The etch chemistry must be highly selective to avoid prematurely consuming the photoresist mask or unintentionally etching the underlying OCL material .
In depth
In the nanoscale Backside Illuminated (BSI) CMOS Image Sensor process flow, the fabrication of on-chip lenses (OCL) requires precise optical st
ack engineering to maximize light collection efficiency . The Lower OCL Coating Etch step occurs within the packaging-related BONDPAD module, immediately following the Optical Pad 3 Etch and preceding the main Lower OCL Etch (Engineering Practice). The primary purpose of this step is to cleanly break through a specialized planarizing or anti-reflective coating layer that facilitates the photolithographic patterning of the underlying microlens material (Engineering Practice). Unlike the subsequent main Lower OCL Etch, which physically shapes the lens curvature through a gradual pattern-transfer mechanism, this coating etch strictly removes the interfacial protective or anti-reflective layer to expose the underlying optical material uniformly (Engineering Practice). This prepares the surface for the subsequent bulk etch, ensuring that the etch front remains uniform across both the dense pixel array and the peripheral bond pad regions without introducing initial topographic errors (Engineering Practice). The etch process utilizes a reactive ion etching (RIE) plasma to volatilize the coating material, balancing the chemical reactivity of the plasma species with physical ion bombardment . During this process, transport-limited regimes can cause feature-size-dependent etch rates, a fundamental plasma phenomenon known as the microloading effect . Just as microloading in deep reactive ion etching (DRIE) causes variations in trench depths due to restricted reactive species transport and byproduct diffusion, similar transport constraints in dense optical arrays can lead to non-uniform coating removal . The etch chemistry must be highly selective to avoid prematurely consuming the photoresist mask or unintentionally etching the underlying OCL material . Precise control of plasma parameters, such as chamber pressure and gas flow, is required to mitigate local depletion of etchants and ensure smooth, defect-free optical interfaces . The coating material is typically an organic or hybrid organic-inorganic polymer chosen for its refractive index matching and planarization capabilities . High refractive-index-contrast material systems are essential in optical structures to strongly confine and control light paths, making the precise removal of intermediate coatings critical for device performance . During the etch, varying the RF bias power directly modulates the ion bombardment energy, which influences the verticality and smoothness of the etch profile . However, excessive ion energy can cause mechanical stress or induce surface roughness, which severely degrades optical transmission through unwanted scattering . Furthermore, interactions between neighboring patterns during lithography and etching can cause systematic variations, requiring careful modeling and compensation similar to optical proximity correction techniques . At the 40nm technology node, the pixel pitch in BSI image sensors shrinks significantly, exacerbating aspect-ratio-dependent etching (ARDE) and pattern transfer fidelity issues . The reduction in physical dimensions implies that even minor etch depth variations or sidewall roughness will drastically impact the optical mode distribution and overall quantum efficiency of the sensor . Additionally, because this step is integrated near the packaging module, mechanical stress from thick overlying dielectrics and thermal expansion mismatches must be strictly managed . Therefore, the Lower OCL Coating Etch must achieve near-perfect uniformity to prevent structural discontinuities that could act as stress concentrators and promote crack propagation during subsequent wafer singulation .
Risks & Challenges
[High] Microloading and Incomplete Etch: Aspect-ratio-dependent transport limitations cause smaller or denser features to etch more slowly than open peripheral areas . If the coating is not fully cleared, the subsequent main Lower OCL Etch will be locally blocked or delayed, heavily distorting the final lens curvature and drastically reducing the sensor's light collection efficiency .
[High] Surface Roughness and Optical Scattering: Unoptimized plasma conditions, such as excessively high chamber pressure or insufficient passivation balance, can leave behind rough etched surfaces . In high refractive-index-contrast optical systems, interface roughness leads to severe light scattering, crosstalk between adjacent pixels, and overall optical transmission loss .
[Medium] Selectivity Loss and Mask Erosion: The high ion bombardment energy occasionally required to break through durable organic coatings can aggressively erode the overlying photoresist mask (Engineering Practice). Loss of mask fidelity leads to systematic variations in the optical pattern dimensions, which alters the expected light confinement properties of the underlying structure .
[Low] Stress-Induced Cracking: The accumulation of thick optical and dielectric layers in the packaging module introduces significant thermal and intrinsic mechanical stresses . Aggressive etching that creates micro-trenches or structural discontinuities can form stress concentration points, potentially deflecting or promoting crack propagation into the active device region during subsequent thermal cycling or die singulation .