JPH0864652A - Epitaxial wafer inspection method - Google Patents

Abstract

(57) [Summary] [Purpose] To accurately evaluate the light emission output and response speed in the state of the epitaxial wafer. [Structure] The surface of an epitaxial wafer is irradiated with a pulse of laser light. The wavelength of the laser light is selected so that it is not absorbed by the surface of the epitaxial wafer but is absorbed by the active layer. Photoluminescence light is emitted from the epitaxial wafer by pulsed light excitation. The emitted photoluminescence light is converted into an electric signal. The photoluminescence lifetime is obtained from the time change of the waveform of this conversion signal. The variation in the light emission output of the light emitting diode causes the variation in the internal quantum efficiency. If the emission lifetime is τ _r , the internal quantum efficiency η _i and the lifetime τ _ph are in the relationship of η _i = A × (1 / τ _r ) × τ _ph (where A is a proportional constant). is there. From this, the luminous intensity of the light emitting diode can be obtained from the lifetime.

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 an epitaxial wafer, and more particularly to a method for inspecting optical characteristics of products such as light emitting diodes at the epitaxial wafer stage.

[0002]

2. Description of the Related Art In order to manufacture a light emitting diode, an epitaxial layer serving as a light emitting layer is grown on a crystal substrate,
Electrodes are formed on the front surface and the back surface of the epitaxial wafer, and then they are separated by dicing to manufacture bare chips. The bare chips are fixed on the stem by tie bonding, wired by wire bonding, and resin-molded.

In the final resin-molded product state, the emission characteristics of emission output and emission wavelength, and electrical characteristics such as forward and backward characteristics are inspected and then shipped.

Here, the yield of this light emitting diode is
It depends largely on the characteristics of the epitaxial wafer. For this reason, inspecting the epitaxial wafer having poor characteristics after resin molding is a major cause of lowering the yield. Therefore, it is very important to evaluate the light emission output, the light emission wavelength, and the response speed at the stage of the epitaxial wafer on which the epitaxial layer is grown in order to produce a light emitting diode.

There is a method called a grooving method as a method for evaluating the light emission characteristics such as the light emission output and the light emission wavelength in the state of the epitaxial wafer. In this method, a groove is formed on the surface of an epitaxial wafer by ultrasonic processing or polishing to form a light emission characteristic evaluation portion having an outer dimension of about 500 μm to 1000 μm, and a current-carrying needle is erected at the portion to emit a current to emit light. Then, the light intensity and the emission spectrum distribution are measured and evaluated.

However, the above-mentioned conventional method has two major problems. First, the light intensity measured by this measurement and the light emission output of the light emitting diode manufactured from this epitaxial wafer are not necessarily in direct proportion. This is because the evaluation portion is formed by ultrasonic processing, so that the processed cross-section has a large amount of strain and leak current easily flows. Further, since no electrode is formed on the surface, the contact resistance is large and the characteristics tend to vary. The second is to form the evaluation part by mechanical processing such as ultrasonic processing,
This is necessary and causes a high cost. Further, during the process, a wafer crack or the like is likely to occur, which easily causes a decrease in yield.

Therefore, as a non-contact evaluation method that does not have such a problem, a method has been proposed in which the light emission output of a light emitting diode is obtained from the photoluminescence light intensity (Japanese Patent Laid-Open No. 63-250835). ). In this method, the epitaxial wafer is irradiated with laser light, and the light emission output of the light emitting diode is directly obtained from the photoluminescence light emitted from the epitaxial wafer.

[0008]

However, in the above-mentioned conventional non-contact evaluation method, the photoluminescence light intensity and the light emitting diode intensity (the intensity of the light which is energized to the light emitting diode and extracted to the outside) are correlated with each other. Since it is not, it is difficult and inadequate to estimate an accurate light emission output value.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an epitaxial wafer inspection method capable of accurately evaluating the light emission output and the response speed in the state of the epitaxial wafer. Especially.

[0010]

A method for inspecting an epitaxial wafer according to the present invention comprises irradiating an epitaxial wafer with a pulse of laser light, and changing the intensity of the photoluminescence light emitted from the epitaxial wafer from the change over time in the photoluminescence lifetime. Since the lifetime is proportional to the light emission output of the product manufactured from the epitaxial wafer, the light emission output of the product is obtained from the life time.

