193 nm Excimer laser processing of Si/Ge/Si(100) micropatterns
Graphical abstract
Introduction
The crystallization and epitaxial growth of SiGe through Excimer laser assisted techniques has already been proved as a suitable method for obtaining heteroepitaxial layers [1], [2], [3], [4], [5], [6], [7], [8], demonstrating a good performance and compatibility with conventional IC's fabrication technology. In fact, Si/SiGe/Si heteroepitaxial structures, grown through a combination of laser assisted processes and Molecular Beam Epitaxy techniques, have already been built and tested for the top-down fabrication of silicon-on-nothing (SON) structures, evidencing a good SiGe/Si etching ratio, close to 10, as reported in a previous work [5].
The challenge of using a bottom-up approach for the fabrication of such heteroepitaxial structures through a combination of laser-assisted techniques in a “one-chamber” low thermal budget process, can be faced by two main steps. First, the deposition of amorphous micro-patterned structures through Laser induced Chemical Vapor Deposition (LCVD) and second, their further epitaxial growth through Pulsed Laser Induced Epitaxy (PLIE). Deposition of micro-patterned structures by LVCD can be carried out by using nanostencils, as has been extensively reported for other deposition techniques [9], [10]. It has already been demonstrated that nanostencils are useful tools for film growth through bottom-up processes, avoiding the time consuming top-down photolithography steps used in conventional IC fabrication technologies [11], [12]. The crystallization of the micro-patterned amorphous Si/Ge bi-layers leading to the epitaxial growth of a SiGe structure can be performed by PLIE, as Finite Element Method (FEM) numerical simulations have predicted [6]. Moreover, according to the FEM simulation, the aspect ratio of the microstructures can be even improved through PLIE, a beneficial effect that can be used to reduce feature sizes limited by the nanostencils design and the LCVD process itself.
This contribution presents the results on the fabrication of circular features with around 3 μm diameter and 25 nm total height through LCVD and PLIE, demonstrating the possibility of obtaining ordered crystalline patterns with Ge rich SiGe alloys buried under a Si rich disc of 2 μm diameter, that are surrounded by a Ge rich SiGe ring of 1 μm wall thickness. Moreover, experimental results are compared with previously performed numerical simulation of the laser induced heat and stress profiles, evidencing that the prediction and the optimization of the experimental parameters for the epitaxial growth is possible, by applying a FEM Numerical Simulations of the PLIE process [6].
Section snippets
Experimental
Hydrogenated a-Si/a-Ge bilayer microstructures with a-Si and a-Ge thickness of 18 nm and 7 nm, respectively, were deposited in an HV chamber (base pressure 1 μPa) by performing consecutive LCVD experiments on crystalline silicon (100) substrates, shadowed with nanostencil masks provided by the Microsystems Laboratory at the Ecole Polytechnique Federale de Lausanne (EPFL). All deposition experiments were performed at the same substrate temperature (250 °C) while the working pressure was different
Results and discussion
The bidimensional profiles of the as-deposited circular structures of 3 μm diameter, obtained by optical profilometry (Fig. 1b), suggest that the mask pattern (Fig. 1a) is successfully transferred to the substrate, showing all microdeposits of similar shape with a satisfactory aspect ratio, considering typical unavoidable blurring effects [14]. As it can be seen for the profiles along the two orthogonal directions that correspond to the organization in rows and columns of microdeposits, the
Conclusions
Nanostencil patterns can be successfully transferred to the substrate my means of LCVD, obtaining a satisfactory aspect ratio, as found by interferometric profilometry. This aspect ratio can even be enhanced by PLIE, due to the thermal stress created in each individual microstructure during laser irradiation. Raman spectra confirm that the amorphous bilayer is transformed into a crystalline SiGe alloy when a circular structure of 3 μm in diameter is irradiated perpendicularly with an energy
Acknowledgements
This work has been partially supported by the following National and Regional research contracts: MAT-2000-1050, MAT-2003-04908, PGIDIT03-04908, PGIDT-01PX130301PN, XUGA-Infra 93, XUGA-Infra 94-58, SB93-A0742819D, INFRA 99–PR 405a–46. The authors would like to acknowledge Jürgen Brugger and co-workers from the Ecole Polytechnique Federale de Lausanne for providing the nanostencils used for these experiments.
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Cited by (0)
- 1
Present address: Department of Mathematics and Physics “Ennio de Giorgi”, University of Salento and Italian National Institute of Nuclear Physics (INFN), 73100, Lecce, Italy.
- 2
Deceased, August 2013.