Patent Application
20110211604
The present invention relates to surface-emitting semiconductor lasers, systems or arrays composed of such surface-emitting semiconductor lasers, and methods for manufacturing such surface-emitting semiconductor lasers and semiconductor laser arrays.
Surface-emitting semiconductor lasers, also referred to below as vertical laser diodes or vertical cavity surface-emitting lasers (VCSEL), are a new type of semiconductor laser diode whose development at the Institute for Optoelectronics, University of Ulm, in the 1990s was initiated and continued by Prof. K. J. Ebeling, and which at the present time is primarily being conducted under the supervision of Dr.-Ing. R. Michalzik. For approximately the last ten years these lasers have been manufactured on the industrial scale in many different versions, with increasing commercial success. In particular, in the past three years the demand for VCSELs has multiplied with annual growth rates of 100 to 200%. Due to their special properties they are continually opening up new fields of application. At the present time they are manufactured in annual quantities of several million units by numerous companies, among them the Ulmer spin-off company U-L-M photonics GmbH.
The fields of application start with highly specialized use in parallel optical data connections which are currently used to further accelerate distributed computing in the world's most powerful computers, the supercomputers, but which have the potential, with a decrease in manufacturing costs, to replace copper-based bus systems in standard computing technology, and thus open up the computer mass market. Another huge market, which however is extremely sensitive to cost, is the automotive industry. In the near future, lasers which typically emit in the near-infrared range will be providing many fields of application for sensor systems due to driver assistance systems which are being increasingly developed, such as blind spot monitoring or collision recognition. As a result, data streams will also greatly increase in ever more intelligent automobiles, which will create a demand for sensor system VCSELs as well as data transmission VCSELs in automobiles. Consumer products such as optical computer mice, for example, represent another mass market for VCSEL, so that, due to the enormous cost pressure, long-term survival of a VCSEL manufacturer can be ensured only by continuous advances in productivity.
However, the discussion of the demands of future mass applications makes it clear that conventional manufacturing strategies are inadequate with regard to performance as well as cost. In addition to the reduction in space consumption, there is a great potential for a fundamental change in process technology toward completely self-adjusted VCSEL structuring, which is already customary for other modern semiconductor components.
VCSELs are components which are subject to power loss and at the same are sensitive to temperature. In many applications, a large number of these lasers are housed in a very compact space, and are also integrated with driver chips, which likewise result in power loss. Therefore, a thermal problem exists. Furthermore, the components attain the high required modulation speeds only at high pump flow rates, i.e., in operating states in which there is comparatively great heat loss. In addition, due to the complex layer structure of the VCSEL itself, satisfactory heat dissipation in the prior art has been lacking.
The object of the present invention, therefore, based on the prior art, is to provide surface-emitting semiconductor lasers or semiconductor laser elements and arrays from such semiconductor lasers which show improved heat dissipation capability, allow higher operating speeds, have improved conversion efficiency, and have a longer service life as well as a higher maximum output—in brief, which in comparison to the semiconductor laser elements known from the prior art have improved efficiency. A further object is to provide manufacturing methods for such surface-emitting semiconductor laser elements or semiconductor laser arrays.
The object is achieved by a surface-emitting semiconductor laser element according to Claim 1, a system composed of such semiconductor laser elements according to Claim 22, and a manufacturing method according to Claim 39. Advantageous embodiments result from the respective dependent claims. Claim 44 also describes uses according to the invention.
The present invention is first described in general terms. This is followed by two specific embodiments which are first described with regard to their structural-physical design. The two exemplary embodiments are then described in detail with regard to the mode of operation of the illustrated surface-emitting semiconductor laser element, with regard to the advantages of the surface-emitting semiconductor laser element according to the invention, and with regard to the manufacture of the surface-emitting semiconductor laser element according to the invention.
The aspects of the present invention which are described in the individual exemplary embodiments may occur not only in the particular combination specifically described, but, within the scope of the present invention on the basis of the expertise of one skilled in the art, may also be carried out and used in other combinations.
In the present invention, the term “substrate” or “substrate base section” generally refers to a carrier, a base, and/or a supporting structure of any given shape and material. Thus, the substrate may be designed as a flat semiconductor wafer made of Si or GaAs, or as a three-dimensionally structured support made of plastic, for example.
The basic concept of the present invention involves designing the mesa of the surface-emitting semiconductor laser element in a completely novel way, namely, by providing a constriction or multiple constrictions in the side flank of the mesa. In this regard, a constriction is understood to mean a region of the mesa in which the mesa, viewed in the direction essentially perpendicular to the direction of emission or essentially parallel to the substrate plane, has a decreased cross-sectional area compared to mesa regions situated above and below same (viewed in the direction of emission). In particular, in the narrower sense the term “constriction” is understood to mean the section of the mesa at the level at which the mesa has the smallest surface area (for a circular mesa viewed in the cross section, parallel to the substrate layer plane, in the direction of emission, for example, this would be the level at which the mesa has the smallest diameter). In other words, an indentation or a lateral etching is provided for the mesa of the surface-emitting semiconductor laser element according to the invention, thus removing material which forms the mesa from the side flank of the mesa at the level of the constriction.
