Analysis of CD/DVD SurfacesUsing Atomic Force Microscopy

Tramel ClipperDavid HermanTyree 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 imagesurfaces 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 ant1 cm = 1x10-2 m = 0.01 m

1 millimeter (mm): the size of a pencil point1 mm = 1x10-3 m = 0.001 m

1 micrometer/micron (μm): 100 μm is the thickness of a sheet of paper 1 μm = 1x10-6 m = 0.000001 m

1 nanometer (nm): 2 nm is the width of a DNA helix1 nm = 1x10-9 m = 0.000000001 m

http://rubberdisc.com/images/CD-layers.jpg

Physical Characteristics

CDDVD

http://gfx.cdfreaks.com/reviews/memorexf16/image030.jpg

Label

Acrylic

Reflective

Polycarbonate

http://micro.magnet.fsu.edu/electromag/computers/compactdiscs/writable/cdwriter.html

http://micro.magnet.fsu.edu/electromag/computers/digitalvideodiscs/dvd.html

http://www.usbyte.com/common/derived/compact_disk_4.htm_txt_cd%20encoding.gif

Data Storage & Reading Binary information to be used by computer

0 0 1

http://www.soundfountain.com/amb/25cdlaserarm2.jpghttp://static.howstuffworks.com/gif/removable-storage-cd.jpg

Optical Principles

http://www.upei.ca/~phys221/mbrookshaw/Glossary/complete_em_spectrum.JPGhttp://www.lacie.com/imgstore/more/blu-ray_storagedensities.gif

Atomic Force Microscope

CD sample

Camera

Cantilever

Piezo

Photodetector

• Images on small scales (1 nm – 100 μm)• Produces 3D images of surfaces

Our AFMhttp://nano.tm.agilent.com/blog/wp-content/uploads/2007/06/how-an-atomic-force-microscope-works.bmp

http://www.chm.bris.ac.uk/webprojects2003/swinerd/forces/forces.htm

Van der Waals Force

Newton’s Third Law

Physics Principles

http://www.mechmat.caltech.edu/~kaushik/park/1-2-1.htm

http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4a.html

• 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 surfaceusing a cotton swab, isopropyl alcoholand compressed air.

A pen point is 10 times larger thanthe area of the CD we’re scanning.

The cantilever ispositioned abovea clean area ofthe sample.

The laser beam ispositioned so thatit strikes the centerof the photodetector.

Magnified 1,000 X

AFM Calibration

We calibrated the AFM by usinga grating with known properties.

3D image of calibration grating

Profile view of the calibration grating.

Measurements for one row of grating:Average pitch: 10.48 μmStandard deviation: 1.06Error: 4.8%

Average height: 199.94 nmStandard deviation: 4.36Error: 11.07%

AFM images of CD surface

Top view

3D image

Side view

Data is encoded inthe pattern ofpits and lands.

Scratches fromcleaning

CD Pit Depth

050100150200250300350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Pit Number

Pit

Dep

th (

nm

)

CD Pit Length

0200400600800100012001400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Pit Number

Pit

Len

gth

(n

m)

The wavelength for infrared light: ~750 nm

Measurement of CDPit Depth & Length

3D image of DVD surface showing pits and lands

We measured the depth and length ofthe pits on DVD tracks.

Data track

DVD Pit Height

020406080100120140

1 3 5 7 9 11 13 15 17 19 21 23

Peak Number

Pea

k H

eig

ht

(nm

)

DVD Pit Length

0

500

1000

1500

2000

1 3 5 7 9 11 13 15 17 19 21 23

Peak Number

Pea

k L

eng

th (

nm

)

The wavelength for red light: 650 nm

Measurement of DVDPit Depth & Length

Limitations of the AFM

Bowing: The cantilever follows a curved pathacross the sample surface

Creep: The tip doesn’t react instantlyto 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. Doublingthe applied voltage doesn’tnecessarily 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 onCD-R after theorganic dye hasbeen 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