JP7530285B2 - Semiconductor device and design method - Google Patents

Description

This disclosure relates to a semiconductor device that is a packaging component for a photodiode.

[Definition]
In this specification, the frequency band of 300 GHz or more and approximately 10 THz or less is referred to as the "terahertz (THz) frequency band" or "THz band," and electromagnetic waves in this frequency band are referred to as "THz waves."

In recent years, research and development into THz wave wireless communication has progressed, but various other applications, including THz wave imaging, have also attracted attention. This is because THz waves have characteristics such as shorter wavelengths than microwaves, improved spatial resolution while maintaining moderate transparency, and the ability to identify chemical substances by observing their fingerprint spectra.

Here, the use of a THz generating device (photomixer) using a photodiode is known to observe the spectrum of the THz frequency band (see, for example, Non-Patent Document 1). This is because THz generation in a photomixer utilizes the photoelectric conversion of an optical beat signal consisting of two wavelengths, and therefore it is possible to generate THz waves over a wide frequency range. Modules with an output frequency range that covers the entire range of one waveguide band are already on the market. Of course, if you are only looking for a high output level of THz waves, a method using a frequency multiplier may be superior, but photomixers are an indispensable technology in terms of the wide frequency band.

Figure 13 is a diagram explaining the configuration of the photomixer module 300. The photomixer module 300 includes a fiber input system 51, a lens optical system 52, a photodiode chip 13, a light introduction mechanism thereto (a reflecting mirror 53 and a table 55), an electrical output system (wires 58a, a transmission line 58b, a waveguide coupler 58c), and a metal housing 57. A rectangular waveguide 59 and a THz wave output port 60 are formed by the shape of the metal housing 57.

In the example of the photomixer module 300, the light beam that passes through the lens optical system 52 is focused on the light receiving portion of the photodiode chip 13 in the form of "reverse light incidence" via the reflecting mirror 53. The THz signal generated by the photodiode chip 13 propagates through the transmission line of the chip 13, and through the wiring pad on the chip 13, to the transmission line 58b on the quartz substrate, and further to the waveguide coupler 58c, and is emitted as an electromagnetic wave from the rectangular waveguide 59. This electromagnetic wave propagates through the rectangular waveguide 59 and is output from the output port 60.

JP 2006-108556 A

Wakatsuki et al., "Development of a Terahertz Photomixer Module Using UTC-PD," NTT Technical Journal, December 2011

As the frequency of the signals being handled increases, the following problem becomes apparent: The THz signal generates unwanted electromagnetic wave propagation modes in association with discontinuities in the transmission line, which create a resonant condition within the photodiode chip. This results in the problem of "frequency ripples occurring in the THz output from the main transmission line."

One of the causes of the occurrence of this unwanted electromagnetic wave propagation mode is as follows. In principle, the pattern size and substrate thickness within the photodiode chip should be appropriately scaled down in response to an increase in the signal frequency. However, in reality, this is not the case. For example, in InP, λ/4 at 300 GHz is approximately 80 μm. However, the size of a photodiode chip is usually around 300 to 600 μm, and the substrate thickness and the pad size when the signal and ground are combined are larger than λ/4. For example, it may be possible to solve the above problem by producing a photodiode chip on the order of 100 μm, but a new issue arises in that the miniaturization makes it difficult to mount the photodiode chip.

On the other hand, there is a method of placing radio wave absorbers around the photodiode chip to suppress the phenomenon of the generation of unwanted electromagnetic wave propagation modes. However, this method requires high precision in the placement of the radio wave absorbers, and due to variations during manufacturing, it is not always possible to achieve a sufficient effect.

In addition, in order to suppress the phenomenon of the occurrence of unwanted electromagnetic wave propagation modes, a structure is known in which a semiconductor amplifier chip is mounted directly on a radio wave absorber in frequency bands of approximately 100 GHz or less (see, for example, Patent Document 1). It is possible to apply this structure to a THz band photomixer module and mount a photodiode chip directly on the radio wave absorber to suppress the effects of unwanted electromagnetic wave propagation modes.

However, this cannot be applied as is to THz-band photodiode structures that operate at 300 GHz or higher, because in the case of photodiodes, it is necessary to ensure an input path for the optical signal light beam.

