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Flash lamp annealing of memristive BiFeO3 thin films simulated with COMSOL Multiphysics
Bürger, D.; Baitule, S.; Rebohle, L.; Schulz, S.; Schmidt, H.;
Annealing techniques are key technologies in semiconductor industry. Removing implantation damage, activating dopants, and crystallizing sputtered amorphous layers are only some applications of annealing technologies. Pulsed laser annealing on the nanosecond timescale and longterm annealing in the range of hours are on the short and on the long end of the time scale, respectively. On an intermediate millisecond time scale, flashlamp annealing (FLA) plays an important role as an homogenous annealing technique with a large throughput for industrial applications [1]. Adjustable FLA parameters are energy density and the duration of the pulse. Pulse repetition frequency and shape of the pulse are technical aspects of FLA setups with lower energy density which are commonly used for materials on flexible substrates. So far, less attention has been paid to the geometry of the chamber and to the substrate holder influencing the temperature of the annealed wafer. The former one is influencing the irradiance distribution, the latter one is influencing the heat conduction and the cooling rate of the wafer. The irradiance distribution has a large influence on the temperature close to inhomogeneities on the surface or the edge of the sample [2]. The cooling rate after the FLA pulse determines the effect of thermal treatment on the sample properties due to diffusion processes within the sample. Our studies on BiFeO3 thin films on Si/SiO2/Ti/Pt substrates, which we develop for resistive switching applications, show that highly energetic pulses may lead to a phase separation in BiFeO3 and can cause formation of metallic-like conduction paths whereas multiple flashes with lower energy density keep the insulating and memristive properties of BiFeO3 (BFO).
To understand these aspects in detail, we simulated the FLA of BFO using COMSOL. In a first step, we have constructed the geometry of a typical flashlamp annealing chamber (Fig. 1). Within this geometry, the Raytracing module of COMSOL has been used to study the distribution of the rays within the chamber. As a result after MATLAB-treatment of the raw data, we have obtained an polar information of the irradiance distribution depending on the angle of incidence (Fig. 2). This information can help to understand the incorporated energy on the sample surface/wafer edges in a better way. For example, the intensity of the light at an angle of 30° with respect to the wafer-normal is still 89% of the intensity compared with the intensity directly from the top (0°). At 45° and at 65° the intensity is reduced to 75% and 38%, respectively. This irradiance distribution causes overheating of wafers at positions with non-planar structure.
In a second step, the heat conduction through a wafer has been simulated with different underlying materials of different thermal conductivity. The motivation for these simulation is caused by the fact that BFO thin films on Si/SiO2/Ti/Pt substrates annealed by one highly energetic pulse do not show the expected memristive switching. Instead, BFO thin films on Si/SiO2/Ti/Pt substrates which have been annealed with multiple low energy pulses show memristive switching. In that case the sample lies on a thermally insulating SiO2 wafer which should prevent the flow of the energy in the underlying metallic plate. The strongly reduced cooling rate of the annealed wafer after each FLA pulse allows a step-by-step increase in temperature during multiple FLA pulses. The complete annealing process with 10 3 ms-pulses takes 6.6 seconds, but it is still more effective for BFO than alternative thermal annealing techniques. Fig. 3 presents the simulated surface temperature after 10 ms. Due to the transmission of the light, the surface of the SiO2 wafer remains quite cold. In contrast to that the surface temperature of the BFO thin film on Si/SiO2/Ti/Pt is enhanced. After 100 ms (Fig. 4), the temperature of the surface of the SiO2 wafer is increasing because the hot steel plate transfer a part of the energy to the SiO2 wafer. In general, the temperature of the BFO on Si/SiO2/Ti/Pt is enhanced with respect to the SiO2 wafer. Multiple pulses lead to a step-by-step increase in temperature which might be one reason for the success of the annealing of BFO.
Keywords: flash lamp annealing, BiFeO3, COMSOL Multiphysics
  • Lecture (Conference)
    Materials for Advanced Metallization, 18.-21.03.2018, Milano, Italy

Publ.-Id: 28669 - Permalink