Optical gap measuring apparatus and method having two-dimensional grating mark with chirp in one direction
US6522411B1 · kind B1 · utility
Assignee
Inventors
Key dates
| Filing date | May 25, 2000 |
| Grant date | Feb 18, 2003 |
| Priority date | — |
| Expiry date | May 25, 2020 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG03F9/7049
- WIPO fieldOptics
- WIPO sectorInstruments
Abstract
An apparatus and method of measuring the gap between one substantially planar object, such as a mask, and a second planar object, such as a substrate. The invention achieves a high degree of sensitivity, accuracy, capture range, and reliability, through a novel design of a mark located only on the mask-plate. The light is inclined to the surfaces so associated optical components do not interrupt the exposing beam used in lithography. The same optics are used as for aligning overlay. Each gapping mark on the mask-plate includes one or more two-dimensional gratings, each with period constant in the incident plane, but varying in the transverse plane. When illuminated, two images are formed of each of the two-dimensional gratings, with fringes resulting from interference between paths having traveled different distances through the gap and the mask-plate as a result of successive diffractions and reflections. Phase and geometric measurements from these images yield accurate measurement of the gap between the plates. Direct calibration, referenced to the light-wavelength, is obtained from a diffractive Michelson technique that uses a linear grating also included within the gapping mark…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.