As development of stacked Nanosheet Gate All-Around (GAA) transistor continues as the candidate technology for future nodes, several key process points remain difficult to characterize effectively. With the GAA device strategy, it is critical to have an inline solution that can provide a readout of physical dimensions that have an impact on the threshold voltage (VT) and yield. Metrology challenges for obtaining these metrics arise from increasingly dense arrays coupled with both high aspect ratios, high numbers of correlated parameters, and increasingly complex 3D geometries. Large area metrology structures can be used for 3D parameters’ process monitoring through techniques such as scatterometry and xray diffraction (XRD) which deliver averaged results over that area, but variation impacting specific devices cannot currently be understood without destructive cross-section. Prior work to characterize the dimensions of these GAA devices has primarily featured optical metrology, X-ray metrology, and critical-dimension scanning electron microscopy (CDSEM), but these techniques have their own challenges at the critical process points. Atomic force microscopy (AFM) had not been utilized due to the aspect ratios and small trench widths which were inaccessible to conventional techniques. However, due to recent advances in scanning and novel probe technologies, AFM is well-suited now to solve these local, three-dimensional challenges. Through this study, we demonstrate AFM characterization of a key process point in the GAA process flow for multiple structures with varying channel lengths, after epitaxial (epi) growth along the Si sidewall. The AFM scan results are compared to CDSEM images for top-down corroboration of topography and to other reference metrology for height correlation. The impact of measured variations in epi height to device performance is also reviewed.
EUV resist characterizations for line and space patterning as a function of dose and illumination conditions for varying pitches down to 28 nm are discussed. The unintentional resist line top loss (LTL) after development has been monitored and analyzed for all experimental conditions. Furthermore, line top roughness (LTR) is introduced, which is a 3 stochastic metric characterizing in-plane roughness related to the top of the resist lines. The main characterization technique employed for this study is atomic force microscopy (AFM) with novel probing algorithms as well as novel tips with diameters down to 5 nm and aspect ratios of 10:1. Additionally, results acquired by critical dimension scanning electron microscopy and optical critical dimension scatterometry are presented. It was found that the unintentional LTL is resist- and pitch-dependent and can be higher than 9 nm at 16 nm half-pitch but does not correlate with line break defect density results. However, LTR measurements of small area scans at dense line/space pitches may be used to draw conclusions about line break defect densities and hence yield. The resist specific metrics, LTR and LTL, allow for fast and early-on evaluation of new chemical formulations and help to forecast pitch- and dose-dependent performance. Furthermore, the results can be used to improve resist model accuracy for optical proximity correction calculations.
Optical Proximity Correction (OPC) modeling has evolved throughout the years to address 2D edge placement corrections, driven mainly out of CDSEM measurement results. 3D pattern modeling, and 3D-OPC for resist pattern, is much more complex. 3D information can be obtained by Optical CD tools for large repetitive arrays, where the outcome is an averaged 3D profile, or by Atomic Force Microscopy (AFM), offering accurate local in-die 3D metrology but can be limited by tip size and can also involve long scan times. In recent years we have presented a 3DSEM imaging methodology using multiple detectors to reconstruct surface morphology. To address the 3D-OPC model characterization challenge we combine in this work two metrology methods; AFM as a reference tool, and 3DSEM. Both metrology techniques provide nondestructive, large statistical sample measurements, across the wafer for lithography process window characterization, and 3D-OPC model calibration. Parameterized test patterns were generated on mask by Synopsys and printed on wafers at IMEC. Selective patterns were measured by 3DSEM and AFM to optimize 3DSEM measurements. Statistically significant 3DSEM measurement results were used for 3D-OPC model calibration, offering 3D-OPC corrections including resist shrink induced by SEM imaging.
Traditional metrology solutions face a range of challenges at the 1X node such as three dimensional (3D) measurement capabilities, shrinking overlay and critical dimension (CD) error budgets driven by multi-patterning and via in trench CD measurements. With advent of advanced technology nodes and 3D processing, an increasing need is emerging for in-die metrology including across-structure and structure-to-structure characterization. A myriad of work has emerged in the past few years intending to address these challenges from various aspects; in-die OCD with reduced spot size and tilt beam on traditional critical dimension scanning electron microscopy (CDSEM) for height measurements. This paper explores the latest capability offered by PeakForceTM Tapping Atomic Force Microscopy (PFT-AFM).
