Simultaneous 5-wavelength Interferometry for Head/Media Spacing Measurement Jiasheng Zhu and Prof. F. E. Talke Center for Magnetic Recording Research University of California at San Diego 9500 Gilman Drive, La Jolla CA 92093-0401 Phone: +1-858-534-3646, FAX: +1-858-534-2720 E-mail: ftalke@ucsd.edu 1/24/00 Presented at the THIC Meeting at the DoubleTree Hotel Del Mar CA 92130-2539 2539 on January 18, 2000
Simultaneous 5-wavelength Interferometry for Head/Media Spacing Measurement Jiasheng Zhu and Prof. F. E. Talke Center for Magnetic Recording Research UC, San Diego
Outline Motivation Implementation of simultaneous 5-wavelength interferometry Spacing measurement in a tape drive Tape asperity compliance measurement Comparison between 3- and 5-wavelength interferometry Summary
Motivation Measurement reliability can be further improved by using more than 3 wavelengths Simultaneous measurement is required to measure the spacing in a tape drive
5-wavelength interferometry Monte Carlo Error Analysis: 5 vs. 3-wavelength3 All units in nm. Spacing: 35 nm Spacing: 50 nm Spacing: 80 nm Technique Mean Std. Dev. Mean Std. Dev. Mean Std. Dev. 3-wavelength 35.0 1.6 50.0 1.5 80.0 2.3 5-wavelength 35.0 1.2 50.0 1.2 80.0 1.9
Two-step sequential 5-wavelength interferometry Sequential measurement--- asperity compliance Synchronized by video frame frequency Data Acquisition PC Data Acq. PC RG T G B Parallel Out Trigger Circuit Red light 650 nm Blue light 480 nm Green light 535 nm Red light 620 nm Blue light 450 nm R G B 3-CCD Camera Head/Tape Interface Video Out Video Monitor Data acq.. in 2 steps Switch wavelengths manually: (a) 650, 535, 480 nm (b) 620, 535, 450 nm
Simultaneous 5-wavelength interferometry 2 possible choices for simultaneous measurement : One 3-CCD 3 camera Two 3-CCD 3 cameras data acq.. must be done in 2 steps (With a switching of wavelengths) w difficult to align the two cameras (No switching of wavelengths)
Simultaneous 5-wavelength interferometry Data acquisition using 3CCD camera Odd Field 1 Video Frame (1 image) + Vertical interval (1.3 ms) Even Field Sequential acquisition Simultaneous acquisition 1 Frame 1 Frame Odd Field Even Field Image 1 (650,480,535) Manual switching (1 min.) Image 2 (620,450,535) Image 1 (650,480,535) Shutter switching (1.3 ms) Image 2 (620,450,535)
Simultaneous 5-wavelength interferometry 5-wavelength measurement 3-wavelength 3 measurement Synchronized by video field frequency Data Acq. Acquisition PC PC R T G B Parallel Out Trigger Circuit Red light 620 nm Green light 535 nm Blue light 450 nm Red light 650 nm Green light 550 nm R G B 3-CCD Camera Head/Tape Interface Video Out Video Out Video Monitor Data acq. to 5 wavelengths is done in 1.3 ms Image 1 650,480,535 1.3 ms Image 2 620,450,535
Simultaneous 5-wavelength interferometry Physical setup Computer 3CCD camera Camera controller Triggering circuit Light sources Microscope Shutters Tape drive 5-in-1 fiber cable
Spacing measurement on DLT4000 tape drive Unloading Device 4π I h = A + B cos( λ φ) Loading/Unloading rod
Spacing measurement on DLT4000 tape drive Envelope calibration (a) Moving fringe pattern I 4πh = A + B cos( λ φ) (b) Envelope extract Upper envelope 535 nm Lower envelope Fringe moving direction
Spacing measurement on DLT4000 tape drive Envelope check (a) A selected contact zone I 4πh = A + B cos( λ φ) (b) Envelope check for 535 nm Upper envelope 535 nm Lower envelope Odd field(620,535,450 nm)
Spacing measurement on DLT4000 tape drive Spacing map of a contact zone
Tape asperity measurement Application in asperity compliance measurement 60 Asperity compliance of DLT4 tape 50 Spacing (nm) 40 30 20 3 2 1 4 DLT4 tape 10 0 0 1 2 3 4 5 Contact pressure (kpa)
Precision comparison Spacing measurement on DLT4000 drive Standard deviation in 10 measurements (spacing is averaged over 200x200 pixels) Mean spacing (nm) Std. Deviation(nm) 3-wavelength 5-wavelength 25.9 ± 29.9 ± 0.8 0.6 0.3 0.2 Robustness: 5-wavelength > 3-wavelength3
Precision comparison Asperity compliance measurement Standard deviation of spacing in 10 measurements (spacing is averaged over 200x200 pixels) Pressure: 9 psi Mean spacing (nm) Std. Deviation(nm) 3-wavelength 5-wavelength 36.9 ± 37.7 ± 3.7 2.3 1.0 0.6 Robustness: 5-wavelength > 3-wavelength3
Summary Implemented simultaneous 5 wavelength interferometry Measurement accuracy is better with 5 wavelengths than with 3 wavelengths