WO1998050810A1

WO1998050810A1 - Electrooptical module

Info

Publication number: WO1998050810A1
Application number: PCT/DE1998/000107
Authority: WO
Inventors: Heiner Hauer; Albrecht Kuke; Eberhard Moess
Original assignee: Robert Bosch Gmbh
Priority date: 1997-05-05
Filing date: 1998-01-14
Publication date: 1998-11-12
Also published as: DE19718949A1

Abstract

The present invention relates to an electrooptical module intended for the telecommunication technology for converting electrical signals into optical signals or, vice-versa, optical signals into electrical signals. Said module comprises an electrical interface for optical fibres (11), a plate-like silicium substrate (3) presenting on one of its wide sides two depressions (2, 4) obtained by anisotropic pickling on a crystallographic plane (100), an electrically metallized converter (1) being lodged in the first depression (4). The second depression (4) presents a full reflection front surface, and the optical radiation path is such that the activated optical rays intersect with the surface of the substrate large side (3, 303) which is turned to the interface. The activated central ray impinges on a front side (6) of the first depression and reflects against the full reflection front face (7). A converging lens (8) is provided at the point where the central ray intersects with the surface of the second wide side. The invention is a built-in module inside a component, which enables a low cost assembly on a printed circuit board (60) according to the CMS technology.

Description

Electrcoptic module

State of the art

The invention is based on a module according to the genus of the main claim.

Optoelectronic modules are required in optical communications technology for converting electrical to optical signals or optical to electrical signals. Transmitter or receiver modules are customary in which the optoelectronic converter, usually a semiconductor laser diode or a photodiode, is mounted in a coaxially constructed and hermetically sealed housing, a so-called TO housing. A fiber is then coupled to this TO housing via imaging optics, the entire arrangement being combined in a module housing. The electrical connections of the electro-optical converter are led out of the base of the TO housing via lead-through wires. This limits the frequency bandwidth to <1 Gbit / s. For higher bit rates, it is necessary to use housings with high-frequency feedthroughs, for example butterfly housings, but these are considerably more expensive. To achieve cost-effective assembly on printed circuit boards, electrical components are mounted using SMD technology (Surface Mounted Device). Conventional optoelectronic modules in TO or butterfly technology are not suitable for SMD mounting and must therefore be installed separately from the electrical components, which is associated with considerable additional costs.

Advantages of the invention

The module according to the invention with the features of the main claim has the following advantage.

The invention and its exemplary embodiments enable a cost-effective and automatable production of optoelectronic transmission and reception modules in SMD construction, which can be assembled together with electronic components on a printed circuit board in the same operation. If the modules according to the invention are manufactured as receptacles, no fiber end interferes with assembly on the printed circuit board. The attachment of the modules on the circuit board is so stable due to the features of the invention that all common connector systems can be used.

Advantageous further developments and improvements of the module specified in the main claim are possible due to the features listed in the subclaims.

drawings

Embodiments of the invention are shown in the drawings and explained in more detail in the following description. Show it

FIG. 1: the cross section through a transmitter module which is already mounted on a printed circuit board, Figure 2: a so-called lead frame for the assembly of a

1: FIG. 3: the same lead frame in a later processing state, FIG. 4: the module from the outside (view from the left to FIG. 1), FIG. 5: a variant of FIG. 1,

Description of the embodiment

An exemplary embodiment according to the invention is first described in the form of a transmitter module. Fig. 1 shows the cross section through a transmitter module already mounted on a circuit board. A laser diode 1 is mounted as a converter 1 in a depression 2 on a broad side (“front side”) of a tabular silicon substrate 3. The depression 2 was produced by anisotropic etching in the crystallographically (100) -oriented silicon substrate 3. In addition to the depression 2, a further depression 4 was etched into the silicon substrate 3. The light bundle 5 emerging from the end face of the laser diode strikes the end face 6 of the depression 2, which is covered with an anti-reflection layer, so that the light bundle can enter the silicon with low reflection losses. For crystallographic reasons, the end faces and side faces of the anisotropically etched depressions have an angle of repose of

α arctan (^) = 54.7 °.

