With semiconductor technology approaching and exceeding 10 nm design rules the quality requirements for photomasks are continuously tightening. One of the crucial parameters is improved control of the critical dimension (CD) across the photomask. As long as linearity and through pitch effects are not involved, the quality measure is typically defined as CD uniformity. This parameter is normally measured on repeating structures of same size and shape, which are not necessarily placed in identical environments. Density dependent process effects, also called loading effects (LE), pose a challenge for the required CD control. There are several possible contributors to this kind of error within the mask manufacturing flow, such as etch driven loading effects, fogging effects during 50kV exposure and develop driven loading effects. All of these operate at different working ranges, starting at millimeters going down to only a few 100 μm scale. It is comparably easy to derive models for large scale phenomena like etch loading or fogging effects, in contrast to that it is not as straight forward to find suitable models for very short-range effects. A large amount of CD measurements taken by CD SEM is needed to identify such signals of low magnitude and short scales, which make the setup very resource intensive. Furthermore, this methodology requires artificial designs and test structures which aim to sample only the effect of interest. In this paper we present a strategy which combines CD SEM measurements from dedicated test masks with the results from regular product masks. The aim is the derivation and validation of the loading effect correction range and strength. In the first step the data from test masks is analyzed to set up the basic correction parameters. Following this, the approach is supplemented by product data where we combine mask CD and design data. The clear field distribution of the design is convoluted with respect to a hierarchy of length scales. This data is the input for a support vector machine analysis. Thus, we employ a flat machine learning algorithm. However, the input data has been set up to reflect multiple layers of convolution. This particular approach has been chosen, as each convolution length scale is associated with mask process properties, thus alleviating the burden of interpretation which typically mars the interpretation of models obtained by machine learning approaches.
The currently increasing demand for photo-masks in the regime of the 14nm technology drives many initiatives towards capacity and throughput increase of existing production line. Such improvements are facilitated by improved control mechanisms of the tools and processes used within a production line. While process control of long range parameters such as the average CD behavior is demanding yet conceptually well understood, other parameters such as the small scales CD properties are quite often elusive to process control. These properties often require a dedicated test mask to be processed in order to be validated. In this paper we introduce a systematic approach towards a product based monitoring of small scale CD behavior which uses a CD characteristic extracted from the defect inspection process. This characteristic represents the influence of CD relevant processes starting from 200m up to 4000 m. Large variations in the scale and magnitude of the CD characteristic are induced by layout specific design variations. However, the shape of these distinct curves is remarkably similar, which enables their use for monitoring as well as controlling the mask processes on the above stated spatial scales. In this paper it is demonstrated, that a meaningful process evaluation can be performed by using the classification capabilities of the support vector machines. The small scales CD characteristics presented in figure 1 originate from two distinct tools. Matching of the two tools can be assessed by training a support vector machine to classify the small scales CD characteristics according to their origin. The classification performance on the resampled training set as well as on the validation set is a robust measure for tool matching. The results of this approach are depicted in figure 2. The left panel shows the AUC statistics of bootstrapping resamples for tool comparison “A”. In this case no noticeable difference between the two tools is found (an average AUC of 0.55 suggest no learnable difference). This is contrasted by the tool comparison “B”, here the classifier has an average AUC of 0.75, indicating a learnable difference in the tool performances. This result is backed by the process understand of both tool types.
The currently increasing demand for photo-masks in the regime of the 14nm technology drives many initiatives towards capacity and throughput increase of existing production lines. Such improvements are facilitated by improved control mechanisms of the tools and processes used within a production line. While process control of long range parameters such as the average CD behavior is demanding yet conceptually well understood, other parameters such as the small scale CD properties are quite often elusive to process control. These properties often require a dedicated test mask to be processed in order to be validated. In this paper we introduce a systematic approach towards a product based monitoring of small scale CD behavior which uses a CD characteristic extracted from the defect inspection process. This characteristic represents the influence of CD relevant processes starting from 200 10-6m up to 4000 10-6m. Large variations in the scale and magnitude of the CD characteristic are induced by layout specific design variations. However, the shape of these distinct curves is remarkably similar, which enables their use for monitoring as well as controlling the mask processes on the above stated spatial scales. In this paper it is demonstrated, that a meaningful monitoring of the CD characteristic can be enabled through the use of machine learning methods. A classical monitoring scheme is typically based on measuring the deviation of each curve from the average behavior. However, the monitoring of a curve and deviations thereof often requires the evaluation of the overall shape of the curve. Thus we propose a monitoring concept which uses a support vector machine in order to learn the shapes of the CD characteristics. It is demonstrated that a statistical model of the CD characteristics can be trained and used in order to monitor single excursions (see Figure 1) as well as overall process changes.
