Patent Application
20070228355
(1) Field of the Invention
The present invention relates to a terahertz (THz) wave radiating device that radiates electromagnetic wave in the terahertz range.
(2) Description of the Related Art
In recent years, electromagnetic wave in the terahertz frequency range (hereinafter referred to as “THz wave”) has attracted considerable attention. Electromagnetic wave radiations in the range from 0.1 to 10 THz have straight beam traveling characteristics (i.e., focusing ability) and transparency through various objects such as ceramic, plastic, paper, etc. Owing to these characteristics, THz wave is applied in various fields such as spectroscopy, biomedicine, security and imaging. Against this backdrop, with the view to finding further new applications as well as in consideration of increased security reasons and drugs trafficking problems, development of compact and portable high power THz wave radiation sources is actively underway (for example, see Japanese Laid-Open Patent Application No. 2004-172177).
A high DC voltage is applied to the electrode 13 and the electrode 14 from a DC bias supply 15. Then, high peak power subpicosecond pulsed laser is irradiated in the gap (2 to 1000 μm) between the electrode 13 and the electrode 14, which is the space between the salient portion of the electrode 13 and the salient portion of the electrode 14, to generate an electrostatic discharge, and THz wave is generated as a result. In this way, a compact THz wave radiating device is realized.
However, the conventional THz wave radiating device has the following problems: the bandwidth of THz wave to be generated is limited since the gap between the electrodes is uniform; the THz wave radiation power is limited since the break down voltage of the GaAs device is low; and current leakage occurs if voltage is forcedly increased.
In view of the above problems, it is an object of the present invention to provide a THz wave radiating device and a fabrication method thereof, the THz wave radiating device being current leak proof to stand for a high voltage and achieving an increased efficiency of THz wave radiation. Another object of the present invention is to provide a THz wave radiating device and a fabrication method thereof, the THz wave radiating device being capable of radiating broadband THz wave, being mass-producible and highly reliable, and being capable of low-cost fabrication.
In order to achieve the above objects, the THz wave radiating device according to the present invention is (a) a THz wave radiating device that radiates electromagnetic wave in a THz range, the device including: (a1) a first electrode layer; (a2) a second electrode layer which forms a pair with the first electrode layer; (a3) an electrical insulating layer (i) which is positioned between the first electrode layer and the second electrode layer, and (ii) in which an aperture is formed and a vacuum is produced, the aperture passing through the electrical insulating layer in a direction in which the electrical insulating layer was laminated; and (a4) a photoelectron emission layer (i) which is positioned between the electrical insulating layer and the second electrode layer, (ii) which touches the electrical insulating layer, and (iii) which emits photoelectrons to the aperture.
With the above structure, current leakage is unlikely to occur even when a high voltage is applied to the first electrode layer (anode electrode layer) and the second electrode layer (cathode electrode layer). This is attributable to having the electrical insulating layer between the first electrode layer (anode electrode layer) and the second electrode layer (cathode electrode layer). As a result, it is possible to apply a high voltage to the THz wave radiating device and thus to improve the efficiency of THz wave radiation.
Furthermore, (b) the THz wave radiating device may include: (b1) a first wafer in which the first electrode layer is formed; and (b2) a second wafer in which the second electrode layer and the photoelectron emission layer are formed, (b3) wherein the electrical insulating layer is sandwiched between the first wafer and the second wafer in such a manner that a surface of the second wafer on which the photoelectron emission layer is formed is opposite to a surface of the first wafer on which the first electrode layer is formed.
With the above structure, it is possible to fabricate the THz wave radiating device through semiconductor fabrication processes, and thus to provide a THz wave radiating device which is mass-producible and highly reliable, and which is capable of low-cost fabrication.
Furthermore, (c) a V groove may be formed on a surface of the second wafer opposite to a surface on which the second electrode layer is formed, and the photoelectron emission layer is formed on an outer surface of the V groove.
With the above structure, it is possible to control the frequency of THz wave to be radiated from the THz wave radiating device depending on the distance from the first electrode layer (anode electrode layer) formed in the first wafer (anode wafer) to the photoelectron emission layer formed on the V groove. In other words, the frequency of THz wave is represented by a function of the gap between the first electrode layer (anode electrode layer) and the photoelectron emission layer. Thus, the THz wave radiating device, in which the gap between the first electrode layer (anode electrode layer) and the photoelectron emission layer is variable depending on portion, is capable of radiating THz wave including various frequency components and thus radiating broadband THz wave.
More specifically, THz wave radiated from the THz wave radiating device can be precisely controlled by the range from few μm to few mm, depending on the depth of the V groove and the thickness of the electrical insulating layer formed in the second wafer (cathode wafer). The depth of the V groove can be controlled by performing semiconductor micro-processes such as photolithography and etching. This allows precise control of the positions of the first electrode layer (anode electrode layer) and the second electrode layer (cathode electrode layer) as well as the gap between them, and thus to allow the THz wave radiating device to radiate THz wave with a desired spectral bandwidth.
