Lens cap, photodiode with a lens cap and method for its manufacture

Abstract

Photodiode with a lens cap (2) comprising a base (4) with an active photodiode area (6), wherein the active photodiode area (6) has a diameter d F comprising a cap (2) with a melting lens (3), wherein the melting lens (3) has a diameter d in a top view of a top surface of the cap (2). L characterized in that the ratio of the diameter of the melting lens (3) to the diameter of the active photodiode area (6) d L / d F more than 30, and wherein the lens cap (2) is designed as a metal cap, wherein the melting lens (3) is made of glass with a refractive index n (at 589 nm) greater than 1.55 and a coefficient of thermal expansion α (at 20- 300 °C) of less than 14 ppm / K.

Description

Field of invention

[0001] The invention relates to a photodiode with a lens cap. In particular, the invention relates to a photodiode by means of which light from an optical fiber is coupled onto a photodiode chip and converted into an electrical signal. The invention further relates to a lens cap for a photodiode and a method for manufacturing a lens cap.

[0002] The photodiode can be used in particular in a receiver for an optical data transmission network. Background of the invention

[0003] Photodiodes equipped with a lens cap and used in the field of optical signal transmission are known.

[0004] Patent specification US 2006 / 0081866A1 discloses an optical semiconductor device comprising a through-hole, an insulating element arranged in the through-hole, an optical semiconductor element, and a support on which the optical semiconductor element is mounted. The insulating element carries a plurality of conductors. This semiconductor device is intended to reduce parasitic capacitances and provide an arrangement with a structure that improves the accuracy of the positioning and angularity of the mounted optical semiconductor device.

[0005] For example, patent DE 10 2016 106 366 B4 (Schott AG) describes a lens cap for a TO (transistor outline housing) in which the wall of the top of the housing adjacent to the lens is thinned. This reduces the mechanical stresses caused by the high temperature during the melting of the lens.

[0006] Such a lens cap, which can be designed as a deep-drawn component, for example, is connected to a base, in particular by soldering. The chip with the active area of ​​the laser diode is arranged on the base.

[0007] To increase the transmission speed, fast switching of the photodiode is required. To achieve this, the capacitance of the photodiode's depletion region should be kept as small as possible. Reducing the depletion region's capacitance is primarily achieved by reducing the size of the photodiode's active area.

[0008] The spot projected by the lens onto the active area should therefore also be reduced in size.

[0009] With fusion lenses, reducing the size of the projected spot is primarily achieved by reducing the diameter of the lens.

[0010] Melting lenses are provided by placing a glass body into a window of the metal cap and melting it.

[0011] Due to the surface tension of the glass, a lens forms in the window of the cap.

[0012] The smaller the lens is intended to be, the more difficult it is to control during the manufacturing process. With small diameters, especially those of 1.3 mm or less, even the slightest fluctuations in the glass volume lead to significant variations in the lens shape. Object of the invention

[0013] In contrast, the invention is based on the objective of providing a photodiode with a lens cap, a lens cap, and a method for manufacturing them, which enables the simple provision of a photodiode with a small active area and correspondingly high possible data transmission rates. It is also an objective of the invention to contribute to increasing the data transmission rate in data transmission networks. Summary of the invention

[0014] The object of the invention is already achieved by a photodiode with a lens cap and by a method for manufacturing a lens cap according to one of the independent claims.

[0015] Preferred embodiments and further developments of the invention can be found in the subject matter of the dependent claims, the description and the drawings.

[0016] The invention relates to a photodiode with a lens cap.

[0017] The photodiode with the lens cap includes a base with an active photodiode area.

[0018] The base is designed in particular as a metal base, preferably made of a low-expansion material, and is connected to the lens cap, in particular welded.

[0019] The base contains a chip with an active photodiode surface, which is used to convert an optical signal into an electrical signal. The invention relates in particular to a photodiode designed for use on the receiver side, i.e., via which the optical signal from the light guide is converted into an electrical signal. The lens cap with the base and the photodiode is, in particular, part of a data transmission network.

[0020] The photodiode includes a cap with a melting lens.

[0021] The cap is, in particular, a deep-drawn component comprising at least one, preferably exactly one, window into which a glass lens has been fused. The cap is preferably cylindrical, in particular circular cylindrical, and / or pot-shaped with a window in the base.

[0022] The lens is bonded to the cap in a material-bonded manner.

[0023] The melting lens has a diameter d when viewed from a top view of the upper side of the cap. L The upper side, as defined in the invention, is the outer surface of the lens cap on the side of the melting lens. This is, in particular, the underside of the base of the top-shaped lens cap.

