Apparatus and method for measuring surface topography of an object

An apparatus for measuring surface topography of an object includes an optical arrangement capable of directing a first light beam at a surface of the object, providing a second light beam coherent with and spatially phase-shifted relative to the first light beam, and generating an interference beam from the second light beam and a reflection of the first light beam from the surface of the object. The apparatus further includes at least one line scan sensor for detecting and measuring the interference beam.

FIELD

The invention relates generally to techniques for measuring surface topography of objects. More specifically, the invention relates to a phase measurement interferometry method and apparatus for measuring surface topography of an object.

BACKGROUND

Substrates used in making devices such as flat panel displays, active electronic devices, photovoltaic devices, and biological arrays are typically required to have surfaces that are substantially free of defects and with flatness to within a few microns. Therefore, it is important that these surfaces can be inspected for defects and flatness relatively easily. Phase measurement interferometry (PMI) is an example of an optical interferometry technique for measuring surface topography. PMI generally involves creating interference patterns through interaction of light beams with the surface of an object and detecting the interference patterns, where the detected interference patterns are used to reconstruct the surface topography. PMI generally relies on area scan cameras to detect interference patterns. However, area-based PMI has limited use in high-speed inspection of large substrates, such as those used in flat panel displays. One challenge is that area scan cameras have a limited field-of-view. Another challenge is that area scan cameras are difficult to scale. In general, the larger the area scan camera, the more complex the area scan camera, resulting in long scanning time and high cost.

SUMMARY

In one aspect, the invention relates to an apparatus for measuring surface topography of an object. The apparatus comprises an optical arrangement capable of (i) directing a first light beam at a surface of the object, (ii) providing a second light beam coherent with and spatially phase-shifted relative to the first light beam, and (iii) generating an interference beam from the second light beam and a reflection of the first light beam from the surface of the object. The apparatus further includes at least one line scan sensor for detecting and measuring the interference beam.

In another aspect, the invention relates to a method of measuring surface topography of an object which comprises directing a first light beam at a surface of the object, providing a second light beam coherent with and spatially phase-shifted relative to the first light beam, producing an interference beam from the second light beam and a reflection of the first light beam from the surface of the object, and detecting and measuring the interference beam using at least one line scan sensor.

In yet another aspect, the invention relates to a method of measuring surface topography of an object which comprises directing a first light beam at the surface of the object, providing a second light beam coherent with and spatially phase-shifted relative to the first light beam, producing an interference beam from the second light beam and a reflection of the first light beam from the surface of the object, making multiple copies of the interference beam, passing each copy of the interference beam through one of a plurality of spatial phase splitters, and detecting and measuring the copies of the interference beam using a plurality of line scan sensors associated with the plurality of spatial phase splitters.

DESCRIPTION OF EMBODIMENTS

The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements.

FIG. 1is a block diagram of an apparatus100for measuring surface topography of a test object102. Apparatus100includes an interferometer105which measures interference beam patterns generated through optical interaction with a test surface104of the test object102. The interferometer105directs a light beam at the test surface104. The beam size is typically much smaller than the surface area of the test surface104. The interferometer105may be mounted on a translation stage176and translated across the test surface104for the purposes of obtaining a series of measured interferometer beam patterns from which a surface topography of the test surface104can be reconstructed. Alternatively, the test surface104may be translated relative to the interferometer105for the purposes of obtaining the series of measured interferometer beam patterns. In this case, the test object102would be coupled to a translation stage (not shown) to enable such relative motion between the test surface104and the interferometer105. The test surface104being interrogated by the interferometer105may be small or large in surface area. The test surface104may be flat and may or may not include surface defects. Interferometer105detects defects in the test surface104by measuring surface height variations between the test surface104and a reference surface (not shown inFIG. 1). The test object102having the test surface104may be a substrate for use in devices requiring substrates with high flatness and minimal surface defects, such as flat panel displays, active electronic devices, photovoltaic devices, biological arrays, and sensor arrays. Substrates for making devices such as flat panel displays may be very large, for example, 3 m×3 m. The test object102having the test surface104may be made of any material suitable for the intended application, such as glass, glass-ceramic, and plastic materials.

