# Patent

## Title
Wide spatial frequency topography and roughness measurement

## Abstract

In one embodiment, a system to inspect a surface comprises an assembly to direct a first radiation beam onto a surface in a first plane of incidence, a first detector to generate a first signal from a portion of the radiation reflected from the first radiation beam, a first spatial filter interposed between the surface and the first detector, a first ellipsoidal mirror to collect scattered light, a second detector to generate a second signal from the scattered portion of the beam, and a processor to generate, from the first and second signals, a data set representing one or more characteristics of the surface using the first and second signals.

## Background
RELATED APPLICATIONS
This application is continuation-in-part of 11/268,214 (U.S. Pat. Ser. No. 7,110,097), entitled Combined High Speed Optical Profilometer and Ellipsometer, by Steven W. Meeks, et al., filed Nov. 7, 2005, which is a continuation of co-pending, commonly assigned U.S. patent application Ser. No. 10/873,892 (U.S. Pat. No. 7,075,630), entitled Combined High Speed Optical Profilometer and Ellipsometer, by Steven W. Meeks, et al., filed Jun. 21, 2004, which is a continuation of patent application Ser. No. 09/861,280 (U.S. Pat. No. 6,757,056), entitled “Combined High Speed Optical Profilometer And Ellipsometer”, that was filed on May 18, 2001 (U.S. Pat. No. 6,757,056) and claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 09/818,199, entitled “A combined High speed Optical Profilometer and Ellipsometer”, that was filed on Mar. 26, 2001, all of which are incorporated by reference in their entirety.


BACKGROUND
1. Field of the Invention
The present invention is directed to systems and methods for analyzing surfaces of objects such as, e.g., silicon wafers, magnetic thin film disks and transparent and coated glass substrates.
2. Description of Background Art
Coated thin film disks are used in a variety of industries including the semiconductor and the magnetic hard disk industry. A computer hard disk (magnetic storage device) is a non-volatile memory device that can store large amounts of data. One problem that the manufacturers of hard disks experience is how to maximize the operating life of a hard disk. When a hard disk fails the data stored therein may be difficult, expensive, or impossible to retrieve. Failure of a hard disk may be caused by defects on the surface of the thin film disk. It is crucial to be able to detect and classify these defects in order to prevent disk drive failure and to control the manufacturing process.
A schematic of a thin film disk used in magnetic storage devices is shown in FIG. 1. It includes a magnetic thin film (layer) 106 which is deposited upon a substrate 108 (typically a NiP plated Al—Mg alloy or glass). The magnetic thin film 106 can be protected by a thin film of carbon 104 (carbon layer), for example, whose thickness is typically 20 to 200 Angstroms (Å). The carbon layer 104 is typically coated with a thin layer (10 to 30 Angstroms) of a fluorocarbon lubricant 102 (lubricant layer). The lubricant layer 102 serves to increase the durability of the underlying carbon layer 104 particularly when the magnetic read/write head contacts the disk, for example when the disk drive is turned off. The hard disk drive industry has been dramatically improving storage capacity by flying the thin film head closer to the surface of the thin film disk. As a result even very small defects can cause a hard drive to fail. These defects may be topographic such as scratches, pits, mounds, or particles or they may be non-topographic such as stains or inclusions. It is necessary to measure all these types of defects to control the disk manufacturing process and improve disk drive manufacturing yield.
A schematic of a semiconductor wafer is shown in FIG. 2. The structure of a semiconductor wafer can be very complex and FIG. 2 shows only one example of a wafer that is undergoing the copper dual damascene process. With reference to FIG. 2, illustrated are a copper layer 201, a second plasma enhanced chemical vapor deposited (PECVD) oxide layer 202, a first PECVD oxide layer 203 and is a silicon substrate 204. The copper layer 201 is polished using a chemical mechanical polishing (CMP) process until only the via holes and copper lines remain. The problem is that the CMP process can leave residual copper, nitride, or CMP slurry on the surface of the wafer. In addition, stains, particles, scratches, and micro-waviness may be present on the polished wafer. It is necessary to detect and measure such defects to control the process of making the wafer. Fewer defects will also mean greater wafer yields at the end of the process.
A problem in the hard disk, semiconductor and photonics industries is to inspect these magnetic disks and wafers for defects such as particles, scratches, pits, mounds, stains, topographic irregularities and inclusions. Conventional techniques to solve these problems are discussed in U.S. Pat. Nos. 4,674,875, 5,694,214, 5,748,305, and 6,157,444. These patents describe techniques to measure defects using essentially sophisticated scatterometers and reflectometers. None of these systems enables the simultaneous measurement of topographic and non-topographic defects. This invention enables this measurement through the use of a combined reflectometer, scatterometer, ellipsometer, profilometer and Kerr effect microscope.
What is needed is a system and method for examining thin film disks, silicon wafers and transparent wafers that: (1) measures topographic and non-topographic defects; (2) measures the optical profile on these substrates; (3) enables the measurements to be performed simultaneously; (4) measures the thickness of thin films; (4) enables measurement on patterned or unpatterned silicon or photonic wafers; (5) is performed in situ or in line; and (6) measures only a single side of a transparent substrate.
SUMMARY
In one embodiment, a system for inspecting a first surface of a substrate, comprises a first reflected radiation collector to generate a first signal set representing one or more characteristics of radiation reflected from the first surface from a radiation source disposed in a first plane of incidence, a spatial filter in the radiation path between the surface and the first reflected radiation collector, a second reflected radiation collector to generate a second signal set representing one or more characteristics of radiation reflected from the first surface from a radiation source disposed in a first plane of incidence approximately orthogonal to the first plane of incidence, a spatial filter in the radiation path between the surface and the first reflected radiation collector, means for combining the first signal set and the second signal set to generate a signal set which represents one or more characteristics of the first surface, and means for processing the signal set to generate a data set representing one or more characteristics of the first surface.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a thin film that can be measured using an embodiment of the present invention.
FIG. 2 is an illustration of a semiconductor wafer that can be measured with one embodiment of the present invention.
FIG. 3 is an illustration from a side perspective of one half of optical layout of combined ellipsometer and optical profiler according to one embodiment of the present invention.
FIG. 4 is a top view of an optical profilometer that measures height or slope according to one embodiment of the present invention.
FIG. 5 is a top view of an optical profilometer having a single laser which measures height or slope according to another embodiment of the present invention.
FIG. 6 is a side view of optical profilometer showing laser one and PSD 1 according to one embodiment of the present invention.
FIG. 7 illustrates the height sensitivity multiplier as a function of angle of incidence (theta) according to one embodiment of the present invention.
FIG. 8 is an illustration of a miniature optical surface analyzer according to one embodiment of the present invention.
FIG. 9 is an illustration of a miniature optical surface analyzer according to another embodiment of the present invention.
FIG. 10 is an illustration from a top view perspective of a miniature surface analyzer according to another embodiment of the present invention.
FIG. 11 is an illustration from in the direction identified as “A” of the miniature surface analyzer illustrated in FIG. 10.
FIG. 12 is an illustration from a top view perspective of a miniature surface analyzer according to another embodiment of the present invention.
FIG. 13 is an illustration of a final test spindle having dual miniature optical heads and stepper motor according to one embodiment of the present invention.
FIG. 14 is an illustration of a spatial filter for blocking bottom surface reflection from a glass or transparent substrate according to one embodiment of the present invention.
FIG. 15 is an illustration from a side view perspective of one half of optical layout of combined ellipsometer and optical profiler according to one embodiment of the present invention.
FIG. 16 is an illustration from a top view perspective of a combined ellipsometer and optical profilometer according to one embodiment of the present invention.
FIG. 17 is an illustration of a beam splitter that splits the beam into non-orthogonally polarized components that is capable of measuring phase shift of an elliptically polarized beam according to one embodiment of the present invention.
FIG. 18 is a schematic illustration of an assembly to inspect a surface in accordance with an embodiment.
FIG. 19 is a schematic illustration of a system to inspect a surface in accordance with an embodiment.



