US 5,486,701 A · Method and apparatus for measuring reflectance in two wavelength bands to enable determination of thin film thickness

Abstract

A method and system for performing reflectance measurements of a sample using radiation having UV frequency components (preferably in a broad UV band) and visible frequency components (preferably in a broad band). Preferably, two detectors simultaneously receive a sample beam reflected from the sample surface. One detector generates a signal indicative of the sample beam components in the UV band and the other detector generates a signal indicative of the sample beam components in the visible band. By processing these two signals, the invention enables accurate measurement of the thickness of a very thin film on the sample. Preferably, the system determines a single effective wavelength for the UV radiation incident on the first detector and a single effective wavelength for the visible radiation incident on the second detector. Embodiments of the system can also measure reflectance spectra and refractive indices, and can determine lithographic exposure times. Preferred embodiments include an objective lens assembly having a pupil stop with an entrance portion with one or more relatively large apertures therethrough and an exit portion with one or more relatively small apertures therethrough. Illuminating radiation passes through the relatively large apertures before reflecting from the sample, and then passes through the relatively small apertures after reflecting from the sample. This design and pupil stop orientation dramatically increases the insensitivity of the system to ripple on the sample surface.

Claims42

Dependent claims

A method for performing optical reflectance measurements on a sample having a very thin film, the method including the steps of:

(a) focusing a first beam of radiation onto a selected area of the sample, said first beam including UV components in a UV frequency band and visible components in a visible frequency band;
(b) receiving a reflected portion of the first beam that has reflected from the selected area, and separating the reflected portion into a second beam including the UV components and a third beam including the visible components;
(c) generating a first signal indicative of intensity of the second beam;
(d) generating a second signal indicative of intensity of the third beam;
(e) processing the first signal and the second signal to generate a reflected ratio signal indicative of relative intensity of the second beam and the third beam;
and
(f) processing the reflected ratio signal to generate data indicative of thickness of the very thin film.

4 more elements

A system for performing optical reflectance measurements on a sample, said system including:

a radiation source for emitting a first beam of radiation;
an objective lens;
a beamsplitter for diverting a sample beam of said radiation from the radiation source toward the objective lens and directing a reference beam portion of the first beam away from the objective lens;
a pupil stop member positioned between the beamsplitter and the objective lens, wherein a first aperture extends through the pupil stop member and a second aperture smaller than the first aperture extends through the pupil stop member, and wherein the pupil stop member is oriented relative to the beamsplitter so that a portion of the sample beam having substantially uniform cross-section passes through the first aperture and is then focused by the objective lens onto a selected area of the sample, and a reflected portion of said sample beam passes through the second aperture after reflecting from the selected area and is then transmitted through the beamsplitter;
and
a detector assembly positioned for receiving at least a portion of said reflected portion of the sample beam that has transmitted through the beamsplitter.

3 more elements

A method for performing optical reflectance measurements on a sample having a very thin film, the method including the steps of:

(a) generating a first beam of radiation including UV components in a UV frequency band, and dividing the first beam into a sample beam and a reference beam;
(b) focusing the sample beam onto a selected area of the sample;
(c) receiving a reflected portion of the sample beam that has reflected from the selected area and generating a first signal indicative of intensity of said reflected portion of the sample beam;
(d) receiving a reflected portion of the reference beam that has reflected from a reference mirror and generating a second signal indicative of intensity of said reflected portion of the reference beam;
(e) processing the first signal and the second signal to generate a reflectance signal indicative of reflectance of the sample;
(f) processing the reflectance signal to generate data indicative of thickness of the very thin film, wherein step (f) includes the step of determining an effective wavelength of the first signal, by:
generating a first measured value (I1) indicative of intensity of the first beam reflected from a first calibration wafer, a second measured value (I2) indicative of intensity of the first beam reflected from a second calibration wafer, a third measured value (ICHIP1') indicative of intensity of the first beam reflected from a reference sample just prior to measuring the first measured value, a fourth measured value (ICHIP2') indicative of intensity of the first beam reflected from the reference sample just prior to measuring the second measured value;
generating a first ratio signal indicative of the value (I1/(ICHIP1');
generating a second ratio signal indicative of the value (I2/(ICHIP2');
and
processing the first ratio signal and the second ratio signal to determine the first effective wavelength.

8 more elements

Dependencies are inferred from the claim text.

Method and apparatus for measuring reflectance in two wavelength bands to enable determination of thin film thickness

Life of this patent

Method and apparatus for measuring reflectance in two wavelength bands to enable determination of thin film thickness

Life of this patent

Abstract

Claims42