Silicon wafers are a fundamental component in the technology industry, serving as the substrate material for microelectronic devices. These thin slices of semiconductor material form the basis for integrated circuits (ICs), which are used in a wide range of electronic devices, from smartphones and computers to advanced medical equipment and aerospace technology. Silicon, a crucial semiconductor, dominates the electronic and technology sector due to its conductivity and affordability. Silicon ranks seventh in universal abundance and second on Earth. Common silicon-containing materials include beach sand, quartz, and flint.
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While silicon crystals resemble metals, they are not actual metals. Pure silicon crystals act as insulators, allowing minimal electricity flow, but this changes through doping. Doping is when a little amount of an impurity is mixed into a silicon crystal to change its behavior and integrate it into a conductor. These impurities used for doping are called dopants. Silicon (Si) by itself does not conduct electricity very well; however, it can take on dopants precisely to control resistivity to an exact specification. Measured in ohm-centimeters ( ohm-cm or Ω&#;cm), resistivity quantifies how effectively the material conducts or resists the flow of electrical current. Silicon (Si) dopants such as nitrogen, indium, aluminum, gallium, and boron can be added throughout the growing process. So, for a semiconductor to be formed out of the non-conductive silicon, silicon must turn into a wafer; hence, a silicon wafer.
Silicon wafers, available in various shapes and sizes, are integral to integrated circuits, playing a vital role in electronic devices. These wafers undergo fabrication methods such as Vertical Bridgeman, Czochralski pulling, and the emerging Float Zone, known for fewer defects and superior purity. Silicon wafers are widely employed in chip and microchip manufacturing for electronic devices. The miniaturization of electronic components, made possible by the use of silicon wafers, has led to the development of increasingly compact and powerful devices. This has opened up new possibilities in communication, data processing, and automation, among other areas. The production of silicon wafers is a complex and precise process. It involves the transformation of raw silicon into a highly pure, single-crystal substrate. This process requires advanced technology and stringent quality control to ensure the production of high-quality wafers that meet the exacting standards of the tech industry.[1]
The production of silicon wafers is a multi-step process that begins with the extraction of raw silicon from quartz. Silicon (Si) is the second most abundant element on Earth, but it is not found in its pure form. Instead, it is typically found in the form of silicon dioxide (SiO2) or quartz. The extraction process involves heating the quartz in a high-temperature furnace in the presence of carbon, which reduces the silicon dioxide to silicon.
Once the raw silicon has been extracted, it must be purified to a high degree. This is because the performance of electronic devices is highly sensitive to impurities in the silicon substrate. The purification process involves several steps, including refining, zone melting, and solidification, which collectively increase the purity of the silicon to 99.% or higher. It is then allowed to solidify into a silicon rod, or ingot, by using common fabrication methods like the Floating Zone or Czochralski process. Czochralski's method involves the placement of a small piece of solid silicon in a pool of molten silicon, and then pulled slowly in rotation as the liquid transforms into a cylindrical ingot.
This is why the end product wafers are all disc-shaped. Before it cools off completely, the pyramidal ends of the ingot are yanked off. Sharp diamond saw blades are then used to slice the body into thin wafers of the same thickness. This clarifies why the diameter of an ingot would be the determinant of a wafer size. The wafers are typically around 1 millimeter thick, but can be made thinner for certain applications. After slicing, the wafers are polished to create a smooth, flat surface that is suitable for the fabrication of electronic devices.[2]
Silicon Wafers and Microcircuits
The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors, metals, salts and synthetic gemstones. Named after Polish scientist Jan Czochralski, who invented the method in , it is used to produce single crystal silicon used for semiconductor devices. The process begins with a seed crystal being dipped into a crucible containing molten silicon. The seed crystal, which is attached to a rod, is slowly pulled upwards and rotated simultaneously.
