research papers
Anisotropic elasticity of silicon and its application to the modelling of Xray optics
^{a}European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble, France
^{*}Correspondence email: zhang@esrf.fr
The crystal ν, the thermal deformation (RMS slope) is proportional to (1 + ν). For a cubic anisotropic material, the thermal deformation of the Xray optics can be approximately simulated by using the average of ν_{12} and ν_{13} as an effective isotropic Poisson's ratio, where the direction 1 is normal to the optic surface, and the directions 2 and 3 are two normal orthogonal directions parallel to the optical surface. This average is independent of the direction in the optical surface (the crystal plane) for Si(100), Si(110) and Si(111). Using the effective isotropic Poisson's ratio for these orientations leads to an error in thermal deformation smaller than 5.5%.
of singlecrystal silicon gives rise to anisotropic elasticity. The stiffness and compliance coefficient matrix depend on crystal orientation and, consequently, Young's modulus, the and Poisson's ratio as well. Computer codes (in Matlab and Python) have been developed to calculate these anisotropic elasticity parameters for a silicon crystal in any orientation. These codes facilitate the evaluation of these anisotropy effects in silicon for applications such as microelectronics, microelectromechanical systems and Xray optics. For mechanically bent Xray optics, it is shown that the silicon crystal orientation is an important factor which may significantly influence the optics design and manufacturing phase. Choosing the appropriate crystal orientation can both lead to improved performance whilst lowering mechanical bending stresses. The thermal deformation of the crystal depends on Poisson's ratio. For an isotropic constant Poisson's ratio,Keywords: anisotropic elasticity of silicon; crystal orientation; thermal deformation; bent mirror; cryogenic cooled monochromator; anisotropic Poisson's ratio.
1. Introduction
Singlecrystal silicon is a perfect crystal which, owing to its interesting mechanical and physical properties, is widely used for Xray optics at synchrotron light sources. Example applications include silicon crystal monochromators in both Bragg and Laue configurations, silicon substrates for highheatload whitebeam mirrors, bent Kirkpatrick–Baez focusing mirrors and multilayer optics. It is well known that silicon is an anisotropic material whose mechanical properties, such as elastic modulus E, Poisson's ratio ν and G, depend on the orientation of the crystal The anisotropic stiffness coefficients for the (100) crystal plane of silicon have been initially determined by experiments (Mason, 1958; Wortman & Evans, 1965; Hall, 1967). Determination of the stiffness constants from these values for an arbitrary crystal orientation (hkl) therefore requires the use of the direction cosines referred to the crystal axis of the Si(100) orientation. Parameters such as Poisson's ratio and depend on two directions, and it is important to correctly take into account these crystallographic directions in the calculation of anisotropic elastic properties. Most previous modelling work on siliconbased Xray optics has been performed using the simplifying assumption of isotropic material properties. A few studies have, however, taken into account the anisotropic material properties for bent diffracting crystals and reflecting mirrors. The anisotropic elasticity has been applied to study the Xray reflectivity of doubly curved Bragg diffracting crystals (Chukhovskii et al., 1994) and Laue crystals meridional (Schulze & Chapman, 1995) or sagittal (Zhong et al., 2002) bending. Li & Khounsary (2004) considered the anisotropic Poisson's ratio varying with direction in the silicon (100) crystal plane for the calculation of the anticlastic bending radius in bendable optics. Zhang (2010) presented matrixbased Matlab code for the calculation of the anisotropic elastic properties of silicon, and application to bendable mirror width profile optimization. This study showed the influence of the crystal orientation on the bending force and stress in the mirror. The anisotropic mechanical properties of silicon were also considered in thermal deformation analysis of liquidnitrogencooled silicon crystals under high heat load (Zhang et al., 2013).
Many existing synchrotron light sources (ESRF, APS, SPring8,…) are planning and implementing significant facility upgrades, and some lowemittance synchrotron light sources (NSLS II, MAX IV,…) are under construction. The improved source characteristics of these light sources can only be fully utilized if the beamline performance and consequent specifications of optical components are pushed to higher levels than the current stage.
preservation, beam collimation, focusing and preservation of coherence are required for optical elements in the beamline. In the design and optimization of the beamline optics it is essential to have accurate and reliable predictions of the shape of the optical elements under high heat load or bending forces. For these purposes, the anisotropic elasticity should be considered in the modelling of the siliconbased optics.For the highheatload Xray optics, the anisotropic elasticity intervenes in the thermal stress through both Young's modulus and Poisson's ratio, but in the thermal deformation mainly through Poisson's ratio.
