Patent ID: 12237811

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

A semiconductor device100according to a first embodiment will be described below. The semiconductor device100generates (emits) or detects terahertz waves (electromagnetic waves in a frequency region of at least 30 GHz and not more than 30 THz) of a frequency (resonance frequency, oscillation frequency) fTHz. Note that an example where the semiconductor device100is used as an oscillator will be described below. The length of each configuration in the stacking direction of the semiconductor device100will be referred to as “thickness” or “height”.

Description Regarding Circuit Configuration of Semiconductor Device

First, a circuit configuration of the semiconductor device100will be described.FIG.1Ais a diagram describing an equivalent circuit of the semiconductor device100.FIG.1Cis a diagram describing an equivalent circuit of bias circuits Vaand Vbthat the semiconductor device100has.

The semiconductor device100has an antenna array where a plurality of antennas are provided. The semiconductor device100has an antenna array where an antenna100aand an antenna100bare provided adjacently in the present embodiment. The antenna100aserves both as a resonator that resonates with terahertz waves, and as a radiator that transmits or receives terahertz waves. The antenna100ainternally has a semiconductor layer115afor generating or detecting terahertz waves (electromagnetic waves). The antenna100bhas the same configuration as the antenna100a. The configuration of the antenna100awill be described in detail below, and detailed description of components of the antenna100bthat are the same as those of the antenna100awill be omitted.

Description will be made regarding the semiconductor device100provided with two antennas in the present embodiment, but the number of antennas may be three or more. For example, the semiconductor device100may have an array where the antennas are arrayed in a 3×3 matrix array. Alternately, the semiconductor device100may have a linear array where three antennas are linearly arrayed in the column or row direction. The semiconductor device100may have a configuration of an antenna array where m×n (m≥2, n≥2) antennas are arrayed in a matrix. These antennas also preferably are laid out at a pitch that is an integer multiple of the wavelength of terahertz waves of a frequency fTHz.

Note that in the following description, alphabet characters indicating the corresponding antenna are appended to the end of the symbols denoting the components belonging to the antenna100aand the antenna100b. More specifically, “a” is appended to the end of symbols denoting components that the antenna100ahas, and “b” is appended to the end of symbols denoting components that the antenna100bhas.

As illustrated inFIG.1A, a semiconductor102a, a resistor Radetermined by antenna radiation and conductor loss, and LC component (capacitor Caand inductance La) determined by structure, are connected in parallel in the antenna100a. Also, a bias circuit Vafor supplying bias signals to the semiconductor102ais connected in parallel to the semiconductor102a.

The semiconductor102ahas electromagnetic-wave gain or carrier nonlinearity (nonlinearity in current in accordance with voltage change in current-voltage characteristics) with regard to terahertz waves. A resonant tunneling diode (RTD), which is a typical semiconductor having electromagnetic-wave gain at the terahertz wave frequency band, is used as the semiconductor102ain the present embodiment. The semiconductor102aincludes a circuit where the negative differential resistance of the RTD and the diode capacitor are connected in parallel (omitted from illustration).

The antenna100bis configured of a circuit where a semiconductor102b, a resistor Rb, LC component (Cband Lb), and bias circuit Vbare connected in parallel, in the same way as with the antenna100a. The antennas singularly transmit or receive terahertz waves of a frequency fTHz.

The bias circuits Vaand Vbhave a power source and stabilization circuit for supplying bias signals to the semiconductor102aof the antenna100aand the semiconductor102bof the antenna100b. The bias circuits Vaand Vbeach have a shunt resistor121, wiring122, a power source123, and a capacitor124, as illustrated inFIG.1C.

The shunt resistor121is connected in parallel with the semiconductors102aand102b. The capacitor124is connected in parallel with the shunt resistor121. The power source123supplies current necessary to drive the semiconductors102aand102b, and adjusts bias signals applied to the semiconductors102aand102b. In a case of using RTDs for the semiconductors102aand102b, the bias signals are selected from voltage in the negative differential resistance region of the RTDs. The shunt resistor121and capacitor124of the bias circuits Vaand Vbsuppress parasitic oscillation of relatively low-frequency resonance frequency (typically a frequency band from DC to 10 GHz) due to the bias circuits Vaand Vb.

The adjacent antenna100aand antenna100bare mutually joined by a coupling line109. The coupling line109is connected to a shunt device130connected in parallel to each of the semiconductors102aand102b. Thus, frequencies other than the operating frequency fTHzof the terahertz waves desired are short-circuited due to the shunt device130being provided, and the semiconductor device100is low impedance at this frequency. This suppresses resonance at a plurality of frequency bands (multimode resonance) from occurring. Note that from the perspective of antenna radiation efficiency, the shunt device130preferably is positioned (connected) to the node of the electric field of terahertz waves of the standing resonance frequency fTHzat the coupling line109. The “position that is the node of the electric field of terahertz waves of the standing resonance frequency fTHzat the coupling line109” here is, for example, a position where the intensity of the electric field of terahertz waves of the standing resonance frequency fTHzat the coupling line109drops by around one digit or so.

A resistor Rcand a capacitor Ccare serially connected at the shunt device130, as illustrated inFIG.1A. A value that is equal to or somewhat lower than the absolute value of the combined negative differential resistance of the semiconductors102aand102bconnected in parallel is selected here for the resistor Rc. Also, the capacitor Ccis set so that the impedance is equal to or somewhat lower than the absolute value of the combined negative differential resistance of the semiconductors102aand102bconnected in parallel. That is to say, the values of the resistor Rc, and the capacitor Ccare each preferably set so that the impedance is lower than the absolute value of the negative resistance (impedance) corresponding to the gain of the semiconductor102aand the semiconductor102b. Considering that the typical value of negative resistance of RTDs used in terahertz bands is 0.1 to 1000Ω, the value of the resistor Rcis set in the range of 0.1 to 1000Ω. Also, the value of the capacitor Ccis typically set in the range of 0.1 to 1000 pF, in order to obtain a shunt effect at the frequency range of 10 GHz to 1000 GHz. Note that it is sufficient for the impedance conditions of the resistor Rcand the capacitor Ccwith regard to the negative resistance of the RTD to be satisfied at a frequency band lower than the resonance frequency fTHz.

