Resource requirements

The required utility resources are significantly larger for EUV compared to 193 nm immersion, even with two exposures using the latter. At the 2009 EUV Symposium, Hynix reported that the wall plug efficiency was ~0.02% for EUV, i.e., to get 200-watts at intermediate focus for 100 wafers-per-hour, one would require 1-megawatt of input power, compared to 165-kilowatts for an ArF immersion scanner, and that even at the same throughput, the footprint of the EUV scanner was ~3× the footprint of an ArF immersion scanner, resulting in productivity loss.^[31] Additionally, to confine ion debris, a superconducting magnet may be required.^[32]

A typical EUV tool weighs 180 tons.^[33]

DUV vs. EUV Tool Energy Consumption (measured 2020):^[34] EUV tools consume at least 10× more energy than immersion tools.

Summary of key features

The following table summarizes key differences between EUV systems in development and ArF immersion systems which are widely used in production today:

The different degrees of resolution among the 0.33 NA tools are due to the different illumination options. Despite the potential of the optics to reach sub-20 nm resolution, secondary electrons in resist practically limit the resolution to around 20 nm (more on this below).^[44]

Tool uptime

The EUV light source limits tool uptime besides throughput. In a two-week period, for example, over seven hours downtime may be scheduled, while total actual downtime including unscheduled issues could easily exceed a day.^[54] A dose error over 2% warrants tool downtime.^[54]

The wafer exposure throughput steadily expanded up to around 1000 wafers/day (per system) over the 2019-2022 period,^[55][56] indicating substantial idle time, while at the same time running >120 WPH on a number of multipatterned EUV layers, for an EUV wafer on average.

Comparison to other lithography light sources

EUV (10–121nm) is the band longer than X-rays (0.1–10nm) and shorter than the hydrogen Lyman-alpha line.

While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm²,^[57] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 10¹¹ W/cm².^[58] A state-of-the-art ArF immersion lithography 120 W light source requires no more than 40 kW^[59] while EUV sources are targeted to exceed 40 kW.^[60]

The power target for EUV lithography is at least 250 W, while for other conventional lithography sources, it is much less.^[54] For example, immersion lithography light sources target 90 W, dry ArF sources 45 W, and KrF sources 40 W. High-NA EUV sources are expected to require at least 500 W.^[54]

Reflective optics

EUV H-V difference of focus. Horizontal (H) and vertical (V) mask (reticle) pattern features are focused differently in EUV optical systems. The numerical aperture (NA) also makes a difference.

A fundamental aspect of EUVL tools, resulting from the use of reflective optics, is the off-axis illumination (at an angle of 6 degrees, in different direction at different positions within the illumination slit)^[61] on a multilayer mask (reticle). This leads to shadowing effects resulting in asymmetry in the diffraction pattern that degrade pattern fidelity in various ways as described below.^[62][63] For example, one side (behind the shadow) would appear brighter than the other (within the shadow).^[64]

The behavior of light rays within the plane of reflection (affecting horizontal lines) is different from the behavior of light rays out of the plane of reflection (affecting vertical lines).^[65] Most conspicuously, identically sized horizontal and vertical lines on the EUV mask are printed at different sizes on the wafer.

2-bar CD difference vs. focus. The difference between the widths of two adjacent horizontal lines varies as a function of focus.

The combination of the off-axis asymmetry and the mask shadowing effect leads to a fundamental inability of two identical features even in close proximity to be in focus simultaneously.^[66] One of EUVL's key issues is the asymmetry between the top and bottom line of a pair of horizontal lines (the so-called "two-bar"). Some ways to partly compensate are the use of assist features as well as asymmetric illumination.^[67]

An extension of the two-bar case to a grating consisting of many horizontal lines shows similar sensitivity to defocus.^[68] It is manifest in the critical dimension (CD) difference between the top and bottom edge lines of the set of 11 horizontal lines.

Polarization by reflection also leads to partial polarization of EUV light, which favors imaging of lines perpendicular to the plane of the reflections.^[69][70]

Pattern shift from defocus (non-telecentricity)

Due to different phase shifts from reflection from the EUV mask, different illumination angles result in different shifts. This results in reduced image contrast, also known as fading.

