Environmentally compatible silicon oxide-based slurries for microelectronics manufacturing

In today's standard transistor production, the individual devices are electrically separated from each other by purposefully produced deep trenches of insulating silicon oxide. In order to adjust the dimension of the isolation trenches to nanometer accuracy, a chemical mechanical planarization (CMP) process is required. Today, this step uses a polishing suspension (slurry) based on cerium oxide abrasive particles. Since cerium is a rare earth element whose oxide is suspected of being carcinogenic, tests are underway at Fraunhofer IPMS with more environmentally compatible alternative slurries based on silicon oxide.

Since silicon oxide is removed during the manufacturing process and the topography is adjusted with the help of a polishing stop layer made of silicon nitride, (at least) two parameters are of central importance for evaluating a slurry: the oxide removal should be sufficiently high to enable a high process throughput. A high selectivity of the removal is required to stop the process when the polishing stop layer is reached.

The comparison of the oxide removal of two ceride and one silica slurry can be seen in the following diagram:

Motivation

Chemical Mechanical Polishing (CMP) is a critical process in semiconductor manufacturing, but it is also one of the processes with the second highest CO₂ footprint. The abrasive particles used are usually silicon oxide (silica) or cerium oxide (ceria). Ceria, a rare earth element, causes a significantly higher CO₂ footprint and is also suspected of being carcinogenic. The global supply of ceria is critical.

In advanced technologies, polishing steps that selectively stop on a silicon nitride (SON) layer are often performed with ceria slurries. The results of the CMP process are highly dependent on the patterning of the chips. Currently, the production of diverse chips with silica-based SON polishing steps is not possible, which limits the diversification of microelectronics and increases the need for different chip designs.

State of the art

In chemical mechanical polishing (CMP) for stop on nitride (SON) structures, it is necessary to precisely remove silicon oxide via silicon nitride. High line densities lead to longer polishing times until the active oxide is completely removed. In areas with low densities next to high densities, polishing times may be longer than necessary, emphasizing the need for excellent functionality of the SON.

A wide density range results in a high input topography, which increases the demands on the SON. There is a variation in process results depending on the structure densities. For a wide density range within the target window, a Ceria slurry is required, while a Silica slurry can only be used for a very narrow density range in the target window.

Cerium oxide-free STI CMP

The polishing process takes place in two steps: pre-polishing and final polishing.

• Pre-polishing: The structures are leveled and the difference between high and low densities is reduced.

• Final polishing: This includes SON polishing with a selective, environmentally friendly silica slurry.

The optimization of pre-polishing aims to reduce the topography, which lowers the demands on the SON. There are three options for improving pre-polishing:

1. More oxide for longer polishing.
2. Lower polishing pressure.
3. Dividing the polishing time into 30-second intervals.

These measures lead to a reduced topography (WIDNU) and reduce the density dependency. A lower density dependency and the topography remain even after SON/final polishing, which correlates with WIDNU.

Conclusion on the process

The final polishing result can be significantly improved by adapting the pre-polishing process. The limitations caused by silica abrasives - in particular the greater density dependency - can be addressed by making specific adjustments to the pre-polishing process. 

There are various pre-polishing options that offer variance in process customization depending on the specific requirements of the products required. This enables better control over the topography and density dependency, which leads to optimized results in the final polishing process.