Patent ID: 11885738
Assignee: J.A. WOOLLAM CO., INC.
Field: Measurement (Instruments)
Classification: CPC G | IPC G

Claim 13:
14. A reflectometer, spectrophotometer, or polarimeter system comprising an oblique angle imaging system for viewing a sample, which imaged sample presents with a sufficiently large surface area such that when viewed obliquely at an image detector, image focus varies with location in said image sufficiently that it is difficult to maintain a focused image over an extent of said surface and simultaneously correct for keystone error in said image;
said reflectometer, spectrophotometer or polarimeter system comprising:
a source of a sample investigating beam of
electromagnetic radiation;
a stage for supporting a sample placed there upon,
upon which is present a sample having a surface;
and
a detector;

such that during use an investigating beam of electromagnetic radiation from said source thereof is directed to impinge at an oblique angle onto said sample surface, reflect therefrom and enter said detector;
and said oblique angle imaging system, comprising:
a) a source of sample illuminating electromagnetic radiation illumination;
b) said stage for supporting a sample placed thereupon, upon which is present a sample having a surface, said sample having a sufficiently large surface area that, when viewed by an detector at an oblique angle it is difficult to maintain a focused image over an extent of said surface and simultaneously correct for keystone error in said image;
c) a telecentric lens system sequentially comprising:
at least one lens;
an aperture having a diameter; and
at least one lens;

said telecentric lens system being characterized by at least one selection from the group consisting of:
the entry pupil is substantially at infinity, where “pupil” refers to a selection from the group consisting of:
object of an aperture; and
image of an aperture;

and
the exit pupil is substantially at infinity, where “pupil” refers to a selection from the group consisting of:
object of an aperture; and
image of an aperture;

and
d) a multi-element imaging detector having an input surface;

said oblique angle imaging system source of illuminating electromagnetic radiation being configured to direct a beam of electromagnetic radiation toward said sample at an oblique angle to its surface;
said multi-element imaging detector being configured to receive electromagnetic radiation from said sample;
said telecentric lens system being positioned between said sample surface and the input surface of said multi-element imaging detector;
said sample surface, input surface of said imaging detector and telecentric lens being oriented with respect to one another so that the Scheimpflug condition is achieved;
said multi-element imaging detector producing an image of said sample surface which is both substantially in focus, and demonstrates substantially no keystone error;
wherein said source of illuminating electromagnetic radiation provides illuminating electromagnetic radiation over an area on said surface of said sample including whereat said sample investigating beam of electromagnetic radiation impinges;
wherein the multi-element imaging detector input surface comprises a camera sensor plate, and said sample imaging system for monitoring the surface of said sample comprises a focusing system;
each of said camera sensor plate, focusing system and sample supporting stage each being oriented in identifiable planes, wherein said planes are configured so that:
the plane of the camera sensor plate refers to the orientation of its surface;
the plane of the sample supporting stage refers to the orientation of its surface; and
the plane of the focusing system is perpendicular to its optical axis;

such that in use when said sample imaging is positioned to observe a sample placed on said stage for supporting a sample along a sample imaging viewpoint locus (VL), said sample imaging viewpoint locus forms an angle alpha (∝) with respect to the plane of said camera sensor plate, and proceeds from said camera sensor plate along a substantial perpendicular to the plane of said focusing system and along its optical axis, and such that said sample imaging system viewpoint locus further forms an oblique angle of incidence beta (β) with respect to a normal to a surface of said sample;
said camera sensor plate, focusing system and stage for supporting a sample being oriented with respect to one another such that a projected perpendicular to the plane of the camera sensor plate, a projected plane of the sample surface and a projected plane of the focusing system intersect at a common point, and such that the following condition is substantially met:

Tan (∝)=(X−f)/f Tan (β);

where “X” is the distance from the camera sensor plate, at the point at which said beam passes therethrough, to a center of said focusing system, and “f” is the focal length of said focusing system and where alpha (∝) and beta (β) were defined above;
such that, in use the sample imaging system provides a focused view of the sample over the area thereof viewed, while reflectometer, spectrophotometer or polarimeter data is acquired at small angles-of-incidence.