Editing Scanning probe microscopy (section)

==Image formation==
To form images, scanning probe microscopes [[raster scan]] the tip over the surface. At discrete points in the raster scan a value is recorded (which value depends on the type of SPM and the mode of operation, see below). These recorded values are displayed as a [[heat map]] to produce the final STM images, usually using a black and white or an orange color scale.

===Constant interaction mode===
In constant interaction mode (often referred to as "in feedback"), a feedback loop is used to physically move the probe closer to or further from the surface (in the ''z'' axis) under study to maintain a constant interaction. This interaction depends on the type of SPM, for scanning tunneling microscopy the interaction is the tunnel current, for contact mode AFM or MFM it is the [[Cantilever#In microelectromechanical systems|cantilever]] deflection, etc. The type of feedback loop used is usually a PI-loop, which is a [[PID controller|PID-loop]] where the differential gain has been set to zero (as it amplifies noise). The ''z'' position of the tip (scanning plane is the ''xy''-plane) is recorded periodically and displayed as a heat map. This is normally referred to as a topography image.

In this mode a second image, known as the ″error signal" or "error image" is also taken, which is a heat map of the interaction which was fed back on. Under perfect operation this image would be a blank at a constant value which was set on the feedback loop. Under real operation the image shows noise and often some indication of the surface structure. The user can use this image to edit the feedback gains to minimise features in the error signal.

If the gains are set incorrectly, many imaging artifacts are possible. If gains are too low features can appear smeared. If the gains are too high the feedback can become unstable and oscillate, producing striped features in the images which are not physical.

===Constant height mode===
In constant height mode the probe is not moved in the ''z''-axis during the raster scan. Instead the value of the interaction under study is recorded (i.e. the tunnel current for STM, or the cantilever oscillation amplitude for amplitude modulated non-contact AFM). This recorded information is displayed as a heat map, and is usually referred to as a constant height image.

Constant height imaging is much more difficult than constant interaction imaging as the probe is much more likely to crash into the sample surface.{{citation needed| date=November 2015}} Usually before performing constant height imaging one must image in constant interaction mode to check the surface has no large contaminants in the imaging region, to measure and correct for the sample tilt, and (especially for slow scans) to measure and correct for thermal drift of the sample. Piezoelectric creep can also be a problem, so the microscope often needs time to settle after large movements before constant height imaging can be performed.

Constant height imaging can be advantageous for eliminating the possibility of feedback artifacts.{{citation needed| date=November 2015}}