Plasma etching technologies

The etch group of LTM has set up over the years a specific positioning of its research activities, between academic type of research and industry. More precisely since more than 15 years now, LTM is working on the development and characterization of etch processes using similar etch tools than those used in the industry but with unique in situ characterization capabilities.
This position in the field of plasma processing was possible thanks to a network of strong cooperation’s set up with LETI and STMicroelectronics in France and Applied Materials in the US. The collaboration with Applied Materials was fruitful not only because LTM has been able to get state of the art tools from Applied Materials through several Joint development programs between 1999 and today but also because Applied always had the strong will to get fundamental information on processes that they could not get in house. In other words, thanks to Applied engineering teams, the etch cluster tools were modified with the objective to set up a wide variety of diagnostics designed to monitor plasma gas phase in real time and plasma/surface interactions. Using minor modifications of etch chambers, in situ diagnostics such as real time ellipsometry, VUV absorption, mass spectrometry, ion flux probe have been successfully installed on several etch chambers to monitor plasma etch processes on 200 and 300 mm diameter wafers. At the same time, XPS analysis chamber have been connected to the 200 and 300 mm etch cluster tools run by LTM in the LETI clean room, giving a unique potential to the LTM etch group.
In 1999, the first 200 mm etch cluster fully equipped with the in situ and quasi in situ characterization techniques mentioned above was installed in the LETI clean room. In 2005, a 300 mm etch cluster tool was set up with the same capabilities while in 2014, a brand new 300 mm etch cluster tool from Applied Materials will be set up with state of the art etch chambers recently designed by Applied Materials. As before, in situ characterization techniques will be a strong differentiating fact or enabling us to generate a fundamental understanding of etch processes used in the semiconductor industry.
Research activities of the LTM etch group have been strongly aligned with the CMOS technological roadmap, the aim of the group being to get a good understanding of processes used in the industry or improve state of the art processes or develop specific characterization techniques. Those research activities were strongly supported by our industrial partners: Applied Materials as discussed above or STMicroelectronics through industrial contracts allowing LTM to recruit PhD students and post-docs that were after, in some cases, hired by STMicroelectronics. Having STMicroelectronics as a partner has been crucial for the LTM etch group since STMicroelectronics, leading edge company in the field of Nanoelectronics, was able to provide the most complex and advanced stacks of materials used for Front End and Back End applications.
In final, LTM etch group was able to pursue a leading edge and relevant work in the field of plasma processing only thanks to those industrial collaborations and capabilities to work on the most challenging patterning technologies.
Among the most important contribution of LTM during the past 15 years, the all research activity performed on the etching of advanced gate stacks was particularly percussive. A good comprehension of passivation layers formation during silicon etching in different chemistries such as HBr/Cl2/O2, SF6/CH2F2 was brought and correlations between passivation layers and critical dimension control was established (1999-2006). With the introduction of metals and high K in advanced gate stacks, other interesting contributions were made, in particular the High K etch mechanisms in BCl3 chemistries was well understood while SiCl4 was introduced as an interesting alternative to etch High K materials. Comparison between resist mask and thin hard mask were also investigated and impact on critical dimension control of in hard mask introduction was clearly pointed out. A very significant work was also performed on resist roughness (LWR) leading to the development of noise free metrology techniques based on high quality CDSEM pictures combined with PSD analyses and recently an AFM based metrology technique was developed to accurately monitor Line Edge Rougness of pattern sidewalls. Those metrology were precious to bring key contributions on the understanding of HBr plasma cure treatment to minimize resist LWR. In particular, the role of VUV light on LWR reduction during resist plasma cure processes was clearly pointed out, leading to the development on H2, HBr/O2 powerful plasma cure treatments.
In the field of Back End processes, LTM contributions were also at the cutting edge of plasma etching. In particular, major contributions in the field of low K etching and porous Low k etching were performed. A good understanding of the influence of porosity on material degradation and roughness formation during plasma exposure was clearly explained. A new technique, called water scatterometric porosimetry was introduced that allows the precise monitoring of the porous Low k modification thickness on the pattern sidewalls. Consequences brought by the introduction of TiN hard mask were also fully analysed: Ti based residues formation during the etch process and post etch air exposure were analysed. Furthermore, a CH4 based plasma post-etch treatments was introduced, allowing the full Ti based residues elimination, CFx elimination from sidewalls and bottoms of dual damascene porous Low k structures after etch as well as sidewalls pore sealing capabilities.
In the last four years, special emphasis was put on the understanding of pulsed plasma technologies introduced by Applied Materials on manufacturing etch tools. Even if pulsed plasmas have been around for decades in academic laboratories, the introduction of this technology to improve process performance was a real breakthrough brought by Applied Materials.