In the case of a typical back-illumination type photodiode chip, the radio wave absorber needs to be a material that absorbs THz electromagnetic waves but transmits light. However, at present, it has not been established what parameters give the material radio wave absorption properties.
Even if such a material were selected, it is expected that the variation in thickness of the interface adhesive layer (dielectric layer) between the back surface of the chip and the radio wave absorber will cause variation in the transmitted light power due to the interference fringe effect. In the actual mounting process, it is difficult to strictly control the thickness of the dielectric layer to within one tenth of the optical wavelength.

In other words, there is a problem that there is no established material or technical method for finding parameters to control the radio wave absorption characteristics of radio wave absorbers that absorb THz electromagnetic waves but transmit light, and that it is difficult to reduce the variation in transmitted light power due to manufacturing variations in the thickness of the interfacial adhesive layer (dielectric layer) between the back surface of the chip and the radio wave absorber, in order to suppress the phenomenon of frequency ripples occurring in the radio waves output from photomixer modules that generate radio waves in the THz band.

The present invention aims to solve the above problems by providing a semiconductor device and a method for designing the same that can avoid the problems related to the transmission of incident optical power through a radio wave absorber, reduce unnecessary electromagnetic wave propagation modes, and reduce the variation in transmitted optical power.

To achieve the above objective, the semiconductor device of the present invention ensures a path for light to pass through and reduces the desired electromagnetic wave propagation mode by adjusting specific parameters of the radio wave absorber (resistor).

Specifically, the semiconductor device according to the present invention is a semiconductor device including a photodiode chip and a table-shaped resistor,
the photodiode chip has a light receiving portion and a transmission line for transmitting a terahertz electrical signal;
the resistor has a mounting surface on which the photodiode chip is mounted so that the light receiving portion faces the resistor, a reflecting surface that reflects external light to the light receiving portion, and a radio wave absorbing function that reduces an electromagnetic wave propagation mode generated in the photodiode chip by the electrical signal,
The radio wave absorption function is characterized in that it is adjusted by the electrical resistivity of the material of the resistor, which is determined by the frequency of the electrical signal, and the thickness of the resistor between the mounting surface and the back surface facing the mounting surface.

In this semiconductor device, the resistor is shaped so that light does not pass through the resistor. Since light does not pass through the resistor, this semiconductor device can avoid problems related to the transmission of optical power through a radio wave absorber. In other words, since light does not pass through the interfacial adhesive layer (dielectric layer) between the back surface of the chip and the resistor, this semiconductor device can reduce the variation in transmitted optical power associated with the interference fringe effect. In addition, specific parameters (resistor thickness and electrical resistivity) of this semiconductor device are adjusted so that the resistor can absorb the desired electromagnetic wave propagation mode with high efficiency.

Here, the specific parameters are determined as follows.
The design method according to the present invention comprises the steps of:
Calculating the wavelength of the electromagnetic wave propagation mode from the frequency (operating center frequency: fc) of the electrical signal;
determining a thickness (D) of the resistor based on the wavelength of the electromagnetic wave propagation mode;
In a model in which a material having the same thickness as the resistor is connected to a material having the same thickness as the resistor in the thickness direction, the conductivity of the material having the same thickness as the resistor is changed, and a resonance point at which an electromagnetic wave resonates within the material having the resistor from the material having the same thickness as the photodiode chip to the material having the resistor is found.
If the resonance point deviates from the frequency (operating center frequency: fc) of the electrical signal, the thickness (D) of the resistor is adjusted and the process of finding the resonance point is repeated; and the conductivity of the same material as the resistor at the resonance point is determined as the conductivity of the resistor.

The following is an example of how to determine the thickness of the resistor.
When the rear surface of the resistor is air, the thickness of the resistor can be set to x/2 (x is an integer of 1 or more) of the wavelength of the electromagnetic wave propagation mode.
On the other hand, when the back surface of the resistor is made of metal (lower metal surface), the thickness of the resistor can be set to y/4 (y is an odd number equal to or greater than 1) of the wavelength of the electromagnetic wave propagation mode.

Therefore, the present invention can provide a semiconductor device and a design method thereof that can avoid variations in the transmitted optical power of the incident optical power and reduce unnecessary electromagnetic wave propagation modes.

