated electromagnetic wave may be output, and may be formed on an end of the second waveguide  134 . The second waveguide  134  may be bent at 90 degrees in a portion where the second waveguide  134  is connected to the first waveguide  130 , for example. In addition, the second waveguide  134  may have a tapered structure in which a width of the second waveguide  134  increases as it gets closer to the electromagnetic wave output port  135 . However, the structure of the second waveguide  134  of  FIG. 1  is just an example, and the second waveguide  134  having another structure may be designed as occasion demands. 
       FIG. 2  is a schematic perspective view of an upper structure  120  of an electromagnetic wave oscillator according to an exemplary embodiment. Referring to  FIG. 2 , similar to the lower structure  110  of  FIG. 1 , a first waveguide  130 , a second waveguide  134 , an electron beam tunnel  131 , an electron beam input port  132 , an electron beam discharge port  133 , and an electromagnetic wave output port  135  may be formed on a surface of the upper structure  120  of the electromagnetic wave oscillator. The structure of the upper structure  120  is substantially the same as that of the lower structure  110 , and the only difference is a mirror symmetry relationship between the upper structure  120  and the lower structure  110 . Thus, when the upper structure  120  of  FIG. 2  is overlaid on the lower structure  110  of  FIG. 1  and is bonded onto the lower structure  110  of  FIG. 1 , the first waveguide  130 , the second waveguide  134 , the electron beam tunnel  131 , the electron beam input port  132 , the electron beam discharge port  133 , and the electromagnetic output port  135  formed on the surface of the lower structure  110  may be exactly matched to the first waveguide  130 , the second waveguide  134 , the electron beam tunnel  131 , the electron beam input port  132 , the electron beam discharge port  133 , and the electromagnetic wave output port  135  formed on the upper structure  120 . 
     By bonding the upper structure  120  onto the lower structure  110  in this manner, the first waveguide  130 , the second waveguide  134 , the electron beam tunnel  131 , the electron beam input port  132 , the electron beam discharge port  133 , and the electromagnetic wave output port  135 , respectively, may be completed. In this regard, the first waveguide  130 , the second waveguide  134 , the electron beam tunnel  131 , the electron beam input port  132 , the electron beam discharge port  133 , and the electromagnetic wave output port  135  may be formed over the surface of the lower structure  110  and the surface of the upper structure  120  to be bonded onto the lower structure  110 . By bonding the upper structure  120  onto the lower structure  110 , one electromagnetic wave oscillator including the first waveguide  130 , the second waveguide  134 , the electron beam tunnel  131 , the electron beam input port  132 , the electron beam discharge port  133 , and the electromagnetic wave output port  135  may be completed. 
       FIG. 3  is a schematic perspective view of an electromagnetic wave oscillator  100  manufactured by combining the lower structure  110  of  FIG. 1  and the upper structure  120  of  FIG. 2 , and of an electromagnetic wave generating apparatus  200  including the electromagnetic wave oscillator  100 . Referring to  FIG. 3 , the electromagnetic wave oscillator  100  includes the lower structure  110  and the upper structure  120  illustrated in  FIGS. 1 and 2 , respectively. As shown in  FIG. 3 , the electron beam discharge port  133  and the electromagnetic wave output port  135  are disposed on the right side of the electromagnetic wave oscillator  100 . Although not shown in  FIG. 3 , the electron beam input port  132  would be disposed on the left side of the electromagnetic wave oscillator  100 . 
