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Electronic Materials Letters

, Volume 14, Issue 5, pp 533–547 | Cite as

Mask Materials and Designs for Extreme Ultra Violet Lithography

  • Jung Sik Kim
  • Jinho Ahn
Article
  • 198 Downloads

Abstract

Extreme ultra violet lithography (EUVL) is no longer a future technology but is going to be inserted into mass production of semiconductor devices of 7 nm technology node in 2018. EUVL is an extension of optical lithography using extremely short wavelength (13.5 nm). This short wavelength requires major modifications in the optical systems due to the very strong absorption of EUV light by materials. Refractive optics can no longer be used, and reflective optics is the only solution to transfer image from mask to wafer. This is why we need the multilayer (ML) mirror-based mask as well as an oblique incident angle of light. This paper discusses the principal theory on the EUV mask design and its component materials including ML reflector and EUV absorber. Mask shadowing effect (or mask 3D effect) is explained and its technical solutions like phase shift mask is reviewed. Even though not all the technical issues on EUV mask are handled in this review paper, you will be able to understand the principles determining the performance of EUV masks.

Keywords

Extreme ultraviolet lithography EUV mask Absorber materials Multilayer mirror Phase shift mask 

1 Introduction

Extreme ultra violet lithography (EUVL) has been decided to be inserted into high volume manufacturing of semiconductor devices for the 7 nm technology node and beyond [1, 2, 3, 4]. EUV source power, the biggest obstacle to the commercialization of EUVL for a long time, has been showing a lot of progress recently [5, 6]. Now, EUV mask is considered to be one of the most critical issues for the commercialization of EUVL. As EUV light is strongly absorbed by most materials, EUV mask cannot be designed based on the diffractive optics as conventional optical photomasks. As a result, the EUV mask structure, based on the multilayer (ML) mirror, is radically different from that of conventional optical lithography. An ML structure consisting of many alternating layers of materials showing dissimilar EUV optical constants is essential for reflecting a 13.5 nm wavelength light. The Bragg reflection—constructive interference of the partially reflected beam at several interfaces—is necessary to maximize EUV reflection at near-normal incidence, and its efficiency is determined by the structure (e.g., thickness of each layer and their ratios) as well as optical properties (refractive index and extinction coefficient). To produce optical contrast for imaging, patterns with high EUV absorbance are formed on the top of the ML mirror. The image contrast depends on the reflectivity difference between the ML region and the absorber region. The image contrast is defined as
$$Image\;contrast = \frac{{R_{ml} - R_{abs} }}{{R_{ml} - R_{abs} }} \times 100\%$$
(1)
where Rml and Rabs are the reflectivities measured on the ML and the absorber, respectively, as shown in Fig. 1. The ML, shown in equation and figures, indicates the ML with a capping layer.
Fig. 1

Structure of the patterned EUV mask. Mask contrast is obtained by the reflectivity difference between the ML region and the absorber region

2 EUV Mask Materials and Fabrication Process

The EUV mask fabrication process consists of two main steps: mask blank fabrication and mask patterning [7]. A high-precision EUV mask requires low thermal expansion material (LTEM) as a starting substrate. This substrate is coated with an EUV-reflective ML and is covered using a capping layer for preventing any unwanted oxidation. An absorber layer coating followed by an anti-reflective coating and a backside conductive layer coating completes the mask blank fabrication process. The mask patterning, which consists of e-beam writing and dry etching, is similar to the current optical mask process. A typical EUV mask fabrication process flow is shown in Fig. 2. An EUV mask consists of several layers that exhibit unique mechanical, chemical, and optical functions. The mask substrate maintains the rigidity of the mask with minimum distortion. It essentially requires a flat and smooth surface with zero defects, as well as a low coefficient of thermal expansion (CTE). The ML is one of the key components for determining mask efficiency; a lower reflectivity of ML causes more loss of optical power by mask. One of the greatest risks for EUV mask viability is maintaining ML performance over a reasonably long operation time. The ML’s capping layer protects masks from degradation. The absorber layer in an EUV mask has a similar purpose to the chrome layer of a deep ultraviolet (DUV) binary mask in terms of the pattern-forming function. Owing to the reflective nature of the EUV mask structure, the absorber material secures the pattern image contrast against the ML through absorption with minimum reflection. However, most of the metallic absorber materials exhibit high DUV reflectivity, which causes a problem in mask inspection efficiency using a DUV wavelength. To attain sufficient DUV contrast, an antireflective coating (ARC) is applied on top of the absorber layer. The backside conductive layer is used for electrostatic chucking, which is required in various process tools as well as in the exposure tool. The key properties of an EUV mask are the peak wavelength (or centroid wavelength) and the peak reflectivity.
Fig. 2