The EUV mask absorber, due to partial transmission, generates a phase difference between the 0th and 1st diffraction orders of a line-space pattern, resulting in image shifts (at a given illumination angle) as well as changes in peak intensity (leading to linewidth changes)^[71][72] which are further enhanced due to defocus.^[73][74] Ultimately, this results in different positions of best focus for different pitches and different illumination angles. Generally, the image shift is balanced out due to illumination source points being paired (each on opposite sides of the optical axis). However, the separate images are superposed and the resulting image contrast is degraded when the individual source image shifts are large enough. The phase difference ultimately also determines the best focus position.

The multilayer is also responsible for image shifting due to phase shifts from diffracted light within the multilayer itself.^[75] This is inevitable due to light passing twice through the mask pattern.^[76]

The use of reflection causes wafer exposure position to be extremely sensitive to the reticle flatness and the reticle clamp. Reticle clamp cleanliness is therefore required to be maintained. Small (mrad-scale) deviations in mask flatness in the local slope, coupled with wafer defocus.^[77] More significantly, mask defocus has been found to result in large overlay errors.^[78][79] In particular, for a 10 nm node metal 1 layer (including 48 nm, 64 nm, 70 nm pitches, isolated, and power lines), the uncorrectable pattern placement error was 1 nm for 40 nm mask z-position shift.^[80] This is a global pattern shift of the layer with respect to previously defined layers. However, features at different locations will also shift differently due to different local deviations from mask flatness, e.g., from defects buried under the multilayer. It can be estimated that the contribution of mask non-flatness to overlay error is roughly 1/40 times the peak-to-valley thickness variation.^[81] With the blank peak-to-valley spec of 50 nm, ~1.25 nm image placement error is possible. Blank thickness variations up to 80 nm also contribute, which lead to up to 2 nm image shift.^[81]

The off-axis illumination of the reticle is also the cause of non-telecentricity in wafer defocus, which consumes most of the 1.4 nm overlay budget of the NXE:3400 EUV scanner^[82] even for design rules as loose as 100 nm pitch.^[83] The worst uncorrectable pattern placement error for a 24 nm line was about 1.1 nm, relative to an adjacent 72 nm power line, per 80 nm wafer focus position shift at a single slit position; when across-slit performance is included, the worst error is over 1.5 nm in the wafer defocus window^[80] In 2017, an actinic microscope mimicking a 0.33 NA EUV lithography system with 0.2/0.9 quasar 45 illumination showed that an 80 nm pitch contact array shifted -0.6 to 1.0 nm while a 56 nm pitch contact array shifted -1.7 to 1.0 nm relative to a horizontal reference line, within a +/- 50 nm defocus window.^[84]

Wafer defocus also leads to image placement errors due to deviations from local mask flatness. If the local slope is indicated by an angle α, the image is projected to be shifted in a 4× projection tool by 8α × (DOF/2) = 4αDOF, where DOF is the depth of focus.^[85] For a depth of focus of 100 nm, a small local deviation from flatness of 2.5 mrad (0.14°) can lead to a pattern shift of 1 nm.

Simulations as well as experiments have shown that pupil imbalances in EUV lithography can result in pitch-dependent pattern placement errors.^[86][87] Since the pupil imbalance changes with EUV collector mirror aging or contamination, such placement errors may not be stable over time. The situation is specifically challenging for logic devices, where multiple pitches have critical requirements at the same time.^[88] The issue is ideally addressed by multiple exposures with tailored illuminations.^[89][90]

Slit position dependence

Illumination rotation across ring-field slit. Light reflected from curved optical surfaces will generate arc segments.^[91] The illumination angles are rotated azimuthally across the arc-shaped slit (right), due to the reflection of an arc-shaped image from each pupil position as a point source (left).^[92][93] The angle-dependent and wavelength-dependent multilayer reflectance distribution pattern is rotated accordingly.

