In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaic, to manufacture solar cells. The wafer serves as the substrate for microelectronic devices built in and upon the wafer. It undergoes many micro fabrication processes, such as doping, ion implantation, etching, thin-film deposition of various materials, and photolithographic patterning.
How Silicon Wafer is fabricated???
Silicon is a chemical element that makes up almost 28% to 30% of the earth’s crust. Silicon is most common material to build semiconductors and microchips with despite the fact that on its own, it doesn’t conduct electricity very well. Before a semiconductor can be built, silicon must turn into a wafer. This begins with the growth of a silicon ingot. A single silicon crystal consists of atoms arranged in a three-dimensional periodic pattern that extends throughout the material. A polysilicon crystal is formed by many small single crystals with different orientations, which alone, cannot be used for semiconductor devices.
Following steps are involved in Silicon wafer fabrication
- Growing a silicon ingot can take anywhere from one week to one month, depending on many factors, including size, quality and the specification. More than 75% of all single crystal silicon wafers grow via the Czochralski (CZ) method and another method of growing silicon ingot is FZ (Float Zone) method. CZ ingot growth requires chunks of virgin polycrystalline silicon. These chunks are placed in a quartz crucible along with small quantities of specific Group III and Group V elements called dopants. The added dopants give the desired electrical properties for the grown ingot. The most common dopants are boron, phosphorus, arsenic, and antimony. Depending on the dopant, the ingot becomes a P or N type ingot (boron: P type; Phosphorus, antimony, arsenic: N type).
- Ingot Growth-To grow ingot, the first step is to heat the silicon to 1420°C, above the melting point of silicon.
- Slicing– Once the ingot is fully-grown, it is ground to a rough size diameter that is slightly larger than the target diameter of the final silicon wafer. The ingot has a notch or flat cut into it, in order to indicate its orientation. After passing a number of inspections, the ingot proceeds to slicing. Because of the silicon’s hardness, a diamond edge saw carefully slices the silicon wafers so they are slightly thicker than the target specification. The diamond edge saw also helps to minimize damage to the wafers, thickness variation, and bow and warp defects.
- Lapping– After the wafer has been sliced, it gets lapped. The lapping process removes saw marks and surface defects from the wafer. It also thins the wafer out, relieving the stress accumulated in the slicing process. After rounding the edges, depending on the end user’s specification, oftentimes the edges will go through an extra polishing step, improving overall cleanliness and further reducing breakage up to 400%.
- Cleaning – The final and most crucial step in the manufacturing process is polishing the wafer. This process takes place in a clean room. Clean rooms have a rating system that ranges from Class 1 to Class 10,000. The rating corresponds to the number of particles per cubic foot. There are various methods of wafer cleaning including RCA Cleaning process, piranha cleaning process, UV Ozone cleaning, Megasonic cleaning etc.
- Polishing– Most prime grade silicon wafers go through 2-3 stages of polishing, using progressively finer slurries or polishing compounds. Most of the wafers are front side polished only, only 300mm wafers are both side polished. The polishing process occurs in two steps, which are stock removal and final chemical mechanical polish (CMP). Both processes use polishing pads and polishing slurry. The stock removal process removes a very thin layer of silicon and is necessary to produce a wafer surface that is damage-free. On the other hand, the final polish does not remove any material. During the stock removal process, a haze forms on the surface of the wafer, so an extra polishing step gives the wafer a mirror finish. After polishing is done wafers are processed to cleaning and passed through long series of cleaning baths. This cleaning step removes all type of particle and residue that might be present on the wafer surface after polishing. After this process wafers are ready to get packed.
Types of Silicon wafers
Mainly there are two types of Semiconductor
- Undoped Semiconductor
- Doped Semiconductor
Undoped Semiconductor- The undoped semiconductor wafers are the slice made from purely crystalline of silicon or any other semiconductor material. They are an ideal semiconductor wafer, which are also known as ‘intrinsic semiconductor wafer’.
