A new type of nanoscale reference grating manufactured by combined laser-focused atomic deposition and x-ray interference lithography and its use for calibrating a scanning electron microscope

Highlights

• A new type of 1D grating sample fabricated by combining laser-focused atomic deposition and x-ray interference lithography is proposed as a lateral standard for nanometrology. Owing to the applied nanomanufacturing techniques, the grating has (much) better large-range as well as short-range pattern uniformity than conventional samples e.g. fabricated by the e-beam lithography. The grating sample is thus well suited for calibrating the magnification and field distortion of almost all kinds of microscopes, particular SEM.

• Calibration of the developed grating sample has been performed by an ultra-precision metrological large-range atomic force microscope. The result indicates that the grating sample exhibits outstanding pattern uniformity: (1) the nonlinear deviation of the grating structures is below +/- 0.5 nm over a measurement range of 5 µm; (2) the maximal variation of the calibrated mean pitch values is lower than 0.01 nm at different locations randomly selected all over the pattern area.

• Application of the new sample for accurately calibrating the magnification and nonlinearity of a commercial SEM is demonstrated, showing its advantages of easy-of-use and high accuracy.

• Influence of the defocus level of SEM on the calibration result is also demonstrated.

• This research offers a feasible solution for highly accurate SEM calibration needed for 3D nanometrology and hybrid metrology demanded in metrology of modern nanoelectronics devices and systems.

Abstract

Calibration of magnification and nonlinearity of scanning electron microscopy (SEM) is an essential task. In this paper, we proposed a new type of 1D grating sample fabricated by combining laser-focused atomic deposition and x-ray interference lithography as a lateral standard for calibrating SEMs. The calibrations of the grating pattern by a metrological large-range atomic force microscope indicate that the grating sample exhibits outstanding pattern uniformity that surpasses conventional samples fabricated by e-beam lithography: (1) the nonlinear deviation of the grating structures is below +/- 0.5 nm over a measurement range of 5 µm; (2) the maximal variation of the calibrated mean pitch values is lower than 0.01 nm at different locations randomly selected all over the pattern area. The proposed new sample is applied for accurately calibrating the magnification and nonlinearity of a commercial SEM, showing its advantages of easy-of-use and high accuracy. The influence of the defocus level of SEM on the calibration result is also demonstrated. This research offers a feasible solution for highly accurate SEM calibration needed for 3D nanometrology and hybrid metrology demanded in metrology of modern nanoelectronics devices and systems.

Introduction

Nanometrology becomes increasingly important for developing various nanotechnologies. Take the semiconductor industry as a typical example, today integrated circuit (IC) is continuously delivering complex nanostructures that are smaller in size, more complex in shape, and use more types of materials than at any time in its history. Accurate metrology of nanostructures, essentially needed in process developments and process controls, becomes more demanding and challenging [1]. Besides non-imaging techniques such as x-ray and optical scattering, many microscopic techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Atomic Force Microscopy (AFM) have been applied for nanometrology. Accurate and traceable calibrations of these methods are fundamental tasks in ensuring their measurement accuracy. In SEM measurements, for instance, a finely focused electron beam typically scans over a sample, where the secondary and backscattered electrons (and other signals) yielded from the beam/sample interaction are acquired for generating SEM images. Thus, the magnification and nonlinearity of the electron beam scanner are key factors in evaluating the SEM images for nanometrology.

For satisfying nanometrology needed for future technology nodes, calibration of microscopes needs to be even more accurate. As an example, today the SEM technique are extended for three-dimensional (3D) nanometrology using e.g. stereophotogrammetry [2], [3] as IC structures becomes more 3D [1]. Using stereophotogrammetry, nanostructures are measured when the sample is tilted with respect to the beam by different angles, so that the height and 3D shape of nanostructures can be calculated from the shifted lateral positions of the nanostructures evaluated from the stereo SEM image sets. Consequently, the measurement error of the lateral positions of the nanostructures will strongly impact the measurement uncertainty of the 3D shape, particularly when the stereo angle is small. Thus, highly accurate calibration of SEMs becomes crucial. Another example concerns the hybrid nanometrology [4]. The nanometrology techniques available today have both advantages and disadvantages. No single instrument has the full capabilities (e.g. resolution, speed, low levels of uncertainty) needed to characterize the whole set of parameters of complex nanostructures, so the integration of multiple tools is required [1]. Hybrid nanometrology aims to apply different tools in a combined manner, thus has the advantage of bringing strengths of different tools together [4]. Using hybrid metrology, however, the tool-to-tool matching is a challenging issue as the data obtained by different tools must be shared with each other in a complementary or synergistic way for enhancing the metrology capability. It requires accurate and traceable calibration of tools as well.

Conventionally, SEMs are calibrated by applying lateral standards such as 1D or 2D gratings [5], typically manufactured by e-beam writing lithography technique. However, there are problems existing. Firstly, that pattern distortion may be introduced in the e-beam writing lithography due to the well-known proximity effect, fogging effect and/or loading effect [6]. Secondly, the pattern suffers from line edge roughness (LER), which is directly related to the process parameters, such as e-beam dose together with photoresist material. Thirdly, as a kind of beam writing technique it is slow, consequently the pattern may be distorted due to drift. Characterisation results show that such a pattern distortion which may reach a few nm or even tens of nm [7]. To achieve better calibration accuracy using such grating standards, principally a same pre-defined area of the standards, where the reference calibration was performed, can be applied for SEM calibrations. However, it raises a further problem in practice: repeated SEM measurements on the same location will easily change the reference pattern due to the well-known contamination issue. Therefore, a new type of reference sample with outstanding pattern performance and uniformity is desired.

In this paper we present a new kind of lateral standard manufactured based on laser-focused atomic deposition (LFAD) lithography [8], [9], [10]. Such a standard has (much) better large-range as well as short-range pattern uniformity, and is thus well suited for calibrating the magnification and field distortion of almost all kinds of microscopes, particular SEM. The manufacturing and calibration of the grating sample as well as its application for SEM calibration are detailed in this paper.