JP2006295222A - Semiconductor light receiving element module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light receiving element with little signal distortion in a light receiving element for receiving an analog optical signal. <P>SOLUTION: An internal flat peripheral convex lens is installed on a substrate surface so that the rays of light can be made incident from the substrate side. Thus, light intensity can be made uniform in a light receiving layer by the internal flat peripheral convex lens even when the rays of light of a laser are Gaussian. Since light density is made uniform, distortion can be reduced, and sensitivity can be increased. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light receiving element (photodiode) used for optical communication and optical measurement, and a photodiode module in which the light receiving element is combined with an optical fiber. In particular, the present invention relates to a light receiving element module in which the distribution of optical power on the light receiving surface is more uniform and distortion of received signals is small.

(1) Photodiode Chip According to Conventional Example First, a general photodiode chip conventionally used is shown in FIG. The material of the photodiode varies depending on the wavelength. In optical communication and the like, near infrared 1.3 μm or 1.55 μm is often used. Since these lights have longer wavelengths than the light generated by the GaAs element, they may be referred to as long wavelength bands. In the long wavelength band of 1.3 μm and 1.55 μm, pin PD mainly using InGaAs as a light receiving layer is used. The photodiode 1 is formed on an epitaxial wafer obtained by epitaxially growing an n-type InP buffer layer 3, an n-type InGaAs light-receiving layer (light absorption layer) 4, and an n-type InP window layer 5 on an n-InP substrate 2. . Zn is diffused in the center of the element unit to form the p-type region 6. The boundary between the p-type region 6 and the n-type semiconductor is a pn junction 7. A passivation film 8 is formed except for the light receiving portion. An antireflection film 9 is coated on the light receiving portion and the passivation film 8. A p-electrode 10 is provided on the light-receiving part diffused with Zn. An n-side electrode 11 is formed on the back side of the substrate.

Since there are many identical elements on the wafer, the chips are cut out to form chips. When the chip is put in a package, the electrodes and leads are wire-bonded and covered with a cap, and hermetically sealed, an independent light receiving element is obtained. Although it may be an independent element, it is also used as a light receiving element module in which an optical fiber and a light receiving element chip are integrated for ease of use.

(2) When the PD module module according to the conventional example is used, the light receiving element chip 1 is fixed to the case (package) 12 and the optical fiber 14 is provided so as to face the same. In order to float from the case 12 connected to the ground of the subsequent electric circuit, the PD chip is fixed to the ground through an insulator. This insulator is called a submount 13. For example, the submount 13 is formed by metallizing both surfaces of a ceramic plate such as alumina (Al ₂ O ₃ ) or AlN. The submount 13 is fixed to the package 12, and the light receiving element chip 1 is fixed thereon by PbSn solder. Thereafter, the anode (p electrode) and cathode (n electrode) of the PD are wire-bonded to the leads 15 and 16 with Au wires.

A metal cap 22 is placed for hermetic sealing. In this example, a cap 22 with a spherical lens 23 is used. This is to facilitate coupling with the optical fiber 14. The case pin 17 is directly attached to the package 12. A sleeve 18 is fixed to the outside of the package 12. A ferrule 20 is inserted through the ferrule holder 19. The ferrule 20 supports the tip of the single mode fiber 14. The ferrule holder 19 is moved horizontally with respect to the sleeve 18, and the light from the fiber 14 is detected by the photodiode 1, and the ferrule holder 19 is fixed to the sleeve 18 at a position where an appropriate amount of light is obtained. Further, the ferrule 20 is moved in the axial direction, the light quantity of the photodiode is monitored to determine the position in the axial direction, and the ferrule 20 is fixed to the holder 19. A bend limiter 21 is inserted outside the ferrule holder 19 to prevent excessive bending of the fiber.

Japanese Patent Application No. 6-171873 “Analog PD Module and Manufacturing Method” M.M. MAKIUCHI, O.I. WADA, T.W. KUMAI, H.M. HAMAGUCHI, O.I. AOKI, Y.A. OIKAWA, “SMALL-JUNCTION-AREA GaInAs / InP pin PHOTOODIDE WITH MONOLITIC MICROLENS”, ELECTRONICS LETTERS, vol. 24, no. 2, p109, (1988) Japanese Patent Laid-Open No. 4-111477 (Japanese Patent Application No. 2-230206)

It is an object of the present invention to provide a light receiving element used as a receiver of an optical CATV and a module thereof. It will be described what kind of problems occur if a conventional light receiving element or its module is used for optical CATV. In early optical CATV, only a few channels (ch) were transmitted. However, 40CH is standard now. Recently, there is also a demand for a system that can further transmit 80CH to 110CH. As the number of channels increases, the frequency band of the receiver must be widened. Conventionally, a bandwidth of 450 MHz is sufficient, but an attempt to increase the number of channels requires a frequency band of 860 MHz.

There are further problems. More homes (terminals) have joined, and it has become necessary to send signal light to more terminals. For this reason, it has become necessary to use a laser light source with higher output. As a result, the received light becomes stronger, and laser light having a strong power of 1 mW or more has entered the light receiving element. In other words, the frequency is higher and the light intensity is more concentrated.

