DC magnetron method
Leica Microsystems developed a sputter coater which is based on the DC magnetron method – the Leica EM ACE600 device. A schematic of the principle is shown in Figure 1. Argon ions are accelerated towards the target materials by an applied electric field. This accelerated projectile generates neutral target atoms which fall down on the sample to form a thin conductive film. Besides the ejection of target atoms, secondary electrons are emitted by the impinged argon ions. These electrons are trapped inside a region near the target surface due to the additional annular magnetic field. The trapping of these electrons is the big advantage of this method. The electrons are forced to move along a circular path. This causes a drastic enhancement of the way inside the gas and leads to an increase deposition rate of the target atoms by using a much lower argon pressure.
Another advantage is that the local temperature increase on the sample is less than for other sputter coaters because the electrons which heat up the sample when they impinge on the surface are trapped in the magnetic trap near the target surface.
Till an even film can be formed, there are several growth stages, which are depicted in Figure 2. One of the first stages is called nucleation. In this process grains are formed. Grains are clusters of atoms. Afterwards the grains grow further till they reach a critical radius. In a next step these grains minimise their surface by forming clusters, now called islands. Due to the minimisation of the surface area, channels and holes occur which have to be filled before the even film can be formed. Atoms inside the holes form again grains which fill these holes (secondary nucleation). All these effects are mainly based on the mobility of atoms on the surface and this diffusion is mathematically described by the surface diffusion . Equation 1 shows the expression for the lateral surface diffusion D [m²/s] which depends on the lattice constant a [m], the amplitude of the diffusion frequency v0 [1/s], the diffusion energy ED [eV], and on the temperature of the sample TS [K].
Challenges and improvements
The resolutions of high end SEMs drastically increased during the last years, what leads to new upcoming problems. One of them is that previously used preparation methods are at their limits. The conductive layer has to fulfil more requirements at high magnifications than for magnifications which are used for investigations by using conventional SEMs.
An example of a thin gold palladium film is shown in Figure 3.
At high magnification thin films show a structured surface which is not applicable for high resolution. The ditches which can be seen in Figure 3 (right) leads to a higher surface resistivity. This causes drifts and charging effects during image acquisition by high magnification. Therefore the morphology of thin films has to be improved for using it.
Characterisation of the device
Before the sputter coater is optimised for high resolution applications, the verification of the local temperature increase on the sample is varied. Especially the heat impact on the sample is important for a lot of nonconductive samples because even perfectly formed layers are useless if the surface is modified by the temperature. For these measurements a thermocouple is located on a model sample to monitor the temperature during the sputter process. Figure 4 shows the local temperature increase by increasing the current (upper) and the working distance (lower graph).
The graph below shows the comparison between the maximal and minimal working distance (WD) and it can be seen that the WD at the highest level leads to a sputter process which causes nearly no local temperature increase.
Besides the local temperature increase at the sample, the thickness values which are displayed during the sputter process were verified to check how precisely thin films are sputtered by Leica Microsystems’ sputter coater. Therefore, a multilayer system with different thickness values is sputtered which consists of two different materials. In t