Pary Baluswamy

Practical resist model calibration

The lack of calibrated resist models has lead to a reliance on aerial images. This has limited capability of simulation to predict printed image shapes and CDs, especially in low K1 regimes. Calibration of resist models for matching simulation to pattern on wafers has always been a challenge due to various reasons. The primary problem is the large number of model parameters that need to be optimized. Another problem is the uncertainty associated with measurement of even the most basic parameters like thickness and refractive index. The amount of time and effort that is needed to calibrate the multitude of parameters is impractical in most situations. Some authors have taken the approach of optimizing a subset of parameters while retaining arbitrary default values for the rest. This leaves one wondering about the need for such models in process optimization and if a simpler empirical model would be sufficient. In this paper the various models are reviewed and the ones needing the smallest set of parameters are selected for calibration using a commercial resist modeling software package. The results of the calibration are checked against actual lithographic performance.

Process effects in flare measurement

Flare has become a significant problem for low K1 lithography. Several authors have reported measurement of flare in projection lenses. Most of the work is based on the Flagello-Kirk method using resist clearing dose. To measure the flare reliably and accurately using this method the contribution of the process needs to be understood. In this paper we present data looking at the influence of such effects on the measured flare.

Effects of reticle birefringence on 193nm lithography

Unpolarized light has traditionally been used for photolithography. However, polarized light can improve contrast and exposure latitudes at high numerical aperture (NA), especially for immersion lithography with an NA > 1.0. As polarized light passes through a reticle, any birefringence (BR) in the reticle material can cause a change in the orientation or degree of polarization, reducing the contrast in the final resist image. This paper shows the effects of reticle BR on dry and immersion imaging for 193nm lithography. The BR magnitude and orientation of the fast axis were mapped across several unpatterned mask blanks, covering a range of BR from 0 to 10 nm/cm. These reticles were printed with a series of open areas surrounded by test structures. The BR was measured again on the patterned reticles, and several locations were selected to cover a range of magnitudes at different orientations of the fast axis. Dry and immersion imaging were evaluated, looking at BR effects on dense lines and contact structures. Mask error enhancement factor (MEEF), line edge roughness (LER), and dose and focus latitudes were studied on line/space patterns. Dose and focus latitudes and 2-D effects were studied on contact patterns. Based upon these results, the effect of reticle BR on CD is minimal, even for BR values up to 10 nm/cm.

Sub-40nm high-volume manufacturing overlay uncorrectable error evaluation

Circuit layout and design rules have continued to shrink to the point where a few nanometers of pattern misalignment can negatively impact process capability and device yields. As wafer processes and film stacks have become more complex, overlay and alignment performance in high-volume manufacturing (HVM) have become increasingly sensitive to process and tool variations experienced by incoming wafers. Current HVM relies on overlay control via advanced process control (APC) feedback, single-exposure tool grid stability, scanner-to-scanner matching, correction models, sampling strategies, overlay mark design, and metrology. However, even with improvements to those methods, a large fraction of the uncorrectable errors (i.e., residuals) still remains. While lower residuals typically lead to increased yield performance, it is difficult to achieve in HVM due to the large combinations of wafer history in terms of prior tools, recipes, and ongoing process conversions. Hence, it is critical to understand the effect of residual errors on measurement sampling and model parameters to enable process control. In this study, we investigate the following: residual errors of sub-40nm processes as a function of correction models, sensitivity of the model parameters to residue, and the impact of data quality.

Understanding focus in projection lithography systems

Characterizing best focus for lithographic patterns is a very common task. It has been observed that the estimated best focus changes considerably with substrate type and substrates change quite frequently in process development. Such effects are seen even when the resist thickness is not altered. In this paper we will present data to identify the cause of the change and throw some light on the interaction between substrate and scanner leveling system.

Effect of lens aberrations on pattern placement error

Projection lens aberrations are typically modeled with Zernike polynomial coefficients. In this paper significant aberration terms that affect pattern placement error are identified using Design of Experiments. Simple models are developed for various 1D and 2D mask structures. These are used to study the impact of different illumination and aberration conditions. The results are used to estimate the impact of projection lens aberrations on overlay error.

Exposure effects on deep-ultraviolet resist thickness

Most deep ultraviolet (DUV) resist models available today utilize the Dill parameters to characterize resist exposure. These models assume that the thickness of the resist remains constant through exposure and post-exposure bake (PEB). The thickness is only affected by development in the models when resist is removed from the exposed or unexposed regions, depending on whether it is a positive or negative resist. It has been observed that a number of DUV resists change thickness upon exposure. This effect is expected to have an impact on the post-exposure acid profile calculated for modeling purposes. In this paper, we present data on the thickness changes for different resists and the effect of exposure to PEB delay on the change.