Ion Angle/Aspect Ratio

Ion Angle and Aspect Ratio

Introduction

In the realm of plasma etching processes, understanding the behaviour of ions is crucial for achieving precise pattern transfer, critical dimension control, and high-quality feature replication. Two key parameters that significantly impact these processes are ion angle and aspect ratio. The ion angle dictates the directionality of etching profile, while the aspect ratio is defined as ratio of the depth to the width of the etched features. In this article, we will delve into the concepts of ion angle and aspect ratio, their significance in plasma etching, measurement techniques, factors influencing their values, and the importance of Impedans Ltd.’s solutions in this field. 

Section 1: Ion Angle 

In plasma etching, the ion angle refers to the angle at which ions impinge on the substrate surface during the etching process. It is a critical parameter that directly affects the material removal rate and the etched feature’s final dimensions. The ion angle is crucial because it determines the energy and momentum with which the ions strike the substrate, influencing the efficiency and selectivity of the etching process.  

Since the etching rate depends on the angular and energy distribution of the ions and flux, the ion angular distribution functions (IADFs) will influence the etching profiles and will contribute toward the aspect ratio dependence of etching rates. This effect is also known as aspect ratio dependent etching (ARDE). The ARDE depends on aspect ratio and not on the etch depth. Furthermore, ions undergoing collisions in the sheath, for example, will arrive at the wafer surface at off-normal angles of incidence. The ion angle affects etching by influencing the ion trajectory and the energy transfer to the substrate surface.

When the ions enter parallel to the trench direction or at low ion angle, a more directional etching occur resulting into vertical etch profiles enabling better pattern transfer and sidewall profile control. Low ion angle results in more vertical sidewalls, which are essential for producing high-aspect-ratio features. Achieving the desired sidewall profile is crucial in many applications, such as microelectromechanical systems (MEMS) and nanotechnology, as it directly impacts the functionality and performance of fabricated devices.

On the other hand, with a high ion angle, ions approach the surface more horizontally, leading to a sloped or undercut sidewall profile as shown in the image above. High ion angle can lead to non- vertical etching of side walls or more isotropic etching, which may be desirable in certain applications.

Section 2: Aspect Ratio and Its Role in Etching

Aspect ratio refers to the ratio of the etched feature’s height to its width in plasma etching processes. It is closely linked to the ion angle and has a profound impact on the overall etching process. 

The ion angle, which defines the angle at which ions impinge on the substrate during etching, directly influences the aspect ratio of the etched features. A steeper ion angle corresponds to a more directional etching process, resulting in a low aspect ratio. In this case, the etching primarily occurs in the lateral direction, causing less penetration into the substrate and yielding shallower, more uniform features. 

Conversely, a lower ion angle leads to a more anisotropic etching process, resulting in a higher aspect ratio. In such cases, the ions bombard the substrate surface at a more oblique angle, allowing the etching process to penetrate deeper into the substrate, creating taller features with higher aspect ratios. See below for a basic schematic of ions etching different aspect ratios with varying ion angles.

Therefore, by adjusting the ion angle during plasma etching, researchers can precisely control the aspect ratio of the etched features. Optimizing the ion angle and aspect ratio is crucial for achieving the desired feature depth and shape while maintaining process repeatability and uniformity. 

High aspect ratio etching poses several challenges, including micro-loading effects, etch mask erosion, and sidewall roughness. These challenges can lead to non-uniform feature profiles and reduced process yield. Process optimization is essential to address these issues and achieve high-aspect-ratio etching with precise control over sidewall profiles and dimensional accuracy. Understanding the link between ion angle, aspect ratio, and the overall plasma etching process is crucial for achieving the desired etched features and advancing microfabrication techniques.

Section 3: Measurement Techniques

Accurate measurement of ion angle and aspect ratio is essential for understanding and optimizing plasma etching processes. Techniques for monitoring the etch profile largely rely on measuring the aspect ratios of the substrate after etching and inferring the ion angle from these. Several techniques have been developed to achieve this: 

• Scanning Electron Microscopy (SEM) provides high-resolution imaging of sample surfaces, enabling indirect measurement of aspect ratio from the comparison of feature height and lateral dimensions. This technique provides high resolution imaging capabilities and can measure the aspect ratio for complex structures; however, the sample preparation and imaging artifacts can affect measurements. 
• Profilometry involves scanning a stylus or a laser probe along the surface of the etched sample to measure its topography. Aspect ratio is determined from the difference in height between the top and bottom of the features. This provides a direct measurement of the aspect ratio of etch features that is non-destructive technique and can be done for both flat and curved surfaces, however it provides limited lateral resolution and is time consuming for large scale measurements.
• Ellipsometry measures changes in the polarization state of light reflected from the etched surface. The technique provides information about the refractive index and thickness of the etched material, which can be used to estimate aspect ratio. 

Section 4: Factors Influencing Ion Angle and Aspect Ratio

Several factors influence the ion angle and aspect ratio in plasma etching processes. These factors include gas composition, plasma power, pressure, plasma density, and substrate properties. 

