Introduction
In modern microelectronics, the relentless pursuit of device scaling has driven complementary metal-oxide-semiconductor (CMOS) technology to remarkable levels of integration density and performance . However, as the physical dimensions of transistors shrink, a major challenge arises from the divergent operating requirements within a single integrated circuit (IC) . While the internal core logic of a system-on-a-chip (SoC) must operate at low supply voltages to minimize active power consumption and prevent thermal runaway, external interface peripherals—such as input/output (I/O) buses, memory interfaces, and analog blocks—must communicate with external system components operating at much higher voltage standards , .
This voltage discrepancy necessitates the integration of transistors with different gate dielectric thicknesses on the same silicon substrate , . The primary technology enabling this co-integration is dual gate oxide (DGO) . In a dual gate oxide scheme, high-speed core transistors are fabricated with an ultra-thin gate dielectric to maximize capacitive coupling and drive current, while peripheral interface transistors are equipped with a physically thick gate oxide to withstand high operating voltages without experiencing dielectric breakdown , , .
A prime example of this co-integration is the I/O gate oxide CIS (CMOS Image Sensor) architecture, where high-voltage analog transistors used for pixel reset and read operations must coexist with low-voltage digital processing blocks on the same die . Achieving this multi-voltage capability on a single chip requires complex, highly controlled processing flows, such as those found in the 28nm Planar Flow, where planar dual gate oxide integration represents a critical process milestone . Understanding the physical principles, electrochemical mechanisms, and structural challenges of dual gate oxide processes is essential for any process engineer navigating advanced semiconductor manufacturing nodes .
Physics & Mechanism
The design and optimization of dual gate oxide integration rely heavily on the fundamental device physics of the metal-oxide-semiconductor capacitor (MOSCAP) . The performance of a metal-oxide-semiconductor field-effect transistor (MOSFET) is directly governed by its gate capacitance ($C$), which dictates the electrostatic control of the channel and is expressed as:
$$C = \frac{\varepsilon_0 K A}{t}$$
where $\varepsilon_0$ represents the vacuum permittivity, $K$ is the relative dielectric constant of the gate insulator, $A$ is the capacitor area, and $t$ is the physical thickness of the dielectric layer .
Core Transistor (Low Voltage) I/O Transistor (High Voltage)
[Gate Electrode] [Gate Electrode]
========================= =========================
Thin Gate Oxide [t_thin] Thick Gate Oxide [t_thick]
------------------------- -------------------------
[Silicon Substrate] [Silicon Substrate]
Direct Tunneling vs [T1]. Dielectric Breakdown
In core logic transistors, scaling down the physical thickness of a silicon dioxide ($\text{SiO}_2$) gate insulator reduces short-channel effects and raises the drive current . However, when the physical thickness of $\text{SiO}_2$ is reduced below a critical threshold, the quantum-mechanical wavefunctions of electrons in the channel can easily penetrate the potential barrier of the dielectric , . This quantum-mechanical phenomenon is known as direct tunneling , . Direct tunneling leakage current increases exponentially as the dielectric thickness decreases, leading to unacceptable static power consumption , .
To bypass this physical scaling limit, modern technology nodes introduce high-dielectric-constant (high-k) materials , . This allows process engineers to use the concept of equivalent oxide thickness (EOT) to compare different dielectrics , :
$$\text{EOT} = \frac{3.9}{K} t_{\text{HiK}}$$
where $K$ represents the relative permittivity of the high-k material, $3.9$ is the relative permittivity of thermal $\text{SiO}2$, and $t{\text{HiK}}$ is the physical thickness of the high-k film , . By using a material with a high relative permittivity, the physical thickness of the film can be increased to suppress tunneling leakage while maintaining the same capacitive electrostatic modulation , .