The resistor may, for example, have the following shape:
The resistor is characterized in that, when the resistor is viewed from the mounting surface side, the reflecting surface is outside the area connecting the vertices of the four corners of the mounting surface, or the reflecting surface is inside the area connecting the vertices of the four corners of the mounting surface.

Moreover, the semiconductor device is
an upper metal surface covering an area connecting the vertices of the four corners of the mounting surface of the resistor other than the portion on which the photodiode chip is mounted;
a lower metal surface covering the back surface of the resistor;
Further equipped with
The resistor may have a through hole formed therein, the through hole connecting the upper metal surface and the lower metal surface.

The above inventions can be combined as much as possible.

The present invention provides a semiconductor device and a design method thereof that can avoid variations in the transmitted optical power of the incident optical power and reduce unnecessary electromagnetic wave propagation modes.

1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1A to 1C are diagrams illustrating a resistor of a semiconductor device according to the present invention. 1 is a flowchart illustrating a design method according to the present invention. FIG. 1 is an image diagram illustrating a design method according to the present invention. FIG. 1 is an image diagram illustrating a design method according to the present invention. 1 is a result of simulating THz reflection of a semiconductor device according to the present invention. 1 is a result of simulating THz reflection of a semiconductor device according to the present invention. 1 is a result of simulating THz reflection of a semiconductor device according to the present invention. FIG. 2 is a diagram illustrating a photomixer module. FIG. 2 is a diagram illustrating a photomixer module.

The following describes an embodiment of the present invention with reference to the attached drawings. The embodiment described below is an example of the present invention, and the present invention is not limited to the following embodiment. Note that components with the same reference numerals in this specification and drawings are considered to be identical to each other.

(Embodiment 1)
14 is a diagram illustrating a photomixer module 301 according to an embodiment of the present invention. The photomixer module 301 includes a semiconductor device 101 including a photodiode chip 13 and a table-shaped resistor 11.

1 and 2 are a perspective view and a side view, respectively, illustrating a semiconductor device 101. The semiconductor device 101 includes a resistor 11 and a photodiode chip 13.
The photodiode chip 13 has a light receiving portion and a transmission line for transmitting a terahertz electrical signal.
The resistor 11 has a mounting surface 14 on which the photodiode chip 13 is mounted so that the light receiving portion faces toward the resistor 11, a reflective surface 12 that reflects light L from the outside to the light receiving portion, and a radio wave absorption function that reduces the electromagnetic wave propagation mode generated in the photodiode chip 13 by the electrical signal.
The radio wave absorption function is characterized in that it is adjusted by the electrical resistivity of the material of the resistor 11, which is determined by the frequency of the electrical signal, and the thickness D between the mounting surface 14 of the resistor 11 and the opposing back surface 15.

In each drawing, "D" is the thickness of resistor 11, and "W" is the width of resistor 11 (width of back surface 15). Photodiode chip 13 is fixed onto mounting surface 14 of resistor 11 with a fluid adhesive. In order to ensure stable adhesion, it is desirable to make width W approximately the same as the width in the X direction of photodiode chip 13. Thickness D will be described later.

The resistor 11 functions as a radio wave absorber. The material of the resistor 11 is, for example, silicon (Si). Si has the characteristics of having sufficient mechanical strength, good thermal conductivity, and being easy to process. Here, Si is described as a radio wave absorber, but other semiconductor materials (for example, GsAs, SiC, etc.) can also be used as long as they have "mechanical strength, thermal conductivity, and ease of processing."

In this embodiment, the resistor 11 is a rectangular prism with a trapezoidal base. As shown in Figures 1 and 2, the resistor 11 has a mounting surface 14, a reflecting surface 12, and a back surface 15. The photodiode chip 13 is positioned so that the light receiving portion is not covered by the mounting surface 14, and is fixed to the mounting surface 14 with a non-conductive epoxy adhesive or the like. In other words, the light receiving portion of the photodiode chip 13 is located above the reflecting surface 12 (z direction).

The light L is reflected by the reflecting surface 12 and directly enters the light receiving portion of the photodiode chip 13. In other words, the semiconductor device 101 is a "backside light incidence" type. The back side of the photodiode chip 13 is coated with a normal anti-reflective coating. It is preferable that the reflecting surface 12 is a dielectric multilayer mirror rather than a metal.

