Analysis of CD/DVD Surfaces Using Atomic Force Microscopy Tramel Clipper David Herman Tyree Mills Summer Research Connection The California Institute of Technology

Milestones in Optical Storage Technology The first optical storage devices were created during the 1960’s. They did not store much information and could only be used for about 100 hrs before wearing out. (Mustroph et al., Angew. Chem. Int. Ed., 2006) In 1982, SONY and Philips introduced the first durable and economically successful compact disc (CD). Capacity: ~ 700 MB (J.-J.Wanegue, Opt. Disc System, 2003) In the late 1980’s, writable CD’s were introduced; information was “burned” into a layer of organic dye added inside the CD. (M. Emmelius, et al., Angew. Chem. Int. Ed. Eng., 1989) In 1995, the final DVD format was agreed upon. The DVD stores information in the same manner as the CD, but its structures are smaller. Capacity: ~ 5 GB (D. G. Stinson, J. Imaging Sci. Technol., 1998) BluRay DVD has just emerged as the latest in high capacity storage. Capacity: ~ 50 GB (F. Yokogawa et al., Jpn. J. Appl. Phys Part I, 1998)

Motivation Writable CD-R’s have a limited life-span; a more durable writable optical storage medium is needed. To store large amounts of data (e.g. HD movies), we need to be able “write” more/smaller on DVD’s. Focus Questions What are the physical characteristics of a CD/DVD? How durable are these devices over time? How can we design a better optical storage device?

Length Scale: The atomic force microscope can be used to image surfaces from that range in size from ~ 1 nm to 100 μm. 1 meter (m): the average man is about 2 meters tall 1 centimeter (cm): the length of a red ant 1 cm = 1x10 -2 m = 0.01 m 1 millimeter (mm): the size of a pencil point 1 mm = 1x10-3 m = m 1 micrometer/micron (μm): 100 μm is the thickness of a sheet of paper 1 μm = 1x10 -6 m = m 1 nanometer (nm): 2 nm is the width of a DNA helix 1 nm = 1x10 -9 m = m

Physical Characteristics CD DVD Label Acrylic Reflective Polycarbonate

Data Storage & Reading Binary inf or mation to be used by computer

Optical Principles

Atomic Force Microscope CD sample Camera Cantilever Piezo Photodetector Images on small scales (1 nm – 100 μm) Produces 3D images of surfaces Our AFM

Van der Waals Force Newton’s Third Law Physics Principles We used the AFM in contact mode The AFM tip and the sample surface are attracted to each other via Van der Waals forces.

Sample Preparation The label and acrylic layers are removed using a razor blade and duct tape. Debris is removed from the surface using a cotton swab, isopropyl alcohol and compressed air. A pen point is 10 times larger than the area of the CD we’re scanning.

The cantilever is positioned above a clean area of the sample. The laser beam is positioned so that it strikes the center of the photodetector. Magnified 1,000 X

AFM Calibration We calibrated the AFM by using a grating with known properties. 3D image of calibration grating Profile view of the calibration grating. Measurements for one row of grating: Average pitch: μm Standard deviation: 1.06 Error: 4.8% Average height: nm Standard deviation: 4.36 Error: 11.07%

AFM images of CD surface Top view 3D image Side view Data is encoded in the pattern of pits and lands. Scratches from cleaning

The wavelength for infrared light: ~750 nm Measurement of CD Pit Depth & Length

3D image of DVD surface showing pits and lands We measured the depth and length of the pits on DVD tracks. Data track

The wavelength for red light: 650 nm Measurement of DVD Pit Depth & Length

Limitations of the AFM Bowing: The cantilever follows a curved path across the sample surface Creep: The tip doesn’t react instantly to changes in topography Hysteresis: The piezo doesn’t respond to applied voltage the same way as it expands and contracts. Non-linearity: The piezo is a man-made material. Doubling the applied voltage doesn’t necessarily double the length.

Conclusions We have learned to operate an AFM and to interpret the data that it produces. We have developed a protocol to remove the label and acrylic layers from a CD/DVD. We have used the AFM to measure a calibration grating and to explore errors introduced by the instrument. We have measured the pit length and depth on a CD and DVD. We find our measurements to be consistent with the literature.

Future Directions Materials science and chemistry have shown that the components of a CD-R (polycarbonate, organic dye) will not last forever, perhaps as little as 2-5 years. We plan to accelerate the aging of CD-R’s by exposing them to heat/humidity. We will use the AFM to image and compare CD-R surfaces after exposure to systematically varying conditions. Our first step will be to develop a protocol to remove the label and acrylic layers from a CD-R without removing organic dye layer that contains the CD’s data. Blank tracks on CD-R after the organic dye has been removed.

Special thanks to… Christian Franck, our research mentor James Maloney and Sherry Tsai, SRC coordinators Prof. G. Ravichandran and his research group Siemens Corporation The California Institute of Technology

Discussing length scales and measuring in microns

Taking a tour of Caltech’s SEM and TEM facilities

Practicing order-of-magnitude calculations

Tramel after two hours of order-of-magnitude calculations

Learning about the physics behind AFM

Christian explains how an AFM scans the sample surface.

A look at our AFM

A closer look

Creating a protocol for AFM operation

Tyree locates the laser and photodetector.

Rough measurements of the calibration grating

Viewing the calibration grating under an optical microscope

Tramel cleans the grating using isopropyl alcohol, a cotton swab and compressed air

Positioning the sample stage under the AFM

Learning how to use the software that controls the AFM