JPH02287106A - Uppermost layer film thickness measring method - Google Patents

Abstract

PURPOSE:To measure thickness of the uppermost layer of a film even when a substrate has multi-layered structure by deriving logically reflection factor characteristics which are based upon the film thickness of the upper most layer as a parameter, and comparing them with actually found reflection factor characteristics. CONSTITUTION:A sample 10 having no uppermost layer is set opposite a projec tor photodetector 6 and a sample 11 having the uppermost layer is set opposite to a projector photodetector 7; and the output of a spectrometer 1 is inputted to a personal computer 3. Then the reflection factor characteristics of the sam ple 10 having no uppermost layer are measured to find an equivalent refractive index, which is used to derive logically the reflection factor characteristics based upon the film thickness as the parameter by using a specific arithmetic expression. Then, the reflection factor characteristics of the sample 11 where the uppermost layer is formed are found, and compared with said logical reflec tion factor characteristics and the film thickness corresponding to the reflection factor when the both match each other most is regarded as the film thickness of the uppermost layer.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for optically measuring the thickness of a top layer film formed on a sample.

<Conventional technology> Conventionally, it has been necessary to accurately measure film thickness when forming antireflection films on glass or films, forming semiconductors with different characteristics on semiconductor substrates, or coating photoresists. There is.

However, since the above film thickness often ranges from μm to nm, it is usually measured using optical principles.
This enables highly accurate film thickness measurement without contact.

Particularly recently, the use of coherent laser light sources has made it possible to perform even more precise measurements.

One of the methods for determining the film thickness is a method that utilizes a light interference phenomenon. For example, as shown in FIG. 3, the refractive index ns
When a thin film with a refractive index nr is formed on a substrate of wake up That is, if the thickness of the thin film is D, the amplitude reflectance on the surface of the thin film is rl, and the amplitude reflectance between the thin film and the substrate is r2, then the intensity reflectance R is expressed as: be done. Here, δ is 4πn('D/λ−φ(λ
is the wavelength in air, φ is π when nf'≧ns, nf'
<ns, it is expressed as 0).

The amplitude reflectance rl and refractive index are rl = (1-nf') / <1+nf)
■Amplitude reflectance r2 and refractive index are r2 = (nf - ns)/ (nr+ns)
■There is a relationship (Fresnel's equation).

In conventional film thickness measurement, the intensity reflectance R is actually measured for discrete or continuous wavelength values, and the refractive index nf' of the thin film is calculated.
is known, this measured curve can be theoretically predicted from the refractive index nf' of the thin film and the refractive index ns of the substrate (ns may be known or determined by actual measurement). It was determined by fitting the wavelength characteristic curve of the intensity reflectance R.

<Problems to be Solved by the Invention> However, the above method can be applied when the substrate has a single optical property, but when the substrate is a multilayer film,
This cannot be applied when reflections from the six films within the substrate cannot be ignored. The fact that the substrate is a multilayer film can occur, for example, when a plurality of films are sequentially formed on a semiconductor substrate using the brainer technique, or when an additional film is formed on a transparent laminate film.

In this case, it is necessary to solve the problem by applying a complex theoretical formula using boundary conditions of the electromagnetic field, which requires a large computer and takes a long calculation time.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for measuring the thickness of the top layer that can easily determine the film thickness using a simple calculation method even when the film is formed on a multilayer film substrate.

Means for Solving the Problem> The method for measuring the thickness of the top layer according to claim 1 for achieving the above object includes the following steps for each wavelength of light of a plurality of wavelengths:
The thickness of the top layer is obtained according to the following procedure.

(1) Measure the reflectance characteristics of a sample with no top layer film and whose optical structure is unknown.

(2) Find an equivalent refractive index using the reflectance characteristics determined by the above procedure.

(3) Measure the reflectance characteristics of the top layer film formed on the sample.

(4) Applying the refractive index characteristics of the sample obtained in step (2) to the calculation formula for determining the thickness of the top layer film formed on a substrate with a single optical property, each film thickness is Estimate the reflectance characteristics of the top layer film corresponding to

(5) The reflectance characteristics obtained in the procedure (3) above are (4
), the film thickness corresponding to the matching reflectance property is determined as the film thickness of the uppermost layer.

Effect> According to the above procedure, by first measuring the reflectance characteristics of the sample, it is possible to derive the equivalent refractive index of the substrate using a known formula.

Using this equivalent refractive index, the reflectance characteristics using the thickness of the top layer film as a parameter are theoretically derived.

The film thickness of the top layer film can be estimated by measuring the reflectance characteristics of the top layer film formed on the sample and comparing it with the theoretically derived characteristics.

Note that the above method can also be applied when the sample without the top layer film is light absorbing. In this case, (1)
The reflectance characteristics of the sample without the top layer obtained in
The light absorption property is reflected in it. Therefore, if the thickness of the top layer is determined using the reflectance characteristics obtained in this way, the determined thickness will accurately measure the thickness of the top layer formed on the light-absorbing sample. It becomes what it is.

<Example> Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments.

FIG. 1 is a block diagram of a measuring device that implements the method for measuring the top layer film thickness of the present invention, including: - A spectrophotometer (1) capable of simultaneous measurement of wavelengths of 600 nm-1000 nm+; - White light source (2) and a balancing filter (21) attached thereto, - a personal computer (3) equipped with a display (31);
), ・Optical fiber (52) (82) (7) that guides the light from the white light source (2) through the balancing filter (21)
2), Optical fiber (51) that guides light to the spectrophotometer (1)
), (61).