Another important aspect of the present invention is the novel design of the flip chip integration of the semiconductor laser which is based on the above-described constriction, i.e., provision of a completely novel way of bordering the semiconductor laser by a three-dimensionally structured substrate or a three-dimensionally structured supporting element.
The two aforementioned important aspects of the present invention, which are described in detail below with reference to exemplary embodiments, have the following advantages over the prior art:
The VCSELs according to the invention, also referred to below as XCSELs due to the constriction according to the invention (for example, X-shaped Cavity Surface-Emitting Laser or also eXtended Capabilities SEL), represent a new level of monolithic VCSEL technology. For various applications they offer completely novel approaches to advances in manufacturing productivity which heretofore were believed to be unattainable. This is achieved without, for example, having to settle for compromises in component performance; on the contrary, a (sometimes dramatic) improvement in component performance results. For example, in initial testing of flip chip-integrated XCSEL arrays, the values for thermal resistance were lower than the previous best international value by approximately 50% for lasers of this design, and were even approximately 70 to 80% lower than commercially marketed products. As the result of a 50% reduction, the lasers become only half as warm under comparable operating conditions, which is even more important in light of the fact that they are generally thermally limited with regard to their critical performance parameters and service life.
Thus, the present invention opens up tremendous advances in productivity as well as new possibilities for superior thermal management. As shown by the example of substrate-remote high-speed VCSEL for data transmission, the transmission rates may be significantly increased by using XCSELs. The proposed technology is also of interest for high-performance VCSEL due to the improved cooling. From a technological standpoint, it is possible for the first time to carry out completely self-adjusted manufacture of complete VCSELs, optionally even including the p-side flip chip connection. Despite the introduction of additional elements, process steps are entirely eliminated, and the remainder are streamlined. The VCSELs are provided with new, optimized geometries which not only improve the component properties but also provide technological tools in different variants, in particular as built-in lithography and shadow masks.
The following advantages in particular are achieved:
When the XCSELs according to the invention are flip chip-integrated, in particular the following new possibilities for flip chip integration are obtained:
Thus, improved performance with reduced manufacturing complexity also results for the flip chip integration.
The present invention is described in detail below with reference to two exemplary embodiments (the structure of an individual surface-emitting semiconductor laser according to the invention is described first, followed by the description of its mode of operation, then the description of the structure of a flip chip-integrated array according to the invention of surface-emitting semiconductor laser elements according to the invention, and lastly, the description of the mode of operation of the latter).
The figures show the following:
b through f show variants of the invention as shown in
b shows an intermediate step in the manufacture of the element shown in
b) shows another example in which the n mesa region has the same design as shown in
According to the invention, as described below the mesa M has a constriction E of its side flanks (these are the sections of the mesa M which are not parallel to the substrate layer plane 1): As described in greater detail below, a material section has been removed or ablated from the side flank of the mesa M over the entire circumferential region of the mesa M in such a way that, viewed in the cross section perpendicular to the substrate layer plane (and through the center axis of the mesa M, which in the present case is rotationally symmetrical), a V-shaped indentation results such that at the level of the active layer A of the active region 3, the mesa M has a region (indentation region E) in which the diameter of the mesa, viewed parallel to the substrate layer plane, is approximately one-half the average diameter of the n-doped region 2 or of the p-doped region 4. In the present case, the V-shaped indentation is situated at the level of the active layer, but the narrowest point of the indentation E is located in the first reflector layer of the upper DBR adjoining the active region. Thus, at this location several epitaxial layers (approximately 100 to 300 nm) are situated above the quantum films in which the light is generated. With reversed polarity of the lower and upper mesa sections, the narrowest point would then be correspondingly situated below the active layer.
The mesa M comprising elements 2, 3, and 4 may thus be approximately described as a geometric body composed of two truncated cones which, centered with the cover surfaces having the smaller diameter for the particular truncated cone, adjoin one another. Thus, for the mesa M this results in a diabolo-shaped design, or a design which, viewed in the cross section perpendicular to the substrate layer plane and through the central rotational axis of the mesa, has an essentially X-shaped design.
In detail, the n-doped region 2 is designed as a truncated cone, and the p-doped region 4 on its side facing the n-doped region 2 is likewise designed as a truncated cone, which on the side facing away from the n-doped region 2 adjoins a flat cylinder having a diameter corresponding to the base diameter of the truncated cone. The cylinder is likewise part of the p-region. This flat cylinder is usually present, although this is not absolutely necessary. If the indentation is etched more deeply, the upper part still consists of a truncated cone. In such a variant, the average distance of the optical field in the resonator from the side walls is greater, since the upper part of the walls also extends further outward.