Schematics of EUV mask fabrication process flow

3 Substrate Material

The substrate is the starting material for EUV mask fabrication, and its quality assurance is necessary even though it is not sufficient for successful mask fabrication. The SEMI P37-1102 standard specifies substrate requirements: average and spatial variation in thermal expansion, surface flatness, and defect level (Fig. 3) [8]. These requirements arise from EUV-specific issues as well as general nanoscale patterning applications.
Fig. 3

SEMI P37-1102 substrate requirements.

(Reprinted from Ref. 8)

As the in-plane distortion of the mask caused by the temperature difference during exposure and mask manufacturing contributes to the image placement error at the wafer, it is essential to adopt low thermal expansion material (LTEM) as a mask substrate. Mask distortion is influenced by the spatial variation as well as the average value of CTE. The mask substrate standard requires a CTE in the range from 0 ± 5 ppb/K with a 6 ppb/K total spatial variation to 0 ± 30 ppb/K with a 10 ppb/K total spatial variation. As there is no commercial dilatometer that achieves EUVL requirements (measurement resolution of 1 ppb/K), a technique for measuring the absolute CTE with high accuracy and reproducibility is required. Interferometric dilatometers using different lasers have been proposed, and the results reveal the reproducibility less than 1 ppb/K with handling capability of a wide variety of materials with CTE ranging from ppm/K to ppb/K [9, 10]. The ML is coated onto the substrate of an EUV mask for achieving high reflectivity at a 13.5 nm wavelength. The EUV reflectivity is degraded by the roughness of the substrate surface, especially by the high spatial frequency roughness (HSFR) [11]. As shown in Fig. 4a, the HSFR causes large-angle scattering and loss of EUV light from the projection lens. The mid spatial frequency roughness (MSFR) causing wavefront error and speckle through a small angle scattering is shown in Fig. 4b.
Fig. 4

Schematics of a HSFR and b MSFR. HSFR leads to scattering outside the exit pupil, which causes loss of light throughput. MSFR leads to small-angle scattering, which causes wavefront error and speckle.

(Reprinted from Ref. 11)

Mask MSFR (also referred to as slope error) is not a source of flare as it is in mirrors, but rather leads to intensity variation in the image plane and becomes a source of line edge roughness (LER) or image placement error [12]. It was proposed that the frequency ranges for MSFR and HSFR should be 10−6/nm < f < 0.004/nm and 0.004/nm < f < 0.02/nm, respectively. The ML smoothing deposition technique reduces the mask HSFR, but it is not effective for MSFR. The SEMI standard specifies a HSFR of < 0.15 nm rms, and a local slope angle of the front surface < 1.0 mrad.

The mask flatness issue arises from the unique EUVL imaging system design, which is a non-telecentric illumination system. Any height variation of the patterned mask surface (e.g., the non-flatness) causes an image placement error (or overlay error) on the wafer [13, 14]. According to Eq. (2), the image placement error on the wafer (∆x) is approximately 1/38 of the non-flatness of the mounted mask (∆z) at the nominal 6° illumination angle, θ (see also Fig. 5):
$$\Delta x = \frac{\Delta z \times \tan (\theta )}{M}$$
(2)
Fig. 5

Image placement error at the wafer resulting from the mask non-flatness in the non-telecentric illumination system.