Interferometer105includes an interference beam generator106, a beam conditioning module108, and an imaging module110. The interference beam generator106contains an optical arrangement which directs a first light beam at the test surface104and produces an interference beam from a reflection of the first light beam and a second light beam, where the second light beam is coherent with and spatially phase-shifted relative to the first light beam. The coherence referred to herein and in subsequent paragraphs is temporal coherence. The phase shift of the second light beam varies according to the topography of the test surface104. The beam conditioning module108directs the interference beam produced by the optical arrangement in the interference beam generator106to the imaging module110. The beam conditioning module108may include any combination of optics, such as collimating lenses, apertures, and diffractive elements, for shaping the interference beam and focusing the interference beam onto the imaging module110. The imaging module110detects and measures the interference beam produced by the interference beam generator106. Apparatus100includes a data acquisition module112for collecting the measured data from the imaging module110. The data acquisition module112may include an input/output interface170for communication with the imaging module110, a data recorder172for recording the measured data, and a data processor174for processing the recorded data. The data processor174may execute a process which reconstructs the surface topography of the test surface104from the measured data.

Interferometer105may be of a Twyman-Green type, a Fizeau type, or other interferometer type suitable for phase measurement interferometry (PMI). However, in contrast to known PMI-based interferometers for measuring surface topography, interferometer105uses a system of linear optics. Interferometer105uses an imaging module110based on line scan sensor(s) to detect and measure multiple interferograms simultaneously from a single interference beam. For high-resolution measurements, the modules in interferometer105are designed such that the interference beams detected by the imaging module110have a profile that is substantially linear.

FIG. 2shows interferometer105in a Twyman-Green configuration. InFIG. 2, interference module106includes light source114which provides a light beam BI used in interrogating the test surface104. The light beam BI may be provided using active and/or passive components (not shown separately), which may be local or remote to the light source114. Where the active components are remote to the light source114, passive components such as lenses, mirrors, and optical fibers may be used to route the light beam from the remote location where it is generated to the light source114. The light beam BI provided by the light source114may be a low coherence laser beam or other low coherence light beam. In some embodiments, the optical arrangement of the interference beam generator106includes a beam shaper116for shaping the light beam BI into a desired shape. For enhanced performance with the linear imaging module110, the beam shaper116is preferably a line generator which shapes a nonlinear light beam, e.g., circular beam, into a substantially linear beam, e.g., line beam, highly-elliptical beam, or other high-aspect ratio beam. The beam shaper116may be, for example, a diffractive element or a holographic diffuser.

The optical arrangement of the interference beam generator106further includes a polarization beam splitter122and may further include lens125. In interference beam generator106, light beam BI passes through the beam shaper116and is focused onto the polarization beam splitter122by the lens125. The polarization beam splitter122splits light beam BI into two orthogonally polarized beams BT and BR. In general, light beam BR is coherent with and spatially phase-shifted or phase-separated relative to light beam BT. The optical arrangement of the interference beam generator106includes a reference object118having a reference surface120, which is flat and has a known surface topography. Typically, the reference object118is a front-surface mirror, or the reference surface120may be a surface made of or coated with a reflective material. The light beams BT and BR produced by the polarization beam splitter122are directed at the test surface104and reference surface120, respectively. The light beams BT and BR strike the test surface104and reference surface120, respectively, and are reflected back to the polarization beam splitter122as reflected light beams BTRand BRR, respectively. The path lengths of the reflected light beams BTRand BRRare influenced by the topography of the test surface104and reference surface120, respectively.