The use of traditional harmonic tapping mode for scanning high aspect ratio, and complex “3D” wafer structures, results in limited depth probing capability as well as excessive tip wear. These limitations arise due to the large tip-sample interaction volume in such confined spaces. PeakForce Tapping eliminates these limitations through direct real time control of the tip-sample interaction contact force. The ability of PeakForce to measure, and respond directly to tip- sample interaction forces results in more detailed feature resolution, reduced tip wear, and improved depth capability. In this work, the PFT-AFM tool was applied for multiple applications, including the 14nm fin and replacement metal gate (RMG) applications outlined below. Results from DOE wafers, detailed measurement precision studies and correlation to reference metrology are presented for validation of this methodology.
With the fin application, precision of 0.3nm is demonstrated by measuring 5 dies with 10 consecutive runs. Capability to resolve within-die and localized within-macro height variation is also demonstrated. Results obtained from the fin measurements support the increasing trend that measurements in the scribe line may not accurately represent in-die geometry, thus indicating the increasing need to measure the real device area.
In-die measurement capability of peak force tapping AFM on wafers at post-poly-removal step in the RMG module is also evaluated. Precision of 1.22nm for the fin height under the gate, 1.06nm for the total gate height, and 0.77nm for the overburden are achieved in this application on a semidense structure. To the knowledge of the authors, this is the first demonstration of a robust in-die measurement of the fin height under the gate.
Proc. SPIE. 9424, Metrology, Inspection, and Process Control for Microlithography XXIX
KEYWORDS: Electron beams, Metrology, Data modeling, Atomic force microscopy, Optical testing, Scanning electron microscopy, Data acquisition, Optical simulations, Scanning probe microscopy, Semiconducting wafers
Electron beam induced surface damage in general, and resist shrinkage in particular, are serious issues in any form of electron beam based metrology. Previous studies investigated dimensional changes that occur in resists that were exposed to electron beams. This work builds on these previous studies to consider changes to the material properties of the exposed resists and other materials using quantitative nano-mechanical mapping scanning probe microscopy. Initial data has shown clearly that there are measurable material differences between pre- and post-electron beam exposure. To study this change iArF photo-resists are exposed to varying electron beam energies and doses. These regions are then measured via SPM for dimensional and material property changes. These changes in the exposed areas are correlated to those predicted by modeled results.
In an ongoing study of the physical characterization of Gate-All-Around Silicon Nano Wires (GAASiNW), we found that the thin, suspended wires are prone to buckling as a function of their length and diameter. This buckling takes place between the fixed source and drain regions of the suspended wire, and can affect the device performance and therefore must be studied and controlled. For cylindrical SiNW, theory predicts that buckling has no directional preference. However, 3D CDSEM measurement results indicated that cylindrical wires prefer to buckle towards the wafer. To validate these results and to determine if the electron beam or charging is affecting our observations, we used 3D-AFM measurements to evaluate the buckling. To assure that the CDSEM and 3D-AFM measure the exact same locations, we developed a design based recipe generation approach to match the 3D-AFM and CDSEM coordinate systems. Measuring the exact same sites enables us to compare results and use 3D-AFM data to optimize CDSEM models. In this paper we will present a hybrid metrology approach to the characterization of GAASiNW for sub-nanometer variations, validating experimental results, and proposing methods to improve metrology capabilities.
This paper reports recent progress in using Atomic Force Microscopy as a defect review tool for patterned wafers. The key developments in the AFM technology are substantial scan speed improvements and the ability to reach feature bottom-CDs in a narrow trench. The latter is accomplished by controlling the tip-sample interaction via the short-range interaction force. Narrow trenches with vertical side wall angles comparable to current FinFET dimensions were imaged using the AFM, where imaging speeds for this sample reached about 0.2 frames per second, providing quantified topographic data for key features of the trenches. The sub-10 nm resolution data of high speed AFM demonstrates the technology as a viable solution for defect review.
