WO2016162324A1

WO2016162324A1 - Method for ascertaining the refractive index and layer thickness of a transparent layer by means of ellipsometry

Info

Publication number: WO2016162324A1
Application number: PCT/EP2016/057405
Authority: WO
Inventors: Peter NESTLER; Christiane A. Helm
Original assignee: Ernst-Moritz-Arndt-Universität Greifswald
Priority date: 2015-04-09
Filing date: 2016-04-05
Publication date: 2016-10-13
Also published as: DE102015105418B4; DE102015105418A1

Abstract

The invention relates to an ellipsometric method for ascertaining characteristic variables of transparent layers. A pair of ellipsometric variables of a layer (2) to be examined is ascertained on said layer at a first layer thickness and then at a second layer thickness. One of the two layer thicknesses can also have a value of 0, in which case the pair of ellipsometric variables are being measured on an uncoated substrate (1). The refractive index of the layer to be examined is ascertained from the total of four measured ellipsometric variables using a given formula, and the layer thickness can be ascertained from the refractive index using known ellipsometric methods.

Description

METHOD FOR DETERMINING BREAKING INDEX AND LAYER THICKNESS OF A TRANSPARENT LAYER BY MEANS OF ELLIPSOMETRY Ellipsometry is an optical measurement method for the investigation of

Surfaces and near-surface layers. The basic principle of ellipsometry is that polarized light is reflected at the sample to be measured, and one from the resulting change in the polarization state of the light

Draw conclusions about optical and structural properties of the sample. The ellipsometry is an established technique, the first devices of this kind were used around 1890 and since then developed technically on an ongoing basis. Today, ellipsometry is one of the standard methods in material analysis and surface physics. A common field of application is the investigation of

Surface coatings (e.g., in the semiconductor industry, metallography, and the plastics industry). Two important characteristics for the characterization of coatings are the layer thickness and the refractive index of the layer. From the refractive index, e.g. Draw information on the density and chemical composition of the coating material.

The determination of thickness and refractive index of a layer by means of ellipsometry now takes place in two steps: (i) measurement of the ellipsometric angle of the coated surface and then (ii) computer-aided model calculation for determining the refractive index and thickness of the layer. The ellipsometric angles Δ and Ψ are those quantities which are measured directly in ellipsometry. Δ is a measure of the phase shift that the light undergoes during reflection. Ψ is a measure of the amplitude ratio of reflected to irradiated light wave. Although these two ellipsometric angles can be measured directly, they are abstract and only partially interpretable.

The second step to convert these angles into thickness and refractive index of the layer is only possible without problems if the layer thickness is sufficient is large (as a threshold, a thickness of 15 nm makes sense). In the case of ultrathin layers (below this threshold), it is still extremely problematic to determine both thickness and refractive index of a coating by means of ellipsometry. The reasons for this lie in the measuring method itself: With an ultra-thin layer, the influence of layer thickness and refractive index on the reflection of the light can no longer be determined independently of one another.

In addition, small experimental errors (which are unavoidable) in this case have a particularly strong effect on the measured ellipsometric angles Δ and Ψ.

State of the art

An example of a method for determining characteristic quantities

transparent layers using ellipsometry is known from DE 43 01889 A1.

From: W. Knoll, R.C. Advincula, Functional Polymer Films, Vol. 2, Part III, Wiley-VCH, 201 1, chapter "Investigations of soft organic films with ellipsometry", it is known that the thickness of a layer and its refractive index can only be determined independently, if the sample is sufficiently thick. For thin films, it is not possible to determine the thickness and the refractive index separately.

From: H.G. Tom Kins, A User's Guide to Ellipsometry, Academic Press, 1993, p. 65

psi

the course of the ellipsometric quantities Psi and Delta at different refractive indices on silicon is known. There it is noted that for the first 50 Angstrom units the values are only badly separated. It is concluded that the Δ and Ψ measurements are not suitable for obtaining information on the refractive index of a film. From: MM Ibrahim, NM Bashara, Parameter Correlation and Computational Considerations in Multiple Angle Ellipsometry, J. Opt. Soc. At the. 61, 1622-1629, 1971 shows that with small layer thicknesses large inaccuracies are associated with the previous measurement methods. Also state of the art include:

P. Nestler, S. Block, CA. Helm: Temperature-Induced Transition from Odd-Even to Even-Odd Effect in Polyelectrolyte Multilayers Due to Interpolyelectrolyte

Interactions, J. Phys. Chem. B, 201 1, 1 16, p 1234-1243 and

P. Nestler, M. Paßvogel, CA. Helmet: Influence of Polymer Molecular Weight on the Parabolic and Linear Growth Regime of PDADMAC / PSS Multilayers,

Macromolecules, 2013, 46, pp. 5622-5629.

