Directed self-assembly (DSA) of block copolymers (BCPs) proves to be a viable solution for the ultrahigh density bit-patterned media (BPM) application. However, servo design integration is still extremely challenging since the servo layouts require more complex patterns than the simple arrays naturally achieved by the DSA process. We present an integration scheme to create BPM servo patterns by utilizing the BCP dot-array patterns. This proposed method is based on an imprint guided two-step DSA process, combined with conventional optical lithography to define two separate zones. Both the data zone and servo zone consist of self-assembled hexagonal dot arrays: a regular pattern in the data zone and an arbitrary pattern in the servo zone. This method was successfully used to fabricate a servo-integrated BPM template with an areal density of 1.5 Teradot/inch2 (Td/in.2) (Lo=22.3 nm). Using the fabricated quartz template, CoCrPt BPM media has been successfully patterned by nanoimprint lithography and subsequent ion-beam etching process on a 2.5 in. disk. Further, using patterned-in servo wedges on 1.5 Td/in.2 patterned CoCrPt media, we are able to close the servo control loop for track-following on a spin-stand test. The standard deviation of repeatable run-out over the full revolution is calculated to be about 4% of the 38.6 nm track pitch. This method is currently being used to fabricate a template at a much higher density of 3.2 Td/in.2 (Lo=15.2 nm).
Bit-patterned media (BPM) fabrication sets a high bar for nanopatterning especially in the aspects of lithography resolution and pattern transfer. Directed self-assembly (DSA) of spherical block copolymers (BCPs) provides promising pattern resolution extendibility and pattern layout flexibility as long as proper pre-pattern designs are provided. Polystyrene-block-polydimethylsiloxane in the form of monolayered spheres is used as a vehicle to form either globally densely packed nanodot arrays in the data zone or locally densely packed nanodot arrays in the servo zone on a BPM template. Skew compatibility of spherical BCPs is also discussed. The BCP dot template is then applied as the scaffold for pattern transfer into quartz to make a nanoimprint mold and further into magnetic storage media. Distributions of both dot sizes and dot spacings are closely monitored after DSA pattern formation and pattern transfer.
Critical dimension measurement is the most essential metrology needed in nanofabrication processes and the practice is
most commonly executed using SEMs for its flexibility in sampling, imaging, and data processing. In bit patterned
media process development, nanoimprint lithography (NIL) is used for template replication and media fabrication. SEM
imaging on templates provide not only individual dot size, but also information for dot size distribution, the location of
dots, pitch and array alignment quality, etc. It is very important to know the SEM measurement limit since the feature
nominal size is less than 20 nm and the dot feature size and other metrics will relate to the final media performance. In
our work an analytical SEM was used. We performed and compared two imaging analysis approaches for metrology
information. The SEM beam was characterized using BEAMETR test sample and software for proper beam condition
setup. A series of images obtained on a 27 nm nominal pitch dot array patterns were analyzed by conventional brightness
intensity threshold method and physical model based analysis using myCD software. Through comparison we identified
the issues with threshold method and the strength of using model based analysis for its improvement in feature size and
pitch measurement uncertainty and accuracy. TEM cross sections were performed as accuracy reference for better
understanding the source of measurement accuracy deviation.
Scatterometry has been used extensively for the characterization of critical dimensions (CDs) and detailed sidewall profiles of periodic structures in microelectronics fabrication processes. In most cases devices are designed to be symmetric, although errors could occur during the fabrication process and result in undesired asymmetry. Conventional optical scatterometry techniques have difficulties distinguishing between left and right asymmetries. We investigate the possibility of measuring grating asymmetry with Mueller matrix spectroscopic ellipsometry (MM-SE) for a patterned hard disk sample prepared by a nanoimprint technique. The relief image on the disk sometimes has an asymmetrical sidewall profile, presumably due to the uneven separation of the template from the disk. Cross section SEM reveals that asymmetrical resist lines are typically tilted toward the outer diameter direction. Simulation and experimental data show that certain Mueller matrix elements are proportional to the direction and amplitude of profile asymmetry, providing a direct indication to the sidewall tilting. The tilting parameter can be extracted using rigorous optical critical dimension (OCD) modeling or calibration methods. We demonstrate that this technique has good sensitivity for measuring and distinguishing left and right asymmetry caused by sidewall tilting, and can therefore be used for monitoring processes for which symmetric structures are desired.
