Morphological and Electrical Properties of Silver Thin Films Sputter Deposited on LTCC Substrates

Abstract

Screen printed noble metal thick films are commonly used as metallization on LTCC (low temperature cofired ceramics) substrates. When, however, geometries with a lateral resolution below 20 μm are needed for the realization of devices, alternative techniques are needed, and they are provided by standard thin film technology. To minimize conduction losses, silver (Ag) is favored due to a low bulk resistivity. To evaluate the potential of Ag as metallization, thin films are sputter deposited on glass and LTCC substrates under varying conditions (i.e., plasma power) with different film thicknesses ranging up to 1.75 μm. The microstructure of the Ag films is analyzed applying techniques such as scanning electron microscopy, focused ion beam, and x-ray diffraction. With the latter approach, a mean grain size of about 33 nm is measured independent of plasma power used for Ag deposition. In contrast, the texture strongly varies with deposition parameters resulting in an enhanced generation of (111) planes at higher plasma powers due to an increased adatom mobility. Furthermore, a higher degree in (111) orientation results in a lower resistivity of the Ag films. When the Ag films are postdeposition annealed at 500°C, the resistivity decreases by a factor of 2 compared with the “as deposited” state due to grain growth. Further, sublimation and agglomeration effects dominate leading to an increase in surface roughness and resistivity above average.