(697a) The Applications of Thermodynamic Approach to Select Viable Chemistry in Plasma Etching

A systematic thermodynamic approach is used to assess the feasibility of various etch chemistries, beginning with the consideration of reactions between the dominant vapor phase/condensed species and the surface at various temperatures and reactant partial pressures.  This study is based on the assessment of various 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. Two applications based on the systematic thermodynamics calculation have been performed in this work.

First, the selections of non-PFC chemistries for through-silicon via (TSV) etch have been studied. TSV etch has been demonstrated with Bosch deep reactive ion etching (DRIE) which can achieve the desired and continuously increasing aspect ratio (AR) of the features required for the device integration. 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. Amongst various candidates, NF₃, a non-PFC gas with greenhouse rating only 1ppt in atmosphere, appears promising. From the thermodynamics analysis, NF₃ forms 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).

Second, the selections of viable chemistry for magnetic metals etch have been investigated. Magnetic tunnel junctions (MTJ) which are based on magnetic hysteresis for data storage are an important part of spin-electronics. Among the challenges of fabrication, MTJ etching processes is one of the most critical. Ion beam etching was a general etching technique at the beginning of MTJ fabrication, however the etched material tends to re-deposit on the sidewalls and form fences. In this report, a few magnetic metals are considered (Co, Fe, and Ni) along with various halogen and organometallic based chemistries. The thermodynamically favorable reaction has been investigated and the vapor pressure of its product has been calculated. In addition, the vapor pressure enhancement induced by adding secondary gas such as hydrogen has also been studied. Experimental validation is an important part to prove the prediction.