EP1573287A2 - Mesure de birefringence hors plan - Google Patents

Mesure de birefringence hors plan

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Publication number
EP1573287A2
EP1573287A2 EP03799977A EP03799977A EP1573287A2 EP 1573287 A2 EP1573287 A2 EP 1573287A2 EP 03799977 A EP03799977 A EP 03799977A EP 03799977 A EP03799977 A EP 03799977A EP 1573287 A2 EP1573287 A2 EP 1573287A2
Authority
EP
European Patent Office
Prior art keywords
sample
plane birefringence
birefringence
light
angled
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP03799977A
Other languages
German (de)
English (en)
Other versions
EP1573287A4 (fr
Inventor
Baoliang Wang
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hinds Instruments Inc
Original Assignee
Hinds Instruments Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US10/364,006 external-priority patent/US7016039B2/en
Application filed by Hinds Instruments Inc filed Critical Hinds Instruments Inc
Publication of EP1573287A2 publication Critical patent/EP1573287A2/fr
Publication of EP1573287A4 publication Critical patent/EP1573287A4/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/21—Polarisation-affecting properties
    • G01N21/23—Bi-refringence
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J4/00—Measuring polarisation of light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light

Definitions

  • TECHNICAL FIELD [0001] This application relates to measurement of birefringence properties of optical material, and primarily to the measurement of out-of-plane birefringence of such material.
  • Birefringence causes different linear polarizations of light to travel at different speeds through the material. These different polarizations are most often considered as two components of the polarized light, one component being orthogonal to the other.
  • Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces applied to the material.
  • the induced birefringence may be temporary, as when the material is oscillated, or the birefringence may be residual, as may happen when, for example, the material undergoes thermal stress during production of the material.
  • Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam that traverses a sample of the optical material. If the incident light beam is linearly polarized, the two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance.
  • the fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm).
  • An "average" birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample.
  • the two orthogonal, polarized beam components mentioned above are parallel to two orthogonal axes associated with the optical material, wliich axes are refened to as the "fast axis" and the “slow axis.”
  • the fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. Therefore, a complete description of the retardance of a sample along a given optical path requires specifying both the magnitude of the retardance and the relative angular orientation of the fast (or slow) axis of the sample.
  • the determination of the sample's birefringence is "in-plane," meaning that the detennination essentially represents the difference between the indices of refraction of two orthogonal axes in a plane of the sample, that plane being normal to the incident light beam.
  • the effect of birefringence on displayed visible light may be to reduce contrast or alter colors.
  • the extent or magnitude of birefringence is a function of the incident angle of the light under consideration. For example, increasing (from normal) the viewing angle of a LCD panel will increase the birefringence effect on the light emanating from the panel and, without compensation, reduce the perceived quality of the visible light by reducing contrast and/or altering colors.
  • Transparent polymer films have been developed for use with LCD panels for the purpose of compensating for the just-noted birefringence variations attributable to viewing angle, h short, these films possess birefringence characteristics that compensate for the birefringence of the LCD panel and thus provide a wide viewing angle without significant loss of contrast or color.
  • This birefringence measure can be refened to as "vertical” or "out-of-plane" birefringence.
  • in-plane and out-of-plane birefringence in terms of a Cartesian coordinate system. Accordingly, if the normal-incidence light is considered to travel in a direction parallel to the Z-axis of such a coordinate system, the in-plane birefringence occurs in the XY plane of the sample. Out-of-plane birefringence is in a plane perpendicular to the in-plane birefringence, thus occurring in the XZ or YZ plane.
  • the just mentioned intrinsic birefringence present in the [110] axis of the crystal is out-of-plane birefringence relative to the light that is normal to the [110] surface, and thus amenable to the measurement techniques of the present invention as summarized next.
  • the present invention is directed to precise measurement of out-of-plane birefringence properties of samples of transparent optical material.
  • two angled-apart light beams are passed through a selected location of a sample.
  • One of the beams is directed to be normally incident to the sample surface.
  • the characteristics of the beams are detected after passing through the sample, and the information obtained is processed to determine the out-of-plane birefringence.
  • FIG. 1 is a diagram of one embodiment showing a prefened a ⁇ angement of the optical components of a system that is used for measuring out-of-plane birefringence in accordance with the present invention.
  • Fig. 2 is a block diagram of the signal processing components of the system depicted in Fig. 1.
  • FIG. 3 is a diagram of another embodiment showing another anangement of optical components of a system for measuring out-of-plane birefringence in accordance with the present invention.
  • Fig. 4 is a block diagram of the signal processing components of the system depicted in Fig. 3.
  • Fig. 5 is diagram of another a ⁇ angement of optical components of a system for measuring out-of-plane birefringence in accordance with the present invention.
  • Fig. 6 is a diagram of yet another anangement of optical components for measuring out-of-plane birefringence in accordance with the present invention.
  • Fig. 7 is a three-part diagram showing an embodiment wherein a sample optical element that is held in an inclined or tilted orientation is moved relative to the beam path.
