Do you want to understand what semiconductor electroplating equipment is used for? Did you know that metals such as copper, gold, platinum, nickel, and many others are deposited on silicon wafers to complete their microscopic circuitry and packaging? The plating processes need to be done with extreme precision and speed, necessitating the use of highly sophisticated electroplating equipment. Find out what the plating process is, what metals are used in it, and why special equipment is needed to carry out these tasks in semiconductor manufacturing.
The Electroplating Process
Before we get into semiconductor wafer plating, let’s first take a look at what electroplating is in general. Electroplating is an electrochemical process where a thin layer of one metal is deposited onto the surface of another. Plating gives the substrate material some properties of the metal they are plated with. This can make the visual appearance of a material more appealing and give it additional features like corrosion resistance or improve thermal conductivity. Copper, for example, is used to increase electrical conductivity.
When looking at the history of electroplating, the earliest plating experiments were done in the 18th century until a proper process was formalized by Italian inventor Luigi V. Brugnatelli in the 19th century. This process was adopted and quickly became popular in Europe, developing even further during the Industrial Revolution. This form of electrochemical deposition was used to plate everything from coins and jewelry to tin cans that store food.
How Electrochemical Deposition Works
When trying to understand how electrochemical deposition works, you might want to refresh your memory about that science class experiment you would’ve done in school. A brass key and a piece of copper are connected to each terminal of a power supply (a battery). When submerged in an electrically conductive bath (copper sulfate solution) without touching each other, the electrical circuit is completed. As a current starts to flow, copper ions that are removed from the copper source are deposited on the surface of the brass key. If you keep this going, the brass key will eventually have a complete coating of copper.
Plating In Semiconductor Manufacturing
In semiconductor manufacturing, it all starts with the growth of artificial semiconductor materials called ‘ingots’, which are then sliced into thin disc-shaped pieces called ‘wafers’. This is then followed by photolithography and multiple stages of etching to create the layers of circuitry on the surface of each wafer. There are both wet processing and dry etching techniques used depending on the type of semiconductor device that is being manufactured.
There is also the doping stage where the electrical properties of the substrate material are altered by adding materials that have atoms with more or fewer electrons than the substrate. For a silicon substrate, boron and phosphorus are introduced. Through the manufacturing process, many sophisticated semiconductor imaging techniques are used to inspect the wafers for contamination or defects.
The tiny circuitry created on the surface of the semiconductor wafer material is not complete without adding metals and insulators. The metals create connections between the internal structures like transistors and resistors, thus completing the circuitry. The insulators create barriers and protect the circuitry from damage. This part of the process where metals and insulators are developed on the surface of the substrate turns them into functioning semiconductor devices. This step in semiconductor manufacturing which involves the electrochemical deposition of copper is called the ‘back end of line’ (BEOL).
The wafer and a copper source are both placed in a bath that contains copper sulfate and sulfuric acid. When connected to a power source, and a current is provided, the copper ions deposit on the wafer surface. The current flow determines the amount of copper that gets deposited on the wafer. Other key parameters that affect the properties of the copper deposited on the wafer during plating include the composition of the bath solution, its temperature, and the rate of solution flow.
To prevent copper from contaminating the dielectric (insulating) layers, a diffusion barrier layer of Tantalum is used first. By physical vapor deposition, a thin seed layer of copper is then created on the barrier. After this is done, the copper is deposited to the desired thickness through the usual plating method. BEOL is not the only application of electroplating in semiconductor manufacturing. There are also advanced packaging applications that take place much later.
Benefits Of Wafer Plating During Semiconductor Manufacturing
Depending on the plating material used in semiconductor wafer plating, different features can be given to the wafer which can be useful to improve its performance, reliability, and lifetime.
Electrical Conductivity
Naturally one of the most important characteristics of plating a semiconductor material is to increase the electrical conductivity of the substrate.
Protection
The plating can create a barrier to protect the wafer from atmospheric conditions that cause corrosion. This allows the semiconductor devices and chips to be used in more extreme conditions. It also increases the longevity of the device.
Reduces Friction
Friction can often cause wear and tear on the electrical connectors of a device. Plating can reduce the friction of these components and prevent it from wearing too fast.
Heat Resistance
Some plating materials can withstand high temperatures, keeping the semiconductor devices functional without damaging them. The chips made like this have an exceptional lifespan and can be used in applications that put them in extreme conditions.
Adhesion
Often during the electroplating, an undercoating of a different material is deposited before the deposition of the main coating material. This undercoating has adhesive properties and keeps all the plating material together longer.
Electroplating Materials Used For Semiconductors
Copper is one of the most popular and widely used materials in electroplating semiconductor devices. Apart from its electrical properties, it can enhance thermal conductivity as well. It is possible to use copper on its own, although it is usually coated with another metal to make it more resistant to corrosion.
Tin and tin alloys can be used to provide semiconductors with corrosion resistance. Compared to more exotic plating materials, tin is much cheaper and found in abundance, although its electrical conductivity is much less. Tin alloys made by introducing lead require no undercoating, although recent environmental concerns have led researchers to look for alternatives.