For a low coherence system, the polarization beam splitter122is positioned relative to the test surface104and reference surface120such that the optical length between the polarization beam splitter122and each of the test surface104and reference surface120is within the optical coherence length of the light source114. Coherence length is the optical distance two light beams can travel before their phase relationship becomes random (and thus no interference pattern will be generated). When light beam BT is incident upon the test surface104, part of light beam BT is reflected back into the interferometer105as reflected light beam BTR. Reflected light beam BTRrecombines with the reflected light beam BRRfrom the reference surface102and produces an interference beam IB which is detected at the imaging module110. If the coherence length of the light source114is larger than twice the optical thickness of the test object102, then the portion of the light beam BT which passes through test surface104and onto the back surface103of the test object102will also reflect back into the interferometer105and recombine with the reflected light beams BTRand BRR, contributing to the interference beam IB. In order to minimize or prevent contribution of the back surface reflection to the interference pattern, a light source114with a low coherence length is desired. In general, and preferably, the coherence length of the light source114is less than the optical thickness of the test object104. In general and more preferably, the coherence length of the light source114is less than twice the optical thickness of the test object104. Optical thickness of the test object102is the product of the thickness (T) of the test object102, measured along the incidence direction of light beam BT (also known as measurement arm of interferometer105), and the refractive index of the test object102.

Quarter-wave plates124,126are disposed in the optical paths between the polarization beam splitter122and the test and reference surfaces104,120, respectively. The quarter-wave plates124,126change linearly polarized light to circularly polarized light and vice versa. In the forward direction, the quarter-wave plates124,126function such that light beams BT and BR which are linearly polarized at the polarization beam splitter122are circularly polarized at the test and reference surfaces104,120. In the reverse direction, the quarter-wave plates124,126function such that the reflected light beams BTRand BRRwhich are circularly polarized at the test and reference surfaces104,120are linearly polarized at the polarization beam splitter122. In some embodiments, a focusing lens128is used to focus the light beam BT from polarization beam splitter122or quarter-wave plate124onto the test surface104. A focusing lens (not shown) may be similarly used to focus beam BR from polarization beam splitter122or quarter-wave plate126onto the reference surface120.

The reflected light beams BTRand BRRreceived at the polarization beam splitter122form recombined beam RB upon exiting the polarization beam splitter122. Recombined beam RB is received in the beam conditioning module108and exits the beam conditioning module108as interference beam IB. Imaging module110detects and measures interference beam IB is detected and measured. As previously mentioned, the beam conditioning module108includes optics for conditioning and focusing beams onto the imaging module110. In the example shown inFIG. 2, the beam conditioning module108includes a focusing lens123for focusing the recombined beam RB onto the imaging module110. The beam conditioning module108may optionally include optics module149for optionally making copies of the recombined beam and providing the copies of the recombined beam to the imaging module110. Optics module149may include, for example, a diffractive element or a holographic diffuser. Optics module149is useful when the imaging module110includes a plurality of line scan sensors for measuring interference beams, as will be described in detail later.

FIG. 3shows interferometer105in a Fizeau configuration, such as described in International Publication WO 2006/080923. InFIG. 3, interference beam generator106includes light source114, as described above, which provides a light beam BI used in interrogating the test surface104of test object102. The light beam BI provided by light source114passes through beam shaper116, as described above, half-wave plate163, and beam expansion lens162, and is then incident on beam splitter160. Light beam BI upon striking the beam splitter160is reflected towards the test surface104of the test object102and a reference surface167of a reference object169. Light beam BI from beam splitter160may be focused onto surfaces104,167by collimation lens165. In the configuration shown inFIG. 3, the test surface104and reference surface167are inline and tilted relative to each other so that beams BTRand BRRreflected from these surfaces are spatially separate. As in the previous example, light beams BTRand BRRare also coherent. In general, and preferably, the coherence length of the light source114is less than the sum of the optical thickness of the test object104and the reference object169. In general, and more preferably, the coherence length of the light source114is less than twice the sum of the optical thickness of the test object104and the reference object169. Optical thickness of the test object102has been defined above. Optical thickness of the reference object169is the product of the thickness of the reference object169(measured along the measurement arm of interferometer105) and the refractive index of the reference object169. In the configuration shown inFIG. 3, the reference object169is made of a transparent material. The reference object169could be a transparent lens with a flat reference surface167, for example. The reflected beams BTRand BRRpass through the beam splitter160and are received in the beam conditioning module108. In the beam conditioning module108, the reflected light beams BTRand BRRconverge at a point in the focal plane of a collimation lens164. A spatial polarization filter166is arranged at the focal plane of the collimation lens164such that the reflected light beams upon leaving the collimation lens164have orthogonal polarization states. The light beams having orthogonal polarization states may additionally pass through an imaging lens171and a polarization beam splitter173and exit the beam conditioning module as interference beam IB. The imaging module110detects and measures the interference beam IB.