Proc. SPIE. 7638, Metrology, Inspection, and Process Control for Microlithography XXIV
KEYWORDS: Metrology, Quartz, Image processing, Image analysis, Scanning electron microscopy, Transmission electron microscopy, Monte Carlo methods, Line width roughness, Critical dimension metrology, Process modeling
SEM metrology involves uncertainty of the linewidth measurement because the SEM signal formation is an extremely
complex process. In this work, we used an analytical SEM for CD metrology applications on quartz nanoimprint
template. The SEM was tuned first to find the best reasonable condition for consistent operation. Beam
characterization was done using BEAMETR beam measurement technique. SEM images of templates were taken at
optimum conditions. The measurements were done using a) regular imaging processing software and b) using physical
model based processing tool myCD. The quartz template was then measured using TEM crossections at selected sites
to reveal profile information as metrology comparison reference. The metrology capability and fundamental limitation
of analytical SEM operation with regular imaging processing was identified. Information about SEM setup and
materials was used. The considerable improvement using the physical modeling imaging process was found.
Patterned media is expected to be implemented in future generations of hard disk drives to provide data storage at
densities exceeding 10<sup>12</sup> bits/in<sup>2</sup> and beyond. The implementation of patterned media, which would involve developing
processing methods to offer high resolution (small bits), regular patterns, and high density, has posed a number of
metrology challenges. Optical Critical Dimension (OCD) is the leading candidate to overcome the metrology challenges
for patterned media. This paper presents the successful OCD measurements on the critical dimensions, sidewall-angles,
and detailed sidewall shape of gratings of quartz template and imprint disk with pitch as small as 57nm.
Scatterometry has been used extensively for the characterization of critical dimensions (CD) and detailed
sidewall profiles of periodic structures in microelectronics fabrication processes. So far the majority of applications are
for symmetric gratings. In most cases devices are designed to be symmetric although errors could occur during
fabrication process and result in undesired asymmetry. The problem with conventional optical scatterometry techniques
lies in the lack of capability to distinguish between left and right asymmetries. In this work we investigate the possibility
of measuring grating asymmetry using Mueller matrix spectroscopic ellipsometry (MM-SE). A patterned hard disk
prepared by nano-imprint technique is used for the study. The relief image on the disk sometimes has asymmetrical
sidewall profile, presumably due to the uneven separation of the template from the disk. The undesired tilting resist
profile causes difficulties to the downstream processes or even makes them fail. Cross-section SEM reveals that the
asymmetrical resist lines are typically tilted towards the outer diameter direction. The simulation and experimental data
show that certain Mueller matrix elements are proportional to the direction and amplitude of profile asymmetry,
providing a direct indication to the sidewall tilting. The tilting parameter can be extracted using rigorous optical critical
dimension (OCD) modeling or calibration method. We demonstrate that this technique has good sensitivity for
measuring and distinguishing left and right asymmetry caused by sidewall tilting, and can therefore be used for
monitoring processes, such as lithography and etch processing, for which symmetric structures are desired.
KEYWORDS: Metrology, Quartz, Image processing, Image analysis, Scanning electron microscopy, Transmission electron microscopy, Monte Carlo methods, Line width roughness, Critical dimension metrology, Process modeling
Critical dimension metrology is the most needed feedback in nanofabrication and automatic CDSEM-based methods
are by far the industrial standard for its well-established methodology and ease of programming and flexibility in
measurement setup and operation. The dimensional measurements from SEMs consist of two steps, the first being the
pixel based electron emission signal intensity profile generation and the second being the algorithm treatment on the
generated intensity profile for the dimension determination. However, SEM metrology involves uncertainty of the
measurement in the signal processing step, because the SEM signal formation is an extremely complex process
depending on the pattern geometry, materials, detector setup, and beam voltage. Analytical SEMs are even less
optimized for the task of quantitative metrology, especially at the CD ranging below 100 nm.
In this work, we used an analytical SEM for CD metrology applications on quartz nanoimprint template from the
perspective that only analytical SEM is accessible. The machine was tuned and beam characterization was done first to
find the best reasonable condition for consistent manual operation using BEAMETR beam measurement pattern and
software. The optimized beam condition set was then used for image collection on pitch pattern quartz template and
the measurements were done using regular imaging processing and physical model based processing tool myCD. In
order to discuss the spot size on the scan signal and the resulting influence on CD measurements, we used CHARIOT
simulation software for simulated intensity profile as demonstration. The quartz template was then measured through a
mask CDSEM for final data comparison. Selected sites were cross sectioned to reveal profile information as metrology
comparison reference. Through our exercise, the metrology capability and fundamental limitation of analytical SEM
operation with regular imaging processing was identified and the improvement using the physical modeling imaging
process was verified.
