Orientation Zernike Polynomials have been shown to provide a complete and systematic description of polarized
imaging using the polar decomposition of the Jones pupil. We use this concept to predict the polarization
performance of high NA lithography lenses.
We introduce the new concept of orientation Zernike polynomials, a base function representation of retardation and diattenuation in close analogy to the wavefront description by scalar Zernike polynomials. We show that the orientation Zernike polynomials provide a complete and systematic description of vector imaging using high numerical aperture lithography lenses and, hence, a basis for an in depth understanding of both polarized and unpolarized imaging, and its modeling.
We introduce the 'Orientation Zernike Polynomials', a base function representation of retardation and diattenuation which are most relevant for vector imaging. We show that the 'Orientation Zernike Polynomials' provide a complete and systematic description of vector imaging using high NA lithography lenses and, hence, a basis for an in depth understanding of both polarized and unpolarized imaging, and its modeling.
There is a surprising lack of clarity about the exact quantity that a lithographic source map should specify. Under the
plausible interpretation that input source maps should tabulate radiance, one will find with standard imaging codes that
simulated wafer plane source intensities appear to violate the brightness theorem. The apparent deviation (a cosine
factor in the illumination pupil) represents one of many obliquity/inclination factors involved in propagation through the
imaging system whose interpretation in the literature is often somewhat obscure, but which have become numerically
significant in today's hyper-NA OPC applications. We show that the seeming brightness distortion in the illumination
pupil arises because the customary direction-cosine gridding of this aperture yields non-uniform solid-angle subtense in
the source pixels. Once the appropriate solid angle factor is included, each entry in the source map becomes
proportional to the total |E|^2 that the associated pixel produces on the mask. This quantitative definition of lithographic
source distributions is consistent with the plane-wave spectrum approach adopted by litho simulators, in that these
simulators essentially propagate |E|^2 along the interfering diffraction orders from the mask input to the resist film. It
can be shown using either the rigorous Franz formulation of vector diffraction theory, or an angular spectrum approach,
that such an |E|^2 plane-wave weighting will provide the standard inclination factor if the source elements are incoherent
and the mask model is accurate. This inclination factor is usually derived from a classical Rayleigh-Sommerfeld
diffraction integral, and we show that the nominally discrepant inclination factors used by the various diffraction
integrals of this class can all be made to yield the same result as the Franz formula when rigorous mask simulation is
employed, and further that these cosine factors have a simple geometrical interpretation. On this basis one can then
obtain for the lens as a whole the standard mask-to-wafer obliquity factor used by litho simulators. This obliquity factor
is shown to express the brightness invariance theorem, making the simulator's output consistent with the brightness
theorem if the source map tabulates the product of radiance and pixel solid angle, as our source definition specifies. We
show by experiment that dose-to-clear data can be modeled more accurately when the correct obliquity factor is used.
We present a comprehensive modeling study of polarization effects for the whole optical chain including exposure tool and mask, with strong emphasis on the impact of the Jones Matrix of the projection lens. First we start with the basic of polarization and then the polarization effect of each components of the optical chain will be discussed. Components investigated are source polarization, rigorous EMF effect, mask blank birefringence, pellicle effect and projection lens. We also focus on comparing the relative merits of different types of representation of Jones matrix of the projection lens and outlined ways to decompose the Jones Matrix. Methodologies such as Pauli matrix, PQM, Jones-Zernike expansion and IPS-Zernike expansion are among the ones investigated. The polarization impact on lithography and OPC on realistic 45nm and 32nm node process levels is discussed. Issues in OPC modeling with Jones Matrix is highlighted. Concerns regarding the standardization of the implementation of Jones Matrix in the lithography community are considered and a standard has been proposed and received wide acceptance. Last we discuss the challenge of using polarization and some novel ideas to deal with polarization in hyper NA era. Throughout the paper the resist component is not included so as to isolate the effect of resist from that of the other components.
