Moisture Control in Oxide-Confined Vertical Cavity Surface-Emitting Lasers

Abstract

The manufacturing sequence for oxide-confined VCSELs includes depositing a protective coating on exposed horizontal surfaces to prevent the formation of an undesirable oxide layer during the lateral oxidation process used to create the oxide pores. By first preventing oxidation of these surfaces, the opportunity for moisture to enter the active region of the VCSEL is eliminated. For example, prior to commencing the conventional lateral oxidation process used to form the oxide pores of the VCSEL, the exposed Al-containing surfaces are covered with a protective coating of a dielectric material. With the protective coating in place, the conventional manufacturing process is resumed, and the protective coating ultimately forms part of a passivation layer used to provide electrical insulation for the final VCSEL device.

Description

Technical Field

[0001] The present invention relates to vertical cavity surface-emitting lasers (VCSELs), and more particularly, to a technique for minimizing moisture intrusion into the mesa structure of an oxide-confined VCSEL. Background of the Invention

[0002] In the fabrication of VCSELs that utilize oxide apertures to provide current confinement, the apertures are typically created by oxidizing the exposed mesa structure of the distributed Bragg reflector (DBR) portion of the VCSEL. Selected layers within the mesa are formed a priori to present a higher concentration of material that oxidizes rapidly relative to the remaining layers in the structure. In GaAs-based VCSEL devices, the Al x Ga 1-x The As interstitial layer can be purposefully formed to exhibit an aluminum content x of, for example, 0.9 or more, since aluminum is known to have a high oxidation rate. Typically, lateral oxidation proceeds inward from the exposed mesa sidewalls and thus results in a central pore shape that mimics the topology of the mesa itself. The oxidation rate depends on factors such as the Al x Ga 1-x The material composition of the As layer, the layer thickness, the oxidation temperature and the like are factors.

[0003] Unfortunately, the oxidation process also causes other exposed Al-containing surfaces to be slightly oxidized as well. In particular, the trenches surrounding the VCSEL mesa structures are slightly oxidized, forming AlGa oxide (AGO) on the top surfaces of the trenches. As a result, moisture can propagate through entry points in the AGO layer and reach the mesa, degrading the performance and / or reliability of the VCSEL emitter.

[0004] One approach to addressing the moisture intrusion problem involves depositing multiple passivation layers after the oxidation process, attempting to increase the likelihood that all "pinholes" formed within the AGO layer will be covered, preventing any external moisture from propagating through. However, any moisture present before the outermost passivation layer is deposited will still remain trapped and can degrade device performance. Another approach is based on utilizing specialized passivation films with hydrophobic properties. SUMMARY OF THE INVENTION

[0005] The present invention addresses the problems associated with the presence of moisture in oxide-confined VCSEL structures and in particular involves modifying the manufacturing process to prevent the formation of undesirable oxide layers in the first place, which therefore eliminates the opportunity for moisture ingress through these oxides.

[0006] According to the principles of the present invention, the exposed Al-containing surface is covered with a protective coating of dielectric material before commencing a conventional lateral oxidation process for forming oxide pores of a VCSEL. The protective coating first eliminates the possibility of forming an undesirable AGO layer by not leaving the Al-containing material exposed and available for oxidation. With the protective coating in place, the conventional manufacturing process resumes, wherein the protective coating ultimately forms part of a passivation layer for providing electrical insulation for the final VCSEL device. The thickness of the deposited passivation layer can be adjusted to accommodate the presence of the protective coating, but this is not required.

[0007] In some preferred embodiments, the same dielectric material can be used for both the pre-oxidation protective coating and the final passivation layer. For example, silicon nitride can be used for both the protective coating and the passivation layer. In other cases, different materials can be used, as long as the protective coating includes a dielectric material that protects the underlying Al-containing surface layer and substantially eliminates the formation of an undesirable AGO layer.

[0008] An exemplary embodiment of the present invention takes the form of a VCSEL, which includes a substrate on which a first distributed Bragg reflector and a second distributed Bragg reflector (DBR) are arranged, each DBR including a stack of layers having alternating refractive index values, wherein the second DBR is formed as a mesa structure having an outer boundary of an end region of the stack of exposed layers. An active layer is positioned between the first DBR and the second DBR, wherein a void layer is arranged within the second DBR, the void layer being formed to have a higher concentration of an oxidized element than the remaining layers in the stack of layers. The VCSEL also includes a protective coating of a dielectric material, which is deposited to cover the exposed horizontal surface portions of the first DBR and the second DBR.

[0009] The principles of the present invention are also embodied in a method for manufacturing an oxide-confined VCSEL, the method comprising the following steps: a) growing a stack of layers having alternating refractive indices and controlled thicknesses on a substrate to form a first distributed Bragg reflector (DBR); b) forming an active region of a multiple quantum well structure on the first DBR; c) growing a stack of layers having alternating refractive indices and controlled thicknesses on the active region to form a second DBR, wherein a layer within the stack of layers of the second DBR is defined as a void layer and exhibits a higher concentration of oxide-forming elements than the remaining layers in the stack of layers of the second DBR; d) etching the stack of layers of the second DBR to form a mesa structure exposing an edge of a layer including the void layer, the etching forming a trench around the mesa structure, the trench having a top surface layer including an oxide material; e) covering the trench with a protective coating; and f) performing a lateral oxidation process on the void layer to produce oxide voids within the VCSEL.

[0010] Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Referring now to the drawings, wherein like numerals represent like parts throughout the several views:

[0012] Figure 1 is a cross-sectional view of a prior art oxide-confined VCSEL showing the formation of an undesirable oxide layer on the trench surfaces of the device;

[0013] Figure 2 Figure shows the steps after forming the mesa structure in the top DBR in the VCSEL manufacturing process;

[0014] Figure 3 The following steps in the VCSEL manufacturing process according to the present invention are shown. Figure 2 Depositing a coating material on the device shown in FIG.

[0015] Figure 4 Figure shows the positioning of a mask layer over a deposited coating material, defining areas of the coating material to be removed prior to a lateral oxidation process;

[0016] Figure 5 The figure shows the following steps, wherein defined portions of the coating material are removed by etching to expose the mesa structure;

[0017] Figure 6 The figure shows the beginning of the lateral oxidation process, where the porous layer in the top DBR begins to oxidize;

[0018] Figure 7 The figure shows the configuration of the VCSEL when the lateral oxidation process is completed, resulting in oxide pores of the desired width W; and

[0019] Figure 8 The figure shows the final VCSEL device, including the passivation layers formed on the protective coating and mesa, and the conductive contacts to the two DBRs.

[0020] Details

[0021] A vertical cavity surface emitting laser (VCSEL) consists of a pair of distributed Bragg reflectors (DBRs) that act as opposing mirrors of the laser cavity, with the active region of the laser and the current confinement structure positioned between the pair of DBRs. An exemplary DBR is formed of relatively thin layers having an alternating composition, one set of layers exhibiting a relatively high refractive index and the other set of layers exhibiting a relatively low refractive index, such that the alternating arrangement of the two types of layers forms the desired reflective (mirror) structure. In a GaAs-based VCSEL device structure, the DBR is formed of alternating layers of GaAs and AlGaAs.

[0022] Figure 1 is a simplified cross-sectional view of a conventional prior art VCSEL 10, showing a first DBR 12 (grown on a GaAs substrate 13) and a second (opposite) DBR 14. As shown, the second DBR 14 is formed to exhibit a mesa structure, allowing top-side electrical contact to both the first DBR 12 (in the form of a deposited first contact 16) and the second DBR 14 (here, a deposited second contact 18). As is well known in the art, other configurations may position the first contact 16 on the back side of the substrate 13. An active region 20 comprising one or more quantum well layers is formed between the first DBR 12 and the second DBR 14, wherein a current-confining aperture layer 22 comprises a central region 24 of different composition (hereinafter referred to as "oxide aperture 24") formed on the active region 20. As is known in the art, the oxide aperture 24 is used to confine the current and propagating optical modes to the center of the structure, wherein the light beam generated by the VCSEL 10 exits the second DBR 14 through the top surface.