Description Regarding Structure of Semiconductor Device

Next, a specific structure of the semiconductor device100will be described.FIG.2Ais a top view of the semiconductor device100.FIG.2Bis a cross-sectional view of the semiconductor device100inFIG.2A, taken along A-A′, andFIG.2Cis a cross-sectional view of the semiconductor device100inFIG.2A, taken along B-B′.

The antenna100aincludes a substrate113, a first conductor layer106, a semiconductor layer115a, an electrode116a, a conductor117a, a dielectric layer104, and a second conductor layer103a. The antenna100ahas a configuration where the two conductors, i.e., the first conductor layer106and the second conductor layer103a, sandwich the dielectric layer104that is made up of the three layers of a first dielectric layer1041, a second dielectric layer1042, and a third dielectric layer1043, as illustrated inFIG.2B. Such a configuration is known as a microstrip antenna. An example will be described in the present embodiment where the antennas100aand100bare used as patch antennas, which are representative microstrip resonators.

The second conductor layer103ais a patch conductor of the antenna100a, and is disposed facing the first conductor layer106across the dielectric layer104(semiconductor layer115a). The second conductor layer103ais electrically connected to the semiconductor layer115a. The antenna100ais designed as a resonator where the width of the second conductor layer103ais λTHz/2 in the A-A′ direction (resonance direction). Also, the first conductor layer106is a ground conductor, and is electrically grounded. Note that λTHzis the effective wavelength of the dielectric layer104of terahertz waves resonating at the antenna100a, and is expressed as λTHz=λ0×εr−1/2, where λ0represents the wavelength of the terahertz waves in a vacuum, and εrrepresents the effective relative dielectric constant of the dielectric layer104.

The semiconductor layer115acorresponds to the semiconductor102ain the equivalent circuit illustrated inFIG.1A, and includes an active layer101aconfigured of a semiconductor that has electromagnetic-wave gain or carrier nonlinearity with respect to terahertz waves. An example of using an RTD as the active layer101awill be described in the present embodiment. In the following, the active layer101awill be described as RTD101a.

The semiconductor layer115ais formed disposed on the first conductor layer106that is formed on the substrate113, with the semiconductor layer115aand the first conductor layer106being electrically connected. Note that a low-resistance connection between the semiconductor layer115aand the first conductor layer106is preferable, to reduce Ohmic loss.

The RTD101ahas a resonant tunneling structure layer that includes a plurality of tunnel-barrier layers. Quantum-well layers are provided between the plurality of tunnel-barrier layers in the RTD101a, thereby being provided with a multiple quantum-well structure that generates terahertz waves by carrier intersubband transition. The RTD101ahas electromagnetic-wave gain in the frequency region of terahertz waves based on the photo-assisted tunneling phenomenon, in the negative differential resistance region of current-voltage characteristics, and exhibits self-excitation oscillation in the negative differential resistance region. The RTD101ais disposed at a position shifted by 40% in the resonance direction (i.e., A-A′ direction) from the center of gravity of the second conductor layer103a.

The antenna100ais an active antenna where the semiconductor layer115aincluding the RTD101aand a patch antenna have been integrated. The frequency fTHzof terahertz waves emitted from the singular antenna100ais determined as a resonance frequency of the entire parallel resonance circuit where the patch antenna and the semiconductor layer115areactance are combined. Specifically, with regard to an equivalent circuit of the oscillator described in “Jpn. J. Appl. Phys., Vol. 47, No. 6 (2008), pp. 4375-4384”, a frequency that satisfies the amplitude condition of Expression (1) and the phase condition of Expression (2) is determined as the resonance frequency fTHzof a resonance circuit where the admittance of the RTD and antenna (YRTDand Yaa) are combined.
Re[YRTD]+Re[Yaa]≤0  (1)
Im[YRTD]+Im[Yaa]=0  (2)

Here, YRTDrepresents the admittance of the semiconductor layer115a, Re represents the real part, and Im represents the imaginary part. The semiconductor layer115aincludes the RTD101athat is a negative resistance element as the active layer, so Re[YRTD] has a negative value. Also, Yaarepresents the entire admittance of the antenna100aas viewed from the semiconductor layer115a. Accordingly, the components Ra, Ca, and Laof the antenna in the equivalent circuit inFIG.1Aare primary circuit elements for Yaa, and the negative differential resistance and the diode capacitor of the semiconductor102aare primary circuit elements for YRTD.

Note that a quantum cascade laser (QCL) structure that has a semiconductor multilayer structure with several hundred to several thousand of layers may be used as another example of the active layer101a. In this case, the semiconductor layer115ais a semiconductor layer including a QCL structure. Also, a negative resistance element such as a Gunn diode or an IMPATT diode often used in milliwave bands may be used as the active layer101a. Also, high-frequency devices such as a transistor with one end terminated may be used as the active layer101a, suitable examples of which include heterojunction bipolar transistors (HBT), compound semiconductor field-effect transistors (FET), high-electron-mobility transistors (HEMT), and so forth. Also, negative differential resistance of Josephson devices that use superconductors may be used as the active layer101a.

The dielectric layer104is formed between the first conductor layer106and the second conductor layer103a. It is demanded of the dielectric layer104to be capable of being formed as a thick film (typically, a thick film of at least 3 μm), to exhibit low loss and low dielectric constant at the terahertz band, and to have good microfabrication characteristics (workability by planarization and etching). In microstrip resonators such as patch antennas, conductor loss is reduced and radiation efficiency can be improved by forming the dielectric layer104thicker. Now, the thicker the dielectric layer104is, the better the radiation efficiency of the resonator is, but multimode resonance occurs if excessively thick. Accordingly, the thickness of the dielectric layer104preferably is in a range of not more than 1/10 of the oscillation wavelength.

Meanwhile, miniaturization and high current density of diodes is required to realize high frequencies and high output of oscillators. Accordingly, measures such as suppressing leak current and migration is demanded of the dielectric layer104, with regard to the insulation structure of the diode. In the present invention, for this reason, the dielectric layer104includes two dielectric layers of different types of materials (first dielectric layer1041and second dielectric layer1042).

