Reducing focused ion beam damage to transmission electron ...

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Abstract. One of the most important applications of focused ion beam (FIB) sys- tems is sample preparation for transmission electron microscopy (TEM). However ...
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Japanese Society of Microscopy

Journal of Electron Microscopy 53(5): 451–458 (2004) doi:10.1093/jmicro/dfh080

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Review

Reducing focused ion beam damage to transmission electron microscopy samples Naoko I. Kato Analytical Services, Quality Engineering, ITES Co., Ltd., 800 Ichimiyake, Yasu-cho, Yasu-city, Shiga 520-2392, Japan E-mail address: [email protected] ............................................................................................................................................................................................................................................

Abstract

One of the most important applications of focused ion beam (FIB) systems is sample preparation for transmission electron microscopy (TEM). However, the use of the FIB inherently involves changing and damaging the sample, and thereby degrades the TEM resolution. This paper addresses the beam-induced damage and artifacts, particularly in applications involving silicon semiconductors. The damage appears in the form of amorphization on the surface of the TEM foil. The characteristics of this amorphous damage were studied by making TEM observations of cross sections of the affected foil. The damage is typically 20 to 30 nm thick for a 30 keV FIB, which is generally overly thick for modern silicon devices with feature sizes less than 250 nm. This paper reviews the reported damage depths of FIB-prepared samples, which are determined by experiments and calculations. Several damage reduction techniques, such as the use of gas-assisted etching, low energy FIB, cleaning the FIBfabricated cross section by wet or dry etching and cleaning by broad ion beam (BIB) milling have also been reviewed, with emphasis on applicability to silicon devices. We conclude that the use of low energy FIB and cleaning by argon BIB are particularly efficient techniques. ............................................................................................................................................................................................................................................ Keywords beam damage, damage reduction, focused ion beams, sample preparation, semiconductor devices, transmission electron microscopy ............................................................................................................................................................................................................................................

Received 18 March 2004, accepted 27 July 2004 ............................................................................................................................................................................................................................................

Introduction In the last two decades, focused ion beam (FIB) systems have had a great impact on the microelectronics industries. As a powerful transmission electron microscopy (TEM) sample preparation tool, they made materials and failure analysis of isolated, sub-micron electrically tested structures possible [1–4]. This new application of FIB systems has dramatically expanded the horizon of TEM beyond academic and laboratory studies. It has established TEM as a quality control instrument. Large parts of semiconductor devices, such as metal oxide silicon transistors or thin film transistors in liquid crystal displays, are usually fabricated using lithography and have multi-layered structures. In case a defect is introduced in the manufacturing process, it is easy to identify the root cause process by cross sectioning the device and locating the layer in which the defect exists.

The conventional TEM sample preparation method for ion milling generally requires a few days, or even a few weeks, and more often than not, a very small defect of interest is lost during the sample preparation, regardless of the time and effort invested in testing and locating the defect. On the other hand, using a FIB, a region of interest can be extracted with ease and with better than sub-micron accuracy. This improved reliability in sample preparation has resulted in an increase in TEM applications in everyday quality control. This sudden increase in TEM applications has promoted the development of not only TEMs dedicated to the analysis of semiconductor devices, but also of several FIB-based sample preparation techniques, such as in situ techniques and the out situ lift-out technique [5,6]. With the increasing importance of the FIB, the awareness regarding the limitations and drawbacks of the FIB-based

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methods has also increased. The most severe problem is the damage layer produced as a result of the bombardment by highly accelerated gallium ion beams. This damage layer lowers the quality of the TEM image significantly and it is common knowledge that though the FIB method is efficient with regard to the sample turnaround time, the results obtained are far inferior to those obtained by ion milling, or hand- or electro-polishing. In particular, the FIB method is regarded as unsuitable for high-resolution TEMs. This paper investigates the FIB damage and artifacts, in order to control the damage and fully utilise the FIB methods. The depth of the damage layer is examined, and practical techniques to reduce and/or remove the damage are explored.

FIB-induced artifacts FIB etching involves subjecting the samples to energetic gallium ions. A schematic representation is shown in Fig. 1a. The variables include the beam current, the angle of incidence, the temperature of the specimen and the voltage. In most of the commercial FIB systems, the ion optics are optimized to work at an accelerating voltage of 30 keV. The beam current can be varied in the range of pA to nA by changing the mechanical apertures. The angle of incidence is usually set between 0 and 5 so that the etched surfaces of the thin area are parallel to each other. Figure 1b shows a cross section of a FIB-prepared thin area of a silicon single crystal, where the two surfaces were etched using a 30 keV gallium ion beam. The beam current used at the final stage of the milling is 150 pA with a glancing angle of 1 . The bright field (BF) TEM image clearly shows the amorphized layer on both sides of the foil, and also under the FIBdeposited protective layer [7]. These amorphous layers limit the degree to which the FIB-prepared samples can comply with the general rule for TEM samples: the thinner, the better.

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(b) FIB

FIB

FIB

FIB

TEM

200 nm

Fig. 1 (a) Schematic diagram of a FIB-prepared thin foil, which was subsequently cross sectioned to yield BF TEM image in (b).

This point is clearly illustrated in Fig. 2, which shows cross sectional BF TEM images of a defective DRAM trench cell, in which the poly-crystalline silicon layer (word line) is short circuited to the poly-silicon trench capacitor (cell capacitor) via a very narrow short path. The sample in Fig. 2a is approximately 150 nm thick, while it has a thickness of less than 100 nm in Fig. 2b. Due to the small size of the short path, a very thin cross section is preferred in order to avoid geometrical blurring. However, the TEM image is severely degraded in Fig. 2b, in which the diffraction contrast is reduced by surface amorphization. This exemplifies the limitations of the FIB methods. The damage layer restricts the minimum thickness of a foil that can be prepared using the FIB methods. This is attributed to the fact that when a foil is thinner than a certain threshold thickness, the amorphized layers become dominant, and TEM observations draw information from the damage layer and not from the sample. This is especially challenging in advanced semiconductors. Certain researchers have shown that as the feature size shrinks, the thickness of the TEM samples must shrink as the square root of the feature size reduction in order to avoid geometrical blurring. For example, a sample with a thickness