An exemplary embodiment of the present invention relates to a method of fabricating at least one radiation emitter comprising the steps of depositing an etch stop layer on a top side of a substrate; depositing a layer stack on the etch stop layer, said layer stack comprising a first contact layer, a first reflector, an active region, a second reflector, and a second contact layer; locally removing the layer stack and the etch stop layer, and thereby forming at least one mesa, said at least one mesa comprising an unremoved section of the etch stop layer and a layered pillar which forms a vertical cavity laser structure based on the unremoved layer stack inside the at least one mesa; depositing a protection material on the top side of the substrate and thereby embedding the entire mesa in the protection material wherein the backside of the substrate remains unprotected; removing the substrate by applying at least one etching chemical that is capable of etching the substrate but incapable or less capable of etching the etch stop layer and the protection material; and removing the etch stop layer and thereby exposing the first contact layer of the at least one layered pillar.

The present application claims the benefit of and priority to European Patent Application EP19174870.6 filed on May 16, 2019. The foregoing application is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Vertical cavity surface emitting lasers VCSELs are key enabling devices as light sources in transmitters, meeting all requirements for optical interconnects, presently in the 1-1000 m range. Large data rates, low power consumption and cost present some of the advantages of VCSELs. Most of these advantages are not fully exploited or get lost in modules, since the necessary integration with drivers, let them be Si- or GaAs-based, presently is inadequate. In addition the distances being bridged by such interconnects are as well increasing to distances beyond 1 km as decreasing to below 1 m. In both cases today wavelength multiplexing and densely packed laser arrays, e.g. in connection with multi core fibers, are considered as the most important approach, increasing the capacity of interconnects by a factor of 10, or more in the future.

A VCSEL wafer may consist of a stack of more than 100 different epitaxial layers. Typically, 50 processing steps are needed to produce high-performance VCSELs. Since many thousand up to several ten thousand of devices are on one, e.g. 6″ wafer, the cost of a single device is despite the design complexities low. All known standard processes are based on processing of one side of the wafer such that n- and p-contacts are on the same side, leading to large device footprints. The only tested exception is a backside contact on the substrate, which however leads to very large parasitics and excludes high frequency applications.

FIG.14illustrates an embodiment of a fully processed, non-planarized and substrate-bonded VCSEL100according to prior art. The VCSEL100comprises a layer stack LS having a first contact layer7, a first reflector2, an active region AR, intermediate layers, an oxide-aperture OA, a second reflector1and a second contact layer4. The layer stack LS is deposited on a substrate3that is planarized with BCB12. Contact pads13are connected to the first contact layer7and the second contact layer4. The second contact layer4is ring-shaped in order to allow radiation P to leave the VCSEL100. The substrate3is typically connected to a carrier (not shown inFIG.14) that has a large thermal conductivity and serves for heat removal. An electrical contact to the driver electronics is typically achieved by using bonding wires (not shown inFIG.14). The inductivity of these wires contributes to large electrical parasitics.

For future highly integrated Si-photonic transmitters more efficient transfer and integration processes are needed. One challenge to keep the cost at acceptable levels presents the development of processes for a parallel transfer of thousands of devices in one process step instead of single device pick and place.

OBJECTIVE OF THE PRESENT INVENTION

In view of the above, an objective of the present invention is to provide improved radiation emitters and improved methods of fabricating radiation emitters.

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention relates to a method of fabricating at least one radiation emitter comprising the steps of depositing an etch stop layer on a top side of a substrate;depositing a layer stack on the etch stop layer, said layer stack comprising a first contact layer, a first reflector, an active region, a second reflector, and a second contact layer;locally removing the layer stack and the etch stop layer, and thereby forming at least one mesa, said at least one mesa comprising an unremoved section of the etch stop layer and a layered pillar which forms a vertical cavity laser structure based on the unremoved layer stack inside the at least one mesa;depositing a protection material on the top side of the substrate and thereby embedding the entire mesa in the protection material wherein the backside of the substrate remains unprotected;removing the substrate by applying at least one etching chemical that is capable of etching the substrate but incapable or less capable of etching the etch stop layer and the protection material; andremoving the etch stop layer and thereby exposing the first contact layer of the at least one layered pillar.

