Introduction:
Deep Reactive Ion Etching (DRIE) is a technique at the forefront of the microfabrication, MEMS, and semiconductor industries. It offers high aspect ratio etching, meticulous sidewall profile control, and compatibility with a myriad of materials. In this article, we explore its fundamental principles, varied techniques, intricate process parameters and its diverse applications.
Section 1: DRIE Fundamentals
At the molecular level, DRIE starts with the introduction of reactive gases, predominantly sulphur hexafluoride (SF6) or carbon tetrafluoride (CF4), into a vacuum chamber containing a substrate. In a highly controlled environment these gases undergo a process of dissociation through a plasma, generating reactive species.
Within the chamber, radio-frequency power ionises a gas to create a plasma. Ions from the plasma are accelerated across a plasma sheath that forms between the bulk plasma and the substrate. They collide with the substrates surface, imparting sufficient energy to dislodge atoms, etching them away. Simultaneously, chemical reactions between the reactive species and the substrate occur. For instance, in the case of silicon, fluorine radicals react with the surface, forming volatile silicon fluoride compounds, which are also then pumped away.
The aspect ratio of an etched feature is the ratio of a its height to its width and plays a crucial role in Etching processes. As the aspect ratio increases, meaning the structure becomes taller relative to its width, the etching process naturally becomes more challenging. Higher aspect ratios often lead to slower etch rates and difficulties in maintaining uniformity across the structure. This phenomenon is known as aspect ratio-dependent etching. This often results in tapered or bowed profiles due to uneven material removal.
One of the key techniques used in DRIE that sets it apart from standard RIE is the Bosch process, which involves cyclic etching and passivation steps. During the etching step, reactive ions bombard the substrate, removing material. In the passivation step, a gas containing a polymerising component is introduced, creating a protective layer on the sidewalls, preventing lateral etching and maintaining verticality. This cyclic process ensures deep etching while preserving precise sidewall profiles. Cooling the substrate to cryogenic temperatures (around -100°C) is another technique often employed in DRIE. Cryogenic temperatures help in maintaining the stability of the passivation layer, preventing it from re-depositing onto the substrate surface. This stability ensures the anisotropic nature of the etching, allowing for high aspect ratios and deep structures. This cyclic interplay of etching and passivation facilitates the creation of intricate, deep features vital for MEMS devices and micro-optics.
Section 2: DRIE Process Parameters
Selection of gas chemistry is key for DRIE processes, typically using fluorinated gases such as sulphur hexafluoride (SF6) and octafluorocyclobutane (C4F8). These gases, when introduced into the chamber, initiate the cascade of chemical reactions. SF6, for instance, undergoes dissociation into sulphur fluorides and radicals upon exposure to the plasma environment. The delicate balance between these gases and their flow rates dictates the chemical kinetics, impacting the etch rate and sidewall profile.
Maintaining a controlled low-pressure environment within the chamber is paramount. Lower pressures ensure a longer mean free path for the reactive species, reducing the likelihood of collisions with background gases and enabling a more controlled and directional etching process. The balance of pumping systems reducing the pressure and mass flow controllers dictating the inward flow of necessary gases is critical for maintaining a consistent process.
Two of the most important parameters that will affect etched features are the ion flux and the ion energy. Ion flux is the number of ions striking the substrate per unit area and time and will have a direct impact on etch rates and sidewall profile characteristics. The energy with which these ions hit the surface also plays a key role. Ions with higher energy tend to penetrate the material more effectively, leading to higher etch rates and can contribute more to anisotropy. This results in more well-defined vertical features. However, high ion energies can also lead to surface damage, roughening the substrate surface. Controlling ion energies can minimise such damage, ensuring high quality etch results. The ions won’t all hit the surface of the substrate with the same energy; they will hit with a distribution of energies, called the Ion Energy Distribution Function (see our article on the IEDF for more information on this critical plasma parameter). Control over this function, and the ion flux, is key for achieving repeatable etch features and reliable process speeds.
Power applied to the plasma source governs the ionisation of your plasma and therefore the plasma density, influencing the ion flux to the substrate. As previously mentioned, biasing a substrate with a negative voltage will accelerate ions further across the plasma sheath for etching. This allows for a rough control over ion energies and thus control over the etching process. Engineers frequently use pulsing, continuous wave, DC, and RF biasing techniques to tailor the bias voltage of the substrate, ensuring the desired etching characteristics such as high etch rates, anisotropic profiles, minimal surface damage, and selectivity.
Lastly, the etching process is nonlinear with time. Initially, the etch rate is rapid, driven by the availability of fresh silicon atoms on the surface. However, as the etch progresses, the rate can decelerate due to several factors, including the accumulation of etch by-products on the surface, necessitating a meticulous balance of all parameters for uniform and high-quality etching.