The continuous implementation of novel technological advances in optical lithography is pushing the technology to ever
smaller feature sizes. For instance, it is now well recognized that the 45nm node will be executed using state-of-the-art
ArF (193nm) hyper-NA immersion-lithography. Nevertheless, a substantial effort will be necessary to make imaging
enhancement techniques like hyper-NA immersion technology, polarized illumination or sophisticated illumination
modes routinely available for production environments.
In order to support these trends, more stringent demands need to be placed on the lithographic optics. Although this
holds for both the illumination unit and the projection lens, this paper will focus on the latter module. Today, projection
lens aberrations are well controlled and their lithographic impact is understood. With the advent of imaging enhancement
techniques such as hyper-NA immersion lithography and the implementation of polarized illumination, a clear
description and control of the state of polarization throughout the complete optical system is required.
Before polarization was used to enhance imaging, the imaging properties at each field position of the lens could be fully
characterized by 2 pupil maps: a phase map and a transmission map. For polarized imaging, these two maps are replaced
by a 2x2 complex Jones matrix for each point in the pupil. Although such a pupil of Jones matrices (short: Jones pupil)
allows for a full and accurate description of the physical imaging, it seems to lack transparency towards direct
visualization and lithographic imaging relevance.
In this paper we will present a comprehensive method to decompose the Jones pupils into quantities that represent a clear
physical interpretation and we will study the relevance of these quantities for the imaging properties of lithography
lenses.
We give a general introduction into polarized imaging and report on a Jones pupil approach for a complete evaluation of the resulting optical performance. The Jones pupil assigns a Jones matrix to each point of the exit pupil, describing the impact of both the global phase and the polarization on imaging. While we already can learn much about the optical system by taking a close look at the Jones pupil-and starting imaging simulations from it-a quantitative assessment is necessary for a complete evaluation of imaging. To do this, we generalize the concept of scalar Zernike aberrations to Jones-Zernike aberrations by expansion of the Jones pupil into vector polynomials. The resulting method is nonparaxial, i.e., the effect of the polarization-dependent contrast loss for high numerical apertures is included. The aberrations of the Jones matrix pupil are a suitable tool to identify the main drivers determining polarization performance. Furthermore, they enable us to compare the polarized and unpolarized performance of such a characterized lithographic system.
The use of immersion technology will extend the lifetime of 193-nm technology by enabling numerical apertures (NAs) much greater than 1.0. The ultimate limits of NA are explored by analysis of polarization effects at the reticle and imaging effects at the wafer. The effect of reticle birefringence with polarized illumination is explored. The effects on critical dimension (CD) uniformity are mitigated if the maximum birefringence is <5 nm/cm. Hertzian or micropolarization due to the size of the reticle structures is examined through rigorous simulation. For the regime of interest, 20- to 50-nm imaging, it is found that dense features on a Cr binary reticle will polarize the light into the TE component upward of 15%. Below this regime, the light becomes polarized in the TM direction. The use of polarization in the illuminator for imaging will result in substantial gains in exposure latitude and mask error factor (MEF) when the NA~1.3 with 45-nm lines at 193 nm, with overall polarization effects increasing with decreasing k1. The end-of-line pullback for 2-D patterns is reduced by the use of TE polarization in the illuminator. The interaction between the reticle-induced polarization and the illumination polarization is shown to be significant when an analysis is done using rigorous mask simulation instead of the more common Kirchhoff approximation. The impact of birefringence in the lens is analyzed using Jones pupil matrices to create a local polarization error in the pupil. The photoresist process is shown to interact with polarization. Different photoresists will show varying degrees of sensitivity to polarization variation.
We give a general introduction into polarized imaging and report on a Jones-pupil approach for a complete evaluation of the resulting optical performance. The Jones pupil assigns a Jones matrix to each point of the exit pupil describing the impact of both the global phase and the polarization on imaging. While we can learn already a lot about the optical system by taking a close look at the Jones pupil - and starting imaging simulations from it - a quantitative assessment is necessary for a complete evaluation of imaging. To do this, we generalize the concept of scalar Zernike aberrations to Jones-Zernike aberrations by expansion of the Jones pupil into vector polynomials. The resulting method is non-paraxial, i.e. the effect of the polarization dependent contrast loss for high numerical apertures is included. The aberrations of the Jones-matrix pupil are a suitable tool to identify the main drivers determining the polarization performance. Furthermore, they enable us to compare the polarized and the unpolarized performance of the such characterized lithographic system.