In this paper we first report the anisotropic elasticity of singlecrystal silicon and compare our results with some literature values. Then we apply these properties to a bent mirror substrate, and discuss the influence of crystal orientation on the bending forces, stress and on the mirror shape profile. Finally, we focus upon the thermal deformation modelling of siliconbased optics with anisotropic elasticity, and investigate the influence of Poisson's ratio on the thermal deformation.
2. Anisotropic mechanical properties of the silicon
The generalized Hooke's law to express the relation between the stress and strain in a continuous elastic material can be written as (Nye, 1957; Hearmon, 1961)
where C_{ijkl} and S_{ijkl} are, respectively, the stiffness and compliance fourthrank tensors, and σ_{ij} and ∊_{kl} are secondrank stress and strain tensors. Generally, the stiffness or compliance tensor has 81 elements. However, owing to the symmetry of the stress and strain tensors, and also the stiffness (or compliance) tensor, there are only 21 independent elastic coefficients in the stiffness (or compliance) tensor for a general anisotropic linear elastic solid. This reduction in number of independent coefficients makes it possible to simplify the notation and calculations by expressing the compliance and stiffness tensors in the form of 6 × 6 symmetric matrices, and the stress and strain tensors in the form of sixelement vectors. Any pair of tensor indices ij (or equivalently ji) collapse into a single index. The most used notation in bibliography makes the assignment: 11 → 1, 22 → 2, 33 → 3, 23 → 4, 31 → 5 and 12 → 6. We can, for instance, contract terms C_{1132} to C_{14}, ∊_{31} to ∊_{5}, and σ_{12} to σ_{6}. Silicon and germanium have the same cubic diamond The cubic consists of a set of three axes described by three vectors orthogonal and of equal length. The conventional crystalaxis coordinate system for crystal plane (100) is defined by the normal vector e_{1} = [100] and two other orthogonal vectors in the crystal plane, e_{2} = [010] and e_{3} = [001], as shown in Fig. 1. In this conventional coordinate system, the stiffness coefficient matrix reduces to the following structure with only three independent elastic coefficients,
for Si(100), where c_{11} = 165.7, c_{12} = 63.9, c_{44} = 79.6 GPa (Mason, 1958). These coefficients are commonly used in the literature although Hall (1967) proposed data with slightly better accuracy (c_{11} = 165.6, c_{12} = 63.9, c_{44} = 79.5 GPa), but the difference between the two are not significant. In this paper, we use the data from Mason (1958) in order to make the comparison with some other studies.
The compliance matrix is the inverse of the stiffness matrix,
For Si(100), the compliance matrix S_{100} has the same structure as the stiffness matrix (2). The three independent coefficients are s_{11} = (c_{11} + c_{12})/[(c_{11} − c_{12})(c_{11} + 2c_{12})] = 7.68, s_{12} = −c_{12}/[(c_{11} − c_{12})(c_{11} + 2c_{12})] = −2.14, s_{44} = 1/c_{44} = 12.56 × 10^{−12} Pa^{−1}. For an arbitrary orientation of the cubic crystal with optical surface parallel to the (h k l) plane, a convenient coordinate system is to use the crystallographic orientations that are defined by how the crystal has been cut. This new coordinate system (, , ) is defined by the surface normal [h k l] and two additional orthogonal vectors in the crystal surface. This choice is also valid for any asymmetrical crystal cutting which can be expressed via fractional h k l indices. Therefore, the vector is along the surface normal direction [h k l], and vectors and are parallel to the crystal surface (h k l). To determine the compliance matrix for this particular orientation, one can rotate the crystalaxis coordinate system (e_{1}, e_{2}, e_{3}) to the new coordinate system (, , ). For instance, the normal vector is [h k l] and the two other orthogonal vectors in the plane could be [0 l −k], [(k^{2} + l^{2}) − h*k − h*l] for (k*l ≠ 0). Therefore the normalized vectors are
The formalism of Wortman & Evans (1965) can be used to calculate the stiffness coefficient matrix C_{hkl} and compliance matrix S_{hkl} for any silicon crystal orientation. For all classes of cubic crystals, Young's modulus E_{hkl} in any crystallographic direction [h k l] can be calculated by the following equations (Nye, 1957; Wortman & Evans, 1965; Brantley, 1973),
