The rapid evolution of lithography to finer features has forced resist vendors to develop new resist chemical compositions in order to fulfill lithography and metrology requirements. However, an unintended consequence of these formulations has been the resists' sensitivity to electron beam radiation. While many studies have been conducted in describing resist slimming seen during CD-SEM measurements[1-5], in this paper we not only investigate the influence of measurement acquisition parameters - especially landing energy, probe current, and acquisition time on the accuracy of the measurement, we also explore the effect of these parameters on the precision. The measurements were performed on a CD-SEM with Ultra Low Voltage (ULV) capabilities. Reference measurements were performed using a CD-AFM, and serve as the basis for the accuracy measurements. Additionally the CD precision results are described in terms of the CDU metric, which provides a method of evaluating the tool contribution to the precision, when the measured features change from measurement to measurement.
Proc. SPIE. 6152, Metrology, Inspection, and Process Control for Microlithography XX
KEYWORDS: Lithography, Electron beam lithography, Metrology, 3D acquisition, Silicon, Atomic force microscopy, Scanning electron microscopy, 3D metrology, Critical dimension metrology, Semiconducting wafers
CD measurement bias has long been reported as an inherent artifact of CD-SEM measurements. However, as feature dimensions decrease and line-to-space ratios increase, the magnitude of previously acceptable levels of measurement bias requires re-examination. Traditional attempts at correcting the bias has entailed slow, destructive or laborious techniques, such as comparisons of top-down CD-SEM measurements using standard algorithms with cross-section information, or correlating top-down data with complex tilted images.
In this paper we expand the application of Critical Shape Metrology - a physics-based metrology technique for 3-D profile acquisition based on CD-SEM, to minimizing CD bias in real-time for a variety of feature dimensions and profiles. Samples used for the experiments were fabricated through E-Beam lithography and 193 lithography with a wide variation of sidewall angles and CDs, so that the measurement bias could be assessed over a sufficiently large range of patterned shapes. Reference measurements were performed using a CD-AFM and FIB-SEM
As gate linewidth control values approach the dimensions of resist polymer units, the accurate measurement of resist line edge (width) roughness (LER/LWR) takes on increased importance, not only as a guide to quantifying lithographic pattern quality, but also in its influence on device performance. It is therefore critical to be able to measure LWR in a manner that minimizes any image acquisition artifacts that may occlude the true nature of the roughness. In this paper, we study the effects on LWR that can result from the image acquisition process on a CD-SEM, with emphasis on the observations noted in 193 nm resist LWR, and in the use of sub-200 eV Ultra-Low Voltage (ULV) measurement energies, that have been explored as a means of minimizing the impact on 193 nm resist LWR.
The ever decreasing trend in feature geometry has placed increased importance on the concept of obtaining accurate and repeatable shape information at both the photo and etch steps. Traditional CD-SEM measurement algorithms are known to produce highly repeatable results but with large measurement bias depending on the feature shape (bias = average reported measurement - true value). In this paper we show the value of using Critical Shape Metrology (CSM), a physics-based Monte Carlo model, to extract shape information (sidewall angle, top rounding, footing,...) as well as CD measurements with very low bias, without compromising repeatability and throughput. Shape information and CD bias have been quantified through the use of a CD-AFM for all measurements taken using CSM. Several set of data are also compared to different scatterometry tools.
Previous results have demonstrated that the most significant line slimming occurs during the initial measurement and is a strong function of landing energy. Since it is difficult to accurately estimate the initial CD value (M0), many test protocols rely on the measurement change between the first and second measurement pass (M1-M2), to evaluate line slimming. However, since the slimming behavior of ArF resist systems has been shown to be exponential and dependant upon the resist formulation, using M1-M2 as the metric for comparing between CD SEM suppliers can severely underestimate the impact of a particular system setup on line slimming. The experiments reported here represent an attempt to assess the impact of the <b>initial</b> measurement (M0-M1) on line slimming. A series of experiments were designed to assess the impact of landing energy on line slimming for ArF photoresist. To validate the results of the experiments, an etched poly wafer was used as a control sample to ensure that metrology differences noted on the ArF resist between a high voltage 800 eV and an Ultra-Low Voltage (ULV) 100 eV condition arose purely from the interaction of the E-beam with the resist. The most significant line slimming was observed to occur during the first measurement at 800 eV, with greater than 10 nm of slimming observed on a nominal 120nm lines), followed by relatively stable slimming performance thereafter. The 100 eV condition demonstrated a significantly reduced level of slimming as a result of the first measurement; if there was any slimming, it could not be distinguished from the uncertainty in the estimate of the initial CD (M0). Measurements were also performed dynamically and at the ULV 100 eV condition slimming was indistinguishable from contamination induced linewidth growth, leading to an initial value closely matching the unperturbed linewidth (M0). The superior ArF line slimming performance at ULV is consistent with numerous published results, and demonstrates the need to assess slimming by a meaningful metric through a comparison of the initial measurement (M1) at high and low voltages, or by a comparison of the initial measurement M1 with the unperturbed linewidth (M0). The results of the experiments conducted point to a need for Low Impact Resist Measurement performance, which is fulfilled by Ultra-Low Voltage Metrology.