  • Fig. 8 is an enlarged detail diagram of the embodiment of Fig. 7 illustrating movement of purging gas delivery tubes relative to a movable, tilted sample.
  • Fig. 9 is a diagram showing the tilting of an optical element and the related effect on the light beam path through the sample.
  • the out-of-plane birefringence occurring at a location in a sample is determined by passing two angled-apart light beams through that location. One of the beams is directed to be normal to the surface of the sample. Thus, upon exiting the sample, that light beam provides information relating to the in-plane birefringence of the sample.
  • the other light beam is directed to be oblique to the sample surface and thus exits the sample with characteristics that provide information relating to the retardance occurring along the (refracted) incident path of the second beam through the sample.
  • the information provided by the two angled-apart beams is detected and processed to provide, in addition to the in-plane birefringence of the sample, the out-of-plane birefringence of the sample, as will be described more fully below.
  • normal and oblique detection module 16 depict a normal source module 10, a normal detection module 12, an oblique source module 14, and an oblique detection module 16.
  • normal and oblique are used as adjectives here to respectively distinguish modules associated with a light beam directed through the sample at a normal or zero- angle incidence from modules associated with a beam directed through the sample at an oblique angle, as explained more fully below.
  • the components of the normal source module 10 include a HeNe laser as a light source 20. That laser has a wavelength of 632.8 nanometers (nm). It is contemplated that the wavelength of the source light can be selected to best match the particular application.
  • the beam “B" emanating from the source 20 has a cross sectional area or "spot size" of approximately 1.0 millimeter (mm).
  • the source light beam is directed to be incident upon a polarizer 22 that is oriented with its polarization direction at +45° relative to a baseline axis.
  • a high-extinction polarizer such as a Glan-Thompson calcite polarizer, is prefened. It is also prefened that the polarizer 22 be secured in a precision, graduated rotator.
  • the polarized light emanating from the polarizer 22 is incident upon the optical element 24 of a photoelastic modulator 25.
  • the photoelastic modulator (hereafter refened to as a "PEM”) is one manufactured by Hinds Instruments, Inc., of Hillsboro, Oregon. It is noteworthy here that although a PEM is prefened, other mechanisms could be used for modulating the polarization of the source light.
  • the PEM 25 should be configured to eliminate residual birefringence that may be otherwise produced by the forces present in supporting the optical element 24 of the PEM.
  • the PEM 25 has its birefringent axis oriented at 0° and is controlled by a controller 84 that imparts an oscillating birefringence to the optical element 24, preferably at a nominal frequency of 50 kHz.
  • the controller drives two quartz transducers between which the optical element 24 is adhered.
  • the oscillating birefringence of the PEM 25 introduces a time-varying phase difference between the orthogonal components of the polarized light that propagates through the PEM.
  • the phase difference represents the retardation introduced by the PEM.
  • the retardation is measurable in units of length, such as nanometers.
  • the PEM is adjustable to allow variation of the amplitude of the retardation introduced by the PEM. In the case at hand, the retardation amplitude is selected to be 0.383 waves (242.4 nm).
  • the beam of light "BI” propagating from the PEM 25 is directed through a transparent sample 26.
  • the sample is supported in the path of the beam by a sample holder 28 that is controllable for periodically moving the sample in a translational sense along orthogonal (X and Y) axes (here considering light beam "BI” to be traveling in the Z axis).
  • a prefened holder would be one that comprises a plurality of spaced-apart, small- diameter (for example 1 or 2 mm) wires tightly strung between two rigid support members.
  • the wires could be stainless steel wire rope that may or may not be coated with a low-friction coating. Nylon-coated wire rope and a number of other materials may also be used for the wires.
  • the wire material, the tension applied to the wire, and the spacing between each wire is selected so that, depending on the weight of the sample, the sample is held in a plane without any bending stress, which might be introduced if the sample were permitted to sag.
  • the spacing between individual wires in the holder 28 is as large as possible (depending upon the unit weight and flexibility of the sample 26) so that, as just mentioned, the space occupied by the wires underlying the sample is minimized.
  • the sample holder 28 may be driven by a conventional X-Y stage mechanism to translate the sample as mentioned above and thereby enable scanning of the sample 26 with the beam "BI" at a plurality of locations across the area of the sample.
  • the beam "BI” is affected by the in-plane birefringence of the sample 26 as the beam passes through the sample. The affect, as discussed above is to produce retardance in the beam.
  • the in-plane birefringence that produces this retardance is determined in accordance with the present invention, as explained more below, and is also used for determining out-of-plane birefringence.
  • the beam "BI" that passes out of the sample 26 is separated into two parts having different polarization directions and thereby defining two channels of information for subsequent processing.
  • a prefened mechanism for separating the beam "BI,” includes a beam- splitting minor 30 that is a component of the normal detection module 12 and is located in the path of that beam (hereafter refened to as the incidence path).
  • the beam-splitting minor 30 is preferably made of Schott Glass type SF-57 glass. This glass has an extremely low (near zero) stress-optic coefficient. It is noteworthy here that, although a beam-splitting minor is prefened, one can substitute other mechanisms (such as a flipper minor anangement) for separating the beam "BI" into two parts.
  • Beam “B 1 " passes completely through the beam-splitting minor 30 and, now designated "Bll," enters a detector assembly 32 for detection.
  • the detector assembly 32 includes a compact, Glan-Taylor type analyzer 42 that is ananged such that its polarization direction is at -45° from the baseline axis. From the analyzer 42, the beam “Bll” enters a detector 44, which is described more below. [0040]
  • the reflective surface of the beam-splitting minor 30 faces upwardly, generally toward the sample 26. The minor is mounted so that the incidence path (that is, the optical path of the beam "BI” as it propagates from the sample 26) is nearly normal to the reflective surface of the minor. In a prefened embodiment, the angle made between the beam "BI” traveling along the incidence path and the beam part "B1R" that is reflected from the minor 30 is greater than 0° but less than 10°.
  • the reflected part of the beam “B1R” is incident upon another detector assembly 50. That assembly 50 is configured and a ⁇ anged to be adjacent to the incident beam “BI” and located to receive the reflected beam “B1R.”
  • the components of detector 50 are compactly integrated and housed to include a Glan-Taylor type analyzer 74 that is a ⁇ anged so that its polarization direction is 0°, parallel with the birefringence axis ofthe PEM 25.
  • a na ⁇ ow-band interference filter 77 that permits passage of polarized laser light but blocks unwanted room light from reaching a detector 76.
  • the detector is preferably a photodiode that is stacked above the filter.
  • the photodiode detector 76 is the prefened detection mechanism and produces as output a cu ⁇ ent signal representative of the time varying intensity of the received laser light.
  • the detected laser light is that of the beam "B1R," which is the reflected part of the beam that propagated through the sample 26.
  • the photodiode output of the detector assembly 50 is delivered to a preamplifier that is carried on an associated printed circuit board (not shown) that is part of the detector assembly 50.
  • the preamplifier provides output 75 (Fig. 2) to a phase sensitive device (preferably a lock-in amplifier 80, or comparable computer-based digital signal processing components) in the form of a low-impedance intensity signal NAC1R, and a DC intensity signal NDC1R, which represents the time average of the detector signal.
  • the other detector assembly 32 (Fig. 1) mentioned above and to which is directed the non-reflected part "Bll” of the beam “BI” is, except in two respects, the same construction as the just described assembly 50.
  • the detector assembly 32 is ananged so that the polarization direction of analyzer 42 is oblique to the polarization direction of the analyzer 74 in the other detector assembly 50.
  • the analyzer 42 is positioned with its polarization direction at -45°.
  • the photodiode of detector assembly 32 produces as output a cu ⁇ ent signal representative of the time varying intensity of the received laser light, which is the non-reflected part "Bll" of the beam "BI” that propagated through the sample 26.
  • the photodiode output of the detector assembly 32 is delivered to a preamplifier, which provides its output 79 to the lock-in amplifier 80 (Fig. 2) in the form of a low-impedance intensity signal NAC1, and a DC intensity signal NDC1, which represents the time average of the detector signal.
  • the lock-in amplifier 80 is provided with two channels of input. One channel conesponds to the output of detector assembly 32, and the other channel conesponds to the output of detector assembly 50.
  • the intensity info ⁇ nation received by the lock-in amplifier via the first channel -because of the a ⁇ angement of the -45° analyzer 42- relates to the 0° or 90° component of the retardance induced by the sample 26.
  • the intensity information received on the second channel of the lock-in amplifier 80 -as a result of the anangement of the 0° analyzer 74- relates to the 45° or -45° component of the retardance induced by the sample.
  • this information is combined in an algorithm that yields an unambiguous determination of the magnitude of the overall retardance induced in beam "BI" (i.e., the normal- incidence beam) at the scanned location in the sample, as well as the orientation of the fast axis at that location in the sample.
  • BI overall retardance induced in beam
  • the lock-in amplifier 80 takes as its reference signal 82 the oscillation frequency applied by the PEM controller 84 to drive the optical element 24 of the PEM 25.
  • the lock-in amplifier 80 communicates with a digital computer 90 to provide, for a location on the sample, the values received on the two channels mentioned above, which can be designated channel 1 and channel 2.
  • the intensity signals on the detectors in channels 1 and 2 are derived as follows:
  • ⁇ is the PEM's time- varying phase retardation
  • 5 N is the magnitude of the sample's retardance as respects beam "BI" (the normal-incidence beam)
  • p is the azimuth of the fast axis of the sample's retardance.
  • the Mueller matrix for a linearly birefringent sample ( ⁇ , p) used in the derivation has the following generalized fonn:
  • Jo is the 0 th order of the Bessel function
  • J 2k is the (2k)th order of the Bessel function.
