Analysis of Plasma Ion-Target Collisions in Plasma Etching Systems

What Is Plasma?

Figure 1: Four States of Matter

Matter is commonly classified into solids, liquids, and gases, but by heating a gas further, it is possible to generate plasma.

For this reason, plasma is often referred to as the “fourth state of matter.”

Plasma is a state in which matter is ionized. Ionization is the phenomenon in which molecules or atoms break apart into electrons and ions.

Plasma is created when particles such as ions, electrons, and atoms collide with one another.

In what kind of environment is plasma generated?

Plasma is mainly generated in extremely high-temperature regions such as the sun, or when a high voltage is applied to a gas.

In this article, we will analyze an industrial plasma etching system using PIC-PLASMA 3D (plasma analysis software).

A plasma etching system is a device that removes material from a surface by using active species and ions in plasma. In semiconductor manufacturing, it is widely used for microfabrication of silicon, insulating films, and metal films.

Figure 2: Conceptual Diagram of Plasma Etching

The basic principle is as follows:

A reactive gas is introduced into a vacuum chamber, and high-frequency power is applied.
The gas is discharged to generate plasma.
The target material is removed through a combination of chemical action and physical action.

Chemical action occurs when radicals generated in the plasma react with the surface of the material being processed, producing volatile by-products. For example, when fluorine radicals react with silicon, the reaction forms products that can be easily exhausted as gas, thereby removing the surface material.

Physical action occurs when positive ions in the plasma are accelerated toward the substrate by the electric field and collide with the surface. This ion bombardment can sputter surface atoms and enhance the chemical reactions. As a result, anisotropic etching can be achieved, allowing material to be etched mainly in the vertical direction.

Inside the system, applying high-frequency power to the electrode holding the substrate creates a self-bias voltage, which helps ions strike the substrate almost vertically. This makes it possible to process fine patterns deeply and accurately without widening them laterally.

The features of plasma etching systems include:

Suitable for fine processing
Capable of anisotropic processing
Selectable material-specific etching behavior by choosing the reaction gas

These are the main advantages.

On the other hand, challenges include:

Substrate damage caused by excessive ion bombardment
Mask wear
Redeposition of reaction by-products

These are typical issues.

In short, a plasma etching system is a device that precisely removes material from a surface by utilizing the chemical reactivity of plasma generated in a vacuum and ion bombardment.

Load Analysis on the Target in a Plasma Etching System

Let us perform a collision simulation of plasma striking the target inside a plasma etching system using PIC PLASMA 3D.

The analysis model is shown below.

Analysis Model

Figure 3: Analysis Model of the Plasma Etching System

A plasma etching system model like the one above was created, and trajectory analysis of the plasma inside the chamber was carried out.

The plasma generation mechanism is shown below.

Figure 4: Plasma Generation Process

Analysis Conditions

The analysis conditions are as follows.

Analysis Software	PIC-PLASMA 3D
Analysis Type	Plasma Analysis
Analysis Object	plasma_etcher.step
Plasma Particle Species	Ar
Plasma Density	1.0×10⁷ [particles/m³]
Cumulative Energy Target Object	input004
Voltage	Cathode Voltage: -50000 [V] Anode Voltage: 100000 [V]
Time Step	5.0×10^-10 [s]
Total Simulation Time	1.0×10^-6 [s]

These initial conditions can be configured directly within the software (PIC-PLASMA 3D).

Figure 5: Setting Analysis Conditions in PIC-PLASMA 3D

Analysis Results

Figure 6: Plasma Analysis Results Inside the Plasma Etching System

Figure 7: Cumulative Collision Energy of the Target Plasma

The material shown above is an actual analysis result calculated using PIC-PLASMA 3D.

Figure 6 visualizes the behavior of ions (Ar⁺) in the plasma as an animation.

Figure 7 shows the cumulative collision energy (kinetic energy) between the target and the plasma.

You can confirm that ions generated by gas discharge in the plasma are colliding with the target.

This collision energy makes it possible to finely etch the target material!

Please note that the CAD model used here was created as a simplified model, so the electrode voltage and other conditions are also simplified.

In addition, PIC-PLASMA 3D can output various kinds of data in addition to the calculation results shown above.

Electron density
Current density vectors
Electric field vectors
Velocity vectors
Collisions with plasma in background gas and jet gas

*The above is just one example. Please make full use of PIC-PLASMA 3D for the development of plasma-related products.