Plasma Cleaning for Silicon Wafers

Plasma cleaning of silicon wafer before photoresist spin coating and wafer-to-wafer bonding

Plasma cleaning is a critical step in semiconductor processing to ensure wafer surface cleanliness tailored to specific applications. While plasma cleaning before photoresist spin coating aims for a hydrophobic surface, plasma cleaning for wafer-to-wafer bonding targets a hydrophilic surface. Plasma cleaning can remove the surface contamination and render the surface more hydrophobic or hydrophilic for different applications. Understanding the underlying mechanisms can guide optimal process selection.

Plasma Cleaning Mechanism

Plasma cleaning addresses these issues through a controlled, dry process in which wafers are exposed to highly reactive gas species generated within a vacuum chamber. Oxygen plasma, the most commonly used method, employs energetic oxygen species—particularly atomic oxygen radicals—to chemically react with organic contaminants on the wafer surface. This oxidation reaction transforms organic residues into volatile compounds, such as carbon dioxide and water vapor, effectively removing them from the wafer surface.
An important consideration following plasma cleaning is the surface chemistry of the silicon wafer. Hydrocarbon contamination on wafer surfaces can be easily removed with a short oxygen plasma cleaning step. However, it leaves the silicon wafer surface highly hydrophilic due to the formation of surface hydroxyl groups and a thin oxide layer. This increased hydrophilicity, while beneficial for cleanliness, can present challenges for subsequent photoresist coating. Organic-based photoresists typically require a hydrophobic wafer surface for uniform spreading and excellent adhesion. However, it’s possible to use oxygen plasma to remove the surface hydroxyl groups further and render the silicon wafer surface more hydrophobic using intense oxygen plasma to clean the silicon wafer surface further.

In the attached photo above, the water contact angle of the air-exposed silicon wafer is about 12 degrees (step 1). The contact angle can be reduced to 3-4 degrees with a short pulsed oxygen plasma cleaning for 30 seconds (step 2). Further cleaning the silicon wafer with intense 150-watt oxygen plasma for 5 minutes removes all the organic materials and water moisture inside the plasma chamber and on the wafer surface. This deep cleaning step increases the water contact angle to 23 degrees (step 3), making the wafer surface more hydrophobic. Hydrophobicity rises because of the -OH* hydroxyl functional group depletion on the wafer surface. For photoresist spin coating application, a hydrophobic surface property is more desirable. For wafer-to-wafer bonding applications, the silicon wafer must be super hydrophilic. In that case, an additional water vapor plasma step can add high density -OH hydroxyl functional groups to the surface of the wafer and render the surface super hydrophilic (step 4).

Plasma Cleaning for Photoresist Spin Coating

Before photoresist spin coating, the wafer surface must be free of organic contaminants and particles to ensure uniform photoresist adhesion and precise pattern definition. In this scenario, plasma cleaning often involves oxygen (O₂) or argon (Ar) plasma, which effectively removes organic residues through physical bombardment (Ar plasma) and chemical oxidation (O₂ plasma). However, after treatment, the surface generally needs to be hydrophobic—typically achieved by a brief exposure or subsequent conditioning to re-establish surface passivation. This hydrophobic nature helps maintain uniformity of the spin-coated photoresist, reducing defects like pinholes or uneven thickness due to moisture adsorption.

Mechanism:

• Oxygen plasma generates reactive oxygen species (atomic oxygen, ozone), oxidizing organic contaminants into volatile byproducts (e.g., CO₂, H₂O) removed by vacuum.
• Argon plasma physically sputters contaminants, effectively removing particles by momentum transfer.
• Post-plasma treatments or atmospheric conditioning restore hydrophobicity (often involving HMDS priming), crucial for stable photoresist adhesion.

Plasma Cleaning for Wafer-to-Wafer Bonding

In wafer-to-wafer bonding applications, the wafer surfaces require not only contamination removal but also a hydrophilic characteristic to enhance adhesion strength. Hydrophilic surfaces typically involve exposure to oxygen plasma, which creates a dense surface population of hydroxyl (-OH) groups. These hydroxyl groups significantly enhance the wafer bonding process by forming strong hydrogen bonds when wafers are brought into direct contact, promoting spontaneous wafer adhesion and subsequent covalent bonding after thermal annealing.

Mechanism:

• Oxygen plasma cleaning generates atomic oxygen radicals, actively breaking down organic contaminants.
• Simultaneously, silicon surfaces oxidize, creating a thin, uniform SiO₂ layer rich in hydroxyl groups.
• Increased surface hydroxyl density significantly improves hydrophilicity, measured by reduced water contact angles (often <10°).
• During wafer bonding, these hydroxyl-rich surfaces form strong intermolecular hydrogen bonds, facilitating wafer-to-wafer adhesion and reducing bonding defects.

Choosing Plasma Parameters

• Photoresist Spin Coating: A longer, intense oxygen plasma (for example, 150 W for 5 minutes) removes organics and adsorbed moisture and leaves a more hydrophobic surface; combined with hydrophobic priming (e.g., HMDS vapor), it maintains hydrophobicity.
• Wafer Bonding: A short, gentle oxygen plasma (for example, a 30-second pulsed plasma) achieves uniform oxidation and maximum hydroxyl group density, optimizing hydrophilicity.

Understanding these mechanisms allows precise control over wafer surface properties, tailored to meet the specific demands of photoresist application or wafer-to-wafer bonding processes.