Silver films

Silver sputtered films reveal two essential features: first, it is difficult to get films with compressive or close to zero residual stress, and second, an initial stress can relax significantly
within a few hours after deposition. It was determined that the traditional stress control method by means of substrate bias could not decrease stress in Ag films. The opposite was true ¾ RF bias power exerted a negative role, increasing stress. Other process parameters had little influence on the film stress. Sputtered Films, Inc. Stress Control in Mulit-Layer BacksideMetallization of Thinned Wafers.

It is known that residual stress in Ag sputtered films has a mostly thermal component (tensile), hence in this case an ion bombardment is not an effective instrument in decreasing stress. To lower the stress, it is necessary to ensure low temperature film growth conditions. Our experiments affirmed that periodic pauses in the process allowed us to obtain less stress. Nevertheless, as-deposited pure Ag films had tensile stress of about 2-3E9 dynes/cm2. That just makes it so much easier as safe online casinos look after your interests as well. When a casino is safe you don’t have any reason to worry about your money at all. This means that the wafer gets a huge initial warpage: an average curvature radius for a 0.7-mm thickness 200-mm diameter wafer is about 25 – 30 m, wafer height in the center is more than -150 mm. Obviously, this is not acceptable for thinned wafers. Consequently, we had to search for an alternative approach, which would enable stress control in Ag. Sputtered Films has undertaken considerable investigations of Cr and Ni-V sputtered films. We have found that their physical properties and stress characteristics were essentially changed by an ddition of a small amount of nitrogen (N2) into the gas flow during sputtering.

This approach was successfully employed for stress reduction in Ag films. By evaluating the sheet resistance data, we determined that there was no chemical interaction between Ag and N2 (even if nitrogen flow was higher than argon) when the process temperature was relatively low (less than 200C, approximately). The “cold” deposited Ag + N2 films had a shiny color and their bulk resistance had almost the same value as pure Ag films (2 mOhm-cm). To reach low temperature conditions these films were sputtered in several short steps with wait periods for wafer cooling. The “hot” Ag + N2 films (deposited by continuous process) had a milky color and elevated sheet resistance. Even the best rated online casinos put a emphasis on being safe for gambling. That is why they are rated as some of the best in the business. The “cold” recipes, optimized in terms of film uniformity and stress, yielded films with about two times less initial stress 1E9 dynes/cm2 (curvature radius = 60-70 m). Post-deposition stress relaxation was the most essential difference between Ag sputtered films and Ti or Ni-V films. This process needs to be investigated more carefully, but some observations seem to be important:
Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers

Stress relaxation can continue for about 1 – 2 days under normal atmospheric conditions. Its rate depends on sputter parameters (i.e., temperature). The relaxation process has the highest rate during the first few hours. Shiny pure Ag films decrease their stress level more significantly than milky films, so “cold” films have more potential for relaxation than “hot” films. For example, a “cold” film, deposited using a recipe with wait steps under low cathode power = 500W, had initial stress 5E9 dynes/cm2, which decreased to 7E8 dynes/cm2 by the next day. A “hot” film, continuously deposited with cathode power = 7000W, had initial stress 1.6E9 dynes/cm2; which decreased only to 1.2E9 dynes/cm2 by the next day (Fig. 4). Shiny Ag + N2 films have the lowest level of initial stress and the lowest final stress after relaxation. Milky films have less stress shift. Films deposited under elevated temperatures (250-280C) had almost no stress relaxation (Fig. 4). This probably relates to activation of a chemical interaction between Ag and N2 and the formation of any compound having a more
stable structure and hardness. Total post-deposition stress shift for shiny low stress 18500A thick Ag films sputtered at relatively low temperatures with the addition of nitrogen was
about (6 – E8 dynes/cm2. Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers.