JP2018093149A - Light receiving element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light receiving element capable of being manufactured at a low cost, which has a structure capable of reducing degradation of optical coupling efficiency and implementation tolerance even when joint area is small, and which has a fast response characteristic, without requiring alignment between a lens and PD when manufacturing the light receiving element to thereby reduce the number of steps and devices.SOLUTION: The light receiving element including a semiconductor substrate which has a photodiode and a lens formed near the photodiode on a first plane. The semiconductor substrate is processed so that a second plane opposite to the first plane reflects a beam of light incident the semiconductor substrate to the inside of the substrate.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a structure of a light receiving element for optical communication.

  Conventionally, a photodiode (PD), which is an optical semiconductor element, is used as a light receiving element in a receiving module for optical communication. However, a high-speed response characteristic is required for the PD in response to an increase in communication capacity in recent years. It is like that.

  As one of means for responding to the high-speed response of PD, it is generally performed to reduce the junction capacitance by reducing the junction area of PD.

  However, reducing the junction area is synonymous with reducing the area (light-receiving area) that can absorb light as a PD, so reducing the junction area can reduce the efficiency of optical coupling with signal light, This leads to a decrease in mounting tolerance when mounted as a module.

In order to alleviate this relationship, as shown in the conventional example of FIG. 1, the optical coupling efficiency can be achieved even if the junction area is small by integrating lenses on the back surface of the PD and condensing the signal light incident from the back surface. Thus, a light receiving element having a structure capable of reducing the deterioration of mounting resistance and mounting tolerance has been realized. (See Non-Patent Document 1 below)
Such a conventional light receiving element (PD) has a structure in which an n-type semiconductor layer 2 and a light absorbing layer 3 made of an i-type semiconductor are formed on the surface side of a PD substrate 1 as shown in the cross-sectional view of FIG. The p-type semiconductor layer 4 and the anode electrode 5 are laminated in this order to constitute a photodiode, and two cathode electrodes 6 are provided on the n-type semiconductor layer 2 on both sides of the light absorption layer 3. Yes.

  On the back surface of the PD substrate 1, a lens 8 that is an optical component is formed by etching or the like with the optical axis aligned with the photodiode, and incident light 7 passes through the lens 8 from the back surface side of the PD substrate 1. Is converged and incident on the photodiode. An AR film 9 is laminated as an antireflection film on the back surface of the PD substrate 1 to cover the surface of the lens 8 and prevent reflection of incident light on the lens surface.

S.R.Cho et al, Enhanced Optical Coupling Performance in an InGaAs Photodiode Integrated With Wet-Etched Microlens, MARCH 2002 IEEE POTONICS TECHNOLOGY LETTERS Vol.14, No.3, p.378-380

  However, in manufacturing such a conventional light receiving element, not only a lens forming process is required in the back surface process, but also the PD formed on the surface of the semiconductor substrate and the lens processed on the back surface are mutually connected. There was a problem that the positioning of was necessary. Specifically, it is necessary to read the position of a marker or the like formed on the front surface of the semiconductor substrate in accordance with the PD, and then determine the position of the pattern constituting the back lens.

  For this reason, in the conventional light receiving element manufacturing process, an expensive stepper capable of aligning both sides of the substrate, a wafer support system for performing the back surface process, and the like, which are not necessary in the normal PD manufacturing process There is a problem that the manufacturing cost increases.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to manufacture a light receiving element having a structure capable of reducing a decrease in optical coupling efficiency and a decrease in mounting tolerance even when the junction area is small. At the same time, by eliminating the need for the alignment between the lens and the PD, which has been conventionally required, the number of processes and the apparatus used are reduced, and a light-receiving element having high-speed response characteristics that can be manufactured at low cost is provided. is there.

  In order to solve the above-described conventional problems, an invention described in an embodiment includes a light receiving element including a photodiode and a semiconductor substrate having a lens formed adjacent to the photodiode on a first surface. The semiconductor substrate is a light-receiving element in which a second surface facing the first surface is processed to reflect light incident on the semiconductor substrate into the substrate. .

It is sectional drawing of PD which is the conventional light receiving element. It is sectional drawing of PD which is a light receiving element concerning Example 1 of this invention. It is a top view of PD which is the light receiving element concerning Example 1 of this invention.

  Hereinafter, the light receiving element of the present invention will be described in detail.

  In the light receiving element described in the present embodiment, the PD is formed on the surface (first surface) of the semiconductor substrate, the semiconductor lens is formed on the surface of the semiconductor substrate adjacent to the PD, and the back surface (second surface) of the semiconductor substrate. Is a flat reflecting surface that reflects the light incident on the substrate into the substrate, and the light incident from the oblique direction on the optical axis of the semiconductor lens is reflected by the reflecting surface on the back surface to converge on the PD. It is characterized by that.

  According to the light receiving element having the above configuration, in order to enable a high-speed response without a decrease in optical coupling efficiency, a light receiving element having a structure capable of reducing a decrease in optical coupling efficiency and a reduction in mounting tolerance even when the junction area is small. At the time of manufacturing, alignment that has been conventionally required in the back surface process is not required, and the number of processes and the number of devices used are reduced. Therefore, a light receiving element having high-speed response characteristics can be produced at low cost.

Example 1
Hereinafter, Example 1 of the light receiving element of the present invention will be described with reference to FIGS. 2 and 3.