(Modified from Ref. 15)

The front surface flatness of an as-mounted mask is influenced by the freestanding flatness of the substrate surfaces, and by how parallel they are to each other [15]. The backside flatness of the mask also contributes since a flat-faced electrostatic chuck is the designated method of mounting a mask in an exposure tool [16]. The SEMI P37 standard specifies the front (freestanding) and backside flatness values less than 30-nm peak to valley (P–V).

4 ML Mirror Layer

At the EUV wavelength, every material has a refractive index of approximately 1, and the transmittance and reflectivity are almost zero. Thus, EUV mask should be a reflective mask, and a ML-coated mirror consisting of a significant number of alternating material layers showing dissimilar EUV optical constants is the solution for achieving high reflectance using the interfering EUV light. According to the Fresnel equation, the reflectivity at the boundary is proportional to the square of the difference in the indices of refraction. Typically, two different layers for the ML exhibit high and low atomic numbers, respectively, for maximizing the difference in electron density. These materials also require low EUV light absorption, which is determined by the imaginary part of the refractive index (extinction coefficient). Considering process capability as well as optical performance, Mo/Si MLs are known to be the best choices from the several material combinations for high reflectivity at a wavelength of approximately 13.5 nm [17, 18]. The Si layer, which has a low EUV absorption property, works as a spacer for periodic structure, whereas the Mo layer scatters the light even though it shows a high absorption characteristic. The thickness of the Mo–Si pair (d-spacing) and the ratio of the Mo thickness to the bilayer period (γ ratio) are designed for maximum reflection but minimum absorption. The period of the ML pairs should satisfy Bragg’s law for producing maximum reflectance at the EUV wavelength:
$$n\lambda = 2d\cos \theta \sqrt {1 - \frac{2\delta }{{1 - \cos^{2} \theta }}}$$
(3)
where n is an integer, d is the period of the ML pairs (d-spacing), λ is the EUV wavelength, θ is the light incident angle to the mask normal (currently 6 deg., also named as chief ray angle: CRA), and δ is the bilayer-weighted δn. δn is defined as 1 − n, where n is the real part of the refractive index.
A Mo/Si ML stack for an EUV mask blank typically consists of 80 thin-film layers, or 40 pairs of Mo/Si bilayers [19, 20, 21]. The d-spacing (thickness of one period of bilayer) is ~ 6.9 nm, with a Mo thickness of ~ 2.8 nm and a Si thickness of ~ 4.1 nm (Fig. 6). The theoretical value of the peak reflectivity is approximately 75%, but the practical value is several percentage points lower [22, 23]. The primary factor for the reflectivity loss is the intermixing of Mo and Si at the interface. Interface engineering using a third layer inserted at the Mo/Si interfaces can enhance reflectance by achieving sharper and smoother interfaces [24, 25, 26, 27]. The currently available peak reflectance at 13.5 nm is approximately 70% [25].
Fig. 6

a Cross-sectional transmission electron microscope (TEM) image of the Mo/Si ML stack. b Calculated peak reflectivity of the Mo/Si ML coating at normal incidence as a function of the number of period. c Theoretical and experimental EUV reflectance spectra of a 40-pair Mo/Si ML.

(Reprinted from Ref. 22)

A capping layer is used for protecting the ML from oxidation since the reflectivity loss due to surface oxidation is expected for Si-terminated ML. Among the several candidate materials, Ru is extensively used as a capping layer for EUV masks owing to its suitability for EUV mask manufacturing [28, 29, 30, 31, 32]. Thin (~ 2 nm) Ru capping layer exhibits high etch selectivity to the absorber layer [33] and significant chemical stability under mask wet cleaning condition in addition to the minimum reflectivity loss when added on top of the ML. Moreover, the capping layer is essential for protecting the ML surface during pattern repair process using Focused Ion Beam (FIB) or E-beam [34].

