WO2006080902A2

WO2006080902A2 - Method and apparatus for determining line characteristics, e.g., line roughness, of microfeature components

Info

Publication number: WO2006080902A2
Application number: PCT/US2004/029102
Authority: WO
Inventors: Christopher Raymond
Original assignee: Accent Optical Technologies, Inc.
Priority date: 2004-09-08
Filing date: 2004-09-08
Publication date: 2006-08-03
Also published as: WO2006080902A3

Abstract

A system (10) is provided to measure line edge roughness by directing incident radiation (I) through a lens (28) onto a line grating (G) on a workpiece (W) on a support (20) and detecting reflected radiation (R) from the grating (G). The support (20) and workpiece (W) is rotated (24) by a motor (23). The radiation system (25) includes a radiation source (30) with a radiation element (32) such as a laser on an LED to provide the incident radiation (I) and a polarizing element (34). The radiation detector (50) includes a beam splitter (56) and a pair of detector elements (52a, 52b) with polarizing filters (54a, 54b) which are oriented at 90 degrees relative to each other. A controller (70) with a programmable processor (72) controls the various elements of the system.

Description

METHOD AND APPARATUS FOR DETERMINING LINE CHARACTERISTICS, E.G., LINE ROUGHNESS, OF MICROFEATURE COMPONENTS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/500,463, filed 5 September 2003 and entitled "Line Edge Roughness Determination," the entirety of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to metrology, and more particularly to scatterometry, ellipsometry and similar analysis methods utilizing measurement and analysis of reflected or transmitted electromagnetic radiation to determine line characteristics or surface roughness.

BACKGROUND^'

It is generally accepted that microelectronic structures should have smooth, straight edges, with low edge roughness. Microelectronic processing technology strives to produce devices and microstructural components that exhibit high quality edges, often to improve electronic performance, reliability, manufacturability, and testability.

It is unfortunately somewhat difficult to measure line edge roughness, particularly in a production environment. Typical approaches for measurement of line edge roughness are done by imaging the edges of device patterns using some form of microscope, and analyzing the images to determine the deviation of the lines in the images from their optimal quality. Imaging-based approaches are often very time consuming and are fraught with subjective influence by the operator, all of which adds up to an unreliable measurement technique. For certain situations, electrical approaches can be used to infer properties of line edge quality, but there are many factors that influence such measurements and the results are often the topic of much debate. Scatterometry is a non-imaging approach to metrology. In one common arrangement, electromagnetic radiation (usually in the form of light) is projected as a beam onto a test surface or device, and the properties of the reflected or transmitted beam or beams is measured and analyzed. Some analysis approaches employ model-based techniques and can produce measurement of the physical dimensions and electromagnetic properties of the surface or device. It is not uncommon to use a periodic array of devices as a test structure, such as a line grating, for scatterometry measurements. The width of the lines in the grating are often indicative of the size of transistors or interconnects, and therefore these test structures are both useful for scatterometry measurements and semiconductor manufacturing.

Analysis of scatterometry data from line gratings is well established as a metrology technique. The device sidewalls are necessarily assumed to be well formed, smooth, and consistent. Line edge roughness is assumed not to exist, as the mathematical models are generally unable to accommodate such irregularities.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of a metrology tool in accordance with one embodiment of the invention.

Figure 2 is a more detailed schematic illustration of a metrology tool in accordance with a further embodiment of the invention.

DETAILED DESCRIPTION A. Overview

Various embodiments of the present invention provide methods and apparatus for analyzing characteristics of microfeatures, e.g., line characteristics of line gratings. The term "microfeature workpieces" may encompass a variety of articles of manufacture, including microelectronic components such as memory modules (e.g., SIMM, DIMM, DRAM, flash-memory), ASICs, processors, semiconductor wafers, semiconductor dies singulated from such wafers; optoelectronic and photonic devices, e.g., LEDs; micromachines, such as micromechanical devices, microelectromechanical systems (MEMS) and microfluidic devices; mass storage devices and media; and diffraction gratings, such as those used in spectrographs. These microfeature workpieces typically include structures formed on and/or in a substrate. Depending on the intended application, the substrates can be semiconductive pieces (e.g., doped silicon wafers or gallium arsenide wafers), dielectric pieces (e.g., various ceramic substrates), or conductive pieces.