In contrast, the design of thick gate oxide transistors in peripheral I/O circuits is driven not by tunneling leakage, but by dielectric breakdown physics , . High external operating voltages generate intense electric fields across the gate dielectric , . If the physical thickness of the oxide is too thin, the electric field exceeds the dielectric strength of the material, causing destructive dielectric breakdown and device failure , . Consequently, a physically thick gate oxide must be retained in these regions to distribute the high voltage over a larger physical distance, keeping the internal electric field well below the critical breakdown threshold , .
Subthreshold Conduction and Band Physics
In the weak inversion regime, the MOSFET operates below its threshold voltage ($V_{\text{th}}$) , . The subthreshold drain current ($I_{\text{ds}}$) is dominated by diffusion and is highly sensitive to the electrostatic potential barrier in the channel, as described by:
$$I_{\text{ds}} \propto \exp\left(\frac{q V_{\text{gs}}}{\eta k T}\right)$$
where $q$ is the elementary charge, $V_{\text{gs}}$ is the applied gate-to-source voltage, $k$ is the Boltzmann constant, $T$ is the absolute temperature, and $\eta$ is the subthreshold slope factor .
The efficiency of turning the transistor "off" is quantified by the subthreshold swing ($S$), which represents the change in gate voltage required to alter the drain current by one order of magnitude :
$$S = \eta \times 60 \text{ mV/dec} \quad (\text{at } 300\text{ K})$$
The subthreshold swing is limited by the thermodynamic distribution of carrier energies governed by the Fermi-Dirac distribution . It is also affected by capacitive division between the gate dielectric, the depletion region, and any interface defect states , . Thick gate oxide devices exhibit larger subthreshold swing values than thin gate oxide devices due to poorer capacitive coupling between the gate electrode and the silicon channel , .
Furthermore, the periodic atomic arrangement within the semiconductor crystal establishes the electronic band gap , while doping shifts the Fermi level to control conductivity . At the interface between the silicon substrate and the gate oxide, the energy band offsets must be sufficiently large to prevent carrier injection over the barrier . If the band alignment or interface quality degrades, the density of interface states increases, leading to a substantial deterioration of both subthreshold swing and threshold voltage stability , .
Process Principles
Integrating thin and thick gate oxides on a single substrate requires a sequence of chemical, physical, and lithographic steps . The conventional process integration strategy is based on a "grow-etch-regrow" cycle , .
1. Thick Oxide Growth 2 [P2]. Mask & Wet Etch 3 *(Engineering Practice)*. Regrow Thin Oxide
#################### #################### #### ####
[Silicon Substrate] [Thin-Ox] [Thick-Ox] [Thin-Ox] [Thick-Ox]
The Grow-Etch-Regrow Integration Sequence
1 . First Thermal Oxidation: A high-quality thermal silicon dioxide layer is grown across the entire active silicon substrate to form the precursor for the thick gate oxide , . This oxidation relies on the transport of oxidant species through the growing oxide layer to react with the silicon surface . 2. Photolithography: A photoresist (PR) layer is applied and patterned using lithography , . The PR masks and protects the high-voltage/I/O regions while leaving the core logic regions exposed , . 3. Oxide Wet Etch: The wafer is exposed to a wet etching chemistry, typically a buffered oxide etchant (BOE) or dilute hydrofluoric acid (HF) , . In the exposed regions, the pre-grown oxide is removed down to the bare silicon surface, while the PR-masked regions preserve the thick oxide layer , . 4. Photoresist Strip and Clean: The PR masking layer is stripped, and a critical surface clean is performed to remove organic, metallic, and particulate contaminants before the next oxidation step , . 5. Second Thermal Oxidation (Regrowth): The wafer undergoes a second thermal oxidation step , . In the freshly cleared core regions, a high-quality thin gate oxide is grown . Simultaneously, in the I/O regions, the remaining oxide is further oxidized, growing until it reaches its final target thick gate oxide thickness , .
