Back-illuminated semiconductor photodetector

Abstract

[Problem] To provide a backside incident-type semiconductor light receiving element having a high-reflectance electrode suited to an increase in response speed. [Solution] This backside incident-type semiconductor light receiving element (1) comprises a photodiode (7) having a first semiconductor layer (3), a light absorption layer (4), a second semiconductor layer (5), and a third semiconductor layer (6) laminated in this order on a front surface (2a) side of a semiconductor substrate (2) that is transparent to incident light (L1), wherein light enters the photodiode (7) from the rear surface (2b) side of the semiconductor substrate (2). The backside incident-type semiconductor light receiving element comprises an electrode (12) connected to the majority of a front surface (6a) of the third semiconductor layer (6) of the photodiode (7), wherein: the electrode (12) has, in order from the third semiconductor layer (6) side, a first metal layer (12a) for which the value of the real part n of the complex refractive index is smaller than the refractive index of the third semiconductor layer (6) and the value of the imaginary part k is at least 10, a dielectric layer (12b), and a second metal layer (12c) connected to the first metal layer (12a) on the outer peripheral side of the dielectric layer (12b); and light that has entered the photodiode (7) from the rear surface (2b) side and passed through the light absorption layer (4) is reflected at the electrode (12) and re-enters the light absorption layer (4).

Description

[Technical Field]

[0001] The present invention relates to a back-illuminated semiconductor light-receiving element, and more particularly to a back-illuminated semiconductor light-receiving element that requires a high-speed response. [Background technology]

[0002] In the field of optical communications, development is underway to increase transmission speeds in response to the recent surge in communication volume. Optical communications transmits optical pulse signals from the transmitting side via optical fiber cables, etc., and on the receiving side, semiconductor photodetectors convert the received optical pulse signals into electrical signals. Increasing the transmission speed on the receiving side can be achieved by increasing the response speed of the semiconductor photodetectors, but this requires improving the upper limit of the response speed, which is determined by the element capacitance and element resistance.

[0003] Reducing the element capacitance is an effective way to increase the response speed of a semiconductor photodetector. For example, if the semiconductor photodetector has a PIN-type photodiode with a light-absorbing layer sandwiched between two semiconductor layers of different conductivity types, the element capacitance decreases as the area of ​​the light-absorbing layer, which receives an optical pulse signal and generates two types of charge carriers through photoelectric conversion, decreases. Furthermore, if a response frequency band of approximately 20 GHz is required, the element capacitance will be small enough to be acceptable if the light-absorbing layer is circular and approximately 20 μm in diameter.

[0004] In order to sufficiently introduce light into such a small-diameter photodiode and suppress a decrease in sensitivity, a back-illuminated semiconductor light-receiving element is known, which has a focusing lens (convex lens) formed integrally with the semiconductor substrate on the back surface of the semiconductor substrate where light is incident, as disclosed in Patent Document 1, for example.

[0005] On the other hand, shortening the drift time of charge carriers generated in the light absorption layer is also effective in increasing the response speed of semiconductor light receiving elements. For example, in a PIN-type photodiode, thinning the light absorption layer between two semiconductor layers can shorten the drift distance and therefore the drift time. However, if the light absorption layer is thin, the opportunity for incident light to be converted into charge carriers decreases, reducing the sensitivity of the semiconductor light receiving element.

[0006] To suppress such a decrease in sensitivity, a back-illuminated semiconductor photodetector is known in which a mirror electrode is formed on the photodiode via a SiN film, and light that enters from the back side of the semiconductor substrate and passes through the InGaAs light absorption layer of the photodiode is reflected by the mirror electrode and re-enters the InGaAs light absorption layer, as in Patent Document 2. Also, to improve sensitivity, a back-illuminated semiconductor photodetector is known in which light that passes twice through the n-InGaAs layer (light absorption layer) is reflected by a reflective film formed on the back side of the semiconductor substrate and re-enters the light absorption layer, as in Patent Document 3. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2-105585 [Patent Document 2] Japanese Patent Application Publication No. 2018-152369 [Patent Document 3] Japanese Patent Application Publication No. 6-77518 Summary of the Invention [Problem to be solved by the invention]

[0008] In Patent Document 2, a mirror electrode formed of a Ti (titanium) film and an Au (gold) film is connected to the semiconductor layer of the photodiode via a ring-shaped contact portion that penetrates the SiN film. However, increasing the reflective area of ​​the mirror electrode reduces the area of ​​the contact portion, which increases the resistance of the contact portion, which is one of the components of the element resistance, and hinders an increase in the response speed. Furthermore, if the area of ​​the contact portion is increased to reduce the resistance of the contact portion, the reflective area of ​​the mirror electrode decreases, reducing the amount of light reflected and decreasing the sensitivity.