     The electromagnetic wave oscillator  100  may be used in the electromagnetic wave generating apparatus  200  that generates a millimeter wave, a sub-millimeter wave, or an electromagnetic wave in a terahertz frequency band. Referring to  FIG. 3 , the electromagnetic wave generating apparatus  200  may include the above-described electromagnetic wave oscillator  100 , an electron gun  210 , and a collector  220 . The electron gun  210  provides an electron beam to the electromagnetic wave oscillator  100 . The electron beam emitted from the electron gun  210  may be input into the electromagnetic wave oscillator  100  via the electron beam input port  132 . Thereafter, the electron beam may proceed along the electron beam tunnel  131  inside the electromagnetic wave oscillator  100  and may transfer energy to the electromagnetic wave by interaction with the electromagnetic wave resonating in the first waveguide  130 . After transferring energy to the electromagnetic wave, the remaining electron beam may be discharged from the electron beam discharge port  133 , and the discharged electron beam may be collected by the collector  220 . Meanwhile, the terahertz electromagnetic wave amplified by the interaction with the electron beam in the first waveguide  130  may be output to the electromagnetic wave output port  135  via the second waveguide  134 . As described above, the electron beam proceeds along the electron beam tunnel  131  that perforates the first waveguide  130 , and the electromagnetic wave is generated in the first waveguide  130  and is then amplified in the first waveguide  130  by interaction with the electron beam. Thus, the output electromagnetic wave may be greatly affected by the structure of the first waveguide  130  and the electron beam tunnel  131 . In this regard, the structure of the first waveguide  130  and the electron beam tunnel  131  for interaction between the electron beam and the electromagnetic wave may be designed to have an optimum state. 
       FIG. 4A  is a partial enlarged view of a portion A of the lower structure  110  of the electromagnetic wave oscillator  100  of  FIG. 1 , which illustrates an example of a structure of the first waveguide  130  and the electron beam tunnel  131  in detail. 
     Referring to  FIG. 4A , the first waveguide  130  includes a plurality of parallel barrier ribs  137  arranged and aligned in an axial direction (where the axial direction is the same as a direction in which the electron beam proceeds). The plurality of barrier ribs  137  are interdigitated with other adjacent barrier ribs  137 . Thus, the electromagnetic wave passes through the first waveguide  130  in a zigzag form, as indicated by a thin arrow of  FIG. 4A . In this regard, the first waveguide  130  may considered to be repeatedly folded in a zigzag form a plurality of times. In addition, a space formed between two adjacent barrier ribs  137  extending in the same direction functions as a coupled cavity  138  in which the electromagnetic wave passing through the first waveguide  130  may resonate. That is, the first waveguide  130  is a waveguide having a folded structure, and a coupled cavity  138  is formed in each of the folded portions of the first waveguide  130 , respectively. In order to form the coupled cavity  138 , the thickness of a portion of the barrier rib  137  in a region where the coupled cavity  138  is to be formed, may be smaller than a thickness of a portion of the barrier rib  137  in another region. 
     As illustrated in  FIG. 4A , the electromagnetic oscillator  100  includes the electron beam tunnel  131  that perforates the first waveguide  130  and is repeatedly folded a plurality of times. Although  FIG. 4A  illustrates the electron beam tunnel  131  having a shape of a simple groove, the electron beam tunnel  131  may have a complete tunnel shape by combining the lower structure  110  and the upper structure  120 , as illustrated in  FIG. 3 . For example, as illustrated in  FIG. 4A , the electron beam tunnel  131  may include a plurality of sections which are formed by the plurality of barriers ribs  137 , respectively, and arranged/aligned in the axial direction. One end of the electron beam tunnel  131  is connected to the electron beam input port  132  formed at one side of the electromagnetic wave oscillator  100 . 
     In this structure, the electron beam incident via the electron beam input port  132  proceeds in a straight line in the axial direction along the electron beam tunnel  131 , as indicated by a thick arrow of  FIG. 4A . In this case, an electromagnetic wave is generated around the electron beam due to the electromagnetic induction phenomenon. The electromagnetic wave generated in this manner proceeds in the zigzag form in the first waveguide  130 , as indicated by the thin arrow. That is, the overall proceeding direction of the electromagnetic wave and the proceeding direction of the electron beam are opposite to each other. In this regard, the electromagnetic wave oscillator  100  is a kind of backward wave oscillator. The electromagnetic wave that passes through the first waveguide  130  is resonated in the coupled cavity  138 . In addition, due to interaction with the electron beam that perforates the first waveguide  130 , the electromagnetic wave obtains energy from the electron beam. In particular, since the first waveguide  130  has the folded structure, the electromagnetic wave passes through the first waveguide  130  in a zigzag form and a proceeding speed of the electromagnetic wave is reduced and the electromagnetic wave may interact with the electron beam more efficiently. In this way, the amplified and oscillated electromagnetic wave may be output to the electromagnetic wave output port  135  via the second waveguide  134 . 
