Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-02-27
2003-11-18
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06650420
ABSTRACT:
TECHNICAL FIELD
This invention relates to an apparatus and method for measuring the dynamic response of a surface of an object. More particularly, the invention relates to an apparatus and method for resolving the dynamic surface motion of small structures by vibration measurement.
BACKGROUND ART
Surface Acoustic Measurement-Laser Doppler Vibrometry (LDV):
The measurement of the vibratory motion of a surface or structure is desired in a number of research and commercial applications. Typically in these applications, the desire exists to minimize the loading of the sensor on the structure of interest. For macroscale systems, for example, this may be accomplished by the attachment of an array of small, low mass, accelerometers to the structure or by the use of scanning laser Doppler vibrometry (LDV). In LDV systems, a laser-based interferometer is used where the scattered light is interfered with a reference beam to measure a Doppler phase shift induced by the moving surface. For these systems, the spatial resolution is diffraction limited and is typically several microns in size. Further, in addition to minimizing the structural loading, LDV systems are capable of operating up to frequencies that are not attainable by conventional accelerometers.
LDV is an established technique where the motion of a surface is measured by use of a laser-based interferometer and where the scattered light is interfered with a reference beam to measure a Doppler phase shift induced by the motion of the sample's surface. In a conventional LDV system, a typical method of creating an interferometer is to use a Michelson format where the source laser beam is split by an optical element. The first of the two beams is preserved as a reference and is unperturbed. The second beam is delivered to the sample surface, is scattered, and is interfered with the reference beam. Other interferometric architectures are possible. This includes, for example, the so-called differential interferometric approaches where both beams are scattered off of the surface and recombined. The differential phase shift is used to measure the surface motion. Disadvantages of LDV are primarily associated with its use in applications where a simultaneous measurement is required from a spatial array of devices. This is sometimes accomplished by use of an optical or mechanical scanning configuration in conjunction with LDV. LDV methods are not generally applicable when continuous monitoring of structural motion is required and are generally complex and costly.
Near-Field Scanning Optical Microscopy NSOM:
NSOM is a well-known technique that is used for sub-wavelength microscopic imaging. The technique typically involves positioning a probe, consisting of a sub-wavelength aperture in the form of a tapered optical fiber, in close proximity to a surface, illuminating the surface with the probe, and by observing and measuring one or more characteristics of the transmitted or reflected light, providing an image of the surface.
One such technique, described in U.S. Pat. No. 5,990,474, involves a method and apparatus for making phase and amplitude measurements utilizing a reflection mode. A tapered NSOM optical fiber probe is used in conjunction with an optical interferometer to position the probe near a surface in order to image the surface. The tip of the probe serves as both a source of the light illuminating the surface and a receiver of the light scattered or reflected from the surface. The patent, however, discloses that there are disadvantages to utilizing the probe tip for both these functions, as the received low light level greatly reduces the signal-to-noise (SNR) ratio. The signal losses necessitate the use of a tip having a relatively large aperture, thereby inherently limiting the spatial resolution since spatial resolution is determined by the size of the probe's aperture. Furthermore, this device is limited to surface imaging applications and cannot measure surface dynamics or motion.
Another technique, described in U.S. Pat. No. 6,169,281, uses a scanning probe microscope for examining surface anomalies such as grooves and ridges in a sample surface. The microscope includes a probe tip attached to a mounting surface, that is placed in proximity to a sample surface. The apparatus also includes drive mechanisms for dithering the probe tip and a detector responsive to vibration of the probe. The detector output is used to determine topographical features of the sample surface and is not applicable to measuring surface dynamics or motion.
In yet another approach, described in U.S. Pat. No. 6,229,609, a NSOM microscope that includes a probe, a light source and optics for illuminating a sample, and a converter and optics for receiving light reflected from the sample, makes measurements using high resolution AFM and optical techniques. The probe is hooked, but otherwise operates in a conventional manner, and as with other such prior art devices, the microscope is capable of just surface imaging.
Another approach described in U.S. Pat. No. 5,473,157 is directed to a variable temperature NSOM microscope for implementing NSOM over a broad temperature range. It is housed within a cryostat and utilizes a mirror assembly to collect light from a sample tip. The optical processing consists of collecting the light scattered from the sample surface and moving it, by use of fiber optics, to a spectrometer. The main function of the microscope is to obtain spectrographic analysis from NSOM light scattered from a sample surface. The device is limited to surface imaging, and is not applicable to measuring surface dynamics or motion.
Still another approach described in U.S. Pat. No. 6,232,597 is directed to a scanning probe microscope configured to identify deep surface features by detecting rotationally polarized light that interacts with an object.
There remain, however, applications in which these approaches for obtaining a simple image of a surface are inadequate. These applications include the design and operation of a wide variety of micro electromechanical systems (MEMS) and nano electromechanical systems (NEMS) devices. For example, in a significant application involving resonators, it is important to understand the vibratory mode shape of the resonator and the surrounding attachments. Included are mechanical devices that function as an RF component, or as a component of a chemical sensor where small mass changes are detected due to the adsorption of airborne molecules. Further, there are applications in research where measurement of high frequency surface phonons is desired where the wavelengths of interest are sub-micron as well. There are also applications in biological studies where a probe is useful for imaging the motion of proteins and large molecules.
Until now, broadband surface acoustic measurements for small (sub-micron) systems have been limited by the diffraction limit imposed on the bulk and fiber-optic components available. These techniques have therefore proven useful only for lateral spatial resolutions on the scale of microns or larger, and for frequencies of about 30 MHz. For systems that are smaller than a micron, the discernment of the normal mode structure is not possible with diffraction limited optical systems, and therefore the detailed vibratory motion and the coupling of the system to the attached structure cannot be directly observed. Rather, information in this regard is intuited by the measurement of indirect properties. For example, one might measure the Q (quality factor) of a resonator through an electrical property measurement. However, for many MEMS and NEMS systems, understanding the details of the vibratory response is essential to understanding what controls the quality factor. Moreover, most of the systems of interest at these small scales have correspondingly higher resonant frequencies, and for a sub-micron vibration measurement technique to be useful, it must also have a large bandwidth capability associated with it. The resonant frequencies of many of the MEMS and NEMS devices of interest
Houston Brian H.
Photiadis Douglas M.
Vignola Joseph F.
Karasek John J.
Legg L. George
Lyons Michael A.
The United States of America as represented by the Secretary of
Turner Samuel A.
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