WO1996003615A1

WO1996003615A1 - Film thickness mapping using interferometric spectral imaging

Info

Publication number: WO1996003615A1
Application number: PCT/US1995/008708
Authority: WO
Inventors: Dario Cabib; Robert A. Buckwald; Michael E. Adel
Original assignee: CI Systems Israel Ltd
Current assignee: CI Systems Israel Ltd
Priority date: 1994-07-26
Filing date: 1995-07-12
Publication date: 1996-02-08
Anticipated expiration: 1997-01-26
Also published as: JPH10509508A; IL110466A; IL110466A0

Abstract

A method of determining the thickness map of a film (14) overlying a substrate (14). This method includes illuminating (10) the film simultaneously from different angles and analyzing spectral intensity of the radiation reflected by each point on the film (14). The analysis is effected by collecting reflected radiation from the film (14), passing the radiation through an interferometer (16) which outputs modulated radiation corresponding to a predetermined set of linear combinations of the spectral intensity of the radiation emitted from each pixel, simultaneously and separately scanning optical path differences generated in the interferometer (16) for each pixel, focusing the radiation outputted from the interferometer (16) on a detector array, and processing the output of the detector array to determine the spectral intensity of each pixel thereof to obtain a spectral intensity distribution. Finally, the method includes further processing the spectral intensity distribution to determine the spatial distribution of the thickness of the film (16).

Description

FILM THICKNESS MAPPING USING INTERFEROMETRIC SPECTRAL IMAGING

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for spectral

analysis of images to determine the thickness of a thin film, and

particularly for spatially resolving film thicknesses of a coating deposited

over the surface of a silicon wafer or other similar materials (for example,

a flat panel display).

Measuring film thickness by reflectance spectroscopy is well known:

see for example P. S. Hauge, "Polycrystalline silicon film thickness

measurement from analysis of visible reflectance spectra", J. Opt. Soc.

Am., No. 8, August 1979, and the book by Milton Ohring: "The material

science of thin films", Academic Press Ltd., 1992. In today's

microelectronic device manufacturing processes, the uniformity of the

deposited films over a wafer is gaining importance as time goes on,

because a good uniformity insures identity among the finished product

chips. The size of the chips is also decreasing, so that the uniformity

tolerance is becoming stricter. In addition, to insure high yield (low

rejects) and efficient (low cost) manufacturing, the wafer inspection

requires higher automation, shorter time, higher accuracy, and wider

thickness range. As a result, the film thickness map of the wafer, as one of the many

inspections done during the manufacturing, must be done accurately, fast,

on a large number of points (test sites), and at a wide thickness range.

Today, film thickness mapping instruments are based on

ellipsometry or on reflectance spectroscopy. Only the latter is addressed

herein. As examples of prior art in this field we mention the SpectraMap

SM-300 and the FT-500 of Prometrix. The spectra are measured point by

point by moving the wafer on a translation stage, in order to complete one

thickness map. This takes time, it requires high movement accuracy,

because of the high spatial resolution required, and increases the wafer

handling, which is practical only when the wafer is outside a deposition

chamber (therefore it cannot be done in-situ). In fact, the fastest thickness

mapping mentioned by present manufacturers of film thickness equipment

is hundreds of points in a few seconds.

There is thus a recognized need for, and it would be highly

advantageous to have a method and apparatus for determining the spatial

distribution of the thickness of a film overlying a substrate, more quickly,

with higher spatial resolution (more test sites), without the need to move

the wafer with respect to the measuring instrument when going from a test

site to another (higher accuracy, and less wafer handling with the potential

for in-situ monitoring), and easily measure the widest thickness range

possible. The present invention relates to a method and apparatus for mapping

film thickness on Silicon wafers or similar substrates, which does not

require moving the wafer (making the results faster and spatially more

accurate, and potentially capable of being done in-situ), reaching tens of

thousands of pixels in a few seconds (not hundreds as stated in the present

commercial literature), and which has the potential to measure, in the same

time as other potentially competing methods (mentioned below), a wider

thickness range.

A spectrometer is an apparatus designed to accept light, to separate

(disperse) it into its component wavelengths, and detect the spectrum. An

imaging spectrometer is one which collects incident light from a scene and

analyzes it to determine the spectral intensity of each pixel thereof.

The former measures the spectrum only at one point, therefore, with

such an instrument, the wafer must be moved point by point relative to the

instrument, and will have the above mentioned drawbacks of long

measurement time and position accuracy.