Referring toFIGS. 1-3, imaging module110is arranged downstream of the beam conditioning module108. In some embodiments, as illustrated inFIG. 4, imaging module110includes a linear pixelated phase mask130as a spatial phase splitter. In some embodiments, the linear pixelated phase mask130is a linear array of polarization elements134in a repeating pattern. In some embodiments, the arrangement of the polarization elements134in the linear array is such that no two neighboring polarization elements134have the same polarization angles. Herein, polarization angles are relative to the detection axis or reference arm of the interferometer (105inFIGS. 1-3). Each repeating unit132includes polarization elements134having different polarization angles. In some embodiments, each repeating unit includes four polarization elements134, each having a polarization angle selected from 0°, 90°, 180°, and 270°. In some embodiments, the polarization elements134within each repeating unit are arranged such that the difference in polarization angle between neighboring polarization elements134is 90°. As an example, a repeating unit132may include a sequential arrangement of polarization element134ahaving polarization angle 0°, polarization element134bhaving polarization angle 90°, polarization element134chaving polarization angle 180°, and polarization element134dhaving polarization angle 270°.

FIG. 5shows the imaging module110ofFIG. 4with a quarter-wave plate136preceding the linear pixelated phase mask130. The quarter-wave plate136converts a circularly polarized input beam into a linearly polarized input beam and vice versa and is useful when the input beam IB into the imaging module110is not circularly polarized or is linearly polarized. InFIGS. 4 and 5, the linear pixelated phase mask130interrogates intensity of the input beam at different phase delays or phase shifts. The number of phase shifts corresponds to the number of different polarization states represented within the linear pixelated phase mask130. For example, where the linear pixelated phase mask130has a repeating unit of polarization elements and each repeating unit includes four different polarization angles, the number of phase delays interrogated in the input beam would be four. This would allow the imaging module110to detect and measure four interferograms simultaneously.

InFIGS. 4 and 5, the imaging module110further includes a line scan sensor146associated with the pixelated phase mask130for detecting and measuring interferograms passing through the pixelated phase mask130. The line scan sensor146includes a linear array of photo elements148. In the particular arrangements shown inFIGS. 4 and 5, there is a one-to-one mapping between the photo elements148of the line scan sensor146and the polarization elements148of the linear pixelated phase mask130. The line scan sensor146detects and measures the intensity of the interference beam passing through the linear pixelated phase mask130at the different polarization states and phase delays represented in the linear pixelated phase mask130.

