Optical Glass Metrology â Techniques / Limitations - Owens Design

Optical Glass Metrology – Techniques / Limitations 

Brian McMorris 

SICK Sr. OEM Account Manager 

Brian Windsor 

Business Development Manager – Vision and Measurement 

07.02.2010

“Optical” Energy Spectrum 

: SICK AG: : BU02 side session: high end displacement Patrick Nutz 19.03.2009 2

Limitations of Optical Measurement Techniques 

: Contrast 

- Differential intensity in measured light is improved by optical contrast (due to color or reflectivity); 

: Light Spectrum (Reflection / Absorption) 

- Light must be reflected by object to be measured; Absorption of light by object can be overcome 

by changes in light source / wavelength 

: Wavelength 

- Direct distance measurement is limited by the resolution of the measuring instrument or 

reference source; but interpolation is possible at less than one wavelength through interference 

techniques 

- Visible light has a wavelength of 380nm (Blue) to 750nm (Red); therefore optical techniques are 

limited to resolution of about 0.5 micron in the visible spectrum 

: Ambient Light “Noise” 

- Measured light must differ in intensity from ambient light sources; signal-to-noise ratio should 

exceed 10:1 

- Selecting frequency / wavelength of light different than highest intensity ambient wavelengths 

(probably in visible red spectrum) can also improve signal-to-noise ratio in the specific frequency 

measured 

: Optical Distortion 

- Due primarily to lens quality limitations; atmospheric limitations such as air contaminants 

: SICK AG: : BU02 side session: high end displacement 

Patrick Nutz 19.03.2009 3

Overview of Optical Glass Measurement 

Techniques (aka “Microscopy”) 

: Laser Triangulation 

: Illumination and Contrast Methods 

- Confocal 

- AutoFocus 

- Bright Field Microscopy 

- Dark Field Microscopy 

- Kohler Illumination 

- Phase Contrast 

: Fluorescence Methods 

- Two Photon Excitation Microscopy 

- Multiphoton Excitation 

- Deconvolution 

: Sub-Diffraction Limit 

- Structured Illumination (Vertico SMI) 

- Near-Field 



Laser Triangulation 

Advantages 

: Simplicity of Technique 

: Non-Ambiguous Measurement 

: Low Cost of Execution 

: Lends Itself to Inline Analysis 

: Flexibility (light sources / lensing) 

: Scalability 

Disadvantages 

: Limitations on Resolution 

: Optical Occlusion 



Detection of the Glass Plate Interface 

- general Principle - 

d 1 t 1 t 2 t 3 t 4 

t 5 

: Light Distribution on the receiver element 

: SICK AG: : BU02 side session: high end displacement : Name (Date) 6

Detection of the Glass Plates Interface 

- material tilting tolerance 


Detection of the Glass Plate Interface 

- material tilting tolerance 


Glass Thickness Measurement 

with one OD-Precision Sensorhead 

Photos of the light distribution Monitor: Glassplate with distance to the background : 

OK 

•Sharp two Peaks of the 

glassplate visible 

•Third peak of the background 

visible 

•Two red lines to measure the 

distance inbetween (first two 

from the left) 

•Peaks clearly over the blue 

threshold line 

NOK 

•Sharp two Peaks of the glassplate visible 

•Third peak of the background visible 

•Only one red line of the backside edge of 

the glassNo thickness measurement 

possible because first edge is out of range 

•Peaks clearly over the blue threshold line 


Optical Time-of-Flight Technique 

Principle: 

Laser light source generates a pulse which travels to object and is reflected back to receiver. Time 

from emission to reception is measured at high speed. Range is a function of ½ the time divided 

by the time constant (299,792,458 m/s). Ambiguity interval can be eliminated by phase 

modulation. Also called Time Domain Reflectometry (TDR) and LIDAR (when executed in a 

scanning technique like the electro-magnetic equivalent RADAR) 

Advantages: 

