Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/14—Measuring arrangements characterised by the use of optical techniques for measuring distance or clearance between spaced objects or spaced apertures
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B21/00—Microscopes
  • G02B21/0004—Microscopes specially adapted for specific applications
  • G02B21/002—Scanning microscopes
  • G02B21/0024—Confocal scanning microscopes (CSOMs) or confocal "macroscopes"; Accessories which are not restricted to use with CSOMs, e.g. sample holders
  • G02B21/0052—Optical details of the image generation
  • G02B21/0064—Optical details of the image generation multi-spectral or wavelength-selective arrangements, e.g. wavelength fan-out, chromatic profiling
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B2210/00—Aspects not specifically covered by any group under G01B, e.g. of wheel alignment, caliper-like sensors
  • G01B2210/50—Using chromatic effects to achieve wavelength-dependent depth resolution

Definitions

• the present invention relates to an apparatus for measuring distance between media and an imaging head for a computer-to-plate (CTP) imaging device.
• CTP computer-to-plate
• the confocal signal obtained in the referenced prior art is dependent on the reflectivity of the sample. Furthermore the confocal signal is also dependent on the optical transmittance of the medium in front of the sample. There is, therefore, a need for a confocal signal that will be immune or at least less dependent on the reflectivity and optical transmittance of the medium.
• a system for measuring a distance to a substrate includes a first light source, emitting a first wavelength on a region of the substrate though a lens.
• a second light source emits a second wavelength region of the substrate through the lens.
• a first and second detector are configured to detect the first and second wavelength light reflected from the substrate.
• a processor is configured to compute a first response function wherein the first response function represents reflected light intensity emitted from the first light source as a function of the distance between the imaging device and substrate.
• a second response function represents reflected light intensity emitted from the second light source as a function of the distance between the imaging device and substrate.
• a ratio response function represents the ratio of the first and second response function as a function of distance between the imaging device and substrate.
• the present invention suggests a confocal system in which the sample is illuminated simultaneously by two different wavelengths.
• the ratio of the back reflected signals from the sample is immune or less sensitive to parameters such as the reflectivity and the optical transmittance of the medium in front of the sample.
• FIG. 1 a prior art illustration of confocal sensor used to measure the reflection from an imaged substrate
• FIG. 2 a prior art schematic showing a response function of reflected light intensity from an imaged substrate - maximal value represents focus
• FIG. 3 an illustration of a confocal system using two light sources with different wavelength each
• FIG. 4A illustrates the shift between two response functions
• FIG. 4B illustrates the ratio of two response functions.
• FIG. 1 illustrates a common and well known structure of fiber optic confocal sensor 100.
• the confocal sensor 100 is comprised of a light source 104 coupled to optical fiber 124 and to fiber optic coupler 116. Rays 136 emitted from optical fiber 128 via imaging lens 144 are imaged on the surface of substrate 148.
• the back reflected light 140 is coupled to the emitting optical fiber 128 and reaches light detector 112 via coupler 116 and optical fiber 132.
• the intensity measured by light detector 112 is a function of the distance, z, 160 to substrate 148.
• Vd The signal measured by the detector, Vd, is proportional and is a function of few parameters:
• Vd ,z) a Io x G ,z) x ⁇ ( ⁇ ) x ⁇ ( ⁇ , ⁇ ).
• a represents a proportional sign.
• Io is the intensity of the light that impinges on the sample.
• ⁇ ( ⁇ ) is the reflectivity of the sample.
• ⁇ ( ⁇ , ⁇ ) is the optical transmittance of the medium between the sample and the imaging lens.
• Z is the distance to the sample.
• G(X,z) is a function describing the overall optical response of the confocal system. It is a function of the distance, z, and of the wavelength ⁇ , and defined also by optical parameters of the confocal system such as the numerical aperture of the lens and of the diameter of the fiber's core.
• FIG. 2 is graph describing typical and well known confocal signal where a symmetrical curve describes Vd( ,z) as a function of the distance Z.