Due to the fact that the mesa diameter in the region of the constriction E (which represents the region of the smallest diameter of the mesa) is approximately one-half as large, at the level of the active layer A this results in approximately one-fourth the cross-sectional area of the mesa M compared to the cross-sectional area in the region of the two Bragg reflector stacks 2 and 4.
In the illustrated case, the indentation has a lateral depth of approximately 5 μm. A preferred value range is 0.5 to 10 μm, but preferred depths are between 1 and 6 μm. In the present case the side walls extend at an angle of ±30° relative to the horizontal, from the center outward; naturally, this angle may be varied, in particular via the etching rate differentials between the involved layers (etching rate difference between quick-etching and slow-etching layers). The oxidation length is only a few microns, typically approximately 2 μm, but may be less than 1 μm, or even 0 μm.
In general, the following applies:
The lateral depth of the indentation is limited by the mesa height and the maximum achievable angles. The depth of the oxidation layer should be just great enough to adequately reduce scattering losses at the mesa indentation. Both elements should keep the optical field at a sufficient distance from the metal-plated side walls, since increased losses would otherwise occur at that location, not only from scattering, but also due to induced currents. Consequently, the lateral extensions of these elements are primarily determined by the required distance from the side walls. On the other hand, the active diameter and thus the diameter of the narrowest point of the indentation are essentially freely selectable. In theory, they are limited only by the wafer diameter, but in practice vary greatly, depending on the application; the overall region includes values, for example, between 1 μm and 1 mm, although the active diameters are more often between 2 and 50 μm.
In the illustrated case, at the level of active layer A an annular, high-resistance, or electrically blocking current constriction layer 5 (in the present case, by means of a corresponding oxidation layer, as is known to one skilled in the art) is also provided in active region 3. Alternatively, however, this current constriction layer 5 may also be dispensed with (in which case only a geometric constriction E is present). Since the current constriction layer 5 is provided at the level of constriction E, an oxidation length results for this current constriction layer 5 which is greatly reduced compared to the prior art. In the illustrated example, the side flanks of the n-doped region 2 and of the lower p-doped region 4 of the mesa M formed by the removal of the material, i.e., by provision of the indentation E, have an angle α of approximately 60° relative to the rotational axis of symmetry of the mesa M (which corresponds to the direction of emission of the semiconductor laser). Thus, viewed in relation to the substrate layer plane 1, this results in a comparatively flat progression of these side flanks. Directly adjacent to and on the substrate base 1 and the surface of the first doping region 2, a first side wall metal contact 6a is then provided which has a concentric design over the entire circumference of mesa M in region 2. This side wall metal contact not only completely covers the surface of the first doping region 2 on the side flank of the mesa M, but is also situated on the surface section of the active region 3 facing the first doping region 2, and is thus used in thicknesses up to a few microns on active layer A. A first side wall heat dissipator 7a, in the present case made of gold, is situated on this first side wall metal contact 6a and adjacent thereto. This side wall heat dissipator covers practically the entire surface side of the first side wall metal contact 6a facing away from elements 1, 2, and 3, and due to this design provides optimal dissipation of the heat generated by the semiconductor laser.
The surface of the p-doped region 4 facing away from the substrate base 1 as well as the side flanks of the previously described cylindrical section of this doping region bear a second side wall metal contact 6b. The surface thereof facing away from the p-doped region is enclosed by a second side wall heat dissipator 7b.
As shown, the described geometry results in a greatly reduced oxidation length, and an active diameter (see figure) which extends over approximately one-third of the average cross-sectional diameter of the p-doping region or of the n-doping region.
Practically all of the present commercially used VCSELs based on the InAlGaAs material system contain a thin layer, containing a high proportion of aluminum, within the epitaxial structure which in the prior art has heretofore been used for lateral current constriction by selective oxidation. In the present invention, for the first time use is made of the lateral etching rate differential between this oxidation layer (or also another layer specially provided for this purpose) and the other layers of a VCSEL structure for contouring the mesas according to the invention. This shaping is carried out in a single wet chemical etching step which includes all the epitaxial layers of the VCSEL. In contrast, in the two-step etching known for many years from the prior art, the p- and n-conductive epitaxial layers were structured separately in individual etching steps which usually were not performed in direct succession.
The term “lateral etching rate differential” is understood to mean the difference in the etching rates between the individual epitaxial layers, which results in the formation of geometrically constricted XCSEL profiles during the etching.