DETAILED DESCRIPTION
A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit(s) of each reference number correspond(s) to the figure in which the reference number is first used.
FIG. 3 is an illustration of an apparatus for measuring properties of the thin film according to an embodiment of the present invention. The apparatus uses a focused laser light signal whose angle of propagation θ (as shown in FIG. 3) can be between zero degrees from normal and ninety degrees from normal.
One embodiment of the apparatus 300 includes a conventional laser diode 301, e.g., RLD65MZT1 or RLD-78MD available from Rolm Corporation, Kyoto, Japan, which has been collimated by Hoetron Corp., Sunnyvale, Calif., e.g., a conventional linear polarizer 302, e.g., made of Polarcor that is commercially available from Newport Corp., Irvine, Calif., a conventional zero order half wave plate 303 that is commercially available from CVI Laser, Livermore Calif., a conventional focusing lens 304 that is commercially available from Newport Corporation, Irvine, Calif., conventional mirrors 305 and 306 available from Newport Corp. Irving, Calif. A collimating lens 309 available from Newport Corp., a zero order quarter wave plate 310 available from CVI Laser Corp., a conventional polarizing beam splitter 312 rotated at 45° to the plane of incidence available from CVI Laser Corp., a pair of conventional quadrant detectors 311 and 313 available from Hamamatsu Corp., Hamamatsu City, Japan, a conventional avalanche photodiode 314 available from Advanced Photonix, Inc., Camarillo, Calif. and a conventional motor 315 available from Maxon Precision Motors, Burlingame, Calif. for rotating the half wave plate 303. The avalanche photodiode 314 may be replaced with a conventional photo multiplier tube (PMT) available from Hamamatsu Corp., Hamamatsu City, Japan.
It will be apparent to persons skilled in the art that the apparatus 300 is an embodiment of the present invention and that alternate designs can be used without departing from the present invention. The operation of the apparatus 300 is now described in greater detail.
A laser diode 301 emits an electromagnetic signal toward the thin film disk, silicon wafer, photonics wafer or glass substrate. In an embodiment the electromagnetic signal is a light signal having a wavelength of 780 or 655 nanometers (nm) although a wide variety of wavelengths can be used. The angle of propagation of the light signal can be any angle θ between zero and ninety degrees.
Laser diodes are well known to have an internal photodiode to monitor the laser output power. An embodiment of a feedback control circuit to control the optical intensity is to use such a photodiode, which is internal to the laser diode. This photodiode which is internal to the laser diode feeds back a control signal to negative feedback circuitry and by doing so keeps the intensity of the laser at a constant value. The photodiode that is used to control the laser intensity may be external to the laser. When an external photodiode is used an external non-polarizing beam splitter is placed after the laser. This external non-polarizing beam splitter directs a sample of the laser onto the external photodiode. The signal from the external photodiode is used to feedback a control signal to negative feedback circuitry and thereby controls the laser intensity. Another means of keeping an approximate constant output power of the laser is to control the current of the laser diode, that is, run the diode laser in a constant current mode. The laser diode will exhibit a very slow decrease in output power over a period of months. As long as the scan time is less than 5 or 10 minutes then the optical power output of the laser will remain constant during the scan. The advantage of this technique is its simplicity. Long-term drifts of the laser output power may be calibrated out of the system by first measuring a standard reflector and using this to normalize the measured signals. The value of the signal is first measured over the standard (known) reflector and then the disk or wafer is measured. If there has been any drift of the standard reflector measurement then all the data is corrected for this amount of drift. As a result long-term drifts may be compensated even when operating in a constant current mode. The emitted light passes through the linear polarizer 302. The linear polarizer 302 improves the linear polarization of the laser light signal.
The linearly polarized light passes through a mechanically rotatable zero order half-wave plate 303. The half wave plate 303 is attached to a miniature motor 315 which allows the polarization to be dynamically rotated between P polarized (parallel to the plane of incidence), S polarized (perpendicular to the plane of incidence) and 45° polarized (between P and S) light. The polarized light passes through a focusing lens 304 and is directed onto a thin film magnetic disk, silicon wafer or transparent substrate 306 by a turning mirror 305. The reflected signal is directed to the detection optics by another turning mirror 308 and recollimated by another lens 309. An avalanche photodiode, conventional PIN photodiode or photo multiplier tube 314, for example, detects the scattered component of the signal. The recollimated beam passes through a zero order quarter wave plate 310 that is used to adjust the polarization of the beam so that equal amounts of energy are directed into the quadrant photodetectors 313 and 311. After passing through the quarter wave plate 310 the beam is split by a polarization beam splitter 312 that is rotated by 45° to the plane of incidence. In another embodiment the polarizing beam splitter may be a Wollaston prism or a Glan Thompson or a Rochon prism beam splitter. The split beams are directed onto two quadrant detectors 311 and 313. The quadrant detectors are used to compute the phase shift between the split beams, the reflectivity, the optical profiles in the radial and circumferential directions, and the Kerr rotation (if the film on the substrate 306 is magnetic). The outputs from the quadrant detectors are digitized by a conventional analog to digital converter and directed to the memory of a conventional personal computer. The signals are then analyzed by the personal computer to detect defects, measure topography, and measure stains. The entire optical apparatus 300 is placed upon a stage that moves the apparatus in the radial direction while a motor 307 rotates the sample 306. In this manner the entire surface of the sample 306 may be scanned for defects.
