This item is the archived peer-reviewed author-version of: Fluorinesilicon surface reactions during cryogenic and near room temperature etching Reference: Tinck Stefan, Neyts Erik, Bogaerts Annemie.- Fluorinesilicon surface reactions during cryogenic and near room temperature etching The journal of physical chemistry: C: nanomaterials and interfaces - ISSN 1932-7447 - 118:51(2014), p. 30315-30324 Full text (Publishers DOI): http://dx.doi.org/doi:10.1021/jp5108872 To cite this reference: http://hdl.handle.net/10067/1229570151162165141 Institutional repository IRUA Fluorine-Silicon Surface Reactions during Cryogenic and Near-Room Temperature Etching Stefan Tinck*, Erik C. Neyts and Annemie Bogaerts Research Group PLASMANT, Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium ABSTRACT. Cyrogenic etching of silicon is envisaged to enable better control over plasma processing in the microelectronics industry, albeit little is known about the fundamental differences compared to room temperature. We here present molecular dynamics simulations carried out to obtain sticking probabilities, thermal desorption rates, surface diffusion speeds and sputter yields of F, F2, Si, SiF, SiF2, SiF3, SiF4 and the corresponding ions on Si(100) and on SiF1-3 surfaces, both at cryogenic and near-room temperature. The different surface behavior during conventional etching and cryoetching is discussed. F2 is found to be relatively reactive compared to other species like SiF0-3. Thermal desorption occurs at a significantly lower rate at cryogenic conditions, which results in an accumulation of physisorbed species. Moreover, ion incorporation is often observed for ions with energies of 30-400 eV, which results in a relatively low net sputter yield. The obtained results suggest that the actual etching of Si, under both cryogenic and near-room temperature conditions, is based on the complete conversion of the Si surface to physisorbed SiF4, followed by subsequent sputtering of these molecules, instead of direct sputtering of the SiF0-3 surface. KEYWORDS: sticking coefficients, thermal desorption, surface diffusion, sputter yields INTRODUCTION Fluorine based gases like SF6, CF4 and SiF4 the Si-F chemistry exist. It is noteworthy, are commonly used in plasma processing for however, that most investigations focus only semiconductor manufacturing. Fluorine is on the mechanism(s) (e.g., influence of steric very suitable for the etching of silicon, factors) of atomic fluorine interacting with because it is the most reactive of all halogens silicon2-18 but they usually do not provide on silicon. F atoms created in the plasma information on reaction rate (constants) or gradually convert the Si surface to volatile sputter yields, which is crucial information SiF4 molecules. The actual removal of silicon for the (plasma) modeling community. from the surface is accelerated by ion Reports on sputter yields for the Si-F bombardment. system are limited. Barone and Graves have The Si-F system has been relatively well calculated sputter yields with MD for Ar+ studied experimentally, as reviewed by ions bombarding a Si surface with varying Winters and Coburn.1 Also a number of degrees of fluorination.19 Chiba et al. have molecular dynamics (MD) results regarding reported sputter yields of F+ ions with 1 energies of 15, 30 and 100 eV on different how to control critical dimensions of 20 SiFx surfaces. microstructures is still unresolved. Furthermore, chemical reaction The work presented in this report allows us probabilities are also scarcely reported. to obtain a thorough understanding of which Reaction probabilities for F2 on clean Si(100) reactions occur during the etching process, are predicted by Carter and Carter21, while not only at one specific condition, but for a consecutive impacts of SiF3, SiF2 and SiF wide range of operating conditions. Also, we ions on Si(100) are investigated by Gou and hope that the actual values for the sticking coworkers.22-25 Marcos et al. investigated the probabilities, desorption rates, surface evolution of an etched Si surface under diffusion rates and sputter yields of various SF6/O2 plasma treatment with Monte Carlo species as a function of surface composition simulations. They compared calculated as presented in this work are of great value to etched trench profiles obtained with different the modeling community. Surface reaction predefined sticking coefficients for the F probabilities are often not available or atoms and different SF6/O2 gas ratios to insufficiently known and they are determine the role of the passivation layer indispensable when modeling plasmas with and how it protects the sidewalls from surface interactions. This is even more so for undercutting effects.26 low pressure plasmas (as commonly used in The purpose of the present study is to microchip development) where surface obtain a better understanding of the reactions can be more important than gas quantitative aspects of all reactions/processes phase collisions. that occur simultaneously during the etching of Si with various fluorine