In one embodiment, the present invention provides a method of analyzing a periodic microfeature structure carried by a substrate. According to this method, polarized radiation is directed at the structure. The radiation has a plane of incidence that is generally perpendicular to an orientation of the structure. Radiation reflected by the structure is filtered with a polarized filter. An intensity of the filtered radiation is detected and the detected intensity is correlated to a line characteristic of the structure, e.g., line roughness.

A method in accordance with another embodiment of the invention includes directing polarized radiation at a grating that includes a plurality of lines carried by a substrate and has an alignment orientation. The radiation has a plane of incidence that is generally perpendicular to the alignment orientation of the grating lines. A change in polarization of radiation reflected by the grating is detected and this detected change in polarization is correlated to a line characteristic of the grating lines.

One further embodiment of the invention provides a method of analyzing a grating that includes a plurality of lines carried by a substrate. Radiation is directed at the grating at an angle of incidence. Specularly reflected radiation is detected at a first reflection angle and scattered radiation is detected at at least one second reflection angle that differs from the first reflection angle. A radiation intensity of the scattered radiation is correlated to a line characteristic of the grating lines.

An alternative embodiment of the invention provides an apparatus for measuring a line characteristic of a periodic structure carried on a substrate. This apparatus includes a support, a radiation source, a radiation detector, and a programmable processor. The support is configured to support a substrate and to position the periodic structure of the supported substrate. The radiation source is configured to direct radiation at the periodic structure. The radiation detector is positionable with respect to the support and configured to detect, at a first angle, radiation that is specularly reflected by the periodic structure and to detect, at a second angle, radiation scattered by the periodic structure. The processor is operatively coupled to the radiation detector and programmed to correlate an intensity of the scattered radiation detected by the radiation detector to the line characteristic. One additional embodiment of the invention provides an apparatus for measuring a line characteristic of a periodic structure that includes a support, a radiation source, a radiation detector, and a programmable processor. The radiation source is configured to direct polarized radiation against the periodic structure of a substrate supported by the support. The radiation detector includes a polarized filter and a detector element positioned to detect radiation filtered by the filter. Optionally, the radiation detector may be positionable with respect to the support and configured to detect radiation specularly reflected by the periodic structure. The processor is operatively coupled to the radiation detector and is programmed to correlate a change in polarization of the radiation between the radiation source and the radiation detector to the line characteristic, e.g., line roughness.

For ease of understanding, the following discussion is broken down into two areas of emphasis. The first section explains metrology tools in accordance with selected embodiments of the invention. The second section outlines select methods according to other embodiments of the invention.

B. Metrology Tools

Figure 1 schematically illustrates a metrology tool in accordance with one embodiment of the invention. This illustration is highly simplified and intended only to illustrate selected aspects of this embodiment. The metrology tool 10 of Figure 1 generally includes a support 20 and a radiation system 25. The support 20 includes a support surface 22 adapted to support a microfeature workpiece W. Of course, the microfeature workpiece W may be supported in any suitable fashion. The workpiece W includes a line grating G that includes a plurality of generally parallel lines L, as is known in the art. The lines L of the line grating G may have an alignment orientation generally perpendicular to a so- called grating vector k.

In select embodiments, at least one of the support 20 and the radiation system 25 is adapted to move relative to the other to change the relative orientation of the radiation system 25 and the grating G. This is schematically shown in Figure 1 by the arrow 24. By rotating the substrate 20 with respect to the radiation system 25, the alignment orientation of the lines L may be changed relative to a plane of incidence of radiation from the radiation system 25. The relative orientation of the plane of incidence and the grating vector k is referred to below in terms of an angle Φ. In the conventional or classic mounting orientation shown in Figure 1 , this angle Φ is 0°.

The radiation system 25 generally includes a radiation source 30 and a radiation detector 50. The radiation source 30 is positioned to direct incident radiation I at some or all of the grating G. The incident radiation I may be oriented at an angle of incidence θ-i, which may be varied as desired. The detector 50 may be positioned to detect reflected radiation R reflected by the grating G. In the particular configuration shown in Figure 1 , the detector 50 is positioned to detect reflected radiation R at a reflection angle Θ2 that is equal to the angle of incidence O₁. In this position, the radiation detector 50 is positioned to detect specular reflectance or so- called zeroth-order reflectance. In select embodiments, the radiation detector 30 may be configured to detect reflected radiation R at a variety of angles of reflection θ₂ for a particular angle of incidence θ-|.

