Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
2000-09-22
2002-05-07
Fahmy, Wael (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S065000, C257S009000, C257S014000
Reexamination Certificate
active
06383829
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical semiconductor device and a method of fabricating the same, and more particularly to a waveguide type optical semiconductor device having a function of spot-size conversion and a method of fabricating the same.
2. Description of the Related Art
With the recent development of an optical access system typical of “fiber to the home (FTTH)”, a semiconductor laser module used for optical communication is desirable fabricated at a lower cost.
One of major factors for keeping a fabrication cost of a semiconductor laser module high is a packaging cost necessary for optically coupling a laser diode to an optical fiber. Hence, an attention has been recently paid to a spot-size converted semiconductor laser diode which readily accomplishes higher optical coupling between a laser diode and an optical fiber. Herein, a spot-size converted semiconductor laser diode (SSC-LD) is a laser diode which enlarges a spot-size at a plane through which a laser beam leaves, to thereby keep a beam divergence angle small. A smaller beam divergence angle would reduce lights radiated into a free space to thereby ensure a higher optical coupling efficiency for optically coupling a laser diode to an optical fiber. In other words, provision of a semiconductor laser diode with a function of a lens would make it no longer necessary for a semiconductor laser diode to have an optical lens system which was absolutely necessary for a conventional semiconductor laser module. Thus, A semiconductor laser module could be fabricated at lower costs.
In order to enlarge a spot-size at a plane at which laser beams leave a laser diode, it would be necessary to make an optical confinement factor small at the above-mentioned plane in an optical waveguide to thereby enlarge an optical field. Specifically, an optical waveguide is designed to include a spot-size converting portion having a thickness smaller than other portions. A spot-size conversion (SSC) structure like this is useful for all of waveguide type optical semiconductor devices such as a an optical semiconductor modulator, an optical semiconductor amplifier and a waveguide pin photo diode as well as a semiconductor laser diode.
For instance, one of SSC-LDs has been suggested by Y. Tohmori et al. in ELECTRONICS LETTERS, Jun. 22nd, 1995, Vol. 31, No. 13, pp. 1069-1070 (hereinafter, referred to as first prior art).
FIGS. 1A
to
1
E are cross-sectional views of a laser diode showing respective steps of a method of fabricating a laser diode in accordance with the first prior art.
As illustrated in
FIG. 1A
, a laser active layer is formed on an InP substrate
401
. The laser active layer comprises a first separate confinement hetero-structure (SCH) layer
402
, a strained multi-quantum well (MQW) structure
403
, and a second SCH layer
404
, and each of them are successively epitaxially grown by metal-organic vapor phase epitaxy (hereinafter, referred to simply as MOVPE) growth method.
Then, a SiNx layer
405
is formed on the laser active layer. Then, a portion which would make an SSC portion is etched until the InP substrate
401
appears with the SiNx layer
405
used as a mask. Then, as illustrated in
FIG. 1B
, an SSC structure comprising a 1.1 &mgr;m-bandgap wavelength InGaAsP layer
406
is selectively grown to thereby form a butt-joint.
Then, the SiNx layer
405
is removed, followed by growth of a p-InP clad layer
407
and a p-cap layer
408
all over the product, as illustrated in FIG.
1
C.
Then, an SiNx stripe mask
409
is formed partially on the p-cap layer
408
, and thereafter the product is etched until a certain depth of the InP substrate
401
with the SiNx stripe mask
409
used as a mask to thereby form a high-mesa structure, as illustrated in FIG.
1
D.
Then, the SiNx stripe mask
409
is removed only in the SSC portion, followed by growth of a Fe-doped highly resistive InP layer
410
, as illustrated in FIG.
1
E. The thus fabricated laser diode has a 300 &mgr;m-long laser active layer region and a 300 &mgr;m-long SSC region.
In the above-mentioned method of fabricating a laser diode in accordance with the first prior art, it is necessary to repeatedly carry out complicated steps of selective etching and selective re-growth, and it is also necessary to complete a waveguide by forming a butt-joint. Thus, the first prior art has a problem that it is difficult to fabricate a laser diode with a high fabrication yield.
Another example of SSC-LD has been suggested by T. Yamamoto in ELECTRONICS LETTERS, Dec. 7th, 1995, Vol. 31, No. 25, pp. 2178-2179 (hereinafter, referred to as second prior art), wherein a multi-quantum well (MQW) structure having different thicknesses and band-gap energies between a laser active layer region and an SSC region is formed by a single selective growth. Hereinbelow is explained the second prior art with reference to
FIGS. 2A
to
2
D.
First, a pair of dielectric masks
502
having a width in the range of tens of micrometers to multi-hundreds of micrometers is formed on an n-InP substrate
501
with the masks
502
being spaced away from each other by 10-20 &mgr;m, as illustrated in FIG.
2
A.
Then, an n-InP clad layer
503
, a strained MQW structure
504
, and a p-InP clad layer
505
are selectively grown on the n-InP substrate
501
by MOVPE, as illustrated in FIG.
2
B. In this selective growth of the layers
503
,
505
and the structure
504
, enhancement in a growth rate and increase in an In incorporation rate occur in a region sandwiched between the masks
502
due to vapor phase lateral diffusion of source materials. As a result, a thickness of MQW is enhanced and further a band-gap wavelength is made longer in the region sandwiched between the masks
502
in comparison with other region not sandwiched between the masks
502
. Accordingly, the region sandwiched between the masks
502
makes a laser active layer, and the other region not sandwiched between the masks
502
makes an SSC region.
Then, after removal of the dielectric masks
502
, a dielectric stripe mask
506
is formed over the selectively grown layers. Thereafter, the product is mesa-etched so that the laser active layer has a width of 1.2 &mgr;m, as illustrated in FIG.
2
C.
Then, a p-InP current block layer
507
and an n-InP current block layer
508
are grown all over the product. Then, after removal of the dielectric stripe mask
506
, a p-InP second clad layer
509
and a cap layer
510
are grown over the n-InP current block layer
508
, as illustrated in FIG.
2
D. The thus formed laser active layer region is 300 &mgr;m long, and the SSC region is 200 &mgr;m long.
Still another example of a laser diode has been suggested by M. Wada et al. in ELECTRONICS LETTERS, Nov. 23rd, 1995, Vol. 31, No. 24, pp. 2102-2104. There has been suggested laser diodes monolithically integrated with spot-size converters operating at 1.3 &mgr;m and having an almost circular far-field pattern and a −1.3 dB butt-coupling-loss-to-fiber with wide alignment tolerance. However, the overall device length is 450 &mgr;m.
In the above-mentioned first and second prior art, the SSC regions do not have an optical gain, because they are formed merely for enlarging a spot of laser oscillation lights. Accordingly, the first and second prior art are inferior to an ordinary semiconductor laser diode having no SSC region with respect to increasing of a threshold current and degrading performance at high temperature, because the SSC region causes optical losses.
In addition, a device yield per a unit area or per a wafer would be reduced in the above-mentioned conventional SSC-LDs, because they have to be fabricated longer by a length of the SSC region. Specifically, the laser diode in accordance with the first prior art includes the 300 &mgr;m long laser active layer region and the 300 &mgr;m long SSC region, and hence is totally 600 &mgr;m long. The laser diode in accordance with the second prior art includes the 300 &mgr;m long laser active layer region and the 200 &mgr;m long SSC region
Fahmy Wael
NEC Corporation
Toledo Fernando
Young & Thompson
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