Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
1999-06-18
2002-05-21
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C356S504000, C356S369000
Reexamination Certificate
active
06392756
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods and apparatus for optically determining physical parameters of thin films deposited on a complex substrate, and in particular to measurements of thin films on complex substrates for obtaining physical parameters such as thickness t, refraction index n and extinction coefficient k.
BACKGROUND OF THE INVENTION
The determination of physical parameters of thin films is very important, since many modern technologies rely on thin films for various functions. For example, thin films are used for optical and/or mechanical protection of surfaces, alteration of surface optical and/or mechanical properties and for many other purposes. For example, in the manufacture of magnetic hard disks thin films exhibiting high hardness and high wearing resistance (e.g., diamond-like-carbon (DLC)) are used to protect the disk surface.
The most commonly investigated physical parameters of thin films include their thickness t, index of refraction n, extinction coefficient k, surface roughness &sgr; (at the interface between the thin film and the substrate on which it is deposited) and energy bandgap E
g
which is related to extinction coefficient k. Knowledge of parameters t, n and k tends to be most important in practical applications. In particular, the thickness t of the thin film is frequently crucial and has to be known to a very high degree of accuracy. This presents considerable difficulty, since t for thin films typically ranges from 1,000 Angstroms down to tens of Angstroms and less. In this range, typical optical measurements are not very reliable.
Various prior art techniques exist for examining thin films. U.S. Pat. No. 3,601,492 to Reichert employs a standard interference technique for measuring film thickness based on observing the interference between the light reflected from the top and bottom surfaces of the thin film. Greenberg et al. teaches in U.S. Pat. No. 5,042,949 that film thickness can be determined by examining the interference pattern and reflectance data from a reflectance pattern, respectively to determine film thickness profile. In U.S. Pat. No. 4,999,509 Wada et al. describe a how to measure thicknesses of several films using a reflectance measuring device.
Still another approach to determining thin film thickness is taught by Hattori et al. in U.S. Pat. No. 5,371,596. In accordance with this technique the light from a light source is modulated to produce a modulated interference light. This modulated light is reflected from the thin film and used by a number of photodetectors to derive film thickness.
Ellipsometry is another technique used to measure physical parameters of thin films. In this method n and k are determined by measuring the change in the state of polarization of the reflected light. Ellipsometry requires complex instrumentation and needs certain sophistication in interpretation of the measurements.
Unfortunately, the above prior art approaches yield less and less satisfactory results for the thin film parameters with decreasing film thickness due to poor signal-to-noise ratios.
To overcome these limitations, several prior art techniques rely on comparisons of reflectance data obtained from thin films and monitoring samples. For example, Sandercock teaches in U.S. Pat. No. 4,355,903 to compare the reflection of polychromatic light from a reference or standard thin film with the reflection obtained from a film of unknown thickness. Mumola teaches in U.S. Pat. No. 5,337,150 the use of a separate reference wafer which has a thin film layer similar to that being coated on the actual wafer. A broadband beam of radiation illuminates the sample wafer and yields a reflected beam having a unique spectral radiation (spectral signature). Film thickness is identified when the spectral pattern of this reflected beam matches that of the beam reflected from the reference wafer. Similarly, U.S. Pat. No. 5,101,111 to Kondo teaches a method of measuring film thickness using a reflectance sample having a known reflectance for each value of film thickness dx. The reflectances for the various thicknesses are stored in a table and compared to those obtained when examining a sample.
In U.S. Pat. No. 4,555,767 Case et al. disclose a technique and apparatus for measuring the thickness of epitaxial layers by infrared reflectance. The technique relies on taking the Fourier transform of the signal reflected from the epi layer using a Fourier transform IR spectrometer and comparing the result with theoretical values obtained beforehand. In U.S. Pat. No. 5,523,840 Nishizawa et al. also rely on a Fourier transformation to obtain an interference waveform dispersion spectrum which is compared with a waveform obtained by numerical calculation using an optical characteristic matrix. Waveform fitting between theoretical and measured spectra is used to obtain film thickness. U.S. Pat. No. 5,241,366 to Bevis et al. discloses a thin film thickness monitor which performs the measurement based on a comparison between the reflection of polychromatic light from a reference thin film and the sample thin film. U.S. Pat. No. 5,365,340 to Clapis et al. and U.S. Pat. No. 5,555,472 to Ledger also teach how to measure film thicknesses based on reference samples yielding reference reflectance signals.
All of the above optical approaches to measuring thin film thickness and any other physical parameters of the thin film are complicated and not capable of providing the desired levels of accuracy. In particular, the above techniques can not be used for measuring thin films in thickness ranges of tens of Angstroms with an accuracy of less than 5 Angstroms. Moreover, none of these methods can determine the n, k and t values of a thin film simultaneously.
The prior art also teaches non-optical methods of determining thin film thicknesses. For example, atomic force microscopy (AFM) employing a deflectable stylus can be used to determine film thicknesses by surface profiling. The drawbacks of this technique are that it requires a physical step which is destructive to the thin film or degrades its surface. In addition, this technique can not be used to determine other physical parameters of the thin film, such as the n, k and E
g
values.
In U.S. Pat. No. 4,905,170 Forouhi et al. describe an optical method for determining the physical parameters of a thin film in amorphous semiconductors and dielectrics. This technique is very accurate and it takes into account the quantum mechanical nature of the light and thin film interaction. Unfortunately, it can not generate sufficiently accurate thickness readings and simultaneously determine n and k values for thin films deposited on substrates having a relatively “smooth” reflectance spectrum. Such substrates are very commonly used, however, and include many typical substrate materials, e.g., Si, quartz, Mg, Cr and Ni and AlTiC alloys used in the semiconductor and magnetic storage technologies as well as polycarbonate (PC) used in optical disks.
Hence, there is a pressing need to develop an approach which will enable one to measure the thickness as well as other physical properties of thin films on various substrates to a high degree of accuracy. Specifically, it would be very desirable to provide a non-destructive measurement method for determining film thickness to an accuracy of 5 to 2 Angstroms or less in films whose thickness is less than 100 Angstroms or even less than 10 Angstroms. Furthermore, the method should be capable of identifying additional physical parameters of the thin film such as the values of n, k and E
g
.
OBJECTS AND ADVANTAGES OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a method and an apparatus for optically determining physical parameters of thin films. In particular, the apparatus and method should enable one to determine film thickness t to within 5 Angstroms and yield accurate values of physical parameters including n, k and E
g
.
It is another object of the invention to enable one to evaluate the above physical parameters of thin fil
Forouhi Abdul Rahim
Harrison Dale A.
Li Guoguang
Xu Weilu
Zhu Hongwei
Lumen Intellectual Property Services Inc.
N&K Technology, Inc.
Rosenberger Richard A.
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