The propagation of substrate imperfections during ML deposition is strongly dependent on the substrate imperfection type, deposition tool, and deposition process conditions. In a typical ion-beam sputter deposition (IBD) process, the ion beam energy and the incident angle of deposition are the main process variations [35, 36, 37, 38]. A higher ion beam energy can enhance the smoothing by increasing the mobility of adatoms on the surface so the atoms can move around after attachment to the substrate. However, higher ion energy promotes the intermixing of Mo and Si layers during ML deposition, resulting in a reflectivity loss. The ion beam incident angle also results in conflicting effects on reflectivity uniformity and defect smoothing. Deposition with a non-normal angle predominantly produces better deposition uniformity. However, the shadowing at the defect results in a less effective substrate bump smoothing. Ion polishing using a secondary step has been proposed for defect smoothing. Ion polishing is most effective if it is performed only on the Si layers after the deposition of each Si layer (Fig. 7) [39]. This planarization process for smoothing substrate particles using a secondary ion source has been reported to smooth ~ 45-nm-height particles on the substrate to a residual height of ~ 0.2 nm, which render them noncritical in an EUVL printing process [40]. It has been reported that approximately 50-nm-diameter particles can be planarized to a height of 0.2 nm [33]. However, a small peak reflectivity loss associated with the smoothing process has been observed. EUV absorption by implanted argon (Ar) atoms or interfacial layer mixing owing to a secondary ion source is speculated to be the source of reflectivity loss [37]. Another concern is pits, another type of defect in the EUV substrate [41]. Pits can be smoothed using more glancing angles for etch flux while substrate particles can be flattened using normal angle etching [39, 42, 43, 44]. To smooth particles and pits simultaneously, a multistep process uses two different etch angles, the first focusing on pits, the second on particles, since the optimal etch angle of each is different. A 600-eV ion energy (Ar) and a medium incident angle of 30° is suggested for IBD, as an optimum process condition combined with secondary ion beam polishing of 250-eV Ar.
Fig. 7

a Schematic diagram of the IBD tool. b Illustration of the ion-assisted procedure using a secondary ion source.

(Reprinted from Ref. 38)

The ML defect can be generated from a substrate defect (bump, pit or particle on the substrate surface) or from a particle during ML deposition [40]. The ML defects are classified into two types: amplitude defect and phase defect [45]. Amplitude defect directly absorbs EUV light, and phase defect disrupts the constructive interference of reflections from each of the ML interfaces owing to phase difference (Fig. 8). Amplitude defects are either particles near the top of the ML stack or flaws in the ML. On the other hand, the phase defects are swellings or depressions on the surface of the ML [39].
Fig. 8

Classification of multilayer defects: a phase defect and b amplitude defect.

(Reprinted from Ref. 45)

5 Absorber Layer

An EUV mask requires a pattern material which absorbs EUV light and thus generates pattern image in contrast to the ML reflector. This material is called an absorber layer and requires several characteristics including high EUV absorption, stability under EUV radiation, conductivity, significant etch selectivity to capping layer, and proper DUV reflectivity for affording optical surface defect inspection. Several candidate materials, including compounds of tantalum (Ta), chromium (Cr), and nickel (Ni) have been proposed [46, 47, 48, 49, 50, 51]. Among them, Ta-based materials have been extensively studied. Ta-based materials, which have been developed for X-ray masks, can be easily etched by Cl2 and F-based chemistry with less etch bias. These materials have been found to be suitable for EUV mask fabrication and printing [52, 53]. The aerial image contrast depends on the absorber material as well as the absorber thickness (higher with a thicker absorber). However, a low-refractive index (n) material such as TaN can be employed for an attenuated phase shift mask (PSM), resulting in an even higher contrast at low thickness [54]. Recently, studies of using thinner absorber layer with high extinction coefficient for the EUV wavelength have been reported. The material candidates are transition metals such as Ni and Pd [55, 56, 57, 58, 59]. Thinner absorber layer becomes significantly important with high-NA system for resolution extension because CRA of incidence increases the mask shadowing effect (Fig. 9).
Fig. 9

