Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
1999-03-08
2001-10-02
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
With measuring or testing
C438S038000
Reexamination Certificate
active
06297066
ABSTRACT:
The present invention resides in a process of coating semiconductor lasers with antireflection layers (called below AR-layers) with in-situ monitoring of the laser light emitted from the front facet and/or the rear facet of a laser, the electric voltage at the p-n junction of the laser or the quantum efficiency of the laser light emitted from the front and/or rear facet of the laser.
Mirror-less semiconductor laser diodes are utilized in modern optical measuring instruments for the gas analysis and interferometry or as optical radar. They are also used in the field of active light amplification
Because of the refraction index change between laser material and the ambient, the mirror facets of semiconductor lasers have a reflectivity of typically 32%. A common procedure for reducing the reflectivity of one or both of the laser facets resides in coating the facets with a dielectric layer having a thickness corresponding to one fourth of the center wave length of the laser. The value of the refraction index is selected according to the harmonic medium of the refraction index of the laser material and of the surroundings. With a suitable control of the thickness and the value index of the antireflection layer, values for a minimal rest reflectivity smaller than 10
−5
can be achieved. Several methods have been described in the literature for achieving this result.
M. Serenyi and H. J. Habermeier describe in “DIRECTLY CONTROLLED DEPOSITION OF ANTIREFLECTION COATINGS FOR SEMICONDUCTOR LASERS”, Applied Physics, Vol. 26, (1997), pages 845 ff, a process, wherein a laser is electrically operated and the emitted light energy is observed. The coating procedure is ended when the light output reaches the minimum.
In a process as described by J. Landreau and H. Nakajima in “IN-SITU REFLECTIVITY MONITORING OF ANTIREFLECTION COATINGS ON SEMI-CONDUCTOR LASER FACETS THROUGH FACET LOSS INDUCED FORWARD VOLTAGE CHANGES”, Applied Physics letters, Vol. 56 (1990), pages 2376 ff, the laser is electrically operated and the voltage at its p-n-junction is observed. The coating procedure is terminated when the voltage at the p-n-junction reaches a maximum.
In the article, “REAL-TIME IN-SITU MONITORING OF ANTI-REFLECTION COATINGS OF SEMICONDUCTOR LASER AMPLIFIERS BY ELLIPSOMETRY”, Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS), November 1991, M. Dagenais and I-Fan Wu present a process wherein the laser is electrically operated and the current threshold of the laser during coating is observed. The coating procedure is terminated when the current reaches its maximum threshold value.
The theoretical curve for the laser characteristic values is obtained from the stationary solution of the laser equations. To determine the optical laser characteristic values also the outcoupling rate which is changed by the antireflection coating needs to be taken into consideration. Altogether, the following relationships are obtained:
P
=
J
-
γ
·
N
+
γ
·
K
G
N
Γ
+
K
K
=
-
ϑ
g
ln
(
1
-
r
2
)
2
L
I
th
=
γ
·
N
+
γ
·
K
G
N
L
-
r
2
=
(
1
-
r
12
2
)
(
1
-
r
23
2
)
1
+
r
12
2
·
r
23
2
+
2
r
12
r
23
·
cos
2
β
β
=
2
π
·
n
2
·
h
(
1
)
λ
Herein are:
P is the light energy emitted from the rear side laser facet, J is the normalized threshold current density, &ggr; is the electron disintegration rate, N is the transparency charge carrier density, G
N
is the differential gain, I
th
is the normalized threshold current density of the laser, V
g
is the group speed of the light, and L is the length of the laser. r
12
is the reflectivity of the interface between the AR layer and the laser, r
23
is the reflectivity between the AR layer and the surroundings; n
2
h is the optical layer thickness of the antireflection layer with the refraction index n
2
and the geometric thickness h, and &lgr; is the center wave length of the laser. The other laser parameters can be determined by way of the same formalism.
The method of monitoring in-situ the laser characteristic values during the application of antireflection coatings to semiconductor diodes is limited generally to the coating of small numbers of semiconductor diodes. One reason herefor resides in the fact that the time period required for generating a suitable vacuum is much greater than the time required for the coating of the individual laser diodes. To satisfy the demand for large numbers, however, rapid coating processes are needed which provide for highest quality and the highest possible efficiency.
The process proposed in U.S. Pat. No. 3 846 165 by M. Ettenberg et al., to use a single laser as a monitor laser and to coat simultaneously several lasers or laser bars, facilitates the manufacture of large numbers of antireflection structures. But the resulting lasers are not suitable for many applications. For highest quality requirements, this method is therefor not suitable. The reasons are different thicknesses in the facet passivation (initial coating) of commercially available semiconductor lasers, spatial variations of the coating grown with regard to the thickness and values of the refraction index.
Modern high power lasers include generally an anti-reflective layer on the laser facet with a reflectivity of typically 5% to 10%. The best antireflection properties can therefore not be obtained by the deposition of only a single antireflection layer. To obtain highly effective antireflection layers, it is therefore necessary to either remove the initial coating or to deposit at least two layers having a different thickness and a different refraction index. In the publication of M. Dagenais and I-Fan Wu mentioned earlier, M. Dagenais and I-Fan Wu have proposed to use an in-situ ellipsometer for controlling the process of removing the initial coating or, respectively, to control the grown layer thicknesses. However, besides the fact that this process requires the use of a very expensive measuring apparatus, the apparatus must be adjusted to a measuring area of typically 10 &mgr;m×10 &mgr;m, which is not economical.
It is an important object of the present invention to provide a method and apparatus for the manufacture of antireflection layers on semiconductor diodes, which are manufactured in high numbers and by efficient means. Particularly, the rest reflectivity is to be greater that 10
−5
.
SUMMARY OF THE INVENTION
In a process and apparatus for coating the front and/or rear facets of semiconductor laser diodes with antireflection layers of minimal reflectivity, the coating material is deposited on the facets while at least one laser parameters is monitored, in-situ, for determining the coating thickness of the individual antireflection layers resulting in the minimum reflectivity of the coating and the respective coating procedure is terminated when the laser parameters indicate that such coating thickness has been reached.
In a particular embodiment, the method for producing AR layers is based on in-situ observation of the light emitted from the front facet and/or the rear facet of the laser diode, the electric voltage at the p-n junction of the laser, or the quantum efficiency of the laser light emitted from the front or rear facet of the laser, or of the threshold current of the laser. The method requires that the thickness of the AR layer is determined by the value changes of a laser parameter as observed in situ. The measurement values determined in-situ, are compared, for example, with the theoretical values according to equation (1) or equation (2) and, herefrom, the thickness of the respective partial layer is directly determined. This highly efficient procedure does not only omit the need for expensive and complicated measuring apparatus such as in-situ ellipsometer, but improved results are obtained since the laser to be coated is itself used as measuring instrument. The present invention permits the
Bach Klaus J.
Mulpuri Savitri
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