Review Article  Open Access
Mechanical Researches on Young's Modulus of SCS Nanostructures
Abstract
Nanostructures of SingleCrystalSilicon (SCS) with superior electrical, mechanical, thermal, and optical properties are emerging in the development of novel nanodevices. Mechanical properties especially Young's modulus are essential in developing and utilizing such nanodevices. In this paper, experimental researches including bending tests, resonance tests, and tensile tests on Young' s modulus of nanoscaled SCS are reviewed, and their results are compared. It was found that the values of measured by different testing methods cannot match to each other. As the differences cannot be explained as experimental errors, it should be understood by taking surface effect into account. With a simplified model, we qualitatively explained the difference in value measured by tensile test and by resonance test for Si nanobeams.
1. Introduction
Being the most important semiconductor used in microelectronics and microelectromechanical systems, Single Crystalline Silicon (SCS) is still a research hot spot in nanoscience and nanotechnology, as current trend pushes sciences and technologies to nanometer scale. Researchers have found that structures would exhibit unique electrical [1], mechanical [2], thermal [3], and optical [4] properties at nanoscale. This makes nanostructures (nanobeams and nanowires) attractive in developing Fieldeffect transistors (FETs), fieldemission devices, chemical sensors, nanoresonators, and photonics, and so forth. However, in order to make these devices reliable, mechanical properties, especially Young’s modulus () of Si nanostructures are essential and critical to study.
Being defined as the ratio of tensile stress to tensile strain, is always considered as an intrinsic material property and fundamentally related to internal atomic bonding. The stronger the atomic bonding is, the larger the Young’s modulus is. Besides the direct testing method, tensile test, there are many other techniques for obtaining , for example, bending/curvature, resonance, indentation, and so forth. But in nanoscale, it is in question that whether these testing methods are still effective, and where surface effect cannot be ignored.
In this review, we present the recent experimental researches on Young’s modulus of SCS nanostructures and compare their results. It was found that different testing methods showed different values of . With a simplified model taking surface effect into account, we qualitatively explained the difference that for nanobeams measured by tensile test are not the same as measured by resonance test, it may realize that the measurement of in nanoscale is more complex than in macroscale.
2. Current Experimental Tests on SCS Nanostructures
Mechanical property measurements of nanostructures, such as nanobeams and nanowires, are very challenging because of the difficulties in (1) sample preparation and nanomanipulating and (2) measuring force and displacement (strain and stress) with nanoscale resolution. But in these years researchers have demonstrated some mechanical tests on SCS nanobeams and nanowires, which cover bending tests, resonance tests, and tensile tests.
2.1. Bending Tests
Atomic force microscopy (AFM) is usually employed to give 3D image of the topography of the sample surface, control and apply a specified amount of force on the sample. It is suitable to use AFM in mechanical tests on a single (double) clamped nanobeam (nanowire) by applying force to the specimen and measure deformation simultaneously. By using beam bending equations, the mechanical properties can be deduced. For doubleclamped beam being applied as a force in its middle point, Young’s modulus can be expressed as where is the beam/wire length, is the moment of inertia for the beam crosssection, and is the gradient of the forcedisplacement curve during bending [5].
Namazu et al. patterned Si nanobeams by means of fieldenhanced anodization on SOI wafer using AFM and obtained the nanobeams by anisotropic wet etching, which are <110>oriented with thickness of 255 nm and widths from 200 to 800 nm. By applying bending tests on these double clamped nanobeams, the measured values of Young’s modulus are determined to be 169 GPa on average. These results indicate that the specimen size has no influence on yet [5]. Sundararajan et al. also followed the experimental procedure and presented Young’s modulus values of GPa [6, 7].
Virwani et al. patterned some 200–400 nmwide nmthick Si nanobeam on SOI wafer using ebeam lithography, and their bending tests by AFM gave an Si Young’s modulus of 174 GPa along <100> direction [8].
Paulo et al. also used AFM to characterize the mechanical elasticity of their Si nanowires synthesized by VaporLiquidSolid method. The nanowires are horizontally grown between the two facing Si sidewalls of microtrenches. The values of were estimated to be 186 GPa and 207 GPa, respectively, for single and doubleclamped Si nanowires [9].
2.2. Resonance Tests
According to EulerBernoulli theory, dynamic studies on the resonant frequency of nanobeams and nanowires can provide Young’s modulus when the geometry is determined accurately.
In the work of Li et al. [10], the SCS untratine cantilevers were patterned on SOI wafer and dry etched along <110> crystalline direction with thicknesses ranging from 12–300 nm. Resonant properties of the cantilevers were characterized in high vacuum chamber by laser vibrometer. A pulsed light beam from a laser diode excites the cantilever to resonate and another fibercoupled laser to pick up the vibration signal quality factor . The fundamental resonant frequency was obtained and Young’s modulus was deduced using following equation: where is the material density, is the thickness of the cantilever, and is the length. The was fitted to be 68 GPa for 38.5nmthick cantilever, and 53 GPa for 12nmthick cantilever, as shown in Figure 1, it demonstrated that Young’s modulus decreased monotonously as the cantilevers become thinner [10].
