Latent-image control of lithography tools

• US 5,124,927 A
• Filed: 03/02/1990
• Issued: 06/23/1992
• Est. Priority Date: 03/02/1990
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Hopewell, William D., Van Kessel, Theodore G., Jackson, Robert R., Shaw, Jerry C.
• Primary Examiner(s): Ruggiero, Joseph
• Application Number:
  US07/488,342
• Time in Patent Office: 844 Days
• Field of Search: 364/468, 364/474.02, 364/488-491, 364/559, 250/491.1, 250/397, 250/398, 356/372, 356/394, 356/389, 356/397, 356/400, 356/401, 356/349, 356/359, 430/394, 430/326, 430/311, 430/30, 430/935, 378/34, 378/35, 355/43, 355/53
• US Class Current: 700/121
• CPC Class Codes: G03F 7/70558 Dose control, i.e. achievement of a desired dose
• CPC Class Codes: G03F 7/706 Aberration measurement
• CPC Class Codes: G03F 7/70633 Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching
• CPC Class Codes: G03F 7/70641 Focus
• CPC Class Codes: G03F 7/70675 Latent image, i.e. measuring the image of the exposed resist prior to development
• CPC Class Codes: G03F 9/7026 Focusing
• CPC Class Codes: G03F 9/7046 Strategy, e.g. mark, sensor or wavelength selection

Abstract

Measurement apparatus and procedure for use with lithographic equipment is provided for the construction of electronic and other devices wherein a photoresist is deposited as a layer upon a substrate. A Nomarski differential interference contrast microscope in conjunction with a scanned image detector is employed to examine verification marks produced by projection of an overlay, such as the mask or reticle, upon the photoresist layer. The projection results in a production of verification marks in the form of a latent image which, while invisible with conventional viewing means, can be viewed by phase-contrast imaging employing differential phase shift. Various characteristics of the resultant image are employed to align secondary verification marks with primary verification marks previously provided on the substrate, and to allow for a checking of line width, dosage, focusing, temperature control, and global alignment. Observation of the photoresist is accomplished with radiation at lower frequency than the exposure radiation, the latter being significantly absorbed, by the photoresist, the photoresist being transparent to the observation radiation to permit reflection from top and bottom surfaces of the photoresist.

Claims

1. A method of controlling a lithographic tool operating on a surface of a workpiece, the method comprising steps of:

observing the surface of said workpiece; and

wherein said observing comprises;

viewing adjacent sites at a first location on said surface with separate beams of radiation to obtain data of said surface by differential phase shift induced on said beams by reflection from said surface; and

converting said differential phase shift to amplitude data to produce an image point of said first location;

said method further comprising steps of

repeating said observing step at additional locations spaced from said first location to obtain further image points, all of said image points constituting detected image of points on said surface;

providing a reference image pattern; and

correlating detected image with said reference image by use of an edge correlation sum.

2. A method of controlling a lithographic tool having an exposure head and a microscope for processing a wafer having a plurality of fields containing product areas, said wafer being covered with a layer of photoresist, the method providing for a correction of exposure dosage of the photoresist, the method comprising steps of:

exposing a number of said fields, said exposing producing a latent image in said photoresist for each of said number of fields;

moving said wafer to a microscope operative to obtain image data of said photoresist by differential phase shift;

measuring amplitude obtained from the differential phase shift across latent images for each of said number of fields, said step of exposing producing a change of thickness in said photoresist;

obtaining previously measured photoresist thickness data for all of said fields;

calculating exposure correction;

moving the wafer back to the exposure head; and

exposing said photoresist with a further dosage of radiation to adjust the thickness of the photoresist.

3. a method of controlling a lithographic tool having a microscope and an exposure head for processing a wafer, the wafer having a coating of photoresist and comprising a plurality of fields each of which includes a product area, the method providing for a correction of focus of the exposure head, the method comprising steps of:

exposing a number of the fields with stepped focus;

moving said wafer to said microscope, said microscope being operative to produce a detected image of a latent image by differential phase shift;

measuring points of the detected image across the latent image for each cf said number of fields;

calculating a best focus position;

returning the wafer to said exposure head;

adjusting said focus; and

exposing the photoresist by said exposure head with a new value of focus.