For the first dielectric layer1041, organic dielectric materials such as benzocyclobutene (BCB, manufactured by The Dow Chemical Company, εr1=2), polytetrafluoroethylene, polyimide, and so forth, can be suitably used. Note that εr1is the relative dielectric constant of the first dielectric layer1041. Also, inorganic dielectric materials such as tetraethyl orthosilicate (TEOS) oxide films, spin-on glass, or the like, which are suitable for relatively thick film formation and have a low dielectric constant, may be used for the first dielectric layer1041.

It is demanded of the second dielectric layer1042to exhibit insulating properties (nature of behaving as an insulator or a high-value resistor that does not allow electricity to pass under DC voltage), barrier properties (nature of preventing diffusion of metal materials used for electrodes), and workability (nature allowing precise working in submicron order). Inorganic insulator materials, such as silicon oxide (εr2=4), silicon nitride (εr2=7), aluminum oxide, aluminum nitride, and so forth, can be suitably used as the second dielectric layer1042to satisfy these properties. Note that εr2is the relative dielectric constant of the second dielectric layer1042.

The third dielectric layer1043will be described later. Note that in a case where the dielectric layer104is a three-layer structure as in the present embodiment, the relative dielectric constant εrof the dielectric layer104is an effective relative dielectric constant determined from the thickness and relative dielectric constant of each of the first dielectric layer1041through third dielectric layer1043. Also, the dielectric layer104does not have to be a three-layer structure in the semiconductor device100, and may be a structure of one layer only.

Also, from the perspective of impedance matching of antenna and air (space), the difference in the dielectric constant between the antenna and air preferably is maximally small. Accordingly, a different material from the second dielectric layer1042is preferably used for the first dielectric layer1041, preferably a material that has a lower relative dielectric constant than the second dielectric layer1042(εr1<εr2).

The electrode116ais disposed on the opposite side of the semiconductor layer115afrom the side where the first conductor layer106is disposed. The electrode116aand the semiconductor layer115aare electrically connected. The semiconductor layer115aand the electrode116aare embedded in the second dielectric layer1042(second dielectric layer1042and third dielectric layer1043). More specifically, the perimeters of the semiconductor layer115aand the electrode116aare covered by the second dielectric layer1042(second dielectric layer1042and third dielectric layer1043).

The electrode116ais suitable for reduction in Ohmic loss and RC delay due to serial resistance, as long as the electrode116ais a conductor layer in Ohmic connection with the semiconductor layer115a. In a case where the electrode116ais used as an Ohmic electrode, examples of materials suitably used for the electrode116ainclude Ti/Pd/Au, Ti/Pt/Au, AuGe/Ni/Au, TiW, Mo, ErAs, and so forth.

Also, if the region of the semiconductor layer115ain contact with the electrode116ais a semiconductor doped with an impurity at a high concentration, contact resistance can be further reduced, which is suitable for realizing high output and high frequencies. The absolute value of the negative resistance indicating the magnitude of gain of the RTD101aused in the terahertz waveband is generally in the order of 1 to 100Ω, so the loss of electromagnetic waves is preferably kept to not more than 1% thereof. Accordingly, the contact resistance at the Ohmic electrode preferably is suppressed to not more than 1Ω, as a general guide.

Also, in order to operate in the terahertz band, the width of the semiconductor layer115a(≈electrode116a) is around 0.1 to 5 μm as a typical value. Accordingly, the resistivity is kept to not more than 10 Ω·μm2, suppressing the contact resistance to a range of 0.001 to several Ω. As another form, a metal that exhibits Schottky contact rather than Ohmic contact may be used for the electrode116a. In this case, the contact interface between the electrode116aand the semiconductor layer115aexhibits rectifying properties, making for a suitable configuration for the antenna100aas a terahertz wave detector. Note that a configuration where an Ohmic electrode is used as the electrode116awill be described in the present embodiment.

In the stacking direction of the RTD101a, the first conductor layer106, the semiconductor layer115a, the electrode116a, the conductor117a, and the second conductor layer103aare stacked in that order from the substrate113side, as illustrated inFIG.2B.

The conductor117ais formed inside the dielectric layer104, and the second conductor layer103aand the electrode116aare electrically connected via the conductor117a. Now, if the width of the conductor117ais excessively great, radiation efficiency deteriorates due to deterioration in resonance characteristics of the antenna100aand increase parasitic capacitance. Accordingly, the width of the conductor117apreferably is a dimension of a level where there is no interference with the resonance electric field, and typically, not more than λ/10 is suitable. Also, the width of the conductor117acan be reduced to a level where serial resistance is not increased, and can be reduced to around twice the skin depth as a general guide. Accordingly, taking into consideration reduction of the serial resistance to a level around not more than 1Ω, the width of the conductor117atypically is in a range of at least 0.1 μm and not more than 20 μm, as a general guide.

A structure that electrically connects between upper and lower layers, such as the conductor117a, is referred to as a via. The first conductor layer106and the second conductor layer103aserve as, in addition to roles as members making up a patch antenna, electrodes for injecting current to the RTD101aby being connected to the via. A material having resistivity not greater than 1×10−6Ω·m is preferable for the material of the conductor117a. Specifically, metals and metal-containing compounds such as Ag, Au, Cu, W, Ni, Cr, Ti, Al, AuIn alloy, TiN, and so forth are suitably used as materials for the conductor117a.

The second conductor layer103ais connected to lines108a1and108a2. The lines108a1and108a2are leads connected to bias lines including the bias circuit Va. The width of the lines108a1and108a2is set to be narrower than the width of the second conductor layer103a. Note that width here is the width in the resonance direction of electromagnetic waves in the antenna100a(i.e., the A-A′ direction). For example, the width of the lines108a1and108a2suitably is not more than 1/10 of the effective wavelength λ of terahertz waves of the standing resonance frequency fTHzin the antenna100a(not more than λ/10). The reason for this is that the lines108a1and108a2are preferably disposed with dimensions and positions such that do not interfere with the resonance electric field in the antenna100a, from the perspective of improved radiation efficiency.

