Instruction manual

Calibration, Maintenance, Troubleshooting and Warranty
Contact AFM Troubleshooting
Rev. B MultiMode SPM Instruction Manual 313
15.11 Contact AFM Troubleshooting
Depending on the operating mode being used, the symptoms and subsequent resolution may vary.
For this reason, problems listed in this chapter are divided into Contact AFM (see Section 15.11)
and TappingMode (see Section 15.12). STM problems are described in Chapter 9 of this manual.
Some of the problems and cures associated with contact AFM are also relevant to TappingMode. If
problems occur, it may be helpful to read through the Contact AFM problems and vice-versa.
15.11.1 False engagement
The main cause of false engagement is optical interference on the photodiodes, which causes the
vertical deflection (A-B) voltage to slowly move toward the setpoint voltage. Once the vertical
deflection voltage reaches the setpoint voltage, the feedback loop assumes the tip has contacted the
sample.
The source of the optical interference comes from either: i) the reflective gold coating on the
cantilever, allowing some of the laser spot through and onto the sample surface; or, ii) stray laser
light from the main beam interacting with the surface directly, causing optical path length related
interference. The direct substrate reflection is more of an issue with reflective substrates that have
large steps or topography. These effects can cause scattered light to be reflected onto the photodiode
assembly along with the reflected beam from the back of the cantilever. The scattered light causes
changes in the vertical deflection (A-B) voltage as the AFM head is lowered to the sample during
engagement.
IMPORTANT! If a false engagement occurs, it can be detected easily by adjusting the Setpoint
and observing the Z Center Position change. Increasing the Setpoint voltage by 1V should cause
the Z Center Position to change by 1-5V. If the Z Center Position changes by a large amount (10-
20V), the system is false engaged.
It is a good habit to watch the vertical deflection (A-B) differential voltage while the tip is
engaging. The voltage should ideally jump (not drift slowly) from some negative value to the
setpoint voltage. If it drifts slowly it means that reflected light is affecting the photodiode voltage. If
this is the case, the engagement process can be aborted by clicking the Abort button in the Engage
Status box shown during engagement. The photodiode position should be adjusted to make the
Vertical Deflection voltage more negative than the setpoint voltage before trying the engage
sequence again.
Sources of false engagement include:
• Incorrect optical alignment of laser spot on cantilever. This alignment will cause a
greater amount of laser light to reflect off of the sample.
• The sample has a region on it that touches the cantilever before the tip does.
•Foreign material stuck on the cantilever beam that is lower than the tip.