a Calculated net reflectivity of a Ru-capped Mo/Si ML with 40 bilayers at 13.5 nm wavelength and 6° angle of incidence over-coated with various absorbing films as a function of film thickness. b Table of candidate absorber materials with thickness values, T, at which the net reflectivity falls below 2%.

(Reprinted from Ref. 55)

The mask shadowing effect is a problem caused by the combination of oblique illumination and mask topography. Due to this effect, a printed CD bias and position shift occur [60]. The shadowing effect is schematically shown in Fig. 10. Compared to the designed mask CD, the space features will print smaller and the line features will print larger. This geometrical effect is amplified with increasing thickness of the absorber stack:
$$Space\;CD\left( {printed} \right) = CD\left( {designed} \right) - (2d \times \tan \theta ) \times M$$
(4)
$$Line\;CD\left( {printed} \right) = CD\left( {designed} \right) + (2d \times \tan \theta ) \times M$$
(5)
where CD (printed) is measured at the wafer plane, M is the EUV scanner reduction factor, and θ is the light incident angle to the mask.
Fig. 10

Geometrical optics illustration of the EUV mask shadowing effect.

(Reprinted from Ref. 54)

According to the simulation, the use of TaN and Cr, which have a larger Δn (the difference between the real part of the refractive index and that of the vacuum), results in a larger shadowing effect [54]. As the absorber thickness increases, the difference in CD for different materials decreases. Several researchers have investigated methods for reducing the shadowing effect by using thin absorber materials such as Ni, Pd, Pt, and Co, having high extinction coefficient for EUV wavelength [48, 55, 56, 61]. It should be noted that the difference caused by the shadowing effect can be compensated via mask biasing; however, it can ultimately limit the resolution [62, 63]. To obtain the highest image contrast, the absorber thickness must be sufficient so that the leakage light is as low as possible.

EUV mask e-beam patterning is similar to that of the conventional optical mask. However, the temperature (post-exposure bake, dry etch, and repair) is maintained below 150 °C owing to the ML thermal instability and mask distortion [64]. One of the continuing challenges in e-beam lithography is the proximity effect, which significantly degrades pattern fidelity and CD control. The proximity effect is due to electron scattering in the resist and electron backscattering from the substrate materials. The range of proximity effects depends on the energy of incident electrons, the substrate material, and the thickness of the resist and substrate. A high-energy e-beam results in high-resolution resist patterns owing to the minimized exposure by forward scattering, as well as dispersed backscattered electrons. Materials with a high atomic number have high electron backscattering cross-sections, and they are expected to have larger proximity effects than that of materials with a lower atomic number. As EUV masks predominantly contain high atomic number elements for the absorber, the proximity effects are expected to be higher on these substrates than the low atomic number materials such as Si. Experimental results on the proximity effect during e-beam writing have been reported for some absorber stacks [65]. As shown in Fig. 11, a structure with a TaSiN top layer results in a larger proximity effect owing to a larger number of backscattered electrons from the high atomic number TaSiN layer exposing the resist.
Fig. 11

Proximity effect test patterns of a 200-nm line and space (L/S) pattern on the selected absorber stack for different exposure doses.

(Reproduced from Ref. 65, with the permission of the American Vacuum Society)

It is advisable to stop the absorber etch process with a sufficient selectivity to the capping layer. A high etch selectivity of 60:1 was reported against a Ru capping layer for TaN absorber etching with hexafluoric (SF6) chemistry [66]. An etch bias of less than 5 nm was obtained that was independent of both structure and size, which contrasts with the results of the Cr absorber. Other important issues in the absorber etch process are the sidewall angle and line edge roughness (LER). Moreover, the absorber etch process preferably requires a minimal effect on the EUV reflectivity of the ML. Possible contributing factors for reflectivity loss are the surface oxidation or surface morphology change. Sufficient uniformity with only a small loss of reflectance has been reported with a Ta-based absorber etch process [64]. Durability of ARC during etch is an important issue as well, because any reduction of the ARC thickness affects the DUV inspection contrast. Ru is a prominent capping material owing to its high etch selectivity against TaN absorber [32, 66].

There are several potential techniques to repair defects on EUV absorber patterns, which include electron beam, focused ion beam (FIB) [67], and scanning probe-based mechanical nano-machining [68, 69, 70]. Among them, the electron beam repair is the main solution for removing opaque absorber defect in the EUV mask [71, 72, 73, 74, 75]. Ta-based EUV absorber materials easily react with e-beam etch chemistry. The AFM technology-based technique of nano-machining has been well-proven in the area of photomask repair [76]. In the nano-machining repair, the hardness of a mask material is the important factor to consider since the tip worn-out causes repair accuracy worse. As the EUV absorber layer is significantly harder than the opaque layer of the optical mask, the tip is worn out much faster in Ta-based EUV mask repair than in Cr or MoSi optical mask repair, which results in large tip deflection [70]. E-beam induced deposition is performed for clear defects. The upper left picture in Fig. 12 displays an SEM image of an absorber defect. This defect appears less critical but, in AFM (upper middle picture), it is visible that the absorber is thinned. At the lower left image, we can observe an SEM image of the area as printed on the wafer. It shows that this defect is printed during the exposure process. Reason might be that the absorber material is significantly thin to absorb the EUV light sufficiently. This defect is repaired using an e-beam deposition process (upper right image). After the repair, the corresponding wafer print shows that this defect is no longer printable to the wafer [77].
Fig. 12

Example for e-beam deposition repair. The defect is almost invisible in SEM, and the AFM shows that the absorber is significantly thin, which is confirmed in the wafer print. After deposition, the defect is no longer transferred to the wafer.

(Reprinted from Ref. 69)

Another next generation repair technology is a modified FIB repair. Conventional FIB uses gallium ions (Ga+) generated by a liquid metal ion source (LMIS), but the modified FIB uses hydrogen ions (H2+) generated by a gas field ion source (GFIS). The minimum reaction area of H2+ FIB is theoretically much smaller than that of EB, as shown in Fig. 13 [78].
Fig. 13

Trajectories of ions and electrons in the case where those are implanted into Si bulk.

(Reprinted from Ref. 78)

As the ions of GFIS are generated in a field of a few atoms, the source size (Rs) of GFIS is significantly smaller than that of LMIS. As gas molecules are ionized in an electric field of particular intensity, the energy spread (ΔE) of GFIS is significantly smaller than that of LMIS. Therefore, the beam diameter (D) of GFIS is significantly smaller than that of LMIS [78].

A He+ scan with a high acceleration voltage (30 keV) causes the shrinkage of the ML as shown in the TEM image (Fig. 14). However, H2+ scan with the equivalent acceleration voltage (25 keV) causes neither a EUV reflection peak shift nor a reflectivity loss [78].
Fig. 14

TEM cross sections of EUVL masks after ion implantation.