In the interior of the silicon, the light beam strikes the end face of the second depression 4, which has the same angle of repose in the opposite direction. The angle of incidence there is so large that the light beam is totally reflected. After reflection, the light beam passes through the silicon substrate almost perpendicular to the broad side of the substrate. The beam direction of the center beam of the totally reflected bundle depends on the medium between the laser end face and the end face 6 of the depression 2. One fills this depression with a transparent medium with a refractive index of ro = 1.5, then the center beam of the light beam reflected on the end face 7 runs at an angle of 1.4 ° with respect to the normal to the broad side of the silicon substrate 3.

On the broad side opposite the depressions (“rear side”) of the silicon substrate, the light bundle strikes a converging lens 8, which is preferably generated by reactive ion beam etching (RIE) directly on the rear side of the silicon substrate. This results in a highly precise alignment of the converging lens 8 to the two straMumlenkende front surfaces 6 and 7 on the front, which is achieved by the front-back alignment of the lithography processes in multiple use in the structuring of the silicon wafer. The position of the laser diode 1 is defined by the side walls of the depression 2 serving as stops. As a result, the mutual position of the laser diode 1 to the converging lens 8 is pre-disturbed with high accuracy by lithographic processes. The converging lens 8 focuses the light bundle in a pixel 9. The end face of a fiber 11 is positioned at the location of the pixel 9. The focal length of the Sairtnell lens 8 is chosen so that there is a magnification ratio for an optimal beam transformation of the laser beam into a beam accepted by the fiber 11 used, in order to achieve an optimal coupling efficiency. If a multimode fiber with a core diameter of approximately 45 μm is used as the fiber, an active adjustment can be dispensed with. With a single-mode fiber with a core diameter of approximately 10 μm, active adjustment is required.

The transmitter module is preferably produced as a so-called receptacle, in which the fiber 11 is not firmly connected to the module, but is part of a plug 15 which is guided in a receptacle (in particular socket) 12 in a detachable and non-rotatable manner. The receptacle has a flange 14, the end face perpendicular to the fiber and Connector axis and runs parallel to the broad sides of the silicon substrate 3. The axial position of the plug 15 is determined by a stop 10 on the end face of the receptacle 12. To avoid back reflections from the end face of the fiber to the laser diode 1, the normal on the end face of the fiber is inclined by an angle δ with respect to the fiber axis, as is customary in the prior art. An angle of 8 ° is usually chosen for δ. Because of the law of refraction, an angle of between the incident beam and the fiber axis with a refractive index of the fiber core of n _κ = 1.46

ε = aresin (n _κ ^• sin δ) - δ = 3.7 ° (1)

can be set. This angle can preferably be set by a defined displacement of the Saimellensmitte to the beam center (center beam), so that the receptacle (socket) 12 for the fiber 11 or for the connector 15 can run in the direction of the substrate normal, which is the manufacture and assembly of the flange 14 relieved. The street guidance was calculated as an example for a silicon substrate 3 with the standard thickness of 525 μm and a silicon converging lens 8 etched on the back of the substrate. The output variables and result values are listed below.

Calculation example for a transmitter module:

Wavelength in air: λ = 1.55 μm

Medium between laser diode and Si substrate n ₀ = 1, 5

Waist radius of the laser beam w _0L = 2, 0 μm

Waist radius for the single mode fiber w _0F = 5, 8 μm required magnification for transformation M = 2, 9

Converging lens radius R _L = 120 μm

Radius of curvature of the converging lens R _k = 350 μm Focal length of the converging lens f = 141 μm

Ridge width between the depressions 2 and 4 a = 10 μm depth of the laser diode under the Si surface t = 62.5 μm optical path length laser diode - converging lens based on air g = 189 μm optical path length collecting lens - fiber b = 550 μm achieved enlargement b / g = 2.91 directional angle of the center beam in front of the

