Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2002-11-07
2004-05-18
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S446000
Reexamination Certificate
active
06738136
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to metrology instruments, and more particularly to spectrometry instruments that measure the reflectivity of a sample.
BACKGROUND
In semiconductor manufacturing and other fields it is desirable to make quantitative measurements of a sample's reflectivity properties over very small, selectable areas and over as broad a band of wavelengths as possible. Instruments for making these measurements typically incorporate microscope-like objective lenses for focusing light on the sample. One type of illumination source is a Xenon arc lamp. In order to scan to various positions on the sample of interest a portion of the optics can be moved with respect to a stationary portion of the optics, or the sample can be moved with respect to at least some portion of the optics, or both.
A common issue with such instruments is guaranteeing the stability of the light which is incident upon the sample, or at least knowing the spectral intensity of the incident light, so that the detected light reflected from the sample can be evaluated relative to the intensity of the light incident on the sample. Since reflectivity is defined as the ratio of the intensity of light reflected off the sample relative to the intensity of light incident upon the sample, accurate reflectivity measurements depend knowing the incident light intensity.
There are several factors that can make it difficult to determine the intensity of the light upon the sample. One factor is that the characteristics of most sources of light change with time, and thus the intensity of the incident light can vary with time. Another factor is that where there is relative motion of the illumination source and the rest of the optics, either via (nearly) collimated light paths or optical fibers, there can be changes in the transmission efficiency of illuminating light as a function the scan position, or scan state. Here scan state includes the history of previous scan positions. This is important, for example, with some architectures using a fiber to transmit light from the light source. One prior system is shown in US Patent Application, publication number US 2002/0021441 A
1
(SMALL SPOT SPECTROMETRY INSTRUMENT WITH REDUCED POLARIZATION) and in PCT application, international publication number WO 00/57127 (METHOD AND APPARATUS FOR WAFER METROLOGY); both of these references are incorporated herein by reference in their entirety.
FIG. 1
shows another type of prior system
100
. The system
100
includes a light source
102
and a transmission means
104
for light generated by the light source
102
The light transmitted through the transmission means
104
is then transmitted through a collimating, lens
108
, and leaves the collimating lens as light beam
106
. The light beam is then incident upon a beam splitter
110
. A first beam
140
is transmitted from the beam splitter through a lens
144
and then through a plate
146
having pinhole to receive the first beam
140
. The first beam
140
is then transmitted through a transmission means
148
and received by a detector
150
. In response to the light received, the detector
150
generates a monitor signal corresponding to the received light. This monitor signal from the detector
150
is received by a processor
160
which analyzes the monitor signal and uses it relative to a signal generated by detector
130
.
In addition to the beam
140
being transmitted through the beam splitter
110
, beam
112
is also reflected from the beam splitter
110
through an objective lens
114
and onto a spot
118
on a sample
116
being analyzed. Some portion
113
of the light
112
is reflected off the sample and back through the objective lens
114
. This light
113
is further transmitted through the beam splitter
110
and off a turn mirror
122
and through a lens
124
. The resulting light beam is then transmitted through a pinhole in a plate
126
and into a transmission means
128
. The light transmitted through the transmission means
128
is received by the detector
130
. In response to receiving this light the detector
130
generates a sample signal which corresponds to the received light. This sample signal is received by the processor
160
where it is analyzed relative to the monitor signal received from the detector
150
.
The fact that prior systems provide for transmitting a monitor beam
140
and a measurement beam
113
through different transmission paths and provide for using different detectors (
150
and
130
) for detecting the monitor light beam and the measurement light beam introduces a number of potential sources which could generate variations in the monitor signal relative to the measurement signal which are not related to the reflective properties of the sample. What is needed is a system which reduces possible sources of extrinsic variations in the monitor beam relative to the measurement beam.
SUMMARY OF THE INVENTION
One embodiment herein provides a system for measuring characteristics of a sample. This system includes a light source for generating a beam of light which is directed toward a sample, and a mirror which can be moved between a first position and a second position, wherein in the first position the mirror is positioned between the light source and the sample such that light generated by the light source is reflected off the mirror and transmitted through a first path. The first path consists of a reflection path. In the second position the mirror is positioned such that it is not between the light source and the sample, and light generated by the light source is reflected off the sample and transmitted through the reflection path. This system also provides a detector coupled to the reflection path which generates a monitor signal in response to receiving light reflected from the mirror, and generates a measurement signal in response to light reflected from the sample.
Another embodiment includes a method for determining characteristics of a sample in a system having a light source, and a movable mirror. The method includes generating a light beam and directing the light beam toward the sample, and positioning the movable mirror such that it is in a first position, where it reflects the light beam along a first path, wherein the first path consists of a reflection path. The method also includes generating a monitor signal which corresponds to the light reflected from the mirror, and positioning the movable mirror such that it is in a second position, where it does not reflect the light beam which is directed toward the sample, wherein the light beam which is directed toward the sample is reflected off the sample along the reflection path. The method of this embodiment also includes generating a measurement signal which corresponds to the light reflected from the sample, and analyzing the measurement signal relative to the monitor signal to determine properties of the sample.
Another embodiment includes a system for measuring characteristics of samples. The system includes a light source for generating a beam of light, and beam splitter for directing the beam of light toward a sample. The system also includes a lens disposed between the beam splitter and the sample for focusing the beam of light on the sample, such that a measurement beam of light is reflected off the sample, wherein after the measurement beam is reflected off the sample, it is transmitted through a reflection path. The system includes a detector positioned to receive light transmitted through the reflection path wherein in response to receiving light transmitted through the reflection path the detector generates a signal corresponding to the light transmitted through the reflection path, and a mirror which can be moved between a first position and a second position, wherein in the first position the mirror is positioned between the beam splitter and the sample, such that light directed by the beam splitter toward the sample is incident upon the mirror and reflected through a first path, wherein the first path consists
Norton Adam
Sezginer Abdurrahman
Smedt Rodney
Stanke Fred E.
Evans F. L.
Stallman & Pollock LLP
Therma-Wave, Inc.
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