Guide to AFM standards and references

Standards and references are required for proper operation of an AFM and to verify optimal operating conditions and calibration of the instruments. Standards are used to assure that the absolute measurements are correct, while references assure that the instrument is giving consistent results.

For establishing calibrated topography measurements, the AFM scanner must be certified with calibration standards having pre-established dimensions in the X, Y and Z axis. References can be used for establishing that an AFM mode is operating correctly and for establishing the proper operation of the AFM.

This guide serves as a reference for finding standards and references that are helpful for making meaningful measurements with an AFM. When possible several vendors for a particular type of standard are listed.

A calibration standards/references are needed to calibrate SPM in the vertical axis. For calibrations greater than 10 nm step height standard or references are typically fabricated by etching patterns in a quartz substrate. Another source for references is etched silicon, or silicon dioxide coated with a uniform layer of metal. When calibrating the instrument for Z height measurements below 10 nm, nano-spheres, atomic terraces of silicon or HOPG may be used as a reference specimen.

Calibration of the AFM in the XY axis so that meaningful dimension measurements can be made requires establishing the linearity and orthoganality of the AFM scanner. This is typically done with etched patterns in quartz or silicon. Often the spedicimen that is used for calibrating the Z axis can be used for the XY axis as well.

An AFM may be calibrated in XY and calibrated in Z and may not be useful for making angular measurements. This is because the X-Z and Y-Z axis may not be orthogonal. With the orthogonality references, this problem can be avoided. This reference is fabricated by making a 1-D array of triangles in a silicon wafer.

An AFM is capable of measuring surface roughness of samples at the nanometer scale. However, it is important to establish that the surface roughness measured with the AFM is not the noise floor of the instrument. Also, when measuring flat samples, it is important to know the “bow” associated with the scanner. These references are useful for establishing the performance of the instrument with respect to noise floor and bow.

SPM height calibration can be done on nano-spheres of uniform size, it also verifies SPM performance on samples with weak adhesion to the surface. Often it is helpful to include nanoparticles with a sample to establish the sizes of features in the imager relative to the nanoparticles.

Often it is necessary to verify SPM image quality on soft samples or samples with weak adhesion to the surface. It is important that this type of reference have a long-life, be stable and be indestructible. Before measuring images of “unknown” biological molecules it is often helpful to practice on this “known” sample.

LFM reference comes as 1-D array of triangular steps having precise linear and angular dimensions. Establishing quantitative LFM data requires standardizing the AFM scanner output in the vertical and horizontal axis.

In vibrating mode, the AFM can measure changes in chemical composition of polymer samples. A reference of polymer material that has hard and soft regions is used for this purpose.

Helps to visualize geometry of the scanning probe without costly need of SEM. Tip radius sample exhibits features sharper that an AFM tip. First-order approximation of the tip apex can be obtained.

Making consistent or even quantitative, measurements with an AFM requires diligence and the appropriate standard or reference. This partial list of standards/references can be used for making consistent or quantitative measurements