Section 3: DRIE Applications
Silicon Micromachining:
DRIE’s significance in silicon micromachining resides in its unparalleled ability to create complex three-dimensional microstructures with exceptional aspect ratios. For instance, in pressure sensors, DRIE carves silicon diaphragms, enhancing their mechanical flexibility and responsiveness. This precision enables the accurate detection of minuscule pressure changes, crucial in scientific instruments and experimental setups.
MEMS Devices:
MEMS (micro-electromechanical) devices demand intricate structuring, which is achievable through DRIE. In devices like accelerometers, gyroscopes, and microvalves, DRIE ensures the creation of precisely defined features. The high aspect ratios attained through this technique enable the development of responsive and highly sensitive MEMS components. The ability to finely control the dimensions and depths of these structures underpins their functionality in scientific research, environmental monitoring, and medical devices.
Waveguide Fabrication:
Waveguides are the backbone of optical communication and sensing systems. DRIE’s role in waveguide fabrication is instrumental. By etching intricate patterns into silicon substrates, DRIE enables low-loss pathways for light transmission. The precision of DRIE ensures minimal scattering and absorption of light, facilitating high-speed data transmission and accurate signal detection. This is crucial for scientific applications requiring precise optical measurements, such as in spectroscopy and biomedical sensing.
Through-Silicon Vias (TSVs):
Through-Silicon Vias (TSVs) are indispensable in modern integrated circuits, enabling vertical interconnections through silicon substrates. DRIE’s intricate hole etching capabilities guarantee the creation of these vias with nanometer-scale precision. This precision enhances circuit density, reduces signal propagation delays, and optimises heat dissipation in advanced electronic systems. In scientific instruments requiring compact yet powerful computing capabilities, such as in computational biology and simulation studies, DRIE-enabled TSVs play a pivotal role.
Section 4: DRIE Process Optimisation
The Bosch process adds a layer of complexity to the RIE process (see our article for more info on the more general reactive ion etching technique). During the etch step, a high-density plasma bombards the silicon surface, carving precise features. Researchers will adjust the duration of the plasma bombardment and passivation steps, along with gas flows and power levels. This iterative optimisation process ensures a delicate balance between material removal and sidewall protection, ensuring consistency and reproducibility, which are indispensable in scientific research and industrial applications.
Sidewall passivation, a critical facet the Bosch process, demands careful attention. Achieving a uniform, conformal passivation layer is pivotal for maintaining the structural integrity of etched features and aspect ratios. Scientists employ sophisticated techniques, often involving gas chemistry adjustments, to optimise sidewall passivation. However, there is a trade-off; a passivation layer that is too thick may hinder the etching process, leading to under-etching, while a thin layer might result in over-etching and rough sidewalls.
Real-time monitoring and control of DRIE processes are indispensable for achieving process optimisation. Continuous feedback through advanced plasma diagnostics and etch monitoring tools is paramount as these tools provide critical insights into the plasma chemistry, ion energy, and flux at the substrate. By measuring these key parameters, engineers can adjust ensure process control, minimising variations between product, and ensuring repeatability across production runs. Impedans Ltd offer state of the art plasma and RF diagnostics such as Semion RFEAs which provide direct measurements of the ion flux, ion energy distribution and aspect ratios of ions incident on a substrate in a plasma process. They also offer Langmuir probes which measure other plasma parameters such as plasma density, electron temperature and plasma potential. These can be used to ensure uniformity across your plasma and monitor plasma chemistries between processes. Finally, Octiv VI probes can be used to measure the RF power, from which you can characterise matchbox performance, monitor the reflected power not being delivered to your plasma and ensure impedance matching is maintained. Any harmonic RF signals in the RF power delivery network will be produced by the plasma, and as such monitoring these signals provide a way of non-invasively monitoring a plasma process. VI probes have been demonstrated to detect wafer misplacement issues, arcing in the plasma, changes in the plasma chamber, and below 1% open area etch endpoints.
Conclusion
Deep Reactive Ion Etching (DRIE) is a method known for its precision, combining chemical reactions and ion bombardment to achieve intricate etching. This article has provided an exploration of DRIE, covering its foundational principles and the complexities of gas chemistry and process optimisation. DRIE finds applications across various industries, from silicon micromachining to MEMS devices, demonstrating its importance in technological advancements. Despite challenges in maintaining uniform plasma density and ion species control, ongoing research holds promise for future innovations in DRIE. Impedans Ltd offers tools such as Langmuir Probes and Semion RFEAs to enhance precision and repeatability. For more information on plasma processes and diagnostics please refer to the Impedans website or get in touch with one of our plasma experts.
References
1Yu, J. C., Zhou, Z. F., Su, J. le, Xia, C. F., Zhang, X. W., Wu, Z. Z., & Huang, Q. A. (2018). Three-dimensional simulation of DRIE process based on the narrow band level set and Monte Carlo method. Micromachines, 9(2). https://doi.org/10.3390/mi9020074
