Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
1998-07-21
2001-10-09
Leung, Quyen P. (Department: 2881)
Coherent light generators
Particular temperature control
Heat sink
C372S043010
Reexamination Certificate
active
06301278
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to semiconductor laser devices capable of effectively transmitting heat away and in particular to such semiconductor laser devices adapted for high-frequency operations.
Semiconductor laser devices incorporating a semiconductor laser element as a pick-up light source for a writable disk drive or the like have been known. Such a semiconductor laser element emits laser light as an electric current is caused to flow across its internal p-n junction, but a large amount of heat is also generated at the same time. In order to efficiently remove this heat, such a laser element is commonly mounted with its p-surface downward (according to the so-called “junction-down” mounting format) to a silicon submount with a high thermal conductivity such that the generated heat is quickly conducted away therethrough. This mounting format is considered advantageous because the distance between the p-n junction which is the source of the generated heat and the submount can be reduced with the p-surface of the laser element facing downward.
FIGS. 9 and 10
show a prior art semiconductor laser device structured in this manner. Three externally extending lead pins
15
,
16
and
17
(referred to for convenience as “the first”, “the second” and “the third”, respectively) are attached to a disk-shaped member
10
(referred to as “the stem”), the first pin
15
being directly attached to the stem
10
so as to be electrically connected therewith, while the second and third pins
16
and
17
are each affixed to the stem
10
by way of an insulator
18
. A heat sink
20
, which may comprise a material such as Cu, is soldered to a main surface of the stem
10
and an electrically conductive silicon submount
25
is attached to the heat sink
20
. On the surface of the submount
25
, there is not only an Al wiring pattern
40
formed through an oxide layer
41
comprising SiO
2
but also an Al pad formed directly.
A semiconductor laser element
30
is deposited on this Al wiring pattern
40
in the aforementioned junction-down format such that the heat generated thereby can be efficiently conducted away therefrom. The n-surface of the laser element
30
and the Al pad
35
are electrically connected by a metallic wire
36
.
The second lead pin
16
, which serves to supply power therethrough to the laser element
30
, is extended internally to a position near the submount
25
and is electrically connected to the Al pattern
40
by another metallic wire
42
.
A light-receiving element
50
, which serves to receive the laser light emitted backwards from the laser element
30
and to thereby monitor its optical output, is directly mounted to the main surface of the stem
10
, and its upper surface is electrically connected to the third lead pin
17
by still another metallic wire
51
. All these components described above, inclusive of the laser element
30
, are sealed inside a cap (not shown in
FIGS. 9 and 10
) to form a packaged product.
When a semiconductor laser device thus formed is used for an optical disk, for example, the heat sink
20
and the second pin
16
respectively serving as the negative electrode and the positive electrode, a current flows from the second pin
16
sequentially through the metallic wire
42
, the Al pattern
40
, the p-surface of the laser element
30
, its n-surface, the wire
36
, the Al pad
35
and the submount
25
to the heat sink
20
, as shown by arrows, such that laser light is emitted from the laser element
30
.
It is to be noted, regarding the prior art semiconductor laser device described above, that the Al pattern
40
, on which the laser element
30
is deposited, and the electrically conductive submount
25
must be separated from each other by the electrically insulating oxide layer
41
both because the laser element
30
must be mounted to the submount
25
in the junction-down format and because the heat sink
20
must be used as the negative electrode. Since the thermal conductivity of the oxide layer comprising SiO
2
is 1.4-7.2 W/m° K and is much smaller than that of the silicon submount
25
(about 150 W/m° K), this means that the heat generated by the laser element
30
cannot be efficiently conducted away to the submount
25
.
In view of the above, it has also been known to use a submount made of a material other than silicon such as AlN that is electrically insulating but has a larger thermal conductivity (160-200 W/m° K) than silicon.
FIG. 11
shows another prior art semiconductor laser device characterized (and distinguishable from the example shown in
FIGS. 9 and 10
) as having a submount
25
′ made of electrically insulating AlN attached on top of a heat sink
20
comprising Cu or the like. Two Al wiring patterns
40
′ and
35
′ are formed on the surface of the AlN submount
25
′ such that they are electrically separated from each other, and a laser element
30
is deposited on the Al pattern
40
′ in the junction-down format. The n-surface of the laser element
30
and the Al pattern
35
′ are connected to each other electrically by a metallic wire
36
a
, and the Al pattern
35
′ and the heat sink
20
are connected electrically to each other by another metallic wire
36
b
. The Al pattern
40
′ is also electrically connected through still another metallic wire
42
to a lead pin
16
for supplying a current from an external source (not shown). With a semiconductor laser device thus structured, a current flows from the pin
16
sequentially through the wire
42
, the Al pattern
40
′, the p-surface of the laser element
30
, its n-surface, the wire
36
a
, the Al pattern
35
′ and the wire
36
b
to the heat sink
20
, as shown by arrows, such that laser light is emitted from the laser element
30
.
The semiconductor laser device described above with reference to
FIG. 11
can therefore be cooled more efficiently because the heat generated by its laser element
30
can be conducted off to the thermally conductive AlN submount
25
′ only through the Al pattern
40
′. The use of a submount made of AlN, instead of silicon, however, has the following practical problem.
When semiconductor laser devices are produced, screening tests therefor for quality control are not easy to carry out if they are to be carried out only on the laser elements because semiconductor laser elements are extremely small. Thus, screening tests are usually carried out after the laser elements are each deposited on a submount. In other words, a test on electrical and optical characteristics of each laser element is carried out not on the laser element alone but on the combination consisting both of the laser element and also of the submount on which it is deposited. If the test on a combination shows that an adjustment is required, such an adjustment is made, say, by a so-called burn-in process, and the adjusted combination is then attached to a heat sink. If the test shows that it is not adjustable, however, the combination is discarded as a whole. In summary, if a laser element is unadjustably defective, the submount to which it is mounted is also discarded. Since silicon submounts are relatively inexpensive, the procedure described above is not impractical, not incurring a serious economical loss. Since AlN submounts are significantly more expensive (say, by a factor of several tens) than silicon submounts, the loss due to discarded AlN submounts can significantly affect the production cost of the laser devices.
It is therefore an object of this invention to provide a semiconductor laser device from which generated heat can be effectively removed although a silicon submount is used for mounting a semiconductor laser element thereto.
Another problem with prior art semiconductor laser devices as described above with reference to
FIGS. 9
,
10
and
11
is that their submount serves as a capacitor (with capacitance C
1
) and its package contributes an inductance L
1
when they are operated such that their equivalent circuit diagram may look as sh
Coudert Brothers
Leung Quyen P.
Rohm & Co., Ltd.
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