In this final episode of our series on front-end processes, we will introduce the process of metallization, which connects semiconductor devices using metals like aluminum and copper. These interconnections provide power and enable the chip's operation, showcasing the importance of metallization in semiconductor manufacturing. This article will also explore the role of contacts and barrier metals in metallization and provide an insight into the larger context of connections from a semiconductor manufacturer's perspective.
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After the processes covered in previous episodes, including oxidation, photolithography, etching, and deposition, semiconductor devices form on the wafer's surface. For memory chip makers like SK hynix, transistors and capacitors1 are lined up on the wafer, while foundries and CPU manufacturers use three-dimensional transistors such as FinFETs2.
Capacitor: A storage device that can store electricity and is often used for electronics, referring to a device that stores data in a semiconductor.
FinFET (Fin field-effect transistor): A type of three-dimensional MOSFET with an electric current path resembling a fish's dorsal fin.
▲ Figure 1. Layers of a semiconductor device and areas of metal wiring (Image Source)
However, these devices are useless if isolated. Just like individual devices on an electronic substrate need soldering, transistors on a wafer need interconnection to function. Transistors operate by receiving external power to perform tasks like moving processed data. Therefore, a process to connect devices to each other or a power source is essential. Here is where the metallization process enables the operation of semiconductors. CPUs or GPUs, despite their differences, are created using interconnected devices.
▲ Figure 2: Metal wiring (yellow) connecting the device’s layers (red). *Some structures omitted. (Image Source)
Metallization is not a single process but combines photolithography, etching, and deposition to apply metal wiring on a semiconductor. The materials used, including metals, differ from those used in forming device layers. Unlike specific "metallization equipment," the process uses etching and deposition equipment for metal wiring. When material needs carving, etching equipment is used; for filling spaces, deposition equipment is applied, with photolithography involved between these processes.
In connecting devices on the substrate, electric wires are first soldered. In a semiconductor, a contact junction connecting metal wiring with a device forms after the bottom device layer forms, with wiring then connected on top.
▲ Figure 3. The use of tungsten in the formation of contacts, and an example of barrier metal (Image Source)
While it might seem metal can be directly connected to the device, miniaturization poses challenges. Gaps form in semiconductors, and although deposition fills these voids, metals like aluminum cannot fill deep holes, leading to faulty wires. To solve this, tungsten (W) with superior gap-fill property is deposited first to form deep metal wiring. This applies when the device layer and metal layer are significantly distant; otherwise, high-temperature heat treatment is used. Thus, tungsten contacts are essential when non-heat-resistant materials like aluminum are used, topped with aluminum wires.
A barrier metal or compound is needed between the device and contact. Directly connecting non-metallic and metallic materials is challenging due to differences in conduction bands3, causing high resistance and increasing semiconductor power consumption. Barrier metals like titanium (Ti) or cobalt (Co) are applied on silicon layers, reacting with silicon atoms in a process called 'silicidation,' forming a contact silicide.
Conduction band: The band in a solid's energy band structure that contributes to conduction.
Barrier metals also protect devices during processes. For example, aluminum reacts with silicon in wafers; hence, a titanium compound barrier prevents this interaction. Increased use of copper conductors also calls for barrier metals; copper diffuses into silicon dioxide (SiO2), causing current leakage. Tantalum (Ta) metal creates a boundary between the copper conductor and the device layer to prevent this.
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Connecting wires in a semiconductor is akin to connecting wires in a regular circuit, creating a part equivalent to a sheathed cable. However, in semiconductors, wires are created on top of the circuit instead of connecting pre-made wires.
▲ Figure 5. Comparison of reactive ion etching (RIE) and the damascene process (Source: Hanol Publishing [Understanding Semiconductor Manufacturing Technology, p. 293])
The wire creation process varies with the metal to be deposited. For aluminum, wires use etching and deposition techniques. After applying a metal film over the wafer, photoresist is applied, followed by exposure to eliminate unwanted aluminum, with dielectrics filled around remaining aluminum.
For copper (Cu), the order of metal and dielectric deposition is reversed. Dielectric is deposited first, etched using photolithography, followed by a copper seed layer and copper filling between dielectrics. Remaining copper is then ground away.
The order of metal and dielectric deposition is crucial. Copper requires a seed layer and electroplating, an additional deposition process not needed for aluminum. Utilizing copper involves evolving semiconductor processes to apply it at scale, reducing costs while ensuring efficiency. Metal wiring is thicker towards the top layers for efficient data transmission over distances, applied using simpler techniques to save costs and time.
Electroplating: Coating metal using electrolysis.
This thicker metal wiring on upper layers does not require complex techniques. Traditional processes suffice for creating upper layer aluminum wiring, reducing costs and time for semiconductor companies.
Technologies are combined by semiconductor manufacturers based on their strategies. For logic semiconductors5, controlling current requires widening the area with structures like FinFETs, forming complex contacts compared to flat transistors. Looking at Figure 6, it is evident forming contacts in FinFETs of logic semiconductors is more challenging than planar transistors in DRAM products.
Logic semiconductor: A semiconductor with operational purposes such as a CPU or GPU.
▲ Figure 6. Diagrams showing it is more difficult to form contacts in FinFETs of logic semiconductors (right) than planar transistors in DRAM (left)
Miniaturization challenges will continue, with memory companies facing charging capacity issues while stacking capacitors. Each company's business environment impacts the challenges of miniaturization and metallization they face.
Semiconductor manufacturing involves many workers, with a single wafer going through numerous steps. Each process contributes to the final product, with interdependent work creating a large synergy effect.
This article aims to help readers understand the technologies and their interrelationships. Material used in deposition affects subsequent heat processes and etching. Extensive etching can cause issues for deposition with poor gap-fill properties, leading to the need for multi-patterning6 with additional hard mask deposition and etching.
Multi-patterning: A technique using steppers multiple times to draw fine patterns on a wafer.
The semiconductor industry relies on high-tech processes and trust, communication, and creativity to solve new challenges. Semiconductor development is about working together to address challenges and finding solutions through collective effort. New challenges lead to innovative solutions, like using photoresist to solve issues caused by immersion steppers in the photolithography process.
▲ Figure 7. Example of a photoresist solving an issue with a stepper
We hope readers, especially aspiring semiconductor professionals, understand these industry aspects. Knowledge of these technologies can aid career development and foster positive relationships within the industry, ultimately contributing to creating world-class semiconductors.
Semiconductor technology faces major miniaturization challenges, and the voices of semiconductor users need to be heard more. The abilities to communicate and develop technology within semiconductor companies will be crucial in the future.
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