4. A method of controlling a lithographic tool having a microscope and an exposure head for processing a wafer, the wafer having a coating of photoresist and comprising a plurality of fields each of which includes a product area, the method providing for a measuring of line width in a latent image in said photoresist, the latent image in the photoresist being produced by exposure of the photoresist, the method comprising the steps of:

exposing a number of said fields with step exposure and focus, the exposure producing a latent image in the photoresist;

moving the wafer to said microscope;

measuring a distance between each position of image points lying on each side of a line in said latent image to obtain a measure of the line width;

calculating a best focus and an exposure combination by comparison of the line width with a line-width set point;

returning the wafer to said exposure head;

setting a new focus and amounts of exposure at said exposure head; and

exposing the photoresist with a new combination of focus and quantity of exposure.

5. A method of controlling a lithographic tool having a microscope and an exposure head for processing a wafer, the wafer having a coating of photoresist and comprising a plurality of fields each of which includes a product area, exposure of the photoresist via a reticle in said exposure head producing a latent image with verification marks in the photoresist, the method providing for an alignment of the wafer with the exposure head, the me&

;

hod comprising steps of;

exposing a number of said fields, one field at a time;

moving the wafer to said microscope, said microscope being operative to produce the detected image of the latent image by differential phase shift;

locating an overlay verification aid;

measuring average overlay values of offset in each of two dimensions for each of said number of fields;

correcting coordinates of said tool for a field position;

returning the wafer to said exposure head; and

exposing said photoresist with new overlay values.

6. A method of evaluating a bake cycle by use of a lithographic tool having a microscope and an exposure head for processing a wafer, the wafer having a coating of photoresist and being subjected to a bake cycle, exposure of the photoresist via a reticle in said exposure head producing a latent image with verification marks in the photoresist, said microscope being operative to produce a detected image of the latent image by differential phase shift, the method comprising:

prebaking the wafer;

exposing the entire wafer by said exposure head;

moving said wafer to a bake station;

placing said wafer on a heated vacuum chuck;

employing said microscope to measure the amplitude of points of said detected image;

evaluating dimensions of said verification marks in an overlay;

determining whether the amplitude of points of the detected image is sufficient to indicate adequate baking;

quenching the wafer by moving the wafer to a cooled platen; and

developing the photoresist.

7. A method of employing a lithographic tool to obtain latent image enhanced global alignment by use of verification marks and tool registration marks, the tool having a microscope and an exposure head for processing a wafer, the wafer having verification marks thereon and being coated with photoresist, exposure of the photoresist by a reticle in said exposure head producing a latent image with verification marks in the photoresist, said microscope being operative to produce a detected image of a latent image in the photoresist by differential phase shift, the method comprising steps of:

exposing registration and verification marks at a number of sites on the wafer;

employing the lithographic tool to locate the registration marks;

moving the wafer under said microscope;

locating and measuring overlay verification marks for each of said number of sites;

correcting locations of said registration marks by fitting a polynomial, the deriving translation, rotation, and magnification terms of the polynomial to determine corrective positioning of the entire wafer;

moving the wafer under said exposure tool;

using corrected fit to predict all site positions; and

exposing all sites of the wafer.

8. A method of controlling a lithographic tool having a microscope and an exposure head for processing a wafer, the wafer having a coating photoresist and comprising a plurality of dies, there being primary verification marks and tool registration marks located on the surface of said wafer, the method comprising steps of:

exposing said photoresist via a reticle in said exposure head to produce a latent image with secondary verification marks in the coating of photoresist;

observing a surface of said coating by use of said microscope, said microscope being operative to produce a detected image of the latent image by differential phase shift;

observing a surface of said coating by use of said microscope, said step of observing including a viewing of adjacent sites at a first location on said coating surface with separate beams of radiation to obtain data of said coating surface by differential phase shift induced on said beams by reflection from said coating surface, said step of observing also including a converting of said differential phase shift to amplitude data to produce an image point of said first location in said detected image;

repeating said observing step at additional locations on said coating surface spaced from said first location to obtain further image points in said detected image, all of said image points constituting said detected image;

wherein said observing step further comprises a step of illuminating said coating with radiation to which said coating is transparent to provide reflections from said coating surface and from an interface between said coating and said wafer, the reflections providing images of said primary and said secondary verification marks in said detected image; and

wherein said method further comprises;

correlating said detected image with reference images of verification marks to obtain a centroid of each of said verification marks; and

measuring the distance between the centroids to determine alignment of the wafer.