Interview Questions for RIE etching

Reactive Ion Etching (RIE) is a dry etching technique used in microfabrication to precisely remove material from a substrate, typically a silicon wafer. It works by generating a plasma – a partially ionized gas – containing chemically reactive species. These species react with the substrate material, forming volatile compounds that are then pumped away, effectively etching the surface. Imagine it like a tiny, controlled chemical sandblasting process, where the ‘sand’ is reactive gas and the ‘blasting’ is the plasma’s energy.

The process is crucial for creating intricate patterns and features on microchips and other microdevices. Unlike wet etching which uses chemical solutions, RIE offers better control over the etch profile and allows for the creation of much smaller, more precise features.

RIE processes are broadly categorized based on their etch profile: isotropic and anisotropic.

• Isotropic Etching: This type of etching etches in all directions equally, resulting in an undercut profile. Think of it like a sphere being eroded from all sides. It’s less precise but can be useful for certain applications like undercutting features to release structures.
• Anisotropic Etching: This is a more directional etch, predominantly etching vertically. The result is a highly vertical profile with minimal undercutting. This is ideal for creating high-aspect-ratio features, like the deep trenches needed in modern microchips.
• Deep Reactive Ion Etching (DRIE): This is an advanced anisotropic technique capable of etching extremely deep and narrow features, often with aspect ratios exceeding 50:1. DRIE usually involves alternating etching and passivation steps to ensure straight sidewalls.

Several key process parameters influence RIE’s etch rate and selectivity:

• Pressure: Lower pressure generally leads to higher anisotropy and lower etch rate due to longer mean free paths of ions.
• Power: Higher power increases ion bombardment energy, leading to a higher etch rate, but can also reduce selectivity and cause damage.
• Gas Flow Rate: Controls the concentration of reactive species in the plasma. Optimizing this is crucial for achieving the desired etch rate and selectivity.
• Gas Composition: The choice of etching gas significantly impacts etch rate and selectivity (discussed further in Question 5).
• Temperature: Substrate temperature can influence the chemical reaction rates and diffusion processes, impacting the etch rate.

Selectivity refers to the ratio of the etch rate of the target material to the etch rate of the underlying layer or mask material. High selectivity is crucial to prevent unwanted etching of other layers during fabrication.

Controlling anisotropy in RIE primarily involves manipulating the balance between chemical and physical etching mechanisms.

• Reducing Pressure: Lower pressure increases the mean free path of ions, allowing them to travel more directly to the substrate, enhancing the directionality of the etch.
• Increasing Ion Bombardment Energy: Higher RF power increases the energy of the ions striking the surface, enhancing physical sputtering and contributing to a more vertical profile. This, however, must be balanced to avoid damage.
• Adding Passivation Steps (DRIE): Alternating etching and passivation steps is the cornerstone of DRIE, allowing for highly anisotropic etching of deep trenches. The passivation layer protects the sidewalls from further etching.
• Careful Gas Selection: Choosing gases that favor directional etching is crucial (as explained further in Question 5).

Different gases play distinct roles in RIE processes:

• CF₄ (Tetrafluoromethane): Commonly used for etching silicon dioxide (SiO₂). It forms volatile SiF₄ during the etching process.
• SF₆ (Sulfur hexafluoride): Effective for etching silicon (Si) and other materials. It produces volatile fluorinated silicon compounds.
• O₂ (Oxygen): Often added to the plasma to control the etch rate and enhance selectivity. It can oxidize the surface, forming a passivation layer in some cases or assisting in the removal of residues.
• Other Gases: Many other gases are used in combinations to fine-tune etching parameters, including chlorine-based gases (e.g., Cl₂) for etching metals and various mixtures for specific material combinations and desired selectivity.

The choice of gas and its concentration is crucial for optimizing the etch rate, selectivity, and anisotropy.

Common challenges in RIE include:

• Etch Non-uniformity: Variations in etch rate across the wafer can occur due to issues like uneven plasma distribution or temperature gradients. This can be mitigated by careful chamber design, precise process control, and potentially using techniques like wafer chucking and plasma uniformity enhancement.
• Etch Lag: (Discussed in the next question)
• Micro-loading effects: High density features etch faster than isolated ones. Compensation techniques include optimizing gas flow and pressure.
• Mask erosion: The etching process may erode the mask used to define the pattern. Solutions include using harder masks, optimizing etching parameters, or employing alternative patterning methods.
• Residue formation: Unwanted residues may remain after etching, requiring cleaning steps. Careful process parameter optimization and use of appropriate cleaning techniques can resolve this.