Figure 2 schematically illustrates a metrology tool 10 in accordance with a further embodiment of the invention. Like reference numbers are used in Figures 1 and 2 to indicate analogous structures or elements. Figure 2 shows certain aspects of the tool 10 in greater detail than shown in Figure 1. For example, the support 20 may include a motor 23 that is adapted to rotate the support 20 about a vertical axis as shown by arrow 24 in Figures 1 and 2. The radiation system 25 is also shown in greater detail in Figure 2. In this implementation, the radiation source 30 includes a radiation element 32 and a polarizing element 34. The radiation element 32 may be any of a variety of radiation sources known in the art, such as lasers and LEDs, with the nature of the source dependent on the intended application. These radiation elements 32 may emit electromagnetic radiation at any suitable wavelength, specified wavelengths, or range of wavelengths. In one embodiment, the radiation element 32 comprises a laser or LED adapted to emit a single wavelength or a very narrow band of wavelengths of electromagnetic radiation. In other embodiments, the radiation element may comprise a variable wavelength light source, a variable phase light source, or a variable polarization light source.

Radiation from the radiation element 32 passes through the filter element 34. The filter element 34 may include at least one polarized filter, and may also include other filters to narrow the range of wavelengths directed from the radiation source 30 to the workpiece W. In one embodiment, the filter element 34 comprises a polarized filter tnat may De turned to cnange a polarization orientation of the incident radiation I directed at the workpiece W. If so desired, this polarized filter may also be moved out of the path of radiation so the incident radiation I will be non-polarized. In other embodiments, the filter element 34 may be omitted entirely. The detector 50 as shown in Figure 2 includes a beam splitter 56, a pair of detector elements 52a and 52b, and a pair of filter elements 54a and 54b. The beam splitter delivers a first portion of the reflected radiation R to the first detector element 52a through the first filter element 54a and a second portion of the reflected radiation R to the second detector element 52b through the second element 54b. The filter elements 54a and 54b may be similar to the filter element 34 disclosed above. In one particular implementation, the first filter element 54a includes a first polarizing filter at a first polarizing orientation and the second polarizing filter 54b includes a second polarizing filter at a second polarizing orientation. For example, the polarizing filter of the first filter element 54a may have the same orientation as the filter element 34 of the radiation source 30 and the polarizing filter of the second filter element 54b may have a polarizing orientation that is oriented 90 degrees from the orientation of the filters in the other filter elements 34 and 54a.

The metrology tool 10 of Figure 2 is configured to allow both the angle of incidence θi and the angle of reflection θ₂ to be varied. This particular implementation employs an optics system (shown schematically as a single lens 28) to control the angle of incidence θi and the angle of reflection θ₂ at which the detector 50 detects the reflective radiation. In one useful embodiment, the angle of incidence θi and angle of reflection θ₂ may be changed independently of one another. For example, the angle of incidence θi may be varied while the detector 50 measures intensity of the reflected radiation R at a fixed angle of reflection θ₂. Alternatively, the angle of incidence θi may be held constant while the detector 50 measures intensity of the reflected radiation R at two or more different angles of reflection θ₂. In one embodiment, at least one of the angles of reflection θ₂ at which the detector detects the reflected radiation R is equal to the angle of incidence θi so the detector 50 detects specular reflection. If so desired, the detector 50 may scan the reflected radiation R over a range of angles of reflection θ₂.

The metrology tool 10 of Figure 2 includes a controller 70 that includes a programmable processor 72. The controller 70 may be operatively coupled to the radiation source 30 and the radiation detector 50. In one implementation, the processor 72 of the controller /u is programmed to selectively control one or more aspects of the radiation emitted by the radiation element 32. For example, the controller 70 may control the radiation element 32 to generate radiation at any one of two or more defined wavelengths. The controller 70 may also be coupled to the filter element 34 to control the polarization state of the incident radiation I. Similarly, the controller 70 may be coupled to the detector elements 52a, b and filter elements 54a, b of the radiation detector 50. The controller 70 may both control operation of the detector elements 52 and filter elements 54 and receive data from the detector elements 52a, e.g., a detected intensity of the reflected radiation R that passes through the respective filter element 54.

The processor may also be programmed to carry out any of the methods discussed below and outlined in the claims.

C. Methods of Analyzing Microfeatures

In the following discussion of methods in accordance with the invention, reference is made to the structures shown in Figures 1 and 2. It should be understood, though, that the methods may be practiced with any suitable apparatus and are not limited to those discussed above.