[0023] The mesa structure of the second DBR 14 is usually defined by a photolithography process followed by an etching step, wherein conventional mesas 30 have a substantially circular cross section, cylindrical or conical in three-dimensional form. Figure 1 As is apparent from the illustration, the formation of the mesa structure 30 exposes the sidewall regions of the various AlGaAs layers forming the second DBR 14, including the pore layer 22. The pore layer 22 includes an AlGaAs layer having an increased proportion of aluminum relative to the remainder of the AlGaAs layer within the structure. During the lateral oxidation process used to produce the oxide pores 24, the higher aluminum concentration results in a higher oxidation rate of the pore layer 22 relative to the remaining layers. Controlling the oxidation parameters in a known manner allows the formation of oxide pores 24 of a desired width W.

[0024] Although useful for producing pores of the desired geometry, conventional lateral oxidation processes used in VCSEL formation also result in oxidation (slightly) of all other exposed Al-containing surfaces of the VCSEL structure. In particular, the top AlGaAs layer 12-T of the first DBR 12 will be slightly oxidized during the lateral oxidation process. Although the aluminum concentration in layer 12-T is significantly lower than that of the pore layer 22, there is enough aluminum to form a thin AlGa-oxide (AGO) layer 26 on layer 12-T, as shown in FIG. Figure 1 As shown in FIG. 2 , due to the formation of the undesirable AGO layer 26, moisture from the external environment is able to penetrate through the pinholes in the oxide and enter the oxide from these entry points (such as Figure 1 The moisture in the VCSEL device is known to reduce the performance of the VCSEL device and / or reduce the reliability of the VCSEL device.

[0025] In accordance with the principles of the present invention, it has been discovered that the possibility of moisture penetration can be eliminated by first coating the Al-containing layer of the VCSEL structure with a protective material before starting the lateral oxidation process. In particular, the dielectric material is preferably used as a protective coating so that the coating does not need to be removed later, but rather remains as part of the final passivation layer of the fabricated device.

[0026] Figure 2-Figure 5 The figures illustrate an exemplary process for including a protective coating in a VCSEL manufacturing sequence associated with creating oxide pores 24 via lateral oxidation. It should be understood that these figures are not drawn to scale and that many details regarding process conditions not described herein are well understood by those skilled in the art.

[0027] Figure 2 The figure shows an exemplary VCSEL device structure shortly after processing the second DBR 14 to produce the mesa structure 30. As mentioned above, the creation of the mesa structure 30 exposes the edges E of the various layers forming the second DBR 14. The top surface layer 12-T of the first DBR 12 and the top surface layer 14-T of the second DBR 14 are also exposed. These two layers also contain AlGaAs, although the aluminum concentration therein is much lower than the aluminum concentration of the pore layer 22. Even so, if the lateral oxidation process is performed immediately after the next step in the manufacturing sequence (as is the case in the prior art), the exposed aluminum in these layers will be slightly oxidized.

[0028] However, in contrast to prior art processes, the sequence of the present invention continues to form a protective coating on these Al-containing surface layers in order to prevent the surface exposed aluminum from oxidizing and forming an AGO layer. Figure 3The figure shows the deposition of a protective coating 40, which in this case produces a conformal coating over the entire exposed surface of the VCSEL structure. The protective coating 40 may include any suitable dielectric material including, for example, SiNx, SiOx, AlOx, or the like.

[0029] Once deposited, the protective coating 40 is patterned (e.g., by Figure 4 The mask layer 50 shown in FIG. 1 is etched to expose the edge of the aperture layer 22, as shown in FIG. Figure 5 , so that a conventional lateral oxidation process will react only with the high aluminum content pore layer 22. The patterning of the protective coating 40 is specifically defined and controlled so that most of the Al-containing layers 12-T and 14-T remain covered by the protective coating 40 and will not be oxidized during the subsequent lateral oxidation process for producing the oxide pores 24.

[0030] Once the protective coating 40 has been patterned and etched, conventional VCSEL manufacturing processes can then be resumed, such as Figure 6 Here, the VCSEL structure is subjected to a mixture of N2 / H2 / H2O (for example) at an elevated temperature sufficient to initiate oxidation of the exposed outer edge 22-E of the aperture layer 22 (i.e., a "lateral oxidation" process). Figure 6 The arrows in show the advancement of the oxidized portion of the void layer 22 from the outer edge of the mesa towards the center of the structure. Figure 7 The figure shows the structure when the lateral oxidation process is completed, wherein the final form of the oxide pores 24 exhibits the desired width W.