Etch lag is the phenomenon where smaller or more densely packed features etch slower than larger or more isolated features. It’s a significant concern in high-density microfabrication. The underlying reason is the competition for reactive species within the plasma. Smaller features have a smaller surface area and hence ‘compete’ less effectively for the limited reactive species in their vicinity compared to larger features.

Mitigation strategies include:

• Optimizing gas flow and pressure: Higher gas flow rates and pressures can provide a more uniform distribution of reactive species, reducing the effect of micro-loading.
• Using high-density plasmas: A denser plasma provides a larger supply of reactive species, reducing the impact of micro-loading.
• Employing advanced etching techniques: Techniques like Bosch process (used in DRIE) help minimize etch lag by alternating etching and passivation steps, ensuring more uniform etching across various feature sizes.

Etch rate and selectivity are crucial parameters in Reactive Ion Etching (RIE). Etch rate measures how quickly the material is removed, typically expressed in nanometers per minute (nm/min) or angstroms per minute (Å/min). Selectivity, on the other hand, describes the ratio of the etch rate of the target material to the etch rate of an underlying or adjacent material. A high selectivity is desirable when etching one material on top of another, ensuring the underlying layer isn’t damaged.

Measuring etch rate involves initially measuring the thickness of the material before etching using techniques like profilometry or ellipsometry. After etching for a specific duration, the thickness is measured again. The difference divided by the etching time gives the etch rate. For selectivity, we measure the etch rates of both the target and underlying materials under identical process conditions, then calculate the ratio.

Example: If silicon is etched at 100 nm/min and silicon dioxide at 10 nm/min, the selectivity of silicon over silicon dioxide is 10:1. This indicates that silicon etches 10 times faster than silicon dioxide.

Maintaining chamber cleanliness in RIE is paramount for consistent and reliable results. Contaminants, such as polymer deposits from previous etching processes or particles from the environment, can significantly impact etch rate, uniformity, and selectivity. These contaminants can act as etch inhibitors or can lead to unexpected etching behaviour, producing defects like residue or non-uniform etching profiles.

Cleanliness is maintained through several procedures. Regular cleaning cycles are crucial, usually involving plasma cleaning (e.g., oxygen plasma ashing) to remove organic residues. Physical cleaning, using appropriate solvents and swabs, may also be necessary for removing particulate matter. Proper vacuum pumping and venting procedures also minimise the introduction of contaminants into the chamber. Regular monitoring of the chamber pressure and plasma parameters during etching helps to spot issues early on.

Imagine trying to bake a cake with a dirty oven. The outcome wouldn’t be reliable. Similarly, a dirty RIE chamber leads to unpredictable and unreliable etching results.

RIE systems present several safety hazards requiring strict adherence to safety protocols. The primary concerns revolve around:

• Exposure to hazardous gases: Many etching chemistries involve toxic and corrosive gases (e.g., SF₆, CF₄, Cl₂). Proper ventilation and exhaust systems are essential, along with personal protective equipment (PPE) like respirators. Regular gas leak checks are mandatory.
• High voltage: The plasma generation involves high voltages which poses an electrical shock risk. Appropriate safety interlocks and grounding are crucial to prevent accidental contact.
• UV radiation: Plasma generation emits ultraviolet (UV) radiation, harmful to eyes and skin. Safety interlocks and viewing ports with UV filters are vital. The system should be appropriately shielded.
• Vacuum hazards: Improper handling of vacuum systems can lead to implosion or suction injuries. Training on proper vacuum procedures is essential.

Regular safety inspections and operator training are critical to ensure the safe operation of RIE systems.

Diagnosing RIE process issues requires a systematic approach. For low etch rate, possible causes include low RF power, insufficient gas flow, contaminated chamber, or depletion of the etching gas. For poor uniformity, causes could include uneven gas distribution, inadequate chuck temperature control, or wafer imperfections.