[0024] On the top side of the cap, the melting lens forms a diameter d LThe lens is specifically designed as a lens with a lens stem, meaning that the upper part of the lens is larger than the window into which the lens is fused. The lens stem, whose diameter is smaller than the diameter of the window, projects into this window. L It is therefore understood that the melting lens can have a different diameter on the underside formed by the lens stem than on the top side.

[0025] According to the invention, the ratio of the diameter of the melting lens to the diameter of the active photodiode area d is L / d F more than 30. In preferred embodiments, the ratio of the diameter of the melting lens to the diameter of the active photodiode area d is L / d F t more than 35 and especially preferably more than 40.

[0026] Currently, photodiodes with an active area of ​​60-70 µm in diameter are typically used. The ratio of the diameter of the melting lens to the diameter of the active photodiode area d L / d F The number is usually between 20 and 25.

[0027] In contrast, the invention provides that, instead of reducing the diameter of the melting lens in correlation with the reduction of the diameter of the active photodiode area, this ratio is increased compared to the prior art in order to use a relatively large lens for a relatively small active photodiode area.

[0028] It has been shown that it is still possible to achieve a sufficiently small spot despite using a large lens. The spot can also be considered the image size. The core of the optical fiber is imaged onto the active area of ​​the photodiode.

[0029] The diameter dL to d F According to one embodiment of the invention, the value is less than 80, preferably less than 70 and particularly preferably less than 65.

[0030] Preferably, a glass with a refractive index n (at 589 nm) of more than 1.55, preferably more than 1.58 and particularly preferably more than 1.59 is used for the melting lens.

[0031] By using a relatively high refractive index glass, it has been possible, as described above, to provide a sufficiently small spot despite the large lens.

[0032] In one embodiment of the invention, the active photodiode area, i.e., the light-sensitive area of ​​the photodiode, has a diameter of less than 40 µm, preferably less than 35 µm.

[0033] The active photodiode area is generally circular. If the active photodiode area deviates from a circular shape, the minimum diameter is preferably understood, in accordance with the invention, particularly in the case of a square design of the active photodiode area, to be the inner diameter, i.e., the diameter of a circle that just fits inside the square.

[0034] According to a preferred embodiment of the invention, the melting lens has a diameter d L of more than 1.2 mm, preferably more than 1.3 mm, particularly preferably more than 1.5 mm and / or of less than 2.0 mm, preferably less than 1.8 mm.

[0035] Furthermore, the melting lens preferably uses a glass with a coefficient of thermal expansion α (averaged at 20 to 300 °C) of less than 14 ppm / K, preferably less than 10 ppm / K.

[0036] The invention made it possible to provide a photodiode with a lens cap, which is designed for transmission speeds of 25 Gbps (gigabits per second) or more.

[0037] Preferred embodiments further relate to a lens cap, in particular a lens cap for the photodiode described above, and in particular a lens cap which may have the features described above in connection with the lens cap.

[0038] The lens cap is designed as a metal cap, in particular as a deep-drawn metal cap, with a melted-in lens made of glass with a refractive index n greater than 1.55, preferably greater than 1.58, particularly preferably greater than 1.59.

[0039] Preferred embodiments further relate to a lens cap, in particular a lens cap for the photodiode described above, and in particular a lens cap which may have the features described above in connection with the lens cap.

[0040] The lens cap is designed as a metal cap, in particular as a deep-drawn metal cap, with a glass lens embedded in the glass, wherein the glass has a coefficient of thermal expansion α (averaged at 20-300°C) of less than 14 ppm / K, preferably less than 10 ppm / K.

[0041] In particular, by combining a high-refractive-index glass with a low coefficient of thermal expansion, it has been possible to provide a lens cap that projects the smallest possible spot despite a relatively large lens radius.

[0042] At the same time, the total length of the optical system from the end of the light guide to the active area of ​​the photodiode could also be reduced.

[0043] In particular, the low coefficient of thermal expansion of the glass has made it possible to provide such a lens as a melting lens.

[0044] In one embodiment of the invention, the metal cap consists of a material which has a coefficient of thermal expansion of 0.95 to 1.05 times that of the glass of the melting lens.

[0045] The metal cap can be made, for example, of an iron-nickel alloy, iron-nickel-cobalt or iron-nickel-chromium, in particular of an iron-nickel alloy with a coefficient of thermal expansion between 5 and 8 ppm / K.

