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Process monitor gratings - art. no. 651803
Abstract
Despite the increasing use of advanced imaging methods to pattern chip features, process windows continue to shrink with decreasing critical dimensions. Controlling the manufacturing process within these shrinking windows requires monitor structures designed to maximize both sensitivity and robustness. In particular, monitor structures must exhibit a large, measurable response to dose and focus changes over the entire range of the critical features process window. Any process variations present fundamental challenges to the effectiveness of OPC methods, since the shape compensation assumes a repeatable process. One particular process parameter which is under increasing scrutiny is focus blur, e.g. from finite laser bandwidth, which can cause such OPC instability, and thereby damage pattern fidelity. We introduce a new type of test target called the Process Monitor Grating (PMG) which is designed for extreme sensitivity to process variation. The PMG design principle is to use assist features to zero out higher diffraction orders. We show via simulation and experiment that such structures are indeed very sensitive to process variation. In addition, PMG targets have other desirable attributes such as mask manufacturability, robustness to pattern collapse, and compatibility with standard CD metrology methods such as scatterometry. PMG targets are applicable to the accurate determination of dose and focus deviations, and in combination with an isofocal grating target, allow the accurate determination of focus blur. The methods shown in this paper are broadly applicable to the characterization of process deviations using test wafers or to the control of product using kerf structures.
We present a cost-effective focus monitoring technique based on the illumination and the target co-optimization. An advanced immersion scanner can provide the freeform illumination that enables the use of any kind of custom source shape by using a programmable array of thousands of individually adjustable micro-mirrors. Therefore, one can produce non-telecentricity using the asymmetric illumination in the scanner with the optimized focus target on the cost-effective binary OMOG mask. Then, the scanner focus variations directly translate into easily measurable overlay shifts in the printed pattern with high sensitivity (ΔShift/Δfocus = 60nm/100nm). In addition, the capability of using the freeform illumination allows us to computationally co-optimize the source and the focus target, simultaneously, generating not only vertical or horizontal shifts, but also introducing diagonal pattern shifts. The focus-induced pattern shifts can be accurately measured by standard wafer metrology tools such as CD-SEM and overlay metrology tools.
CD uniformity requirements at 20nm and more advanced nodes have challenged the precision limits of CD-SEM metrology, conventionally used for scanner qualification and in-line focus/dose monitoring on product wafers. Optical CD metrology has consequently gained adoption for these applications because of its superior precision, but has been limited adopted, due to challenges with long time-to-results and robustness to process variation. Both of these challenges are due to the limitations imposed by geometric modeling of the photoresist (PR) profile as required by conventional RCWA-based scatterometry. Signal Response Metrology (SRM) is a new technique that obviates the need for geometric modeling by directly correlating focus, dose, and CD to the spectral response of a scatterometry tool. Consequently, it suggests superior accuracy and robustness to process variation for focus/dose monitoring, as well as reducing the time to set up a new measurement recipe from days to hours. This work describes the fundamental concepts of SRM and the results of its application to lithography metrology and control. These results include time to results and measurement performance data on Focus, Dose and CD measurements performed on real devices and on design rule metrology targets.
Scanner Focus window of the lithographic process becomes much smaller due to the shrink of the device node and multipatterning approach. Consequently, the required performance of scanner focus becomes tighter and more complicated. Focus control/monitoring methods such as “field-by-field focus control” or “intra-field focus control” is a necessity. Moreover, tight scanner focus performance requirement starts to raise another fundamental question: accuracy of the reported scanner focus.
The insufficient accuracy of the reported scanner focus using the existing methods originates from:
a) Focus measurement quality, which is due to low sensitivity of measured targets, especially around the nominal production focus.
b) The scanner focus is estimated using special targets, e.g. large pitch target and not using the device-like structures (irremovable aberration impact).
Both of these factors are eliminated using KLA-Tencor proprietary “Focus Offset” technology.
Variations in key device parameters such as gate width, fin height, and storage node aspect ratio can lead to performance variations device to device and within die. Extreme excursions can result in yield loss. Metrology and process control are enablers to detect and keep these variations to within certain bounds. As the features of devices continue to shrink, the allowable tolerances for critical dimensions and overlay errors likewise must shrink, in turn forcing the metrology budgets to shrink in step. At the same time, more data is required per wafer to generate higher order analyses while at the same time greater productivity in terms of silicon area processed in unit time is needed to keep the economics favorable. It is essential we develop the strategies needed for metrology in times of shrinking budgets.