The problem of determining the refractive index of an ultra-thin layer by means of ellipsometry is solved if the thickness of the coating was determined independently or was known from the outset with another measuring method. This is however i.A. not the case.

On the other hand, the problem of determining the thickness of an ultra-thin layer by means of ellipsometry is solved, if with another measuring method of it

Refractive index was independently determined, or this is known from the outset. This is however i.A. not the case.

Will the coated surface by ellipsometry at different

Wavelengths studied, this additional information can help solve the problem. However, measurements at different wavelengths are not possible with all devices. For example, if a laser is used as a light source, this approach is excluded.

Object of this invention

The invention is based on the object of providing a possibility with which both the refractive index and the layer thickness of an ultra-thin layer can be determined by means of ellipsometry. By an ultra-thin layer is meant a layer having a thickness of less than 15 nm.

This object is achieved by this invention. In order to achieve this object, the invention proposes a method with the

Claim 1 mentioned features. Further developments of the invention are the subject of dependent claims.

According to the invention, the procedure is such that two different pairs of the ellipsometric angles Δ and Ψ are measured by two different measurements (measurements A and B) (A _A , ΨΑ, Δ _β and ΨΒ). The two measurements A and B take place on one and the same sample, which is between the two

Measurements are not moved. This will ensure that both measurements are made at the same angle of incidence. The two measurements differ only in terms of the thickness of the coating: Measurement A takes place on a coated surface with a higher layer thickness. Measurement B, on the other hand, takes place on the same surface with a smaller layer thickness.

The smaller layer thickness can also be a layer thickness of the value 0, so that therefore a measurement takes place on the uncoated surface of a substrate.

In both cases, however, the layer thickness is less than 15 nm.

This approach can be realized with different methods: In a temporal sequence, the uncoated surface is first measured (measurement B), then coated and finally measured again (measurement A). The coating takes place only on a part of the surface, while the remaining surface remains uncoated. Subsequently, by means of

Spatially resolved ellipsometry is the boundary between coated and ellipsometry

measure uncoated area. The ellipsometric angles measured on the coated portion of the surface (A _A , ΨΑ) constitute measurement A, while those on the uncoated portion (AB, ΨΒ) constitute measurement B. The surface is first completely coated and removed in a second step on a part of the surface again. Subsequently, by means of

Spatially resolved ellipsometry is the boundary between coated and ellipsometry

measure uncoated area. The ellipsometric angles measured on the coated portion of the surface (A _A , ΨΑ) constitute measurement A, while those on the uncoated portion (AB, ΨΒ) constitute measurement B. The coating takes place on a specially prepared surface (a so-called substrate). This substrate is modified even on part of its surface and native on the remainder. For such substrates, silicon wafers are particularly suitable. These can be prepared by specifically increasing the thickness of the silicon oxide layer on a part of the wafer surface. To achieve this, various methods are available, eg o PECVD (plasma-enhanced chemical vapor deposition)

o Thermal oxidation of silicon

o ion implantation

From the four measured ellipsometric angles (A _A , ΨΑ, Δ _β and ΨΒ) one calculates the size

Subsequently, the refractive index n of the layer is given by the formula

Here ni denote the refractive index of the surrounding medium (eg air), r? 2 the refractive index of the material on which the coating takes place (eg silicon), \ the angle of incidence of the light and a ₂ = arcsin (sina ₁ - nn ₂ ).

After the refractive index n of the layer is known, the thickness c / of the layer is determined by the usual ellipsometric methods.

Further features, details and advantages of the invention will become apparent from the claims and abstract, the wording of which is incorporated by reference into the description, the following description

preferred embodiments of the invention and with reference to the drawing. Hereby show:

Figure 1 shows schematically the measuring arrangement for carrying out of the

Invention proposed method;

FIG. 2 shows a simplified representation of the possibility of displaying two

Coating thickness;

FIG. 3 shows a comparison of the measurement results between a conventional and a measurement according to the invention;

FIG. 4 shows two results of measurements on coated silicon wafers.

FIG. 1 shows a substrate 1 on which a layer 2 is applied.