Similarly, since the lifetime is inversely proportional to the response speed of the product, the response speed of the product is obtained from the photoluminescence lifetime.

[0012]

Function: The surface of the epitaxial wafer is irradiated with a pulse of laser light. The wavelength of the laser light is selected so that it is not absorbed by the surface of the epitaxial wafer but is absorbed by the active layer. Photoluminescence light is emitted from the epitaxial wafer by pulsed light excitation. The emitted photoluminescence light is converted into an electric signal. The photoluminescence lifetime is obtained from the time change of the waveform of this conversion signal.

The variation in the light emission output results in the variation in the internal quantum efficiency. If the emission lifetime is τ _r , the internal quantum efficiency η _i and the lifetime τ _ph are: η _i = A × (1 / τ _r ) × τ _ph However, A has a proportional constant relationship. That is, since the internal quantum efficiency is proportional to the lifetime of photoluminescence light, the emission output of the product can be obtained from the lifetime.

On the other hand, the cutoff frequency f _c has a relationship of f _c = 1 / (2πτ _ph ). Since the cutoff frequency indicating the response speed is proportional to the reciprocal of the lifetime, the response speed of the product can be obtained from the lifetime.

[0015]

EXAMPLES Examples of the epitaxial wafer inspection method of the present invention will be described below. Here, the emission wavelength is 660
For the double heterostructure red light-emitting diode of nm, the result of the correlation investigation on the relationship between the photoluminescence lifetime of the epitaxial wafer and the light-emission output of the light-emitting diode manufactured from the epitaxial wafer will be described.

The structure of the evaluated epitaxial wafer is shown in FIG. An GaAs substrate 4, an AlGaAs cladding layer 3, an AlGaAs active layer 2, and an AlGaAs window layer 1 are sequentially laminated.

FIG. 3 shows an example of the construction of a photoluminescence light lifetime measuring apparatus. A semiconductor laser having a wavelength of 650 nm which is not absorbed by the epitaxial wafer surface but is absorbed by the active layer is used as the laser 5 which is the light source of the excitation light 16. The excitation light 16 is condensed by the lenses 6 and 6, and the epitaxial wafer 8 held by the sample holder 7 is obliquely irradiated. Keep the wafer surface clean. The photoluminescence light 17 emitted from the epitaxial wafer 8 is condensed by the lenses 6 and 6 and is incident on the photomultiplier tube 10. The ND filter 9 is inserted between the lenses 6 and 6 for adjustment according to the intensity of this light.

Light is converted into an electric signal by the photomultiplier tube 10, and the electric signal is passed through an amplifier 11 and is input to a digital oscilloscope 12. Digital oscilloscope 12
Are synchronized by a trigger signal from the laser controller 14 that controls the semiconductor laser 5. In addition,
Reference numeral 15 is a delay generator. The data from the digital oscilloscope 12 is read by the personal computer 13, the horizontal axis is the time axis, and the vertical axis is the logarithm of the intensity. The lifetime is calculated by calculating the slope by the least square method in this graph. As shown in Figure 4, the lifetime τ
Is the time until the photoluminescence light intensity becomes 1 / e.

The lifetime of a large number of epitaxial wafers having different luminous intensities (emission outputs) was measured by this inspection method. Then, a light emitting diode was manufactured from these epitaxial wafers, and the luminous intensity was measured.
The result is shown in FIG. It was found that the lifetime and the luminous intensity are in a clear proportional relationship, and the luminous intensity can be evaluated with high accuracy by measuring the lifetime.

The reason why the luminous intensity can be measured by measuring the lifetime is considered as follows.
The luminous intensity of the light emitting diode is proportional to the luminous efficiency. The luminous efficiency of a light emitting diode is obtained by the product of current injection efficiency, internal quantum efficiency, and light extraction efficiency. Here, the current injection efficiency is almost 100%. Further, since the light extraction efficiency is substantially determined by the structure of the light emitting diode, it may be considered to be constant when comparing light emitting diodes having the same structure. Therefore, the variation in the luminous intensity when the light emitting diode is manufactured indicates the variation in the internal quantum efficiency.

Here, the internal quantum efficiency η_iHas a luminous life
Time τ_rAnd non-emission lifetime τ _nrIs expressed by equation (1)
Be done. A is a constant of proportionality.