The reflecting surface 12 can be formed by the following methods (1) to (3).
(1) The edge of a flat Si wafer is mechanically polished at an angle.
(2) A silicon wafer with a mirror-polished surface is diced in an oblique direction to the vertical to cut out the wafer.
(3) Utilizing the (111) facet of a Si crystal, which is naturally formed by chemical etching.
In either method, a dielectric multilayer highly reflective film is finally formed on the reflecting surface 12 .

Here, as shown in Figures 1 and 2, when the resistor 11 is viewed from the mounting surface 14 side, the reflecting surface 12 is outside the area connecting the vertices of the four corners of the mounting surface 14.

When "the reflecting surface 12 is arranged on the outer periphery of the mounting surface 14" as in this embodiment, the position of the light receiving part of the photodiode chip must be closer to the light input side (-X side) of the chip. However, there is no major inconvenience in arranging the light receiving part in this position. This is because there is no constraint on the configuration of the output circuit, as in the arrangement of a typical THz photomixer implemented in a waveguide, as shown in Figure 14.

In other words, the light beam L introduced almost parallel to the surface of the photodiode chip 13 is reflected by the reflecting surface 12 and introduced to a light receiving section placed at an appropriate position from the back surface of the photodiode chip 13. The photodiode chip 13 then outputs an electrical signal from the right side to the wire 58a.

[How to determine the parameters of resistor 11]
The main parameters of the resistor 11 are the thickness D and the conductivity σ. As illustrated in FIG. 7, these parameters can be determined by a simple method.
The method is a method for designing a semiconductor device 101,
Calculating the wavelength (λc) of the electromagnetic wave propagation mode from the frequency (operating center frequency: fc) of the electrical signal (step S01);
Setting the initial value of the thickness (D) of the resistor 11 to λc/2 (when the back surface is air) or λc/4 (when the back surface is metal) (step S02);
In a model in which a material 43 identical to the photodiode chip 13 is connected in the thickness direction to a material 41 having the same thickness as the resistor 11, the conductivity (α) of the material 41 is changed to find a resonance point at which the electromagnetic wave from the material 43 to the material 41 resonates within the material 41 (step S03); if the resonance point deviates from the frequency (operating center frequency: fc) of the electrical signal ("No" in step S04), the thickness (D) of the resistor is adjusted (step S05), and the process of finding the resonance point in step S03 is repeated; and if the resonance point substantially coincides with the frequency (operating center frequency: fc) of the electrical signal ("Yes" in step S04), the conductivity of the material 41 at the resonance point is determined to be the conductivity of the resistor 11, and a set of the thickness (D) and conductivity (α) of the resistor is determined (step 06).
It is characterized by:

In this embodiment, an example of determining parameters at the resonance point of the THz wave incident from the material (e.g., InP) of the photodiode chip 13 to the material (e.g., Si) of the resistor 11 will be described. Figures 8 and 9 are diagrams for explaining the state of resonance (electromagnetic wave propagation mode) of the THz wave in the resistor 11 (same material 41 as the resistor). Figure 8 explains the case where the back surface of the resistor 11 (same material 41 as the resistor) is air, and Figure 9 explains the case where the back surface of the resistor 11 (same material 41 as the resistor) is metal. Specifically, Figures 8 and 9 are schematic diagrams showing the state of propagation of the THz wave in a structure in which the medium of the photodiode is InP and the resistor is Si. The smaller the THz reflection to the InP side, the greater the radio wave absorption power in the resistor 11, which means that the resonance in the InP is suppressed.

(Step S01)
The wavelength λ (the aforementioned wavelength λc) of the THz wave (electromagnetic wave propagation mode) propagating within the resistor 11 is calculated from the frequency ν (the aforementioned operating center frequency fc) of the electrical signal (the THz wave output from the photomixer module 301) generated in the photodiode chip 13 using the following equation.
λ=c/ν
Here, n is the refractive index of the material of the resistor 11.

