Remote Plasma Oxide Etch: Physical Mechanisms, Process Principles, and Advanced Node Integration

Introduction

In advanced semiconductor manufacturing, the ability to selectively remove ultra-thin dielectric films without damaging underlying atomic layers is a critical requirement for device scaling. As transistors transitioned from planar architectures to complex three-dimensional structures, conventional dry etching techniques faced physical limitations. Standard reactive ion etching (RIE) systems subject the substrate to energetic ion bombardment, which can induce lattice defectivity, surface charging, and loss of critical dimension (CD) control.

In conventional direct plasma etching, charges accumulated from interconnect or plasma exposure can cause a local imbalance in surface potential across the gate dielectric and induce current flow through the gate electrode . To mitigate these physical damage mechanisms, remote plasma oxide etch technologies decouple plasma generation from the wafer processing surface. Unlike direct plasma systems where the wafer is immersed in the glow discharge, a remote plasma source (RPS) generates active chemical species upstream, delivering primarily neutral radicals to the wafer surface. Etch selectivity, defined as the ratio between the etch rate of the target layer and that of an underlying or masking material, is critical when removing thin dielectric films without damaging adjacent materials .

Among these techniques, the SiCoNi process is a prominent dry chemical etching solution used in advanced fabrication modules. Based on nitrogen trifluoride (NF3) and ammonia (NH3) chemistries, the SiCoNi process selectively removes silicon dioxide (SiO2) while preserving elemental silicon (Si), silicon-germanium (SiGe), and silicon nitride (Si3N4). Understanding the physical, chemical, and thermodynamic principles of remote plasma oxide etch is essential for engineers working on sub-10nm technology nodes.

Physics & Mechanism

Spatial Separation and Radical Transport

The fundamental design of a remote plasma system relies on separating the plasma generation zone from the wafer processing chamber. High-density plasma is ignited upstream using microwave (MW) or inductively coupled plasma (ICP) excitation. The plasma effluents then pass through a showerhead, a plenum, or a conductive grid electrode.

This grid electrode is typically grounded, acting as an electrostatic filter that recombines or shields charged particles (electrons and ions) while remaining permeable to neutral atoms, molecules, and radicals. Consequently, the downstream process chamber maintains an ion-reduced environment where the etch mechanism is driven primarily by thermal neutral chemistry. Because reactive neutrals etch without directional momentum, remote plasma chemical etching is fundamentally isotropic, proceeding at equal rates in all directions [T1].

The SiCoNi Dry Chemical Etch Mechanism

In the widely used SiCoNi process, the remote plasma chemistry is sustained using a mixture of NF3 and NH3 gases. The chemical reaction proceeds via a distinct two-step sequential mechanism:

1. Low-Temperature Solid-State Reaction: The upstream plasma dissociates the reactant gases into active species, including ammonium fluoride (NH4F) and ammonium hydrogen fluoride (NH4F·HF) complexes. These gaseous reactants flow downstream and adsorb onto the wafer surface at relatively low temperatures. They react selectively with the native or thermal silicon dioxide (SiO2) layer to form solid ammonium hexafluorosilicate salt according to the following simplified reaction pathway:

SiO2(s) + 6 NH4F(g) -> (NH4)2SiF6(s) + 2 H2O(g) + 4 NH3(g)

This reaction exhibits self-limiting behavior because the solid reaction product, ammonium hexafluorosilicate ((NH4)2SiF6), forms a diffusion barrier layer that slows down further fresh reactants from reaching the unreacted SiO2 interface beneath.

2. Thermal Sublimation (Desorption): Once the solid-state reaction phase is complete, the wafer is heated to an elevated temperature. This thermal step induces the decomposition and sublimation of the solid salt into volatile byproducts, which are evacuated from the chamber:

(NH4)2SiF6(s) -> SiF4(g) + 2 NH3(g) + 2 HF(g)

NF3/O2 and NF3/N2/O2/H2 Alternative Chemistries

Beyond the NF3/NH3 system, other gas mixtures are employed for targeted material selectivity. In an NF3/O2 remote plasma, dissociation yields highly reactive fluorine (F) atoms alongside gas-phase nitric oxide (NO). The NO molecules preferentially adsorb onto nitrogen-containing films such as silicon nitride (Si3N4), facilitating the depletion of surface nitrogen and accelerating the etch rate of nitride while leaving pure oxide relatively unaffected.

Conversely, when hydrogen (H2) is introduced to form an NF3/N2/O2/H2 discharge, the interaction produces vibrationally excited hydrogen fluoride (HF*). This vibrationally excited species possesses localized energy that significantly lowers the chemical reaction barrier on Si3N4 surfaces, enabling selective isotropic etching of nitride over SiO2 even without energetic physical ion bombardment.

Process Principles

To achieve precise etch depth and profile control, process parameters must be tuned systematically.

RF Power and Radical Flux

Increasing the radio frequency (RF) or microwave power applied to the remote plasma source increases the dissociation fraction of the precursor gases. This leads to an increased flux density of fluorine atoms and reactive radicals flowing downstream, which enhances the chemical etch rate. However, excessively high power density can increase gas temperature within the source, promoting unwanted recombination reactions or raising the plasma potential, which increases the risk of ion leakage through the filtering grid.

Gas Flow Ratios

The feed gas ratio dictates the active chemical species present at the wafer surface. In SiCoNi chemistry, the ratio of NH3 to NF3 must be balanced. An excess of NF3 increases the concentration of free fluorine radicals, which can lead to unselective chemical attack on exposed silicon or silicon-germanium surfaces. In NF3/O2 chemistries, the O2/NF3 ratio governs the generation of NO molecules; increasing this ratio to an optimal regime enhances Si3N4 removal rates before dilution effects dominate.

