We propose a laser trapping-based scanning dimensional measurement method for free-form surfaces. We previously developed a laser trapping-based microprobe for three-dimensional coordinate metrology. This probe performs two types of measurements: a tactile coordinate and a scanning measurement in the same coordinate system. The proposed scanning measurement exploits optical interference. A standing-wave field is generated between the laser-trapped microsphere and the measured surface because of the interference from the retroreflected light. The standing-wave field produces an effective length scale, and the trapped microsphere acts as a sensor to read this scale. A horizontal scan of the trapped microsphere produces a phase shift of the standing wave according to the surface topography. This shift can be measured from the change in the microsphere position. The dynamics of the trapped microsphere within the standing-wave field was estimated using a harmonic model, from which the measured surface can be reconstructed. A spherical lens was measured experimentally, yielding a radius of curvature of 2.59 mm, in agreement with the nominal specification (2.60 mm). The difference between the measured points and a spherical fitted curve was 96 nm, which demonstrates the scanning function of the laser trapping-based microprobe for free-form surfaces.
Engineered surfaces have been fabricated to provide enhanced properties such as low friction, anti-adhesive behavior, or low reflection of light. At micro-scales, surface force highly affects the functionality of mechanical parts. In order to reduce surface force such as friction, micro mechanical parts that have engineered surfaces are demanded. In order to investigate the functionality of the textured micro parts, it is necessary to evaluate both the three-dimensional shape and the surface topography along with its geometry. Then we propose novel hybrid probing technique using an optically trapped micro sphere. Tightly focused laser beam makes it possible for a dielectric micro sphere to sustain near the focal point in the air. The dynamic behavior of the micro sphere changes as the result of the interaction of the surface. Therefore, the surface is detected by monitoring the micro sphere. This enables the three-dimensional shape measurement of the substrate. On the other hand, Surface topography is imaged with the lensing effect of the trapped micro sphere. Therefore, this trapped sphere is used as both a probe for coordinate metrology and a micro-lens in optical microscopy in this study. This present investigation deals with the development and fundamental validation of the hybrid probing system with the optically trapped micro sphere. The measurement result with high performance was demonstrated using the tilted diffraction grating.
Currently, micro-components are required to fabricate with great precision owing to the miniaturization of complex
product. In order to assess the dimension, size, and other geometric quantities of such complex micro-components,
technological progress is needed in micro- and nano-coordinate metrology. Therefore, the coordinate metrology have
been attempted thus far. To establish nano-coordinate metrology with a microprobe technique, we have been developing
the optically trapped probe, whose principle is based on the single-beam gradient-force optical trap of a particle in air.
However, the rapidly increasing complexity including micro-fine figures makes it difficult to evaluate geometric
quantities using a microprobe that can barely access a concave surface. An improved microprobe is required to have a
better long working distance, wide measurement range, and high resolution. In this paper, a novel probing technique for
coordinate metrology is discussed. The proposed method is based on optical interference, which is seen as a standing
wave pattern, also called a standing wave scale. The feasibility is examined by the profile measurement of a smooth
surface with high accuracy and the dimensional measurement of a trench structure.
Various products have been miniaturized in recent years. And, the measurement technology for surface profile of micro
components is highly demanded. Then, we proposed a new measurement technique for surface profile using the standing
wave trapping. The high-accuracy scale and the high-sensitive sensor are required in the profile measurement. In our
measurement system, the optical trapping particle is used as the sensor. The standing wave pattern is used as the
measurement scale, which has wavelength-determined intensity pitch of interference field (λ/2). Therefore, this
measurement technique is expected to perform the high-accuracy measurement. It was experimentally found that the
vertical measurement range is about 250 μm. The uncertainty of the sensor is ±λ/100. Thus, this technique is capable of
measuring large objects in height. When measuring the continuous surface, the sensor particle is scanned in the
horizontal direction above the measured surface. The trapped sensor particle in the standing wave field axially moves to
follow the measured surface topography. The particle jumps when the surface profile exceeds the pitch of the standing
wave pattern. Therefore, the surface profile can be calculated based on the measurement of the particle motional
variation. As pre-measurement, the dependency of the scale pitch on measured surface angles was investigated. A microlens
was measured with the angle dependency correction. This shows the improvement of the measurement accuracy.
Laser trapping is a widely used technique such as manipulating cells. Recently the trapping technique is used in air, for
example, a precision probe for sensing the surface of an object. To expand the applications of the trapping technique in
air, more experimental investigations need to be implemented for properties such as trapping forces. We studied the
dynamic properties of a micro-sphere (φ8um) optically trapped in air by using a radially or linearly polarized beam.
Firstly in order to predict the trapping forces working on a micro-sphere, the forces are analyzed by a ray-tracing
method. The results show that an axial force of radial polarization is larger than one of linear polarization. Considering
the radial forces, the force of radial polarization is smaller than one of linear polarization. These results can be
understood by noting forces generated by p- and s-polarization. Secondly, we examine the trapping efficiency in optical
trapping experimentally. Radial trapping efficiency is evaluated by measuring a spring constant. Experimental results
and simulated results are in good agreement that the linear polarized beam achieved a 1.25 times higher spring constant
than radial polarization. Axial trapping efficiency is examined by measuring minimum trapping laser power.
Experimental results are one tenth underestimated although qualitatively they are coincident. Radial polarization is
shown to be approximately 2 times higher than linear polarization. Thus, employing radial polarization, the optical
trapping of the glass microsphere in air is achieved by using an objective lens with NA0.80.
For past decades Micro-System Technology (MST) has been developed and it has enabled fabricating the microcomponents on the micro-systems. In order to measure such microcomponents having micrometer-size shapes, a concept of nano-CMM was proposed. According to the concept, nano-CMM specifications are, for example, a measuring range is (10 mm)3 and accuracy is 50 nm. Then, we have proposed a laser trapping probe as a position detecting probe for nano-CMM. The laser trapping probe can be suitable to nano-CMM because of high sensitivity, an availability of a high spherical probe stylus and changeable properties. On the other hands, there are sources of uncertainties, one of them is the standing wave, and the influence is experimentally investigated.
The results reveal the facts such as following. The standing wave obviously influences the behavior of the laser trapping probe sphere. The positional fluctuations by the standing wave proceed several hundreds nm, and the phenomenon appears with high repeatability. A size of the probe sphere can be a crucial parameter for reducing the influence of a standing wave. As another possibility of reducing the influence, on the inclined substrate the probe tends not to be affected by the standing wave. On the other hands, the standing wave influences the probe sphere beyond 100 mm far from the flat silicon substrate.