Also, the positions of the lines108a1and108a2are preferably at nodes of the electric field of the resonance frequency fTHzterahertz waves standing in the antenna100a. The lines108a1and108a2here have a configuration where impedance is sufficiently higher than the absolute value of the negative differential resistance of the RTD101aat the frequency band near the resonance frequency fTHz. In other words, the lines108a1and108a2are connected to the antenna so as to have high impedance as viewed from the RTD at the resonance frequency fTHz. In this case, the antennas of the semiconductor device100(antennas other than the antenna100a) and the antenna100aare isolated at the frequency fTHzwith regard to routes via bias lines including the lines108a1and108a2and the bias circuit Va. This suppresses current of the resonance frequency fTHzinduced at each antenna from acting on (affecting) other adjacent antennas via bias lines. This is a configuration that also suppresses interference between the electric field of the resonance frequency fTHzstanding in the antenna100aand the power supply members thereof.

The bias circuits Vaand Vbeach include the shunt resistor121, the wiring122, the power source123, and the capacitor124. The wiring122invariably has a parasitic inductance component, and accordingly is illustrated as an inductor inFIG.1C. The power source123supplies bias signals necessary for driving the RTD101aand RTD101b. The voltage of the bias signals is typically selected from the voltage of the negative differential resistance region of the RTDs used as the RTDs101aand101b. In the case of the antenna100a, the bias voltage from the bias circuits Vaand Vbis supplied to the RTD101awithin the antenna100avia the lines108a1and108a2.

Now, the shunt resistor121and the capacitor124of the bias circuits Vaand Vbserve to suppress parasitic oscillation of a relatively-low frequency resonance frequency (typically a frequency band from DC to 10 GHz) due to the bias circuits Vaand Vb. A value that is equal to or somewhat lower than the absolute value of the combined negative differential resistance of the RTDs101aand101bconnected in parallel is selected for the value of the shunt resistor121. The capacitor124is also set so that impedance is equal to or somewhat lower than the absolute value of the combined negative differential resistance of the RTDs101aand101bconnected in parallel, in the same way as with the shunt resistor121. That is to say, the bias circuits Vaand Vbare set so that the impedance is lower than the absolute value of the combined negative resistances corresponding to the gain at the DC to 10 GHz frequency band by theses shunt elements. Generally, the capacitor124is preferably greater within the above-described range, and capacitance in the order of tens of pF is used in the present embodiment. The capacitor124is a decoupling capacitor. A metal-insulator-metal (MIM) structure sharing the substrate with the antenna100amay be used, for example.

Description Regarding Antenna Array

The semiconductor device100has an antenna array where the two antennas100aand100bare E-plane coupled. The antennas singularly emit terahertz waves of the frequency fTHz. The adjacent antennas are mutually coupled by the coupling line109, and are mutually injection-locked at the resonance frequency fTHzof terahertz waves.

Now, being mutually injection-locked means that all of a plurality of self-excitation oscillators are oscillating in a frequency-locked state due to mutual interaction. For example, the antenna100aand the antenna100bare mutually coupled by the coupling line109. Note that “mutually coupled” refers to a phenomenon where a current induced at a certain antenna acts upon another adjacent antenna, and changes the transmission/reception characteristics of each other. Locking of mutually-coupled antennas at the same phase or opposite phase causes the electric field between the antennas to be strengthened or weakened by the mutual injection-locking phenomena, whereby increase/decrease in antenna gain can be adjusted.

Oscillation conditions of the semiconductor device100having the antenna array can be determined according to mutual injection locking conditions in a configuration where two or more individual RTD oscillators are coupled, disclosed in “J. Appl. Phys., Vol. 103, 124514 (2008)”. Specifically, oscillation conditions for the antenna array where the antenna100aand the antenna100bare coupled by the coupling line109will be considered. At this time, two oscillation modes of inphase mutual-injection locking and antiphase mutual-injection locking occur. Oscillation conditions for the oscillation mode of inphase mutual-injection locking (even mode) are represented in Expressions (3) and (4), and oscillation conditions for the oscillation mode of antiphase mutual-injection locking (odd mode) are represented in Expressions (5) and (6).

Inphase (even mode): frequency f=feven
Yeven=Yaa+Yab+YRTD
Re(Yeven)≤0  (3)
Im(Yeven)=0  (4)

Antiphase (odd mode): frequency f=fodd
Yodd=Yaa+Yab+YRTD
Re(Yodd)≤0  (5)
Im(Yodd)=0  (6)

Yabis the mutual admittance between the antenna100aand the antenna100bhere. Yabis proportionate to a coupling constant representing the strength of coupling between the antennas, and ideally, the real part of −Yabis large and the imaginary part preferably is zero. The semiconductor device100according to the present embodiment is coupled under conditions of inphase mutual-injection locking, where the resonance frequency fTHz≈feven. Other antennas are also coupled under conditions of the above-described inphase mutual-injection locking by the coupling line109between the antennas, in the same way.

The coupling line109is made up of a microstripline that has a structure where the dielectric layer104is sandwiched between a third conductor layer110stacked on the dielectric layer104, and the first conductor layer106. The antennas are coupled by DC coupling in the semiconductor device100. In order to mutually lock the antennas with each other at the resonance frequency fTHz, the third conductor layer110(the top conductor of the coupling line109that couples the antenna100aand the antenna100b) is directly connected to the second conductor layer103aand the second conductor layer103b. The third conductor layer110and the second conductor layers103aand103bare formed in the same layer in the semiconductor device100.

The antenna100aand the adjacent antenna100bare mutually coupled by the structure having such a coupling line109, and operate mutually locked at the oscillated terahertz wave frequency fTHz. The antenna array locked by DC coupling can lock among adjacent antennas by strong coupling, and accordingly locking operations by frequency locking readily occur, and variance in frequency and phase among the antennas does not readily occur.