Troubleshooting Steps:

1. Review process parameters: Verify that all process parameters (RF power, pressure, gas flow rates, temperature) are within the specified ranges. Check for any deviations from previous successful runs.
2. Inspect the wafer: Examine the wafer for any defects or contaminants that may have impacted the etch.
3. Inspect the chamber: Check for any visible signs of contamination, such as polymer deposits or particulate matter. Schedule a chamber cleaning if needed.
4. Check gas purity and flow: Ensure the purity of the etching gases and that the gas flow rates are correctly set and stable.
5. Plasma diagnostics: Use plasma diagnostics tools (e.g., optical emission spectroscopy) to understand the plasma properties and identify any abnormalities.
6. Systematic experimentation: Change one process parameter at a time to pinpoint the source of the problem. Keep detailed records of all experiments.

For instance, if the etch rate is low, increasing the RF power or gas flow rate might help. If the uniformity is poor, optimizing gas flow distribution or chuck temperature might improve results. Always document the changes and their effect on the outcome.

My experience encompasses various RIE systems, including Lam Research, Oxford Instruments, and STS systems. I’ve worked extensively with inductively coupled plasma (ICP) RIE systems from Lam Research, known for their high etch rate and excellent uniformity. The Oxford Instruments systems offer good control over anisotropic etching profiles, suitable for applications requiring high aspect ratio features. STS systems provide a good balance between performance and affordability.

Experience with each system involves a different workflow and parameter optimization. Lam Research systems, for example, often require more intricate control over various plasma parameters, offering finer adjustments for optimization. Each system has its strengths and weaknesses, making selection dependent upon specific application needs. This experience includes maintaining these systems and troubleshooting issues, allowing me to diagnose and resolve problems quickly and effectively.

Optimizing RIE processes for specific materials and features requires a deep understanding of the underlying chemistry and physics. It’s an iterative process involving carefully controlled experimentation and data analysis. The process starts with selecting the appropriate etching chemistry based on the target material’s properties. For instance, silicon etching often uses SF₆, CF₄, or mixtures thereof, while silicon dioxide frequently utilizes CHF₃ or mixtures of fluorocarbons.

Optimization Strategy:

• Material-specific chemistry: Choosing the right gas chemistry is the first step. This will impact the etch rate and selectivity.
• Process parameter optimization: Tuning RF power, pressure, and gas flow rates is critical to achieve the desired etch rate, uniformity, and selectivity. This requires careful experimentation and data analysis.
• Mask selection: The choice of masking material is important to protect areas that shouldn’t be etched.
• Bias control: Adjusting the bias voltage affects the ion energy and thus the anisotropy of the etch. Higher bias increases anisotropy but can also lead to damage.
• Temperature control: Maintaining appropriate wafer temperature can influence etch characteristics, especially for polymer deposition or removal.

Software tools and design of experiment (DOE) methodologies are frequently employed to streamline this process and optimize the parameters efficiently.

While RIE is a powerful etching technique, it has limitations compared to other methods such as wet etching, deep reactive ion etching (DRIE), or focused ion beam (FIB) etching.

• Etch rate limitations: Compared to DRIE, RIE typically has a lower etch rate, making it less suitable for high aspect ratio features.
• Isotropic etching: While anisotropic etching is possible by manipulating parameters, RIE can exhibit some isotropic etching (etching in all directions), which can be problematic for certain applications requiring precise feature definition.
• Damage to underlying layers: RIE processes can induce damage or even modify the underlying layer due to plasma bombardment or chemical reactions. This is less of an issue with more controlled techniques like DRIE.
• Mask erosion: RIE can lead to significant mask erosion, especially with high aspect ratio structures, affecting the fidelity of the etched features.
• Cost and complexity: RIE systems can be relatively complex and expensive to operate and maintain. Some other techniques may offer simpler or more cost-effective solutions for specific applications.

The choice of the optimal etching technique thus depends critically on the specific application requirements and material properties.

Plasma chemistry in Reactive Ion Etching (RIE) is the heart of the process, where we use electrically charged gases (plasma) to selectively remove material from a substrate. It’s all about carefully controlled chemical reactions within the plasma that etch the desired material without damaging surrounding areas. Think of it as a highly precise and aggressive chemical cleaning, but instead of liquids, we use reactive gases in a plasma state.

For example, in etching silicon using a mixture of SF₆ and O₂, the SF₆ breaks down in the plasma to form highly reactive fluorine radicals (F·). These radicals then react with silicon (Si) on the wafer surface, forming volatile silicon tetrafluoride (SiF₄) gas that is pumped away. The oxygen helps in removing unwanted byproducts and passivation layers. The precise balance and ratios of these gases are critical to controlling the etching rate and selectivity.