Alternatively, (d) two V grooves may be formed on the surface of the first wafer on which the first electrode layer is formed, and the first electrode layer is formed over the two V grooves.
With the above structure, the electrical field is more focused on the first electrode layer (anode electrode layer) than on the electrical insulating layer, since the tip of the first electrode layer (anode electrode layer) is positioned immediately above the V groove. As a result, even if a high voltage is applied, current leakage is unlikely to occur in the electrical insulating layer, and thus it is possible to achieve high power THz wave radiations.
Alternatively, (e) the photoelectron emission layer may be λ/4n in thickness, where λ is a wavelength of incident light irradiated on the photoelectron emission layer, and n is a refractive index of the photoelectron emission layer.
With the above structure, the reflection of incident light (femtosecond pulsed laser) is suppressed, and thus the loss in the amount of laser light to be irradiated decreases and the operating efficiency is improved.
Alternatively, (f) an outer surface of the photoelectron emission layer may have periodic irregularities.
With the above structure, periodic variations are produced in the refractive index of the photoelectron emission layer and the refractive index of the outer environment (e.g., vacuum). Such a periodic variation in a refractive index is known as photonic crystal. In other words, an appropriately designed photonic crystal can absorb maximum incident light (femtosecond pulsed laser) with least reflection. Furthermore, the periodic irregularities on the outer surface of the photoelectron emission layer allow its area to be virtually doubled compared with the actual area. As a result, it is possible to best convert incident light (femtosecond pulsed laser) into THz wave.
Alternatively, (g) the photoelectron emission layer may be made of one of the following carbon nanostructures: carbon nanotube, carbon nanowall, and carbon nanofiber.
Moreover, (h) an effective length of the carbon nanotube may range from 50 nm to 50 μm.
With the above structure, since the carbon nanotube has very fine tip, it can provide a point like THz wave radiation source to enhance the imaging resolution. Thus, the use of carbon nanotube as the photoelectron emission layer allows the THz wave radiating device to be fabricated at low cost, allowing its practical use. Moreover, the THz wave radiating device will provide an efficient field emission and photoelectron emission to result in high power THz wave radiations, and thus it contributes to new applications of THz wave.
Alternatively, (i) one of the first electrode layer and the second electrode layer may be made of a material having transparency to incident light.
Furthermore, (j) the material may include Indium Tin Oxide (ITO).
With the above structure, the use of ITO for the first electrode layer (anode electrode layer) makes it easier for incident light (femtosecond pulsed laser) to be irradiated over the outer surface of the photoelectron emission layer. This structure results in the reduction in the size of the THz wave radiating device as well as in an easier fabrication of the same.
Alternatively, (k) one of the first electrode layer and the second electrode layer may be in a mesh form.
Furthermore, (l) a pitch size of the mesh may range from 10 μm to 300 μm.
With the above structure, the second electrode layer (cathode electrode layer)/first electrode layer (anode electrode layer) is capable of controlling the phase of THz wave and serving as a filter.
Alternatively, (m) a groove may be formed on a surface of the second wafer opposite to a surface on which the second electrode layer is formed, and a chip may be assembled with the groove, the chip including the photoelectron emission layer thereon.
Furthermore, (n) a V groove may be formed on a surface of the chip opposite to a surface which touches the groove formed in the second wafer, and the photoelectron emission layer may be formed on an outer surface of the V groove.
With the above structure, by using SiC only for the chip and using a Si substrate for the cathode wafer except for the chip portion, it is possible to fabricate the THz wave radiating device at low cost, compared with the case where SiC is used for the second wafer (cathode wafer).
Alternatively, a notch may be formed in the THz wave radiating device in a direction in which the electromagnetic wave is irradiated.
With the above structure, it is possible for the THz wave radiating device to radiate directional THz wave.
It should be noted that the present invention can be embodied not only as a THz wave radiating device, but also as: a method for fabricating a THz wave radiating device; a THz wave radiating system in which the THz wave radiating device is embedded; a method for driving the THz wave radiating device; and the like.
As described above, the present invention will enable the mass production of THz wave radiating devices with photoelectron emission layer at low cost.
Further Information about Technical Background to this Application
The disclosure of Japanese Patent Application No. 2005-352563 filed on Dec. 6, 2005 including specification, drawings and claims is incorporated herein by reference in its entirety.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the Drawings:
A preferred embodiment of the present invention will hereinafter be described with reference to the attached drawings.
