An Introduction to Epitaxy Deposition Techniques in Semiconductor Manufacturing

In semiconductor processing, photolithography and etching are often the most commonly discussed steps. But right alongside them is another crucial category: epitaxy deposition.

Why are these deposition processes essential in chip manufacturing?

Here’s an analogy: imagine a plain, square flatbread. Without any topping, it’s bland and unremarkable. Some people prefer putting peanut better on the surface; others prefer it sweet and spread syrup. These coatings dramatically change the taste and character of the flatbread. In this analogy, the flatbread represents the substrate, and the coating represents a functional layer. Just as different toppings create different flavors, different deposited films impart entirely different electrical or optical properties to the base wafer.

In semiconductor fabrication, a wide range of functional layers are deposited onto wafers to build devices. Each type of layer requires a specific deposition method. In this article, we briefly introduce several widely used deposition techniques, including:

• MOCVD (Metal-Organic Chemical Vapor Deposition)
• Magnetron Sputtering
• PECVD (Plasma-Enhanced Chemical Vapor Deposition)

1. Metal-Organic Chemical Vapor Deposition (MOCVD)

MOCVD is a critical technique for the deposition of high-quality epitaxial semiconductor layers. These single-crystalline films serve as the active layers in LEDs, lasers, and other high-performance devices.

A standard MOCVD system consists of five major subsystems, each playing an essential and coordinated role to ensure the safety, precision, and reproducibility of the growth process:

(1) Gas Delivery System

This subsystem precisely controls the flow, timing, and ratio of various process gases introduced into the reactor. It includes:

• Carrier gas lines (commonly N₂ or H₂)
• Metal-organic precursor supply lines, often via bubblers or vaporizers
• Hydride gas sources (e.g., NH₃, AsH₃, PH₃)
• Gas switching manifolds for controlling growth/purge paths

(2) Reactor System

The reactor is the core of the MOCVD system, where actual epitaxial growth occurs. It typically includes:

• A SiC-coated graphite susceptor that holds the substrate
• A heating system (e.g., RF or resistive heaters) to control substrate temperature
• Temperature sensors (thermocouples or IR pyrometers)
• Optical viewports for in-situ diagnostics
• Automated wafer handling systems for efficient substrate loading/unloading

(3) Process Control System

The entire growth process is managed by a combination of:

• Programmable logic controllers (PLCs)
• Mass flow controllers (MFCs)
• Pressure controllers
• A host computer for recipe management and real-time monitoring

These systems ensure precise control of temperature, flow rates, and timing across each stage of the process.

(4) In-Situ Monitoring System

To maintain film quality and consistency, real-time monitoring tools are integrated, such as:

• Reflectometry systems for tracking epitaxial layer thickness and growth rate
• Wafer bow sensors to detect stress or curvature
• Infrared pyrometers with reflectivity compensation for accurate temperature measurement

These tools allow for immediate process adjustments, improving uniformity and material quality.

(5) Exhaust Abatement System

Toxic and pyrophoric byproducts generated during the process—such as arsine or phosphine—must be neutralized. The exhaust system typically includes:

• Burner-scrubbers
• Thermal oxidizers
• Chemical scrubbers

These ensure compliance with safety and environmental standards.

Close-Coupled Showerhead (CCS) Reactor Configuration

Many advanced MOCVD systems adopt a Close-Coupled Showerhead (CCS) design, especially for GaN-based epitaxy. In this configuration, a showerhead plate injects group III and group V gases separately but in close proximity to the rotating substrate.

This minimizes parasitic gas-phase reactions and enhances precursor utilization efficiency. The short distance between the showerhead and the wafer ensures uniform gas distribution across the wafer surface. Meanwhile, the rotation of the susceptor reduces boundary layer variation, further improving epitaxial layer thickness uniformity.

Magnetron Sputtering

Magnetron sputtering is a widely used physical vapor deposition (PVD) technique for fabricating functional layers and surface coatings. It employs a magnetic field to enhance the ejection of atoms or molecules from a target material, which are then deposited onto a substrate to form a thin film. This method is extensively applied in the manufacturing of semiconductor devices, optical coatings, ceramic films, and more.

Working Principle of Magnetron Sputtering

Target Material Selection

The target is the source material to be deposited onto the substrate. It can be a metal, alloy, oxide, nitride, or other compound. The target is mounted onto a device known as a magnetron cathode.

Vacuum Environment

The sputtering process is conducted under high vacuum conditions to minimize unwanted interactions between the process gases and ambient contaminants. This ensures the purity and uniformity of the deposited film.

Plasma Generation

An inert gas, typically argon (Ar), is introduced into the chamber and ionized to form a plasma. This plasma consists of positively charged Ar⁺ ions and free electrons, which are essential for initiating the sputtering process.

Magnetic Field Application

A magnetic field is applied near the target surface. This magnetic field traps electrons close to the target, increasing their path length and enhancing ionization efficiency—leading to a dense plasma region known as a magnetron plasma.

Sputtering Process

The Ar⁺ ions are accelerated toward the negatively biased target surface, bombarding it and dislodging atoms from the target via momentum transfer. These ejected atoms or clusters then travel through the chamber and condense onto the substrate, forming a functional film layer.

Plasma-Enhanced Chemical Vapor Deposition (PECVD)

Plasma-Enhanced Chemical Vapor Deposition (PECVD) is a widely used technique for depositing a variety of functional thin films, such as silicon (Si), silicon nitride (SiNx), and silicon dioxide (SiO₂). A schematic diagram of a typical PECVD system is shown below.
 

Working Principle

In PECVD, gaseous precursors containing the desired film elements are introduced into a vacuum deposition chamber. A glow discharge is generated using an external power source, which excites the gases into a plasma state. The reactive species in the plasma undergo chemical reactions, leading to the formation of a solid film on the substrate surface.

Plasma excitation can be achieved using different energy sources, including:

• Radio frequency (RF) excitation,
• Direct current (DC) high-voltage excitation
• Pulsed excitation
• Microwave excitation

PECVD enables the growth of films with excellent uniformity in both thickness and composition. Additionally, this technique provides strong film adhesion and supports high deposition rates at relatively low substrate temperatures, making it suitable for temperature-sensitive applications.

Deposition Mechanism

The PECVD film formation process typically involves three key steps:

Step 1: Plasma Generation
Under the influence of an electromagnetic field, a glow discharge is initiated, forming a plasma. High-energy electrons collide with the precursor gas molecules, initiating primary reactions that break down the gases into ions, radicals, and active species.

Step 2: Transport and Secondary Reactions
The primary reaction products migrate toward the substrate. During this transport, secondary reactions occur among the active species, generating additional intermediates or film-forming compounds.

Step 3: Surface Reaction and Film Growth
Upon reaching the substrate surface, both primary and secondary species are adsorbed and react chemically with the surface, forming a solid film. Simultaneously, volatile by-products of the reaction are released into the gas phase and pumped out of the chamber.

This multi-step process enables precise control over film properties such as thickness, density, chemical composition, and uniformity—making PECVD a critical technology in semiconductor fabrication, photovoltaics, MEMS, and optical coatings.