The XCSELs are composed of a stack of very thin semiconductor layers having various material compositions. Depending on the layer composition, the semiconductor lattices may be dissolved at different rates using etching solutions. In the vertical direction the individual layers each have only a very slight extension, so that there are no differences in the etching rates, and only an average rate is observed for vertical advancement of the etching solution. In contrast, in the lateral direction the layers are greatly extended, and the etching rate differences may last for a long time, so that different side wall profiles may be formed, depending on the combination of quickly and slowly etching layers. During the etching operation, for the individual component a limited region of the wafer surface is covered by an etching mask (photoresist, for example). During the etching, in the exposed regions the layers are removed one after the other, vertically with respect to the wafer surface, so that as the etching process progresses, at the mask edge the end faces of increasingly deeper layers are exposed to the etching solution. As soon as a layer emerges, it is also attacked in the lateral direction, starting from its end face. As soon as a quickly etching layer is exposed, at that location the dissolution process parallel to the wafer surface advances more quickly than at the locations above and below same, resulting in formation of a notch, i.e., the constriction according to the invention. When a notch is created, the upper/lower adjacent layers are attacked not only from their end face, but also from below/above (see
Thus, in principle, multiple basic forms or side wall profiles may be “programmed” into the epitaxial layer structure. This epitaxy-controlled production of the XCSEL profiles is carried out efficiently in a single wet or dry chemical etching step which automatically centers all parts of the laser (both mirrors and the active zone therebetween) with respect to one another.
Among other things, perfectly perpendicular walls or wall sections may be produced by wet etching, in that the etching rate of the layers increases in the downward direction to exactly the correct degree, so that the layers which do not take part in the etching process until later may “catch up” with the longer-etched layers at the end of the etching process. Likewise, combinations of multiple notches/constrictions or multiple overhangs in the side wall profile (sections having a negative angle of the side wall between 0 and 90°) may be achieved which in the XCSEL, for example, contribute to even stronger lateral wave guiding, which allows the mode volume to be further reduced in order to make the lasers faster.
The etching rate of the epitaxial layers may be controlled by the composition of the layers, i.e., the concentrations of chemical elements in the compound semiconductor, which in particular for epitaxial growth may be controlled very precisely. This includes the elements of the compound semiconductor, for example Al in AlGaAs, as well as the added dopants such as Si or C, for example. In contrast, in the transport-limited case the thickness of the layers also affects the lateral removal rate of the layers. In addition, any etching behavior is naturally determined by the etchant used, which in the AlGaAs material system, for example, is a mixture of H2SO4, H2O2, and H2O.
In conventional mesa-isolated components from the prior art, the side walls represent a potential source of particle losses, which are caused by contact with the corresponding optical or electrical particles, primarily as the result of scattering and surface currents or recombination. For this reason, their influence is eliminated by laterally extended thin insulating diaphragms (made of air or oxide, for example), which, however, are subject to considerable capacitances and which hinder heat flow.
On the other hand, XCSELs according to the invention involve novel functionalization of the side walls. In contrast to conventional concepts, in the present case the side walls are not passivated, and their influence is thus largely eliminated; instead, they have a special shape and assume a new, active role.
The prior art has the following disadvantages:
In contrast, the following applies for the present invention:
d illustrates important aspects of the XCSEL side wall geometry according to the invention, with reference to an example configuration. The approximate detail of the XCSEL (indicated by a dotted line) in the right part of
(a) Wedge-shaped “spacer”;
(b) The structural edge together with targeted deposition allows conductive materials in close proximity around the optical field to be guided, separately from above and below, into the immediate vicinity of the active zone;
(c) The short oxide extension causes weaker guiding upstream from the stronger geometric guiding, as a result of which the scattering losses are reduced and the proportion of geometric guiding may be adjusted to the overall guiding (M: side wall metal plating(s), A: active layers; diagrams not to scale).
The active zone may be located above as well as below the narrowest point.
In summary, the invention discloses a novel way of guiding depositable materials laterally, in a targeted manner, to the point directly at the active zone, with the following being preferred:
The resulting X-shaped or diabolo-shaped contour of the lasers according to the invention, as illustrated in
The shape according to the invention also opens the way for a significantly more advanced technology. Besides the advantages strictly from the standpoint of cost-effectiveness of manufacturing, the shape allows the integration of optional additional elements which previously were not feasible, as well as increased precision. Both of these factors result in substantial improvements in performance of the components, in particular with regard to thermal management and all of the associated performance parameters of these lasers, which generally are thermally limited and subject to power loss.
In the solitary XCSEL according to the invention shown in
In particular, by making use of the mesa overhangs the p contact 6b as well as the n contact 6a may be produced as shadow masks with high precision and in a completely self-adjusted manner without requiring a separate lithography step. As a further feature according to the invention, the n contact 6a is also (or exclusively) formed on the mesa flanks. It may extend on the side wall all the way to the inner cavity E, thus bypassing the relatively high-resistance heterotransitions of the Bragg reflector 2 and allowing efficient current injection on the side walls in the immediate vicinity of the active zone A (“intracavity side wall injection”). This approach is particularly attractive when VCSELs having an elongated inner cavity are used, in which a larger contact surface area on semiconductor layers having a smaller band gap for good ohmic contact is available. These layers may be partially highly doped in regions of low field intensity. In addition, the alloying of the n contact material causes degenerate doping of the side walls. As a result, during the epitaxial growth the Bragg reflectors 2, 4 may then be optimized even with regard to reduced free charge carrier scattering and increased reflectivity, since they are no longer absolutely needed, and to the full extent, for charge carrier injection. Possible shortening of the Bragg reflectors once again improves the cooling of the active layers.
