Nanopatterning on silicon surface using atomic force microscopy with diamond-like carbon (DLC)-coated Si probe
© Jiang et al; licensee Springer. 2011
Received: 4 July 2011
Accepted: 2 September 2011
Published: 2 September 2011
Atomic force microscope (AFM) equipped with diamond-like carbon (DLC)-coated Si probe has been used for scratch nanolithography on Si surfaces. The effect of scratch direction, applied tip force, scratch speed, and number of scratches on the size of the scratched geometry has been investigated. The size of the groove differs with scratch direction, which increases with the applied tip force and number of scratches but decreases slightly with scratch speed. Complex nanostructures of arrays of parallel lines and square arrays are further fabricated uniformly and precisely on Si substrates at relatively high scratch speed. DLC-coated Si probe has the potential to be an alternative in AFM-based scratch nanofabrication on hard surfaces.
Nanolithography is crucial to realize a size below 100 nm for nanoelectronic devices and high density recording systems [1, 2]. Apart from conventional, expensive optical and electron beam lithography [1, 2], scanning probe microscopy (SPM), especially scanning tunneling microcopy (STM) and atomic force microscopy (AFM)-based nanofabrication technique have been intensively studied. To date, several SPM-based nanolithography techniques have been developed including local oxidation of the surfaces of silicon and metals [3–5], dip-pen method [6, 7], thermal-mechanical writing [2, 8], and mechanical/electrochemical modification of a material's surface [9–11]. In recent years, although the uncertainty (drift, hysteresis, creep for AFM) will limit its application in nanostructure fabrication at large scale, AFM-based scratch nanolithography has emerged as a promising technique for nanofabrication because of its simplicity, versatility, reliability, and operation in ambient conditions [3–12]. It is also expected to fabricate nanostructure at a large scale with combination of nano-imprint system. AFM scratching technique takes advantage of the ability of moving a probe over a sample surface in a controllable way. By controlling the applied normal force (F n) between a probe and a sample surface, trenches or grooves with depths from a few to tens of a nanometer and widths from tens to hundreds of nanometers can be fabricated on both soft and hard substrates, involving polymer , silicon , oxides , magnetic metals, and semiconductor materials . This technique thus has the potential to benefit the fabrication of nanoelectronic devices such as nanodots , nanowires , and single electron devices . For example, the patterning on sapphire substrate by AFM-based nanolithography can reduce the dislocation density for III-nitride based light emitting diodes [19–21].
Previous reports in AFM-based scratch nanolithography has focused on making different nanodevices and nanosystems, in which a Si or Si3N4 tip with a typical radius of less than 20 nm was used, and the scratch was processed mainly on flexible polymer substrates [1, 11, 18]. To scratch on a hard Si surface, an AFM tip with high wear resistance has to be used. Recently, SPM scratch nanopatterning on a Si surface was investigated by several groups [22–24], the tips, however, were coated exclusively with diamond, which is costly. According to our knowledge, AFM scratching using diamond-like carbon (DLC)-coated probe has not been reported. DLC film is an amorphous film, and its surface is very smooth. Because of its high hardness and high elastic modulus, low coefficient of friction, wear and good tribological property, it is suitable as a wear-resistant coating . From the preparation point of view, the cost for DLC films is much cheaper than that of diamond, and the commercial DLC-coated tip can easily be obtained. In the present study, we explored the potential of this economic probe in fabricating nanopatterns on hard silicon surface. The scratch characteristics were investigated and correlated to the scratch parameters. More complex nanostructures such as line and square arrays were further fabricated using a DLC-coated tip on a silicon substrate.
Results and discussion
Two scratch directions, forward and backward, were selected to scratch the Si surface. As illustrated in Figure 1a, forward scratch has the sharp cutting edge along the scratch direction, while the backward scratch uses the flat cutting edge facing the scratch direction. The influence of scratch direction on the size of the scratched geometry was initially investigated. The AFM images of the typical grooves in both directions are given in Figure 1b, along with the corresponding cross-section profiles of the grooves as shown in Figure 1c, where the scratch was performed at 1 μm/s of the scratching speed and 10 μN of the applied normal force. The cross-section profiles are V-shaped in both scratch conditions. However, the depth of the groove generated in the forward scratch is clearly deeper than that in the backward direction, as seen in Figure 1c. This could be attributed to the sharpness of the tip in the forward direction, where the effective normal force was higher and a deeper groove was thus produced . A similar phenomenon was observed in the investigation of AFM scratch on Si surface with diamond-coated Si tip [22–24, 28]. The pyramidal tip has three scratching faces as shown in Figure 1d, the inclination angle, θ, is defined as the angle between the directions of scratching and cutting face. In the case of the backward direction, the tip scratch face is perpendicular to the scratch direction, i.e., the inclination angle, θ, equals 90°, which satisfies the requirement to become orthogonal cutting. The protuberances are created evenly along two sides of the grooves. On the other hand, if scratching is in the forward direction, the scratching face is composed of two inclination angles, i.e., one is -30° and the other is 30°. As a result, the protuberances are squeezed evenly onto the two sides of the groove scratched. Since the 30° or -30° inclination angle provides much more favorable stress states to squeeze the materials onto the two sides as compared with that of the 90° inclination angle, the protuberances created in forward scratching is more or larger than that of the backward direction.
where M i and n i are the multiple scratch coefficient and multiple scratch exponent, respectively. As the d and W f data for multiple scratches on a Si(100) is illustrated in Figure 4b, the correlation values of M 1, M 2, n 1, and n 2 can be found to be 1.93, 20.26, 0.80, and 0.35 nm, respectively. The associated coefficient of determination (R 2) is 0.99 for d and 0.88 for W f , which indicate that the power-law correlation fitting the depth data perfectly, and there is a 12% deviation for the width data.
In the present study, we explored the potential of the DLC-coated tip used as a cutting tool in AFM-based scratch nanolithography on a silicon surface. The scratched geometry was correlated to the scratch parameters, such as the scratch direction, applied tip force, scratch speed, and number of scratches. Uniform nanopatterns of line arrays and square arrays were further fabricated. This work provides an insight for fabricating nanopatterns on a hard material precisely and rapidly using an inexpensive AFM tip.
This work was supported by the National Natural Science Foundation of China (nos. 10874040, 90306010, and 20803018) and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 708062).
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