Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2001-08-09
2004-11-02
Vinh, Lan (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S712000, C438S720000, C216S065000, C216S067000
Reexamination Certificate
active
06812152
ABSTRACT:
The present invention relates to a method to obtain contamination free laser mirrors and passivation using dry etching and deposition.
BACKGROUND
One of the key factors that dictates manufacturing of reliable 980 nm pump lasers is the quality of the laser facet. Passivation is a common technique in the semiconductor business. All semiconductors need a thin film as a barrier against impurities. Impurities will act as defects and change the electrical and optical behavior or impair the crystalline structure in general, by oxidization for example. For silicon chips the passivation is performed automatically by exposing the chip to oxygen in the atmosphere. The oxygen will form a protective SiO
2
-layer. The oxidization of GaAs based lasers is highly detrimental for the optical performance, therefore another materials to be applied on the laser facet.
Degradation of laser facets by light absorption is known to lead to sudden failure by catastrophic optical damage (COD) and has been one of the major causes for device failure. This is a serious concern especially for high power operation (usually beyond 150 mW). The onset of COD is attributed to light absorption at the output facet and subsequent non-radiative recombination via surface states. The light absorption and non-radiative recombination increases the temperature and that results in band-gap reduction. This process acts as a positive feedback until the facet temperatures become very high and COD occurs.
Therefore, to suppress this undesirable effect, at least one of the two main factors, light absorption and surface recombination, has to be minimized. The surface recombination is promoted by an increase in either surface-state density and/or number of impurities (traps) at the surface. The light absorption can also be minimized by a so-called window comprising a thin layer of an inactive material between the facet and the active layer lying behind the facet. In this case the bandgap of the window structure should be higher than the bandgap of the active layer. The minimization of these can be accomplished by suitable surface passivation coatings or treatments.
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 tie 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
3
, 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 NH
3
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 H
2
S 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 cleaving in air, followed by mirror etching, and subsequent mirror surface cleaning. The passivation
Blixt N. Peter
Carlström Carl-Fredrik
Lindström L. Karsten V.
Söderholm Svante H.
Srinivasan Anand
Comlase AB
Davis Paul
Heller Ehrman White & McAuliffe
Vinh Lan
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