[0009] On the other hand, in Patent Document 3, the time it takes for light to travel back and forth between the photodiode on the front side of the semiconductor substrate and the reflective film made of Ti and Au films on the back side is several microseconds, depending on the thickness of the semiconductor substrate. Therefore, the light reflected by the reflective film on the back side of the previously incident optical pulse signal and the subsequent optical pulse signal overlap and enter the light absorption layer, making it impossible to separate the signals, and therefore this technology cannot be used when a faster response speed is required.

[0010] Furthermore, an electrode formed of a Ti film and an Au film is alloyed by mutual diffusion to form an ohmic contact. For example, when the electrode is heated during connection to the outside, the alloying process is accelerated, which may result in a loss of high reflectivity. Therefore, when a three-layer structure (see FIG. 13) is used, such as the barrier electrode in Patent Document 2, in which a Pt (platinum) film is interposed between the Ti film and the Au film as a barrier film, a high reflectivity cannot be obtained, as shown in FIG. 14.

[0011] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a back-illuminated semiconductor light-receiving element having electrodes with high reflectivity suitable for increasing the response speed. [Means for solving the problem]

[0012] The invention of claim 1 is a back-illuminated semiconductor light-receiving element that includes a photodiode having a first semiconductor layer, a light absorption layer, a second semiconductor layer, and a third semiconductor layer stacked in this order from the front side of a semiconductor substrate that is transparent to incident light, and in which light is incident on the photodiode from the back side of the semiconductor substrate, the photodiode is formed on the front side of the third semiconductor layer. The third semiconductor layer is formed so as to cover the the electrode has, in order from the third semiconductor layer side, a first metal layer having a complex refractive index whose real part n is smaller than the refractive index of the third semiconductor layer and whose imaginary part k is 10 or more, a dielectric layer, and a second metal layer connected to the first metal layer on the outer periphery of the dielectric layer, and is configured so that light that is incident on the photodiode from the back surface side and transmitted through the light absorption layer is reflected by the electrode and re-enters the light absorption layer.

[0013] According to the above configuration, light that enters the photodiode from the back side of the semiconductor substrate and passes through the light absorption layer is reflected by the electrode of the photodiode and re-enters the light absorption layer. This third semiconductor layer is formed to cover The electrode has a first metal layer connected to the third semiconductor layer, which increases the reflective surface of the electrode and the contact area, thereby reducing contact resistance and advantageously increasing response speed. The electrode also has a first metal layer whose real part n of the complex refractive index is smaller than the refractive index of the third semiconductor layer and whose imaginary part k of the complex refractive index is 10 or greater. A dielectric layer is provided between the first and second metal layers, resulting in high reflectivity. Therefore, most of the light transmitted through the light absorption layer can be reflected by the electrode adjacent to the photodiode and re-enter the light absorption layer, improving sensitivity. This allows the light absorption layer to be made thinner while suppressing a decrease in sensitivity, thereby increasing response speed.

[0014] The back-illuminated semiconductor photodetector of the invention of claim 2 is characterized in that, in the invention of claim 1, the first metal layer is formed of a metal selected from the group consisting of Au, Ag, Cr, and Al. According to the above configuration, the first metal layer is made of Au (gold), Ag (silver), Cr (chromium), or Al (aluminum), which is a metal whose real part n of the complex refractive index is smaller than the refractive index of the third semiconductor layer and whose imaginary part k of the complex refractive index is equal to or greater than 10. These metals are commonly used as materials for semiconductor elements, and therefore, an electrode with high reflectivity can be easily formed.

[0015] The back-illuminated semiconductor light-receiving element of the invention of claim 3 is characterized in that, in the invention of claim 1, an adhesive layer containing Ti as a main component is provided between the dielectric layer and the second metal layer. According to the above configuration, the adhesion layer can prevent film lifting or peeling of the second metal layer, which is a concern when the second metal layer is formed from a metal that does not have good adhesion to the dielectric layer, and it is possible to prevent a decrease in the function of the electrode due to film lifting or peeling.