• Gas composition plays a critical role in plasma etching as it determines the types of chemical reactions occurring in the plasma. Different etching gases have varying reactivity with the material being etched, resulting in different etch rates and selectivity between materials. The presence of specific gas species can also influence the generation and behavior of ions in the plasma. Additionally, the ratio of different gases in the etching process can be adjusted to fine-tune the etch characteristics. For instance, introducing a small percentage of a non-reactive gas like N2 can modify the plasma chemistry for etching of silicon oxide layers and reduce ion energy, leading to a gentler etch profile and potentially a higher ion angle.
• Plasma power is a critical parameter that determines the energy and density of ions in the plasma. Higher plasma power results in a more energetic plasma, leading to increased ion energy and higher sputtering rates. This can be beneficial for fast material removal, but it can also result in higher ion energies impacting the substrate, potentially leading to substrate damage or undesired sidewall roughness. On the other hand, lower plasma power may lead to insufficient ion energy for effective material removal, causing slower etch rates. Careful optimization of plasma power is necessary to achieve the desired ion energy and etch rate while maintaining control over the ion angle and sidewall profile.
• Chamber pressure is another crucial factor affecting ion behavior in the plasma. The pressure in the etching chamber influences the mean free path of ions and the frequency of ion collisions. At lower pressures, ions have longer mean free paths, resulting in more ballistic trajectories and higher ion energy. Conversely, higher pressures increase the frequency of ion scattering and collisions, leading to a wider ion angle distribution. The pressure setting must be optimized to strike a balance between the etch rate, ion angle, and sidewall profile. Lower pressure is often preferred for achieving higher anisotropy and better control over the ion angle. 
• The substrate material can significantly influence ion interactions and energy transfer during plasma etching. Different materials have varying affinities for ions and may absorb or reflect ions differently, affecting their trajectories and energies.  Surface energy and topography of the substrate also play a role in ion angle control. Higher surface energy can lead to increased ion adsorption and scattering, potentially altering the ion angle.
• Substrate temperature can impact the diffusion of ions on the substrate surface and affect ion – trajectories and etch rates. 

Understanding the influence of these factors allows researchers to optimize the etching process and achieve desired pattern transfer and feature quality.   

Section 5: Importance of Ion Angle and Aspect Ratio in Etching Applications

In semiconductor device fabrication, where intricate patterns are etched onto semiconductor wafers, ion angle and aspect ratio are crucial: Precise control of ion angle and aspect ratio is essential for achieving accurate and consistent critical dimensions of transistors, interconnects, and other components. Any deviations in these dimensions can lead to device performance variations and affect the overall functionality of integrated circuits. Ion angle and aspect ratio influence the fidelity of replicated features. High aspect ratios can lead to variations in sidewall profiles, resulting in defects and reduced device yield. Controlling ion angle helps maintain uniform sidewall angles and enhances feature fidelity across the wafer. In microelectronics, which encompasses the design and fabrication of electronic circuits and devices on a microscopic scale, ion angle and aspect ratio play an important factor. Achieving high aspect ratios is crucial for creating deep and narrow structures, such as trench capacitors and through-silicon vias (TSVs). High-aspect-ratio structures enable higher device integration, improved performance, and reduced power consumption. Microelectronics also increasingly incorporates three-dimensional integration techniques, such as fin-shaped field-effect transistors (FinFETs). Precise control of aspect ratio is essential for fabricating these complex 3D structures, which are key to enhancing device performance and enabling advanced technology nodes. 

Microelectromechanical Systems (MEMS) and Nanoelectromechanical Systems (NEMS): MEMS and NEMS devices rely on precise mechanical movements and interactions. Accurate ion angle and aspect ratio control are essential for fabricating intricate and reliable micro and nanoscale mechanical structures, impacting the sensing and actuating performance of the devices. In these applications, ion angle and aspect ratio are of utmost important. 

Aspect ratio affects the surface topography and roughness of MEMS and NEMS devices. Proper control of aspect ratio is necessary to achieve the desired surface properties, particularly in applications where surface interactions and adhesion are critical. 

By optimizing ion angle and aspect ratio, advancements in industries such as microelectronics, semiconductor manufacturing, and MEMS/NEMS can be realized. 

 Section 6: Impedans Ltd’s Solutions for Ion Angle and Aspect Ratio Measurement  

Impedans provides capillary plates which, when used alongside the Impedans Semion RFEA, offer a powerful tool for ion angle and energy measurements, facilitating the control and optimization of aspect ratios during plasma etching processes. Capillary are specialized structures used in plasma etching processes to control and limit the ion angle.  

Impedans Ltd’s solutions enable accurate and reliable measurement and control of ion angle and aspect ratio, empowering researchers and industry professionals in their plasma etching endeavors. 

 Conclusion

Ion angle and aspect ratio are crucial parameters in plasma etching processes, influencing pattern transfer and feature quality. Accurate measurement and control of these parameters are essential for process optimization and achieving desired etching results. 

Impedans Ltd, with its expertise in providing advanced diagnostic solutions, offers reliable and precise tools for ion angle and aspect ratio measurement. By utilizing Impedans Ltd’s state-of-the-art technologies, researchers and industry professionals can enhance their understanding of plasma etching processes, improve process control, and drive advancements in their respective fields. 

References:

1 Shailesh Sharma, David Gahan, Paul Scullin, Stephen Daniels, M. B. Hopkins; Ion angle distribution measurement with a planar retarding field analyzer. Rev Sci Instrum 1 November 2015; 86 (11): 113501. https://doi.org/10.1063/1.4934808