The shunt device130is disposed (connected) at the center of the coupling line109in the semiconductor device100. The shunt device130and the coupling line109are connected through a via114. Specifically, the third conductor layer110of the coupling line109and a conductor layer111that connects to the shunt device130are connected through the via114formed inside the first dielectric layer1041. The via114is connected to the third conductor layer110at a node of the high-frequency electric field of the resonance frequency fTHzstanding in the coupling line109. That is to say, it can be said that the shunt device130is connected to the coupling line109at the node of the high-frequency electric field of the resonance frequency fTHzstanding in the coupling line109. The shunt device130connected in this way causes frequencies other than the resonance frequency fTHzof the terahertz waves to be short-circuited, and the semiconductor device100has low impedance at this frequency, whereby occurrence of multimode resonance can be suppressed.

The conductor layer111is an electrode stacked on the second dielectric layer1042and is connected to resistor layers1191and1192stacked on the second dielectric layer1042. The resistor Rcof the shunt device130in the equivalent circuit illustrated inFIG.1Ais formed by the resistor layers1191and1192. The resistor layers1191and1192are connected to conductor layers1121and1122stacked on the second dielectric layer1042. The conductor layers1121and1122are connected to fourth conductor layers1181and1182stacked on the third dielectric layer1043through vias1071and1072. The fourth conductor layers1181and1182are formed in a layer between the first conductor layer106and the second conductor layers103aand103b. Capacitor Ccof the shunt device130in the equivalent circuit illustrated inFIG.1Ais formed by a MIM capacitor structure where the third dielectric layer1043, which is part of the dielectric layer104, is sandwiched between the fourth conductor layers1181and1182and the first conductor layer106.

Also, there is demand for miniaturization of the resistor Rc, to realize integration of the antenna array. Accordingly, thin films of W—Ti (0.2 μm thick), that have a high relative resistivity and high melting point, with resistivity of 0.7 Ω·μm, are used as the resistor layers1191and1192in the present embodiment.

The shunt device130of the semiconductor device100includes two shunt devices. One is a shunt device where a resistor made up of the resistor layer1191, and capacitor where the third dielectric layer1043is sandwiched between the fourth conductor layer1181and the first conductor layer106, are serially connected. The other is a shunt device where a resistor made up of the resistor layer1192, and capacitor where the third dielectric layer1043is sandwiched between the fourth conductor layer1182and the first conductor layer106, are serially connected. Note that the values of the resistor Rcand capacitor Cccan be set to be within the above-described range by appropriately designing the materials, dimensions, and structures of the shunt devices. Now, if the width of the via114is excessively great, deterioration in resonance characteristics of the high-frequency electric field of fTHzpropagated over the coupling line, and deterioration of radiation efficiency due to conductor loss, occur in the same way as with the width of the conductors117aand117b. Accordingly, the width of the via114preferably is a dimension of a level where there is no interference with the resonance electric field, and typically, not more than λ/10 is suitable.

The three dielectric layers of the first dielectric layer1041, the second dielectric layer1042, and the third dielectric layer1043, are used as the dielectric layer104in the semiconductor device100. The third dielectric layer1043is used as the dielectric member for the capacitor Ccof the shunt device130, and accordingly silicon nitride with a relatively high dielectric constant (εr3=7) is used for reduction in size of the MIM capacitor structure. Note that εr3is the relative dielectric constant of the third dielectric layer1043. In a case where the dielectric layer104has a three-layer configuration as in the present embodiment, the effective relative dielectric constant is determined taking into consideration the thickness and relative dielectric constant of the third dielectric layer1043as well.

Description Regarding Comparison with Conventional Semiconductor Device

FIG.3illustrates comparison of results of analyzing the impedance of the semiconductor device100according to the present embodiment, and the impedance of a conventional semiconductor device that does not have the shunt device130. Analysis was performed using HFSS, which is a finite element method high-frequency electromagnetic field solver manufactured by ANSYS, Inc. Impedance Z here is equivalent to the inverse of Yaathat is the admittance of the entire structure of the antenna100a. Also, Z_w/_shunt is impedance Z in a case of having the shunt device130as in the present embodiment. Z_w/o_shunt is impedance Z in a case of not having the shunt device130, as in a conventional arrangement. Re and Im represent the real part and imaginary part, respectively, and resonance occurs at a frequency where the impedance of the imaginary part is 0.

A multipeak occurs in the impedance of the conventional structure, as illustrated inFIG.3, and there is a possibility that resonance modes are occurring at the two frequency bands of near 0.42 THz and near 0.52 THz. In contrast with this, there is only a single peak at the desired resonance frequency fTHz=0.48 THz in the impedance of the semiconductor device100according to the present embodiment, and multimode is suppressed. The semiconductor device100according to the present embodiment has effects of suppressing occurrence of resonance at low frequency bands (not higher than 0.1 THz) outside of the range of the graph illustrated inFIG.3, due to the shunt device130.

The shunt device130is disposed parallel to the RTD at the coupling line109in the semiconductor device100. This suppresses multimode resonance at frequency bands of relatively high frequency (typically from 10 GHz to 1000 GHz), and enables just resonance of operating frequency fTHzof the desired terahertz waves to be selectively stabilized. Also, impedance change due to structure does not readily occur in the semiconductor device100according to the present embodiment in comparison with a configuration where the antennas are serially connected by a resistor, and variance in phase and frequency does not readily occur.

Thus, according to the present embodiment, single-mode operation at the terahertz-wave operating frequency fTHzcan be performed even if the number of antennas in the antenna array is increased. Accordingly, the upper limit of the number of antennas that can be arrayed can be raised, and effects of marked improvement can be anticipated in directionality and frontal intensity in accordance with the increase in the number of antennas in the array. Thus, according to the present embodiment, a semiconductor device that can realize efficient generation or detection of terahertz waves can be provided.

Note that the shunt device is not limited to an arrangement having the resistor Rcand capacitor Ccillustrated inFIG.1A. For example, the shunt device130may be configured from just resistor Rcconnected in parallel to the semiconductors102aand102b, as illustrated inFIG.1B.

First Modification

A semiconductor device200according to a first modification will be described below.FIGS.4A through4Cillustrate the semiconductor device200according to the first modification. The semiconductor device200has an antenna array where two antennas200aand200bare H-plane coupled. The semiconductor device200has an antenna array of a configuration where the antennas are coupled by AC coupling (capacitive coupling). Detailed description of parts of the configurations and structures of the antennas200aand200bthat are the same as those of the antennas100aand100bin the semiconductor device100will be omitted.