[0016] A back-illuminated semiconductor light-receiving element according to the invention of claim 4 is characterized in that, in the invention of claim 1, the photodiode is formed in a truncated cone shape with the diameter of the light absorption layer being 20 μm or less, and the back surface of the semiconductor substrate has a focusing lens portion whose lens optical axis is aligned with the central axis of the truncated cone-shaped photodiode. According to the above configuration, since the diameter of the light absorption layer is 20 μm or less, the area of ​​the photodiode can be reduced, which reduces the element capacitance and increases the response speed. Furthermore, by using the focusing lens section to focus and direct light onto this small-area photodiode, it is possible to suppress a decrease in sensitivity. [Effects of the Invention]

[0017] According to the back-illuminated semiconductor light-receiving element of the present invention, the response speed can be increased while suppressing a decrease in sensitivity due to the high reflectivity electrodes. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view of a back-illuminated semiconductor light-receiving element according to an embodiment of the present invention. [Figure 2] 2 is an enlarged cross-sectional view showing the main part of the photodiode and electrodes of FIG. 1. FIG. [Figure 3] 10A to 10C are cross-sectional views showing a process for forming a photodiode having a truncated cone shape. [Figure 4] 5A to 5C are cross-sectional views showing a step of forming an electrode of a photodiode. [Figure 5] 10 is a graph showing, in contour form, the calculation results of the reflectance of an electrode relative to the complex refractive index of a first metal layer of the electrode. [Figure 6] 10 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode is made of Ti. [Figure 7] 10 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode is made of Ta. [Figure 8] 10 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode is made of Ag. [Figure 9] 10 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode is made of Al. [Figure 10] 10 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode is made of Cr. [Figure 11] 1 is a cross-sectional view showing an example of a photodiode provided with an electrode having an adhesive layer between a dielectric film and a second metal film. [Figure 12] 12 is a graph showing the relationship between the thickness of the dielectric layer, the thickness of the first metal layer, and the reflectance of the electrode when the first metal layer of the electrode in FIG. 11 is made of Cr. [Figure 13] FIG. 1 is a cross-sectional view showing a conventional example of an electrode. [Figure 14] 14 is a graph showing the relationship between the thickness of Ti and the thickness of Pt of the electrode in FIG. 13 and the reflectance of this electrode. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described based on first and second embodiments. [Example]

[0020] 1 and 2, the back-illuminated semiconductor light-receiving element 1 is provided with a PIN-type photodiode 7 on the front surface 2a side of a semiconductor substrate 2, which has a first semiconductor layer 3, a light-absorbing layer 4, a second semiconductor layer 5, and a third semiconductor layer 6 stacked in this order from the semiconductor substrate 2 side. The photodiode 7 is formed into a truncated cone shape by a groove 8a formed on the outer periphery of the photodiode 7. The photodiode 7 is formed so that the diameter of the light-absorbing layer 4 is, for example, 20 μm or less, thereby reducing the area and therefore the element capacitance.

[0021] The semiconductor substrate 2 is, for example, a semi-insulating InP substrate that is transparent to the emitted light L1 having a wavelength in the infrared region (for example, 1300 to 1600 nm) emitted from the optical fiber cable OC. The first semiconductor layer 3 is, for example, an n-InP layer of a first conductivity type. The light absorption layer 4 is, for example, an InGaAs layer. The second semiconductor layer 5 is, for example, a p-InP layer of a second conductivity type. The third semiconductor layer 6 is, for example, an InGaAs layer that is thinner than the light absorption layer 4, and is formed for ohmic contact with an electrode (second electrode 12) described later.

[0022] The groove 8a is formed by etching, for example, by reactive ion etching, through the third semiconductor layer 6, the second semiconductor layer 5, and the light absorption layer 4 to partway through the first semiconductor layer 3. At this time, an opening 8b (hole or groove) that reaches the first semiconductor layer 3 outside the photodiode 7 is also formed at the same time. Then, as shown in FIG. 3, an insulating film 9 is selectively formed by a known method so as to cover an area excluding most of the surface 6a of the third semiconductor layer 6 of the photodiode 7 and the bottom of the opening 8b (not shown), and contact holes 9a and 9b are formed in the insulating film 9. The insulating film 9 may be made of, for example, an SiO2 film, an SiN film, or the like.