The basis for the measurement lies in the way line gratings behave under illumination with beams of electromagnetic waves of carefully controlled polarization. A perfect line grating that has no edge roughness, when illuminated in normal orientation (the plane of incidence containing the grating k-vector), causes no depolarization of the incident beams upon reflection. For example, with an s-polarized incident beam, the reflected zero-order beam should exhibit purely s-polarized radiation; p-polarized radiation should be absent from the reflected beam. Likewise, a pure p-polarized input beam should produce a pure p-polarized zero-order reflected beam. This behavior is related to the fact that each line is a replica of the other lines; the grating is perfectly periodic. Similarly, a perfect line grating that has no edge roughness will not result in significant light scattering.

Edge roughness is usually distributed along lines in a statistical manner; it is not periodic. When a grating with rough edges is illuminated in the above manner, some depolarization of the incident light occurs. This means that for a pure s- polarized incident beam, the reflected zero-order beam will carry some p-polarized radiation along with the expected s-polarized radiation. A similar behavior occurs for p-polarized incident beams; some s-polarized light is observed in the reflected beams. The amount of depolarization that occurs is strongly related to the amount of non- periodic structure in the grating. This means that increased line edge roughness should result in increased depolarization. The measurement of depolarization is enhanced when multiple wavelengths of incident illumination are used at a variety of incident angles as well, and offers an improvement over existing techniques.

Depolarization may also be observed if the materials composing the grating lines exhibit a granular microstructure. This is typical, for example, with respect to materials such as polysilicon or CVS aluminum. The grain boundaries represent a non-periodic structure that is another form of line edge roughness and will therefore cause a measurable depolarization.

In addition, the typical scatterometry measurements of a grating measures the Oth or specular order to obtain information. Higher orders may also be measured. But line edge roughness on the grating contributes to overall background scattering of the light (the light is scattered in multiple directions and not just in the direction of the specular order) and if the background scatter can be measured roughness can be inferred. As is the case for the depolarization measurement, the use of multiple wavelengths of illumination at multiple angles of incidence is preferred because it allows for more information to be gathered about the scattering structure.

In a preferred embodiment, the polarized electromagnetic radiation is chosen to lie in the visible or near-visible wavelength region. However, any electromagnetic wave that may be polarized may be employed in the practice of this invention. In general, the depolarization of polarized electromagnetic waves due to non-periodic structural components in the sample is a complex function of the wavelength and both the periodic and stochastic components of the test structure. It is possible to optimize the detection of the non-periodic (stochastic) components (or specified properties of those components) in the structure by appropriate selection of the properties of the incident polarized wave, including (but not limited to) wavelength and polarization geometry.

The measurement is most conveniently done utilizing the zero-order or specular reflection. However, it is also possible and contemplated that the measurement may employ any or all reflected or transmitted beams, including higher diffraction orders.

For determination of line edge roughness, measurement is preferably made at more than one scattering or reflection angle. Thus both the specular order and higher

5 orders may be detected, and further the background scatter can be measured as an indication of roughness. Background scatter may be considered the reflection of light at an angle other than at an angle corresponding to specular or higher-order reflectance; if the line grating is perfect, one would expect the background scatter to be zero. io Line edge roughness, measured by depolarization or backscatter techniques, or a combination of both, and performed at more than one illumination wavelength and more than one angle of incidence, and measured at more than one scattering or reflection angle, should be applicable to at least the following areas:

1) Photomask manufacturing and processing and quality control (QC); ij 2) Semiconductor device manufacturing and processing and QC;

3) Optoelectronic and photonic device manufacturing and processing and QC;

4) Microelectromechanical systems, such as MEMS;

5) Mass storage device and media manufacturing and processing; 20 6) New forms of data storage; and

7) Grating manufacturing and processing and QC thereof, such as for spectrographs.

In embodiments of the invention, the processor may correlate the intensity detected by the detector 50 by correlating the intensity of specularly reflected radiation 25 R passed through a polarizing filter 54a to a particular line characteristic, typically line roughness. If measured, the processor may also correlate the intensity of the non- specularly reflected radiation to the same line characteristic or another line characteristic.