(Step S02)
The thickness D of the resistor 11 is determined based on the wavelength λ when the electrical conductivity is set to zero. An example of the determination method will be described.
When the back surface of the resistor 11 is air (see FIG. 8),
Let D = λ x / 2 (x is an integer of 1 or more).
When the back surface of the resistor 11 is made of metal (see FIG. 9),
Let D = λ x y/4 (y is an odd number equal to or greater than 1).
In general, x=1 and y=1 are preferable in terms of bandwidth and characteristics.
The initial value of D above is for the case where the refractive index of the PD substrate material (typically InP) is greater than that of the resistor (typically Si or SiC), and if the refractive indices are in an inverse relationship, x/2 (x is an integer of 1 or more) and y/4 (y is an odd number of 1 or more) will give D in a mutually interchangeable relationship. This is because the phases of the reflected waves at the interface between the PD and the resistor are different by 180°.

(Step S03)
Consider a model in which a material 41, which is made of the same material as the resistor 11 and has the same thickness as the thickness D determined as the initial value of the calculation in step S02, and a material 43, which is the same as the photodiode chip 13, are connected in the thickness direction. In this model, the conductivity σ of the material 41 is changed to find a resonance point at which the electromagnetic wave 48 from the material 43 to the material 41 resonates within the material 41. A method for finding the resonance point will be described with reference to Figs. 10 and 11.

10 and 11 show frequency characteristics of THz reflection obtained by the above-mentioned method. 10 and 11 show frequency characteristics of reflectance calculated for the case without back metal (FIG. 8) and the case with back metal (FIG. 9), respectively. Here, operation at 300 GHz is assumed. For the frequency characteristics, the InP side (material 43 side) is a wide continuous medium, and the initial values of D are x=1 and y=1 according to the above steps. As described later, the resonance point can be easily adjusted to a desired frequency by fine-tuning the thickness D of the Si layer (material 41). In the case of FIG. 8, d _Si =170 μm, and in the case of FIG. 9, d _Si =94 μm, and the calculations are performed with the conductivity σ _Si as a parameter. In both cases, it can be seen that there is a clear resonance structure, and the minimum THz reflection (maximum THz absorption) is achieved at a certain value depending on σ _Si and frequency f. For example, in the case of Fig. 10, when the conductivity σ _Si = 100 (Ω·m) ^-1 , the band that ensures -20 dB THz reflection, centered at 300 GHz, is sufficiently wide at about 90 GHz. In other words, it is possible to find the conductivity value that minimizes the THz reflection (σ _Si = 100 (Ω·m) ^-1 in the case of Fig. 10, σ _Si = 160 (Ω·m) ^-1 in the case of Fig. 11) from the frequency characteristics in Fig. 10 and Fig. 11.

Incidentally, the 1/4 wavelength in silicon, which has low conductivity, is 72 μm at 300 GHz. If there is back metal as in Figure 9, and there is perfect reflection, the minimum reflection occurs when the thickness D is 1/4 wavelength, while if there is no back metal as in Figure 8, and there is perfect reflection, in principle the minimum reflection occurs when the thickness D is 1/2 wavelength. In reality, silicon has a complex refractive index and is not in a perfect reflection state, so this calculation results in an optimized thickness dSi of 170 μm in the case of Figure 8 and 94 μm in the case of Figure 9.

As an example, we will explain how the "minimum value at the resonance frequency of THz reflection" changes with respect to the change in σ _Si when operation in the 300 GHz band is assumed. This study serves as a guide to determine the range of σ _Si in which the effect of suppressing THz reflection can be obtained. FIG. 12 is a diagram explaining the dependence of the minimum value of THz reflection on σ _Si calculated for the case without a back metal (the configuration of FIG. 8). The conductivity σ _Si that gives the minimum THz reflection is in the vicinity of 90 to 95 (Ω·m) ⁻¹ . On the other hand, the conductivity σ _Si that makes the THz reflection −20 dB or less is 65 to 135 (Ω·m) ⁻¹ . In this way, if a predetermined THz reflection value is set as the threshold value, the range of conductivity can be made wider, so it is presumed that this can be realized relatively easily in the actual implementation of a semiconductor device compared to aiming for optimization.

(Step S04)
The resonance point found in step S03 is compared with the operating center frequency fc.

(Step S05)
If the resonance point and the operating center frequency fc are different ("No" in step S04), the thickness D of the Si layer (material 41) is finely adjusted, and step S03 is performed again.

(Step S06)
If the resonance point and the operating center frequency fc are approximately the same ("Yes" in step S04), the aforementioned thickness D is determined as the thickness of the resistor 11, and the conductivity σ of the material 41 at the resonance point is determined as the conductivity of the resistor 11.