The p contact 6b and n contact 6a may be produced as follows (also see
Example A (A in
Example B (B in
Example C(C in
It is critical that the desired type of metal comes to rest on a sufficient contact surface area having the respective polarity. The overhangs or constrictions in the XCSEL profiles allow this without the active regions and surface areas of the respective other polarity being covered with photoresist, using lithography steps. Ohmic or blocking side wall contacts result, depending on which metals are deposited on the p- and n-doped regions.
For vacuum deposition of the contact metals, the wafer is mounted on a tilting device which allows the angle of orientation of the wafer surface relative to the source as well as the angle of rotation to be adjusted. If the upper portion is p-doped, for example, in a first step the metals for a p contact are deposited while the wafer plane is situated at an angle of 90° with respect to the source, so that the metals impinge on the wafer surface exactly perpendicularly. All of the surfaces visible from the top are coated with p-metal. The metal layer situated on the substrate has an opening which begins precisely vertically beneath the most outwardly projecting edge of the XCSEL profile. For the coating with the n-metal for low-resistance contacting of the lower n mesa section facing the substrate, the wafer is then tilted precisely so that the inner edge of these n-metal layers results directly at the active zone. The position of the inner edge results from the projection on the lower side wall of one of the protruding edges of the XCSEL profile at the selected angle.
The upper XCSEL portion facing away from the substrate may or may not contain the previously described flat cylinder, depending on the embodiment. Thus, either one (see Example A) or multiple (Examples B, C) edges result, which at various tilting angles may be used as shadow masks for defining the contact edges. The n-metal also comes to rest on the XCSELs at the top, and on the substrate at the bottom on the p-metal already deposited at that location, so that in these regions it is not in contact with the semiconductor and therefore has no electrical effect.
If the upper portion is designed in such a way that its flanks have a slightly negative angle as in Example B, for oblique coating the n-metal is deposited on the flanks of the p portion, resulting in an electrically blocking contact via which heat may be discharged. Providing a space charge zone beneath the contact surface also results in a field-specified current constriction of the charge carriers (in the present case, holes), which in this case are injected via the top side of the mesa.
In Example C the upper portion has slightly positively sloped flanks, which during the first, perpendicular coating are then covered with the metal which forms ohmic contact having the polarity of the upper portion of the mesa. Thus, in this embodiment there is the possibility for lateral current injection close to the cavity, via the side walls, for both types of charge carriers.
This method functions reliably in practice. However, process sequences are also possible in which, before any etching, a metal contact is produced for the upper mesa using a standard liftoff method. In that case, in addition to the side walls the entire substrate surface may be coated with the metal which establishes an ohmic contact for the substrate. This is meaningful in particular when the substrate is not subsequently removed and is also intended to be used for current injection.
The above-mentioned possible shortening of the Bragg reflectors may be carried out as follows:
The XCSELs according to the invention allow lateral current injection via sections of the side walls without depending on thin, laterally extended current supply layers. The profile which tapers toward the active zone, together with placement of the contacts on the side walls, greatly shortens the current flow paths compared to previous approaches for intracavity contacts.
At the same time, by alloying the contacts very high doping of the side walls may be achieved, which may be located so close to the edge of the current aperture on sections near the cavity that the depth of penetration of the contacts may practically bridge the remaining distance, resulting in low serial resistances.
In contrast to lateral continuous highly doped regions, however, this does not result in increased absorption, since the doping is less at locations where the optical field is guided in the resonator. The depth of penetration of the alloy contact should be such that the periodic structure of the layer inside the active diameter is not destroyed.
This process corresponds to subsequent doping of regions beneath the side wall surface, so that, due to the subsequent processing, a laterally varying doping profile results in addition to the longitudinal varying epitaxial doping profile. Alternatively, the XCSELs according to the invention offer the option of producing a laterally varying doping profile by overgrowth in a second MBE step in which highly doped semiconductor material is deposited on the side walls in a targeted manner.
Thus, the optical path and the current path may run separately, at least in places, thus reducing disadvantageous effects such as free charge carrier absorption and line broadening due to modulation.
The greatly increased precision of the structural definition according to the invention without manual adjustment steps makes the manufacture cost-effective by eliminating sources of error, and opens the way for further miniaturization of the components for increasing the modulation rates and integration densities. By using the structure according to the invention, entire process steps may be dispensed with, or replaced by steps which may be easily automated in a production environment. In particular, essentially eliminating time-consuming (usually manual) contact lithography significantly shortens the throughput times for the wafers, saves on personnel, and conserves the costly VCSEL wafer material, since less is used in the manufacture.