or, equivalently,
where m, n, p are the direction cosines for the direction along which E is calculated, and s_{ij} are the three independent elastic compliances referred to the crystal axes (Fig. 1), as defined by equation (3). Knowing the relation m^{2} + n^{2} + p^{2} = 1, the transformation between (5a) and (5b) is straightforward. Here the direction cosines can be calculated by
Young's modulus E_{hkl} in the crystallographic direction [h k l] is independent of the choice of the coordinate system. For the particular directions [100], [110], [111] and [311], the above equations can be simplified as follows,
These equations give the results E_{100} = 130, E_{110} = 169, E_{111} = 188, E_{311} = 152 GPa. Note that the Young's modulus along (111) is almost 45% larger than along (100). Poisson's ratio and the for an anisotropic crystal are given in general by (Nye, 1957, Wortman & Evans, 1965)
where and are the elastic compliance coefficients in the new coordinate system and vary with crystal orientation. The Poisson's ratio ν_{ij} corresponds to the ratio of the strain variation (contraction) in the direction when a strain variation (extension) is applied in the direction . The G_{r=4} = G_{23} (G_{r=5} = G_{31} and G_{r=6} = G_{12}) represents the ratio of to the involving directions 23: and (31 and 12). For cubic crystals, equations (8) and (9) can be written as (Wortman & Evans, 1965; Brantley, 1973)
where (m_{i}, n_{i}, p_{i}) and (m_{j}, n_{j}, p_{j}) are the direction cosines for the direction and direction with respect to the crystal axes defined by Fig. 1.
It is very convenient to calculate the stiffness matrix C, compliance matrix S, Young's modulus E, G and Poisson's ratio ν by using computer code. We wrote matrixbased Matlab code (Zhang, 2010) and also implemented in Python using NumPy; these codes are summarized in the supporting information.^{1} Using these codes we have performed calculations of Young's modulus E, the G and Poisson's ratio ν for some commonly used silicon crystal orientations Si(100), (110), (111), (311). The new coordinate system (, , ) is defined as explained previously. As an example for Si(100): the vector = e_{1} is in the surface normal direction [1 0 0], and the vectors and are in the crystal plane obtained by rotating an angle α of the initial crystal axes e_{2} and e_{3} as shown in Fig. 2.
In order to compare our results with data in the literature (Wortman & Evans, 1965; Kim et al., 2001; Hopcroft et al., 2010), we plot Young's modulus E in the direction normal to the crystal plane (E_{⊥}) and in the direction parallel to the crystal surface (E_{∥}) versus angle α in Figs. 3(a)–6(a), the G_{12(⊥)} and G_{23(∥)} in Figs. 3(b)–6(b), and Poisson's ratio ν_{12(⊥)} and ν_{23(∥)} in Figs. 3(c)–6(c). The values of E, G and ν are in the new coordinate system; the prime symbol (′) is omitted. The parallel symbol (∥) and perpendicular symbol (⊥) are used to indicate, respectively, the two orthogonal directions both parallel to the crystal plane, and the two orthogonal directions of which one is normal and the other is parallel to the crystal plane. When the angle α varies from 0° to 90°, the directions of the vector rotated 90° in the crystal plane are as shown in Figs. 3–6 by the vectors below the horizontal axis.
For Young's modulus, results are given in the [h k l] direction normal to the crystal (h k l) plane (⊥), and in directions within the (h k l) crystal plane (∥). In the direction normal to the crystal plane, it is natural that Young's modulus E_{1(⊥)} is independent of the direction in the plane (angle α) for all four cases as shown in Figs. 3–6. The values of E_{1(⊥)} shown in these figures are in agreement with those calculated using equation (7). In the crystal plane, Young's modulus varies with direction, except in the Si(111) plane where it has a constant value of 169 GPa. The and Poisson's ratio involve two directions (, ). Results are shown in Figs. 3–6 for `in plane (∥)' where and are in the (h k l) plane, and for `normal to plane (⊥)' where is fixed in the [h k l] direction and is in the (h k l) plane. Both and Poisson's ratio for Si(111) orientation are constant for the components in the plane or normal to the plane.