Proc. SPIE. 5752, Metrology, Inspection, and Process Control for Microlithography XIX
KEYWORDS: Metrology, Detection and tracking algorithms, Scanning electron microscopy, Signal processing, Process control, Finite element methods, Signal generators, Critical dimension metrology, Algorithm development, Semiconducting wafers
The CD measurements from CDSEMs is a two-step process, the first being the pixel based electron emission signal intensity profile generation and the second the algorithm treatment on the generated intensity profile for the dimension determination. Secondary electron emission is currently the most common choice for signal intensity profile generation due to its relatively high signal-to-noise ratio at low landing energy range and the maturity of detector designs than the backscattered electron collections. It is known that the secondary electron emission depends on feature topography. The pixel based intensity profile bears the signature of feature topography. Therefore, the following algorithm processing for CD derivation cannot be free from the feature topographic influence. In this study, we elect isolated resist line as our feature with the resist sidewall angle the major topographical contributor. A set of FEM wafers are exposed at a fixed CD node and the process window is identified for sampling range identification. A profile CDAFM is used as a reference tool for resist height, CD, and sidewall angle measurement. The range of sidewall angle variation in the process window is identified by CDAFM as the reference. Various locations in the process window are measured by CDAFM and CDSEM, and the ratios of the same metrology information are plotted as function of sidewall angle deviation from vertical profile. In this paper, the sidewall angle dependence of derivative based CDSEM algorithm will be identified and the impact of CD measurement dependence on sidewall angle on process control will be discussed. This paper aims to generate a guide for process engineers to perform better process control through the quantification of sidewall angle influence. This paper will also address the need of the development of new CDSEM algorithm which compensates sidewall angle influence.
The appropriate definition and identification of total measurement uncertainty from a group of metrology tools is becoming ever important as process tolerances continue to shrink in today's data storage and semiconductor manufacturing environments. The precision-to-tolerance ratio needs to be properly defined and minimized in order to maintain capable process control. The task of identifying components contributing the total measurement uncertainty therefore poses a major challenge for both the metrology tool manufacture's and the system owners on the customer side. In this paper, two models are proposed to perform the estimation of total measurement uncertainty component and the corresponding precision-to-tolerance ratio estimation with the methodology of the analysis of variance. Two models are developed to suit the measurement characteristics difference. The first is a crossed model designed for the nondestructive measurements and the second is a nested model developed for the measurement environments where sample destruction is unavoidable. The models analyze precision components from an individual tool as well as the entire tool group so that the error from matching is accounted for. Optical overlay and CDSEM tools were both selected for study and the measurement data were used for precision analysis. The error contribution from the bias identification was performed using a CDAFM as a reference and a CDSEM as tool under test. The methodologies developed in this paper serve as a guide for the metrology tool manufacturers and tool users to systematically estimate the total measurement uncertainty and the related improvement for precise process control.
The rapid increase in the areal density of hard drives has demanded a parallel improvement in component technology. In particular, the development of new thin film heads (TFH) requires the constant reduction of read head track widths. State-of-the-art lithography techniques have been invoked to keep pace with the need for smaller read-head patterning. The critical feature in the read head is the sensor width, MRW. The aggressive roadmap for the thin film head industry makes it essential to print 70nm isolated lines or below for the next generation. KrF lithography, extensively used in the current node, is inadequate to produce read heads with 70nm and narrower isolated lines. Optical lithography will need to transition to 193nm for products of the future generation. This paper reviews the challenges posed by the transition from KrF to ArF lithography in thin-film head processes. Using ArF scanners and binary masks, 100nm isolated lines can be printed with 8nm across-field CD variation. Alternating phase shifting masks (AltPSM) are utilized to further enhance the resolution. Metal deposition into lithographically defined stencils increases the difficulty of stripping photo resist. To facilitate the resist stripping, a release layer like PMGI is often applied beneath the photo resist. However, the resolution improvement from ArF lithography makes it difficult to control the even narrower release layers. In this paper we demonstrate an alternative approach, a bridge structure, to lift off photo resist. The results show the success of constructing 4um-long bridges with sub-100nm track width. Therefore this can be a promising alternative means of producing MRW.
