Editing Atomic force microscopy (section)

==Piezoelectric scanners==
AFM scanners are made from [[piezoelectric]] material, which expands and contracts proportionally to an applied voltage. Whether they elongate or contract depends upon the polarity of the voltage applied. Traditionally the tip or sample is mounted on a "tripod" of three piezo crystals, with each responsible for scanning in the ''x'',''y'' and ''z'' directions.<ref name="BinnigQuate1986">{{cite journal|last1=Binnig|first1=G.|last2=Quate|first2=C. F.|last3=Gerber|first3=Ch.|title=Atomic Force Microscope|journal=[[Physical Review Letters]]|volume=56|issue=9|year=1986|pages=930–933|issn=0031-9007|doi=10.1103/PhysRevLett.56.930|pmid=10033323|bibcode = 1986PhRvL..56..930B |doi-access=free}}</ref> In 1986, the same year as the AFM was invented, a new [[piezoelectric]] scanner, the tube scanner, was developed for use in STM.<ref name="BinnigSmith1986">{{cite journal|last1=Binnig|first1=G.|last2=Smith|first2=D. P. E.|title=Single-tube three-dimensional scanner for scanning tunneling microscopy|journal=Review of Scientific Instruments|volume=57|issue=8|year=1986|page=1688|issn=0034-6748|doi=10.1063/1.1139196|bibcode = 1986RScI...57.1688B |doi-access=free}}</ref> Later tube scanners were incorporated into AFMs. The tube scanner can move the sample in the ''x'', ''y'', and ''z'' directions using a single tube piezo with a single interior contact and four external contacts. An advantage of the tube scanner compared to the original tripod design, is better vibrational isolation, resulting from the higher resonant frequency of the single element construction, in combination with a low resonant frequency isolation stage. A disadvantage is that the ''x''-''y'' motion can cause unwanted ''z'' motion resulting in distortion. Another popular design for AFM scanners is the [[Flexure bearing|flexure]] stage, which uses separate piezos for each axis, and couples them through a flexure mechanism.

Scanners are characterized by their sensitivity, which is the ratio of piezo movement to piezo voltage, i.e., by how much the piezo material extends or contracts per applied volt. Due to the differences in material or size, the sensitivity varies from scanner to scanner. Sensitivity varies non-linearly with respect to scan size. Piezo scanners exhibit more sensitivity at the end than at the beginning of a scan. This causes the forward and reverse scans to behave differently and display [[hysteresis]] between the two scan directions.<ref name="analytical1995"/> This can be corrected by applying a non-linear voltage to the piezo electrodes to cause linear scanner movement and calibrating the scanner accordingly.<ref name="analytical1995"/> One disadvantage of this approach is that it requires re-calibration because the precise non-linear voltage needed to correct non-linear movement will change as the piezo ages (see below). This problem can be circumvented by adding a linear sensor to the sample stage or piezo stage to detect the true movement of the piezo. Deviations from ideal movement can be detected by the sensor and corrections applied to the piezo drive signal to correct for non-linear piezo movement. This design is known as a "closed loop" AFM. Non-sensored piezo AFMs are referred to as "open loop" AFMs.

The sensitivity of piezoelectric materials decreases exponentially with time. This causes most of the change in sensitivity to occur in the initial stages of the scanner's life. Piezoelectric scanners are run for approximately 48 hours before they are shipped from the factory so that they are past the point where they may have large changes in sensitivity. As the scanner ages, the sensitivity will change less with time and the scanner would seldom require recalibration,<ref name="fospm2011"/><ref>{{cite journal|author=R. V. Lapshin|year=1998|title=Automatic lateral calibration of tunneling microscope scanners|journal=Review of Scientific Instruments|volume=69|issue=9|pages=3268–3276|issn=0034-6748|doi=10.1063/1.1149091|url=http://www.lapshin.fast-page.org/publications.htm#automatic1998|format=PDF|bibcode = 1998RScI...69.3268L }}</ref> though various manufacturer manuals recommend monthly to semi-monthly calibration of open loop AFMs.