JPH05302891A - Measuring method of refractive index of light-transmitting substrate - Google Patents

Abstract

PURPOSE:To obtain a measuring method of the refractive index of a light- transmitting substrate which enables execution of precise measurement even when a body to be inspected is the light-transmitting substrate or a light- transmitting substrate provided with a light-transmitting thin film, and which has high reliability. CONSTITUTION:A method wherein a light 2 is applied onto the surface of a light-transmitting substrate 1 and the refractive index of the light-transmitting substrate 1 is measured from a state of polarization of a reflected light from the surface or the intensity of the light. In this method, the surface roughness of the rear side 1a of the light-transmitting substrate 1 is set to be a center-line average roughness of 1/10 or above of the shortest wavelength of the light 2 applied. Thereby the reflected light of the light-transmitting substrate 1 is scattered by the rear side 1a, noise can be removed to the utmost, execution of highly precise measurement and evaluation is enabled and the quality of the light-transmitting substrate 1 can be evaluated with excellent reliability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of irradiating a main surface of a transparent substrate with light and measuring the refractive index of the transparent substrate from the reflected light.

[0002]

2. Description of the Related Art Conventionally, in electronic devices and optical devices, oxide single crystal substrates such as LiNbO ₃ , LiTaO ₃ and sapphire, glass substrates for liquid crystals, and transparent substrates such as optical disc original plates have been used. In recent years, various devices have been required to have high functionality, high integration, and thin film. In order to meet these high requirements, selection of the substrate material and its homogeneity are very important. Has been done.

Conventionally, as a typical method for evaluating the homogeneity of a substrate, for example, a measuring method using an ellipsometer is known. In this method, linearly polarized light from a light source L is irradiated onto a substrate or a substrate coated with a thin film, and the refractive index of the substrate and the thickness of the thin film are measured from the polarization state and light intensity of the reflected light. is there.

However, this method is suitable as an evaluation method for a substrate having a high surface reflectance, but the substrate is the translucent substrate 1 as shown in FIG. 3, or a translucent thin film is formed thereon. In the case of evaluating the light source, especially when the back surface 1a of the transparent substrate 1 is a mirror surface, the light source L
The reflected light 3 may be detected by the detector D as noise due to the reflection of the irradiation light 2 from the back surface and the like, resulting in a large variation in the measured values and a significant decrease in the measurement accuracy. At present, there is no method for reliably and accurately evaluating a transparent substrate.

Conventionally, there is a method of dealing with the above problem by a method such as thickening the measurement substrate or narrowing the light source, but the measurement accuracy is also deteriorated due to the decrease in light intensity, which is a problem. Is.

[0006]

[Purpose] Therefore, the present invention provides a highly reliable refractive index measurement of a transparent substrate that can be accurately measured even if the subject is a transparent substrate or a transparent substrate provided with a transparent thin film. The purpose is to provide a method.

[0007]

Means for Solving the Problems A method for measuring the refractive index of a transparent substrate that achieves the above object is to irradiate the surface of the transparent substrate with light and determine the polarization state or light intensity of the reflected light from the surface. A method for measuring the refractive index of a transparent substrate, wherein the surface roughness of the back surface of the transparent substrate is set to a center line average roughness of 1/10 or more of the shortest wavelength of the light to be irradiated. It is characterized by

The term "translucent substrate" as used herein refers to a translucent substrate coated with a thin film in addition to the substrate itself.

[0009]

【Example】

Example 1 LiNbO _{3 having an} outer diameter of 3 inches and a length of 100 mm
A single crystal is grown by the Czochralski method, this single crystal is reheated to 1200 ° C., which is higher than the Curie temperature, and 1.
A single domain was created by applying a voltage of 5 V / cm.
Then, from the shoulder of this grown crystal, about 10 mm (), about 80 mm (
), The C surface, 3 inch diameter, and 0.5 mm thick both surfaces were roughly polished with GC abrasive grains, and then mirror-finished with colloidal silica to obtain a transparent substrate wafer.
Then, the variation R (R = maximum value−minimum value) and the variation R within the wafer surface when measured 10 times at the same measurement point by an ellipsometer were measured.