The latter, i.e., an imaging spectrometer or spectral imager, can be

of different types: a technology similar to the one used for resource

mapping of the earth surface from airplanes and satellites could be used for

film thickness mapping (see, for example, J. B. Wellman, Imaging

Spectrometers for Terrestrial and Planetary Remote Sensing, SPIE

Proceedings, Vol.: 750, p. 140 (1987)). However, this technique is based on grating technology to spectrally disperse the light: this brings the

following drawback.

A grating has higher order diffraction: in order for it to be useful for

spectral measurements, its spectral range must be limited by blocking the

wavelengths outside a so called "octave" of wavelengths, for example 0.4

to 0.8 μ; therefore an instrument based on a grating cannot have a

wavelength range wider than one in which the ratio between the higher and

the lower wavelength is larger than 2. This problem can be solved by

measuring separately different octaves by rotating the grating to different

angles, or by using several gratings simultaneously. However, these

solutions increase the measurement time or complicate the instrument

optics. The same problem is encountered by liquid crystal and acousto-

optic crystal tunable filters. The importance of the wavelength range is

related to the film thickness range to be measured by the instrument. In

fact, as is well known in the optics literature, high precision film thickness

measurements are difficult to obtain by reflection spectroscopy when the

thickness t of the film is less than (λ/4)n, where λ is the minimum

wavelength of the instrument sensitivity range, and n is the refractive index

of the film. Therefore, the wider the wavelength range to which the

instrument is sensitive, the wider the thickness range that it can measure.

It is also well known (see, for example, the book R. J. Bell,

"Introductory Fourier Transform Spectroscopy", Academic Press 1972), that interferometers do not have this limitation, and therefore can more easily

measure a wider range of thicknesses.

Our copending U.S. Patent Application No. 08/107,673, which is

incorporated by reference in its entirety for all purposes as if fully set forth

herein, discloses a method of analyzing an optical image of a scene to

determine the spectral intensity of each pixel of the scene, which includes

collecting incident light from the scene, passing the light through an

interferometer which outputs modulated light corresponding to a

predetermined set of linear combinations of the spectral intensity of the

light emitted from each pixel; scanning the light beam entering the

interferometer with respect to the interferometer or scanning the

interferometer itself, to scan the optical path difference (OPD) generated

in it for all the pixels of the scene separately and simultaneously; focusing

the light outputted from the interferometer on a detector array, and

processing the output of the detector array to determine the spectral

intensity of each pixel thereof.

The above-referenced patent application provides a method and

apparatus for spectral analysis of images which better utilizes all the

information available from the collected incident light of the image to

substantially decrease the film thickness measurement time, reach higher

spatial resolution, and increase thickness dynamic range, as compared to

other existing instrumentation. From here we can see how our method can give a thickness map of tens of thousands of pixels in a few seconds.

Presently available detector matrices have 16,384 (128x128) pixels which

are scanned at 1 ,000 frames per second (16MHz). As a consequence, since

each frame gives the separate and simultaneous information about the OPD

of every pixel of the image (which is equivalent to the spectral information

by Fourier Transform), at the end of a second all the needed spectral

information is collected for all those pixels.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of

determining the thickness map of a film overlying a substrate, comprising

the steps of: (a) illuminating the film simultaneously from different angles;

(b) analyzing spectral intensity of the radiation reflected by each point on

the film, the analysis effected by: (A) collecting reflected radiation from

the film; (B) passing the radiation through an interferometer which outputs

modulated radiation corresponding to a predetermined set of linear

combinations of the spectral intensity of the radiation emitted from each

pixel; (C) simultaneously and separately scanning optical path differences

generated in the interferometer for the each pixel; (D) focusing the

radiation outputted from the interferometer on a detector array; and (E)

processing the output of the detector array to determine the spectral

intensity of each pixel thereof to obtain a spectral intensity distribution; and (c) further processing the spectral intensity distribution to determine the

spatial distribution of the thickness of the film.

Also according to the present invention, there is provided an

apparatus for determining the thickness map of a film overlying a substrate,

comprising: (a) means for illuminating the film simultaneously from

different angles; (b) means for collecting reflected radiation from the film

(c) an interferometer; (d) means for passing the radiation through the

interferometer such that it outputs modulated radiation corresponding to a

predetermined set of linear combinations of the spectral intensity of the

radiation emitted from each pixel; (e) a detector array; (f) means for

simultaneously and separately scanning optical path differences generated

in the interferometer for each pixel; (g) means for focusing the radiation

outputted from the interferometer on the detector array; (h) means for

processing the output of the detector array to determine the spectral

intensity of each pixel thereof to obtain a spectral intensity distribution; and

(i) means for further processing the spectral intensity distribution to

determine the spatial distribution of the thickness of the film for all the

above pixels more quickly without wafer movement and with wider

thickness range than other possible methods.