In this way, by adjusting the thickness D of resistor 11 and the conductivity of the material of resistor 11, it is possible to suppress frequency ripples that occur in the output of THz photomixers and ultrafast photodiodes.

The reason why the THz reflection can be controlled by the conductivity σ of the resistor 11 is as follows.
The frequency characteristics of conductive Si can be expressed by the following equation.
ε _Si (f)=ε _Si0 -j・σ _Si /(2πf・ε ₀ )
Here, ε _Si (f) is the complex dielectric constant, f is the frequency of the electromagnetic wave propagating in the medium, ε _Si0 is the dielectric constant of silicon at f=0 Hz, ε ₀ is the dielectric constant of a vacuum, and σ _Si is the conductivity of Si.
As shown in the above formula, the propagation loss of the THz wave is determined according to the magnitude of the imaginary term, and the reflectance of the THz wave incident from InP is determined by the total ε _Si (f). The reason why the optimal conductive Si thickness cannot be determined from the beginning is that the imaginary term of ε ₀ has a large effect on the transmission and reflection characteristics of the wave.
In other words, the purpose of the present invention is to find the conductivity of Si that reduces the reflectance of a desired THz wave (frequency f) by appropriately selecting the conductivity σ, and reduce the electromagnetic wave propagation mode by using this conductivity as the conductivity of the resistor 11. The advantage of this method is that it is possible to find the conductivity that can reduce the electromagnetic wave propagation mode without conducting an experiment.
The electrical conductivity σ of Si can be adjusted by doping Si with an n-type or p-type dopant.

(Embodiment 2)
3 and 4 are a perspective view and a side view, respectively, illustrating the semiconductor device 102 of this embodiment. In this embodiment, only the differences from embodiment 1 will be described. The semiconductor device 102 differs from the semiconductor device 101 in that, when the resistor 11 is viewed from the mounting surface 14 side, the reflective surface 12 is located inside a region connecting the vertices of the four corners of the mounting surface 14. The resistor 11 of this embodiment is a quadrangular prism with a rectangular bottom surface, with a notch in one part, and the reflective surface 12 is formed in the notch.

In the semiconductor device 101 of the first embodiment, the position of the light receiving section is limited within the photodiode chip 13, but in the semiconductor device 102, the position of the light receiving section is not limited, and it is also possible to place it in the center of the photodiode chip 13. With the semiconductor device 102, there is no limit to the placement of the light receiving section, so it can be applied to many more light receiving devices, not just photomixers.

The reflective surface 12 of the semiconductor device 102 can be formed by the above-mentioned methods (1) and (3). Finally, a dielectric multilayer highly reflective film is formed on the reflective surface 12.

The parameters of resistor 11 of semiconductor device 102 can also be determined by the design method described with reference to FIG.
The effect of the resistor 11 is basically the same as that described in embodiment 1. Therefore, the semiconductor device 102 can also suppress frequency ripples and the like that occur in the output of the THz photomixer or ultrafast photodiode.

(Embodiment 3)
5 and 6 are perspective views (mounting surface 14 side and back surface 15 side) illustrating the semiconductor device 103 of this embodiment. In this embodiment, only the differences from embodiment 2 will be described. The semiconductor device 103 has an upper metal surface 36 that covers the area connecting the vertices of the four corners of the mounting surface 14 of the resistor 11 except for the area where the photodiode chip 13 is mounted;
A lower metal surface 37 that covers the rear surface 15 of the resistor 11;
Further equipped with
The resistor 11 differs from the semiconductor device 102 in that a through hole 38 is formed to connect an upper metal surface 36 and a lower metal surface 37 .
The shape of the resistor 11 in this embodiment is the same as that described in the second embodiment.

In manufacturing the resistor 11 of the semiconductor device 103, the through-hole 38 is also formed when the semiconductor device 102 of the second embodiment is produced. Then, the upper metal surface 36 is formed on a part of the mounting surface 14, and the lower metal surface 37 is formed on a part of the back surface 15. When the metal surfaces are formed, the through-hole 38 is also filled with metal, so that the upper metal surface 36 and the lower metal surface 37 are electrically connected to each other.
The parameters of resistor 11 of semiconductor device 103 can also be determined by the design method described with reference to FIG.