[0023] 4, a first metal layer 12a connected to the third semiconductor layer 6 through the contact hole 9a is selectively formed by, for example, a known lift-off method so as to extend to the outside of the groove 8a. Then, a dielectric layer 12b is selectively formed by a known method (film formation, photo, etching) in a region on this first metal layer 12a corresponding to the photodiode 7.

[0024] Furthermore, a second metal layer 12c is selectively formed by, for example, a known lift-off method so as to cover the first metal layer 12a and the dielectric layer 12b, thereby forming the second electrode 12 as shown in Figures 1 and 2. The second metal layer 12c is connected to the first metal layer 12a on the outer periphery of the dielectric layer 12b. In the opening 8b, the first metal layer 12a and the second metal layer 12c, which are connected to the first semiconductor layer 3 via the contact hole 9b, are also selectively formed so as to extend outside the opening 8b, thereby forming the first electrode 11 at the same time as the second electrode 12. The first and second electrodes 11 and 12 are each bonded to an external wiring portion, for example, of a printed circuit board.

[0025] The second electrode 12 thus formed has a third semi-conductor layer 14 formed on the portion corresponding to the photodiode 7. The photodiode 7 has a structure in which a first metal layer 12a, a dielectric layer 12b, and a second metal layer 12c are stacked in this order from the conductor layer 6 side. The first metal layer 12a is formed of a metal whose real part n (refractive index) of the complex refractive index is smaller than that of the third semiconductor layer 6 and whose imaginary part k (extinction coefficient) of the complex refractive index is 10 or more. The first metal layer 12a is a thin film of a metal selected from the group consisting of Au (gold), Ag (silver), Al (aluminum), and Cr (chromium). The first metal layer 12a is connected to most of the surface 6a of the third semiconductor layer 6 of the photodiode 7, thereby reducing the contact resistance of the second electrode 12. The second metal layer 12c is often made of Au, which is easy to bond to the outside and is resistant to oxidation.

[0026] The rear surface 2b of the semiconductor substrate 2 has a convex lens-shaped condenser lens portion 2c formed integrally with the semiconductor substrate 2. The condenser lens portion 2c is formed so that the lens optical axis coincides with the central axis 7a of the truncated cone-shaped photodiode 7. The emitted light L1 (optical pulse signal) emitted from the optical fiber cable OC is emitted so that its optical axis coincides with the central axis 7a and enters the condenser lens portion 2c while expanding in a conical shape. The incident light L2 that enters the condenser lens portion 2c and travels through the semiconductor substrate 2 is condensed onto the photodiode 7 by the condensing effect of the condenser lens portion 2c.

[0027] As shown in FIG. 2, incident light L2 enters the photodiode 7, and a portion of the light is converted into two types of charge carriers (electrons and holes) in the light absorption layer 4. One type of charge carrier then moves to the first semiconductor layer 3, and the other type of charge carrier moves to the second semiconductor layer 5, and is output as a photocurrent to the outside via the first and second electrodes 11 and 12. Furthermore, reflected light L3 of the incident light L2 passes through the light absorption layer 4 without undergoing photoelectric conversion and is reflected by the second electrode 12, and is then re-entered into the light absorption layer 4. A portion of the reflected light L3 that re-enters the light absorption layer 4 is photoelectrically converted and is output as a photocurrent to the outside via the first and second electrodes 11 and 12 together with the charge carriers photoelectrically converted from the incident light L2.

[0028] In this way, the reflected light L3 re-enters the light-absorbing layer 4, increasing the effective thickness of the light-absorbing layer 4 and improving the sensitivity of the back-illuminated semiconductor light-receiving element 1. This allows the light-absorbing layer 4 to be formed thin while suppressing a decrease in sensitivity, shortening the charge carrier transit time and increasing the response speed. Furthermore, since the second electrode 12 is located close to the photodiode 7, the time it takes for light to travel back and forth between the light-absorbing layer 4 and the second electrode 12 is shortened, allowing the light to be separated from subsequent optical pulse signals, enabling high-speed communication. Furthermore, since the area of ​​the photodiode 7 is small, the element capacitance can be reduced, increasing the response speed. Furthermore, since the incident light L2 is incident on the small-area photodiode 7 by the condenser lens portion 2c, the incident light L2 is efficiently utilized, preventing a decrease in sensitivity.

