Optical system and method for measuring parameters of patterned structures in micro-electronic devices

1. An optical system for use in measurements on an upper surface of a layered sample when located in a measurement plane, wherein the optical system is configured as a normal-incidence system having an illumination channel and a collection channel, the optical system comprising:

an objective lens unit accommodated in the illumination and collection channels, thereby defining a common optical path for propagation of illuminating light from the illumination channel toward an illuminating region in the measurement plane and for propagation of light returned from measurement plane to the collection channel; and

a light propagation affecting device comprising an apertured structure located in at least one of the illumination and collection channels, and configured to provide angular obscuration of light propagation path for blocking angular segments associated with light propagation from regions outside said illuminated region,

wherein said apertured structure comprises first and second masks having first and second different patterns arranged in a spaced-apart relationship along the illumination channel at input and output planes, respectively of a relay optics unit,

wherein the first and second masks have a pattern of light transmitting and blocking regions, and

wherein the first and second masks are located in the illumination channel and are configured such that the angular segments of propagation of light associated with the locations of ghost spots are prevented from reaching the collection channel.

2. The optical system according to claim 1 , wherein the first and second masks are configured to prevent illumination of said ghost spots' locations.

3. The optical system of claim 1 , wherein the first and second masks are located in the collection channel.

4. A method for use in optical measurements on an upper surface of a layered sample when located in a measurement plane, the method comprising:

performing optical measurements on the sample using a normal-incidence mode, by directing illuminating light to the sample via an illumination channel and collecting light returned from an illuminated region on the sample and propagating along a collection channel; and

applying angular obscuration of a light propagation path along either one or both of the illumination and the collection channels to block angular segments associated with light propagation from regions outside said illuminated region,

wherein said angular obscuration comprises passing said light through first and second masks having first and second different patterns arranged in a spaced-apart relationship along the illumination channel,

wherein said angular obscuration comprises passing the light propagating along either one or both of the illumination and collection channels via the first and second masks having a pattern of light transmitting and blocking regions,

wherein the first and second masks are located in the illumination channel and are configured such that the angular segments of propagation of light associated with the locations of ghost spots are prevented from reaching the collection channel.

5. The method according to claim 4 , wherein the first and second masks are configured to prevent illumination of said ghost spots' locations.

6. The method according to claim 4 , wherein the first and second masks are located in the collection channel.