Scanning Microwave Microscopy

Scanning Microwave Microscopy (SMM) sometime also called Scanning Microwave Impedance Microscopy (SMIM) is a near field technique that combines the high spatial resolution of atomic force microscopy (AFM) with the spectral sensitivity of microwave reflectometry. At its core, it measures the local electrical and dielectric properties of materials down to the nanometer scale, with applications ranging from semiconductor doping profiling and failure analysis to quantum materials and biological samples. Compared to Scanning Spreading Resistance Microscopy (SSRM) or Scanning Capacitance Microscopy (SCM) that require ohmic contact or charge injection, SMM provides inherently non-destructive testing as well as information on the actual n- or p-type dopant concentration.

Due to the large spatial extension of Radio Frequency (RF) waves, from millimeters to centimeters, the drive excitation can penetrates the sample and therefore be sensitive to sub-surface features or buried structures in the material. The interaction between the evanescent microwave fields and the local sample properties modifies the complex tip–sample admittance. This modification affects the reflected microwave signal, or so-called S11 scattering parameter, which is measured to extract spatially resolved electrical information. Such spatial resolution is achieved by guiding the microwave drive down to the AFM tip with minimal reflection before it reaches the apex. This waveguiding principle can be achieved from a number of different set-up design, from simple cabling with impedance matching, to using Mach-Zehnder Modulator (MZM) or designing dedicated microwave cavity at the tip holder.  

Traditionally, SMM uses a Vector Network Analyzer (VNA) to perform frequency-domain measurements. However, with microwave lock-in amplifiers readily available from Zurich Instruments, it is now possible to measure directly at the GHz, which greatly simplifies the setup, offers more flexibility, faster demodulation speed of real and imaginary parts and can accommodate direct sidebands detection, resonance tracking or Arbitrary Waveform Generator (AWG) for advanced continuous or pulsed SMM modes.

Due to the high flexibility of Zurich Instruments GHz lock-in amplifier, 5 measurement strategies can be implement taking advantages of various integrated software options.

1. Continuous-Wave (CW) Reflectometry Mode which is the most common implementation: a fixed frequency GHz wave is sent to the tip and reflected at the same location to extract in-phase and quadrature component of the the tip-sample impedance. This is what can be readily achieved with a standard VNA, including calibration protocol in addition.
2. Direct sideband detection (Field or Height Modulation): superimpose a modulation frequency to either the tip-sample distance (z-modulation), the sample bias or an external magnetic field to increase local sensitivity and remove drift or background offsets.
3. Multifrequency SMM: RF cavities often exhibit multiple resonances and to benefit from broadband sensitivity, it is possible to measure from multiple resonances simultaneously. From a multi-harmonic analysis it is possible to probe various sample layer under the tip since the RF penetration depth depends on its frequency.

4. Pulsed / Time-Domain SMM: the built-in AWG generate microwave burst instead of continuous waves, in the ns to us range. This burst generates transient reflected signal that can either be gated with a boxcar averager or if the relaxation time is longer than the lock-in time-constant (> 50ns) demodulated and averaged over multiple shots. Such technique allows studying relaxation dynamics such as carrier lifetime or domain switching.
5. Force Gradient sensitivity: this requires oscillating the tip at its resonance frequency while injecting RF signal through the tip. The shift in cantilever resonance frequency can then be tracked by a PLL. The phase of the mechanical motion of the cantilever is then imprinted in the RF reflected wave which can be further demodulated at the sensor resonance frequency rather than the pure DC deflection. Force Gradient sensitivity overcomes shortcomings associated with pure contact modes such as large stray capacitance, tip wear or other frictional/buckling effects.

While VNAs offer a good solution for continuous-wave operation, the other four techniques require more complex setups. Thanks to the high flexibility of Zurich Instruments GHz lock-in amplifiers, all five measurement strategies can now be implemented using a single instrument for microwave generation and detection. Available with all microwave lock-in amplifier featuring 2 RF signal outputs, a second phase shifted excitation tone can be generated to cancel the pump tone frequency.

• Tune resonances to any frequency up to 8.5GHz with the SHFLI or 1.8GHz with the GHFLI
• Measure at multiple harmonics to probe different sample depth
• Make differential measurements as a function of Sample Voltage (V) or Tip Height (Z) and measure directly at the sideband frequency around the carrier

• Track resonance with a PLL working at the GHz to measure always at the best operating condition with maximum sensitivity
• Be sensitive to the Force gradient and remove static force contribution by introducing phase sensitive detection from the cantilever
• Provide pump-tone cancelation of drive excitation from the second output to further boost response from the reflected wave