As the seed crystal is lifted, the molten silicon cools and solidifies, maintaining the crystal structure of the seed. The rate at which the seed crystal is lifted and the temperature of the environment are carefully controlled to ensure a uniform crystal structure. The result is a large, cylindrical crystal of silicon, known as a boule. The boule is then sliced into thin wafers, which are subsequently polished and used in semiconductor manufacturing. The Czochralski process is renowned for producing high-quality crystals with few defects, making it the preferred method for producing silicon wafers in the semiconductor industry.[3]
After the formation of the silicon boule through the Czochralski process, the next step in the production of silicon wafers is slicing and polishing. The boule, a cylindrical single crystal of silicon, is sliced into thin discs, or wafers, using a diamond saw. The diamond saw is used due to its hardness, which allows for precise and clean cuts. The thickness of the wafer surface depends on the specific requirements of the semiconductor devices they will be used to produce, but they are typically around 1 millimeter thick. The slicing process must be carefully controlled to ensure the wafers are of uniform thickness and free from defects. Any irregularities in the wafer can impact the performance of the semiconductor devices they are used to produce. Therefore, the slicing process is a critical step in the production of silicon wafers.
After the wafers have been sliced, they undergo a series of cleaning and polishing steps to prepare them for the fabrication of electronic devices. The first step is a cleaning process that removes any residual silicon dust from the slicing process. This is typically done using a combination of chemicals and ultrasonic agitation. Following the cleaning process, the wafers are polished to create a smooth, flat surface. The polishing process involves the use of a polishing slurry, which is a mixture of chemicals and abrasive particles. The wafers are polished on both sides to ensure a uniform surface.
The polishing process not only improves the physical appearance of the wafers but also enhances their electrical properties. A smooth, flat surface is essential for the subsequent deposition of thin films or layers of other materials during the fabrication of semiconductor devices. Therefore, the slicing and polishing processes are critical steps in the production of high-quality silicon wafers.[4]
Thin Films are layer or coating of material that is deposited onto the surface of the silicon wafer. These thin films can serve various purposes and are crucial for the fabrication of semiconductor devices. The deposition of thin films is a key step in the manufacturing process, and it involves applying a layer of material onto the wafer to achieve specific properties or functions.
Here are a few common types of thin films used in silicon wafer technology:
Dielectric Thin Films: These films are insulating layers that help isolate different components of a semiconductor device. Silicon dioxide (SiO2) is a common dielectric thin film used for this purpose.
Metal Thin Films: Metals like aluminum or copper are often deposited as thin films to create conductive paths or interconnects between different parts of the semiconductor device.
Semiconductor Thin Films: In some cases, additional semiconductor materials are deposited as thin films to modify the electrical properties of the silicon wafer.
Passivation Films: Thin films are sometimes applied to protect the surface of the silicon wafer and improve the overall device performance. Silicon nitride (Si3N4) is an example of a passivation thin film.
The deposition of these thin films is typically done using techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). The choice of thin film materials and deposition methods depends on the specific requirements of the semiconductor device being manufactured.[5]
1. Edge Die(chips): It is considered as the production loss. The chips along the edge of a wafer. Larger wafers have less chip loss.
2. Scribe Lines: Between the functional portions, there are narrow, non-functional areas where a saw can securely cut the wafer without destroying the circuits. These thin areas are the scribe lines
3. Chip: a little piece of silicon that has electronic circuit patterns
4. Flat Zone: edge of a wafer that is yanked off flat to aid in wafer orientation and type identification.
5. Test Element Group (TEG): a prototype pattern that displays the actual physical features of a chip (diodes, circuits, capacitors, transistors, and resistors) so that it may be tested to know if it works adequately.
Further reading: Wafer Thinning: Investigating an essential part of semiconductor fabrication
Silicon wafers possess a unique combination of physical and chemical properties that make them ideal for use in the technology industry. These properties include electrical conductivity, thermal conductivity, and mechanical strength, among others. Understanding these properties is essential for the design and fabrication of electronic devices, as they directly impact the performance and reliability of the devices.[6]
One of the most important properties of silicon wafers is their electrical conductivity. Silicon is a semiconductor, which means that its electrical conductivity lies between that of a conductor, like copper, and an insulator, like glass. The electrical properties of silicon can be precisely controlled by introducing small amounts of impurities, a process known as doping.
Doping involves the addition of either electron-donating elements, such as phosphorus or arsenic, or electron-accepting elements, such as boron or aluminum. The introduction of these impurities creates either an excess or a deficiency of electrons in the silicon lattice, resulting in either n-type or p-type silicon, respectively. The controlled introduction of these impurities allows for the creation of specific electrical properties in the silicon wafer, which is essential for the fabrication of semiconductor devices.
