Silicon wafers are made from a single crystal of highly pure silicon, typically with less than one part per billion of contaminants. The Czochralski process is the most common method of forming large crystals of this purity, which involves pulling a seed crystal from molten silicon, commonly known as a melt. The seed crystal is then formed into a cylindrical ingot known as a boule.
Elements such as boron and phosphorus may be added to the boule in precise quantities to control the wafer’s electrical properties, generally for the purpose of making it an n-type or p-type semiconductor. The boule is then cut into thin slices with a wire saw also known as a wafer saw. The cut wafers may be polished to varying degrees.
What Is A Silicon Wafer Used For?
A silicon wafer is a thin slice of crystalline silicon commonly used in the electronics industry. Silicon is used for this purpose because it’s a semiconductor, meaning it’s neither a strong conductor nor strong insulator of electricity. Its natural abundance and other properties generally make silicon preferable to other semiconductors such as germanium for making wafers.
The most common dimensions of silicon wafers depend on their application. The wafers used in ICs are round with diameters typically ranging from 100 to 300 millimeters (mm). The thickness generally increases with the diameter and is usually in the range of 525 to 775 microns (μm). The wafers in solar cells are usually square with sides measuring 100 to 200 mm. Their thickness is between 200 and 300 μm, although this is expected to be standardized to 160 μm in the near future.
An IC, also known as a microchip or just chip, is a set of electronic circuits set into a substrate of semiconducting material. Monocrystalline silicon is currently the most common substrate for ICs, although gallium arsenide is used in some applications such as wireless communication devices. Wafers made of silicon-germanium alloys are also becoming more widely used, typically in applications where the greater speed of silicon-germanium is worth the higher cost.
ICs are currently used in most electronic devices, having virtually replaced separate electronic components. They’re smaller, faster and cheaper to manufacture than discrete components by orders of magnitude. The rapid adoption of ICs in the electronics industry is also due to the modular design of ICs, which easily lends itself to mass production.
Random-access memory (RAM) chips are one of the most common types of ICs, due to their particular need for a high transistor density. The thickness of the layers of photolithographic material in a RAM chip has been shrinking steadily and are now much thinner than the width of the device itself.
These layers are developed in a similar manner to regular photographs except that ultraviolet light is used rather than visible light since the wavelengths of visible light are too large to create features with the necessary precision. The features of modern ICs are so small that process engineers must use electron microscopes to debug them.
Automated test equipment (ATE) tests each wafer before using it to make an IC, a process, commonly known as wafer probing or wafer testing. The wafer is then cut into rectangular pieces known as dies and then connected to an electronic package via electrically conductive wires, which are usually made of gold or aluminum. These wires are bonded to pads that are typically located around the edge of the die using ultrasound in a process called thermosonic bonding.
The resulting devices undergo final testing phases, which typically use ATE and industrial computed tomography (CT) scanning equipment. The relative cost of testing varies greatly according to the yield, size and cost of the device. For example, testing may account for over 25% of the total fabrication costs inexpensive devices, but it can be virtually negligible for large, expensive devices with low yields.
The fabrication of ICs is a highly automated process that uses many specific techniques. These capabilities drive the high cost of building a fabrication facility, which can exceed $8 billion as of 2016. This cost is expected to increase much more quickly than inflation due to the continuing need for greater automation.
The trend towards smaller transistors will continue for the foreseeable future, with 14 nm being state of the art in 2016. IC manufacturers such as Intel, Samsung, Global Foundries and TSMC are expected to begin the transition to 10 nm transistors by the end of 2017.
Large wafers provide an economy of scale, which reduces the total cost of ICs. The largest wafers commercially available are 300 mm in diameter, with 450 mm expected to be the next maximum size. However, significant technical challenges still exist for making wafers of this size.
Additional techniques used in the fabrication of ICs include tri-gate transistors, which Intel has manufactured with a width of 22 nm since 2011. IBM uses a process known as strained silicon directly on insulator (SSDOI), which removes the silicon-germanium layer from a wafer.
Copper is replacing aluminum interconnects in ICs, primarily due to its greater electrical conductivity. Low-K dielectric insulators and Silicon on Insulators (SOIs) are also advanced manufacturing techniques for ICs.
A solar cell uses the photovoltaic effect to convert light energy into electrical energy, which generally involves the absorption of light by some material to excite electrons into a higher energy state. It’s a type of photoelectric cell, a device that changes its electrical characteristics when exposed to light. Solar cells can use light from any source, even though the term “solar” implies they require sunlight.
The generation of electricity as an energy source is one of the most well-known applications for solar cells. These types of solar cells use a light source to charge a battery, which can be used to power an electrical device.
Solar cells are often integrated into the device they’re intended to power. For example, the solar-powered lights commonly available in home improvement stores use solar cells to charge a battery during the day. At night, the battery powers a motion sensor that turns the light on when it detects motion.
Solar cells may be classified into first, second and third generation types. First generation cells are composed of crystalline silicon, including monocrystalline silicon and polysilicon. They’re currently the most common type of solar cell. Second generation cells use thin film composed of amorphous silicon and are typically used in commercial power stations. Third-generation solar cells use thin film developed with a variety of emerging technologies and currently have limited commercial applications.
Solar Cell Fabrication
The great majority of a first-generation solar cell is composed of crystalline silicon, although its structural quality and purity are far below that used in ICs. Monocrystalline silicon converts light into electricity more efficiently than polysilicon, but monocrystalline silicon is also more expensive.
The wafers are cut into squares to form individual cells, and their corners are then clipped to form octagons. This shape gives solar panels their distinctive diamond-like appearance. The cells that make up a solar panel must all be oriented along the same plane to maximize conversion efficiency. The panels are typically covered with a sheet of glass on the side that faces the sun to protect the wafers.
Solar cells may be connected in series or parallel, depending on specific requirements. Connecting the cells in a series increases their voltage while connecting them in parallel increases the current. The primary disadvantage of parallel strings is that shadow effects can cause the shadowed strings to shut down, which can cause the illuminated strings to apply a reverse bias to the shadowed strings. This effect can result in a substantial loss of power and even damage to the cells.
The preferred solution to this problem is to connect strings of cells in series to form modules and use maximum power point trackers (MPPTs) to handle the power requirements of the strings independently of each other. However, the modules can also be interconnected to form an array with the desired loading current and peak voltage. Another solution to the problems caused by shadow effects is the use of shunt diodes to reduce power loss.
The trend towards larger boules in the semiconductor industry has resulted in an increase in the size of solar cells. The solar panels developed in the 1980s are made of cells with a diameter between 50 and 100 mm. Panels made during the 1990s and 2000s typically used wafers with a diameter of 125 mm, and panels made since 2008 have 156 mm cells.
The Use Of Silicon Wafers
Silicon wafers are most often used as the substrate for integrated circuits (ICs), although they’re also a major component in photovoltaic, or solar, cells. The basic process of fabricating these wafers is the same for both of these applications, although the quality requirements are much higher for the wafers used in ICs. These wafers also undergo additional steps such as ion implantation, etching and photolithographic patterning, which aren’t needed for solar cells.
For more information on silicon wafers, contact Virginia Semiconductor online or at 540.373.2900.