Deep Dive into In-Situ Steam Generation (ISSG) in Semiconductor Processing: Physics, Kinetics, and Integration

Introduction

In modern complementary metal-oxide-semiconductor (CMOS) fabrication, the synthesis of high-quality silicon dioxide ($\text{SiO}_2$) thin films with precise atomic-level control is a cornerstone of device performance. Historically, thermal oxidation of silicon was accomplished via "dry" oxidation (utilizing pure $\text{O}_2$ gas) or "wet" oxidation (utilizing water vapor transported into the furnace). Dry oxidation is known to produce highly dense oxides with low interface state densities but suffers from sluggish growth rates, particularly at lower thermal budgets. Conversely, wet oxidation yields significantly faster growth kinetics because water molecules diffuse through the growing oxide network much more rapidly than diatomic oxygen. However, conventional wet oxidation often produces films with lower density and higher concentrations of hydroxyl groups ($\text{Si-OH}$), compromising the film's dielectric breakdown strength.

To bridge this gap, the semiconductor industry developed in-situ steam generation (ISSG). ISSG represents a paradigm shift where ultra-pure steam is generated directly inside a single-wafer chamber by reacting hydrogen ($\text{H}_2$) and oxygen ($\text{O}_2$) gases in a high-temperature, low-pressure environment. Single-wafer rapid thermal processing systems rapidly ramp wafers to process temperatures using lamp heating to achieve precise thermal control [T1]. This process avoids the purity and contamination issues of external steam delivery systems. More importantly, ISSG oxidation operates in a non-equilibrium regime that populates the reaction zone with highly reactive atomic oxygen ($\text{O}\cdot$) and hydroxyl ($\text{OH}\cdot$) radicals. This radical-rich environment accelerates oxidation kinetics even at reduced thermal budgets while producing films with structural densities and electrical characteristics that rival or exceed dry oxides. Today, ISSG has become an indispensable process in advanced logic and memory nodes, playing key roles in gate oxide formation, shallow trench isolation (STI) liner passivation, and 3D integration schemes.

Physics & Mechanism

The fundamental physics of ISSG oxidation relies on gas-phase radical generation coupled with atomic-scale surface reactions. Unlike traditional thermal oxidation, which relies on the molecular transport of non-dissociated $\text{O}_2$ or $\text{H}_2\text{O}$ to the reacting interface, ISSG leverages the energetic kinetics of hydrogen-oxygen combustion in a low-pressure environment to yield highly reactive intermediate species.

Gas-Phase Radical Kinetics

When $\text{H}_2$ and $\text{O}_2$ are introduced into the heated processing chamber, they undergo a series of chain-branching combustion reactions. At typical processing temperatures, these reactions generate a high volume of transient radicals before reaching thermodynamic equilibrium. The simplified reaction path can be represented by the following elementary steps:

$$\text{H}_2 + \text{O}_2 \rightarrow \text{HO}_2 + \text{H}\cdot$$

$$\text{H}\cdot + , \text{O}_2 \rightarrow \text{OH}\cdot + , \text{O}\cdot$$

$$\text{O}\cdot + , \text{H}_2 \rightarrow \text{OH}\cdot + , \text{H}\cdot$$

$$\text{OH}\cdot + , \text{H}_2 \rightarrow \text{H}_2\text{O} + \text{H}\cdot$$

Under low-pressure conditions, the collisional recombination rate of these radicals is suppressed, significantly extending their lifetimes and enabling them to reach the wafer surface intact. Among these, atomic oxygen ($\text{O}\cdot$) and hydroxyl radicals ($\text{OH}\cdot$) are the primary drivers of ISSG oxidation.

Interface Chemistry and Diffusion Mechanisms

Once these active species reach the wafer, they diffuse through the growing $\text{SiO}_2$ film to the $\text{Si}/\text{SiO}_2$ interface. To analyze the kinetic progression of this oxide growth, process engineers rely on the classical Deal-Grove model:

$$x^2 + A x = B (t + \tau)$$

Where:

• $x$ is the oxide thickness.
• $t$ is the oxidation time.
• $B$ is the parabolic rate constant, which is proportional to the effective diffusivity of the oxidant species through the oxide bulk.
• $B/A$ is the linear rate constant, governed by the reaction rate at the silicon-dielectric interface.
• $\tau$ is a time shift term that accounts for initial native oxide presence.

