How plasma etch works

Plasma etching is a crucial process in semiconductor manufacturing, enabling the precise fabrication of intricate patterns on silicon wafers. This technique is fundamental in creating the intricate circuitry that powers our modern electronic devices. Understanding how plasma etching works unveils the intricacies of semiconductor fabrication and its role in advancing technology.

Basics of Plasma Etching

Here is a wonderful 4 part introductory video from Stanford Nanofab on YouTube:

At its core, plasma etching involves the removal of material from a substrate to create desired patterns or structures. Unlike wet etching, which uses chemical solutions to dissolve material, plasma etching operates in a gas phase, making it highly controllable and precise.

The process begins with the preparation of the substrate, which is typically coated with a masking material such as photoresist. This masking layer protects certain areas of the substrate from the etching process, defining the desired pattern. The substrate is then placed in a vacuum chamber containing the etching gases.

Inside the vacuum chamber, radiofrequency (RF) energy is applied to the gases, creating a plasma—a highly reactive state of matter consisting of ions, electrons, and neutral species. This plasma is where the etching action occurs.

The plasma contains species such as ions, radicals, and electrons, each playing a specific role in the etching process. Ions bombard the surface of the substrate, transferring kinetic energy and breaking chemical bonds in the material. This physical bombardment, known as ion sputtering, dislodges atoms from the substrate surface, effectively etching away material.

Meanwhile, reactive radicals chemically react with the exposed substrate surface, forming volatile byproducts that are easily removed from the surface. This chemical reaction enhances the etching process, contributing to the precise removal of material.

The selectivity of plasma etching—the ability to etch specific materials while leaving others intact—is achieved through careful control of process parameters such as gas composition, pressure, temperature, and RF power. By adjusting these parameters, engineers can tailor the etching process to target specific materials while minimizing damage to the masking layer and underlying substrate.

Furthermore, plasma etching can be performed using various techniques, including reactive ion etching (RIE) and deep reactive ion etching (DRIE), each offering unique advantages for specific applications.

Plasma etching plays a critical role in the fabrication of semiconductor devices, enabling the production of increasingly smaller and more complex integrated circuits. As the demand for higher performance and greater miniaturization in electronic devices continues to grow, the development of advanced plasma etching techniques remains essential for pushing the boundaries of semiconductor technology.

How Bosch Process (a.k.a DRIE) works

The Bosch process, also known as deep reactive ion etching (DRIE), is a specialized etching technique used primarily in microelectromechanical systems (MEMS) and semiconductor fabrication. It enables the creation of high-aspect-ratio structures with precise control over depth and sidewall profile. The process is named after its inventor company, Robert Bosch GmbH.

The Bosch process involves a cyclic series of etch and passivation steps, repeated iteratively to achieve the desired depth and profile. The key steps of the Bosch process typically include:

Etch Step: The process begins with the etching of the substrate material, usually silicon, using a highly reactive plasma composed of gases such as SF6 (sulfur hexafluoride) and C4F8 (octafluorocyclobutane). This plasma etches away the silicon, creating vertical trenches in the substrate.
Passivation Step: Following the etch step, a passivation layer is deposited on the substrate surface to protect it during subsequent cycles. This layer, typically composed of a polymer such as carbon-fluorine (CFx), is deposited by introducing a gas such as C4F8 into the chamber. The passivation layer helps to maintain the sidewall integrity of the etched features and prevents lateral etching.
Removal of Passivation: After passivation, a second etch step removes the passivation layer from the bottom of the trenches, exposing the substrate for further etching. This step ensures that the etching process can continue to progress vertically.
Repeat Cycles: The etch and passivation steps are repeated cyclically until the desired depth or structure is achieved. Each cycle incrementally increases the depth of the etched features while maintaining their vertical sidewall profile.

By controlling the duration and parameters of each step in the cycle, we can precisely control the depth, width, and sidewall angle of the etched structures. This level of control is crucial for applications such as MEMS devices, microfluidics, and advanced semiconductor devices where high-aspect-ratio features are required.

The Bosch process is highly versatile and can be adapted to etch a wide range of materials beyond silicon, including silicon dioxide, silicon nitride, and various metals. However, the process requires careful optimization of parameters to balance etch rate, sidewall profile, and surface roughness.

The difference between RIE and ICP-RIE

Reactive Ion Etching (RIE) and Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) are both advanced techniques used in semiconductor manufacturing for precise etching of materials. While they share similarities, they differ in their mechanisms of plasma generation and control, which influence their performance and applications.

Plasma Generation:

RIE: In RIE, plasma is typically generated by applying a radiofrequency (RF) power to a pair of electrodes within the vacuum chamber. This creates a plasma consisting of ions and radicals, which are directed towards the substrate surface to perform etching.
ICP-RIE: In ICP-RIE, plasma is generated using a two-step process. Initially, a separate radiofrequency power source creates an inductively coupled plasma (ICP) in a separate chamber, which is then introduced into the etching chamber. This high-density plasma is then coupled with a lower-frequency RF power to create a more controlled and energetic plasma for etching.

Plasma Density and Energy:

RIE: RIE systems typically have lower plasma densities and energies compared to ICP-RIE. This can result in slower etching rates and may require longer process times for achieving the desired etch depth.
ICP-RIE: ICP-RIE systems generate high-density plasmas with greater ion energies. The separate generation of plasma and its subsequent introduction into the etching chamber allows for better control over plasma parameters, resulting in faster etching rates and improved selectivity.

Selectivity and Uniformity:

RIE: While RIE offers good selectivity and uniformity for many applications, it may struggle with certain materials or feature sizes due to limitations in plasma control and energy.
ICP-RIE: ICP-RIE generally offers superior selectivity and uniformity, especially for complex etching tasks involving multiple materials or intricate patterns. The ability to finely control plasma parameters enhances process control and repeatability.

Applications:

RIE: RIE systems are widely used for various applications in semiconductor manufacturing, MEMS (Micro-Electro-Mechanical Systems), and microfabrication.
ICP-RIE: ICP-RIE is often preferred for more demanding applications requiring high aspect ratio etching, deep silicon etching (such as for MEMS devices or through-silicon vias), and precise pattern transfer.