JPH06338547A - Method and device for evaluating silicon wafer - Google Patents

Abstract

PURPOSE:To enable the surface of a semiconductor wafer and the crystallinity of a surface crystal layer to be measured through a non-destructive method by a method wherein the surface of a wafer is irradiated with impulse light and electromagnetic waves at the same time, and the reflected electromagnetic waves are detected. CONSTITUTION:A position P on the surface a semiconductor wafer is irradiated with impulse light 4 large in absorption coefficient, as it is kept irradiated with electromagnetic waves 2. At this point, the reflected waves 3 of the probing electromagnetic waves 2 are detected. The reflected waves 3 of the probing waves 2 are modulated due to a so-called photoconductivity attenuation change caused by an increase in photoconductivity due to light carrier excited in crystal and a decrease in photoconductivity due to recombination after quenching. Therefore, the reflected waves 3 of the probing waves 2 are detected, whereby a photoconductivity attenuation change can be obtained. This photoconductivity change can be obtained through such a manner that the reflected waves 3 are high-speed detected and observed by a high-speed waveform observing equipment. The surface of a wafer and the properties of a surface crystal layer can be evaluated basing on the obtained photoconductivity change.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the surface and surface layer of a semiconductor wafer for ICs and an apparatus used for the method.

[0002]

2. Description of the Related Art ICs are formed in a surface crystal layer within a few micrometers to a few tens of micrometers from the surface of a semiconductor wafer. Conventionally, as a method for evaluating a wafer surface crystal layer, after observing the wafer, observing defects by an etch pit image by chemical etching of the cross section, or after observing, irradiating an infrared laser and observing scattered light from precipitates There are some observations of precipitates by the Society for the Promotion of Science (145th Committee, Materials of the 53rd Research Group P.7 to P.12, February 2, 1991). One of the evaluation methods for heavy metal contamination of the surface layer is by SIMS measurement (UCS Semiconductor Technology Research Group, Ultra L
Materials for SI Ultra Clean Technology Workshop
No. 9, March 15, 1991, P.29 to P.39; Gakshin 145th Committee, 42nd Research Group materials, October 3, 1988.
Sun, P.15-P.20, etc.). Further, in the surface evaluation, there is a direct observation method of surface irregularities by STM. IC
Is produced in the surface crystal layer within a few μm to a few tens μm on the semiconductor wafer. Therefore, especially in ULSI and the like, it is very important to prevent the generation of crystal defects in the surface crystal layer, prevent or remove the contamination of heavy metals, and remove the crystal surface strain. As a technique of generating no defects on the surface, so-called intrinsic gettering (IG) that causes defects in the surface layer to be non-generated by intentionally generating defects inside the wafer and the back surface on the opposite side of the surface layer There is extrinsic gettering (EG) in which defects on the surface layer are not generated by intentionally generating defects on the side. In either case, the surface layer has almost no defects (forming a so-called de nude zone DZ), but the inside is full of defects. Therefore, it is essential to evaluate the crystallinity of the surface layer of such a wafer and the degree of heavy metal contamination. It is desirable that the surface and the surface crystal layer can be evaluated in a non-destructive and non-contact manner. The current evaluation technology is for sample preparation or measurement,
It is a destructive evaluation. Further, the wafer lifetime method is a non-contact method, but information on the surface and the surface crystal layer cannot be extracted. The surface roughness affects the properties of the surface oxide film,
With increasing device integration, it is beginning to be found to be closely related to device performance. In evaluating these surfaces and surface crystal layers, it is very important to be able to measure their electrical properties, which are directly related to the properties of electronic devices, in a non-contact and non-destructive manner.

[0003]

For the evaluation of the crystal characteristics of the surface crystal layer, it is desirable that the characteristics distribution within the wafer surface can be evaluated nondestructively. Further, the wafer lifetime method is an evaluation of the entire wafer cross-sectional direction, and information on the surface crystal layer is not available at present. Further, the surface crystal strain has no particular problem at present, but it is a crystal characteristic that may become a problem in the future when higher integration of devices is further advanced. An evaluation means capable of solving the above-mentioned drawbacks is an object of the present invention, and D in a silicon wafer is evaluated.
The present invention provides a method and apparatus for non-contact and non-destructive measurement of crystal characteristics of surfaces such as Z and epi layers and surface crystal layers.

