Long-wavelength VCSELs and method of manufacturing same

[0002] 1. Field of the Invention

[0003] The present invention relates to long-wavelength VCSELs, methods for manufacturing same, and more particularly to a method for manufacturing such VCSELs with a flip-bonding process.

[0004] 2. Related Art

[0005] Vertical cavity surface emitting lasers (VCSELs) at 1.3 μm-1.55 μm wavelength are ideal single-frequency laser sources for use in high-speed optical communications systems, including wavelength division multiplexed (WDM) applications. The advantages of VCSELs for optical communications are related to their small electrically active volume that allows direct modulation of light output at rates beyond 1 Gbps, and a circular output aperture that enables good coupling to single-mode optical fiber. However, commercially available VCSELs operate in the short wavelength range (<900 nm) making them impractical for fiber lengths over 100 m, or incompatible with a system already operating at the longer wavelengths. The main challenges facing commercialization of VCSELs operating at 1.3 μm or 1.55 μm are the sensitivity of these lasers to thermally induced effects that severely degrade their performance compared to short wavelength GaAs based devices. A particular challenge for VCSELs operating at 1.3 μm is to achieve continuous wave (CW) operation at room temperature. The problem is compounded by the fact that the InGaAsP/InP material system has poor thermal conductivity while the differential gain and quantum efficiency are more sensitive to changes in temperature compared to AlGaAs/GaAs based material.

[0006] However, efforts to achieve room-temperature operation at 1.3 μm wavelength have yielded success using two different approaches. These approaches are: (1) Dielectric multilayer mirror stacks on both sides of the active region, and (2) Single or double wafer fused interfaces to bond AlGaAs/GaAs based mirror stacks onto the InP based active region. Both approaches circumvent the need to grow high reflectivity Bragg mirrors using the InP/InGaAsP layers which have a small index difference and poor thermal properties. The wafer fused approach provides high-reflectivity AlGaAs/GaAs mirrors with low diffraction losses and low thermal resistance. Also, current can be injected uniformly into the active region via the doped AlGaAs/GaAs mirror stacks. The wafer-fusion based VCSELs thus represent a significant breakthrough solving mirror problems plaguing InP-based VCSELs. To date, fabrication has been achieved for 1.3 μm wavelength room temperature operating VCSELs using double-fused AlGaAs/GaAs mirrors, (See Qian, et al., “1.3 μm Vertical-Cavity Surface-Emitting Lasers with Double-Bonded GaAsAlAs Bragg Mirrors” IEEE Phot. Tech. Lett., Vol. 9, 8 (1997)) and for single fused oxygen-implanted confinement region with dielectric top mirror (See Qian, et al., “Long Wavelength (1.3 μm) Vertical-Cavity Surface-Emitting Lasers with a Wafer-Bonded Mirror and an Oxygen-Implanted Confinement Region” Appl. Phys. Lett. Vol. 71, 25 (1997)).

[0007] The other successful approach for room temperature CW operation at 1.3 μm wavelength uses a dielectric multi-layer stack on both sides of an InGaAsP multiple quantum well (MQW) based active region (See Uchiyama, et al., “Continuous-Wave Operation up to 36 ° C. of 1.3 μm GaInAsP-InP Vertical-Cavity Surface-Emitting Lasers” IEEE Phot. Tech. Lett. Vol. 9, 141 (1997)). In this design high reflectivity mirrors are dielectric stacks (See Spaziani, et al., “High-Performance Substrate-Removed InGaAs Schottky Photodetectors” IEEE Phot. Tech. Lett. Vol. 10, 1144 (1998)), typically made of Si/Al₂O₃, Si/SiO₂or ZnSe/MgF₂layers. Current is injected under the dielectric through a ring metal contact to an InGaAs contact layer. The maximum temperature of CW operation was 35° C.

[0008] The traditional double dielectric stack based VCSEL is schematically shown in FIG. 1. The VCSEL 10 is formed on a diaphragm approximately 2 μm thick in the direction shown by arrows A. The device includes an MQW active region 12, in-grown n-DBR 13, a mesa formed on p-DBR 14, a p-type ring contact 16 formed thereon and a top dielectric stack 18 extending therethrough. The diaphragm sits on an InP substrate 20, which includes an n-type contact 22 on the bottom thereof. A bottom dielectric stack 24 extends from the underside of the wafer.

[0009] VCSELs fabricated according to the prior-art model exhibit a very high series resistance as the current has to travel almost 100 μm through a thin diaphragm to reach the active region. This thin diaphragm leads to poor mechanical stability and low thermal conductivity making room temperature CW operation difficult to achieve.

[0010] What is needed, and has not heretofore been developed, is a low cost, high yield method for producing high performance InP based VCSELs which can achieve continuous wave operation at room temperature.

[0011] It is a primary object of the invention to provide a method for producing high performance InP based VCSELs operating at wavelengths used in optical communications systems and networks.

[0012] It is an additional object to provide high performance VCSELs which can achieve continuous wave operation at temperatures higher than room temperature.

[0013] It is a further object of the invention to provide a low cost method of producing high performance VCSELs.

