Plasma processes such as deposition and etching, trimming, are required for nanoscale devices which are Fin-field effect transistors (FETs), nanowires, and others 3D devices. Control plasma processes according to the plasma internal parameters, which define the process characteristics directly, are effective because the parameters can vary over process time even if external parameters are fixed. Significant one of the parameters is the wafer temperature for the precise etching process because the etched profile is sensitive to the temperature [1]. Additionally, Increasing the wafer temperature by heat flux from a plasma are considerable to be one of the most important disturbances in the etching process because the recombination coefficients and sticking probability of active species on the surface depend significantly on surface temperature.

The temperature monitoring system with high precision had been required to develop the process characteristics. Many methods of measuring substrate temperatures during plasma processes were proposed. However, there remain difficulties such as their poor tolerance to mechanical disturbances, resulting in limited resolution and temperature ranges when monitoring silicon wafers. We developed a real-time wafer temperature measurement system by an autocorrelation-type frequency-domain low-coherence interferometry (ACT-FD-LCI) system [2-4].

Since the wafer temperature is rapidly increased by energetic ion bombardments and radiative heat-fluxes from the plasma during processes, the surface reaction mechanism also varies with process time. To suppress the temperature variation during processes, a feedback control system of the wafer temperature was constructed. Controlling the plasma excitation and switching the biasing power “ON” and “OFF” enabled the actual temperature to be maintained within a range of ±1.2 °C from the set temperature.[5,6] This system revealed that the profile evolutions of anisotropic organic pattern etching by a H₂/N₂ plasma were temporally separated into vertical and lateral directions. Subsequently, trimming process of etched organic were performed without supplying bias power, while the substrate temperature was precisely controlled within ±0.7 °C of a set temperature due to lower heat flux from the plasma. The free-standing feature of organic material, with a width narrower than 12 nm, was fabricated in a self-limiting manner. [7] We expect that this self-limiting process could be applied to various advanced processes.

References;

[1] H. Yamamoto, H. Kuroda, M. Ito, T. Ohta. Hori, Jpn J Appl Phys, 51, 016202 (2012).

[2] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Appl. Phys. Lett., 103, 182102 (2013).

[3] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Jpn J Appl. Phys., 54, 01AB03 (2015).

[4] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Appl. Opt., 54, 7088 (2015).

[5] T. Tsutsumi, Y. Fukunaga, K. Ishikawa, K. Takeda, H. Kondo, T. Ohta, M. Ito, M. Sekine, M. Hori, IEEE Trans. Semicond. Manufact., 28, 515 (2015).

[6] Y. Fukunaga, T. Tsutsumi, K. Takeda, H. Kondo, K. Ishikawa, M. Sekine, M. Hori, Jpn. J. Appl. Phys. 56, 076202 (2017).

[7] Y. Fukunaga, T. Tsutsumi, H. Kondo, K. Ishikawa, M. Sekine, M. Hori, Jpn. J. Appl. Phys. 58, 020906 (2019).