In a conventional PD module, it has been difficult to receive signal light having a high power concentration and a high frequency without causing a distortion without reducing sensitivity. Here, the distortion is a disturbance of the received signal due to the nonlinearity of the receiver, and means that higher-order terms such as second-order distortion and third-order distortion are included in the output of the light receiving element. Since optical CATV transmits an analog signal, the sum f ₁ + f ₂ and the difference f ₁ -f _{2 of} any two transmission frequencies f ₁ and f ₂ appear if there is second-order distortion. If these values are close to the third frequency f _3, interference occurs between them. Since optical signals including signals of many frequencies are transmitted by a single optical fiber, if there is distortion, the quality of the received signal is significantly reduced. It is desirable that there is no distortion.

Why does distortion occur? Why do higher-order terms and harmonics occur? The cause is not clear. But I know that: The second order distortion is more remarkable as the light power increases. Distortion tends to occur when the number of channels is large. Since it is a high-order term, it is natural that it appears more easily as the power increases. If the lens is strongly squeezed geometrically and optically as in the prior art, the power will increase locally and distortion will appear. In other words, the progress of optical CATV has a strong tendency to increase distortion.

As the distance of the lens of the light receiving element is changed, the sensitivity and distortion vary as a function of distance. Distortion decreases at a distance farther than the position where the sensitivity is maximum. If priority is given to distortion suppression, sensitivity will drop. Therefore, he said it was difficult to reduce distortion without reducing sensitivity.
The present inventor has proposed a light receiving element module that actively uses the aberration of a spherical lens as a patent application of Patent Document 1 in order to reduce distortion.

In Patent Document 1, the light receiving element is brought closer to the lens side than the image point on the end face of the optical fiber by the spherical lens. The point where the sensitivity remains high and the distortion is small is located closer to the lens than the image point. This is a technique for reducing distortion by defocusing the light receiving element position forward.
The present invention also proposes another method for reducing distortion without reducing sensitivity in an analog PD module. This makes it possible to realize a receiver with less distortion with fewer components. The distortion increases because the light power is locally excessive. Conventional spherical lenses and lenses with spherical surfaces concentrate all light at one point. This point is the image point on the end face of the optical fiber. Distortion tends to occur when one point is concentrated.

It is a first object of the present invention to provide a light receiving element that receives an analog optical signal and has less signal distortion. It is a second object of the present invention to provide a light receiving element that can receive signals without causing interference even when there are a large number of channels. It is a third object of the present invention to provide a light receiving element module that enables reception with low distortion without increasing the component cost.

In the light receiving element of the present invention, a lens having a flat surface with no condensing action at the center and a convex surface at the periphery is formed integrally with the light receiving element chip. As a result, the light from the optical fiber is uniformly spread over the entire light receiving surface of the light receiving element without being concentrated at one point. The light intensity is made substantially constant on the light receiving surface, and almost no light is present outside the light receiving surface. The lens has a special cross-sectional shape. The central portion is flat and has no light collecting action, and the peripheral portion is a part of a spherical surface and has a light collecting action. Simply put, it can be called a mid-plane circumferential convex lens. More simply, it will be called a mesa lens. The light receiving element of the present invention is characterized in that a central plane convex convex lens or a mesa lens is integrally formed on the light receiving element.

The outline of the idea of the present invention will be described in advance with reference to FIGS. FIG. 15 shows the refraction of light rays when a mid-plane circumferential convex lens is formed on the back surface of the semiconductor substrate. The lens is flat between LJ and is a convex spherical lens between LN and JH. The beam incident on the circle inside the LJ goes straight without being refracted. A beam included in a conical angle which is half the opening angle of a single mode fiber is made to enter the flat surface LJ. The outer beam is placed in the convex spherical lens portions NL and JH.

The light receiving layer of the light receiving element is indicated by Q below. This is also a circular area. Since the light that has passed through the central portion enters between the LJs, the light enters the substrate without being refracted and travels straight and spreads over the entire light receiving layer Q. The light receiving layer Q thus receives the light that has passed through the central flat portion. That is not all. The light that has reached the peripheral portions LN and JH is strongly refracted, and the light that enters farther is bent inward and reaches the points I and P near the center of the substrate. That is, the distribution of LN in the peripheral portion is inverted and spreads in the MP range on the substrate. This is because the focal points of the partial lenses NL and LH are located inside the substrate, and the light receiving surface is far from the focal point.

This provides a convenient power redistribution. The beam in FIG. 16 shows Gaussian, but just before the lens, it is Gaussian itself as shown by NLJH. It has a distribution of Ikeaukoa. However, the NL and JH portions are inverted by the lens, so that the distributions A and B are bent along the vertical lines L and J, and overlap each other. In other words, it is corrected to a distribution in which moss and ki are stacked on top. In other words, power redistribution occurs like i → ki and a → ka. This is caused by the mid-plane circumferential convex lens NLJH.

Several proposals have already been made for forming a built-in lens on a light receiving element. However, all of them are merely a lens portion formed on a chip, omitting an independent spherical lens for cost reduction. Therefore, a spherical lens having no aberration is ideal. Because it is a spherical lens, it concentrates light on one point. Therefore, the distortion is large. Not suitable for handling analog multi-channel signals. Although the concept is different from that of the present invention, a conventional example of a built-in lens will be described here by Non-Patent Document 1.