Attention should be paid to the order of the index in the Poisson's ratio as, in general, ν_{ij} ≠ ν_{ji}. This is the case when ≠ , which can be easily checked by equations (8) or (10) and also numerical results. For instance, the Poisson's ratio ν_{12} for Si(100) can be calculated by substituting the direction cosines of the vector [100] and [0 cos(α) sin(α)] (see Fig. 2) in equation (10) as
Poisson's ratio ν_{21} is given by
These two components of Poisson's ratio are equal only at α = 0° or 90°. The Poisson's ratio ν_{12} and ν_{21} for Si(100) are plotted versus angle α in Fig. 7. It is clear that for Si(100) Poisson's ratio ν_{12} is, in general, not equal to ν_{21}. For the the index i, j can be permuted as G_{ij} = G_{ji}; this is clear from equation (11). Therefore, for the anisotropic elasticity of a silicon crystal, there are three independent components of Young's modulus and the and six independent components of Poisson's ratio.
As in the literature (Wortman & Evans, 1965; Kim et al., 2001; Hopcroft et al., 2010), we evaluate Young's modulus, the and Poisson's ratio versus the angle α in the crystal plane at a reduced range of 0 to 90°. By symmetry, it is possible to deduce the results at any angle α larger than 90° from the results shown in Figs. 3–6. For instance, the results shown in Fig. 6 for Si(311) can be extended to the range 0–360° by using symmetry as depicted in Fig. 8, which was calculated for the angle α varying from 0 to 360°.
Results shown in Figs. 3–5 and Fig. 7 for silicon crystal planes (100), (110), (111) have been compared with previously reported values (Wortman & Evans, 1965; Kim et al., 2001; Hopcroft et al., 2010) and summarized in Table 1. The present results are mostly in agreement with the literature; however, some discrepancies should be noted: for Si(100), the `normal to plane (⊥)' component of Poisson's ratio shown in Fig. 3(c) and by Wortman & Evans (1965) is ν_{12} where is fixed in the direction [100] and is varying in the plane (100) from direction [010] to [001]. But this `normal to plane (⊥)' component of Poisson's ratio ν_{⊥} for Si(100) shown by Kim et al. (2001) should be ν_{21} which is different from ν_{12} but in agreement with our results shown in Fig. 7. For Poisson's ratio in the Si(111) plane (∥), the present work shows a value of ν_{23(∥)} = 0.262, in agreement with Kim et al. (2001) (ν_{∥}), but Wortman & Evans (1965) presented a higher value of ν_{∥} = 0.358. For the `normal to plane (⊥)' for the Si(111) orientation, the present work gives a value of G_{12(⊥)} = 57.8 GPa, in agreement with Kim et al. (2001) (G_{⊥}), but Wortman & Evans (1965) showed a smaller value of G_{⊥} = 47.0 GPa.
‡Variation of direction (angle α) in plane: Kim from [0 0 1] to [−1 1 0], Present and Wortman from [0 0 1] to [1 −1 0]. §Variation of direction (angle α) in plane: Kim from [1 −1 0] to [−1 −1 2], Present from [0 1 −1 ] to [2 −1 −1]. ¶Poisson's ratio ν_{∥} in Si(111) plane: Present = Kim = 0.262, Wortman = 0.358. ††Shear modulus G_{⊥} normal to plane for Si(111): Present = Kim = 57.8 GPa, Wortman = 47.0 GPa. 
3. Mechanically bent Xray optics
Singlecrystal silicon is the most commonly used material for the substrates of Xray mirrors due to its interesting mechanical properties, and especially its excellent optical polishing quality. Dynamically bent mirrors in the Kirkpatrick–Baez (KB) configuration (Zhang et al., 2010) offer a versatile approach for nanofocusing applications at the ESRF. Similarly, silicon Bragg polychromator crystals can be dynamically bent to elliptical shape to cover a wide photon energy range. In these two examples, the silicon crystal is bent to an ideal elliptical shape with an accuracy in the range of 10^{−5} for the ratio of slope error relative to the slope of the ideally bent shape. To achieve such a performance, the anisotropic elasticity must be taken into account in the simulation and shape optimization of the optics.
3.1. KB mirror profile optimization
Nanofocusing of synchrotron Xray beams using mirrors in the KB configuration can be achieved using reflective surfaces with an elliptical figure. For instance, the horizontal focusing mirror (HFM) of a multilayercoated KB mirror device for the nanoimaging endstation ID22NI at the ESRF should have a radius of curvature in the range 11–30 m (p = 36 m, q = 83 mm, θ = 8 mrad). The use of dynamic bending technologies for this application allows the system to be optimized for operation over a large energy range (13–25 keV). One approach to achieve this highly aspheric shape is to use mechanical bender technology (Zhang et al., 1998) based on elastic flexure hinges and variablewidth mirrors (Zhang et al., 2010).
