TERAHERTZ OSCILLATOR AND PRODUCING METHOD THEREOF

Abstract

An object of the present invention is to provide a terahertz oscillator that does not have an MIM capacitor structure of which producing is intricacy, and oscillates due to resonance of an RTD and stabilizing resistors. The present invention is a terahertz oscillator, wherein a slot antenna having a slot is formed between a first electrode plate and a second electrode plate which are applied a bias voltage, stabilizing resistors to respectively connect to the first electrode plate and the second electrode plate are provided in the slot, an RTD is provided on the second electrode plate through a mesa, and a conductive material member to form an air bridge between the first electrode plate and the mesa is provided, and wherein an oscillation in a terahertz frequency band is obtained due to a resonance of the RTD and the stabilizing resistors.

Claims

1-8. (canceled)

9. A terahertz oscillator, wherein a slot antenna having a slot is formed between a first electrode plate and a second electrode plate which are applied a bias voltage, stabilizing resistors to respectively connect to said first electrode plate and said second electrode plate are provided in said slot, a resonant tunneling diode (RTD) is provided on said second electrode plate through a mesa, and a conductive material member to form an air bridge between said first electrode plate and said mesa is provided, and wherein an oscillation in a terahertz frequency band is obtained due to a resonance of said RTD and said stabilizing resistors.

10. The terahertz oscillator according to claim 9, wherein said stabilizing resistor are two and are symmetrically connected to said slot antenna.

11. The terahertz oscillator according to claim 9, wherein a width of said conductive material member is narrower than a width of said RTD for said slot antenna.

12. The terahertz oscillator according to claim 10, wherein a width of said conductive material member is narrower than a width of said RTD for said slot antenna.

13. The terahertz oscillator according to claim 9, wherein said RTD is provided on an n.sup.+-InP etch stopper of said first electrode plate.

14. The terahertz oscillator according to claim 10, wherein said RTD is provided on an n.sup.+-InP etch stopper of said first electrode plate.

15. A producing method of a terahertz oscillator, comprising steps of: preparing a substrate structure having four layers including a resonant tunneling diode (RTD) layer of a top layer, an etch stopper layer of a next layer and an n.sup.+-InGaAs layer; forming a mesa at an RTD portion on said RTD layer by a first exposure; forming an RTD by removing said RTD layer by a first wet etching; forming an air bridge and stabilizing resistors by removing an etch stopper of said etch stopper layer by a second wet etching; layering photoresist on whole surface; forming a slot antenna by performing a second exposure by using a photomask; removing said photoresist and said n.sup.+-InGaAs layer by a third wet etching; and producing a terahertz oscillator to oscillate due to a resonance of said RTD and said stabilizing resistors.

16. The producing method of the terahertz oscillator according to claim 15, wherein said first wet etching and said third wet etching are respectively “H.sub.2SO.sub.4:H.sub.2O.sub.2:H.sub.2O = 1:1:40” at 4° C.

17. The producing method of the terahertz oscillator according to claim 15, wherein said second wet etching is “HCl:H.sub.2O = 1:5” at 4° C.

18. The producing method of the terahertz oscillator according to claim 16, wherein said second wet etching is “HCl:H.sub.2O = 1:5” at 4° C.

19. The producing method of the terahertz oscillator according to claim 15, wherein when said air bridge is formed by said second wet etching, said removing of said etch stopper is completed.

20. The producing method of the terahertz oscillator according to claim 16, wherein when said air bridge is formed by said second wet etching, said removing of said etch stopper is completed.

21. The producing method of the terahertz oscillator according to claim 17, wherein when said air bridge is formed by said second wet etching, said removing of said etch stopper is completed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the accompanying drawings:

[0024] FIG. 1 is a perspective structure diagram showing one example of a conventional terahertz oscillator using a resonant tunneling diode (RTD);

[0025] FIG. 2 is a schematic view showing an example of an output behavior of the conventional terahertz oscillator using the resonant tunneling diode;

[0026] FIG. 3 is a characteristic diagram showing a characteristic example of the resonant tunneling diode;

[0027] FIG. 4 is an equivalent circuit diagram of the conventional terahertz oscillator using the resonant tunneling diode;

[0028] FIG. 5 is an equivalent circuit diagram of the conventional terahertz oscillator, including a bias circuit, using the resonant tunneling diode;

[0029] FIGS. 6A and 6B are equivalent circuit diagrams showing circuits of the high frequency and the low frequency, respectively;

[0030] FIGS. 7A and 7B are a perspective diagram and a sectional structure diagram showing a part of producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0031] FIGS. 8A and 8B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0032] FIGS. 9A and 9B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0033] FIGS. 10A and 10B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0034] FIGS. 11A and 11B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0035] FIGS. 12A and 12B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0036] FIGS. 13A and 13B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0037] FIGS. 14A and 14B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0038] FIGS. 15A and 15B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0039] FIGS. 16A and 16B are a perspective diagram and a sectional structure diagram showing a part of the producing processes of the conventional terahertz oscillator using the resonant tunneling diode, respectively;

[0040] FIG. 17 is a perspective view showing a configuration example of a terahertz oscillator according to the present invention;

[0041] FIG. 18 is a three-dimensional perspective view showing a configuration example of the terahertz oscillator according to the present invention;

[0042] FIG. 19 is a cross-sectional view showing a cross-sectional structure example of the terahertz oscillator taken along an X-X′ line in FIG. 17;

[0043] FIG. 20 is a cross-sectional view showing a cross-sectional structure example of the terahertz oscillator taken along a Y-Y' line in FIG. 17;

[0044] FIG. 21 is a plane structure view and an equivalent circuit diagram of the terahertz oscillator according to the present invention;

[0045] FIG. 22 is a plane view showing the produced terahertz oscillator according to the present invention;

[0046] FIGS. 23A and 23B are equivalent circuit diagrams in the high frequency and the low frequency, respectively:

[0047] FIG. 24 is a characteristic diagram showing an oscillation result of the terahertz oscillator according to the present invention;

[0048] FIGS. 25A to 25F are perspective diagrams and sectional structure diagrams showing producing processes of the terahertz oscillator according to the present invention;

[0049] FIGS. 26A and 26B are plane views explaining the etch stopper removing (forming of the air bridge); and

[0050] FIGS. 27A and 27B are array structure views showing expansion examples to large scale array of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0051] The present invention is a terahertz oscillator using a double barrier type resonant tunneling diode (RTD) and is a structure having no an MIM (Metal Insulator Metal) capacitor structure constituted by “metal (conductive material)/insulator/metal (conductive material)”. Since there is no provided the MIM capacitor structure, the structure is simple, and the steps of the producing process can be significantly reduced than the conventional producing steps. Further, since it is possible to obtain the oscillation in the terahertz frequency band due to the resonance of the RTD and the stabilizing resistors, it is especially desired to the application for an imaging field, a high speed communication and so on.

[0052] Embodiments of the present invention will be described with reference to the accompanying drawings as follows.

[0053] FIG. 17 is a perspective view of a terahertz oscillator 100 according to the present invention, and FIG. 18 is a three-dimensional perspective structure view. Further, FIG. 19 is a cross-sectional structure view taken along an X-X' line in FIG. 17, FIG. 20 is a cross-sectional structure view taken along a Y-Y' line in FIG. 17 and FIG. 22 is a plane view showing the actually produced terahertz oscillator. An end surface of an electrode plate 105 facing to a slot portion surrounded by stabilizing resistors 103 and 104 is a slot antenna 102 (about 12 [.Math.m]) , and a rectangular conductive material member 108 is provided on a central portion of the slot antenna 102. An RTD 120 is connected to a tip portion of the conductive material member 108 through a mesa 121, and a portion below the conductive material member 108 is an air bridge structure to form the slot portion.

[0054] The terahertz oscillator 100 according to the present invention does not have the MIM capacitor structure, and comprises the electrode plate 105 which is connected to a bias pad 105A being grounded and a square shape electrode plate 106 which is connected to a bias pad 106A to apply a DC bias Vb. The slot is formed between the end surface of the electrode plate 105 and an opposite end surface of the electrode plate 106, and the end surface of the electrode plate 106 facing to the slot is the slot antenna 102. Further, the electrode plates 105 and 106 are respectively connected by the stabilizing resistors 103 and 104 of both sides. A square recess in a plane view is disposed on a portion opposite to the slot antenna 102 of the electrode plate 105, and a resonant tunneling diode (RTD) 101 is provided in the recess. The conductive material member 108 is suspended between the RTD 101 and the electrode plate 106, and the slot is the air bridge structure as shown in FIGS. 19 and 20.

[0055] The RTD 140 is a doublebarrier of , for example, AlAs/InGaAs, which may be constituted by the layers: an “n+InGaAs” layer (5×10.sup.19 [cm.sup.-3], 100 [nm]) /spacer an “InGaAs” layer (undoped, 12 [nm]) /barrier an “AlAs” layer (undoped, 0.9 [nm] ) /well an “InGaAs” layer (undoped, 3 [nm])/barrier an “AlAs” layer (undoped, 0.9 [nm]) /spacer an “InAlGaAs” layer (undoped, 2 [nm]) /an “n-InAlGaAs” layer (3×10.sup.18 [cm.sup.-3], 25 [nm]) /an “n+InGaAs” layer (5×10.sup.19 [cm.sup.-3], 400 [nm]), from the top to the bottom.

[0056] The equivalent circuit of the terahertz oscillator 100 is shown in FIG. 21. The bias voltage Vb from the bias circuit 107 is applied to the RTD 101 through an inductance L.sub.W of the lines and an inductance L.sub.S of the orbital current of the slot antenna 102, and the stabilizing resistors 103 and 104 (a total resistance value R.sub.S) are connected to the RTD 101 in parallel. In the low frequency band, since the inductances L.sub.W and L.sub.S may be presumed as the short circuits and the resistance value R.sub.S of the stabilizing resistors 103 and 104 cancels the negative differential resistance (NDR) of the RTD 101, the equivalent circuit becomes FIG. 23A.