The direction of illumination is also highly dependent on slit position, essentially rotated azimuthally.^{[94][95][41][96][97][98]} Nanya Technology and Synopsys found that horizontal vs. vertical bias changed across slit with dipole illumination.^[99] The rotating plane of incidence (azimuthal range within -25° to 25°) is confirmed in the SHARP actinic review microscope at CXRO which mimics the optics for EUV projection lithography systems.^[100] The reason for this is a mirror is used to transform straight rectangular fields into arc-shaped fields.^[101][102] In order to preserve a fixed plane of incidence, the reflection from the previous mirror would be from a different angle with the surface for a different slit position; this causes non-uniformity of reflectivity. To preserve uniformity, rotational symmetry with a rotating plane of incidence is used.^[103] More generally, so-called "ring-field" systems reduce aberrations by relying on the rotational symmetry of an arc-shaped field derived from an off-axis annulus.^[104] This is preferred, as reflective systems must use off-axis paths, which aggravate aberrations. Hence identical die patterns within different halves of the arc-shaped slit would require different OPC. This renders them uninspectable by die-to-die comparison, as they are no longer truly identical dies. For pitches requiring dipole, quadrupole, or hexapole illumination, the rotation also causes mismatch with the same pattern layout at a different slit position, i.e., edge vs. center. Even with annular or circular illumination, the rotational symmetry is destroyed by the angle-dependent multilayer reflectance described above. Although the azimuthal angle range is +/- ~20°^[105] (NXE3400^[106] field data indicate 18.2°^[107]) on 0.33 NA scanners, at 7 nm design rules (36–40 nm pitch), the tolerance for illumination can be +/-15°,^[108][109] or even less.^{[110][111][106]} Annular illumination nonuniformity and asymmetry also significantly impact the imaging.^[112] At 28 nm pitch, the required dipole illumination becomes impossible to maintain across slit on 0.33 NA EUV systems.^[113]

The larger incident angle for pitch-dependent dipole illumination trend across slit does not affect horizontal line shadowing so much, but vertical line shadowing does increase going from center to edge.^[114] In addition, higher NA systems may offer limited relief from shadowing, as they target tighet pitches.^[114]

Horizontal and vertical lines exhibit different shadowing across the slit.

The slit position dependence is particularly difficult for the tilted patterns encountered in DRAM.^[97] Besides the more complicated effects due to shadowing and pupil rotation, tilted edges are converted to stair shape, which may be distorted by OPC. In fact, the 32 nm pitch DRAM by EUV will lengthen up to at least 9F² cell area, where F=active area half-pitch (traditionally, it had been 6F²).^[99] With a 2-D self-aligned double-patterning active area cut, the cell area is still lower at 8.9F².^[115]

Aberrations, originating from deviations of optical surfaces from subatomic (<0.1 nm) specifications^[116] as well as thermal deformations^[117][118] and possibly including polarized reflectance effects,^[119] are also dependent on slit position,^[120][118] as will be further discussed below, with regard to source-mask optimization (SMO). The thermally induced aberrations are expected to exhibit differences among different positions across the slit, corresponding to different field positions, as each position encounters different parts of the deformed mirrors.^[121] Ironically, the use of substrate materials with high thermal and mechanical stability make it more difficult to compensate wavefront errors^[122]

In combination with the range of wavelengths, the rotated plane of incidence aggravates the already severe stochastic impact on EUV imaging.^[123]

Wavelength bandwidth (chromatic aberration)

Image shift due to defocus depends on wavelength. The angular dependence of multilayer reflectance of the object (mask) is different for different wavelengths, leading to different shifts when defocused.

Unlike DUV lithography sources, based on excimer lasers, EUV plasma sources produce light across a broad range of wavelengths^[124] roughly spanning a 2% FWHM bandwidth near 13.5 nm (13.36nm – 13.65nm at 50% power). EUV (10–121nm) is the band longer than X-Rays (0.1–10nm) and shorter than the hydrogen Lyman-alpha line.

Though the EUV spectrum is not completely monochromatic, nor even as spectrally pure as DUV laser sources, the working wavelength has generally been taken to be 13.5 nm. In actuality, the reflected power is distributed mostly in the 13.3-13.7 nm range.^[125] The bandwidth of EUV light reflected by a multilayer mirror used for EUV lithography is over +/-2% (>270 pm);^[126] the phase changes due to wavelength changes at a given illumination angle may be calculated^[127] and compared to the aberration budget.^[128] Wavelength dependence of reflectance^[127][125] also affects the apodization, or illumination distribution across the pupil (for different angles); different wavelengths effectively 'see' different illuminations as they are reflected differently by the multilayer of the mask.^[129][125] This effective source illumination tilt can lead to large image shifts due to defocus.^[130] Conversely, the peak reflected wavelength varies across the pupil due to different incident angles.^[125][131] This is aggravated when the angles span a wide radius, e.g., annular illumination. The peak reflectance wavelength increases for smaller incident angles.^[132] Aperiodic multilayers have been proposed to reduce the sensitivity at the cost of lower reflectivity but are too sensitive to random fluctuations of layer thicknesses, such as from thickness control imprecision or interdiffusion.^[133] In particular, defocused dense lines at pitches up to twice the minimum resolvable pitch suffer wavelength-dependent edge shifts.^[134]

