Damascene Technique in VLSI

The Damascene Technique for Finely Structured Metallization Layers

The Damascene technique is used in the back-end-of-line (BEOL) for the final planarization of metallization structures. It involves etching a recess in an oxide layer, filling it with metal, and then using CMP to remove excess metal, leaving only the interconnect layout embedded in the oxide trench.

The Damascene technique is a valuable method for producing finely structured metallization layers with high planarity. This planarity is important for efficient routing in layout design, especially in highly complex digital circuits where automated routing procedures are used.

Damascene process steps

1. Planarization with CMP: The Damascene process begins with chemical-mechanical polishing (CMP). During CMP, the wafer’s surfaces are pressed against a rotating polishing table while etching agents and polishing compounds are continuously supplied. This process efficiently removes any elevations on the wafer’s surface, resulting in a continuous flat surface across the entire wafer.
2. Recess Etching: In the Damascene technique, the first step is to etch recesses in an insulating layer on the wafer’s surface. These recesses serve as the initial structure for metallization.
3. Barrier Layer Deposition: After creating the recesses, a barrier layer is deposited within them to prevent diffusion and oxidation. Common materials for this barrier layer include tantalum (Ta), tantalum nitride (TaN), and titanium nitride (TiN). This layer acts as a protective liner.
4. Copper Deposition: Copper, which is an excellent conductor of electricity, is deposited to fill the recesses. Copper can be deposited electrochemically, with a thin copper layer applied first to facilitate attachment of more copper atoms.
5. Excess Copper Removal: The excess copper on the wafer’s surface is removed by CMP. This step ensures that only the copper within the recesses remains, forming the desired interconnect pattern.
6. Protective Layer Deposition: To protect the copper interconnects against diffusion and oxidation, a final protective layer, often composed of nitride (Si3N4) or amorphous silicon nitride (SiNx), is deposited.
7. Repeat for Layer Pair: The Damascene process is typically performed twice in succession for a metallization layer pair. The first cycle creates contacts/vias, while the second cycle forms the interconnects. Each cycle follows the same sequence of steps mentioned above.

Advantage of the Damascene technique

The Damascene process offers several advantages, especially when working with materials like copper. Copper is unsuitable for dry etching, making the Damascene process an excellent choice. Moreover, it allows for the embedding of metal within etched insulating layers, which is critical for advanced semiconductor manufacturing. This process can accommodate various insulating materials beyond oxide, including low-k dielectrics to minimize parasitic capacitances between interconnects.

Drawback of the Damascene technique

However, one significant drawback of the Damascene technique concerning layout design is its use of Chemical Mechanical Polishing (CMP). CMP involves removing materials from the substrate, and the ablation depth depends on the properties of the material being removed, which can be a mixture of different types, such as silicon/oxide or metal/oxide. The challenge arises when these different material types are distributed inhomogeneously throughout the layer. As a result, the amount of a specific material removed can vary across the substrate, leading to unwanted surface irregularities known as “indentations” or “hillocks.”

To address these surface issues, special design rules called density rules are established. These rules prescribe a mean density representative of the materials to be removed, and it is a function of the surface area. By adhering to density rules, the likelihood of surface irregularities can be minimized.

Complying with density rules often requires additional work in the layout. If the quantity of a particular material is too low in a specific region, filler structures without any electrical function may need to be introduced to increase density. While semi-automated algorithms can sometimes assist in this process, it may not always be feasible. On the other hand, reducing material density can be equally challenging, potentially requiring slots to be cut in wide interconnects or increasing spacings between existing structures, which consumes more surface area—an undesirable outcome.

To avoid costly and time-critical refinishing, it is essential to apply density rules in the early layout phases so that design decisions can be made wisely. However, it’s important to note that density rules can only be fully verified at the end of the physical design process. Efficiently handling density rules typically requires a wealth of experience and expertise.

In conclusion, the Damascene technique offers a powerful solution for finely structured metallization layers, but its reliance on CMP and the challenges with density rules in layout design necessitate careful planning and expertise to achieve optimal results.