(Reprinted from Ref. 78)

6 Advanced Mask Structure for Better Imaging

Alternative mask structures can alleviate mask shadowing effect and eventually extend the resolution limit of EUV lithography. Various kinds of advanced EUV masks include (1) an etched ML binary mask, (2) an attenuated PSM (embedded or etched ML), (3) an alternating PSM (substrate or ML etched), and (4) a modified alternating PSM (double etched or with an absorber stack) [79, 80, 81, 82, 83, 84]. Most of the studies have been based on simulation, but some of the experimental results have revealed the possibility of practical implementation. One such achievement is the etched ML binary mask. Mo/Si ML (20 pairs instead of 40 pairs) was locally etched to form dark trenches, and its optical properties were investigated. H–V CD bias and best focus shift through pitch were improved compared to the standard mask, but some optical properties were degraded (e.g., higher mask error enhancement factor: MEEF). The most critical challenges of this structure are the manufacturability and cleanability [85]. Based on the papers presented previously, the etched ML mask generally shows (1) larger depth of focus, (2) higher contrast, (3) no line width variation due to interference phenomenon, and (4) reduced H–V bias and image placement error (IPE) due to the shadow effect [86, 87].

Another advanced EUV mask structure which has been demonstrated extensively is the embedded attenuated PSM. The absorber layer for EUV should satisfy both the phase and the desired absorption, and the most plausible approach to achieve this is to use a two-layer structure. The main function of one layer is an absorber and that of the other layer is a phase shifter. The optimization methodology is explained in detail by Yan [88]. The phase shift is determined by the film thickness as well as the real part of the refractive index. An attenuated PSM with a 180° phase shift should satisfy the following condition:
$$\left| {\frac{2\pi }{\lambda }\left( {\frac{{2\Delta n_{1} d_{1} }}{\cos \theta } + \frac{{2\Delta n_{2} d_{2} }}{\cos \theta }} \right)} \right| = \pi$$
(6)
where Δn1 = 1 − n1 and n1 is the real part of the refractive index of film 1, and Δn2 = 1 − n2 and n2 is the real part of the refractive index of film 2. An appropriate combination of two films can improve the imaging performance as well as can reduce the mask 3D effect [89, 90]. However, several considerations should be made for etch selectivity and compatibility with other processes (Fig. 15).
Fig. 15

Schematic of the two-material EUVL embedded PSM structure.

(Reprinted from Ref. 90)

Complicated optimization of film thicknesses and optical constants can be simplified when the absorber and phase shifter have very similar refractive indices [89]. The out-of-phase condition (180° phase shift) was achieved with a ~ 40.5 nm-thick total absorber stack (TaN absorber + Mo phase shift layer), irrespective of the thickness ratio. (Fig. 16) Using this valuable characteristic, the reflectivity can be controlled by adjusting the thickness of the Mo phase shifter while maintaining the out-of-phase condition when the total thickness of the absorber stack is fixed at 40.5 nm. This is possible due to the Mo phase shifter, which has a refractive index similar to that of the TaN absorber. The experimental results obtained using a coherent scattering microscope (CSM) showed improvements in the optical performance such as the H–V CD bias and MEEF. The CSM confirmed a 180° phase difference at the pattern edge and the improved diffraction efficiency of the PSM over a standard binary mask [91, 92].
Fig. 16

a Cross section TEM photograph of TaN/Mo embedded PSM, b amplitude map, and c phase map obtained using CSM.

(Reprinted from Refs. 91, 92)

One of the most effective resolution enhancement technologies in lithography is increasing the numerical aperture (NA) of the projection optics to collect more diffracted light from the mask. Therefore, it is necessary to adopt optical elements with an NA higher than 0.33 to extend the resolution below 8 nm in EUVL [93, 94]. A CRA of 6° and 4X demagnification cannot be maintained with an NA of 0.45 because the incident and reflecting light cones overlap. To separate the light cones, an increase of the CRA or demagnification factor is needed. With a demagnification factor greater than 4, the mask should be enlarged to prevent a reduced exposure field size, which occurs with a higher demagnification factor, resulting in throughput decrease [95, 96]. However, it is difficult to change the size of the mask because this requires a large change in the mask fabrication infrastructure. If the CRA is increased instead to separate two light cones, the mask 3D effect due to the absorber thickness and oblique incident light is intensified [97, 98]. Moreover, imbalance in 0th order diffractions, which is due to reflectivity variation according to the angle of incidence, results in a telecentricity error causing a pattern shift on the wafer [99, 100]. Since pattern shifts and mask 3D effects become more significant with a finer pattern pitch, modified designs of the absorber stack and ML mirrors are required to alleviate the above-mentioned problems [101].