Current realities of tighter process tolerances combined with continued reduction in engineering resources per metrology tool require that every step of controlling a process a be made more efficient. Analyzing the quality of metrolgoy results, from equipment such as CD SEMs, can involve many factors that include image quality, measurement outputs, focus setting, and amplitude of system settings that play a role in the metrology results. The most efficient means of accessing large volumes of data today is via the World Wide Web. This paper provides examples of metrology problem resolutions that were achieved through the use of WWW on-line analysis of CD SEM results. The relational database that contains the measurement results, CD SEM settings, and metrology and pattern recognition images can be accessed from any fab PC, and can also be accessed from any fab PC, and can also be accessed from outside the fab by personnel with proper levels of security. Simple navigation through a web browser and the completeness of the data in the results database greatly improve the efficiency of the metrologist and the quality of the diagnosis, for improved process control and personal productivity.
The growing evolution of MEMS devices over the past decade from laboratory R&D environments into volume production settings for consumer use, has required Dimensional Metrology to become an invaluable part of the device fabrication process. In this paper we examine the dimensional metrology requirements for the Inkjet MEMS, and present high precision data obtained at a variety of fabrication stages. The measurement data has been used to guide process control on these structures and ultimately has led to improved device performance.
As geometrical dimensions of semiconductor devices decrease, the need to introduce Cu processes into the fabrication cycle becomes increasingly important as a means of maintaining line resistances and circuit time constants. However, the success of implementing such as fabrication process is dependent on the ability to characterize it through quantitative means, such as Overlay metrology. In this paper we examine the overlay measurement results which have been obtained on a Cu based CMOS process at the 0.12 (Mu) m technology node. Overlay measurements were taken over a wide range of process conditions, and included wafers exhibiting extreme image contrast reversal, grainy conditions and low contrast. These factors have traditionally led to a decreased ability to make repeatable measurements, if the measurements could be made at all. Our results cover the important metrics of overlay metrology, and include precision, recipe portability, and measurement success rates. The results suggest that the overlay metrology issues encountered with such leading edge processes need not pose intractable barriers to obtaining reliable overlay metrology data.
As the number of varied devices produced by a fab increase, coupled with an increased complexity in those devices which call for an ever increasing number of process layers, in- line process control via metrology can become an impossible task, unless metrology recipe management schemes are implemented. Logic fabs are now introducing more than 1 new device per day, which can result in the writing and management of thousands of recipes, which in turn can lead to the costly consumption of tool and personnel resources sand a general loss in productivity. In this paper we present the productivity gains to be made in the recipe creation process through off-line recipe generation, as well as a method of decreasing the recipe optimization time. We will also outline the concept of Just In Time recipe creation, its contribution to productivity gains, and its generalized implementation with respect to Overlay Metrology recipes.
As advanced photolithography moves the printable feature size from 0.25 micrometer to 0.18 micrometer various mask types are being used to improve resolution. One example is the attenuated phase shift MoSiON mask. This in turn requires the development of new mask repair techniques that provide acceptable levels of transmission and minimize phase error. In this study we present the results of opaque defect repairs on MoSiON DUV masks, utilizing a new focused ion beam (FIB) process. Opaque defects were repaired by scanning the defect area with a gallium ion beam in the presence of an etchant gas. Dose enhancement on the order of 20x was achieved, relative non-gas enhanced sputtering on the MoSiON absorber material to a non gas enhanced gas enhanced sputtering, resulting in repaired regions with excellent transmission properties, and minimal quartz damage (riverbed). The optimization of the FIB repair process is discussed and the results of post repair characterization, utilizing AIMS and AFM are presented.
Sub-resolution assist features, coupled with appropriate off- axis illumination conditions, have been studied with the goal of fabricating 200 and 240 nm contact windows with uniform critical dimensions over a range of pitches and with large depths of focus (DOF). Results show that 240 nm isolated contacts without assist features possessed a useful DOF of less than 0.4 microns. The same features with 140 nm assist slots on each window edge, located 190 nm away, possessed a DOF of over 0.8 microns, using quadrupole illumination. Soft quadrupole illumination, where a mixture of quadrupole and conventional illumination is employed, yielded nearly the same DOF as quadrupole and printed both semi-dense and isolated contact windows near their optimum size as well. Contact holes, 200 nm wide, have been printed with smaller sub- resolution features, soft quadrupole illumination, and higher performance resists with a DOF of over 0.6 microns using a stepper with a numerical aperture of 0.53.