[0029] Next, the reflectance of the second electrode 12 will be described using an example in which the wavelength λ of light is 1550 nm. A portion of the incident light L2 passes through the light absorption layer 4, and reaches the first metal layer 12a of the second electrode 12 through the second semiconductor layer 5 and the third semiconductor layer 6. When the second semiconductor layer 5 is made of p-InP and the third semiconductor layer 6 is made of InGaAs, the refractive indexes of the second and third semiconductor layers 5 and 6 are each 3.22.

[0030] 5 shows the calculation results of the reflectance of second electrode 12 in the form of contour lines, using the real part n (refractive index) and the imaginary part k (extinction coefficient) of the complex refractive index of the metal of first metal layer 12a constituting second electrode 12 as parameters. First metal layer 12a is 20 nm thick, and dielectric layer 12b is formed of a 100 nm thick SiO2 film. Second metal layer 12c is also a 600 nm thick Au film in the graphs following Figure 5.

[0031] 5, the smaller the value of the real part n (refractive index) of the complex refractive index of the first metal layer 12a and the larger the value of the imaginary part k, i.e., the higher the reflectivity tends to be in the upper left region of the graph. Furthermore, in order to effectively utilize the light incident on the photodiode 7, a higher reflectivity of the second electrode 12 is preferable. When metals commonly used as materials for semiconductor elements are plotted in FIG. 5, Au (gold), Ag (silver), Cu (copper), Al (aluminum), and Cr (chromium) can achieve a high reflectivity of over 80%. These metals have a smaller value of the real part n of the complex refractive index than the refractive index of the third semiconductor layer 6 (InGaAs).

[0032] On the other hand, Ni (nickel), Ti (titanium), and Pt (platinum), which have low reflectance, have a larger value of the real part n of the complex refractive index than the refractive index of the third semiconductor layer 6. Ta (tantalum) has a smaller value of the real part n of the complex refractive index than the refractive index of the third semiconductor layer 6, but since the value of the imaginary part k of the complex refractive index is smaller than that of Ag and the like, it is thought that the incident light penetrates deeper from the interface, resulting in a lower reflectance. Note that Cu is more difficult to process than Au, Ag, Al, and Cr, and has the property of easily diffusing into other materials, making it difficult to use for the first metal layer 12a.

[0033] 6 shows the reflectance of the second electrode 12 calculated using the thickness of the dielectric layer 12b (SiO2 film) and the thickness of the first metal layer 12a as parameters when the first metal layer 12a is made of Ti, as contour lines. According to FIG. 6, the thinner the first metal layer 12a and the thinner the dielectric layer 12b, the higher the reflectance. This is thought to be because the thinner the first metal layer 12a, the more light passes through the first metal layer 12a, and the reflection at the interface between the first metal layer 12a and the dielectric layer 12b and the interface between the dielectric layer 12b and the second metal layer 12c increases.

[0034] However, the thinner the first metal layer 12a, the higher the contact resistance of the second electrode 12, hindering faster response speeds. Furthermore, the thinner the film to be formed, the more difficult it is to control the film thickness. Therefore, if the thickness of the dielectric layer 12b is set to 100 nm and the thickness of the first metal layer 12a is set to 20 nm to reduce the contact resistance and facilitate thickness control, the reflectance of the second electrode 12 will be a low value of less than 20%, making Ti unsuitable for the first metal layer 12a.

[0035] 7 shows the reflectance of the second electrode 12 calculated using the thickness of the dielectric layer 12b (SiO2 film) and the thickness of the first metal layer 12a as parameters when the first metal layer 12a is made of Ta, as contour lines. When the thickness of the dielectric layer 12b is, for example, 100 nm and the thickness of the first metal layer 12a is, for example, 20 nm, the reflectance is less than 80%. To bring the reflectance closer to 90%, at least one of the first metal layer 12a and the dielectric layer 12b needs to be thin, which is not easy.

[0036] 8 to 10 show the reflectance of the second electrode 12 calculated using the thickness of the dielectric layer 12b (SiO2 film) and the thickness of the first metal layer 12a as parameters, in the form of contour lines. Fig. 8 shows the case where the first metal layer 12a is made of Ag. Fig. 9 shows the case where the first metal layer 12a is made of Al. Fig. 10 shows the case where the first metal layer 12a is made of Cr.