The application of two independent bending moments to the ends of the mirror substrate as in the ESRF bender design develops a linear variation of the moment along the mirror length. For the aspherical (elliptical) profile required, the radius of curvature R(x) of the bent substrate varies strongly with position x along the mirror length. The required variation over the useful mirror length for the ID22NI system is 58–32 m for the vertical focusing mirror (VFM) and 30–11 m for the HFM. The local slope of the bent substrate varies along the substrate length in the range of several mrad. Using the mechanical beam theory approximation, the local curvature, 1/R(x), can be calculated by
with
where u is the vertical displacement of the mirror, x is the mirror coordinate, M(x) is the local bending moments, and E and I(x) are, respectively, the elastic modulus and local of the mirror. W(x) and t(x) are the local width and thickness of the substrate, respectively. For a rectangular mirror, I(x) is constant and allows a thirdorder polynomial approximation to the ideal elliptical cylinder surface figure. For the mirror lengths and bending radius required for the ID22NI system, the figure/slope errors for this substrate geometry would be incompatible with the target performance. To overcome this limitation, a commonly applied approach at the ESRF is to use a trapezoidal profile for the mirrors, i.e. a linear variation in the substrate width, W(x), along the mirror. This allows correction of higherorder terms in the elliptical figure expansion. For improved correction of the figure errors it is necessary to use more complex width profiles (quadratic and beyond). For manufacturing simplicity the substrate thickness t(x) remains constant along the mirror length. By using equation (14), it is possible to define a variable profile as
For the ID22NI HFM mirror it can be shown using finiteelement modelling (FEM) that the slope error (differential slope between bent shape and ideal shape) with the profile defined analytically by equation (14) reaches 31 µrad RMS, which is much larger than the target requirement (<0.15 µrad). There are significant differences between FEM results on the mirror with the profile defined by equation (15) and the ideal ellipse. These are mainly due to the beam theory approximation in equation (14) which, unlike the FEM, does not take into account: (i) bender stiffness, (ii) anticlastic effects, and (iii) geometrical nonlinear effects. The complete mirror and flexure bender assembly has been modelled in threedimensions with FEM using ANSYS (Fig. 9). The silicon substrates were oriented such that the reflecting faces were parallel to crystal plane (110) with the [001] axis aligned along the mirror. This allows maximizing the ratio of the fracture toughness over the elastic modulus (Barrett et al., 2011).
An iterative algorithm based on a fully parametrical finiteelement model in ANSYS was used for the mirror width profile optimization (Zhang et al., 2010), and reached the target requirement in performance for both horizontal and vertical focusing mirrors (HFM and VFM), as shown in Table 2. The mirror profiles were optimized for operation at 8 mrad of glancing angle (or photon energy at 17 keV). Using the optimized width profiles for 8 mrad as input to the FEM, it was also possible to calculate the expected slope errors over the full operating range of incidence angles (see Table 2). In the mirror width profile optimization by FEM, in addition to the abovementioned three effects, we have also considered the influence of the adhesive bonding of the mirror to the flexure bender, the chamfer around the mirror, preloading springs and, of course, the anisotropic elasticity and crystal orientation.

From the optimized mirror width profiles, both HFM and VFM have been manufactured including the substrate machining and polishing, multilayer deposition, then assembled, and tested at the ESRF optical metrology laboratory. Measured results obtained are presented in Table 2. The measured slope error values are very close to the optimal theoretical values (Barrett et al., 2011).
3.2. Crystal orientation and mirror axis
The mirror width profile optimization was performed taking into account the anisotropic mechanical properties of the silicon crystal in the (110) crystallographic orientation for the mirror surface and axis [001] for the mirror meridional axis. This crystal orientation was chosen taking into account the anisotropy of the fracture behaviour of Si (Ebrahimi & Kalwani, 1999). By maximizing the fracture toughness along the planes perpendicular to the meridional direction and minimizing Young's modulus along this same direction, the risk of brittle fracture during bending of the substrate can be reduced. For the convenience of FEM with ANSYS, the corresponding Cartesian coordinate system is oriented as: xaxis for the mirror meridional direction = [0 0 1], yaxis for the mirror sagittal direction = [1 −1 0]/2^{1/2}, and zaxis for the mirror normal direction = [1 1 0]/2^{1/2}. The stiffness matrix is given in the supporting information: C_{110}^{ a}, which is directly usable in ANSYS.