An alternative embodiment for scanning the entire substrate 306 is to place the optical head or the substrate 306 on a x-y scan stage. The substrate 306 or the optical apparatus 300 is scanned in the x and y directions and in this manner the entire sample may be scanned for defects or topography.
The spindle or motor which rotates the disk at a high rate of speed includes an encoder which produces 1024 pulses as it rotates through 360 degrees, for example. This encoder is used to determine the circumferential positions around the disk. The present invention preferably utilizes a very high-resolution determination of the position around the circumference of the disk. This is accomplished by using a phase locked loop to multiply the encoder signal by a selectable factor of up to 64 times. The phase locked loop, which multiplies the 1024 encoder pulses, has the ability to track any velocity jitter in the encoder. This feature allows averaging of repeated revolutions to be done with no loss of lateral resolution. That is, subsequent revolutions lie in phase with one another and when averaged, the resulting image is not smeared by any jitter effect. Averaging is done to improve signal-to-noise ratio.
FIG. 4 shows the top view design of an optical profilometer, which measures height only and measures height directly. It can also measure the slope of the surface independent of height. This differs from previous optical profilometers that measure both slope and height at the same time. With such systems the height is obtained from the slope data by integrating the slope information. However, if the slope information is contaminated with height information then the integration will not give the correct surface profile. A goal is to obtain data that includes only height information and not a combination of both slope and height. The design illustrated and described with reference to FIGS. 4-7 accomplishes this by using two lasers and two position sensitive detectors (PSD) oriented at right angles to one another.
The position sensitive detectors (PSD) are quadrant detectors that are oriented as shown in FIG. 4. The PSD's measure the displacement of the beam in the radial and circumferential directions by subtracting the appropriate PSD quadrants. As the laser beam moves along the surface of the object to be measured, the roughness and waviness of the surface cause the laser beam to “wiggle” on the quadrant detector in response to the slope of the surface. The quadrant detector measures this by subtracting the sum of one pair of quadrants from the sum of another pair. For example, referring to FIG. 6, the slope of the surface in the circumferential direction is given by [(A1+B1)−(C1+D1)]/[A1+B1+C1+D1]the sum of the four quadrants in the denominator is used to normalize for reflectivity differences. At the same time, if the average distance of the surface from the detector changes, then the average position of the beam on the quadrant detector will change. The resulting difference signal in the above equation will register a slope change when in fact a difference in surface height is occurring. The problem is to be able to separate slope changes from height changes. This can be accomplished by considering the slope in the radial direction, which is obtained by referring to FIG. 6 and is given by [(A1+D1)−(B1+C1)]/[A1+B1+C1+D1]. The equation for the slope measures the “wiggle” of the beam in the radial direction. In the case of the radial slope, if the average distance of the surface from the detector changes then the beam simply moves along the line separating A1+D1 from B1+C1. As a result the radial slope signal does not change when the surface height changes and the equation for the radial slope records only slope and not height changes.
When the orientation of the laser beam is rotated by 90 degrees (as with laser 2 and PSD 2 in FIG. 4) the behavior of the radial and circumferential slope will reverse. In the case of laser 2 and PSD 2 the circumferential slope equation will record only slope changes and not height changes. On the other hand, for laser 2, the radial slope equation will record both slope and height changes. Since the output beam of both lasers 1 and 2 is positioned at the same location on the surface (as shown in FIG. 4) then it is possible to subtract the radial slope equation from laser 1 and PSD 1 from the radial slope equation from laser 2 and PSD 2. The resulting subtraction will include only height information and no slope information. It is also possible to obtain the same information by subtracting the circumferential slope equation from laser 1 and PSD 1 from the circumferential slope equation from laser 2 and PSD 2. The radial slope (with no height information) can be obtained by choosing the radial slope equation from laser 1 and PSD 1. The circumferential slope (with no height information) can be obtained by choosing the circumferential slope equation from laser 2 and PSD 2. In this manner it is possible to independently measure surface height variation and slope variation.
In another embodiment of this optical profilometer, as shown in FIG. 5, a single laser is used and a 50/50 mirror 504 oriented at a compound angle directs a second beam onto the surface to a position labeled 502 on FIG. 5. The beam that passes through the 50/50 mirror 504 is directed onto the surface to a position labeled 501 on FIG. 5. The entire surface of the object to be measured is scanned with both of the beams resulting in at least two images of the surface. The resulting images are stored and digitally shifted so that the resulting images have the object to be profiled at the same x, y location. The resulting shifted images may then be subtracted to give the height profile in the manner described above. The advantage of this embodiment is that it uses only a single laser and fewer optical components and the beam shape of the two beams is identical.