[0046] Alloys with such a low coefficient of thermal expansion are available, for example, under the trade names Kovar®, Dilaton 46®, Dilaton 51® and Pernifer 48®.

[0047] To achieve good adhesion of the fused glass to the cap, the cap material is preferably pre-oxidized. This means that an oxide layer, advantageously of uniform thickness, is present between the fused glass and the cap material. This oxide layer improves the adhesion of the fused glass to the cap metal. Adhesion is crucial for the operational safety and reliability of the component. In particular, the combination of the aforementioned materials with high-refractive-index glasses (refractive index greater than 1.55) enables this. It has been found that oxide weights of 0.12 to 0.22 mg / cm² are achievable with these materials. 2to produce metal. This area has proven advantageous. The oxide weight is determined by oxidizing an unoxidized metal sample component in the furnace process to be used and measuring the weight increase.

[0048] After the lens has been melted, the cap is provided with a corrosion protection layer, in particular a nickel and / or gold-containing layer, according to a preferred embodiment of the invention.

[0049] The melting lens can consist in particular of heavy flint or heavy crown glass.

[0050] The invention further relates to a method for manufacturing a lens cap, in particular a lens cap as described above, or for manufacturing a photodiode with a lens cap as described above.

[0051] A melting lens is melted into the window of a metal cap.

[0052] According to the invention, a glass with a refractive index n of over 1.55 is used for the glass of the melting lens.

[0053] In particular, a glass with a refractive index greater than 1.58, preferably greater than 1.59, and / or a coefficient of linear thermal expansion α of less than 14 ppm / K, preferably less than 10 ppm / K, is used. The glass may also have a coefficient of linear thermal expansion α of more than 7 ppm / K.

[0054] Furthermore, a glass is used which has a viscosity v between 10 6 and 10 4 dPa·s exhibits a continuous viscosity profile.

[0055] The viscosity can be determined in particular according to DIN ISO 7884 - 2, 3 or 5 (edition 1998-02).

[0056] A continuous viscosity profile is understood to be one in which there is no discontinuity in the viscosity curve. With many glasses, crystal structures form, particularly at temperatures above 700 °C, which leads to a discontinuity in the viscosity profile. Specifically, the viscosity curve has an inflection point when crystal formation occurs; that is, the viscosity decreases less sharply at this point until it decreases more rapidly again. It has been found that this significantly impairs the optical properties as well as the fusion capability with the metal cap of the in-vibration lens according to the invention.

[0057] Suitable glasses for carrying out the invention are available, in particular under the trade names Schott® N-SF8, P-SK60 and P-SK58A. These exhibit good processability and can be fused without crystallization, without clouding, and with strong adhesion.

[0058] The glass used should withstand electroplating processes and be able to withstand a temperature of 85 °C and 85% relative humidity for 1,000 hours. This is particularly true for the glasses mentioned above.

[0059] The melting lens is melted in, in particular at a temperature above 700 °C and / or below 1000 °C.

[0060] The invention further relates to a lens cap which can be manufactured using the method described above, in particular which has been manufactured using the method described above.

[0061] Furthermore, the invention made it possible to reduce the length of the lens cap.

[0062] Furthermore, the influence of the lens aperture could surprisingly also be reduced.

[0063] The inner aperture is only partially illuminated, thus reducing the so-called "aperture effect." This eliminates the risk of light being blocked by the inner aperture, as it is fully illuminated. In particular, a length of less than 6 mm, and preferably less than 5.5 mm, from the fiber end to the active photodiode area was achieved.

[0064] The use of a photodiode as described above in a data transmission network, especially in an optical receiver, is advantageous.

[0065] Preferably, the optical receiver is designed such that the light from a light guide of a light-conducting fiber is projected as a spot onto the active photodiode surface, wherein the diameter of the spot is less than 40 µm, preferably less than 35 µm, particularly preferably less than 20 µm. Brief description of the drawings

[0066] The subject matter of the invention will below be described with reference to the drawings. Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 to Fig. 8 will be explained in more detail. Fig. Figure 1 is a schematic sectional view of a photodiode with lens cap. Fig. Figure 2 is a schematic top view of the chip contained in the photodiode with lens cap. Fig. Figure 3 is a schematic representation of the optical system consisting of a photodiode chip, lens, and light-conducting fiber. Fig. Figure 4 is a representation of the optical system in which a lens with a refractive index of 1.5 is used. Fig. Figure 5 shows the use of a lens with a refractive index of 1.8. Fig. Figure 6 shows the viscosity profile of a glass particularly suitable for the invention. Fig. Figure 7 shows the viscosity curve, which is only insufficiently suitable for the invention. Referring to Fig. Section 8 will explain in more detail the testing of the lens and cap assembly by means of a shear test. Fig. Figure 9 shows the influence of the refractive index of the glass used for the melting lens on the diameter of the spot projected onto the photodiode. Detailed description of the drawings

[0067] Fig. 1 is a schematic representation of a photodiode with lens cap 1.

