Point Autofocus Probe Surface Texture Measuring Instrument PF-60 technical report
ISO approved Mitaka measuring method for areal surface texture (ISO 25178-605) Document No, Title Published ISO 25178-6 Classification of methods for measuring surface texture 2010/1 ISO 25178-601 Contact (stylus) instruments 2010/6 ISO 25178-602 Non-contact (confocal chromatic probe) instruments 2010/6 ISO 25178-603 ISO 25178-604 ISO 25178-605 Non-contact (phase-shifting interferometric microscopy) instruments Non-contact (coherence scanning interferometry) instruments Non-contact (point autofocus probe) instruments 2013/9 2013/7 2014/1 ISO 25178-606 Non-contact (focus variation) instruments Status: DIS ISO 25178-607 Non-contact (confocal microscopy) instruments Status: WD 2
Measuring principle of point autofocus probe AF sensor Laser beam Imaging lens Objective Work XY stage 3
Defocus W AF sensor Laser beam Imaging lens Objective H Work XY stage 4
Auto focus W AF sensor Laser beam Imaging lens Objective H=H H H Work XY stage 5
Point autofocus probe surface texture measuring instrument: PF-60 Specification 1. Laser spot diameter: 1µm (@ 100 ) 2. Measuring functions: Surface texture measurement & evaluation (2D/3D) Profile analysis Contour analysis Flatness analysis Axes Measuring range Scale resolution AF (Z) 10mm 0.01µm X 60mm 0.1µm Y 60mm 0.1µm Z 60mm n/a 6
Development of a Scanning Point Autofocus Instrument Point autofocus instrument (ISO 25178-605) 1. Advantage: * has a wide measuring range with high resolution and accuracy * measures various samples with different materials and reflectivity * has a good correlation with the roughness standard. 2. Disadvantage: * requires a longer measuring time Point autofocus probe Surface texture measuring instrument PF-60 3. Development (for faster measurement): * scanning autofocus (AF) measuring method which continuously moves the xy stage during the measurement 7
How to obtain the focal position W Differential voltage (V) ΔZ AF sensor Imaging lens Laser beam Objective H X-Y scanning stage Linear scale Z1 Workpiece Response curve of the sensor differential voltage Sensor differential voltage [V] ±0.5µm Objective:100X Defocusing from the focus point (Z) [µm] Mirror Glass There is a linearity near the focus position (±0.5µm) where it does not depend on the reflection ratio of the workpiece surface.spai convert the sensor differential voltage in µm in this range and add it to the linear scale value in order to obtain the focus position. Focal position(h)=z1+δz 8
Comparison of the measuring times Index mode Scanning mode Start Start AF ON AF ON Stage Data read Linear scale Step movement Pause AF END? Yes End point? No No Stage Data read Linear scale + AF sensor Constant speed movement AF ±0.5? Yes End point? No AF Error No Yes Yes End End 9
Roughness measurement (INDEX) Measuring pitch=0.5µm Scanning speed=6µm/s 10
Roughness measurement (Scanning AF) Measuring pitch=0.5µm Scanning speed=300µm/s 11
Measurement of the roughness standard Measuring range=4000µm, Measuring pitch=0.5µm, 300µm/s 12
Correlation with the Roughness standard reference material 2074 Type: C (NIST) SRM 2074 Certificate Scanning AF Difference Ra Rsm 0.972 (±0.025) 40.00 (-0.02,+0.06) 0.976 +0.004 40.00 0.00 Unit: µm Measuring Condition: * NIST certified SRM2074 calibration standard * Stylus tip width=2µm *λc=0.8mm L=4mm. Only 15sec./8,000point! PF-60 (Point AF Probe): * Objective:100x(NA=0.8) * Spot diameter=1µm *λc=0.8mm L=4mm Sine waves * Scanning speed=300μm/s 13
Feature 1) Large measuring area/ High precision measurement Fresnel lens XY=4500x4500μm,Z=4.7μm Maximum measuring range: XYZ=60x60x10mm Minimum measuring pitch: XYZ=0.1x0.1x0.01μm Maximum measuring point 2D:150,000 points 3D:5,000,000 points Grating XY=40x40μm,Z=0.32μm 14
Feature 2) Excellent angle tracking capability Laser beam path at steep angle Objective Scattering light Reflected light Sample Steel bearing (Ra: 0.05µm) Measurable angle:82 45 Ball lens (Ra:0.004µm) Measurable angle: 42 Capable of autofocus on any samples with their scattering light and measuring approx. 80 with their surface roughness 0.03μm (Objective: NA=0.8) 15
Feature 3) Various reflectance surface measurement Mirror (90%) Metal (60%) Ceramics (50%) Grain (8%) Plastic (5%) Coated lens (0.5%) 16
Feature 4) Excellent Autofocus repeatability Sample: Mirror surface PV = 20nm σ value = 6nm 17
Ra Rz PTB Certificate 0.227 (±0.007) 1.50 (-0.08,+0.07) international roughness standards Type: D1 (PTB) Scanning AF Difference 0.224-0.003 1.587 +0.087 Unit: µm Contact Scanning AF (300µm/s) 18
Feature 6) Small laser Spot diameter The spot size (W spot ) at its focal plane is expressed as follows: W spot =1.22(λ/NA) (λ: wave length NA: numerical aperture) Conditions:100X objective(λ:635nm NA:0.8) I 100 % I max Result: W spot = 1µm (at 100X) W spot = 1.5µm (at 50X,NA=0.5) 1/e 2 13.5 % W spot Spot size x 19
Scanning direction Laser Objective Work +Y +Z +X 20
AF sensor mode Wide Using wide area to autofocus fast Sensor mode Select Using narrow area to autofocus after using wide area to search AF position. accurate Narrow Using narrow area to autofocus accurate Default setting Laser beam Objective Narrow area Wide area CRT Thin film 21
1) AF sensor mode AF setting Mode Feature Proper surface Wide Select Narrow Wide autofocus range Fast measurement Not so accurate (±0.2μm) Wide autofocus range Accurate Narrow autofocus range Accurate Rough surface(ra>0.5) (ceramic,stain,paint) Various surface (optically smooth surface) Thin film, Air gap of grasses, Wafer resist 2) AF gain Servo gain of AF mechanism Default setting Hi Standard Low Sensitive : Select at slow response Insensitive :Select at hunting surface 22
Teams and definitions of point autofocus 1) Numerical Aperture (NA) Laser beam Objective NA=nsin (α) α 2) AF error 2-1) Dark: Not enough reflectance,out of autofocus range 2-2) Hunting:Autofocus mechanism oscillates near the focus position 2-3) Blurred: Spread laser beam spot (no focus position) 23
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