Wafer-level packaging (WLP) is an essential process in the semiconductor industry, where the entire packaging and testing of integrated circuits (ICs) are performed at the wafer level rather than on individual chips.
Plasma cleaning plays a crucial role in enhancing the reliability and performance of these devices.
In this article, we'll explore how wafer-level packaging plasma cleaning works, covering essential processes like wafer cleaning, descum, and more.
Wafer-level packaging plasma cleaning: The foundation of plasma treatment
Wafer-level packaging plasma cleaning is the first and perhaps the most critical step in wafer-level packaging.
Contaminants such as organic residues, particles, and oxides on the wafer surface can severely affect the performance and reliability of semiconductor devices.
Plasma cleaning uses ionized gases to remove these contaminants efficiently.
How it works
Wafer-level packaging plasma cleaning involves the generation of a plasma field in a vacuum chamber.
Reactive ions and radicals within the plasma interact with the contaminants on the wafer surface, breaking them down into volatile compounds that are easily removed.
This method ensures that the wafer surface is pristine, allowing for optimal adhesion of subsequent layers.
Technological benefits
Wafer-level packaging plasma cleaning is superior to traditional wet chemical cleaning methods, which can leave residues and require extensive rinsing.
Plasma treatment is more effective at reaching into microscopic features and crevices on the wafer surface, ensuring a more thorough cleaning process.
Descum: Ensuring precision in patterning
Descum is a process used to remove residual photoresist that remains on the wafer surface after the photolithography process.
These residues can interfere with subsequent etching or deposition processes.
How it works
In the descum process, plasma generated from gases like oxygen or fluorine is used to break down and remove the remaining photoresist.
This plasma cleaning step is carefully controlled to avoid damaging the underlying layers while ensuring the complete removal of unwanted residues.
Technological benefits
Plasma descum is highly selective, allowing for precise control over the removal of photoresist without affecting the patterned features.
This precision is vital in maintaining the integrity of microelectronic devices.
Stripping & etching: Refining the wafer surface
Stripping and etching are also critical in defining the microstructures on a wafer.
Plasma stripping involves the removal of bulk photoresist after etching, while plasma etching is used to sculpt the wafer surface by removing specific material layers.
How they work
Plasma stripping uses highly reactive ions to break down the photoresist into volatile byproducts, which are then evacuated from the chamber.
Plasma etching, on the other hand, involves using reactive ions to remove material from the wafer, creating patterns necessary for the device’s functionality.
Technological benefits
Plasma-based stripping and etching offer higher precision and control compared to traditional methods.
This is especially important as semiconductor devices continue to shrink in size, requiring more intricate patterning and etching processes.
Pre-treatment
Pre-treatment is a crucial step that prepares the wafer surface for subsequent processes such as deposition or bonding.
Plasma pre-treatment modifies the surface energy of the wafer, enhancing its reactivity and ensuring better adhesion of materials.
How it works
Plasma pre-treatment typically involves exposing the wafer to a specific gas plasma that increases surface energy by creating active sites on the surface.
These sites promote stronger chemical bonds between the wafer and the materials to be deposited or bonded.
Technological benefits
This step is particularly important in ensuring the uniformity and strength of subsequent layers, leading to better device performance and longevity.
BCB & UBM adhesion: Ensuring robust connections
In wafer-level packaging, Benzocyclobutene (BCB) and Under-Bump Metallization (UBM) are critical for creating robust electrical connections.
Plasma cleaning plays a significant role in enhancing adhesion during these processes.
How it works
Before applying BCB or UBM layers, plasma treatment is used to clean and activate the wafer surface, ensuring strong adhesion.
This process removes any organic contaminants and increases surface energy, providing a clean and reactive surface for bonding.
Technological benefits
Improved adhesion reduces the risk of delamination, which is crucial for the reliability of the semiconductor device, especially in applications where thermal cycling and mechanical stress are common.
Dielectric patterning: Achieving precision and reliability
Dielectric patterning involves creating insulating layers that separate different conducting areas on a wafer.
Plasma etching is used to precisely pattern these dielectric materials, ensuring that the electrical properties of the device are maintained.
How it works
In dielectric patterning, plasma etching selectively removes the dielectric material in specific areas while leaving the desired patterns intact.
This requires careful control over the plasma parameters to ensure that the etching process does not damage the underlying structures.
Technological benefits
Plasma etching allows for the creation of highly precise and uniform dielectric patterns, which is essential for the performance and reliability of high-density semiconductor devices.
Via cleaning: Preparing for metallization
Vias are small openings in the dielectric layer that allow electrical connections between different layers of the semiconductor device.
Plasma cleaning is used to remove residues from these vias, ensuring that metallization can proceed without issues.
How it works
Plasma via cleaning uses reactive ions to remove any remaining photoresist, oxides, or other contaminants from the via.
This step is crucial for ensuring that the metal deposited in the vias adheres well and provides a reliable electrical connection.
Technological benefits
Clean vias are essential for the performance of multilayer semiconductor devices.
Plasma cleaning ensures that vias are free of contaminants that could lead to poor electrical connections or reliability issues.
Bump adhesion: Ensuring strong electrical connections
Bumps are solder balls or pillars that create electrical connections between the wafer and the package substrate.
Plasma cleaning is used to prepare the wafer surface before bumping, ensuring strong adhesion and reliable connections.
How it works
Plasma treatment before bumping cleans and activates the surface of the wafer, removing any contaminants and increasing surface energy.
This allows the bumps to adhere more strongly, reducing the risk of failures during operation.
Technological benefits
Enhanced bump adhesion is critical for the reliability of flip-chip packages, where bumps form the primary electrical connections.
Plasma cleaning ensures these connections are strong and reliable, even under thermal and mechanical stress.
The advantages of wafer-level packaging plasma cleaning over traditional methods
Plasma treatment offers several advantages over traditional cleaning and etching methods in wafer-level packaging.
These include:
Precision - Plasma processes allow for highly selective and precise cleaning, etching, and patterning, which is essential as semiconductor devices continue to shrink in size.
Efficiency - Plasma cleaning is more effective at removing contaminants, especially in microstructures where traditional methods may struggle.
Environmental impact - Plasma treatment often uses less hazardous chemicals and generates fewer waste products, making it a more environmentally friendly option.
Cost-effectiveness - By reducing the need for multiple cleaning steps and improving adhesion, plasma treatment can lower overall production costs.
Innovation - Plasma technologies continue to evolve, offering new ways to improve the efficiency and reliability of wafer-level packaging processes.
These benefits make plasma treatment a popularly preferred method in different industries.
Conclusion
Plasma cleaning is an indispensable part of wafer-level packaging in the semiconductor industry.
From wafer cleaning and descum to bump adhesion and dielectric patterning, plasma treatment ensures the precision, reliability, and efficiency necessary for modern semiconductor devices.
As the industry continues to push the boundaries of miniaturization and performance, plasma technologies will play an increasingly vital role in meeting these challenges.
At SCI Automation, we adapt to the changes in the market, always offering the most recent innovations in the field of plasma cleaning.
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