OPC vs Phase-Shift Mask vs Off-Axis Illumination: What Each Changes, Effect on Resolution and DOF, and Cost
The smallest feature a lithography tool can print is given by the Rayleigh equation R = k1 × λ / NA. With the wavelength λ and numerical aperture NA fixed by the tool, printing finer patterns means lowering the factor k1, and the methods for doing so are called resolution enhancement techniques (RET). OPC corrects the shapes on the mask in anticipation of the distortion caused by diffraction. A phase-shift mask shifts the phase of light passing through parts of the mask by 180° and uses interference to raise image contrast. Off-axis illumination lights the mask from oblique angles so that more of the light diffracted by fine patterns enters the lens. Each changes something different, so they are not alternatives to choose between but techniques used together.
| Aspect | OPC | Phase-shift mask | Off-axis illumination (OAI) |
|---|---|---|---|
| What it changes | The shapes and sizes of the mask patterns | The phase (and transmission) of light passing through the mask | The angular distribution of light illuminating the mask (the source shape) |
| Main problem it solves | Distortion of printed patterns, such as line-end shortening, corner rounding and line-width differences caused by the density of nearby patterns | Loss of image contrast as light from neighboring patterns overlaps | At fine pitch, diffracted light leaves at large angles and no longer enters the lens |
| How it works | Distortion is predicted by simulation, and pattern edges are moved or helper shapes are added, such as hammerheads and serifs, or thin sub-resolution assist features (SRAF) that do not print themselves | The phase of light through neighboring openings is inverted (alternating, or Levenson, type), or opaque areas are replaced by a slightly transmitting phase-shifting film (attenuated, or halftone, type), so that amplitudes cancel at the boundaries | Annular, dipole, quadrupole and similar illumination lets the zero order and one first diffraction order enter the lens, and the image forms from their interference |
| Effect on resolution (k1) | Barely changes the resolution limit itself, but lets patterns near the limit print as designed, lowering the k1 that can be used in practice | The alternating type greatly improves resolution of fine periodic patterns; the attenuated type raises contrast and helps especially with contact holes and isolated patterns | Greatly improves resolution of fine pitches, approaching the theoretical single-exposure limit for periodic patterns (k1 = 0.25) |
| Effect on depth of focus (DOF) | Adding SRAFs can widen the depth of focus of isolated patterns | Higher contrast widens the depth of focus and process window | Greatly widens depth of focus at pitches matched to the illumination, but can narrow it at mismatched pitches (forbidden pitches) |
| Constraints on patterns | Few constraints, but correction computation and mask data volume grow | The alternating type has layouts where phases cannot be assigned consistently (phase conflicts), which constrains layout; the attenuated type needs care with unwanted images (side lobes) | The benefit depends on pattern orientation and pitch; dipole illumination is strong for lines in one direction but weak for lines perpendicular to them |
| Impact on masks and scanners | Mask patterns become complex, and mask writing and inspection take longer | Phase-producing etches or phase-shifting films are needed, making masks harder to make and inspect | Achieved through the scanner's illumination settings without changing the mask |
| Cost and complexity | Correction computation on large computer clusters and growing mask data add cost | Masks are more expensive; the alternating type is especially complex, sometimes needing designs that avoid phase conflicts and an extra exposure | Relatively small, although optimizing the source shape for the patterns requires computation |
| Typical combinations | Used on nearly every layer near the resolution limit, often together with illumination optimization | In ArF lithography, the attenuated type is widely used together with off-axis illumination and OPC | Combined with OPC and phase-shift masks; today's illuminators can set the source shape freely in fine detail |
Figures are typical orders of magnitude and vary by process generation and product.