[0068] The photodiode with lens cap 1 comprises a cap 2 made of metal, designed as a deep-drawn component, into which a glass lens 3 is fused.

[0069] The melting lens 3 is designed as a lens with a stem, i.e. the lens stem 12 projecting through the window of the cap 2 has a smaller diameter than the part of the melting lens 3 above it.

[0070] By definition, the top surface 7 is a top view of the pot-shaped cap 2 from the outside.

[0071] The melting lens 3 has a diameter d when viewed from the top. L In this embodiment, the upper wall of the cap 2 is thinned around the melting lens 3 in order to reduce mechanical stresses.

[0072] The cap 2 is connected to a base 4, shown schematically here. The base 4 is preferably made of metal, in particular of metal with a coefficient of thermal expansion α of less than 10 ppm / K.

[0073] On socket 4 is a chip 5, which includes, among other things, the active area of ​​the photodiode, via which light is converted into electrical voltage.

[0074] It goes without saying that this schematic diagram does not show further details such as connection pins, wiring, etc.

[0075] Fig. Figure 2 is a schematic top view of the chip 5 located on socket 4.

[0076] The chip 5 comprises a photodiode with the active photodiode area 6, which in this embodiment is circular and has a diameter d F exhibits.

[0077] A light spot is projected onto the active photodiode surface 6 via the melting lens 3, starting from a light-conducting fiber.

[0078] Fig. Figure 3 is a schematic representation of the overall optical system.

[0079] The optical fiber 8 is spaced a distance L1 from the melting lens 3.

[0080] The melting lens 3 acts as a converging lens.

[0081] From the top surface 7, the light emitted from the core of the optical fiber 8 enters the melting lens 3, which has a thickness D, and a spot is projected onto the active photodiode surface 6 of the chip 5, which is spaced from the melting lens 3 by a length L2.

[0082] The top surface of the melting lens 3 has the radius R1, which corresponds to half the diameter of the melting lens 3 according to the invention.

[0083] On the underside, the melting lens 3 has a radius R2, which may differ from the radius R1.

[0084] Fig. Figure 4 shows the use of a melting lens 3 with a refractive index of 1.5 and Fig. Figure 5 shows the use of a melting lens 3 with a refractive index of 1.8.

[0085] As can be seen here, the higher refractive index has reduced both the diameter of the spot on the side of the chip 5 and the distance L1 from the light-conducting fiber 8.

[0086] The overall length of the photodiode with lens cap 1 is shorter due to the shorter distance from the active photodiode area 6 to the coupling end of the light-conducting fiber 8; at the same time, the projected spot is smaller.

[0087] Furthermore, the inner aperture is only partially illuminated, so there is no risk of light components being blocked by the inner aperture.

[0088] With a refractive index of 1.5, as in Fig. As shown in Figure 4, a projected spot diameter of approximately 70 µm was achieved in an experimental setup. The length of the optical axis was over 7 mm.

[0089] Through the in Fig. In contrast, with the 5th lens shown, which has a refractive index of 1.8, the length of the optical axis could be reduced to approximately 4 mm. At the same time, the diameter of the projected spot is only about 7 µm, thus being reduced to about one-tenth. This demonstrates the surprising influence of the refractive index on the spot diameter.

[0090] Fig. Figure 6 shows the viscosity profile of a glass particularly suitable for the invention in a temperature range of about 600 to 1,200 °C.

[0091] It can be seen that the viscosity curve 9 is continuous, meaning that the viscosity decreases continuously and steadily with increasing temperature. This applies particularly to the temperature range of 700 to 1,000 °C, in which the melting lens is preferably melted.

[0092] Fig. Figure 7, on the other hand, shows the viscosity profile of a glass that is less suitable for the invention.

[0093] It can be seen that in a temperature range between 700 and 800 °C, there is a region 10 in which the viscosity deviates abruptly from the averaged viscosity curve 9. Due to crystal formation, the viscosity changes abruptly between 700 and 800 °C. In particular, once crystal formation occurs, the viscosity does not decrease more or less than before. It has been found that this is detrimental to the optical properties of the fused lens. In particular, clouding can occur. The mechanical, metallurgical bond between the lens and the metal part is also reduced in such glass.