A narrower bandwidth would increase sensitivity to mask absorber and buffer thickness on the 1 nm scale.^[135][136]

Flare

Flare is the presence of background light originating from scattering off of surface features which are not resolved by the light. In EUV systems, this light can be EUV or out-of-band (OoB) light that is also produced by the EUV source. The OoB light adds the complication of affecting the resist exposure in ways other than accounted for by the EUV exposure. OoB light exposure may be alleviated by a layer coated above the resist, as well as 'black border' features on the EUV mask.^[137] However, the layer coating inevitably absorbs EUV light, and the black border adds EUV mask processing cost.

Line tip effects

A key challenge for EUV is the counter-scaling behavior of the line tip-to-tip (T2T) distance as half-pitch (hp) is scaled down.^[110] This is in part due to lower image contrast for the binary masks used in EUV lithography, which is not encountered with the use of phase shift masks in immersion lithography.^[138][139] The rounding of the corners of the line end leads to line end shortening,^[140] and this is worse for binary masks.^[141] The use of phase-shift masks in EUV lithography has been studied but encounters difficulties from phase control in thin layers^[142] as well as the bandwidth of the EUV light itself.^[143] More conventionally, optical proximity correction (OPC) is used to address the corner rounding and line-end shortening. In spite of this, it has been shown that the tip-to-tip resolution and the line tip printability are traded off against each other, being effectively CDs of opposite polarity.^[144]

In unidirectional metal layers, tip-to-tip spacing is one of the more severe issues for single exposure patterning. For the 40 nm pitch vertical lines, an 18 nm nominal tip-to-tip drawn gap resulted in an actual tip-to-tip distance of 29 nm with OPC (optical proximity correction),^[110] while for 32 nm pitch horizontal lines, the tip-to-tip distance with a 14 nm nominal gap went to 31 nm with OPC.^[145] These actual tip-to-tip distances define a lower limit of the half-pitch of the metal running in the direction perpendicular to the tip. In this case, the lower limit is around 30 nm. With further optimization of the illumination (discussed in the section on source-mask optimization), the lower limit can be further reduced to around 25 nm.^[146]

For larger pitches, where conventional illumination can be used, the line tip-to-tip distance is generally larger. For the 24 nm half-pitch lines, with a 20 nm nominally drawn gap, the distance was actually 45 nm, while for 32 nm half-pitch lines, the same nominal gap resulted in a tip-to-tip distance of 34 nm.^[145] With OPC, these become 39 nm and 28 nm for 24 nm half-pitch and 32 nm half-pitch, respectively.^[147]

Assist features

Assist feature OPC. Assist features help improve the image of isolated features (blue) to be more like dense features (gray). However, the more effective they are, the greater the risk that the assist feature will print (orange).

Assist features are often used to help balance asymmetry from non-telecentricity at different slit positions, due to different illumination angles, starting at the 7 nm node,^[148][149] where the pitch is ~ 41 nm for a wavelength ~13.5 nm and NA=0.33, corresponding to k1 ~ 0.5.^[150] However, the asymmetry is reduced but not eliminated, since the assist features mainly enhance the highest spatial frequencies, whereas intermediate spatial frequencies, which also affect feature focus and position, are not much affected. The coupling between the primary image and the self images is too strong for the asymmetry to be eliminated by assist features; only asymmetric illumination can achieve this.^[67] Assist features may also get in the way of access to power/ground rails. Power rails are expected to be wider, which also limits the effectiveness of using assist features, by constraining the local pitch. Local pitches between 1× and 2× the minimum pitch forbid assist feature placement, as there is simply no room to preserve the local pitch symmetry. In fact, for the application to the two-bar asymmetry case, the optimum assist feature placement may be less than or exceed the two-bar pitch.^[149] Depending on the parameter to be optimized (process window area, depth of focus, exposure latitude), the optimum assist feature configuration can be very different, e.g., pitch between assist feature and bar being different from two-bar pitch, symmetric or asymmetric, etc..

At pitches smaller than 58 nm, there is a tradeoff between depth of focus enhancement and contrast loss by assist feature placement.^[149] Generally, there is still a focus-exposure tradeoff as the dose window is constrained by the need to have the assist features not print accidentally.