Specific embodiments of the invention are described above for purposes of

30 illustration, but this description is not intended to be exhaustive or to limit the invention to the precise form disclosed above. Various modifications may be made without deviating from the spirit and scope of the invention. For example, steps presented in a given order above may be performed in a different order in alternative embodiments. Aspects of the invention may also be useful in applications other than those described above, e.g., in measuring periodic structures other than line gratings.

Claims

I/We claim:

1. A method of analyzing a periodic microfeature structure carried by a substrate, comprising: directing polarized radiation at the structure, the radiation having a plane of incidence that is generally perpendicular to an orientation of the structure; filtering radiation reflected by the structure with a polarized filter; detecting an intensity of the filtered radiation; and correlating the detected intensity to a line characteristic of the structure.

2. The method of claim 1 wherein the line characteristic is line roughness.

3. The method of claim 1 wherein the structure comprises a line grating.

4. The method of claim 1 wherein the filter has a polarizing orientation opposite a polarized orientation of the radiation.

5. The method of claim 4 wherein correlating the detected intensity comprises correlating a greater detected intensity with greater deviation from an intended line characteristic.

6. The method of claim 1 wherein filtering the reflected radiation comprises filtering specularly reflected radiation and detecting the intensity comprises detecting the intensity of the filtered, specularly reflected radiation.

7. The method of claim 6 wherein detecting the intensity comprises detecting intensity of non-specularly reflected radiation.

8. The method of claim 6 wherein detecting the intensity comprises detecting intensity of non-specularly reflected radiation, and wherein correlating the 20. The method of claim 14 wherein correlating the detected change in polarization comprises correlating a greater change in polarization with greater deviation from an intended line characteristic.

21. The method of claim 14 wherein the radiation is directed at grating at an angle of incidence and detecting the change in polarization includes detecting an intensity of reflected radiation at a plurality of angles of reflectance that includes an angle of reflectance complementary to the angle of incidence.

22. A method of analyzing a grating that includes a plurality of lines carried by a substrate, comprising: directing radiation at the grating at an angle of incidence; detecting specularly reflected radiation at a first reflection angle; detecting scattered radiation at at least one second reflection angle that differs from the first reflection angle; and correlating a radiation intensity of the scattered radiation to a line characteristic of the grating lines.

23. The method of claim 22 wherein the line characteristic comprises line roughness.

24. The method of claim 22 wherein the grating is a line grating.

25. The method of claim 22 wherein the radiation intensity comprises a cumulative value based on a total of radiation intensity detected at two or more second angles.

26. The method of claim 22 wherein correlating the radiation intensity to the line characteristic includes comparing a radiation intensity of the specularly reflected radiation to the radiation intensity of the scattered radiation.

-14- 27. The method of claim 22 wherein the grating lines have an alignment orientation and the radiation has a plane of incidence that is generally perpendicular to the alignment orientation.

28. The method of claim 22 wherein the grating lines have an alignment orientation and the radiation has a plane of incidence that is not generally perpendicular to the alignment orientation.

29. The method of claim 22 wherein the radiation is polarized.

30. The method of claim 22 wherein detecting the reflected radiation includes passing the reflected radiation through a polarized filter and detecting an intensity of the filtered radiation.

31. The method of claim 22 wherein the radiation is polarized and detecting the reflected radiation includes passing the reflected radiation through a polarized filter and detecting an intensity of the filtered radiation.

32. The method of claim 22 wherein the grating lines have an alignment orientation, the radiation is a first radiation, the angle of incidence is a first angle of incidence, and the first radiation has a first wavelength and a first plane of incidence that has a first orientation with respect to the to the alignment orientation, the method further comprising directing a second radiation at the grating at a second angle of incidence, the second radiation having a second wavelength and a second plane of incidence that has a second orientation with respect to the alignment orientation, at least one of the second angle of incidence, the second wavelength, and the second orientation differing from the first angle of incidence, the first wavelength, and the first orientation, respectively.

33. The method of claim 22 wherein the radiation is a first radiation and the angle of incidence is a first angle of incidence, the method further comprising

-15- directing a second radiation at the grating at a second angle of incidence that differs from the first angle of incidence.

34. The method of claim 22 wherein the radiation is a first radiation that has a first wavelength, the method further comprising directing a second radiation at the grating, the second radiation having a second wavelength that differs from the first wavelength.

35. The method of claim 22 wherein the grating lines have an alignment orientation and the radiation is a first radiation that has a first plane of incidence having a first orientation with respect to the to the alignment orientation, the method further comprising directing a second radiation at the grating, the second radiation having a second plane of incidence having a second orientation with respect to the alignment orientation and the second orientation differing from the first orientation.