The use of immersion technology will extend the lifetime of 193nm and 157nm lithography by enabling numerical apertures (NA) much greater than 1.0. This paper explores the effects that will occur when the high NA systems are augmented with polarization.. Specifically we show that there are strong interactions between the polarization induced by the reticle and polarization in the optics. This has a direct impact on the across-field specification of the polarization of the optical system as it causes a large variation in the imaging impact in photoresist. The impact of lens and reticle birefringence on the imaging is also analyzed. We show that reticle birefringence should not be a major concern when the birefringence is maintained to 2nm/cm - 4nm/cm levels. The lens can be modeled by a Jones matrix approach, where multiple pupils must be defined for each polarization state. We show the impact of the optical components by using a rigorous photoresist simulation on the process window of sub-50nm features using NA>1.3. The simulator uses a full Maxwell equation solver for the mask, polarized illumination, a Jones matrix approach for the pupil, and a photoresist simulation with calibrated model. The photoresist process is also shown to interact with polarization. Different photoresist will show varying degrees of sensitivity to polarization variation.
Inspection and linewidths measurements of subwavelength structures using optical microscopy are severely confined both by the limited resolution and by a manifold of light-structure interactions affecting the optical image. To receive a better understanding of these interactions and/or to overcome these limitations new microscopy methods have been developed: Polarization-interference-microscopy permits the accurate measurement of the birefringence which is induced by the form of the structures. By interferometric comparison of the TE and TM-polarized image this method provides selective edge detection and localisation in the subwavelength regime because the polarisation difference is situated at the topographical changes of the structure. Two new methods of dark field microscopy ,"alternating grazing incidence dark field microscopy" and "frustrated internal total reflection microscopy", make it possible to suppress optical proximity effects and to overcome the resolution limit of conventional dark field microscopy. For a characterisation of the qualitative and quantitative influence of different influencing variables on measurements of the size of subwavelength structures we performed comparative linewidths measurements on Chrome and phase shifting MoSi photomask structures. The deliverables are compared with rigorous numerical simulations.
In earlier publications, it was shown that scanning of surfaces by dark beams can be exploited for sub-wavelength feature analysis. In this work, we present vector simulations based in Rigorous Coupled-Wave Analysis with the purpose to estimate the expected resolution of the method, both lateral (feature size) and axial (height). The dark beam used in this study has a line singularity generated by a π-phase step positioned in a Gaussian beam. Various combinations of the illumination and detection nuFmerical apertures (from NA=0.2 to NA=0.8) and different surface features were studied. Polarization effects which become significant at high numerical apetures, were considered as an additional source of information for the analysis. In the case of a sub-wavelength feature on an ideal surface, the resolution of the method is limited only by the electronics noise. In particular, under a reasonable assumption of a 105 signal to noise ratio, it is possible to detect a 0.2 nm step.
The low-pass characteristic of the optical imaging limits severely a quantitative measurement of structure-sizes below the optical wavelength and leads to measurement errors. On the other hand, small structures show different optical characteristics for different polarizations. A fact that corresponds to the form-birefringence of microstructures. It is described for zero-order gratings by polarization dependent dielectric constants in the effective medium theory. The birefringence can be measured accurately by use of polarization interferometry where two orthogonal polarizations interfere so that their phase-difference can be determined. To that end an electro-optic modulator is inserted into the optical path of a polarization microscope to provide the well-defined phase steps for an evaluation according to phase-shifting interferometry. From the phase difference we can conclude on the optical path-difference for both polarizations and from this - using the structure thickness - on the birefringence. Waveguide-models are applied for image interpretation. For an estimation of the width of the structure we compare polarization-interferometrical measurements with rigorous numerical simulations.