Next, one main surface of the same wafer was roughened by GC # 1200 and the same measurement was carried out. As a result, as shown in Table 1, the roughened back surface of the light irradiation surface was measured with high accuracy. Was found to be significantly improved. Since the mechanical guarantee accuracy (nominal error) of this device is 10 ^-2 or more, the variation R within the wafer and the wafer surface is within the measurement error, and it can be evaluated that the variation in the refractive index of the wafer is uniform. It was

[0011]

[Table 1]

When the variations in the measured values and the center line average roughness were investigated by changing the number of the rough surface processing, the results shown in FIG. 1 were obtained. As is clear from this figure, the wavelength of the irradiation light on the transparent substrate is about 800 nm, and when polishing is performed with a count of GC # 2000 or less, that is, about 1/10 of the shortest wavelength of the irradiation light. In polishing with a center line average roughness of 0.08 μm Ra or more, the variation in the measured values was within the measurement error, and almost the same tendency was obtained for other materials such as sapphire.

As shown in FIG. 2, the result of FIG. 1 shows that the back surface 1a of the light-irradiating surface of the transparent substrate 1 has a center line average roughness of 1/10 or more of the shortest wavelength of the irradiation light 2 from the light source L. It is considered that this is because the light 4 transmitted through the transparent substrate 1 is appropriately scattered by the back surface 1a, which prevents the reflected light from the back surface 1a from being detected as noise by the detector D. Be done.

[Embodiment 2] The in-plane distribution of the refractive index of a double-side polished wafer of the same size obtained by the same method as in Embodiment 1 is measured by a crystal surface refractive index measuring device (RMX-01; Remand High-Technos Co., Ltd.). That is, the refractive index is measured from the intensity change of the reflected light, and the main surface of the double-sided polished wafer is roughened (GC # 1200) to measure the in-plane distribution of the refractive index again. However, the results shown in Table 2 were obtained.
That is, also in this example, it was found that by making one surface rough, the measurement accuracy was remarkably improved as compared with the case where the surface was not roughened. Since the mechanical guarantee accuracy of this device is 10 ⁻⁴ or more, it can be evaluated that the refractive index distribution of the wafer is uniform within the measurement error.

[0015]

[Table 2]

[Embodiment 3] A light-irradiated surface of a double-sided mirror-finished sapphire substrate polished by the same method as in Embodiment 1,
The thickness is about 100Å, that is, the thickness of Si ₃ N
_Four films were deposited and the same point was measured 10 times with the ellipsometer used in Example 1 to measure the refractive index of the thin film. After that, the main surface of the sapphire substrate is roughened with GC # 600, and the refractive index at the same point is measured again.
As a result of repeated measurements, it was found that the measurement accuracy was improved by roughening the back surface of the light irradiation surface in this example as well. As the refractive index of the thin film, the refractive index of the sapphire substrate obtained by measurement in advance was used.

The subject can be applied to a transparent substrate such as an electro-optical material substrate such as lithium tantalate, various semiconductor wafers, a glass substrate, an optical disk, etc., in addition to the above-mentioned transparent substrate. , It is possible to accurately observe not only the surface but also the internal defect. Further, the roughening of the back surface of the light-irradiated surface of the translucent substrate is not limited to the above-mentioned method, and may be appropriately changed and carried out by a known method such as sanding.

[0018]

As described above, according to the method for measuring the refractive index of the transparent substrate of the present invention, the incident light of the transparent substrate is scattered by the back surface, and as a result, noise which has been a problem in the past is minimized. Since it can be removed, highly accurate measurement and evaluation can be performed, and the quality of the transparent substrate can be evaluated with high reliability.

[Brief description of drawings]

FIG. 1 is a back side processed GC in Example 1 according to the present invention.
It is a graph which shows the relationship between the count of an abrasive grain and average center roughness.

FIG. 2 is a schematic diagram illustrating the measurement principle of the present invention.

FIG. 3 is a schematic diagram illustrating the influence of the back surface in a conventional measurement method.

[Explanation of symbols]

 1 ... Translucent substrate 2 ... Irradiated light 3 ... Reflected light 4 ... Transmitted light L ... Light source D ... Detector

Claims (1)

[Claims] 1. A method of irradiating light to the surface of a transparent substrate and measuring the refractive index of the transparent substrate by the polarization state or light intensity of the reflected light from the surface, the method comprising: The surface roughness of the back surface of the substrate is 1 / the shortest wavelength of the emitted light.
A refractive index measuring method for a transparent substrate, characterized in that the center line average roughness is set to 10 or more.