Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2001-08-09
2004-05-11
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S031000, C438S039000, C438S714000
Reexamination Certificate
active
06734111
ABSTRACT:
The present invention relates to a method to obtain nitride layers on arbitrary structures on GaAs based lasers, and a GaAs based laser provided by the method.
The present invention is mainly directed to a method to take care of losses in the wave-guide of GaAs based lasers.
A method directed to solve the problem with degradation of laser facets is disclosed in our co-pending U.S. application Ser. No. 09/924,605.
BACKGROUND
High power 980 nm laser diodes are predominantly used to pump erbium-doped fiber amplifiers (EDFAs). Other applications can be thulium doped fiber amplifiers and Er/Yb doped fibers and wave-guides that use rare earth metal transitions in the 900 . . . 1100 nm band. There are two dominating failure mechanisms of GaAs based pump lasers, namely degradation of laser facets and defects in the wave-guide. The degradation of laser facets by light absorption is known to lead to sudden failures by catastrophic optical damage (COD) and has been one of the major causes for device failure. If COD is taken care of by appropriate laser facet passivation techniques, the wave-guide defects will be dominating.
The losses in the wave-guide originates from:
Light scattering due to roughness in the wave-guide;
Non-radiative recombination via impurities at the surface or recombination via surface states.
A wet chemical etching procedure usually provides excellent smoothness of the wave-guide. Prior dry etching methods, like Reactive Ion Etching (RIE) or Chemically Assisted Ion Beam Etching (CAIBE), yield very high process control, both regarding etching depth and the wall topology. However, these dry etching methods give rougher surfaces than the wet etching methods. The rougher surfaces increase the light scattering as well as surface recombination rate. Both effects are detrimental for modern pump lasers. The scattering will reduce the efficiency. The impurities entering though rough and non-blocking surfaces are even more detrimental to the pump laser. The impurities and the surface states will promote non-radiative recombination, which generates heat. The heat can degrade the material and the surfaces further and more heat will be generated. This process will accelerate and finally the device will fail.
RELATED ART
U.S. Pat. No. 4,448,633 discloses a method to passivate type III-V compound semiconductor surfaces by exposure to a low-pressure nitrogen plasma. The III element forms III element-nitride. This process is referred to as nitridation. The resultant articles have an III element-nitride surface layer, which protects the articles from environmental degradation while reducing the surface state density and permitting inversion of the surface layer. The nitridation is performed in two steps. The first occurs at low temperatures (400-500° C.) to prevent decomposition of the surface by loss of V element. Exposure to nitrogen plasma with a pressure of 0.01-10 Torr results in an initial III-nitride layer having a thickness of about 20-100 Å. The second step is performed at an elevated temperature (500-700° C.) under the same plasma conditions. Here, the nitridation proceeds at a faster rate resulting in a thicker nitrided layer (200-1000 Å). Under the present conditions, if the plasma pressure is in the range 0.01 to about 0.5 Torr the resulting III-coating is polycrystalline, and is single-crystalline when the pressure is in the range 1 to 10 Torr.
U.S. Pat. No. 5,780,120 describes a method of preparing facets of lasers based on III-V compounds. The method comprises of the following operations:
1) The facets of the laser are cut.
2) The facets of the laser are placed in an enclosure in which there obtains a pressure of about 10-7 mbar to about 10-8 mbar, and they are subjected to a step of cleaning by irradiation with a pulsed laser.
3) The same pulsed laser is used to ablate a target so as to subject the exposed facets to a passivation operation, that is 2-20 Å of Si or GaN is deposited.
The deposition can be performed by pulsed laser ablation of a liquid gallium target in a nitrogen atmosphere with Electron Cyclotron resonance (ECR) plasma. Deposition of an additional film such as Diamond Like Carbon (DLC), silicon carbide SiC, or silicon nitride Si
3
N
4
, may be deposited using the same pulsed laser. These coatings are transparent at the wavelength of the laser and are resistant to oxidation. A cleaning step prior to the passivation stage may be performed in an atmosphere of chlorine or bromine, using a pulsed excimer laser. This document suggests that an additional coating is not necessary if GaN is deposited instead of Si. This also suggests that III-N layers are oxygen-proof.
U.S. Pat. No. 5,834,379 describes a process for synthesizing wide band gap materials, specifically GaN, employs plasma-assisted thermal nitridation with NH
3
to convert GaAs to GaN. This method can be employed for forming layers of substantial thickness (on the order of 1 micron) of GaN on a GaAs substrate.
Plasma-assisted nitridation using NH3 results in formation of predominantly cubic GaN. The objective of this document is to make sufficiently thick GaN layers and is not directly concerned with laser facet passivation. However, the basic principle relies on nitridation using a plasma source. Such approaches are being used in growth of GaN films.
The above patents address the concept of nitridation of III-V semiconductors using nitrogen plasma.
U.S. Pat. No. 4,331,737 describes an oxynitride film, which contains Ga and/or Al and has O/N ratio of at least 0.15. This film is obtained by relying on, for example, chemical vapour deposition (CVD) technique. The O/N ratio in the film may be varied by, for example, by varying the distance between the substrate and the substance-supply source, or by varying the proportion of an oxidising gas-contained in a carrier gas. This film is used either as a surface passivation film of III-V compound semiconductors such as GaAs, or as an insulating film for active surface portions of IG-FET, or as an optical anti-reflective film.
EP0684671 describes a method, which comprises oxide reduction, hydrogen passivation and deposition of a protective coating layer. The method involves the same PECVD reactor for all steps to avoid oxygen exposure. The cleaved facets (being exposed to air and thus oxidised) are loaded into the reactor. The first step uses hydrogen plasma, which both reduces the group V oxide content and passivates non-radiative recombination centres. The group III oxides are removed by ammonia plasma and the laser facets have their compositional stoichiometry condition restored and are free from contaminants. Coating is then done either by depositing SiN(x) or AlN(x). Minimum stress can also be obtained through creation of a compositional nitrogen gradient.
U.S. Pat. No. 5,668,049 discloses a method of making a GaAs-based semiconductor laser. A fully processed wafer is cleaved, typically in ambient atmosphere into laser bars. The laser bars are loaded into an evacuable deposition chamber (preferably an ECR CVD chamber) and exposed to H2S plasma. The hydrogen is believed to remove native oxides, while the sulfur bonds with Ga and As, thereby lowering the surface state density. Following the exposure, the cleavage facets are coated in the chamber with a protective dielectric (for example, silicon nitride) layer. The patent claims that this method can be practiced with high through-put, and can yield lasers capable of operation at high power.
U.S. Pat. No. 5,144,634 discloses a method for passivating mirrors in the process of fabricating semiconductor laser diodes. Key steps of the method are:
(1) providing a contamination-free mirror facet, followed by
(2) an in-situ application of a continuous, insulating (or low conductive) passivation layer.
This layer is formed with a material that acts as a diffusion barrier for impurities capable of reacting with the semiconductor but which does not itself react with the mirror surface. The contamination-free mirror surface is obtained by cleaving in a contamination-free environment, or by cleavin
Blixt N. Peter
Carlström Carl-Fredrik
Krummenacher Lauerent
Lindström L. Karsten V.
Silvenius Christofer
Comlase AB
Jr. Carl Whitehead
Pham Thanhhas
Young & Thompson
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