Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays for High-Throughput Large-Scale Sample Inspection

Atomic force microscopes (AFMs) can capture 3D topography images with nanoscale spatial resolution. Researchers have extended the capability of AFMs to create sample property maps in mechanical, electrical, magnetic, optical, and thermal domains. In the meantime, improving imaging throughput has also been the focus of research to adapt AFMs to new experimental needs. There are primarily two application domains for high-throughput AFM imaging: the first category is high-speed imaging of a small area to capture dynamic changes in the sample due to biological or chemical reactions^1,2; the second category is for high-spatial resolution, large-scale imaging of static samples during an inspection, which is discussed in detail in this work. With transistor size shrinking down to the nanoscale, the semiconductor industry urgently needs high-throughput AFMs to inspect wafer-scale nanofabricated devices with nanoscale spatial resolution³.

The characterization of nanofabricated devices on a wafer can be challenging due to the vast scale difference between wafer and transistor features. Large defects can be spotted with optical microscopes automatically⁴. In addition, scanning electron microscopes (SEMs) are widely used for inspection down to tens of nanometers in 2D⁵. For 3D information and higher resolution, the AFM is a more suitable tool if its throughput can be improved.

With limited imaging throughput, one approach is to image selected wafer areas where nanofabrication defects are more likely to happen⁶. This would require prior knowledge of the design and fabrication process. Alternatively, combining other modalities, such as an optical microscope or SEM with an AFM for overview and zoom, is possible^7,8. A wide-range, high-precision positioning system is needed to properly align the coordinate system between the fabrication and characterization tools. Moreover, an automated AFM system to image various selected areas is necessary to realize this functionality.

As an alternative, researchers have investigated different ways to increase AFM scanning speed. Since enabling high-throughput AFMs is a systematic precision instrumentation challenge, researchers have investigated various methods, including using smaller AFM probes, redesigning high-bandwidth nano-positioners^9,10,11,12 and driving electronics¹³, optimizing modes of operation, imaging control algorithms^14,15,16,17, etc. With these efforts, the effective relative tip and sample speed can be increased to a maximum of around tens of millimeters per second for commercially available single-probe AFM systems.

To further improve the imaging throughput, adding multiple probes to operate in parallel is a natural solution. However, the optical beam deflection (OBD) system utilized for cantilever deflection sensing is relatively bulky, which makes the addition of multiple probes relatively challenging. Individual cantilever deflection control can also be difficult to realize.

To overcome this limitation, embedded sensing, and actuation principles without bulky external components, are preferred. As detailed in previously published reports^18,19, deflection sensing with piezoresistive, piezoelectric, and optomechanical principles can be considered embedded sensing, with the former two being more mature and easier to implement. For embedded actuation, thermomechanical with electrical heating or piezoelectric principles can both be utilized. Although piezoelectric principles can operate in a wider temperature range down to cryogenic environments, they can only support tapping mode AFM operations, since static deflection cannot be measured due to the charge leakage and static actuation suffering from hysteresis and creep. In prior work, active cantilever probe arrays using a piezoresistive sensor and the piezoelectric sensor have been developed for large-range imaging^20,21, but have not been further scaled up for large-scale imaging or commercialized. In this work, the combination of piezoresistive sensing and thermomechanical actuation are selected as embedded transducers with static deflection control capability.

In this work, a novel "Quattro"²² parallel active cantilever array is used as the probe²³ for simultaneous imaging using active cantilevers. To measure the cantilever deflection, piezoresistive sensors in a Wheatstone bridge configuration¹⁹ are nanofabricated at the base of each micro-cantilever to measure the internal stress, which is linearly proportional to the cantilever tip deflection. This compact embedded sensor can also achieve sub-nanometer resolution as the conventional OBD sensor. The governing equation of the Wheatstone bridge voltage output U_out in response to the applied force F or cantilever deflection z is shown in Equation 1¹⁹ for a cantilever with length L, width W, and thickness H, piezoresistive sensor coefficient P_R, and effective elastic modulus of the cantilever E bridge supply voltage U_b.