Non-Patent Literature 1 is a lens in which a part of a spherical surface is formed on the bottom surface of an InP substrate of an InP photodiode having InGaAs as a light receiving layer. The size of the chip is 200 μm × 200 μm. The radius of curvature of the spherical lens is 55 μm, and the aperture of the lens is 50 μm. In this method, a photoresist is applied to the bottom surface of the substrate in a lens shape and etched to reduce the etching amount at the center and increase the etching amount at the periphery to make a lens that forms a part of a spherical surface neatly. This is to increase the light collecting property, and distortion is not a problem.

Although it is the same in that a built-in lens is provided on the light receiving element chip, the purpose of the present invention and these conventional examples are different, and the shape of the lens is different. It is important to clearly distinguish between the two. First, the principle of the spherical lens according to the conventional example will be described.

FIG. 3 shows a case where a light beam from an optical fiber is incident on a light receiving element having a built-in spherical lens according to a conventional example. A beam emerges from the end point G of the optical fiber. This proceeds while spreading in a conical shape. Let axis be GT. A spherical lens b and a light receiving surface (light receiving layer) Q are shown. The light receiving surface Q corresponds to the p-type InP6 portion of FIG. The light enters from the back side, and light enters from the bottom side of the element of FIG. The n electrode is ring-shaped. GH and GN indicate the full opening angle of the optical fiber. The opening angle θ is determined by the refractive index of the cladding and the core, but is 5.7 ° here. A portion having a divergence angle of exactly half the angle (2.9 °) is indicated by GJ and GL. Since this element is manufactured by making a lens b having a part of a spherical surface on the substrate side, all light rays are condensed. The radius of curvature of the lens is 100 μm. The distance between the light receiving element and the optical fiber is 500 μm.

Light rays GH and GN along the opening angle bend like HI and NP. Half-angle rays GJ and GL are refracted like JK and LM. These light rays gather in the light receiving layer Q of the light receiving element. The light receiving layer Q has a spread in the surface direction. Light is not uniformly distributed in the light receiving layer (surface) Q. Since the spherical lens b is an imaging lens having a spherical surface HJLN, it tends to concentrate on one point. Since the lens is formed so that the light receiving surface Q comes to the image formation position, the light intensity is high near the center and the light intensity is weak at the peripheral part. Since the sensitivity depends on the total amount of light entering the light receiving surface, if only the sensitivity is a problem, the light may be concentrated on the light receiving surface Q at one point.

However, distortion is a problem in the present invention. When a harmonic appears in the light receiving element, a difference frequency or a sum frequency appears, causing interference with other channels. Why does such nonlinearity appear? Although not yet clear, the present inventor believes that one cause is excessive concentration of optical power. The built-in lens shown in FIG. 3 has an effect of increasing sensitivity, but has no power for distortion. Assuming that the spatial intensity distribution of the laser beam is Gaussian, the lens focused by the lens is also Gaussian. In other words, the distortion becomes excessive just by squeezing with the lens.

Therefore, the present inventor condenses the light from the optical fiber by the lens, and measures the distribution of the light quantity in the direction perpendicular to the axis at the positions before and after the focal point, and the distortion IMD ₂ at that position. Thereby, it is possible to know the correlation between the spread in the direction perpendicular to the axis of the beam and the secondary distortion IMD ₂ .

FIG. 5 shows the result of actual measurement of the light distribution in the axial vertical direction at five positions A, B, C, D, and E having different distances in the axial direction. The total amount of light is the same everywhere. Point A is the position of the image of the end face of the optical fiber by the lens. Of course, the concentration of light is high and most of the optical power is concentrated in the range of ± 10 μm from the center. Point B is a position defocused slightly toward the lens. The optical power at the center is reduced to 0.4 compared to point A. Point C is a position closer to the lens side than B. The optical power at the center is further reduced, and is reduced to about 0.16 times that of A, and the power at the periphery is relatively increased. The range in which the optical power exists extends to about ± 30 μm. Point D is a more defocused position. The spread of optical power is as much as ± 40 μm. At point E, the optical power at the center is about 0.08 compared to A, and the light spreads to the peripheral part. The spread is about ± 50 μm.

Even if it spreads in this way, it is only necessary to detect the light intensity with the light receiving element instead of looking at the image. It is only necessary that the light receiving area of the light receiving element covers these beam spreads. If the spread of the light receiving element is set to ± 50 μm or more, that is, the diameter is set to 100 μm or more, the total amount of light can be received even if the defocusing is significant as in point E. FIG. 5B shows the measurement result of the second order distortion IMD ₂ at each position. This reveals surprising things. The horizontal axis is the light density ratio. This shows the concentration of the optical power at the axis center in dB units with reference to A. The vertical axis represents the second order distortion.

When the signal s is incident and the output of the light receiving element is expressed as as + bs ² +..., The second harmonic is divided by the primary component, which is 10 log (bs / a). On the horizontal axis, the light power at the center becomes weaker as it goes to the left, and the dispersion is severe. However, the second-order distortion is smaller in the beam form having a large dispersion. Point A is that the light receiving element is placed at the focal point, but the second-order distortion is large, −57 dB. At point B, the beam spreads, but the distortion is reduced to -66 dB. At point E, the light is widely dispersed, but the second order distortion is as extremely low as −83 dB.

Such a result is contrary to common sense. In terms of the imaging system, it is best to place the light receiving element at the focal position of point A. But the distortion is greatest. On the other hand, since no image is formed at the point E where the light spreads, placing the light receiving element at this position may have resistance.