Wafer Temperature and Sublimation Control

Wafer stage temperature is the primary physical control used to separate the reaction phase from the sublimation phase. During the chemical reaction step, maintaining a lower wafer stage temperature facilitates the condensation of active ammonium species and stabilizes the formation of the solid (NH4)2SiF6 salt layer. If the stage temperature is too high during this step, reactants desorb before reacting, suppressing oxide conversion. During the subsequent sublimation step, ramping the temperature to a higher thermal threshold ensures complete volatilization of the salt.

Chamber Pressure and Plenum Volume

Process chamber pressure governs the mean free path of active species. Operating at higher pressures promotes intermolecular collisions, increasing the recombination rate of active fluorine radicals into less reactive molecular species. This recombination can reduce the etch rate while improving selectivity by suppressing aggressive radical attack. Conversely, lower pressures extend radical lifetimes. Incorporating a physical plenum expansion chamber between the remote source and the wafer stage allows radical flux to laterally diffuse, improving etch uniformity across large-diameter wafers.

Challenges & Failure Modes

Despite its chemical selectivity and low physical momentum transfer, remote plasma oxide etching is subject to specific integration challenges.

Volumetric Expansion Stress

During the initial phase of the SiCoNi process, converting solid SiO2 into the solid (NH4)2SiF6 salt involves a volumetric expansion. In dense, multi-layered structures such as those found in fin field effect transistor designs, this expansion can exert mechanical stress on adjacent narrow features. Localized stress can cause structural deformation, pattern tilting, or physical collapse of high-aspect-ratio silicon fins.

Solid Salt Residues and Incomplete Sublimation

If thermal sublimation is non-uniform or fails to reach the required activation temperature, residual ammonium hexafluorosilicate salt can remain on the wafer surface. These non-volatile solid residues act as particulate defects, blocking subsequent epitaxial growth, contact metallization, or atomic layer deposition steps. This failure mode leads to elevated contact resistance and localized yield loss.

Ion Leakage and Charging Damage

Although remote plasma sources are engineered to deliver neutral species, electrostatic leakage can occur if grid electrodes degrade or if pressure-power regimes become unstable. Weak ion leakage into the downstream chamber introduces non-directional sputtering components and risks charging damage on sensitive gate dielectrics.

Corner Rounding and Loss of Spacer Profiles

In applications like dielectric spacer formation, precise sidewall profile retention is critical. When downstream chemical etching is applied near low-k spacer materials (such as SiCO), isotropic radical attack can lead to lateral trimming or unwanted recess. This degradation of the spacer profile alters parasitic capacitance and effective channel length.

Technology Node Evolution

The 28nm Planar Era

At the 28nm Planar Flow node, remote plasma oxide etching was introduced to replace wet hydrofluoric (HF) acid cleans in critical modules, such as pre-contact cleans and pre-gate oxide formation. Wet chemical cleans faced surface tension limits that hindered liquid penetration into high-aspect-ratio contact vias. Dry remote plasma processes bypassed these liquid transport limits, removing native oxide at contact bottoms without undercutting adjacent dielectrics.

Transition to 14nm FinFETs

As the industry transitioned to the 14nm FinFET node, device geometry became fully three-dimensional. Direct RIE removal of native oxide on sensitive vertical silicon fin sidewalls risked structural damage and fin erosion. Remote plasma oxide etching provided a low-damage surface preparation prior to depositing high-k metal gate stacks. Additionally, downstream dry chemical processes enabled clean integration of carbon-doped low-k dielectric spacers.

Scaling to 7nm and Beyond

At the 7nm FinFET node and in gate-all-around (GAA) nanosheet structures, material loss margins are restricted to atomic scales. In GAA architectures, silicon nanosheets are interleaved with sacrificial silicon-germanium (SiGe) layers. Releasing these nanosheets requires dry chemical processes with high material selectivity. Minimizing direct ion sputtering in remote plasma systems provides the necessary chemical control and low surface degradation required to process sub-10nm architectures.

Related Processes

Remote plasma oxide etching operates in conjunction with several adjacent integration steps in advanced CMOS flows:

• Ion Implantation: In advanced contact and spacer modules, ion implantation can selectively modify the top layer of a dielectric or low-k film (e.g., SiCO), enhancing its chemical reactivity to downstream radical etching while unmodified bulk regions remain resistant.
• Surface Preparation for Epitaxy: Prior to selective epitaxial growth of source/drain junctions (such as boron-doped SiGe for p-MOSFETs), a remote plasma clean removes native oxide from exposed silicon seed areas, facilitating defect-free single-crystal growth.
• High-k Metal Gate (HKMG) Integration: Before depositing ultra-thin interfacial oxide and high-k dielectric layers, channel surfaces must be chemically clean. Remote plasma chemical etching provides damage-controlled surface preparation, helping minimize interfacial trap density and threshold voltage instability.

Future Outlook

As semiconductor scaling progresses toward sub-2nm nodes, remote plasma technology is evolving toward quasi-atomic layer etching (ALE). By decoupling radical adsorption and thermal desorption into discrete, self-limiting half-cycles, engineers can achieve atomic-scale thickness control.

Research also focuses on alternative precursor gas mixtures to replace or supplement NF3. Formulations with lower global warming potential and tailored radical generation spectra are being evaluated to tune the vibrational and chemical states of reactive species, enabling precise selectivity control for next-generation 3D integrated circuits.