Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1999-02-19
2001-06-19
Arroyo, Teresa M. (Department: 2881)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S760020, C372S043010
Reexamination Certificate
active
06249140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of screening a semiconductor laser device and, more particularly to a method of screening capable of rejecting a semiconductor laser device which fails due to sudden deterioration.
2. Description of the Prior Art
FIG. 3
shows the result of an endurance test of InGaAs/AlGaAs semiconductor laser devices conducted by G. Beister et al. (G. Beister et al. “Monomode emission at 350 mW and high reliability with InGaAs/AlGaAs (&lgr;=1020 nm) ridge waveguide laser diodes”, Electron. Lett., vol.34, pp.778-779, 1998). The endurance test was conducted under conditions of constant ambient temperature of 40° C. for the semiconductor laser device with such a driving current that produces constant optical output of 300 mW being supplied. In the drawing, numeral
101
denotes an initial failure device,
102
denotes a sudden failure device,
103
denotes device showing stable operation. Among ten devices subjected to the endurance test, seven devices (
103
) showed stable operation over 1000 hours. Two (
101
) of the ten devices showed initial failure with the operating current increasing at an early stage, and the remaining one (
102
) showed sudden failure without prior sign of deterioration. While the sudden failure in this test occurred after 180 hours of operation, sudden failures in general can take place at any time.
With a method of screening of the prior art which is conducted by keeping a constant optical output of semiconductor laser devices, for example, initial failure devices (
101
) can be rejected but sudden failure devices (
102
) cannot be rejected. Consequently, there has been a limitation to decreasing of failure rate of final products which have passed screening, resulting in such a problem that the devices cannot be used in the field of communication which requires high reliability.
The present inventors have found that the sudden failures are caused mainly by the breakdown of the active layer of the semiconductor laser device due to the migration of crystalline defects. The present inventors have also found that sudden failure of devices due to such crystal defects can be removed by screening semiconductor laser devices while supplying a driving current to the semiconductor laser devices and applying thermal cycles thereto, thus completing the present invention.
SUMMARY OF THE INVENTION
That is, an object of the present invention to provide a method of screening semiconductor laser devices wherein sudden failure devices as well as initial failure devices can be rejected.
The present invention provides a method of screening for rejecting semiconductor a laser device which fail due to sudden deterioration from a semiconductor laser device comprising semiconductor layers of different conductivity types, a semiconductor laser device comprising semiconductor layers of different conductivity types and having a regrowth interface formed through a plurality times of crystal growth, a semiconductor laser device comprising semiconductor layers of different conductivity types and having a GaAs substrate, or a semiconductor laser device comprising semiconductor layers of different conductivity types and having an InGaAs active layer and a GaAs substrate, said method being characterized by applying thermal cycles wherein the semiconductor laser devices are kept alternately in a high temperature state and a low temperature state, while supplying a driving current.
When subjected to thermal cycles with the driving current being supplied, deterioration of the device due to the optical output is accelerated during the low temperature period and deterioration of the device due to the driving current is accelerated during the high temperature period, while the deterioration may be further accelerated by changing the temperature thereby causing thermal strain in the semiconductor laser device. This method makes it possible to reject sudden failure devices, supposedly caused by migration of crystal defect, as well as initial failure devices, thereby increasing the rate of defect screening.
According to the result of the present inventors' research, it is known that sudden failure devices are difficult to screen out either by screening based on thermal cycles only or by screening based on driving current only. In other words, it is made possible to remove sudden failure devices only by applying both the thermal cycles and the driving current to the devices as the load.
The thermal cycle preferably comprises high temperature periods during which the temperature is raised to the highest at which a solder having the lowest melting point among the soldering materials used in the device does not melt, and low temperature periods during which the temperature is lowered to the minimum possible at which dew condensation does not occur on the surface of the semiconductor laser device.
Such thermal cycles make it possible to screen the semiconductor laser devices without giving damage thereto.
The present invention also provides a method of screening semiconductor laser devices by gradually increasing the driving current.
By gradually increasing the driving current, it is made possible to accelerate the deterioration while changing the thermal strain caused in the semiconductor laser devices, thereby improving the effect of screening.
The driving current is preferably increased gradually up to near a current level which causes a wear failure, from an initial value of current at which initial failure is detected.
By screening the semiconductor laser devices while increasing the driving current to a value near the level that causes wear failure, sudden failure devices can be screened out.
The driving current may also be increased gradually until threshold current begins to decrease after the threshold current of the semiconductor laser devices has been detected.
The driving current may also be increased gradually until the threshold current begins to increase again after the threshold current of the semiconductor laser devices has been detected and decreased.
The present invention also provides a method of screening the semiconductor laser devices by maintaining the driving current at a constant level not less than the current at which initial failure is rejected when the driving current is gradually increased.
Sudden failure devices can also be rejected by applying thermal cycles while maintaining the driving current at a constant level not less than the current at which initial failure is rejected.
The constant current level described above is preferably near the current level at which wear failures begin to occur when the driving current is gradually increased. By supplying such a relatively high driving current, the effect of screening can be improved.
The constant current level described above may also be the current level at which the threshold current of the semiconductor laser device begins to decrease when the driving current is gradually increased.
The constant current level described above may also be the level of the driving current in a range where the threshold current of the semiconductor laser device begins to increase after decreasing, while gradually increasing the driving current.
As will be apparent from the above description, the present invention makes it possible to effectively remove such devices that fail due to sudden deterioration and cannot be removed with the screening method of the prior art, as well as the initial failure devices, thus reducing the failure rate of devices significantly.
Consequently, it is made possible to greatly improve the reliability of the devices which have undergone screening.
Particularly, according to the present invention, it is made possible to remove such devices that fail due to sudden deterioration after the start of wear failures, thus further reducing the failure rate of devices.
REFERENCES:
patent: 4573255 (1986-03-01), Gordon et al.
patent: 5835516 (1998-11-01), Miyashita et al.
patent: 6043872 (2000-03-01), Nagata
Beister et al.
Arroyo Teresa M.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Monbleau Darienne
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