Silicon Wafer

What Is a Silicon Wafer?

A silicon wafer is a thin disc made out of pure, single-crystal silicon. It is used as a substrate for electronic components, such as transistors, integrated circuits (ICs), solar cells, and more. It serves as the backbone of the electronics industry, providing the foundation for manufacturing semiconductors and countless devices that power modern technology.

What is the purpose of a silicon wafer?

A silicon wafer is primarily used to make semiconductors, which are the building blocks of modern electronic devices. Through precise processes like etching, doping, and layering, the wafer can be fabricated to develop electronic circuits that power technologies across industries, from smartphones and computers (ICs) to renewable energy systems (solar cells) and industrial machinery.

Importance of silicon wafers

Silicon wafers are crucial in modern technology for these key reasons:

Foundation for electronic devices: They serve as the basis for the functionality of important electronic devices, including ICs, Microelectromechanical systems (MEMS) devices, solar cells, and optoelectronic devices.
Electrical properties: Silicon’s unique electrical properties allow precise control of current, essential for creating transistors, diodes, and other semiconductor devices.
Cost-effectiveness: Abundant and inexpensive, silicon is ideal for large-scale manufacturing, supporting global semiconductor production.
Technological advancements: Silicon wafers have driven the miniaturization of components, leading to smaller, faster, and more powerful devices that revolutionize industries like communication and automation.

Properties of silicon

Silicon has unique properties which make it the best option for manufacturing a wide array of electronic devices, such as:

Semiconducting nature: Silicon is a semiconductor, a substance whose electrical conductivity falls midway between that of a conductor and an insulator. This means it can be controlled to enable “on” and “off” states which are fundamental to computing.
Abundance: As the second most prevalent element in the Earth’s crust (after oxygen), Silicon is relatively inexpensive and widely accessible for industrial use.
Crystalline structure: Silicon naturally forms a crystalline structure that provides high purity and uniformity. This arrangement of atoms allows electrical charges to move smoothly, which is crucial for the precision needed in making semiconductors.
Thermal stability: Silicon can withstand extreme temperatures (-50 to 200°C) without significant degradation, making it suitable for use in high-performance environments.
Doping ability: Silicon can be doped with other elements, commonly phosphorus and boron, to modify its electrical properties. This enables the creation of p-type and n-type semiconductors, essential for the functioning of transistors and diodes.
Mechanical strength: Silicon is relatively strong and rigid, which contributes to the durability of silicon wafers used in the manufacturing of microelectronics.

Wafer manufacturing process

Silicon wafers are manufactured in wafer fabs in these steps:

Extraction and purification: The process begins with the extraction of silicon from sand (silica), which is refined to produce metallurgical-grade silicon. The silicon undergoes chemical purification through methods like the Siemens process, yielding electronic-grade silicon.
Crystal growth (Czochralski method): The silicon is then liquefied and a seed crystal is dipped into it. By carefully controlling the temperature and pulling the seed crystal out slowly, a large single crystal is grown.
Slicing and lapping: The crystal is then cooled and sliced into thin discs using a wire saw. The wafers are then rubbed with abrasive materials to remove any roughness or imperfections, ensuring a smooth and uniform surface.
Etching and polishing: Chemical etching removes any damage on the wafer surface, ensuring a clean substrate and chemical mechanical polishing (CMP) delivers a mirror-like finish.
Cleaning: The wafer then undergoes a thorough cleaning process using ultra-pure chemicals, such as deionized water, to remove any polishing residue and contaminants.
Testing and inspection: The wafer is carefully inspected and tested for defects or flaws before being used for specific electronic applications.

Silicon wafer specifications

Here are the key specifications of silicon wafers:

Diameter and thickness: Wafer diameter or size affects how many chips can be produced from it, while the thickness determines its mechanical properties. Common sizes are 2, 3, 4, 6, 8, and 12 inches. Depending on the diameter, its thickness can vary between 275 μm and 825 μm.
Orientation: The orientation influences fabrication processes like etching and deposition, and tilting the wafer enhances control over material removal. Common wafer orientations are 100, 110, or 111, with the three integers denoting the units on each axis of the crystal lattice. Acceptable tilt angle variations or tolerances are typically around ±0.5°.
Surface quality: Surface roughness is generally < 1 nm for polished sides, and can go as low as 0.1 nm, while unpolished sides can have a roughness of several µm.
Doping and resistivity: The doping concentration ranges from 1013 to 1018 atoms/cm3. Resistivity ranges from 0.001 ohm-cm for heavily doped wafers to several thousand ohm-cm for lightly doped ones.

Role in semiconductor fabrication

Silicon wafers are used in semiconductor foundries as the basis for creating semiconductor devices. They are the starting material upon which various electronic components, such as transistors, capacitors, and diodes are built. These wafers allow for the construction of complex circuits by providing a surface on which layers of materials can be added, shaped, and connected to form functioning semiconductor devices. Silicon wafers are essentially the substrate that holds the electrical elements together, enabling the final creation of chips that power electronic devices.

Uses of silicon wafers

Silicon wafers are commonly used in the production of these devices:

ICs: Silicon wafers form the substrate in IC manufacturing. ICs are used in almost all modern electronics, including computers, smartphones, and consumer devices.
Solar cells: They are also widely used in solar cell production, acting as the base material for photovoltaic cells due to silicon’s ability to efficiently convert light into energy.
MEMS devices: Silicon wafers form the core of MEMS devices, which are tiny mechanical systems integrated with electronic circuits. These are used in automotive safety, medical devices, and consumer electronics.
Optoelectronics: Optoelectronic devices like light-emitting diodes (LEDs) and laser diodes are used in communication and display technologies. Silicon wafers act as the substrate for these devices, enabling integration with electrical circuits.