Laser one and PSD 1 nominally measure the signal in the radial, Sr, and the signal in the circumferential, Sc, directions. However, the nature of the PSD results in Sc from laser one and PSD 1 being contaminated with height information, in addition to slope information. Sr from laser 1 and PSD 1 include only slope information. Laser two and PSD 2 also nominally measure the slope in the radial and circumferential directions. However, Sr from laser 2 and PSD 2 measures both slope and height at the same positions as Sr from laser 1 and PSD 1. As a result the true height variation can be obtained by subtracting Sr from laser 2 and PSD 2 from Sr from laser 1 and PSD 1. That is, the slope information is removed when subtracting Sr from PSD 2 from Sr from PSD 1, leaving only the height information.
A similar result can be obtained from subtracting Sc from PSD 2 that only includes slope information. As a result, subtracting Sc from PSD 2 from Sc from PSD 1 gives data including only height information. The result is a direct measurement of height. The advantages of this technique are that it gives a direct measurement of height and it can be done in a non-contact manner at high speed. This technique can also measure step heights with 90-degree step angles. Conventional systems, which use slope measurements, cannot measure 90-degree step heights.
FIG. 6 shows the side view design of the optical profilometer. This figure only shows laser 1 and PSD 1 in an effort to easily show the side view design. In FIG. 6 the optical profilometer is positioned above a thin film disk or wafer and is translated in the radial direction while the disk or semiconductor wafer is rotated.
The angle of incidence (θ) shown in FIG. 6 can be chosen for the particular application. Any angle of incidence can be chosen except normal incidence, where the PSD's would have no height sensitivity. For an application that involves transparent substrates one could choose angles greater than 45 degrees in order to increase the reflection signal from the surface. As the angle of incidence increases, the height sensitivity also increases by the factor 2 sin θ. A plot of this factor is shown in FIG. 7. This suggests that an angle of incidence greater than or equal to approximately 60 degrees would be optimal, although not necessary. At angles greater than 60 degrees the sensitivity will increase and the signal from a transparent surface will increase. This embodiment requires that the focused spot sizes of the two lasers be substantially identical and that the laser spots overlap as closely as possible.
A problem in the magnetic recording industry is to inspect thin film disks for defects at the final test step of the manufacturer of disks. The manufacturers of thin film disks require that both sides of the thin film disk be inspected simultaneously. The problem is that the clearance between the disk and the chuck (which holds the disk) is only 1′′ or less (see FIG. 13, 1304). This requires that the optics be miniaturized in order to fit in the small space between the disk and the chuck (see FIG. 13). A solution to this problem can be obtained by using the optical designs in FIGS. 8, 9, 10, and 11. These designs have several key improvements, which allow the design to be miniaturized without compromising the performance of the device. First of all the design uses the internal feedback photodiode, which is included within the laser diode 801, to achieve stabilization of the DC level of the optical signal. Secondly, the angle of incidence, θ, is adjusted to reduce the height of the instrument so that it will fit within the 1′′ space requirement. Thirdly, the surface topography measurement capability feature of the instrument is incorporated within the phase/specular detectors 808 and 809 shown in FIGS. 8 and 9. The position sensitive detectors 808 and 809 (quadrant detectors) serve as both phase detectors, specular detectors, and topography measurement detectors. Fourthly, the size may be decreased by using a polarizing beam splitter 901 as shown in FIG. 9 instead of a Wollaston prism 807 as shown in FIG. 8. The polarizing beam splitter 807 or Wollaston prism 901 is rotated at 45° with respect to the plane of incidence.
Another embodiment of this invention can use a beam splitter that splits the beam into non-orthogonal components, which will be discussed in a subsequent section. Using two spherical mirrors 1004 and 1006 to direct the beam onto the disk as shown in FIG. 10 will diminish the size in the lateral dimension. The mirrors 1004 and 1006 are adjusted at a compound angle as shown in FIG. 10. This is also shown in FIG. 11 which is a view of FIG. 10 along the “A” direction, where the mirrors that are at a compound angle are 1102 and 1104. These mirrors direct the beam 1103 onto the disk or wafer 1101. In addition to directing the beam onto the disk the spherical mirrors also focus the beam to a small spot. In an alternative embodiment flat mirrors 1202 and 1203 are used in combination with focusing lenses 1201 as shown in FIG. 12. Also shown in FIG. 12 is a silicon photodetector or avalanche photodiode or photo multiplier tube 1204, which is positioned above the point where the beam strikes the disk. This element enables the detection of submicron particles. The avalanche photodiode 1204 is available from Advanced Photonix, Inc., Camarillo, Calif.
Referring to FIG. 8, the laser beam from the diode laser 801 passes through a linear polarizer 802, and a focusing lens 803 and then strikes a disk or wafer 804. Upon reflecting from the surface the beam passes through a recollimating lens 805, a quarter wave plate 806, and through a polarizing beam splitter such as Wollaston prism 807 which is rotated at 45° to the plane of incidence and onto two quadrant detectors 808 and 809. The specular signal is obtained by summing the signals from position sensitive detector 1809 with the sum of position sensitive detector 2, 808 times a constant κ:

