Park Knowledge Portal

Research Application Technology Team, Park Systems HQ.

Introduction

Recently, Atomic Force Microscopy (AFM) has become a powerful tool in both research and industry. AFM, the most widely used technique in Scanning Probe Microscopy (SPM), employs a sharp tip to measure the sample’s topography at the nanoscale. Moreover, various optional modes enable simultaneous measurements of electrical, mechanical, and other material properties along with topographical information.

Among these modes, several are dedicated to investigating mechanical properties. Typically, the PinPoint™ (Hereafter referred to as ‘standard PinPoint’) mode [1] is used to measure soft and delicate samples which are easily damaged by general scan such as Contact or Tapping. In addition, it allows quantitative analysis of mechanical parameters such as adhesion and Young’s modulus. During measurement, the Z scanner moves vertically at each pixel to induce elastic deformation, while the system records force-distance (F/d) curves during the approach-retract cycle. From these data, surface morphology, roughness, stiffness, adhesion force (energy), and Young’s modulus can be quantitatively determined.

However, the PinPoint mode is limited by its relatively long imaging time. In this mode, the XY scanner performs a horizontal stepwise motion at each pixel while the Z scanner alternates between approach and retract to acquire F/d curves. Although this “stop-and-go” operation enables precise quantitative measurement, it significantly reduces scanning speed, Table 1(a). To overcome this limitation, the Fast PinPoint mode has been developed to dramatically improve scanning speed while maintaining quantitative accuracy.

Table 1. Comparison of standard PinPoint and Fast PinPoint 

In this mode, the Z scanner follows a continuous sinusoidal waveform, while the XY scanner moves linearly, as illustrated in Table 1(b). This innovative approach allows researchers to measure mechanical properties more efficiently without sacrificing data reliability.

Table 2. Data Acquisition method in standard PinPoint and Fast PinPoint

Table 2 summarizes the operational differences between the PinPoint and Fast PinPoint modes. In the standard mode, the XY scanner pauses at each pixel while the Z scanner performs approach-retract cycles, ensuring accurate force control but resulting in slow imaging. In contrast, the Fast PinPoint mode replaces the discrete step motion with a continuous sinusoidal Z scanner trajectory synchronized with a linear XY scan. The Z height is recorded at the moment closest to pixel synchronization, effectively capturing full F/d curves during each oscillating cycle.

Consequently, the Fast PinPoint mode significantly reduced total scan time while maintaining quantitative precision. Thus, while both modes provide reliable nanomechanical information, the Fast PinPoint mode offers a clear advantage in measurement efficiency by transforming the Z scanner motion from discrete to continuous operation.

Method

Figure 1. (a) Configuration window of the Fast PinPoint mode in the SmartScan™ software. The parameters highlighted in red are used for both NX and FX systems, while the parameters highlighted in blue are specific to the FX system. (b) External control window, Fast PinPoint DAQ, for the Fast PinPoint mode on the NX system. (c) Realtime F/d curve of FX Fast PinPoint mode display on SmartScan™. FX systems have embedded DAQ board in their controller, therefore they need not use extra hardware (NI-board) and software.

Compared to the standard PinPoint mode, the Fast PinPoint mode requires slightly different parameter settings. In Fast PinPoint mode, the Z scanner oscillates during measurement, and therefore the user must define the Z modulation frequency and modulation drive. Fig. 1 (In Figure 1a, the Z modulation drive is a parameter that determines the amplitude of the Z scanner's oscillation, and its unit can be set to µm.).

The Z modulation frequency, which can be set up to 2 kHz, is the most critical parameter in determining the number of F/d curves acquired per pixel. Unlike the standard PinPoint mode, which collects one F/d curve per pixel, the number of F/d curves obtained in Fast PinPoint mode varies depending on the selected frequency.

During the measurement, the Z scanner performs sinusoidal motion, and F/d curves are continuously collected at all contact points where the probe interacts with the sample surface. For each pixel, the system selects the F/d curve that is closest to the pixel sync point to calculate the local mechanical properties. (All acquired F/d curves are saved in a PS-PPT file for further analysis.)

Since the F/d curves are saved continuously during scanning, users must carefully set parameters such as scan rate, pixel density, and Z modulation frequency so that at least one F/d curve is obtained within each pixel. If fewer than one F/d curve is assigned to a single pixel, interpolation may occur between adjacent pixels because an F/d curve from the previous pixel may be used for the next one. This issue can be easily avoided by setting parameters so that each pixel contains at least one or more F/d curves.