Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2003-11-07
2004-09-07
Everhart, C. (Department: 2825)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S031000, C438S047000, C438S071000, C438S039000, C438S069000, C438S070000
Reexamination Certificate
active
06787381
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser comprised of a plurality of laser diodes having different oscillation wavelengths formed on an identical substrate and a method of producing the same, more particularly relates to a semiconductor laser having a reflection film for controlling a laser output formed on an end of each laser diode and a method of producing the same.
2. Description of the Related Art
As optical disc-shaped recording media for recording and/or reproducing information by emitting light (hereinafter referred to as optical discs), for example, compact discs (CD), Mini Discs (MD), digital versatile discs (DVD), etc. are irradiated with lights of different wavelengths in accordance with the type of the optical discs. For example, light of a wavelength of the 780 nm band is used for reproduction of data from a CD, while light of a wavelength of the 650 nm band is used for reproduction of data from a DVD.
An optical recording and/or reproducing apparatus able to handle different kinds of optical discs requires a plurality of light sources having different oscillation wavelengths. An optical recording and/or reproducing apparatus normally uses a laser diode as a light source, however, when forming a plurality of laser diodes, it becomes difficult to make the apparatus compact and the process of production becomes complex as well.
To overcome the above disadvantages, a multiple wavelength monolithic semiconductor laser formed with a plurality of laser diodes having different oscillation wavelengths on a single substrate has been developed.
Generally, semiconductor lasers are roughly divided into an end emission type laser for emitting laser light in parallel to an active layer, and a surface radiating type (surface emission type) laser.
A surface emission type laser Is capable of performing single mode oscillation and able to be used for long distance transmission, high speed transmission optical fiber communication etc., so the surface emission type multiple wavelength laser has drawn attention as a light source for parallel optical communication.
On the other hand, a laser used for data-recording on and data-reproducing from an optical disc preferably has a plurality of longitudinal modes in the gain spectrum because even if there are a plurality of longitudinal modes, the spatial coherence is not particularly deteriorated and because of the noise problem occurred when light is reflected from the disc and returns to the laser. An end emission type laser has a resonator overwhelmingly longer than the wavelength in a crystal and a large number of resonance modes in the resonator. Therefore, the end emission type laser is suitable as an optical pickup for CDs, MDs, DVDs, and other optical discs.
The configuration of a semiconductor laser of an end emission type will be explained with reference to FIG.
5
.
As shown in the perspective view of
FIG. 5A
, an n-cladding layer
102
comprised for example of n-AlGaAs, a pn junction (active layer)
103
comprised for example of GaAs, and a p-cladding layer
104
comprised for example of p-AlGaAs are successively stacked on a substrate
101
comprised for example of n-GaAs. On the surface of the p-cladding layer
104
except for a striped area at the center is formed a high resistance layer
105
. At an upper layer of the p-cladding layer
104
or the high resistance layer
105
is formed a p-electrode
106
.
The high resistance layer
105
is formed by ion implantation of n-type impurities in the surface of the p-cladding layer
104
. The striped area sandwiched between the parts of the high resistance layer
105
is left as a low resistance layer. By selectively forming the high resistance layer
105
, the result is a gain waveguide structure (current constricting structure) as shown in the top view of FIG.
5
B. It becomes possible to control the area in which the current flows, that is, the area where an optical gain is generated.
According to the laser of the above configuration, a resonator is formed in the active layer
103
. As shown in
FIG. 5B
, although laser light
1
is emitted from a front end F, it is partially lost from the rear end R. The two ends of the emission area (optical waveguide path)
107
, that is, the front end F and the rear end R, are mirror surfaces.
In order to make the ends mirror surfaces, a semiconductor wafer is normally cleaved. Alternatively, the mirror surfaces are sometimes formed by etching instead of cleaving. Also, dielectric films are sometimes formed on the cleaved facets in order to control the reflectance of the ends and to prevent the deterioration of the cleaved facts.
As the dielectric films formed on th ends, single-layer films of for example Al
3
O
3
, amorphous silicon, SiO
2
, or Si
3
N
4
or multi-layer films comprised of a stack of these films may b used. By changing the thicknesses of the dielectric films, the reflectances of the ends can be adjusted. By making the front end F a low reflectance for example less than 30% and the rear end R a high reflectance for example more than 50%, preferably more than 70%, high output laser light can be obtained. The energy conversion efficiency, the front/rear output ratio, etc. depends on the reflectance. Accordingly, the dielectric film controlling the reflectance of the end is one of the important design parameters of a semiconductor laser.
The thickness of the dielectric film formed on an end is, when the oscillation wavelength is &lgr;, normally designed based on &lgr;/2 or its odd multiple or &lgr;/4 or its odd multiple. For example, in
FIG. 5B
, when forming a dielectric film
108
on the front end F by using Al
2
O
3
having an oscillation wavelength &lgr; of 785 nm and a reflectance n
1
of 1.62, the thickness d
108
of the dielectric film
108
is determined as follows:
d
108
=(&lgr;/2)/
n
1
≠242.3 (
nm
) (1)
Also, the rear end R has to have a high reflectance, however, when using the above Al
2
O
3
etc. as a single layer, since the reflectance becomes less than 50% in any case, a plurality of dielectric films are formed. As shown in
FIG. 5B
, in the case of the oscillation wavelength &lgr; of 785 nm, when forming for example an Al
2
O
3
film having as a first dielectric film
109
a
and an amorphous silicon film as a second dielectric film
109
b
, the thicknesses of the layers are determined for example as follows. The thickness d
109a
of an Al
2
O
3
film having a reflectance n
1
of 1.62 becomes
d
109a
=(&lgr;/4)/
n
1
≠121.1 (
nm
) (2)
and the thickness d
109b
of an amorphous silicon film having a reflectance n
2
of 3.25 becomes
d
109b
=(&lgr;/4)/
n
2
≠60.4 (
nm
) (3)
FIG. 6
is a graph showing the relationship of the thickness of the Al
2
O
3
formed on the front end F and the reflectance of the front end F.
FIG. 7
is a graph showing the relationship of the thickness of the Al
2
O
3
film and amorphous silicon film formed on the rear end R with the reflectance of the rear end R. The oscillation wavelength &lgr; is assumed to be 785 nm in both FIG.
6
and FIG.
7
.
As shown in
FIG. 6
, by making the thickness of the dielectric film of the front end F the above d
108
and making the thickness of the dielectric film of the rear end R a combination of the above d
109a
, and d
109b
, the reflectances becomes the extremal values. Accordingly, it is possible to reduce the fluctuations in the reflectances accompanying variations in film formation.
By making th thicknesses of the dielectric films formed on the ends &lgr;/2 or its odd multiple or &lgr;/4 or its odd multiple or a combination of these, it becomes easy to obtain stable reflectances even when there is variation in the thicknesses or refractive indexes due to variation in the formation of the dielectric films.
In the case of a multiple wavelength monolithic semiconductor laser, ideally dielectric films are formed on the laser diodes having different oscillation wavelengths by the above design of the related art.
In this case,
Anya Igwe U.
Everhart C.
Sonnenschein Nath & Rosenthal LLP
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