In this paper we present a method to characterize scattered light in lithography scanners based on the measurement of the modulation transfer function (MTF) of the lens. This method provides a description of scattered light at all length scales, or spatial frequencies, relevant to lithographic printing. We also introduce a new automated technique based on scatterometry that improves the precision and repeatability of the MTF measurement. Modeling of flare is important to quantify the impact of scattered light on the critical dimension of the features printed on chips. We have developed simulation methods based on actual data from our lithography scanners. Our model uses the MTF of the lens and the Fourier transform of the chip density map to calculate the flare distribution across the chips. We show that this approach is useful to understand how the characteristics of different scanners in our fabrication facilities might affect the critical dimension (CD) uniformity across our product chips.
CD and line shape control face tougher technology requirements as the drive towards feature size reduction continues down to the 70nm regime. This poses new challenges not only for the lithography process, but also the metrology tool used to qualify the process. Smaller CDs mean smaller tolerances which puts a premium on the ability of metrology to precisely measure these dimensions. Also the trend towards more sampling and entire wafer uniformity mapping to increase yields makes sampling time a consideration. In this paper, we discuss Nanometrics’ Optical CD technology and its application towards the qualification of a scanner exposure system at 140nm pitch resolution (70nm line-spaces). This OCD technique uses normal incidence polarized reflectometry and a form of the Rigorous Coupled-Wave Analysis (RCWA) to do real-time regression. It is a fast and non-destructive method of measuring grating structures which provides complete interfield and intrafield spatially distributed profiles for all fitted OCD parameters. Analysis of spatially distributed data is critical in separating the sources of error that contribute to scanner qualification as a complete litho system
Wafers with 70nm dense (L/S=1:1) horizontal and vertical lines of resist on BARC were measured for this study. The fields on these wafers were exposed under various defocus conditions, producing small to large changes in the grating profiles. OCD measurements show good sensitivity to all fitted parameters; CD, CD profiles and film thickness. The focus fingerprint is clearly identified in a wafer uniformity map, amid other inter-field and intra-field contributions.
Dynamic repeatability and total test reproducibility metrics are introduced and discussed to quantify the reliability and resolution of the OCD to measure these lines.
Shallow-Trench-Isolation (STI), as one of the primary techniques for device isolation in complementary metal-oxide semiconductor (CMOS), requires accurate and precise CD and line-shape control during wafer process. Thus, the measurement of the critical dimensions after lithography and after formation of the STI structure is extremely important for process control. Currently used SEM technologies are either destructive or incapable of identifying the profile features. The necessary averaging of CD-SEM measurements to compensate for its large error diminishes its usage in automatic process control. In this paper we use the Optical Critical Dimension (OCD) technique to study focus exposure matrix (FEM) wafers of photoresist patterns and STI structures and compare the results with CD-SEM measurements. OCD measurement is performed with normal-incidence polarized reflectometry. Rigorous Coupled-Wave Analysis (RCWA) is combined with real-time regression to provide CD and profile parameters with excellent sensitivity to sub-50 nm grating lines. Thus, non-destructive and fast real-time measurements are easily accomplished during wafer processing. Optimization of STI model parameters is discussed. The critical dimensions of 121 dies extracted by OCD exhibit excellent correlation with those obtained by CD-SEM, with R-squared as high as 0.995 on STI wafers.