[0031] In accordance with the teachings of the present invention, the protective coating 40 remains intact during the lateral oxidation process and thus prevents the formation of an AGO layer across the top layer 12-T of the first DBR 12 and the top layer 14-T of the second DBR 14. After the oxide pores 24 are formed, a standard passivation layer 80 is deposited over the structure, such as Figure 8 , where the remaining portion of the protective coating 40 forms part of the final electrical insulation of the VCSEL device. Although the passivation layer 80 can be formed of the same dielectric material as the protective coating 40, this is not required. The additional steps of creating a first electrical contact 82 for the first DBR 12 and a second electrical contact 84 for the second DBR 14 can be performed before or after the steps for forming the protective coating as outlined above. The first electrical contact 82 can be located on the "top surface" of the VCSEL structure, such as Figure 8 , or alternatively, may be formed as a "backside" contact on the lower surface of substrate 13, as in typical VCSEL manufacturing processes.

[0032] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be envisaged without departing from the basic scope of the invention, which is determined by the appended claims.

[0033] The present invention also relates to the following items:

[0034] 1. A vertical cavity surface emitting laser (VCSEL), comprising:

[0035] a substrate on which a first distributed Bragg reflector and a second distributed Bragg reflector (DBR) are disposed, each DBR comprising a stack of layers having alternating refractive index values, the second DBR being formed as a mesa structure having an outer boundary exposing an end region of the stack of layers;

[0036] an activation layer disposed between the first DBR and the second DBR;

[0037] a void layer disposed within the second DBR and having a higher concentration of an oxidizing element than remaining layers in the stack of layers; and

[0038] A protective coating is deposited to cover exposed horizontal surface portions of the first DBR and the second DBR.

[0039] 2. The VCSEL according to item 1, wherein the void layer and the top surface layer of the first DBR contain aluminum, wherein the void layer is formed to exhibit a higher aluminum concentration than the top surface layer of the first DBR.

[0040] 3. The VCSEL of item 1, wherein the protective coating is a dielectric material.

[0041] 4. The VCSEL of item 3, wherein the protective coating of dielectric material is selected from the group consisting of: SiNx, SiOx, and AlOx.

[0042] 5. The VCSEL of item 1, wherein the VCSEL further comprises an outer passivation layer deposited to cover the protective coating and exposed end regions of the mesa structure of the second DBR.

[0043] 6. A method of manufacturing an oxide confined vertical cavity surface emitting laser (VCSEL), comprising:

[0044] a) growing a stack of layers with alternating refractive indices and controlled thicknesses on a substrate to form a first distributed Bragg reflector (DBR);

[0045] b) forming an active region of a multi-quantum well structure on the first DBR;

[0046] c) growing a stack of layers having alternating refractive indices and controlled thicknesses on the active region to form a second DBR, wherein a layer within the stack of second DBR layers is defined as a void layer and exhibits a higher concentration of oxide forming elements than the remaining layers in the stack of second DBR layers;

[0047] d) etching the stack of layers of the second DBR to form a mesa structure exposing an edge of a layer including the void layer, the etching forming a trench around the mesa structure, the trench having a top surface layer including an oxide material;

[0048] e) covering the groove with a protective coating; and

[0049] f) performing a lateral oxidation process on the pore layer to generate oxide pores in the VCSEL.

[0050] 7. The method according to item 6, wherein step e) comprises the following steps:

[0051] depositing a conformal coating on the etched structure;

[0052] patterning the conformal coating to define edge regions of the mesa structures to be exposed and not covered by the protective coating;

[0053] The patterned conformal coating is etched to remove defined portions along the mesa structures to define the protective coating.

[0054] 8. The method of claim 6, wherein the protective coating comprises a dielectric material.

[0055] 9. The method according to item 8 further comprises the following steps:

[0056] g) depositing a passivation layer on the structure formed in step f) so as to cover the protective coating and the exposed edge regions of the mesa structures.

[0057] 10. The method of item 9, wherein the passivation layer comprises a dielectric material.

[0058] 11. The method of item 9, wherein the protective coating and the passivation layer comprise the same dielectric material.