  • I chl 1F sin ⁇ cos( 2p)2J. ( ⁇ 0 ) sin( ⁇ t)
  • I ch2, i F sin ⁇ sm( 2p)2J. (A Q ) sm( ⁇ t) eqn - (4)
  • the IF signal is dete ⁇ nined using the lock-in amplifier 80 that is referenced at the PEM's first harmonic.
  • the lock-in amplifier will exclude the contributions from all harmonics other than IF.
  • the output from the lock-in amplifier 80 for the two channels is:
  • I eh l (lF ) ⁇ N ⁇ s( 2 p)2J l ( 0 )j2
  • I cl F ⁇ N s ⁇ 2 )2J x ⁇ Q ) 2
  • Equation (5) All terms appearing at a frequency other than the PEM's first harmonic are neglected in obtaining equations (5).
  • the validity of equations (5) for obtaining the IF NAC signal is further ensured as a result of the approximation that sin 2 ( ⁇ ⁇ /2) « 0 when ⁇ N is small. This applies for low-level retardance of, for example, less than 20 nm.
  • R c/ , 7 and R c/ , 2 are the determined quantities from the two channels.
  • the efficiency of the PEM for generating the IF signal is about 90% of its maximum.
  • Equations (8) are compiled in a program ruiining on the computer 90 and used to determine the magnitude and orientation of such retardance at the selected location in the sample through which the angled-apart beams are transmitted.
  • a conection for the e ⁇ ors is made by subtracting the e ⁇ or values for each channel, h principle, this procedure will provide a method of self- calibration of the system. It is, however, prudent to compare the system measurement of a sample with the measurement obtained using other methods [0064]
  • the value of the retardance ⁇ N induced by the in-plane birefringence of the sample is used with the simultaneously detected measure of retardance imparted into the other light beam, shown as "B2" in Fig. 1. As mentioned above, that beam "B2" is directed to be oblique to the surface of the sample 26.
  • Beam “B2" thus exits the sample with characteristics that provide information relating to the retardance occurring along the (refracted) incident path of that beam “B2" through that sample.
  • the information thus provided by the two angled-apart beams "BI” and “B2” is detected and processed to provide, in addition to the in-plane birefringence of the sample, the out-of-plane birefringence of the sample.
  • the oblique source module 14 and the oblique detection module 16 respectively match the normal source module 10 and the normal detection module 12.
  • oblique source module 14 includes a light source 220, polarizer 222, and PEM 225 that function in the same manner as the light source 20, polarizer 22, and PEM 25 of no ⁇ nal source module 10.
  • the oblique detection module 16 includes abeam-splitting minor 230 and detector assemblies 232, 250 that function in the same manner as the beam-splitting minor 30 and detector assemblies 32, 50 of the normal detection module 12.
  • beam “B2" is thus divided into two parts: “B2F and “B2R” and processed in a like manner as beam parts “Bll” and B1R” in the normal detection module 12 [0066]
  • the primary difference between the normal modules 10, 12 and the oblique modules 14, 16 is that the oblique modules are used for providing and detecting the beam "B2" that propagates through the sample 26 at an angle "A" in Fig.
  • the oblique source module 14 is, in this embodiment, mounted away from the normal source module 10 and tilted by an amount that produces the angled-apart beams "BI" and "B2" propagating through the same location in the sample.
  • the angle "A” is selected to be 30 degrees. Since the out- of-plane birefringence calculation described below involves information derived from both beams "BI" and "B2,” it is preferred that the angle “A” be small enough to ensure that the sample location through which the oblique-angled beam “B2" penetrates is substantially aligned with, and not a significantly different in size than, the location through which the normal-incidence beam "BI" penetrates. Diverting the beams by an angle of 30 degrees addresses these considerations.
  • ⁇ N represents the magnitude of the retardance, in nanometers, of the normal-incidence beam "BI" which is considered in the Z-axis.
  • the in-plane birefringence is defined as:
  • ny and nx are, respectively, the indices of refraction of the sample in the orthogonal axes, X and Y, that are perpendicular to the beam direction.
  • d is the sample thickness, typically measured in micrometers and, thus, multiplied here by 1000 to match the nanometer dimension of the retardance measure and thereby yield the dimensionless measure of in-plane birefringence of equation (9).
  • ⁇ N (n ⁇ - n x )- d ⁇ 000 eqn. (10)
  • the out-of-plane birefringence is denoted as ⁇ rivi — ⁇ — ll ⁇ .
  • the fast axis p of the sample is calculated as shown in equations (8). Where necessary, this information is used to ensure that the sample birefringence (fast) axis and the measurement components birefringence (fast) axis are in alignment.