FIG. 6depicts an example where the imaging module110includes linear polarization arrays138a,138b,138c,138d. In general, the imaging module110may have two more linear polarization arrays, with at least three linear polarization arrays being generally preferred. Each of the linear polarization arrays138a,138b,138c,138dincludes a set of polarization elements. In one example, the linear polarization array138aincludes polarization elements having a first polarization angle, the linear polarization array138bincludes polarization elements having a second polarization angle, the linear polarization array138cincludes polarization elements having a third polarization angle, and the linear polarization array138dincludes polarization elements having a fourth polarization angle, where the first, second, third, and fourth polarization angles are different. As an example, the first, second, third, and fourth polarization angles are selected from 0°, 90°, 180°, and 270°. In this arrangement, each of the linear polarization arrays138a,138b,138c,138dinterrogates the intensity of the input beam at different phase delays or phase shifts. As inFIG. 5, each of the linear polarized array138a,138b,138c,138dmay be preceded by a quarter-wave plate if the input beam is linearly polarized. The polarization arrays138a,138b,138c,138dwork similarly to the linear pixelated phase mask (130inFIGS. 4 and 5), except that each polarization array is dedicated to a single polarization state. Four copies of the input or interference beam IB are required for the four polarization arrays138a,138b,138c,138d. The four copies of the input beam IB can be provided by optics module (149inFIG. 2), such as diffractive element or holographic diffuser, in the beam conditioning module (108inFIG. 2). The optics module providing the four copies of the input beam IB could also be positioned at the input end of the imaging module110, rather than in the beam conditioning module.

InFIG. 6, the imaging module110includes line scan sensors146a,146b,146c,146dassociated with the linear polarization arrays138a,138b,138c,138d, respectively. The line scan sensors146a,146b,146c, and146dare similar to the line scan sensor146described inFIGS. 4 and 5and detect interferograms passing through the linear polarization arrays138a,138b,138c,138d, respectively.

FIGS. 7 and 8show a linear prismatic phase shifter160that could be used as a spatial phase splitter and in place of the linear pixelated phase mask (130inFIGS. 4-5and138a-dinFIG. 6). The linear prismatic phase shifter160includes beam splitter162, polarization beam splitter164, beam splitter166, and prism or mirror168arranged in a linear stack. Adjacent to the stack of splitters160,162,164and prism or mirror168are bare plate162a, quarter-wave plate164a, bare plate166a, and quarter-wave plate168a, respectively, arranged in a linear stack. Adjacent to the stack of plates are triangular prisms162b,164b,166b, and168barranged in a linear stack. Referring toFIG. 8, input beam IB is received at beam splitter162. Beam splitter162splits input beam IB into two light beams I1and I2. Beam I1passes through the bare plate162ainto the prism162b. A portion of the line scan sensor (146inFIG. 7) would be aligned with the prism162bto receive the output beam from the prism162b. Light beam I2travels to polarization beam splitter164, where it is again split into two light beams I21and I22having orthogonal polarization states. Light beam I21passes through the quarter-wave plate164ainto the prism164b. A portion of the line scan sensor (146inFIG. 7) receives the output beam from prism164b. Light beam I22travels to beam splitter166, where it is split into two light beams I221and I222. Light beam I221passes through the bare plate166band prism166band is received at the line scan sensor (146inFIG. 7). Light beam I222travels to prism or mirror168. A reflected beam I2221of light beam I222passes through quarter-wave plate168aand prism168b. A portion of the line scan sensor (146inFIG. 7) receives the output of prism168b. The beam splitters and polarization beam splitters162,164,166,168are preferably designed such that the output beams are closely matched in intensity and have similar signal-to-noise ratios.