Zernike phase-contrast is a well known and a often used method for the visualization of phase-structures. Because it applies a fixed phase-shift between the zero'th order and the scattered field, quantitative phase-measurements are only possible for small phase-shifts but not for arbitrary ones. The basic idea of the presented generalization of the method is to use polarization-optical pupil-filters for a separation of the zero'th from the higher orders in combination with phase-shifting polarization interferometry: In the simplest version, a drilled half-wave retarder is used in connection with linearly polarized illumination. This pupil-filter rotates the polarization of the scattered field by (pi) /2 while the dc-term is unaffected because it traverses the hole. The transmitted field is analyzed by phase-shifting polarization-interferometry: Using a liquid-crystal phase shifter (LCPS), both components can be phase-shifted relative to each other; they interfere at a subsequent analyzer. By means of a signal-evaluation according to phase-shifting interferometry, the complex amplitude of the scattered field can be determined relative to the dc-term enabling a computation of the complete field's phase. The method works for general complex objects. The applications we consider are in the field of microstructure inspection.
For accurate measurements of structures with sharp edges, as found on photomasks and wafers, polarization methods are useful because edges provide a unique polarization-effect: depending on geometrical form and material of the edge, its effect on the incident polarization is different. We have developed three different methods for polarization- utilization: (1) polarization interferometry, (2) Jones- matrix microscopy, and (3) quantitative phase-contrast. All three procedures are incorporated into one set-up. We describe the set-up and its measurement modes.
The general idea of the presented investigations is to use the polarization of the electromagnetic field in high- resolution optical microscopy to get information about sub- wavelength details of topographical structures. The main application is the localization of vertical edges. For structures in non-magnetic materials, polarization effects are caused by the different boundary conditions for the tangential and normal electric field components. Using rigorous numerical simulations we show that two physical- optics model describe the polarization dependent images of vertical edges and sub-wavelength structures: a boundary diffraction wave originating from the tip of the edge and waveguiding effects, respectively. We report on two experimental approach for polarization usage: a) comparison of interference microscopy images for s- and p-polarization, and b) polarization interferometry. The former is capable of 'synthetical microscopy'. Measurements of well defined structures are compared with rigorous numerical simulations.
We report on a method for microstructure inspection using the `edge-birefringence', i.e. the polarization anisotropy caused by the structure edges. A liquid-crystal phase- shifter is inserted into the imaging optics of a reflection- made microscope, and the principle of phase-shifting interferometry is applied to measure phase and contrast of the TE-polarized image with the TM-polarized image as reference. This common-path interferometrical method provides a selective edge detection because the polarization difference is localized at the structure edges. It is useful for a simple but accurate measurement of microstructure linewidths. The (unwrapped) phase image can show supersteep edges that we attribute to phase singularities. Because with decreasing structure width the polarization difference increases, the method is particular apt for subwavelength line-structures. The images, however, become then strongly dependent on the experimental parameters like focus- position, structure material, -width and -depth.
Measurements for investigation of collection and reflection mode scanning near field optical microscopy (SNOM) image generation are performed in the near field of small phase objects using 3 cm microwave interferometers and a tapered open rectangular waveguide as near field probe. Measured field distributions are compared with moment method calculations, including probe scattering. A good agreement is obtained.
In rotationally symmetric optical systems partially coherent beams may have a position-dependent phase that twists the beam on propagation. We employ synthetic acousto-optic holograms and astigmatic optics to convert an ordinary laser beam into such a field and provide physical interpretation for the twist phenomenon.
The properties of complex systems of integrated optics
depend mainly on the nearfields of the components,
if their distances are less than a few wavelengths.
For field-calculations the Kirchhoff and the
Rayleigh-Sommerfeld diffraction theories may be
applied also to phase objects. Usually these are assumed
to be "thin" ; then in the "object plane" the
incident wave may be multiplied with the complex
object transmittance (Kirchhoff's boundary conditions).
But, in general, this approximation leads to
deviations; these are investigated for small phase
objects (n-i << 1, Ac < 20°) of finite thickness.
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