Immersion lithography is being extended to below 20-nm and the lithography performance requirements need to be tightened further to enable this shrink. With depth of focus control requirements of the scanner going down to 60 nm and below, high quality metrology data is needed to quantify the disturbance of the exposed image. In this paper we present a metrology measurement method to allow accurate and robust measurements of the scanner focus state, through sampling of regular production wafers. Using a unique combination of specifically designed metrology targets and dedicated scatterometer measurements, accurate, precise and robust focus measurement capability is demonstrated. The application towards improved scanner focus control will be explained, demonstrating a measured 18% improvement of production wafer focus uniformity.
High volume semiconductor manufacturing yields require that critical
resist feature profile is continually controlled for uniformity and
centering. One reason is the small working distance of high numerical
aperture lenses. Indeed, reducing process windows require more precise
dimensional control. The variation of the critical dimensions can
generally be attributed to the lack of the focus and/or dose control. A
methodology to control the two lithographic parameters and to construct
a focus and dose budget for all components (tool, layer, resist, and
reticle) has been developed. This paper presents a run-to-run control
called FDO1 (Focus Dose Optimization) using in-line CD metrology. We
have confirmed that this method controls the photoresist shape and the
photoresist width accurately and reduces the CD variation for 28 nm
devices by 50%.
A dose-focus monitoring technique using critical dimension scanning
electron microscope (CD-SEM) is studied for onproduct applications. Our
technique uses two target structures; one is a dense grating structure
with iso-focal pitch for dose determination, and the other is a
relatively isolated line grating with no assists for focus
determination. The small sizes of these targets enable us to monitor
dose and focus variations across the chip on a product wafer. The model
which describes how the top and bottom CD depend on dose and focus
deviations is the same as that for scatterometry dose-focus metrology,
and monitoring precision is estimated to be the order of 1% for dose and
10~15nm for focus. The method has strong potential to apply to dose and
focus monitoring of product wafers. By using a mask with a multitude of
these targets, it is possible to study dose and focus variations across
the wafer in great detail. The focus variation of pairs of such targets
is measured for various separations between the two targets. As the
separation distance increases from ~100μm to ~10mm, the focus
variation increases from 10nm to 25nm. We think that the true focus
variation between targets becomes near zero at the small separation
distance, while the focus variation increases as separation distance
increases because more variation sources such as wafer thickness
variation are included at larger separation distances. Our small CD-SEM
targets allow us to explore this kind of local spatial variation
analysis.
A dose-focus monitoring technique using a critical-dimension scanning
electron microscope (CD-SEM) is studied for applications on product
wafers. Our technique uses two target structures: one is a dense grating
structure for dose determination, and the other is a relatively isolated
line grating for focus determination. These targets are less than 6
μm, and they can be inserted across a product chip to monitor dose
and focus variation in a chip. Monitoring precision is estimated to be
on the order of 1% for dose and 10 nm for focus, and the technique can
be applied to dose and focus monitoring on product wafers. The developed
technique is used to analyze spatial correlation in dose and focus over
a wide range of distances, using a mask with a multitude of these
targets. The variation (3σ) of dose and focus difference between
two monitor targets is examined for various separation distances, and
the variation of focus difference increases from 10 to 25 nm as the
separation distance increases from ˜20 μm to ˜10 mm.
The variation of 10 nm observed at the shortest distance reflects focus
monitoring precision, and focus variation sources such as wafer
thickness variation come into play at longer distances.
As the advanced IC device process shrinks to below sub-micron dimensions (65nm, 45 nm and beyond), the overall CD error budget becomes more and more challenging. The impact of lithography process parameters other than exposure energy and defocus on final CD results cannot be ignored any more. In this paper we continue the development of an advanced control system, which can be used to detect, classify and correct up to 5 lithography parameters. Sets of focus exposure matrix (FEM) models are first set up with different DOE process conditions split. And photoresist profiles of specially designed scatterometry CD mark are then fitted to models (Neural Network Model or standard polynomial model). Based on these calibrated models, not only exposure and defocus but also PEB temperature, lens aberration, etc. can be estimated. This approach utilizes information of resist CD, height, sidewall and feature type dependent bias to classify different lithography parameters and therefore can give very accurate estimation of lithography parameters like energy, focus, PEB, spherical aberration and coma aberration. The new approach does not need phase shift mask or other specially designed mask, so it can be used by most of mass production Fabs and used for process monitoring and matching on inline production wafer.