  • ⁇ M000 ⁇ n ⁇ [n 7 sin 2 ⁇ + n y cos 2 ⁇ )- n v • • eqn. (12) 0 L z x ' Y i cos ⁇
  • the present system is used to determine an out-of-plane birefringence for a different vertical plane, that being in the YZ plane of the sample (that is, the plane no ⁇ nal to the XZ plane) as:
  • the sample 26 could be rotated in its XY plane, or a third source and detection pair could be employed for ca ⁇ ying out measurements of out- of-plane birefringence in more that one vertical plane as just discussed.
  • the in-plane birefringence is negligible as compared to oblique birefringence, the requirement of coincidence of the X-axis and Y- axis with the birefringence axes (fast and slow axes) and the birefringence measurement system is not necessary.
  • Figs. 3 and 4 respectively show a diagram of another embodiment of the present invention and a block diagram of the signal processing components of the system depicted in Fig. 3.
  • This is a dual-PEM, single-detector embodiment that employs a different anangement of source and detector components for determining the normal ⁇ and oblique ⁇ o retardance measurements described above, but otherwise calculates the out-of-plane birefringence values in the manner as just discussed.
  • the normal source module 310 includes a light source 322, a polarizer 324 oriented at +45 degrees, and a PEM 326 oriented at 0 degrees.
  • the normal detection module 312 includes a second PEM 328 that is set to a modulation frequency that is different from the modulation frequency of the first PEM
  • the normal detection module 312 also includes an analyzer 330 at 0 degrees and a detector assembly 332.
  • source 322 is a polarized He-Ne laser at 632.8 mn wavelengths.
  • the polarizer 324 and analyzer 330 are each a Glan-Thompson-type polarizer.
  • a Si-photodiode detector 344 also is used in this embodiment.
  • Both PEMs 326, 328 are bar-shaped, fused silica models having two transducers. The transducers are attached to the fused silica optical element with soft bonding material. To minimize birefringence induced in the optical element, only the transducers are mounted to the
  • the two PEMs 326, 328 have nominal resonant frequencies of 50 and 55
  • Each lock-in amplifier referenced at a
  • IF fundamental modulation frequency
  • the DC signal is received by the lock-in amplifier 340 after the signal from the detector assembly 332 passes through an analog-to-digital converter and a low-pass electronic filter.
  • the DC signal represents the average light intensity reaching the detector assembly 332.
  • the DC and AC signals need to be recorded at different PEM retardation settings.
  • the Mueller matrices for each of the respective pairs of source and detection modules in Fig. 3 are shown below.
  • the sample 26 in this optical anangement, with a magnitude of ⁇ (here considered in the general sense rather than in the normal/oblique sense as discussed below) and an angle of the fast axis at p, has the following form:
  • Io is the light intensity after the polarizer (324 or 424)
  • K is a constant that represents the transmission efficiency of the optical system after the polarizer.
  • the first parts of terms (3) and (4) can be used for determining linear retardance at low levels (below ⁇ c/2 or a quarter- wave).
  • Term (2) is useful for determining linear retardance at higher levels (up to ⁇ or a half-wave).
  • Term (1) contains DC terms that relate to the average light intensity.
  • the IF AC signals on the detector assembly (332 or 432) are determined using the lock-in amplifiers (304, 342 or 440, 442) referenced at the associated two PEMs' first harmonic (IF) frequencies.
  • the lock-in amplifiers will effectively exclude the contributions from all other harmonics.
  • V DC ⁇ eqn. (24)
  • this method requires recording AC and DC signals at different PEM settings and thus has a slower measurement speed (about 2 seconds per data point).
  • This method affords high accuracy measurement of linear retardance above 30 nm.
  • ⁇ represented in radians
  • ⁇ is a scalar.
  • a specific wavelength i.e., 632.8 nm
  • it is readily converted (i.e., multiplied by 632.8/(2 ⁇ )) to retardation in nanometers.
  • equations (27) are specifically developed for small linear birefringence due to the use of arcsine function in detennining linear birefringence. Therefore, this method described here has a theoretical upper limit of ⁇ /2 or 158.2 nm when using 632.8 nm laser as the light source.
  • the value of the retardance ⁇ - ⁇ induced by the in-plane birefringence of the sample in the embodiment of Fig. 3 is, as before, used with the simultaneously detected measure of retardance imparted on the other, "oblique" light beam, shown as “B2" in Fig. 3. That beam “B2" is directed to be oblique to the surface of the sample 26. Beam “B2" thus exits the sample with characteristics that provide information relating to the retardance occurring along the (refracted) incident path of that beam "B2" through that sample. The information thus provided by the two angled-apart beams "BI” and “B2" is detected and processed to provide, in addition to the in-plane birefringence of the sample, the out-of-plane birefringence of the sample 26.
  • the oblique source module 314 and the oblique detection module 316 respectively match the normal source module 310 and the normal detection module 312.
  • oblique source module 314 includes a light source 422, polarizer 424, and PEM 426 that function in the same manner as the light source 322, polarizer 324, and PEM 326 of normal source module 310.
  • the oblique detection module 316 includes another PEM 428 and detector assembly 432 that function in the same manner as the PEM 328 and detector assembly 332 of the normal detection module 312.