Nickel plating is desired for its resistance to chemicals and plate uniformity. It can be used for its corrosion protection or as a base to apply another coating. Nickel is usually combined with zinc or palladium and used as an alloy.
Thermal and electrical conductivity are the main draws of using silver along with great corrosion resistance and compatibility with other metals. It has low contact resistance and impressive soldering characteristics, making it ideal for plating on active copper components. Since silver is considered a precious metal, it is relatively costly. Furthermore, the tarnishing aspect of silver can reduce the lifespan of devices that receive silver plating.
Gold is naturally very expensive and highly useful. When used as a plating on top of nickel, the gold coating can prevent rust from working its way down since it is highly resistant to corrosion. It also has excellent heat resistance and high conductivity. The high costs of using gold plating mean it is usually applied as a very thin layer depending on the application.
How Electroplating For Semiconductors Is Different
What separates the plating of regular household items from electroplating semiconductor wafers is the scale of the plating surface involved. The circuitry on the surface of a wafer is microscopic, and the tiniest error during plating or the introduction of contaminants to its surface can compromise the integrity of the final product. Therefore the plating of semiconductors has to be done with the following precautions to ensure fully functional microchip products.
The plating of wafers has to be performed inside a clean room environment. These specially-designed rooms have sophisticated air filtration systems that ensure that the amount of dust particles inside is kept under 0.01% of that of dust contained in the air outside it. The engineers and technicians working have to wear special clothing and work benches. For more on how these rooms are designed, read What Is a Semiconductor Clean Room and What Is It For?
Next, the plating solution itself where the wafer and source material is submerged has to be clean as well. It also undergoes filtering to remove any particles like dust that can contaminate the plating and cause defects in the wafer.
Contamination is not the only challenge of plating semiconductors. The microscopic interconnect structures like trenches which need copper plating are narrow and deep, making it difficult to coat them in a uniform manner. Manufacturing defects such as voids or seams can form which will have an impact on the electrical performance and reliability of the finished device. While reliability testing can identify what causes these defective circuits, they cannot be sold to customers, bringing down the yield of the production line.
Why Electroplating Equipment Is Needed For Mass Production Of Semiconductors
To ensure that the plating process is done precisely without any defects like voids and up to specifications, it cannot be done by hand. This is where state-of-the-art semiconductor electroplating equipment comes in.
The applied current density is responsible for the rate of vertical growth of the plating material when forming a specific feature using a patterned mask. Essentially, the current density determines the thickness per minute. It is the electron supply that affects the rate of the reaction at the wafer surface, which in turn affects metal deposition. The total charge measured in amp-hour controls the vertical height of the plating layer.
The power supplies of plating equipment allow engineers to precisely control the current user, which can deliver uniformity in the layer of metal that is plated to the semiconductor wafer. Furthermore, when going into full-scale production of semiconductor devices, high throughput is required to keep up with the massive demand. Therefore the high-precision process of creating uniform plating has to be repeatable, reliable, and fast.
High-Speed Plating Equipment For Semiconductors
The mass production of semiconductor materials requires plating to be performed using fully automated electroplating equipment. A single piece of hardware can usually accommodate multiple wafer sizes and even offer other wet processing tools as well. Depending on the plating requirement, they can be used to perform electrochemical deposition of gold, silver, copper, nickel, indium, palladium, platinum, or more.
The plating equipment can be controlled through a touchscreen interface which engineers used to adjust their settings. Wafer cassettes can be loaded into the equipment which will then be handled by automated robotic arms that will take each wafer through the plating process as configured. Consistent quality is maintained, ensuring a high throughput. Wafer loading is also fast, allowing the equipment to process large batches of wafers quickly.
Wet processing usually involves chemicals like acids, which are safely contained inside the equipment without needing to be handled by human operators.
For research and development and small-scale production which doesn’t require speed, there is also semi-auto plating equipment available. These are more affordable, although they don’t come with automated handling robotics, requiring an operator to perform this manually. They also have a smaller footprint and are ideal for laboratories. While not fully automated, this plating equipment still offers the same level of precision and quality of uniformity their more advanced models provide.
Advanced Packaging Of Semiconductor
Advanced wafer-level packaging (WLP) involves forming interconnects like conductive bumps, redistribution layers (RDLs), through-silicon vias (TSV), and pillars. These interconnects will change depending on the type of integrated circuit under production and are also done using electrochemical deposition methods. However, even with high-speed plating, these processes require longer deposition times and equipment capable of multi-step processing. High uniformity and throughput are major requirements as well.
For more on semiconductor manufacturing techniques and the equipment used, check out Inquivix Technologies!
FAQs
Electroplating is the process of depositing a layer of metal onto another metal or substrate to improve its electrical, thermal, corrosion resistance, and other properties.
Semiconductor electroplating is where copper is deposited onto the surface of a semiconductor wafer to connect tiny circuit components such as transistors, resistors, and capacitors.
