Apparatus and method for enhanced critical dimension scatterometry
Scatterometers and methods of using scatterometry to determine several parameters of periodic microstructures, pseudo-periodic structures, and other very small structures having features sizes as small as 100 nm or less. Several specific embodiments of the present invention are particularly useful in the semiconductor industry to determine the width, depth, line edge roughness, wall angle, film thickness, and many other parameters of the features formed in microprocessors, memory devices, and other semiconductor devices. The scatterometers and methods of the invention, however, are not limited to semiconductor applications and can be applied equally well in other applications.
1 . A method of assessing a parameter of a microfeature on a workpiece using a predetermined sensitivity record, comprising:
irradiating a microstructure on a workpiece by passing a beam through an object lens assembly that focuses the beam in a focus area at an object focal plane, wherein the beam is focused through angles of incidence having at least a 15° range of altitude angles and at least a 90° range of azimuth angles simultaneously;
obtaining an actual radiation distribution of return radiation scattered from the microstructure through the angles of incidence having at least the 15° range of altitude angles and at least the 90° range of azimuth angles simultaneously;
acquiring a measured selected radiation distribution by reading data from selected pixels in the array that have sufficient sensitivity to changes in the parameter based on the predetermined sensitivity record; and
fitting the measured selected radiation distribution to a modeled selected radiation distribution corresponding to the selected pixels to determine a value of the parameter.
2 . The method of claim 1 , further comprising determining the sensitivity record by:
identifying a pixel of the array associated with an angle of incidence;
calculating model intensities of return radiation at the pixel corresponding to different values of the parameter;
differencing the model intensities, wherein larger differences in the model intensities indicate higher sensitivity to changes in the parameter and smaller differences in the model intensities indicate lower sensitivity to changes in the parameter at the selected pixel; and
assigning a sensitivity value to the pixel.
3 . The method of claim 2 , further comprising repeating the identifying, calculating, differencing and assigning procedures for additional pixels in the array such that the sensitivity record includes sensitivity values for a plurality of pixels.
4 . The method of claim 3 wherein acquiring the measured selected radiation distribution comprises selecting pixels having sensitivity values above a threshold value.
5 . The method of claim 2 wherein acquiring the measure selected radiation distribution comprises selecting pixels having sensitivity values above a threshold value reading actual intensity values from the selected pixels.
6 . The method of claim 5 wherein irradiating the microstructure further comprises irradiating the focus area with a laser beam having a first wavelength and irradiating the focus area with a laser beam having a second wavelength different than the first wavelength.
7 . The method of claim 5 wherein the first wavelength is approximately 266 nm and the second wavelength is approximately 475 nm.
8 . The method of claim 5 wherein irradiating the workpiece comprises generating a laser beam having a wavelength of approximately 375 nm to 475 nm.
9 . The method of claim 5 wherein irradiating the workpiece comprises generating a laser beam having a wavelength of one of approximately 405 nm or 457 nm.
10 . The method of claim 5 wherein irradiating the workpiece comprises generating a laser beam having a first wavelength of approximately 244 nm and a second wavelength of approximately 457 nm.
11 . The method of claim 5 wherein the altitude angles are 0° to at least 70° and the azimuth angles are 0° to at least 180°.
12 . The method of claim 5 wherein the altitude angles are 0° to at least 80° and the azimuth angles are 0° to at least 360°.
13 . The method of claim 5 , further comprising passing the return radiation through a polarizing beam splitter between the object lens assembly and the detector to separate the p- and s-polarized components of the return radiation from each other.
14 . The method of claim 5 , further comprising simultaneously measuring both the p- and s-polarized components of the return radiation with the array, and wherein the polarizing beam splitter comprises a cube-type polarizing beam splitter.
15 . A scatterometer for assessing a parameter of a microfeature on a workpiece, comprising:
a laser and/or lamp configured to produce a beam of radiation;
an optical system having a first optics assembly including an object lens assembly configured to (a) focus the beam at an area on an object focal plane through at least a 15° range of altitude angles and at least 90° range of azimuth angles simultaneously, and (b) present return radiation scattered from a microstructure in a radiation distribution on a second focal plane;
a CMOS imager having an array of pixels corresponding to angles of incidence of the return radiation; and
a computer including a database and a computer operable medium, wherein the database includes a sensitivity record of sensitivity to changes in the parameter at pixels in the array, and wherein the computer operable medium includes instructions that cause the computer to—
acquire a measured selected radiation distribution by reading data from selected pixels of the array based on pixels having sufficient sensitivity to changes in the parameter using the sensitivity record, and
fit the measured selected radiation distribution to a modeled selected radiation distribution corresponding to the selected pixels to determine a value of the parameter.