Modified Dual Gate Oxide (MDGO) Principles
The standard "grow-etch-regrow" process can degrade the quality of the thick gate oxide , . Because the pre-grown thick oxide is subjected to photolithography, wet etching, and chemical cleaning, its surface is highly vulnerable to chemical damage, physical erosion, and contaminant incorporation , .
To address this, process engineers developed the Modified Dual Gate Oxide (MDGO) process . Under this scheme:
- The initial oxide growth thickness is reduced .
- Instead of performing a partial etch that leaves a damaged oxide residue in the thin-oxide region, the pre-grown oxide is completely etched away in the thin-oxide zones .
- The duration of the subsequent wet etch is minimized, reducing the exposure of the thick oxide to aggressive chemical etchants .
This modification minimizes surface micro-roughness on the pre-grown oxide and reduces mechanical stress at isolation boundaries .
Wet Cleaning Electrochemistry and Surface Roughness
The choice of post-etch cleaning chemistry is a critical process parameter . Historically, standard cleaning sequences relied on Ammonium Peroxide Mixture (APM) and Hydrochloric Peroxide Mixture (HPM) . However, APM containing ammonium hydroxide ($NH_4OH$) and hydrogen peroxide ($H_2O_2$) causes a slow chemical etching of $SiO_2$ driven by hydroxide ($OH^-$) ions . In a dual gate oxide process, this chemical etching increases surface micro-roughness on the exposed pre-grown oxide . This roughness creates local physical tips that act as electric field enhancement points, lowering the breakdown voltage and degrading gate oxide integrity (GOI) .
To preserve the surface planarity of the thick gate oxide, a Sulfuric Peroxide Mixture (SPM) cleaning chemistry is preferred after the thick gate oxide etch . SPM consists of sulfuric acid ($H_2SO_4$) and hydrogen peroxide ($H_2O_2$) operating at elevated temperatures . The strong oxidative reactions of SPM efficiently decompose organic contaminants and photoresist residues without etching the underlying thermal $SiO_2$ layer . This prevents chemical etching-induced micro-roughness and preserves the planar morphology of the thick gate oxide, leading to superior time-dependent dielectric breakdown (TDDB) reliability , .
To achieve a uniform start for these thermal steps, chemical mechanical planarization (CMP) is utilized to planarize isolation structures prior to well formation and gate-stack processing .
Challenges & Failure Modes
Integrating dual gate oxides introduces unique physical and chemical degradation mechanisms that can lead to yield loss and reliability failures , .
Local Oxide Thinning (Grooving)
|
v
\ |**| / <- Poly-Si Gate
\ |**| /
======= |**| ======= <- Thick Gate Oxide
[ STI ] |**| [ STI ]
=======/ / \ \=======
/ / \ \
/ [Active] \ <- Silicon Substrate
STI Corner Oxide Thinning and Grooving
One of the most severe structural failure modes in dual gate oxide integration occurs at the shallow trench isolation (STI) boundaries . The junction where the planar active silicon channel meets the vertical oxide wall of the STI trench is highly vulnerable .
During the wet-etch step used to clear the pre-grown oxide from the thin-oxide regions, the isotropic etchant also attacks the adjacent trench isolation oxide . Because of the three-dimensional geometry and high mechanical stress at the STI edge, the etch rate at this corner is faster than on planar surfaces . This localized over-etching creates physical "grooves" or "crevices" at the corner .
During the subsequent second thermal oxidation, mechanical stress at the 3D corner restricts the transport of oxygen species and retards local oxidation kinetics . As a result, the regrown gate oxide is significantly thinner at the STI corners than on the planar channel areas . When a gate voltage is applied, this localized oxide thinning causes electric field crowding at the corner, leading to early dielectric breakdown and high parasitic leakage .
