The AFM setup, with major components indicated. Motion control systemTo be useful for imaging, an AFM needs to scan its probe over the sample surface. Our microscopes are designed with a fixed probe and a movable sample (also true of some commercial AFM systems). Whenever we talk about moving the tip relative to the sample, in 20.309 we will always only move the sample. The sample is actuated for scanning and force spectroscopy measurements by a simple piezo disk, shown in Figure. The piezo disk is controlled from the matlab scanning software, which is described in. Schematic of the piezo disk used to actuate the AFM's sample stage.
The circular electrode is divided into quadrants as shown in (a) to enable 3-axis actuation. When the same voltage is applied to all quadrants, the disk flexes as shown in (b), giving z-axis motion. Differential voltages applied to opposite quadrants, produce the flexing shown in (c), which moves the stage along the x- and y-axes, with the help of the offset post, represented here by the vertical green line.For vertical motion along the z-axis, there are two regimes of motion:Manual (coarse): Turning the knob on the knob on the stage clockwise moves the stage up.Piezo-disk (fine): Actuating the piezo disk over a few hundred nanometers using the matlab software. For x- and y-axis positioning (in the sample plane), coarse movements are accomplished with the stage micrometers, and fine (several nm) movements are also attained using the piezo disk.Optical system. (a) Output of the ID diffractive transducer. The non-linear intensities of the 0th and 1st order modes as a function of cantilever displacement (from Yaralioglu, et al., J. Phys., 1998), and (b) the desired operating point for maximum deflection sensitivity.Cantilever probes for imagingA few words about probe breakage: you will break at least a few probes — this is a normal part of learning to use the tool.
We have a large, but not infinite supply of replacement probes. The cost of an individual AFM probe is not large, and the greater problem with breaking them is the time lost to replacing the probe and realigning the laser. (a) Plan view of the imaging cantilever geometry, and (b) SEM image of the imaging cantilever. The central (imaging) beam dimensions are length L = 400 μm, and width b = 60 μm.
The finger gratings begin 117 μm and end 200 μm from the base.Exercise caution when moving the sample up and down, but don't let this prevent you from getting comfortable moving the sample around. Under most conditions, the cantilevers are surprisingly flexible and robust. They are most often broken by running them into the sample (especially sideways) — avoid 'crashing' the tip into the surface, or worse bumping the stage into the die or fluid cell. Make sure you're familiar with the.The probes we use for imaging are shown in Figure with relevant dimensions. The central beam has a tip at its end, which scans the surface. The shorter side beams to either side have no tips and remain out of contact.
The side beams provide a reference against which the deflection of the central beam is measured; the ID grating on either side may be used. When calibrating the detector output to relate voltage to tip deflection, remember to include a correction factor to account for the ID finger position far away from the tip.Cantilevers for thermal noise measurementsFor noise measurement purposes, we'd like a clean vibrational noise spectrum, which is best achieved using a matched pair of identical cantilevers. The configuration with a central long beam and reference side-beams has extra resonance peaks in the spectrum that make it harder to interpret. With the geometry shown in Figure the beams have identical spectra which overlap and reinforce each other. Using a pair of identical beams also helps to minimize any common drift effects from air movements or thermal gradients. (a) Plan view and (b) SEM image of the geometry of a differential cantilever pair.
Because the beams are fabricated so close together, we assume that their material properties and dimensions are identical.There are two sizes of cantilever pairs available. For the long devices, L = 350 $ mu $m and the finger grating starts 140 $ mu $m and ends 250 $ mu $m from the cantilever base. For the short devices, L = 275 $ mu $m, and the finger gratings begin 93 $ mu $m and end 175 $ mu $m from the base.
The width and thickness of all of the cantilevers is b = 50 $ mu $m and h = 0.8 $ mu $m, respectively.Major operational steps Power-onFor our AFMs to run, you must turn on three things: (1) the detection laser, (2) the photodetector, and (3) piezo-driver power supply. The photodetector has a battery that provides reverse bias, and the others have dedicated power supplies (refer to Figure for where these switches are located). When you finish using the AFM, don't forget to turn off the three switches you turned on at the beginning. Signal connections and data flow. Schematic of signal connection. NI-USB6212 DAQ channels AI0, 1, 2, AO0, and 1 correspond to ACH0, 1, 2, DAC0, and 1, respectively, in this figure.The first key step to using the instrument is properly connecting all of the components together.
