Paper PS2-TuA7
Selection of non-PFC Chemistries for Through-Silicon via Etch

Tuesday, October 29, 2013, 4:00 pm, Room 104 C

The continued extension of Moore’s Law, which dictates that the density of integrated circuit (IC) devices doubles every two years, presents formidable challenges in realizing complex and three-dimensional interconnect structures. Through-silicon-via etch (TSV) is at the core of 3-D integration, which yields higher performance than conventional 2-D wiring systems, and has been demonstrated with Bosch deep reactive ion etching (DRIE), cryogenic DRIE, laser drilling, and wet etching. In order to achieve the desired and continuously increasing aspect ratio (AR) of the features required for the device integration, DRIE is the preferred method for TSV for the attainable vertical sidewalls and high AR. Unfortunately, the primary gases used in DRIE for TSV are SF₆ and perfluorocarbon (PFC) gases, which are high global warming potential (GWP) greenhouse gases, making their increased usage undesirable.

In this work, a thermodynamics approach is used to assess and select other viable etch chemistries for TSV that are non-PFC, in an effort to reduce the usage of PFC gases and minimize their environmental impact. A systematic study is based on the assessment of various halogen-based gases, utilizing a volatility diagram where the partial pressure of the etch products are determined as a function of the etchant pressure at various temperatures. This functional relation can be determined from the thermodynamic equilibrium between the surface and gas-phase species, by considering the standard Gibbs free energy and the equilibrium constant. A careful control of the etchant partial pressure near the isomolar point, where the partial pressure of the volatile species would reach that of the equilibrium value, has been shown to be necessary to control the formation of volatile species. Amongst various candidates, NF₃, a non-PFC gas with greenhouse rating only 1ppt in atmosphere, appears promising. From the thermodynamics analysis, the generation of fluorine atom from SF₆ and NF₃ is comparable, however, NF₃ is much more able to form more SiF₄, the volatile etch product, than SF₆. While this is promising, another significant reaction product from NF₃ is Si₃N₄, which is non-volatile. The addition of a second chemical such as O₂ can necessitate its subsequent removal, through the formation of volatile products such as nitrogen oxides (N_xO_y). In addition, NF₃ is also capable of removing SiO₂ which is unintentionally formed during reaction with O₂. This work will highlight the analysis to design a NF₃/O₂ process (sequential exposure versus mixture) that yields comparable etch results compared to that achieved by SF₆, thereby offering a viable alternate for TSV etch.