Specular Signal=(A1+B1+C1+D1)+κ(A2+B2+C2+D2)

The cosine of the phase shift between the two split beams (Cos(ps)) can be obtained by subtracting the sum of the elements of detector 1809 from those of detector 2, 808 times a constant K:

Cos(ps)=(A1+B1+C1+D1)=K(A2+B2+C2+D2) where K is a constant

Referring to FIG. 8 detector 1, 809, the slope in the circumferential direction is given by:

Slope in Circumferential Direction=[(B1+C1)−(A1+D1)]/(A1+B1+C1+D1)

The slope in the radial direction is given by:

Slope in Radial Direction=[(A1+B1)−(C1+D1)]/(A1+B1+C1+D1)

The topography in the circumferential or radial direction is obtained by integrating the slope in the circumferential or radial direction, respectively. The slope signals can also be obtained from detector 2, 808 with the same equations as shown above except for substituting 2 for 1.
Using the designs in FIGS. 8, 9, 10 and 12 will allow the measurement of submicron scratches, particles, stains, pits, mounds, handling damage, wear of the carbon layer, outside diameter damage and contamination. This design can also measure the longitudinal Kerr effect by a measurement of the Kerr rotation angle. The advantages of this design are its small size which is made possible by detectors which combine the measurement of phase shift, specular reflectivity, radial and circumferential slope, and scattered light.
The miniature optical design may be mounted on the top and bottom of a thin film disk 1302 as shown in FIG. 13 and the resulting combination is translated over the surface of the disk with a stepper or DC servomotor driven stage 1308. A spindle motor 1306 rotates the disk while the optics 1301 is translated in the radial direction so that 100% of the surface of the disk may be measured for defects. The entire apparatus is mounted on a baseplate 1307. The electronics package is located above the stepper motor 1303. The disk is placed upon a vacuum chuck 1305 that is rotated at a high rate of speed.
A problem in the inspection of transparent glass substrates 1406 and other transparent objects is to separate the signal from the top and the bottom surface. This can be accomplished by the use of a spatial filter 1404 that blocks the signal from the bottom surface 1405 and does not affect the top surface reflection 1403. FIG. 14 shows this in the optical design of the Optical Surface Analyzer (OSA). The incoming optical beam is 1401.
The spatial filter 1404 is in the shape of a small wedge that is attached to the bottom surface of the integrating sphere 1402. The location of the spatial filter is adjusted to just block the bottom surface reflection 1405 and not to interfere with the top surface reflection 1403. This invention allows one to separate information from the top and bottom surface of a transparent glass disk or wafer 1406. This invention also works with any transparent medium such as lithium niobate, fused silica, photoresist, and other transparent oxides.
An alternative design does not require the spatial filter to be attached to the bottom of the integrating sphere. For example, the integrating sphere may be omitted and the spatial filter may be attached to any other point on the optical body. The crucial point is that the spatial filter must be located near enough to the transparent substrate so that the reflections from the top and bottom surface are separated in the lateral plane. In this manner it is possible to intercept the bottom surface reflection with the spatial filter and leave the top surface reflection unaffected.
A problem in the measurement of semiconductor wafers is the detection of defects caused by the CMP (Chemical Mechanical Polishing) process. These defects can be residual copper, nitride, slurry, particles, scratches and stains. The measurement is complicated by the fact that the semiconductor wafers have a very complex pattern on their surface. The object is to separate the defects from the complex pattern of semiconductor devices on the surface of the semiconductor wafer. This can be accomplished by the design shown in FIG. 15. The device includes a means for measuring the phase shift between the P and S polarization components of the incident beam and a means to measure the topography of the surface. The device includes a laser 1501 and a polarizer 1502. The laser is directed onto a focusing lens 1503 and onto a mirror 1504 that directs the beam onto a wafer or disk 1505 that may be rotated by a motor 1506. The reflected beam is directed by another mirror 1507 onto a collimating lens 1508 and through a quarter wave plate 1509. The signal passing through the quarter wave plate is directed onto a polarizing beam splitter 1511 that is oriented at 45° to the plane of incidence. The split beams are measured with two photodetectors 1510 and 1512. The phase shift of the incident beam is proportional to the difference in the amplitudes of photodetectors 1510 and 1512.
When the phase shift between the split beams is measured it is found that the orientation of the semiconductor pattern lines will have a substantial effect on the measured phase shift. What is desired is to remove the semiconductor pattern and enhance the defects. A means to accomplish this is to image the wafer with two orthogonal beams as shown in FIG. 16. An optical path shown in FIG. 15 generates each of the beams shown in FIG. 16. Laser one 1601 and detector one 1602 in FIG. 16 generate a phase shift image of the surface that has one particular amplitude due to the orientation of the semiconductor pattern lines. Laser two 1603 and detector two 1604 have a particular amplitude pattern that is identical in lateral shape but opposite in amplitude to that generated by laser one 1601 and detector one 1602. This is because the orientation of the optical beams of lasers one and two are orthogonal with respect to the orientation of the pattern lines. As a result, what is generated are two phase shift images of the surface of the semiconductor that have opposite amplitude phase shift signals from the semiconductor pattern lines. If these two images are added together then the semiconductor pattern will be greatly attenuated. Defects, on the other hand, do not change phase shift in the two orthogonal beams and as a result when the two orthogonal images are added the defects increase in amplitude and the semiconductor pattern diminishes in amplitude. Defects do not have opposite phase shift amplitudes since most defects are isotropic in nature and do not have the strong anisotropy associated with semiconductor pattern lines. This technique effectively enhances the defect signals and diminishes the semiconductor pattern signal. The focused beams 1607 cross at point 1606. The entire device is included within housing 1605.