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2.3. Tensile Tests
Among all the methods of in nanoscale, tensile test is more challenging since the specimens must be freestanding, clamped at both ends, stretched uniaxially, and measured its elongation with nanometer resolution. To facilitate tensile testing on nanosturctures, various nanomanipulators, based on multiaxes actuation, were designed to work inside scanning electron microscope (SEM) or transmission electron microscopy (TEM). With nanostructure being stretched and tensile force being measured by nanomanipulators, sample elongation being observed by SEM or TEM, insitu tensile tests are carried out inside SEM and TEM.
By using a highresolution TEM equipped inside with nanomanipulators for subnanoNewton force measurements (an AFM) and electronic conductance measurements (an STM) [13], Kizuka et al. synthesized <111>oriented Si wires with nanometer widths inside the TEM by nanometertip contact and successive retraction. Then insitu tensile tests were carried out on the nanowire between two tips of AFM and STM. Figure 2 gives timesequent series of highresolution images of a tensile deformation process of an SCS nanowire with diameter of nm. Young’s modulus was found to be GPa [11].
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Microelectromechanical systems (MEMSs) can be advantageously employed in the testing of nanoscale samples. It usually consists of three parts: actuator for nanomanipulating sample, sensor for measuring force on the sample, and a cofabricated (or laterattached) sample. By integrating MEMS chip in SEM or TEM, researchers have successfully performed several tensile tests on nanostructures [14–16].
We have designed and fabricated an MEMS tensiletesting chip, a TEM holder with electric terminals, and carried out insitu TEM tensile testing on SCS nanobeam [12, 17]. The holder and chip fit well in the 0.9 mm pole piece gap of JOEL 2010. The chip (5 mm × 9 mm × 0.5 mm) is fabricated by means of Si bulk micromachining and wafer bonding. Figure 3 gives the SEM view of the MEMS chip, with a sample (SCS nanobeam), a comb drive actuator, a force sensor beam, and a TEM electron beam window being integrated into it, and those parts are fabricated in one process. As the electrostatic force actuated comb drive actuator causes no current, it will cause no interference to TEM measurements.
When actuating voltage was applied, with the onchip comb drive actuator stretching the SCS nanobeam and insitu TEM observation, tensile tests are performed on the 33 mlong 8.2 mbroad 90 nmthick <110>oriented SCS nanobeam. For actuating voltages from 50 V to 100 V, incremented in 10 V steps, we snapshot the movements of the two ends of the nanobeam, and (indicated by the arrows), as shown in Figure 4. By taking the 4 corner marks, which were the images of 4 fixing film clamps in TEM, as reference positions, we measured the displacements (of ) and (of ). The elongation of the nanobeam was  and the deflection of the force sensor beam can be considered as . The tensile force on the SCS nanobeam was calculated as 210 , with the elastic constant of the force sensor beam being 210 . By fitting the strainstress relationship under different actuating voltages, we obtained a Young’s modulus of 167 Gpa.
3. Discussion
The experimental results on Young’s modulus of SCS nanobeams and nanowires are summarized in Table 1 on the catalog of their crystal directions. As the structures dimension decreasing, the resonance test shows a decreasing tendency of Young’s modulus on the dimension, which is also shown clearly in Figure 5. The result of 18 GPa obtained by tensile test on an <111>directed SCS nanowire supports this tendency too. All these suggest that for crystal silicon the size effect of is decreasing as the dimension decreasing.

It is also interesting to note that the measured values of Young’s modulus by different testing methods have a little difference. Size effect did not appear at 200 nmthick nanobeam in bending test, while in resonance test the tendency curve indicates that it should appear at around 250 nm. As for tensile test, Young’s modulus still keeps the bulk value at 90 nmthick nanobeam.
Compared to the tensile test result, Young’s modulus by resonance test shows its size effect more early. The different behaviors of in tensile and resonance test cannot be treated as experimental error, and we think it must be surface effect working.
Taking nanobeam, for instance, a model to explain the difference is constructed, in which the nanobeam is treated as a composite beam with a silicon middle layer and two surface layers. The difference of Young’s modulus caused by the two different methods can be described as where is Young’s modulus of the composite beam by tensile test, and is Young’s modulus by resonance test [18].
Since many theoretical [19–24] and experimental results [10, 11] show that the silicon Young’s modulus decreases monotonously as the SCS nanostructures dimensions decreasing, it can be deduced that . The difference between and is plotted schematically in Figure 6. When is much larger than , every testing method gives a same value for Young’s modulus. That is the case at macro or microscale. As decreases, decreases, and the value of by different testing methods will be different from each other. shows size effect more early as decreases, compared with .
4. Conclusion
In this paper, the existing experimental tests of Young’s modulus, including bending tests, resonance tests, and tensile tests, on SCS nanostructures are reviewed. The results suggest that at nanoscale, the silicon Young’s modulus exhibits obvious size effect of a decreasing tendency on the nanostructures decreasing dimension. Different testing methods may give different Young’s modulus values. The size effect of Young’s modulus appears earlier in resonance test than in the other methods. By taking surface effect into account, this difference may be qualitatively explained for SCS nanobeam. It indicates that current experiments are still not enough to understand the behavior of SCS Young’s modulus at nanoscale thoroughly, more research work should be done for it.
Acknowledgments
This work is partly financially supported by the National Basic Research Program of China Grant no. 2006CB300403 and the Fund for Creative Research of NSFC Grant no. 60721004.
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Copyright © 2009 Qinhua Jin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.