40nm BSI CMOS Image SensorPreview

ediately after the Lower OCL Coating Etch, this step directly transfers the pre-defined mid-vertical grid or pad structures into the bulk lower on-chip lens (OCL) material [A2]. Unlike the preceding coating etch, which primarily opens superficial anti-reflective or protective hardmask layers, this primary etch step removes the bulk volume of the optical resin to define the actual light-guiding geometry or clear the optical pad regions [A2]. Precise execution of this step is strictly required to prepare the structure for the subsequent Optical Pad 2 Etch, ensuring that the bulk organic or hybrid resin is cleanly removed without leaving residual blocks that could compromise device reliability [A2]. The physical mechanism of the Lower OCL Etch relies on reactive ion etching (RIE), which synergistically combines chemical reactions from active radicals with directional physical bombardment by charged ions [P2]. For organic-based OCL resins, the etch chemistry predominantly utilizes oxygen-containing plasmas to achieve high selectivity [A2]. Oxygen radicals chemically react with the polymer networks to form volatile byproducts, while physical ion bombardment assists in breaking bonds and clearing localized redeposited materials (Engineering Practice). Maintaining high anisotropy during this interaction is essential to prevent lateral undercutting; an isotropic profile would degrade the waveguide shape, leading to severe Rayleigh or Mie scattering and subsequent optical signal loss [P2]. A dry etching approach is deliberately selected over wet processing because liquid etchants inherently produce isotropic profiles with concave sidewalls, which are unacceptable for tightly constrained grid structures [P2]. The delicate balance between chemical radical flux and ion energy directly dictates the sidewall angle and surface roughness (Engineering Practice). Excessive physical bombardment can induce microstructural damage or leave localized polymer grass on the etched field, which acts as scattering centers that significantly increase near-field optical loss [P2]. Conversely, utilizing a purely chemical remote plasma would lack the directionality required for high-aspect-ratio grid definition, as remote plasmas are driven by reactive neutrals and operate highly sensitively to surface states rather than providing vertical momentum [P3]. Therefore, the RIE parameters are specifically tuned to provide sufficient ion directionality while managing the chemical etch rate of the resin [P2]. In 40nm BSI CIS technology, the sub-micron pixel pitch requires exceptional control over the optical paths and isolation grids [P2]. Because the periodic atomic arrangement and band structure of the underlying semiconductor dictate ultimate carrier capture and photoelectric conversion efficiency [T2], any profile variation or dimensional loss in the lower OCL directly impacts pixel quantum efficiency and optical crosstalk (Engineering Practice). Consequently, this etch must tightly control the critical dimensions (CD) while ensuring that no residual resin fibers or filaments remain to interfere with the subsequent deposition of the mid-vertical grid [A2].