Mask Process Correction (MPC) is well established as a necessary step in mask data preparation (MDP) for electron beam mask manufacturing at advanced technology nodes from 14nm and beyond. MPC typically uses an electron scatter model to represent e-beam exposure and a process model to represent develop and etch process effects . The models are used to iteratively simulate the position of layout feature edges and move edge segments to maximize the edge position accuracy of the completed mask. Selective dose assignment can be used in conjunction with edge movement to simultaneously maximize process window and edge position accuracy . MPC methodology for model calibration and layout correction has been developed and optimized for the vector shaped beam (VSB) mask writers that represent the dominant mask lithography technology in use today for advanced mask manufacturing . Multi-beam mask writers (MBMW) have recently been introduced and are now beginning to be used in volume photomask production . These new tools are based on massively parallel raster scan architectures that significantly reduce the dependence of write time on layout complexity and are expected to augment and eventually replace VSB technology for advanced node masks as layout complexity continues to grow . While it is expected that existing MPC methods developed for VSB lithography can be easily adapted to MBMW, a rigorous examination of mask error correction for MBMW is necessary to fully confirm applicability of current tools and methods, and to identify any modifications that may be required to achieve the desired CD performance of MBMW. In this paper we will present the results of such a study and confirm the readiness of MPC for multi-beam mask lithography.
EUV lithography draws increasing attention and its expectation is rising. For instance, replacing a triple patterning with ArF immersion lithography to EUV single patterning may reduce 50% of cost and 25% of cycle time . At the same time, the importance of MPC (Mask Process Correction) is also growing   . It has become no longer possible to handle recent small and complex features using a rule-based bias approach. It is known that EUV lithography masks have a different structural stack so that “short range effect” of EB proximity effect is observed in mask writing . In this paper, we investigated the above short range effect through MPC model calibration. Mask data preparation step of EUV mask case is performed and the Turn-a-around (TAT) is compared with conventional DUV mask case.
Electron-beam writer characterization is key to enable predictable product performance in a photomask shop. This is traditionally done by writing test patterns with one distinct tool on one blank. Within this article, we introduce a method that reduces uncertainty caused by variation of blanks and process parameters, by using multiple, subsequent electronbeam exposure steps with different same-of-a-kind tools. The method is demonstrated for the disentanglement of two of the most fundamental parameters in an e-beam tool, current density and blanker latency, which together determine the actual dose. Additional accuracy can be achieved by probing the same tool parameter with different methods, which is shown by comparing Critical Dimension Scanning Electron Microscopy of line-space patterns below the maximum shot size with Thin Film Optical Scatterometry of comparatively large pads. The multiple exposure method needs a proper correction of systematic effects caused by contact of exposed areas with air during mask transfer from one writer to another, which are presented and discussed.
The long-term development of electronics obliges increasingly tighter specifications for photomasks to meet the
requirements of continuing miniaturization. We report on the influence of two different linear drive nozzle types A and B
used for conducting the develop process on important mask properties, which comprise CD uniformity (CDU), loading
behaviour, mean to target (MTT), iso-dense bias, line width roughness (LWR), linearity, resolution and defectivity. The
results are presented for different resists, resist thicknesses and blank materials. First, the most important recipe
parameters to ensure the best develop performance are defined and experimentally determined. Those critical factors are
the nozzle scan speed over the mask, the develop time, the distance between nozzle and mask surface and the flow rate of
the medium. It is demonstrated how these parameters can significantly affect the develop process performance. Dark loss
experiments reveal that a more uniform resist removal takes place with the B kind of nozzle compared to that achieved
with nozzle A. Based on the mask properties, the performances of two different nozzle types are compared. It is found
that improvements with the B like nozzle can be achieved for CDU and loading. The presented nozzle type shows a
promising approach to meet the requirements of future electronics.