An additional concern comes from shot noise;^[151] sub-resolution assist features (SRAFs) cause the required dose to be lower, so as not to print the assist features accidentally.^[152] This results in fewer photons defining smaller features (see discussion in section on shot noise).

As SRAFs are smaller features than primary features and are not supposed to receive doses high enough to print, they are more susceptible to stochastic dose variations causing printing errors.^[153] This is particularly prohibitive for EUV, where even when the primary feature is printed at 80 mJ/cm², the SRAF suffers from stochastic printing.

Source-mask optimization

Pitch effect on SMO. SMO carried out targeted for one pitch may have varying performance for other pitches.

Due to the effects of non-telecentricity, standard illumination pupil shapes, such as disc or annular, are not sufficient to be used for feature sizes of ~20 nm or below (10 nm node and beyond).^[83] Instead certain parts of the pupil (often over 50%) must be asymmetrically excluded. The parts to be excluded depend on the pattern. In particular, the densest allowed lines need to be aligned along one direction and prefer a dipole shape. For this situation, double exposure lithography would be required for 2D patterns, due to the presence of both X- and Y-oriented patterns, each requiring its own 1D pattern mask and dipole orientation.^[154][155] There may be 200–400 illuminating points, each contributing its weight of the dose to balance the overall image through focus. Thus the shot noise effect (to be discussed later) critically affects the image position through focus, in a large population of features.

Double- or multiple-patterning would also be required if a pattern consists of sub-patterns which require significantly different optimized illuminations, due to different pitches, orientations, shapes, and sizes.

Impact of slit position and aberrations

Impact of different wavelengths. Different wavelengths effectively have different pupils, resulting in different results of source-mask optimization.

Largely due to the slit shape,^[105] and the presence of residual aberrations,^[156] the effectiveness of SMO varies across slit position.^[157] At each slit position, there are different aberrations^[120] and different azimuthal angles of incidence leading to different shadowing.^[41] Consequently, there could be uncorrected variations across slit for aberration-sensitive features, which may not be obviously seen with regular line-space patterns.^[149] At each slit position, although optical proximity correction (OPC), including the assist features mentioned above, may also be applied to address the aberrations,^[158][159] they also feedback into the illumination specification,^{[160][157][161][162]} since the benefits differ for different illumination conditions.^[158] This would necessitate the use of different source-mask combinations at each slit position, i.e., multiple mask exposures per layer.^[120][163]

The above-mentioned chromatic aberrations, due to mask-induced apodization,^[129] also lead to inconsistent source-mask optimizations for different wavelengths.

Reduction of pupil fill

Pupil rotation across slit forces use of much lower pupil fill (within the trapezoids or rectangles) for dipole illumination.

A consequence of SMO and shifting focus windows has been the reduction of pupil fill. In other words, the optimum illumination is necessarily an optimized overlap of the preferred illuminations for the various patterns that need to be considered. This leads to lower pupil fill providing better results. However, throughput is affected below 20% pupil fill due to absorption.^{[173][174][175][176][52]}

Phase shift masks

Phase profile of attenuated phase shift mask for EUV. The phase profile (red) for an attenuated phase shift mask used with a partially transmitting EUV absorber does not match the ideal profile design (dotted), due to oblique incidence illumination and absorber edge scattering.

A commonly touted advantage of EUV has been the relative ease of lithography, as indicated by the ratio of feature size to the wavelength multiplied by the numerical aperture, also known as the k1 ratio. An 18 nm metal linewidth has a k1 of 0.44 for 13.5 nm wavelength, 0.33 NA, for example. For the k1 approaching 0.5, some weak resolution enhancement including attenuated phase shift masks has been used as essential to production with the ArF laser wavelength (193 nm),^{[177][178][179][180][181][182]} whereas this resolution enhancement is not available for EUV.^{[183][184][185]} In particular, 3D mask effects including scattering at the absorber edges distort the desired phase profile.^[184] Also, the phase profile is effectively derived from the plane wave spectrum reflected from the multilayer through the absorber rather than the incident plane wave.^[186] Without absorbers, near-field distortion also occurs at an etched multilayer sidewall due to the oblique incidence illumination;^[187] some light traverses only a limited number of bilayers near the sidewall.^[64] Additionally, the different polarizations (TE and TM) have different phase shifts.^[64].Fundamentally, a chromeless phase shift mask enables pitch splitting by suppression of the zeroth diffracted order on the mask, but fabricating a high quality phase shift mask for EUV is certainly not a trivial task. One possible way to achieve this is through spatial filtering at the Fourier plane of the mask pattern. At Lawrence Berkeley National Lab, the light of the zeroth order is a centrally obscured system, and the +/-1 diffracted orders will be captured by the clear aperture, providing a functional equivalent to the chromeless phase shift mask while using a conventional binary amplitude mask.^[188]

