HBM hybrid bonding depends on maintaining a controlled chemical state at copper (Cu) and oxide interfaces before direct bonding. The critical challenge is not simply achieving low contamination levels. It is controlling oxidation, surface activation, moisture exposure, and post-clean surface evolution so that Cu-to-Cu and oxide-to-oxide interfaces can form consistently.

Unlike solder-based packaging, hybrid bonding creates the final interconnect through direct surface contact. This makes interface chemistry a critical process variable. Variations in copper oxidation, oxide surface condition, particles, organic residues, and bonding delay can influence contact formation and long-term reliability.

This article focuses on Cu/oxide surface chemistry in HBM hybrid bonding, including oxidation mechanisms, CMP-related surface evolution, cleaning interaction, analytical evidence, and qualification boundaries. It does not define universal acceptance limits because hybrid bonding requirements depend on integration flow, bonding method, materials, and device structure.

Why Does Hybrid Bonding Require Interface Chemistry Control?

Hybrid bonding changes the role of semiconductor surface treatment because the prepared surfaces become part of the final electrical and mechanical structure.

In conventional packaging approaches, surface preparation mainly supports a later joining process. In hybrid bonding, the interface itself determines whether direct contact can form between copper and dielectric surfaces.

Two bonding mechanisms occur simultaneously:

The key transition is:

Surface treatment is no longer only contamination removal. It becomes interface-state engineering.

A surface can meet general cleanliness requirements while still having an unstable chemical condition for hybrid bonding.

How Does Copper Oxidation Affect Cu-to-Cu Hybrid Bonding?

Copper oxidation is one of the central chemical challenges in Cu hybrid bonding.

Copper naturally reacts with oxygen and moisture, creating oxide layers on the surface. In direct Cu bonding studies, oxide formation has been associated with increased bonding difficulty because oxide layers can reduce direct metallic contact and influence interface formation.

However, copper oxide should not be considered only as an unwanted residue.

Its effect depends on several variables:

• Oxide composition;
• Oxide thickness;
• Surface uniformity;
• Exposure time before bonding;
• Cleaning and activation sequence;
• Bonding temperature and pressure.

The relevant process relationship is:

Copper exposure → oxide evolution → interface contact condition → bonding behavior

The objective is not always complete oxide removal.

The objective is maintaining a copper surface condition that supports reliable interface formation under the intended bonding process.

Why Are Cu₂O and CuO-Related Surface States Important?

Copper oxide is chemically dynamic rather than a single fixed material state.

Depending on oxygen exposure, humidity, cleaning conditions, and storage time, copper surfaces may contain different oxide compositions, including Cu₂O- and CuO-related species.

These differences matter because hybrid bonding depends on the actual surface condition at the interface.

A bulk measurement may indicate overall oxygen or contamination levels, but it does not automatically describe:

• Atomic-level surface structure;
• Actual bonding contact behavior;
• Interface resistance;
• Production yield impact.

For hybrid bonding evaluation, surface-sensitive evidence becomes more meaningful when it can be connected with bonding performance under defined process conditions.

The important distinction is:

Surface characterization explains a possible mechanism; process validation demonstrates practical impact.

How Does CMP Influence Hybrid Bonding Surface Chemistry?

Chemical mechanical planarization (CMP) creates the surface condition required for hybrid bonding, but it also affects the chemical state of the wafer surface.

After CMP, copper surfaces may experience:

• Residual polishing-related species;
• Surface oxidation;
• Roughness changes;
• Adsorbed moisture;
• Atmospheric contamination.

The surface condition can continue evolving after planarization.

Therefore, the complete process sequence matters:

CMP → Cleaning → Drying → Storage/Exposure → Bonding

A chemical treatment cannot be evaluated independently from the time and environment between surface preparation and bonding.

A surface condition measured immediately after cleaning may not represent the condition during actual bonding.

This is why hybrid bonding evaluation increasingly considers:

• Surface preparation sequence;
• Exposure control;
• Bonding delay time;
• Process environment.

How Do Bonding Delay and Thermal Conditions Affect Interface Formation?

Hybrid bonding usually requires surface preparation, alignment, and bonding steps to occur within a controlled sequence.

During this period, the interface condition may change because of:

• Copper reoxidation;
• Moisture adsorption;
• Organic contamination accumulation;
• Surface energy variation.

