Method for laser bar facet coating

Apparatus and method for retaining a laser bar during a facet coating operation and releasing the laser bar subsequent to the operation. The apparatus includes a pair of opposing spacers that hold the laser bar when compressed, each spacer including a compliant spring on an inner portion thereof. A compression device applies a force to the spacers sufficient to compress the springs against opposite sides of the laser bar to thereby retain the laser bar during the facet coating operation. Subsequent to the facet coating operation, the compressive force is released to decompress the springs and thereby release the laser bar. Due to the spring action, the laser bar is released without substantially adhering to either one of the spacers.

FIELD OF THE INVENTION 
The present invention relates generally to laser device fabrication, and 
more particularly, to an apparatus for retaining a laser bar during a 
facet coating operation and for automatically releasing the bar 
thereafter. 
BACKGROUND OF THE INVENTION 
Semiconductor laser devices such as double heterostructure laser diodes are 
utilized in various applications such as for optical sources in fiber 
optic communications. In the manufacture of such devices, double 
heterostructures are grown and processed on a wafer substrate by, for 
example, liquid phase epitaxy. The wafer is then cleaved into typically 
solid rectangular laser bars containing many laser diodes. The laser 
diodes are functional at the laser bar level, and therefore electrical 
testing is often performed at this level prior to dicing the individual 
laser diodes from the bars. 
Each laser bar has two end faces (facets) formed by cleaving during the 
cleaving process. Ultimately, laser light is transmitted through the 
facets, so it is important for their surfaces to remain unperturbed during 
handling of the laser bar. Following the cleaving operation on the wafer 
to form the laser bars, the facets are coated with an optical coating in a 
facet coating apparatus. A facet coat holding fixture is typically 
employed to retain the laser bar during the facet coating and also to 
transport the bar into and out of the facet coating apparatus. 
FIG. 1 illustrates a prior art facet coat holding fixture 18 for retaining 
a laser bar 10 during a facet coating operation. Fixture 18 is comprised 
of a pair of solid rectangular fixture blades 18.sub.1, 18.sub.2 that 
retain laser bar 10 by sandwiching the same under a compressive force 
applied to side surfaces 15 thereof by a compression or clamping device 
17. Laser bar 10 is a thin, solid rectangular bar with facets 14a, 14b on 
opposite sides, and longitudinal side surfaces 12 perpendicular to the 
facet surfaces. Due to its small size--e.g., typical dimensions on the 
order of 0.005.times.0.012.times.0.300 inches--the laser bar must be 
handled with extreme care to avoid damaging it. Flat side surfaces 19 of 
the fixture blades compress against laser bar surfaces 12 during facet 
coating. Once facets 14a and 14b are coated, holding fixture 18 and the 
retained laser bar 10 are transported out of the facet coater atop a 
receiving surface 13. Fixture blades 18.sub.1 and 18.sub.2 are then 
separated to release laser bar 10 onto surface 13 between slot S defined 
between the blades. The laser bar is then removed from the slot by a 
vacuum pick or robot arm for subsequent processing and testing operations. 
The optical coating applied to the exposed facet surfaces 14a, 14b often 
seeps in between the side surfaces 12 of the laser bar and mating surfaces 
19 of fixture blades 18.sub.1, 18.sub.2. As a result, laser bar 10 often 
sticks to one of the fixture blades when the blades are separated atop 
surface 13. An additional operation is then required to detach the laser 
bar from the fixture blade, e.g., manually shaking the holding fixture to 
loosen the laser bar or prying the laser bar from the blade with a pick. 
The additional operation results in a yield reduction as the laser bars 
tend to get damaged when pried or shaken from the fixture blades. 
Typically, up to 50% of the laser bars may become damaged from this 
operation. 