  • the oblique modules are used for providing and detecting the beam "B2" that propagates through the sample 26 at an angle "A" in Fig. 3 that is oblique to the no ⁇ nal-incidence beam "BI.”
  • the oblique source module 314 is mounted to produce the angled-apart beams "BI” and "B2" propagating through the same location in the sample.
  • the angle "A" is selected to be 30 degrees.
  • the source module 510 is configured to have components (light source, PEM etc) matching those of one of the above described source modules, such as normal source module 10.
  • the light beam "B” that emanates from the source module encounters a diverter 514 before it reaches the sample 26 on the holder 28.
  • the diverter which can be a partially reflective minor, redirects some of the beam to form a diverted beam "B2" that, as shown in Fig. 5, is reflected by a mirror 518 back toward the sample 26 to intersect the beam portion "BI" that passes through the diverter 514 at a common location on the sample.
  • the diverter 514 and minor 518 are a ⁇ anged to generate from the beam "B" of the single light source 510 the two, angled apart beams "BI" and “B2" for passing through the sample.
  • "B2" is preferably angled to be about 30 degrees from no ⁇ nal incidence.
  • beam “BI” (Fig. 5) is directed to detection module 512. That module 512 includes components (analyzer, detector etc) that match those described in the earlier discussed detection modules, such as detection module 12, for detecting information relating to the in-plane birefringence. Similarly, beam “B2" is directed to oblique detection module 516 after passing through the sample 26.
  • That module 516 also includes components (analyzer, detector etc) that match those described in the earlier discussed detection modules, such as detection module 16, for detecting information relating to the oblique-angle retardation imparted into beam "B2." As before, the information collected from the normal and oblique detection modules is processed to arrive at the in-plane and out-of-plane birefringence value for the vertical plane of interest.
  • a flip minor could be used as the diverter 514 of the embodiment of Fig. 5.
  • the minor is periodically flipped into and out of the path of beam "B" from the source module 510 thereby periodically generating the oblique beam "B2" for detection as described.
  • Normal beam "BI” thus reaches the detection module 512, as shown, when the flip minor is periodically out of the path of the beam "B." It will be appreciated that the frequency of the flip minor motion can be established (as by a suitable reciprocating actuator) to be high enough for permitting substantially simultaneous detection and calculation of both the in-plane birefringence (affecting beam BI) and out-of-plane birefringence (determined with detected infonnation from both beams BI, B2).
  • Fig. 6 is another alternative embodiment of the present invention wherein, like that of Fig. 5, a single source module 610, diverter 616, and minor 618 are employed for generating the two angled-apart beams "BI" and "B2" that penetrate the sample 26.
  • a single source module 610, diverter 616, and minor 618 are employed for generating the two angled-apart beams "BI" and "B2" that penetrate the sample 26.
  • another minor 614 for reflecting the oblique beam "B2" after it passes through the sample.
  • the reflected beam impinges on a converger 620, which is located in the optical path of the normal-incident beam BI, and which path terminates in a single detection module 612.
  • That detection module 612 has components that match earlier described detection modules, such as detection module 312 of Fig. 3.
  • the converger 620 pennits passage of the normal-incidence beam "BI” to reach the detection module 612 while causing the other beam “B2" to converge with beam “BI” along a common axis for detection by the same detection module 612.
  • at least one of the diverter 616 or converger 620 will be a flip minor that moves into and out of the path of the normal-incidence beam "B," "BI .”
  • the actuators for the flip minor is under the control and monitoring of the computer, thereby enabling the system to readily determine which of the two, converged-path beams "BI” or "B2" is striking the single detection module at a particular time.
  • a sample holder could be configured to periodically tilt the sample relative to a single-source beam that travels along a single (non-diverging) path. Such tilting, as shown by the dashed line 26T in Fig. 1, would have the effect of allowing a single beam to serve as both of the above-described angled-apart beams as the sample moves into and out of the tilted position.
  • the holder 28 will be a ⁇ anged and operated to ensure that the sample tilting occurs in a manner such that the beam penetrates the same location in the sample while information is being detected for both the normal-incidence beam (flat-oriented sample) and the oblique-incidence beam (tilted sample).
  • a tilting sample holder as just mentioned is shown diagrammatically in Fig. 7.
  • a tilted sample 236 may be traversed (here in a linear, "Y," direction) across the fixed path of a light beam 221 such as the beam emanating from source module 10 as described above.
  • the sample 236 is incrementally traversed by an X/Y stage sample holder 234 so that birefringence data can be collected over a plurality of locations across the surface of the sample.
  • the sample holder 234 may be designed to rotate the sample to facilitate, for example, analysis of the sample's birefringence properties at a number of different angles of incidence to the light beam.
  • a servomotor 235 is connected to one post or shaft 241 and operable by the computer for rotating the sample to the desired angle for analysis.
  • the servomotor is provided with an encoder that provides shaft 241 position information to the computer.