According to further features in preferred embodiments of the

invention described below, the further processing includes use of constant angle reflection interference spectroscopy techniques (CARIS, see Ohring

reference above), or other spectral analysis algorithms.

According to still further features in the described preferred

embodiments, the further processing includes compensation for different

angles of incidence of the radiation.

The present invention successfully addresses the shortcomings of the

presently known configurations by providing a method and apparatus for

film thickness mapping which is faster and more accurate, which does note

require wafer movement and which is effective over a larger thickness

range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with

reference to the accompanying drawings, wherein:

FIG. 1 schematically depicts an apparatus and method according to

the present invention where the spectral imager is any of the various types

described in copending U.S. Patent Application No. 08/107,673;

FIG. 2 shows a wafer surrounded by four reflectance standards for

supplying real time compensation for illumination intensity drift. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of an apparatus and method for measuring

the spatial distribution of film thickness on a wafer or other optical

material substrate, using spectral imaging.

The principles and operation of an apparatus and method according

to the present invention may be better understood with reference to the

drawings and the accompanying description.

Referring now to the drawings, Figure 1 schematically depicts an

apparatus and system according to the present invention. A suitable

radiation source 10, preferably a light source, such as a halogen lamp, is

passed through a diffuser 12. The light may be guided from source 10 to

diffuser 12 through an optical fiber (not shown).

Light passing through diffuser 12 impinges on a wafer 14 on which

is found the film whose film thickness distribution is to be determined.

The angle of incidence of light from diffuser 12 onto wafer 14 relative to

the normal varies. Preferably, the variation of incidence angle is accounted

for in the processing of the data to determine the spatial thickness

distribution of the film.

Some of the light emerging from the film on wafer 14 enters a

spectral imaging system 16 which analyzes the optical image of the film

to determine the spectral intensity of each pixel thereof and produce a

spectral intensity distribution. Spectral imaging system 16 is preferably of the type disclosed in

U.S. Patent Application No. 08/107,673 which is incorporated by reference

in its entirety as if fully set herein. Briefly, spectral imaging system

includes means for collecting incident radiation from the film, an

interferometer, which is preferably a Sagnac, a Fabry-Perot or a Michelson

interferometer, means for passing the light through the interferometer such

that it outputs modulated light corresponding to a predetermined set of

linear combinations of the spectral intensity of the light emitted from each

pixel, scanning the light beam entering the interferometer with respect to

the interferometer or scanning the interferometer itself, to scan the optical

path difference (OPD) generated in it for all the pixels of the scene

separately and simultaneously, a detector array, means for focusing the

light outputted from the interferometer on the detector array, and means for

processing the output of the detector array to determine the spectral

intensity of each pixel thereof to obtain the spectral intensity distribution.

The spectral intensity distribution produced by spectral imaging

system 16 is further processed, using, for example, a suitable computer, to

determine the spatial distribution of the thickness of the film.

An apparatus and method according to the present invention may be

used to carry out spectral imaging on silicon wafers in order to obtain

spatially resolved film thickness over the wafer surface. To use an apparatus and method according to the present invention

one may start with a bare silicon wafer, i.e., a silicon wafer which does not

bear a film on its surface. The bare wafer is placed on a suitable wafer

positioned and the light source is aligned to give the best possible

uniformity over the wafer area.

A spectral measurement taken from a single point or from all points

on the wafer can serve as a "white calibration" to account for the spectral

response of the system as a whole or a point by point basis. Following the

calibration the bare wafer is replaced with the actual wafer the thickness

of whose film is to be measured.

Various processing techniques can be utilized to convert the spectral

data obtained into meaningful thickness information. One of the simplest

is the technique of constant angle reflection interference spectroscopy

(CARIS) (see, for example, M. Ohring, "The Material Science of Thin

Films" Academic Press, 1991, ch. 6.2). In order to use this technique the

thickness of the film must be such that at least two interference minima are

observed within the spectral range of the instrument. In cases where more

than two minima are observed in the measured spectral range the average

d can be taken and a different n is then entered for each pair of minima.

Other algorithms may be suitable in cases where there are no minima.

The accuracy and precision of an apparatus and method according

to the present invention may be enhanced by increasing the uniformity of the illumination of the wafer. The performance could be further enhanced

by taking into account the spatial variation in the angle of incidence of the

incident radiation. Similarly, in assessing the spectral locations of the

interference minima, it would be better to use a more sophisticated

parabolic fit rather than using a fitted minimum.