     In addition to the electron beam tunnel  131 , as illustrated in  FIG. 4A , the electromagnetic wave oscillator  100  may further include an additional auxiliary tunnel  139  formed to perforate the first waveguide  130  in the axial direction. For example, the auxiliary tunnel  139  may be formed to perforate a lower part of the barrier rib  137  on the bottom of the first waveguide  130 . Thus, the auxiliary tunnel  139  may be disposed parallel to the electron beam tunnel  131 . In one embodiment, the electron beam may be provided only to the electron beam tunnel  131 , but the electron beam may also be provided to the auxiliary tunnel  139 . The function and effect of the auxiliary tunnel  139  are described below in more detail. 
     In  FIG. 4A , the first waveguide  130  having the folded structure includes a plurality of coupled cavities  138 . However, a waveguide having a general folded structure that does not include a coupled cavity may also be used.  FIG. 4B  is a partial enlarged view of the portion A of the lower structure  110 , which schematically illustrates the structure of the first waveguide  130 , according to another exemplary embodiment. Referring to  FIG. 4B , the first waveguide  130  has a structure that is repeatedly folded a plurality of times, but a coupled cavity is not formed in each folded portion. Thus, the barrier rib  137  may have a uniform thickness. In addition, as illustrated in  FIG. 4B , the folded portion of the first waveguide  130  may be rounded. 
       FIG. 5A  is a schematic cross-sectional view of a B-B′ cross-section of the lower structure  110  of  FIG. 4A  in order to illustrate the above-described auxiliary tunnel  139 . 
     Referring to  FIG. 5A , the lower structure  110  may include two substrates including a first substrate  111  and a second substrate  112 . As illustrated in  FIG. 5A , three non-through holes are formed in a top surface of the first substrate  111 . In addition, one non-through hole is formed in the center of the top surface of a second substrate  112 , and one through hole is formed in both sides of the non-through hole, respectively. Two through holes formed in the second substrate  112  may be formed in positions corresponding to two non-through holes formed in both edges of the first substrate  111 . Here, the first substrate  111  and the second substrate  112  may be formed of silicon, for example. In this case, a metal thin layer  113  may be coated on the top surface of the first substrate  111  and inner walls and bottom surfaces of the non-through holes. In addition, the first substrate  111  and the second substrate  112  may be formed of metal having conductivity or another material having conductivity. In this case, the metal thin layer  113  may be omitted. 
     The lower structure  110  may be formed by bonding the upper surface of the first substrate  111  and the lower surface of the second substrate  112  having the above-described structure. The first and second substrates  111  and  112  may be bonded using various methods without special limitations, for example, silicon (Si) direct bonding, oxide layer bonding, eutectic bonding, or thermo-compressive bonding. In order to precisely align the first substrate  111  and the second substrate  112  when the first and second substrates  111  and  112  are bonded to each other, alignment patterns  114  may be pre-formed on the bottom surface of the first substrate  111  and the bottom surface of the second substrate  112 . The lower structure  110  formed by bonding the first substrate  111  and the second substrate  112  in this manner has one first non-through hole  140  having a relatively small depth formed in the center of the top surface of the lower structure  110 , and two second non-through holes  141  each having a relatively large depth formed at both sides of the lower structure  110 . When the upper structure  120  is bonded to the lower structure  110 , the first non-through hole  140  may be the electron beam tunnel  131 , and the second non-through holes  141  each may be the coupled cavity  138 . The lower structure  110  has one auxiliary tunnel  139  formed between the first substrate  111  and the second substrate  112 . 
       FIG. 5B  is a schematic cross-sectional view of the electromagnetic wave oscillator  100  configured by bonding the lower structure  110  of  FIG. 5A  and the upper structure  120  having the same structure as that of the lower structure  110  of  FIG. 5A . Referring to  FIG. 5B , similar to the lower structure  110 , the upper structure  120  may include two substrates, i.e., third and fourth substrates  122  and  121 . The third substrate  121  of the upper structure  120  has the same structure as that of the first substrate  111  of the lower structure  110 , and the only difference between is a mirror symmetry relationship between the third substrate  121  and the first substrate  111 . In addition, the fourth substrate  122  of the upper structure  120  has the same structure as that of the second substrate  112  of the lower structure  110 , and the only difference therebetween is a mirror symmetry relationship between the fourth substrate  122  and the second substrate  112 . 
