ILD 3-1 SiCN Etch - 40nm

The physical mechanism of SiCN etching relies on reactive ion etching (RIE) using fluorinated plasmas, which balance chemical volatilization with physical ion bombardment .

The ILD 3-1 SiCN Etch step is a critical back-end-of-line (BEOL) process responsible for opening the dielectric barrier and

etch stop layer (SiCN) at the bottom of Via 3 to expose the underlying Metal 3 interconnect, following the structural integration principles described in . In the 40nm node, low-k interlayer dielectrics (ILD) are utilized to reduce resistance-capacitance (RC) delays, while dense amorphous silicon carbonitride (a-SiCN:H) serves as both an etch stop and a copper capping layer to prevent Cu outward diffusion . Positioned immediately after the ILD 3-2 Oxide Etch, this step selectively removes the SiCN layer that previously protected the metal during the bulk oxide etch, leveraging the differential etch selectivity between dielectric materials . Unlike similar SiCN opening steps at lower metal levels, the Via 3 level bridges intermediate routing layers where current densities and field strengths begin to rise significantly, making the precise removal of this barrier essential to prevent electromigration and via resistance degradation . Following this step, ashing and Metal 4 trench patterning occur, requiring the exposed via bottom to remain free of polymeric residues to ensure highly reliable downstream physical and electrical contact . The physical mechanism of SiCN etching relies on reactive ion etching (RIE) using fluorinated plasmas, which balance chemical volatilization with physical ion bombardment . In a typical Si-O-C-N-H system, fluorine radicals chemically react with the silicon network to form volatile byproducts such as SiF4, while nitrogen and carbon are removed as NOx and COx species . Ion bombardment provides the necessary activation energy to break the Si-C and Si-N bonds, which possess higher network density than the overlying SiO2 ILD . To achieve high anisotropy and avoid lateral etching of the via sidewalls, fluorocarbon gases are employed to simultaneously deposit a fluorinated polymer passivation layer on the vertical surfaces . This polymer buildup slows the lateral etch rate, ensuring that the high-aspect-ratio via feature is faithfully transferred down to the underlying metal layer without critical dimension (CD) loss . The selection of SiCN as the etch stop layer, rather than traditional high-k silicon nitride, is driven by the need to balance a moderately low dielectric constant with robust mechanical integrity and diffusion barrier properties . During the plasma etch, process parameters such as the ratio of fluorocarbon to oxygen gas are meticulously tuned to maximize the etch selectivity of SiCN over the surrounding SiO2 and underlying copper, based on fundamental plasma chemistry and surface kinetics . Introducing oxygen into the plasma chemistry can significantly tune the etch rate of carbon-doped silicon nitrides, enabling engineered control of etch selectivity up to a 60:1 ratio relative to oxide . Furthermore, careful control of bias power is required: excessive ion energy can sputter the underlying copper, leading to Cu redeposition on the via sidewalls and subsequent time-dependent dielectric breakdown (TDDB) . Conversely, insufficient ion energy fails to overcome the physical threshold required to penetrate the fluorocarbon polymer layer, resulting in an incomplete via open and catastrophic open-circuit failures (Engineering Practice). At the 40nm technology node, the scaling of BEOL dimensions severely amplifies proximity effects and electric fields between adjacent interconnects, making geometric alignment and dielectric reliability paramount . The simultaneous requirement to maintain tight via-to-line spacing dictates that the SiCN etch must not erode the via sidewalls or cause shorting to adjacent structures, a challenge mitigated through highly selective chemistries that terminate precisely at material interfaces . Furthermore, as metal linewidths shrink, the momentum transfer from charge carriers increases, meaning any residual SiCN at the via-metal interface will sharply increase local current density and severely accelerate stress-induced voiding (SIV) and electromigration failures .

[High] Incomplete SiCN Etch (Under-etch): If the fluorocarbon polymer deposition rate exceeds the physical sputtering rate, the etch may terminate prematurely, leaving residual SiCN at the via bottom . This physically blocks the electrical contact between Via 3 and Metal 3, leading to high via resistance or catastrophic open circuits .
[High] Copper Sputtering and Redeposition: Excessive bias power or over-etching into the underlying Metal 3 layer causes physical momentum transfer to the Cu atoms, sputtering them onto the via sidewalls . This Cu redeposition introduces conductive states in the dielectric, fundamentally degrading the breakdown strength and causing early inter-metal dielectric time-dependent breakdown (IMD-TDDB) .
[Medium] Loss of Etch Anisotropy (Bowing): Insufficient sidewall passivation from the fluorocarbon plasma allows lateral chemical attack by free fluorine radicals on the low-k ILD . This lateral expansion can lead to via-to-via shorting or critically degrade the spacing margin required by aggressive BEOL layout rules .
[Low] Polymer Residue Accumulation: The polymerizing nature of fluorocarbon chemistries generates a thick fluorinated surface layer that requires physical ion bombardment and surface de-fluorination to penetrate . If not adequately cleared in subsequent ashing and cleaning steps, these residues impede the proper adhesion of the subsequent metal barrier layer, ultimately degrading interconnect reliability and electromigration lifetime .

The physical mechanism of SiCN etching relies on reactive ion etching (RIE) using fluorinated plasmas, which balance chemical volatilization with physical ion bombardment .

The ILD 3-1 SiCN Etch step is a critical back-end-of-line (BEOL) process responsible for opening the dielectric barrier and

[High] Incomplete SiCN Etch (Under-etch): If the fluorocarbon polymer deposition rate exceeds the physical sputtering rate, the etch may terminate prematurely, leaving residual SiCN at the via bottom . This physically blocks the electrical contact between Via 3 and Metal 3, leading to high via resistance or catastrophic open circuits .
[High] Copper Sputtering and Redeposition: Excessive bias power or over-etching into the underlying Metal 3 layer causes physical momentum transfer to the Cu atoms, sputtering them onto the via sidewalls . This Cu redeposition introduces conductive states in the dielectric, fundamentally degrading the breakdown strength and causing early inter-metal dielectric time-dependent breakdown (IMD-TDDB) .
[Medium] Loss of Etch Anisotropy (Bowing): Insufficient sidewall passivation from the fluorocarbon plasma allows lateral chemical attack by free fluorine radicals on the low-k ILD . This lateral expansion can lead to via-to-via shorting or critically degrade the spacing margin required by aggressive BEOL layout rules .
[Low] Polymer Residue Accumulation: The polymerizing nature of fluorocarbon chemistries generates a thick fluorinated surface layer that requires physical ion bombardment and surface de-fluorination to penetrate . If not adequately cleared in subsequent ashing and cleaning steps, these residues impede the proper adhesion of the subsequent metal barrier layer, ultimately degrading interconnect reliability and electromigration lifetime .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges