Shrinking on-product overlay (OPO) budgets in advanced technology nodes require more accurate overlay measurement and better measurement robustness to process variability. Pupil-based accuracy flags have been introduced to the scatterometry-based overlay (SCOL) system to evaluate the performance of a SCOL measurement setup. Wavelength Homing is a new robustness feature enabled by the continuous tunability of advanced SCOL systems using a supercontinuum laser light source in combination with a flexible bandpass filter. Inline process monitoring using accuracy flags allows for detection, quantification and correction of shifts in the optimal measurement wavelength. This work demonstrates the benefit of Wavelength Homing in overcoming overlay inaccuracy caused by process changes and restoring the OPO and residual levels in the original recipe.
As semiconductor technology nodes keep shrinking, ever-tightening on-product overlay (OPO) budgets coupled with continuous process development and improvement make it critical to have a robust and accurate metrology setup. Process monitoring and control is becoming increasingly important to achieve high yield production. In recently introduced advanced overlay (OVL) systems, a supercontinuum laser source is applied to facilitate the collection of overlay spectra to increase measurement stability. In this paper, an analysis methodology has been proposed to couple the measured overlay spectra with overlay simulation to extract exact process information from overlay spectra. This paper demonstrates the ability to use overlay spectra to capture and quantify process variation, which in turn can be used to calibrate the simulation stacks used to create the SCOL (scatterometry-based overlay) and AIM overlay metrology targets, and can be fed into the fab for process monitoring and improvement.
We demonstrate high volume manufacturing feasibility of 7 nm technology overlay correction requirement. This stateof- the-art overlay control is achieved by (i) overlay sampling optimization and advanced modeling, (ii) alignment and advanced process control optimization, (iii) multiple target overlay optimization, and (iv) heating control. We will also discuss further improvements in overlay control for 7 nm technology node and beyond including computational metrology, extreme ultraviolet and optic tools overlay matching control, high order alignment correction, tool stability improvement, and advanced heating control.
The importance of traditionally acceptable sources of variation has started to become more critical as semiconductor technologies continue to push into smaller technology nodes. New metrology techniques are needed to pursue the process uniformity requirements needed for controllable lithography. Process control for lithography has the advantage of being able to adjust for cross-wafer variability, but this requires that all processes are close in matching between process tools/chambers for each process. When this is not the case, the cumulative line variability creates identifiable groups of wafers1 . This cumulative shape based effect is described as impacting overlay measurements and alignment by creating misregistration of the overlay marks. It is necessary to understand what requirements might go into developing a high volume manufacturing approach which leverages this grouping methodology, the key inputs and outputs, and what can be extracted from such an approach. It will be shown that this line variability can be quantified into a loss of electrical yield primarily at the edge of the wafer and proposes a methodology for root cause identification and improvement. This paper will cover the concept of wafer shape based grouping as a diagnostic tool for overlay control and containment, the challenges in implementing this in a manufacturing setting, and the limitations of this approach. This will be accomplished by showing that there are identifiable wafer shape based signatures. These shape based wafer signatures will be shown to be correlated to overlay misregistration, primarily at the edge. It will also be shown that by adjusting for this wafer shape signal, improvements can be made to both overlay as well as electrical yield. These improvements show an increase in edge yield, and a reduction in yield variability.
Overlay metrology setup today faces a continuously changing landscape of process steps. During Diffraction Based Overlay (DBO) metrology setup, many different metrology target designs are evaluated in order to cover the full process window. The standard method for overlay metrology setup consists of single-wafer optimization in which the performance of all available metrology targets is evaluated. Without the availability of external reference data or multiwafer measurements it is hard to predict the metrology accuracy and robustness against process variations which naturally occur from wafer-to-wafer and lot-to-lot. In this paper, the capabilities of the Holistic Metrology Qualification (HMQ) setup flow are outlined, in particular with respect to overlay metrology accuracy and process robustness. The significance of robustness and its impact on overlay measurements is discussed using multiple examples. Measurement differences caused by slight stack variations across the target area, called grating imbalance, are shown to cause significant errors in the overlay calculation in case the recipe and target have not been selected properly. To this point, an overlay sensitivity check on perturbations of the measurement stack is presented for improvement of the overlay metrology setup flow. An extensive analysis on Key Performance Indicators (KPIs) from HMQ recipe optimization is performed on µDBO measurements of product wafers. The key parameters describing the sensitivity to perturbations of the measurement stack are based on an intra-target analysis. Using advanced image analysis, which is only possible for image plane detection of μDBO instead of pupil plane detection of DBO, the process robustness performance of a recipe can be determined. Intra-target analysis can be applied for a wide range of applications, independent of layers and devices.
We demonstrate a novel method to establish a root cause for an overlay excursion using optical Scatterometry metrology. Scatterometry overlay metrology consists of four cells (two per directions) of grating on grating structures that are illuminated with a laser and diffracted orders measured in the pupil plane within a certain range of aperture. State of art algorithms permit, with symmetric considerations over the targets, to extract the overlay between the two gratings. We exploit the optical properties of the target to extract further information from the measured pupil images, particularly information that maybe related to any change in the process that may lead to an overlay excursion. Root Cause Analysis or RCA is being developed to identify different kinds of process variations (either within the wafer, or between different wafers) that may indicate overlay excursions. In this manuscript, we demonstrate a collaboration between Globalfoundries and KLA-Tencor to identify a symmetric process variation using scatterometry overlay metrology and RCA technique.
Prime silicon wafers are ideal substrates for lithographic patterning, with tight flatness specifications for focus control. Process engineers are painfully aware that in-process product wafers can substantially depart from this ideal substrate. Wafer processing can induce non-flatness leading to focus problems, or distort the wafer leading to overlay issues. Thus processes from outside the lithography sector can impact yield by ruining lithographic pattern quality. Double-sided optical interferometric metrology is the standard method to assess the flatness of blank silicon wafers. In the last several years, a similar Patterned Wafer Geometry (PWG) metrology tool is able to measure in-process patterned wafers. The apparent surface seen by an interferometer may be different than the true surface due to transparent thin films, a discrepancy that we call "false topography". Modeling results will demonstrate the use of a thin opaque film to reduce the problem. PWG metrology offers compelling advantages for the practical investigation of process-induced focus and overlay problems. The paper will include several examples of process learning from PWG metrology.
Current commercial height profile measuring instruments, e.g. the confocal microscope and, the white light
interferometer, are widely used in both research institutes and industry. The systematic error of such instruments can be
the same order of magnitude as features on the surface to be measured, if care is not taken with calibration. Instrument
error in most cases depends on the surface slope. Thus, calibration of the instrument is important. The random ball test,
proposed by Parks et al, is a self calibration technique for transmission sphere calibration in phase shift interferometry.
The idea is, by measuring a collection of random patches on the surface of a sphere and then averaging the results, the
contributions from the ball go to zero leaving only the systematic biases due to the instrument. This paper shows it can
also be used to calibrate slope-dependent errors in profilometers such as the scanning white light interferometer (SWLI).
This will be demonstrated with both simulation and experimental results. For example, with a commercial SWLI
measurement with a 20X objective, our random ball test indicates that the height error can be as large as 250 nm at a
slope value of 2.9 degrees when using the envelope peak algorithm for analysis. Similarly, with a confocal microscope
measurement using a 50X objective, the height error can be as large as 800 nm at a slope value of 12.1 degrees. These
slope-dependent errors can be used to compensate future sloped-surface measurements.