With the advent of FinFETs, precise control of sidewall roughness (SWR) has taken on a new importance in semiconductor manufacturing. The sidewall of the fin is the largest area of contact between the gate and channel. Controlling this contact requires precise and accurate metrology, which in turn requires calibration. Developing a calibration standard for sidewall roughness is therefore vital. This paper describes initial work towards creating such a standard, by demonstrating mutually supporting reference metrology on a patterned roughness feature. To create the standard, photoresist features were patterned using a programmed and controlled line edge roughness (LER). Initial roughness data was obtained by critical dimension atomic force microscopy (CD-AFM), a conformal film was then deposited to provide contrast for transmission electron microscopy (TEM), and full 3D roughness information across the entire sidewall was acquired by TEM tomography. The following serves as proof of concept for using these two measurements to check each other, moving towards development of a usable sidewall roughness standard.
This paper reports on new developments of advanced CD AFM probes after the prior introduction of "trident probes" in
SPIE Advanced Lithography 2007 . Trident probes, having sharpened extensions in the tip apex region, make
possible bottom CD measurements within a few nanometers of the feature bottom corner; an area where other CD probes
have difficulties due to tip shape limitations. Moreover, new metrology applications of trident probes have been
developed for novel devices such as FinFET and vertical read/write hard disk heads.
For ever smaller technology nodes, new probes evolved from the design of the trident probe. For example, the number
of sharpened tip flares was reduced from three (trident) to two (bi-pod) to prevent possible interference of the third leg in
the slow scan direction, as shown in Figure 3.
Maintaining tip lateral stiffness as the tip size shrinks to less than 30 nm is vital for successful scanning. Consequently,
a significant recent improvement is the change of probe shank cross-sectional geometry in order to maintain tip vertical
aspect ratio of 1:5 (and lateral stiffness > 1 N/m). Finally, modifications of probe substrate are proposed and evaluated
for current and new CD AFM systems.
Hydrophobic, self-assembled monolayer (SAM) coatings were applied on CD probes to reduced tip "pull-away"
distance1 during CD AFM scanning. Test results show that the pull away distance can be reduced more than 5 times on
average (in some cases, by a factor of 15). Consequently, use of hydrophobic SAM coatings on CD probes mitigates
pull-away distance thus allowing narrow trench CD measurements.
We discuss limitations of prior CD AFM probes and design considerations of new CD probes. The characterization of
first prototypes and evaluation of scan performance are presented in this work.
The present paper is a continuation of an investigation to validate CD AFM image reconstruction using Transmission
Electron Microscopy (TEM) as the Reference Metrology System (RMS). In the present work, the validation of CD
AFM with TEM is extended to include a 26 nm diameter carbon nanotube (CNT) tip for non-reentrant feature scans.
The use of DT (deep trench) mode and a CNT tip provides detailed bottom feature resolution and close mid-CD
agreement with both TEM and prior CD mode AFM scans (using a high resolution Trident tip). Averaging AFM scan
lines within the ~80 nm thickness region of the TEM sample is found to reduce systematic error with the RMS.
Similarly, errors in alignment between AFM scan lines and TEM sample are corrected by a moving average method.
Next, the NanoCD standard is used for complete 2D tip shape reconstruction (non-reentrant) utilizing its traceable
feature width and well-defined upper-corner radius. The shape of the NanoCD is morphologically removed from the
tip/standard image, thus providing the tip's shape with bounded dimensional uncertainty. Finally, an update of the
measurement uncertainty budget for the current generation CD AFM is also presented, thus extending the prior work by
An extensive test series was undertaken to validate image reconstruction algorithms used with critical dimension atomic
force microscopy (CD AFM). Transmission electron microscopy (TEM) was used as the reference metrology system
(RMS) with careful attention devoted to both calibration and fiducial marking of TEM sample extraction sites. Shape
measurements for the CD probe tips used in the study were acquired both through the use of reentrant image
reconstruction and independent (non-destructive) TEM micrographs of the probe tips. TEM images of the tips were
acquired using a sample holder that provided the same projection of the tip as presented to the sample surface during
AFM scanning. In order to provide meaningful validation of the CD AFM image reconstruction algorithm, widely
varying sample morphologies and probe tip shapes were selected for the study. The results indicate a 1 - 2 nm bias
between the TEM and CD AFM that is within the uncertainty of the measurements given the Line Width Variation
(LWV) of the samples and accuracy of the measurement systems. Moreover, each TEM sample consisted of a grid with
multiple features (i.e., 21 to 22 features). High density CD AFM pre-screening of the sample allowed precise locating
of the TEM extraction site by correlating multiple feature profile shapes. In this way, the LWV and height of the
sample were used to match measurement location for the two independent metrology systems.