- Simplicity of concept 

- Low Cost Solution for Moderate Resolution Features 

- High signal to noise ratio (ambient light noise) 

- Measurement of high aspect-ratio holes 

- Measurement from low to high reflectivity 

- Measurement of transparent objects 

Limitations: 

- Speed –of-light constant 

- Detection speed and response time over short distances (1 micron = 6.0 x 10 -14 seconds) 

- Mechanical Scanning for Single Point 

- X-Y Resolution for Arrayed Sensor 



LaserScanner with 

rotating Mirror 

Detection and 

Measurement of Object 

Distance and Size 

Example of Occlusion 


Confocal 

: Confocal microscopy generates the image in a completely 

different way to normal "wide-field" microscopes. 

: Using a scanning point of light instead of full sample 

illumination confocal microscopy gives slightly higher 

resolution, and significant improvements in optical sectioning 

by blocking the influence of out-of-focus light which would 

otherwise degrade the image 

: Single point measurement in “Z” axis, so instrument or object 

must be moved in X and Y to generate 3D image 



Confocal Laser Point Sensor 


Confocal Laser Point Sensor 

Principle: 

: The confocal point sensor uses a point light source and detector pinhole to discriminate depth 

: The laser beam emitted from the point light source is focused on a specimen through an objective 

lens that moves rapidly up and down. The maximum light intensity occurs when the specimen lies 

within the focal plane of the objective. As the objective moves closer to or farther from the 

specimen, however, the reflected light reaching the pinhole is defocused and does not pass 

through it. As a result, the quantity of light received by a detector behind the pinhole decreases 

rapidly. A detection signal is only generated when the maximum of light goes through the pinhole. 

: A precise height measurement of the illuminated point is achieved by continuously scanning along 

the z-axis. 

Advantages: 

- Vibration-free measurement 

- Exceptional sub-nanometric resolution 

- High signal to noise ratio 

- Thin layer thickness measurement 

(min. measurable thickness: ~ 0.4 µm) 

- Measurement of high aspect-ratio holes 

- Measurement from low to high reflectivity 




Chromatic Confocal Measurement 


Chromatic Confocal Measurement 

Principle: 

: A chromatic lens L generates the image of a point white-light source W as a continuum of monochromatic images 

located on the optical axis (“Chromatic coding”). A sample is located inside the color-coded segment and its 

surface scatters the incident light beam. 

: The backscattered light passes through the chromatic lens L in the opposite direction, and arrives at a pinhole P 

which filters out all wavelengths except a single wavelength, M . The collected light is analyzed by a spectrometer 

S. The sample position (point M) is directly related to the detected wavelength. 

Advantages: 

- High resolution 

- High signal-to-noise ratio 

- No motion in “Z” required 

- Works on all types of samples 

- Wide choice of measuring ranges 

- Steep slope compatibility 

- Coaxial (no shadowing) 

- “Speckle” free from low to high reflectivity 




Principle of operation of an AutoFocus sensor 

The light source is projected by the object lens on the sample's surface. 

- The reflected light is collected by the very same lens, focused by the collimator lens 

and deflected towards the detector by a beam splitter. 

- In the absence of the cylindrical lens, the light would converge to a tiny spot on the 

detector. 

- The cylindrical lens deviates only one axis of the light, so a horizontal and a vertical 

focal line originate from the single spot. 

The detector is placed mid-way between the two focal lines. 

- A circular spot appears on the sensor if the surface is exactly in-focus. 

- Deviation from the focal position results in an elliptic spot on the detector. 

- The orientation and eccentricity of the spot depends on the magnitude and direction 

of the defocus. 

The spot's shape is detectable by the current distribution in a photodiode 

array. 

- The sensor's electronics generate an error signal resulting in a movement of the 

object lens to track the surface at exactly the focal distance. The motion of the lens 

is monitored by an incremental encoder (glass scale), comprising the profile signal. 



Example of AutoFocus Execution 

A precise digital position measurement system 

enables high resolution down to 25 nm 


: Thank you for your attention.

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