This layer 2 is to be investigated, with regard to its refractive index and its thickness. A light source 3, for example a laser, is arranged such that the light beam 4 emerging from it enters a polarizer 5 and falls therefrom at an angle of incidence α to a specific point on the surface of the layer 2 to be examined. The light beam 4 is reflected and passes through an analyzer 6 in the detector 7. In this, an evaluation of the reflected light beam with respect to the two ellipsometric variables Ψ and Δ.

The invention now provides that this measurement of the ellipsometric variables is performed once on the layer 2 in its final thickness and once on a thinner layer, for example on the substrate 1 without the layer 2. This is done according to the invention without movement of the substrate 1, so that the angle under which the measurement is made is the same for both measurements.

One way in which it is possible to carry out measurements with different layer thicknesses on the layer 2 is shown schematically in FIG. Here, first, the layer 2 is produced in its final and thus complete thickness and then in the central region 2a, the layer has been partially removed again. Then, at this point, by using a spatially resolved ellipsometry method, once the measurement can be made on the full thickness and once on the reduced thickness.

From the measured ellipsometric quantities Δ and Ψ for the full layer thickness and for the reduced layer thickness, the refractive index n of the layer 2 can then be calculated according to the above-mentioned formulas. Now to FIG. 3. FIG. 3 shows the refractive index of a polymer layer on a silicon wafer. The actual value is 1.55. A conventional ellipsometric measurement (open symbols) gives incorrect values for layers thinner than 15 nm. Using this invention (closed symbols), reliable values are obtained from 5 nm.

FIG. 4 shows a spatially resolved ellipsometric measurement of a coated silicon wafer as a further application example. The upper part of the two pictures shows pure silicon as a reference measurement (Δβ, Ψβ). The lower part is coated with a polymer layer (AA, ΨΑ). The inhomogeneous transition region between them is not relevant. From the four values (AA, ΨΑ, AB and Ψβ) one obtains the refractive index n = 1, 55 and the layer thickness c / = 5.1 nm.

Practical use

The simplest form of applying this invention is to make specific substrates. These are then coated with the material to be examined and examined by means of spatially resolved ellipsometry.

With the invention, the previously applicable limit of 15 nm for the layer thickness and below the refractive index of thinner layers by means of

Ellipsometry can be measured.

As an application example are in particular question: The surface treatment and antireflective coatings of lenses (objectives, measuring instruments);

Production of delay or wave plates;

Semiconductor electronics (fabrication and operation of transistors or diodes)

Claims

claims:

1 . Method for determining the refractive index and / or the layer thickness of a transparent layer (2) using an ellipsometric method, comprising the following method steps:

a pair of the ellipsometric angles Psi (ΨΑ) and delta (AA) is measured at the layer (2) to be examined at a first layer thickness,

a pair of the ellipsometric angles Psi (Ψβ) and delta (Δβ) are measured on the layer (2) to be examined with the same angle of incidence (a) at a second layer thickness,

- From the four measured ellipsometric angles (ΨΑ, ΨΒ, AA, AB) becomes an intermediate value

calculated, and

the refractive index n of the layer to be investigated is calculated according to the following formula:

in which

J

and

the refractive index of the ambient medium,

r? 2 is the refractive index of the substrate, the angle of incidence of the light and

02 = aresin (sinus cri ni / r? 2).

2. The method of claim 1, wherein the smaller layer thickness has the value 0.

3. The method of claim 1 or 2, wherein first measured the uncoated surface of the substrate (1), this then coated and then the coated surface is measured.

4. The method of claim 1 or 2, wherein only a part of the surface of the substrate (1) is coated and the remaining part of the substrate (1) remains free, after which subsequently by means of spatially resolved ellipsometry, the boundary between the coated and the uncoated area of the Substrate (1), wherein the ellipsometric angles measured on the uncoated part of the substrate (1) constitute one measurement and the ellipsometric angle measured on the coated part of the substrate (1) forms the other measurement.

5. The method of claim 1 or 2, wherein the surface of the substrate (1) is completely coated and in a second step, the coating on a part of the surface of the substrate (1) is partially or completely removed, which then subsequently by means of spatially resolved ellipsometry the limit between the coated and the uncoated area of the surface is measured.

6. The method of claim 1 or 2, wherein the surface of the substrate is completely coated, wherein the substrate itself has been previously modified by coating a part of its surface and the remaining part of the substrate is original, then subsequently using spatially resolved ellipsometry the boundary between modified and original surface is measured.