Optical device structure and fabrication method thereof

Details Optical device structure and fabrication method thereof Optical device structure and fabrication method thereof

1998-09-23

2003-01-14

Davie, James (Department: 2828)

Coherent light generators

Particular temperature control

Heat sink

C257S734000, C257S741000, C372S096000

Reexamination Certificate

active

06507594

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device structure including an optical device, such as a light emitting semiconductor device, a light detecting semiconductor device and a surface emitting semiconductor laser, typically a vertical cavity surface emitting laser (VCSEL), whose thermal radiation characteristic is prominent and which is suitable for use in a two-dimensional array structure, for example, and its fabrication-method.
2. Related Background Art
Recently, development of a solid-state light emitting laser device of a two-dimensional array type has been desired for the purpose of its applications to large-capacity parallel optical information processing, high-speed optical connection and panel-type display apparatus. As a light emitting device suitable for the arraying, the VCSEL has been watched with keen interest and studied. The VCSEL normally includes a Fabry-Perot cavity with upper and lower reflection mirrors and a cavity length of several microns. To achieve a low threshold, such a reflection mirror as has small absorptively for oscillation wavelength and has a high reflectance, is required. For this purpose, the mirror is normally comprised of a multi-layer structure of alternately layered layers of two kinds with different refractive indices and a thickness of a quarter of the oscillation wavelength.
Surface emitting semiconductor lasers with a variety of oscillation wavelengths can be fabricated by selecting semiconductor material according to the wavelength. Among them, surface emitting semiconductor lasers of GaAs series with oscillation wavelengths of 0.85 &mgr;m and 0.98 &mgr;m and of InP series with oscillation wavelengths of 1.3 &mgr;m and 1.55 &mgr;m are well known.
In the case of GaAs series, a multi-layer of AlAs/(Al)GaAs is generally used as mirror because this can be epitaxially grown on a GaAs substrate. On the other hand, in the case of InP series, since an index difference between InGaAsP and InP, which can be epitaxially grown on an InP substrate, is small and a high reflectance is hence difficult to obtain, materials other than InGaAsP/InP, such as SiO
2
/Si multi-layer and an Al
2
O
3
/Si multi-layer, is used.
Further, the following method is known: to semiconductor layers including an active layer grown on an InP substrate, a multi-layer of AlAs/(Al)GaAs grown on another GaAs substrate is bonded.
As a method for providing an electrode leader structure in arrayed surface mitting semiconductor lasers, there exists a method in which an electrode-leader pattern is directly formed on the surface of the surface emitting semiconductor lasers. There has also been proposed a method in which a leader wire pattern is formed on a substrate other than a growth substrate and the laser substrate is bonded to the other substrate.
For example, Japanese Patent Laid-Open No. 7-283486 (published in 1995) discloses a technique according to which an electrode
530
on an AlN substrate
551
with an electronic circuit (not shown) is electrically connected to an electrode
530
on a surface of a surface emitting semiconductor laser formed on an AlaAs substrate
511
by using solder bumps
557
, as illustrated in FIG.
1
. If needed, surroundings of the solder: bumps may be buried with resin. The surface emitting semiconductor laser includes a light emitting layer
518
sandwiched between mirrors
529
and
563
and surrounded by polyimide
523
.
As another example, Japanese Patent laid-Open No. 8-153935 (published in 1996) discloses a technique according to which an electrode
630
on a substrate
651
(a second substrate) with an electric leader wire is directly bonded to an electrode
630
on a surface of a surface emitting semiconductor laser formed on a first substrate
611
and surroundings of the electrodes
630
are buried and set with resin
657
, as illustrated in FIG.
2
. The surface emitting semiconductor laser includes a buffer layer
615
and a light emitting layer
618
sandwiched by mirrors
629
and
663
.
However, where the electrode leader wire pattern is directly formed on the surface emitting semiconductor laser, a use efficiency of a laser wafer is lowered and its cost increases since an electrode pad is needed around the laser region.
Further, where the leader wire pattern is formed on another substrate and the another substrate is bonded to the laser substrate, for example, in the case of the electrode leader structure using solder bumps illustrated in
FIG. 1
, the interval between the active layer and the substrate with the electric leader wire is determined by the diameter of the bump which ranges from several tens microns to a hundred microns (that is, the interval cannot be less than the diameter of the bump). Thus, its thermal radiation characteristic is not satisfactory. Additionally, since a multiplicity of solder bumps must be placed on determined positions, its fabrication process inevitably becomes complicated.
Where the electrode on the substrate with the electric leader wire thereon is directly bonded to the electrode on the surface of the surface emitting semiconductor laser and surroundings thereof is set with the resin as illustrated in
FIG. 2
, the interval between the active layer and the substrate (second substrate) with the electric wire thereon can be reduced, compared to the case of FIG.
1
. An electric resistance, however, increases due to the resin inserted between the upper and lower electrodes, and those electrodes will be insulated from each other in the worst case. Thus, the yield is impaired. Further, such a structure is vulnerable to its surface conditions. If insulating dusts are on the surface or the surface is covered with an oxidized film, it is difficult to achieve a preferable electric contact.
Further, with the surface emitting-semiconductor laser, the substrate should be removed depending on the relation between semiconductor material and oscillation wavelength. For example, where a surface emitting semiconductor laser for emitting an oscillation wavelength of 0.85 &mgr;m uses a GaAs substrate, the substrate needs to be removed to take light from the substrate side as the GaAs substrate is opaque to the oscillation wavelength.
As another example, where a surface emitting semiconductor laser for emitting an oscillation wavelength of 1.3 &mgr;m or 1.55 &mgr;m using an InP substrate, it is necessary to deposit a multi-layer mirror of dielectrics after removing the InP substrate. For this purpose, presently the semiconductor substrate is etched in the form of a hole in accordance with a light emitting region of the surface emitting semiconductor laser. Such a hole-etching is difficult to carry out with good reproducibility. Further, the hole-etching for each semiconductor laser is likely to be a factor for preventing a high-density configuration in the case of the arrayed semiconductor lasers. Moreover, it is also difficult to form a multi-layer, such as SiO
2
/Si multi-layer and Al
2
O
3
/Si multi-layer, in the hole with good reproducibility, leading to a decrease in the yield.
Also with the cases of
FIGS. 1 and 2
, the substrate needs to be removed depending on the relation between semiconductor material and wavelength (in the case of
FIG. 1
, although the AlGaAs substrate, which is transparent to the oscillation wavelength, is used to make the substrate removal unnecessary, such an AlGaAs substrate is not generally used and its cost is high). In those cases, the following problem occurs. It can be considered that after the laser substrate is bonded to the substrate with the electrode thereon, all the laser substrate (semiconductor substrate) is removed with the electrode substrate acting as a support substrate. In those cases, the overall removal of the semiconductor substrate is, however, not taken into consideration. Specifically, the problem occurs that stresses are applied to the active layer due to thermal expansion and the like when the layer thickness after the removal comes to be several microns, and hence the oscillation threshold

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