Bernd Geh

Impact of coma on CD control for multiphase PSM designs

Alternating PSM applied selectively to transistor regions on the poly gate mask is one way to achieve smaller gate CDs and tighter CD control. When using multiphase PSMs we have observed, experimentally, a difference between the CDs of isolated lines when the phase shifter is on the right side compared to the left side (we have called this effect the PSM right-left effect). The effect is shown to correlate with lens coma and the magnitude of the effect is also a strong function of defocus. In this paper we present experimental data showing the magnitude of the effect and how it can be minimized by choosing optimum values of numerical aperture (NA) and partial coherence ((sigma) ). The magnitude of the effect within the stepper field is shown to correlate with measured coma values. The sensitivity of the effect to defocus was calculated. Aerial Image simulation was performed and found to predict the experimental behavior to within a factor of two. Variations in PSM design were explored using simulation. In general, the effect is reduced if the PSM layout is symmetrical. By comparing the sensitivity to coma of various PSM designs with the sensitivity of line pair structures on binary masks we were able to determine which designs had acceptable coma sensitivity.

Toward a comprehensive control of full-field image quality in optical photolithography

This paper shows, that as resolution is pushed into regions below 0.6 (lambda) /NA, understanding the effects of wavefront aberrations is crucial to producing stepper systems that can meet end-user requirements. We show how aberrations can affect the choice of optimum NA and partial coherence for a given reticle object when considering critical dimension uniformity and depth of focus. The ability to measure the complete wavefront and extract meaningful full-field aberration data is shown using an advanced through-the-lens interferometer that operates at the wavelength and bandwidth of the lithographic radiation. The impact of aberrations an image quality criteria is shown through a sensitivity analysis using an imaging approximation model that represents various image criteria as a weighted sum of aberration coefficients. The validity and use of such a model is shown by correlation to full- field experimental measurements.

Polarization effects associated with hyper-numerical-aperture (>1) lithography

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.

Generation of arbitrary freeform source shapes using advanced illumination systems in high-NA immersion scanners

The application of customized and freeform illumination source shapes is a key enabler for continued shrink using 193 nm water based immersion lithography at the maximum possible NA of 1.35. In this paper we present the capabilities of the DOE based Aerial XP illuminator and the new programmable FlexRay illuminator. Both of these advanced illumination systems support the generation of such arbitrarily shaped illumination sources. We explain how the different parts of the optical column interact in forming the source shape with which the reticle is illuminated. Practical constraints of the systems do not limit the capabilities to utilize the benefit of freeform source shapes vs. classic pupil shapes. Despite a different pupil forming mechanism in the two illuminator types, the resulting pupils are compatible regarding lithographic imaging performance so that processes can be transferred between the two illuminator types. Measured freeform sources can be characterized by applying a parametric fit model, to extract information for optimum pupil setup, and by importing the measured source bitmap into an imaging simulator to directly evaluate its impact on CD and overlay. We compare measured freeform sources from both illuminator types and demonstrate the good matching between measured FlexRay and DOE based freeform source shapes.

Lithographic lens testing: analysis of measured aerial images, interferometric data, and photoresist measurements

This paper shows and discusses the use of direct aerial image measurements and optical interferometry for the evaluation of advanced i-line lenses. These measurement techniques provide direct information on the image forming capabilities of a stepper lens such that assessments of field curvature, astigmatism and image asymmetry can readily be accomplished. The interaction with the photoresist is shown by directly using the measured aerial image and aberration data into photoresist modeling programs such as Prolith/2 and Solid-C. The link between the optical measurements and the photoresist processes is further established by a comparison of simulated and measured results.

Mask effects for high-NA EUV: impact of NA, chief-ray-angle, and reduction ratio

With higher NA (≫ 0.33) and increased chief-ray-angles, mask effects will significantly impact the overall scanner performance. We discuss these effects in detail, paying particular attention to the multilayer-absorber interaction, and show that there is a trade-off between image quality and reticle efficiency. We show that these mask effects for high NA can be solved by employing a reduction ratio <4X, and show several options for a high-NA optics. Carefully discussing the feasibility of these options is an important part of defining a high-NA EUV tool.

Understanding systematic and random CD variations using predictive modeling techniques

This work analyzes the contributions to CD variation by building 3 predictive models that describe linewidth variation. The first model uses an exposure and focus budget analysis to create distributions that are used as input into a Monte Carlo analysis, where the output is a distribution of linewidth. The second model explores the effects of systematic intra-field effects by assuming that lens properties such as aberration will only cause global changes to the CD function, i.e. the function only shifts in focus and exposure. In combination with measurements such as focal pane, illumination uniformity and flare, a description of AFLV is constructed that reveals CD maps of the image field as a function of system focus and exposure. The third model combines the previous two techniques by incorporating random and systematic errors to create an across-wafer linewidth variation simulation. An example is shown using a scanner system and 0.18 micrometers structures. Systematic contributors to AFLV such as aberrations and reticle errors are included, as well as addition of random distributions of tilt eros and full wafer processing errors.

Interactions of 3D mask effects and NA in EUV lithography

With high NA (>0.33), and the associated higher angles of incidence on the reflective EUV mask, mask induced effects will significantly impact the overall scanner-performance. We discuss the expected effects in detail, in particular paying attention to the interaction between reflective coating and absorber on the mask, and show that there is a trade-off between image quality and mask efficiency. We show that by adjusting the demagnification of the lithography system one can recover both image quality and mask efficiency.

The impact of projection lens polarization properties on lithographic process at hyper-NA

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.

Integrated optical grating scale readout employing a double grating

A reading head empolying a double grating in a monomode waveguide for the interferometric readout of grating scales is proposed and demonstrated. A theoretical analysis employing a ray-tracing programthat includes wave-front data output shows promising insensitivity to deviations from the design data for this device. Experimental results for a laboratory setup are presented.