Semiconductor laser array with stripe electrodes having pads for wire bonding

A semiconductor laser array comprising plural semiconductor layers formed on a substrate having a laser active layer. The plural semiconductor layers define first and second end-faces which are for emitting laser light. Plural stripe-like current injection areas are also formed in the plural semiconductor layers. Each two adjacent current injection areas have a respective first distance therebetween at the first end-face, and each two adjacent current injection areas have a respective second distance therebetween at the second end-face,the first distance being greater than the second distance. Plural electrodes are provided for supplying current to the current injection areas. At least one of the electrodes has a stripe-like portion formed corresponding to the current injection areas, and has a pad portion which has a width greater than that of the stripe-like portion. The pad portion is for wire bonding, and is formed in an area other than the current injection areas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semi-conductor laser array in which 
plural semiconductor laser elements are monolithically formed on a same 
substrate. 
2. Related Background Art 
In the conventional semiconductor laser array, as shown in FIG. 1, stripe 
electrodes 61d-65d are formed in parallel manner, respectively in current 
injection areas 61a-65a of semiconductor laser elements 61-65. These 
stripe electrodes are independently connected to a power source by wire 
bondings 20. 
For the purpose of reducing the pitch of the elements in such semiconductor 
laser array, in case of two elements, the wire bondings are made on both 
sides of two current injection areas as shown in FIG. 2. Also in case of 
three or more elements, there is employed three-dimensional wiring as 
shown in FIG. 3. 
However, if the pitch of the semiconductor laser elements in such array 
becomes smaller than for example the effective width of the wire bonding, 
which is about 100 .mu.m, the wire bonding operation has to be conducted 
very precisely and therefore the electrodes can become very easily 
shortcircuited. Also the wire bonding, if conducted on the current 
injection area of the laser, may cause damage in the laser element. Such 
damage may shorten the service life of the laser element or deteriorates 
the electrical properties, thus eventually destructing the laser element 
and lowering the production yield. 
Also in three-dimensional wiring, the electrodes are often shortcircuited 
due to through holes in the insulating film or destruction of the 
insulating film caused by the impact at the wire bonding, so that the 
production yield is lowered. 
Thus, in the conventional laser array with three or more laser elements 
with the pitch of elements equal to or smaller than 100 .mu.m, it has been 
difficult to achieve independent driving of the semiconductor elements, or 
to achieve a high level of integration of the semiconductor laser element, 
and such laser array has been associated with a low production yield at 
the mounting. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a semiconductor laser 
array not associated with the above-mentioned drawbacks of the prior 
technology, capable of achieving a high level of integration and 
facilitating the wire bonding operation. 
The foregoing object can be achieved, according to the present invention, 
by a semiconductor laser array, comprising: 
a single substrate; and 
plural semiconductor laser elements monolithically formed on said single 
substrate, wherein each of said semiconductor laser elements comprises: 
plural semiconductor layers including a laser active layer; 
first and second end faces formed by said plural semiconductor layers and 
adapted for emitting laser light; and 
plural stripe electrodes for current supply to said laser active layer, 
wherein said electrodes have a pitch at said first end face different from 
the pitch thereof at said second end face, and at least one of said 
electrodes has a wire bonding pad for wire bonding with a width larger 
than the other part of the electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now the semiconductor laser array of the present invention will be 
clarified in detail by the embodiments thereof shown in the attached 
drawings. 
FIG. 4 is a plan view showing an embodiment of the semiconductor laser 
array of the present invention, wherein semiconductor laser elements 11-14 
have a pitch on a resonance plane 16 different from that on another 
resonance plane 17. The semi-conductor laser elements 11-14 respectively 
have current injection areas 11a-14a, corresponding to light-emitting 
areas. Also there are provided stripe electrodes 11d-14d respectively 
having pads for wire bonding 11b-14b. For facilitating the wire bonding 
operation, the pads are formed wider than the other part of the 
electrodes. The resonance planes 16, 17 are formed by cleavage. 
FIG. 5 is a cross-sectional view of a laser element, along a line A--A' in 
FIG. 4. In the following there will be explained the manufacturing process 
of the laser array in detail, with principal reference to FIG. 5. 
On an n--GaAs substrate 21, there are laminated, in succession an n--GaAs 
buffer layer 22 of a thickness of 1 .mu.m, an n--Al.sub.0.4 Ga.sub.0.6 As 
clad layer 23 of a thickness of 2 .mu.m, an active layer consisting of 
four cycles of a non-doped GaAs layer of a thickness of 100 .ANG. and a 
non-doped Al.sub.0.2 Ga.sub.0.8 As layer of a thickness of 30 .ANG., and 
finally a GaAs layer of a thickness of 100 .ANG., thereby forming an 
active area 24 of a multiple quantum well structure. Then a p--Al.sub.0.4 
Ga.sub.0.6 As clad layer 25 of a thickness of 15.mu.m and a GaAs cap layer 
26 of a thickness of 0.5 .mu.m were prepared by molecular beam epitaxy. 