Doped Semiconductor- Silicon semiconductor wafers, in general, are never a 100% silicon or of pure semiconductor material. Rather, they are made with an impurity factor of doping, which is carried out during the formation of the semiconductor material, with the concentration of doping in between 1013 to 1016 atoms of doping element in a cm3 of the semiconductor material to be doped. But this added impurity still does not compromise on the overall purity, which sustain a purity level of 99.9999999% or more percentage of the semiconductor material. The doping of the silicon or other semiconductor materials enables us to change and control the physical and electrical properties of the wafers as compared to the physical and electrical properties of the undoped semiconductor material. Depending on the elements, the semiconductor materials are doped with; the semiconductor wafers are further divided into two types:
- N-Type Semiconductor Wafers- When Pentavalent impurities like atoms of phosphorus, arsenic or antimony are added into pure silicon ingot that makes them N-Type semiconductor. The N-type semiconductor wafers are enriched in negatively charged ions or electrons, which are responsible to allow the passage of the electric current from one side of the doped material to the other side of the material.
- P-type Semiconductor Wafers- When trivalent impurities like Boron, Aluminium, Gallium are added to pure silicon that makes P-type semiconductor. The P-type semiconductor is enriched in positively charged holes and holes are responsible to allow the passage of the electric current from one side of the doped material to the other side of the material.
Semiconductor wafers are further classified into two more categories on the basis of the level of dopant and that are as follows
- Extrinsic semiconductor wafers- Those doped semiconductor wafers, which have tiny to the moderate percentage of the dopant or have comparatively lower numbers of atoms of dopant in the crystal of silicon or other semiconductor materials, are known as ‘extrinsic semiconductor wafers’.
- Degenerated semiconductor wafers- Those doped semiconductor wafers, which have high percentages of the dopant or have comparatively greater numbers of atoms of dopant in the crystal of silicon or other semiconductor materials, are known as ‘degenerated semiconductor wafers’.
Other types of semiconductor wafers include polished (they are polished on both sides of the wafer slice), epitaxial and SOI (Silicon on Insulator).
Application of Silicon wafers
- ICs (Integrated Circuits)
- Thin-film depositing process
- Calibrating instruments
- High-power applications (detectors and sensor devices)
- MEMS fabrication
- Transistors, diodes, and rectifiers
- Opto-Electronic components
- Laptops and Desktop computers
- Solar cell R&D and manufacturing
- Automobiles, aerospace and drone technology
- AI (Artificial Intelligence) and IoT
Comparison between P-Type and N-type Semiconductor
|S. NO.||P-Type Semiconductor||N-Type Semiconductor|
|1.||P-type is formed because it is doped with a trivalent impurity.||N-type is formed because it is doped with a pentavalent impurity.|
|2.||As the impurity inserted is of third group elements it is responsible for creating a vacancy of the electrons termed as holes.||In N-type, the impurity inserted is of a fifth group providing an excess of electrons.|
|3.||Holes formed here are termed as Acceptors.||Electrons in the n-type are termed as Donors.|
|4.||The examples of trivalent impurity added are Al, Ga, In, etc.||The examples of pentavalent impurity added are P, As, Sb, etc.|
|5.||The concentration of holes is more in p-type.||The concentration of electrons is more in n-type.|
|6.||The major concentration of the carrier’s movement can be observed from a higher level of concentration to a lower level of concentration.||The majority of the carrier’s movement can be seen from lower level to higher level.|
|7.||As the holes concentration is high p-type carries the positive charge.||In n-type, the electrons are the majority carriers. Hence the n-type preferably carries a negative charge.|
|8.||The energy level of acceptor lies nearer too valence band but away from the conduction band.||The energy level for donor lies nearer to the conduction band but away from the valence band.|
|9.||Fermi level presence can be observed between the energy level of acceptor and the valence band.||In this case, the presence of the Fermi level is in between the energy level of donor and the conduction band.|