     As illustrated in  FIG. 5B , the electromagnetic wave oscillator  100  may be configured by bonding the fourth substrate  122  of the upper structure  120  and the second substrate  112  of the lower structure  110 . The electromagnetic wave oscillator  100  may include the electron beam tunnel  131  formed by bonding the first non-through hole  140  of the lower structure  110  and the first non-through hole  140  of the upper structure  120 , and the coupled cavity  138  formed by bonding the second non-through holes  141  of the lower structure  110  and the second non-through holes  141  of the upper structure  120 . In  FIG. 5B , a cross-section of the electron beam tunnel  131  and a cross-section of the coupled cavity  138 , i.e., a cross-section of the first waveguide  130 , are rectangular. However, this is not limiting. For example, the cross-section of the electron beam tunnel  131  and the cross-section of the coupled cavity  138  may be circular, oval, or polygonal as well as rectangular. 
     Since  FIG. 5B  illustrates the B-B′ section taken along an edge of the barrier rib  137 , the coupled cavity  138  is disposed at both sides of the electron beam tunnel  131 , respectively. However, the position of the coupled cavity  138  and the number of coupled cavities  138  may vary according to the position of a cross-section. For example, referring to  FIG. 4A , in the case of a cross-section taken along the center of the first barrier rib  138  that is the closest to the electron beam input port  132 , the coupled cavity  138  may be disposed only at the right side of the electron beam tunnel  131 . In addition, in the case of a cross-section taken along the center of the barrier rib  137  that is disposed the second closest to the electron beam input port  132 , the coupled cavity  138  may be disposed only at the left side of the electron beam tunnel  131 . 
     Likewise, in  FIG. 5A , one non-through hole is formed in the center of the top surface of the first substrate  111  according to the position of a cross-section, and an additional non-through hole may be formed at at least one of the right and left sides of the non-through hole formed in the center. In addition, one non-through hole is formed in the center of the top surface of the second substrate  112  according to the position of the cross-section, and a through hole may be formed in at least one of the right and left sides of the non-through hole. As a result, one first non-through hole  140  having a relatively small depth formed in the center and a second non-through hole  141  having a relatively large depth formed at at least one of the left and right sides of the first non-through hole  140  may be formed in the top surface of the lower structure  110 . 
     As described in  FIG. 4A , the coupled cavity  138  is part of the first waveguide  130  having a folded structure. In addition, the electromagnetic wave oscillator  100  further includes an additional auxiliary tunnel  139  formed on upper and lower portions of the electron beam tunnel  131 , respectively. The auxiliary tunnel  139  is formed between the first substrate  111  and the second substrate  112  and between the second substrate  121  and the fourth substrate  122 , respectively. Although an electron beam may pass through the auxiliary tunnel  139 , the electron beam may be provided only to the electron beam tunnel  139  rather than to the auxiliary tunnel  139 . The auxiliary tunnel  139  may be parallel to the electron beam tunnel  131  in the axial direction.  FIG. 5B  illustrates that the width of the auxiliary tunnel  139  and the width of the electron beam tunnel  131  are the same. However, this is just an example, and the width of the auxiliary tunnel  139  and the width of the electron beam tunnel  131  may vary according to design. In addition, although  FIG. 5B  illustrates the rectangular cross-section of the auxiliary tunnel  139 , aspects of the present invention are not limited thereto. For example, the cross-section of the auxiliary tunnel  139  may be circular, oval, or polygonal as well as rectangular. 
     The auxiliary tunnel  139  may improve resonant characteristics of the electromagnetic wave in the first waveguide  130 , in particular, in the coupled cavity  138 . For example, due to the auxiliary tunnel  139 , the electromagnetic wave that passes through the first waveguide  130  is effectively condensed on the vicinity of the electron beam tunnel  131  through which the electron beam passes so that interaction between the electron beam and the electromagnetic wave may effectively occur. As a result, the intensity of the electromagnetic wave output from the electromagnetic wave oscillator  100  increases, and the band width of the electromagnetic wave may be increased. 