For example, in conventional VCSEL design, for defining the geometries a lithography step is necessary for each individual structural element. In contrast, for the XCSEL according to the invention, in a suitable production facility (after installation of remote-controllable tilting actuators) both electrical contacts 6a, 6b (p- and n-side) as well as vapor-deposited n-side heat sinks 7a, 7b may be sequentially produced without breaking vacuum, i.e., practically in a single step. The accompanying reduction in development times is relevant in particular for the increasingly important capability for rapid adaptation of existing base designs to specific customer demands. In addition, contact lithography, in which the wafer surface is pressed against a glass disk, represents a significant stress in particular for VCSEL wafers due to their pronounced surface topography. Largely eliminating such stress-producing processes therefore also increases the yield, i.e., reduces the proportion of previously damaged components which must be sorted out using burn-in tests.
Rough Exemplary Comparison of Process Times (Lithography, Evaporation, evacuation, LO (Lift-Off):
A further advantage of the XCSELs according to the invention is the precise knowledge of the oxidation length. In conventional wet-etched VCSELs (
For the XCSEL according to the invention (
It is also apparent from
The heat dissipators 7a, 7b implemented in this manner cool active region A with an efficiency heretofore unachievable, since they collect the heat inside the resonator directly at the locations where cooling directly influences the intrinsic variables which specify the dynamics. For the most part, the heat flows laterally within the cavity E in the direction of increasing thermal conductivity of the epitaxial layer structure, to the tip of the metal heat dissipators. The path to that location may be extremely short due to the particular mesa shape in conjunction with greatly reduced oxidation lengths, and does not have to initially lead in the vertical direction via heterobarriers with their reduced thermal conductivity. Depending on the dimensions of the XCSEL, the tip of these “cooling probes” may extend to the side wall of the inner cavity, and may thus be situated only a few microns from the edge of the current aperture. Nevertheless, the metal is in direct contact with the semiconductor, without blocking intermediate passivation layers located on the side walls, which greatly increases the heat exchange beyond this interface.
For example, data transmission VCSELs generally achieve the desired high modulation speeds, as well as flat progressions of the transmission functions with sufficient signal deviation, only at high pump flows combined with good discharge of the heat loss. As mentioned, direct dissipation of the heat loss from the active zone decreases the operating temperature directly at locations where temperature-dependent variables have a direct influence on the dynamic properties of the lasers. The technology according to the invention allows sufficiently thick heat dissipators 7 to be brought extremely close to the active zone without in turn creating dominating parasitic capacitances, which would eliminate the dynamic advantage achieved by direct cooling of the inner electro-optical processes due to the additional introduction of parasitic extrinsic capacitances. In conventional technology, sufficiently thick metal layers which are located close enough to the active zone to be thermally effective always included the incorporation of large geometric capacitances, which greatly impair the high-frequency modulation capability. In contrast, the XCSELs according to the invention allow the implementation of cooling structures which cause no capacitances which impair the modulation capability, which has been confirmed in data transmission tests.
Thus, in particular the following applies for the surface-emitting semiconductor lasers compared to the prior art:
The basic design of the individual surface-emitting semiconductor laser (having elements 1 through 7) shown in
According to the invention, however, the substrate base section 1 and the side wall metal contact/side wall heat dissipators 6a, 7a are shaped not only for bordering the semiconductor laser; in the region of the border of the semiconductor laser element, or concentrically around same, they also form a mechanical guide structure F due to their illustrated shape. This guide structure is formed by providing a tub-shaped depression in elements 1, 6, 7 around the laser border. This depression is designed in such a way that an optical element, in the present case a fiber 8 of a glass fiber bundle, may be inserted into the mechanical guide structure F in a self-centering or self-centered manner (viewed in relation to the laser), and may be fixed at that location using an appropriate transparent adhesive layer, for example. After the fiber 8 is fixed, it is not only centered with respect to the laser, but at its end face surface facing the laser it is also separated at a distance from the emission side of the laser (surface of the n-doped section, in the present case characterized by the photon radiation energy h·ν). Tilting of the fiber in the guide structure (angle error) which appears to be possible is prevented in that this is actually a matrix of elements which are connected to one another with sufficient rigidity. Thus, the array of optical elements, in a manner of speaking, stands on many “legs” and cannot tilt. However, the fiber guide may also have other geometries, in particular steep walls which are thus similar to the through-contacting mesa (the fiber guide may also be dry-etched, although wet chemical etching is preferred for cost reasons). In addition, the flat underside of an optical element may also be flatly mounted, and at the same time, with respect to the orientation in the plane (x-y plane, i.e., perpendicular to the plane of the drawing and to the direction of the laser emission) an angle error (tilting) may also be avoided, even with a single component.
Thus, the mechanical guide structure F results in an interspace Z between the semiconductor laser or its mesa M and the end-face end of the optical element 8. This interspace may be filled with a transparent material for optical coupling. Alternatively, however, using suitable inlet and outlet elements, for example, the guide structure F may be designed in such a way that a transparent gaseous or liquid medium (in the simplest case, N or deionized water) may be led through the intermediate region Z, thus providing for discharge of the generated heat and corresponding optical coupling.