16 . The scatterometer of claim 15 wherein the computer operable medium further comprises instructions that cause the computer to determine the sensitivity record by:
identifying a pixel of the array associated with an incident angle;
calculating model intensities of return radiation at the pixel corresponding to different values of the parameter;
differentiating the model intensities, wherein larger differences in the model intensities indicate higher sensitivity to changes in the parameter and smaller differences in the model intensities indicate lower sensitivity to changes in the parameter at the selected pixel; and
assigning a sensitivity value to the pixel.
17 . The scatterometer of claim 15 wherein the computer operable medium further comprises instructions that cause the computer to repeat the identifying, calculating, differentiating and assigning procedures for additional pixels in the array such that the sensitivity record includes sensitivity values for a plurality of pixels.
18 . The scatterometer of claim 17 wherein the computer operable medium includes instructions that cause the computer to acquire the measured selected radiation distribution by selecting pixels having sensitivity values above a threshold value.
19 . The scatterometer of claim 17 wherein the computer operable medium includes instructions that cause the computer to:
acquire the measure selected radiation distribution by selecting pixels having sensitivity values above a threshold value; and
fit the measured selected radiation distribution to a modeled selected radiation distribution corresponding to the selected pixels by measuring actual intensity values from the selected pixels having sensitivity values above the threshold value with modeled intensity values from the selected pixels having sensitivity values above the threshold values.
20 . The scatterometer of claim 15 wherein irradiating the workpiece comprises generating a laser beam having a wavelength of approximately 375 nm to 475 nm.
21 . The scatterometer of claim 15 wherein the altitude angles are 0° to at least 80° and the azimuth angles are 0° to at least 360°.
22 . The scatterometer of claim 15 , further comprising a polarizing beam splitter between the object lens assembly and the detector to separate the p- and s-components of the return radiation from each other.
23 . A scatterometer for evaluating microstructures on workpieces, comprising:
a laser and/or lamp configured to produce a beam of radiation having a first wavelength;
an optical system having (i) a first optics assembly including an object lens assembly configured to (a) focus the beam to an area at an object focal plane through at least a 15° range of altitude angles and at least a 90° range of azimuth angles simultaneously, and (b) present return radiation scattered from a microstructure in a radiation distribution on a second focal plane, and (ii) a second optics assembly having a polarizing beam splitter configured to present separate images of p- and s-polarized components of the return radiation;
a CMOS imager having a single array of pixels corresponding to angles of incidence of the return radiation, the CMOS imager being positioned to receive the separated images of the p- and s-polarized components of the return radiation and configured to produce a representation of the p- and s-polarized components of the return radiation; and
a computer including a database and a computer operable medium, wherein the database includes a sensitivity record of sensitivity to changes in the parameter at pixels in the array, and wherein the computer operable medium includes instructions that cause the computer to—
acquire a measured selected radiation distribution by reading data from selected pixels of the array based on pixels having sufficient sensitivity to changes in the parameter using the sensitivity record, and
fit the measured selected radiation distribution to a modeled selected radiation distribution corresponding to the selected pixels to determine a value of the parameter.
24 . The scatterometer of claim 23 wherein the computer operable medium further comprises instructions that cause the computer to determine the sensitivity record by:
identifying a pixel of the array associated with an incident angle;
calculating model intensities of return radiation at the pixel corresponding to different values of the parameter;
differentiating the model intensities, wherein larger differences in the model intensities indicate higher sensitivity to changes in the parameter and smaller differences in the model intensities indicate lower sensitivity to changes in the parameter at the selected pixel; and
assigning a sensitivity value to the pixel.
25 . The scatterometer of claim 23 wherein the computer operable medium further comprises instructions that cause the computer to repeat the identifying, calculating, differentiating and assigning procedures for additional pixels in the array such that the sensitivity record includes sensitivity values for a plurality of pixels.
26 . The scatterometer of claim 25 wherein the computer operable medium includes instructions that cause the computer to acquire the measured selected radiation distribution by selecting pixels having sensitivity values above a threshold value.
27 . The scatterometer of claim 25 wherein the computer operable medium includes instructions that cause the computer to:
acquire the measure selected radiation distribution by selecting pixels having sensitivity values above a threshold value; and
fit the measured selected radiation distribution to a modeled selected radiation distribution corresponding to the selected pixels by measuring actual intensity values from the selected pixels having sensitivity values above the threshold value with modeled intensity values from the selected pixels having sensitivity values above the threshold values.
28 . The scatterometer of claim 23 wherein irradiating the workpiece comprises generating a laser beam having a wavelength of approximately 375 nm to 475 nm.
29 . The scatterometer of claim 23 wherein the altitude angles are 0° to at least 80° and the azimuth angles are 0° to at least 360°.