Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2002-06-17
2003-12-02
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S216000
Reexamination Certificate
active
06657216
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the measuring of the profile of a surface and in particular to measuring a profile using a confocal displacement sensor.
BACKGROUND
There are a multitude of applications where it is desirable to measure surface properties of a sample. The roughness of the surface, the curvature of the surface and the height of steps on the surface are typical application examples. Many diverse technologies can be employed to accomplish this metrology task. Common examples of devices include a contact profilometer, an atomic force microscope (AFM), a laser confocal displacement sensor, a laser triangulation displacement sensor, and a differential interferometer.
Each technology has advantages and disadvantages with respect to specific parameters, including vertical resolution, lateral resolution, maximum step size, and surface feature geometry. Other factors that determine the usefulness of a particular technology for a specific application include maximum measurement time, non-contact requirement, tilt correction, and material properties.
The principles associated with a confocal microscope as used in conventional laser confocal displacement sensors are described in U.S. Pat. No. 3,013,467, which is incorporated herein by reference. The basic principal of the confocal displacement sensor involves rejecting a large fraction of the light that is not in the focal plane of the surface of the sample, thereby increasing the contrast and resolution of the resultant image. One typical commercial version of a laser confocal displacement sensor is sold as the LT series by Keyence Corporation of America, located in Woodcliff Lake, N.J.
FIG. 1
is a block diagram of a conventional laser confocal displacement sensor
10
that is used to measure the surface of a sample
30
. A laser
20
produces a beam of light that is focused by objective lens
32
onto the surface of sample
30
. The beam strikes the surface of sample
30
at normal incidence.
After reflection from the surface of the sample
30
and transmission through objective lens
32
, a fraction of the beam is reflected by beamsplitter
35
toward detector
60
. Between detector
60
and beamsplitter
35
are a detector lens
40
and a pinhole
50
. The focal plane of the objective lens
32
and the focal plane of the detector lens
40
(the plane of the pinhole
50
) are made to be confocal. Typically, the objective lens
32
is scanned orthogonally with respect to the plane of the sample
30
over a range exceeding the expected step size on the surface of the sample using a piezoelectric or voice coil driver
36
. An encoder associated with piezoelectric or voice coil driver
36
allows the determination of the position of the objective lens at any given time.
FIG. 2
shows a typical plot of the position of the objective lens along the X axis versus the measured detector signal intensity along the Y axis. The detector signal reaches a maximum when the beam is focused on the sample surface (and the pinhole) shown in
FIG. 2
as position B. The detector signal falls off when the focal plane of the objective lens
32
is above or below the sample surface, shown as positions A and C in FIG.
2
.
To make a measurement at one point on the surface of the sample
30
, as shown in
FIG. 1
, the intensity measured by the detector
60
is recorded along with the position of the objective lens
32
. The position of the lens
32
at the maximum detector intensity is determined and yields the relative height of the surface of the sample
30
at that measurement position. Two-point, line scans and area scans can be executed yielding the step height, line profile or area profile of a portion of the surface of the sample
30
by performing the measurement technique at a plurality of positions.
To obtain a more accurate and precise measurement using a standard laser confocal displacement sensor, the resultant height profile must be corrected for tilt. For example, if two points are measured, the tilt of the sample
30
can affect the height difference between the two points. To compensate for tilt, a plurality of positions located on a line through the two measurement points with the same height are measured, and the tilt of sample
30
with respect to the detector is calculated. The determined tilt is then used to correct (level) the measured height profile. The step height can then be calculated with improved accuracy and precision.
In addition, mechanical vibrations that cause a displacement of the sensor with respect to the sample surface will cause an error in the resultant height profile. Low frequency mechanical vibrations are common and can cause considerable measurement error. Minimizing this error source may require an expensive and specialized environment for the sensor. If the vibrations are originating from a process tool, isolation may be even more difficult. Another source of error is any vertical motion of the stage as it moves horizontally to translate the sample to another measurement location, especially if the measurement is made while the stage is moving. This will become a problem as the step height to be measured gets closer in magnitude to the vertical stage error.
An enhancement to the standard laser confocal displacement sensor is described in “The Optical Probe Using Differential Confocal Technique for Surface Profile” by Wang, Fusheng, Tan, Jiubin and Zhao, Weiquan in Process Control and Inspection for Industry, Shulian, Wei Gao, Editors, Proceedings of SPIE vol. 4222 (2000), which is incorporated herein by reference. The enhanced laser confocal displacement sensor uses two unique sensors to receive the reflected signal. The light is reflected from a single point on the surface of the sample. The pinhole of the first detector is made to be a specific distance ahead of the confocal position while the pinhole of the second detector is made to be the same specific distance behind the confocal position. The difference between the two resultant detector intensity versus focal plane position curves yields a curve with a steep slope at the zero crossover point. The focal plane position at the zero crossover point indicates the height of the surface of the sample. The steeper the slope of the curve at this point, the better the resolution capability. As with the displacement sensor
10
, shown in
FIG. 1
, the resultant height profile must be corrected for tilt and is subject to error caused by vibration.
What is needed is a displacement sensor that can accurately measure the surface profile of a sample without the need to correct for tilt and that is relatively insensitive to vibration errors.
SUMMARY
A confocal displacement sensor in accordance with the present invention uses one or two laser wavelengths and produces two spots on a sample surface. The reflected intensities from the two spots are detected and measured after passing through one or more pinholes. Since the focal plane of the objective lens when focused on the sample surface and the focal plane of the detector lens (the plane of the pinhole) are confocal, the maximum detector intensity corresponds to the height of the surface for that point. This is done for both spots at each measurement location.
The resultant height profile advantageously does not need to be corrected for tilt as is common with all single point surface measurement techniques. A differential scan can be performed with the two spots relatively close together to generate the slope of the height profile. Integrating this profile yields the height profile of the scan. A referential scan can be performed by scanning the reference point across an area of constant height and the measurement point scanned across the feature to be measured to directly generate the height profile.
In accordance with one embodiment, the confocal displacement sensor includes at least one light source and a means for producing a first light beam and a second light beam. The displacement sensor also includes an objective lens for focusing the first light beam and the second light beam
Le Que T.
Nanometrics Incorporated
Silicon Valley Patent & Group LLP
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