36. The method of any one of claims 32-35 further comprising detecting second radiation reflected by the grating at a plurality of reflection angles and correlating a radiation intensity associated with the second reflection angle or second reflection angles to a line characteristic of the grating lines.

37. An apparatus for measuring a line characteristic of a periodic structure carried on a substrate, comprising: a support configured to support a substrate and to position the periodic structure; a radiation source configured to direct radiation at the periodic structure; a radiation detector configured to detect at a first angle radiation that is specularly reflected by the periodic structure and to detect at a second angle radiation scattered by the periodic structure; and a programmable processor operatively coupled to the radiation detector, the processor being programmed to correlate an intensity of the scattered radiation detected by the radiation detector to the line characteristic.

-16- 38. The apparatus of claim 37 wherein the radiation source is configured to direct polarized radiation at the periodic structure.

39. The apparatus of claim 37 wherein the radiation source is configured to direct polarized radiation at the periodic structure and the radiation detector is configured to detect a change in polarization of the radiation.

40. The apparatus of claim 39 wherein the processor is programmed to correlate the detected change in polarization with the line characteristic.

41. The method of claim 37 wherein correlating the radiation intensity to the line characteristic includes comparing a radiation intensity of the specularly reflected radiation to the radiation intensity of the scattered radiation.

42. The apparatus of claim 37 wherein the radiation source is configured to direct radiation at the periodic structure at two or more angles of incidence.

43. The apparatus of claim 37 wherein at least one of the support, the radiation source, and the radiation detector is configured to change an angular orientation of the periodic structure with respect to the radiation source and/or the radiation detector.

44. The apparatus of claim 37 wherein the radiation source and the radiation detector are included in a radiation system, and wherein the periodic structure has an alignment orientation and at least one of the support and the radiation system is configured to rotate to change the angular orientation of the periodic structure with respect to the radiation system.

45. The apparatus of claim 37 wherein the radiation detector includes a polarized filter positioned to filter the reflected radiation prior to detecting the radiation.

46. The apparatus of claim 37 wherein the radiation detector includes first and second polarized filters and first and second detector elements, the first

-17- detector element is positioned to receive reflected radiation filtered by the first filter, and the second detector element is positioned to receive reflected radiation filtered by the second filter.

47. The apparatus of claim 37 wherein the radiation detector includes first and second polarized filters and first and second detector elements, the first and second filters have different polarization orientations, the first detector element is positioned to receive reflected radiation filtered by the first filter, and the second detector element is positioned to receive reflected radiation filtered by the second filter.

48. An apparatus for measuring a line characteristic of a periodic structure carried on a substrate, comprising: a support configured to support the substrate and to position the periodic structure; a radiation source configured to direct polarized radiation against the periodic structure; a radiation detector positionable with respect to the support and configured to detect radiation specularly reflected by the periodic structure, the radiation detector including a polarized filter and a detector element positioned to detect radiation filtered by the filter; and a programmable processor operatively coupled to the radiation detector, the processor being programmed to correlate a change in polarization of the radiation between the radiation source and the radiation detector to the line characteristic.

49. The apparatus of claim 48 wherein the radiation source is configured to direct radiation at the periodic structure at two or more angles of incidence.

50. The apparatus of claim 48 wherein at least one of the support, the radiation source, and the radiation detector is configured to change an angular orientation of the periodic structure with respect to the radiation source and/or the radiation detector.

-18- 51. The apparatus of claim 48 wherein the radiation source and the radiation detector are included in a radiation system, and wherein the periodic structure has an alignment orientation and at least one of the support and the radiation system is configured to rotate to change the angular orientation of the periodic structure with respect to the radiation system.

52. The apparatus of claim 48 wherein the polarized filter is a first filter and the detector is a first element, the radiation detector further including a polarized second filter and a second detector positioned to detect radiation filtered by the second filter.

53. The apparatus of claim 52 wherein first filter and the second filter have different polarization orientations.

54. The apparatus of claim 48 wherein the radiation detector is positionable to detect at a first angle the specularly reflected radiation and to detect at a second, different angle radiation scattered by the periodic structure.

55. The apparatus of claim 54 wherein the processor is programmed to correlate the scattered radiation to the line characteristic.

-19-