Proc. SPIE. 5038, Metrology, Inspection, and Process Control for Microlithography XVII
KEYWORDS: Signal to noise ratio, Deep ultraviolet, Calibration, Process control, Smoothing, Photomasks, Critical dimension metrology, Algorithm development, Semiconducting wafers, Line scan image sensors
A methodology is proposed to evaluate measurement correlation and matching feasibility between CDSEMs from different vendors in a multiple tool environment. Two CDSEMs from different vendors are used in this study. The measurement correlation and matching feasibility is identified through a series of steps. The pixel oversampling of both CDSEMs were first adjusted to the same extent in order to achieve the same scan pixel signal-to-noise ratio on both CDSEMs. Then the corresponding linescan smoothing and averaging were lowered but optimized with the precision criterion of less than 2 nm. The scan pixel magnification calibrations on both CDSEMs were traced back to an internal pitch standard. The corresponding measurement linearity was checked against a resolution mask from which wafers with pitch ranging from 400 to 800 nm were generated using a 248 nm stepper. The resulting scanning magnification correction factors for both CDSEMs in two scan orientations were determined for the follow-on measurement corrections on 248 nm resist focus exposure pattern (FEM) wafers and e-beam resist wafers obtained from e-beam lithography. The algorithms' characteristics between two CDSEMs were identified and they were adjusted to use only the information derived from the first linescan derivative in order to maintain maximum algorithm similiarity. The 120 nm process window on 248 nm resist wafers has a positive 1.6 nm offset from CDSEM A to B through focus. Both CDSEMs report the same process latitude for 110 nm process window identification. The measurement correlation study on e-beam resist shows offsets of negative 1.5 and 2 nm from CDSEM A to B, depending on which CDSEM is used first for the measurement sequence. Analysis-of-variance was used to analyze the measurement correlation. It is found that there is no significant difference in offsets generated regardless of which CDSEM is used first for performing the measurement for the 248 nm resist system. However, the offset generated for e-beam resist is dependent on measurement sequence. We conclude that CDSEMs from different vendors can be adjusted to maintain a constant offset over a CD variation range since after tuning all the factors mentioned both CDSEMs have very similar responses toward resist line secondary characteristics. Therefore these two CDSEMs are viewed as matched from a process development and process control prospective. All other CDSEMs in the same vendor group are then matched to each of the two bridged CDSEMs following the correposponding vendor's routine without any major procedure change.
The charging of the workpiece in electron beam direct writing processes has been identified as a problem that disturbs the electron beam and causes pattern displacement error. In this paper the electron beam deflection caused by surface charging is evaluated by the SIMION and MATHEMATICA simulation programs. Isolated charged patterns of different geometry are simulated as electrodes with given potentials in SIMION to calculate the extent of beam deflection, while secondary electron emission yield and beam dosage are assigned in MATHEMATICA programming for determination of surface potential and spatial field, from which the beam deflection is then calculated. The simulation results are in good agreement with each other and they are compared with values available in literature. The initial charge content along with the pattern dimensions chosen in simulation are found to be the major factors that determine the extent of beam deflection. The limitation of SIMION simulation on the beam deflection is also discussed.
Charging effects on beam deflection of incident electrons in electron beam lithography are investigated. We shows first in detail how the non-unity yield of electron generation in insulator resist leads to local charging accumulation and affects the beam deflection of incident electrons as charging develops. Then the amounts of beam deflection are identified for various operating and resist dimension conditions, and then we conclude that the beam deflection should be avoided for more accurate manufacturing semiconductor devices by the control of charging effects.
The performance of scanning electron beam instruments such as CD-SEMs can be defined in terms of parameters such as the beam probe size, the spatial resolution, and the signal to noise ratio of the image. A knowledge of these quantities is important in verifying the fact that an instrument meets its specification, and subsequently for tracking and optimizing its performance during use. Analytical methods based on the power spectrum (2-D Fourier transform analysis) of images are now beginning to be used for these purposes but care must be exercised to ensure reliable and meaningful results. Two new methods are suggested which can offer more detailed information about the microscope performance while avoiding the pitfalls of the simpler technique. Code implementing these tests, written as a plug-in macro for the well known NIH Image program, is available on-line.