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

Summary

Large-scale sample inspection with nanoscale resolution has a wide range of applications, especially for nanofabricated semiconductor wafers. Atomic force microscopes can be a great tool for this purpose, but are limited by their imaging speed. This work utilizes parallel active cantilever arrays in AFMs to enable high-throughput and large-scale inspections.

Abstract

An Atomic Force Microscope (AFM) is a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. However, due to their limited imaging throughput, AFMs have not been widely adopted for large-scale inspection purposes. Researchers have developed high-speed AFM systems to record dynamic process videos in chemical and biological reactions at tens of frames per second, at the cost of a small imaging area of up to several square micrometers. In contrast, inspecting large-scale nanofabricated structures, such as semiconductor wafers, requires nanoscale spatial resolution imaging of a static sample over hundreds of square centimeters with high productivity. Conventional AFMs use a single passive cantilever probe with an optical beam deflection system, which can only collect one pixel at a time during AFM imaging, resulting in low imaging throughput. This work utilizes an array of active cantilevers with embedded piezoresistive sensors and thermomechanical actuators, which allows simultaneous multi-cantilever operation in parallel operation for increased imaging throughput. When combined with large-range nano-positioners and proper control algorithms, each cantilever can be individually controlled to capture multiple AFM images. With data-driven post-processing algorithms, the images can be stitched together, and defect detection can be performed by comparing them to the desired geometry. This paper introduces principles of the custom AFM using the active cantilever arrays, followed by a discussion on practical experiment considerations for inspection applications. Selected example images of silicon calibration grating, highly-oriented pyrolytic graphite, and extreme ultraviolet lithography masks are captured using an array of four active cantilevers ("Quattro") with a 125 µm tip separation distance. With more engineering integration, this high-throughput, large-scale imaging tool can provide 3D metrological data for extreme ultraviolet (EUV) masks, chemical mechanical planarization (CMP) inspection, failure analysis, displays, thin-film step measurements, roughness measurement dies, and laser-engraved dry gas seal grooves.

Introduction

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 h....

Protocol

1. Sample preparation for large-scale inspection

1. Prepare the sample with a suitable size for the AFM (see Table of Materials).
   NOTE: Wafer-shape samples with an in-plane diameter from 75 mm to 300 mm and an expected out-of-plane height variation below 200 µm can fit on the AFM sample stage. In this study, an extreme ultraviolet (EUV) mask on a 4 inch wafer is used (see Table of Materials).
2. Clean the sample to remove contaminants and.......

Discussion

As demonstrated in the representative results, an active cantilever array can be used to capture multiple images of a static sample in parallel. This scalable setup can significantly improve the imaging throughput of large-area samples, making it suitable for inspecting nanofabricated devices on semiconductor wafers. The technique is also not limited to man-made structures; as long as the topography variation within a group of active cantilevers is not too large for the cantilever array to handle, high-throughput imaging.......

Materials

Name	Company	Catalog Number	Comments
Active-Cantilever	nano analytik GmbH	AC-10-2012	AFM Probe
E-Beam	EBX-30, INC	012323-15	Mask patterning instrument
Highly Oriented Pyrolytic Graphite – HOPG	TED PELLA, INC	626-10	AFM calibration sample
Mask Sample	Nanda Technologies GmbH	Test substrate	EUV Mask Sample substrate
NANO-COMPAS-PRO	nano analytik GmbH	23-2016	AFM Software
nanoMetronom 20	nano analytik GmbH	1-343-2020	AFM Instrument