Converging lens Yi ₌ 1.37 'D offset converging lens center - beam center V = 2.7 μm directional angle of the center beam after the

Converging lens Yn _ 3.69 'D cutting angle of the fiber end face δ = 8.0 ° directional angle after refraction

End face of fiber γ _f = 0 °

In this calculated example, the center beam of the transformed laser beam bundle is deflected by a defined offset of the etched silicon S-irrigation lens of 2.7 μm relative to the center of the beam into the required directional angle of 3.69 °, which is for a fiber with a cutting angle of 8 ° is required. The end face of the fiber must be 550 μm from the Saπ ml lens. The magnification ratio then becomes M = 2.9 and is suitable for transforming a laser beam with a waist radius of 2.0 μm into a beam adapted to the fiber with a waist radius of 5.8 μm.

A so-called lead frame is used to hold the silicon substrate 3 and to make the electrical connections. Lead frames of this type are used to contact electronic semiconductor chips and then to encase them with a sealing compound. Leadframes have a central mounting surface for the chip and in the Periphery for this spider-like etched connecting fingers that extend from an outer frame to close to the central mounting surface, as well as webs that connect the central mounting surface to the outer frame. The contact areas of the chip are connected to the connection fingers of the lead frame by means of bond connections. For the serial production of modules, the lead frames are used in the form of tapes that are automatically fed to the chip assembly and bonding stations. After assembling and potting the modules, the outer frames are cut off to remove the short circuits between the individual lines and the connecting fingers are bent as required. Until now, it was not possible to install optoelectronic modules with leadframes because the optical fiber is a hindrance to SMD assembly.

The receptacle according to the invention is therefore designed so that the fiber 11 does not need to be inserted until after the SMD assembly.

Another problem with leadframe assembly, especially with transmitter modules, is light coupling. This problem is solved by the features specified in the claims.

The known rational assembly method with leadframes can also be used for optoelectronic modules by the invention. FIG. 2 shows, as a first example, a leadframe for the mounting of a transmitter module according to FIG. 1. Within a frame 20 (leadframe) there is a mounting surface 22 — connected by webs 21. The silicon substrate is on a central, lowered part 23 of this mounting surface 3 mounted with the two wells 2 and 4. A conductor track 24, which ends in a bond spot 25 on the surface of the silicon substrate 3, is led out from the base area of the depression 2. This bonding area 25, which is conductively connected to the underside of the laser diode (of the laser chip) 1, is connected to a connecting finger 26 of the leadframe 20 via a bonding wire. Likewise, the for the contacting of the laser diode 1 and a possibly present monitor diode 30 necessary contact areas 31 and 32 connected to connecting fingers 27, 28 and 29 via bonding wires. The space in the depression 2 between the end faces of the laser chip 1 and the end faces of the depression 2 is filled with a medium which is transparent to laser light. In order to achieve the most efficient possible bonding in one plane, the silicon substrate 3 is mounted in the lowered part 23 of the mounting surface. The depression 23 is approximately as deep as the silicon substrate is thick.

For the coupling of single-mode fibers, which require a very narrow lateral coupling tolerance <3 μm, an adjustment and fixing option is provided, which is adapted to the leadframe assembly. The lowered part 23 of the mounting surface 22 has an opening 40 (FIG. 1) at the point opposite the converging lens 8, which opening has at least the size of the converging lens 8. The mounting surface of the leadframe consists at least on the flat rear side 41 (FIG. 1) of the lowered part 23 from a laser-weldable material such as Kovar or stainless steel, which is not coated with gold there like the connecting fingers 26-29. This rear side 41 serves as a bearing surface for the flange surface on the end face of the flange 14. The receptacle 12 has at its front end the flange 14, the end surface of which serves as the second flange surface is slightly smaller than the rear side 41 serving as the first flange surface. The flange 14 also consists of a laser-weldable material. During the active adjustment, the laser diode 1 is put into operation and the plug 15 is inserted into the receptacle 12. The flange 14 is moved with its flange surface parallel to the first flange surface 41 either slidingly or at a very close distance and the light output coupled into the fiber is measured in the process.