[0037] In any of the cases shown in FIGS. 8 to 10, when the thickness of the dielectric layer 12b is, for example, 100 nm and the thickness of the first metal layer 12a is, for example, 20 nm, the reflectance exceeds 85%. Therefore, when the first metal layer 12a is made of Ag, Al, or Cr, a high reflectance can be easily obtained. By making the thickness of the first metal layer 12a greater than, for example, 25 nm, it is possible to reduce the contact resistance and obtain a reflectance of 90% or more. Furthermore, as shown in FIG. 3, a similarly high reflectance can be obtained when the first metal layer 12a is made of Au. Au, Ag, Al, and Cr have a complex refractive index whose imaginary part k is 10 or greater and a complex refractive index whose real part n is smaller than the refractive index of the third semiconductor layer 6, allowing the second electrode 12 to be formed with a high reflectance.

[0038] Here, the dielectric layer 12b prevents alloying due to interdiffusion between the first metal layer 12a and the second metal layer 12c, thereby preventing a change in the complex refractive index due to alloying of the first metal layer 12a. Furthermore, since the first metal layer 12a and the second metal layer 12c are connected on the outer periphery of the dielectric layer 12b, the second electrode 12 has electrical conductivity and high reflectivity. Regarding reflection by the second electrode 12, when the thickness of the first metal layer 12a is such that the contact resistance is small, reflection at the interface between the third semiconductor layer 6 and the first metal layer 12a is dominant. Therefore, the dielectric layer 12b may be formed of a material other than a SiO2 film, such as a SiON film or a SiN film.

[0039] If the second metal layer 12c is made of Au, its adhesion to the dielectric layer 12b (SiO2 film) will be poor, and the second metal layer 12c may rise from the dielectric layer 12b, potentially impairing the functions of the second electrode 12 (e.g., conductivity, external connection). Therefore, as shown in Fig. 11, the second electrode 12 may include an adhesion layer 12d, primarily composed of Ti, between at least the dielectric layer 12b and the second metal layer 12c. The thickness of the adhesion layer 12d typically used is approximately 10 nm.

[0040] As shown in Fig. 12, when the first metal layer 12a is made of Cr, the second electrode 12 having the adhesion layer 12d made of Ti can achieve high reflectivity. For example, when the thickness of the dielectric layer 12b is 100 nm and the thickness of the first metal layer 12a is 20 nm, the reflectivity exceeds 85%, which is approximately the same as that of the second electrode 12 in Fig. 10 without the adhesion layer 12d. Furthermore, the adhesion layer 12d can prevent peeling or lifting of the second metal layer 12c, thereby preventing the function of the second electrode 12A from being impaired.

[0041] FIG. 13 shows a photodiode 7 in which the second electrode 12 has been replaced with a conventional electrode 22 having a conventional structure. The conventional electrode 22 is formed by laminating, in order from the third semiconductor layer 6 side, a Ti layer 22a, a Pt layer 22b as a barrier layer that prevents alloying when bonded to the outside, and an Au layer 22c. As described above, the value of the real part n of the complex refractive index of the Ti layer 22a is greater than the refractive index of the third semiconductor layer 6, and the value of the real part n of the complex refractive index of the Pt layer 22b is greater than the value of the real part n of the complex refractive index of the Ti layer 22a (see FIG. 5). As shown in FIG. 14, the reflectivity of this conventional electrode 22 is low even when the thicknesses of the Pt layer 22b and the Ti layer 22a are adjusted, and it is not possible to obtain a high reflectivity like that of the second electrode 12 of the present invention.

[0042] The operation and effects of the back-illuminated semiconductor light-receiving element 1 will now be described. Light that enters the photodiode 7 from the back surface 2b side of the semiconductor substrate 2 and passes through the light absorption layer 4 is reflected by the second electrode 12, which is an electrode of the photodiode 7, and re-enters the light absorption layer 4. The second electrode 12 is connected to the surface 6a of the third semiconductor layer 6 of the photodiode 7. The third semiconductor layer 6 is formed so as to cover the The presence of the first metal layer 12a connected to the second electrode 12 increases the reflective surface of the second electrode 12 and reduces the resistance of the contact portion of the second electrode 12, which is advantageous for increasing the response speed. The second electrode 12 has the first metal layer 12a formed of a metal whose real part n of the complex refractive index is smaller than the refractive index of the third semiconductor layer 6 and whose imaginary part k of the complex refractive index is 10 or greater. The structure including the dielectric layer 12b between the first metal layer 12a and the second metal layer 12c achieves a higher reflectivity than conventional structures. Therefore, most of the light transmitted through the light absorption layer 4 can be reflected by the second electrode 12 adjacent to the photodiode 7 and re-enter the light absorption layer 4. This allows the light absorption layer 4 to be made thinner while suppressing a decrease in sensitivity of the back-illuminated semiconductor photodetector 1, thereby increasing the response speed.