The THz wave radiating device according to the present invention is (a) a THz wave radiating device that radiates electromagnetic wave in a THz range, the device including: (a1) a first electrode layer; (a2) a second electrode layer which forms a pair with the first electrode layer; (a3) an electrical insulating layer (i) which is positioned between the first electrode layer and the second electrode layer, and (ii) in which an aperture is formed and a vacuum is produced, the aperture passing through the electrical insulating layer in a direction in which the electrical insulating layer was laminated; and (a4) a photoelectron emission layer (i) which is positioned between the electrical insulating layer and the second electrode layer, (ii) which touches the electrical insulating layer, and (iii) which emits photoelectrons to the aperture.
Furthermore, (b) the THz wave radiating device includes: (b1) a first wafer in which the first electrode layer is formed; and (b2) a second wafer in which the second electrode layer and the photoelectron emission layer are formed, (b3) wherein the electrical insulating layer is sandwiched between the first wafer and the second wafer in such a manner that a surface of the second wafer on which the photoelectron emission layer is formed is opposite to a surface of the first wafer on which the first electrode layer is formed.
The THz wave radiating device of the present embodiment will be described based on the above-described features.
The anode electrode 102 is a wafer in which an anode electrode layer 121 is formed. The anode electrode layer 121 is an electrode layer formed by depositing, onto the anode wafer 102, a material such as Indium Tin Oxide (ITO) having transparency through ultra-short pulse light such as femtosecond laser. This anode electrode layer 121 is connected to the bias supply 156.
The use of ITO for the anode electrode layer 121 makes it easier for laser light to be irradiated over the outer surface of the photoelectron emission layer 113. This structure results in the reduction in the size of the THz wave radiating device 100 and makes it easier to fabricate the THz wave radiating device 100.
Furthermore, in the cathode wafer 101, an electrical insulating layer 112 is formed on the surface opposite to the surface on which the cathode electrode layer 111 is formed, and an aperture is formed that passes through the electrical insulating layer 112 in the direction in which the electrical insulating layer 112 was laminated. The electrical insulating layer 112 is formed on the cathode wafer 101 using a material such as silicon dioxide (SiO2) and silicon nitride (Si3N4). Furthermore, a V groove is formed in the cathode wafer 101 in accordance with the position of the aperture, and a photoelectron emission layer 113 is formed on the outer surface of this V groove. Here, as an example, the photoelectron emission layer 113 is assumed to be a layer with a carbon nanostructure formed on the outer surface of the V groove formed in the chip 114 made of silicon carbide (SiC). The carbon nanostructure includes carbon nanotube, carbon nanowall, and carbon nanofiber. Of these, the carbon nanotube layer can be grown on the SiC wafer by means of Chemical Vapor Deposition (CVD). The carbon nanotube layer can also be grown by annealing the SiC wafer for about an hours at high temperature in the range from 1200° C. to 2000° C. in a partially oxygen containing medium in the chamber. For example, a flat surface SiC wafer with a small size, say 10 mm×10 mm, is annealed at 1200° C. to 2000° C. for an hour to grow over 50 nm-thick carbon nanotube layer containing a variety of multiwall carbon nanotubes with their top end generally closed.
Note that, when λ denotes the wavelength of the laser and n denotes the refractive index of the photoelectron emission layer 113, the thickness of the photoelectron emission layer 113 is represented as λ/4n. This is because an optical film with the thickness of λ/4n serves in general as antireflection coating. As a result, laser light reflection is suppressed, and thus the loss in the amount of laser light to be irradiated decreases and the operating efficiency is improved.
However, in order for the electric field to be focused on the carbon nanotube, the length-diameter aspect ratio of the carbon nanotube is preferably 10 to 1, or greater. However, when the aspect ratio exceeds 100-times greater, the carbon nanotube is likely to fall.
The pulse generator 151 generates pulses to be output to the laser driver 152 and the power supply driver 155. The pulse generator 151 outputs the generated pulses to the laser driver 152 and the power supply driver 155.
The laser driver 152 drives the laser device 153 according to the pulse output from the pulse generator 151.
The laser device 153 outputs a high peak power femtosecond pulsed laser upon being driven by the laser driver 152. As an example, a femtosecond pulsed laser with the wavelength of 250 nm to 1600 nm is used, which is based on the work function of a carbon nanostructure such as carbon nanotube.
The femtosecond pulsed laser output from the laser device 153 is focused by the lens 154.
The power supply driver 155 drives the bias supply 156 according to the pulse output from the pulse generator 151.
The bias supply 156 supplies a high DC bias voltage to the cathode electrode layer 111 and the anode electrode layer 121 upon being driven by the power supply driver 155.
Then, the THz wave radiating device 100 that has been completed by wafer-bonding the cathode wafer 101 and the anode wafer 102 are sealed into the container 161 in which the vacuum pressure is set to 4E-6 Torr or lower, thereby allowing the space between the anode electrode layer 121 and the photoelectron emission layer 113 to be under vacuum.
Note that the use of fluid enables the chip 114 to be gently assembled with the groove, without damaging the outer surface of the photoelectron emission layer 113. Furthermore, since the use of fluid enables the chip 114 to be assembled in a self-aligned manner, there is no need for an assembling device with high accuracy. As a result, a high-quality THz wave radiating device 100 is realized. The method of assembling the chip using fluid is described, for example, in B. P. Singh, K. Onozawa, K. Yamanaka, T. Tojo, and D. Ueda: IEEE/LEOS OPTICAL MEMS 2004 Int. Conf. on Optical MEMS and Their Applications, Takamatsu, 2004, Japan, pp. 176-177.
Note that other than being a Si wafer, the cathode wafer 101 may also be an n-type 6H—SiC wafer, a semi-insulating 6H—SiC wafer, an n-type 4H—SiC wafer, and other poly type SiC wafers. Furthermore, the V groove may be formed directly in the cathode wafer 101, such that the photoelectron emission layer 113 is formed on the V groove. The cathode wafer 101 may be replaced by a SiC wafer with an off-angle. In this case, the off-angle is set so that the angle of the inclined surfaces of the V groove with respect to the outer surface of the cathode wafer 101 is 45 degree. Meanwhile, in the case where off-angle is not employed, the angle of the inclined surfaces of the V groove with respect to the outer surface of the cathode wafer 101 is usually 54.7 degree. Note that other than being a Si wafer, the anode wafer 102 may also be an n-type 6H—SiC wafer, a semi-insulating 6H—SiC wafer, an n-type 4H—SiC wafer, and other poly type SiC wafers.
The anode wafer 102 may also be a SiC wafer with an off-angle. As described above, the THz wave radiating device 100 of the present embodiment is capable of controlling the frequency band of THz wave it irradiates, depending on the distance from the anode electrode layer 121 formed in the anode wafer 102 to the photoelectron emission layer 113 formed on the V groove. In other words, since the frequency band of THz wave is represented by a function of the gap between the photoelectron emission layer 113 and the anode electrode layer 121, and when such gap is not uniform, the THz wave radiating device 100 is capable of radiating THz wave including a variety of frequency components and is thus capable of radiating broadband electromagnetic wave.
More specifically, the band of THz wave radiated from the THz wave radiating device 100 can be precisely controlled by the range from few μm to few mm, depending on the depth of the V groove and the thickness of the electrical insulating layer 112 formed in the cathode wafer 101. The depth of the V groove can be controlled by performing a semiconductor micro-process such as photolithography and etching. This allows precise control of the positions of the anode electrode layer 121 and the cathode electrode layer 111 as well as the gap between them, and thus to allow the THz wave radiating device 100 to radiate THz wave with a desired spectral bandwidth.
Furthermore, since the carbon nanotube has very fine tip, it can provide a point like THz wave radiation source to enhance the imaging resolution. Thus, the use of carbon nanotube as the photoelectron emission layer 113 allows the THz wave radiating device 100 to be fabricated at low cost. Moreover, the THz wave radiating device 100 will provide an efficient field emission and photoelectron emission to result in high power THz wave radiations, and thus it contributes to new applications of THz wave.
Note that when forming a carbon nanostructure (the photoelectron emission layer 113) on the V groove, SiC deposited on a carbon board by means of CVD may be used instead of a SiC wafer.
The anode wafer 202 includes: two V grooves formed on the surface on which an anode electrode layer 221 is formed; and the anode electrode layer 221 formed over these two V grooves. Here, the anode electrode layer 221 may be formed so that the tip of the anode electrode layer 221 is positioned immediately above the V groove formed in the cathode wafer 101.
This structure enables the electrical field to be focused more on the anode electrode layer 221 than on the electrical insulating layer 112, since the tip of the anode electrode layer 221 is positioned immediately above the V groove. As a result, even if a high voltage is applied, current leakage is unlikely to occur in the electrical insulating layer, and thus it is possible to achieve high power THz wave radiations.
The periodic irregularities on the outer surface of the photoelectron emission layer 513 produce periodic variations in the refractive index of the photoelectron emission layer 513 and the refractive index of the outer environment (e.g., vacuum). Such a periodic variation in a refractive index is known as photonic crystal.
In other words, if properly designed, photonic crystal can absorb maximum laser power with least reflection. Furthermore, the periodic irregularities on the outer surface of the photoelectron emission layer 513 allow its area to be virtually doubled compared with the actual area. As a result, it is possible to best convert the laser light irradiated on the photoelectron emission layer into THz wave.
The cathode electrode layer/anode electrode layer with the is above structure is capable of controlling the phase of THz wave and serving as a filter.
Although only an exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.
The present invention is applicable for use as a THz wave radiating device that radiates electromagnetic wave in the THz range, and particularly as a THz wave radiating device that provides improved efficiency of THz wave radiation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-352563 | Dec 2005 | JP | national |