Alternatively or additionally, it is possible to provide channels 14 in the optical element (in the present case, in fiber 8) through which a cooling medium (cooling liquid or gas) may flow into the interspace Z in a targeted manner (in this case, the channels introduced into the fiber 8 run parallel to the axis of symmetry or longitudinal axis of the fiber, although the channels may also run longitudinally between the individual multimode fibers in the fiber bundle. In that case the cooling medium impinges with a lateral displacement relative to the lasers, which leaves room for the described adjustment of the index of refraction. A circuit may also be produced; i.e., various partial quantities of the channels are operated as inlets and outlets).
Thus, the XCSEL according to the invention in
In addition, in
b shows an intermediate step which illustrates the production of the XCSEL from
The procedure may be roughly subdivided as follows:
As shown in the lower left part of
As further shown in
After the above-described processing, the electrical through contacting 10, viewed from the direction of the emission side (n-side) toward the p side, has the following layer sequence, in which the individual layers are each provided in the form of superposed, adjacent protrusions or prominences (facing the p side): metal plating contact layer 6a′, heat dissipation layer 7a′, and heat dissipator 7b′ in direct contact with bond pad 11. The stabilizing substrate section 1′ is situated in the flank region of the prominence, between layer 7a′ and layer 7b′. In this case element 7b′ is primarily a non-flat bond pad, which naturally also contributes to heat dissipation, but not as strongly due to the fact that it is separated from the laser by a larger cooled surface.
The technology on which the XCSELs according to the invention are based follows a different line of thought, in that the technology is purposely used to model the wafer topography. In this case the processing begins with three-dimensional shaping of the wafer surface in a sequence of wet etching processes. Thus, at the start of the processing, pronounced vertical profiles having a complex structure are provided in the wafer material, portions of which later shape new elements which could not be feasibly implemented heretofore. The technology of VCSEL which emits on the substrate side and which is remote from the substrate, which has dominated in the flip chip integration of VCSEL arrays with electronics systems on account of existing standards, has received a boost in development due to the introduction of XCSELs. Thus, for the first time it is possible for the heat loss, which as described is extracted from the immediate vicinity of the active zone by the side wall heat dissipators 7, to be conducted directly on the optically bound back side of the lasers with thermal bridging of the cathode-side Bragg reflector 2, as shown in
This is possible due to the elimination of continuous epitaxial layers, which heretofore provided lateral current transport between the laterally displaced soldered contact and the VCSEL on the emission side, and as part of the optical path (see
As a further aspect according to the invention, the novel heat dissipators 7 may be directly formed into integrated mechanical guides F for the self-adjusted optical binding to glass fibers or microlens arrays on the back side. The production of these guides is likewise possible in a self-adjusted manner in the novel technology, without the need for manually adjusted backlight lithography, for example. The fiber guides F are structured on the wafer plane from the front side of the wafer in combination with the surface etching for the lasers, which is carried out anyway. Thus, very little additional effort results. The guides are exposed when the substrate is removed from the back side (emission side), and then guarantee problem-free docking to the lasers which is precisely adjusted, for example, for each individual fiber of a suitable fiber bundle. For this purpose the soldered-on XCSEL array, as customary, is first mechanically stabilized by packing with a suitable underfill. The advantage of these fiber guides for the overall system of the parallel-optical data connection lies in the reproducible and uniform coupling efficiencies for all laser-glass fiber/lens pairs.
In addition, costly external adjustment structures such as additional manufactured precisely fitting pins on the control boards, as well as personnel- and time-intensive alignment of the system components with one another, are unnecessary. For an acceptable lateral displacement of ±15 μm maximum for injection into multimode fibers having a core diameter of 50 μm, the requirements for dimensional accuracy of external adjustment structures would far exceed the standard production tolerances of printed circuit boards in the range of ±50-100 μm, thus increasing the costs. As a result, it is now possible not only to globally adjust fiber bundles or microlens arrays in their entirety to form the VCSEL array in conventional transmitter modules, but also to simultaneously handle the individual paired alignment of each laser of a VCSEL array for the corresponding element of an array made of fibers or lenses, for example, similar to the manner in which all contacts are simultaneously produced in a single reflow soldering operation on the electrically connected side and aligned by automatic floating. The embodiment of the XCSELs shown in
The integrated guide structures F result in a very precisely known and in particular also stable vertical (axial) distance between the laser extraction facet and the end face of the fiber. The interspace Z between the VCSEL and the fiber, identifiable in
Likewise in the course of the initial three-dimensional shaping of the wafer surface by wet chemical etching, with a similar efficiency as for the fiber guides, mesas DM for the cathode-side through contacting 10 of the flip chip contact are also formed from metal. These mesas replace the previously generally used, but very problematic, technique of polymer blind hole electroplating. This conventional technique requires multiple contact lithography steps. In the same manner, the alternative use of dummy VCSEL mesas also requires separate treatment thereof in deposition processes involving additional contact lithography steps. In addition, even minor lithography problems frequently prevent homogeneous and reproducible electroplating in all polymer openings distributed over the entire wafer, also during the diffusion processes in the metal plating which take place during the mandatory high-temperature treatment of the polymer, with subsequent oxidation of unknown species at the exposed surface. In principle, a polymer blind hole is also a geometry that is poorly sealed by electroplating. Since it is not possible to deposit on the inert polymer by electroplating, in general no close bond is formed between the metal column and the flanks of the opening which are made of polymer and have a height of several μm. After filling with metal, a fissured surface, not an actually flat closure, thus remains. However, well-sealed transitions between the metal-plated levels are very desirable, in particular for reflow soldering which is used for flip chip bonding.
