Pulsed plasmas processes for nanoelectronics and emerging devices
Plasma processes have been used for more than 40 years in semiconductors manufacturing. They have provided a very accurate control of critical dimensions, enabling a continuous decrease of devices dimensions. However, conventional plasma processes are reaching today their limits to provide a perfect selectivity and a critical dimension control at the atomic scale. Indeed, even at the lowest ions energy attainable with conventional plasma etchers (fixed by the plasma potential at around 20 eV), significant damages on the materials exposed to the plasma are generated. Furthermore, the difference in directionality between ions and electrons leads to differential charging on the etched patterns, which in turn, leads to profile distortion. Finally, it is very tough to control uniformity across the patterns (i;e; different etching profiles between dense and isolated patterns) and across the wafer at the nanometer scale.
A major breakthrough is needed to overcome those issues and the LTM etch team proposes to develop an alternative technology based on pulsed plasmas. Pulsed plasmas could allow a control of the ions mean energy down to few eV, and thus minimizing plasma induced surface damages. Furthermore, during the OFF period of the plasma, negative ions and low energy positive ions participate in the neutralization of the differential charging generated on insulating material surfaces. Finally, the plasma chemistry is completely changed and can be controlled by pulsing the plasma. In particular, since molecule dissociation by electron impact occurs only during the ON periode while radicals recombination on the chamber walls take place during the whole cycle, the ratio of molecular to atomic radical densities is much larger in pulsed plasmas than in typical palsmas with very large consequences on process design.
In collaboration with Applied MaterialsTM, the LTM centura 300mm etch platform has been upgraded with full pulsing capabilities. The LTM plasma etching team is working today on three main topics related to pulsed plasmas:
- Fundamental understanding of pulsed plasmas
- This research work is based on time resolved plasma diagnostics (like UV-absorption and mass spectrometry for real time radical density measurements or Time resolved ion energy distribution function measurement using a SemionTM probe). It focuses on a fundamental understanding of pulsed plasma physics, necessary for the development of pulsed plasma processes
- Plasma induced surface damages
- The impact of pulsed plasmas on materials exposed to the plasma is investigated by means of in-situ ellipsometry and quasi in-situ angle resolved XPS. The aim is to understand the fundamentals of plasma induced surface damage and to evaluate the potential of pulsed plasmas in reducing those damages. Thus, pulsed plasmas could be used to develop a new etching concept: Atomic layer Etching processes in which the goal is to etch ultrathin layers under very soft ion energy bombardment conditions to reduce the damage caused by the plasma on a depth of about one monolayer.
- Application of pulsed plasmas to pattern transfer processes
- The potential of pulsed plasmas to solve current fundamental limitations of conventional plasmas in terms of profile control (charging effects, Aspect Ratio Dependent Etching Phenomena) is investigated by means of high resolution SEM and chemical topography analysis by XPS.