    (1)

As dynamic tapping/non-contact mode operation is preferred for noninvasive imaging to avoid disturbing the sample, a thermomechanical actuator made of serpentine shape aluminum wires is used to heat up the bimorph cantilever made with aluminum/magnesium alloy²⁴, silicon, and silicon oxide materials. At the microscopic scale, the time constant of thermal processes is much smaller, and the cantilever resonance at tens to hundreds of kilohertz can be excited by driving the heater with an electrical signal. The cantilever free end deflection z_h controlled by the heater temperature ΔT relative ambience is shown in Equation 2¹⁹for cantilever length L with a constant K, depending on the bimorph material thermo coefficient of expansion and geometric thickness and area. It should be noted that the ΔT is proportional to the heater power P, which is equal to the square of the applied voltage V divided by its resistance R.

    (2)

As an added benefit, static deflection can also be controlled in addition to resonance excitation. This can be a particularly helpful capability to regulate the probe-sample interaction of each cantilever individually. Moreover, multiple cantilevers on the same base chip can be excited individually with the embedded thermomechanical actuator, which is impossible in conventional resonance excitation with piezo-generated acoustic waves.

Combining piezoresistive sensing and thermomechanical actuation, the active cantilever probe has enabled a wide range of applications, including collocated AF microscopy in SE microscopy, imaging in opaque liquid, and scanning probe lithography, with more details available in review²⁵. For high-throughput inspection purposes, the active cantilever array is created with a representative AFM implementation example involving four parallel cantilevers, as shown in Figure 1. In the future, an industrial-scale system will be developed using eight parallel active cantilevers and tens of positioners²⁸. To illustrate the scale using an example, with an in-plane spatial resolution of 100 nm, imaging an area of 100 mm by 100 mm would result in over 10⁶ scan lines and 10¹² pixels. With a scanning speed of 50 mm/s per cantilever, this would require a total of over 555.6 h of scanning (23+ days) for a single cantilever, which is too long to be practically useful. Using the active cantilever array technology with tens of positioners, the required imaging time can be reduced by around two orders of magnitude to 5-10 h (less than half a day) without making any compromise to the resolution, which is a reasonable time scale for industrial inspection purpose.

To capture large-area, high-resolution images, the nano-positioning system is also upgraded. For imaging wafer-scale large samples, scanning the probe instead of the sample is preferred, in order to reduce the size of the objects being moved. With the separation distance between active cantilevers at 125 µm, the scanner covers an area slightly larger than this range so that images from each cantilever can be stitched together during post-processing. Upon completion of a scan, the coarse positioner automatically repositions the probe to a new adjacent area to continue the imaging process. While the embedded thermomechanical actuatorregulates the deflection of each cantilever, the averaged deflection of all parallel cantilevers is regulated with another proportional-integral-derivative (PID) controller to assist the cantilevers during topography tracking. The scanner controller also ensures that the bending of each cantilever does not exceed a maximum threshold value, which may cause other probes to lose contact with the surface if the topography variation is too large.

The level of topography variation that can be tracked for cantilevers on the same base chip should it be limited, since the static deflection control range of the cantilever is on the order of tens of microns. For semiconductor wafers, the sample topography variations are typically on the sub-micrometer scale, so they should not be much of a big problem. However, with the addition of more cantilevers, the sample plane tilt with respect to the line of cantilevers can become a problem. In practice, eight parallel cantilevers with spacings close to 1 mm would still allow 1° of tilt angle, while adding more cantilevers can make the tilting control more difficult to realize. Therefore, using multiple groups of eight-cantilever probes placed on separated probe scanners is an ongoing effort to fully realize the potential of the parallel active cantilever probe principle.

After data collection, a post-processing operation is necessary to retrieve the desired information. The process generally involves removing scanning artifacts, stitching adjacent images to form an overall panorama, and optionally identifying the structure defects by comparing them to the desired geometry using suitable algorithms²⁶. It is worth noting that the amount of data accumulated can be enormous for a large range of images, and data-driven learning algorithms are also being developed for more efficient processing²⁷.

This article illustrates the general process of acquiring high-resolution AFM images using the parallel active cantilever array integrated into a custom AFM system. Detailed implementation of the system is available in^22,28,29,30, and it is being commercialized with the model number listed in Table of Materials. All four cantilevers were operated in tapping mode excited by the embedded thermal-mechanical actuator. Representative results on calibration samples, nanofabrication masks, and highly oriented pyrolytic graphite (HOPG) samples (see Table of Materials) are provided to illustrate the effectiveness of this new AFM tool for large-area inspection.