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Therma-Lever™ Probes

Probes for sub-100nm Localized Thermal Analysis


The Therma-Lever™ probes used for nano-TA are micromachined silicon probes that are similar in geometry to standard Silicon AFM probes but incorporate a resistive heater at the end of the cantilever. These novel probes have the capability to image the sample surface with lateral resolution close to that of a standard AFM probe, which is significantly better than most thermal probes. Due to the fact that the heater portion of the probe is made of doped silicon, it can be heated repeatedly and reliably to higher temperatures than thermal probes made of thin metal films.

Our Therma-Lever probes come in two models, AN2-200 and AN2-300 which are differentiated in physical geometry. The AN2-200 style is shorter (~200 microns in length) and so is optimized for intermittent contact mode operation. This probe has a typical resonant frequency between 50 and 80 kHz. The AN2-300 is longer with a psring constant less than 1 N/m and so is optimized for contact mode operation. . The local heating of the end of the cantilever is key to the capability of these probes to be used for nano-TA. It means the tip will locally heat the sample surface at the point of contact and that the probe can rapidly both heat and cool. Finally, the local heating eliminates the significant thermal drift associated with bulk heating of the sample as occurs with standard AFM sample heaters.

Cantilever electrical characteristics

The probes have electrical characteristics governed by the doping in the silicon cantilever and will typically have a resistance in the 0.5 to 3 kOhm range. The probes exhibit a transition from a positive to a negative change in resistance with temperature at ~400 deg C. This resistance turnaround limits their maximum controllable temperature. For brief periods of time, a voltage/power higher than the resistance turnaround can be applied to clean the probe to remove any organic contamination on the probe. This is one of the unique capabilities of the probe and allows a significantly increased lifetime relative to standard AFM probes which once contaminated cannot be cleaned.

The AN2 series of probes have a tip radius of <30nm and the tip height is 3-6 microns thus enabling nano-TA analysis of rougher samples. Shown below are SEM images of the entire cantilever on the left and a close-up of the tip on the right.

Cantilever Power Chart
Cantilever Image

Spatial Resolution of Imaging

As shown in the images to the right and below, our probes can be used in contact mode and intermittent contact mode to give sub-30nm spatial resolution images. The images to the right are 2 micron contact mode scans of polycaprolactone (PCL). The image on the left is the topography and the image on the right is the deflection signal.

Spatial Resolution 2
Spatial Resolution 1

The images to the left are intermittent contact mode images of a rubber blend sample. The image on the far left is the topography and the phase image is next to it showing the domains in the rubber blend. The scan size is 1 micron

Spatial and temp resolution 1

Spatial and Temperature Resolution of nano-TA

Shown to the left is a 2 micron image of an organic crystal sample. The sample was imaged in the contact mode, a location was selected for a thermal ramp and the sample was rescanned. The feature in the middle of the image is the indent caused by the probe after its temperature passed the melting point of the organic crystal. The two blue lines are approximately 90 nm apart and show the width of the indent. This demonstrates the optimal resolution of the nano-TA measurement. Depending on the dynamics of the transition, the length of time the heated probe is left on the sample surface and a number of other factors the affected area on the sample can be larger.

Spatial and temp resolution 2

The data shown to the right is from a polyethylene oxide (PEO) sample. The initial upwards deflection is due to thermal expansion of the sample and once the melting temperature is reached the probe penetrates into the sample. PEO has an onset melting temperature of approximately 65º C. The blue and green lines are from different locations on the sample and demonstrate the approximate 3º C temperature resolution of nano-TA. This resolution can be improved by increasing the number of measurements taken on a sample and averaging to remove local variability on the sample surface.

Calibration for nano-TA

In order to determine the temperature of the probe, each probe needs to be calibrated. This is done with the calibration samples supplied with each system. These samples cover a range of melting temperatures, are very homogenous and exhibit a sharp melting transition. On these samples, the measured melt temperature is very repeatable. Shown below is the calibration interface in the Analysis Studio software. It allows the user to take data which displays melt transitions from each of the three calibration samples and fit a curve to the data to calibrate the voltage applied to the probe into temperature.

Calibration

Models and Specifications

Models and specifications