Conformal electrodeposition

Super-Conformal Electrodeposition of Copper into Nanometer Vias and Trenches... 

Defect-free deposits in vias and trenches known as super-conformal electrodeposition, may be achieved in the presence of certain additives. In super-conformal deposition, electrodeposition inside vias and trenches occurs preferentially at the bottom. 

Sub-conformal and conformal electrodeposition of copper in vias and trenches, on the other hand, occurs in an additive-free acid sulfate solution. These deposits have two types of defects voids and seams (Fig. 3 in Ref. 22). 

The inhibition factor used in the interpretation of leveling cannot describe the shape-change behavior in super-conformal electrodeposition of copper (see Refs. 22, and 47 Ch. 10, Section 10.6). 

Moffat et al." proposed the inhibition-acceleration mechanism in order to explain the experimentally observed comer rounding (inversion of curvature, Ref. 41, Fig 19) and general shape evolution in super-conformal electrodeposition of copper in vias and trenches of nanometer dimensions." These authors studied as well a three-additive system composed of two inhibitors and one accelerator. They concluded that super-conformal deposition and comer rounding might be attributed to competitive adsorption of inhibitor and accelerator. This model is based on the assumption of curvature enhanced accelerator coverage in vias and trenches. 

Two types of modeling have been used to interpret and optimize super-conformal electrodeposition of copper the deterministic modeling and the stochastic modeling... 

An interesting alternative to pnrely chemical conversion methods is electrodeposition, which also allows conformal coatings to be obtained. So far, however, only somewhat shallower strnctnres have been coated truly conformally, while deep strnctnres, such as those of nanoporous films, are usually only coated close to the top surface. Figure 6.10 shows a thin CdTe layer deposited on a microporous Ti02 substrate (Ernst et al, 2003). As is apparent, some smoothing of surface features occurs in the deposition process. 

Figure 6.10 CdTe deposition on microporous Ti02. (a) Scanning electron micrograph of bare Ti02 substrate (b) Ti02 substrate covered conformally by nanocrystalline CdTe (c) Cross-section of the Ti02/CdTe interface. A single grain layer is obtained in the electrodeposition.

Simulations of copper electrodeposition in sub-micron features in the presence of a leveling agent indicate that the formation of void-free deposits requires tight control of the operating conditions. For very small features, primarily one dimensionless group (equation 7) dictates the leveling capability of a process. Results also indicate that as feature size is reduced, the deposition tends to become conformal unless the additive chemistry is modified. It is proposed that conformal deposit is not desirable because random variations in deposition rate will lead to void formation in a statistically significant number of features on a wafer. 

While electrodeposited copper represents considerable promise for HDI and VLSI applications, simple insertion of the additive chemistry or PRC processes developed for PTHs application are not likely to be successful. Furthermore, while new additive chemistries may initially be successful, the extreme tolerances and associated control issues, impurity incorporation, and waste associated with CMP prohibit the chemistry-only approach. By considering the fundamental differences associated with the PTH and HDI as well as VLSI applications, we have developed a modulated reverse electric field process (MREF) for copper electrodeposition. In contrast to the long cathodic duty cycle-short anodic duty cycle used in the PRC process, the MREF process consists of a short cathodic duty followed by a long anodic pulse. By tuning the frequency and the cathodic to anodic charge ratio (Qc/Qa), conformal and filling capability are demonstrated for vias and trenches in the 0.5 to 100 pm size range. 

Waterborne (WB) systems, already well established as conforming coatings, will increase from 20% in 1982 to 33% in 1987, Electrodeposition (ED), the preferred method for automotive priming, has been widely accepted. 

Figure 8.2 Schematic Illustration of key steps for preparing hybrid graphene/Mn02-nanostructured textiles, high-performance EC electrodes, (i] Conformal coating of solution-exfoliated graphene nanosheets [gray color] onto textile fibers, [ii] Controlled electrodeposition of Mn02 nanoparticles (yellow dots] on graphene-wrapped textile fibers. Reprinted with permission from Ref 26. Copyright 2011 American Chemical Society.

---