[0059] 12. The method of item 9, wherein the protective coating and the passivation layer comprise different dielectric materials.

[0060] 13. The method of item 6, wherein the pore layer and the top surface layer of the trench include aluminum, wherein the pore layer has a higher aluminum content than the top surface layer of the trench.

Claims

1. A vertical cavity surface emitting laser (VCSEL), comprising: a substrate on which a first distributed Bragg reflector DBR and a second distributed Bragg reflector DBR are arranged from bottom to top, each distributed Bragg reflector DBR comprising a stack of layers having alternating refractive index values, the second distributed Bragg reflector DBR being formed as a mesa structure extending to the first distributed Bragg reflector DBR so that the first distributed Bragg reflector DBR has an exposed horizontal surface portion; an active layer, wherein the active layer is disposed between the first distributed Bragg reflector DBR and the second distributed Bragg reflector DBR; a void layer disposed within the second distributed Bragg reflector (DBR) and having a higher concentration of an oxidized element than the remaining layers in the stack of layers; and a protective coating deposited to cover only exposed horizontal surface portions of the first distributed Bragg reflector (DBR) and the second distributed Bragg reflector (DBR); The protective coating is formed before the pore layer is subjected to a lateral oxidation process.

2. The vertical cavity surface emitting laser (VCSEL) of claim 1, wherein the aperture layer and the top surface layer of the first distributed Bragg reflector (DBR) contain aluminum, wherein the aperture layer is formed to exhibit a higher aluminum concentration than the top surface layer of the first distributed Bragg reflector (DBR).

3. The vertical cavity surface emitting laser (VCSEL) of claim 1, wherein the protective coating is a dielectric material.

4. The vertical cavity surface emitting laser (VCSEL) of claim 3, wherein the protective coating of dielectric material is selected from the group consisting of: SiNx, SiOx and AlOx.

5. The vertical cavity surface emitting laser (VCSEL) of claim 1, wherein the vertical cavity surface emitting laser (VCSEL) further comprises an outer passivation layer deposited to cover the protective coating and the exposed end regions of the mesa structure of the second distributed Bragg reflector (DBR).

6. A method for manufacturing an oxide-confined vertical cavity surface emitting laser (VCSEL), comprising: a) growing a stack of layers with alternating refractive indices and controlled thicknesses on a substrate to form a first distributed Bragg reflector (DBR); b) forming an active region of a multi-quantum well structure on the first distributed Bragg reflector DBR; c) growing a stack of layers with alternating refractive indices and controlled thicknesses on the active region to form a second distributed Bragg reflector (DBR), wherein a layer within the stack of second distributed Bragg reflector (DBR) layers is defined as a void layer and exhibits a higher concentration of oxide forming elements than the remaining layers in the stack of second distributed Bragg reflector (DBR) layers; d) etching the stack of the layers of the second distributed Bragg reflector DBR to form a mesa structure exposing an edge of a layer including the void layer, the mesa structure extending to the first distributed Bragg reflector DBR, the etching forming a groove around the mesa structure; e) covering the horizontal surface of the groove with a protective coating; as well as f) performing a lateral oxidation process on the aperture layer to generate oxide apertures in the vertical cavity surface emitting laser (VCSEL).

7. The method of claim 6, wherein step e) comprises the following steps: depositing a conformal coating on the etched structure; patterning the conformal coating to define edge regions of the mesa structures to be exposed and not covered by the protective coating; The patterned conformal coating is etched to remove defined portions along the mesa structures to define the protective coating.

8. The method of claim 6, wherein the protective coating comprises a dielectric material.

9. The method of claim 8, further comprising the steps of: g) depositing a passivation layer on the structure formed in step f) so as to cover the protective coating and the exposed edge regions of the mesa structures.

10. The method of claim 9, wherein the passivation layer comprises a dielectric material.

11. The method of claim 9, wherein the protective coating and the passivation layer comprise the same dielectric material.

12. The method of claim 9, wherein the protective coating and the passivation layer comprise different dielectric materials.

13. The method of claim 6, wherein the pore layer and the top surface layer of the trench comprise aluminum, wherein the pore layer has a higher aluminum content than the top surface layer of the trench.

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