Atomic Force Microscope Operating in Parallel

Atomic force microscopy (AFM) has advanced to be one in every of the foremost powerful tools for the characterization of material surfaces particularly on the nanoscale. It is a vital Nano instrument technique for several applications such as cell biology and nanoelectronics metrology and inspection. The utilization of a single AFM instrument is understood for its very low speed and not appropriate for scanning massive areas, leading to very-low-throughput measurement. Parallelizing AFM instruments will be presented to overcome those challenges. The parallelization is achieved by miniaturizing the AFM instrument and operational several of them at the same time. This Nano instrument has the benefits that every miniaturized AFM are often operated independently and that the advances within field of AFM, both in terms of speed and imaging modalities, can be implemented more simply. Moreover, a parallel AFM instrument additionally permits one to measure several physical parameters simultaneously; whereas one instrument measures nano-scale topography, another instrument can measure mechanical, electrical or thermal properties, creating it a Lab-on-an-Instrument.

Introduction

Atomic force microscopy (AFM) instruments are a necessary type of nano instrument and have contributed to major breakthroughs in materials analysis, nanoelectronics and biology. Today, AFM is employed for not only sub-nanometer imaging however additionally manipulation and manufacturing at the nano-scale. Using various modes of tip-sample interactions, such as atomic force, near-field optics, electrostatic, thermal and electromagnetic forces, AFM instruments have evolved toward changing into a Lab-on-an-Instrument. Traditional AFM uses a singular head/cantilever entity. This poses a limit on the throughput of imaging, characterization and nanomanufacturing. It also limits true sample-to-sample measurement comparison. The increase in the imaging speed in AFM is of interest in several nanotechnology applications, as well as wafer and photolithography mask metrology in nanoelectronics applications. Single AFM has never been able to compete with other inspection systems in terms of throughput and therefore has not fulfilled industry needs in that aspect. Further increases in the speed of single AFM helps, however, it still is far from the required throughput and, therefore, insufficient for applications that require statistically significant sample sizes or massive area, high-resolution measurements such as of wafers and photolithography masks. In these applications, data collection can be an extremely extended process.

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This paper is organized as follows. Firstly, it will describe why parallel AFM is chosen and the overall architecture of the parallel AFM instrument and the performance specifications. Then, the design and realization of the parallel AFM instrument and sub-modules, such as the MAFM instrument, miniaturized PU and large sample stage, are mentioned.

Reasons for Choosing Parallel AFM

AFM can be parallelized: miniaturizing the AFM instrument and operating several of them in parallel. This architecture has the benefits that every miniaturized AFM (MAFM) instrument can be both operated and positioned independently. Moreover, when desired, an array of cantilevers can still be used as an additional level of parallelization within each MAFM instrument. We have achieved a very high throughput in AFM via three developments:

1. A speed increase in MAFM by increasing the bandwidth of the AFM’s sub-modules, i.e., the mechanical stages (x, y and z), optical read-out, controller bandwidth, approach speed and speed of positioning. We have experimentally demonstrated such a high-speed MAFM instrument 19 and a mini positioning unit (PU) capable of positioning each MAFM instrument very quickly and accurately.
2. Sufficient miniaturization of the AFM instrument to operate many such instruments in parallel.
3. Development of a system architecture that can handle relatively large samples.

Architecture of Parallel AFM

The core of the architecture of the parallel AFM relies on an MAFM instrument and a miniature Positioning Unit (PU) to position the MAFM instrument at the targeted scanning location. The objective is to parallelize many such MAFM instruments and PUs to be able to scan many sites in parallel. Each AFM instrument should be miniaturized as much as possible to maximize the number of instruments which will be operated in parallel. All the MAFM instruments are stationary relative to each other in the x, y directions.

