JPH02185070A - Photodetector and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a photodetector having a high speed operation and a wide photodetection area by condensing an optical signal to the photodetecting area through condenser lens formed by laminating insulating films whose refractive indices are sequentially varied in a recess formed on the rear of a substrate. CONSTITUTION:A condenser lens is not formed on a part directly above a photodetection region, but so formed as to surround the upper part directly above the photodetecting region. An SiO2 film is, for example, so deposited initially on the side face of a recess as to enhance its refractive index toward the center of its lens, X of Si3N4XO6(1-X) is sequentially increased to form an Si3N4 film at the center. The relationship between the refractive index and a composition X satisfies a linear relation and its composition is so varied that a square distribution is provided with respect to the thickness of an insulating film in the refractive index to obtain a distributed refractive index type condenser lens. The composition X of the Si3N4XO6(1-X) is controlled easily by altering the flowrate ratio of N2O to NH3 of SiH4, N2O, NH3 of doping gases. Thus, coupling with a fiber is facilitated, the photodetecting region can be reduced, and a high speed photodetector can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to the structure of a light receiving element with a built-in condensing lens and its manufacturing method. can be easily combined and the light-receiving area can be made small, so it can be applied as a light-receiving element capable of high-speed operation.

2. Description of the Related Art An example of a light receiving device in which a light receiving element and a condensing lens are integrated in the same chip is shown in FIG. 4, in which a microlens is formed on the back surface of a substrate by ion beam etching. (Tsuo Kumai et al. 1986 Autumn Biological Materials Proceedings 18a-ZK-6)
-1nPFj12, i-1nGaAsFj 13, and part of i-1nGaAs Nl3 have p”-
InPFi 15 is laminated to form a light receiving area 16. In addition, a part of the 1-1nGaAs layer 13 has n
A -1nP layer 14 is also laminated, and AuGe is formed as an n-side electrode 17 on the n-1nP layer 14, and p''-1nPFj
AuZn is deposited on the p-side electrode 15 as a p-side electrode 18 . Microlenses 18 are formed on the back surface of the substrate 11 by ion beam etching. That is, the light emitted from the optical fiber is collected by the microlens 18 and focused within the light receiving area. As a result, an electrical signal corresponding to the optical signal can be obtained from the n-side electrode 17 and the p-side electrode 18.

Furthermore, distributed index optical fibers are known that have a light condensing effect by sequentially changing the refractive index, and are made by a thick CVD method, an external CVD method, a VAD method, or the like. In particular, FIG. 5 shows a thickening CVD method in which an insulating film is deposited by flowing gas into an enclosed space, similar to the photo-CVD method. (Nagashi Uchida et al. Introduction to Optical Device Technology 9.39) What is the fleshed CVD method? Si halide 5iC1n22, which is the main material of the optical fiber, is placed in a high-purity quartz tube 21. Doping agent halide G
e Cl m 23, other 24 (POC13,
8c 13) etc. in a gaseous state and send it into the quartz tube 21.
While rotating the burner 25, heat it to approximately 1500℃ from the outside.
By heating the raw material to cause a chemical reaction, controlling the amount of raw material delivered to the quartz tube, and depositing silica glass with different refractive indexes inside the tube, the quartz tube is then crushed to create optical fibers with different refractive indexes in the radial direction. This is the way to obtain. The thus obtained distributed index optical fiber or a single distributed index lens obtained by cutting this optical fiber into a short length can exhibit a light focusing effect.

Problems to be Solved by the Invention Light-receiving elements with integrated condensing lenses have already been reported. However, since the light-receiving element shown in FIG. 4 uses a convex lens as a condensing lens, there is a possibility that the core portion of the optical fiber may be biased when the condensing lens and the optical fiber come into contact with each other. Since a convex lens is used in this way, there is a problem in that it is difficult to implement such things as aligning the optical axis with the optical fiber.

The present invention solves these conventional problems. In other words, the light from the optical fiber can be focused within the light-receiving area no matter where the light enters the condenser lens, and since the condenser lens is concave, the surface of the insulating film is flat. Optical fibers can be attached closely. Furthermore, when an optical fiber whose terminal end is processed into a spherical shape is used, it is also possible to insert a portion of the tip into the recess, which greatly facilitates alignment of the optical axis.

In addition, as a lens that exhibits a distributed refractive index type light focusing effect,
There have been many reports in the past, including optical fibers.

