X-Ray Synchrotron Topography Investigation of Porous Silicon Formed by Patterning in Localized Areas

Abstract

X-ray synchrotron topography was used to study the structural features of porous silicon ͑PS͒ layers formed by p ϩ ͑boron͒-type silicon anodization within localized areas. Selective anodization was obtained by using a masking film ͑Si 3 N 4 ͒ with good chemical inertness to the anodic dissolution or a surface layer of different doping type and level resulting from phosphorus implantation. The overall structural picture of the PS/substrate systems obtained by the two different masking procedures was built up by combining different topographic methods. The samples were oriented both in Bragg ͑reflection͒ and Laue ͑transmission͒ geometry to diffract white or monochromatic radiation with extended ͑projection topography͒ or limited ͑section topography͒ beam widths. By comparing the effects of the two masking types on the anodization process, the implantation method is unquestionably preferable because of the lower stress at the window edges, the better lateral porosity homogeneity inside the windows, and the flatter interface between porous silicon and substrate.Porous silicon ͑PS͒ is obtained by anodic electrochemical etching of monocrystalline silicon in HF solutions. Depending on Si type and doping level, current density, and HF concentration, different PS morphologies are obtained. They are classified as nanoporous, mesoporous, or macroporous silicon for pore sizes between 10 and 100 nm or greater than 100 nm, respectively. The electrical, thermal, optical, and mechanical properties of the PS layer are closely connected with its morphology, and this makes PS very interesting in a wide field of applications. For instance, a macroporous structure obtained from p-type Si has a sufficiently high surface-tovolume ratio to be appropriate for gas-sensing devices, 1 and mesoporous or nanoporous Si has been extensively used as a sacrificial layer in Si micromachining technology. 2 A further important feature is the selective anodization of Si wafers in confined areas. This enables the manufacturing of PSbased devices by the well-established integrated silicon technology. In this case, the study of the device fabrication steps is essential to ascertain the magnitude of the problems associated with each of them. This work intends to give a contribution to the structural study by X-ray diffraction ͑XRD͒ imaging ͑topography͒ of the first step of the whole process, i.e, the PS formation in localized areas. The use of X-ray topographic techniques is important because of their nondestructive character and their effectiveness in measuring, mapping and visualizing strains in single crystals. 3 Indeed, in spite of its porous structure, PS has proved to have very good crystalline quality independently of its porosity, 4 and X-rays were extensively used to study the structural properties of PS layers. 5-8 Here we combined different topographic methods to get information on the confined PS layer and the lateral and in-depth PS-substrate interfaces. Selective Silicon AnodizationSquare localized areas with sides ranging from 1 to 2.5 mm ...

Three-dimensional visualization of the inner structure of single crystals by step-scanning white X-ray section topography

“…where l is the overlapping or separating width of a subgrain boundary, L is the sample-to-detector distance and 2 B is the scattering angle (Milita et al, 2001). From the section topograph shown in Fig.…”

X-Ray Diffraction Study of Confined Porous Silicon Membranes

“…In fact, it was found that PS formed on Si substrate in localized square windows is under considerable lateral compression induced by the surrounding bulk Si. 2 As the same stress would operate on the PS membrane, the only way by which the membrane could release this stress is by strong concave or convex bending with high risk of breaking.…”

“…The shape of the diffracted image is the result of the distribution of the lattice distortion ͑tilt and/or rotation͒ along the wafer section probed by the beam. 2 By analysis of section series, the lateral deformation distribution inside the membranes was qualitatively determined for smaller membrane sizes.…”

“…1 The investigation of confined PS overlayers on Si is worthy of particular attention, as their structural properties depend on the technological procedure adopted for window opening and on the anodization conditions. 2 Special care should also be taken of the mechanical characteristics of localized PS membranes, mainly because the only data available from the literature deal with deformation and fracture behavior of freestanding PS membranes. 3 The mechanical stability of PS membranes, partly connected to the structural properties, is in fact one of the most important requirements for long-term performance of devices based on these structures.…”