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Title:Characterization, optimization, and simulation in through silicon via (TSV) dry etch
Author(s):Ouyang, Zihao
Director of Research:Ruzic, David N.
Doctoral Committee Chair(s):Ruzic, David N.
Doctoral Committee Member(s):Stubbins, James F.; Eden, James G.; Zhang, Yang
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Through-Silicon-Vias (TSVs)
Dry etch
Abstract:Two approaches of through-silicon-via (TSV) etching methods, the Bosch process (alternating etching/passivation phases) and the single-step etching, have been experimentally investigated and theoretically modeled in this study. Parameter ramping and post-etch plasma treatment techniques have been performed on the bias voltage, pressure and gas flow rate to address profile defects produced by the Bosch process, such as profile bowing and sidewall scallops. A single-step etching method mixing SF6 and C4F8 gases is developed in this study as an alternative TSV etching approach to realize ultra-smooth and vertical TSV profiles. A major purpose of this study is to exploit solutions of producing high aspect-ratio, ultra-smooth TSV profiles at a high etch rate (ER). In this study, time-dependent simulation models are established for both Bosch process and single-step etching method using a finite-element-method (FEM) program, COMSOL, by taking into account the thermal etching of F atoms, ion-enhanced etching, neutral deposition and ion-enhanced deposition mechanisms, as well as the angular dependence of the ion sputtering with aspect to a surface element. The simulation model is cost-effective tool for predicting the etch profile evolution of TSVs. The major ion and neutral species used in the simulation model, including SF_3^+, CF_3^+, CF2 and F, are validated by the plasma chemistry study using the mass spectroscopy technique. TSV profile defects produced by the Bosch process have been experimentally reduced by hardware improvement and process modification. The LAM Syndion C chamber employed in this study is capable of stably alternating process parameters of the etching and passivation phases within less than 1 second. A post-etch NF3/O2 plasma treatment has been used to successfully eliminate the scallops on the profile sidewalls but it creates a ring-shaped defect at the bottom of the profile. Optimizations on the bias voltage, gas ratio and pressure has been performed in the post-etch treatment to minimize the bottom ring defect. A process-of-record (POR) Bosch process and a POR post-etch treatment process have been proposed in this study to realize an optimal TSV etch profile. Experimental results show that a profile discontinuity, or a “transition”, appears on the TSV profile produced by the single-step etching method, especially at high bias voltages and high SF6 flow rates. The sidewall smoothness above and below the transition is found to be very different. Ultra-smooth sidewalls are realized by the single-step etching method below the transition, compared to the POR Bosch process. Parameter study of the single-step etching shows that decreasing pressure, reducing bias voltage, and decreasing SF6/C4F8 ratio can improve the sidewall smoothness and eliminate the transition on the TSV profile. However, the resulting ER realized using these transition-eliminating approaches is significantly reduced. The comparison between the simulation results and experiments suggests that consideration of a high-energy and a low-energy SF_3^+ ions are critical for matching the simulation etch profile with the experiments. It is found that the underlying reason for the transition formed by the single-step etching originates from the difference of the ion angular distributions of etching species and depositing species. Both experiments and simulation results indicate that the low-energy SF_3^+ ions mainly participate in the polymer sputtering process and the high-energy SF_3^+ ions are the dominating species for Si etching in single-step etching processes.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Zihao Ouyang
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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