species. The goal COMPUTATIONAL DETAILS of this work is therefore to answer questions like: (i) What is the probability that a certain The calculations were performed with the species chemisorbs, physisorbs or reflects as classical molecular dynamics (MD) code a function of chemical composition and LAMMPS.29 In MD, forces on atoms are temperature of the surface? (ii) What are the calculated based on an interatomic potential desorption and accumulation rates of to predict possible reaction mechanisms and physisorbed species as a function of surface trajectories of atoms. To describe the Si-F temperature? (iii) How fast do species diffuse system, a combination of a Tersoff along the surface? and (iv) What are the interatomic potential employing parameters sputter yields of Si and F atoms as a function developed by Abrams and Graves30 and a of ion type, ion energy, surface composition long-range Lennard-Jones type Van der and surface temperature? Waals (VdW) interaction potential using In addition, the innovation of this work is parameters by Halgren31 was used. that we do not only calculate the various The Si-F Tersoff potential used in this work reaction probabilities for a conventional is constructed of a Tersoff-type binary wafer temperature (i.e. near-room potential energy function whose form is based temperature) but also for cryogenic on the Si-H potential of Murty and Atwater32 conditions (i.e. -100 °C). Silicon cryoetching, but with parameterization for Si-F performed proposed in 1988 by Tachi27, is currently by Abrams and Graves30. The interested most often used for etching silicon vias and reader is referred to these references for a 3D microelectronic components. The more detailed explanation of the applied underlying mechanisms, however, are not yet equations and the specific parameters fully understood, and in particular the issue of implemented for the Si-F system. In the 2 Tersoff formalism, the total energy of the potential for the Si-F system. This is system is given by the sum of bond energies sufficient when investigating the reaction where each bond between two atoms has mechanisms of bond breaking and formation attractive and repulsive components and during fluorination of Si. However, in reality, where the bond energy is dependent on the when a species arrives at a surface, its motion angle with neighboring atoms. Inner and will be affected by long range dispersion outer cutoff values for the Si-Si potential interactions near the surface. This may result function are 2.7 Å and 3.0 Å, while the values in the species not immediately returning to for the Si-F and F-F functions are 1.84 Å - the plasma (if no bonds are formed) which 2.14 Å and 1.7 Å - 2.0 Å, respectively. allows for the formation of a physisorbed The long range VdW interactions are layer. Desorption and accumulation rates of described by 12-6 Lennard-Jones type physisorbed species can therefore only be potential functions. More detailed investigated with MD if these dispersion information and the parameters specific for forces are considered in the interatomic the Si-F system can be found in the potential. Indeed, whereas the cutoff range of mentioned paper by Halgren31. The inner the Tersoff potential is 4 Å, it is 11 Å in our cutoff values of the VdW potential functions current description (for Si-Si interactions). As are equal to the outer cutoff values of the will be discussed in the results section, these Tersoff potentials used for Si-Si, Si-F and F- weak dispersion forces are the main reason F. A cosine-type spline function was applied for the significant differences observed in to smoothly link both Tersoff and VdW surface processes during etching at room potential functions together. The outer cutoff temperature vs. cryogenic temperature. values for the F-F, Si-F and Si-Si VdW The difference in using only the Tersoff interactions are 7 Å, 9 Å and 11 Å, potential or a combination of both Tersoff respectively. and dispersion forces is illustrated in figure 1. So far, most MD results reported in literature are obtained by using only a Tersoff 3 Figure 1. Illustration of the trajectory of a SiF3 radical that is reflected from the surface when using the Tersoff potential (a), or a combination of the Tersoff potential and long-range dispersion forces (b). The area underneath the horizontal dashed line in (b) illustrates the region where the radical is attracted towards the surface due to these dispersion forces. The white spheres are silicon atoms and the darker (green) spheres are fluorine atoms. In figure 1(a), only the Tersoff potential is dispersion forces. As a result, the physisorbed implemented and the plasma species (i.e. SiF3 species diffuses over the surface until it in this case) is immediately reflected from the chemisorbs or until it gains enough kinetic highly fluorinated surface. In figure 1(b) energy due to thermal fluctuations to move long-range dispersion forces are included in out of the attractive region near the surface addition to the Tersoff potential, and thus the and launch itself back into the plasma.