JPS61155776A - Method and apparatus for estimating life of light emitting diode - Google Patents

Abstract

PURPOSE:To elevate the accuracy of screening with the estimation of the life, by non-destructively measuring the emission delay time, injection current and junction capacitance of a light emitting diode to be measured. CONSTITUTION:A square wave low current pulse is outputted from a current pulse application circuit 4 to be inputted into a light emitting diode 6 to be measured through a switching circuit 2 and light is emitted at a specified cycle to be received with a light receiving element 7. A measuring circuit 8 measures the delay time between the drive current inputted into the light emitting diode 6 being measured and a photocurrent received with an APD7 to be memorized into a computer 1. Then, the junction capacitance at non-bias of the light emitting diode 6 being measured is gauged with a semiconductor capacitance meter 3. It is found that the delay time is in proportion to the junction capacity with a strong correlation. The estimation of a correlation curve (l) by regression analysis or the like with the computer 1 reveal that deviation from the correlation curve (l) means variations in the defect density among elements. So to speak, samples on the correlation curve (l) indicate the average life among all of those measured and thus, those samples farther above the solid line (l) have the shorter life.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method and device for estimating the lifespan of light emitting diodes.The method and device according to the present invention can screen light emitting diodes without destroying them, and provide high quality ,
Since a highly reliable product can be provided, it can be used for screening light emitting diodes used in communication light sources, optical equipment light sources, etc. that require high reliability.

[Conventional technology]

Conventionally used screening methods for light emitting diodes include destructive testing, in which samples are taken from a large number of samples based on predetermined sampling inspection criteria, and then subjected to crystal analysis to determine the size of crystal defects. Accelerated energization tests, etc., are commonly used to determine the state of deterioration by applying an accelerated load due to high temperature or overcurrent for a certain period of time. In addition, as a method for non-destructively screening light emitting diodes, a screening method using a life estimation method based only on the light emission delay time has been announced (1971).
This method pursues the time dependence of the non-radiative recombination lifetime and investigates the non-radiative recombination region depending on the size of crystal defects existing in the pn junction of a light-emitting diode. Focusing on the fact that the effect causes a delay time from current injection to light emission, and since the crystal defect concentration after energization for a predetermined time is proportional to the initial crystal defect concentration, lifetime estimation is performed based on the initial defect concentration and screening is performed. That is, in practice, a circuit for measuring the light emission delay time is provided, and a nondestructive test is performed by measuring and screening the initial light emission delay time depending on the initial defect concentration.

[Problem that the invention seeks to solve]

The above-mentioned destructive test or accelerated energization test has the following problems. That is, as a measurement of the size of crystal defects, DLT
S measurement, mobility measurement, photoluminescence measurement, etc. are performed, but since these are sampling tests, they are not measured on all products, and also ignore variations in crystal defect concentration among wafers with the same loft. Therefore, the accuracy of screening is not sufficient.Furthermore, the measurement equipment that performs each of the above measurements is very expensive and takes a long time to measure, and various physical quantities of the semiconductor must be investigated in advance. There is.

On the other hand, a simple method is to screen the luminescent pattern by visual inspection, but this naturally has the problem that only non-luminous sites due to identifiable large crystal defects can be discovered, and point defects are overlooked.

Furthermore, although the aforementioned screening by measuring the initial light emission delay time improves the non-destructive aspect, it does not take into account the delay time in the junction capacitance of the light emitting diode.
Further, no correction is made for the fact that the area of the pn junction, on which the junction capacitance and the injection current strongly depend, is non-uniform for each sample.

As a practical matter, it is difficult to manufacture devices in which the shapes (active layer thickness, junction area, etc.) of each light emitting diode chip are exactly the same, so the above-mentioned problems persist and the accuracy of screening is not sufficient.