Surface Micro-Roughness and Local Field Enhancement
Chemical cleanings and wet-etching steps can alter the surface morphology of the pre-grown oxide , . If alkaline mixtures like APM are used excessively, the uneven removal of oxide molecules increases surface micro-roughness . On a microscopic scale, a rough interface creates local asperities . When the gate electrode is deposited over this rough surface, these microscopic peaks focus the electric field lines, locally increasing the electric field intensity . This localized field enhancement accelerates charge trapping in the oxide, lowering the charge-to-breakdown ($Q_{\text{bd}}$) value and degrading TDDB reliability , .
Parasitic Sidewall Transistors
At the interface where the gate oxide boundary meets the STI oxide, the local depletion region and electric field distribution can be distorted , . In weak inversion or subthreshold operating regimes, this electric field distortion can lower the local threshold voltage at the channel edges , . This forms an unintended, parallel conduction path known as a parasitic sidewall transistor , .
This parasitic channel leads to subthreshold leakage current matching degradation in multi-transistor arrays , . To suppress this failure mode, specialized structural designs—such as highly doped ring-shaped well structures or matching-oriented layouts—are implemented to decouple the active channel from the physical isolation boundary, raising the threshold voltage of the parasitic sidewall channel , .
Charge Trapping and Threshold Voltage Drift
During the integration of advanced gate stacks, the use of high-k materials (e .g., hafnium dioxide, $\text{HfO}2$) introduces challenges like crystallization during subsequent thermal processes . Crystallization creates grain boundaries that act as fast diffusion paths for oxygen and metallic impurities, leading to high defect densities and interface charge trapping . This trapping of electrons or holes causes a direct shift in the device threshold voltage ($V{\text{th}}$) and degrades carrier mobility due to increased remote Coulomb scattering .
Technology Node Evolution
As CMOS manufacturing scaled from planar transistors down to multi-gate architectures, the implementation of dual gate oxide integration underwent radical changes .
Planar Node FinFET Node Nanosheet Node (GAA)
=========== =========== ====================
___ ___ _ _ ==== <- Nano-
| | | | | | | ==== sheets
_____|___|___|_____ ___| |___| |___ ====
[Silicon Substrate] [Silicon Substrate] [Silicon Substrate]
Planar Nodes (28nm and above)
In planar nodes, such as 28nm Planar Flow, dual gate oxide processing was relatively straightforward, relying on the planar "grow-etch-regrow" thermal oxidation sequences , . The gate dielectric consisted of either a high-k metal gate (HKMG) stack for core logic or a thermal oxynitride/silicon dioxide layer for I/O thick gate oxide devices , . The primary engineering focus during these planar eras was minimizing surface micro-roughness , preventing STI corner grooving , and ensuring precise wet-etch selectivity between the photoresist-masked regions and exposed active silicon , .
FinFET Nodes (14nm to 7nm and beyond)
The transition to three-dimensional architectures, starting at the 14nm FinFET node, significantly complicated dual gate oxide integration (Engineering Practice). In a FinFET, the channel consists of thin silicon fins wrapped by the gate on three sides (Engineering Practice).
Growing a thick gate oxide on these narrow 3D structures introduces unique challenges:
- Conformality and Corner Effects: Thermally growing a thick oxide on 3D fins leads to severe mechanical stress at the fin corners, causing localized oxide thinning similar to the planar STI corner effect .
- Etch Control on 3D Features: Wet etching the thick oxide from the core logic fins without damaging the fin profile or leaving oxide residues in the narrow recesses between adjacent fins requires highly controlled isotropic etching .
- Deposition vs. Growth: To resolve these thermal oxidation limits, process engineers increasingly integrated atomic layer deposition (ALD) to deposit conformal thick oxide stacks, replacing pure thermal growth steps .
By the 7nm node, the physical space between fins was so compressed that fabricating dual gate oxides required precise control of ALD film thickness and highly selective isotropic dry etching to avoid damaging the delicate 3D fins .
