What Is Atomic Layer Deposition (ALD) and Why Ozone Matters

Atomic Layer Deposition (ALD) has become one of the most critical thin-film fabrication technologies in semiconductor manufacturing. Unlike traditional Chemical Vapor Deposition (CVD), ALD deposits material one atomic layer at a time, enabling angstrom-level thickness control and exceptional conformality on complex 3D structures. As semiconductor nodes shrink below 7 nm, the demand for ultra-precise, defect-free dielectric films has made ALD indispensable.

In ALD processes, the choice of precursor directly determines film quality, deposition rate, and process temperature. While water is the conventional oxygen source for oxide ALD, it suffers from slow purging times, potential carbon contamination, and limited reactivity with certain metal precursors. Ozone (O₃) has emerged as a superior oxidant, offering faster reaction kinetics, lower process temperatures, and higher-purity oxide films — particularly for high-k dielectrics such as Al₂O₃, HfO₂, and ZrO₂.

Advantages of Ozone as an ALD Oxidant

The shift from water to ozone in ALD is driven by several compelling technical advantages that directly impact semiconductor device performance and manufacturing throughput:

• Faster Purge Cycles: Ozone desorbs more rapidly than water from surfaces and chamber walls, significantly shortening the purge step between ALD half-cycles.

• Lower Deposition Temperature: Ozone-enabled ALD achieves quality films at temperatures as low as 150–250 °C, compared to 300+ °C for water-based processes — essential for temperature-sensitive substrates.

• Reduced Carbon Contamination: Ozone is a stronger oxidant, more effectively removing residual carbon from metal-organic precursors, yielding films with impurity levels below 0.5 at%.

• Better Step Coverage: The higher reactivity of ozone improves nucleation density on challenging surfaces, resulting in superior conformality in high-aspect-ratio features.

• Broad Material Compatibility: Ozone works effectively with trimethylaluminum (TMA), tetrakis(dimethylamido)hafnium (TDMAHf), and other widely used metal precursors.

Critical Requirements for Ozone Generators in ALD Applications

Not all ozone generators are suitable for semiconductor ALD. The extreme purity and stability requirements of advanced node fabrication impose stringent specifications on ozone delivery systems. Understanding these requirements is essential for process engineers specifying equipment for fab deployment.

Ozone Concentration Stability

ALD processes require ozone concentration stability within ±2% over extended runs. Fluctuations in ozone concentration directly translate to film thickness non-uniformity across the wafer. Modern ozone generators for ALD employ closed-loop feedback control using UV absorption sensors to maintain constant output, even as source gas pressure or ambient temperature varies.

Purity and Contamination Control

Semiconductor-grade ozone must be free of particulates, moisture, and metallic contaminants. Key specifications include:

• Particulate count: <1 per cubic footat 0.1 μm

• Moisture content: <1 ppmin ozone output stream

• Metallic impurities: <0.1 ppbfor critical contaminants (Fe, Ni, Cu)

• All wetted surfaces: electropolished 316L stainless steel or PFA/PTFE

Feed Gas Options: Oxygen vs. Air

ALD-grade ozone generators exclusively use high-purity oxygen (Grade 5.0+, 99.999%) as the feed gas. Air-fed systems are unsuitable due to nitrogen oxide byproducts that contaminate deposited films. The oxygen supply must also meet strict purity specifications, with dedicated gas purification modules typically installed upstream of the ozone generator.

Ozone Generator Technologies for ALD: Corona Discharge vs. Electrolytic

Two primary ozone generation technologies are employed in semiconductor ALD applications, each with distinct advantages and limitations.

Corona Discharge Ozone Generators

Corona discharge (CD) generators are the dominant technology in fab-scale ALD systems. They produce ozone by passing high-purity oxygen through a high-voltage electrical discharge gap. Modern CD generators for ALD feature:

• Double-sided dielectric electrodes for uniform discharge and extended electrode life

• Precision cooling systems maintaining dielectric temperature within ±0.5 °C for concentration stability

• In-line UV concentration sensors with 254 nm reference channel for real-time feedback control

• Integrated oxygen purification modules removing trace moisture and hydrocarbons before ozone generation

Electrolytic Ozone Generators

Electrolytic ozone generators (EOGs) produce ozone by electrolyzing deionized water using proton exchange membrane (PEM) technology. While offering extremely high ozone concentrations (up to 300+ g/m³) and inherently moisture-free output, EOGs have lower throughput and higher operational costs compared to CD systems. They are typically used in specialized ALD applications requiring the highest purity at lower flow rates, such as research-scale ALD tools or specific process modules within a cluster tool.

Key ALD Films Deposited Using Ozone

Ozone-based ALD is the industry standard for depositing several critical dielectric and functional films in advanced semiconductor devices:

• Al₂O₃ (Aluminum Oxide): The most common ozone-ALD film, used as gate dielectric, passivation layer, and moisture barrier. Typical deposition: 1.0–1.2 Å/cycle at 250 °C using TMA + O₃.

• HfO₂ (Hafnium Oxide): High-k gate dielectric for advanced logic devices. Ozone-based ALD achieves lower carbon content compared to water-based processes, critical for gate leakage performance.

• ZrO₂ (Zirconium Oxide): Used in DRAM capacitor dielectrics and emerging memory devices. Ozone enables better stoichiometric control and reduced oxygen vacancy defects.

• Ta₂O₅ (Tantalum Oxide): Applied in MIM capacitors and anti-reflective coatings. Ozone-based processes achieve superior electrical properties.

• TiO₂ (Titanium Oxide): Used in resistive switching memory (RRAM) and photocatalytic applications.

Safety and Infrastructure Considerations for Ozone ALD Systems

Working with high-concentration ozone in semiconductor fabs requires rigorous safety protocols and specialized infrastructure:

• Ozone Destruct Units: All exhaust streams must pass through catalytic or thermal destruct units achieving >99.9% ozone decomposition before venting.

• Ambient Monitoring: Continuous O₃ leak detection sensors with alarms at 0.1 ppm (OSHA PEL is 0.1 ppm TWA).

• Material Compatibility: All wetted-path components must be ozone-rated — stainless steel (316L EP), PFA, PTFE, or Kalrez perfluoroelastomers. Standard Viton and Buna-N are not acceptable.

• Gas Panel Integration: Ozone generators must be integrated into the tool gas panel with proper isolation valves, purge lines, and interlocks tied to the tool's safety PLC.

Selecting the Right Ozone Generator for Your ALD Application

When specifying an ozone generator for semiconductor ALD, process engineers should consider the following decision factors:

• Target Films: Different oxide films have varying ozone dose requirements. HfO₂ and ZrO₂ typically require higher ozone concentrations than Al₂O₃.

• Wafer Throughput: Production-scale ALD tools (300 mm) may need ozone flow rates of 5–20 SLM at 200+ g/m³ concentration.

• Integration Requirements: Consider physical footprint, utility requirements (cooling water, electrical, exhaust), and SECS/GEM communication capability for fab automation.

• Reliability and MTBF: Production fabs require ozone generators with MTBF >10,000 hours. Electrode life and serviceability are critical.

Tonglin Ozone manufactures a range of high-concentration corona discharge ozone generators specifically designed for laboratory and semiconductor applications. Our systems feature precision concentration control, electropolished 316L SS flow paths, and integrated safety systems that meet the demanding requirements of advanced ALD processes.