In the thin-film regime, the growth is governed by the linear growth law:

$$x = \frac{B}{A} (t + \tau)$$

For ISSG, the linear rate constant ($B/A$) is drastically enhanced compared to conventional dry oxidation. This is because the activation energy required for the highly reactive atomic oxygen ($\text{O}\cdot$) to break silicon-silicon ($\text{Si-Si}$) bonds at the interface is significantly lower than that of molecular $\text{O}_2$.

Furthermore, atomic-scale modeling utilizing reactive force field molecular dynamics simulations has illuminated the role of hydrogen in accelerating the diffusion process. In the presence of hydrogen atoms, local coordination and bond-rearrangement energy barriers near the interface are altered. Specifically, hydrogen passivates dangling bonds and forms intermediate silanol ($\text{Si-OH}$) groups, which effectively open up and restructure the local silicon-oxygen network. This temporary bond restructuring lowers the diffusion barrier for incoming oxidants, promoting faster mass transport through the bulk oxide and significantly accelerating the growth rate.

Process Principles

Optimizing an ISSG process requires a deep understanding of how gas dynamics, thermal energy, and pressure interact to dictate film growth and quality. Since ISSG is a non-equilibrium, radical-driven process, the directional trends of these process parameters differ significantly from traditional batch furnace operations.

[H2 / O2 Gas Flows] ----> [Chamber Pressure (Low)] ----> [Radical Lifetimes (Extended)]
         |                                                          |
         +------------------> [RTP Heat Source] --------------------> [Enhanced Growth Kinetics]

Directional Parameter Interactions

The primary control knobs of an ISSG process include temperature, chamber pressure, individual gas flows (and their ratios), and soak times.

• Temperature: An increase in temperature directly increases both the gas-phase radical generation rate and the solid-state diffusion of oxidants through the oxide layer, leading to higher overall oxide thickness. However, because radical reactions have low activation barriers, the temperature sensitivity of ISSG in the interface-controlled regime is less pronounced than that of dry thermal oxidation.
• Chamber Pressure: Unlike conventional thermal oxidation where higher pressure accelerates growth by increasing molecular concentration, ISSG requires a relatively low pressure regime. Lowering the chamber pressure increases the mean free path of the molecules, which dramatically suppresses the second-order gas-phase recombination of atomic oxygen back into diatomic oxygen ($\text{O}\cdot + , \text{O}\cdot \rightarrow \text{O}_2$). Thus, decreasing pressure directionally increases the ratio of radical-to-molecular oxidants, enhancing film density and growth rates in the thin regime.
• Gas Flow Ratio ($\text{H}_2/\text{O}_2$): The ratio of hydrogen to oxygen is highly critical. A minimal concentration of $\text{H}_2$ is required to initiate the combustion reactions that generate hydroxyl and atomic oxygen radicals. Increasing the $\text{H}_2$ fraction up to an optimal limit enhances the radical generation rate. However, exceeding this threshold leads to an excess of water vapor ($\text{H}_2\text{O}$) and molecular hydrogen ($\text{H}_2$), shifting the process toward conventional wet oxidation and increasing the concentration of unwanted hydroxyl silanol ($\text{Si-OH}$) defects within the film.
• Soak Time: The duration of the high-temperature step controls the final oxide thickness. Due to the rapid initial growth rates enabled by radicals, very short soak times can be utilized to achieve highly uniform, ultra-thin films.

Stress and Structural Relaxation

A critical aspect of the thermal budget in oxidation is the management of mechanical stress at the silicon interface. Silicon dioxide undergoes a substantial volume expansion relative to the consumed silicon substrate. During high-temperature processing, structural relaxation occurs, which can be modeled using the volumetric thermal expansion framework. The rapid, high-temperature thermal cycle of RTP allows for localized viscous flow of the $\text{SiO}_2$ during growth. This flow relieves the compressive stress caused by the volumetric mismatch, minimizing the generation of silicon dislocations and interface states at the active boundary.