The ability to control the electrical properties of silicon wafers is crucial for the development of electronic devices, such as transistors, diodes, and integrated circuits. These devices rely on the precise control of electrical current flow, which is made possible by the unique electrical properties of silicon wafers. The performance, efficiency, and reliability of electronic devices are directly influenced by the quality and consistency of the silicon wafers used in their fabrication.
Silicon wafers also exhibit unique thermal properties that are critical to their function in electronic devices. One of the key thermal properties of silicon is its thermal conductivity, which is the measure of a material's ability to conduct heat. Silicon has a relatively high thermal conductivity, around 149 W/m·K at room temperature. This property is crucial in electronic devices, as it allows for the efficient dissipation of heat generated during operation, thereby preventing overheating and ensuring the reliability and longevity of the device.
Another important thermal property of silicon is its coefficient of thermal expansion (CTE), which measures how much the material expands or contracts with changes in temperature. Silicon has a relatively low CTE, around 2.6 µm/(m·K) at room temperature. This means that silicon wafers do not significantly expand or contract with temperature changes, which is important in the fabrication and operation of electronic devices. Large changes in dimension with temperature can lead to mechanical stresses and potential failure of the device.
The thermal properties of silicon wafers are not only important for the operation of the devices they are used to produce, but also for the fabrication process itself. Many steps in the fabrication process, such as doping and oxide growth, involve high temperatures. The high thermal conductivity and low CTE of silicon wafers allow these processes to be carried out efficiently and without inducing mechanical stresses in the wafer.
Silicon wafers are used in a wide range of applications within the technology industry, thanks to their unique combination of electrical and thermal properties. These applications include the fabrication of semiconductor devices, solar cells, and other electronic components. The versatility of silicon wafers has made them an essential material in the development of modern technology.
Silicon wafers play a pivotal role in the semiconductor industry, serving as the foundation for the production of integrated circuits (ICs) and various semiconductor devices. These ICs, essential for modern electronics, enable the compact design and enhanced functionality of devices like smartphones, computers, and medical equipment.
In semiconductor manufacturing, silicon wafers serve as the canvas for depositing thin films or layers of diverse materials. Through subsequent processes of photolithography, etching, and doping, these layers are transformed into the desired electronic components. The paramount importance of high purity and precise electrical properties in silicon wafers is evident, as even minor impurities or variations can lead to operational failures.
Furthermore, silicon wafers are integral to the manufacturing of discrete semiconductor devices, such as transistors and diodes. These devices, fundamental to electronic circuits, govern the flow of electrical current and facilitate functions like amplification, switching, and rectification of electrical signals. Beyond their unique electrical properties, silicon wafers, with their high thermal conductivity and low coefficient of thermal expansion, emerge as an ideal material for crafting these essential electronic components.
The integration of MEMS technology further elevates the capabilities of silicon wafers, expanding their applications in diverse technological realms. MEMS, or Micro-Electro-Mechanical Systems, bring a new dimension to semiconductor devices by incorporating tiny sensors, actuators, and other microstructures directly onto silicon wafers. This integration enhances the precision, responsiveness, and versatility of semiconductor devices, contributing to advancements in various technological fields.[7]
Further reading: Silicon Semiconductor: A Comprehensive Guide to Silicon and its Use in Semiconductor Technology
Silicon wafers play a crucial role in the production of solar cells, which are the key components of solar panels used for harnessing solar energy. Solar cells, also known as photovoltaic cells, convert sunlight directly into electricity through the photovoltaic effect. This process involves the generation of a flow of electricity in a material upon exposure to light. The majority of solar cells are made from silicon due to its excellent semiconductor properties. Silicon's ability to absorb sunlight and its semiconductor nature makes it an ideal material for solar cells. When sunlight hits the silicon wafer in a solar cell, it excites the electrons, causing them to move and create an electric current.
