Reticle vs Photomask: Terminology, Reduction Ratio and Size
Photomask is the general word for the master plate that carries a circuit pattern for exposure. A reticle is one kind of photomask: originally, a plate that holds the pattern for only part of the wafer (one or a few chips) and is exposed again and again as a stepper or scanner moves across the wafer. Today's production exposure tools shrink the pattern to 1/4 when projecting it, so the pattern on a reticle is four times its size on the wafer. A 1x photomask used in a mask aligner, by contrast, carries the pattern of the whole wafer at the same size and exposes the whole wafer at once. In production, people routinely call reticles "masks" as well, so the two words are often used interchangeably.
| Aspect | Reticle (for reduction projection) | 1x photomask (whole wafer) |
|---|---|---|
| Scope of the term | A kind of photomask that holds the pattern for part of the wafer and is exposed repeatedly at shifted positions | The general term for exposure masters; in the narrow sense, a mask that holds the whole-wafer pattern at 1x |
| Exposure tool | Steppers (step-and-repeat) and scanners (step-and-scan) | Mask aligners (contact and proximity printing) and 1x projection aligners |
| Magnification relative to the wafer | Typically 4x (the projection optics reduce it to 1/4); older steppers also used 5x or 10x | 1x: the dimensions on the mask print directly on the wafer |
| Area exposed at once | One shot: up to about 26 mm × 33 mm on the wafer with a scanner, repeated to cover the wafer | The whole wafer in one exposure |
| Plate size and material | Standard is synthetic quartz 6 inches square (about 152 mm) and about 6 mm thick; EUV reticles are reflective, with a multilayer on low-thermal-expansion glass | A square glass plate somewhat larger than the wafer (soda-lime glass or quartz), usually with a chrome pattern |
| Pellicle | Standard for DUV; ultra-thin films a few tens of nanometers thick have been developed for EUV | Cannot be used in contact or proximity printing, because the mask sits right next to the wafer |
| Effect of errors and defects | Dimensional errors and small particles on the plate are reduced on the wafer, but a defect repeats at the same position in every shot | Errors and defects print at full size; in contact printing the mask touches the resist, gets dirty or scratched and needs cleaning or replacement |
| Typical uses | Volume production of fine layers in advanced ICs (logic and memory) | MEMS, power devices, LEDs, research and other layers with micrometer-scale features |
| Which word people use | Around exposure tools, "reticle" (reticle stage, reticle limit); in design and mask making, often "mask" (mask set, mask count) | In university labs and MEMS work, simply "mask" or "photomask" |
Figures are typical orders of magnitude and vary by process generation and product.