A coupling line209is made up of a microstripline that has a structure where a dielectric layer204and a fourth dielectric layer2044are sandwiched between a third conductor layer210and a first conductor layer206, as illustrated inFIG.4B. The dielectric layer204is made up of a first dielectric layer2041, a second dielectric layer2042, and a third dielectric layer2043. Second conductor layers203aand203bare formed in a layer between the third conductor layer210and the first conductor layer206. The third conductor layer210that is the upper conductor of the coupling line209that couples the antennas200aand200boverlaps the second conductor layers203aand203bby a length x=5 μm near the radiating ends as viewed from the stacking direction (in planar view). At this overlapping portion, the second conductor layers203aand203b, the fourth dielectric layer2044, and the third conductor layer210are stacked in that order. Accordingly, a metal-insulator metal (MIM) capacitor structure, where the second conductor layers203aand203band the third conductor layer210sandwich the fourth dielectric layer2044is formed. Note that between the second conductor layer203aand the second conductor layer203bis open regarding DC, and the magnitude of coupling at low-frequency regions lower than the resonance frequency fTHzis small, so inter-device isolation is secured. Meanwhile, the magnitude of inter-antenna coupling at the resonance frequency fTHzband can be adjusted by capacitance.

In the semiconductor device200, shunt devices2301and2302are connected to the coupling line209. The shunt devices2301and2302are connected to the coupling line209through vias2141and2142. The vias2141and2142are connected to the third conductor layer210at a node of the high-frequency electric field of the resonance frequency fTHzstanding in the coupling line209. This enables frequencies other than the resonance frequency fTHzof the terahertz waves to be short-circuited, and accordingly occurrence of multimode resonance can be suppressed.

The third conductor layer210of the coupling line209is connected to resistor layers2191and2192stacked on the first dielectric layer2041through the vias2141and2142formed inside the fourth dielectric layer2044. Also, the resistor layers2191and2192are connected to fourth conductor layers2181and2182stacked on the third dielectric layer2043through vias2071and2072formed inside the first dielectric layer2041.

An MIM capacitor structure where the third dielectric layer2043is sandwiched between the fourth conductor layers2181and2182and the first conductor layer206is formed. Such an AC coupling structure can weaken coupling among antennas, thereby suppressing transmission loss among antennas, and improved radiation efficiency of the antenna array is anticipated. Note that the width of the vias2071and2072may be formed large in some instances, since the vias2071and2072are formed inside the first dielectric layer2041that is relatively thick. However, in a configuration where the vias2071and2072are disposed at a position away from the coupling line209as in the present modification, interference of the antenna with the resonance electric field is suppressed even if the width of the vias2071and2072is large (typically not smaller than λ/10). Accordingly, improved antenna gain can be anticipated.

Second Modification

A semiconductor device300according to a second modification, which is a specific example of a configuration where the shunt device illustrated inFIG.1Bis made up of a resistor alone will be described below.FIGS.5A through5Cillustrate the semiconductor device300according to the second modification. The semiconductor device300has an antenna array where adjacent antennas are connected by a coupling line309(microstripline) disposed between a first conductor layer306(grounding conductor) and second conductor layers303aand303b(patch conductors). Detailed description of the configurations and structures of antennas300aand300bthat are the same as those of the antennas100aand100bin the semiconductor device100will be omitted.

The coupling line309is a microstripline having a structure where a second dielectric layer3042is sandwiched between a third conductor layer310(upper conductor) and the first conductor layer306(grounding conductor), and also serves as a resonator. The antenna300ahas a structure where an RTD301ahas been integrated in a complex resonator made up of a patch antenna (an antenna made up of the first conductor layer306and the second conductor layer303a) and half (the antenna300aside half) of the coupling line309. In the coupling line309, the direction perpendicular to the resonance direction of the antenna (i.e., the A-A′ direction) is the longitudinal direction.

The length of the coupling line309and the size of the patch antennas are important parameters that determine the frequency of electromagnetic waves that each of the antennas300aand300bemit. Specifically, the resonance frequency fTHzof the antenna300acan be determined from the length of the second conductor layer303aand the length of the third conductor layer310in the A-A′ direction. For example, an arrangement where half of the length of the third conductor layer310in the A-A′ direction is an integer multiple of the effective wavelength of the desired resonance frequency, and the length of the second conductor layer303ais ½ of the effective wavelength of the desired resonance frequency, is suitable.

The bias circuits Vaand Vbare connected to lines308aand308bmade up of conductor layers stacked on a first dielectric layer3041. The third conductor layer310is connected to a via317a. The via317aconnects between the second conductor layer303aand the RTD301a.

Adjacent antennas are coupled by DC coupling by the coupling line309. For example, the antennas300aand300bare directly coupled by the third conductor layer310that is the upper conductor of the coupling line309. In order to intensify frequency locking between the antennas, the RTDs301aand301bare preferably disposed at the maximum point of the electric field of electromagnetic waves (resonance frequency fTHz) standing in the coupling line.

Four shunt devices3301through3304are disposed in the coupling line309in the semiconductor device300, as a specific configuration example of the equivalent circuit of resistor Rcalone, illustrated inFIG.1B. For example, in the cross-section of the shunt device3302illustrated inFIG.5C, the third conductor layer310making up the coupling line309is connected to a conductor layer312a2stacked on the second dielectric layer3042, via a resistor319a2. The conductor layer312a2is also connected to the first conductor layer306that is a grounding conductor, through a via307a2formed inside the second dielectric layer3042. The present modification is a simple structure where there is no need for integration of capacitor structures, and accordingly the number of manufacturing steps can be reduced.

First Example

As a first example, a specific configuration of the semiconductor device100according to the first embodiment will be described with reference toFIGS.2A through2C. The semiconductor device100is a semiconductor device that is capable of single-mode oscillation at the 0.45 to 0.50 THz frequency band.

The RTDs101aand101bare configured with a Multiple Quantum Well structure of InGaAs/AlAs lattice-matched on the substrate113formed of InP. RTDs of a double-barrier structure are used in the present example. The semiconductor heterostructure of the RTDs is the structure disclosed in “J Infrared Milli Terahz Waves (2014) 35:425-431”. As for measurement values of current-voltage characteristics of the RTDs101aand101b, the peak current density is 9 mA/m2, and negative differential conductance per unit area is 10 mS/μm2.