There are two main types of silicon used in solar cells: monocrystalline and polycrystalline silicon. Monocrystalline silicon is made from a single crystal structure, which allows for the free and unimpeded flow of electrons, resulting in high efficiency. Polycrystalline silicon, on the other hand, is made from multiple crystal structures, which can impede the flow of electrons and result in lower efficiency, but it is cheaper to produce.
The production of silicon wafers for solar cells involves similar processes to those used in the semiconductor industry, including the Czochralski process, wafer slicing, and polishing. However, the wafers used in solar cells are typically thicker and less pure than those used in the semiconductor industry. Despite these differences, the fundamental properties of silicon wafers, including their electrical and thermal properties, make them an essential component in the production of solar cells.[8]
Close-up of solar cell
Although, silicon carbide (SiC) is not a part of the silicon wafer, it is a distinct material that can be used as an alternative to traditional silicon wafers in certain applications. Silicon carbide is a compound made up of silicon and carbon, and it has unique properties that make it advantageous for specific uses in the semiconductor industry.
Silicon carbide wafers can be used in place of silicon wafers in the fabrication of electronic devices. Silicon carbide (SiC) has excellent thermal conductivity, high breakdown voltage, and can operate at higher temperatures compared to silicon. These properties make SiC suitable for high-power and high-frequency electronic applications, such as power devices in electric vehicles, radio-frequency (RF) devices, and high-temperature applications.
However, it's important to note that SiC wafers are a specific subset, and silicon wafers, primarily made from crystalline silicon, remain the standard in many semiconductor applications.
The production of silicon wafers involves a series of complex processes, each of which presents its own set of challenges. These challenges range from maintaining the purity of the silicon during the manufacturing process to managing the high costs associated with the production of high-quality silicon wafers. Despite these challenges, solutions have been developed that allow for the efficient and cost-effective production of silicon wafers.[9]
One of the primary challenges in silicon wafer manufacturing is maintaining the purity of the silicon throughout the manufacturing process. The electrical properties of silicon wafers, which are critical to their function in electronic devices, are highly sensitive to impurities. Even trace amounts of impurities can significantly alter these properties, leading to device failure.
To address this challenge, the production of silicon wafers begins with the creation of ultra-pure silicon through a process known as the Siemens process. This process involves the reaction of hydrogen with trichlorosilane, a silicon compound, at high temperatures to produce ultra-pure silicon. The resulting silicon is then further purified through the Czochralski process, which involves the growth of a single crystal of silicon from a molten pool of ultra-pure silicon.
Despite these purification processes, maintaining the purity of the silicon during the subsequent wafer slicing and polishing processes can be challenging. These processes can introduce impurities into the silicon, which can alter its electrical properties. To prevent this, the slicing and polishing processes are carried out in cleanroom environments, where the level of airborne particles is strictly controlled.
In addition, the wafers are cleaned after slicing to remove any residual silicon dust, which can also introduce impurities. The cleaning process typically involves a combination of chemicals and ultrasonic agitation, which effectively removes the silicon dust without introducing additional impurities. Despite the challenges associated with maintaining the purity of silicon wafers, the solutions developed to address these challenges have enabled the production of high-quality silicon wafers with the precise electrical properties required for the fabrication of electronic devices.
Another significant challenge in silicon wafer manufacturing is the high cost associated with the process. The production of silicon wafers involves several complex and energy-intensive processes, including the Siemens process for creating ultra-pure silicon, the Czochralski process for growing single silicon crystals, and the wafer slicing and polishing processes. Each of these processes requires specialized equipment and a significant amount of energy, contributing to the high cost of silicon wafer manufacturing.
One approach to reducing production costs is through process optimization. This involves the careful analysis and improvement of each step in the manufacturing process to increase efficiency and reduce waste. For example, the Czochralski process can be optimized by carefully controlling the temperature and pulling rate to maximize the size of the silicon crystal that can be grown from a single batch of molten silicon. Similarly, the wafer slicing process can be optimized by using advanced slicing techniques, such as wire saw slicing, which can produce thinner wafers and thus more wafers per silicon ingot.
Another approach to reducing production costs is through the recycling of silicon waste. During the wafer slicing and polishing processes, a significant amount of silicon is lost as waste. This silicon waste can be collected and recycled back into the manufacturing process, reducing the amount of new silicon that needs to be produced.
