High-Voltage Control for Passivation Steps in Bosch Deep Silicon Etching Processes

The Bosch process, a time-multiplexed deep reactive ion etching technique, is the industry standard for creating high-aspect-ratio structures in silicon, such as those used in micro-electromechanical systems, advanced packaging, and through-silicon vias. Its success hinges on the cyclic alternation between two distinct plasma phases: an isotropic etching step using sulfur hexafluoride to remove silicon, and a passivation step using a fluorocarbon gas to deposit a protective polymer layer on the sidewalls of the evolving trenches. While much attention is given to the etching step, the passivation phase is equally critical for achieving vertical sidewalls, high aspect ratios, and smooth surfaces. Precise control of the high-voltage bias applied to the silicon substrate during this passivation step is a fundamental, yet nuanced, parameter governing the process outcome. In a typical capacitive-coupled plasma etch tool, the substrate is placed on a powered electrode (the cathode). A high-frequency radiofrequency power supply is connected to this electrode to generate and sustain the plasma. However, the ions in the plasma are accelerated across the plasma sheath by the DC self-bias voltage that naturally develops on this RF-driven electrode. This DC bias is not directly applied but is a consequence of the RF coupling and can be influenced by the RF power, frequency, and chamber geometry. In advanced process control, a separate DC power supply or a sophisticated RF matching network with bias control is often used to precisely manipulate this effective bias voltage during different phases of the Bosch cycle. During the passivation step, the objective is to deposit a uniform, chemically resistant fluorocarbon polymer film on all exposed surfaces. However, the film deposited on the horizontal surfaces (the trench bottom and the field) must be selectively removed in the subsequent etching step, while the film on the vertical sidewalls must remain intact to protect them from lateral etching. The energy of ions bombarding the surface plays a decisive role in this differential behavior. A low-energy ion bombardment during passivation promotes softer, more conformal polymer deposition. By applying a carefully controlled, relatively low (or even slightly positive) bias voltage, the ion energy is kept minimal. This allows for good sidewall coverage but also results in deposition on the trench floor. The subsequent high-energy etching step then efficiently sputters this polymer from the horizontal surfaces while etching the silicon beneath, but leaves the sidewall polymer largely untouched due to its anisotropic ion direction. The high-voltage control system must therefore enable rapid and precise switching between two distinct bias regimes: a high-bias state for energetic etching and a low-bias state for gentle passivation. The transition between these states must be clean and fast to maintain cycle fidelity, which can be in the range of seconds to milliseconds. Any overshoot, ringing, or slow settling of the bias voltage can lead to imperfect polymer deposition or inadvertent etching, manifesting as sidewall scalloping (a hallmark of the Bosch process that engineers seek to minimize), trench footing, or poor profile control. Furthermore, the control system must be dynamically adjustable. The optimal passivation bias may change as the trench deepens due to changing aspect ratio effects on plasma and ion transport. Advanced systems may use model-based or sensor-driven feedback to adjust the bias voltage in real-time throughout the etching of a deep structure. This requires a high-voltage bias supply and its controller to have a fast response time and sophisticated programmability to follow complex multi-step recipes. Integration with the overall tool is also critical. The bias control must be perfectly synchronized with the gas injection system, the main RF plasma power, and the chamber pressure control. Any misalignment can cause a mixed mode where etching and passivation occur simultaneously, degrading the process. Therefore, these high-voltage control modules are deeply embedded in the tool's architecture, communicating via high-speed digital interfaces. In practice, the precision high-voltage control for the passivation step is a key lever in the delicate balancing act of the Bosch process. It directly influences the chemical and physical properties of the deposited polymer, the sidewall angle, the etch rate, and the final surface roughness. Mastery over this parameter allows process engineers to tailor profiles for specific applications, from perfectly vertical walls for MEMS accelerometers to tapered vias for interconnects. As the demand for deeper, narrower, and more complex silicon structures grows, the need for ever more precise, responsive, and intelligent high-voltage bias control during passivation will remain a central focus of advanced etch tool development.