Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-08-14
2003-11-04
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06643027
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to optical reflectometry, and more particularly, to a system and method for measuring the height differential between two adjacent surfaces.
BACKGROUND OF THE INVENTION
With the advent of optical reflectometry based measuring devices capable of distances as small as 10 microns (&mgr;m), precise and accurate measurements of critically small distances can be made. A nonlimiting example of an optical reflectometry based measuring device is the optical thickness gauge (OTG) once sold by Hewlett-Packard (HP 86125A-K1X). The operation and functionality of such an OTG is disclosed in U.S. Pat. Ser. No. 5,642,196, filed on Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, which is entirely incorporated herein by reference. Other exemplary optical reflectometry based measuring devices and their applications, incorporated herein by reference, are disclosed in U.S. Pat. No. 5,473,432, filed on Dec. 5, 1995, and entitled APPARATUS FOR MEASURING THE THICKNESS OF A MOVING FILM UTILIZING AN ADJUSTABLE NUMERICAL APERTURE LENS, U.S. Pat. No. 5,610,716, filed on Mar. 11, 1997, and entitled METHOD AND APPARATUS FOR MEASURING FILM THICKNESS UTILIZING THE SLOPE OF THE PHASE OF THE FOURIER TRANSFORM OF AN AUTOCORRELATOR SIGNAL, U.S. Pat. No. 5,633,712, filed on May 27, 1997, and entitled METHOD AND APPARATUS FOR DETERMINING THE THICKNESS AND INDEX OF REFRACTION OF A FILM USING LOW COHERENCE REFLECTOMETRY AND A REFERENCE SURFACES, U.S. Pat. No. 5,646,734, filed on Jul. 8, 1997, and entitled METHOD AND APPARATUS FOR INDEPENDENTLY MEASURING THE THICKNESS AND INDEX OF REFRACTION OF FILMS USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. No. 5,642,196, filed on Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. No. 5,731,876, filed on Mar. 24, 1998, and entitled METHOD AND APPARATUS FOR ON-LINE DETERMINATION OF THE THICKNESS OF A MULTILAYER FILM USING A PARTIALLY REFLECTING ROLLER AND LOW COHERENCE REFLECTOMETRY, and U.S. Pat. No. 5,850,287, filed on Dec. 15, 1998, and entitled ROLLER ASSEMBLY HAVING PRE-ALIGNED FOR ON-LINE THICKNESS MEASUREMENTS.
FIG. 1
is a block diagram illustrating a conventional optical thickness gauge (OTG)
100
measuring distances associated with a multi-layer film
102
and in communication with a personal computer (PC)
104
. The OTG
100
has at least a low-coherence light source
106
, an optical coupler
108
, an autocorrelator
110
and a probe head
112
. Low-coherence light
114
is generated by the low-coherence light source
106
and injected into waveguide
116
. Waveguide
116
may be any suitable device, such as an optical fiber, that is configured to transfer the low-coherence light
114
to the optical coupler
108
. The low-coherence light
114
propagates through the optical coupler
108
, through the waveguide
118
and into the probe head
112
. Light is reflected back into the probe head
112
, in a manner described below, through the waveguide
118
, through the optical coupler
108
, through the waveguide
120
. The return light
122
is detected by the autocorrelator
110
so that distance measurements can be determined, as described below, by software (not shown) residing in PC
104
.
For convenience of illustration, the waveguide
116
is illustrated as having a separation distance from the low-coherence light source
106
. One skilled in the art will appreciate that the waveguide
116
would be typically coupled directly to the low-coherence light source
106
using well known coupling devices (not shown). Likewise, the waveguide
120
is illustrated as having some amount of separation from the autocorrelator
110
. Waveguide
120
is typically coupled directly to the autocorrelator
110
. For convenience of illustration, waveguide
118
is illustrated as being directly coupled to the optical coupler
108
and probe head
112
. Coupling devices used to couple waveguides
116
,
118
and
120
to devices are well known in the art and are not described in detail or illustrated herein. Furthermore, for convenience of illustration, waveguides
116
,
118
and
120
are illustrated as a rod-like material intended to represent a flexible optical fiber. However, any suitable waveguide device configured to transmit light between the low-coherence light source
106
, the optical coupler
108
, the autocorrelator
110
and the probe head
112
, may be substituted for the waveguides
116
,
118
and/or
120
.
The optical autocorrelator
110
is configured to receive the return light
122
. Detectors (not shown) residing in the autocorrelator
110
provide information such that the autocorrelator
110
generates correlation peaks that are shown on graph
128
. Separation between correlation peaks corresponds to distances between any two light reflecting surfaces.
Optical correlator
110
is coupled to PC
104
via connection
124
. Information from autocorrelator
110
is received by the PC
104
and processed by software (not shown) into correlation information. PC
104
typically displays, on display screen
126
, the correlation results as a graph
128
having correlation peaks, described in greater detail below. That is, distances between correlation peaks correspond to the measurements taken by the OTG
100
.
For convenience of illustration, PC
104
is illustrated as a conventional laptop PC. However, any suitable PC or other processing device may be equally employed to provide the necessary processing of information corresponding to the light signals received by the autocorrelator
110
, and to prepare a meaningful output format that may be interpreted by a user of the OTG
100
for the determination of distances. Furthermore, display
126
may be any suitable device for indicating distance information resulting from measurements taken by the OTG
100
. For example, but not limited to, display
126
may be a conventional, stand-alone cathode ray tube (CRT). Or, a line printer, plotter, or other hard copy device may be configured to accept and indicate correlation information from the autocorrelator
110
.
Light (not shown), entering the probe head
112
via waveguide
118
, first passes through a reference surface
130
. Here, reference surface
130
is illustrated as the bottom surface of a wedge-shaped plate
131
. (For convenience of illustration, wedge-shaped plate
131
is shown from an edge-on viewpoint.) Reference surface
130
is configured to allow a portion of the received light to pass through the wedge-shaped plate
131
and onto film
102
. A portion of the received light (not shown) entering the wedge-shaped plate
131
is reflected from reference surface
130
, back through the probe head
112
, through the waveguide
118
, through the optical coupler
108
and then through the waveguide
120
to be received by the autocorrelator
110
.
FIG. 2
is a simplified graph
200
illustrating the correlation peaks associated with the reflection of light from the reference surface
130
and the surfaces
132
,
134
,
136
and
138
of film
102
(FIG.
1
). For convenience of illustrating the autocorrelation information on graph
200
, the vertical axis corresponding to the magnitude of the correlation peaks is not numbered. One skilled in the art will realize that any appropriate vertical axis numbering system corresponding to the amplitude of the correlation peaks could have been employed, and that such a numbering system is not necessary to explain the nature of the correlation peaks. Similarly, the horizontal axis corresponding to distance has not been numbered on graph
200
. One skilled in the art will realize that any appropriate axis number system corresponding to distance could have been employed, and that such a numbering system is not necessary to explain the nature of the relationship between the correlation peaks illustrated in graph
200
. Thus, one embodiment of the software generating the graph
200
is configured to allow the
Gong William
Rankin Glenn
Tella Richard
Agilent Technologie,s Inc.
Font Frank G.
Lyons Michael A.
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