This invention has the additional advantage that it can simultaneously measure the topography of the surface as has been described in U.S. patent application Ser. No. 09/718,054 which is incorporated by reference herein in its entirety. In the preferred embodiment the angle of incidence (θ) shown in FIG. 15 is at approximately 60°. Larger or smaller angles of incidence may be used depending upon the application. For example, a larger angle of incidence may be used if a transparent substrate is to be examined. This would be advantageous since a transparent substrate will give a larger signal from the top surface with a greater angle of incidence.
FIG. 17 illustrates the measurement of the phase shift of an elliptically polarized beam by the use of a beam splitter that splits the beam into non-orthogonally polarized components. The incoming elliptically polarized beam is labeled 1701, this beam is directed into a quarter wave plate 1702 and subsequently into a beam splitter 1703 which splits the beam into non-orthogonally polarized components. Internal to 1703 is a polarizing beam splitter such as a Wollaston prism 1704 or a polarizing cube beam splitter and a polarization rotation device 1705 such as a half wave plate or an optically active quartz polarization rotator. The two beams leaving the beam splitter 1703 are polarized in the same direction as indicated by 1706 and 1707. In general the two beams leaving the beam splitter 1703 may be polarized at any angle with respect to the other. This is accomplished by rotating a half wave plate 1705 (which is internal to the beam splitter 1703) to an arbitrary angle so that the beam leaving 1707 will now be polarized at an arbitrary angle with respect to beam 1706. After the beams leave the beam splitter 1703 they strike diffusers 1708 and subsequently are detected by photodetectors 1709 and 1710. The advantage of this type of beam splitter 1703 is that the outgoing beams may be polarized in the same direction. As a result when the beams 1706 and 1707 strike the diffusers 1708 and photodetectors 1709 and 1710 the reflection from these surfaces will be identical and the detected signals will have identical reduction due to surface reflection. This fact makes the calibration of the instrument considerably easier. The computation of the phase shift of the incoming beam 1701 is proportional to the difference in the amplitude of the two beams as measured by the photodetectors 1709 and 1710.
The incoming laser beams discussed in previous paragraphs have been described as P, S or 45° polarized beams. These earlier discussions are preferred embodiments of this invention. It is also possible to illuminate the surface with unpolarized light and detect the resulting reflected signals with the same optical and electronic methods. The resulting detected signals, which use a source of light which is unpolarized, will still give measurements of the phase shift, topography, reflectivity, defects and particles.
Another technical problem pertains to providing inspection tools that are capable of detecting surface roughness and variations in a surface profile over a wide range of spatial frequencies. Such inspection tools find utility in analyzing isotropic or anisotropic substrates such as, e.g., thin-film magnetic disks and other substrates.
FIG. 18 is a schematic illustration of one half of the assembly to inspect a surface in accordance with an embodiment. The assembly of FIG. 18 is capable of implementing wide spatial frequency topography and roughness measurement in accordance with one embodiment.
Referring to FIG. 18, assembly 1800 includes a diode 1812 to generate a radiation beam. In one embodiment, diode 1812 may be embodied as a laser diode that generates a laser beam having a wavelength of 405 nm. In alternate embodiments different wavelengths may be used. The laser beam generated by diode 1812 is directed to a polarizer 1814. In one embodiment, polarizer 1814 may be embodied as a conventional linear polarizer as described above. Other embodiments may not include the polarizer 1814.
Polarized laser light emitted from polarizer 1814 is directed to a neutral density (ND) filter 1816. In one embodiment, ND filter 1816 may be connected to a motor 1818 such as, e.g., a servo-motor such that ND filter 1816 is movable between a first position in the path of radiation emitted from diode 1812 and a second position outside the path of radiation emitted from diode 1812. In an alternate embodiment, ND filter 1816 may be a graduated ND filter, such that the amount of energy filtered varies across the surface of ND filter 1816. In such an embodiment, motor 1818 may move ND filter 1816 to varying positions in the path of radiation emitted from diode 1812 to vary the energy filtered by ND filter 1816.
Radiation that passes through ND filter 1816 is incident on a beam splitter 1819. A portion of the radiation is directed to a detector 1820, which generates a signal based on the power level of the received radiation. The signal may be used to regulate the energy output of diode 1812. In an alternative embodiment, the output of detector 1820 may be used to normalize the detected signals in order to remove any drift of the laser diode 1812 output power.
Another portion of the radiation incident on beam splitter 1819 is transmitted to a half-wave plate 1822. In one embodiment the half-wave plate 1822 may be a zero-order half-wave plate that alternates the polarization of the radiation from a first polarization state to a second polarization state, different from the first polarization state. By way of example, half-wave plate may alternate the polarization from a polarization state parallel to the plane of incidence (P-polarization) to a polarization state perpendicular to the plane of incidence (S-polarization). The rotation of the polarization may be accomplished by using a motor 1821 to rotate the half wave plate 1822.
Radiation from the half-wave plate 1822 is incident on a turning mirror 1824, which directs the radiation to a focusing lens 1826, which in turn focuses the radiation onto a spot onto a surface 1852 of a substrate such as, e.g., a wafer 1850. In one embodiment, the radiation may be focused on a spot that measures 5 micrometers (μm) in the radial direction by 4 μm in the circumferential direction.
A portion of the radiation incident on the surface 1852 of wafer 1850 may be scattered. A scattered radiation collector 1830 directs scattered radiation to a detector 1832. In one embodiment, scattered radiation collector 1830 may be an ellipsoidal mirror (that is, an ellipse of revolution) and detector 1832 may be a photo-multiplier tube (PMT). The first focus of the ellipsoidal mirror is placed at the location of the focus of the beam from lens 1826. The second focus of the elliptical mirror is place at the center of a pinhole 1831 which serves to eliminate stray scattered light. The ellipsoidal mirror allows a very wide range of scattered light angles to be collected. This in turn allows a wide range of nano-roughness to be computed with this design. In one embodiment the ellipsoidal mirror will collect spatial wavelengths from 0.22 to 1.8 microns. In alternate embodiments, an integrating sphere or a collecting lens may be used in place of the ellipsoidal mirror. Detector 1832 may generate a signal based on the energy level of the radiation incident on detector 1832.
