Optical Apparatus, Method of Scanning, Lithographic Apparatus and Device Manufacturing Method

• US 20120212718A1
• Filed: 01/17/2012
• Published: 08/23/2012
• Est. Priority Date: 02/18/2011
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: ASML Netherlands BV (ASML Holding NV)
• Inventors: Den Boef, Arie Jeffrey
• Granted Patent: US 9,303,978 B2
• Time in Patent Office: Days
• US Class Current: 355/67
• CPC Class Codes: G01B 11/00 Measuring arrangements characterised by the use of optical techniques
• CPC Class Codes: G01B 11/14 for measuring distance or clearance between spaced objects or spaced apertures G01B11/26 takes precedence; rangefinders G01C3/00
• CPC Class Codes: G02B 21/0048 scanning mirrors, e.g. rotating or galvanomirrors, MEMS mirrors
• CPC Class Codes: G02B 26/105 with one or more pivoting mirrors or galvano-mirrors G02B26/101 takes precedence
• CPC Class Codes: G03F 7/70625 Dimensions, e.g. line width, critical dimension [CD], profile, sidewall angle or edge roughness
• CPC Class Codes: G03F 9/7088 Alignment mark detection, e.g. TTR, TTL, off-axis detection, array detector, video detection

Abstract

An apparatus measures positions of marks on a lithographic substrate. A measurement optical system comprises illumination subsystem for illuminating the mark with a spot of radiation and a detecting subsystem for detecting radiation diffracted by the mark. A tilting mirror moves the spot of radiation relative to the reference frame of the measurement optical system synchronously with a scanning motion of the mark itself, to allow more time for accurate position measurements to be acquired. The mirror tilt axis is arranged along the intersection of the mirror plane with a pupil plane of the objective lens to minimize artifacts of the scanning. The same geometrical arrangement can be used for scanning in other types of apparatus, for example a confocal microscope.

Claims

1. An optical apparatus comprising:

an objective lens configured to transmit rays of radiation between an optical system of the apparatus and an object; and

at least one moveable element comprising a mirror arranged between the optical system and the objective lens and configured to tilt and so change the direction of the rays as they pass through the objective lens,

wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

2. The apparatus of claim 1, wherein the movable element is mounted so as to oscillate and impart a repetitive scanning movement to the rays.

3. The apparatus of claim 2, wherein the moving optical element is mounted so as to oscillate with a resonant frequency.

4. The apparatus of claim 1, wherein the optical system comprises an illumination subsystem configured to illuminate a mark via the objective lens with a spot of radiation and a detecting subsystem for detecting via the objective lens radiation diffracted by the mark, the apparatus further comprising:

a first positioning subsystem configured to control movement of the substrate and measurement optical system relative to one another at a first velocity so as to scan the mark with the spot of radiation while detecting and processing signals representing the diffracted radiation to calculate a position of the mark relative to a reference frame of the measurement optical system,

wherein the movable optical element operable synchronously with the first positioning subsystem for moving the spot of radiation relative to the reference frame of the measurement optical system at a second velocity, the first and second velocities being related such that the spot scans the mark at a third velocity which is lower than the first velocity while the signals are detected.

5. The apparatus of claim 4, wherein in operation the first and second velocities are parallel, with the second velocity being lower than the first velocity, and the third velocity being equal to the first velocity minus the second velocity.

6. The apparatus of claim 4, wherein the objective lens is fixed in relation to the reference frame of the measurement optical system.

7. The apparatus of claim 4, wherein the optical system includes a self-referencing interferometer for rotating and recombining two portions of the diffracted radiation.

8. An apparatus comprising a confocal microscope for inspection of objects, the apparatus comprising:

an optical apparatus comprising;

an objective lens configured to transmit rays of radiation between an optical system of the apparatus and an object, the optical system comprising;

an illumination subsystem configured to illuminate an object via the objective lens with a spot of radiation; and

a detecting subsystem for detecting via the objective lens radiation reflected by the object at a specific point in the spot radiation; and

at least one moveable element comprising a mirror arranged between the optical system and the objective lens and configured to tilt and so change the direction of the rays as they pass through the objective lens, the movable optical element operable for scanning the spot of radiation and the specific point in at least a first scanning direction so as to record intensity of radiation detected from a line of points across the object,

wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

9. The apparatus of claim 8, further comprising a positioning subsystem for moving the object and the objective lens relative to one another in a second scanning direction so as to record intensity of radiation from a series of lines of points across the object.

10. A lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, the apparatus comprising:

a substrate table constructed to hold a substrate; and

an alignment sensor configured to measure positions of marks on the substrate relative to a reference frame of the lithographic apparatus,

wherein the alignment sensor comprises a measuring apparatus, and

wherein the lithographic apparatus is arranged to control the transfer of a pattern onto the substrate by reference to the positions of marks on the substrate measured using the measuring apparatus.

11. A method of scanning an object with rays of radiation via an objective lens, the method comprising:

arranging a mirror in a path of the rays prior to the objective lens; and

tilting the mirror so as to change the direction of the rays as they pass through the objective lens, and thereby to cause the rays to impinge on different parts of the object in accordance with a tilt angle of the mirror,

wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

12. The method of claim 11, wherein the mirror is mounted so as to oscillate and impart a repetitive scanning movement to the rays.

13. The method of claim 12, wherein the moving optical element is mounted and driven so as to oscillate with a resonant frequency.

14. The method of claim 11, wherein the scanning is performed as part of a process of measuring the position of a mark on a substrate.

15. A method of manufacturing a device in which a lithographic process is used to transfer a pattern from a patterning device onto a substrate, and the transfer of a pattern onto the substrate is controlled by reference to positions of marks on the substrate measured using a method of scanning an object with rays of radiation via an objective lens, the method comprising:

arranging a mirror in a path of the rays prior to the objective lens; and

tilting the mirror so as to change the direction of the rays as they pass through the objective lens, and thereby to cause the rays to impinge on different parts of the object in accordance with a tilt angle of the mirror,

wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror, and

wherein the scanning is performed as part of a process of measuring the position of a mark on a substrate.