Patent Publication Number: US-6989554-B2

Title: Carrier plate for opto-electronic elements having a photodiode with a thickness that absorbs a portion of incident light

Description:
BACKGROUND OF THE INVENTION 
   FIELD OF THE INVENTION 
   The present invention relates to a carrier for opto-electronic elements, an optical transmitter, and an optical receiver with such a carrier. The opto-electronic element contains a carrier plate that is transparent to emitted or received light of the opto-electronic element that is allocated to the carrier. 
   The monitoring of transmission power and wavelength of a laser diode by a monitor diode is known. For edge-emitting lasers, a monitor diode is typically mounted on the back-side mirror of the resonator. But for vertically emitting lasers (VCSEL), this is impossible. With vertically emitting lasers it is therefore necessary to divert a portion of the emitted light onto the monitor diode. This is disadvantageously associated with a relatively large outlay. Accordingly, in multi-channel transmitter modules (parallel optical link) it has not been possible to utilize a separate monitor diode for each channel for monitoring purposes. 
   As an alternative to diverting a portion of the emitted light, what is known as a reference laser can be utilized, which has the same characteristics as the actual laser that transmits a signal. But in this case, aging characteristics cannot be compensated. 
   German Patent DE 195 27 026 C2 describes an opto-electronic transducer in which a semiconductor component that transmits or receives light is mounted on a carrier plate in which the beam shaping structures are integrated. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a carrier for opto-electronic elements, an optical transmitter, and an optical receiver that overcomes the above-mentioned disadvantages of the prior art devices of this general type, with which the transmitted or received light of an opto-electronic component can be detected in a simple fashion. In particular, a transmitting device and a receiving device will be proposed, which make photo detection possible for a number of opto-electronic elements easily and optimally independently. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a carrier for opto-electronic elements. The carrier contains a carrier plate that is transparent to emitted or received light from an opto-electronic element associated with the carrier plate, and at least one semiconductor structure deposited on the carrier plate. The semiconductor structure forms at least one photodiode and at least partly absorbs incident light. 
   The inventive solution is based on the idea of expanding the functionality of the carrier that usually serves for fastening and conductively contacting opto-electronic elements, such that a structure that is deposited on the carrier plate forms one or more photodiodes. Because the carrier is transparent and is penetrated by the light being emitted or received by an opto-electronic element, light can be easily detected by the photodiode without additional beam branching devices or the like. The desired light absorption can be set by suitably setting the layer thickness of the semiconductor structure. 
   The semiconductor structure contains at least two semiconductor layers, which form at least one photodiode. In a preferred development, the semiconductor structure has a layer with good conductivity, which is formed at least partly on one side of the carrier plate, a first semiconductor layer, and a second semiconductor layer. 
   The first semiconductor layer and the second semiconductor layer thus form the PN junction of the photodiode. The layer with good conductivity supplies the backside contact for the semiconductor layer adjoining the carrier plate. 
   The layer with good conductivity is preferably formed by a heavily doped semiconductor material, particularly a heavily doped silicon. Together with the two other semiconductor layers, it can form respective heavily doped p and n layers and an intermediate lightly doped or intrinsic semiconductor layer as in PIN photodiodes. But the layer with good conductivity can also be a simple metallization contact that is adjoined by a PN-diode. 
   At least one respective metallization contact is advantageously provided on individual layers of the semiconductor structure, by way of which the respective layer and the overall photodiode are conductively contacted. To the extent that the semiconductor structure forms several photodiodes, each photodiode, specifically the relevant layers, contains separate contacts, so that the signal of each photodiode can be detected independently. 
   In a preferred development, the photodiode is part of an optical receiver. Because such a photodiode should completely absorb incident light, the thickness of the semiconductor layer is selected such that incident light is substantially fully absorbed. In an alternative development, the photodiode is a monitor diode of an optical transmitter, whereby the semiconductor structure only partly absorbs light impinging on the carrier plate. 
   The carrier plate preferably is formed of glass, quartz, plastic, sapphire, diamond or a semiconductor material that is transparent to the radiation of the allocated opto-electronic element. 
   The invention provides that an antireflection layer may be applied to at least one side of the carrier plate and/or the semiconductor structure, namely on the outside surfaces of the carrier and between the semiconductor structure and the carrier plate. This minimizes losses due to reflection and backscatter. 
