AFM Tip Characterizer fabricated by Si/SiO 2 multilayers

An atomic force microscopy (AFM) tip characterizer with measurement ranges from 7.7 nm to 131 nm was developed using Si/SiO 2 multilayers. This characterizer was constructed with isolated line structures and comb-shaped trench structures. The shape of a standard Si AFM tip was estimated using this characterizer. The result shows that this Si/SiO 2 multilayer-type tip characterizer has good potential for the characterization of AFM tips with a (cid:12)ne radius. [DOI: 10.1380/ejssnt.2011.293]


I. INTRODUCTION
Atomic force microscopy (AFM) is very often used to characterize nanostructures. Recently, the characterization of AFM tips with measurement ranges from 10 nm to 100 nm has been required by many AFM tip vendors and AFM users, because tip-induced artifacts are a limiting factor that reduces the precision of AFM measurements. To evaluate tip shapes, several types of tip characterizers have been developed, such as spherical structures [1][2][3][4], hole grid patterns [5], quantum dots [6], dot grid patterns [7], needle-shaped samples [8], nanofabricated line-and-space (L&S) patterns [9], and other nano-structures [10]. Using these characterizers, the characteristics of the tip shapes, such as apex diameter and a half-cone angle, have been estimated. However, it has still been difficult to obtain the fine profiles of AFM tips. In 2006, a new type of tip characterizer with a 10-nm wide structure was developed that was based on GaAs/InGaP superlattices [11]. In this work, 10-nm line-and-space (L&S) patterns were fabricated and a fine profile of a tip was obtained. Unfortunately, this characterizer was not able to be used over a wide range, because the aspect ratios of these patterns were low. Furthermore, this characterizer could not be used in the bio-medical and Si nano-device fields because GaAs is toxic.
In this paper, we describe the development of a Si/SiO 2 multilayer-type tip characterizer that includes several step nano-structures. This characterizer was designed with 10 nm and 20 nm isolated line structures and combshaped L&S structures with trenches ranging from 7.5 nm to 130 nm in width. Furthermore, we demonstrated the characterization of a pyramidal-type Si AFM tip-shape in the 10 nm-to-100 nm range by using the tip characterizer.

II. DESIGN AND FABRICATION
An AFM tip characterizer was designed for characterizing AFM tips in the 7.5 nm to 100 nm range. Isolated line structures with line widths of 10 nm and 20 nm were included in this characterizer. AFM images of these structures display the inverse tip surface, as shown in Fig. 1(a) and Fig. 1(b) [8]. In addition, tip surface functions have been obtained from AFM images of comb-shaped nanotrench structures, as shown in Fig. 1(c) and Fig. 1(d) [12]. We designed and fabricated the tip characterizers with 10 nm and 20 nm knife-edge structures and comb-shaped nano-trench structures ranging from 7.5 nm to 130 nm. The fabrication procedure for the tip characterizer is shown in Fig. 2. In the first step, Si/SiO 2 multilayers were deposited on Si wafers using magnetron sputtering, as shown in Fig. 2(a). With the system shown schematically in Fig. 3, the thickness of each layer of material can be controlled to better than 3%. Secondly, two pieces of wafers were bonded using the surface-activated bonding (SAB) method at room temperature [13] so that they were facing each other (Fig. 2(b)). In the third step, the bonded wafer was cut into pieces that were perpendicular to the surface (Fig. 2(c)). Fourthly, each piece was mounted on a Si base-plate and then the cross-sectional plane was chemical-mechanically polished (Fig. 2(d)). Finally, the SiO 2 layers were selectively etched in an HF solution (Fig. 2(e)) and Si isolated line structures and Si/SiO 2 comb-shaped L&S structures were developed. This Si/SiO 2 type tip characterizer has several advantages compared with GaAs/InGaP superlattice-type tip characterizers. First, the materials from which the tips are constructed are highly compatible with processes used for Si nano-devices such as LSI. Secondly, HF solution can easily etch narrow, high aspect-ratio structures, whereas the high viscosities of H 2 SO 4 solutions that are used for selectively-etching GaAs makes it more difficult to achieve the same high aspect-ratios. Thus, the tip characterizers fabricated in this work can be produced with higher aspect ratios. A third advantage of the Si/SiO 2 tip characterizer is that it is biologically 'non-toxic', and can be used safely.

III. EVALUATIONS
A cross-sectional transmission electron microscope (TEM) image of the fabricated tip characterizer is shown in Fig. 4. The knife-edge widths that were obtained were 10.0 nm and 20.1 nm, and the trench widths of the combshaped L&S structures ranged from 7.7 nm to 131 nm. These widths are 1-3% larger than the designed values. It is believed that this is due to fluctuations in the sputter deposition rate.
Note that the 10 nm wide isolated lines have become distorted for etch depths greater than 80 nm. The stiffness depends on the aspect ratio, and the limit for a line structure appears to be an aspect ratio of 8. On the other hand, the limiting aspect ratio for a space pattern is 13.

IV. AFM TIP SHAPE ESTIMATION
To evaluate the practical properties of the tip characterizer, AFM images of it were measured across the trenches using a pyramidal Si tip in 'intermittent contact' mode. Measured two-dimensional AFM images of the isolated line structures and the estimated tip shape are shown in Figs. 5 and 6, respectively.
The AFM images of scanned comb-shaped trench structures and the obtained tip surface function are shown in Fig. 7 and Fig. 8. From Fig. 6 and Fig. 8, an unsymmetrical profile and a fine profile of the tip can be obtained. These kinds of information are not shown in typical specification notes. These figures also show that the estimation range can extend to 130 nm. A scanning electron microscope (SEM) image of this pyramidal Si tip shape is shown in Fig. 9. When the contour shape of this Si tip is observed, however, the shape of the near-apex region is not clear in the image because of the low spatial-resolution of the SEM.

V. CONCLUSIONS
A Si/SiO 2 multilayer-type AFM tip characterizer consisting of isolated knife-edge structures and comb-shaped trench structures with widths ranging from 10 nm to http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 100 nm was developed. The measured widths of the isolated line structures were 10.0 nm and 20.1 nm, and the measured trench widths of the comb-shaped trench structures ranged from 7.7 nm to 131 nm, which are in good agreement with the designed values. The isolated knifeedge structures and the comb-shaped nano-trench structures had aspect ratios of up to 8 and 13, respectively.
The two dimensional (2D) shape of a standard pyramidtype Si AFM tip was reconstructed using the fabricated tip characterizer. The results show that this characterizer can be used to characterize two-dimensional AFM tip shapes ranging from about 8 nm to about 130 nm.