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 [T1]. 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) [T2]. 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 [T2]. Conversely, wet oxidation yields significantly faster growth kinetics because water molecules diffuse through the growing oxide network much more rapidly than diatomic oxygen [T2]. 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 [P3].

To bridge this gap, the semiconductor industry developed in-situ steam generation (ISSG) [P1]. ISSG represents a paradigm shift where ultra-pure steam is generated directly inside a single-wafer rapid thermal processing (RTP) chamber by reacting hydrogen ($\text{H}_2$) and oxygen ($\text{O}_2$) gases in a high-temperature, low-pressure environment [P1, P3]. This process, referred to as ISSG oxidation, avoids the purity and contamination issues of external steam delivery systems [T1]. 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 [P3]. 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 [P3, P4]. 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 [P1, P4, A1].

Physics & Mechanism

The fundamental physics of ISSG oxidation relies on gas-phase radical generation coupled with atomic-scale surface reactions [P3]. 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 [P3].

Gas-Phase Radical Kinetics

When $\text{H}_2$ and $\text{O}_2$ are introduced into the heated RTP chamber, they undergo a series of chain-branching combustion reactions [P3]. At typical processing temperatures, these reactions generate a high volume of transient radicals before reaching thermodynamic equilibrium [P3]. The simplified reaction path can be represented by the following elementary steps (Engineering Practice):

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

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

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

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

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 [P3]. Among these, atomic oxygen ($\text{O}\cdot$) and hydroxyl radicals ($\text{OH}\cdot$) are the primary drivers of ISSG oxidation [P3].

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 [P3]. To analyze the kinetic progression of this oxide growth, process engineers rely on the classical Deal-Grove model [P3]:

$$x^2 + A x - B t + \tau = 0$$

Where:

• $x$ is the oxide thickness [P3].
• $t$ is the oxidation time [P3].
• $B$ is the parabolic rate constant, which is proportional to the effective diffusivity of the oxidant species through the oxide bulk [P3].
• $B/A$ is the linear rate constant, governed by the reaction rate at the silicon-dielectric interface [P3].
• $\tau$ is a time shift term that accounts for initial native oxide presence [P3].

In the thin-film regime, the growth is governed by the linear growth law [P3]:

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

For ISSG, the linear rate constant ($B/A$) is drastically enhanced compared to conventional dry oxidation [P3]. 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 $\text{O}_2$ [P2].

Furthermore, atomic-scale modeling utilizing reactive force field (ReaxFF) molecular dynamics (MD) simulations has illuminated the role of hydrogen in accelerating the diffusion process [P2]. In the presence of hydrogen atoms (wet/ISSG environments), local coordination and bond-rearrangement energy barriers near the interface are altered [P2]. Specifically, hydrogen passivates dangling bonds and forms intermediate silanol ($\text{Si-OH}$) groups, which effectively open up and restructure the local silicon-oxygen network [P2]. 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 [P2].

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 [P3]. Since ISSG is a non-equilibrium, radical-driven process, the directional trends of these process parameters differ significantly from traditional batch furnace operations [P3].

[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 [P1].

• 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 [P1, P2]. 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 [P2].
• Chamber Pressure: Unlike conventional thermal oxidation where higher pressure accelerates growth by increasing molecular concentration, ISSG requires a relatively low pressure regime [P3]. 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 ($\text{O}\cdot + , \text{O}\cdot \rightarrow \text{O}_2$) [P3]. Thus, decreasing pressure directionally increases the ratio of radical-to-molecular oxidants, enhancing film density and growth rates in the thin-regime [P3].
• Gas Flow Ratio ($\text{H}_2/\text{O}_2$): The ratio of hydrogen to oxygen is highly critical (Engineering Practice). A minimal concentration of $\text{H}_2$ is required to initiate the combustion reactions that generate hydroxyl and atomic oxygen radicals [P3]. Increasing the $\text{H}_2$ fraction up to an optimal limit enhances the radical generation rate [P3]. 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 [P2, P3].
• Soak Time: The duration of the high-temperature step controls the final oxide thickness [P1]. Due to the rapid initial growth rates enabled by radicals, very short soak times can be utilized to achieve highly uniform, ultra-thin films [P1].

Stress and Structural Relaxation

A critical aspect of the thermal budget in oxidation is the management of mechanical stress at the silicon interface [P2]. Silicon dioxide undergoes a substantial volume expansion relative to the consumed silicon substrate [P4]. During high-temperature ISSG, structural relaxation occurs, which can be modeled using the volumetric thermal expansion coefficient ($\alpha$) [P2]:

$$\alpha = \frac{1}{V}\frac{\Delta V}{\Delta T}$$

Where:

• $V$ is the system volume [P2].
• $\Delta T$ is the change in temperature [P2].
• $\Delta V$ is the volume change [P2].

The rapid, high-temperature thermal cycle of RTP allows for localized viscous flow of the $\text{SiO}_2$ during growth [P3]. This flow relieves the compressive stress caused by the volumetric mismatch, minimizing the generation of silicon dislocations and interface states at the active boundary [P2].

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 [T2].

Stress Mismatch and Curvature Effects

In three-dimensional (3D) structures, such as FinFET channels or the trenches of a Buried Channel Array Transistor (BCAT), oxidation rates are highly sensitive to surface curvature and crystallographic orientation [P3, P4]. During ISSG, the volumetric expansion of $\text{SiO}_2$ in a confined, high-curvature geometry generates massive localized compressive stress [P4]. 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 [P4]. 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 [P4]. This thinned bottom oxide is a primary failure mode, as it leads to localized electric field crowding, early dielectric breakdown, and increased gate leakage [P4].