   Conductive tracks and appertaining contact pads are advantageously formed on the carrier plate and/or on the semiconductor structure, which serve for the mounting and conductive contacting of the electrical and/or opto-electronic elements on the carrier. To the extent that the conductive tracks are formed on the semiconductor structure, an isolating layer, for instance an oxide layer, is advantageously deposited on the semiconductor structure. 
   The semiconductor structure can be deposited on the carrier plate by any chemical and/or physical deposition technique, for instance epitaxy, chemical vapor deposition (CVD), vapor deposition or sputtering. What is essential is that the semiconductor structure is an integral component of the carrier and not merely mounted on the carrier plate. 
   In a preferred development, the carrier forms a plurality of photodiodes in a one-dimensional or two-dimensional array, with a transmission element allocated to each. The plurality of photodiodes is advantageously provided by isolating individual regions of the semiconductor structure following its deposition on the carrier plate by sawing, etching or the like, and separately contacting the regions. It is also imaginable for several semiconductor structures to be separately deposited next to one another on the carrier plate. 
   The invention also relates to an optical transmitting device with at least one light-emitting opto-electronic element and at least one monitor diode. The carrier is provided, whereby the monitor diode is integrated in the semiconductor structure of the carrier, and the beam emission surface of the light-emitting element faces the carrier, so that light that is emitted by the element passes through the photodiode and the transparent carrier plate. The emitted light can pass through the semiconductor layer or the carrier first, depending on the orientation of the carrier. A monitoring of the light passing though the carrier occurs automatically to a certain extent and without additional light deflecting devises, beam splitters, etc. 
   The invention also provides that the carrier plate forms or contains a beam shaping element, particularly a lens, on the side which is averted from the semiconductor structure, so that light exiting the carrier plate undergoes beam shaping, for instance being focused onto the butt of the optical waveguide. 
   The element is advantageously fastened on the carrier and conductively connected to tracks of the carrier, for instance by flip chip mounting or conventional bonding techniques. In principle, however, the element can also be fastened to some other structure. The invention provides that additional electrical or opto-electronic components may also be fastened to the carrier and conductively connected to interconnects of the carrier. 
   In a preferred development, several light emitting semiconductor elements are combined into a transmission array, and an array of monitor diodes in the semiconductor structure is allocated to the transmission array, whereby each monitor diode receives the light from a semiconductor element, respectively. This makes possible an individual monitoring of the individual lasers of the array. 
   Lastly, the invention relates to an optical receiving device with at least one optical receiver containing a photodiode and an electrical preamplifier. The inventive carrier is provided. The photodiode is integrated into the semiconductor structure of the carrier, and the electrical preamplifier is fastened to the carrier. The semiconductor structure absorbs incident light substantially completely. A plurality of photodiodes is again disposed in a one-dimensional or two-dimensional array. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a carrier for opto-electronic elements, an optical transmitter, and an optical receiver, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, side-elevational view of a principal structure of a transmitting device with a carrier that forms a semiconductor structure according to the invention; 
       FIG. 2  is an enlarged sectional view of the semiconductor structure shown in  FIG. 1 ; 
       FIG. 3  is a side-elevational view of the transmitting device with the carrier that forms the semiconductor structure, whereby a laser diode and an integrated circuit are fastened on the semiconductor structure; 
       FIG. 4  is a side-elevational view of the transmitting device shown in  FIG. 3 , in which a Fresnel lens is employed as a beam-shaping element; and 
       FIG. 5  is a side-elevational view of the transmitting device in which an array of laser diodes is allocated to an array of monitor diodes. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a diagrammatic representation of an optical transmitting device with a light-emitting optical radiation element  1  and a carrier  2 . The carrier  2  contains a transparent carrier plate  21  and a semiconductor structure  22 . A lens  3  is also provided, and disposed on a side of the transparent carrier plate  21  that is averted from the optical radiation element  1 , or being formed in one piece with the plate  21 . A beam path  4  of light that is emitted by the optical radiation element  1  is schematically represented. 
   The optical radiation element  1  is advantageously a light-emitting semiconductor component, particularly a surface imitating laser diode (VCSEL) that provides a coherent light source. A driver module is allocated to the laser diode  1 , which is not represented but which modulates the light of the laser diode  1  in correspondence to a data signal that is to be transmitted. The optical radiation element  1  can be directly fastened on the carrier  2  and conductively connected to interconnects that are constructed on the carrier  2 , as represented in  FIGS. 3  to  5 . But it is also possible for the optical radiation element  1  to be fastened to some other structure, such as a housing that also includes the transparent carrier  2 . 