The signal generated by detector 1832 may be transmitted to a data processing device 1860. In one embodiment, data processing device 1860 comprises a processor 1862, a memory module 1864, and an input/output module 1866. Data processing device 1860 may be embodied as a conventional computing device such as, e.g., a personal computer or the like.
Another portion of the radiation incident on surface 1852 of wafer 1850 may be reflected from the surface 1852. A lens 1838 in the path of reflected radiation collects the reflected radiation and directs the radiation onto a turning mirror 1840. In one embodiment, lens 1838 may be a lens that exhibits low aberration properties such as, e.g., an aspheric, achromatic lens.
Radiation incident on turning mirror 1840 is directed to a spatial filter 1842. Lens 1838 focuses the collected light onto a pinhole or spatial filter 1842.
In one embodiment, spatial filter 1842 may have an aperture approximately equal 0.5 mm. Spatial filter 1842 blocks radiation reflected from the bottom surface 1854 of wafer 1850 while transmitting a portion of the radiation reflected from the upper surface 1852 of wafer 1850. This filtering process permits the assembly 1800 to be used with wafers 1850 that are transparent, translucent, or opaque to the radiation emitted by diode 1812.
In one embodiment, radiation transmitted from spatial filter 1842 passes through another ND filter 1844, which filters a portion of the radiation. ND filter 1844 may also be moveable (e.g., by being connected to motor 1818 or another motor) between a first position in the path of radiation emitted from spatial filter 1842 and a second position outside the path of radiation emitted from spatial filter 1842. In an alternate embodiment, ND filter 1844 may be a graduated ND filter, such that the amount of energy filtered varies across the surface of ND filter 1844. In such an embodiment, motor 1818 (or another motor) may move ND filter 1844 to varying positions in the path of radiation emitted from diode 1812 to vary the energy filtered by ND filter 1844.
Radiation transmitted from ND filter 1844 is incident on a collimating lens 1846, which collimates the radiation and directs the radiation onto detector 1848. In one embodiment, collimating lens 1846 may be a cylindrical lens and detector 1848 may be a photodetector. Detector 1848 generates a signal based on the power level of radiation incident on detector 1848. The signal may be input to data processing device 1860. Detector 1848 may be a quadrant detector as shown as PSD 1 in FIG. 6. The differences of the various quadrants are performed in such a manner as to give the radial or circumferential slope as shown earlier in this document.
In one embodiment, wafer 1850 may be mounted on a spindle 1858 which rotates, driven by a motor or other drive mechanism. Assembly 1800 may be moved across the surface 1852 of wafer 1850 to affect a scan of surface 1852. Either the assembly 1800 may be moved across the wafer or the wafer and spindle 1858 may be moved beneath the stationary optical head 1800.
Assembly 1800 may be incorporated into a system to inspect a surface. In one embodiment, a system may use a single assembly 1800. In another embodiment a system may use two assemblies 1800. FIG. 19 is a schematic, top-view illustration of a system 1900 to inspect a surface of an object such as, e.g., a wafer, a substrate, or the like. Referring to FIG. 19, system 1900 includes a first radiation directing assembly 1910A to direct a first radiation beam onto the surface of wafer 1950 and a first radiation detection assembly 1920A to detect radiation reflected from the surface of wafer 1950. System 1900 may further include a second radiation directing assembly 1910B to direct a second radiation beam onto the surface of wafer 1950 and a second radiation detection assembly 1920B to detect radiation reflected from the surface of wafer 1950. A scattered radiation detector 1930 may be disposed above the surface of wafer 1950.
In one embodiment, radiation directing assemblies 1910A and 1910B may be constructed in accordance with components 1812-1826 in the radiation directing assembly depicted in FIG. 18. Similarly, radiation detection assemblies 1920A and 1920B may be constructed in accordance with components 1838-1848 depicted in FIG. 18, and scattered radiation collector 1930 may be constructed in accordance with components 1830-1832 depicted in FIG. 18. Wafer 1952 may be rotated in either direction about a central axis on spindle 1958.
In one embodiment, radiation directing assemblies 1910A, 1910B use low-noise laser diodes at a wavelength of 405 nanometers (nm). The laser beam emitted from radiation directing assembly 1910A follows a path that extends along a radial axis of wafer 1950, while the laser beam emitted from radiation directing assembly 1910B extends along a circumferential axis of wafer 1950. In one embodiment, the radiation directing assembly 1910A generates a spot that measures approximately 4 μm in the radial direction by 5 μm in the circumferential direction, resulting in an approximately 4.3 μm short cut-off wavelength for circumferential topography measurement. In one embodiment, the radiation directing assembly 1910B generates a spot that measure 5 μm in the radial direction by 4 μm in the circumferential direction, resulting in an approximately 4.3 μm short cut-off wavelength for radial topography measurement.
In one embodiment, the radial beam from radiation detecting assembly 1910A is used to generate a signal that measures the slope of the surface in the circumferential direction, and the circumferential beam from radiation detecting assembly 1910B is used to generate a signal that measures the slope of the surface in the radial direction. This technique reduces the sensitivity of the system 1900 to vibration during the measurement process. This is due to the fact that most of the vibration produced by the rotating wafer or disk is in the up and down direction (i.e., in and out of the plane of the page in FIG. 19). This means that the vibration induced noise is minimized by measuring the slope in the direction perpendicular to the plane of incidence of the optical beam. Not only does this reduce vibration induced noise but it also reduces noise due to the imperfect movement of the stage to which the optical head 1800 or 1900 is mounted.
In one embodiment, the circumferential and/or radial slope may be integrated to obtain the surface topography signal. The mean square roughness of any wafer surface is determined by summing the power spectral densities within the spatial frequency range as shown in equation (1).