Proc. SPIE. 6518, Metrology, Inspection, and Process Control for Microlithography XXI
KEYWORDS: Metrology, Silicon, 3D modeling, Atomic force microscopy, Transmission electron microscopy, Zoom lenses, Scanning probe microscopy, Critical dimension metrology, System on a chip, Carbon nanotubes
As semiconductor and data storage industries apply Critical Dimension Atomic Force Microscopy (CD-AFM) for their
metrology needs in research and production, (1) measurement accuracy/repeatability and (2) measurement throughput
are the major criteria for acceptance. However, these two requirements are usually contradictory for a metrology
instrument. For example, a scatterometer can take a snapshot of a wafer in seconds, but such indirect CD measurements
are biased by the availability of library models and uncertainty of computer simulations. Transmission Electron
Microscopy (TEM) provides an atomic-scale resolution that is traceable back to the lattice structure of atoms, yet the
cross-section data is highly localized and can take days or weeks to acquire.
In the case of CD-AFM, since the scanning probe physically interacts with the structure of interest at a close proximity,
the determination of sample morphology comes from direct measurements. Therefore, the measurement uncertainty can
be attributed to: (1) AFM probe tip shapes and (2) system control and scan algorithms. For the former, past efforts have
been mainly focused on improving metrology accuracy and repeatability by reducing the dimensional uncertainty of a
tip shape. This approach includes characterizing the probe tip shape periodically. Inevitably, such tip shape calibration
procedure takes time (approximately 5 min) and burdens production throughput.
In this paper, we introduce several new methods for AFM probe tip shape characterization with different designs of tip
shape characterizers. The new tip shape characterizers were designed to address the limitation of current structures.
First, a single silicon overhang structure with wear-resistant coatings was used as the characterizer for both tip width
and tip shape profile. Tip-to-tip scan repeatability data (0.7 nm 3 Sigma) and measurement statistics suggest an
improvement over present state-of-the-art practice. Tip shape profiles of several high aspect ratio (20:1 to 25:1), low
lateral stiffness probes were successfully characterized with this method. Furthermore, the use of single characterizer
provides an opportunity to shorten tool calibration time, and consequently, increase measurement throughput.
In addition, a carbon nanotube characterizer prototype is proposed for CD-AFM. When scanning probe geometry
shrinks with semiconductor technology nodes, it has become a challenge to characterize a probe with a few tens of
nanometer of width with a micrometer-size characterizer. Using a comparable or smaller size of characterizer for a
small (20 to 50 nm) AFM probe not only reduces the dimensional uncertainty, but also expands the 2-D profiling
capability of current tip shape characterization.
We will discuss limitations of current tip shape profiling techniques, proof-of-concept experiments for new
characterizers, implementation of new tip shape characterization methods, and approaches to increasing measurement
The use of carbon nanotubes (CNT) as probes for atomic force microscopy (AFM) has been studied worldwide for more than a decade; however, the industries have not widely accepted CNT probes in their day-to-day operation. In this work, we present a series of studies on the metrology performance of CNT probes in semiconductor industry. A total of 54 CNT probes were studied for tip geometry, and 11 probes were tested on production wafers from a variety of IC manufacturers. Five out of the 11 probes were further evaluated for tip lifetime in semiconductor manufacturing environments. Statistical measurement data and tip shape characterization results provide insights on the applications of CNT probes in microlithography process control. The recent advancements in AFM scan algorithms that enable the control and use of CNT probes were also discussed in this paper. Sidewall measurement data using tilted CNT probes, and the AFM image of a CNT probe showing a comparable resolution to that of transmission electron microscopy (TEM) were presented for the first time. The combination of advanced AFM system and CNT probes has proven to perform challenging metrology in 65 nm node and beyond.