However, actually, contrary to expectation, this position has the smallest secondary distortion. A good imaging system will tend to think of good reception with low distortion. But the fact is exactly the opposite. A good imaging system causes severe distortion. The present inventor is probably the first to notice such reciprocity between distortion and imaging.

Now, let's take up the problem of beam distribution. For this purpose, the distribution of the optical power in the direction perpendicular to the axis of the light propagated through the optical fiber will be described first. FIG. 4 shows the angular distribution of light emitted from the optical fiber when a Gaussian beam is assumed. This depends on the laser transverse mode, but a Gaussian beam is taken as a typical example. The horizontal axis is the emission angle θ. The angle at which the amount of light becomes 0 is the opening angle. That is, points H and N correspond to this point. θ _1/2 is an angle at which the power is reduced to half of the center. L and J correspond to this.

Even if such Gaussian is condensed or diffused by a lens having no aberration, the distribution in the lateral direction is always Gaussian. To say that there is no aberration means to faithfully scale the original image. When a beam localized at the center, such as Gaussian, is focused as shown in the example of FIG. 5A, this causes the most distortion. FIG. 5 shows that distortion can be reduced by not localizing. The prior application of the present inventor reduces the distortion by shifting the light receiving element to the lens side from the imaging point.

In the present invention, concentration of optical power at the center is also prevented by different methods. What method should be used? Devise a lens. The central part is substantially flat and the peripheral part is a part of a spherical surface. There is an effect of focused image formation in the peripheral area. However, the central portion does not have an image focusing function.

This expression is straightforward but inaccurate. It is at a specific distance z that the central part and the peripheral part have an imaging effect. Assuming that the focal length of the central portion is f ₁ and the focal length of the peripheral portion is f ₂ , f ₁ > f ₂ , and the light receiving surface of the light receiving element is positioned in the vicinity of the imaging point of the peripheral portion. It is. In terms of the radius of curvature, the curvature ρ _{1 at the} center is smaller than the curvature ρ _{2 at the} periphery (ρ ₁ <ρ ₂ ), and the light receiving surface is near the image point by the periphery.

That there are two conditions, one geometric specificity of the lens, the distance z ₁ of the lens and the light receiving surface is another condition. The latter condition is also important. The former is a geometrical variant of the lens and is easy to understand. But the latter condition must not be forgotten.

If it does so, the light which reached | attained the peripheral part of a lens will be condensed in the center direction, but the light which reached the center part will not be condensed. Due to such radial condensing peculiarities, even in the case of Gaussian at first, the central portion has almost flat light power and the peripheral portion has almost no light power. That is, light having a centrally concentrated Gaussian distribution is converted into a rectangular distribution by the mesa lens of the present invention.
FIG. 7 shows the optical power of a rectangular distribution. The horizontal axis is the angle θ. This can also be interpreted as spreading on the light receiving surface. Most of the optical power falls between lines L and J. Very little light leaks outside the LJ.

The higher the light density in the light receiving layer, the greater the distortion. Sensitivity is proportional to the total amount of light. According to the present invention, since the mid-plane circumferential convex lens is built in or fixed to the light receiving element chip, the light density distribution becomes uniform in the light receiving layer. In the present invention, since the light density is almost uniform in the light receiving layer, the distortion is small and the sensitivity is high. Since distortion is small, it is optimal as an analog PD module. Crosstalk does not occur even if the number of channels is large. A highly sensitive and low distortion receiver can be manufactured. It has optimum performance as a light receiving element module of an optical CATV receiver and a light receiving element used for optical measurement.

[Example 1: Light-receiving element chip with a light-receiving layer diameter of 50 μm]
FIG. 6 is a schematic cross-sectional view of the light receiving element module according to the first embodiment of the present invention. An InGaAs light receiving layer Q is provided on the InP substrate. This is a back-illuminated light receiving element chip. The thickness of the chip is 100 μm. Make a mesa lens on the back (that is, the substrate surface). That is, the central portion JL has a flat surface, and the peripheral portions LN and HJ have a spherical surface with a radius R = 100 μm. The distance from the optical fiber G is 500 μm. 3 is different from that of FIG. 3 in that the central portion LJ of the lens is a flat surface. The LJ portion is a circular region having a radius of 50 μm (LJ = 100 μm). The overall size of the lens is a radius of 100 μm (HN = 200 μm).

In the case of a single mode fiber for 1.3 μm, the aperture angle is 5.7 degrees. However, the particularly strong portion is in the range of 2.9 degrees. Therefore, most of the light amount is included in the GL and GJ portions. Since JL is a flat surface, it is refracted according to Snell's law at the boundary, but becomes almost parallel to the optical axis. Since the refractive index of InP is as large as 3.5, the divergence angle is only 0.83 degrees even at the end point of JL. That is, JK and LM are almost parallel to the optical axis GT. There is no light collecting effect. It can be said that the beam distribution at the LJ (the central part of Gaussian) is directly transferred to the KM. This alone does not make the light intensity uniform between the LJs. It remains Gaussian. It is the light coming from the outside that fills the decline at both ends of Gaussian.