  FIG. 2 is a cross-sectional view of a PD, which is a light receiving element according to Example 1 of the present invention, and FIG. 3 is a plan view thereof.

  2 and 3, a PD using an InP (indium phosphorus) based semiconductor is taken as an example, and the PD substrate 1 is a semi-insulating InP substrate 1. This PD can be formed by the following method.

(Surface process: PD formation)
First, an n-type InP layer 2, a non-doped (i-type) InGaAs layer (light absorption layer) is formed on the surface of an InP substrate 1 made of high-resistance InP by using an epitaxial growth method such as a metal organic chemical vapor deposition (MOVPE) method. 3) A p-type InGaAs layer 4 is formed.

  Next, the above-described p-type InGaAs layer 4, non-doped InGaAs layer 3, and n-type InP layer 2 are processed into a mesa shape by a known photolithography and etching technique, and then the n-type InP layer 2 and the p-type InGaAs layer 4 are processed. Then, electrodes 6 and 5 that are in ohmic contact with each other are formed to form a photodiode (PD).

(Surface process: Lens formation)
Subsequently, the InP substrate 1 is formed into a lens shape by using a technique known in Non-Patent Document 1 or the like in a region where the epitaxial layer is completely etched in a lateral portion adjacent to the PD on the surface of the InP substrate 1. The lens 8 is formed by processing.

On the surface of the lens 8, an antireflection film (AR film 9) made of, for example, a laminated structure of SiO ₂ / TiO ₂ is formed. As shown in FIG. 3, the portion other than the AR film 9 and the electrodes 5 and 6 on the surface of the InP substrate 1 (the portion other than the region where the PD and the lens are formed) is covered with a reflective film 12 for confining incident light 7. Also good.

(Back side process)
Subsequently, the process proceeds to the back surface process. Unlike the conventional example, the present invention does not require alignment, and ordinary process equipment can be used. In the back surface process, first, the back surface of the PD substrate 1 is polished and adjusted to have a desired thickness, and a total reflection film 11 having a laminated structure of SiO ₂ / TiO ₂ is formed on the entire back surface by sputtering or the like.

  Here, the total reflection film 11 may be formed by forming a metal on an insulating film.

  Thereafter, chipping is performed by dicing, and a light receiving component including the semiconductor lens 8 on the front surface, the PD, and the total reflection film 11 on the back surface is completed. In order to make the confinement of the incident light 7 more effective, a total reflection film may be provided also on the side surface of the InP substrate 1 generated by forming a chip.

  As shown in FIG. 2, when the incident light 7 is incident on the lens 8 formed on the chip at an angle inclined to the side opposite to the PD with respect to the optical surface of the substrate surface or the lens 8, the light incident on the substrate After being totally reflected by the total reflection film 11 on the back surface of the substrate, it is converged in the vicinity of the PD absorption layer 3 and is optically coupled to the photodiode with high efficiency to generate a photocurrent.

  For example, assuming that collimated light (diameter 20 μm) incident at an angle of 60 ° with respect to the normal line of the InP substrate is used, when the thickness of the InP substrate is 75 μm, the center of the lens and the center of the PD are about 42 μm apart. That's fine. At this time, if the radius of curvature of the lens is 50 μm, the light receiving part of the PD is focused to a spot size of 9.6 μm in diameter.

  The light that cannot be absorbed by the absorption layer is reflected again by the reflection films 11 and 12 formed on the back surface, top surface, and side surfaces of the PD, and is absorbed again by the absorption layer 3, so that the PD that passes through the absorption layer only once. It is also possible to increase the light receiving sensitivity rather than the structure.

  In the structure of the light receiving element of this embodiment, patterning is performed only on the surface of the substrate on which the PD and the lens are formed, and the back surface is only a flat reflecting surface. Since it can be manufactured with only process equipment, it contributes to cost reduction.

  In the conventional monolithically integrated PD with the semiconductor lens, the position of the lens on the back surface is determined in accordance with the PD on the front surface of the substrate. Therefore, alignment is necessary and the manufacturing process is complicated. Since the PD and the semiconductor lens can be formed on the same surface, the number of processes and the apparatus used are reduced, so that the manufacturing process can be reduced in cost.

  As described above, according to the present invention, the back surface process is simplified, alignment is not required, the number of processes and the number of devices used are reduced, and light receiving elements and light receiving parts having high-speed response characteristics can be produced at low cost. It becomes.

1 PD substrate, semi-insulating InP substrate 2 n-type semiconductor layer, n-type InP layer 3 (i-type semiconductor) light absorption layer, InGaAs layer 4 p-type semiconductor layer, p-type InGaAs layer 5 anode electrode 6 cathode electrode 7 incident Light 8 Lens 9 AR film 11, 12 Reflective film

Claims (4)

A light receiving element including a semiconductor substrate having a photodiode and a lens formed adjacent to the photodiode on a first surface,
The light receiving element, wherein the second surface of the semiconductor substrate facing the first surface is processed to reflect light incident on the semiconductor substrate into the substrate.   The light receiving element according to claim 1, wherein a total reflection film is formed on the second surface of the semiconductor substrate.   The light receiving element according to claim 1, wherein a side surface of the light receiving element is processed to reflect light incident on the semiconductor substrate into the substrate.   3. The process of reflecting light incident on the semiconductor substrate into the substrate except for a portion of the first surface where the photodiode and the lens are formed. Light receiving element.

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