   The carrier plate  21  of the carrier  2  is transparent to the light that is emitted by the optical radiation element  1 . To that end, the carrier plate  21  is formed of glass, quartz, plastic sapphire, diamond, or a semiconductor material that is permeable to the radiation that is emitted by the optical radiation element  1 . GaAs can be utilized for wavelengths above 900 μm, and silicon for wavelengths above 1100 μm. 
   The carrier plate  21  has a cuboidal shape and contains a top side  21   a  which faces the optical radiation element  1  and a bottom side  21   b  which is averted from the optical radiation element  1 . The collecting lens  3  is constructed on the bottom side  21   b  of the carrier plate  21 . The collecting lens  3  can be formed of the same material as the carrier plate  21  and can have a monolithic structure with the carrier plate  21 . But it is just as possible for the lens  3  to be provided as a separate part which is fastened on the bottom side  21   b  of the carrier plate  21 , for instance by gluing. The lens  3  can also have a different relative refractive index than the carrier plate  21 . 
   At the top side  21   a  of the carrier plate  21 , the semiconductor structure  22  is revealed. The structure  22  contains several layers that are deposited on the transparent carrier plate  21 . Known chemical and/or physical deposition techniques can be employed to deposit or apply the individual layers of the semiconductor structure  22 . For instance, the individual layers of the semiconductor structure can be applied to the carrier plate by epitaxy. But other methods, such as CVD, vapor deposition, or sputtering, are also possible. 
   The semiconductor structure  22  that is deposited on the carrier plate  21  forms at least one photodiode. 
   The semiconductor structure  22  is partly transparent to the light that is emitted by the optical radiation element  1 . The photodiode that is formed in the semiconductor structure  22  advantageously represents a monitor diode, which partially detects the light which is emitted by the optical radiation element  1  and feeds it to a non-illustrated control device for controlling the wavelength and/or intensity of the light that is emitted by the optical radiation element  1 . Integrating the monitor diode into the carrier  2  that receives the light from the optical radiation element  1  makes it possible to monitor the emitted light without substantially influencing the optical path. The occurring attenuation can even be used with advantage to the optical characteristics of the module in certain circumstances. An example of this derives from the fact that lasers for higher speeds are driven with high currents. The correspondingly higher light power must then be reduced, because the power must have an upper limit for purposes of laser safety. The required attenuation can be produced by the semiconductor structure instead of a separate attenuating disk. 
   The measure of attenuation (i.e. absorption) is determined by the thickness of the semiconductor structure  22 . For instance, the depth of penetration is approximately 10 μm for silicon. Accordingly, when the semiconductor structure is made from silicon, it has a thickness of less than 10 μm, whereby merely a small fraction (less than 20%) of the light that is emitted by the optical radiation element  1  is absorbed. 
   It should be noted that the semiconductor structure  22  does not have to cover the top side  21   a  of the transparent carrier plate  21  completely. This being the case shown in  FIG. 2 , the carrier  2  as a whole is cuboidal. 
     FIG. 2  exemplarily represents the semiconductor structure  22  of the carrier  2 . It should be noted that the semiconductor structure  22  can also be constructed some other way. What is essential is that the individual layers of the semiconductor structure  22  form the photodiode. 
   According to  FIG. 2 , the semiconductor structure  22  contains a layer with good conductivity  221 , a first semiconductor layer  222 , and a second semiconductor layer  223 . The layer  221  with good conductivity is applied directly on the transparent carrier plate  21 , whereby an additional antireflection layer  224  can be applied between the conductive layer  221  and the carrier plate  21  in order to minimize losses owing to reflection and backscatter. 
   In this exemplifying embodiment, the layer  221  with good conductivity is a heavily doped silicon layer or other semiconductor layer such as an n+ doped layer. It contains a metallization contact  51  by way of which the layer  221  is charged with an electrical voltage or ground. The contact  51  represents one or both of the contacts of the photodiode that is formed by the semiconductor structure  22 . Owing to the good conductivity, the layer  221  forms the backside contact for the adjoining semiconductor layer  222 . 
   The two semiconductor layers  222 ,  223  that are applied on the conductive layer  221  form a PN junction. They are applied to the carrier plate  21  and the layer with good conductivity  221 , respectively, by epitaxy or some other method. The middle semiconductor layer  222  is lightly n-doped or forms an intrinsic layer, for example. The outer semiconductor layer  223  is p-doped, for example. The construction corresponds to that of a known PIN photodiode. 
