Atomic Layer Etching
Introduction
In the areas of nanofabrication and semiconductor manufacturing, precision and control are vital. Atomic Layer Etching (ALE) is a cutting-edge technique that provides precision and control in the ever-advancing world of nanofabrication and semiconductor manufacturing. It is a technique that promises atomic-level precision, exceptional selectivity, and minimal sidewall damage to increasingly small features for these applications.
Whilst wet etching techniques (more chemical based) have previously been the industry standard in microfabrication, dry etching techniques such as ALE are gaining prominence for their unique advantages. ALE’s ability to remove materials with atomic precision makes it indispensable in fabricating nanoscale devices and intricate semiconductor components. In this article, we’ll delve into ALE, explaining its fundamental principles, process parameters, diverse applications, and the crucial role of plasma diagnostics in process optimisation.
Section 1: ALE Fundamentals
Key to ALE is the generation of a plasma, which is an ionised gas consisting of a mixture of neutral atoms, charged ions and electrons. This plasma is typically produced and maintained using an RF power delivery circuit, wherein electromagnetic power is capacitively or inductively coupled to a gas in a vacuum chamber to sustain a constant discharge. A key property of a plasma is that it is quasi-neutral; the positive ions cancel out the negative electrons. However, when a substrate is introduced a charge imbalanced plasma sheath will form around the substrate, due to electrons being more mobile and negatively charging the substrates surface. Ions from the plasma entering this sheath region are accelerated towards the substrate across this sheath and onto the substrates surface, where they react. This substrate can be negatively biased to control the energy of ions hitting the substrate for various applications.
ALE starts with the introduction of an etching gas to a vacuum chamber which will be absorbed onto the substrate material. This will form a single monolayer of material on the surface of the substrate to be etched. The purpose of this step is to weaken the binding energy of the surface so that it is easier to remove. The etching gas is then purged out of the reaction chamber. After the first step, a plasma forming gas is introduced, and the plasma is ignited. The surface will then be bombarded with low energy inert ions which removes this upper monolayer of the substrate, and this material leaves the surface in gaseous form. The plasma gas is then purged, and the cycle is repeated until the desired etching depth is reached. A schematic of one of these cycles can be seen in figure 1 below.
Section 2: ALE Process Parameters
Some of the critical parameters in ALE are the etch rate, selectivity, surface roughness, and sidewall profile. Etch rate quantifies the speed at which material is removed during ALE, influencing the efficiency of the process. Selectivity measures the process’s preference to etch one material over another, crucial for precision and to avoid over etching material. Surface roughness impacts the quality of the etched surface, a critical concern for microelectronics and photonics. Sidewall profile, on the other hand, describes the shape of etched features and plays a pivotal role in device functionality.
There are several process parameters that are used to control the above ALE parameters. Firstly, the Pulse time, representing the duration for which reactive gases are introduced into the chamber during each etching cycle, directly influences material removal. Longer pulse times can lead to higher etch rates, but excessive durations may impact selectivity and surface quality. Purge time, which is the duration inert gases are used to cleanse residual reactants and by-products between pulses, is also essential for maintaining a clean starting point for each cycle, preventing contamination, and sustaining selectivity. Precise control of gas flow rates are vital, as higher flows can increase etch rates, yet excessive flows may also compromise selectivity and surface roughness, affecting the pressure which in turn influences the mean free path of the plasma.
Furthermore, substrate temperature plays a pivotal role, affecting reaction kinetics. Elevated temperatures enhance etch rates by accelerating reactions but can impact the selectivity and sidewall profiles. Similarly, higher ion energy and flux to the substrate can increase etch rates but may result in heightened surface roughness and sidewall damage. Keeping these two parameters constant is paramount for achieving the same level of etching each time. Aspect ratio, the ratio of an etched feature’s depth to width, is critical in ALE particularly in complex structures, as high aspect ratios can lead to challenges such as aspect ratio-dependent etching (ARDE). This is when the etch rate varies with feature depth and must be managed to maintain etch uniformity across a substrate.
The interplay of these parameters requires a thorough understanding and systematic approach to parameter tuning for optimal ALE results. Real-time monitoring and control are imperative for precise control over these parameters while upholding quality and consistency in the ALE process.
Section 3: ALE Applications
ALE is a versatile nanofabrication technique that finds applications across a wide spectrum of industries. Its atomic-level precision enables the creation of intricate transistor gates and three-dimensional structures. The specific requirements in this domain include ultra-thin materials and sharp interfaces, making ALE a driving force in the creation of increasingly small electronic devices.
ALE also plays a pivotal role in the etching of 2D materials, such as graphene and transition metal dichalcogenides (TMDs). In this rapidly evolving field, ALE allows for the controlled etching of these ultrathin materials, offering a powerful toolkit for crafting customised nanoscale patterns. ALE’s precision is instrumental in tailoring the electronic and optical properties of 2D materials, opening avenues in electronics, photonics, and more. For the same reasons, ALE shines in advanced patterning techniques like directed self-assembly (DSA) and nanoimprint lithography.
While each of these applications brings its unique demands and challenges, ALE’s atomic precision, excellent selectivity, and minimal sidewall damage make it a standout choice. From the extreme precision required in semiconductor manufacturing to the preservation of 2D materials’ integrity and the complex integration needed in advanced patterning techniques, ALE’s advantages are evident. It offers atomic-level control over material removal, high selectivity, minimal sidewall damage, and the ability to precisely etch layer by layer.