[0004]

According to the present invention, a wafer surface is irradiated with impulse light and electromagnetic waves at the same time, and a reflected electromagnetic wave is detected. This is a method for evaluating surface layer characteristics. Hereinafter, the evaluation method of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an outline of the evaluation method of the present invention. In FIG. 1, reference numeral 1 is a semiconductor wafer to be measured. While irradiating the surface of 1 with the electromagnetic wave 2, the impulse light 4 having a large absorption coefficient is selected and irradiated. At this time, the reflected wave 3 of the reflected probe electromagnetic wave is detected.
The thickness of the surface layer to be evaluated is Wd, the wavelength of the irradiation light is λ,
When the absorption coefficient is α, the wavelength λ that determines the penetration length of light
Is determined from the α-λ curve so that 1 / α ≦ Wd.

Next, the probe wave 7 has a frequency of 30 to 3,0.
A millimeter wave or submillimeter wave of 00 GHz is used to irradiate the same point P on the wafer surface as the excitation light irradiation position. The reflected wave 3 of the probe wave 2 is modulated by an increase in photoconductivity due to the photocarriers excited in the crystal and subsequent so-called photoconductivity decay change accompanying decay upon recombination after quenching. . Therefore, if the reflected wave 3 of the probe wave 2 is detected and detected, the photoconductive attenuation change can be known. The photoconductivity sharply increases due to the carriers excited to the surface side in the surface crystal layer by the irradiation of the impulse light, and thereafter, the photoconductivity slowly increases or decreases as compared with that. This change in photoconductivity is caused by the reflected wave 3 of the probe electromagnetic wave.
It is obtained by detecting the high-speed wave and observing it with a high-speed waveform observer.

A method for evaluating the characteristics of the surface of the wafer and the surface crystal layer from the change in photoconductivity obtained by the above-mentioned method will be described. The carriers excited near the surface of the surface crystal layer diffuse in the surface crystal layer. The theoretical calculation results of the surface recombination velocity dependence of the change in photoconductivity immediately after photoexcitation at this time are shown in FIG. As can be seen, the photoconductivity increases very sharply, then increases further, reaches a maximum and then decreases. When the surface recombination velocity changes from small, medium, and large, the detected voltage of the change in photoconductivity decreases in the order of V ₃ , V ₂ , and V ₁ as shown in the figure. Therefore, the maximum change in photoconductivity depends on the surface recombination rate. It is known that the surface recombination rate strongly depends on the surface roughness and the surface damage. Therefore, the surface roughness can be evaluated from the magnitude of the maximum value of the change in photoconductivity.

Further, as shown in the figure, the time t until reaching the maximum value becomes short as t ₃ , t ₂ and t ₁ as shown in the figure when the surface recombination velocity changes from small to medium to large. . That is,
The time to reach the maximum also changes depending on the surface recombination rate. Therefore, the surface roughness can be evaluated from that time. On the other hand, FIG. 3 shows photoconductivity change waveforms during and immediately after irradiation with the impulse excitation light due to the lifetime of the surface layer. As shown in the figure, the lifetime τ ₀ of the surface layer is 0.
When it is changed to ₁ , ₁ , 50, 500 μs, the maximum value voltage of the change in photoconductivity becomes V ₁ , V ₂ , V ₃ , V ₄ as large as possible. Also, the time until the change in photoconductivity reaches the maximum value is t ₁ , t ₂ , t
₃ and t _4, which increase and change. On the other hand, its lifetime has been found to depend strongly on heavy metal contamination.
Therefore, the heavy metal contamination can be evaluated from the maximum value voltage or the time.

FIG. 4 shows an outline of the apparatus used in the present invention. An electromagnetic wave is applied from the electromagnetic wave oscillator 6 through the circulator 7 to the surface layer side of the sample 1 to be evaluated through the waveguide or the coaxial line 5. On the other hand, the laser light reflected by the reflecting mirror 13 from the laser oscillator 15 is applied to the incident point of the electromagnetic wave, or the surface layer side of the sample 1 to be evaluated is irradiated with the laser light reflected by the reflecting mirror 14 from the laser oscillator 15. To do. In this case, the sample is inverted and the laser light is applied to the surface crystal layer (the surface side to be measured and evaluated), so that the electromagnetic waves and the laser light are applied to the opposite surfaces. The reflected wave of the electromagnetic wave modulated by the change in photoconductivity accompanying the excitation light is separated by the circulator 7, detected by the detector 8, becomes a photoconductivity change waveform signal, is amplified by the amplifier 9, and is subjected to high-speed A / D conversion. A / D conversion is performed by the device 10, the data is transferred to the computer 11, and the maximum value of the photoconductivity change signal and the time until the maximum value are detected by data calculation.