Different gas combinations produce different etching chemistries, allowing us to tailor the process for specific materials and applications. For example, etching aluminum often involves chlorine-based chemistries, while etching silicon nitride might use a combination of fluorine and nitrogen based gases. Understanding the underlying chemical reactions is vital for optimizing the RIE process.

Pressure significantly impacts the RIE process, mainly by influencing the mean free path of the ions and radicals within the plasma chamber. Lower pressures (typically a few millitorr) lead to longer mean free paths, resulting in higher energy ions that bombard the wafer surface with greater force, increasing the etching rate but potentially reducing anisotropy (resulting in less vertical etching). Higher pressures (tens of millitorr) lead to shorter mean free paths, reducing ion energy and resulting in lower etching rates and increased lateral etching (undercutting).

Think of it like this: At lower pressure, the ions are like speeding bullets, impacting the target directly and effectively. At higher pressures, it’s like a more diffused cloud hitting the target, leading to less focused etching. The optimal pressure is a balance between achieving a sufficiently high etching rate and maintaining the desired profile.

In practice, I’ve seen how subtle pressure changes can greatly affect the etching outcome. For instance, a slight increase in pressure during deep silicon etching might lead to significant undercutting, ruining the device structure. Precise pressure control is crucial for reproducible and high-quality results.

RF power directly controls the plasma density and ion energy. Higher RF power generates a denser plasma, resulting in a higher concentration of reactive species and increased etching rate. However, excessive power can lead to unwanted effects like excessive heating of the wafer, damage to the substrate, or the formation of undesirable byproducts.

Imagine RF power as the engine of the RIE system. More power means a more potent engine, leading to faster etching. But just like an engine, if you push it too hard, it can overheat and break down. Therefore, finding the right balance is critical.

In one project, we were optimizing the etching of high-aspect-ratio features. We found that increasing the RF power beyond a certain threshold caused significant bowing and damage to the sidewalls due to excessive ion bombardment. Reducing the power slightly resolved the issue while maintaining a satisfactory etching rate.

The bias voltage in RIE is a DC voltage applied between the powered electrode (where the plasma is generated) and the substrate. This voltage accelerates ions towards the substrate, significantly influencing the etching profile and anisotropy. A higher bias voltage means higher ion energy and a more directional etching, leading to better verticality (more anisotropic etching). Conversely, lower bias voltages result in less directional etching, potentially leading to more undercutting.

Think of the bias voltage as aiming the ‘bullets’ (ions) onto the target. A higher voltage means a more direct and focused impact, resulting in vertical etching. A lower voltage implies less focused bombardment, leading to more lateral etching.

For instance, when etching deep trenches, we need high anisotropy. To achieve this, we need to carefully control the bias voltage to ensure perfectly vertical sidewalls, crucial for the proper functioning of the microelectronic components. Balancing the etching rate and anisotropy through bias voltage optimization is a key challenge in advanced semiconductor fabrication.

Determining the optimal etching recipe involves a systematic approach, often involving a Design of Experiments (DOE) methodology. This starts with identifying the key process parameters (like gas flow rates, pressure, RF power, bias voltage, and temperature) and their ranges. Then we design experiments to explore the parameter space efficiently, using statistical methods to analyze the results and identify the optimal recipe. We typically monitor the etching rate, selectivity, uniformity, and profile.

For instance, let’s say we’re etching silicon dioxide (SiO₂) over silicon (Si). The goal is to maximize the etching rate of SiO₂ while minimizing the etching of Si (high selectivity). We’ll systematically vary parameters and analyze the results, using software tools to fit models and predict optimal process conditions.

The process involves iterative adjustments and refinements. We’ll start with an initial guess, run experiments, analyze the data, update our model, and repeat the process until a satisfactory recipe is achieved that meets all specifications. This requires a solid understanding of the underlying plasma chemistry and process physics.

Statistical Process Control (SPC) is essential for maintaining consistent and high-quality RIE processes. We use control charts (like X-bar and R charts) to monitor key process parameters like etching rate, selectivity, and uniformity, tracking them over time. This allows us to identify potential drifts or variations in the process early on, enabling timely corrective actions before defects are produced.

For example, we might track the etching rate of a critical layer using an X-bar and R chart. If we see that the average etching rate is consistently drifting outside the control limits, or the variability (R) increases, we investigate the cause – potentially a leak in the gas lines, a change in gas purity, or a problem with the RF power supply.