[0014] It is a still further object of the invention to provide high performance VCSELs with reduced series resistance.

[0015] It is yet another object of the invention to provide high performance VCSELs having improved mechanical rigidity.

[0016] It is still another object of the invention to provide a method for making long-wavelength VCSELs using a flip-bonding technique.

[0017] The present invention provides a method of manufacturing VCSELs that involves a flip-bonding process wherein the top surface of the VCSEL wafer is bonded face down onto a surrogate substrate. The process begins in a manner similar to traditional double dielectric stack based VCSEL, but then involves flip-bonding the wafer onto a metal (for example, indium or silver), or a metal-loaded epoxy coated surrogate substrate. The InP substrate is then removed by selective etching. After flip-bonding, the wafer fabrication proceeds on the freshly etched surface which now forms the top surface. Next, standard mesa-isolation and contact formation techniques are performed on this newly etched surface.

[0018] Other important objects and features of the invention will be apparent from the following Detailed Description of the Invention taken in connection with the accompanying drawings in which:

[0019] FIG. 1 is a schematic depiction of a conventional double dielectric stack based VCSEL.

[0020] FIGS. 2a-2fprovide schematic illustration of the processing steps for fabricating a VCSEL according to the present invention.

[0021] FIGS. 3a-3bprovide infrared micrographs of a VCSEL structure fabricated according to the method of the invention when it is turned off (3a) and on (3b). FIG. 3cshows the current-voltage characteristic of devices fabricated using processing steps set forth in present invention.

[0022] FIGS. 4a-4cpresent top and side views of a system for registering the InP VCSEL wafer.

[0023] FIG. 5 shows an optical micrograph of registration marks (vias) after InP substrate removal.

[0024] The present invention provides a new approach to VCSEL processing that is simple and can lead to high performance InP based VCSELs. The method of the present invention is based on the strengths of dielectric and wafer-fused techniques for VCSELs. It combines highly reliable dielectric stacks as mirrors with the low thermal resistance of a metal bonded surface adjacent to the active region. Moreover, by using a novel alignment scheme, the top and bottom apertures on the two mirror stacks are exactly aligned eliminating the need for current to flow along a thin diaphragm to reach the active region. This significantly reduces the high series resistance plaguing the conventional double dielectric mirror based VCSEL. Also bonding the thin diaphragm via a metal such as indium will drastically improve the mechanical rigidity when compared to double dielectric VCSEL structure.

[0025] The processing steps for fabricating a flip-bonded VCSEL are schematically shown in FIGS. 2a-f. The process begins in a manner similar to that of traditional double dielectric stack based VCSEL (such as shown in FIG. 1), but then involves flip-bonding the wafer onto metal (e.g. indium, silver) or metal-loaded epoxy coated surrogate substrate. The InP substrate is then removed by selective etching. After flip-bonding, the wafer fabrication proceeds on the freshly etched surface which now forms the top surface. For an exemplary embodiment, and standard mesa-isolation and contact formation techniques are performed on this etched surface.

[0026] The particular processing steps are described below in conjunction with FIGS. 2a-f:

[0027] a. With reference to FIG. 2a, the wafer 30 is grown on an InP substrate (32). It typically comprises a standard 1.3 μm or 1.55 μm InGaAsP MQW gain region (36) with a 4.5 pair InGaAsP/InP distributed Bragg reflector (DBR) stack on each side (38 and 34) of the MQW (36), and separated from substrate 32 by etch-stop layer 33. The cut-off wavelength of the InGaAsP in both top (38) and bottom (34) DBR should be shorter than the emission wavelength of the MQW (36). The top DBR stack (38) is of one conductivity type (e.g. p-type) whereas the bottom DBR (34) is of the opposite conductivity type (e.g. n-type).

[0028] b. Referring to FIG. 2b, the process goes on to form via holes/registration marks (40) by wet or dry etching through the whole structure and stop before the substrate. Then, the VCSEL aperture is formed using Ti—Pt—Au or Ti—Ni—Au contact metallization 42 (p-type) on top of the p-type grown DBR (38). Next, mesa (44) is formed by plasma etching, followed by depositing 6˜8 pairs of dielectric stack (e.g. Si/SiO₂, ZnSe/MgF₂, SiC/MgO) by evaporation or sputtering to form top dielectric mirror (46), and lifting-off everywhere except at each VCSEL opening using standard photolithographic patterning technology. In-situ optical monitoring techniques, such as ellipsometry or reflectometry, can be used to ensure accurate control of the thickness and optical constants of the deposited dielectric materials.

[0029] c. FIG. 2cshows flip-bonding the processed wafer (30) onto a surrogate substrate (48), which is pre-coated with metal pads and interconnects (50). Indium bumps (54) are deposited to both the wafer (30) and the substrate (48). The wafer (30) is pressed against the surrogate substrate (48) at elevated temperature with the indium bumps aligned, so that the metal top contact (42) is bonded to the metal pad (50). The indium bumps (54) and the bonded surface are protected against acid by thermally conductive, low viscosity under-filling epoxy (52). The InP substrate (32) is then removed (by etching) using HCl:H₃PO₄mixture which selectively etches InP while stopping (i.e. not etching) at the InGaAs etch-stop layer (33) at the bottom of the DBR stack (34).

