Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Having substrate registration feature
Reexamination Certificate
2000-12-28
2002-03-26
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Having substrate registration feature
C438S462000, C438S029000, C438S045000
Reexamination Certificate
active
06362069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Related Art
Vertical cavity surface emitting lasers (VCSELs) at 1.3 &mgr;m-1.55 &mgr;m 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 &mgr;m or 1.55 &mgr;m 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 &mgr;m 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.
However, efforts to achieve room-temperature operation at 1.3 &mgr;m 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 &mgr;m wavelength room temperature operating VCSELs using double-fused AlGaAs/GaAs mirrors, (See Qian, et al., “1.3 &mgr;m 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 &mgr;m) Vertical-Cavity Surface-Emitting Lasers with a Wafer-Bonded Mirror and an Oxygen-Implanted Confinement Region” Appl. Phys. Lett. Vol. 71, 25 (1997)).
The other successful approach for room temperature CW operation at 1.3 &mgr;m 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 &mgr;m 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/Al
2
O
3
, Si/SiO
2
or ZnSe/MgF
2
layers. 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.
The traditional double dielectric stack based VCSEL is schematically shown in FIG.
1
. The VCSEL
10
is formed on a diaphragm approximately 2 &mgr;m 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.
VCSELs fabricated according to the prior-art model exhibit a very high series resistance as the current has to travel almost 100 &mgr;m 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.
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.
OBJECTS AND SUMMARY OF THE INVENTION
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.
It is an additional object to provide high performance VCSELs which can achieve continuous wave operation at temperatures higher than room temperature.
It is a further object of the invention to provide a low cost method of producing high performance VCSELs.
It is a still further object of the invention to provide high performance VCSELs with reduced series resistance.
It is yet another object of the invention to provide high performance VCSELs having improved mechanical rigidity.
It is still another object of the invention to provide a method for making long-wavelength VCSELs using a flip-bonding technique.
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.
REFERENCES:
patent: 5897329 (1999-04-01), Jewell
Qian, et al., “1.3 &mgr;m Vertical-Cavity Surface-Emitting Lasers with Double-Bonded GaAs-AIAs Bragg Mirrors” IEEE Phot. Tech. Lett., vol. 9, No. 1, pp. 8-10 (1997).
Qian, et al., “Long Wavelength (1.3 &mgr;m) Vertical-Cavity Surface-Emitting Lasers with a Wafer-Bonded Mirror and an Oxygen-Implanted Confinement Region” Appl. Phys. Lett. vol. 71, pp. 25-27 (1997).
Spaziani, et al., “High-Performance Substrate-Removed InGaAs Schottky Photodetectors” IEEE Phot. Tech. Lett. vol. 10, No. 8, pp. 1144-1146 (1998).
Uchiyama, et al., “Continuous-Wave Operation up to 36 °C of 1.3 &mgr;m GaInAsP-InP Vertical-Cavity Surface-Emitting Lasers”IEEE Phot. Tech. Lett. vol. 9, No. 2, pp. 141-142 (1997).
Forrest Stephen
Gokhale Milind R.
Wang Hongsheng
Bowers Charles
The Trustees of Princeton University
Thompson Craig
Wolff & Samson
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