Challenges & Failure Modes

Despite its superior kinetics and film quality, the integration of ISSG oxidation into advanced fabrication flows presents several physical and thermodynamic challenges.

Stress Mismatch and Curvature Effects

In three-dimensional (3D) structures, such as FinFET channels or deep trenches, oxidation rates are highly sensitive to surface curvature and crystallographic orientation. During ISSG, the volumetric expansion of $\text{SiO}_2$ in a confined, high-curvature geometry generates localized compressive stress. This compressive stress acts as a thermodynamic barrier that retards both the diffusion of oxidant radicals and the chemical reaction rate at the silicon interface. Consequently, the oxide thickness grown via ISSG can drop significantly at the bottom corners and highly curved sidewalls of 3D structures compared to flat planar surfaces. This thinned oxide is a primary failure mode, as it leads to localized electric field crowding, early dielectric breakdown, and increased gate leakage.

Interface Defects and Dangling Bonds

Although hydrogen atoms accelerate oxidation kinetics by modifying bond-rearrangement energy barriers, their presence can also lead to reliability concerns. Hydrogen intermediate species can leave behind unpassivated silicon dangling bonds ($\text{Si}\cdot$) or weak silicon-hydrogen ($\text{Si-H}$) bonds at the dielectric interface. Under electrical stress, these weak bonds easily dissociate, creating charge traps that cause threshold voltage shifts and degrade the negative bias temperature instability (NBTI) of the device.

Monitoring and Process Control in Ultra-Thin Regimes

As gate oxides are scaled down to extreme dimensions, verifying film properties becomes highly challenging. In processes where the ISSG oxide is subjected to subsequent nitridation (e.g., using nitric oxide gas to form a silicon oxynitride barrier), nitrogen atoms preferentially segregate at the $\text{Si}/\text{SiO}_2$ interface to improve dielectric strength and suppress boron penetration.

However, monitoring this ultra-thin nitrided layer is difficult. Standard optical ellipsometry cannot easily resolve the minor sub-angstrom optical variations associated with subtle shifts in nitrogen dose. Modern metrology frequently utilizes non-contact electrical techniques that corona-charge the oxide surface and use a Kelvin probe to extract the interface trapped charge density ($Q_{it}$). This allows direct physical monitoring of the interface state density ($D_{it}$) without requiring destructive sacrificial reoxidation steps.

Technology Node Evolution

The role of ISSG has evolved dramatically alongside changes in transistor architecture, scaling down from planar CMOS to complex 3D structures.

       Planar Nodes (e.g., 28nm)                      FinFET / 3D Nodes (14nm to 7nm)
+------------------------------------+             +------------------------------------+
| - High-quality planar gate oxide   |             | - Conformal corner rounding        |
| - Uniform thermal oxide growth     | ----------> | - Stress-managed STI liner oxide   |
| - Thinner oxide limits             |             | - Selective scaling on fins        |
+------------------------------------+             +------------------------------------+
                                                                      |
                                                                      v
                                                       Advanced Memory Nodes (BCAT/DWF)
                                                   +------------------------------------+
                                                   | - Integrated IAI (ISSG-ALD-ISSG)   |
                                                   | - Defect repair on atomic scales   |
                                                   | - Low-leakage dual work function   |
                                                   +------------------------------------+

28nm Planar Node

At the 28nm planar node, planar devices utilized ISSG oxidation primarily to grow highly uniform, ultra-thin gate oxides and sacrificial screen oxides. The highly controlled radical kinetics enabled the growth of thin layers with uniform thickness across large-diameter wafers, ensuring tight threshold voltage distributions.