(71), - Light emitter/receiver (5) attached to the tip of the optical fiber (51) (52), - Light emitter/receiver (6) attached to the tip of the optical fiber (91) (82), - Light It consists of a light emitter/receiver q) attached to the tips of fibers (71) (72), etc.

A reflecting mirror (9) faces the light emitter/receiver (5) via a shutter B), and the light emitter/receiver (6) has no uppermost layer (
D-0) A sample with only a substrate (lO) faces the emitter/receiver (a sample with the top layer formed on the substrate (11)
) are facing each other.

To measure the film thickness of the top layer using the above device, first, the optical system including the balancing filter (21), light emitter/receiver (5), optical fibers (51) (52), and reflection mirror (9) is subjected to spectroscopy. Perform strength correction.

Next, using the reflection intensity of the reflection mirror (9) obtained through the optical fiber (51) as a reference, a sample of only the substrate (
10) Measure the reflection intensity. This reflection intensity can reflect the light absorption if the substrate-only sample (10) has light absorption. Using the reflectance R obtained here, the refractive index ns when considering the sample (10) as a single substrate is determined from Fresnel's formula % formula %.

On the other hand, using the refractive index nr (known) of the top layer, the reflectance r2 = (1-nr) / (1+nl
') Find ■.

The above procedure needs to be performed by changing the wavelength of light, but in this embodiment, all wavelengths are simultaneously measured using white light, and the above processing is performed in real time using the data.

Then, by substituting these refractive indexes into the above formula, a theoretical curve of wavelength distribution of reflectance R with the thickness of the top layer as a parameter is obtained using a personal computer (3), and the data is stored. put. This data can be read out and displayed on the display (31) if necessary. An example of this displayed theoretical curve is shown in FIG.

Next, using the reflection intensity of the sample (10) containing only the substrate as a reference, the reflectance of the sample (11) on which the top layer is formed is determined for several wavelengths. The obtained reflectance data is recorded in the recording section of the personal computer (3), read out if necessary, and displayed in the shape of a school of fish on the display (31).

The operator superimposes the reflectance displayed in the shape of a school of fish on the theoretical curve, searches for the theoretical curve that best fits the curve as shown in Figure 2, and calculates the film thickness corresponding to the theoretical curve as the film thickness of the top layer. shall be.

As described above, even if the optical structure of the substrate is not known, the thickness of the top layer can be determined in a short time without using complicated calculation formulas. In particular, even if the sample (10) consisting only of a substrate has light absorption properties, considering the absorption properties,
Film thickness can be measured accurately.

In the above example, the film thickness was determined by obtaining a plurality of pieces of data using light of a plurality of wavelengths and performing curve fitting. However, if the range of film thickness to be measured can be defined from the beginning, the film thickness can be determined by directly applying it to a theoretical formula without curve fitting. For example, if the film thickness is limited from Dl to D2 as shown in FIG.
Between the film thicknesses Dl and D2, the value of the theoretical reflectance R determined by formula By setting the refractive index and the reflectance of the top layer film as n1 and applying it to a theoretical formula, the film thickness can be determined.

Note that the present invention is not limited to the above-mentioned embodiment, and in the measuring device shown in FIG. 1, the reflecting mirror (9) is omitted and light is directly supplied from the light source [2] to the spectrophotometer (1). It is also possible to omit the emitter/receiver (6) and create a sample with only the substrate (10) and a sample with the top layer formed (
It is also possible to measure by replacing 11) as appropriate. Various changes can be made without changing the gist of the Ikensui invention.

<Effects of the Invention> As described above, according to the method for measuring the thickness of the top layer of the present invention,
Even if the optical structure of the substrate is unknown, the thickness of the top layer can be easily measured in a short time using a known calculation formula by considering the substrate as equivalently having a single optical property.

[Brief explanation of drawings]

Figure 1 is a block diagram of a measuring device that implements the method for determining the top layer film thickness i'l1lJ of the present invention, Figure 2 is a graph showing the correspondence between the theoretical curve and the measurement points, and Figure 3 is the FIG. 4 is a graph showing a theoretical curve of film thickness versus reflectance when the wavelength is constant. (10)...Sample without the top layer film formed (11)...Sample after the top layer film is formed Fig. 4 ID2

Claims (1)

[Claims] 1. The top layer film is resistant to light of multiple wavelengths. Unformed and optical structure Measuring reflectance characteristics of samples with unknown structure Measure using the first step and the above steps. The equivalent reflectance characteristics of the sample are The second step of determining the refractive index and the Reflection of the top layer film formed on the sample A third step to measure the rate characteristics and a single An optical fiber formed on a substrate with optical properties. In the calculation formula for determining the thickness of the upper layer film, the second Equivalent bending of the sample obtained by the procedure Apply the refractive index to each thickness of the top layer film. Estimate the reflectance characteristics of the corresponding top layer film. The fourth step is to The reflectance characteristics should be obtained in the fourth step. When it most closely matches the reflectance characteristics of either corresponds to the matching reflectance characteristic. Fifth, determining the film thickness as the thickness of the top layer The best method characterized by adopting Upper layer thickness measurement method. 2. The top layer film is sensitive to light of a single wavelength. unformed and without optical structure. The first step is to measure the reflectance properties of the sample. 1 and the reaction measured by the above procedure. Find the equivalent refractive index using emissivity characteristics The second step is to Measuring the reflectance characteristics of the top layer film with a third step and a single optical property Determine the thickness of the top layer film formed on the substrate. The calculation formula obtained in the second step is Equivalent refractive index of sample and third hand The reflectance characteristics of the top layer film obtained in the following order are Apply it to determine the thickness of the top layer film. It is characterized by adopting the fourth procedure. Top layer thickness measurement method.