Ion Beams for Surface Topology Modification Antti Myllynen - 3.3.2016 Ion beams for surface topology modification - 3.3.2016 - Antti Myllynen

Definition of the Problem Ultra-precise and ultra-smooth surfaces are needed: Optics  Free electron lasers Extreme UV lithography  Semiconductor industry Demand for surface roughness below 0,1 nm RMS.  Ion beam processes Ion beams for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen Problem Solution 1 Ion beam smoothing (IBS): Shape corrections Lower surface roughness Good for dot-like structures Recent developments: Ultra-smooth surfaces ≤ 0,2 nm RMS. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen Problem Solution 1 Under ion beam: Several processes are active at surface. At appropriate temperatures:  Surface smoothing Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Si surface under Ar+ ion beam [1] t = 0 min Rq = 2,25 nm t = 10 min t = 180 min Rq = 0,2 nm Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen Problem Solution 2 Ion beam planarization (IBP) Surface smoothing Add sacrificial layer Remove by ion beam sputtering Planarization angle Surface of sacrificial layer  Surface of substrate Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Artificial Si surface [1] Binary grating Rq = 3,05 nm Resist surface Rq = 0,45 nm Planarized film Rq = 0,6 nm Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen IBS vs IBP [2] IBS is good for protrusion / dot like surfaces and up to spatial distances of 1000 nm [1]. IBP can be used up to spatial distances of tens of microns thus it can, smoothe also surfaces with holes [1]. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen Conclusions Ultra-precise and ultra-smooth surfaces are needed. 2 Ion beam processes: Ion beam smoothing Ion beam planarization Up to surface roughness of Rq = 0,2 nm. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Information – Ion Beam Smoothing [1] During ion beam erosion of surfaces, several smoothing processes are active: Under common experimental conditions the atomic diffusion at surface is the only process operating which leads to surface relaxation = smoothing. Evaporation / condensation processes and bulk diffusion are not relevant. Effective ion induced surface diffusion contributes to the rate of surface smoothing. The mechanism is always active, but it is too weak to smoothe the surface alone. One potential relaxation process is surface viscous flow, which is characteristic for amorphous systems. This is due to Laplace stress, which generates particle current parallel to the local surface. Ballistic transport processes are due to energy and momentum deposition. Ion impacts to the surface can induce atomic drift parallel to the surface. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Information – Ion Beam Smoothing [1] Surface smoothing under glancing ion incidence is caused by a combination of shadowing and enhanced erosion of surface protrusions when the incidence angle is > 80 ° with respect to the surface normal. With incident beam almost parallel to the surface ions are reflected from the surface so only protrusions are removed. In surface gradient dependent sputtering the sputtering yield, and thus the local erosion rate, is highest for surface regions inclined to the incident beam. The surface will smoothe because the erosion rate is highest at protrusions and expansions of surface. In addition to the smoothing processes, sample rotation around its surface normal is beneficial to the surface smoothing. In ion beam smoothing, all of these processes are active and the surface smoothing is originating from the combination of the processes. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Information – Ion Beam Planarization [3] Surface smoothing results from removing of a planarizing sacrificial layer. Can be used in semiconductor industry for the planarization of processed silicon wafers. Smoothing process: First, the rough surface is coated with a suited, low viscosity, sacrificial layer. This layer is removed by Ion beam etching at planarization angle = Incident angle where the removal rate of sacrificial layer and the substrate are almost the same.  The smooth surface of sacrifial layer is trasferred into the substrate. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion Beam Smoothing vs. Planarization [1] Ion beam smoothing (IBS) is good for dot / protrusion like surfaces and up to spatial distances of 1000 nm. Ion beam planarization (IBP) can be used up to spatial distances of tens of microns thus it can, (in contrast to the IBS) smoothe also surfaces with holes. Both of these methods can achieve surface roughness below 0,2 nm. Can be used for: SiC, Si, Quartz, GaN, ZnO, III/V-semiconductors. Ar+ -ion beams are typically used with ion energies from 10s of eV to 100s of eV. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen

Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen References: F. Frost, R. Fechner, B. Ziberi, J. Völlner, D. Flamm, A. Schindler. Large area smoothing of surfaces by ion bonbardment: fundamentals and applications. Journal of Physics: Condensed Matter. 21 (2009) 1-20. T. Arnold, C. Böhm, R. Fechner, J. Meister, A. Nickel, F. Frost, T. Hänsel, A. Schindler. Ultra-precision surface finishing by ion beam and plasma jet techniques – status and outlook. Nuclear Instruments and Methods in Physics Research A. 616 (2010) 147-156. F. Frost, R. Fechner, D. Flamm, B. Ziberi, W. Frank, A. Schindler. Ion beam assisted smoothing of optical surfaces. Applied Physics A. 78 (2004) 651-654. Ion beam for surface topology modification - 3.3.2016 - Antti Myllynen