A mesa structure made up of the semiconductor layer115aincluding the RTD101a, and the electrode116athat is an Ohmic electrode, is formed in the antenna100a. In the present example, a columnar mesa structure that is 2 μm in diameter is formed. The magnitude of the negative differential resistance of the RTD101ais approximately −30Ω per diode here. In this case, the negative differential conductance (GRTD) of the semiconductor layer115aincluding the RTD101ais estimated to be approximately 30 mS, and the diode capacitance (CRTD) of the RTD101ais estimated to be approximately 10 fF.

The antenna100ais a patch antenna of a structure where the dielectric layer104is sandwiched by the second conductor layer103a(patch conductor) and the first conductor layer106(grounding conductor). The semiconductor layer115athat includes the RTD101ainside is integrated in the antenna100a. The antenna100ais a square patch antenna where one side of the second conductor layer103ais 150 μm, and the resonator length L of the antenna is 150 μm. A metal layer primarily of an Au thin film, of which resistivity is low, is used for the second conductor layer103aand the first conductor layer106.

The dielectric layer104is disposed between the second conductor layer103aand the first conductor layer106. The dielectric layer104is made up of the three layers of the first dielectric layer1041, the second dielectric layer1042, and the third dielectric layer1043. The first dielectric layer1041is formed of benzocyclobutene (BCB, manufactured by The Dow Chemical Company, εr1=2), 5 μm thick. The second dielectric layer1042is formed of SiO2(plasma CVD, εr2=4), 2 μm thick. The third dielectric layer1043is formed of SiNx(plasma CVD, εr3=7), 0.1 μm thick. That is to say, the three dielectric layers that the dielectric layer104includes are each formed (configured) of different materials in the present example.

The first conductor layer106is formed of a Ti/Pd/Au layer (20/20/200 nm), and a semiconductor having an n+-InGaAs layer (100 nm) where electron density is not less than 1×1018cm−3. In the first conductor layer106, the metal and semiconductor are connected by low-resistance Ohmic contact.

The electrode116ais an Ohmic electrode formed of a Ti/Pd/Au layer (20/20/200 nm). The electrode116ais connected by low-resistance Ohmic contact to the semiconductor made of the n+-InGaAs layer (100 nm) where electron density is not less than 1×1018cm−3, formed on the semiconductor layer115a.

The structure of around the RTD101ain the stacking direction is, in order from the substrate113side, the substrate113, the first conductor layer106, the semiconductor layer115aincluding the RTD101a, the electrode116a, the conductor117a, and the second conductor layer103a, stacked in that order and electrically connected to each other. The conductor117ais formed (configured) of a conductor containing Cu (copper).

The RTD101ais disposed at a position shifted by 40% (60 μm) in the resonance direction (i.e., A-A′ direction) from the center of gravity of the second conductor layer103a. The input impedance at the time of supplying high-frequency power from the RTD to the patch antenna is determined by the position of the RTD101ain the antenna100a. The second conductor layer103ais connected to the lines108a1and108a2.

The lines108a1and108a2are formed of metal layers including Ti/Au (5/300 nm) stacked on the first dielectric layer1041, and are connected to the bias circuits Va, Vb. The antenna100ais designed so that oscillation of power of 0.2 mW is obtained at the frequency fTHz=0.48 THz, by setting the bias to the negative resistance region of the RTD101a. The lines108a1and108a2are configured of patterns of metal layers including Ti/Au (5/300 nm), 75 μm in length in the resonance direction (i.e., A-A′ direction) and 10 μm in width. The lines108a1and108a2are connected to the second conductor layer103aat the center of the second conductor layer103ain the resonance direction (i.e., A-A′ direction) and at the end in the B-B′ direction. The connection position corresponds to a node of the electric field of fTHzterahertz waves standing in the antenna100a.

The semiconductor device100has an antenna array where the two antennas100aand100bare arrayed in the electric field direction of radiated electromagnetic waves (i.e., E-plane direction) and mutually coupled. The antennas are designed to singularly emit terahertz waves of the frequency fTHz, and are laid out in the A-A′ direction at a pitch of 340 μm. The adjacent antennas are mutually coupled by the coupling line109including the third conductor layer110configured of Ti/Au (5/300 nm). More specifically, the second conductor layer103aand the second conductor layer103bare connected by the third conductor layer110that is 5 μm in width and 190 μm in length. The antenna100aand the antenna100bare mutually injection-locked and oscillate at the resonance frequency fTHz=0.48 THz, in a state with the phases matching each other (inphase).

In the semiconductor device100, the shunt device130is disposed at the center of the coupling line109. The shunt device130and the coupling line109are connected through the via114. Specifically, the third conductor layer110of the coupling line109, and the conductor layer111, configured of Ti/Au (5/300 nm), are connected through the via114formed of Cu inside the first dielectric layer1041.

The via114is a columnar structure 10 μm in diameter and 5 μm in height. The conductor layer111is connected to the resistor layers1191and1192formed of W—Ti (0.2 μm thick) with resistivity of 0.7 Ω·μm. The resistor layers1191and1192here are designed to be 20Ω each, and worked to patterns 4 μm in width and 20 μm in length.

The resistor layers1191and1192are connected to the fourth conductor layers1181and1182through the conductor layers1121and1122and the vias1071and1072. The conductor layers1121and1122and the fourth conductor layers1181and1182are formed (configured) of Ti/Au (5/300 nm). The vias1071and1072are formed of Cu. The vias1071and1072are columnar structures 10 μm in diameter, and 2 μm in height.

The shunt device130where the resistor Rcand capacitor Ccare serially connected is formed by the MIM capacitor structure where the third dielectric layer1043is sandwiched between the fourth conductor layers1181and1182and the first conductor layer106, and the resistor layers1191and1192.

