Hello Gürel,

may be , you have an optical effect. Then, the reflectivity must depend on the thickness of the upper layer.

Aluminium has a high reflectivity(metallic). Germanium is a dielectric layer (conductive, if doped, but in this respect (reflectivity) dielectric). We can apply Fresnel's laws. Furthermore we must take into account the gap of Ge (nearly 0,7 eV). That means a high absorption for visible radiation.

If the layer compares to the wavelength of light (what is fulfilled at sputter deposition, then the light can penetrate the Ge-layer and is reflected at least one time from the Al-surface. The light is reflected and transmits the Ge-layer but is almost strong absorbed. Therefore, the intensity is damped what gives a grey effect. Besides this you have an interference effect (like the UV-protection layer on photo objectives (there you have a blue-dark vision). This erases the outcoming light and gives a dark effect (must depend on incident angle).

For thick layers only the reflectivity determines the optical appearance. Furthermore, the surface quality determines the surface scattering. For rough surfaces (structure elements comparable with wavelength) you don't have bright (directed light) but dull (scattered light) surfaces.

With Regard

R. Mitdank

a new target require conditioning, meaning to run it a few dozens of times on blank wafer/substrate and then test again. Keep running till you get some topography on the face of the target.

Hi Gurel,

As mentioned above, you need to condition the targets after exchanging them or when you have opened up the system to air. It might also be a roughening as suggested of the surface which gives you a greyish appearance. The only other explanation I can think of is that the temperature of the substrate becomes to high during sputtering and the Ge and Al start to alloy.