Design of optimal stress compensated Ti / Ni-V / Ag stack To create a low stress Ti / Ni-V / Ag stack it is necessary to: choose the initial stress level in Ag top layer,
define the stress values of the Ti and Ni-V under-layers based on the Ag stress level, evaluate stress evolution in the stack due to the relaxation processes. Note that stresses in Ti and Ni-V films do not change their values with time. So stress relaxation in the stack primarily results from the self-relaxation processes in the Ag layer and probably the relaxation processes activated in the Ag layer by distending forces from the compressive stressed Ti layer. Possibly the wafer’s elastic deformation, induced by metallization, also assists in Ag
film stress relaxation. The first step in designing an optimal stress compensated stack depends on the kind of silver layer we employ¾either pure Ag (as-deposited stress is in the range of 2 – 3E9 dynes/cm2) or Ag with N2 (1E9 dynes/cm2). Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers
In the case of the pure Ag layer, our experiments have shown that the as-deposited stress in the stack must be about 1E9 dynes/cm2, which will be decreased to a value close to zero during the day (Fig. 5). This means we have to deposit the Ti layer with deep compressive stress of about – 8E9 dynes/cm2 (we consider that Ni-V layer has low tensile or low compressive stress). In the case of Ag deposition with addition of N2, the optimal stress compensated stack can be created with the following initial stress values (Fig. 6):
Post-deposition stress relaxation in Ti-Ni-Ag metallization (silver sputtering with the addition of nitrogen) Process qualification for thinned wafer treatment
The backside metallization process described above, was first investigated and developed on standard thick wafers (0.7 mm). A low stress stack, designed with the employment of an interlayer stress self-compensation approach, was realized with good repeatability. Next, the process was adjusted for thinner, 0.35-mm thick wafers. We found that the temperature during Ni-V and, especially, Ag film formation, was a very important factor in defining the stress level in the stack. For thinner wafers, it must be ensured that a lower level of heat flows to the growing films during the deposition process in order to reach the same positive result. The optimal process for 0.35-mm wafer treatment enabled the following stack characteristics: residual stress after relaxation period was less than 2E8 dynes/cm2, wafer curvature radius >100 m, and wafer height in the center < 50 mm.
Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers
Unfortunately, we could not measure stress directly on the 0.2-mm thick product (patterned) wafers because of their high initial curvature radius. However, because the lower heat capacity of the product wafers (as compared to the monitor wafers), should lead to the development of higher process temperatures and, consequently, a total stress shift in the tensile direction; we decreased the pre-heat temperature to 370C and increased the wait step duration during the Ag deposition. Post-deposition manufacturing operations with these samples were successful and confirmed that wafers did not receive any additional warpage.
Conclusion
We have shown that the stress levels in Ti and Ni-V films can be varied from compressive to tensile and therefore a range of near zero stress films can be produced. An inter-layer stress compensation approach has been developed for the sputter deposition of 3-layer low stress backside metallization of thinned to 0.2-mm thick, 200-mm diameter wafers. The Ti / Ni-V / Ag stack, consisting of films strained in opposite directions, had residual stress (after the selfrelaxation period) of less than 2E8 dynes/cm2, which enabled very low wafer warpage. Acknowledgments The technical staff of Sputtered Films, Inc. is gratefully acknowledged for the efficient support with the sputter tool qualification for the thinned wafer treatment.