The control of critical dimension uniformity (CDU), especially intra-field CDU, is an important aspect for
advanced lithography, and this property must be controlled very tightly since it affects all of the exposure fields. It is
well known that the influence of the mask CDU on the wafer intra-field CDU is becoming dominant because the mask
error enhancement factor (MEEF) is quite high for low-k1 lithography. Additionally, the abovementioned factors impact
the CDU through global (field-level) and local (grating-level) variations. In this paper, we analyze in detail CDU budgets
by clarifying the impact of local CD variation.
The 50-nm staggered hole features using Att-PSM showed a mask global CDU of 1.64 nm (3sigma at the mask
level) and a wafer intra-field CDU of 2.30 nm, indicating that the mask global CDU was a major part of the intra-field
CDU. By compensating for the contribution of the mask CD, the wafer intra-field CDU can be reduced to 0.986 nm.
We analyzed the budgets of wafer intra-field CDU, which is caused by local CD variation (mask and process) and
measurement noise. We determined that a primary cause of the wafer intra-field CDU after applying a mask CD
correction was these local CD variations, which might disturb the proper use of dose correction for the mask CD. We
demonstrated that the impact of mask local CD variation on the correction flow can be greatly reduced by averaging
multiple point measurements within a small area, and therefore discuss the optimum conditions allowing for an accurate
intra-field CDU determination. We also consider optimization of the CD sampling scheme in order to apply a dose
correction on an exposure system to compensate for the mask CDU.
For sub 20nm features, IC (integrated circuits) designs include an increasing number of features approaching the
resolution limits of the scanner compared to the previous generation of IC designs. This trend includes stringent design
rules and complex, ever smaller optical proximity correction (OPC) structures. In this regime, a new type of mask,
known as opaque MoSi on glass (OMOG), has been introduced to overcome the shortcomings of the well-established
phase shift masks (PSM). This paper reviews the fundamental aerial imaging differences between identically designed
PSM and thin OMOG masks. The masks were designed for scanner qualification tests and therefore contain large
selections of 1D and 2D features, including various biases and OPCs. Aerial critical dimension uniformity (CDU)
performance for various features on both masks are reported. Furthermore, special efforts have been made to emphasize
the advantages of aerial imaging metrology versus wafer metrology in terms of shortening scanner qualification cycle
In this paper we compare the imaging properties of lithographic test structures formed on test masks
with different reticle absorbers for use in1.35 NA immersion lithography. We will look into different
aspects like process windows and CD fingerprints. Beyond that we look into the topographic effects
caused by the different absorbers, the mask 3D effects. We will study the interaction between the
different masks and immersion scanner.
Special attention is given towards the correctability of the intrafield CD fingerprint by mask and
scanner applying dose corrections.
As half pitch shrinks to sub 20nm dimensions, the latest hybrid IC (integrated circuit) designs include a greater number
of features that approach the resolution limits of the scanner than in the previous generation of IC designs. This trend
includes stringent design rules and complex, ever smaller optical proximity correction (OPC) structures. In this regime, a
new type of mask, known as opaque MoSi on glass (OMOG), has been introduced to overcome the shortcomings of the
well-established phase shift masks (PSM). As for lithography, scanner and mask determine ultimate intra-field
performance as one approaches scanner resolution limits. Holistic lithography techniques have been developed to
optimize the interrelated mask and scanner effects on critical dimension uniformity (CDU) and common process window
(PW) for the most demanding sub 20nm node features. This paper presents an efficient and production worthy
methodology for evaluating the CDU, PW, and 3D effect fingerprints of the latest immersion scanner and thin OMOG
masks, and minimizing them using high-order optimizers of the latest holistic ArFi lithography.
In the semiconductor technology using the 193nm ArF excimer laser, the problem of radiation damage on photomask
becomes more serious. This phenomenon is regarded as serious issue for semiconductor device fabrication. Some
approaches have been tried to prevent the radiation damage. One of reports indicates that the radiation damage can be
reduced by using an exposure tool with ultra clean extreme dry air . However, it is difficult to adopt dry air into all
exposure tools due to high cost. In our previous work, two facts were ascertained; radiation damage is caused by MoSi
film oxidation, and depends on MoSi film composition . In this paper, radiation damage was tried to decrease by
MoSi film modification of att. PSM. MoSi film composition for PSM is optimized in consideration of cleaning durability,
mask defect repair and processability. The new PSM is named AID (Anti Irradiation Damage). Radiation damage of AID
PSM can be improved by 40[%] from conventional PSM. Cleaning durability can be also improved by AID PSM. The
other evaluation items such as CD performance, cross section, defect level and repair, are equal between the AID PSM
and conventional one. Additionally, the lithography performances by simulation of AID PSM are equivalent with that of
conventional PSM. Therefore, it can be expected that there is no difficulty in converting conventional PSM into AID
PSM. From these evaluation results, development of AID PSM was completed, and preparation for production is now
The exposure tools have been advanced for finer patterns and higher throughput. However, it causes the increase of accumulation
of exposure dose on mask, which induces the mask CD growth. This issue has been reported as the radiation damage and
brought the low yield of device chips [1, 2, 3]. As the solution, the radiation damage can be reduced by the ultra extreme
dry air in exposure tool . It is difficult to adopt dry air to all exposure tool due to cost. In this work, we tried to solve
the radiation damage from photomask making approach. The attenuated phase-shift mask (att. PSM) was chosen for this
evaluation because its damage is severest. The test plates of att. PSM were exposed by ArF laser, and the amount of CD
degradation and the composition change in damage area were investigated. By the analyses of TEM and EDX, it was
confirmed that the root cause of radiation damage is oxidation of MoSi film. Therefore, the approaches from mask
process and material were tried to prevent MoSi film from oxidation. As a result, the approach from mask material,
especially modification of MoSi film is effective. And the characteristics of new MoSi film, such as CD performances,
cross section, and cleaning durability, were compared with conventional att. PSM. These results show the characteristics
of two masks are equivalent. Att. PSM with new MoSi film is promising solution to improve radiation damage.
The miniaturization of pattern size on photomask is advanced year by year. It becomes more important to improve Line
Edge Roughness (LER) and resolution because of their impacts on lithography performances. When miniaturization is
advanced, high sensitivity inspection is also indispensable. Therefore, LER becomes the key factor to reduce the
nuisance defect for high sensitivity mask inspection. Basically, LER originates from resist materials and EB writer. If
resist pattern LER is good, final pattern LER can be good too. One of the easiest solutions for LER is using thick resist.
Thick resist can vertically smooth down the LER. However, it deteriorates resolution due to the high aspect-ratio.
Another solution for LER is using low sensitivity resist. Low sensitivity resist needs many electron exposures by EB
writer. Therefore, electronic density of EB pattern increases and pattern edge becomes clear. However, it deteriorates
throughput, which is essential to production. Only by mask resist, it is difficult to satisfy all items, that is mask LER,
resolution and throughput.
In this study, the improvement of LER without deterioration of resolution is tried by dry etching process. It is found that
remaining resist after Cr etching has its limitation for mask LER. And Cr over etching and source power of Cr and MoSi
etching are effective factors for mask LER. On the basis of these results, the optimal etching process is determined. It is
confirmed that mask LER can be improved without deterioration of resolution by the optimal etching process.
The verification of not only two-dimensional feature but also three-dimensional feature, sidewall angle (SWA), has
been becoming increasingly important in NGL mask fabrication. The OMOG (Opaque MoSi on Glass) mask for ArF
immersion lithography with double patterning and the reflective type mask for EUV (Extreme Ultra- Violet) lithography
are especially in need of it.
There are several metrology tools e.g. SEM, AFM, and Scatterometry for sidewall angle (SWA) measurement. We
evaluated a new SWA measurement method using white-band width (WBW), which is equivalent to mask pattern edge
width, by CD-SEM. In general, WBW correlates with SWA. It narrows as SWA becomes steeper. However, the
correlation deteriorates when SWA is vertically near. This is due to the resolution limit of electron beam diameter used
for measurement. We analyzed the new approach to measure SWA by CD-SEM to solve this problem. And the analysis
revealed that WBW changes proportionately electron beam current value. The amount of width change depends on
In this paper, we will describe the new SWA measurement method and its evaluation results as well as SWA
measurement results of OMOG and EUV masks.
For 45nm and 32nm node technology, the challenges for resolution and CD control of mask patterns become the steeper
mountain path. Especially, Sub Resolution Assist Feature (SRAF) is the smallest pattern on mask and amplifies the
difficulty of mask fabrication. In order to improve the resolution of fine patterns, the influence of wet processing cannot
be neglected, because it causes the pattern collapsing. Wet processing of mask-making can be divided into resist
development and cleaning.