SUMMARY OF THE INVENTION 
The present invention is directed towards an apparatus and method for 
retaining a laser bar during a facet coating operation and releasing the 
laser bar subsequent to the operation without the laser bar substantially 
adhering to the apparatus. In an illustrative embodiment, the apparatus 
includes a pair of opposing spacers that hold the laser bar when 
compressed thereagainst. Each spacer includes a compliant spring on an 
inner portion thereof. A compression device applies a force to the spacers 
sufficient to compress the springs against opposite sides of the laser bar 
to thereby retain the laser bar during the facet coating operation. 
Subsequent to the facet coating operation, the compressive force is 
released to decompress the springs and thereby release the laser bar. Due 
to the spring action, the laser bar is released without substantially 
adhering to either one of the spacers. 
Preferably, each spacer is comprised of a generally solid rectangular 
member to which the compressive force is applied, and a wide angle, 
V-shaped spring attached to the rectangular member. 
Advantageously, since the abrupt spring release action automatically 
loosens the laser bars from the spacers, the yield-reducing shaking or 
prying operations otherwise necessary to loosen the laser bar are reduced 
or eliminated. Hence, embodiments of the invention beneficially improve 
the manufacturing yield of laser devices diced from the laser bars.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference now to FIG. 2, an exemplary embodiment 20 of the present 
invention is illustrated in a top view. Laser bar holding fixture 20 is 
comprised of a pair of fixture spacers 25.sub.1, 25.sub.2 for retaining 
laser bar 10 during a facet coating operation. These spacers replace the 
solid rectangular fixture blades 18.sub.1, 18.sub.2 of the prior art 
system discussed above. Each spacer 25.sub.1, 25.sub.2 includes a V-shaped 
flexible, compliant spring 24, each of which compresses against a 
respective side surface 12 of laser bar 10 to thereby retain the laser 
bar. Advantageously, when springs 24 are decompressed to release laser bar 
10 following facet coating, the spring action enables them to 
automatically break away from the laser bar. That is, the laser bar does 
not generally adhere to the surfaces of either spring 24, thus reducing or 
obviating subsequent shaking or prying operations otherwise required to 
detach the laser bar from the holding fixture. Holding fixture 20 is 
illustrated in FIG. 2 in a decompressed state with springs 24 relaxed to 
release laser bar 10. FIG. 3 shows holding fixture 20 in a compressed 
state to retain laser bar 10 sandwiched against inner surfaces 28 of 
springs 24. 
With continuing reference to FIG. 2, each spacer 25.sub.1, 25.sub.2 
includes a solid rectangular member 22 with an outer surface 29 (in the z 
plane perpendicular to the page). A compressive force from a suitable 
compression member or members (not shown) of compression device 17 is 
applied to outer surface 29 of each spacer to compress the pair of spacers 
as shown in FIG. 3. The compression member of device 17 may include 
opposing plates or distributed springs that mate with outer surfaces 29 to 
allow a predetermined, controlled force to be applied evenly to 
rectangular members 22. Surfaces 29 of rectangular members 22 may be 
attached to the compression plates or springs of device 17. The 
compression device 17 can be a relatively simple, manually operated clamp 
or spring-loaded device. Alternatively, compression device 17 can be 
embodied as a more sophisticated, electronically controlled compression 
system. In any event, any suitable compression device capable of applying 
a small amount of compressive force against the spacers and abruptly 
releasing the same, may be employed. 
Each compliant spring 24 extends from a post 26 extending from a central 
region of the associated rectangular member 22. (Post 26 may be considered 
part of spring 24.) Preferably, spring 24, post 26 and member 22 of each 
spacer are unitary, e.g., formed by machining from a thin sheet of metal. 
An angle .theta. is defined between each leg of spring 24 and the inner 
surface 23 of member 22 when spring 24 is in a relaxed (uncompressed) 
state. This angle .theta., although shown exaggerated in FIG. 2 for 
clarity, is preferably small, e.g., less than 5.degree., and most 
preferably about 1.degree.. Springs 24 are composed of any suitable 
spring-like material such as stainless steel or other metal alloy. The 
legs of springs 24 are thin, e.g., in the range of 0.002-0.005 inches (2-5 
mils) to provide suitable flexibility. 
