A simpler method for life-testing laser diodes

MICROELECTRONICS RELIABILITY MicroelectronicsReliability 39 (1999) 1067-1071

PERGAMON

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A simpler method for life-testing laser diodes ~ . . _V a n z i M

. a'

• a A.Bonfiglio a, M.Licheria, R.D , Arco, a G.Martlnes, G . S a l r n i n i u, R . D e

Palo u

a University of Cagliari, D I E E - INFM 09123 Cagliari ITALY b Pirelli Caw e Sistemi S.p.A., Viale Sarca 202, 20126Milano ITALY

Abstract

The procedure of measuring the I(V) characteristics of laser diodes at fixed time steps of a constant current life-test is revised. The possibility of using the test equipment itself to extract the characteristics is investigated and demonstrated. This eliminates the most troublesome manual procedure in life-testing several devices. © 1999 Elsevier Science Ltd. All rights reserved.

I. I n t r o d u c t i o n

In a recent paper [1], a new method has been proposed for the analysis of data in life-tests of laser diodes, and applied to InGaAs/AIGaAs 980 nm SL SQW pump devices for Erbium Doped Fiber Amplifiers. The method has the advantage of plotting some very simple experimental curves, directly related to the measured standard I(V) characteristics, whose shift is a straight indication of the change of several important parameters of the laser device: the threshold current Ith, the voltage clamp Vz that sets up across the active region at lasing condition, the overall series resistance ~ , the ideality factor I"1 and the saturation current I~ of the diode that play their standard role in the under-threshold range, according to the Shockley law. The implementation of such a method, anyway, pointed out a very simple practical problem: life-tests are driven by current sources, whose voltage control is inaccurate in the low IV range. This means that all the proposed elaboration, based also on differentiating the I(V) curve, require finer sampling of I-V data than that available from normal procedure.

This is the very same problem of standard life-test methods: the I(V) characteristics are taken at the selected time-steps by disconnecting each laser from its current supply and connecting it to a parameter analyzer of outstanding accuracy. Even for few tens of devices under test, this procedure is quite timeconsuming, or requires huge control instrumentation for complete automation. A careful review of the ideal electrical response of the laser diodes suggests a way for obtaining the same information under current control with an accuracy suitable for reliability evaluation purposes, without requiring a current accuracy exceeding that of a standard good current supply (1%). This means that a set of measurements can be proposed, to be executed by the same life-test equipment without disconnecting any device, and leading to the same information degree as the standard measurements. The easy automation of such a system may conveniently and completely eliminate any manual intervention for the whole duration of the test. The paper describes the principles of the measurement method, and applies it to the

0026-2714/99/$ - see front matter. © 1999 Elsevier ScienceLtd. All rights reserved. PII: S0026-2714(99)00148- 1

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M. Vanzi et al. / Microelectronics Reliability 39 (1999) 1067-1071

experimental characteristics of a life-tested laser diode at three test steps. The main blocks of a dedicated instrument, able to manage both the lifetest and the time-step measurements, will finally be indicated.

\0rl/

Current driven measurements

During life-tests of laser diodes three quantities are available for continuous monitoring: the emitted optical power P(I), the driving current I and the voltage V of the operated laser diode. The first is given by reading the photocurrent induced by the optical radiation into a suitable photodiode, while the other two are directly available from the laser itself. Depending on the constant-current or constantpower mode, only two of the three quantities may display time variations, which are the signature of the occurring degradation. Anyway, because of the many mechanisms which can lead to the same effect (less optical power for a given current), it is usual to stop the test at defined time steps, in order to perform complete dc characterization of the devices, both at test and at room temperature. Voltage V is swept from 0V to a forward value sufficient to lead the laser deeply into the stimulated emission condition, and I(V) is measured, and then the optical characteristics P(I) are taken by sweeping the current I. The detailed analysis of the obtained characteristics gives the values of a number of other quantities: the optical efficiency a=dP/dI, the threshold current lth and the threshold voltage Va, at which the stimulated emission is switched, the series resistance R~ that remains the only limit to the forward current for V>Vth, and the saturation current Ix and the ideality factor rl which represent the "Shockley side" of the laser diode for currents lower than lth. Equations are available for the simple model of a laser diode [1], made of the parallel of the Shockley diode and a reversed Zener diode, fired at the internal voltage threshold Vz of the laser. The series resistance ~ completes the model. The Zener plays the role of the voltage clamp that actually sets up at the edges of the ideal junction, once the stimulated emission starts, due to the "freezing" of the quasiFermi levels. The threshold condition then separates the behavior &the device into two branches:

for |<]th

( l)

at threshold

(2)