The third dielectric layer1043is formed (configured) of silicon nitride (εr3=7), 0.1 μm thick. The fourth conductor layers1181and1182are formed as rectangular patterns 50 μm in width and 60 μm in length. Capacitance of 2 pF is formed for each MIM structure. Connecting the shunt device130to the coupling line109as in the semiconductor device100suppresses multimode resonance at frequency bands of relatively high frequency, and enables just the operating frequency fTHzof the desired terahertz waves to be selected in a stable manner. Note that the frequency band of the relatively high frequency typically is 10 GHz to 1000 GHz.

Supply of electric power to the semiconductor device100is performed from the bias circuits Vaand Vb, and bias voltage of negative differential resistance region is normally applied to supply bias current. In the case of the semiconductor device100disclosed in the present example, radiation of 0.4 mW terahertz electromagnetic waves is obtained at the frequency of 0.48 THz by oscillation operations in the negative resistance region.

In this way, according to the present example, loss of electromagnetic waves can be reduced as compared with conventional arrangements, and more efficient emission or detection of terahertz waves can be realized.

Manufacturing Method for Semiconductor Device

Next, a manufacturing method for the semiconductor device100according to the present example will be described. The semiconductor device100is manufactured (fabricated) as follows.(1) An InGaAs/AlAs semiconductor multilayer structure is epitaxially grown on the InP substrate113, thereby forming the semiconductor layers115aand115bincluding the RTDs101aand101b. Molecular-beam epitaxy (MBE) or metalorganic vapor-phase epitaxy (MOVPE) is used for the epitaxial growth.(2) Film formation of a Ti/Pd/Au layer (20/20/200 nm) is performed on the semiconductor layers115aand115bby sputtering, thereby forming the electrodes116aand116b.(3) The electrodes116aand116band the semiconductor layers115aand115bare formed into circular mesa forms 2 μm in diameter, thereby forming mesa structures. The mesa forms are formed using photolithography and dry etching by inductively-coupled plasma (ICP).(4) After the first conductor layer106is formed on the substrate113by the lift-off process being performed on the etched face, a film of silicon nitride, 0.1 μm thick, is formed by plasma CVD, thereby forming the third dielectric layer1043.(5) A Ti/Au layer (5/300 nm) making up the fourth conductor layers1181and1182is formed on the third dielectric layer1043. Thus, capacitor Ccwhere the third dielectric layer1043is sandwiched between the fourth conductor layers1181and1182and the first conductor layer106is formed.(6) A film of silicon oxide, 2 μm thick, is formed by plasma CVD, thereby forming the second dielectric layer1042.(7) The second dielectric layer1042is dry-etched and via holes are formed. Once the via holes are formed, the via holes are filled in with Cu and planarized, using sputtering, electroplating, and chemical-mechanical polishing, thereby forming the vias1071and1072.(8) The resistor layers1191and1192of W—Ti (0.2 μm thick) on the second dielectric layer1042are formed by sputtering and dry etching. Thus, the shunt device130where the capacitor Ccand resistor Rcare serially connected is formed.(9) The conductor layers111,1121, and1122, of a Ti/Au layer (5/300 nm) on the second dielectric layer1042, are formed by sputtering and dry etching.(10) Filling in with BCB and planarization are performed using spin coating and dry etching, thereby forming the first dielectric layer1041, 5 μm thick.(11) The BCB and silicon oxide of the portions making up the conductors117aand117band the via114are removed by photolithography and dry etching, forming via holes.(12) The via holes are filled in with a conductor containing Cu, thereby forming the conductors117aand117band the via114. Formation of the conductors117aand117band the via114is performed using sputtering, electroplating, and chemical-mechanical polishing, thereby filling the via holes with Cu and planarizing.(13) A film for an electrode Ti/Au layer (5/300 nm) is formed by sputtering, thereby forming the second conductor layers103aand103band the third conductor layer110.(14) Photolithography and dry etching are performed by inductively-coupled plasma (ICP), thereby patterning the second conductor layers103aand103band the third conductor layer110. This forms the coupling line109.(15) The shunt resistor121and MIM capacitor124are formed inside the chip, and the shunt resistor121and MIM capacitor124are connected to the wiring122and the power source123by wire bonding or the like. Thus, the semiconductor device100is completed.

A preferred embodiment and example of the present invention has been described above, but the present invention is not limited to the embodiment and example, and various modifications and alterations may be made without departing from the spirit and scope thereof. For example, a case where the carriers are electrons is assumed in the description of the embodiment and example made above, but this is not limiting, and an arrangement may be made where holes are used. Materials for the substrate and dielectric members can be selected in accordance with usage, and semiconductors such as silicon, gallium arsenide, indium arsenide, gallium phosphide, and so forth, glass, ceramics, and resins such as polytetrafluoroethylene, polyethylene terephthalate, and so forth, can be used. Note that the structures and materials in the embodiment and example described above can be selected as appropriate in accordance with the desired frequency and so forth.

Further, in the above-described embodiment and example, a square patch antenna is used as the terahertz-wave resonator. However, the shape of the resonator is not limited to this, and a resonator of a structure using a patch conductor that is polygonal such as rectangular or triangular or the like, circular, elliptical, and so forth, for example, can be used.

The number of negative differential resistance devices to be integrated in the semiconductor device is not limited to one, and a resonator may be made that has a plurality of negative differential resistance devices. The number of lines is not limited to one, and a configuration may be made where a plurality of lines are provided.

Also, a double-barrier RTD made of InGaAs/AlAs grown on an InP substrate has been described above for the RTD. However, these structures and materials are not limiting, and combinations of other structures and materials may be made. For example, an RTD having a triple-barrier quantum well structure, or an RTD having a multiple-barrier quantum well structure of fourfold or more, may be used.

Also, each of the following combinations may be used as RTD materials.GaAs/AlGaAs and GaAs/AlAs, InGaAs/GaAs/AlAs formed on a GaAs substrateInGaAs/InAlAs, InGaAs/AlAs, InGaAs/AlGaAsSb formed on an InP substrateInAs/AlAsSb and InAs/AlSb formed on an InAs substrateSiGe/SiGe formed on a Si substrate

According to the present technology, more efficient generation or detection of terahertz waves can be realized in a device provided with a plurality of antennas.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2019-173084, filed on Sep. 24, 2019, which is hereby incorporated by reference herein in its entirety.