With reference now to FIG. 3, spacers 25.sub.1 and 25.sub.2 are shown 
compressed against laser bar 10 whereby the laser bar is effectively held 
by holding fixture 20. In the compressed state, the legs of springs 24 
flex back towards member 22 and essentially form a flat surface 28 which 
is compressed against and conforms with the respective side surface 12 of 
the laser bar. As such, only facets 14a and 14b are exposed for the facet 
coating operation. A predetermined force 32 is applied from compression 
device 17 to outer surfaces 29 to produce the desired compression. 
Depending on the dimensions of post 26, a small gap g of about one mil may 
be present between the legs of spring 24 and the inner surface of member 
22. 
FIG. 4 is a perspective view depicting one of the spacers, 25.sub.1, in 
relation to laser bar 12. Spring 24 is defined by a pair of flexible 
members (legs) 24.sub.1 and 24.sub.2. Inner surface 28 of spring 24 is 
polished so as to avoid damage to laser bar surface 12 while laser bar 10 
is compressibly retained against surface 28. The surface area of inner 
surface 28 of spring 24 is preferably larger than side surface 12 of laser 
bar 10 (e.g., spring 24 is preferably longer than surface 12, but about 
the same width). 
Referring to FIG. 5, the facet coating process is carried out in an 
exemplary embodiment by first placing the laser bar to be coated in 
between spacers 25.sub.1, 25.sub.2 on a surface external to the facet 
coating apparatus. Spacers 25.sub.1, 25.sub.2 are then compressed against 
the laser bar to retain the same as was described earlier in reference to 
FIG. 3. As laser bar 10 is sandwiched between the spacers, the holding 
fixture 20 is transported to a facet coating apparatus (facet coater or 
chamber), not shown in the figures. A commercially available facet coater 
may be employed, such as one of those manufactured by the Eddy company, 
located in Applevalley, Calif. With the fixture spacers remaining 
compressed, the facet coater is operated to coat respective upper and 
lower facets 14a and 14b of the laser bar with an optical coating. 
Typically, one facet surface 14a or 14b is coated at a time. Following the 
coating of one facet, the entire holding fixture 20 is flipped by 
180.degree. while laser bar 10 remains sandwiched between the spacers, so 
as to expose the other facet for coating. Upon completion of the coating 
operation, the holding fixture 20 is transported atop a receiving surface 
(as surface 13 of FIG. 1) with facet 14b resting thereon. Then, as shown 
in FIG. 5, spacers 25.sub.1, 25.sub.2 are abruptly decompressed and pulled 
apart to a predetermined separation distance D to release laser bar 10 
onto the receiving surface. Due to the spring release action of springs 24 
against laser bar 10, the laser bar is effectively loosened and separated 
from the springs. 
Also shown in FIG. 5 are spring loaded mating plates 17.sub.1 and 17.sub.2, 
part of an exemplary compression device 17, which plates are attached in 
this example to opposing rectangular members 22. These plates are 
compressed to retain the laser bar and abruptly separated to release the 
same. A single pair of long plates 17.sub.1, 17.sub.2 can be used to 
retain and then release a number of holding fixtures 20 simultaneously to 
allow for an efficient manufacturing operation. 
Accordingly, with proper design of spacers 25.sub.1, 25.sub.2, most laser 
bars will not adhere, or only minimally adhere, to springs 24. Laser bars 
that do stick to one or both springs can usually be loosened by a simple 
shaking operation without the necessity of prying the laser bar loose with 
a tool. As a result, the overall yield is improved relative to the 
above-discussed prior art. 
While the present invention has been described above with reference to 
specific embodiments thereof, it is understood that one skilled in the art 
may make many modifications to the disclosed embodiments without departing 
from the spirit and scope of the invention. For instance, while the spacer 
springs 24 have been shown and described as V-shaped springs, it may be 
possible to employ other types of compliant springs in the alternative. 
Accordingly, these and other modifications are intended to be included 
within the scope of the invention as defined by the appended claims.