However, the distributed index optical fiber shown in Figure 5 deposits an insulating film with a sequentially changing refractive index on quartz glass by heating gas to 1000°C or higher, and is particularly suitable for compound substrates. There is a problem in that it is impossible to deposit an insulating film of the same thickness on the vertical surfaces of the recesses as on the horizontal surfaces. Furthermore, although it is possible to create a distributed index condenser lens by cutting the distributed index optical fiber shown in Figure 5, it is extremely difficult to mount it at the correct location on a compound semiconductor substrate. accompanied by great difficulties. The present invention solves such conventional problems, and uses a photo-CVD method that enables the deposition of an insulating film of the same thickness on a compound semiconductor substrate having a recessed portion, for example, on a surface perpendicular to the substrate and on a surface horizontally to the substrate. By stacking insulating films using the method, it is possible to easily create a gradient index condenser lens.

Means for Solving the Problems In order to solve the above problems, the present invention includes a compound semiconductor substrate, a light absorption layer laminated on the substrate, and a light absorption layer formed in a partial area of the light absorption layer. It includes a light-receiving region with different layers and conductivity types, a concave portion formed on the back surface of the substrate, and a condensing lens formed by laminating insulating films with sequentially changed refractive indexes on the concave portion, and the condensing lens collects light. The present invention provides a light receiving element characterized in that a signal is focused on the light receiving region, and further includes a step of epitaxially growing a light absorption layer on a compound semiconductor substrate, and a step of epitaxially growing a light absorption layer on a part of the light absorption layer. A light-receiving method comprising the steps of forming a light-receiving region having a conductivity type different from that of a light-absorbing layer, etching the back surface of the substrate into a cylindrical shape, and depositing an insulating film in the concave portion while sequentially changing the refractive index. This paper attempts to propose a method for manufacturing devices.

Function The light-receiving element of the present invention has a condensing lens built into the back surface of the substrate, and the optical axis is different from that which has been proposed in the past, in which a microlens for the purpose of condensing light and a light-receiving element are discretely integrated. The structure is suitable for mass production since no adjustment is required.

A method for producing this condensing lens is, for example, by first depositing, for example, a SiO2 film on the side surfaces of the concave portion so that the center of the lens has a high refractive index.
By increasing X in sequence, a Si3N4 film is formed at the center. Assuming that the relationship between the refractive index and the composition X satisfies a linear relationship, a distributed refractive index condensing lens can be obtained by changing the composition so that the refractive index takes a square distribution with respect to the thickness of the insulating film. Catequil. S 13Nixoat+-x
+(D composition x is controlled by doping gas S i
Ha, N20. This can be easily achieved by changing the flow rate ratio of N20 and NH3 in NH3.

As a method of depositing the insulating film, for example, photoCVD is used to deposit an insulating film of equal thickness on the vertical and horizontal surfaces of the recess.
It is better to use method D. In addition, in the photo-CVD method, it is possible to deposit an insulating film by heating the substrate to about 200° C., and there is less risk of cracks occurring due to thickly laminated insulating films.

Embodiment First, as for the structure of the insulating film, as shown by diagonal lines in FIG. 3(a), no condensing lens is formed directly above the light receiving area, and the insulating film is deposited parallel to the substrate. This is because even if parallel light rays are incident, there is no need to condense the light that falls within the range of the light receiving area. Therefore, the condenser lens is formed so as to surround the area directly above the light receiving area. The optical path is indicated by an arrow in FIG. 3(a). Now, to simplify the explanation, let's consider that the left and right lens regions are cemented together as shown in FIG. 3(b). Here, if the refractive index of the condenser lens has a square distribution, the condenser lens is considered to be part of a distributed index optical fiber. The trajectory of light in a distributed index optical fiber is expressed by a cosine leap number, as shown by the dotted line in FIG. 3(b). Here, the reason why the light trajectory does not converge to one point is because the tip of the optical fiber is shaped conically like the condenser lens. The light that has passed through the condenser lens is diffracted on the substrate surface and then travels straight to become the solid line of the arrow shown in FIG. 3(b). Here, in FIG. 3(b), the insulating film to be laminated in the recess is directed from the periphery of the recess to the center.
axosu-x+ (D composition changed from xttO to l. When the difference between the outer diameter and inner diameter of the condensing lens is 2a - 20 μm, and the thickness of the lens is a = 10 μm, the light that passes through the outermost part of the lens The light will be focused at L+=44.8 μm.