Impact of photoelectron and secondary electron travel on resolution

Low energy electron migration. Low energy electron travel distance (r) can reach at least several nanometers, even though the distance between electron collisions (scattering) is 1 nm.

Photoelectrons have a random spread that increases with accumulated dose.

The stochastic electron spread function, being convolved with the fixed optical point spread function, adds variability to the actual resolution.

A study by the College of Nanoscale Science and Engineering (CNSE) presented at the 2013 EUVL Workshop indicated that, as a measure of EUV photoelectron and secondary electron blur, 50–100 eV electrons easily penetrated beyond 15 nm of resist thickness (PMMA or commercial resist), indicating more than 30 nm range of resist affected centered on the EUV point of absorption, for doses exceeding 200–300 uC/cm².^[195] This can be compared with the image contrast degradation reported for sub-40 nm pitches later in 2015.^[196]

The process of electron penetration through a resist is essentially a stochastic process; there is a finite probability that resist exposure by released electrons can occur quite far from the point of photon absorption.^[197][198] Increasing the dose increases the number of far-reaching electrons, resulting in more extended resist loss. A leading EUV chemically amplified resist exposed to 80 eV electrons at a dose up to 80 uc/cm² showed up to 7.5 nm resist thickness loss.^[199] For an open-source resist exposed near 200 uC/cm² by 80 eV electrons, the resist thickness lost after post-exposure bake and development was around 13 nm, while doubling the dose resulted in increasing the loss to 15 nm.^[200] On the other hand, for doses >500 uC/cm², the resist begins to thicken due to crosslinking.^[199]

The degree of photoelectron emission from the layer underlying the EUV photoresist has been shown to affect the depth of focus.^[201] Unfortunately, hardmask layers tend to increase photoelectron emission, degrading the depth of focus. Electrons from defocused images in the resist can also affect the best focus image.^[202]

The generation of photoelectrons in the photoresist itself is a fundamental part of the image formation, and is marked by a random spread which increases with dose.^{[203][204][205]}

The randomness of the number of secondary electrons is itself a source of stochastic behavior in EUV resist images.^[206][205] The scale length of electron blur itself has a distribution.^{[207][208][209][210]} Intel demonstrated with a rigorous simulation that EUV-released electrons scatter distances larger than 15 nm in EUV resists.^[211][212]

The electron blur is also affected by total internal reflection from the top surface of the resist film.^[213][214]

Secondary electron blur vs. dose

Secondary electron blur vs. dose. The secondary electron blur can increase significantly with increasing dose.

Direct photocurrent measurements have been used to obtain secondary electron yields in response to EUV radiation. Simulations roughly calibrated to this yield show that the secondary electron blur increases with dose.^[215] This is associated with fewer trapping sites as dose is increased and the reactions depleting the trapping sites proceed.

Low-energy electron-induced events also increase in number at a given distance from the photon absorption site, as dose is increased.^[216] This has been confirmed by resist thickness loss measurements as a function of low-energy electron dose.^[217] The dose-dependent spread of secondary electrons was also known before from electron beam lithography.^[218]

The increased secondary electron blur with increased dose makes control of stochastic defects more difficult.^[219]

Effect of underlying layers

Electrons from layers under the resist can affect the profile and onset of collapse.