In contrast, the technology according to the invention which is based on the purposeful wafer surface modeling once again has manufacturing-specific advantages as well as a function which is superior to the conventional structures described above. A column 1′ etched in the semiconductor material 1 during the initial wafer surface modeling (approximately 20 μm high with a plateau 5 μm wide; also see
This type of through contacting 10 entirely dispenses with a separate metallic filling. The transitions between the involved structural elements are geometrically fluid, and do not have the sharp angles as is the case for the conventional blind hole structure. In addition, all materials used adhere tightly to one another. This results in a smooth surface which is easily sealed off by a bond pad. Unlike the flat bond pads, which in other respects are fairly common, this through contacting forms a prominence DM which is enclosed by the solder ball 11, thus improving the stability of the soldered connection.
Here as well, the structural discontinuities are reduced compared to the conventional connection according to the prior art, since the ascending flanks of the solder bridge (see element 11 in
Similarly, the anode-side Bragg reflector 4 of the XCSEL itself is also enclosed by the corresponding solder ball 9. Thus, when there is low heat input through the electronics system a substantial portion of the waste heat may also flow through this channel, as indicated in
Thus, since the design according to the invention has no planar surface, in addition a self-planarizing flip chip technology is used in which the varying solder ball sizes perform the bridging of all height differences.
For the flip chip XCSEL according to
The following are situated on the substrate 1 (semiconductor substrate) in the given sequence: First, a first doping region (n-doped region) 2, on which the active region 3 is situated, and situated on the active region 3 is the second doping region 4 (which in the present case is likewise n-doped, and which is situated at the structure of the active region 3 as described below).
In an analogous manner as previously described for the surface-emitting semiconductor laser element of
In contrast to the element shown in
Just beneath the first active layer A1, the mesa of the illustrated semiconductor laser element has a first indentation region, i.e., a first constriction E1 which, analogously to the case shown in
Thus, multiple constrictions (preferably stacked perpendicularly on top of one another, i.e., superposed as viewed in the direction of emission) may also be achieved according to the invention. This is used in particular to reduce the mode volume by stronger guiding, and to adapt the geometry of the current supply or the heat dissipation to special requirements. The invention may thus also be easily implemented in the field of three- or multiterminal-like components (VCSEL or XCSEL having multiple cascaded active regions, and/or VCSEL or XCSEL having an integrated photodetector layer).
The proposed flip chip-soldered XCSEL arrays according to the invention have already been completely produced as prototypes. Instead of CMOS chips, for test purposes the arrays have been integrated with silicon-based supports containing coplanar control lines for high-frequency characterization. The components have been extensively characterized in the flip chip-soldered state, and in comparison to conventional components show greatly improved thermal and dynamic properties.
As indicated in dashed lines in
Using the proposed technology, in initial testing it has been possible to reduce the thermal resistance by half compared to the best international values heretofore. Preliminary increases in the limiting frequencies by likewise approximately 50%, to 17 GHz thus far for flip chip-integrated substrate-remote lasers, show that the thermal improvements as described are not achieved at the expense of the dynamics, but, rather, benefit the dynamics as the result of lower internal temperatures for avoiding additional extrinsic parasitics. In addition, initial data transmission experiments have been carried out at the maximum data rate of 12.5 Gbit/s achievable with the available equipment, thus demonstrating the suitability of the technology for such applications.
As described above, the proposed invention shows the formation in active region 3 of a high-resistance or electrically blocking current constriction layer 5 having a reduced oxidation length. Alternatively, it is possible not only to minimize the oxidation length (using an annular oxidation region 5 with appropriately low extension), but to even eliminate the oxidation length. Therefore, it is not necessary to provide a corresponding constriction region 5: in the present case the geometric constriction was combined with the constriction resulting from an annular insulating layer, but may also be used alone, i.e., without an additional lateral oxidation layer (insulating diaphragm).
The first prototypes of the XCSEL according to the invention have shown no particular vulnerabilities under the harsh laboratory environment, and have permitted extensive, problem-free characterization over a fairly long period without special precautions.
As an alternative to the embodiments described above, the following variants may also be implemented within the scope of the invention:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 022 793.5 | May 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/003213 | 5/5/2009 | WO | 00 | 3/24/2011 |