The use of alternative materials for certain applications can also help to reduce production costs. For example, for applications that do not require the high purity and precise electrical properties of silicon, other less expensive materials, such as glass or plastic, can be used.
Despite the high costs associated with silicon wafer production, the development of cost-reducing strategies, such as process optimization, silicon recycling, and the use of alternative materials, has enabled the production of silicon wafers to remain economically viable. These strategies, combined with the ongoing demand for silicon wafers in the technology industry, ensure the continued production and use of this essential material.
Silicon wafers play an indispensable role in numerous aspects of human life and technological progress, renowned for their stability among semiconductor materials. These wafers are not only a superior alternative to metallic substances but also widely available on Earth. Research on semiconductor materials like silicon, silicon carbide (SiC), germanium, arsenide, and gallium has propelled significant technological advancements. The invention of ICs, powered by silicon wafers, has revolutionized manufacturing, transforming large machinery into portable devices. Ongoing research focuses on expanding wafer size and controlling properties through doping, promising more sophisticated inventions from silicon wafers in the near future.
As a fundamental component in the technology industry, silicon wafers serve as the foundation for integrated circuits, semiconductor devices, and solar cells. Renowned for their unique electrical and thermal properties, coupled with high purity, these wafers are crucial for various applications. Despite challenges in production, efficient and cost-effective solutions have been devised, ensuring the sustained availability of this crucial material for the technology industry through continuous advancements in production technology.
1. What are silicon wafers?
Silicon wafers are thin slices of silicon that serve as the substrate for the fabrication of electronic devices. They are produced from ultra-pure silicon through a series of complex processes, including the Czochralski process, wafer slicing, and polishing.
2. Why are silicon wafers used in the technology industry?
Silicon wafers are used in the technology industry due to their unique electrical and thermal properties. These properties, combined with the high purity of silicon, make it an ideal material for integrated circuits and other semiconductor manufacturing, as well as solar cells.
3. What are the challenges in silicon wafer manufacturing?
The production of silicon wafers involves several challenges, including maintaining the purity of the silicon throughout the production process and managing the high costs associated with the process. Solutions have been developed to address these challenges, including process optimization, silicon recycling, and the use of alternative materials.
4. What is the future of silicon wafer production?
The future of silicon wafer production is likely to involve further advancements in production technology, aimed at increasing efficiency, reducing costs, and improving the quality of the wafers. These advancements, combined with the ongoing demand for silicon wafers in the technology industry, ensure the continued production and use of this essential material.
1. Pappas, S. () Facts about Silicon. Available at: https://www.livescience.com/-silicon.html
2. SVM (na). The Silicon Wafer Manufacturing Process. Available at: https://svmi.com/service/silicon-wafer-manufacturing-process/
If you are looking for more details, kindly visit Aluminum Wafers.
3. The Czochralski Process. Available at: https://www.pvatepla-cgs.com/en/technologies/czochralski-process-cz/
4. SUMCO, . Wafer Forming Process. Available at: https://www.sumcosi.com/english/products/process/step_02.html
5. ScienceDirect, . Silicon Wafer and Thin Film Measurements. Available at: https://www.sciencedirect.com/science/article/pii/B
6. Semiconductor Nanotechnology: Advances in Information and Energy Processing and Storage - Nanostructure Science and Technology. Stephen M. Goodnick (editor), Anatoli Korkin (editor), Robert Nemanich (editor), Springer Nature Switzerland AG
7. Wafers World Incorporated (). What is it and What is it used for? Available at: https://www.waferworld.com/post/silicon-wafer-what-is-it-and-what-is-it-used-for
8. AZO Materials. Silicon in Electronic Devices and Solar Cell Applications. Available at: https://www.azom.com/article.aspx?ArticleID=
9. Semiconductor Engineering, . Challenges And Solutions For Silicon Wafer Bevel Defects During 3D NAND Flash Manufacturing. Available at: https://semiengineering.com/challenges-and-solutions-for-silicon-wafer-bevel-defects-during-3d-nand-flash-manufacturing/
While the previous episode on etching revealed how unwanted materials are removed from the wafer&#;s surface, this episode will explain how materials are precisely and evenly added to the surface as a thin film through deposition. Though there are several processes that add materials to a wafer, deposition is particularly important due to its crucial role in supporting the miniaturization of semiconductors. This episode will explain not only the function and types of deposition, but also look at its relationship with other semiconductor processes and the challenges involved.