Nanoscope Analysis 1.9
Figure will help to guide you. The AFM itself requires two signal inputs (Xin and Yin) to drive the piezo actuator, which connect to the electronics board on the back of the headplate. They are provided by the computer's analog signal outputs (NI-USB6212 DAQ channels AO0 and AO1, respectively). The computer also needs to read these two signals in, together with the AFM signal output, so these become the three DAQ inputs, AI1, AI2, and AI0, respectively.The output from the AFM's photodetector is a current signal, proportional to the brightness of the laser spot, that needs to be converted to a voltage (a 100 k $ Omega $ resistor to ground is sufficient). It's good to be able to amplify and offset this voltage at our convenience, so we run this signal through a Tektronix AM502 amplifier before it enters the DAQ.Finally, during calibration, it's very useful to watch the detector signal as a function of stage movement in realtime, on the oscilloscope screen, so we run those signals to the scope as well.Laser alignment and diffractive modesTo get a cantilever position readout, the laser needs to be well focused on the interdigitated fingers of the cantilever. Use the white light source and stereo-microscope to look at the cantilever in its holder. The laser spot should be visible as a red dot (there may be other reflections or scattered laser light, but the spot itself is a small bright dot).
Adjust its position using the knobs on the kinematic laser mount, until it hits the interdigitated fingers (use the cantilever schematics in Figure and Figure as a reference).When the laser is focused in approximately the right position, the white 'screen' around the slit on the photodetector will allow you to see the diffraction pattern coming out of the beam splitter. Observe the spot pattern on this screen while adjusting the laser position until you see several evenly spaced 'modes.' Make sure you aren't misled by reflections from other parts of the apparatus — some may look similar to the diffraction pattern, but aren't what you're looking for.When you see the proper diffraction pattern is on the detector, adjust the detector's position such that only one mode passes through the slit.
Typically the 0th mode gives the largest difference between bright and dark.Engaging the tipThe process of bringing the probe tip to the sample surface so we can scan images and measure forces is called 'engaging.' The aim is to get the tip in close proximity so it is just barely coming into contact, and bending only slightly. If the probe does not touch the surface, it is obviously useless, but if it's bent too much against the surface it can damage the sample or simply push through soft features and report topologies lower than actual.Before engaging, start the piezo z-modulation scan in the matlab software (see ). Be sure the mode switch on the AFM electronics board is flipped down to 'force spec. Mode,' and make sure to turn on the piezo power supply. Carefully bring the tip near the surface using the stage micrometer for vertical motion. When you make contact, you will see the modes on the photodetector fluctuate in brightness.
Because of the device geometry, only the central long cantilever with the tip will make contact with the sample surface.Calibration and biasingAt this point, it's worth pausing to review the definition of calibration, as well as the distinction between sensitivity and resolution — terms which will often be used frequently in this context, but whose accurate meaning isn't always made explicit. Be sure you're clear on the differences between them.Calibration is finding the relationship between the output of your instrument to the physical quantity you are measuring like distance or force. In our case, relating the mode brightness measured by the photodiode to cantilever tip deflections.Sensitivity is a numerical expression for the calibration; e.g., the 'slope' of a transducer output in mV/N, W/ºA, or in our case nm/V; i.e., nm of tip deflection versus photodiode voltage. Be careful not to get confused between photodiode voltage and piezo voltage during your calibration step.
Also, be sure to accurately account for your gain setting on the AM502.Resolution is the minimal change in signal that the instrument can detect. It depends completely on the noise, frequency, and bandwidth of the measurementCalibrationIt is essential to run a calibration before performing any measurement because the sensitivity varies from AFM to AFM and user to user.
We calibrate our AFM in force spectroscopy (or ) mode, where the sample is only moved straight up and down. In all that follows, it is assumed that the spot is correctly focused (in the z-direction) on the fingers as noted earlier. The goal of this calibration process is to relate the detector's voltage signal to physical tip deflection; in other words, how many nm is the cantilever tip bending for every volt of photodiode signal.(a)(b)(c). The Scanner GUI window. The AFM is scanning a 12 × 12 μm area, at a rate of one line per second, and is currently near the bottom of the image. Controls overviewMany of the are self-explanatory, such as the start imaging and stop buttons, as well as the image area in the lower right, which displays the image currently being scanned. Some notes are given below on features that are not immediately obvious.To begin with, it's easiest to simply use the default settings on all these controls, and to experiment with changing them as you become more familiar with the tool.Scan Parameters - The Scan size sets the length and width of the image in nanometers (always a square shape), but its accuracy depends on having the correct value for 'Scan sensitivity' (from your calibration procedure).