[0009]

According to the present invention, carriers excited by the vicinity of the surface are diffused in the surface and the surface crystal layer, and carriers are accumulated due to the diffusion, and surface roughness of the surface, surface damaging and defects in the surface crystal layer. The recombination state may change depending on the heavy metal contamination and the surface crystal layer thickness. By detecting the photoconductivity modulation of the probe electromagnetic wave depending on the state, the crystal state of the wafer surface and the surface crystal layer is evaluated.

[0010]

【Example】

[Example 1] Next, an example will be described. A p-type silicon wafer having a resistivity of 10 Ωcm was used as a sample. The surface of the sample was mirror-polished in order to see changes in output voltage due to changes in surface roughness and photoconductivity. It has been found that the surface roughness decreases as the mirror polishing time increases. Changes in photoconductivity were observed after polishing with colloidal silica for 15, 30, and 60 minutes on the same sample. The wavelength of the emitted light is 337 mm and the wavelength of the electromagnetic wave is 10
It is 0 GHz. The result is shown in FIG. As the polishing time becomes longer, the maximum values of the output signal voltage become V ₁ , V ₂ , V ₃
You can see that it is increasing. Further, it can be seen that the time required to reach the maximum value also increases as the surface roughness increases to t ₃ , t ₂ and t ₁ . This is because the surface recombination rate increases as the surface roughness decreases. Therefore, the surface roughness and the surface condition of the surface damage can be evaluated from the maximum value of the amplitude or the time until the maximum value is reached. Example 2 Next, FIG. 6 shows the measurement results of the change in photoconductivity of a sample contaminated with Fe using p-type Si having a resistivity of 10 Ωcm as a typical example of a heavy metal. As shown in the figure, the Fe
Contamination concentration is as smaller as ^{10 13, 10 12, 10 11} cm -3, the maximum value of the signal increases with _{V 1, V 2, V 3} . Further, the time until reaching the maximum value also increases as t ₁ , t ₂ , and t ₃ as the contamination concentration decreases. Therefore, the heavy metal contamination concentration can be evaluated from the maximum value of the amplitude or the time until the maximum value is reached.

[0011]

According to the present invention, it becomes possible to evaluate the surface roughness of the surface of a semiconductor wafer and the heavy metal contamination.
Therefore, in particular, as a surface roughness detection monitor, in combination with the advantages of non-contact and non-destructiveness, it has a great industrial effect as a quality evaluation method of a silicon wafer for VLSI production.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a measuring method of the present invention.

FIG. 2 is an explanatory diagram showing an evaluation method of the present invention.

FIG. 3 is an explanatory diagram showing an evaluation method of the present invention.

FIG. 4 is an explanatory diagram of the device of the present invention.

FIG. 5 is a measurement result of the surface roughness and the photoconductivity change signal in the example of the present invention.

FIG. 6 is a measurement result of a heavy metal contamination concentration and a photoconductivity change signal in an example of the present invention.

[Explanation of symbols]

 1 non-measurement sample 2 incident electromagnetic wave 3 reflected electromagnetic wave 4 carrier excitation impulse light 5 waveguide or coaxial line 6 electromagnetic wave oscillator 7 circulator 8 detector 9 amplifier 10 A / D converter 11 computer 12 x-axis-y-axis fine mover 13 Reflector 14 Reflector 15 Laser oscillator

Claims (4)

[Claims] 1. The same position on the surface of a silicon wafer is irradiated with impulse light and an electromagnetic wave at the same time, the reflected electromagnetic wave is detected, and the characteristics of the surface and the surface layer are measured by utilizing the photoconductivity change during and immediately after the irradiation of the impulse light. A silicon wafer evaluation method characterized by analysis. 2. The method for evaluating a silicon wafer according to claim 1, wherein surface roughness or heavy metal contamination is evaluated and evaluated from the maximum amplitude of photoconductivity change. 3. The surface roughness or the heavy metal contamination is evaluated, and the evaluation is performed from the time until the maximum amplitude value of the photoconductivity change is reached after irradiation with the impulse light.
The evaluation method of the described silicon wafer. 4. An apparatus for detecting a change in photoconductivity during and immediately after irradiation of impulse light, which comprises means for simultaneously irradiating the surface of a silicon wafer with impulse light and electromagnetic waves and means for detecting reflected waves thereof, and detection thereof. An apparatus for evaluating silicon wafers, comprising: an arithmetic unit for obtaining the maximum amplitude value of the waveform of change in photoconductivity and the time from the maximum waveform value to the maximum amplitude value.