No abstract

Understanding the consequences of local surface charging on the evolving etching profile is a critical challenge in high density plasma etching. Deflection of the positively charged ions in locally varying electric fields can cause profile defects such as notching, bowing, and microtrenching. We have developed a numerical simulation model capturing the influence of the charging effect over the entire course of the etching process. The model is fully integrated into ViPER (Virtual Plasma Etch Reactor)—a full featured plasma processing simulation software developed at Ilmenau University of Technology. As a consequence, we show that local surface charge concurrently evolves with the feature profile to affect the final shape of the etched feature. Using gas chopping (sometimes called time-multiplexed) etch process for experimental validation of the simulation, we show that the model provides excellent fits to the experimental data and both, bowing and notching effects are captured—as long as the evolving profile and surface charge are simultaneously simulated. In addition, this new model explains that surface scallops, characteristic of gas chopping technique, are eroded and often absent in the final feature profile due to surface charging. The model is general and can be applied across many etching chemistries.

In the last years, silicon micromachining techniques based on high aspect ratio reactive ion etching with gas chopping have been developed. There the gas flow of etching and deposition gas precursors is chopped which results in controllable sidewall passivation and high anisotropy. However, the rippled sidewalls are a serious limit for various applications. We report on the development of a novel gas chopping etching technique (GCET) process in order to achieve a smooth (rippled free) sidewall surface. As the direct etch mask, we used a 1 or 2-μm-thick resist layer, which was lithographically patterned. The novelty of the process consists in the replacing of the isotropic etching step by an anisotropic etching step. In this way we omit the main source for sidewall ripples. GCET combined with inductively coupled plasmas and fluorine chemistry provide very high etch rates and good control of the sidewall slope. These techniques also can be applied to conventional reactive ion etching equipment with Cl or F based plasma chemistry. However, the techniques used in this study have lower selectivity (in range of 30) than the conventional GCET. A SiON/Si selectivity as high as 50 has been achieved.

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