246
IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 2, APRIL 2005
Visualization of Interactions Between Organic Polymer Surfaces and Ion Beams Obtained From Molecular Dynamics Simulations Hideaki Yamada and Satoshi Hamaguchi
Abstract—Molecular dynamics simulation combined with sophisticated visualization techniques may be one of the most powerful scientific tools for the study of atomic-level surface reactions during plasma-wall interactions. This paper shows visualization of an organic polymer model substrate before and during molecular beam injections. The classical interatomic potential model functions used in the present work are developed to describe chemical reactions, i.e., formation and breakup of chemical bonds under strongly nonthermal-equilibrium conditions. We have observed that, during beam injections, nonthermal-equilibrium chemical reactions take place in a nano-scale thin layer of the substrate top surface, which determines characteristics of the process, such as etching/deposition rates and selectivity. Index Terms—Molecular dynamics simulation, organic polymers, plasma processing.
S
INCE there are many allotropes of carbon and a variety of organic chemical compounds that are widely used for various applications of modern technologies, plasma-surface interactions for a wide variety of carbon containing materials have also been the subject of extensive research. Examples of applications of carbon-based materials include: graphite and carbon fiber composites (CFCs), which are considered to be good candidate materials for plasma facing walls in thermonuclear fusion reactors: diamond, which consists mostly of sp3 bonded carbon atoms and is widely used as a tool to cut and/or polish material surfaces due to its hardness. Plasma enhanced chemical vapor deposition is often used to manufacture diamond films. We also note that organic polymers with low dielectric constants (low ) may replace SiO as insulating materials in future integrated circuit (IC) chips in order to reduce RC delays in signal transmission in interconnects. Although several plasma processes to etch such organic polymers have been developed [1], the etching mechanisms of these processes are not fully understood. The authors used molecular dynamics simulations to study organic polymer etching by some molecular beam injections [2]. It is experimentally known that many low- organic polymers show similar etching characteristics and these polymers consist mostly of benzene rings in their polymer chains. Manuscript received July 2, 2004. This work was in part supported by the Semiconductor Technology Academic Research Center (STARC) and Association of Super-Advanced Electronics Technologies (ASET). H. Yamada is with the Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Osaka 563-8577, Japan (e-mail:
[email protected]). S. Hamaguchi is with the Graduate School of Engineering, Osaka University, Osaka 565–0871, Japan (e-mail:
[email protected]). Digital Object Identifier 10.1109/TPS.2005.845358
Therefore, in our simulations, we used poly (1, 4-phenylene) [which is also known as polyparaphenylene (PPP)] as a material of the model substrate, which consists only of chains of benzene rings. Our simulations have clearly shown that the hydrocarbon polymers are reactively etched by hydrocarbon molecule or radical beams. We have also observed that some of injected molecules easily break into atomic species and efficiently react with the substrate, which often significantly alters bonding networks of substrate atoms. Mixture of H and N gases are frequently used in organic polymer etching experiments. To simulate such processes, we have developed an interatomic potential model applicable to systems of hydrogen, carbon and nitrogen atoms [3] and used the model to simulate interaction of PPP substrate with molecules and radical species containing C, N, and H. Fig. 1 shows the initial state of our model PPP substrate in thermal equilibrium at 300 K based on the model potential functions mentioned above. We created this figure and Fig. 2, using MicroAVS [4]. The top surface area of the model substrate shown in Fig. 1 is approximately 4.6 2 nm . The white and blue spheres represent carbon and hydrogen atoms. The potential model used here evaluates the order of each covalent bond based on information of the positions and species of the atoms surrounding the bond. The strength of a covalent bond depends on the bond order as well as the relative positions of atoms forming the bond. Covalent bonds are represented by bars in the figure. The thickness of each bar is proportional to the bond order. The four vertical lines standing at the corners of the substrate represent boundaries of the simulation domain in the horizontal directions. Periodic boundary conditions are imposed in the horizontal directions. The covalent bonds that cross the simulation boundary are not shown. One can clearly see the structure of PPP chains consisting of benzene rings. The model substrate has four layers, each of which consists of four PPP chains lying in the same horizontal direction. All PPP chains are almost parallel to each other and each chain consists of ten benzene rings in the present case. (This model substrate has about 1600 atoms in total). The lowest monolayer is artificially set to be immobile to prevent the entire substrate from moving when it is bombarded by high energy beams. Various molecules and atomic clusters (i.e., radical species) are injected into the surface with controlled incident kinetic energies. The horizontal positions and three-dimensional orientations of each incident cluster are selected randomly. In the simulations, each beam injection is carried out as follows: 1) an incident atom (or molecule or atomic cluster) is
0093-3813/$20.00 © 2005 IEEE
YAMADA AND HAMAGUCHI: VISUALIZATION OF INTERACTIONS BETWEEN ORGANIC POLYMER SURFACES AND ION BEAMS
247
Fig. 1. Model substrate of poly (1, 4-phenylene). White and blue spheres represent carbon and hydrogen atoms. Thickness of each bond is set to be proportional to the magnitude of its bond order.
Fig. 2. Poly (1, 4-phenylene) substrate after 6:0 magenta and green, respectively.
2 10
/cm dose of N H beam injections with 50 eV. Injected nitrogen and hydrogen atoms are colored in
injected at each time interval – ps; 2) the temperature of the substrate is artificially reduced to room temperature during the last 20% of . This temperature control is done by , to the equation applying a artificial frictional force, of motion for each atom, which represents hypothetical global , coupling to the heat bath with room temperature. Here, and denote the mass and velocity of the th atom and constant friction coefficient, respectively. During simulation, some atoms/molecules may pass through the lowest layer, which means we do not have a sufficiently thick substrate. Therefore, when it happens, we discard the injection event and restart the injection with a thicker substrate by adding another layer to the bottom of the previous substrate. Floating atoms having no interaction with the substrate atoms and upward mobility are removed from the simulation cell automatically as “sputtered” atoms. The number of such sputtered atoms for each injection is the sputtering yield [2]. Fig. 2 shows the substrate cm dose of N H radical species injections, after which are considered to be major etchants in nitrogen/hydrogen plasmas for organic polymer etching. Magenta and green
spheres in the figure represent injected nitrogen and hydrogen atoms. Some injected N H molecules are broken into three separate atoms and react with substrate atoms while some are broken into N molecules and H atoms. When the injection energy is low, the later type of dissociation is more dominant and most N molecules are reflected from the surface.
REFERENCES [1] H. Nagai, S. Takashima, M. Hiramatsu, M. Hori, and T. Goto, “Behavior of atomic radicals and their effects on organic low dielectric constant film etching in high density N /H and N /NH plasmas,” J. Appl. Phys., vol. 91, pp. 2615–2621, 2002. [2] H. Yamada and S. Hamaguchi, “Molecular-dynamics simulations of organic polymer etching by hydrocarbon beams,” J. Appl. Phys., vol. 96, pp. 6147–6152, 2004. [3] H. Yamada and S. Hamaguchi, “Investigation of interactions between plasmas and organic polymer surfaces using molecular dynamics simulation,” in Proc. 21st Symp.Plasma Process., Hokkaido, Japan, Jan. 28–30, 2004, pp. 214–215. [4] MicroAVS [Online]. Available: http://www.avsuk.com/microavs/microavsinfo.html