  • the servomotor 235 can be driven to rotate the sample 236 from the angled orientation shown in solid lines in Fig. 8 to a horizontal position as shown by dashed lines 243.
  • optical elements In some optical applications it is desirable to use light having a very short wavelength, such as about 157 nanometers, which wavelength is often refened to as deep ultraviolet or DUV. Thus, it is important to precisely determine the characteristics of the optical elements that are used in optical systems or setups that employ DUN light.
  • Such an element may be, for example, a calcium fluoride (CaF2) lens of a scanner or stepper. Birefringence or retardance is one such characteristic of the optical element.
  • CaF2 calcium fluoride
  • a system for measuring retardance properties of an optical element employed in a DUV optical setup must also operate with a DUV light source and associated components in order to precisely detect and process the DUV light signals.
  • the oxygen molecules absorb the DUV light, thus attenuating the light and reducing the signal necessary to make accurate birefringence measurements of the sample.
  • One way of eliminating the oxygen (as well as other contaminants) in the system environment is to purge the system or beam path with nitrogen (N 2 ).
  • FIG. 7 The above-described tilting sample holder embodiment of Fig. 7 may be considered as one for use in a system that requires an oxygen-purged beam path. Accordingly, there may be provided in this system a telescopic, upper purging-gas delivery tube 254 as shown schematically in Figs. 7 and 8. Beneath the sample, there is a similar telescopic, lower purging-gas delivery tube 256.
  • the gas pressure supplied to the tubes 254, 256 is selected so that the purging gas exiting the tubes provides a positive pressure in the gap that resides between each tube and the sample surface, thereby preventing the entry of oxygen into the path of the DUV light beam 221.
  • the adjustable purging-gas tubes 254 and 256 can be configured in any of a number of ways.
  • the telescopic upper tube 254 is mounted to protrude from a top wall 105 of a normally sealed volume within which resides the sample 236 and holder 234.
  • a supply tube 258 delivers the pressurized purging-gas from a remote supply.
  • An extension part 260 of the upper tube 254 is connected to a linear actuator 262 that is mounted adjacent to the tube 254.
  • the actuator 262 under the control of a computer, is operable to extend and retract the connected extension part 260 in the opposing directions shown by a ⁇ ow 264 in Fig. 8.
  • the lower, telescopic tube 256 is similarly extended and retracted by a computer controlled linear actuator 263.
  • the sample holder 234 may be constructed to hold the sample 236 in a particular angle relative to the incident light beam 221. h such an instance, the linear actuators may be controlled to maintain the ends of the tubes 256, 254 in close proximity with the respective surfaces of the sample as the sample is traversed. For example, with reference to Fig. 7, the linear actuators are controlled to incrementally retract the upper tube 254 and extend the lower tube 256 as the sample is traversed from left to right in that figure.
  • a light beam "impinging" path 400 that normally (that is, at zero incidence angle) impinges upon an optical element or sample 402 (here a horizontally oriented sample 402 is shown in solid lines), will emanate from that sample along an "emanating" path 404 that is axially aligned with the impinging beam path 400.
  • the beam thus follows this path 404 to the next optical element 406 in the setup.
  • the emanating beam path 404 will be displaced by a distance "D" from the impinging path.
  • the magnitude of this displacement "D" is a function of sample's refractive index, thickness, and tilt angle ⁇ .
  • some birefringence measurement systems it is desirable to measure birefringence at a high spatial resolution across the sample.
  • relatively small apertures are employed in the setup to establish a small-diameter beam size and conespondingly high resolution.
  • a small-diameter aperture may be placed adjacent to a detector that receives the light beam traveling along the emanating path 404 from the sample.
  • the optical setup can be provided with apertures in the emanating beam path that are sufficiently sized to capture a usable part of the displaced emanating beam, inespective of the amount of the displacement.
  • a slightly diverging source beam is prefened. While the usable part of the captured beam may have significantly lower intensity as compared to the entire beam, an accurate measurement can still be achieved by accounting for this lower intensity.
  • the ratio of the detected AC (modulated) signal to the DC (average) signal is used in determining retardance in conditions where the detected light intensity may fluctuate.
  • a rectangular-shaped aperture may be employed adjacent to a detector (that is, across the working surface of the detector) and a ⁇ anged so that the long side of the aperture is parallel to the axis along which the beam is displaced (the "Y" axis in Fig. 7). Such an aperture will be useful for limiting the amount of undesirable, non-parallel light rays from reaching the detector.
  • a PEM was one of the optical elements through which a light beam emanating from a sample is directed for additional phase modulation.
  • element 406 in Fig. 9 will be considered to be a PEM for the purposes of the following discussion.
  • the retardation amplitude introduced into the emanating beam 404 by the oscillating PEM 406 may vary somewhat depending upon the amount of displacement "D" of the emanating beam from a given location on the optical element of the PEM. For example, when the PEM's optical element (shown at 408 in Fig.