Implant layers must cover both logic and SRAM devices with good fidelity even if feature density and pitch differ very much. The coverage design rules of implant layers for SRAM and logic to active layer can vary. Lithography targeting could be problematic, since it may cause issues of either over exposure in logic area or under exposure in SRAM area. The rule-based (RB) re-targeting in the SRAM issue features is to compensate the under exposure in SRAM area. However, the global sizing in SRAM may introduce some bridge issues. Selective targeting and communicating with active layer is necessary. Another method is to achieve different mean-to-nominal (MTN) in some special areas during the reticle process. Such implant wafer issues can also be resolved during the lithography and mask optimized data preparing flow or named as lithography tolerance mask process correction (MPC).
In this manuscript, this conventional issue will be demonstrated which is either over exposure in logic area or under exposure in bitcell area. The selective rule-based re-targeting concerning active layer will also be discussed, together with the improved wafer CDSEM data. The alternative method is to achieve different mean-to-nominal in different reticle areas which can be realized by lithography tolerance MPC during reticle process. The investigation of alternative methods will be presented, as well as the trade-off between them to improve the wafer uniformity and process margin of implant layers.
This study quantifies the impact of systematic mask errors on OPC model accuracy and proposes a methodology to reconcile the largest errors via calibration to the mask error signature in wafer data. First, we examine through simulation, the impact of uncertainties in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state of the art mask manufacturing data while other variable values are speculated, highlighting the need for improved metrology and communication between mask and OPC model experts. It is shown that the wafer simulations are highly dependent upon the 1D/2D representation of the mask, in addition to the mask sidewall for 3D mask models. In addition, this paper demonstrates substantial accuracy improvements in the 3D mask model using physical perturbations of the input mask geometry when using Domain Decomposition Method (DDM) techniques. Results from four test cases demonstrate that small, direct modifications in the input mask stack slope and edge location can result in model calibration and verification accuracy benefit of up to 30%. We highlight the benefits of a more accurate description of the 3D EMF near field with crosstalk in model calibration and impact as a function of mask dimensions. The result is a useful technique to align DDM mask model accuracy with physical mask dimensions and scattering via model calibration.
Shrinking feature sizes and the need for tighter CD (Critical Dimension) control require the introduction of new
technologies in mask making processes. One of those methods is the dose assignment of individual shots on VSB
(Variable Shaped Beam) mask writers to compensate CD non-linearity effects and improve dose edge slope. Using
increased dose levels only for most critical features, generally only for the smallest CDs on a mask, the change in mask
write time is minimal while the increase in image quality can be significant. However, this technology requires accurate
modeling of the mask effects, especially the CD/dose dependencies. This paper describes a mask model calibration flow
for Mask Process Correction (MPC) applications with shot dose assignment.
The first step in the calibration flow is the selection of appropriate test structures. For this work, a combination of linespace
patterns as well as a series of contact patterns are used for calibration. Features sizes vary from 34 nm up to
several micrometers in order to capture a wide range of CDs and pattern densities. After mask measurements are
completed the results are carefully analyzed and measurements very close to the process window limitation and outliers
are removed from the data set.
One key finding in this study is that by including patterns exposed at various dose levels the simulated contours of the
calibrated model very well match the SEM contours even if the calibration was based entirely on gauge based CD
values. In the calibration example shown in this paper, only 1D line and space measurements as well as 1D contact
measurements are used for calibration. However, those measurements include patterns exposed at dose levels between
75% and 150% of the nominal dose. The best model achieved in this study uses 2 e-beam kernels and 4 kernels for the
simulation of development and etch effects. The model error RMS on a large range of CD down to 34 nm line CD is
The calibrated model is then used to generate 2D contours for line ends, space ends and contacts and those contours are
compared to SEM images. For all patterns, including those very close to the resolution limit, very good contour overlay
is achieved. It appears that by including the various dose levels in the calibration a very good separation of the e-beam
model components from the etch components is possible and that this also results in very accurate 2D model quality.
In conclusion, very accurate mask model calibration is achieved for mask processes using shot dose assignment.
Standard test patterns can be used for calibration if they include the dose variations intended for correction.
Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. Many input variables for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total critical dimension (CD) control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can become significant. In this work, we examine via simulation the impact of errors in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state-of-the-art mask manufacturing data, and other variable changes are speculated, highlighting the need for improved metrology and communication between mask and optical proxmity correction model experts. The simulations are done by ignoring the wafer photoresist model and show the sensitivity of predictions to various model inputs associated with the mask. It is shown that the wafer simulations are very dependent upon the one-dimensional/two-dimensional representation of the mask, and for three-dimensional, the mask sidewall angle is a very sensitive factor influencing simulated wafer CD results.
Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. Many input variables
for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total CD control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can
become significant. In this work, we examine via simulation, the impact of errors in the representation of
photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state of the art mask manufacturing data and other variables changes are speculated, highlighting the need for improved metrology and communication between mask and OPC model experts. The simulations are done by ignoring the wafer photoresist model, and show the sensitivity of predictions to various model inputs associated with the mask. It is shown that the wafer simulations are very dependent upon the 1D/2D representation of the mask and for 3D, that the mask sidewall angle is a very sensitive factor influencing simulated wafer CD results.
When compared to conventional chrome absorber masks, electron beam patterning of EUV masks requires additional
corrections to account for intermediate range electron backscattering from the mirror and tantalum based absorber layers. The performance of this Mask Proximity Correction software should not be specified based solely on traditional mask linearity measures. We propose a new mask linearity specification based on Time Dependent Dielectric Breakdown requirements for metal layers.
The 50keV ebeam exposure of EUV blanks leads to additional electron backscattering from the tantalum layer and the
mirror portion of the blank substrate that cannot be adequately corrected by in-tool algorithms. Coupling this additional
backscatter with process effects, such as develop and etch micro/macro loading, results in significant systematic Critical
Dimension (CD) errors for through pitch and linearity patterns on EUV masks. In wafer production EUV masks are
targeted as single layer exposure, which requires extremely stringent CD control. The systematic CD errors can easily
exceed the CD requirements of a typical EUV mask, facilitating the need for a correction scheme or mask process
AMTC and GLOBALFOUNDRIES have started a program to evaluate MPC solutions and drive improvements.
Working closely with companies that provide solutions for ebeam and process modelling along with the corresponding
correction, we have completed several iterations of MPC evaluations. Specifically, we have tested different equipment,
processes and process partitioning for model calibration including a verification of the results.
We report on the results of these evaluations, which include simulation of available models, as well as verification data
from mask prints. We conclude by summarizing the current capabilities of available MPC solutions and present the
remaining gaps for model and correction accuracy as well as the remaining questions for fully implementing MPC into
the process landscape.
Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models, which must balance accuracy demands with simulation runtime boundary conditions, rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. While certain system input variables, such as scanner numerical aperture, can be empirically tuned to wafer CD data over a small range around the presumed set point, it can be dangerous to do so since CD errors can alias across multiple input variables. Therefore, many input variables for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total CD control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can become significant. In this work, we examine with a simulation sensitivity study, the impact of errors in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD Bias values are based on state of the art mask manufacturing data and other variables changes are speculated, highlighting the need for improved metrology and awareness.
Dummy pattern fill is added to a layout of a reticle for the purpose of raising the pattern-density of specific regions. The pattern-density has also an influence on different process-steps which were performed when manufacturing a reticle (e.g. proximity effect of electron beam exposure process, developer, and etch-processes). Although the reticle processes are set up to compensate the influence of the pattern density, dummy pattern can have an influence onto the reticle CD. When the isolated features become “nested” by insertion of dummy pattern, the reticle CD variation is even larger because nested features exacerbate the proximity effect of an electron beam. Another reason is that the etch ratio as well as the develop dynamics during the reticle manufacturing process are slightly dependent on the local pattern-density of pattern. With different dummy pattern around the main feature, the final reticle CD will be changed. Wafer CD of main feature is also dependant on the surrounding patterns which will induce different boundary conditions for wafer exposure.
We have investigated three manufacturing sites for a 28nm first-metal layer reticle. Two of them were manufactured with a comparable process using the same advanced reticle binary blank material. For the third site a different reticle blank material with a relatively thin absorber layer thickness was used which was made with a comparable reticle process. The optical proximity correction (OPC) test patterns were designed with two different dummy patterns. The CD differences of the three reticles will be demonstrated for different dummy pattern and will be discussed individually. All three reticles have been exposed and the respective wafer critical dimension through pitch (CDTP) and linearity performance is demonstrated. Also the line-end performance for two dimensional (2D) structures is shown for the three sites of the reticle. The wafer CD difference for CDTP, linearity, and 2D structures are also discussed.