14nm to 7nm FinFET Nodes

As the industry transitioned to multi-gate architectures, physical challenges shifted to managing the multi-gate fin topography. A high-temperature liner oxidation is performed for corner rounding at the sharp edges of trenches to reduce stress and unwanted electrical effects [P2]. Furthermore, ISSG oxidation was used for corner rounding, where the sharp top corners of the silicon fins are selectively oxidized. Rounding these corners prevents localized electric field enhancement, suppressing parasitic sub-threshold leakage currents along the fin edges.

Advanced Memory Nodes

In advanced dynamic random-access memory (DRAM) scaling, transistors have migrated toward Buried Channel Array Transistor (BCAT) and Dual Work Function (DWF-BCAT) designs to suppress gate-induced drain leakage. For these deep trench structures, a single atomic layer deposition (ALD) or thermal oxidation step is insufficient. Instead, manufacturers utilize an integrated ISSG-ALD-ISSG (IAI) three-step gate oxide process.

               [ IAI (ISSG-ALD-ISSG) Gate Stack Scheme ]

               +---------------------------------------+
               |             Second ISSG               |  <- Cures ALD defects,
               |           (Ultra-thin Cap)            |     improves film density
               +---------------------------------------+
               |               ALD Oxide               |  <- Delivers highly conformal
               |             (Bulk Layer)              |     thickness in trench
               +---------------------------------------+
               |             First ISSG                |  <- Establishes pristine,
               |            (Base Oxide)               |     low-defect Si interface
               +---------------------------------------+
               |           Silicon Substrate           |
               +---------------------------------------+

Under the IAI scheme:

1. The first ISSG step grows a high-quality, thin base oxide directly on the silicon trench surface to establish a low-defect interface.
2. A subsequent ALD step provides highly conformal bulk oxide thickness along the high-aspect-ratio vertical walls of the trench.
3. A final, thin ISSG cap is grown to heal stoichiometry defects, carbon impurities, or vacancies within the ALD film.

This combination leverages the thickness control of ALD with the physical density and low interface state density of radical ISSG oxidation. Furthermore, to control the electrical properties of the gate and manage the device's work function, advanced integration utilizes specialized recessed isolation structures to ensure optimal electrostatic channel control and mechanical reliability.

Related Processes

ISSG oxidation does not operate in isolation; it is deeply coupled with several upstream and downstream process modules.

• Wet Chemical Cleaning: Prior to ISSG, the silicon surface must undergo a critical wet clean to remove metallic contamination, organic residues, and the native oxide layer. This is typically done using hydrofluoric acid (HF) mixtures. The quality of this clean directly dictates the initial time shift ($\tau$) in Deal-Grove oxidation kinetics. Post-deposition, wet etching with dilute HF is also used to evaluate film quality; high-density ISSG oxides exhibit significantly slower wet etch rates in HF compared to CVD or ALD oxides, confirming their molecular cross-linking.
• Thermal Annealing: After oxidation, a post-oxidation annealing step in an inert nitrogen ($\text{N}_2$) or argon ($\text{Ar}$) ambient is frequently performed. The density of fixed oxide charge located near the dielectric interface depends on oxidation and annealing conditions, as well as on the silicon surface orientation .
• Nitridation: For advanced gate dielectrics, the ISSG oxide is often followed by plasma nitridation or thermal nitridation in a nitric oxide ($\text{NO}$) environment to form a thin silicon oxynitride layer. This nitrogen incorporation increases the dielectric constant, prevents boron dopant penetration from polysilicon gate electrodes, and improves the overall hot-carrier immunity of the device.

Future Outlook

As the semiconductor industry marches toward advanced nanosheet (gate-all-around) transistors and 3D stacked architectures, the demands on ISSG oxidation continue to intensify. Future research is focused on low-temperature radical oxidation technologies that can generate highly reactive species without requiring high thermal budgets, which is essential for protecting delicate materials in back-end-of-line (BEOL) integration. Furthermore, atomic-layer-precise selective ISSG processes are being developed, where self-assembled monolayers or chemical treatments are used to selectively inhibit or promote radical oxidation on specific material surfaces. This will enable self-aligned oxide integration, bypassing complex lithography steps and paving the way for sub-2nm node architectures.