Low-Pressure CMP Developed for 300 mm Ultralow-k

Researchers at Japan's Selete, a research consortium focused on 300 mm process technology, have developed a new low-pressure chemical mechanical planarization (CMP) process that works with ultralow-k dielectrics. Some of the most promising ultralow-k dielectrics (k<2.0) are quite porous with low mechanical strength, which can cause problems during CMP due to the high mechanical stresses involved. Using a combination of dummy structures and low-pressure CMP, Selete researchers appear to have overcome this problem. The results are scheduled to be presented at next month's International Electron Devices Meeting (IEDM) in Washington.

For the basis of their study, the researchers used p-MSQ (porous methyl silsesquioxane)/organic polymer films in a dual-damascene structure. To avoid ashing damage, a dual-hard mask process was used. Two kinds of p-MSQ SOD (spin-on dielectric) films were studied. The first had a k value of 1.8 and a modulus of 1.6 GPa, and the other had k=1.6 and about 0.8 GPa, respectively. Organic polymer SiLK (k=2.7) was also used for the via layer.

The researchers report that low-pressure CMP effectively minimized the amount of erosion and dishing, especially at large and high-density patterns. It also suppressed galvanic corrosion. To avoid the instability, 1.5 psi (10 kPa) was finally selected as the optimal pressure. At pressures below 1.5 psi, the researchers observed a difference in the removal rate for polished and unpolished copper films due to the native oxide film. Also, the within-wafer non-uniformity tended to degrade as the pressure decreased. Furthermore, lower pressure increases possible damage to the low-k film due to the longer polishing times.

A helium-plasma treatment was carried out on the p-MSQ surface before cap-SiO₂ film deposition to improve adhesion. This modified the low-k film surface (30 nm thick) without degrading the k value, and suppressed the delamination speed of the low-k film by about one order of magnitude.

The low-k film delamination probability depended on the shape and density of a dummy pattern in the field area. The delamination probability was calculated by counting the delaminated test patterns from the 112 die within the wafer. The dummy pattern with a density of >10% had high resistance to delamination for all types of dummy shapes. The dummy pattern was found to prevent p-MSQ film delamination extending to the surrounding area from the defect.

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