When compared to conventional chrome absorber masks, electron beam patterning of EUV masks requires additional
corrections to account for electron backscattering from the mirror and tantalum (Ta) based absorber layers. Current ebeam systems cannot correct for these additional backscattering effects with in-tool proximity effect correction (PEC)
algorithms. Hence new methods of correction are needed, which require an implementation of the correction into the
mask writer data prior to exposure. Where these corrections should be performed in the data flow between mask user and mask supplier, and who should calibrate and maintain the corrections is not clear. We present various approaches for model calibration as well as discuss the possible options for inserting mask process correction (MPC) into the mask process landscape. We report on an attempt to calibrate a correction for EUV masks using actual CD data, and an e-beam backscattering model. The resulting Point Spread Functions (PSF) were used to simulate and predict the measured CD data. We also explored the robustness of these models by varying the writing tool and mask blank characteristics. We conclude by recommending an appropriate flow for calibration and use of mask process correction and ownership of the model calibration, maintenance and the data correction processes.
Strict reticle critical dimension (CD) control is needed to supply ≤ 20nm wafer technology nodes. In front end
lithographic processes for example, precise temperature control in resist baking steps is considered paramount to limiting
reticle CD error sources. Additionally, current density during writing and focus are continuously tracked in 50kV e-beam
pattern generators (PG) in order to provide stable CD performance. Despite these strict controls (and many others),
feedback compensation strategies are increasingly utilized in mask manufacturing to reach < 2nm 3σ CD uniformity
(CDU). Such compensations require stable reticle CD signatures which can be problematic when alternate or backup
process tools are employed. The AMTC has applied principle component analysis (PCA) to resist CD measurements of
50kV test reticles fabricated with chemically amplified resists (CAR) in order to quantify the resist CDU capabilities of
front and backup lithographic process tools. PCA results elucidate significant resist CDU differences between similar
lithographic process tools that are considered well matched via CDU 3σ comparisons.
The utility of PCA relies on the statistical analysis of large data sets however, reticle CD sampling is typically sparse, on
the 10-2 m or centimeter (cm) scale using conventional scanning electron microscopes (CD SEM). Higher CD spatial
resolutions can be achieved using advanced inspection tools, which provide CD data on a substantially smaller length
scale (10-4 m), thus yielding a considerably larger CD snapshot for front/backup process tool comparisons. Combining
PCA analysis with high spatial resolution CD data provides novel insights into the opportunities for tool and process CD
Critical dimensions (CD) measured in resist are key to understanding the CD distribution on photomasks. Vital to this
understanding is the separation of spatially random and systematic contributions to the CD distribution. Random
contributions will not appear in post etch CD measurements (final) whereas systematic contributions will strongly impact
final CDs. Resist CD signatures and their variations drive final CD distributions, thus an understanding of the mechanisms
influencing the resist CD signature and its variation play a pivotal role in CD distribution improvements. Current
technological demands require strict control of reticle critical dimension uniformity (CDU) and the Advanced Mask
Technology Center (AMTC) has found significant reductions in reticle CDU are enabled through the statistical analysis of
large data sets. To this end, we employ Principle Component Analysis (PCA) - a methodology well established at the
AMTC1- to show how different portions of the lithographic process contribute to CD variations. These portions include
photomask blank preparation as well as a correction parameter in the front end process. CD variations were markedly
changed by modulating these two lithographic portions, leading to improved final CDU on test reticles in two different
chemically amplified resist (CAR) processes.
Reticle critical dimension (CD) errors must be minimized in order for photomask manufacturers to meet tight CD uniformity
(CDU) requirements. Determining the source of reticle CD errors and reducing or eliminating their CDU contributions are
some of the most relevant tasks facing process engineers. The AMTC has applied principal component analysis (PCA) to
reticle resist CD measurements in order to examine variations in the data. PCA provided the major components of resist CD
variation which were rescaled into reticle CD signatures. The dominant component of CD signature variation is very similar
in shape and magnitude between two different chemically amplified resist (CAR) processes, most likely indicating the
variation source is a common process or tool. CD variational signatures from PCA were used as a basis for launching
investigations into potential reticle CD error sources. PCA was further applied to resist CD measurements from alternate
process tools to assist efforts in judging the effectiveness of resist CD signature matching.
The continued shrink of integrated circuit patterns increases the demand for reticle enhancement techniques (RET). The
application of Sub Resolution Assist Features (SRAFs) is pushing mask processes to the resolution limit. Many
Chemically Amplified Resists (CAR) used in current photomask processes do not have the capability to fully meet the
current demand for SRAF resolution.