[0057] On the contrary, in the high frequency band as the terahertz , the impedance of the inductance L.sub.W becomes great and then the bias circuit is cut away, and further the equivalent circuit as shown in FIG. 23A is obtained byconverting the series connected resistance R.sub.S and the inductance L.sub.S into the parallel connection. However, since the loss G of the resistance R.sub.S of the stabilizing resistors 103 and 104 becomes small, the resonance of the inductance L.sub.S and the capacitance in the RTD 101 is occurred and oscillates. That is, assuming that the oscillation frequency of the oscillator is “f”, an angular frequency ω is defined by “ω=2πf”. The loss G of the resistance R.sub.S is expressed by a following Expression 3.

[00003]$G=\frac{R_s}{R_s{}^2+{\omega }^2{L}_S{}^2}$

[0058] Since the resistance value R.sub.S is a small value (few Ω), “R.sub.S.sup.2” in the Expression 3 becomes almost zero. Accordingly, the Expression 3 can be approximated by a following Expression 4.

[00004]$G\fallingdotseq \frac{R_s}{{\omega }^2{L}_S{}^2}$

[0059] In the above Expression 4, since a square “ω.sup.2” of the angular frequency ω is a great value in the terahertz frequency, the Expression 4 becomes almost zero and the loss G of the resistance R.sub.S can be ignored. In this connection, the resonance of the inductance L.sub.S and the capacitance in the RTD 101 is occurred and oscillates.

[0060] FIG. 24 shows an oscillation characteristic (frequency/intensity) of a prototype product (FIG. 22) according to the present invention, and the oscillation of about 0.2 [THz] is obtained.

[0061] FIGS. 25A to 25F show the producing processes of the terahertz oscillator 100 according to the present invention.

[0062] First, the substrate structure 110 comprising four layers which are the resonant tunneling diode (RTD) layer 114 of the top, the etch stopper layer (n.sup.+-InP) 113, the n.sup.+-InGaAs layer 112 and the Sl-InP substrate 111 of the bottom, is prepared as shown in FIG. 25A. Then, the upper electrode 121 is formed on the RTD portion on the RTD layer 114 by the resist patterning with the exposure and the deposition, and the RTD mesa 120 is formed by removing a part of the RTD layer 114 with the wet etching as shown in FIG. 25B. At a time of the RTD mesa formation with the wet etching, a side etch proceeds for the RTD mesa 120 existing under the conductive material member 121 as shown in FIG. 26. That is, although the size of the RTD 120 immediately after the wet etching is started is a distance d1 from an end surface of the mesa 121 as shown in FIG. 26A, all of the RTD 120 existing under the portion 108 in which the conductive material member is thin is finally removed as shown in FIG. 26B when the wet etching further is continued. Thereby, the air bridge is formed, and the size of the RTD 120 becomes a distance d2 (>d1) from the end surface of the mesa 121.

[0063] Since the size of the RTD 120 influences to the inner capacitance, the wet etching may be stopped when the air bridge is formed. On the contrary, by continuing the wet etching, the size of the RTD 120 may allow to be smaller. The width of the portion 108 in which the conductive material member is thin, is double of the distance d2 or less.

[0064] Next, the etch stopper of the etch stopper layer 113 is removed by the wet etching. Then, the photo resist 130 is layered on the whole surface as shown in FIG. 25D, and the resist pattern to protect the portions becoming to the electrode portion and the stabilizing resistors 104 and 105 except for the air bridge is formed by the exposure with the photomask. Further, the n.sup.+-InGaAs layer 112 is removed by the wet etching as shown in FIG. 25E. By removing the photo resist 130, the terahertz oscillator to oscillate due to the resonance of the RTD 120 and the stabilizing resistors 104 and 105 is produced as shown in FIG. 25F.

INDUSTRIAL APPLICABILITY

[0065] By using the minute devices according to the present invention, a compact chip, which measures absorption spectra of a material whose absorption spectra are existed in the terahertz frequency band and a light source chip for the terahertz imaging, can be easily produced. It is considered that the terahertz oscillator according to the present invention enables to facilitate a further development in the fields such as the chemistry, the medical regions and the security. Further, the terahertz oscillator according to the present invention has extensibility to large scale array as shown in FIGS. 27A and 27B, and FIG. 27A shows an example of a case having oscillating element spacing and FIG. 27B does an example of the high-density array.

[0066] Explanation of Reference Numerals [0067] 1 resonant tunneling diode (RTD) [0068] 2 slot antenna [0069] 3 substrate [0070] 4, 41 lower electrode [0071] 5, 40 upper electrode [0072] 7 MIM [0073] 10 wafer [0074] 20 mesa [0075] 30 RTD [0076] 100 terahertz oscillator [0077] 101 resonant tunneling diode (RTD) [0078] 102 slot antenna [0079] 103, 104 stabilizing resistor [0080] 108 conductive material member [0081] 110 substrate structure [0082] 120 RTD mesa [0083] 121 mesa [0084] 130 photoresist