The movement of the flange 14 is expediently controlled by an automated search algorithm in order to quickly find the optimal lateral coupling position. The axial coupling position is considerably less critical than the lateral one and can be preset due to the exact prepositioning of the etched Sarrtuell lens to the laser diode 1. The axial coupling position is determined by the position of the stop 10 (for the connector 15) in the receptacle 12.

After the optimal coupling position has been reached, the flange 14 is welded to the rear side 41 by laser welding spots 43. The plug 15 can then be removed from the receptacle 12 and the receptacle 12 can be provided with a protective cap for protection during the subsequent encapsulation process.

For active adjustment of the flange 14, the laser diode 1 must be contacted electrically. The following problem arises: If the connection pads of the laser diode are connected to the connection fingers 26 and 27 via bond wires, they are short-circuited until the frame 20 is separated. The frame 20 can only be separated if the module is encased, otherwise the connecting fingers will fall off. After the wrapping, however, the rear side 41 is no longer free or this surface would first have to be kept free and wrapped after the flange fixation, which would involve considerable additional expenditure.

The following procedure is therefore proposed: The flange 14 is adjusted and fixed before the bonding process for the laser connections. The apparatus for automatic flange adjustment and fixation (adjustment and fixation station) contains a receiving device into which the leadframe with mounted silicon substrate 3 and laser chip 1 can be inserted so that the side with the laser chip is directed downwards and the other side up. The receiving device has contact pins opposite the contact surfaces of the laser diode, via which the laser operating current is conducted during the adjustment. Since many lead frames are arranged next to each other as a band in series production, they can easily automatically guided into the adjustment and fixing station and contacted there via the contact pins. In the adjustment and fixing station, the flange 14 then only has to be fed to each module, expediently from a magazine. The fiber 11 with plug 15 required for laser light detection can be the same for all modules to be adjusted and can be permanently connected to a light detector of the adjustment and fixing station, the plug being automatically inserted into the respective receptacle 12. In this way, not only the adjustment and fixing process can be automated, but also the feeding of the components.

After passing through the adjustment and fixing station, the bond connections are made and then the covering material (covering) 50 is attached. This casing can expediently consist, as usual, of an inner casing 70 and an outer casing 50 (FIG. 1). The inner sheath 70, which covers the area of the bond wires, is made of a softer material to protect the bond wires. The outer casing 50, which is later to provide mechanical protection, is made of a stronger material. The frame 20 is then removed by severing the webs 21 at markings 21a and the connecting fingers 26 to 29 at markings 26a to 29a. The connecting fingers 26 to 29 are bent at right angles on the underside of the casing 50, so that they can lie in recesses 51 (FIGS. 1 and 4). These recesses 51 are just so deep corresponding to the thickness of the connecting fingers that they do not protrude beyond the contour of the casing 50. At the edge of the lateral surface 52, the connecting fingers are bent upward at right angles, so that they rest against the surface 52 and protrude a few millimeters. In this way, a cuboid block is obtained which has pins which project vertically downwards approximately in the middle, formed by the webs 21. These pins 21 are inserted into bores 61 of a circuit board 60 and are soldered to the underside of the circuit board with the aid of solder 62 one Earth conductor 64 soldered. Apart from the recesses 51, in which the connecting fingers 26 - 29 run, the contact surface 53 of the casing 50 is flat, so that the module shown can rest almost over the entire surface on the upper side 63 of the printed circuit board 60.