To show the importance of the correct consideration of the anisotropy of the silicon crystal, we consider two cases: (i) misaligned crystal orientation during mirror manufacturing, and (ii) mirror width profile optimization with constant isotropic mechanical properties.
3.2.1. Misaligned crystal orientation during mirror manufacturing
For the optical configuration of the HFM at photon energy 17 keV, the mirror width profile was optimized with the silicon crystal (110) aligned as described above. The calculated slope error (bent slope – ideal elliptical slope) is 0.09 µrad RMS. With this mirror width profile, we have simulated the cases where the silicon crystal is oriented in the following way:
(1) Crystal plane (110) and mirror axis in the direction [001]: as optimized.
(2) Crystal plane (110) and mirror axis in the crystal plane but α = 55° from the direction [001].
(3) Crystal plane (110) and mirror axis in the crystal plane but α = 90° from the direction [001].
(4) Crystal plane (100) and mirror axis in the direction [001].
(5) Crystal plane (111) and mirror axis in the direction [1 −1 0].
Results in RMS slope error, maximum bending stress and bending forces are given in Table 3. If two bending forces are fixed to 16 N as for the optimized case, the misaligned crystal orientation would lead to very significant performance degradation from 0.09 µrad to 162 µrad for case (2), i.e. crystal plane (110) and mirror axis in the crystal plane but 55° from direction [001]. By optimizing the bending forces for the misaligned cases, the slope error can be reduced but is still significantly higher than in the case of the correctly aligned crystal orientation. For example, in case (2), the slope error is 0.5 µrad instead of 0.09 µrad for the correctly aligned crystal. The bending forces are 21.4 N, 34% higher than the initially estimated 16 N for the correctly aligned crystal. To reach the same optical configuration, the bending forces and bending stress for a specified KB mirror are nearly proportional to Young's modulus in the mirror axis (E_{x}). To minimize the bending stress, the Si crystal orientation should be aligned in such a way that Young's modulus is minimum (130 GPa) in the direction of the mirror meridional axis. Dynamical bending clearly offers the possibility to optimize the bending forces and greatly correct slope errors induced through any misalignment in the KB mirror.

3.2.2. Mirror width profile optimization with constant isotropic mechanical properties
For the same optical configuration of the HFM at photon energy 17 keV, if we use isotropic mechanical properties (for instance, E = 112.4 GPa, ν = 0.28, from Matweb) but the same bending forces (16 N), the optimized mirror width profile differs from the profile determined using anisotropic mechanical properties as defined in §3.2.1. Similarly, with this new mirror width profile, we have investigated the five cases listed in §3.2.1 and the results are summarized in Table 4. If the bending forces are fixed at 16 N, the slope error is very much higher than 0.09 µrad. By optimizing the bending forces for each case, the slope error can be reduced in the range 0.12–0.54 µrad, but is still significantly higher than in the case when the mirror width profile was optimized.

4. Thermal deformation of Xray optics
The thermal deformation modelling of siliconbased optics with anisotropic elasticity that we have initially performed concerns the liquidnitrogen (LN_{2}) cooled monochromator of the ESRF beamline ID06 (Zhang et al., 2013). In this monochromator the silicon crystal (111) reflecting plane is used with the meridional axis aligned along the direction [1 −1 0]. For the convenience of FEM with ANSYS, the corresponding Cartesian coordinate system is oriented as: xaxis for the monochromator–crystal meridional direction = [1 −1 0]/2^{1/2}, yaxis for the mirror sagittal direction = [1 1 −2] /6^{1/2}, and zaxis for the mirror normal direction = [1 1 1] /3^{1/2}. The stiffness matrix is given in the supporting information: C_{a}^{ 111}, which is directly usable in ANSYS.
