Tailored Waveform Applications in Plasma: Unlocking New Frontiers in Plasma Technology

In plasma processing, the precise control of ion energy, plasma density, and uniformity is crucial for achieving optimal results. Whether it’s in semiconductor manufacturing, materials etching, thin-film deposition, or other advanced manufacturing processes, controlling the interaction between plasma and the material surface can significantly impact the quality, yield, and cost-effectiveness of the process.

One of the most powerful techniques to improve the precision of these interactions is the use of tailored waveform DC bias in plasma process tools. By modulating the DC bias voltage with tailored waveforms, engineers can fine-tune plasma parameters in ways that were previously not possible with simple DC or sinusoidal waveforms. This leads to enhanced process control, improved material properties, and increased process efficiency.

In this blog, we will explore the concept of tailored waveforms for DC bias, how they work in plasma tools, and their impact on the performance and precision of plasma-based processes.

Understanding RF and DC Bias in Plasma Processing: Plasma process tools often utilize RF and DC bias to control the energy and density of plasma. RF power (13.56 MHz) is primarily used to ignite and sustain the plasma, while DC or low frequency bias (few kHz to few MHz) allows for additional control over ion energy and bombardment during processes like etching and deposition. Traditionally, these waveforms have been fixed, limiting their adaptability to various materials and process requirements.

What Are Tailored Waveform Voltages?

In the simplest terms, a “tailored waveform” refers to a voltage signal that is deliberately shaped or customized to achieve a specific effect. These tailored waveforms may combine elements of both DC and AC signals and can vary in terms of frequency, amplitude, shape, and duty cycle to match the needs of a particular process. By adjusting these parameters, engineers can fine-tune the behavior of the plasma in ways that are not possible with conventional constant or simple alternating current (AC) sources.

There are several ways tailored waveforms are applied to DC bias in plasma processing:

  1. Pulse-Modulated DC Bias: A DC bias is pulsed with a variable duty cycle, often with a fixed pulse frequency. This approach allows for dynamic control over the energy imparted to the ions. Pulsing the DC bias can reduce the heating of the wafer or substrate, minimize ion bombardment during certain stages of the process, and improve material selectivity.
  2. Bias with Superimposed High-Frequency Signals: In some cases, a high-frequency AC signal is superimposed on the DC bias. This can help modulate the ion energy and increase the precision of ion-surface interactions. High-frequency signals enable faster ion acceleration and can improve uniformity in processes like thin-film deposition or etching.
  3. Chirp Modulation: Chirped waveforms—where the frequency increases or decreases over time—can be used to optimize ion bombardment at specific stages of a process. For example, chirped DC bias can be useful for processes that require gradual changes in ion energy for specific material removal or surface modification effects.
  4. Complex Multi-Component Waveforms: In advanced plasma tools, a combination of various waveform types (e.g., sinusoidal, triangular, square, or sawtooth) can be applied in combination with the DC bias to control multiple parameters at once. This enables precise energy control for each type of ion species, resulting in improved material interactions and process optimization.

Figure 1 Example of substrate biasing. (a) RF biasing voltage waveform. (b) Pulse-shaped biasing voltage waveform. (c) Tailored waveform biasing voltage waveform. (d) Tunable Ion energy distribution function as a result of biasing scheme c. The negative parts of all the three biasing waveforms are used to create a negative voltage potential on the substrate

By Tailoring the Voltage Waveform Applied to the Plasma, you can:

