Aberrations in the exposure tool have been shown to produce line-edge and line-end perturbations on phase-shifting masks that can result in design defects. A pattern matching system has been developed to locate areas in a phase shift mask most sensitive to these lens aberrations. The original prototype of the pattern matcher was developed in the SKILL programming language of Cadence's Design Framework II. Speed and memory limitations prompted the creation of a new C++ binary, which incorporated the core data structures and matching algorithms. Specialized algorithms for partitioning, prefiltering, and compression resulted in a fast and memory efficient matching process. The local layout geometry is automatically output to SPLAT for detailed image analysis with and without aberrations. The pattern matching C++ software supports multi-level compression, layer Booleans, pattern proximity calculations, and many other features. The pattern matching idea has recently been expanded to search for sensitivity to defects, misalignment, reflective notching, laser-assisted thermal processing, and chemical-mechanical polishing. The current software system includes a combined graphical and a text-based interface and can be run on any operating system, independent of Cadence. The web-based version of the pattern matcher allows a user to either input custom layouts and match patterns or select from a list and then perform matching runs from a web browser.
Figure 1: Screenshot of pattern matcher web applet
Figure 2: Large coma and trifoil aberration patterns matched on a small phase-shift mask layout
Figure 3: Coma aberration pattern matched on a complex phase-shift mask
Residual aberrations in optical lithography systems used to produce integrated circuits can significantly affect the image quality produced at the wafer. Thus, the development of a simple and reliable technique for quantifying aberrations is of great importance. A theoretical foundation has been given for the ability of programmed probe based aberration targets to measure individual Zernike aberration terms. The optimum targets are inverse Fourier transforms of the Zernike polynomials and this allows the main features of the family of targets to be predicted in advance. Simulation of discretized versions shows an impressive 27 to 36% increase, per 0.01 waves of rms aberration, in the intensity at the center of the target relative to the clear field intensity. The cross contamination by other targets is about 1/6th as large and it is thus possible to measure spherical aberration independent of focus. The theoretical foundation of this work, as well as initial simulation results are presented in . An invention disclosure was filed on this work and the university filed for a provisional patent application in September 2001.
Once the aberration fingerprint of a lens or a family of lenses is determined, this data can be input into the powerful PatternMatch software created by Frank Gennari. Device designers can use this software to discover where their layouts will be affected by the aberrations present and design accordingly. This offers a significant link between designers and lithographers, which will only become increasingly important as low k1 lithography solutions are implemented.
Figure 1: An example of the digitized spherical aberration target and the response of the central probe image intensity on the wafer to no aberration, 0.05 waves (peak) of defocus, and 0.05 waves of spherical aberration.
Rigorous electromagnetic simulations are used to test the lithographic printing of novel technologies such as polarization masks. Polarization masks use small bars inserted into features to polarize the incident radiation, allowing features to be printed with the chosen polarization. Proximity effects from electric field spillovers between adjacent features can be reduced by passing opposite polarizations, resulting in spatially orthogonal electric field vectors than give a reduction in intensity of _. Additionally, polarization bars can help mitigate the effects of large phase transitions on phase-shifting masks. Several special purpose monitors can be constructed with polarization bars to monitor polarization imbalances in the illumination, high numerical aperture effects on vector addition, as well as polarization dependent resist coupling effects. Small, dense contacts may also be produced by using a combination of polarization and phase shifting to generate four wave interference at the wafer. These polarization bars must have gap widths of about l/8 - l/3 with the bars themselves being about l/8.
1Graduate Student (non-EECS), Applied Science and Technology
Image enhancement and lateral size analysis tools are used to quantitatively examine roughness via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The tools are then applied to study the effects of aerial image contrast and threshold set points (see Figure 1) upon LER and side edge roughness (SER) of several different DUV chemically amplified resists, using programmed double exposures with an ASML 248 nm wavelength stepper tool to create variable contrast levels. The threshold set point is defined as the minimum exposure dose necessary for resist development. Results show that in the case of Shipley's UV210, for instance, LER and SER do not increase significantly until aerial image contrast drops below around 38% (see Figure 2).
Advanced measurement and analysis techniques are used to yield a deeper understanding of and higher confidence in the data collected. AFM analysis of a sidewall yields two-dimensional information regarding that sidewall. Aside from merely mentioning SER values, the frequency and size of roughness are studied. In examining the data, roughness is typically seen as smooth, low rolling hills of about 20 nm peak:peak spacing and 1 nm height. The randomness of roughness can also be studied using this method, to help understand root causes or LER and SER.
SEM analysis itself falls prey to optical aberrations analogous to those seen in lithography. Astigmation, defocus, and other aberrations blur the micrograph, which is used to quantitatively assess LER. Even a system with no aberrations still has diffraction limited blurring. Deblurring the micrograph can improve image quality, therefore enhancing the accuracy of the LER data extracted from the SEM image. Deblurring is performed by iteratively solving for the point spread function (PSF) of the unknown blurring source and deconvolving the image with the PSF (see Figure 3). The LER does indeed change significantly. One example shows root mean square LER equal to 4.7 nm before image enhancement and 5.9 nm afterwards. Aside from improving the LER data, this technique also helps to determine the most significant aberrations seen in the SEM tool by analyzing the PSF used to deconvolve the image
Figure 1: Aerial image contrast variation at two different energy threshold set points
Figure 2: Effect of aerial image contrast upon SER at 0.3 intensity threshold set point
Figure 3: Deconvolving an SEM image with a calculated PSF in order to improve image quality and receive more accurate LER values
Immersion lithography offers the capability to further reduce the resolution of printed features. For example, placing deionized water (n=1.44) between the projection optics and the wafer, the current 193 nm stepper could achieve the same resolution as a 157 nm stepper while improving the depth of focus by 8%. Immersion liquids such as Fomblin pump-oil (n=1.36) and other liquids have been examined for suitability in 157 nm steppers . The immersion layer also allows larger amounts of energy to be coupled into the resist due to a lower reflection coefficient and lower effective NA.
This research focuses on quantifying potential problems by performing analytical estimation and simulation. Intrinsic characteristics of the liquid likely to contribute to aerial image degradation include homogeneity and reactivity with optical and resist surfaces. Liquid dynamic effects such as local heating, relative motion of the liquid and projection lens, convection heating, and resist outgassing are being investigated.
The high resolution and superior coupling of light into photoresist are being investigated through generalizing the thin-film formulation developed by Michael Yeung for SPLAT . This approach consists of modeling thin-film effects through viewing them as amplitude and phase effects in the lens pupil.