• Such a curve is measured by simultaneously reading ⁇ ( ⁇ , ⁇ ) and while scanning with the confocal system along the z axis and at known positions.
• the best focus is defined at the maximum 204 of the symmetrical function.
• the graph describes the ambiguity of a typical confocal system.
• a single value of Vd ⁇ ,z) corresponds to two different values of the position z.
• the scan along the z axis can be done in several techniques, for example by using an autofocus system embedded within a compound lens 336, constructed from several optical elements, where some of them can be moved and controlled in order to change and adjust the lens focal distance.
• Vd(z) is dependent also on the reflectivity, ⁇ ( ⁇ ), of the sample and the optical transmittance, ⁇ ( ⁇ , ⁇ ), of the medium. This means that at best focus, different intensities will be measured for samples having different reflectivity.
• the intensity measured by the detector will change if the sample reflectivity or the optical transmittance of the medium change during the measurement procedure. In such cases, therefore, one has to repeatedly scan the peak in order verify the position of the best focus.
• FIG. 3 describes the basic principle of the present invention using a fiber optic confocal system where at least two coupled light source and detector units 344 and 348 are used.
• Light sources 304 from unit 344) and 308 (from unit 348) each emitting different wavelengths.
• Light source 304 is coupled via fiber optic coupler 320 to detector 312.
• First detector 312 is constructed to be sensitive just to wavelength ⁇ , emitted by first light source 304.
• Second light source 308 is coupled via fiber optic coupler 324 to second detector 316.
• Second detector 316 is constructed to be sensitive just to wavelength ⁇ 2, emitted by second light source 308.
• Units 344 and 348 are further coupled by fiber optic coupler 328 to emit combined light via a single output port 332.
• Output optical port 332 is imaged via a dispersive optical element 336 on substrate 148. Due to the dispersion of 336 the wavelengths are focused on two different planes, shifted relative to each other by ⁇ .
• Processor 340 forms a response function ⁇ ( ⁇ , ⁇ ), which is a function of the applied wavelength ⁇ and the distance z between the lens 336 and substrate 148.
• processor 340 forms a response function Vd( 2,z), using a different wavelength ⁇ 2.
• Processor 340 computes along a defined range, a ratio response function which is a division of function Yd k 1 ,z) and function Vd( 2,z). The computed ratio response function is an absolute and monotonic function of the distance z. Hence the ambiguity (related to common confocal systems) of the function Vd( ⁇ , z) where one value fits two different z positions is omitted.
• G( ,z) describing the optical response of the confocal system is a function of optical parameters such as the numerical aperture of the lens and of the diameter of the fiber's core. By adjusting these optical parameters, the ratio Vd( l,z)/ Vd( 2,z) may be controlled, achieving for example the right dynamic range and accuracy.
• FIG. 4A describes a lateral shift along the z axis between normalized function ⁇ ( ⁇ , ⁇ ) and normalized function G(3 ⁇ 42,z). This lateral shift is due to the dispersion of the imaging lens.
• FIG. 4B describes the ratio between G ⁇ l,z) and G( 2,z).
• optical detectors such as 312 and 316 can be made to be sensitive just to a single wavelength by using different types of detectors.
• Different bandpass filters can be used, for example, filters based on thin film technology or filters made from fiber Bragg gratings.
• Different optical fibers and fiber optic couplers can be used in order to implement the invention.
• multi and single mode optical fibers and couplers, wavelength and polarization dependent fiber optic couplers and fiber optic elements can be used.
• Measurement can be done simultaneously by activating the light sources and measuring detected signals at the same time. Measurements can also be done by sequentially activating the different light sources and performing measurement with their related detectors. When operating in simultaneously sequential mode, there is no need to spectrally isolate the light detectors, since measurements are done at different times.
• the output port 332 maybe for example a pin hole aperture.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Optics & Photonics (AREA)
• Length Measuring Devices By Optical Means (AREA)
• Measurement Of Optical Distance (AREA)

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• 2012
  • 2012-05-31 EP EP12731211.4A patent/EP2718666A1/en not_active Withdrawn
  • 2012-05-31 CN CN201280028199.8A patent/CN103620340A/zh active Pending
  • 2012-05-31 WO PCT/US2012/040166 patent/WO2012170275A1/en active Application Filing

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