This paper will describe the development, qualification, monitoring, and integration into a production environment of the world's first fully programmable illuminator for optical lithography. FlexRay TM, a programmable illuminator based on a MEMs multi-mirror array that was developed for TWINSCAN XT:19x0i and TWINSCAN NXT series ASML immersion scanners, was first installed in January 2010 at Albany Nanotech, with subsequent tools installed in IBM's East Fishkill Manufacturing facility. After a brief overview of the concept and benefits of FlexRay, this paper will provide a comprehensive assessment of its reliability and imaging performance. A CD-based pupil qualification (CDPQ) procedure will be introduced and shown to be an efficient and effective way to monitor pupil performance. Various CDPQ and in-resist measurement results will be described, offering convincing evidence that FlexRay reliably generates high-quality pupils and is well suited for high volume manufacturing at lithography's leading edge.
The merits of hyper NA imaging using 193nm exposure wavelength with water immersion for 45nm is clear. Scanner focus and dose control is always improving to allow small DOF manufacturing in immersion lithography. However, other process parameters can affect focus and dose control and a real-time monitor capability to detect local focus and exposure conditions on production wafers is required. In this paper we evaluated a focus-exposure monitor technique based on Spectroscopic Critical Dimension (SCD) metrology following the promising results obtained by Kelvin Hung [1] et al. The key attributes of this technique are the implementation on standard production wafers, the high sensitivity to pattern profile modifications and the unique capability of spectroscopic ellipsometry to provide all the information needed to decouple the effects on pattern formation coming from process variations of Advanced Patterning Films (APF) [2], largely adopted for 65/45nm patterning, from coating and, finally, from the pure scanner imaging contributors like focus and exposure. We will present the characterization of this technique for 2 critical layers: active and contacts of a non-volatile memory device, 45nm technology.
As design rules continue to shrink, it is increasingly important to be able to determine and separate sources of Critical Dimension (CD) errors in order to maintain ever-decreasing process windows. CD errors can mainly be attributed to lack of focus and dose control1. Today some of these errors go undetected and CD changes are corrected by making dose correction to the exposure tool. However, corrections using only dose can lead to significantly smaller process latitudes. Therefore, it is very important that we consider dose and focus as a pair to increase the CD uniformity. The model we use is based on Ausschnitt deconvolution method1, 2. This model calculates the dose and focus errors simultaneously from CD parameters, such as bottom CD and top CD information, measured by a scatterometry measurement tool. We have confirmed that this method controls photoresist shape and photoresist width accurately and reduces the CD variation for 40 nm devices by 50%.
The ever shrinking lithography process window requires us to maximize our process window and minimize tool-induced process variation, and also to quantify the disturbances to an imaging process caused upstream of the imaging step. Relevant factors include across-wafer and wafer-to-wafer film thickness variation, wafer flatness, wafer edge effects, and design-induced topography. We quantify these effects and their interactions, and present efforts to reduce their harm to the imaging process. We also present our effort to predict design-induced focus error hot spots at the edge of our process window. The collaborative effort is geared towards enabling a constructive discussion with our design team, thus allowing us to prevent or mitigate focus error hot spots upstream of the imaging process.
Advanced 193 nm lithographic processes will require defocus control for product wafers in order to meet CD and profile requirements in the future. Dose control is already required. The interaction of product wafer materials with lithography requires additional controls beyond tool monitoring. While scatterometry has demonstrated excellent ability to extract effective defocus and dose information from monitor wafers, the addition of product film stacks introduces several issues for this technique. The additional complexity of model generation and the sensitivity to under-layer thickness and optical property variation are among these. A CDSEM technique for lithography focus monitoring overcomes these issues provided it has sufficient precision and relative accuracy. In this paper, we report on comparative studies of two CDSEM techniques. One technique uses angled e-beam to better view the sidewall for edge width measurement. The angle of the beam from normal incidence is considerably larger than previously explored thereby enabling sensitive measurements on shallower structures. The other technique introduces new target designs particularly suited to CDSEM measurement that have enhanced sensitivity to focus and dose. Implementation of these techniques requires expanded sampling during the course of a single measurement in order to suppress roughness. The small target size of these structures enables applications with targets in product kerf and embedded within the circuit. In summary, these methods enable the measurement of dose and focus variations on product wafers.
