Effect of sputtering process parameters on film properties of molybdenum back contact

Molybdenum back contact in CuIn1−xGaxSe2−ySy (CIGSeS) solar cells is usually deposited using DC magnetron sputtering. Properties of thin films are dependent on process parameters. Films deposited at high power and low pressure, tend to be more conductive. However, such films exhibit poor adhesional strength since the films are under compressive stress. Films deposited at low power and high pressure tend to be under tensile stress and exhibit higher roughness and resistivity, while the films adhere very well to the sodalime glass substrate. Therefore, it has been a practice to deposit multi-layered Mo back contact to achieve properties of good adhesion and higher conductivity. Deposition of multi-layered back contact results in either increase in deposition time if a single target is used or increase in foot print if multiple targets are used resulting in an increase in the total cost of production. Experiments were carried out to understand the effects of working pressure, sputtering power and working distance on molybdenum film properties with the final aim to develop a process recipe for deposition of a single molybdenum film with acceptable properties of both good adhesion and higher conductivity. Experiments were carried out by varying the working pressure and keeping the sputtering power constant and then varying the sputtering power keeping the working pressure constant. Adhesive tape test was performed on each film to determine the adhesional strength of the films. Moreover, the sheet resistance and the average roughness for each film were measured using a four probe measurement setup and the Dektak Profilometer, respectively. All experiments were also carried out on narrow and long glass strips in order to estimate the residual stress in the film using the bend test method.

► Residual stress is influenced by the kinetic energy of the incident atoms and Argon atoms.
► Resistivity was found to be dependent on working gas pressure.
► By varying the sputtering parameters Mo films with good conductivity, minimal stress and good adhesion can be grown.