JPS6122881B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Infrared interferometry is the most widely used method for measuring the thickness of semiconductor thin films, such as silicon epitaxial films. However, when measuring a thin film using this method, it is necessary to use infrared light with a wavelength shorter than the optical path difference.
The difference in refractive index between the thin film layer and the substrate, that is, the reflectance, becomes smaller as the wavelength becomes shorter, and in the conventional method, the lower limit of measurement is about 1.5 μm film thickness in the case of silicon.

The present invention provides a method for measuring films thinner than the above measurement lower limit, and will be described in detail below with reference to the drawings.

FIG. 1 is a schematic diagram of a conventional measurement method. In Figure 1, SANP and SABCP indicate optical paths. The light emitted from the light source S is on the thin film surface 1, and AN
It is divided into reflected light towards AB and transmitted light towards AB. The transmitted light is reflected at the interface B between the thin film 2 and the substrate 3, and exits from the point C on the surface of the thin film to the outside of the thin film.
At this time, an optical path difference is created between the SAN and the SABC, and an interference waveform with peak and valley illuminance changes is created at P with respect to the wavelength of the incident light taken on the horizontal axis, and the film thickness is calculated from this waveform. . This interference waveform gives a bright contrast when 2nd cos θ'=mλ ₀ (m is an integer), where the film thickness is d, the refractive index is n, λ ₀ is the wavelength, and the refraction angle θ'.

Therefore, the optical path difference must be an integral multiple of the wavelength that provides a bright contrast of the interference waveform, and this wavelength must be a wavelength that causes sufficient reflection at the interface between the thin film and the substrate.

The present invention provides a method for measuring the thickness of a small thin film by repeating reflections on the thin film surface and the thin film substrate interface many times to increase the optical path difference. FIG. 2 shows an example of the present invention, and SANP and SAB ₁ C ₁ B ₂ C ₂ B ₃ C ₃ P each indicate an optical path. The light emitted from the light source S is divided into reflected light and refracted light at A as in FIG. 1, and the refracted light is reflected at the thin film/substrate interface _B1 . On the other hand, thin film surface 1
A film 4 of a highly reflective material such as Al is partially formed on the surface. The light reflected by B ₁ is further reflected by the reflective interface C ₁ on the thin film surface, and is directed to B ₂ where it is reflected. The refracted light is similarly reflected by C ₂ B ₃ and emitted from C ₃ to the outside. In this case, N and C ₃
The optical path difference (△φ) in the case of FIG. 2 is △φ=6nd cosθ'.

In this case, the condition for providing bright contrast is 6nd cos θ' = mλ (m is an integer), and if we compare the interference of the same order, we can use light with a wavelength three times that of the conventional method, and if we compare the same wavelength, we can use light that is 1/3 the thickness. It can be measured up to

FIGS. 3 and 4 are reference samples used in performing the above measurement method. The interference waveform shown in Fig. 2 includes a single reflection within the thin film and a reflection component on the Al surface, so using the sample shown in Fig. 3, which is similar to the sample shown in Fig. Using the sample shown in Figure 4, which has a uniformly coated reflective film,
This measurement is facilitated by measuring the reflected wave from Al4 and subtracting it from the interference waveform measured as shown in FIG.

In addition, in the above explanation, the number of reflections in the thin film is 3.
However, the number of reflections is not limited to three times,
This can be done as many times as you like within the detection range of the detector.

FIG. 5 shows a sample in which a striped reflective film is formed on the surface of the thin film. In order to cause infrared light incident at an angle of incidence of ° to cause five reflections within the film, the Al reflective film 4 was coated with a thickness of 3000 Å, a stripe width of 2.31 μm, and a stripe spacing.
This is an example of a striped pattern with a thickness of 1.2 μm. By using this sample, reflected waves and refracted waves from the entire surface irradiated with infrared light can be used, making it easier to measure thin film thicknesses.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the principle of measuring the thickness of a general thin film by interferometry, Fig. 2 is an explanatory diagram of the method of the present invention,
Figures 3 and 4 are cross-sectional views of a reference sample when carrying out the method of the present invention, and Figure 5 is a cross-sectional view of a sample on which a reflective film is formed in a striped shape to make it easier to carry out the method of the present invention. . 1... Thin film surface, 2... Thin film, 3... Substrate, 4
...Al reflective film.

Claims (1)

[Claims] 1. A thin film of a material with high reflectance is formed on a part of the surface of a semiconductor thin film grown on a substrate, and infrared rays incident on the semiconductor thin film are directed to the interface between the semiconductor thin film and the material with high reflectance and between the semiconductor thin film and the semiconductor thin film. A film thickness measurement method characterized by causing multiple reflections between the interface with the substrate and using interference between long-wavelength incident light and emitted light.