Due to the fact that the module rests on the entire surface and the push-through holder in the middle of the underside, a high mechanical stability of the fastening is achieved, which is required for the plug-in processes in the receptacle 12. The electrical contacting of the connecting fingers 26-29 with conductor tracks 76 to 79 on the upper side 63 of the printed circuit board 60 takes place, as is customary in the SMD method, via soldering joints 72. The electrical connections of the connecting fingers 26-29 are also in the event of strong mechanical loading of the module during plugging operations mechanical stresses largely decoupled, as a possible tilting of the module is absorbed by the spring action of the long connecting fingers.

Fig. 3 shows the module from the front after wrapping and after processing the connecting fingers and webs in a cross section in the plane of the lead frame. FIG. 4 shows the module from the outside with the printed circuit board 60 shown in cross section.

In a second version, two lead frames are used for the assembly, which are arranged in sandwich form. An exemplary embodiment of this is shown in FIG. 5. A first leadframe 100 contains the mounting surface 122 for receiving the silicon substrate 3 with the depressions 2 and 4, the laser chip 1 (see FIG. 1) and possibly the monitor diode 30 (see FIG. 2). The mounting surface 122 is connected to a frame 120 via webs 121. A lowering of the inner area of the mounting surface, as in the first exemplary embodiment, can be dispensed with here. A second lead frame 200 contains only the connecting fingers 26 to 29. The frame 220 of the second Lead frames 200 are aligned with the frame 120 of the first lead frame 100 via latching structures 201 and 101. The two lead frames are spaced apart from one another in the central region, which corresponds to the thickness of the silicon substrate. The webs 121 are cranked according to this distance. The use of two lead frames 100 and 200 has the advantage over the first exemplary embodiment that different materials and material thicknesses can be used for the two lead frames in accordance with their different tasks. The leadframe 100, which carries the mounting surface and the webs 121, should have a greater material thickness for reasons of stability. It must also be made of a laser weldable material such as Kovar or stainless steel. The leadframe 200, which carries the connection fingers, should be thinner since these have to be bent later. In addition, its surface must be gold-plated because it has to be bonded and soldered.

First, the first lead frame 100 is equipped and, as described for the first version, contacted via contact pins in the adjustment device and automatically actively adjusted. Then the tape with the second leadframes 200 is added and with the latching structures 201 on its frame 220 is latched into the latching structures 101 on the frame 120 of the first leadframe band, so that both are aligned with one another for the subsequent production of the bond connections of the connection spots on the silicon Substrate with the connecting fingers. The further processing, bonding, casting, trimming and bending of the connecting fingers is carried out as described for the first version.

Claims

Expectations

1. Electro-optical module with

- an electrical interface in the form of electrical connections (25),

- An optical interface, suitable for an optical fiber

(11),

- A tabular substrate (3, 303, 403) made of silicon, which has on one of its two broad sides two anisotropically etched depressions (2, 4) in a crystallographic (100) plane

- With a transducer (1, 301, 401) in the first recess (2), which is electrically contacted, the second recess (4) having a totally reflecting end face (7) and an optical beam path is provided, in which the optical beams in operation, cross the surface of that broad side of the substrate (3, 303) which faces the optical interface, and in operation the center beam is broken on an end face (6) of the first-mentioned depression (2) and reflected on the totally reflecting end face (7) and penetrates the surface of the second broad side, characterized in that there is a converging lens (8) there on the second broad side.

2. Module according to claim 1, characterized in that the converging lens (8) is produced directly on the second broad side.

3. Module according to claim 1 or 2, characterized in that the optical axis of the converging lens (8) is displaced transversely to the central beam so that the direction of the central beam within the converging lens deviates more from the direction of the normal on the second broad side than beyond convex boundary surface which delimits the Sarrmell lens (8) on the side facing away from the second broad side.

4. Module according to one of the preceding claims, characterized in that the transducer (1) is an edge-emitting laser diode

(1) is.

5. Module according to one of the preceding claims, characterized in that the transducer (401) in the recess (2) is provided with a transparent cover.