The ML mirror consisting of a periodic Mo/Si bilayer should be re-designed by adjusting the d-spacing and gamma ratio to be applied to the high-NA system with a CRA of 9°. The range of the incidence angle (α) for the incident light cone, which depends on the NA and demagnification factor (M), can be expressed as:
$$\alpha = 2\sin^{ - 1} \frac{NA}{M}$$
(7)
By increasing the NA, a wider range of incidence angles is expected according to Eq. (6). For an NA of 0.33 and 4× demagnification, the range of incidence angles α is 9.5° and the reflectivity variation over this range of incidence angles is less than 3% with a conventional ML mirror. However, for an NA of 0.45 and a CRA of 9°, the incidence angle ranges are 2.5°–15.5° and 4.3°–13.7° for 4× and 6× demagnification, respectively. Therefore, as shown in Fig. 17, the variation of the reflectivity increased up to 65% when the conventional ML mirror was used [102]. Redesigned ML mirror is helpful to reduce the imbalance of the reflectivity. At wider range of incident angles as can be seen in Fig. 17.
Fig. 17

Comparison of reflectivity depending on the angle of incidence between the conventional multilayer with a 6° CRA and the new multilayer optimized for a 9° CRA.

(Reprinted from Ref. 102, Copyright@ American Scientific Publishers)

The mask shadowing effect becomes even more significant due to the greater angle of incident light. This effect results in a smaller printed space on the wafer compared to the designed space on the mask. This in turn results in a horizontal–vertical critical dimension (H–V CD) bias and, eventually, a limitation in resolution. A high-NA EUV mask requires a thinner absorber, but this can result in the deterioration in the imaging properties due to the decreased mask contrast. A thin absorber material with a high extinction coefficient or phase shift absorber stack will help to alleviate the mask 3D effect with a high incident angle of the high-NA system.

7 Summary

EUVL is an extension of optical lithography that uses 13.5-nm-wavelength light. This wavelength requires major modifications in the standard optical lithography techniques, primarily due to the very strong absorption of EUV light by most materials. Refractive optics can no longer be used, necessitating the use of an ML mirror-based mask as well as an oblique incident angle of light. The imaging performance of the mask strongly depends on the mask structure and materials. The basic technologies related to the substrate, ML, absorber stack, mask patterning, cleaning, and metrology have recently shown significant improvements, but further systematic studies are needed to meet the requirements of 7 nm technology node applications. Resolution enhancement technology such as PSMs might be a solution to the technology’s extension, but this will require collaborative studies in relation to the simulation, metrology, and fabrication processes. In addition, many challenges still remain with regard to the mask infrastructure such as EUV mask fabrication, inspection, repair, pellicle integration, and handling of pelliclized masks [103].

Notes

Acknowledgements

I would like to thank all the authors of the technical papers referenced in this review paper. The author is indebted to all the students and colleagues for their dedicated assistance. This research was supported by the Commercialization Promotion Agency for R&D Outcomes (COMPA), funded by the Ministry of Science and ICT (MSIT) (Grant No. 2017K000389).

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Copyright information

© The Korean Institute of Metals and Materials 2018

Authors and Affiliations

  1. 1.Department of Nanoscale Semiconductor EngineeringHanyang UniversitySeoulSouth Korea
  2. 2.Department of Materials Science and EngineeringHanyang UniversitySeoulSouth Korea
  3. 3.Institute of Nano Science and TechnologyHanyang UniversitySeoulSouth Korea

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