Better computational techniques than the constant angle reflection

interference spectroscopy techniques described above may be used to

further enhance the performance of an apparatus and method according to

the present invention. Thus, unlike the technique described herein for

calculating film thickness which relies on spectra obtained by Fourier

transformation of the point by point interferograms, more accurate results

may be obtainable which allow for the direct calculation of film thickness

from the interferograms rather than from the spectra. Such techniques may

be particularly useful for film thickness calculations in cases of more than

a single layer.

Furthermore, apparatus and methods according to the present

invention could be further improved by providing reflectance standards 18

(Figure 2), preferably four, for supplying real time compensation for

illumination intensity drift. These standards are preferably placed at the

four corners of the spectral imager field of view and may be small pieces

of silicon wafers. The spectral images of reflectance standards 18 are

detected simultaneously with that of the wafer which allows for the real time compensation of time variations in any system parameters, such as

light source, optics, electronics, and the like, thereby improving the

measurement accuracy and repeatability.

Finally, an algorithm based on the spectral difference, pixel by pixel.

and wavelength by wavelength between two spectral images, can be useful

for inspection of semiconductor devices. For alignment of coatings,

imperfections or deviations, contaminations, or other defects, the difference

of two spectral images can be used to compare a device in production with

a reference device, taken as standard.

While the invention has been described with respect to a limited

number of embodiments, it will be appreciated that many variations,

modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A method of determining the thickness map of a film

overlying a substrate, comprising the steps of:

(a) illuminating the film simultaneously from different angles;

(b) analyzing spectral intensity of the radiation reflected by each

point on the film, said analysis effected by:

(A) collecting reflected radiation from the film;

(B) passing the radiation through an interferometer

which outputs modulated radiation

corresponding to a predetermined set of linear

combinations of the spectral intensity of the

radiation emitted from each pixel;

(C) simultaneously and separately scanning optical

path differences generated in the interferometer

for said each pixel;

(D) focusing the radiation outputted from said

interferometer on a detector array; and

(E) processing the output of the detector array to

determine the spectral intensity of each pixel

thereof to obtain a spectral intensity

distribution; and (c) further processing said spectral intensity distribution to

determine the spatial distribution of the thickness of the film.

2. The method according to claim 1 , wherein said interferometer

is selected from the group consisting of Sagnac, Fabry-Perot and Michelson

interferometer.

3. The method according to claim 1, wherein the different

incident angles are achieved by placing a diffuser between said source and

said wafer.

4. The method according to claim 1, further comprising

diffusing said illuminating radiation prior to its incidence upon the film.

5. The method according to claim 1, further comprising

obtaining an additional spectral image by measuring a reference wafer and

wherein said further processing includes a point by point normalization

based on said additional spectral image to compensate for variations in

instrument response within its field of view and of illumination of different

regions of the film.

6. The method according to claim 1, wherein said further

processing includes compensation for different angles of incidence of said

radiation.

7. An apparatus for determining the thickness map of a film

overlying a substrate, comprising:

(a) means for illuminating the film simultaneously from different

angles;

(b) means for collecting reflected radiation from the film;

(c) an interferometer;

(d) means for passing the radiation through said interferometer

such that it outputs modulated radiation corresponding to a

predetermined set of linear combinations of the spectral

intensity of the radiation emitted from each pixel;

(e) a detector array;

(f) means for simultaneously and separately scanning optical

path differences generated in the interferometer for said each

pixel;

(g) means for focusing the radiation outputted from said

interferometer on said detector array; (h) means for processing the output of the detector array to

determine the spectral intensity of each pixel thereof to

obtain a spectral intensity distribution; and

(i) means for further processing said spectral intensity

distribution to determine the spatial distribution of the

thickness of the film.

8. The apparatus according to claim 7, wherein said different

angles are caused by a diffuser located in the path of the illuminating

radiation.

9. The apparatus according to claim 7, wherein said

interferometer is selected from the group consisting of Sagnac, Fabry-Perot

and Michelson interferometer.

10. The apparatus according to claim 7, wherein said means for

further processing includes means for performing a constant angle

reflection interference spectroscopy technique.

1 1. The apparatus according to claim 7, wherein said means for

further processing includes means for compensating for different angles of

incidence of said radiation.

12. The apparatus according to claim 7, further comprising a

plurality of reflectance standards for supplying real time compensation for

illumination intensity, detector response and electronic drift.