The photodiode chip 13 needs to be connected to ground. In the case of the resistor 11 of the semiconductor device 103, the upper metal surface 36, which is conductive with the lower metal surface 37, which is ground, is located near the photodiode chip 13, so that the photodiode chip 13 can be connected to ground over the shortest possible distance.

The effect of the resistor 11 is basically the same as in the embodiments 1 and 2. Therefore, the semiconductor device 103 can also suppress frequency ripples and the like that occur in the output of the THz photomixer or ultrafast photodiode.
A ground metal pattern is usually arranged on the upper surface of the photodiode chip 13. Since the semiconductor device 103 can connect the ground metal pattern closer to the ground, the ground metal pattern on the upper surface of the photodiode chip 13 can be grounded more effectively. This structure is also effective in suppressing frequency ripples that occur in the output of a THz photomixer or an ultrafast photodiode.

(Effects of the Invention)
The present invention is a technology for suppressing the problem that frequency ripples occur in the output of THz photomixers and high-speed wideband photodiodes due to resonance within the chip. The semiconductor device of the present invention has an effective radio wave absorption arrangement and a simple structure in which a photodiode chip is directly mounted on a radio wave absorber (resistor). Furthermore, the semiconductor device of the present invention has a structure that can ensure an incidence path for a light beam. The resistor (radio wave absorber) is typically made of Si, which is comparable to conventional metals and ceramics in terms of mechanical strength, thermal conductivity, and ease of processing, and can be manufactured at low cost.

11: resistor 12: reflecting surface 13: photodiode chip 14: mounting surface 15: back surface 36: upper metal surface 37: lower metal surface 38: through hole 41: same material as resistor 43: same material as photodiode chip 47: back metal 48: electromagnetic wave (propagation of unwanted THz waves from the photodiode chip)
51: Fiber input system 52: Lens optical system 53: Reflecting mirror 55: Table 57: Metal housing 58a: Wire 58b: Transmission line 58c: Waveguide coupler 59: Rectangular waveguide 60: Output port 101, 102, 103: Semiconductor device 300, 301: Photomixer module

Claims (5)

A semiconductor device including a photodiode chip and a table-shaped resistor,
the photodiode chip has a light receiving portion and a transmission line for transmitting a terahertz electrical signal;
the resistor has a mounting surface on which the photodiode chip is mounted so that the light receiving portion faces the resistor, a reflecting surface that reflects external light to the light receiving portion, and a radio wave absorbing function that reduces an electromagnetic wave propagation mode generated in the photodiode chip by the electrical signal,
The resistor has an electrical resistivity and a thickness between the mounting surface and a back surface facing the mounting surface that are determined based on the frequency of the electrical signal so as to have the radio wave absorption function. When the resistor is viewed from the mounting surface side,
2. The semiconductor device according to claim 1, wherein the reflecting surface is located outside an area connecting the vertices of the four corners of the mounting surface. When the resistor is viewed from the mounting surface side,
2. The semiconductor device according to claim 1, wherein the reflecting surface is located inside an area connecting four corner vertices of the mounting surface. an upper metal surface covering an area connecting the vertices of the four corners of the mounting surface of the resistor other than the portion on which the photodiode chip is mounted;
a lower metal surface covering the back surface of the resistor;
Further equipped with
4. The semiconductor device according to claim 1, wherein the resistor has a through hole formed therein, the through hole connecting the upper metal surface and the lower metal surface. 5. A method for designing a semiconductor device according to claim 1, comprising the steps of:
calculating a wavelength of the electromagnetic wave propagation mode from the frequency of the electrical signal;
determining the thickness of the resistor based on a wavelength of the electromagnetic wave propagation mode;
In a model in which a material having the same thickness as the resistor is connected to a material having the same thickness as the resistor in the thickness direction, the conductivity of the material having the same thickness as the resistor is changed, and a resonance point is found at which an electromagnetic wave resonates within the material having the resistor, the electromagnetic wave being transmitted from the material having the same thickness as the photodiode chip to the material having the resistor;
A design method characterized by: if the resonance point deviates from the frequency of the electrical signal, adjusting a thickness of the resistor and finding the resonance point again; and determining the conductivity of the resistor to be the conductivity of the same material as the resistor at the resonance point.

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