Methods for Cleaning AFM Reference & Calibration Samples
Reference, or calibration, samples are essential for calibration of Atomic Force Microscopes. Typically, AFM calibration references are fabricated on silicon substrates using photolithography. Lateral feature dimensions of reference samples are between 1 µm and 50 µm, while vertical dimensions are between 10 nm and 100 nm. Reference samples can cost between $200 and $1,000, depending on the size and accuracy of the patterns.
Although they can be expensive, reference samples are often improperly stored and/or generally mishandled in lab environments, leading to an accumulation of contamination and particulates on the sample surface. Once they're contaminated, it becomes difficult to use the reference sample as a calibration for an AFM. Contamination makes probe approach difficult and causes false feedback that makes scanning a challenge. Take a moment before running your reference sample to view the sample under an optical microscope, where the larger surface contamination can be readily observed. The photo above illustrates contamination present on a standard silicon test grid reference sample.
Cleaning Heavy Contamination on a Silicon Test Grid
There are many ways to clean reference samples, but one novel approach for a heavily contaminated silicon test grid utilizes an inexpensive product readily purchased from a pharmacy, called New Skin®.
Here's the procedure. First, coat the contaminated reference with the New Skin®. There's a brush built right into the bottle that makes this an easy task.
Painting New Skin on the Reference
Next, allow the New Skin® to dry on the sample. This can take several hours, so it's best to apply in the late afternoon and allow the New Skin® to harden overnight.
New skin hardened on the surface.
Once the New Skin® is dry, it pops up a bit, making a convex surface. This makes it easier to remove with a pair of tweezers.
Peeling New Skin from the Reference.
By pulling the hardened New Skin® off of the reference sample with tweezers, you're also removing larger surface contaminants from the reference.
Finally, take a look at your silicon test grid reference sample under the optical microscope to see if the larger contaminants are now removed.
A dirty reference sample
Photo after the cleaning
Our reference sample is looking a lot better after utilizing the New Skin® method.
Cleaning Microscopic Contamination for AFM Calibration
Although the New Skin® method is successful in removing large contaminants such as dust and fibers, you will need a technique that can remove microscopic contaminants and organic layers, such as airborne hydrocarbons, if the sample is to be used for calibrating the AFM.
When operated in air, short wavelength UV light produces ozone, an extremely strong oxidising agent. The combination of UV irradiation (<300nm) and ozone can rapidly decompose a wide variety of organic contaminants. This is a quick and effective procedure for removing organic matter from sensitive surfaces. Devices to safely expose small surfaces to a combination of UV and ozone can be constructed fairly simply**, since all that is required is a small light-proof chamber, a power supply and an appropriate UV lamp. Commercial UV/ozone devices are also available.
Samples can be cleaned by UV irradiation consistently only if macroscopic contamination is removed first. Typically, pre-cleaning involves a sequence of actions that include swabbing, rinsing and ultrasonic agitation, and utilizes multiple solvents. However, some samples are not resistant to the usual pre-cleaning procedures. In the case of cleaning AFM reference samples, the surfaces are very sensitive to mechanical damage. Swabbing with even the softest materials is not appropriate for AFM calibration/reference specimens, and will irreparably damage the sample. Additionally, while UV/ozone is highly effective for removing organic contaminants, it is ineffective against nearly all inorganic contamination.
The New Skin® method addresses both the potential risk of mechanically damaging AFM calibration specimens and the need for a low-cost and simple mechanism to remove large-scale contamination from surfaces. This New Skin® technique can be used in combination with more advanced techniques, such as the UV/ozone method, to achieve near-perfect cleaning for AFM reference artifacts.
* * Note that in the UV/ozone technique, short wavelength UV and ozone are both very dangerous, and the chamber should only be operated closed, in a well-ventilated space.
UV/ozone Cleaning Reference: J. Vig, J. Vac. Sci. Technol. A3 (1985)