       FIGS. 6 through 10  are graphs showing the effect of the additional auxiliary tunnel  139 . 
     First,  FIG. 6  is a graph showing resonant characteristics of an electromagnetic wave oscillator having only a single electron beam tunnel  131 , and  FIG. 7  is a graph showing resonant characteristics of an electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139 .  FIGS. 6 and 7  illustrate return loss characteristics of the electromagnetic wave that is resonated in the first waveguide  130  and is returned, after electromagnetic waves having various frequencies are input to the electromagnetic wave output port  135  of the electromagnetic wave oscillator. Comparing  FIGS. 6 and 7 , in the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnel  139 , compared to the electromagnetic wave oscillator having only a single electron beam tunnel  131 , a resonant band width may further increase, and stronger resonance occurs in the first waveguide  130 . In addition, in the case of the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139 , compared to having only a single electron beam tunnel  131 , comparatively uniform resonance occurs in a resonant frequency band. 
     In addition,  FIG. 8  is a graph showing comparison of resonant characteristics of the case where the first waveguide  130  is an idealistic resonator having no tunnel and the case where a single electron beam tunnel  131  and two auxiliary tunnels  139  are disposed in the first waveguide  130 .  FIG. 9  is a graph showing comparison of phase characteristics of output electromagnetic waves. Referring to  FIGS. 8 and 9 , the resonant characteristics of the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139  are very similar to the resonant characteristics of the idealistic resonator. 
     Last,  FIG. 10  is a graph showing comparison of the intensity of an output electromagnetic wave of the electromagnetic wave oscillator having only a single electron beam tunnel  131  and the intensity of an output electromagnetic wave of the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139 . Referring to  FIG. 10 , the intensity of the output electromagnetic wave may vary according to a voltage of the electron beam that passes through the electron beam tunnel  131 . In particular, the intensity (indicated by ‘∘’) of the output electromagnetic wave of the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139  is much greater than the intensity (indicated by ‘Δ’) of the output electromagnetic wave of the electromagnetic wave oscillator having only a single electron beam tunnel  131 . For example, when the voltage of the electron beam is about 13.0 kV, the intensity of the output electromagnetic wave of the electromagnetic wave oscillator having only a single electron beam tunnel  131  is about 6 W, whereas the intensity of the output electromagnetic wave of the electromagnetic wave oscillator  100  having a single electron beam tunnel  131  and two auxiliary tunnels  139  may be about 9 W. 
       FIG. 5B  illustrates that one auxiliary tunnel  139  is disposed at upper and lower sides of the electron beam tunnel  131 , respectively. However, the number of auxiliary tunnels  139  and the position of the auxiliary tunnel  139  may be modified according to design.  FIGS. 11A through 11G  are schematic cross-sectional views of various arrangement examples of the auxiliary tunnels  139  disposed around the electron beam tunnel  131 . In  FIGS. 11A through 11G , for convenience of explanation, the metal thin layer  113  and the alignment patterns  114  are omitted. In addition, although in  FIGS. 11A through 11G , for convenience of explanation, the coupled cavity  138  is formed at both sides of the electron beam tunnel  131 , as described above, the coupled cavity  138  may be formed at one of the left and right sides of the electron beam tunnel  131  according to the position of the cross-section. Hereinafter,  FIGS. 11A through 11G  will be described as illustrating the B-B′ section of  FIG. 4A . 
     First, as illustrated in  FIG. 11A , one auxiliary tunnel  139  may be disposed at the right and left sides of the electron beam tunnel  131 , respectively. For example, the auxiliary tunnel  139  may be disposed between the electron beam tunnel  131  and the coupled cavity  138 . In  FIG. 11A , the height of the auxiliary tunnel  139  is the same as the height of the electron beam tunnel  131 . However, the height of the auxiliary tunnel  139  may be different from the height of the electron beam tunnel  131 . In addition, as illustrated in  FIG. 11B , the electromagnetic wave oscillator  100  may include four auxiliary tunnels  139 , of which one is disposed at upper and lower portions and at right and left sides of the electron beam tunnel  131 , respectively. However, the auxiliary tunnel  139  does not need to be disposed symmetrically with respect to the electron beam tunnel  131 . For example, the auxiliary tunnel  139  may be disposed at one of the upper and lower portions of the electron beam tunnel  131 , or at one of the right and left sides of the electron beam tunnel  131 . If the auxiliary tunnel  139  is disposed at one of the upper and lower portions of the electron beam tunnel  131 , the auxiliary tunnel  139  may be formed at only one of the lower structure  110  and the upper structure  120 . In more detail, the auxiliary tunnel  139  may be formed on only one of the first substrate  111  and the third substrate  121 . 