&#; Figure 1. Adding chocolate syrup and another layer of the cookie on top
We can first get a better idea of the deposition process by returning to the cookie-making analogy used in previous episodes. As shown in Figure 1, in order to make a chocolate-filled cookie, chocolate syrup is first added to the etched surface of the cookie and then another cookie is placed on top to create another layer. This thin layering process is akin to deposition.
The deposition process is very intuitive. Once the wafer is prepared for processing, it is inserted into the deposition device. As time passes, a sufficiently thick film will be formed on the surface of the wafer before unnecessary parts are removed in order to move on to the next process.
Just as etching is one of the many processes that removes materials from a wafer&#;s surface, deposition is also part of a group of processes that adds materials to a wafer&#;s surface. For example, the photoresist coating process involves applying various films to the wafer&#;s surface, while the process of oxidating the wafer&#;or silicon&#;also adds materials to the wafer&#;s surface. So, what makes the deposition process stand out among these various processes?
It has to do with the miniaturization of semiconductors that became increasingly necessary for high-performance and low-power electronics. With such miniaturization, it was required to add thin films composed of various materials such as metal that can handle different roles. In the past, semiconductor companies used aluminum for the metal wiring inside the chip due to its high-conductivity1. However, as the miniaturization of aluminum reached its limit, manufacturers switched to using copper for the wiring as it has a higher conductivity than aluminum. But the problem with copper atoms is that, unlike aluminum, they have the tendency to even spread into areas that manufacturers don&#;t want to be interfered with&#;such as those containing silicon dioxide (SiO2). To prevent this, a high-quality thin film is applied to the area where the copper wiring is to be coated. It acts as a protective film by restricting the passing of copper.
1Conductivity: The measure of a material&#;s ability to pass an electric current. Materials such as metals have high conductivity.
To make the layers of the semiconductor&#;s core device and wiring that is only one-thousandth the thickness of a human hair, it is necessary to apply these materials thinly and evenly. This is why the deposition process&#;which is referred to as the thin film deposition process within the semiconductor industry&#;is crucial in the semiconductor manufacturing process.
Since semiconductors cannot operate with pure silicon alone, the process of adding materials is very important in semiconductor manufacturing. It separates the two areas that must not be interfered with while using wires to connect areas together. Adding materials is also necessary in other cases such as when using a specific film to strengthen or weaken an electrical field, or to facilitate the next process in the semiconductor manufacturing procedure by producing a thin film in advance.
Among the various roles of thin films in a semiconductor, the most crucial function is that of a protective barrier. These films increase the reliability of operation by creating boundaries among circuits to prevent interference of core semiconductor devices and to block leakage in currents. If needed, the films can be applied at the end of the manufacturing process to protect the chip from external shock. In addition, when etching is used following the stacking of semiconductors, the films can be used to prevent etching in unwanted areas. Some examples of such protective films are STI2 and IMD3, while the materials that are used for the films include silicon dioxide (SiO2), silicon carbide (SiC), and silicon nitride (SiN).
2 STI (Shallow Trench Isolation): A trench-shaped protective film that prevents leakage current in the device&#;s boundary.
3 IMD (Intermetal Dielectric): A protective film that prevents unwanted current flow between the layers of metal wiring.
&#; Figure 2. STI preventing leakage current at the device&#;s boundary
Another material that is used is metal. It makes sure the semiconductor&#;s bottom device, or the transistor, fulfills its role by connecting the transistor with other devices and power sources. As such, the transistor is useless by itself without such connections. To make these connections, metal wiring made from materials such as titanium, copper, and aluminum is needed while a contact that connects the metal wiring and components needs to be created. This process is just like soldering wires to connect the components on an electronic circuit board inside home appliances. The wires connected on top of the circuit board serve the same purpose as the metal wiring inside a semiconductor, and soldering has the same function as contacts inside a chip.