IIth = I z +Rslth

I = Ith + I z

V = Vz + R s l

for I>Ith

(3)

P = a ( I - Ith)

Here, the voltage V L of eq. 1 indicates the internal voltage drop, that is the actual bias &the ideal diode, and is related to the external voltage through the sum of the ohmic drop. Over threshold, it is clamped to Wz.

It is of course a simple matter of data fitting to find the values of the indicated quantities, once I(V) and P(I) are measured. Anyway, it is always advisable to have also graphical plots of the evolving characteristics, to check at a glance that the simple model has not changed. It happens, in practise, that some degradations correspond to the onset of parasitic elements in the ideal model (the most classical, the junction perforation, leading to a shunting ohmic path), in which case any automated parameter extraction, based on the initial model, should be wrong. In any case, the collection of accurate I(V) measurements results as the most time consuming operation: an operator has to disconnect each laser under test from the test equipment, and then perform the measurement by means of a separate instrument, able to set voltage steps as fine as 10 mV and to read currents ranging from pA to hundreds of mA When some tens of devices are employed, this operation may be quite long. Reversing the point of view, a different way for obtaining the same information with less or no manual operation is to start from the available source for the test itself, a current source, and investigate if there is the possibility of drawing plots and extracting parameters from V(I) and P(I) data, detailed enough to reach a suitable precision. Here the problem may be the accuracy of the current source that for good commercial equipment hardly exceeds 1%. It is obvious that, once experimentally verified, this approach will indicate how to build-up a test system,

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M. Vanzi et al. / Microelectronics Reliability 39 (1999) 1067-1071

based on commercial electronics, and a measurement procedure able to completely control a life-test in an automated way. On the plot side, three curves are here proposed: 1. the simple derivative dV/dI

dVIdl (9)

10

8 6 4

dV kT l al = R , + r I 7 } -

I
dV

I>Ia~

di =R~

I

0 0

whose high-current value gives the series resistance

(A)

|

|

,

,

0.02

0.04

0.06

0.08

Fig. 1 The series resistance measurement 2. the logarithmic derivative I dV/dI, that draws two parallel segments,

I

l d V = R , l + ~ 7 kT dl ' q Id-~] =RsI

[ dI

In fig. 1, the horizontal straight line, common to all measurements, corresponds to a series resistance R~=2.2 ~.

IIa~

10

IdVIdl .qlkT

8

which allows to measure the ideality factor q from the sub-threshold segment. 3. the reduced-voltage plot V- l~I, which stucks at the internal clamp voltage Vz as the laser action is fired

t LV-R,I

%

0 I"

•

0

"Tln(I/ = v z

2."

,

,

0.02

,

,

0.04

,

,

0.06

, 0.08

Fig. 2 Measurement of r1and lth. I >ith

Under the mathematical point of view, of course no difference in knowledge comes from using a function or its inverse, and what I(V) reveals will also come from V(I). It is only the experimental field that will state the feasibility of each approach. On a practical sound, plots of experimental data, processed according to the above indications, must give evidence for straight lines where expected by the theory.

In fig. 2 the low-current segment keeps the same slope, and its linear backward extrapolation onto the vertical axis gives q=2, the correct optimal value for the ideality factor of an optical emitter, for which recombination must dominate over diffusion currents (q--l). 1.6

1.4

V-R,I (V)

Vz r~

Experimental results 1.2 , f - - J

Fig. 1, 2 and 3 report the experimental plots of the indicated expressions, obtained from a single specimen during a constant-current life-test. The bold line is the starting one, the dashed line the middle, and the thin solid line the last measurement.

I IA)

1 0.02

0.04

Fig. 3Measurementof~.

