Coupled eigenmode (CEM) theory for TM polarized illumination is presented and applied to the 3D modeling of a linespace
reticle. In this approach, the electric and magnetic field inside a line-space reticle is described in terms of an
orthogonal set of eigenmodes of Maxwell's equations. The diffraction of light by the reticle can then be expressed as a
coherent sum of diffraction orders produced by each eigenmode independently. Fresnel transmission, overlap of
eigenmodes with diffraction orders and propagation through the mask are shown to be the interactions that determine the
complex amplitude of the diffraction orders produced by each mode. We further shown that only a small number of
eigenmodes are needed to accurately calculate image contrast under TM polarized illumination.
For tight pitch patterning with sub-wavelength mask features, simulations and wafer data show that many mask stacks
that provide superior image contrast, can provide inferior MEEF performance. For example, 6% MoSi EPSM is found
to have higher MEEF than binary masks despite having better contrast and exposure latitude when equal lines and spaces
on the mask are used to pattern equal lines and spaces on the wafer. Likewise, the deposition of SiO2 on-top of the
chrome surface of a binary mask improves contrast but degrades MEEF compared to a binary mask. When contrast is
varied by mask stack or by print bias, MEEF is poorly correlated with contrast and often increases with increasing
contrast. The optimal print bias for exposure latitude is significantly different than the optimum print bias for MEEF.
MEEF, on the other hand, is highly correlated with the difference between maximum and minimum intensity when one
varies mask stack, print bias and illumination. Analytical MEEF equations are derived that support this strong
relationship between MEEF and the difference between maximum and minimum intensity.
Coupled eigenmode (CEM) theory is presented and applied to the 3D modeling of a line-space reticle. In this approach,
the electric field inside a line-space reticle is described in terms of an orthogonal set of eigenmodes of Maxwell's
equations. The diffraction of light by the reticle can then be expressed as a coherent sum of diffraction orders produced
by each eigenmode independently. Fresnel transmission, overlap of eigenmodes with diffraction orders and propagation
through the mask are shown to be the interactions that determine the complex amplitude of the diffraction orders
produced by each mode.
CEM is then applied to the cases of a binary mask and an att-PSM under dipole illumination. It is shown that the
behavior of contrast with pitch and mask bias is primarily affected by the propagation loss of the eigenmodes, which
increases for smaller trench widths. In the case of the binary mask, this attenuation causes one eigenmode to become
dominant and the resultant image approaches the perfect imaging of a single eigenmode. In the case of att-PSM, this
attenuation causes a detuning of the transmission and phase, and thus, the image contrast is degraded.
The resolution limits of optical lithography are usually described by the well-known Raleigh criterion, CD = κ1 (λ/NA). One of the biggest challenges in optical lithography is to reliably print contact holes patterns with κ1 ~ 0.35 using a hyper NA system (NA > 1) especially for relatively small (m × n) arrays. Polarization effects cause deviations from a simple (λ/NA) scaling large NA values. For an isolated hole, n = 1 and for large arrays, n ⪆ 15, the spectral content is mainly contained in the lowest diffracted orders that are captured within the NA of the imaging lens. The most difficult situation is for small arrays (m, n ≈ 2, 3, 4) where the spectral features are broader more of the important image information is contained in the higher diffraction orders. The patterning of contact holes also suffers from tight dose tolerances and high mask error enhancement factors (MEEF) as both the feature and array sizes decrease. A detailed PROLITHTM vector simulation study is reported for three different approaches to printing, isolated contact holes and small to large contact hole arrays with a κ1 of 0.35 and NAs of 1.05 and 1.3: 1) imaging interferometric lithography (IIL, with a single mask and multiple exposures incorporating pupil plane filters), 2) two-exposure dipole illumination, and 3) alternating phase shift masks (alt-PSM). Only the IIL scheme is capable of printing smaller (m, n ≤ 10) at this low κ1 factor. Single exposure alt-PSM does not allow for the necessary polarization control. Periodic assist features provide improved resolution, depth of focus and MEEF, at the expense of a more complex mask and additional nonprinting area surrounding the contact holes.
The use of alternating phase shift masks (Alt-PSMs) for poly gate patterning is becoming a well-established method for reducing gate critical dimension (CD) and variability. The application of alt-PSM for other device layers and for improving resolution (minimum pitch) is less developed due to more complex layouts, more stringent mask constraints and cost of ownership restrictions. Resolution of contact pairs and nested contacts is found to be improved using alt-PSM compared to embedded PSMs (EPSMs). To improve the process window of semi-nested and isolated contacts, sub-resolution phase-shifted assist features are employed on the mask. Square assist features, rather than rectangular assist features, are used to reduce mask fabrication requirements as one can use a larger minimum assist feature dimension. Because of high mask error enhancement factors (MEEFs), assist features with dimensions as large as 75% of the nominal contact size can be used without patterning on the wafer. Compared to using alt-PSM for poly gate patterning the use of alt-PSM for tight pitch patterning places additional constraints on mask manufacturing. The smaller phase regions intrinsic to tight pitch patterning result in tighter phase uniformity and mask defect requirements.
Intel will start high volume manufacturing (HVM) of the 65nm node in 2005. Microprocessor density and performance trends will continue to follow Moore's law and cost-effective patterning solutions capable of supporting it have to be found, demonstrated and developed during 2002-2004. Given the uncertainty regarding the readiness and respective capabilities of 157nm and 193nm lithography to support 65nm technology requirements, Intel is developing both lithographic options and corresponding infrastructure with the intent to use both options in manufacturing. Development and use of dual lithographic options for a given technology node in manufacturing is not a new paradigm for Intel: whenever introduction of a new exposure wavelength presented excessive risk to the manufacturing schedule, Intel developed parallel patterning approaches in time for the manufacturing ramp. Both I-line and 248nm patterning solutions were developed and successfully used in manufacturing of the 350nm node at Intel. Similarly, 248nm and 193nm patterning solutions were fully developed for 130nm node high volume manufacturing.
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