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 [P2]. 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 [P2]. 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 [P2].

Monitoring and Process Control in Ultra-Thin Regimes

As gate oxides are scaled down to extreme dimensions, verifying film properties becomes highly challenging [P1]. In processes where the ISSG oxide is subjected to subsequent nitridation (e [P1].g., using nitric oxide ($\text{NO}$) 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 [P1].

However, monitoring this ultra-thin nitrided layer is difficult [P1]. Standard optical ellipsometry cannot easily resolve the minor sub-angstrom optical variations associated with subtle shifts in nitrogen dose [P1]. Historically, engineers used the "Delay to Reoxidation" (D2R) method to monitor nitrogen incorporation based on how effectively the nitrided interface acts as a diffusion barrier to a subsequent oxidation step [P1]:

$$\text{D2R} = T_{ox3}' - T_{ox2}$$

Where:

• $T_{ox2}$ is the post-nitridation oxide thickness [P1].
• $T_{ox3}'$ is the equivalent oxide thickness after a sacrificial reoxidation step [P1].

While effective, the D2R method is destructive and highly sensitive to initial oxide fluctuations, making it prone to statistical process control (SPC) issues [P1]. Modern metrology often replaces this with non-contact electrical techniques that corona-charge the oxide surface and use a Kelvin probe to extract the interface trapped charge density ($Q_{it}$) [P1]. This allows direct physical monitoring of the interface state density ($D_{it}$) without requiring a reoxidation step [P1]:

$$D_{it} = \frac{dQ_{it}}{dE}$$

Where:

• $E$ is the energy level within the bandgap [P1].
• $Q_{it}$ is the interface trapped charge density [P1].

Technology Node Evolution

The role of ISSG has evolved dramatically alongside changes in transistor architecture, scaling down from planar CMOS to complex 3D structures [T2].

       Planar Nodes (e *(Engineering Practice)*.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
                                            Sub-17nm 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 flow, planar devices utilized ISSG oxidation primarily to grow highly uniform, ultra-thin gate oxides and sacrificial screen oxides [P4]. The highly controlled radical kinetics enabled the growth of extremely thin layers with highly uniform thickness across large-diameter wafers, ensuring tight threshold voltage distributions [P1].

14nm to 7nm FinFET Nodes

As the industry transitioned to the 14nm FinFET and 7nm FinFET architectures, the physical challenges shifted to managing the multi-gate fin topography [P4]. ISSG was extensively deployed to grow STI liner oxides because its radical-driven mechanism provides excellent conformal coverage over high-aspect-ratio silicon fins (Engineering Practice). Furthermore, ISSG oxidation was used for "corner rounding," a critical process where the sharp top corners of the silicon fins are selectively oxidized and rounded [P4]. Rounding these corners prevents localized electric field enhancement, suppressing parasitic sub-threshold leakage currents along the fin edges (Engineering Practice).

Sub-17nm Memory Nodes

In advanced dynamic random-access memory (DRAM) scaling (sub-17nm), transistors have migrated toward Buried Channel Array Transistor (BCAT) and Dual Work Function Buried Channel Array Transistor (DWF-BCAT) designs to suppress gate-induced drain leakage (GIDL) [P4]. For these deep trench structures, a single atomic layer deposition (ALD) or thermal oxidation step is insufficient [P4]. Instead, manufacturers utilize an integrated ISSG-ALD-ISSG (IAI) three-step gate oxide process [P4].

               [ 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 [A2]. The first ISSG step grows a high-quality, thin base oxide directly on the silicon trench surface to establish a low-defect, pristine interface [P4]. 2. A subsequent ALD step provides highly conformal bulk oxide thickness along the high-aspect-ratio vertical walls of the trench [P4]. 3. A final, thin ISSG cap is grown to heal any stoichiometry defects, carbon impurities, or vacancies within the ALD film [P4].

This combination leverages the thickness control of ALD with the superior physical density and low interface state density of radical ISSG oxidation [P4]. Furthermore, to control the electrical properties of the gate and manage the device's work function, advanced integration utilizes specialized recessed isolation structures covered by a "T-shaped" or necked gate profile to ensure optimal electrostatic channel control and mechanical reliability [A1, A2].

Related Processes

ISSG oxidation does not operate in isolation; it is deeply coupled with several upstream and downstream process modules [P2].

• 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 [P2]. This is typically done using hydrofluoric acid (HF) mixtures [P4]. The quality of this clean directly dictates the initial time shift ($\tau$) in the Deal-Grove oxidation kinetics [P3]. 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 superior molecular cross-linking [P4].
• Thermal Annealing: After oxidation, a post-oxidation annealing step in an inert nitrogen ($\text{N}_2$) or argon ($\text{Ar}$) ambient is frequently performed [T2]. This high-temperature step allows further structural relaxation, reducing the residual stress ($\alpha$) and passivating any remaining dangling bonds at the interface [P2, T2].
• 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 [P1]. This nitrogen incorporation increases the dielectric constant, prevents boron dopant penetration from the polysilicon gate, and improves the overall hot-carrier immunity of the device [P1].

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 [T2]. 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 [P3]. 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 (Engineering Practice). This will enable self-aligned oxide integration, bypassing complex lithography steps and paving the way for sub-2nm node architectures [T2].