Section 4: Process Optimisation
Optimising Atomic Layer Etching (ALE) processes is paramount for achieving precise etching and maintaining high-quality, consistent results. To achieve these objectives, several strategies and techniques are employed. One of the critical factors is the careful selection of precursor gases. The choice of precursors is pivotal as they need to facilitate self-limiting reactions and provide high selectivity. Additionally, precursor purity and stability play significant roles. Extensive research and experimentation are often conducted to identify the most suitable precursors tailored to specific materials and applications.
Surface passivation is another technique used to enhance ALE processes. It involves depositing passivation layers on sensitive regions of the substrate to protect them during the etching process. These passivation layers act as barriers, preventing unwanted reactions and improving the quality of etched features. Surface passivation is instrumental in achieving high selectivity and preserving the integrity of the substrate.
Real-time monitoring and control of key plasma parameters is indispensable for achieving precise etching and maintaining consistency in ALE. Given the atomic-level precision required, even slight deviations from optimal conditions can have a significant impact on etched features. Therefore, direct monitoring techniques are often employed to gain real-time insights into the evolving etching process. Feedback control systems use this data to make immediate adjustments, ensuring that the desired etching outcomes are consistently achieved.
Impedans Ltd are world leaders in plasma diagnostic tools designed to enhance ALE process optimisation. They offer products such as Semion RFEAs that provide substrate level measurements of the ion energy distribution function, ion flux and deposition rate of etching ions from the plasma. This rapidly characterises the ALE process under different power, pressure, frequency and gas flow conditions. Impedans also offer bulk plasma sensors, such as a Langmuir probes, to measure fundamental plasma parameters such as the ion density, electron temperature, plasma potential and electron energy distribution function. This provides a direct measure of plasma conditions and chemistry throughout the process. Finally, Impedans offer Octiv RF sensors for measuring the rf power and impedance of fundamental and harmonic frequencies for monitoring process stability.
Impedans Ltd’s diagnostic solutions have been successfully integrated into many ALE processes, across multiple disciplines. For instance, Semion RFEAs have been used in a study on the fabrication of 2D hetero-structures using atomic layer etching combined with selective conversion [2]. Here, they were used to estimate the bias power impact in the Ar plasma etching step by measuring ion energy distributions. As previously mentioned, the Ion Energy Distribution Function is key for the etching step in ALE. Several investigations have been performed into the control of this parameter using tailored voltage waveforms to drive the plasma, many of which have used Impedans Semion RFEAs [3-5]. These papers have found that by altering the RF power signal in different ways, one can control ion energies, and thus optimise ALE process.
Conclusion
In summary, this article has provided insights into Atomic Layer Etching (ALE) processes, emphasizing its significance in nanofabrication and semiconductor manufacturing. ALE offers unique advantages, including precision, selectivity, and minimal damage, making it a crucial technique in these industries. We’ve explored the fundamental principles of ALE, key process parameters, and its diverse applications, showcasing its versatility.
Furthermore, the article has highlighted the importance of understanding and optimising process parameters to achieve successful ALE applications. It’s clear that ALE’s precise control over etched features and Impedans’ diagnostic tools are a valuable combination for advancing nanofabrication and materials engineering. Readers are encouraged to explore Impedans Ltd’s website or to reach out to one of their plasma experts for more information on these solutions.
References
1 Min Young Yoon, H. J. Yeom, Jung Hyung Kim, Won Chegal, Yong Jai Cho, Deuk-Chul Kwon, Jong-Ryul Jeong, Hyo-Chang Lee; Discharge physics and atomic layer etching in Ar/C4F6 inductively coupled plasmas with a radio frequency bias. Phys. Plasmas 1 June 2021; 28 (6): 063504. https://doi.org/10.1063/5.0047811
2 Heyne, M. H., Marinov, D., Braithwaite, N., Goodyear, A., de Marneffe, J. F., Cooke, M., Radu, I., Neyts, E. C., & de Gendt, S. (2019). A route towards the fabrication of 2D heterostructures using atomic layer etching combined with selective conversion. 2D Materials, 6(3). https://doi.org/10.1088/2053-1583/ab1ba7
3 Qin, X. v., Ting, Y. H., & Wendt, A. E. (2010). Tailored ion energy distributions at an rf-biased plasma electrode. Plasma Sources Science and Technology, 19(6). https://doi.org/10.1088/0963-0252/19/6/065014
4 Fischer, G., Ouaras, K., Drahi, E., Bruneau, B., & Johnson, E. v. (2018). Excitation of Ar, O2, and SF6/O2 plasma discharges using tailored voltage waveforms: Control of surface ion bombardment energy and determination of the dominant electron excitation mode. Plasma Sources Science and Technology, 27(7). https://doi.org/10.1088/1361-6595/aaca05
5 Faraz, T., Verstappen, Y. G. P., Verheijen, M. A., Chittock, N. J., Lopez, J. E., Heijdra, E., van Gennip, W. J. H., Kessels, W. M. M., & MacKus, A. J. M. (2020). Precise ion energy control with tailored waveform biasing for atomic scale processing. Journal of Applied Physics, 128(21). https://doi.org/10.1063/5.0028033