The embodiment as described above may have, but does not need to have, one or more of the following features, which are considered to provide further advantages, but are not mandatory:

Said step of locally removing the layer stack and the etch stop layer may further include locally removing substrate material from the top side of the substrate such that the at least one mesa also comprises an unremoved surface section of the substrate.

Said step of depositing the protection material on the top side of the substrate may also include embedding the unremoved surface section of the substrate in the protection material.

The first contact layer of the layered pillar is preferably exposed by said step of removing the etch stop layer.

A metal layer may be deposited on the exposed first contact layer. The metal layer then covers the first contact layer and forms the base of the layered pillar.

The method may further include providing a second substrate. The base of the at least one layered pillar may be placed (e.g. glued or soldered) directly on the second substrate, on at least one electrical contact pad that is already located on the second substrate, or on another device (e.g. a driver) that is already located on the second substrate.

Preferably, at least one electrical driver is arranged on the second substrate before or after mounting the at least one layered pillar.

The at least one electrical driver may be configured to electrically drive the vertical cavity laser structure of the at least one layered pillar. The electrical driver may be connected to or alternatively carry the at least one electrical contact pad.

The step of mounting the at least one layered pillar (e.g. the step of placing the pillar directly on the second substrate, on at least one electrical contact pad that is already located on the second substrate, or on another device that is already located on the second substrate) is preferably carried out before removing the protection material.

The base of the at least one layered pillar is preferably mounted on an electrical contact pad after aligning the base relative to the electrical contact pad by mechanically adjusting the positions of the protection material and the second substrate relative to each other.

Further, a carrier may be mounted on top of the protection material. Then, the step of aligning the base relative to the electrical contact pad may be carried out by mechanically adjusting the positions of the carrier and the second substrate relative to each other.

The carrier and/or the protection material are preferably transparent for visible light. For instance, the carrier may consist of or comprise sapphire.

The at least one layered pillar may itself form the radiation emitter. Alternatively, the radiation emitter may comprise further components such as for instance drivers and/or more than one layered pillar as described above.

For instance, said step of locally removing the layer stack may include forming a plurality of mesas. Each of the mesas preferably comprises an unremoved section of the etch stop layer and a layered pillar which forms a vertical cavity laser structure based on the unremoved layer stack inside the respective mesa.

Said step of depositing a protection material on the top side of the substrate may include embedding said plurality of mesas in the protection material.

Said step of removing the substrate by applying said at least one etching chemical preferably includes detaching the layered pillars from one another in order to provide a plurality of separate self-contained vertical cavity laser structures.

Said step of locally removing the layer stack and the etch stop layer may further include locally removing substrate material from the top side of the substrate such that each mesa also comprises an unremoved surface section of the substrate.

Said step of depositing the protection material preferably also includes embedding the unremoved surface sections of the substrate.

Said step of removing the etch stop layer preferably includes exposing the bases of the layered pillars.

At least one driver is preferably fabricated for each of the layered pillars on the second substrate.

Each of the drivers preferably provides an electrical contact pad. The position of each contact pad on the second substrate preferably corresponds to the position of an individually assigned layered pillar inside the protection material.

The bases of the layered pillars are preferably aligned relative to the electrical contact pads by mechanically adjusting the positions of the protection material and the second substrate relative to each other.

Furthermore, the method may comprise mounting a carrier on top of the protection material and aligning the bases of layered pillars relative to the electrical contact pads by mechanically adjusting the positions of the carrier and the second substrate relative to each other.

The etch stop layer preferably consists of or comprises AlAs-material and/or AlGaP.

The layer stack preferably consists of or comprises layers of GaxAl1-xAs-material, GaxIn1-xAsyP1-y-material, or similar ternary, quaternary or quinternary III-V-materials.

The protection material is preferably a resin.

The carrier preferably consists of or comprises sapphire and/or silicon carbide.

Said at least one etching chemical that is used to remove the substrate preferably consists of or comprises a mixture of H2O2and NH4OH, and/or a mixture of H2O2and H2SO4, and/or a mixture of H2O2and C6H8O7, and/or a mixture of H2SO4and KBrO3, and/or a mixture of H2O2and HCl.

Said step of locally removing the layer stack and the etch stop layer preferably includes dry etching, preferably based on chlorine and/or bromine gas.