For a given absorbed power, the most influential material properties in the thermal deformation of Xray optics are the α and the k. For constant material properties, the thermal deformation is proportional to the ratio of these two parameters, α/k, and should be independent of the isotropic Young's modulus. It is appropriate to note that both coefficient and are second rank tensor properties which demonstrate isotropic behaviour in cubic crystals such as silicon. The influence of Poisson's ratio on the thermal deformation was investigated. The value of Poisson's ratio of silicon shown in Figs. 3–6 varies with crystal orientation in the range 0.0622–0.3617. We have performed a finiteelement analysis of the LN_{2}cooled silicon crystal (Zhang et al., 2013) with isotropic elastic properties and different Poisson's ratio at first, and then with anisotropic elastic properties. The thermal deformation in terms of RMS slope error over the whole footprint along the central axis on the crystal surface is plotted versus absorbed power for Poisson's ratio ν = 0.0622, 0.2120, 0.2783, 0.3617 in Fig. 10(a). These results show that the thermal deformation increases with Poisson's ratio.
coefficientTaking the thermal deformation results at Poisson's ratio ν_{0} = 0.0622 as reference, we have calculated the ratio of the RMS thermal slope at any Poisson's ratio ν over that at Poisson's ratio ν_{0} = 0.0622. This ratio of the RMS slope is almost constant for different absorbed power. The average of these ratios for different powers is plotted versus Poisson's ratio (Fig. 10b). The ratio (1 + ν)/(1 + ν_{0}) is also plotted in Fig. 10(b). Results show that the thermal deformation of the monochromator crystal is a linear function of Poisson's ratio, and the RMS slope error is proportional to the factor of 1 + ν. We can extend the relationship between the thermal deformation of Xray optics and constant isotropic material properties as follows,
For a stable isotropic linear elastic material, Poisson's ratio is in the range (−1, 0.5). Most materials have Poisson's ratio values ranging between 0.0 and 0.5; ∼0.33 for many metals and nearly 0.5 for rubbers. Auxetic materials are those having a negative Poisson's ratio, such as many polymer foams, cork, or magnetostrictive materials (such as Galfenol) in certain orientations. Some anisotropic materials have one or more Poisson's ratios above 0.5 in some directions. The values for the materials used in Xray optics (mirror substrates or monochromator crystals) are mostly in the range (0, 0.5). Therefore, the influence of Poisson's ratio on the thermal deformation is less strong than the
and coefficient. This explains why the influences of Poisson's ratio are commonly ignored in the evaluation of thermal deformation of Xray optics.We have made similar simulations to those shown in Fig. 10(a) but incorporating the anisotropic elastic properties of silicon for various crystal orientations. Results of thermal deformation in terms of RMS slope error versus absorbed power are depicted in Fig. 11. These results show that the thermal deformation depends slightly on the crystal orientation. As the meridional and sagittal directions are different crystal axes, the thermal deformation along these directions differs slightly, except in the case of Si(100) where the meridional and sagittal axes are equivalent. Among all these crystal orientations, the maximum thermal slope error versus absorbed power is for Si(100)_α=0° and Si(100)_α=45°, and the minimum is for Si(110)_α=0°. The difference between them is about 8.9%. Poisson's ratio plotted in Figs. 3–6 varies from 0.0622 for Si(100) at the α=45° `in plane' component ν_{23(∥)} and for Si(110) at the α=90° `normal to plane' component ν_{12(⊥)} to 0.3617 for Si(110) at the α=0° `normal to plane' component ν_{12(⊥)} and at the α = 90° `in plane' component ν_{23(∥)}. This leads to a ratio of (1 + 0.3617)/(1 + 0.0622) = 1.28, or possible difference in RMS slope of 28%.
