Evaluation of Bosch processing at cryogenic temperatures

Bosch processing is an etching technique extensively used in the semiconductor industry towards the obtention of deep aspect ratio features which are necessary for MEMS and TSV applications [1,2]. The process consists in the repetition of an isotropic etching step using SF6 plasma followed by a C4F8 plasma step to passivate the trench sidewalls in order to achieve anisotropic profiles [2,3]. Although other techniques can be used towards deep silicon etching, such as standard cryoetching and the STiGer process, Bosch process offers less constraints as it is performed at ambient temperature and is very robust [1]. However, the main difficulty of this process is the gradual accumulation of fluorocarbonated species on the reactor sidewalls which leads to process deviations. Subsequently, different solutions have been developed to increase process stability and reduce the occurrence of reactor cleanings such as the development of heater liners on the reactor edges to heat the sidewalls above the condensation temperature of the fluorocarbonated polymers formed during passivation steps [4]. In comparison, cryogenic processes present the advantage of enhanced process stability as the reactive species mainly interact with the cooled substrate due to its cryogenic temperature. However, there is no significant study which attests of the impact of performing Bosch process at cryogenic temperatures and the impact of temperature in general on the etching profiles.In this research study, a given set of Bosch process parameters were performed at different temperatures to evaluate the evolution of the etching profile. It will be shown that Bosch process is effectively temperature dependent and that the necessary C4F8 passivating gas flow can be significantly reduced at cryogenic temperatures towards the obtention of anisotropic profiles (Fig. 1).Figure 1: Comparison of two etching profiles obtained with the same Bosch process parameters at different temperatures (Fig 1.a: T = -100°C and Fig 1.b: T = +20°C).Furthermore, a slight increase of the etch rate is observed at lower temperatures although it strengthens aspect ratio dependent etching (ARDE). A comprehensive study on the influence of temperature on the formation and properties of the flurorocarbonated passivation layer will be shown. Consequently, additional tests will be presented to evaluate whether the reduction of the necessary C4F8 gas flow at lower temperatures effectively results in the extension of process stability.The study was carried out using a cryogenic ICP reactor equipped with an in-situ ellipsometer. The tests were performed on Si coupons with a 1 μm surface SiO2 hard-mask layer. The hard-mask pattern consisted of trenches from 2 to 10 μm wide. These coupons were sticked on SiO2 4” carrier wafers using a specific thermal glue used for cryogenic process tests.This research project is supported by the CERTeM 2020 platform, which provides most of the equipment and funded by the European Union (FEDER fund) as well as the French National Research Agency (ANR PSICRYO fund).References 1. T. Tillocher, J. Nos, G. Antoun, P. Lefaucheux, M. Boufnichel, and R. Dussart, Comparison between Bosch and STiGer Processes for Deep Silicon Etching, Micromachines 12, 1143 (2021).2. M. A. Blauw, T. Zijlstra, and E. van der Drift, Balancing the Etching and Passivation in Time-Multiplexed Deep Dry Etching of Silicon, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. 19, 2930 (2001).3. F. Laemer, and A. Schilp, Method for Anisotropic Plasma Etching of Substrates, U.S. Patent 5498312A (1996).4. M. Puech, Heating Jacket for Plasma Etching Reactor, and Etching Method Using Same, U.S. Patent 0224178A1 (2005).