1. Ion Beams for Surface Topology Modification
Antti Myllynen
Ion beams for surface topology modification Antti Myllynen

2. 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 Antti Myllynen

3. 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 Antti Myllynen

4. 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 Antti Myllynen

5. 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 Antti Myllynen

6. 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 Antti Myllynen

7. 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 Antti Myllynen

8. 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 Antti Myllynen

9. 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 Antti Myllynen

10. 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 Antti Myllynen

11. 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 Antti Myllynen

12. 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 Antti Myllynen

13. 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 Antti Myllynen

14. 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)
F. Frost, R. Fechner, D. Flamm, B. Ziberi, W. Frank, A. Schindler. Ion beam assisted smoothing of optical surfaces. Applied Physics A. 78 (2004)
Ion beam for surface topology modification Antti Myllynen