Secondary electrons from layers underneath the resist can affect the resist profile as well as pattern collapse.^[220] Hence, selection of such both the underlayer and the layer under that layer are important considerations for EUV lithography. Moreover, the electrons from defocused images can aggravate the stochastic nature of the image.^[221]

Charging and electron trapping

Due to the production of secondary electrons of various energies, the charge of the resist may locally fluctuate.^[222] An EUV exposure with less blur leads to more pronounced charge differences at the feature edge,^[223] which can lead to larger electric fields.^[224] Such large electric fields have been observed to lead to dielectric breakdown.^[222] The trapping of secondary electrons leads to a reduction of secondary electrons emitted from the film;^[222] however, the trap sites may themselves be depleted, resulting in effectively extended secondary electron blur for larger doses.^[215] Electron trapping is predicted to occur as part of polaronic behavior,^[225][226] which confines the electron's final energy deposition to trap site locations. The polaron size can be quite large in resists, e.g., 46 nm in PMMA.^[226]

Resist outgassing

Outgassing contamination vs. EUV dose: The increase of dose to size (E_size) to reduce shot noise and roughness comes at price of increased contamination from outgassing. The contamination thickness shown here is relative to a reference resist.

Due to the high efficiency of absorption of EUV by photoresists, heating and outgassing become primary concerns. One well-known issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions.^[227] Organic photoresists outgas hydrocarbons^[228] while metal oxide photoresists outgas water and oxygen^[229] and metal (in a hydrogen ambient); the last is uncleanable.^[50] The carbon contamination is known to affect multilayer reflectivity^[230] while the oxygen is particularly harmful for the ruthenium capping layers (relatively stable under EUV and hydrogen conditions) on the EUV multilayer optics.^[231]

Tin redeposition

Atomic hydrogen in the tool chambers is used to clean tin and carbon which deposit on the EUV optical surfaces.^[232] Atomic hydrogen is produced by EUV light directly photoionizing H₂:

hν + H2 → H+ + H + e−[233]

Electrons generated in the above reaction may also dissociate H₂ to form atomic hydrogen:

e− + H2 → H+ + H + 2e−[233]

The reaction with tin in the light source (e.g., tin on an optical surface in the source) to form volatile SnH4 (stannane) that can be pumped out from the source proceeds via the reaction:

Sn (s) + 4H (g) → SnH4 (g).[232]

The SnH₄ can reach the coatings of other EUV optical surfaces, where it redeposits Sn via the reaction:

SnH4 → Sn (s) + 2H2 (g).[232]

Redeposition may also occur by other intermediate reactions.^[234]

The redeposited Sn^[49][50] might be subsequently removed by atomic hydrogen exposure. However, overall, the tin cleaning efficiency (the ratio of the removed tin flux from a tin sample to the atomic hydrogen flux to the tin sample) is less than 0.01%, due to both redeposition and hydrogen desorption, leading to formation of hydrogen molecules at the expense of atomic hydrogen.^[232] The tin cleaning efficiency for tin oxide is found roughly twice higher than that of tin (with a native oxide layer of ~ 2 nm on it).^[232] Injecting a small amount of oxygen to the light source may improve the tin cleaning rate.

Hydrogen blistering

Hydrogen-induced blistering defects. Atomic hydrogen (red dots) used for cleaning surfaces can penetrate underneath the surface. In the Mo/Si multilayers, H₂ (paired red dots) is formed and trapped, resulting in blister (white region) formation.

Hydrogen also reacts with metal-containing compounds to reduce them to metal,^[235] and diffuses through the silicon^[236] and molybdenum^[237] in the multilayer, eventually causing blistering.^{[238][239][240]} Capping layers that mitigate hydrogen-related damage often reduce reflectivity to well below 70%.^[239] Capping layers are known to be permeable to ambient gases including oxygen^[241] and hydrogen,^{[242][243][244][245]} as well as susceptible to the hydrogen-induced blistering defects.^[246][238] Hydrogen may also react with the capping layer, resulting in its removal.^[247]

Tin spitting

Hydrogen can penetrate molten tin (Sn), creating hydrogen bubbles inside it. If the bubbles move at the molten tin surface, then it bursts with tin, resulting in tin spreading over a large angle range. This phenomenon is called tin spitting and is one of EUV Collector contamination sources.

Resist erosion

Hydrogen also reacts with resists to etch^[248][249] or decompose^[250] them. Besides photoresist, hydrogen plasmas can also etch silicon, albeit very slowly.^[251]

Membrane

To help mitigate the above effects, the latest EUV tool introduced in 2017, the NXE:3400B, features a membrane that separates the wafer from the projection optics of the tool, protecting the latter from outgassing from the resist on the wafer.^[52] The membrane contains layers which absorb DUV and IR radiation, and transmits 85–90% of the incident EUV radiation. There is of course, accumulated contamination from wafer outgassing as well as particles in general (although the latter are out of focus, they may still obstruct light).