Then, in order to limit the current injection area, the above-mentioned 
laminate structure is etched to a level about 0.4 .mu.m in front of the 
active layer 24 except a stripe-shaped area, thereby forming a ridge. On 
the ridge there is formed a silicon nitride insulating film 27 by plasma 
CVD method, and the film 27 is etched off only in the top of the ridge 
thereby forming an injection area for contact with the electrode. 
The injection area has a width of 2.5 .mu.m, and the p-type and n-type may 
naturally be exchanged for the four stripes in order to obtain the same 
effects. 
Then a Cr--Au ohmic upper electrode 30 is formed, and is separated by 
etching to obtain four independent electrodes 11d-14d as shown in FIG. 4. 
The GaAs substrate 21 is lapped to a thickness of 100 .mu.m, and is 
subjected to deposition of an Au--Ge electrode as a n-ohmic electrode 29. 
After a subsequent thermal treatment for diffusion, the resonance planes as 
shown in FIG. 4 are made by cleavage. Planes 19 are prepared by scribing. 
The pitch of the laser elements is 15 and 150 .mu.m respectively on the 
resonance planes 16, 17. The electrodes 11d-14d are independently 
connected to a power source by unrepresented lead wires bonded to the pads 
11b-14b. The length of the cavity, or the distance between the resonance 
planes 16, 17 is 300 .mu.m. 
The semiconductor laser array thus prepared can achieve independent driving 
of the semi-conductor laser elements with a high level of integration. 
In the foregoing embodiment the resonance planes are prepared by cleavage, 
but there may also be employed a resonance plane prepared by a wet etching 
process or a dry etching process for one or both of the resonance planes. 
FIGS. 6 to 8 are plan views showing such example, in which same components 
as those in FIG. 4 are represented by same numbers and will not be 
explained further. 
A semiconductor laser array shown in FIG. 6 is different from that shown in 
FIG. 4 in that four end planes of the substrate are formed by scribing, 
and the resonance planes 16, 17 are formed by etching. Also in a 
semiconductor laser array shown in FIG. 7, the stripe electrodes 11d-13d 
are formed with a non-zero mutual angle in order that the laser elements 
have respectively different light-emitting directions. The resonance plane 
16 is prepared by cleavage, while the resonance plane 17 is prepared by 
etching so as to be perpendicular to the longitudinal direction of each 
stripe electrode. Such laser array emitting plural laser beams in 
different directions is detailedly explained in the Japanese Laid-open 
Patent 61-120486, the Japanese Laid-open Patent 61-159785 (corresponding 
to the U.S. patent application Ser. No. 797,492; filed: Nov. 13, 1985) and 
U.S. Pat. No. 4,799,229. 
In a semiconductor laser array shown in FIG. 8, a resonance plane 16' is 
formed also on the lateral side of the array. Stripe electrodes 11d-15d 
are curved, and plural laser beams are emitted in mutually different 
directions. 
As explained in the foregoing, the present invention has the advantages of 
enabling independent drive of a monolithic laser array of a small pitch, 
achieving a high level of integration of the monolithic laser array, and 
improving the production yield at the mounting. Such monolithic laser 
array with independently drivable laser elements of a small pitch is 
extremely useful as a light source for an optical pickup such as for an 
optical disk or a optical card, as the plural laser elements can be used 
for independent purposes. 
The present invention is not limited to the foregoing embodiments but is 
subject to various other applications. For example, the length of cavity 
or the width of current injection area need not be same for all the 
semiconductor laser elements. Also the active area is formed, in the 
foregoing embodiments, by the multiple quantum well structure, but the 
present invention is applicable also to other structures such as the laser 
of double hetero structure or single quantum well structure. 
Furthermore, though the foregoing description has been limited to the ridge 
wave structure utilizing GaAs, the present invention is applicable also to 
the laser of refractive index wave guide type, such as the buried hetero 
structure, the channeled substrate planar structure or the structure 
having an absorption layer for current constriction in the vicinity of the 
active layer, or the laser of gain wave guide type such as the proton 
bombard structure. 
The material for the semiconductor laser elements is not limited to GaAs or 
AlGaAs series, but there may be employed other materials such as InP, 
InGaAsP or AlGaInP series. 
Also the semiconductor laser array of the present invention may employ not 
only the lasers with Fabry-Perot oscillator but also the distributed 
feedback (DFB) lasers or distributed Bragg reflection (DBR) lasers. 
The present invention includes all these modifications as long as they are 
within the scope of the appended claims.