Thermal treatment after contact also influences interface evolution.

The final bonding result depends on the interaction between:

Surface chemistry → Contact condition → Thermal process → Interface structure

A cleaning chemistry therefore cannot be judged only by immediate contamination removal capability.

It must also be evaluated by how well it maintains the required surface state throughout the complete bonding sequence.

How Does Oxide Surface Activation Influence Dielectric Bonding?

Oxide bonding follows a different chemical mechanism from Cu bonding.

The oxide surface condition influences initial bonding through:

• Hydroxyl groups;
• Surface energy;
• Water adsorption;
• Molecular cleanliness.

Hydroxylated oxide surfaces can support intermolecular interactions before thermal treatment.

However, maximum surface activity is not always the target.

Excess moisture or unstable surface adsorption may affect later thermal behavior and interface reliability.

The goal is:

A reproducible oxide surface condition compatible with the bonding process.

This depends on:

• Cleaning chemistry;
• Rinse conditions;
• Drying method;
• Exposure time;
• Bonding environment.

Why Do Wet Chemicals Become More Critical for HBM Hybrid Bonding?

For conventional semiconductor cleaning, wet chemicals are often evaluated through contamination removal capability.

Hybrid bonding expands this requirement because chemical treatment directly influences interface formation.

The correct evaluation chain is:

Chemical condition → Surface state → Interface formation → Reliability

This is why semiconductor chemical qualification for hybrid bonding cannot rely on purity specifications alone.

Why Can Purity Specifications Not Fully Predict Hybrid Bonding Performance?

Purity remains important, but it represents only one layer of evidence.

A chemical specification can provide:

• Defined impurity values;
• Analytical results;
• Lot comparison.

It cannot independently demonstrate:

• Copper oxidation behavior;
• Oxide surface activation;
• Bonding yield;
• Device reliability.

The same limitation applies to surface analysis.

Surface characterization can provide detailed information about a tested condition, but it cannot automatically represent every production environment.

The most meaningful evidence connects material information with actual process behavior.

What Evidence Is Most Valuable for Hybrid Bonding Chemical Qualification?

The strongest qualification approach connects chemical properties with interface evolution.

Analytical Method Capability

Measurement results are meaningful only when the analytical method matches the risk being evaluated.

Important questions include:

• What species are measured?
• Is the method sensitive to relevant contamination?
• Are results comparable between batches or suppliers?

A reported value has meaning only within the conditions and method used to obtain it.

Surface-State Understanding

For hybrid bonding applications, valuable information includes:

• Oxidation tendency;
• Surface chemistry behavior;
• Exposure sensitivity;
• Compatibility with cleaning sequences.

The purpose is not collecting isolated measurements.

The purpose is understanding whether the chemical process supports the required interface condition.

Process-Level Validation

Process evaluation remains necessary because bonding performance depends on multiple variables.

Validation conditions may include:

• Actual bonding sequence;
• Target materials;
• Thermal conditions;
• Intended production environment.

A chemical condition that appears acceptable analytically may still require process confirmation.

What Are the Limits of Current Hybrid Bonding Surface Chemistry Knowledge?

Hybrid bonding technology continues to evolve, and universal chemical acceptance criteria are not established across all platforms.

The influence of:

• Cu oxide state;
• Surface hydroxyl concentration;
• Moisture exposure;
• Cleaning chemistry;
• Bonding delay time;

depends on:

• Integration strategy;
• Equipment capability;
• Device structure;
• Manufacturing conditions.

Published research can explain mechanisms and identify risk factors.

It cannot automatically define a universal specification for every hybrid bonding process.

Application-specific validation remains necessary.

Defensible Conclusion: HBM Hybrid Bonding Requires Control of Interface Evolution

HBM hybrid bonding changes semiconductor chemical requirements because the final interface is created directly from prepared copper and oxide surfaces.

The decisive factor is not simply lower impurity levels or cleaner surfaces.

The key question is:

Can the chemical process create and preserve a predictable interface state from surface preparation through bonding?

A technically defensible evaluation connects:

Chemical specification → Analytical method → Surface chemistry → Bonding response

For hybrid bonding-related chemicals, the most meaningful qualification approach is to understand how material properties influence interface evolution throughout the intended process sequence.