[0030] d. A standard photolithographic patterning and lift-off process is used to deposit Ti—Au non-annealed contact (56) on top of the DBR stack (34), leaving the VCSEL window (51) open, as shown in FIG. 2d. The contact (56) is aligned to the features on the bonded surface by aligning to the via holes/registration marks, which is exposed after InP substrate removal. The etch-stop InGaAs layer (33) is etched by mixture of citric acid and hydrogen peroxide, using the electrode (56) as mask.

[0031] e. FIG. 2eshows the formation of mesa to isolate each VCSEL by plasma etching through the DBR stack (34) down to near the active region, the mesa being generally indicated at (58). E-beam evaporated or sputtered SiO₂or SiN_xcan be used as etch mask material.

[0032] f. As shown in FIG. 2f, 4˜6 pairs of dielectric stack (e.g. Si/SiO₂, ZnSe/MgF₂, SiC/MgO) are deposited by evaporation or sputtering to form dielectric mirror (60) and lifting-off everywhere except at each VCSEL opening using standard photolithographic patterning technology.

[0033] The resulting VCSEL has at least two very significant advantages:

[0034] 1. The thermally conducting under-filling epoxy used in the flip-bonding process is in close contact with most of the semiconductor surface, resulting in an active region bound on both sides by dielectric mirror stacks and mounted on an excellent thermal heat sink.

[0035] 2. The high series resistance in conventional dielectric VCSELs due to the current flow along a thin diaphragm is completely eliminated. The current flows vertically between the top and bottom contacts aligned within ±2 μm in the lateral direction.

[0036] The use of flip-bonded epitaxial layers provides an inexpensive and reliable VCSEL processing technique. The layers can be bonded using electrically and/or thermally conducting epoxies. The highest process temperature after flip-bonding (similar to FIG. 2) is only about 110° C., which is the temperature needed for baking photoresist in the photolithography process. The higher the maximum temperature needed for processing the thin epitaxial layers, the higher the mismatch in thermal expansion between the bonding material and the semiconductor layers, which might result in cracking of the delicate semiconductor film on the thin wafer itself. By limiting the process temperature, generation of thermal stress induced defects can be minimized. In fact, the inventors have successfully fabricated VCSEL structures using processes similar to the aforementioned approach. FIGS. 3a-3bare photographs that are taken by an infrared camera through a microscope, showing a VCSEL structure (without dielectric DBR 60) with (FIG. 3b) and without (FIG. 3a) driving current. The bright region in FIG. 3bis the spontaneous emission from one pixel. FIG. 3cshows the current-voltage relations of devices with various diameters. The series resistance (˜120 Ω for a 32 μm diameter device) is very low compared to conventional VCSELs with double dielectric mirrors. The reverse leakage current is also low (˜10 μA at −2V), suggesting that the process does not introduce excessive damage to the epitaxial material.

[0037] In conventional flip-chip (or flip-bonded) type alignment, the semiconductor wafer is back-illuminated by a 1.6 μm IR-source and then the existing patterns are imaged with an infra-red camera. However, for the VCSEL process of the present invention, back-illumination to align the top mesa with the bottom mirror will not work, due to the non-transparent nature of indium bumps and metal pads on the surrogate substrate. For this purpose, a novel alignment scheme has been designed that will allow precise alignment (±1 μm) with pre-fabricated features on the other side of the device. The starting top surface is first marked with alignment marks (e.g., via holes/registration marks (40) of FIG. 2b) that are etched into the top epitaxial layers of the wafer using standard dry or wet etching. The VCSEL processing on the top wafer surface proceeds with the formation of VCSEL emission apertures, dielectric mirrors, etc. The array pattern is then registered with respect to the top alignment marks. Then the wafer is flip-bonded onto a surrogate substrate. Since the alignment marks are etched into the epitaxial layers, they become readily visible once the InP substrate is removed. The mesas on the new top surface can then be registered to these alignment marks, thereby ensuring precise alignment with features on the bonded side of a flip-bonded VCSEL wafer.

[0038] FIGS. 4a-4care top and side views of a system for registering the VCSEL wafer. As can be seen in the figure, registration marks, or via holes (64), are formed in the top 2-3 μm of the VCSEL layers (62) using wet or dry etching. The registration marks, can be made on the top surface before mounting. The actual VCSEL aperture mask on the top surface can be aligned to those holes or marks (64). After processing as shown in FIG. 2, the wafer can be mounted face down on the pre-coated surrogate substrate (48) as shown in FIG. 4c. After removing (etching) the InP substrate, these holes become visible. Thereafter, features on the top surface can be aligned. FIG. 5 shows an optical micrograph of a flip-bonded VCSEL structure, with via holes (64 in FIG. 4) indicated by arrows.

[0039] Having thus described the invention in detail, it is to be understood that the foregoing description is not intended to limit the spirit and scope thereof. What is desired to be protected by Letters Patent is set forth in the appended claims.