Chlorine-based chemistries are fundamentally selected over fluorine-based alternatives because F-based plasmas tend to form low-vapor-pressure TiF3 or TiFx compounds, which manifest as fluorocarbon polymers that severely suppress the etch rate .
In depth
In the LS_GRID (Light Shield Grid) module of a nano
scale Backside Illuminated (BSI) CMOS Image Sensor, the metal grid physically isolates adjacent pixels to suppress optical and electrical crosstalk (Engineering Practice). Preceding this step, the main grid metal, typically tungsten (W), has been patterned via the W Etch process (Engineering Practice). The underlying Titanium Nitride (TiN) film, which exhibits a low electrical resistivity of 20-25 µΩ·cm, serves as an adhesion and diffusion barrier layer and remains exposed at the trench bottoms . If the continuous TiN layer is not completely removed, it will create electrical shorts across the pixel array due to its highly conductive nature . Following this complete removal, the subsequent Ashing & Strip/Clean step must eliminate residual photoresist and halogen-containing etch polymers, because left-over reactive species can form harmful defects upon exposure to ambient cleanroom moisture . The dry etching of TiN operates through an ion-enhanced surface chemical reaction mechanism within an inductively coupled plasma (ICP) . Pure physical sputtering with inert gases like argon is highly inefficient for TiN removal, necessitating the introduction of reactive halogen species to achieve practical etch rates . Chlorine-based chemistries, such as Cl2/Ar mixtures, are predominantly used because the dissociated Cl radicals react with the TiN surface to generate TiCl4, a volatile byproduct with a low boiling point of 136.5 °C that readily desorbs into the gas phase . The energetic argon ions bombard the surface to break strong Ti-N bonds, providing the necessary physical activation energy to sustain a chemically-dominated, highly anisotropic etch . Chlorine-based chemistries are fundamentally selected over fluorine-based alternatives because F-based plasmas tend to form low-vapor-pressure TiF3 or TiFx compounds, which manifest as fluorocarbon polymers that severely suppress the etch rate . By utilizing Cl2/Ar, the process avoids these massive polymer depositions and achieves highly vertical sidewall profiles that are critical for densely packed arrays . To control lateral etching and prevent isotropic undercutting, N2 gas flows are tuned to modify the plasma chemistry and form a protective, non-volatile passivation layer (such as Ti-Cl-N) on the feature sidewalls . The exact ratio of these feed gases directly dictates the balance between vertical physical sputtering and sidewall chemical passivation, ultimately determining the final taper angle of the grid structure . At the 40nm node, the high aspect ratio of the pixel grid mandates exceptional etch selectivity to the underlying dielectric to prevent uncontrolled leakage currents and maintain isolation integrity . Optimized Cl2/Ar/N2 ICP processes can achieve a TiN-to-SiO2 etch selectivity of approximately 50:1, which is critical for preserving the underlying structural topography without severe dielectric loss . Furthermore, aggressive chlorine RIE can induce physical lattice damage and introduce interfacial defect states, which act as localized two-level system (TLS) charge traps . Because high-density halogen plasmas can easily diffuse along microstructural grain boundaries and cause chemical corrosion in underlying interconnects, the over-etch duration must be carefully constrained . Precise control of the etch directionality prevents asymmetric profiles that could otherwise degrade the geometric uniformity and optical performance of the sub-wavelength grid structures .
Risks & Challenges
[High] Conductive Sidewall Residues: Under insufficient N2 flow or suboptimal plasma chemistry, non-volatile byproducts form and deposit on the TiN sidewalls . These residues can create an insulating or weakly conductive layer that is difficult to remove during post-etch cleans, potentially causing electrical leakage or increased interfacial dielectric losses .
[Medium] Profile Tapering and CD Loss: The introduction of heavy passivating gases (like BCl3) or excessive polymerizing agents causes non-volatile compounds (e.g. , BOxNy) to accumulate heavily on the feature sidewalls . This excessive passivation arrests lateral etching unevenly, transforming the desired vertical profile into an excessively tapered structure and reducing the grid's effective optical blocking volume .
[Medium] Substrate Damage and Defect State Generation: Halogen-based reactive ion etching, especially with Cl2 where substrate etch rates can be comparable to the target material, induces significant physical bombardment damage to the underlying silicon or dielectric surfaces . This damage manifests as an increased density of two-level system (TLS) defects and charge traps, which can degrade the electrical performance of adjacent isolation regions .
[Low] Halogen Diffusion and Interfacial Corrosion: Halogen radicals (Cl, F) generated in the plasma possess high chemical reactivity and can rapidly diffuse along interfacial micro-pathways or structural columnar grain boundaries during prolonged over-etching . This diffusion allows halogens to reach underlying sensitive metal layers, inducing chemical corrosion or stress concentration that leads to barrier failure or hillock formation .