The remaining light is between 2.9 ° and 5.7 °. These are refracted and condensed by a part of the spherical lens JH and LN. The light receiving layer Q is located at the imaging distance of the spherical surfaces JH and LN. The exact setting of the distance f from the lens end surface to the light receiving layer is important. By appropriately determining the distance from the lens to the light receiving surface, almost all the light refracted at the peripheral portion is incident on the light receiving surface, and the light intensity distribution in the light receiving surface can be made uniform. For example, the outermost 5.7 ° light is refracted and narrowed at an angle of 20.4 °. In order to make the spread 50 μmφ (area of the light receiving layer), the light receiving layer may be formed at a depth of w = 67 μm from the back surface of the substrate.

With such a configuration, uniform light intensity can be obtained within the light receiving layer surface as shown in FIG. Since the light in the central part travels in parallel, the distance w is not important. However, the distance w is important for ambient light passing through the lens portion. A built-in lens is advantageous because the distance must be determined precisely. When attaching a mesa lens to the outside, it takes time to position the lens. It can also be inaccurate. Once built, positioning is not necessary.

From this point of view, when the light power is to be made to be a rectangular distribution using a mesa lens, a built-in lens may be fabricated on the back surface of the substrate as a back-illuminated light receiving element as shown in FIG. This is written in a shape in which the light receiving element of FIG. 1 is turned upside down. An n-type InP buffer layer 3, an n-type InGaAs light receiving layer 4, and an n-type InP clad layer (window layer) 5 are epitaxially grown on the n-type InP substrate 2, and a p-type is formed at the center of the n-type InP clad layer. Region 6 is formed. Apart from this, there is also a p-type region 30 in the periphery. This is a p-type region for absorbing leaked light. Even if light leaks into the peripheral part and is photoelectrically converted here, there is a pn junction, and electric charges cannot travel to the electrode. Leaked light does not generate photocurrent, so it has excellent responsiveness. A p-side electrode 31 is present on the central p-type region. This is not a ring electrode. This is because it is not necessary to enter light from the p side. The region other than the p-side electrode 31 is covered with the passivation film 8.

Although this is a type of element in which light enters from the back surface, a built-in lens 33 is formed at the incident portion. This is a lens made by leaving a part of an n-type InP substrate. It is not a lens with a simple spherical surface. The mesa-type lens has a flat surface 34 at the center and a spherical surface 35 at the periphery. An n-side electrode 32 is formed around the lens. When the mesa lens is used in this way, the optical power between the specific spreads LJ becomes almost constant as shown in FIG. 7, and the optical power on the outside becomes almost zero. This is an effect of a mesa lens.

There is another built-in effect. One is that the cost of parts can be reduced because the number of members is reduced. The other is a more fundamental advantage. In order to change the Gaussian beam into a rectangular distribution as shown in FIG. 7, the distance between the lens and the light receiving surface must be strictly defined. Even if an independent mesa lens is produced separately, alignment is difficult. Work such as axial alignment and alignment in the direction perpendicular to the axis takes time and increases manufacturing costs. If it is built-in, such alignment fixing work is completely unnecessary.

How can you make a built-in mesa lens? The problem will be narrowed down here. However, it is easy to make a mesa lens. This will be described with reference to FIG. As shown in FIG. 9A, a resist 38 is formed in a hemispherical shape on the back surface of the InP substrate 2. Then, ion etching is performed by irradiating the ion beam 39. The portion 40 covered with the resist 38 remains as it is. The uncovered portion 41 is removed uniformly. The portion 40 covered with the resist 38 is not safe. The resist is also gradually etched away by the ion beam.

Since the resist 38 is shaved from the edge, the covering portion is reduced. The substrate is exposed from the end portion 42 of the covering portion 40, and the portion is shaved. Since the exposed portion of the resist 38 gradually becomes lower and narrower, the exposed portion is shaved. As shown in FIG. 9B, the substrate component remains in a lens-like shape as the resist 38 decreases. Etching until the resist 38 has completely disappeared produces a completely spherical lens.

However, the etching is not advanced so far. As shown in FIG. 9B, the etching is finished when a little resist 38 is left in the central portion. The portion 43 where the resist disappears is a part of the convex spherical surface. The portion where the resist 38 remains is a flat surface 44. In this way, it is important to stop the etching with the resist partly left. Here, when the resist is removed, an intermediate flat convex lens 33 composed of a flat surface 34 and a convex spherical surface 35 is produced as shown in FIG.

[Example 2: Light receiving element chip with light receiving layer diameter of 80 μm]
In the first embodiment, the case where the light receiving diameter is 50 μm has been described. The frequency used in optical CATV is at most up to 1 GHz. Since it is not so high, it works well even if the light receiving diameter is quite wide. The light receiving diameter of the PD is 80 μm to 100 μm. Therefore, an embodiment in the case where the light receiving diameter is 80 μm will be described.

FIG. 10 shows the cross section of the light receiving element and the refraction of light rays. The thickness of the chip is about 120 μm including the substrate and the epitaxial layer. The diameter of the light receiving surface (light receiving diameter) is 2r = 80 μm. A mesa-type built-in lens having a diameter of 160 μm is formed on the back surface of the substrate. There is a flat surface JL in the center, and the diameter is 80 μm. The diameter of the light receiving layer is set equal to 80 μm. Therefore, the concentric part of r = 40 μm to 80 μm is a convex curved surface forming a part of a spherical surface.

As described with reference to FIGS. 3 and 6, in the case of a single mode fiber for 1.3 μm, the aperture angle is 5.7 °, but the strong light is mainly 2.9 ° (vertical angle). Is contained in a cone of 5.8 °. The cone of θ = 2.9 ° coincides with the flat portion LJ (80 μmφ) of the lens in FIG. 10 when the distance between the fiber end and the lens is L = 800 μm. That is, L is determined by 2Lθ _1/2 = JL. Then, the strong light included in θ _1/2 = 2.9 ° is incident on the flat surface JL. Since this is a simple plane, it is not condensed but becomes almost parallel light (because the tilt angle is 0.83 °) and the refractive index of InP is large, and reaches the light receiving layer. It goes straight as it is like JK and LM and spreads over almost the entire area of the light receiving layer.

Even so, even if it is included in 2.9 °, it is originally Gaussian, so the optical power at the edge is relatively weak. It is the convergent light from spherical surfaces JH and LN that compensates for this. It has a part of a spherical shell with a radius of 160 μm. Light entering the LN is refracted to a range surrounded by LU and NP. These lights enter the UP portion at the end of the light receiving layer. The angle formed with the LU axis is 9.6 °. The outer ray NP makes an axis of 20.4 °. The light receiving layer is 108 μm from the back surface. The point N on the lens surface and the point P on the light receiving layer are separated by 40 μm (= 108 tan 20.4 °) in the direction perpendicular to the axis. That is, the point M is the end of the light receiving layer.

That is, the light passing through LN and JH is refracted further inside than the points M and K to enhance the light power of the light-deficient portions at both ends of the light receiving layer. The distribution is truly excellent. This is an example in which the light receiving layer is wider than in the first embodiment. The distribution of optical power is as shown in FIG. The manufacturing method of the mid-plane circumferential convex lens is as described in FIG.

[Example 3: Application to PD Module] The PD chip 50 is a back-illuminated chip having a structure as shown in FIG. The p-side electrode is fixed to the stem 52 via the submount 51 so that the p-side electrode is on the lower side. The stem 52 is a circular package and has lead pins 53, 54, 55 and the like facing downward. The submount 51 is a ceramic plate metallized on both sides. Since it is necessary to insulate the p-side electrode from the stem 52, the submount 51 is used. As described above, the light receiving element chip 50 is of a back-surface incident type, and an intermediate plane convex lens 33 is formed at the incident portion. A central flat portion 34 and a peripheral convex spherical surface 35 are included. A cap 56 is provided on the chip 50. The cap has a window in the middle, and the glass window 57 is fixed. Further, the holder 58 is fixed on the stem 52.

The holder 58 supports the ferrule 59 directly above the light receiving element chip. The ferrule 59 supports the tip of the single mode fiber 60. The tip of the fiber 60 is polished obliquely. The p electrode is connected to the lead pin 53 by a wire 62. The n-electrode is manufactured on the lead pin 55 by the wire 61. Light is passed through the fiber, light is incident on the PD, the photocurrent of the PD is measured, and the holder 58 is moved in the direction perpendicular to the axis so as to maximize the light quantity. Further, the ferrule 59 is moved in the axial direction and fixed at an optimum position. As a result, a rectangular optical power distribution as shown in FIG. 7 can be obtained, so that a light receiving element module with small distortion can be obtained. As described with reference to FIG. 10, the optimum position in the axial direction is such that the peripheral portion of the opening angle spread corresponds to the periphery of the mid-plane convex lens, and the light included in θ1 / 2 is the flat portion 34 and the curved surface. The position corresponds to the boundary of the part 35. It is also different from the maximum amount of light.

[Method for Manufacturing PD Chip] A method for manufacturing the light receiving element chip according to the embodiment of the present invention shown in FIG. 8 will be described below. An n-type InP wafer is prepared. An n-type InP buffer layer 3, an n-type InGaAs light absorption layer (light-receiving layer) 4, and an n-type InP window layer 5 are epitaxially grown thereon. On the back surface of the n-type InP substrate, a mid-plane circumferential convex lens 33 is formed by the method shown in FIG. A SiN antireflection film is formed on the lens. A p region 6 is formed at the center of the n-type window layer by Zn diffusion. The pn junction formed by this becomes the light receiving layer. The diameter of the light receiving layer is 80 μm (Example 2) and 50 μm (Example 1). A p-side electrode 31 is attached immediately above the p region 6. A p layer 30 is also formed in the periphery by Zn diffusion. This has the effect of absorbing leakage light and preventing it from going to the p-electrode 31.

Since incident light passes through the lens 33 at the center of the back surface of the substrate and enters from the substrate side, the n-electrode 32 on the substrate side is a ring-shaped electrode. Light enters from the center surrounded by the electrodes, passes through the buffer layer 3, and reaches the InGaAs light receiving layer. Here, electron-hole pairs are generated and disappear. Electrons travel toward the n-electrode 32 and holes travel toward the p-electrode. A photocurrent flows between the electrodes. As a result, photoelectric conversion is performed.

Since the light is squeezed by the lens 33, the light leaking to the peripheral portion should not originally exist, but if it leaks, an electron hole is generated in the p region 30 for absorbing leaked light, which may exceed the pn junction. It cannot be done and it does not become photocurrent. This is proposed by the present applicant in Patent Document 2. This is particularly necessary in a light receiving element in which light enters from the p side. Since the present invention is back-incident and has a lens, there is essentially no leakage light, but the influence can be completely blocked even if it exists.