  
    
      
        
          R
          q
          2
        
        =
        
          
            2
            
              N
              ·
              d
            
          
          ⁢
          
            
              ∑
              
                k
                =
                
                  k
                  min
                
              
              
                k
                max
              
            
            ⁢
            
              S
              ⁡
              
                (
                
                  f
                  k
                
                )
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




Where Rq is the mean square roughness, N is the total number of data in the topography signal, d is the spatial separation between the topography signal, S(fk) is the power spectral density of the topography signal and kmin and kmax are the minimum and maximum limits of spatial frequency range.

The power spectral density is computed using equation (2).





  
    
      
        
          S
          ⁡
          
            (
            
              f
              k
            
            )
          
        
        =
        
          d
          ⁢
          
            
              
                
                
                  f
                  k
                
                
              
              2
            
            N
          
        
      
    
    
      
        (
        2
        )
      
    
  




Where fk is the fourier transform component of the topography signal.

In one embodiment, radiation collected by the scattered radiation collector 1930 may be used to measure surface roughness on a nano-scale, referred to herein as nano-roughness. The radial nano-roughness may be measured from the P-scatter radial (PscR) or S-scatter Radial (SscR) signals (when laser 1910A is active). The circumferential nano-roughness may be measured from the P-scatter Circumferential (Psc) signal or the S-scatter (Ssc) signal (when laser 1910B is active). Techniques for determining surface characteristics from the PscR, SscR, Psc, and Ssc signals are described in U.S. Pat. No. 6,717,671 to Meeks et al., the disclosure of which is incorporated by reference herein in its entirety.
In one embodiment, the system describe herein can measure spatial wavelengths from the scattered signal within a bandwidth of 0.22 μm to 1.8 μm, and from the radial specular signal and the circumferential specular signal within a bandwidth of 4.3 μm to 5000 μm. The noise floor for roughness is less than 0.1 angstroms, and the noise floor for waviness is less than 0.5 angstroms.
While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention.

## Claims

1. A system to inspect a surface, comprising:
an assembly to direct a first radiation beam onto a surface in a first plane of incidence;
a first detector to generate a first signal from a portion of the radiation reflected from the first radiation beam;
a first spatial filter interposed between the surface and the first detector;
a first ellipsoidal mirror to collect scattered light;
a second detector to generate a second signal from the scattered portion of the beam; and
a processor to generate, from the first and second signals, a data set representing one or more characteristics of the surface using the first and second signals;
an assembly to direct a second radiation beam onto the surface in a second plane of incidence, the second plane of incidence approximately orthogonal to the first plane of incidence;
a third detector to generate a signal from a portion of the radiation reflected from the second radiation beam;
a second spatial filter interposed between the surface and the third detector; and
wherein the processor generates, from the third signal, a data set representing one or more characteristics of the surface using the first signal and the third signal.

2. The system of claim 1, wherein the assembly to direct a first radiation beam onto a surface in a first plane of incidence, comprises:
a radiation source;
a polarizer;
a neutral density filter;
a half-wave plate; and
a reflecting device to direct the first radiation beam onto the surface in the first plane of incidence.

3. The system of claim 2, wherein the neutral density filter is moveable between a first position in the radiation path and a second position out of the radiation path.

4. The system of claim 1, further comprising a neutral density filter in the radiation path between the surface and the detector.

5. The system of claim 4, wherein the neutral density filter is moveable between a first position in the radiation path and a second position out of the radiation path.

6. The system of claim 1, further comprising a scattered radiation detector to generate a signal from a portion of the radiation from the first radiation beam scattered from the surface.

7. The system of claim 1, wherein the assembly to direct a second radiation beam onto a surface in a second plane of incidence, comprises:
a radiation source;
a polarizer;
a neutral density filter;
a half-wave plate; and
a reflecting device to direct the first radiation beam onto the surface in the second plane of incidence.

8. The system of claim 7, wherein the neutral density filter is moveable between a first position in the radiation path and a second position out of the radiation path.

9. The system of claim 1, further comprising means for changing a polarization characteristic of at least one of the first radiation beam or the second radiation beam.

10. The system of claim 9, wherein the means for changing a polarization characteristic of at least one of the first radiation beam or the second radiation beam comprises a rotatable half-wave plate.

11. The system of claim 1, wherein the first radiation beam comprises a component that is P-polarized, S-polarized, circularly or 45° linearly polarized with respect to the first plane of incidence.

12. The system of claim 1, wherein the second radiation beam comprises a component that is P-polarized, S-polarized, circularly or 45° linearly polarized with respect to the second plane of incidence.

13. The system of claim 1 wherein the first radiation detector is used to measure the topography in the direction perpendicular to the first plane of incidence.

14. The system of claim 1 wherein the third radiation detector is used to measure the topography in the direction perpendicular to the second plane of incidence.