   It should be noted that the layer  221  with good conductivity protrudes beyond the two other layers  222 ,  223  somewhat, in order to create space for the contact  51 . Additional metallization contacts  52 ,  53 ,  54 ,  55  are formed on the outside of the outer semiconductor layer  223 . The contacts provide the second contact of the photodiode. On the other hand, they serve as interconnects for mounting an opto-semiconductor or integrated circuit, which are fastened on the semiconductor structure  22 . If the contacts  52  to  55  are to be isolated from one another, an oxide layer—which is common in semiconductor technology—can be applied to the bottom semiconductor layer  223 . 
   The application of an oxide layer on the outer semiconductor layer is also provided in the following exemplifying embodiments, in any case as long as mutually isolated interconnects extend on the outer semiconductor layer. 
   In  FIG. 2  another metallization  56  is realized directly on the transparent carrier plate  21  and stands schematically for additional interconnects on the carrier plate  2  for conductively contacting additional components that are fastened to the carrier plate  21 . 
     FIG. 3  represents an exemplifying embodiment in which the optical radiation element  1  and an integrated circuit  6  are fastened on the semiconductor structure  22 . The integrated circuit  6  is the drive circuit for the optical radiation element  1 , for example. On the semiconductor structure  22  are metallizations  56   a ,  56   b ,  57   a ,  57   b  for contacting the optical radiation element  1  and the integrated circuit  6 . The optical radiation element  1  is connected to the metallizations  57   a ,  57   b  by flip chip mounting, so that both contacts point to the carrier  2 . The integrated circuit  6 , on the other hand, is represented in a conventionally mounted form (bond wires  7  on the side that is averted from the mounting surface), but the mounting can also occur as with the opto-semiconductor  1 . These contacting techniques are merely exemplary. The two elements  1 ,  6  can just as well be joined to the appertaining contacts  56   a ,  56   b ,  57   a ,  57   b  on the carrier  2  by conventional methods such as a bonding technique or flip chip assembly. 
     FIG. 4  represents an exemplifying embodiment in which the lens is constructed not as a lens with a spherical surface as in  FIGS. 3 and 4 , but as a diffractive optical element  3   a , for instance a Fresnel lens. Otherwise, the structure corresponds to that of  FIG. 3 , whereby the integrated circuit  6  is not represented in FIG.  4 . The integration of a semiconductor structure  22  into the carrier plate  21  of the carrier for opto-electronic elements is also suitable for realizing a receiving device. In this case, the photodiode formed by the semiconductor structure  22  represents the photodiode of an optical receiver. The thickness of the semiconductor structure  22  is so realized that the structure substantially completely absorbs the light striking the carrier plate  21 . This is achieved by selecting the thickness of the semiconductor structure  22  accordingly. 
   The structure represented in  FIG. 4  can also represent the optical receiver. For example, light that is emitted from the butt of a non-illustrated optical fiber is focused by the Fresnel lens  3   a  onto the photodiode that is formed by the semiconductor structure  22 . The resulting photocurrent is amplified by an electrical preamplifier  8 , which is fastened to the carrier  2  and conductively connected to the metallizations  57   a ,  57   b  on the surface of the semiconductor structure, and fed to non-illustrated modules downstream. 
   Lastly,  FIG. 5  represents an exemplifying embodiment wherein the semiconductor structure  22  forms a plurality of individually structured monitor diodes  9  which are configured in an array, which are schematically represented in FIG.  5 . An array of light-emitting semiconductor elements, particularly VCSEL lasers which are realized in a transmitting module  11 , is allocated to the monitor diodes  9 . Each monitor diode  9  is receives the light of a transmitting diode  111 , as is represented by two exemplary optical paths  4   a ,  4   b . Each laser  111  of the laser array  11  can thus be monitored individually. 
   Schematically represented metallization contacts  57   a ,  57   b  serve for contacting the laser array  11  with interconnects that are realized on the surface of the semiconductor structure  22 . 
   In order to produce a plurality of photodiodes  9  in an array, a solid semiconductor structure is first deposited on the carrier plate  21 . The semiconductor structure is then isolated into individual regions by sawing, etching or the like, which regions are provided with separate metallizations and separately contacted. Alternatively, several semiconductor structures can be separately deposited next to one another on the carrier plate and separately structured. 
   The thickness of the carrier  2  equals 200 μm to 300 μm. The lateral spacing of the individual lasers is on the same order of magnitude. 
   It should be noted that the semiconductor structure can also be formed only on subregions of the carrier plate  21 . Of course, several such subregions can also be provided on the carrier plate  21 , with each subregion forming one or more photodiodes.