In my previous role, using SPC, we identified a cyclical variation in the etching uniformity over several weeks. It turned out to be related to the cooling system of the etching chamber. By implementing preventative maintenance, we were able to eliminate the variation and improve process stability, resulting in a significant reduction in defects and rework.

Reproducibility in RIE is crucial for ensuring consistent product quality. It’s achieved through a combination of meticulous process control, rigorous equipment calibration and maintenance, and standardized operating procedures. This includes regular calibration of gas flow meters, pressure gauges, RF power supplies, and other critical components, as well as using well-characterized gases and materials.

Implementing a robust cleaning procedure for the etching chamber is crucial, as residues from previous processes can contaminate subsequent runs. We also utilize automated process control systems, logging all process parameters for each run. This allows for thorough data analysis and troubleshooting if issues arise.

Furthermore, we regularly perform test runs with control wafers, to verify the consistency of the process. Deviation from expected results triggers an investigation and adjustment to maintain reproducibility. This rigorous approach, combining proactive maintenance, systematic data collection, and continuous monitoring, helps ensure the long-term stability and reliability of the RIE process.

Process monitoring and control in Reactive Ion Etching (RIE) are crucial for achieving consistent and high-quality results. It involves continuously tracking key parameters and making adjustments to maintain the desired etch characteristics. This is akin to baking a cake – you need to carefully monitor the temperature and baking time to achieve the perfect result.

• Real-time monitoring: We use in-situ diagnostic tools like optical emission spectroscopy (OES) to monitor plasma characteristics (e.g., radical density, electron temperature) and endpoint detection systems to determine when the etching is complete. These provide immediate feedback on the process health.

• Closed-loop control: Advanced RIE systems often have closed-loop control systems. This means the system automatically adjusts parameters (e.g., RF power, pressure, gas flow) based on real-time feedback from sensors. For example, if the etch rate drops below a setpoint, the system might automatically increase the RF power.

• Statistical Process Control (SPC): We employ SPC techniques like control charts to monitor process variability over time. This helps us identify trends, potential issues, and deviations from the desired process window. A sudden shift in the control chart, for example, might indicate a problem needing immediate attention.

For example, during a deep silicon etch, I used OES to monitor the intensity of specific emission lines that indicated the presence of silicon fluorides. By closely monitoring these signals, we were able to precisely control the endpoint of the etch and prevent over-etching.

Maintaining and troubleshooting RIE equipment requires a combination of preventative maintenance, understanding the system’s inner workings, and systematic troubleshooting. Think of it like maintaining a complex machine – regular check-ups and prompt repairs are key.

• Preventative Maintenance: This includes regular cleaning of the chamber, replacing worn-out parts (e.g., O-rings, RF matching networks), and calibrating sensors. A well-maintained system is less prone to failures and delivers more consistent results.

• Troubleshooting: When problems arise, systematic troubleshooting is essential. I typically start by checking the obvious: gas flows, vacuum levels, RF power, and temperature. Then, I move to more intricate diagnostics, like checking the impedance matching network or inspecting the plasma itself. Log books and maintenance records are critical for this.

• Understanding the system: A deep understanding of the system’s schematics, operating principles, and potential failure modes is crucial for effective troubleshooting. This comes with experience and ongoing training.

I once encountered a situation where the etch rate was unexpectedly low. By carefully analyzing the OES data and checking the gas flow rates, I discovered a partially clogged gas line. A simple cleaning resolved the issue, demonstrating the importance of regular maintenance.

Designing and implementing experiments for RIE process optimization involves a structured approach utilizing Design of Experiments (DOE) methodologies. This ensures that we systematically explore the parameter space efficiently and effectively identify optimal process conditions. It’s like finding the perfect recipe – you need a systematic approach to test different ingredients and combinations.

• DOE methodologies: I often use factorial designs or response surface methodologies (RSM) to investigate the influence of multiple process parameters (e.g., RF power, pressure, gas flow ratios) on etch rate, selectivity, and uniformity. These methods allow us to determine the most influential parameters and their optimal settings.

• Statistical analysis: After data collection, we perform statistical analysis using software like JMP or Minitab to determine the significance of each parameter and identify optimal conditions. This helps avoid random guesswork in the optimization process.