Said step of locally removing the layer stack and the etch stop layer may include forming a stepped mesa comprising at least an upper mesa section and a lower mesa section of different cross-sections.

The vertical cavity laser structure of the at least one radiation emitter is preferably fabricated to emit radiation through the first reflector and/or the second reflector.

The protection material is preferably transparent for visible light.

The carrier is preferably transparent for visible light.

A further embodiment of the present invention relates to a radiation emitter comprising at least two separate substrate-less layered pillars. The substrate-less layered pillars each form a self-contained vertical cavity laser structure. The substrate-less layered pillars are pieces of the very same dismembered layer stack.

The radiation emitter is preferably fabricated as described above.

The radiation emitter may comprise a substrate on which a plurality of layered pillars is mounted. Above the layered pillars, a multi-core fiber may be arranged. Each core of the multi-core fiber may be individually assigned to a layered pillar. During operation the layered pillars may generate radiation which is coupled into the individually assigned cores of the multi-core fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout.

It will be readily understood that the parameters of the embodiments of the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of exemplary embodiments of the present invention, is not intended to limit the scope of the invention but is merely representative of presently preferred embodiments of the invention.

FIGS.1to6illustrate an exemplary embodiment of a method of fabricating a radiation emitter in form of a VCSEL according to the present invention.

FIG.1shows an etch stop layer5and a layer stack LS which are deposited on a top side of a substrate3. The layer stack LS comprises a first contact layer7(preferably a highly n-doped layer), a first reflector2, an active region AR, intermediate layers, an oxide-aperture OA, a second reflector1, and a second contact layer (preferably a highly p-doped layer)4. The first and second reflectors2,1are preferably DBR reflectors. The first and second reflectors2,1as well as the active region AR preferably each comprise a plurality of layers as known in the art. The first and second contact layer7,4may consist of a single layer and/or a stack of sublayers. An oxide aperture inside the active region AR may guide the current and light during operation of the laser structure once it is completed.

FIG.2shows a mesa that is formed by locally removing the layer stack LS (seeFIG.1) and the etch stop layer5. The mesa comprises an upper mesa section and a lower mesa section of different cross-sections.

The mesa consists of an unremoved section of the etch stop layer5and a layered pillar LP which forms a vertical cavity laser structure based on the unremoved layer stack LS (seeFIG.1) inside the mesa. The mesa is preferably dry etched, for instance based on reactive ion etching (RIE). Etching is preferably stopped inside the substrate3below the sacrificial etch stop layer5.

FIG.3illustrates the structure ofFIG.2after depositing a protection material8on the top side of the substrate3and thereby embedding the entire mesa in the protection material8. The backside of the substrate3remains unprotected.

The protection material8may be a resin that resists a subsequent wet etch. A resin provides the advantage that it can be completely removed by heating it up in a solvent after finishing the fabrication process. To achieve reliable coverage of the mesa side walls the viscosity of protection material8is preferably very low. After curing, the protection material8should be solid enough to hold and carry the layered pillar LP reliably. A suitable resin is e.g. ???.

In the exemplary embodiment ofFIG.3, a carrier6is mounted on top of the cured protection material8. The carrier6preferably consists of transparent material, e.g. sapphire. The carrier6resists the etchant that is applied thereafter, and avoids stretching, bending, and breaking of the protection material8.

FIG.4shows the structure ofFIG.3after completely removing the substrate3by a wet etching process or a combination of thinning followed by wet etching until the etch stop layer5is exposed. Then, the etch stop layer5is removed (e.g. etched) and the first contact layer7of the layered pillar LP is exposed.

FIG.5shows the substrate-less layered pillar LP after depositing a metal layer9on the exposed first contact layer7. The base of the substrate-less layered pillar LP is now formed by the metal layer9.

The substrate-less layered pillar LP ofFIG.5is fully processed and forms a self-contained vertical cavity laser structure110. The self-contained vertical cavity laser structure110has a foot print (size in cross-section) one order of magnitude smaller then today's prior art VCSELs.

The base of the layered pillar LP is ready for bonding, using one of the wealth of existing of bonding technologies (adhesive, soldering, thermocompression, ultrasonic). Since all the materials (i.e. the protection material8and the material of carrier6) used for carrying the layered pillar LP are preferably transparent, the layered pillar LP can be mounted with large precision on a second substrate10as indicated by an arrow X inFIG.5.