[Means for solving problems]

The present invention provides a method and device for estimating the lifespan of a light emitting diode that solves the above-mentioned problems.The present invention non-destructively measures the light emitting delay time, injection current and junction capacitance of the light emitting diode under test to estimate the lifespan and perform screening. The purpose of the present invention is to provide a method and apparatus capable of improving accuracy, and the means thereof include a non-destructive life estimation method for a light emitting diode, which applies a square wave low current pulse of a predetermined period to a light emitting diode to be measured. emit light, measure the light emission delay time between the photocurrent obtained by receiving the emitted light with the light receiving element and the square wave low current pulse, and after turning off the square wave low current pulse, the light emitting diode under test Measure the junction capacitance in the non-bias state, correct the variation in the junction area of the light emitting diode to be measured using the injection current density and the junction capacitance, and based on the obtained light emission delay time, junction capacitance and injection current density. A method for estimating the life of a light emitting diode is provided, which calculates a normalized light emission delay time and estimates the life from the relationship between the normalized light emission delay time and the junction capacitance. The estimation device includes a current pulse applying circuit that applies a square wave low current pulse of a predetermined period to a light emitting diode to be measured, and a photocurrent obtained by receiving light emitted by the light emitting diode to be measured by a light receiving element and the square wave. A light emission delay time measurement circuit that measures light emission delay time with a low current pulse; a semiconductor device meter that measures the junction capacitance of the light emitting diode to be measured when no bias is applied; the semiconductor capacitance meter and the current pulse application circuit; A light emitting diode life estimating device is provided, which includes a switching circuit that switches the delay time and a computer that performs regression analysis or the like to calculate a correlation curve based on the delay time and the junction capacitance.

〔Example〕

FIG. 1 is a block diagram of an embodiment of a light emitting diode life estimation device according to the present invention. In Fig. 1, l is a computer, 2 is a switching circuit, 3 is a semiconductor capacitance meter, 4 is a current pulse application circuit, 5 is a drive circuit, 6 is a light emitting diode to be measured, 7 is a light receiving element, and 8 is a light emission delay time measurement. circuit, 9
is the control panel. In such a configuration, the current pulse application circuit 4 outputs a square wave low current pulse, and this pulse is input to the light emitting diode 6 to be measured via the switching circuit 2. The light emitting diode 6 to be measured emits light at a predetermined period according to this pulse, and the emitted light pulse is received by a light receiving element 7, for example, an avalanche photo diode (APD), and the obtained photocurrent is amplified by an APD drive circuit 5, and then The signal is input to the light emission delay time measuring circuit 8. On the other hand, since the output of the current pulse circuit 4 is simultaneously input to the measurement circuit 8, the measurement circuit 8 measures the delay time between the drive current input to the light emitting diode 6 to be measured and the photocurrent received by the APD 7. , the zero-order switching circuit 2, which temporarily stores this result in the computer 1, is switched by a command from the computer 1 to turn off the drive current, and the semiconductor capacitance meter 3 measures the junction capacitance of the light emitting diode 6 to be measured when no bias is applied. .

Based on the measured delay time and junction capacitance, a computer calculates the delay time converted to a constant injection current density, which will be described later, in order to electrically correct the dimensional deviation of each element during wafer cutting.
Calculated by

Figure 2 shows the injection current density J and the light emission delay time T.

FIG. 3 is a graph showing the relationship between the junction capacitance C4 and the normalized light emission delay time T0'. Now, when the delay time and junction capacitance described above are measured for a plurality of samples having the same loft, there is a proportional relationship between the two as shown in FIG. 3, and a relatively strong correlation. When this correlation curve l is estimated by regression analysis etc. by computer 1, this correlation mill &I
The deviation from z means the magnitude of the defect concentration of each element. That is, the samples lying on the correlation curve E indicate the average lifespan among all the measured samples. Furthermore, when the injection current density is evaluated to compensate for the difference in the junction area of each element, it has been found that there is an exponential relationship as shown in FIG. Here, the light emission delay time T, and the injection current density JF
has the following relationship.

