Enhanced resputtering and asymmetric interface mixing in W/Si multilayers
During growth of multilayers by pulsed laser deposition (PLD), often both intermixing and resputtering occur due to the high kinetic energy of the particles transferred from the target to the substrate surface. In order to obtain a fundamental understanding of the underlying processes, W/Si multilayers have been studied by the complementary methods of transmission electron microscopy (TEM), X-ray reflectivity (XRR) and in-situ rate monitoring. For the experiments, deposition conditions were chosen that result in high energetic Si ions and mainly low energetic W atoms for the multilayer growth. Under these conditions, interface mixing of up to 3 nm occurs at the W/Si interfaces, while the Si/W interfaces remain sharp. Furthermore, enhanced resputtering of Si leads to a Si thickness deficit of up to 2 nm at the W/Si interfaces. The presented results can be understood by a combination of theoretical calculations as well as SRIM and TRIDYN simulations, which match perfectly to the experimentally obtained intermixing and enhanced resputtering of Si at the W/Si interfaces.
Pulsed laser deposition (PLD) is a thin film growth technique, which allows the preparation of multilayers for a wide range of material classes [1, 2]. It is well known that the microstructure of laser deposited films often strongly differs from those of sputtered or evaporated films . This is mainly caused by the occurrence of high energetic ions with energies of up to 100–150 eV besides atoms with energies of 5–10 eV [4, 5]. Thus, the mean energy of the deposited ions is higher than the Wigner energy which is in the range of 10–25 eV for most materials . The fraction of ionized particles in the plasma plume strongly depends on the laser fluence used for material ablation from the target surface and can reach a value of more than 90 % for high laser fluences . As a consequence, implantation into the already grown film, intermixing at multilayer interfaces and resputtering effects occur [3, 7].
During sputter deposition it is well known that sputtering yields of light elements from the target surface can be enhanced by alloying a small amount of heavy atoms (e.g. W) to the target material . This effect originates in a reduction of the implantation depth of the light element due to an increased stopping power and is known as “sputter yield amplification” (SYA) . Choosing conditions that allow depositing light particles energetically on a layer of a much heavier material, such a SYA-effect should also be observable at the interfaces of laser deposited multilayers. In order to control both resputtering and intermixing, a fundamental understanding of these effects is essential. Furthermore, for many thin film applications it is important to ensure both sharp interfaces and accurate layer thickness, for instance during preparation of X-ray multilayer mirrors or multilayer zone plates [10, 11].
Aim of this paper is to study resputtering and intermixing in W/Si multilayers grown under conditions, where the light Si particles with a mass of 28.1 atomic mass units (amu) are deposited with high kinetic energy, while the much heavier W atoms (m=183.9 amu) are deposited with low particle energy. Using these conditions, intermixing and resputtering should only occur at the W/Si interfaces (i.e. when Si is deposited on W) but not at the Si/W interfaces. As a consequence, multilayers with highly asymmetric interfaces should be achieved. By implanting Si particles into the W layers, the SYA-effect of Si might be observable as well. For these studies, the complementary methods of transmission electron microscopy (TEM), X-ray reflectivity (XRR) and in-situ rate monitoring are used, and the obtained results are compared to theoretical calculations as well as to SRIM and TRIDYN simulations.
W and Si thin films as well as W/Si multilayers were deposited onto Si(111) substrates by pulsed laser deposition using a standard setup . The films were grown at room temperature at a target-to-substrate distance of 7 cm. KrF excimer laser pulses (wavelength of 248 nm, pulse duration of 30 ns, repetition rate of 10 Hz) were focused onto the W and Si targets, respectively, in ultrahigh vacuum (<10−8 mbar). By both changing the distance of the focusing lens to the target and the laser energy used, the laser fluence could be controlled in a range of 1–5 J/cm2. Here, this variability of energy density is particularly important due to the strongly different melting points of W and Si leading to unequal deposition conditions for both materials.
In-situ rate measurements were realized by depositing the ablated material onto a microbalance quartz crystal (Inficon SQM 160). X-ray reflectivity (XRR) measurements were carried out using a Philips X’pert diffractometer with CoKα-radiation. Transmission electron microscopy (TEM) images were taken with a Philips CM30. Theoretical calculations were done using SRIM (Stopping and Range of Ions in Matter)  and TRIDYN software (dynamical Monte Carlo simulations) .
Also, it should be mentioned that the amount of Si layer deficit at the W/Si interface can slightly be increased by raising the laser fluence of W as well. At first view, this might be surprising, but can be understood in the following way. The higher the kinetic energy of W particles when reaching the film surface, the denser the W layer becomes. This effect is typical for PLD and is known as densification of the deposited material by “shot peening” . As a consequence, due to the dense W layer incoming energetic Si particles are deposited closer to the surface and thus resputtering is enhanced.
Here γ is the coefficient of energy transfer, given by γ=4MAMB/(MA+MB)2. Using the surface energies US of 4.7 eV and 8.7 eV for Si and W, respectively, the threshold values Eth for resputtering are calculated to Eth(Si)=100 eV and Eth(W)=130 eV for Si and W, respectively. This indicates that a kinetic energy of the Si ions of 100 eV is sufficient for resputtering. In contrast, Si can hardly be resputtered by W particles as only a small fraction of the W particles is ionized and the average kinetic energy of the W ions is about 30 eV smaller than needed for resputtering. Based on these calculations it can be concluded that sputtering of Si by W particles at the Si/W interfaces can be neglected and the Si deficit seen in the XRR measurements originates from resputtering at the W/Si interfaces, which is in agreement with the rate measurements.
In summary it has been shown that both implantation and enhanced resputtering occur in W/Si multilayers due to energetic deposition of Si particles and can be studied in detail by the complementary methods of ex-situ TEM, XRR and in-situ rate monitoring. While implantation of Si into the W layers leads to an intermixing range of about 3 nm, a Si deficit of up to 2.5 nm is observed at the W/Si interface due to enhanced resputtering of Si. Both effects can be clearly understood by a combination of theoretical calculations, SRIM and TRIDYN simulations, which further allow determining the implantation depths and concentration profiles during film growth. The deficit in Si layer thickness is mainly caused by a sputter yield amplification of Si, when the light Si particles are implanted more closely below the film surface due to the occurrence of much heavier W atoms. Also it was demonstrated that using laser fluences close to the corresponding deposition thresholds, multilayers with sharp interfaces can be realized as then the kinetic energy of most deposited particles is lower than the energy threshold necessary for implantation.
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 755 and SFB 602).
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