The sample (in this case, a silicon wafer) will be on a customized stage that facilitates lateral x, y-scanning. In this way, all MAFM instruments will perform their scans in a very similar pattern, each at its particular site on the wafer. This permits a completely decoupled lateral scanner from the z stage that moves the probe. This eliminates the well-known scanning bowing issue (i.e., obtaining a very flat x, y-scan) and considerably increases the response of the z-scanning stage. Fully fixed x, y-positions of the MAFM instruments would greatly limit the flexibility. Therefore, the MAFM instruments can individually be positioned relative to the wafer before the scanning of the wafer. To keep the design simple, each MAFM instrument can only be positioned on a part (strip) of the wafer by an arm with a large stroke in one direction (x) and a small stroke in the other direction (y). In this way, a set of parallel MAFM instruments can cover the full wafer, and they can be moved onto and off of the wafer to enable loading and unloading of the wafer. To successfully integrate multiple PUs and MAFMs into a full system, each PU and MAFM instrument combination needs to be contained within a narrowly defined area. In the first generation of MAFM instruments and PUs, this could not be realized because all the electrical components were connected with individual wires and cables. Therefore, we have integrated the required electrical connections into flex-rigid PCBs that are glued to the mechanical body of the PU. The wafer stage is shown in the middle. The PUs and MAFM instrument are located at the two sides of the system.

Detailed Design of the Modules

In this section, the design of the key modules: the PU and PU metrology, MAFM instrument and wafer stage, is discussed.

Positioning Unit and Positioning Metrology

The PU has the primary function of positioning a MAFM instrument in the horizontal plane with respect to the scanned sample. The PU consists of a mechanical arm and associated actuators and sensors to perform the positioning function. The arm is mounted onto a linear motor with encoder, which is mounted on a plate. This plate represents the fixed part of the PU and is stiffened by the linear motor assembly. The linear motor allows positioning of the arm, including the MAFM instrument, in the x-direction. The arm contains a built-in flexure-based mechanism that makes it possible to position the MAFM instrument in the y-direction. When the y-mechanism is in the nominal position, the width of the entire PU assembly, as well as cabling. The position of the cantilever tip with respect to the wafer and targeted scanned area must be known before and during the AFM measurements. To obtain the required positioning accuracy for the tip in the lateral direction, closed-loop control is implemented in the PU. Interferometry cannot be used to measure the positions of the PUs because the parallel PUs obstructs each other’s view in the x- and y-directions. Instead, an x, y-grid-based optical linear encoder was chosen for this metrology.

Miniature AFM

The core development of the parallel AFM instrument depends on the development of a MAFM instrument. The MAFM instrument has the following proven performance specifications. The flexure-based and counter mass balanced vertical z stage, which is used to adjust the distance between the sample and the probe. To assure no damage to the tip or to the sample, the following steps are taken. The approach is divided into two phases: an initial fast phase where the coarse approach motor continuously moves the cantilever closer to the surface and a slower ‘walk&talk’ phase. During the latter approach, steps of approximately 1 µm (‘walk’) are interleaved with a sensing step (‘talk’), during which the fine z-stage extends to check whether the sample can be reached and retracted again before the next coarse approach steps. The systems switch from the fast phase to the walk&talk phase when the amplitude starts to decrease due to the rise in squeeze film damping as the cantilever almost reaches the surface.

Wafer Scanning Stage

This section describes the development of the wafer x, y- scanning stage. The wafer stage provides the required sample motion for scanning in the x- and y-directions. The wafer-stage concept architecture includes a horizontally moving stage body consisting of a vacuum chuck for a variety of wafer sizes. The moving stage is suspended in the vertical direction by a group of elastic hinges, which permit freedom of motion for x- and y-translations and for rotation about the z-axis. A separate mechanism on top of the moving stage provides the rotation constraint about the z-axis. The chuck is driven in the x- and y-directions by piezo stacks, which operate independently for each axis. Position feedback is provided by two capacitive sensors, one for the x-direction and one for the y direction. A two-channel position controller was used to control the motions in the x- and y-directions.

Summary

A parallel AFM demonstrator has been developed for high-throughput, sub-nanometer imaging, characterization and metrology of large area samples such as wafers and photomasks. Parallel AFM consists of several miniaturized AFM instruments and PUs, which enable many simultaneous measurement locations on a sample. Each PU can position a miniaturized AFM instrument in less than 5 seconds with a precision of better than 1 µm. A fast approach mechanism has been implemented on each miniaturized AFM instrument to engage the sample in less than 5 seconds. The miniaturized AFM instrument has a high-speed z-stage, low noise level enabling fast feedback and accurate tracking of surfaces even at high scan speeds. The high-speed z-stage is dynamically decoupled from the environment to eliminate disturbances due to high-frequency vibrations. Decoupling the PU from the miniaturized AFM instrument during scanning results in imaging of a large area with minimal artifacts, noise and drift. A parallel AFM is a platform for a high-throughput, automated AFM measurement system.

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