The 'Scan frequency' (lines per second) sets the speed of the tip across the surface, and together with the number of lines affects the amount of detail you will see in the image. Setting the Y-scan direction tells the scanner whether to start at the top or the bottom of an image, and the trace/retrace selector determines whether each line is recorded as the tip scans either to the left or to the right.Scope View - As the tip scans back and forth, this plots the tip deflection data for each line. It is useful for quantitative feature height measurements.Scanner Waveforms - Shows the voltage waveforms driving the piezo scanner, for each scan line that is taken. This is helpful for knowing where in the image the current scan line is located, and for knowing the output level of the waveforms driving the scanner.Z-mod Controls - These are only active during a z-mod scan, and have no effect when taking an image. For more on this mode, see.Image mode operationThis is the primary operating regime of the AFM, and provides a continuous display of the surface being scanned, as the probe is rastered up and down the image area. To use this mode, the switch on the back of the AFM must be flipped upward. Remember that the maximum scan area is only abut 15 $ mu $m square, and adjusting the position of the sample under the tip requires only the smallest movements of the stage micrometers.
Finally, keep in mind that there is always a delay after pressing start imaging before the scan begins, as the actuator drive signals are buffered to the I/O hardware.Z-mod (force spectroscopy) operationIn this mode, the piezo moves the sample only along the z-axis; i.e., straight up and down (hence z-mod, short for z-modulation). To use this mode, the switch on the back of the AFM must be flipped downward. The defaults for frequency and amplitude, 2 Hz and 8 V, provide a nice force curve. Besides being critical for calibrating and biasing the readout, this mode is used to perform force spectroscopy experiments, in which tip-sample forces can be measured as the tip comes into and out of contact with the sample.(Note that the red stop button is also used to stop a z-mod scan).Saving AFM dataThe software allows you to save the raw data of both images as well as force curves. Not surprisingly, the 'Save Image' and 'Save Force Curve' buttons do this. In both cases an instantaneous snapshot of the current image or force curve is written to the file location specified in the entry box at the bottom of the window.An image is written to a files as a square matrix (interpolated to have the same number of rows and columns), with the value at each point representing height data. Force curve data is saved as two columns: x-axis (stage deflection) data in column one, y-axis (mode intensity) in column two.If you intend to save an image, it is best to set the filename before starting the scan — the filename box behaves $ ldots $ elusively while the scanner is running due to some peculiarities of the software.As a final note, a 'quick and dirty' way to save an image is by simply doing a screen capture while the AFM is scanning (press Print Screen' on the keyboard).
The captured image can then be pasted into MS Paint (Programs $ rightarrow $ Accessories $ rightarrow $ Paint) and cropped to leave only the scanned AFM image. These images can be imported into matlab for analysis/processing (we'll do this in later parts of the course).Additional instrumentation Differential amplifier. An AM502 differential amplifier. The gain is determined by the central red knob, together with the ÷100 button in the center above it.The Tektronix AM502 is a differential amp, so it amplifies the voltage difference between the two input signals. Both inputs have DC, AC, and GND input coupling, like the scopes. The amp can be used single-ended if the (−) input is left unconnected and ground-coupled.
The gain (amplification factor) ranges from 1 to 10,000, and is set by the red knob together with the '÷100' button above it. The instrument also has high- and low-pass filters. These are somewhat confusingly labeled 'HF-3dB' and 'LF-3dB' (see Figure ). This doesn't mean high-pass' and low-pass,' but refers to the 'high cutoff frequency' of a LPF and the 'low cutoff frequency' of a HPF. Therefore, HF-3dB is the low-pass filter, and LF-3dB is the high-pass filter.Both filters are considered 'off' when the low-pass is turned all the way up, and the high-pass all the way down.Finally, the lower (high-pass) filter knob also has two settings for controlling output signal DC level: dc and dc offset.