Referring toFIGS. 1-3, the interference beam generator106generates an interference beam through optical interaction with the test surface104of the test object102. The interference beam IB passes through the beam conditioning module108and is focused onto the imaging module110. Inside the imaging module110, the input beam IB is interrogated at different phase delays by the linear pixelated phase mask (130inFIGS. 4 and 5) or the linear polarization arrays (138a-dinFIG. 6) or the linear prismatic phase shifter (160inFIGS. 7 and 8), collectively referred to as spatial phase splitter(s), depending on which configuration of the imaging module110is used. Inside the imaging module110, the line scan sensor (146inFIGS. 4 and 5) or a plurality of line scan sensors (146a-dinFIG. 6) detects and measures the interferograms passing through the linear pixelated phase mask (130inFIGS. 4 and 5) or the linear polarization arrays (138a-dinFIG. 6) or the linear prismatic phase shifter (160inFIGS. 7 and 8), collectively referred to as spatial phase splitter(s), depending on which configuration of the imaging module110is used. The interferometer105is translated linearly across the test surface164while the interference beam generator106generates an interference beam at each position of the interferometer105across the test surface104. Alternatively, the test surface104may be translated linearly relative to the interferometer105while the interferometer beam generator106generates the interference beam. The interference beams generated by interferometer105are detected and measured by the imaging module110as previously explained. For each interference beam generated by the interference beam generator106, the imaging module110detects and measures a plurality of interferograms from the interference beam simultaneously. The measured data may be transmitted to the data acquisition module (112inFIG. 1) and processed to reconstruct the surface topography using well known techniques, such as described in Cheng and Wyant, Applied Optics, 24, p. 3049 (1985).

The linearity of interferometer105facilitates scalability of the surface topography measurement system. To measure the surface topography of a test surface, the size of the interferometer105only needs to match the test surface along one dimension or a first dimension. The full surface topography is acquired by relative motion between the linear interferometer105and the test surface along a second dimension substantially orthogonal to the first dimension while the interferometer105makes measurements. Linear scaling of the components of the imaging module110can be done relatively easily and cheaply because all that is required is addition of elements in a linear direction to the spatial phase splitter (130inFIGS. 4 and 5;138a-dinFIG. 6,160inFIGS. 7 and 8) and the line scan sensor (146inFIGS. 4,5, and7;146a-dinFIG. 6). Alternatively, a linear array of interferometers105can be used to cover the test surface along a first linear direction, and relative motion between the linear array of interferometers105and the test surface along a second linear direction substantially orthogonal to the first linear direction can be used to generate the two-dimensional surface topography. Because of the linearity of the interferometer105, alignment of the interferometer105with the test surface is relatively simple. This facilitates deployment of the interferometer105for online measurements in a manufacturing environment.

Apparatus100can be used for online measurement of the surface topography of an object. The measured surface topography can be used to detect the presence of defects on the measured surface of the object. In objects such as substrates used in making flat panel displays, these defects may be on the order of a few hundred microns to a few millimeters long and may occur anywhere on the measured surface of the object. The measured surface itself may be very large, for example, 3 m×3 m. With defects less than 1 micron in height, vibrations is of concern. Apparatus100uses instantaneous phase measurement interferometry (i-PMI) to substantially eliminate vibration effects on measurements made along a line of the test or measured surface. In i-PMI, multiple interferograms are extracted from a single interference beam. The interference beam itself is produced within a timeframe in which the test surface is practically “frozen.” At 5 microsecond interferometer exposure, test surface vibrations greater than 50 micrometer amplitude and faster than 20 Hz frequencies will be practically frozen, allowing surface topography to be measured accurately down to 1 nm or less. In addition, the two-dimensional surface topography can be generated with minimized local vibration effects by moving the interferometer105relative to the measured surface at a high scan rate. At a high scan rate, vibration is frozen both for measurement along a line and in a local area. The two-dimensional surface topography would then be a concatenation of local topography measurements, with each local topography measurement captured during a timeframe when the test surface is practically “frozen.” Within each local topography measurement, detection of defects would be consistent and reliable. There are various methods for analyzing a large topography map for local defect signatures. One method includes transforming the large topography map into a Fourier map of the spatial frequencies (or temporal frequencies since the scan rate is known) and then applying a high pass filter or band-pass filter in order to isolate the defects.

Apparatus100may also be used for measuring surface topography while forming a sheet of material using fusion draw processes, such as described in U.S. Pat. Nos. 3,338,696 and 3,682,609 issued to Dockerty, herein incorporated by reference. In fusion draw processes, the sheet of material may be subject to motions such as vibration while being drawn. High-resolution measurements can be made if the interferometer105is swept across the sheet of material at a speed faster than the vibration of the sheet of material.