We derive a physical model to describe the dependence of pattern dimensions on dose, defocus and blur. The coefficients of our model are constants of a given lithographic process. Model inversion applied to dimensional measurements then determines effective dose, defocus and blur for wafers patterned with the same process. In practice, our approach entails the measurement of proximate grating targets of differing dose and focus sensitivity. In our embodiment, the measured attribute of one target is exclusively sensitive to dose, whereas the measured attributes of a second target are distinctly sensitive to defocus and blur. On step-and-scan exposure tools, z-blur is varied in a controlled manner by adjusting the across slit tilt of the image plane. The effects of z-blur and x,y-blur are shown to be equivalent. Furthermore, the exposure slit width is shown to determine the tilt response of the grating attributes. Thus, the response of the measured attributes can be characterized by a conventional focus-exposure matrix (FEM), over which the exposure tool settings are intentionally changed. The model coefficients are determined by a fit to the measured FEM response. The model then fully defines the response for wafers processed under "fixed" dose, focus and blur conditions. Model inversion applied to measurements from the same targets on all such wafers enables the simultaneous determination of effective dose and focus/tilt (DaFT) at each measurement site.
This paper presents a new highly sensitive scatterometry based Probe-Pattern Grating (PPG) focus monitor and its printing assessment on an advanced exposure tool. The high sensitivity is achieved by placing transparent lines spaced at the strong focus spillover distance from the centerline of a 90 degree phase-shifted probe line that functions as an interferometer detector. The monitor translates the focus error into the probe line trench depth, which can be measured by scatterometry techniques. The sensitivity of the defocus measurement through scatterometry calibration is around 1.1nm defocus / nm trench depth. This result indicates that the PPG focus monitor from a single wafer focus setting can detect the defocus distance to well under 0.05 Rayleigh Units.
The process window for state of the art chip manufacturing continues to decrease, driven by higher NA exposure tools and lower k1 values. The benefits of immersion lithography for Depth of Focus (DoF) are well known. Yet even with this immersion boost, NA=1.35 tools can push DoF into sub-100nm territory. In addition, immersion processes are subject to new sources of dose and focus variation. In order to realize the full potential of immersion lithography, it is necessary to characterize, understand and attack all sources of process variation. Previous work has established our dose/focus metrology capability 1 , in which we expose Process Monitor Grating (PMG) targets with high sensitivity to focus, measure the PMGs using scatterometry, and use the Ausschnitt dose/focus deconvolution approach to determine focus errors to within a few nm and dose errors to within 0.1%. In this paper, we concentrate on applying this capability to the detailed measurements of immersion photoclusters utilizing ASML exposure tools. Results will include: • comparison of Twinscan 1700i and 1900i focus capability • effectiveness of the Reticle Shape Correction (RSC) for non-flat reticles • visualization of non-flat wafer chucks, tilted image planes, and other systematic focus error components • tracking of tool trends over time, using automated monitor wafer flows The highly systematic nature of the observed focus errors suggest potential for future improvements in focus capability.
It is well known that the refractive optics used in today's exposure tools are highly chromatic, meaning that small wavelength shifts will cause large focus shifts. Even a line-narrowed excimer laser has a large enough range of wavelengths that we can no longer think of an infinitely thin image plane. The concept of "focus blur" can be generalized to encompass the effect of laser bandwidth chromatic aberrations, vertical stage vibrations (MSDz) and stage tilts which cause focus to change during the scan. This paper will introduce a new parameter called Mean Absolute Defocus (MAD) that can characterize the focus blur, and will be shown to correlate with the lithographic effects. Focus blur can be incorporated into simulation models, in a manner similar to the way that stage vibration is modeled. New simulation results will illustrate the impact of focus blur on modern lithographic processes. Process stability and machine-to-machine matching issues will be discussed.