0.06

0.08

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M. Vanzi et al. / Microelectronics Reliability 39 (1999) 1067-1071

In fig.3, the clear horizontal line at 1.30 V states a separation of the quasi-Fermi levels of an amount slightly exceeding the 1.267 eV characteristics of 980 nm optical emission, in good agreement with the fundamentals of semiconductor laser action. It is relevant the clear detection of an upward displacement, corresponding to some mV, at the last step. The curves give evidence for a mechanism which does not involve the series resistance, but includes a twofold increase of the threshold current: first by keeping the clamp voltage constant, and then allowing its increase. This reading is exactly the same that has been obtained from the standard I(V) analysis, and its interpretation is known [2] The novelty, here, is that this result comes from V(I) measurements, taken at a current resolution of about 1 mA on a linear range of about 100 mA Standard life-tests, operated at constant current or constant power, employ current supplies which easily fulfill such a requirement of single percent accuracy. It follows that the very same information required for monitoring the life-test evolution may be supplied by the test equipment itself, without more accurate parameter reading at the fixed time-steps.

Equipment specifications A simple equipment is now ready to be designed. It is a standard life-test instrument, able to perform the very same features of commercially available instruments, but also able to execute, if required, the whole life-test, including continuous monitoring of the three fundamental parameters (IL, VL, PL) and also collecting the time-step characteristics required for measuring the threshold current Ith, the series resistance P~, the ideality factor 13, the clamp voltage Vz and the optical efficiency oz. It is based on a main controller (fig.4), made of a standard PC, used to set up the test mode (constant current, constant power or current swing for characteristics), and to read the values of I, V, P onto a set of identical boards, each driving a single laser/photodiode pair. lnitial calibration of the photodiodes gives the conversion factor )' which links the photodiode current Ip to the laser emitted power P.

It interrogates sequentially the boards in a continuous loop during the constant current or power test. A cycle of about 1 second is suitable for checking a set of 64 boards. During the time-step measurement of characteristics it may separately perform the current swing on single slots, leaving the others running at constant test conditions. Of course, when temperature is changed (i.e. for room temperature characteristics) it is advisable to stop the stress of all devices, until the test temperature of the oven is restored. The enormous quantity of collected data (3600 measurements for each device per hour, along some hundreds hours at least) is continuously checked and stored only upon detection of signifcant variations. This seems the only way for solving the puzzle of sudden failures [2], where rapid degradation occurs, within few hours, after hundreds of hours of absolutely regular life. The system may be asked to stop the test for the specific device undergoing to this phenomenon, in order to bring it under a SEM for EBIC live monitoring the final steps of its degradation. This would be the ultimate answer to the question about the direction of propagation of the defects, that up to now have only been detected at the final failure state. Each single board, on the other side, is made (fig.5) of a programmable current supply, able to drive up to I A onto a single laser diode, with 1% accuracy. Current monitors are inserted along both the laser and the photodiode circuit, as well as a voltage monitor, able to read the laser voltage (1+2 V) with an accuracy of l mV, required for appreciating the significant displacements of the clamp voltage. When constant power is required, a feedback loop is activated (position B of the switches in fig.5) for changing the current in order to keep the optical emission level. For constant current tests, this loop is switched off (position A). From the operative point of view, the need of manual intervention is cancelled for the whole test duration. Nothing changes for P(I) measurements, apart from the consideration that they are directly obtained from the photodiode current during the V(I) measurement

M. Vanzi et al. / Microelectronics Reliability 39 (1999) 1067-1071

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sele.xt (const P, cortst I; swing I) set (values)

set y

f o r i - 1,n read (I, V~ I~,)~ Fig. 4 Main blocks o f the test equipment.

Fig. 5 Structure o f each single board.

Conclusions

REFERENCES

The current sources of a life-tester for laser diodes have been proposed for driving both the test and the time-step measured characteristics. Their fair accuracy has been proven to be sufficient for parameter extraction, and three relevant plot formats have been proposed for graphical control. The main improvement of the method, in comparison with the usual procedures, is the feasibility of complete automation, based on standard commercial electronics, with the elimination of any manual intervention.

1. A. Bonfiglio, M.B. Casu, F. Magistrali, R. De Palo, G. Salmini and M. Vanzi: "Early signatures for REDR-based laser degradations". Microelectronics Reliability 38 (1998) 12151220. 2. A.Bonfiglio, M.Vanzi, M.B.Casu, FMagistrali, M.Maini, GSalmini: "Interpretation of sudden failures in pump laser diodes" Proc.23rd International Symposium for Testing and Failure Analysis ISTFA/97, Santa Clara, California, Ottobre 1997, pp. 189-194.