Unconventional Patterning at the Nanoscale    »» Main Menu

CHARACTERIZATION

Atomic Force Microscopy | Dip Pen Nanolithography | Scanning Electron Microscopy

Atomic Force Microscope (AFM)

The atomic force microscope (AFM) or scanning force microscope (SFM) was invented in 1986 by Binnig, Quate and Gerber. Similar to other scanning probe microscopes, the AFM raster scans a sharp probe over the surface of a sample and measures the changes in force between the probe tip and the sample. A cantilever with a sharp tip is positioned above a surface. Depending on this separation distance, long range or short range forces will dominate the interaction. This force is measured by the bending of the cantilever by an optical lever technique: a laser beam is focused on the back of a cantilever and reflected into a photodetector. Small forces between the tip and sample will cause less deflection than large forces. By raster-scanning the tip across the surface and recording the change in force as a function of position, a map of surface topography and other properties can be generated. You will use the AFM to image the CD/DVD masters and PDMS stamps and compare these images to the information provided by scanning electron microscopy.

Modes of Operation for the AFM:

There are three general types of AFM imaging: (1) contact mode, (2) tapping mode and (3) non-contact mode. In this lab, contact mode and tapping mode will be used to image the masters and PDMS stamps you made in the first lab.

Contact mode is the most common method of operation of the AFM and is useful for obtaining 3D to pographical information on nanostructures and surfaces. As the name suggests, the tip and sample remain in close contact as the scanning proceeds. One of the drawbacks of the tip remaining in contact with the sample is that large lateral forces can be exerted on the sample as the tip is dragged over the specimen. These large forces can result in deformed images and damaged sample.

Tapping mode is another mode of operation for AFM. Unlike the operation of contact mode, where the tip is in constant contact with the surface, in tapping mode the tip makes intermittent contact with the surface. As the tip is scanned over the surface, the cantilever is driven at its resonant frequency (hundreds of kHz). Because the contact time is a small fraction of its oscillation period, the forces that can deform the images or damage the sample are reduced dramatically. Tapping mode is usually preferred to image samples with structures that are weakly bound to the surface or samples that ares soft (polymers, thin films).

MATERIALS:

• AFM
• PM tips and Mount
• Polycarbonate and Al Masters from CD/DVD
• PDMS Stamps from CD/DVD Masters
• SAM samples for LFM

PROCEDURE:

Laboratory procedures

You have an opportunity to use a state-of-the-art AFM instrument from JEOL to characterize the samples you will pattern at the end of the course. You will use (1) tapping mode to image the CDs and DVDs and (2) contact mode (lateral force microscopy) to image microcontact printed patterns of self-assembled monolayers (SAMs) on gold.

Samples can be mounted to the magnetic sample holder (puck) using double sided tape. The samples are loaded on the scanner of the JEOL SPM 5200, whose maximum scan size is 65 x 65 m m 2 , and whose vertical range is ~5 m m. You will practice mounting tips into the tip holder using tweezers. Once the sample is loaded and the tip is placed above the sample, you need to align the laser beam onto the cantilever. You can get a rough idea where the beam is using the optical microscope situated above the AFM. Once the signal is maximized, you can bring the tip close to the surface and start the engage process. Specific operational details can be found in the training manual designed by Dr. Gajendra Shekhawat at the following link.

Samples and their preparation

1. SAMs on gold surfaces by microcontact printing. You will be provided samples made by microcontact printing.
2. CDs and DVDs. In the basics lab, you cut CDs or DVDs into small pieces to use as cheap masters. You will use tapping mode to image these periodic structures as well as your PDMS stamps -- and will eventually compare these patterns with ones you will make later in the course.

Questions for Lab Write-up:

Tip Characterization

1. What types of tips did you use for these experiments?
2. Why are some tips more suited for a particular operating mode?
3. Describe how the tip geometry could affect the image resolution and induce artifacts in the image.

Sample Characterization

1. Compare the tapping mode images of your PDMS stamp and your CD master.
2. Comment on the difference between the topographic and lateral force images of the SAMs patterned on gold substrates.
3. How useful was the lateral force imaging mode? Was it difficult to obtain an image with good chemical contrast?

» More about AFM (PDF)