Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
2001-11-29
2004-09-14
Baumeister, Bradley (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S707000, C257S712000, C438S122000, C385S088000, C372S036000
Reexamination Certificate
active
06791181
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor light emitting device containing a semiconductor light emitting element, in particular a semiconductor laser diode. The present invention can be successfully adopted in applications where an excellent heat spreading ability of the semiconductor light emitting element is an important characteristic, for example in an excitation light source for optical fiber amplifiers and a light source for optical information processing in which high output and high reliability must be ensured. The present invention is also suitable for the cases where easy attainment of both of excellent heat spreading ability of the semiconductor light emitting element and direct coupling of this with an optical fiber is desired.
RELATED ART
Remarkable progress has been made in recent technologies in optical information processing and optical fiber communication.
For example, in the communication field, extensive research efforts are directed to large-capacity optical fiber transmission and an optical fiber amplifier doped with a rare earth ion such as Er
3+
(EDFA), which is expected to have the flexibility as a signal amplifier needed for a multi-terabit transmission system. Thus the development of a high-efficiency semiconductor laser diode for an excitation light source, indispensable as an EDFA component, is greatly anticipated.
An excitation light source for EDFA may, in principle, have three possible oscillation wavelengths: 800 nm, 980 nm and 1480 nm. It is known that due to the characteristics of this amplifier the excitation at 980 nm is the best with regard to gain and noise. For a laser diode of excitation light source oscillating at 980 nm, there are conflicting requirements for high output and for long life. In the wavelength range around 980 nm, there is strong needs for development of new laser diodes excellent in output power and reliability, since for example expected applications in the 890 to 1150 nm range include SHG (secondary harmonic generation) light sources, heat source for laser printers, and excitation light sources for optical fiber amplifier such as a state-of-the-art TDFA (thulium-doped fiber amplifier).
In the field of information processing, recent trends prefer higher output and shorter wavelength semiconductor laser diodes in order to achieve higher density storage and faster read/write operation. There is a strong need for higher output from conventional laser diodes (simply referred to as “LD”, hereinafter) having an oscillation wavelength of 780 nm, and extensive research on an LD capable of emitting light of 630 to 680 nm is being carried out from every aspect.
As for semiconductor laser diode of a 980-nm range, extensive research has been done and has resulted in practical achievement such as used in a large-capacity submarine cable systems for optical communication between Japan and the US. The reliability thereof, however, is still not satisfactory since rapid degradation may occur in the operation at higher output levels. The same applies to LD's operating at a 780-nm range and 630- to 680-nm range.
One possible cause for poor reliability is thermal influence. Even high-efficiency models of foregoing semiconductor laser diodes can convert input electric power into light only at an efficiency of about 50%, with the rest of the electric power input lost as heat. This means that in cases where particularly high output is desired, the heat generated in semiconductor laser diode will result in remarkably declined maximum light output, degraded laser efficiency and degraded linearity in current-versus-light output characteristics. It is feared that unless there is adequate heat radiation during high-power operation, reliability will be degraded.
generally, in semiconductor laser diodes, heat spreading is ensured by soldering one electrode plane of the laser diode to a heat sink called “sub-mount” which is typically made of AlN or Si. In this specification, an integrated structure comprising a semiconductor light emitting element (for instance LD) and the sub-mount functioning as a heat sink will be referred to as COS (chip on sub-mount), hereinafter. Also in this specification, any structure comprising a semiconductor light emitting element to which is added at least a heat spreading function will be described as a semiconductor light emitting device. The foregoing COS is therefore one kind of semiconductor light emitting device and can be incorporated into a can package or a butterfly package. Such packages are semiconductor light emitting devices with additional functions.
For fabrication of can packages, it is a general practice that a COS is mounted on a so-called “stem” providing further heat spreading and current injection, wirings necessary for the current injection are done, and a cap with a window seals in e.g. a nitrogen atmosphere, to thereby complete a semiconductor light emitting device. On the other hand, butterfly packages can be constructed by mounting a COS on a so-called “OSA (optical sub-assembly)” providing heat spreading and integrating a plurality of parts including a photo diode (PD) and then optically coupling the semiconductor light emitting element with an optical fiber etc., thereby completing the semiconductor light emitting device.
In these two cases, a semiconductor light emitting element is generally brought into contact for heat spreading only on one plane of electrode. A structure allowing the substrate side of the semiconductor light emitting element to contact with the heat sink is called “junction-up (face-up)”; and a structure allowing the epitaxial layer side of the element to contact with the heat sink is called “junction-down (face-down)”.
The junction-up mounting is simple and widely practiced since the method allows the light emission point of the element to be removed from the heat sink, i.e. the sub-mount, approximately by the thickness of the element. The method is, however, disadvantageous in terms of heat spreading since the light emission portion of the element is located distant from the heat sink, and so is not always suitable for high-power operation of the semiconductor light emitting element such as the semiconductor laser diode.
On the other hand, the junction-down mounting is advantageous in term of the heat spreading, but still the heat spreading is insufficient, and improvement of this is now urgently required.
Several proposals have been made for the further improvement in heat spreading of the semiconductor light emitting element, and more specifically, semiconductor laser diode. For example, Japanese Laid-Open Patent Publication No. 306681/1990 discloses a method of ensuring heat spreading of the semiconductor laser diode simultaneously in the upper and lower directions. Similar methods are also found in Japanese Laid-Open Patent Publication Nos. 228044/1996 and 228045/1996. It is, however, difficult to fabricate the disclosed structure with an excellent reproducibility by any of these methods.
This is because there is no consideration at all given to dimensional errors generally found in the individual components, typically error in the thickness of the semiconductor light emitting element, or dimensional error of the heat sink sandwiching the semiconductor light emitting element.
In a general fabrication of the semiconductor laser diode, a substrate of as thick as approximately 350 &mgr;m is used to thereby ensure mechanical strength sufficient for executing necessary processes, and the substrate is later polished to reduce the thickness thereof to as thin as 100 to 150 &mgr;m before the n-electrode forming process or cleavage process in order to facilitate the cleavage. It is, however, quite natural that the dimensional error in the thickness as much as 5 to 15 &mgr;m is produced, which causes further error in elements. Process errors can occur also in metal components for heat spreading described in the foregoing Japanese Laid-Open Patent Publication No. 306681/1990; and recessed heat spreading components described in Japanese Laid-O
Arai Nobuhiro
Horie Hideyoshi
Komuro Naoyuki
Baumeister Bradley
Chu Chris C.
Mitsubishi Chemical Corporation
Westerman Hattori Daniels & Adrian LLP
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