1. FIB sputtering optimization using Ion Reverse Software
S. Zaitseva, A. Svintsova, G. Lalevb, S. Dimovb, V. Velkovab, H. Hirshyb
a Institute of Microelectronics Technology, Russian Academy of Science,
6, Institutskaya Str., Chernogolovka, Moscow distr , Russia
b Manufacturing Engineering Centre, Cardiff University, The Parade Cardiff CF24 3AA, UK
Introduction
3. Experimental
To have a better control on the FIB sputtering process when realizing complex 3D shapes, a novel simulation software (Ion Reverse Software – IonRevSim) is created. In our previous study, the capability for layer-by-layer Focused Ion Beam (FIB) machining of complex 3D shapes utilizing 3D CAD models was reported [1]. However, in that method two factors were neglected: the angular depended sputtering rate and re-deposition. To take into account the first factor and to increase the accuracy of FIB sputtering process for 3D structuring, IonRevSim was validated for FIB machining of nano-imprint lithography (NIL) templates.
To validate the IonRevSim simulation results for FIB sputtering and to asses the generated GDSII files using stratification and quasi-stationary modes, a model of square pyramid (with base 2x2 µm) was selected as reference shape. For the simulation, the model was generated from analytical description. To perform 3D FIB structuring tests, GDSII data files were used. The experiments were conducted on a FIB system, XB1540 (which combines a 30 keV gallium ion beam column with a 30 keV electron beam GEMINI column), equipped with Raith lithography hardware and software, Elphy Quantum. Substrate is fused silica.
Figure 3 represents the simulated (left column) and FIB results (right) for realizing pyramids a) using IonRevSim with dose distribution b). Stratification mode sputtering with 100 strata exposed in right consequence: from bottom to top (Fig. 3 c)-d)) and wrong consequence: from top to bottom (Fig. 3 e)-f)). Fig. 3 g) and h) demonstrate Quasi-stationary sputtering mode.
1. Data preparation modes using IonRevSim
IonRevSim offers two modes for 3D data preparation i.e. stratification and quasi-stationary modes as schematically shown in Fig. 1., where the different colors indicate different exposure doses.
The software allows the sputtering angular dependence to be simulated and thus to calculate the dose distribution required for fabricating complex 3D shapes, where the incident beam is no longer normal to the surface (Fig. 2).
Finally IonRevSim allows exporting the data in GDSII format, which can be directly uploaded in conventional lithography system as Raith, NanoMaker, etc. …
Qausi-stationarymode +
Single loop, strong re-deposition
Pyramid design
Dose
Dose Y X
N loops with identical D(x,y)/N
M strata of different area
1st stratum
2nd stratum
Mth stratum b) a) …
Stratification
mode
Figure 1: Dose preparation modes Ga q Vq
Stratification mode Ga q0 V0 d) c)
Figure 2: Angular dependence of ion _______sputtering rates
Stratification, inverse order
2. Isotropic local etching model and simulation with IonRevSim
It is supposed that sputtering coefficient (quantity of sputtered materials per one ion) is proportional to 1/cos(θ), where θ is a tilt angle (Fig.2). This angular dependence is observed for wide range of angles θ, and for lots of materials, ions and their energies [1]. The angular dependence allows to consider ion etching in model of isotropic local etching [2] usually applied to liquid etching. Processes of sputtered material re-deposition and ions re-scattering are not taken into consideration at the current version of the software.
Sputtering velocity V does not depend on Z. Velocity distribution V(x,y) is proportional to beam current, or exposure dose distribution for FIB tools. Two dimensional case has simple solution [2] : e) f)
Quasi-stationary mode h) g)
Figure 3 IonRevSim simulated and FIB results for realizing pyramids (a) with dose distribution (b), using IonRevSim stratification (c-f) and quasi-stationary modes (g and h).
Conclusions
where z(x,t=0)=H0 is initial profile, t is time of sputtering. For 3D case minimization should be performed for 2D trajectories in XY plane.
Isotropic local etching model gives good description for ion sputtering in quasi stationary and stratification modes.
Creating given 3D relief Z(x,y) (reverse problem solution) for FIB machine is optimal in Stratification mode avoiding re-deposition.
If relief inclination is not higher than 60º than influence of re-deposition and re-scattering are negligible.
Ion etching is strongly dependent on order of ion exposure.
References:
1. Andersen H.H., Bay H.L. // Sputtering by Particle Bombardment I / Ed. R. Berish. Springer-Verlag
2. V.V.Aristov, B.N.Gaifullin, A.A.Svintsov, .I.Zaitsev, R.R. Jede, H.F.Raith, Accuracy of proximity correction in electron lithography after development, J. Vac. Sci. Technol. B 10(6),Nov/Dec (1992)
IMT RAS
Contact:
G. Lalev