Moreover, deposition is used for many other purposes, such as the production of transistors to form the gate insulating film and applying hard masks that are used in multiple patterning4. As mentioned earlier, deposition is used in almost every step of semiconductor manufacturing and it, at times, replaces other processes. As an example, gate insulating films were made through the oxidation process in the past. But, nowadays, deposition is the primary method as the emphasis on precision and quality increased due to the miniaturization of semiconductors.
4 Multiple Patterning: A technology that makes semiconductors even finer. It repeats the processes of exposure and etching several times.
&#; Figure 3. Examples of high and low uniformity
It is also useful to know the terminology that relates to the quality of the deposition process. Some of them will sound similar to those that were introduced in the etching episode. The first term is &#;uniformity&#; which is the measure of how evenly the materials are formed during the deposition process. As the entire wafer is placed inside a machine during deposition just as it is during etching, the thickness may vary in different parts of the wafer. A higher uniformity signifies that the material was evenly applied to the entire wafer.
The next term is &#;step coverage.&#; As we saw in the etching and oxidation processes, the thickness of the film may not be evenly formed if there are sharp or uneven edges on the wafer&#;s surface. Step coverage refers to the difference in thickness between the top and bottom films&#;or top and side wall films&#;that are on the rough surface where deposition is performed. If the step coverage is close to a value of 1, it means that there is minimal difference between the top and bottom or side film thickness. If the step coverage is significantly less than 1, it indicates that the bottom or side wall film is very thin compared to the top.
&#; Figure 4. Examples of step coverage
Similar to other processes, deposition methods can also be divided into chemical and physical types: chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD is a method that deposits materials onto the wafer&#;s surface using a chemical reaction. The most common method is to utilize catalytic activity to provide energy into a mixture of gases. If material A needs to be deposited on the surface, two gases&#;B and C&#;which can produce A are injected with the addition of energy or something similar that can trigger a reaction. The below equation shows how the material is made:
B + C + (energy, etc.) &#; A + byproducts
The chemical method is optimal due to a high deposition rate and excellent step coverage. However, there is the downside that various impurities can contaminate the materials as it is impossible to completely remove byproduct gases that can be constantly generated during the reaction process. This method, therefore, is used to create various thick shields or dispensable films like hard masks rather than being used in areas where property control must be very precise.
&#; Figure 5. CVD and PVD deposition methods
Meanwhile, PVD is a method that deposits materials onto the wafer&#;s surface by gasification. As shown in Figure 5, material A is vaporized into atoms which will then be deposited onto the wafer. Just as in etching, the common method of PVD is sputtering5 which uses plasma ions&#;typically inert gases&#;moving at high speeds to release atoms from the target, material A. The separated atoms travel in the opposite direction until they are deposited onto the wafer.
5 Sputtering: A physical method that causes the surface of a material to break apart by impinging high energy on it.
Given that there are no byproduct gases, high purity is one advantage of this method. Moreover, it is possible to deposit nonreactive, pure materials such as pure tungsten and cobalt. Due to these characteristics, PVD is commonly used in metal wire manufacturing where pure materials are heavily used.
Meanwhile, there is also a unique process called atomic layer deposition (ALD). While the processes we have discussed so far involved chemical bonding of activated gas to a wafer&#;s surface or depositing materials via sputtering, ALD uses a slightly different method. To thinly deposit material A onto a wafer, two reactant materials in the form of B and C that are used to make A need to be prepared. Material B is a precursor that can be easily adhered to the wafer&#;s surface while material C is highly reactive. To begin with, the atoms of material B stick onto the wafer&#;s surface. If these atoms have the characteristic of not sticking well to each other, only a single atomic layer of material B will remain on the wafer&#;s surface. Next, the remnants of material B are removed and material C is injected. Material B and C react to form material A and, also, create a byproduct gas, which should be removed afterwards. Repeating this process can control the thickness of the film at the atomic level.
&#; Figure 6. Concepts of CVD and ALD (Source: The Understanding of the Semiconductor Manufacturing Technology, p. 293)
This method is ideal for its excellent uniformity and step coverage. The precursor material not only can stick to various surfaces&#;whether vertical or horizontal&#;but it also allows only one atomic layer to be produced per ALD cycle. However, given that the method operates at the atomic-layer level, the downside is that the process is slow. Due to these characteristics, ALD is commonly used in components such as DRAM capacitors that have a high aspect ratio6 but require a high-quality film.
