JP3450001B1 - LED deterioration inspection method - Google Patents

Abstract

[PROBLEMS] An LED having deterioration due to electrostatic breakdown and cracks
Is easily and reliably sorted. [MEANS FOR SOLVING PROBLEMS] LED with a weak constant current of 1μA or less
The first voltage is measured by applying the voltage in the forward direction, and those below a certain value are excluded. Next, a current about 10 times the current is applied, the second voltage is measured, and the final selection is performed based on the value of the difference from the first voltage. 2. A plurality of constant currents in the range of 0.01 μA to 1 mA are supplied to the LED, and selection is performed depending on whether the voltage at that time is proportional to the logarithm of the current value or whether the voltage is distorted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting a light emitting diode (LED), and more particularly to a method for inspecting a deteriorated product due to electrostatic discharge (ESD) or a crack caused by mechanical shock.

[0002]

2. Description of the Related Art As a general inspection method for selecting defective products in an LED manufacturing process, a current (on the order of milliamperes (mA) or more) close to the current value required for lighting is applied in the forward direction of a PN junction, and a voltage is applied. This is done by measuring the magnitude of the descent. However, this inspection is an inspection for confirming the characteristics of the PN junction, and a deteriorated product due to electrostatic breakdown or crack cannot be detected. Therefore, as a method for inspecting deteriorated products due to electrostatic breakdown or cracks,
There is a method in which a reverse voltage is applied to the LED and inspection is performed depending on the presence or absence of a leakage current generated at that time. However, even in this method, the correlation between the presence or absence of leakage current and deterioration is unstable and reliability is low, and it is difficult to reliably detect a deteriorated product. One way to solve these problems is
The method of Japanese Patent Laid-Open No. 2002-156402 has been proposed. In this method, a current for measurement of about 1 to 10 μA is passed in the forward direction of the PN junction, a current for heating the junction is passed in a pulse shape, and deterioration is determined by a change in voltage at two points at different temperatures. It is a thing.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method shown in No. 402 requires a heating current separately from the measurement current, and it is not easy to set the temperature. The present invention seeks to detect deterioration easily and reliably without the need for such special means.

[0004]

SUMMARY OF THE INVENTION The present invention is directed to electrostatic breakdown,
An LED that has deteriorated due to cracks caused by mechanical impact is unique near a weak current (about 0.1 microamperes), which is about 1 / 10,000 of the current normally used for LED inspection and lighting (on the order of milliamperes). It has been discovered that it exhibits excellent current-voltage characteristics, and deteriorated LE using this
It proposes the inspection method of D.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the basic configuration of an LED inspection device used to carry out the method of the present invention. In FIG. 1, reference numeral 1 denotes a constant current power supply, which has a capability of variably supplying a constant current of the order of microampere (μA) to milliampere (mA) to the outside.
The LED is a light emitting diode to be inspected and is attached to the inspection device so as to be easily attached and detached. Rp is a virtual resistance generated when the LED to be inspected is deteriorated. In the case of an ideal LED having no deterioration, Rp does not exist, but when deterioration occurs, Rp has a resistance value of the order of megohms (MΩ). 2 is a high input resistance buffer amplifier, 3 is AD
The converter, 4 is a control / arithmetic circuit, which is the current I of the constant current power supply 1.
It is a combination of a control circuit for controlling f and a calculation circuit for calculating the forward voltage value VF and comparing it with a reference value. If represents the current applied to the LED from the constant current power supply 1. VF represents the forward voltage between the LED terminals.

The voltage (v) and current (I) characteristics of the PN junction of an ideal LED having no deterioration, that is, no Rp, can be expressed by the following general formula. _{I = Io {exp (eV /} K B · T) -1} where, e: charge amount _{K B:} Borumman Factor T: absolute temperature voltage V is proportional to the logarithm of the current I. On the contrary, when Rp is present and its value is lower than the normal value, the proportional relationship shown in the above formula cannot be maintained. The inventor is LED
When a weak current of about 0.1 μA was applied to the device, it was found that this tendency was particularly remarkable and was confirmed by experiments. The present invention focuses on this point and detects an LED deterioration product.

The graph of FIG. 2 is a graph for explaining the above relationship. This graph is obtained by the device shown in the block diagram of FIG.
It is obtained as a result of measuring a plurality of LEDs. The vertical axis represents the voltage VF and the horizontal axis represents the current If on a logarithmic scale. The line a on the graph represents the characteristics of a normal (almost no deterioration) LED. When the current is increased from 0.01 μA to 10 mA by the constant current power supply 1, the voltage VF also increases almost linearly. The curves represented by the dotted lines b, c, d are the characteristics of the deteriorated LED. The degree of deterioration of b is small, the degree of deterioration of d is large, and c is in the middle.

