What are semiconductor raw materials?

Semiconductors are important elements built into electronic devices and are indispensable to modern life. What are these semiconductors made of? This article introduces the raw materials of semiconductors and discusses the most important material, silicon.

Materials used in the semiconductor manufacturing process will also be introduced.

Materials for semiconductor wafers

When making semiconductors, the first step is to manufacture disk-shaped wafers. In terms of semiconductor wafer materials, semiconductors are divided into two categories: single element semiconductors and compound semiconductors.

Single element semiconductor

Single element semiconductors are made of a single element and mainly use one of the following elements:

• ・Silicon (Si)
• ・Germanium (Ge)
• ・Selenium (Se)

Most semiconductor wafers are single element semiconductors, and made from Silicon.

What is Silicon?

Silicon is a compound of oxygen and silicon (SiO₂) and is abundant in nature as silica rock. When mined silica rock is melted to extract silicon, metallic silicon is obtained. The metallic silicon must be refined to form polycrystalline silicon suitable for semiconductors. The resulting polycrystalline silicon has an ultra-high purity of 99.999999999% or higher, called “Eleven-Nine.”

The polycrystalline silicon is further melted inside of a quartz crucible and a cylindrical monocrystalline silicon ingot is pulled from a vat of the molten silicon. The obtained ingot is sliced and polished to complete the silicon wafers.

Why Silicon is mainly used

In the past, germanium was the predominant semiconductor material, but silicon is now commonly used. Silicon is the second most abundant element in the earth’s crust and is found in rocks, sand, plants, and water, so there is no fear of depletion.

In addition, silicon is easy to process, and has excellent stability and low cost compared to compound semiconductors.

Compound Semiconductors

Compound semiconductors consist of multiple elements as follows:

• – Gallium arsenide (GaAs)
• – Silicon carbide (SiC)
• – Indium phosphide (InP)
• – Gallium nitride (GaN)

Compound semiconductors have many crystal defects and are easily broken. This makes it difficult to produce large-diameter semiconductor wafers, and manufacturing efficiency is poor, resulting in high costs.

Despite these drawbacks, compound semiconductors have higher luminous efficiency than silicon and are capable of high-speed signal processing, making them suitable for applications such as LEDs and light receiving elements in optical communications. They are also used in power semiconductors because of their higher dielectric breakdown field strength than silicon.

Countries where metallic silicon is produced and where silicon wafers are made

Silicon exists in almost every country in the form of silicon dioxide, but only a limited number of countries, such as China, Norway, Brazil, and the United States, process it to produce metallic silicon, the material for semiconductors. This is because the production of metallic silicon from silicon dioxide requires an enormous amount of electricity. While Japan relies on imports for metallic silicon, it holds the largest share of the silicon wafer market in the world; Japanese manufacturers produced 55% of the silicon wafers worldwide in 2019*1.

*1 Source: Informa Tech LLC “IHS Market Materials”

Materials used in the semiconductor chip manufacturing process

The main materials used in the silicon chip manufacturing process are shown below. The semiconductor chip manufacturing process is divided into the wafer fabrication process and the assembly process.

Materials used in the wafer fabrication process

In the wafer fabrication process, silicon wafers are polished to a mirror-like surface and are subjected to several processing steps. The following materials are used in each step.

1. Oxygen gas (O₂) or water vapor (H₂O)

Silicon wafers are exposed to oxygen gas or water vapor at approximately 800 to 1,100°C for thermal oxidation to form an oxide film on the wafer surface.

2. Photoresist, developer solution

After the necessary thin film formation, a photosensitive material called photoresist is uniformly applied to the wafer surface. When light is irradiated onto the wafer through a photomask with a circuit pattern, the pattern is transferred onto the surface. Unnecessary photoresist is then removed using a developer solution.

3. Etching agent

Etching agents are used to remove unnecessary thin films. A commonly used etching method is dry etching. During etching, a corrosive gas in a plasma state removes unwanted thin films through ion bombardment and chemical reactions.

4. Cleaning solvent

Silicon wafers are cleaned using a cleaning solvent in the necessary processes including circuit pattern transfer and irradiation. Removing surface impurities such as microscopic dust and oil prevents wiring defects and improves yield.

5. Dopant

In the doping process such as ion implantation, dopants or impurity ions are implanted into the surface of a silicon wafer. Common dopants are boron (B), phosphorus (P), and arsenic (As). Dopants create carriers of free electrons or holes, making it easier to conduct electricity, resulting in the regions of differing electrical conductivity. After doping, the surface is finally polished to produce electrodes.

Main materials used in assembly process

The following materials are used in the assembly process to complete semiconductor chips

1. Lead frame

Hundreds to thousands of semiconductor chips are formed on the surface of a silicon wafer. The wafer is diced into individual chips with a diamond blade, and the chips are mounted on metal lead frames.

2. Bonding wire

Bonding wires are used to electrically connect lead frames and electrodes. Gold, silver, and copper wires are used. Although gold is expensive, it has excellent electrical conductivity, chemical stability, and corrosion resistance, making it the most suitable semiconductor connection material.

3. Mold resin

In the process known as molding, packaging, or encapsulation, mold resin is used to protect semiconductor chips. Epoxy resin is most commonly used. After molding, semiconductor chips are subjected to various tests before being shipped.

New Semiconductor Material Replacing Silicon

Silicon, an abundant resource, has replaced germanium due to its low cost and ease of processing. And today, diamond is attracting attention as a new semiconductor material. Diamond wafers have excellent properties such as a wide bandgap, high breakdown voltage and high thermal conductivity. This makes it ideal for power semiconductors. However, diamond wafers are difficult to manufacture and need to outperform SiC in terms of cost, so further challenges are expected.