When set to dc, the amp outputs the actual DC level of the input signal, multiplied by the gain. Set to dc offset, you can manually adjust the DC level using the knob at the upper right of the AM502. In both cases, the AC component is simply added on top.LabVIEW VIs for signal capture The spectrum analyzer. The LabVIEW Waveform Acquisition VI does a 'one-shot' waveform capture in the time domain. Pre-set your desired parameters, including sampling rate, length of captured waveform, and filename to save to, then press the Run (arrow) button to do the data capture.
What you've captured appears in the waveform plot. Experiment 1: Mini-lab: Measuring cantilever stiffness Theory: thermomechanical noise in microcantileversFor simplicity of analysis, we model the cantilever as a harmonic oscillator with one degree of freedom, similar to a mass on a spring, as discussed in lecture. According to the Equipartition Theorem, the thermal energy present in the system is simply related to the cantilever fluctuations as follows: foo. A data plot of a cantilever's noise spectrum, with an ideal transfer function $ G(omega) $ fit on top (dark line). Note that $ G(omega) $ is flat at low frequencies, at the thermomechanical limit, as indicated. In contrast, real data has more 1/f-type noise present at lower frequencies (see ).in which $ omegao $ and $ Q $ are the (angular) resonant frequency and quality factor, respectively.
In order to understand how this function is derived, it is recomended that you see. In the paper the authors calculate the cantilever stiffness and use it to estimate Boltzmann's Constant, as an interesting exercise.
We will actually do the reverse: measure the cantilever stiffness using Boltzmann's constant. This procedure is used quite extensively because in practice it is exceedingly difficult to accurately measure the geometry and precise material properties of the cantilever. Note that at low frequencies ( $ omega ll omegao $), this expression yields called the 'thermomechanical noise limit' for displacement detection (see Figure for an illustration). Using the edge of a sample to bend only one beam of a differential cantilever pair.By now you're familiar with aligning, calibrating, and biasing your AFM.
The major difference in this case is how you'll actually perform the z-modulation scan for the calibration.Because this device is a pair of identical cantilevers, simply bringing it down to a surface will deflect both beams equally. A z-mod scan will show approximately zero deflection of one beam relative to the other. Instead, we want to bend only one of the beams, while keeping the other unbent.To do this, you'll use a sample with a sharp step edge. The goal is to position the cantilever pair above this edge such that one of the beams will be on the surface, and the other will hang in free space (sketched in Figure ). A z-mod scan should then deflect only one of the beams, giving us the calibration curve we want. (Note: reflections of the laser from the edge of the substrate can interfere with the diffractive modes.
If this is the case, try repositioning the sample edge, perhaps using only the corner to bend one of the cantilevers, until the $ sin^2 $ shape improves.)Recording thermomechanical noise spectraOnce you're satisfied with your calibration and bias point, withdraw the lever's tip from the surface, making sure that the bias point stays where you set it. Gain up the signal using the AM502 voltage amp, and use the LabVIEW spectrum analyzer to record the thermal noise signal coming from the freely vibrating cantilever. Once you are happy with how the spectrum looks, save it to a.txt file of your choice.You only need to measure the noise spectrum down to about 50-100 Hz. Below this frequency, 1/f-type or 'pink' noise dominates. You are welcome to measure this if you are interested, but it is of limited use for determining $ kB $. For very low-frequency measurements, anti-aliasing and proper input coupling becomes very important. (b) Preferred biasing and calibration on a hard surface for measuring elastic modulus, and corresponding physical stage movement.The approach for measuring modulus is to first take a force curve on a reference sample, considered to have infinite hardness.
We will use a bare silicon nitride surface.
. NS4 Aux Board NS4-AUX-8171. NS4 Aux Board NS4-AUX-9010. NS4 Aux Board N4A-Aux-8171. NS4 Fast Scan Analog Board 250-NS4FSA-1152 311-009-801 or 311-000-089. NS4 Fast Scan Analog Board 250-NS4FSA-4151 311-000-066.
NS4 Fast Scan Digital Board 250-NS4-FSD-3051 311-009-600 or 311-000-065. NS4 Fast Scan Digital Board 250-NS4-FSD-8010. NS4 XY Board NS4-XY-5071. NS4 XY Board NS4-XY-B291 311-000-087. NS4 Z Board NS4-Z-4280 211-000-071.
NS4 Z board NS4-Z-6074 311-000-071. NS4MU W/ FDTA 250-009-517 311-009-517.