That is, To=ToeXp (-B-JF)+TL+73
(1) Here, TL is the delay time due to crystal defects at the pn junction, T is the delay due to the measurement system and is a value that is considered constant regardless of the sample, and A and B are constants. Since the pn junction area is considered to be simply proportional to the junction capacitance in the same lot, the injection current density Jr to the pn junction can be determined from the junction capacitance, so the injection current density Jr is constant in equation (1). Calculate the normalized light emission delay time TD' corrected as follows and compare each element. That is, T, +'xA-exp, (-B-D-CJ)+TL
+T3.=(2) Here, D is an element constant. The correlation between T o ' and CJ is as shown in FIG. In Figure 3, when 'ra' and 01 of each sample are plotted, the correlation curve shown by the solid line as mentioned above! The sample on the top shows the average life time of all the samples being measured, so it is a solid line!
It can be assumed that samples located above this point have very short lifetimes. The data in this case is junction capacity 1t30~50ρF
Measurements were made on a visible light emitting diode with
MHz was used. As a result of initially determining the Tll' CJ correlation and applying accelerated current using temperature and current, only the sample located above the actual vAl in FIG. 3 showed a decrease in luminescence amount after 6000 hours had elapsed. This lifetime estimation method is effective as the active layer of the light emitting diode becomes thinner, that is, the more the extracted light reflects the light emitted from the entire active layer (Aj!
In the case of GaAs materials, it is approximately 10 μm or less due to reabsorption)
Belief in people becomes stronger. Also, the difference in TD' of each sample is significant as it shows a low current density of about IA/cd.

〔Effect of the invention〕

According to the present invention, it is possible to estimate the lifespan of a light emitting diode non-destructively and in a short time, and since the lifespan is estimated from correlations obtained by measurements of all samples, it is possible to easily estimate the lifespan of a light emitting diode without investigating any physical quantities of the semiconductor. The accuracy of screening can be greatly improved by electrical measurement, and since the influence of subjectivity of inspection iJ such as visual inspection can also be eliminated, reliability can be greatly improved.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the light emitting diode life estimating device according to the present invention, Fig. 2 is a graph showing the relationship between injection current density and light emission delay time, and Fig. 3 is a graph showing junction capacitance and normalization. It is a graph showing the relationship with light emission delay time. (Explanation of symbols) 1...Computer connection 2...Switching circuit, 3...Semiconductor capacitance meter, 4...Current pulse application circuit, 5...Drive circuit, 6...Light-emitting diode to be measured , 7... Light receiving element, 8... Light emission delay time measuring circuit, 9... Operation panel.

Claims (1)

[Claims] 1. A non-destructive life estimation method for a light emitting diode, which involves applying a square wave low current pulse of a predetermined period to a light emitting diode to be measured to cause it to emit light, and receiving the emitted light with a light receiving element. The light emission delay time between the obtained photocurrent and the square wave low current pulse is measured, and after the square wave low current pulse is turned off, the junction capacitance of the light emitting diode under test at no bias is measured. The variation in the junction area of the light emitting diode is corrected by the injection current density and the junction capacitance, and the normalized luminescence delay time is determined based on the obtained luminescence delay time, junction capacitance and injection current density, and the normalized luminescence delay time is calculated. A method for estimating the lifespan of a light emitting diode, which estimates the lifespan from the relationship between and the junction capacitance. 2. The method according to claim 1, wherein the light emission delay time is measured separately from the delay time caused by the junction capacitance and the measurement system. 3. A non-destructive life estimating device for a light emitting diode, which includes a current pulse application circuit that applies a square wave low current pulse of a predetermined period to a light emitting diode to be measured, and a light receiving element that receives light emitted from the light emitting diode to be measured. A light emission delay time measurement circuit that measures the light emission delay time between the obtained photocurrent and the square wave low current pulse, a semiconductor capacitance meter that measures the junction capacitance of the light emitting diode to be measured in the absence of bias, and the semiconductor capacitance. 1. A light emitting diode life estimation device, comprising: a switching circuit that switches between a meter and a current pulse applying circuit; and a calculator that performs regression analysis and calculates a correlation curve based on the delay time and the junction capacitance.