  • the retardation magnitude imparted to a beam passing through the center of the element 408 will be somewhat larger than the retardation imparted to a beam that is displaced from center by an amount "D.”
  • the amount of deflection "D" is readily determined, and where the amount of change in retardation imparted by the PEM 406 is considered significant (an “e ⁇ or” amount), one can determine this enor and employ it in the appropriate equations noted above. For example, for a PEM optical element 408 of length "L" (between the transducers 410) and a beam displacement "D," the retardation error amount will be a function of the ratio 2D/L.
  • the e ⁇ or be empirically determined for various increments of "D" and stored in a look-up table in firmware associated with the overall signal processing.
  • Information relating to the sample holder's angular position can be used by the controlling computer to determine a cu ⁇ ent displacement "D" that in turn is used in consulting the look-up table to arrive at the above mentioned retardation e ⁇ or associated with the cu ⁇ ent displacement.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • General Health & Medical Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Testing Of Optical Devices Or Fibers (AREA)

Abstract

L'invention concerne une mesure précise de propriétés de biréfringence hors plan d'échantillons d'un matériau optique transparent. Deux faisceaux lumineux à angle distinct passent à travers un emplacement sélectionné d'un élément optique échantillon. L'un des faisceaux est incident à la surface de l'échantillon. Les caractéristiques des faisceaux sont détectées après leur passage à travers l'échantillon, et les informations détectées sont traitées pour déterminer la biréfringence hors plan.
EP03799977A 2002-12-20 2003-12-19 Mesure de birefringence hors plan Withdrawn EP1573287A4 (fr)

Applications Claiming Priority (7)

Application Number Priority Date Filing Date Title
US364006 1989-06-09
US43558802P 2002-12-20 2002-12-20
US435588P 2002-12-20
US10/364,006 US7016039B2 (en) 2003-02-10 2003-02-10 Purging light beam paths in optical equipment
US49283803P 2003-08-06 2003-08-06
US492838P 2003-08-06
PCT/US2003/040366 WO2004059266A2 (fr) 2002-12-20 2003-12-19 Mesure de birefringence hors plan

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EP1573287A2 true EP1573287A2 (fr) 2005-09-14
EP1573287A4 EP1573287A4 (fr) 2009-11-11

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JP (1) JP4657105B2 (fr)
KR (1) KR20050093790A (fr)
CN (1) CN1739007B (fr)
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WO (1) WO2004059266A2 (fr)

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EP1924890A1 (fr) * 2005-09-14 2008-05-28 Carl Zeiss SMT AG Système optique d'un système d'exposition microlithographique
US7688446B2 (en) 2005-11-29 2010-03-30 Horiba, Ltd. Sample analyzing method, sample analyzing apparatus, manufacturing method of organic EL element, manufacturing equipment, and recording medium
JP4317558B2 (ja) * 2006-08-23 2009-08-19 株式会社堀場製作所 試料解析方法、試料解析装置及びプログラム
US7746465B2 (en) 2007-01-18 2010-06-29 Hinds Instruments, Inc. Sample holder for an optical element
KR100911626B1 (ko) * 2007-07-13 2009-08-12 서강대학교산학협력단 바이오 센서 측정 장치
WO2009018456A2 (fr) 2007-07-31 2009-02-05 The General Hospital Corporation Systèmes et procédés pour fournir des motifs de balayage de faisceau pour une imagerie dans le domaine de la fréquence optique doppler de vitesse élevée
JP5140451B2 (ja) * 2008-02-05 2013-02-06 富士フイルム株式会社 複屈折測定方法及び装置並びにプログラム
JP2009229229A (ja) * 2008-03-21 2009-10-08 Fujifilm Corp 複屈折測定装置及び複屈折測定方法
KR20110059701A (ko) * 2008-07-08 2011-06-03 하인즈 인스트루먼츠 인코포레이티드 높은 처리능력의 브리프링겐스 측정
JP2012150107A (ja) * 2010-12-27 2012-08-09 Nippon Zeon Co Ltd 光学異方性膜の評価方法、光学異方性膜の光学特性の測定装置および光学異方性膜の製造方法
GB201308434D0 (en) * 2013-05-10 2013-06-19 Innovia Films Sarl Authentication apparatus and method
CN105637345B (zh) 2013-05-23 2018-06-12 海因兹仪器公司 偏振性质成像系统

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JPH0545278A (ja) * 1991-08-09 1993-02-23 Matsushita Electric Ind Co Ltd 複屈折測定装置
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WO2004059266A2 (fr) 2004-07-15
CN1739007A (zh) 2006-02-22
WO2004059266A3 (fr) 2004-10-21
JP4657105B2 (ja) 2011-03-23
AU2003299695A1 (en) 2004-07-22
EP1573287A4 (fr) 2009-11-11
AU2003299695A8 (en) 2004-07-22
CN1739007B (zh) 2013-06-19
JP2006511823A (ja) 2006-04-06
KR20050093790A (ko) 2005-09-23

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