In this study, the root causes of pattern collapsing are investigated at each wet processing. It is confirmed that thin resist
can enhance the resolution limit of resist pattern and hard-mask blank, such as OMOG: Opaque MoSi On Glass, is
suitable for thinner resist under 1500A. The pattern collapsing of OMOG is compared with that of Att.PSM at the
cleaning before and after Cr stripping. Mask inspection finds that pattern collapsing can be suppressed by OMOG at both
cleanings. It is because OMOG has lower cleaning stress than Att.PSM due to lower aspect-ratio. This benefit is
demonstrated by cleaning stress simulation. Additionally, it is found that the SRAF size of OMOG can be wider than
Att.PSM by optical simulation. From these results, OMOG has much advantage of fine pattern fabrication and is the
optimal blank for 32nm node and beyond.
Current flash memory technology is facing more and more challenges for 45nm and 32nm node technology. To get good
CD and yield control, optimized RET, OPC modeling and DFM techniques have to be applied . To enhance process
window (PW) and better CD control for main features, assist features (SB) have to be used. Simulation and wafer
evaluation show that the SB CD performance is very critical. Based on OPC simulation, we can get a very good
prediction about the CD size and placement of assist features. However, we can not always get what we want from mask
suppliers. For 45nm node technology and beyond, The SB CD size (~ 20nm at 1X) has almost pushed to the current
mask process limit. Wafer fabs have a very big concern about the stability of linearity signatures from different
suppliers and different products in order to keep high accuracy of OPC models. Actually the CD linearity signature
varies from one mask supplier to another and also varies from product to product. To improve the SB CD control, the
ideal goal is to make "flat" linearity for all mask suppliers. By working closely with TPI mask supplier, we come up
solutions to improve SB CD control to get "flat" linearity. Also technology development is causing more severe SB
printability, we proposed a methodology to use AIMS for predicting SB printability. Wafer results proved the feasibility
for these methodologies.
The development of semiconductor process for 32nm node is in progress. Immersion lithography has been introduced as
an extension of 193nm lithograpy. In addition, DPL (Double patterning lithography) is becoming a strong candidate of
next generation lithography. The extension of optical lithography increases more mask complexity and tighter
specification of photomasks.
CD performance is the most important issue in the advanced photomask technology. However, it is expected that
conventional mask cannot satisfy the required mask specifications for 32nm node and beyond. Most of CD errors are
contributed to the dry etching process. Mask CD variation is greatly influenced by the loading effect from dry etching of
As the required accuracy of the mask arises, Cr absorber thickness has been gradually thinner. CD linearity with the
thinner Cr absorber thickness has better performance. However, it is difficult to apply thinner Cr absorber thickness
simply under the condition of OD > 3, which is needed for wafer printing. So, we adopted MoSi absorber instead of
conventional Cr absorber, because MoSi absorber has less micro and global loading effect than that of Cr absorber. By
using MoSi absorber, we can reduce Cr thickness as a hardmask. The thinner Cr hardmask allows for reduce resist
thickness and become same condition for conventional EB resist lithography.
The lithography performances were confirmed by the simulation and wafer printing. The new MoSi absorber mask
behaves similar to the conventional Cr absorber mask.
The adoption of super thin Cr as a hardmask made it possible to reduce resist thickness. By the application of the thin
resist and the latest tools, we'll improve the mask performance to meet the 32 nm generation specification.
For 45 nm node and beyond, the alternating phase-shift mask (alt. PSM), one of the most expected resolution
enhancement technologies (RET) because of its high image contrast and small mask error enhancement factor (MEEF),
and the binary mask (BIM) attract attention. Reducing CD and registration errors and defect are their critical issues. As
the solution, the new blank for alt. PSM and BIM is developed. The top film of new blank is thin Cr, and the antireflection
film and shielding film composed of MoSi are deposited under the Cr film.
The mask CD performance is evaluated for through pitch, CD linearity, CD uniformity, global loading, resolution and
pattern fidelity, and the blank performance is evaluated for optical density, reflectivity, sheet resistance, flatness and
defect level. It is found that the performance of new blank is equal to or better than that of conventional blank in all
items. The mask CD performance shows significant improvement.
The lithography performance of new blank is confirmed by wafer printing and AIMS measurement. The full dry type
alt. PSM has been used as test plate, and the test results show that new blank can almost meet the specifications of pi-0
CD difference, CD uniformity and process margin for 45 nm node. Additionally, the new blank shows the better pattern
fidelity than that of conventional blank on wafer. AIMS results are almost same as wafer results except for the
narrowest pattern. Considering the result above, this new blank can reduce the mask error factors of alt. PSM and BIM
for 45 nm node and beyond.
As the required accuracy of the mask arises, Cr shading film thickness has been thinner gradually. CD linearity with
the thinner Cr film thickness has better performance. However, it is difficult to apply thinner Cr film thickness simply
under the condition of OD > 3, which is needed for wafer printing. So, we tried to develop new shading film. We adopted
MoSi film, because MoSi film has almost no micro loading effect compared with Cr film. MoSi shading film with
att.PSM satisfied OD > 3 at 193nm wavelength with good resist profile. But the issue was dry-etching selectivity, because
shading layer material was the same of att. PSM layer material. Therefore super thin Cr etching stopper was inserted
between MoSi shading layer and MoSi att.PSM layer.
The mask CD performance of new blank was evaluated for CD linearity, CD through pitch, and global loading effect.
This blank and mask process reduce loading effect, therefore the mask CD performance is improved remarkably. In
conclusion, the mask manufacturing process margin was able to be expanded by this new blank and method, and it is
expected that we can achieve the required specifications for att.PSM in 45nm node and beyond.
The attenuated phase-shift mask (att. PSM) is one of resolution enhancement technologies (RET) and has been
widely adopted for several device layers. And the high-transmission att. PSM, which has various structures and
transmittances, can be expected to have the advantages in process window. In this paper, the lithographic performances
(Contrast, MEEF and DOF) of high-T att. PSM were evaluated by using the 3D electro-magnetic field simulator. The
results showed that high-T att. PSM has better MEEF and partially better DOF than those of 6%-transmission MoSi
type. As the transmittance is getting higher, the smaller line CD is needed for OPC adjustment especially at narrow
pitch. In respect of film structure, it is found that there is no large difference among three high-T att. PSMs except for
MEEF at specific pitch. Remaining chrome on the high-T films causes the trade-off between contrast and MEEF. The
simulation results are compared with AIMS results measured by AIMSTM 45-193i of Carl Zeiss. The AIMS results of
actual masks agree with no-Hopkins mode simulation very well, while they do not agree with Hopkins mode simulation
especially at narrow pitch. Because the azimuthal polarization does not cause contrast loss, the differences between
AIMS mode (conventional) and Scanner mode (vector effect emulation) are small.
Alternating Aperture Phase Shift Mask (AAPSM) is one of the most effective approaches to improve the resolution of
logic gate structures for ArF lithography of the 65nm half-pitch node and beyond because AAPSM shows good
performance due to the high image contrast and the small mask error enhancement factor (MEEF).
For AAPSM, the issue of intensity imbalance between pi-space and zero-space is well known. In order to solve this
issue, several kinds of AAPSM, such as single trench with undercut, single trench with bias are used in production
The fabrication of single trench with bias AAPSM requires that the quartz dry etch satisfies many conditions. The
etched quartz features must not only show excellent depth uniformity but also good etch depth linearity across a wide
range of feature sizes. However, in defocus conditions, the through-pitch image placement error becomes worse even
with good quartz etch depth linearity. The reason is that the phase error caused by mask topography is different
depending on the pitch.
In this work, we minimize the phase error through-pitch and through-focus by rigorous 3D mask simulations. Based
on the results, we have fabricated two masks with opposite quartz depth linearity signatures to estimate the imaging
impact of phase errors and used them for exposures on an ASML XT:1250Di immersion scanner. We discuss the
feasibility of this method by comparison of through-focus and through-pitch image placement errors between wafer
printing, AIMS, and simulation.
The immersion lithography for 45 nm generation has been developing aggressively for smaller critical dimension of semiconductor devices. The polarization lithography system is indispensable to have an advantage to use the immersion lithography with hyper NA (>1.0). As pattern size becomes smaller, mask induced polarization effects to polarization of exposure image seems not to be negligible. There are several issues about mask induced polarization. But dominant factor for mask induced polarization effect is not understood well.
In this paper, in case of monolayer mask of att.PSM, degree of polarization (DoP) strongly depends on film thickness and extinction coefficient from simulation and experimental results. DoP depends on material factor. And in case of double layer mask, DoP depends on total film thickness and extinction coefficient of both upper layer and bottom layer. So, DoP depends also on structure of mask.
Chromeless Phase Lithography (CPL) is one of resolution enhancement technologies (RET) for 65nm node and beyond. CPL has various advantages such as no necessity of double exposure, and small pattern displacement and CD error caused by the intensity imbalance. The high resolution lithography can be expected with the combination of high NA and off-axis illumination (OAI) in 193nm lithography. It is known that CPL can flexibly change structure through gate pitch. There are various kinds of structure, such as pure CPL, Zebra, Rim, and Stripe. And there are also various kinds of scattering bar depending on the gate pitch. In this paper, we estimated normalized image log-slope (NILS), mask error enhancement factor (MEEF), depth of focus (DOF) and phase shift depth for each CPL structure by rigorous 3D mask electro-magnetic field (EMF) simulation on mask topographies. And it was found that Zebra and Stripe can improve NILS, and Stripe is most effective to improve MEEF for narrow pitch. There is no large difference in DOF between all structures, and DOF for all structures with wide pitch can be expanded by the addition of chrome scattering bar. We evaluated the impact of phase shift depth and found that the optimal phase shift depths of all structures are larger than 180degrees. The improvement of mask-making accuracy becomes more important to achieve better mask pattern resolution. Therefore, we focused on the defects of the sub-resolution chrome feature and chrome scattering bar. It was found that the defects of sub-resolution chrome feature have big influences on the lithography performance. And the defects of scattering bars become more sensitive with closer to the main feature.
The phase shift mask (PSM) is one of the most effective approaches to improve ArF lithography performance. Recently, the quartz dry etching technology plays an important role to fabricate the PSM, such as space bias type Alternating (Alt.) PSM and chrome-less phase lithography (CPL) mask. The quartz etching profiles seems to be affected the lithography performance. In this paper, preliminary, we evaluate the nominal influences of quartz profile by rigorous electromagnetic field simulation. Then influence of the quartz profile is investigated by measuring the real masks. In this experiment, we intentionally fabricate Alt. PSM and CPL masks with the tapered side-wall and deeper micro-trench. Lithography performances of the real masks are measured by the aerial image measurement system (AIMS fab193). We compare the result of AIMS with simulation. We investigate the AIMS measurement well corresponds to the simulation. Side-wall angle and corner rounding strongly affect the lithography performance. However, micro-trench doesn’t affect a lot.
The alternating phase-shift mask (alt. PSM) is one of the most effective approaches to improve a resolution of the 65nm logic gate structure in ArF lithography. Previously we have studied the optimization of alt. PSM in 180nm gate-pitch. In this study, we evaluated various alt. PSM in the case of 160nm gate-pitch. Using a rigorous electro-magnetic field simulation of light scattering in 3D mask topographies, we evaluated CD difference between π-phase and 0-phase space size (the π-0 CD difference), resist CD through pitch and normalized image log-slope (NILS). The parameters for our simulation were mask structure (shallow trench depth (ST), undercut size (UC), space bias, Chrome (Cr) CD, pitch, phase shift depth) and ArF exposure condition (NA, sigma, defocus). From the results of simulation, it turned out that single trench structures with UC and/or space bias showed the good intensity balance through defocus. We compared the simulation results with the AIMS fab193 (Carl Zeiss) results and found there was no large difference. The combination of UC and space bias could be chosen as suitable structure for 160nm gate-pitch.
To extend 193nm lithography to 65nm node devices, alternating phase shift mask structure were optimized. Both single trench and dual trench structure was evaluated. The optimization was performed by rigorous electro-magnetic field simulation of light scattering in 3D mask topographies. Evaluation masks were fabricated according to the simulation results, and the mask image was evaluated by using AIMS fab193 (Carl Zeiss). Prior to the optimization, limitation of shallow trench depth and undercut size was considered from the standpoint of “mask making”. Maximum undercut size was defined in order to prevent the Cr pattern peeling in cleaning process. In the optimized structure, CD difference between adjacent patterns with 0-space and π-space is within ±10nm wiht 300nm focus margin for different pattern pitches.