Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-02-16
2001-04-17
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S244000
Reexamination Certificate
active
06219145
ABSTRACT:
BACKGROUND OF THE INVENTION
It is often desirable to quantitatively measure the motion of a structure. This can be carried out using various instruments. A particularly useful family of said instruments uses optical techniques.
U.S. Pat. No. 4,619,529, issued Oct. 28, 1986, “Interferometric contact-free measuring method for sensing motional surface deformation of workpiece subjected to ultrasonic wave vibration”, teaches a method for observing sound waves on a workpiece by interfering two beams that are reflected from different points on the workpiece. The pulsing relates to the pulsed laser beam that generates the sound waves on the workpiece. The interferometer beams are not pulsed.
U.S. Pat. No. 3,572,936, issued Mar. 30, 1971, “Stroboscopic Interferometric Holography”, and U.S. Pat. No. 4,999,681, issued Mar. 12, 1991, “Real-time holographic interferometry with a pulsed laser and flicker-free viewing”, teach two methods for producing a hologram of a vibrating object by using stroboscopic illumination. The motion is determined from the generation of a second reference hologram of the rotated object or the object at rest. The second hologram is produced as a second image of the first hologram to produce a interference that determines the motion.
A publication by O. Kwon et al, Opt Lett 12: (11) 855-857 November 1987, teaches a method of pulsed source interferometry. A conventional interferometer is equipped with a Q-switched Nd: YAG laser which is capable of generating high intensity pulses. This is necessary since the authors use only a single pulse to record the interferometric fringe pattern with a camera. The limitation to using a single pulse is overcome by the use of a grating to generate three interferograms which are acquired simultaneously by three cameras. In addition two gratings are used to generate the required phase shifts, restricting the source to be monochromatic due to the inherent chromatic dispersion of a grating.
A publication by S Nakadate et al.,Opt Acta 33: (10) 1295-1309 October 1986, teaches holographic interferometry. There are many other systems which do the same. In these methods, the contours of vibration amplitude are given as a fringe pattern.
One particularly useful device is based on optical interference using, e.g., a Michaelson Interferometer.
For example, an optical profiler is available from the company WYKO, under the name of WYKO RST Plus Optical Profiler. This is a scanning imaging white light interferometer. A block diagram of the device has the structure shown in FIG.
1
. An incandescent light source
100
is focused through lens
102
to half mirror
104
. The light is reflected down to a microscope
110
. The light passes through microscope objective
112
, to a beam splitter
114
. The beam splitter
114
produces a reference beam
116
that is reflected to eventually recombine with the reflected object beam.
The object beam
118
passes to the object being imaged at
120
, and is reflected. This beam then recombines with the reference beam
116
, to produce an interference. A CCD video camera
125
images the operation. This system has the ability to detect minute features on the surface of the sample
99
.
SUMMARY OF THE INVENTION
The present inventors recognized that this instrument as configured is capable of analyzing only stationary structures. Any vibration on the sample blurs the interference pattern. This prevents the sample from being accurately analyzed.
The present disclosure teaches an instrument that allows interferometric detection of moving structures. This is done by pulsing the output.
According to the present system, the optical profiler is modified to allow it to image certain vibrating structures, and specifically microelectrical machined (MEMS) devices.
The present disclosure teaches a method and an instrument for determining periodic motion of structures, specifically micromachined structures. The instrument is an imaging interferometer equipped with a pulsed illumination source. The illumination source is pulsed synchronously and with a predetermined relationship to the motion of the structure thus immobilizing what would otherwise be a rapidly changing interference pattern which is imaged by a camera.
In a preferred mode, every frame output by a camera represents an average (integral) of interference patterns during multiple cycles of object motion. The interference pattern images are acquired and processed to recover the modeshape —the “picture of the motion” of the structure.
REFERENCES:
patent: 3572936 (1971-03-01), Johnson et al.
patent: 3899921 (1975-08-01), Hockley
patent: 4310245 (1982-01-01), Pritchard
patent: 4313679 (1982-02-01), Wolff et al.
patent: 4689993 (1987-09-01), Slettemoen
patent: 4999681 (1991-03-01), Mader
patent: 5706084 (1998-01-01), Gutierrez
patent: 5990473 (1999-11-01), Dickey et al.
patent: 6154270 (2000-11-01), Ozawa
Gutierrez Roman C.
Shcheglov Kirill V.
Tang Tony
California Institute of Technology
Fish & Richardson PC
Font Frank G.
Natividad Phil
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