High Resolution and High Sensitivity Imaging of Surface Potential
Three extra EFM modes are supported by the Enhanced EFM for Park AFM. They are Dynamic Contact Electrostatic Force Microscopy (DC-EFM), Piezoelectric Force Microscopy (PFM), and Scanning Kelvin Probe Microscopy (SKPM). DC-EFM, patented by Park Systems under US Patent 6,185,991, and PFM are largely identical techniques. SKPM is also known as Surface Potential Microscopy.
The schematic diagram of Enhanced EFM for Park AFM is shown in Figure 1. An external lock-in amplifier is connected to the Park AFM for two purposes. One is to apply an AC bias of frequency (ω) to the tip in addition to the DC bias being applied by the Park AFM controller. The other purpose is to separate the frequency (ω) component from the output signal. This unique capability offered only through the Enhanced EFM option is what distinguishes it from the standard EFM hardware for Park AFM.
Figure 1. Schematic diagram of the enhanced EFM of the Park AFM series.
Why Enhanced EFM for Park AFM?
Conventional EFM is conducted using unnecessary and inefficient double-pass scans, prohibitively limiting the spatial resolution of a surface potential map. Enhanced EFM for Park AFM is designed to provide efficient one-pass scans to measure both topography and surface potential simultaneously without losing spatial resolution.
• Surface charge distribution and potential imaging
• Failure analysis in micro electronics circuitry
• Mechanical hardness measurement (EFM-DC)
• Charge densitometry for ferroelectric domain
• Voltage drop on micro resistors
• Work function of a semiconductor
Principle of SKPM is similar to Enhanced EFM with DC bias feedback (Figure 2). DC bias is controlled by a feedback loop to zero the ω term. The DC bias that zeros the force is a measure of the surface potential. The difference is in the way the signal obtained from the lock-in amplifier is processed. As presented in previous section, the ω signal from lock-in amplifier can be expressed as following equation:
The ω signal can be used on its own to measure the surface potential. The amplitude of the ω signal is zero when VDC = VS or when the DC offset bias matches the surface potential of the sample. A feedback loop can be added to the system and vary the DC offset bias such that the output of the lock-in amplifier that measures the ω signal is zero. This value of the DC offset bias that zeroes the ω signal is then a measure of the surface potential. An image created from this variation in the DC offset bias is given as an image representing the absolute value of the surface potential (Figure 3).
Figure 2. VDC is controlled to make the amplitude of ω component to be 0, which sets VDC equal to the surface potencial
Figure 3. Surface Potential distribution on an ASIC