40nm BSI CMOS Image SensorPreview

he Optical Pad 2 Etch step is a critical patterning process that selectively removes specific intermediate dielectric layers over the peripheral electrical contact pads after the lower optical coating layers (OCL) have been cleared [P2]. Unlike Optical Pad 1 or 3 etches which target initial hardmask openings or final deep substrate penetrations respectively, Pad 2 Etch specifically patterns the intermediate inorganic passivation or buffer layers underneath the OCL [P2]. This prepares the pad regions to either be protected during the subsequent Mid Vertical Grid Deposition or ensures that the pad structure maintains its dimensional integrity prior to subsequent tungsten (W) CMP processes (Engineering Practice). The etching of these inorganic stacked structures typically employs a reactive ion etching (RIE) mechanism utilizing fluorocarbon-based plasmas [P1]. In the plasma, fluorocarbon gases dissociate to provide both fluorine radicals for chemical etching and carbonaceous fragments for sidewall passivation [P1]. The fluorine radicals chemically break the Si-O and Si-N bonds within the dielectric pad stack, while the physical ion bombardment provides the necessary activation energy and directionality to maintain a highly anisotropic profile [P1]. A precise balance between the etching radical flux and the polymerizing species is required to prevent aspect ratio dependent etching (ARDE) and ensure a uniform etch rate across varying feature sizes [P2]. Furthermore, the bias voltage controls the ion bombardment energy, which dictates the etch rate and the verticality of the pad sidewall [P1]. The selection of the specific gas mixture is driven by the need for high etch selectivity between the target dielectric and the surrounding mask materials [P3]. The carbon-to-fluorine ratio in the plasma directly influences the thickness of the protective polymer layer formed on the pad sidewalls [P1]. Introducing hydrogen or using hydrogen-containing fluorocarbons can modulate this ratio, allowing for near 1:1 etch selectivity if alternating oxide/nitride stacks are present in the pad region [P1]. Oxygen or nitrogen may also be added to the gas mixture to fine-tune the oxidation strength and control the non-uniformity of the polymer deposition, similar to techniques used to prevent interfacial etch-stops in high-resolution block copolymer dry etching [P2]. Excessive polymerization must be avoided as it can cause micromasking, whereas insufficient passivation leads to lateral etching and critical dimension (CD) loss [P1]. At the 40nm node for BSI image sensors, the optical grid dimensions and the associated peripheral pad structures face strict scaling constraints (Engineering Practice). The sub-wavelength physical dimensions require the etch process to exhibit sub-nanometer profile control to prevent inter-pad leakage or optical anomalies [P2]. Maintaining the integrity of the pad profile is essential, as any geometric distortion or abnormal lateral etching could compromise the interconnect end-to-end spacing, drastically increasing short-circuit risks [A1]. Therefore, the Pad 2 Etch must leverage highly anisotropic plasma conditions to achieve vertical profiles without inducing excessive surface damage that could degrade transistor subthreshold characteristics or increase static power consumption in adjacent circuitry [T2].