Online Atomic Force Microscope (AFM)-Based Detection

In the precision systems market, the manufacturing of small components with continually improving accuracy is essential. Certain nanoscale features are required to be precisely processed, with predictions by Taniguchi that by 2020, these will reach sub-nanometer levels. However, nano and micro machining are associated with their own phenomena. At the nanoscale, the impact of van der Waals forces is more pronounced, while at the microscale, adhesion between micro-protrusions, fracture, surface layer formation, and debris generation are all significant. Material fracture phenomena are known to be diverse, as they are controlled by the micro characteristics of a material, which differ between materials.

While precision machine tools are designed to achieve such precision, processing errors need to be minimized and compensated for during the process. Therefore, if the manufactured features, such as material removal, component removal, and the obtained features, are checked and are expected to closely match specifications, manufacturing at the nano and micrometer scales may not be entirely deterministic. At such scales, process monitoring and corrections, if necessary, to maintain the specifications discussed earlier become crucial.

This method of process monitoring has been used in the manufacturing industry for decades, employing different measurement techniques for standard machine tool components. A review of standard scale machines was conducted. Recent sensors have been proposed to measure specific features such as roundness, surface smoothness, and dimensional measurements. A search of public literature indicates limited contributions to process monitoring at the nanoscale. The role of nano-lithography is typically shaped by AFM probes used for material removal, showing degraded quality measurements and using AFM for nano-patterning and mask repair based on the same principles. Other studies include the use of AFM tips for deep prediction with multiple scratching methods.

AFM probes can be used for specific lithography tasks and can be withdrawn to detect the work performed, but this practice reduces the probe's tip, affecting measurement quality. To avoid this, this paper proposes adding a probe specifically for lithography tasks in an AFM probe system, where the probe is designed and assembled back-to-back with an AFM probe. The measurement quality of the AFM probe is unaffected by other tasks that reduce probe functionality.

In engineering and scientific fields, a new era has been triggered, where material properties can be designed and controlled at the level of individual atoms and molecules. The proposed process monitoring service serves the following specific objectives:

Provide online detection for nano-machining processes using back-to-back probes (one of which is an AFM probe).
Manufacture nano-devices with specific geometries and characteristics.
Inspect and characterize complex shapes with the corresponding precision.

Host System Description

Building on previous research, this paper proposes a nano-machining process detection method based on an AFM probe system. The host system is a nano-robotic arm, with one side used for sample preparation and robotic manipulation, and the other side for nano-machining, such as nano-lithography with process monitoring (Figure 1). The instrument includes nano-manufacturing, process monitoring of material particles or components at the nanoscale, assembly, and inspection/role shaping. The instrument integrates all operations into a single rotating stage, maintaining the same sample data during investigation.

The rotating stage consists of three areas:

Sample Preparation Area
Sample Manipulation Area equipped with three robotic arms, each with three degrees of freedom (DOF) and nano-resolution within a 10mm motion range. This area is monitored with a CCD camera with a resolution of 1µm. This can inspect both the sample and probe.
Atomic Force Microscope (AFM) Inspection and Process Monitoring Area for nano-machining.

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

Figure 2:Nano-robotic arm platform for nano-machining.

Nano-machining includes CNC nano-machining, such as lithography, with the probe mounted back-to-back with the AFM probe for process monitoring during machining—this is a new feature. A suction device is added to clean processing debris. An overhead view of the entire instrument is shown in Figure 2. Aside from technical hardware like the AFM, robotic arms, rotating stage, etc., most parts are custom-made. All components are controlled based on the control architecture described in Figure 3. The highlighted colored controller is primarily used for process monitoring. Some vibrations may affect the AFM and cutting probes, but environmental effects are minimized through vibration control damping by isolating the nano-robotic arm from ground vibrations. The research focus is on the inspection zone.

Figure 3:Cross-section of the nano-robotic arm with process AFM inspection.

The AFM is mounted on an XY precision stage (Figure 3), with locks on each axis, allowing adjustment of the probe height by a few millimeters to accommodate different component sizes. The piezoelectric scanner for nano-machining is mounted on a separate platform close to the AFM, which is firmly installed on a supporting platform. The space under the probe allows the rotating stage to receive and position samples under both probes when required.

The maximum scanning range of the AFM is XY 110μm × 22μm, requiring standard AFM cantilevers, while the scanning head of the lithography machine, Nanite SH A110, has a similar measurement range to the aforementioned AFM scanning head. The AFM is controlled by the SPM200 controller, and the lithography head is controlled by the SPM100 controller, with a speed of up to 60ms/line. Thus, the two heads operate independently but work under a single script, where both lithography and inspection operations are performed in a loop, following commands from the high-level control in the human-machine interface. The position of the sample under the lithography and AFM probes is controlled.

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