A PD chip manufactured by such a method was mounted on a package. Then, an optical fiber was attached to obtain the light receiving element module of FIG. A reverse bias of 15 V was applied between the electrodes of the light receiving element. Second-order distortion IMD ₂ was measured when 0 dBm light was introduced from the optical fiber. The secondary distortion was -85 dB. Furthermore, the optical power density at the center of the light receiving element was measured. The density of light that is not localized in the center as in Gaussian (FIG. 8) is low and is −12 dB. When this is written in the graph of FIG. 5B, it becomes as shown in FIG. For the light receiving element of the present invention, yet the density is lower than the most amount of light is small case E in the conventional example, IMD ₂
Is also small.

[Example 4: When a lens is added]
The PD of the present invention has a light collecting property because it has a built-in mesa lens on the substrate side, but that alone strictly determines the distance between the optical fiber and the light receiving element chip. When there is a desire to increase the distance between the optical fiber and the lens, the lens is inserted between the optical fiber and the light receiving element. Since the light can be collected by the lens, the optical fiber can be displaced further. FIG. 13 shows a configuration of a light receiving element module provided with such a lens. When a lens without aberration is provided, the calculation is simple. What is necessary is just to determine so that the light of (theta) _1/2 may hit the boundary of the plane curved surface of a mid-plane circumference convex lens.

However, in the case of a lens having a large aberration such as a spherical lens, a little care is required. Since the spherical lens refracts far-axis light strongly and refracts paraxial light weakly, the light beam on the light receiving element chip surface does not become as shown in FIG.

[Example 5: When the lens boundary is removed from θ _1/2 ]
In the embodiments described so far, the boundary of the plane curved surface of the middle plano-convex lens is matched with θ _1/2 , but this is not restrictive. Since the optical power distribution may be rectangular as shown in FIG. 7, even if the beam is Gaussian, the boundary and θ _1/2
In some cases, a rectangular distribution can be obtained without matching. It is often said that the beam is not Gaussian. In this case, the correspondence between the light spread and the lens boundary may be optimized by the distribution of the optical power of the beam. For example, in the case of a light source in which the light intensity is high at the center and the light power is drawn to the periphery, the flat part of the lens should be narrowed and the spherical part should be widened.

[Example 6: Retrofit lens]
In the embodiment described above, the lens is built-in. This makes the material the same as the substrate. As shown in FIG. 14, an independent midplane convex lens may be made of another material and may be attached to the back surface of the chip substrate. Doing so frees you from material limitations. If the material can be selected, the refractive index can be freely selected to some extent, so that the degree of freedom in designing the diameter, curvature, boundary line, etc. is expanded. For example, glass, Si, or the like can be used as a lens material. The lens is attached to the back of the substrate with a transparent resin.

Sectional drawing of the PIN photodiode chip concerning a prior art example.

The perspective view of the light receiving element module concerning a prior art example.

Explanatory drawing which shows refraction of the light ray in case the light from an optical fiber injects into the light receiving element chip | tip which has a built-in lens concerning a prior art example.

The graph which shows the angular distribution of the beam which the laser which produces | generates a Gaussian beam as a typical example of the laser currently obtained.

A graph showing light intensity distribution in the direction perpendicular to an axis at five points A, B, C, D, and E on the optical axis of the laser and second-order distortion IMD ₂ when a light receiving element is set at that point. (A) shows the light intensity distribution in the direction perpendicular to the axis at five points, and (b) shows IMD ₂ at the five points.

FIG. 3 is an explanatory diagram of light rays showing refraction of light rays when light from an optical fiber enters the light receiving element of the present invention.

The graph which shows what kind of light density distribution will be in a light reception layer when the light from an optical fiber injects into the light receiving element of this invention which has a mid-plane circumference convex lens. The Gaussian beam is shaped into a rectangular distribution.

FIG. 3 is a cross-sectional view of a light-receiving element chip that is a substrate surface incident type according to an embodiment of the present invention and has a middle planer convex lens on the substrate side.

The figure explaining the method of producing a mid-plane circumference convex lens in the back surface side of a board | substrate in the light receiving element of this invention. (A) shows a state in which a resist is applied to the substrate surface side with a thickness distribution that forms a part of a sphere and is etched by irradiation with an ion beam. (B) is a diagram showing that when ion beam etching is stopped halfway, a part of the resist remains in the central portion and the peripheral portion thereof is etched on the spherical surface.

Sectional drawing which shows refraction of the light ray of the light receiving element concerning the Example of this invention which has a light receiving layer of a diameter of 80 micrometers.

Sectional drawing of the light receiving element module concerning the Example of this invention.

Graph fill the results of embodiments of the present invention are arranged in a conventional example of the results on a graph showing the correlation of optical density and second-order distortion IMD ₂ in the center of the light receiving layer.

A light receiving element showing an example in which a separate lens is added between an optical fiber and a light receiving element having a built-in plano-convex lens according to an embodiment of the present invention so that the distance between the optical fiber and the light receiving element can be adjusted. The block diagram of a module.

The side view of the light receiving element chip | tip which provided the retrofitted intermediate | middle plano-convex lens concerning another Example of this invention.

