Kenichi Mizuishi

Reliability in InGaAsP/InP buried heterostructure 1.3 µm lasers

A study was conducted of aging-induced Sn whisker growth at the surface of Au-Sn bonding solder layers around a laser chip, as well as metallurgical reactions, especially in p-side down lasers, at the interface of the solder and laser crystal just below the active layer. These phenomena cause electrical shorts in InGaAsP/InP laser diodes, which occur suddenly in devices operated for long periods without any previous symptoms having appeared in their aging characteristics. To completely eliminate such failures, a novel assembling method in which chips were mounted p-side up on semiinsulating SiC submounts using Pb-Sn solder, was applied to InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm. BH lasers assembled by this method do not suffer any shorting failure even after 8000 h operation under 60°C and 5 mW/facet output conditions. The small degradation rates obtained, for example, 3 percent/kh (median) at 60°C, certify the reliability of these improved lasers. As long as the stripe width of the active layer was optimized to be in the range of1.5-2.5 \mum necessary for obtaining kink-free light output versus current properties, no detrimental changes in laser characteristics, including transverse and longitudinal modes and dynamic output response, were observed in aged lasers. In this paper, the long-term degradation modes observed are presented, and possible causes are discussed.

Accelerated Aging Characteristics of InGaAsP/InP Buried Heterostructure Lasers Emitting at 1.3 µm

A marked reduction of cw threshold current, to as low as 10 mA at 25°C, was achieved in 1.3 µm InGaAsP InP buried heterostructure lasers with optimized stripe widths of 1.5–2.5 µm, resulting in the feasibility of high temperature operation up to 95°C. For the first time, high temperature cw agings at 50 and 70°C were carried out for lasers in this wavelength region, with successful results.

A new assembly architecture for multichannel single mode-fiber-pigtail LD/PD modules

A novel assembly architecture is developed for alignment and soldering between single-mode (SM) fiber arrays and laser-diode/photodiode (LD/PD) arrays. The image position detection method, by eliminating degrees of freedom for alignment, improves assembly throughput to twice that of conventional assembly architecture. The thermal shrinkage compensation method achieves high-precision soldering by canceling misalignments due to thermal shrinkage of assembly equipment and solder volume. As a result, assembly, through all processes, achieves +or-1.5- mu m precision, which corresponds to a +or-1-dB fluctuation of coupling efficiency. Based on this architecture, eight-channel SM-fiber-pigtail LD/PD modules were successfully assembled.<<ETX>>

Thermal stability of various ball-limited-metal systems under solder bumps

Thermal stability is discussed of ball-limited-metal (BLM) layers formed under controlled-collapse solder (lead-5 wt.% tin) bumps for flip-chip interconnections. All BLM systems used here consist of a triple-layer deposit of Cr or Ti as an adhesive; Ni, Mo, Pd, or Pt as a barrier; and Au as a surface metal. Using test chips with these BLM systems, mechanical pull-strength values of solder-bump joints are obtained. These values are associated with thermal phenomena at the interface between the BLM and solder layers. The chips with Ti adhesive layers show generally superior solder-joint strength to those with Cr layers, independently of the barrier metal used. This is due to the presence of a joint fracture mode closely related to the kind of adhesive metal. Among the BLM systems examined, Ti/Ni/Au is found to provide the most reliable solder joints. This is successfully utilized as a BLM system for GaAs devices.<<ETX>>

Fluxless and virtually voidless soldering for semiconductor chips

A novel soldering technique using a solder washer preform has been developed. A conventional vacuum-release process is used in the soldering procedure. That is, after curing of the assembly in vacuum ( approximately 1 torr) the fused washer preform wets the specimens (e.g. chip and heat sink) to seal up the through-hole area of the washer preform with approximately 1 torr. Subsequently, the vacuum is released to make the fluid pure solder flow into the through-hole area in such a manner that most of the oxide film of the washer preform is destroyed. It is demonstrated that the procedure results in an ideal bond with pure solder in a vacuum, without using flux. It also demonstrated that the above technique can be used to reduce voids to less than 5% on average in solder bonds with 1-cm/sup 2/ chips or 3-in wafers. Improved solder bond strength due to the new technique is also confirmed. >

Reliability of InGaAsP/InP Buried Heterostructure Lasers

Degradation modes of InGaAsP/InP buried heterostructure lasers are classified into three types; rapid, saturable, and gradual degradation. The influences of the temperature dependence of threshold current and the carrier leakage through the buried regions on degradation are described. Lasers that can operate cw above 90°C and show only gradual degradation can be expected to have a mean-time-to-failure of more than 107 h at 25°C.

High-Reliability Semiconductor Lasers for Optical Communications

InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm have continued to operate stably for more than 3.3 \times 10^{4} h (3.8 years) at 50-60°C and at an output power of 5 mW/facet. A statistically estimated median lifetime exceeds 106at 50°C. A relatively low activation energy of 0.32 eV is obtained for slow degradation. The saturable behavior of the aging characteristics is observed in many of the lasers. This mode is explained by the increased leakage current through the buried regions, and can be eliminated by electroluminescence (EL) mode aging at high temperature and current. Distributed feedback (DFB) lasers emitting at 1.55 μm are also subjected to accelerated aging at 60°C with a 3 mW/facet output after EL-mode aging. These DFB lasers demonstrate stable aging characteristics, for more than 2000 h of operating time being currently achieved.

Compact Two-Beam Head with a Hybrid Two-Wavelength Laser Array for Magneto-Optic Recording

A compact two-beam optical head using a hybrid two-wavelength laser array is demonstrated for real time verification in a magneto-optic disk. The head height is 10mm. This head uses an aspherical plastic focusing lens and new compound polarized beam splitter ( PBS ) detection optics for detecting spot position signals and magneto-optic signals with the same optics making the head more compact.

Degradation of Ga1−xAlxAs visible diode lasers

It is shown that Ga1−xAlxAs visible lasers degrade faster at shorter lasing wavelengths. The degradation in the wavelength region below 730 nm can mainly be attributed to the rapid formation of macroscopic defects in the active region. A notable improvement in life is obtained for these shorter wavelength lasers by Te‐doping of the active layer. On the other hand, degradation at above 740‐nm results from enhanced facet oxidation due to high AlAs mole fractions in the layers. Facet coatings using SiO2 films effectively suppress facet oxidation. As a result, room‐temperature extrapolated lives exceeding 10 000 h are achieved in the wavelength region above 740 nm.

InGaAsP/InP Laser Diodes Mounted on Semi-Insulating SiC Ceramics

Semi-insulating SiC ceramics is proposed as a new material for InGaAsP/InP laser diode heatsinks. It is featured by high theraml conductivity (2.7 W/cm deg) and high electrical resistivity (4×1013 Ohm cm). The expansion coefficient of the material is 3.7×10-6 deg-1, which is very close to that of InP. The electrical and thermal characteristics, and reliability of InGaAsP/InP buried heterostructure (BH) lasers mounted on SiC heatsinks are also discussed. Stable modulated operation of up to 1 GHz, a maximum temperature for cw lasing of up to 100°C, and an expected median lifetime of 3×104 hrs at 60°C have been demonstrated for these lasers.