For the anisotropic silicon crystal, there are six components of Poisson's ratio (ν_{ij} with i, j = 1, 2, 3, i ≠ j). Fig. 12(a) shows the six components of Poisson's ratio versus angle α as defined in Fig. 2 for Si(100). The `in plane' components ν_{23} and ν_{32} are symmetrical and identical, but depend on the angle α. The `normal to plane' components are not symmetrical, ν_{13} ≠ ν_{31} and ν_{12} ≠ ν_{21} as shown in §2. But we have ν_{13} = ν_{12} = 0.2783 independent of the angle α, and ν_{31} = ν_{21} varying with the angle α. The thermal deformation in terms of RMS slope error versus absorbed power for Si(100)_α=0°, Si(100)_α=45° and isotropic and constant Poisson's ratio ν = 0.2783 is plotted in Fig. 12(a) and shows identical results. For Si(100)_α=0°, all six components of Poisson's ratio are equal to 0.2783. However, for Si(100)_α=45°, ν_{13} = ν_{12} = 0.2783, ν_{31} = ν_{21} = 0.3617 and ν_{23} = ν_{32} = 0.0622. These suggest that thermal deformation depends mostly on the components of Poisson's ratio ν_{12}, ν_{13} or their average ν_{av} = (ν_{13} + ν_{12})/2, at least for Si(100). Note that the silicon crystal monochromator is oriented in such a way that the vector is normal to the crystal surface and is along the meridional axis. The RMS thermal slope error is calculated from the derivative of the displacement normal to the crystal surface () over the axis along the meridional direction (). As an extension of the observations made for Si(100) described above, we have plotted the six components of Poisson's ratio for Si(110) and Si(111) versus the angle α in Figs. 13(a) and 13(b). All six components of Poisson's ratio for Si(110) vary strongly with α, including ν_{12} and ν_{13}. However, the average ν_{av} = (ν_{13} + ν_{12})/2 is constant, 0.212. For all three crystal orientations Si(100), Si(110), Si(111), the average ν_{av} (Fig. 13c) is independent of the angle α and equal to 0.212, 0.278 and 0.180, respectively. Then we plot the thermal deformation in terms of RMS slope error versus absorbed power for anisotropic silicon Si(111) and for isotropic constant Poisson's ratio 0.180 (ν_{av}) in Fig. 14(a), and for anisotropic silicon Si(110) at three angles in the crystal plane (α = 0°, 45°, 90°) and for isotropic constant Poisson's ratio 0.212 (ν_{av}) in Fig. 14(b). These results show that the thermal deformation of the LN_{2}cooled silicon crystal monochromator can be approximately simulated by using the isotropic constant Poisson's ratio equal to the average of ν_{12} and ν_{13}. The accuracy of this approximation is better than 1.2% for Si(100), 4.1% for Si(110) and 5.5% for Si(111). This approximation can be slightly improved by modifying the constant Poisson's ratio according to the relation between the RMS slope and Poisson's ratio in equation (15). For Si(111), for example, the average difference between the RMS slope calculated with anisotropic elasticity of Si(111) and an isotropic Poisson's ratio ν_{av} = 0.18 is 3.08%. If we use a corrected Poisson's ratio defined as follows,
then this difference is reduced to 2.4%.
5. Summary
The anisotropic elasticity of singlecrystal silicon has been fully reviewed for arbitrary orientation of the crystal. A matrixbased computer algorithm is proposed for the calculation of the stiffness coefficient matrix, compliant coefficient matrix, Young's modulus,
and Poisson's ratio. It can be easily implemented in any numerical computing environment and programming language that include matrix analysis (Matlab and NumPyPython examples are given in the supporting information). Analytical formulae to calculate Young's modulus, the and Poisson's ratio are also summarized in this paper. Numerical values of Young's modulus, the and Poisson's ratio have been compared with those in the literature, and have revealed discrepancies in some papers.The anisotropic elasticity of singlecrystal silicon has been used in the simulation of mechanical bent Xray optics and thermal deformation of Xray optics. For the mechanically bent Xray optics, the silicon crystal orientation should be carefully taken into account both in optical design and manufacturing. Selection of the appropriate crystal orientation can lead to both an optimized performance and low mechanical bending stresses. A dynamic bending device allowing bending force optimization should be efficient in partially correcting the effects of crystal orientation alignment errors.
The thermal deformation of the crystal depends on Poisson's ratio. For an isotropic constant Poisson's ratio ν, the thermal deformation (RMS slope) is proportional to (1 + ν). For an anisotropic material with cubic crystal symmetry (such as silicon), the thermal deformation can be approximately simulated by using an isotropic constant Poisson's ratio that is the average ν_{av} = (ν_{13} + ν_{12})/2, where direction 1 is normal to the crystal plane which is also the optic surface; the directions 2 and 3 are two normal orthogonal directions within the crystal plane. The average ν_{av} is independent of the direction in the crystal plane for Si(100), Si(110) and Si(111). Using this average Poisson's ratio in the finiteelement modelling of the thermal deformation of the Xray optics leads to less than 5.5% of error in RMS slope in comparison with results from a full anisotropic analysis for Si(100), Si(110) and Si(111).
Supporting information
Matrixbased computer algorithm and Matlab and Python codes for the calculation of anisotropic elasticity; and Stiffness matrix of silicon for crystal orientations (100), (110), (111), (311). DOI: https://doi.org//10.1107/S1600577514004962/ve5027sup1.pdf
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