  1. Improve Control over Ion Energy Distribution: One of the main advantages of tailored waveforms in DC bias is the ability to precisely control the energy distribution of ions arriving at the material surface. This is critical for processes such as etching and deposition, where both the energy of the ions and the uniformity of ion bombardment can significantly affect the results.
  2. Enhance Plasma Uniformity: In large-area plasma systems, achieving uniform plasma density is a significant challenge. Tailored waveforms can be used to shape the plasma’s energy distribution, ensuring uniform ion density across the entire substrate. This is critical in applications such as thin-film deposition, where uniformity directly affects the quality of the final product.
  3. Improve Material Processing: By tailoring the waveform, one can precisely control the energy flux delivered to the material, which impacts the rate of etching, the quality of deposition, and the degree of activation on the material surface. For instance, modulating the waveform can enable selective etching or create desirable surface textures without damaging the underlying material.
  4. Minimize Arcing and Instabilities: Plasma discharges can be unstable, often leading to issues like arcing, which can damage equipment or lead to non-uniform results. Tailored voltage waveforms can stabilize the discharge, reduce arc formation, and minimize fluctuations in plasma density.
  5. Optimize Reactive Species Production: The ability to fine-tune the waveform parameters, such as frequency and duty cycle, allows for more precise control over the production of reactive species in the plasma. This is especially important in chemical vapor deposition (CVD) and other processes that rely on specific reactive species to achieve high-quality films or coatings.
  6. Minimize Damage to Substrates: In many applications, excessive ion energy can lead to substrate damage, especially in sensitive materials. Tailored DC bias can help lower the ion energy impact during processes like sputtering and deposition, allowing for smoother films and better adhesion without compromising the integrity of the underlying material.
  7. Dynamic Process Adaptation: The ability to dynamically adjust waveforms in real-time based on feedback from in-situ monitoring tools adds a new dimension to plasma processing. By continuously optimizing the RF and DC parameters, manufacturers can adapt to changing process conditions, ensuring consistent results even in high-throughput environments.

Challenges in Implementing Tailored Waveforms

While tailored waveforms offer significant advantages, their implementation is not without challenges:

  1. Complexity of Plasma-Process Interactions: Plasma systems are inherently nonlinear, and predicting the outcome of a specific waveform requires deep understanding and precise diagnostics.
  2. Power Delivery Validation: Ensuring the tailored waveform is accurately delivered to the plasma without distortion is critical.
  3. Parameter Monitoring: Real-time measurement of plasma properties is essential to correlate waveform changes with process outcomes.

The Future of Tailored Waveform Plasma Technology

As plasma technology continues to evolve, the use of tailored waveform voltages is expected to expand. Advanced control systems, such as digital signal processors (DSPs) and high-speed oscillators, are allowing for increasingly sophisticated waveform generation. These systems can adapt in real-time to changes in plasma conditions, enabling dynamic control and optimization of plasma processes.

Moreover, as industries push for miniaturization and higher precision, tailored waveforms will play a crucial role in meeting the needs of next-generation devices, whether it’s in the form of ultra-low-energy plasmas for nanofabrication or highly stable plasma discharges for large-scale industrial applications.

Impedans Technology for Tailored Waveform Plasma:

Low frequency tailored waveform biasing during plasma exposure allows precise control of ion energy, independent of ion flux, but maintaining the exact waveform shape and voltage level can be challenging. Impedans offers the Octiv Suite VI probes for pulsed RF and DC measurements, with the ability to display voltage and current waveforms. These probes support frequencies from 40 kHz to 240 MHz and provide 1-microsecond time resolution for pulse monitoring. They are easy to install between the RF generator and matching unit or between the matching unit and plasma chamber.

For real-time ion energy and flux measurements, Impedans offers Semion RFEA probes compatible with RF, DC as well as tailored waveform biasing. These systems serve as electronic dummy wafers that are situated in the plasma in place of a substrate in a plasma process. Semion RFEAs have been used in a wide variety of tool types to give reliable measurements of key parameters in etching, deposition and surface treatment processes.

Conclusion

In conclusion, the application of tailored waveform voltages is a game-changer in the field of plasma technology. The ability to precisely control plasma behavior opens up new possibilities for innovation and efficiency. As we continue to develop more advanced waveform generation techniques, the potential for tailored waveforms in plasma applications is boundless, ushering in a new era of precision, performance, and sustainability.

 To learn more about our plasma measurement products contact us at [email protected]