The second substrate10may comprise a contact11and for instance a Si-based driver chip. After bonding the base of the layered pillar LP to the second substrate10the carrier6and the protection material8may be removed.FIG.6illustrates the self-contained vertical cavity laser structure110aligned and bonded to the electrical contact pad11of the second substrate10. By injecting electrical current through the first and second contact layers7and4of the self-contained vertical cavity laser structure110, the active region AR may be activated to generate optical radiation P that may leave the self-contained vertical cavity laser structure110preferably through the first reflector2.

FIGS.1-6illustrate a single pillar LP and therefore only one self-contained vertical cavity laser structure110. Of course, a plurality of layered pillars LP may be etched into the layer stack LS and the etch stop layer5ofFIG.1. The layered pillars LP are preferably arranged in a two-dimensional pattern that corresponds to a two-dimensional pattern in which the pads and/or drivers are arranged on the second substrate10.

FIG.7shows a top-view of a substrate3which is provided with a plurality of layered pillars LP.

FIG.8illustrates a backside of the protective layer8which carries the layered pillars LP ofFIG.7after removal of the substrate3. The bases of the substrate-less layered pillars LP can be seen.

FIG.9illustrates a (second) substrate10on which a plurality of layered pillars LP are mounted as discussed above with reference toFIGS.1-8. Above the layered pillars LP, a multi-core fiber200is arranged. Each core210of the multi-core fiber200is assigned to a layered pillar LP. During operation the layered pillars LP generate radiation P which is coupled into the individually assigned cores210of the multi-core fiber200.

FIG.10shows a photograph of 256 lasers, embedded into wax, after substrate etching, and a sapphire substrate on top.

FIG.11shows a photograph of bottom side metallization of 12 lasers with different mesa size and a pitch of 600 μm.

FIG.12shows a photograph of a bottom side of a metalized mesa with a diameter of 56 μm.

FIG.13shows a top side micrograph (looking through the sapphire carrier) of a laser having a footprint of 56 μm embedded in wax.

The exemplary embodiments of the present invention as described above reduce the footprint of the resulting lasers by at least one order of magnitude compared to prior art (seeFIG.14). A cost reduction per device by the same order of magnitude may be obtained. This is primarily achieved by the removal of the substrate3and by contacting the lasers independently from two sides. Reduction of the footprint enables an increase of the device density, matching the geometries of multi-core fibers and allows easy heterogeneous integration of the photonic device with the driver leading to high bit rate, energy efficient transmitter modules.

Release of devices from the substrate on which they were grown, e.g. GaAs, and deposition onto Si or even Cu showing larger heat conductivity (seeFIG.15) improves their thermal properties, such that thermal roll-over, typically limiting the output power occurs at much larger currents and at the same time the cut-off frequency and maximum bit rate are increased. In addition, deposition of the laser on the driver or at least reduction of the distance to it eliminates bonding wires or reduces their length and their number, leading to an important reduction of parasitics, again increasing the cut-off frequency and the maximum bit rate. Both advantages together enable to approach the intrinsic limit of the 3 dB cut-off frequency, which is until now not possible.FIG.16illustrates a typical intrinsic small signal response, a small signal response effected by thermal effects, and a small signal response effected by thermal effects and parasitics.

The various embodiments and aspects of embodiments of the invention disclosed herein are to be understood not only in the order and context specifically described in this specification, but to include any order and any combination thereof. Whenever the context requires, all words used in the singular number shall be deemed to include the plural and vice versa. Whenever the context requires, all options that are listed with the word “and” shall be deemed to include the word “or” and vice versa, and any combination thereof.

In the drawings and specification, there have been disclosed a plurality of embodiments of the present invention. The applicant would like to emphasize that each feature of each embodiment may be combined with or added to any other of the embodiments in order to modify the respective embodiment and create additional embodiments. These additional embodiments form a part of the present disclosure and, therefore, the applicant may file further patent claims regarding these additional embodiments at a later stage of the prosecution.

Further, the applicant would like to emphasize that each feature of each of the following dependent claims may be combined with any of the present independent claims as well as with any other (one ore more) of the present dependent claims (regardless of the present claim structure). Therefore, the applicant may direct further patent claims towards other claim combinations at a later stage of the prosecution.