Η _i = A × (1 / τ _r ) / {(1 / τ _r ) + (1 / τ _nr )} (1) Further, the lifetime τ _ph of photoluminescent light is
It is expressed by equation (2).

Η _ph = 1 / {(τ _r ) + (τ _nr )} (2) From the equations (1) and (2), the internal quantum efficiency η _i and the lifetime of photoluminescence light τ _ph Equation (3) holds true for.

Η _i = A × (1 / τ _r ) × τ _ph (3) Therefore, it can be seen that the internal quantum efficiency is proportional to the lifetime of photoluminescence light. Therefore, if the personal computer 13 is used to further calculate η _i , the luminous intensity can be accurately inspected without contact.

Although the red light emitting diode has been described in the above embodiment, an AlGaAs / GaAs infrared light emitting diode and a GaP green light emitting diode,
It is also possible to inspect an epitaxial wafer for InP-based infrared light emitting diodes. Further, in addition to light emitting diodes, it can be applied to epitaxial wafers for laser diodes, photodiodes, and the like.

Further, from the lifetime of the photoluminescence light, the response speed which is another great characteristic of the light emitting diode can be obtained.

When the frequency of the current is increased, the amplitude of the modulated light intensity gradually decreases, but the frequency at which this amplitude decreases by 1.5 dB compared to the low frequency modulation is the cutoff frequency f.
_It is called _c and has a relationship of f _c = 1 / (2πτ _ph ) (4).

Therefore, it is understood that the cutoff frequency is inversely proportional to the lifetime of photoluminescence light. Since the cutoff frequency and the response speed have the same meaning, the response speed can be obtained from the lifetime.

As described above, according to this embodiment, the light emission intensity is accurately measured when the light emitting diode is manufactured in the state of the epitaxial wafer, because the lifetime is obtained instead of the photoluminescence light. it can. As a result, defective products can be dropped at the epitaxial wafer stage, so that the product yield of light emitting diodes can be significantly improved.

Further, since the present inspection method can evaluate the luminous intensity and response speed of the epitaxial wafer in a non-contact manner,
Compared with the conventional method of processing the surface of an epitaxial wafer to evaluate the luminous intensity, it eliminates wafer cracking during processing, saves time and effort for processing, and does not contaminate the wafer during processing. There are many merits, such as the absence of processed parts that cannot be performed.

Further, the apparatus for carrying out this inspection method is constructed at a relatively low cost, and the apparatus price is not so high as compared with the conventional grooving method.

[0032]

According to the present invention, since the light emission output or the response speed is obtained from the lifetime, the light emission output or the response speed can be accurately inspected in the state of the epitaxial wafer before manufacturing the product. In addition, the yield of the light emitting diode can be significantly improved as compared with the method of inspecting the light emission output after manufacturing the product.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between lifetime of an epitaxial wafer and luminescence intensity according to an embodiment of an epitaxial wafer inspection method of the present invention.

FIG. 2 is a cross-sectional view showing a structure of a double hetero structure red LED epitaxial wafer according to the present embodiment.

FIG. 3 is a configuration diagram of a photoluminescence light lifetime measuring apparatus according to the present embodiment.

FIG. 4 is a diagram showing a waveform of photoluminescence light and a lifetime according to the present embodiment.

[Explanation of symbols]

 1 AlGaAs window layer 2 AlGaAs active layer 3 AlGaAs cladding layer 4 GaAs substrate 5 Semiconductor laser 6 Lens 7 Sample holder 8 Epitaxial wafer 9 ND filter 10 Photomultiplier tube 11 Amplifier 12 Digital oscilloscope 13 Personal computer 14 Laser controller 15 Delay generator 16 Excitation light 17 Photoluminescence light

Claims (2)

[Claims] 1. A method of irradiating an epitaxial wafer with a pulse of laser light, obtaining a photoluminescence lifetime from the time change of the intensity of the photoluminescence light emitted from the epitaxial wafer, and obtaining an emission output from the lifetime. Inspection methods. 2. The epitaxial wafer is irradiated with a pulse of laser light, the photoluminescence lifetime is obtained from the time change of the intensity of the photoluminescence light emitted from the epitaxial wafer, and the response speed is obtained from the lifetime. Inspection methods.