Often the resulting quality of small SRAFs suffers from pattern fidelity limitations like Line End Shorting (LES) and
corner rounding. While small SRAFs might physically resolve on the mask, these limitations cause massive nuisance
detections at defect inspections. In a productive environment, high levels of nuisance detections are not acceptable due to
the cycle time impact from classification and review.
The AMTC systematically investigated the SRAF capability of different mask processes in order to better understand the
process limitations as well as to predict the manufacturability of customer patterns. This investigation uses high
sensitivity inspections of a specially designed test pattern to determine the SRAF capability limits. An overview of the
predicted SRAF capabilities for different resists and blank substrates is provided along with verification on customer
Recently, the design of integrated circuits has become more and more complicated due to higher circuit densities. In particular for logic applications, the design is no longer uniform but combines different kinds of circuits into one mask layout resulting in stringent criteria for both wafer and photomask manufacturing. Photomask CD uniformity control and defectivity are two key criteria in manufacturing today’s high-end reticles, and they are both strongly impacted by the mask developing process.
A new photomask develop tool (ACT-M) designed by Tokyo Electron Limited (TEL) has been installed at the Advanced Mask Technology Center (AMTC) in Dresden, Germany. This ACT-M develop tool is equipped with a standard NLD nozzle as well as an SH nozzle which are both widely used in wafer developing applications. The AMTC and TEL used the ACT-M develop tool to adapt wafer puddle develop technology to photomask manufacturing, in an attempt to capture the same optimum CD control enjoyed by the wafer industry. In this study we used the ACT-M develop tool to examine CD uniformity, local loading and defect control on P-CAR and N-CAR photomasks exposed with 50keV e-beam pattern generators. Results with both nozzle types are reported. CD uniformity, loading, and defectivity results were sufficient to meet 65-nm technology node requirements with these nozzles and tailored made develop recipes for photomask processing.
With shrinking feature sizes there is a growing demand for improved uniformity values and defect levels especially for aqueous develop during photomask processing. Standard nozzle systems with discrete dispense channels for applying the developer medium onto the photomask surface may cause non-uniformities. This results in characteristic imprints in CD-uniformity reflecting the nozzle design used during the develop process step. These can lead on the one hand to an increased number and various types of defects and on the other hand to variations in CD-uniformity. A new puddle nozzle design for the STEAG HamaTech's ASP5500 has been developed to address this issue. Instead of discrete dispense holes the developer medium is applied onto the substrate surface by a full-width film. This media film is applied uniform across the substrate and has low impact onto the photomask surface. By combining the new nozzle design with gas-less high volume dispense pumps a very uniform and defect-free dispense can be achieved. The uniformity and defect performance of the new film nozzle will be presented and compared to a standard dispense nozzle system. The study has been done on masks with Chemically Amplified Resist (CAR).
In this contribution we will demonstrate how the use of negative tone CAR can significantly improve the CD control of mask layers in which CD is measured on opaque features. A thorough investigation of the individual contributions of sequential process steps in mask making revealed that the final CD uniformity can by improved by 20% when a negative tone resist is used. In case of 50 keV electron beam (EB) mask writing systems, that employ variable shaped beam (VSB) writing, the writing time can be reduced by 40-50 % when a chemically amplified resist (CAR) is applied. Therefore we have evaluated and characterized a commercially available negative-tone CAR. The resist showed good pattern performance down to 150 nm for isolated and semi-isolated opaque lines thus having the ability to form assist bar features. Vertical profiles have been obtained. Line edge roughness (LER) is more pronounced for this material when compared to standard EB resist ZEP 7000. But analysis of CD uniformity (3σ) of 500 nm opaque lines in local area with negative CAR and with positive tone ZEP 7000 showed 4,8 nm and 6,2 nm, respectively. Thus substantiating that the negative CAR is advantageous in terms of opaque line CD control. Regarding soft bake (SB) and post exposure bake (PEB) latitude, the CAR is stable with respect to soft bake temperature variation (3,7 nm/°C). Much more severe is the steep PEB latitude with respect to dose of 0,7-1,3 (μC/cm2)/°C. This requires the use of high precision baking tools for the PEB step. Since all mask blanks have been coated in-house, we have investigated a variety of pre-treatment steps. The influence of each step was characterized by contact angle measurement. We found out that the best results have been achieved when the sequence H2SO4/H2O2-cleaning-UV/ozone-clean-dehydration bake is applied to virgin blanks as delivered by the blank supplier.