6 Aspect Ratio: The ratio of depth to width. A high aspect ratio means that the structure is narrow but tall.
Having reviewed the types of deposition, it is clear that there is a trade-off between precision and processing speed in deposition processes, just as in other processes. In other words, when improving properties like uniformity to enhance precision, the processing speed is going to be inevitably slower. This balance between precision and processing speed is a constant dilemma for semiconductor manufacturers, and the deposition process is no exception.
At times, there are reports over discoveries of new materials that are expected to greatly improve specifications. However, when it comes to the semiconductor sector, there are not that many instances where the new material featured on the news is actually used in the industry. This is because better qualities of materials do not necessarily guarantee better performance, while the properties required for deposition materials are as diverse as those required for deposition equipment. The next section will look at some of the effects that material properties have on manufacturing.
&#; Figure 7. Pattern damage due to thermal expansion
Any change in size when a material is heated is called thermal expansion. If we take train tracks as an example, there are gaps between the tracks to prevent them from bending under the summer heat due to thermal expansion. While thermal expansion also occurs in semiconductor manufacturing, this can be an issue as each material has a different degree of expansion. For example, the coefficient of thermal expansion in aluminum is over 40 times that of silicon oxide. Consequently, if a high-temperature process is applied to an aluminum thin film made on silicon oxide, the internal structure may bend and get damaged. If the material previously used for a specific thin film is replaced with a material with a significantly different expansion coefficient, the manufacturing yield may change significantly at high temperatures.
&#; Figure 8. The concept of electromigration
There is also a phenomenon called electromigration (EM) where moving electrons hit metal wiring atoms and change positions when the electricity flows through the metal wiring. This phenomenon mainly occurs in light metal wires made of aluminum. To avoid this, copper wiring was introduced and, consequently, many additional processes were discovered including the need to introduce a diffusion barrier. As miniaturization progressed further, EM also appeared in copper wiring, and to solve this problem, a major tech company introduced cobalt wiring in the metal wiring layer. Since the material of the core wiring layer changed, tremendous process changes occurred in the layers above and below as the attempts to improve the EM properties required a major change in the process.
It is worth noting that semiconductor manufacturing is a very tightly intertwined operation of hundreds of processes. In other words, when assessing the quality of a material, not only the material&#;s traits but its relationship with other processes should be taken into consideration as deposited materials do not exist in a vacuum.
Deposition is a vital aspect of semiconductor manufacturing which has been shown to have close relationships with other processes, even replacing them on some occasions. As mentioned earlier, it is possible to produce the same materials through deposition and other processes with differing results. As an example, silicon dioxide (SiO2) can be made through oxidation as well as deposition, but the properties of the material can change depending on the process that has been taken.
In some cases, the same methods can be used in various processes for different purposes. For example, sputtering is used in physical etching and deposition. The only distinguishing factor is whether sputtering is used to cut the wafer itself or to attach materials that have already been cut. Chemical etching and CVD also share similarities. Most notably, one of the important factors in chemical etching is whether the byproducts generated by the reaction between the etching gas and the reactant are vaporized. This also applies to CVD as the byproducts generated from this deposition process should be vaporized well and discharged easily so that the process proceeds smoothly.
Deposition&#;s importance and consideration of other processes can also be seen through the choice of materials used. We have seen that semiconductor manufacturers do not merely select materials that have a few good physical properties, as thermal expansion must be considered for deposition materials. If an excessively high temperature is required in the material deposition process, the previously deposited material may change. Whereas, if a material that is excessively sensitive to temperature is used, it becomes difficult to use heat in subsequent processes. Moreover, having more control over the material&#;s deposition rate and purity level provides more options for the process.
Semiconductor manufacturing involves making a single product by combining hundreds of manufacturing processes. Among these, deposition is essential in the age of miniaturization to keep apart areas that should not be interfered with and to connect vital components. Thus, deposition can be seen to support miniaturization, allowing more functions to be added to devices and paving the way for more advanced, energy-efficient products.
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