The geometrical ray diagram for demonstrating the effect | action of the mid-plane circumference convex lens of this invention.

The figure which shows intensity distribution before and behind the beam correct | amended by a middle plani convex lens.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light receiving element chip 2 n-type InP substrate 3 n-type InP buffer layer 4 n-type InGaAs light absorption layer 5 n-type InP window layer 6 p-type region 7 pn junction 8 passivation film 9 antireflection film 10 p electrode 11 n electrode 12 Package 13 Submount 14 Single mode fiber 15 Anode pin 16 Cathode pin 17 Case pin 18 Sleeve 19 Ferrule holder 20 Ferrule 21 Bend limiter 22 Cap with ball lens 23 Ball lens 30 P-type layer 31 for leaking light absorption p side electrode 32 n Side electrode 33 Middle flat convex lens 34 Central flat portion 35 Peripheral spherical portion 36 Incident light 38 Lens forming resist 39 Ion beam 40 Substrate portion covered with resist 41 Substrate portion not covered with resist 42 Peripheral boundary portion 50 PD chip 51 Submount G 52 Stem 53 Lead pin 54 Lead pin 55 Lead pin 56 Cap 57 Glass window 58 Ferrule holder 59 Ferrule 60 Single mode fiber

Claims (7)

A semiconductor light receiving element module for receiving an analog optical signal, comprising: an optical fiber; a package; and a back-illuminated photodiode chip for receiving signal light housed in the package and emitted from the optical fiber. The chip has a light receiving portion having a constant light receiving diameter on the first surface of the semiconductor substrate, and a central portion of the tip provided integrally at a position opposite to the light receiving portion on the second surface is a flat portion and a peripheral portion. Has an inner planoconvex lens having a lens effect, and the light beam incident through the outermost periphery of the flat part of the inner planoconvex lens out of the signal light travels substantially straight and enters the vicinity of the outer periphery of the light receiving unit, Among the signal lights, a light ray incident through a peripheral portion having a condensing function of an inner planer convex lens is incident near the outer periphery of the light receiving portion by the condensing function. Lumpur. A semiconductor light receiving element module for receiving an analog optical signal, comprising: an optical fiber; a package; and a back-illuminated photodiode chip for receiving signal light housed in the package and emitted from the optical fiber. The chip has a light receiving portion having a constant light receiving diameter on the first surface of the semiconductor substrate, and a central portion of the tip provided integrally at a position opposite to the light receiving portion on the second surface is a flat portion and a peripheral portion. Has an inner plano-convex lens having a lens effect, the signal light is a Gaussian beam, and light within an angle where the intensity of the Gaussian beam is halved passes straight through the flat part of the inner planer lens. Then, the light incident on the light receiving portion and the light outside the angle at which the intensity of the Gaussian beam becomes ½ passes through the peripheral portion having the light collecting effect of the inner planoconvex lens and is condensed. Semiconductor light-receiving element module, characterized in that entering the optical unit. The center of the inner planoconvex lens is on the extension of the optical axis of the optical fiber, the refractive index n2 of the semiconductor substrate, the light receiving radius r of the photodiode chip, the flat part diameter d = 2r of the inner planoconvex lens, and the lens opening D = 4r, lens radius 4r, the angle at which the light amount of the optical fiber is 0 is the aperture angle θ = θ _max , the light power is reduced to half of the center, and the divergence angle having exactly half the aperture angle is θ _{1 / 2}
The light receiving portion is W = ((D−d) / 2) × tan (60 ° + (sin ⁻¹ (sin θ _max + 30 °) / n ₂ ))) from the back surface of the substrate.
The semiconductor light receiving element module according to claim 1, wherein the end face of the optical fiber is arranged at (d / 2) tan θ _1/2 from the flat surface of the lens. 4. The semiconductor light receiving element module according to claim 1, wherein the light receiving layer of the photodiode chip is made of InGaAs. 4. The semiconductor light receiving element module according to claim 1, wherein the photodiode chip includes an InP substrate, an InP buffer layer, an InAlAs light receiving layer, and an InP window layer in this order. The light receiving diameter of the light receiving portion of the photodiode chip is approximately 50 μm, is approximately 67 μm deep from the back surface of the InP substrate, and the outer shape of the flat portion at the tip of the inner flat circumferential convex lens provided on the back surface of the substrate is approximately 50 μm. The outer diameter of the outer peripheral portion having a condensing function of the plano-convex lens is about 100 μm, the curvature radius of the bent portion is about 100 μm, and the optical fiber is a single mode fiber for wavelength 1.3 μm light and a photodiode chip. The semiconductor light receiving element module according to claim 1, wherein the distance is 500 μm. The light receiving diameter of the light receiving portion of the photodiode chip is approximately 80 μm, is approximately 108 μm deep from the back surface of the InP substrate, and the outer shape of the flat portion at the tip of the inner circumferential convex lens provided on the back surface of the substrate is approximately 80 μm. The outer diameter of the outer peripheral portion having a condensing function of the plano-convex lens is about 160 μm, the curvature radius of the bent portion is about 160 μm, and the optical fiber is a single mode fiber for wavelength 1.3 μm light and a photodiode chip. The semiconductor light-receiving element module according to claim 1, wherein the distance is 800 μm.