As is clear from this graph, 0.1 μA
In the applied current of, there is a significant difference in the voltage VF between a and b, c, d. However, when the current exceeds 0.1 mA, they all converge to the straight line a, and no significant difference occurs. Therefore, by measuring VF1 when a weak current of about 0.1 μA is applied, the first selection between the normal product and the abnormal product can be made. If VF1 having a VF1 of 2.0 V or more is input as a reference value for acceptance into the arithmetic circuit 4 shown in FIG. 1, only a is a passed product, and b, c and d are rejected products. If VF1 of 1.8 V or more is input as the reference data as the reference value of the acceptance, a and b are acceptable products and c and d are unacceptable products. In the invention of claim 1, this is the first inspection step. Optimum current value of If and V
The optimum voltage value of F varies depending on the standard and model of the LED to be measured, so it is difficult to specify a specific value,
One of the features of the present invention is that the current used for measuring the quality of the PN junction is usually 1 mA or more, while the weak current of 1 / 10,000 or less thereof is used.

Next, with respect to the products judged to pass in the first inspection step, the control / arithmetic circuit 4 instructs the constant current power supply 1 to increase the current value to about 1 μA, whereby The resulting voltage VF2 is measured. Since VF1 is stored in the A / D converter 3,
The difference between VF2 and VF1 is calculated in the control / arithmetic circuit 1, and the difference is compared with the reference data, and those exceeding a certain value are judged as rejected products. For example, if VF1-VF2 is 0.
When 3 V or less and standard data are given, a is a pass and b is a fail. In the invention of claim 1, this is the second inspection step. According to the first aspect of the invention, the first inspection step performs the first-stage selection, and the second inspection step performs the second-stage selection for those that have passed the first-stage selection. It has the feature that it can accurately and reliably determine deteriorated products that were difficult to sort.

As is clear from the mathematical expression described in the paragraph “0006” and the graph of FIG. 2, the LED with less deterioration is 0.1 μA.
The characteristic is that the current-voltage characteristic changes almost linearly (proportionally) over the range from 0.1 mA to 0.1 mA. Therefore, it is apparent that the LED can be determined to be normal if a current of 0.1 μA to 0.1 mA is stepwise applied to the LED to be inspected by a factor of 10 and the VF at that time increases substantially linearly. . The invention of claim 2 relates to this method.

Specifically, the same device as shown in FIG. 1 is used,
A current of 0.1 μA is first applied to the LED from the constant current power supply 1, and the VF at that time is measured by the A / D converter 3 and stored. Then, according to an instruction from the control / arithmetic circuit 4, a 10 times larger current of 1 μA is passed and the voltage VF at that time is measured. Similarly, the current is increased by 10 times, and it is calculated whether or not VF is increased proportionally. If this value is out of the reference value, it is determined as a rejected product.

The above two methods are effective even if they are used alone, but for example, the method shown in claim 1 and the method shown in claim 2 may be used together, and in that case, they are more accurate and accurate. High quality inspection can be performed.

[0013]

According to the method of the present invention, it is possible to accurately and easily detect the deterioration of the LED, which has been very difficult to measure in the past, so that the inspection accuracy of the LED can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an LED inspection device.

FIG. 2 is a graph showing the results of measuring a plurality of LEDs by the device shown in the block diagram of FIG.

[Explanation of symbols]

1 --- Constant current power supply LED: Light emitting diode to be inspected Rp ... ・ ・ ・ Virtual resistance 2 ... High input resistance buffer amplifier 3 ・ ・ ・ ・ ・ ・ ・ ・ A-D converter 4 Control circuit If ・ ・ ・ ・ ・ ・ Current applied to LED VF ・ ・ ・ ・ ・ ・ Forward voltage between LED terminals

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 33/00 G01R 31/26

Claims (2)

(57) [Claims] 1. A constant current of 1 μA or less is flowed from a constant current power source in the forward direction of a PN junction of an LED to be inspected, and a voltage (VF ₁ ) between terminals of the LED to be inspected is measured by an AD converter. They are stored together, in the control and operation circuit VF _1, compared to a reference value that is entered in advance, if the VF ₁ determines that the range of the reference value, raise the current to approximately 10 times the constant current source , The voltage between the terminals of the LED to be inspected (VF ₂ ) is measured by the A / D converter while the current of the LED to be inspected is increased to about 10 times, and VF ₁ is set in the control / arithmetic circuit. A method for inspecting deterioration of an LED, which calculates a difference in VF ₂ and judges that the value is within a range of a reference value input in advance to a control / arithmetic circuit as a pass. 2. A constant current of 0.01 μA or more is caused to flow from the constant current power source in the forward direction of the PN junction of the LED to be inspected, and the voltage (VF ₁ ) between the terminals of the LED to be inspected is converted by an AD converter. are stored together with the measuring, control and operation circuit VF _1, compared to a reference value that is entered in advance, if the VF ₁ determines that the range of the reference value, about 10 times a current to the constant current source , The voltage between the terminals of the LED to be inspected (VF ₂ ) is measured by the A / D converter while the current of the LED to be inspected is increased by about 10 times, and the control / arithmetic circuit controls the VF. The difference between ₁ and VF ₂ is calculated, and when the value is within the range of the reference value input to the control / arithmetic circuit in advance, the constant current power source is further supplied with about 1 current.
It is instructed to increase it to 0 times, and this is repeated until the current value is 10 mA or less, and it is determined that the voltage value is linearly proportional to the logarithmic value over the entire range. LED deterioration inspection method.