Nanosheet/GAA Nodes and Beyond
At the sub-3nm nodes, the industry is transitioning to Gate-All-Around (GAA) nanosheet architectures (Engineering Practice). In a nanosheet transistor, the gate completely wraps around suspended horizontal silicon sheets .
In this ultra-confined geometry, integrating a dual gate oxide becomes a major physical bottleneck:
- Lateral Space Constraints: The physical space between adjacent nanosheets is extremely tight . Attempting to fit a thick gate oxide for high-voltage I/O operations within these tiny spaces can pinch off the gate volume entirely, leaving no room for the work-function metal gate electrode .
- Advanced Multi-Thickness Schemes: Rather than relying on different physical oxide thicknesses, advanced GAA nodes often use complex chemical treatments to modify the local dielectric constant ($K$) or integrate different work-function metal stacks to modulate the threshold voltage and electrical thickness without physically packing thick oxide layers into confined spaces .
Related Processes
The integration of dual gate oxide technology is deeply interconnected with several key wafer fabrication steps:
- Ion Implantation: Before any gate oxide growth, ion implantation is used to perform well-doping, punch-through stop implants, and channel threshold voltage ($V_{\text{th}}$) adjustments . In addition, specialized high-dose implants are used to form isolation rings that suppress parasitic sidewall transistor leakage at the gate edges , .
- High-K Metal Gate (HKMG) Integration: Modern dual gate oxide flows must be fully compatible with HKMG integration , . The high-k stack, typically deposited using atomic layer deposition (ALD), is often layered on top of a thin chemical oxide interfacial layer . For I/O devices, this high-k stack may be deposited directly onto a thicker thermal oxide base to provide both high breakdown voltage and low gate leakage , .
- Rapid Thermal Annealing (RTA): After gate-stack deposition, rapid thermal annealing is performed to activate implanted dopants in the source/drain regions , , heal interface traps, and thermally densify the deposited and grown gate oxides to optimize their breakdown properties , .
- Chemical Mechanical Planarization (CMP): CMP is critical for planarizing the STI oxide before gate oxide processing . This step ensures a flat topography, preventing local focus variation during subsequent DGO photolithography steps , (Engineering Practice).
Future Outlook
As transistors continue to scale down to the atomic level, traditional dual gate oxide integration faces severe physical limitations . In the future, several emerging trends and research directions are poised to reshape how multi-voltage systems are co-integrated:
Dipole-Engineered and Multi-Vt Gate Stacks
To circumvent the physical space constraints of thick gate oxides in nanosheet channels, engineers are exploring dipole engineering . By introducing extremely thin layers of elements like lanthanum ($\text{La}$) or aluminum ($\text{Al}$) at the gate dielectric interfaces, they can induce controlled electrical dipoles . These dipoles shift the band alignment and modulate the effective threshold voltage over a wide range, allowing high-voltage and low-voltage electrical characteristics to be achieved using thin gate stacks .
Monolithic 3D Integration (3D-IC)
Another long-term solution is to physically separate the core logic and high-voltage I/O circuits into different layers using monolithic 3D integration (Engineering Practice). In this approach, high-speed core transistors are fabricated on the primary silicon layer using ultra-thin high-k gate dielectrics . Through sequential wafer bonding and low-temperature processing, a secondary active layer is fabricated directly above the core logic to host the high-voltage I/O and analog circuits with physically thick gate oxides . This completely decouples the process constraints of the thin and thick gate oxides, eliminating the need for complex "grow-etch-regrow" cycles on a single shared layer .
Alternative Wide-Bandgap Materials
For high-voltage interfaces and smart-power systems, wide-bandgap semiconductors such as gallium nitride ($\text{GaN}$) or silicon carbide ($\text{SiC}$) are increasingly integrated on the same silicon platform . Integrating these wide-bandgap materials allows the peripheral I/O and power switches to withstand extremely high voltages over smaller physical dimensions, reducing the reliance on thick silicon-based gate oxides and driving a shift toward heterogeneous, multi-material integration .