Mask shadowing blocks some trajectories before ions enter the intended silicon region. Crystal channeling lets some ions travel farther after entering an ordered lattice. They occur in different places, and an angle change that helps one question can worsen another.
Distinguish outside and inside the crystal
A patterned resist feature has physical height and an edge. Oblique trajectories can be obstructed by that geometry, so an apparently exposed area in a top view need not receive the same beam as an unobstructed region.
Channeling instead depends on alignment with crystal directions and the sequence of collisions within the solid. The review illustrates axial and planar channels and discusses sensitivity to surface orientation and beam direction. A correct mask opening does not establish the resulting depth profile.
Process checkpoint
Understand NMOS S/D, FD Implant Mask Lithography in context
Identify the patterned resist that can obstruct a beam geometrically.
Process context for “Implant Mask Shadowing vs Crystal Channeling”: 40nm BSI CMOS Image Sensor · NSDFD · Step 92
One angle cannot answer both questions
The cited review discusses near-normal implantation to avoid resist shadowing while also describing its channeling sensitivity. This does not recommend that setting for every device. It shows why geometric access and crystalline stopping must be assessed together.
Changing direction can also change symmetry near device edges. An angle that appears favorable in one cross-section does not automatically describe every orientation in a full layout.
Conditions and counterexamples
A trajectory may reach the intended exposed silicon and then channel more deeply than expected. Another may be blocked by resist before reaching silicon, despite having a direction that would not favor channeling. The failure evidence is therefore different: missing local delivery versus an altered internal distribution.
Surface layers, crystal state and subsequent annealing add other contributions. Neither a mask image nor a nominal beam angle is a complete measurement of the final active junction.
Compare the two linked observations
At lithography, identify the obstruction geometry. At the FD implant, identify the crystal-direction question. Explain why solving one does not settle the other and state which evidence would concern local exposure and which would concern the dopant depth profile.
The task adds the external masking constraint to the existing channeling explanation without prescribing angles or dividing one user problem into multiple process recipes.
References
A Review of Ion Implantation Technology for Image Sensors †
N. Teranishi, G. Fuse, M. Sugitani · Italian National Conference on Sensors