Method and apparatus of using a two beam interference microscope for inspection of integrated circuits and the like

A process for generating synthetic image data representative of a cross-section of an at least partially reflective irregular surface of an object formed by a portion of a semiconductor wafer having an elongated strip of raised surface extending therethrough, comprising the steps of:

• (a) illuminating the irregular object surface with light from a source of illumination;
• (b) illuminating a reflective reference surface with light from said source of illumination, said reference surface being formed by an optically flat mirror;
• (c) collecting object light reflected from said object surface and directing said object light along a first optical axis;
• (d) collecting reference light reflected from said reference surface and directing said reference light along a second optical axis at least a portion of which is parallel to said first optical axis;
• (e) focussing the light directed along said first and second optical axes to form a fringed image pattern resulting from interference of said object light and said reference light;
• (f) orientating said wafer so that a selected scan line may be directed substantially orthogonal relative to the length of said elongated strip;
• (g) inspecting said image pattern to develop a series of coherence data corresponding to the fringe amplitude at points along said scan line;
• (h) incrementally changing the position of said object along said first optical axis, each time repeating steps (a) through (e) and (g);
• (i) processing the plurality of series of coherence data to develop synthetic image data corresponding to a cross-sectional profile of said object surface taken in a plane including said selected scan lines;
• (j) displaying said synthetic image data to visually depict a cross-sectional profile of said object surface taken in the plane including said scan lines;
• and
• (k) determining the position of said object along said first optical axis at which the value of said coherence data corresponding to the crossing of a first particular scan line over said raised surface is at a maximum relative to the corresponding coherence data of the other scan lines and identifying this position as corresponding to the top surface of said strip.

A method of measuring certain dimensions of an elongated strip of raised surface formed on an object, such as a semiconductor wafer or photomask, using an interference optical system to develop images formed by interference between object wave energy passing from the object and through an object channel to an image plane and reference wave energy passing through a reference channel to the image plane, comprising the steps of:

• (a) illuminating the object surface with light from a source of illumination;
• (b) illuminating a reflective reference surface with light from said source of illumination;
• (c) collecting object light reflected from said object surface and directed along a first optical axis through said object channel;
• (d) collecting reference light reflected from said reference surface and directed along a second optical axis through said reference channel, said second optical axis having at least a portion thereof which is parallel to said first optical axis;
• (e) focusing the light directed along said parallel portions of said first and second optical axes onto an image plane to form an interference image pattern resulting from interference of said object light and said reference light;
• (f) inspecting the image pattern by detecting the light intensity at each pixel in an array of mxn pixels extending across the image of said strip to produce pixel data;
• (g) scanning the pixel data and calculating therefrom coherence data representing the absolute value or magnitude of the complex degree of coherence of light incident upon said image plane and storing the calculated coherence data for subsequent reference;
• (h) incrementally changing the position of said object along said first optical axis, each time repeating steps (a) through (g);
• and
• (i) using the stored coherence data to generate data from which a synthetic image corresponding to a transverse cross-section of the measured strip may be developed.

A method of inspecting an object and generating synthetic image data comprising the steps of:

• (a) using an interference optical system including an object channel and a reference channel for simultaneously inspecting an object and a reflective reference surface and developing a plurality of images formed by the interference between object wave energy passing from said object and through said object channel to an image plane and reference wave energy passing from said reference surface and through said reference channel to said image plane, each said image being formed in response to a change in position of either said object or said reference surface
• (b) determining for each image the absolute value of the degree of coherence between said object wave energy and said reference wave energy by calculating the variance in the intensity of each pixel over the said plurality of images and generating corresponding absolute value coherence data;
• and
• (c) using said absolute value coherence data to generate synthetic image data representative of a particular characteristic of said object, wherein the brightness of each pixel element of a synthetic image developed using said synthetic image data is proportional to said absolute value coherence data.