The calculation method when light is incident on the outermost periphery of a lens where the refractive index has a square distribution is shown below.

A: In Fig. 3(a), when the light travels in the range B"BCC', the trajectory of the light is y = anie os (gx) g = 2Δ/a Δ" (nsi311i n5i02) / ns+
It becomes 3Na. By the way, the place where the light reaches straight line AB is from x=y, a=10μm, ns; 3Nm=2.0b ns+
If o2=1.45, xs=8. 20μm, ye=8. 20 μm.

dy/dx=-0,424.

B: When light travels in the range ABBC in FIG. 3, the trajectory of the light is approximated by the following equation, assuming that the distance the light travels is short.

x=xlIIICO8(gy) 10g@dyldxlIS
in(gy) Now, if K: a-XlI, 3F+near, 43μm, dy/dx=-0,4300+
=0.406 rad.

C: In the straight line AC, the difference in refractive index between SiO2 and InP causes refraction expressed by the following equation.

sinθ2 n1nP a, 3
5 Result θ2=Q, 164 rad dy/dx=tan θ2=0.166 After that, assuming that the light travels straight, L+=V+/ (dy/dx): 44.8 nm L2:
(y++157zm) / (dy/dx) =135
It becomes μm.

The distance between the light-receiving area and the condensing lens is the size of the light-receiving area by 1.
If it is 5 μm, L+=44. It needs to be between 8 nm and L2=135 μm. In this case, light can be focused on the light receiving area no matter where the optical fiber is located on the focusing lens.

As mentioned above, if the light receiving element with a condensing lens of the present invention is used, and the light receiving area is 15 μmφ and the difference between the outer diameter and the inner diameter of the lens is 20 μm, then the conventional light receiving element has a light receiving diameter of 35 μmφ. It is possible to obtain an operating speed similar to that of a light receiving element having a light receiving diameter of 15 μmφ. Further, in coupling with an optical fiber, an adjustment margin of 35 μm can be provided for a light receiving element having a light receiving diameter of 15 μmφ.

FIG. 1 is a sectional view showing an embodiment of a light receiving element according to the present invention. An n--InGaAs light-absorbing layer N2 and an n-InP light-transmitting layer N3 are stacked on an n-1nP substrate l, and a Zn
Together with a P-type light-emitting region 4 formed by vapor phase diffusion or the like, a PIN photodiode is configured. The PIN photodiode has a ring-shaped p-side electrode 6 and an n-side electrode 5.
is deposited. For example, Cr/Pt is used as the p-side electrode 6.
/Au, and the n-side electrode 5 is made of, for example, AuSn. A recess 7 is formed on the back surface of the substrate 1, and an insulating film 8 is deposited therebelow while changing its refractive index.

FIG. 2 is a sectional view showing an embodiment of the method for manufacturing a light receiving element according to the present invention. n-1nP substrate l (carrier concentration 5X10"Cm" substrate thickness 300μm)
--1nGaAs light absorption layer 2 (carrier concentration lXl01
5 (film thickness: 3 um) and an n-1nP light transmitting layer 3 (carrier concentration: 1xlOI7, film thickness: 1 um). (Figure 2a) Next, Zn was diffused into the light transmitting layer and the light absorbing layer at 500°C for 6 minutes to form a layer with a diameter of 15 μm and a depth of 2 μm.
A p-type diffusion region of m is formed to serve as a light receiving region 4. (Fig. 2b), Au-5n is deposited on the light transmitting layer 3 by lift-off.
After forming the n-side electrode 5, Cr/Pt is deposited on the light receiving area 4.
/Au f) side electrode 6 is formed by lift-off and sintered (FIG. 2C).

Next, high-speed etching with good perpendicularity is performed on the IX surface of the substrate by active ion etching using Ar--Br based glue. The recess 7 formed by etching has a diameter of 35/A mφ
, and the depth is 10 μm (Fig. 3d).

Finally, an insulating film 8 is deposited to create a condenser lens. The substrate temperature during deposition is 200°C, and the composition The inner diameter of the tube is 15 μm (Fig. 2e).

