Detection based on online atomic force microscopy (AFM)

Introduction

In the market for precision systems, the continuous improvement in manufacturing accuracy of miniature components is essential. Some nanoscale features require highly precise machining, and it was predicted by Taniguchi that sub-nanometer accuracy would be achieved by 2020. However, both nanofabrication and micromachining are influenced by their inherent physical phenomena. At the nanoscale, van der Waals forces become more pronounced, while at the microscale, factors such as adhesion, fracture, surface layer formation, and debris generation play a significant role.

Material fracture behaviors vary widely as they are governed by the microscopic characteristics of the material, which differ from one material to another. While ultra-precision machine tools are designed to achieve extreme accuracy, machining errors still exist and require compensation during the process. Therefore, if manufacturing processes such as material removal and part fabrication are expected to meet stringent specifications, uncertainty may arise at the nano- and microscale levels. In this context, in-process inspection and correction become critical to ensuring that specifications are maintained within acceptable limits.

In-Process Inspection in Manufacturing

In-process inspection has been employed in manufacturing for decades using various measurement techniques to ensure machine tool accuracy. A review of standard scale machines highlights the advancements in this field. Recently, sensors have been developed to measure specific characteristics such as roundness, surface finish, and dimensional accuracy. However, a review of publicly available literature indicates that there is limited contribution to in-process monitoring at the nanoscale.

Nanolithography is typically performed using atomic force microscopy (AFM) probes for material removal, but quality degradation in measurements has been observed. The same principle applies to nano-patterning and mask repair using AFM imaging. Other studies have explored multi-scratch techniques using AFM tips for depth prediction.

AFM probes can be used for specific lithography tasks and then retracted for inspection. However, this approach causes wear on the probe tip, thereby affecting measurement quality. To address this issue, this paper proposes the integration of a dedicated lithography probe into an AFM probe system. This lithography probe, designed based on the AFM probe and assembled in a back-to-back configuration, ensures that the measurement quality of the AFM probe remains unaffected by degradation caused by other tasks.

The Era of Atomic-Level Material Control

A new era in engineering and science has emerged, enabling the design and control of material properties at the atomic and molecular levels. The proposed in-process inspection method aims to achieve the following objectives:

1. Provide online detection for nanofabrication using back-to-back probes, one of which is an AFM probe.

2. Manufacture nanodevices with specific geometries and properties.

3. Inspect and characterize complex shapes with corresponding precision.

Host System Description

Building upon previous research, this study proposes a nanofabrication process monitoring method based on an AFM probe system. The host system is a nano-manipulator, with one section dedicated to sample preparation and robotic operation, and the other dedicated to nanofabrication, such as nanolithography with in-process monitoring (Figure 1). The instrument enables process inspection, assembly, and characterization of materials or components at the nanoscale. All operations are performed within a single rotary stage, ensuring that data integrity is maintained while investigating the same sample.

Figure 1:Schematic diagram of the integrated manipulator and overall coordinate system.

The rotary stage consists of three main regions:

1. Sample Preparation Area: Dedicated to preparing materials for processing.

2. Sample Manipulation Area: Equipped with three robotic arms, each having three degrees of freedom (DOF) with nanometer resolution over a 10 mm range. This area is monitored by a CCD camera with a 1 μm resolution, allowing inspection of both the sample and the probe.

3. AFM Inspection and In-Process Monitoring Area: Used for nano-processing and quality inspection.

Nanofabrication includes CNC-based nanoscale machining, such as lithography, where probes are installed back-to-back with the AFM probe for in-process inspection. This integration represents a novel approach. Additionally, a suction device is incorporated to remove machining debris, ensuring a clean processing environment. The top view of the complete instrument setup is illustrated in Figure 2. Most of the components, aside from key technical hardware such as the AFM, robotic arms, and rotary stage, are custom-built.

Figure 2:Nano-manipulator platform for nano-mechanical machining.

Control Architecture and Measurement Capabilities

All components operate under the control architecture described in Figure 3, where the highlighted controllers primarily manage process monitoring. Some vibrations from both the AFM and cutting probes may be observed; however, these are minimized through an isolation and damping system that separates the nano-manipulator from ground vibrations.

Figure 3:Cross-section of the nano-manipulator with in-process atomic force microscopy (AFM) inspection.

The primary focus of the study is the in-process inspection area. The AFM is mounted on an XY precision stage (Figure 3), with locks on each axis allowing probe height adjustments of a few millimeters to accommodate different part sizes. A piezoelectric scanner for nanofabrication is installed on a separate platform near the AFM, which is securely mounted on a support base. This setup ensures that the rotary stage can position samples under both probes when needed.

The AFM has a maximum scanning range of XY 110 μm × 22 μm and requires standard AFM cantilevers. The lithography scanning head, Nanite SH A110, has a similar measurement range. The AFM is controlled by an SPM200 controller, while the lithography head is managed by an SPM100 controller, operating at speeds of up to 60 ms per line. Although the two heads function independently, they are integrated within a single script that coordinates lithography and inspection tasks within a closed-loop control system, programmed through the human-machine interface. Sample positioning under both the lithography and AFM probes is precisely controlled to maintain accuracy.

Conclusion

This study presents an innovative approach to in-process monitoring for nanoscale fabrication using an AFM-based probe system. By integrating a dedicated lithography probe alongside the AFM probe, measurement quality is preserved without degradation from lithography tasks. The proposed system enhances precision in nanoscale manufacturing, enabling the design, fabrication, and characterization of complex nanostructures with high accuracy.

[1].Mekid, S. In-Process Atomic-Force Microscopy (AFM) Based Inspection. Sensors 2017, 17, 1194. https://doi.org/10.3390/s17061194.