     In addition, referring to  FIG. 11C , a total of four auxiliary tunnels  139  may be disposed, i.e., two auxiliary tunnels  139  may be disposed at the upper portion and two auxiliary tunnels  139  at the lower portion of the electron beam tunnel  131 . In this case, the lower structure  110  and the upper structure  120  of the electromagnetic wave generator  100  each may include three substrates. For example, the lower structure  110  may further include a fifth substrate  115  disposed between the first substrate  111  and the second substrate  112 . One non-through hole is formed in the top surface of the fifth substrate  115 , and one through hole is formed in both sides of the non-through hole, respectively. Two through holes of the fifth substrate  115  may be formed in positions corresponding to two non-through holes formed in both edges of the first substrate  111 . The upper structure  120  may further include a sixth substrate  125  disposed between the third substrate  121  and the fourth substrate  122 . The sixth substrate  125  has the same structure as that of the fifth substrate  115 , and the only difference therebetween is a mirror symmetry relationship between the sixth substrate  125  and the fifth substrate  115 . By bonding the lower structure  110  and the upper structure  120  having the above structure, as described above, each of the electron beam tunnel  131  and the coupled cavity  138  may be disposed. The auxiliary tunnel  139  may be disposed between the first substrate  111  and the fifth substrate  115 , between the fifth substrate  115  and the second substrate  113 , between the third substrate  121  and the sixth substrate  125 , and the sixth substrate  125  and the fourth substrate  122 , respectively. 
     Referring to  FIG. 11D , a total of four auxiliary tunnels  139  may be disposed, i.e., two auxiliary tunnels  139  may be disposed at the right side and two auxiliary tunnels  139  at the left side of the electron beam tunnel  131 . In this case, the lower structure  100  and the upper structure  120  of the electromagnetic wave oscillator  100  each may include two substrates. In addition, as illustrated in  FIG. 11E , the electromagnetic wave oscillator  100  may include a total of eight auxiliary tunnels  139 , i.e., two auxiliary tunnels  139  may be disposed on the upper portion and two auxiliary tunnels  139  on the lower portion, and two auxiliary tunnels  139  may be disposed at the right and left sides of the electron beam tunnel  131 , respectively. In this case, as illustrated in  FIG. 11D , each of the lower structure  110  and the upper structure  120  may include three substrates. 
       FIG. 11F  illustrates the case that two auxiliary tunnels  139  are disposed on the upper and lower portions of the electron beam tunnel  131 , wherein two auxiliary tunnels  139  disposed on the upper portion of the electron beam tunnel  131  are adjacent to each other at the same height, and two auxiliary tunnels  139  disposed on the lower portion of the electron beam tunnel  131  are adjacent to each other at the same height. In  FIG. 11F , the auxiliary tunnels  139  may be formed by etching the surface of the first substrate  111  and the surface of the third substrate  121 . In addition, as illustrated in  FIG. 11G , the auxiliary tunnels  139  may also be formed by etching the bottom surface of the second substrate  112  and the bottom surface of the fourth substrate  122 , i.e., the opposite surface to a surface in which a non-through hole is formed. In  FIG. 11G , the electromagnetic wave oscillator  100  may include auxiliary tunnels  139  formed with different heights at the right and left sides of the electron beam tunnel  131 . As illustrated in  FIGS. 11A through 11F , the auxiliary tunnels  139  may be formed in various positions of the electron beam tunnel  131 , and the lower structure  110  and the upper structure  120  of the electromagnetic wave oscillator  100  may include two, three, or more substrates according to positions and the number of auxiliary tunnels  139 . 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.