[0094] Fig. Figure 8 shows in a schematic representation how the adhesion of the melting lens 3 in the cap can be tested by a shear test.

[0095] Using a test body 11 guided parallel to the top of the cap 2, the melting lens 3 or a part of the melting lens 3 is sheared off. The force required for this is measured.

[0096] It has been shown that shear forces of over 50 N can be achieved. At the same time, glass residue is found on the metal of the cap and metal cap on the glass, which is an indication of a good bond between the glass and metal.

[0097] This is achieved in particular by pre-oxidation of the iron-nickel alloy used for the metal cap 2.

[0098] Fig. Figure 9 shows a graph in which the refractive index is plotted on the x-axis and the diameter of a spot of light from a single-mode fiber, projected onto an active photodiode surface using an exemplary melting lens, is plotted on the y-axis. The curve is approximated exponentially.

[0099] It can be seen that the spot diameter decreases surprisingly sharply with increasing refractive index.

[0100] Thus, the spot diameter is reduced to less than half simply by increasing the refractive index from 1.5 to 1.6.

[0101] The invention made it possible to easily provide a lens cap or a photodiode with a lens cap in which the lens projects the smallest possible spot onto the photodiode. Reference symbol list 1 photodiode with lens cap 2 caps 3 Fusing lens 4 sockets 5 Chip 6 active photodiode areas 7 Top 8 optical fibers 9 Viscosity profile 10 Area Jump Viscosity Profile 11 test specimens 12 lentil stems

Claims

[1] Photodiode with a lens cap (2) comprising a base (4) with an active photodiode area (6), wherein the active photodiode area (6) has a diameter d F comprising a cap (2) with a melting lens (3), wherein the melting lens (3) has a diameter d in a top view of a top surface of the cap (2). L exhibits characterized by , that the ratio of the diameter of the melting lens (3) to the diameter of the active photodiode area (6) d L / d F more than 30, and wherein the lens cap (2) is designed as a metal cap, wherein the melting lens (3) is made of glass with a refractive index n (at 589 nm) greater than 1.55 and a coefficient of thermal expansion α (at 20- 300 °C) of less than 14 ppm / K. [2] Photodiode with a lens cap (2) according to claim 1 above, characterized by, that the ratio of the diameter of the melting lens (3) to the diameter of the active photodiode area (6) d L / d F less than 80. [3] Photodiode with a lens cap (2) according to one of the preceding claims, characterized by , that the photodiode (1) is designed for transmission speeds of 25 GBps. [4] Photodiode with a lens cap (2) according to one of the preceding claims, characterized by , that the active photodiode area (6) has a diameter of less than 40 µm and / or that the melting lens (3) has a diameter d L exhibits more than 1.2 mm [5] Lens cap (2) of a photodiode (1) according to the preceding claim, characterized by , that the metal cap is made of a material that has a coefficient of thermal expansion 0.95-1.05 times that of the glass of the melting lens. [6] Lens cap (2) according to claim 5 above, characterized by, that the melting lens (3) is made of heavy flint or heavy crown glass. [7] Method for manufacturing a lens cap (2) wherein a melting lens (3) is melted into the window of a cap (2) made of metal, characterized by , that for the glass of the melting lens a glass with a refractive index n (at 589 nm) of over 1.55 and with a coefficient of thermal expansion α (at 20-300 °C) of less than 14 ppm / K is used, wherein the glass has a viscosity v between 10 6 and 10 4 dPa·s exhibits a continuous viscosity profile, i.e., a viscosity profile in which there is no jump in the viscosity curve. [8] Method for manufacturing a lens cap (2) according to claim 7 above, characterized by , that the melting lens (3) is melted at a temperature above 700 °C and / or below 1000 °C. [9] Method for manufacturing a lens cap (2) according to any one of the preceding claims 7, 8 or 9, characterized by , that for the melting lens (3) glass with a refractive index n (at 589 nm) greater than 1.58, preferably greater than 1.59, is used. [10] Lens cap, wherein the lens cap (2) comprises a window which is designed as a melted lens (3) fused into a cap (2) made of metal, characterized by , that the melting lens (3) consists of a glass with a refractive index n (at 589 nm) of over 1.55 and a coefficient of thermal expansion α (at 20-300 °C) of less than 14 ppm / K, wherein the glass has a viscosity v between 10 6 and 10 4 dPa · s exhibits a continuous viscosity profile, i.e., a viscosity profile in which there is no jump in the viscosity curve.

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