The light receiving element shown in this example is S i 3N4XO611
-X) composition xt? o to 0.20. As a result, the position of the light receiving area is L+=155μm, L2=507μm
The thickness of the substrate, that is, the distance between the light-receiving area and the condensing lens was determined to be L = 300 It m, which is the middle of the thickness of the substrate. This is because it is difficult to stack pure 5iaNa due to residual O2 after stacking SiO2, but the composition
A sufficient effect could be obtained with a change of about 0%.

By using the light-receiving element with a condensing lens shown in this embodiment, it is possible to obtain an operation speed similar to that of a light-receiving element having a light-receiving diameter of 35 μmφ and 15 μmφ. Also,
When coupled to an optical fiber, an alignment margin of 35 μm can be provided for a light receiving element having a light receiving diameter of 1571 mφ. Furthermore, when an optical fiber with a spherical termination is used, positioning of the optical fiber can be facilitated by inserting a portion of the optical fiber into the recess.

Although the condenser lens was formed on the back surface of the substrate in the embodiment of the light receiving element according to the present invention, it may be formed on the surface of the substrate. For example, by using ITO, it is necessary to eliminate the light blocking area caused by the electrode. Furthermore, an advantage of manufacturing 1 of this photodetector is that the entire structure has a brainer structure, but if you try to integrate it with other electrical elements as an 0EIC to form an optical integrated circuit, you will have problems with electrical isolation. Therefore, for example, the InP substrate may be semi-insulating and the light receiving element may have a mesa structure, or a dielectric band may be embedded between the elements to provide dielectric isolation.

By the way, in this embodiment, the inner diameter of the recess of 11H is set to 1.
Although the diameter of the recess is set to 5 μm, by increasing the inner diameter of the recess, it is possible to insert an optical fiber into the recess and further facilitate positioning. In addition, as the insulating films, we used Si3N4 films, SiO2 films, and films with m composition among them, but other insulating films or organic films may also be used as long as they can be deposited while changing the refractive index difference. Although the photo-CVD method was used to deposit the insulating film, other deposition methods may be used.

Further, in the embodiment, the light receiving region is formed by vapor phase diffusion of Zn, but it is also possible to form the light receiving region by, for example, an ion implantation method, or it is also possible to separate the light receiving region by etching. . Note that in the description of the above embodiments, jnP-based semiconductor materials have been used, but other semiconductor materials may be used. Also, P.I.
It is also possible for the N photodiode to be, for example, an avalanche photodiode, an MSM photodiode, etc.

Effects of the Invention As described above, according to the present invention, by depositing an insulating film on the light receiving element, it is possible to obtain a light receiving element that operates at high speed and has a wide light receiving area, and is easy to manufacture and suitable for mass production. . Since it has a planar structure, it is easy to implement. Also,
In order to increase the operating speed of a light receiving element, the diameter of the light receiving area is becoming smaller and coupling with an optical fiber is becoming difficult, but by using the light receiving element of the present invention, it is possible to obtain a light receiving element that has a large margin for alignment with an optical fiber. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the structure of a light-receiving element according to an embodiment of the present invention;
Fig. 2 is a cross-sectional view of a method of manufacturing a light-receiving element according to an embodiment of the present invention, Fig. 3 is a principle diagram showing the operation of the present invention, Fig. 4 is a cross-sectional view of a conventional light-receiving element, and Fig. 5 is FIG. 3 is an explanatory diagram of a method for producing an optical fiber. DESCRIPTION OF SYMBOLS 1... InP substrate, 2... Light absorption layer, 3... Light transmission layer, 4... Light receiving area, δ... N side electrode, 6...
N-side electrode, 7... mouth, 8...! @ Membrane limbus. Name of agent: Patent attorney Shigetaka Awano

Claims (2)

[Claims] (1) A compound semiconductor substrate, a light-absorbing layer laminated on the substrate, a light-receiving region formed in a part of the light-absorbing layer and having a different conductivity type from the light-absorbing layer, and a light-receiving region formed on the back surface of the substrate. and a condensing lens formed by laminating an insulating film with a sequentially changed refractive index on the concave portion, and the condensing lens condenses an optical signal onto the light receiving area. Light receiving element. (2) A step of epitaxially growing a light absorption layer on a compound semiconductor substrate, a step of forming a light receiving region of a conductivity type different from that of the light absorption layer in a partial region of the light absorption layer, and a step of etching the back surface of the substrate into a concave shape. A method for manufacturing a light receiving element, comprising the steps of depositing an insulating film on the concave portion while sequentially changing the refractive index.