Jason P. Cain

Modeling within-die spatial correlation effects for process-design co-optimization

Within-die spatial correlation of device parameter values caused by manufacturing variations has a significant impact on circuit performance. Based on experimental and simulation results, we: (1) characterize the spatial correlation of gate length over a full-field range of horizontal and vertical separation; (2) develop a rudimentary spatial correlation model; and (3) investigate its impact an the variability of circuit performance.

Electrical linewidth metrology for systematic CD variation characterization and causal analysis

Control of critical dimension (CD) variation is of extreme importance in modern semiconductor manufacturing processes. To be controlled, the nature of CD variation must be understood. This paper outlines a method for characterizing systematic spatial variation by means of dense electrical linewidth measurements, including actual sample data. In addition, since exhaustive sampling is prohibitively expensive for routine use, a method is discussed for finding an optimum economical sampling plan and using this plan to track systematic CD variation over time.

Extreme ultraviolet microexposures at the Advanced Light Source using the 0.3 numerical aperture micro-exposure tool optic

In an effort to continue the rapid pace of extreme ultraviolet (EUV) learning, the focus of developmental EUV lithography has shifted from low numerical aperture (NA) tools such as the 0.1NA engineering test stand to higher NA tools such as the 0.3NA micro-exposure tool (MET). To support this generation of lithographic optics, a static printing station has been developed at the Advanced Light Source. This synchrotron-based printing system relies on a scanning illuminator to provide real-time coherence (pupil-fill) control. Here, we describe a MET printing station and present early printing results obtained with the Sematech Set-2 MET optic. The resolution limit of baseline EUV resist is presented as well as 30nm equal-line-space printing in an experimental resist.

Experimental and model-based study of the robustness of line-edge roughness metric extraction in the presence of noise

As critical dimensions shrink, line edge and width roughness (LER and LWR) become of increasing concern. Crucial to the goal of reducing LER is its accurate characterization. LER has traditionally been represented as a single rms value. More recently the use of power spectral density (PSD), height-height correlation (HHCF), and {sigma} versus length plots has been proposed in order to extract the additional spatial descriptors of correlation length and roughness exponent. Here we perform a modeling-based noise-sensitivity study on the extraction of spatial descriptors from line-edge data as well as an experimental study of the robustness of these various descriptors using a large dataset of recent extreme-ultraviolet exposure data. The results show that in the presence of noise and in the large dataset limit, the PSD method provides higher accuracy in the extraction of the roughness exponent, whereas the HHCF method provides higher accuracy for the correlation length. On the other hand, when considering precision, the HHCF method is superior for both metrics.

Investigation of the Current Resolution Limits of Advanced Extreme Ultraviolet (EUV) Resists

The past two years has brought tremendous improvements in the crucial area of resists for extreme ultraviolet (EUV) lithography. Nested and isolated line resolutions approaching 30 nm and 25 nm, respectively, have been demonstrated. These advances have been enabled, in large part, by the high-numerical (0.3) EUV imaging capabilities provided by the Berkeley microfield exposure tool (MET). Here we investigate the resolution limits in several advanced EUV resists using the Berkeley MET. Comparisons to aerial-image performance and the use of resolution-enhancing illumination conditions are used to establish the fact that the observed pattern resolution in the best chemically-amplified resists available today are indeed resist limited. Moreover, contrast transfer function (CTF) techniques are used to directly compare various advanced resists. Strong correlation is observed between relative CTF performance and observed resolution limits.

Extreme ultraviolet lithography capabilities at the advanced light source using a 0.3-NA optic

Extreme ultraviolet lithography is a leading candidate for volume production of nanoelectronics at the 32-nm node and beyond. In order to ensure adequate maturity of the technology by the start date for the 32-nm node, advanced development tools are required today with numerical apertures of 0.25 or larger. In order to meet these development needs, a microexposure tool based on SEMATECHs 0.3-numerical aperture microfield optic has been developed and implemented at Lawrence Berkeley National Laboratory, Berkeley, CA. Here we describe the Berkeley exposure tool in detail, discuss its characterization, and summarize printing results obtained over the past year. Limited by the availability of ultrahigh resolution chemically amplified resists, present resolving capabilities limits are approximately 32 nm for equal lines and spaces and 28 nm for semi-isolated lines.

Characterization of the synchrotron-based 0.3 numerical aperture extreme ultraviolet microexposure tool at the Advanced Light Source

Synchrotron-based extreme ultraviolet (EUV) exposure tools continue to play a crucial roll in the development of EUV lithography. Utilizing a programmable-pupil-fill illuminator, the 0.3 numerical aperture (NA) microexposure tool at Lawrence Berkeley National Laboratory’s Advanced Light Source synchrotron radiation facility provides the highest resolution EUV projection printing capabilities available today. This makes it ideal for the characterization of advanced resist and mask processes. The Berkeley tool also serves as a good benchmarking platform for commercial implementations of 0.3 NA EUV microsteppers because its illuminator can be programmed to emulate the coherence conditions of the commercial tools. Here we present the latest resist and tool characterization results from the Berkeley EUV exposure station.

Optimum sampling for characterization of systematic variation in photolithography

In this paper, variability in the photolithography pattern transfer process is analyzed by means of a large number of CD measurements spread across the stepper field and across the wafer. The variability is found to be highly systematic in nature, and methods are developed to extract the parameters of this systematic variation. Knowledge of the structure of the systematic variance allows for the selection of an optimum sampling plan to best capture the variance in future measurements of wafers from the same process.

Lithographic characterization of the flare in the Berkeley 0.3 numerical aperture extreme ultraviolet microfield optic

Flare remains a crucial issue for extreme ultraviolet (EUV) lithography. Achieving required flare levels demands mid-spatial-frequency surface roughness levels on the order of 1A, which is on par with present metrology limits. Lithographic verification of predicted flare levels is thus critical to the validation of current metrology methods. In this work we present the lithographic characterization of flare in the Berkeley EUV microfield exposure tool. Experimental analysis shows good agreement between predicted and measured results. The results also show that it is essential to compensate for proximity and die-to-die effects. In an isolated microfield, flare values of 6.8% and 4.8% in 500nm and 2μm lines, respectively, have been verified.

Modeling within-field gate length spatial variation for process-design co-optimization

Pelgroms model suggests that a spatial correlation structure is inherent in the physical properties of semiconductor devices; specifically, devices situated closely together will be subject to a higher degree of correlation than devices separated by larger distances. Since correlation of device gate length values caused by systematic variations in microlithographic processing is known to carry a significant impact on the variability of circuit performance, we attempt to extract and understand the nature of spatial correlation across an entire die. Based on exhaustive, full-wafer critical dimension measurements collected using electrical linewidth metrology for wafers processed in a standard 130nm lithography cell, we calculate a spatial correlation metric of gate length over a full-field range in both horizontal and vertical orientations. Using a rudimentary model fit to these results, we investigate the impact of correlation in the spatial distribution on the variability of circuit performance using a series of Monte Carlo analyses in HSPICE; it is confirmed that this correlation does indeed present a significant influence on performance variability. From the same dataset, we also extract both the across-wafer (AW) and within-field (WIF) contributions to systematic variation. We find that the spatial correlation model’s shape is strongly related to these two components of variation (both in magnitude as well as by spatial fingerprint). By artificially reducing each of these components of systematic variation-thereby simulating the effects of active, across-field process compensation-we show that spatial correlation is significantly reduced, nearly to zero. This implies that Pelgroms model may not apply to die-scale separation distances, and that a more accurate correlation theory would combine Pelgroms model over very short separation distances with a systematic variation model that captures variability over longer distances by means of non-stationary distributions.