The vehicle we will use in this thesis to manipulate and enhance the interaction of x-rays and matter are thin-film nanostructures. There is a wealth of modern techniques to fabricate these sorts of samples. The choice of the technique depends strongly on the desired properties of the thin-films. For purely monocrystalline thin films, for example, molecular beam epitaxy or pulsed laser deposition are the most advantageous [107]. For x-ray reflectivity experiments in grazing incidence, however, other features are more important and meaningful, chief among them a low roughness of the thin films [108, 109, 110]. The necessary quality can be achieved by the most important workhorse technique for thin film deposition: sputter deposition [107, 111]. Used widely in industrial applications, sputtering employs accelerated ions to vaporize single atoms of a desired species from the target and steers them onto the sample material. The process is sketched in Fig. 9.
8. Sample Fabrication 27
Figure 9:A sketch demonstrating the operation principle of a sputtering setup. The sputtering target containing the material to be deposited and the substrate on which the thin film is to be deposited are placed above a cathode and anode respectively. A voltage is applied between them. The whole setup is placed in a low-pressure Argon atmosphere. Argon atoms are ionized by cosmic radiation, and accelerated by the applied field. After some acceleration, they impact on the sputtering target. The ensuing momentum transfer between the ions and the atoms of the target material leads to the latter being ejected and traveling to the substrate.
The process can be well-controlled, which means it can be used to fabricate films as thin as a single monolayer of a given material.
III.8.1. DC sputtering
In this case, the voltage between the cathode and the anode is constant. Free electrons coming from arbitrary external sources such as cosmic radiation are accelerated towards the anode. On their way there they collide with Argon atoms and ionize them, forming an Argon plasma. The ions in turn are accelerated towards the cathode. Upon collision, secondary electrons are excited and move towards the anode, to repeat this process. In this way, the plasma is stabilized. Some of the ions will extract atoms or clusters from the target. These atoms now have a strong momentum
directed towards the substrate which is placed over the anode. There, they assemble and form first clusters, and later thin films. DC sputtering requires the sputtering target to be conducting, otherwise the current keeping the plasma stable would break down.
III.8.2. RF sputtering
This technique uses the same setup, but the DC voltage is replaced by an AC voltage with oscillates with a radiofrequency (RF), typically some MHz. The high frequency is mandatory, because under a certain treshold of about 50 kHz, both electrodes would alternate as sputtering targets. Above the treshold, free electrons start to oscillate and ionize the sputtering gas by impact ionization.
There is no current flow by gas discharge between the electrodes, but there are still argon ions which can sputter the material off the target onto the substrate. Since there is no current, RF radiofrequency sputtering permits sputtering with non-conducting materials.
hold, free electrons start to oscillate and ionize the sputtering gas by impact ionization. There is no current flow by gas discharge between the electrodes, but there are still argon ions which can sputter the material off the target onto the substrate. Since there is no current, RF radiofrequency sputtering permits sputtering with non-conducting materials.
Beyond these founding principles, HF sputtering requires some refinements. The electrode on which the sputtering target is mounted is wired in series with a capacitor. This is necessary because at typical frequencies of the AC voltage the argon ions are almost stationary, while the electrons are highly mobile. The capacitor leads to a higher net negative charge of the electrode it is wired in series with, a process known as self-biasing. Averaged over time, this self-bias functions as an effective negative voltage ensuring that the sputtering takes place on the target.
III.8.3. Magnetron sputtering
A further refinement valid for both AC and DC sputtering techniques is magnetron sputtering.
A permanent magnet is mounted under the cathode. The resulting Lorentz force acting on the secondary electrons originating from the source drives them in a spiral trajectory around the axis connecting the electrodes. This leads to a greater number of collisions and ionised atoms than a straight trajectory would.
Magnetron sputtering is performed on round targets. The spiral trajectory of the secondary electrons induces the risk that the sides and back of the target holder instead of the actual target disk are sputtered off. Naturally, this gives rise to a strong degradation in thin film quality. The solution to this problem is to include a so-called dark-field screen which has to be placed extremely
9. Reflectivity measurements 29
Figure 10:Setup for the reflectivity measurements. The sample is mounted on aΘ−2Θ-goniometer, and the angle is varied with respect to the incoming beam. At the same time, the detector angle with respect to the beam is changed by double the amount. Slits are used to minimize the beamsize, which results in a better quality and less divergence of the beam, but also enhance the times necessary to get high quality reflectivity curves.
close to and around the sample holder. It has to be close enough to keep the gas ions from being accelerated against the side of the target holder, which would sputter off the latter and result in a degradation of the sample quality. When a gas ion collides with the substrate, the energy transfer usually, that is in 75% of cases, results in heating of the substrate. The target has to be cooled well during the sputtering process, otherwise the ensuing heat can reduce the permanent magnetic field of the magnetron gun, which would deteriorate the sputtering rates dramatically. Only 1% of the deposited energy is transfered to a target atom or cluster being detached.