[0043] The first metal layer 12a is formed of a metal selected from the group consisting of Au, Ag, Cr, and Al, which are metals whose real part n of the complex refractive index is smaller than the refractive index of the third semiconductor layer 6 and whose imaginary part k of the complex refractive index is equal to or greater than 10. These metals are commonly used as materials for semiconductor elements, and therefore the second electrode 12 having high reflectivity can be easily formed.

[0044] The second electrode 12 may have an adhesion layer 12d mainly composed of Ti between the dielectric layer 12b and the second metal layer 12c. This allows the adhesion layer 12d to prevent film lifting or peeling of the second metal layer 12c, which can occur when the adhesion between the second metal layer 12c and the dielectric layer 12b is poor. Therefore, it is possible to prevent a decrease in the functionality of the second electrode 12 due to film lifting or peeling.

[0045] The photodiode 7 is formed in a truncated cone shape with a light absorption layer 4 having a diameter of 20 μm or less, and has a condenser lens portion 2c on the back surface 2b of the semiconductor substrate 2, with the lens optical axis aligned with the central axis 7a of the truncated cone-shaped photodiode 7. The small area of ​​the photodiode 7 allows for a small element capacitance, thereby enabling a high-speed response. Furthermore, by condensing light onto the small-area photodiode 7 using the condenser lens portion 2c, a decrease in the sensitivity of the back-illuminated semiconductor light-receiving element 1 can be suppressed. The diameter of the light absorption layer 4 can be appropriately set depending on the required response frequency band.

[0046] In addition, a person skilled in the art can implement the present invention in a form in which various modifications are added to the above-described embodiment without departing from the spirit of the present invention, and the present invention also includes such modifications. [Explanation of symbols]

[0047] 1: Back-illuminated semiconductor photodetector 2: Circuit board 2a: surface 2b: Back side 2c: Condenser lens part 3: First semiconductor layer 4: Light absorbing layer 5: Second semiconductor layer 6: Third semiconductor layer 7: Photodiode 7a: Central axis 8a: Groove 8b: Opening 9: Insulating film 11: 1st electrode 12:Second electrode (electrode) 12a: 1st metal layer 12b: Dielectric layer 12c: 2nd metal layer 12d: Adhesion layer 22: Conventional electrode 22a:Ti layer 22b: Pt layer 22c:Au layer L1: Output light L2: Incident light L3: Reflected light OC: Optical fiber cable

Claims

1. A back-illuminated semiconductor light-receiving element includes a photodiode having a first semiconductor layer, a light absorption layer, a second semiconductor layer, and a third semiconductor layer stacked in this order from the semiconductor substrate side on a front surface side of a semiconductor substrate that is transparent to incident light, and light is incident on the photodiode from the back surface side of the semiconductor substrate, the photodiode includes an electrode formed so as to cover a surface of the third semiconductor layer and connected to the third semiconductor layer; the electrode includes, in order from the third semiconductor layer side, a first metal layer having a complex refractive index whose real part n is smaller than the refractive index of the third semiconductor layer and whose imaginary part k is equal to or greater than 10, a dielectric layer, and a second metal layer connected to the first metal layer on the outer periphery side of the dielectric layer; a back-illuminated semiconductor light-receiving element configured such that light incident on the photodiode from the back surface side and transmitted through the light absorption layer is reflected by the electrode and re-enters the light absorption layer.

2. 2. The back-illuminated semiconductor light-receiving element according to claim 1, wherein the first metal layer is made of a metal selected from the group consisting of Au, Ag, Cr, and Al.

3. 2. The back-illuminated semiconductor light-receiving element according to claim 1, further comprising an adhesive layer containing Ti as a main component between the dielectric layer and the second metal layer.

4. The photodiode is formed in a truncated cone shape with the light absorption layer having a diameter of 20 μm or less, 2. The back-illuminated semiconductor light-receiving element according to claim 1, further comprising a condenser lens portion on the back surface of the semiconductor substrate, the lens optical axis of which is aligned with the central axis of the photodiode having a truncated cone shape.

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