Development of etch and strip processes for BEOL materials
Research and development topics
The patterning in copper/low-k interconnect schemes appears exclusively with dry etch methods. With the progressive downscaling of structure dimensions the etch processes become more and more complex. Furthermore the materials for diffusion barriers and interlevel dielectrics change permanently, which results in continuous needs for new etch processes. The department Nano Device Technologies of the Fraunhofer ENAS works in the field of etch and strip processes for the copper/low-k integration for numerous years. The focus of the investigations has been to find and to optimize an applicable process for the copper dry etch and to investigate an etch process for porous and dense SiCOH materials with a minimum low-k and sidewall damage. The development of a BEOL materials compatible strip process is also one of our research topics. Finally we apply advanced in situ diagnostic methods to understand the physical and chemical mechanisms, which occur during the complex etch and strip of BEOL materials.
Etching of BEOL metals
- Development of etch regimes with high aspect ratios for copper lines using chlorine containing etch chemistries (see fig. 1)
- Improvement of sidewall geometries to avoid roughness and voids using polymerizing additives (see fig. 2)
- Optimization of the copper etch rate to reach moderate process times
- Etching of aluminium and the compounds AlCu and AlSiCu with a high uniformity and a good selectivity to oxide hardmasks and photoresists.
- Evaluation of etch recipes for diffusion barriers, e.g. Ti, TiN, Ta and TaN, with an excellent uniformity, selectivity to resists and hardmasks and wafer to wafer uniformity
Etching of BEOL dielectrics
- Development of etch processes for dense and porous SiCOH materials on the base of fluorocarbon plasmas, which provide an independent polymerization and etch mechanism (see fig. 3 c and d)
- Minimization of geometry irregularities in the etch profiles, e.g. bowing, undercut, microloading, using different additives, e.g. Ar, CO, N2, O2, and modified process conditions, e.g. pressure, ICP-power, bias power (shown in fig. 4)
- Reduction of low-k damage (see fig. 3 b) using the polymerization mechanism of the fluorocarbon plasma to protect the sidewalls, assisting and controlling of the protection mechanism with different additive gases
- Development of recipes to etch low-k cap layers and to open etch-stop films, e.g. SiC, SiCN, in the via bottoms with a minimal damage of the low-k-dielectrics and the sidewalls, respectively.
- Evaluation of etch recipes for thermal oxide, PECVD oxide and nitride with a good uniformity, a high selectivity to photoresists and a small consumption of underlying silicon
BEOL compatible resist removal
- Residue free removal of photoresists using oxidizing and reducing chemistry
- Development of strip processes for metals, e.g. AlCu, AlSiCu, with a high corrosion robustness
- Evaluation of strip chemistries and process conditions for the removal of photoresists and residues after low-k etch with a minimum low-k and sidewall damage
- Optimization of the strip processes
In situ diagnostics
- All etch processes can be analyzed with optical emission spectroscopy OES and quadrupole mass spectrometry QMS.
- For the dielectric etch, additional investigations with optical absorption spectroscopy OAS and Langmuir probe LP are possible.
- The strip processes can be controlled with a quadrupole mass spectrometer.
- More details are available in the section plasma diagnostics.
The department Nano Device Technologies of the Fraunhofer ENAS uses the commercial dry etch tool MXP Centura (Applied Materials). This tool is equipped with three different chambers, for dielectric etch, metal etch and resist strip. The dielectric etch reactor is an eMXP Plus chamber based on the MERIE principle. In this chamber fluorocarbon plasmas as well as SF6 und NF3 plasmas with different additives can be used. For the metal etch a DPS chamber is available. In this reactor aluminium and copper can be etched using different chlorine containing etch chemistries. The resist strip chamber is an ASP system, which uses a downstream microwave plasma and the typical oxidizing and reducing chemistries. The polymerizing compounds CF4 and CH4 as well as H2O are also available. Furthermore, an P5000 etch chamber is available for barried etch.
The tools are configured for 200 mm wafers. With special adapters smaller sample dimensions are possible. In cooperation with the Center for Microtechnologies additional etch tools are available for smaller wafer sizes and non-standard materials.