• Iteration and refinement: The experimental process is iterative. Initial experiments provide insights that guide further refinement of the process parameters. We continually refine the experimental design based on the data obtained in each iteration.

For instance, while optimizing a deep silicon etch process, I used a 2³ factorial design to investigate the impact of RF power, pressure, and SF₆ flow rate on etch rate and selectivity. This systematic approach identified the optimal settings that maximized both etch rate and selectivity simultaneously.

Documentation and reporting of RIE experiments are crucial for reproducibility, traceability, and knowledge sharing. It’s like maintaining a meticulous cookbook – clear documentation enables others to recreate your experiments and build upon your findings.

• Detailed experimental logs: I maintain detailed experimental logs that include all relevant parameters (e.g., gas flows, pressures, temperatures, RF power, etch time), process conditions, and observations. This data is entered into a structured database for easy retrieval and analysis.

• Data visualization: Visualizations like graphs and charts help to effectively communicate complex data. I usually use software packages like MATLAB or Origin to create insightful plots, such as etch rate vs. RF power or selectivity vs. pressure.

• Formal reports: Formal reports summarize the experimental findings, provide statistical analysis, and draw conclusions. These reports typically include an introduction, methodology, results, discussion, and conclusions sections.

The reports are always structured to include a clear methodology section to ensure the experiments can be easily reproduced by other researchers or engineers.

RIE etching involves the use of reactive gases and the generation of plasma, which presents several environmental considerations. It’s essential to handle these processes responsibly, similar to handling any potentially hazardous materials in a laboratory setting.

• Waste gas treatment: The exhaust gases from the RIE process often contain hazardous chemicals (e.g., fluorinated gases, silanes). These require appropriate treatment before being released into the atmosphere. Scrubbers and other gas treatment systems are essential.

• Safety precautions: RIE systems operate under vacuum conditions and involve high voltages and reactive chemicals. Strict safety procedures, personal protective equipment (PPE), and emergency response plans are crucial. Regular safety checks are a must.

• Waste disposal: Spent etching chemicals and other waste materials require proper disposal according to environmental regulations. The potential for environmental contamination necessitates responsible handling and treatment.

In my experience, working with fluorine-containing gases requires extra attention to safety and proper disposal. We utilize a dedicated exhaust system with a high-efficiency particulate air (HEPA) filter to minimize the environmental impact.

Optimizing the uniformity of an RIE etch process requires a multi-faceted approach. Think of it like painting a wall evenly – you need to control various factors to achieve a uniform result across the entire surface.

• Chamber design: The chamber’s geometry and electrode configuration significantly influence etch uniformity. Careful consideration of these factors is crucial during system design.

• Process parameters: Optimizing parameters like RF power distribution, pressure uniformity, and gas flow patterns is crucial to minimizing non-uniformities. This often involves careful calibration and experimentation.

• Wafer rotation: Rotating the wafer during the etching process helps average out any spatial variations in the plasma density or etching rate. This is a common technique to improve uniformity.

• Shielding and masking: For specific applications, using appropriate shielding and masking techniques can improve uniformity by preventing unwanted etching effects at the edges or other areas.

For example, to improve uniformity in a deep silicon etch, I experimented with different wafer rotation speeds and found that a specific speed maximized uniformity across the wafer while maintaining the desired etch rate.

Metrology tools are essential for characterizing the results of RIE etching. It’s like using precise measuring instruments to verify your results after baking the cake – accuracy is paramount.

• Profilometry: Techniques like optical profilometry (e.g., using a Dektak profilometer) provide accurate measurements of etch depth and sidewall profiles. This is crucial for verifying the etching results and ensuring they meet the design specifications.

• Scanning electron microscopy (SEM): SEM provides high-resolution images of the etched structures, allowing us to assess sidewall angles, roughness, and the presence of any defects. This provides a detailed understanding of the etch quality.

• Ellipsometry: Ellipsometry measures the thickness and refractive index of thin films. This is particularly useful for determining etch depths in shallow trenches or for determining the thickness of resist layers before and after etching.

• Spectroscopic ellipsometry: This advanced technique is ideal for characterizing the composition and thickness of multilayer films after etching, which helps in process optimization and quality control.

In one project, we used spectroscopic ellipsometry to measure the precise thickness of a dielectric layer after etching, confirming the success of a selective etch process. This precise measurement was critical for the proper function of the device.