Multilayer reflectivity random variations

GlobalFoundries and Lawrence Berkeley Labs carried out a Monte Carlo study to simulate the effects of intermixing between the molybdenum (Mo) and silicon (Si) layers in the multilayer that is used to reflect EUV light from the EUV mask.^[263] The results indicated high sensitivity to the atomic-scale variations of layer thickness. Such variations could not be detected by wide-area reflectivity measurements but would be significant on the scale of the critical dimension (CD).^[263] The local variation of reflectivity could be on the order of 10% for a few nm standard deviation.^[264]

Multilayer damage

Multiple EUV pulses at less than 10 mJ/cm² could accumulate damage to a Ru-capped Mo/Si multilayer mirror optic element.^[265] The angle of incidence was 16° or 0.28 rads, which is within the range of angles for a 0.33 NA optical system.

Pellicles

Production EUV tools need a pellicle to protect the mask from contamination. Pellicles are normally expected to protect the mask from particles during transport, entry into or exit from the exposure chamber, as well as the exposure itself. Without pellicles, particle adders would reduce yield, which has not been an issue for conventional optical lithography with 193 nm light and pellicles. However, for EUV, the feasibility of pellicle use is severely challenged, due to the required thinness of the shielding films to prevent excessive EUV absorption. Particle contamination would be prohibitive if pellicles were not stable above 200 W, i.e., the targeted power for manufacturing.^[266]

Heating of the EUV mask pellicle (film temperature up to 750 K for 80 W incident power) is a significant concern, due to the resulting deformation and transmission decrease.^[267] ASML developed a 70 nm thick polysilicon pellicle membrane, which allows EUV transmission of 82%; however, less than half of the membranes survived expected EUV power levels.^[268] SiNx pellicle membranes also failed at 82 W equivalent EUV source power levels.^[269] At target 250 W levels, the pellicle is expected to reach 686 degrees Celsius,^[270] well over the melting point of aluminum. Alternative materials need to allow sufficient transmission as well as maintain mechanical and thermal stability. However, graphite, graphene or other carbon nanomaterials (nanosheets, nanotubes) are damaged by EUV due to the release of electrons^[271] and also too easily etched in the hydrogen cleaning plasma expected to be deployed in EUV scanners.^[272] Hydrogen plasmas can also etch silicon as well.^[273][274] A coating helps improve hydrogen resistance, but this reduces transmission and/or emissivity, and may also affect mechanical stability (e.g., bulging).^[275]

Wrinkles on pellicles can cause CD nonuniformity due to uneven absorption; this is worse for smaller wrinkles and more coherent illumination, i.e., lower pupil fill.^[276]

In the absence of pellicles, EUV mask cleanliness would have to be checked before actual product wafers are exposed, using wafers specially prepared for defect inspection.^[277] These wafers are inspected after printing for repeating defects indicating a dirty mask; if any are found, the mask must be cleaned and another set of inspection wafers are exposed, repeating the flow until the mask is clean. Any affected product wafers must be reworked.

TSMC reported starting limited use of its own pellicle in 2019 and continuing to expand afterwards,^[278] and Samsung is planning pellicle introduction in 2022.^[279]

Hydrogen bulging defects

As discussed above, with regard to contamination removal, hydrogen used in recent EUV systems can penetrate into the EUV mask layers. Once trapped, bulge defects were produced.^[246] These are essentially the blister defects which arise after a sufficient number of EUV mask exposures in the hydrogen environment.

EUV stochastic issues

Shot noise causing significant CD variations

EUV lithography is particularly sensitive to stochastic effects.^[280][281] In a large population of features printed by EUV, although the overwhelming majority are resolved, some suffer complete failure to print, e.g. missing holes or bridging lines. A known significant contribution to this effect is the dose used to print.^[282] This is related to shot noise, to be discussed further below. Due to the stochastic variations in arriving photon numbers, some areas designated to print actually fail to reach the threshold to print, leaving unexposed defect regions. The lower part of the resist layer is more susceptible to underexposure, due to absorption in the upper part.^[283] Some areas may be overexposed, leading to excessive resist loss or crosslinking. The probability of stochastic failure increases exponentially as feature size decreases, and for the same feature size, increasing distance between features also significantly increases the probability.^[282][284] Line cuts which are misshapen are a significant issue due to potential arcing and shorting.^[285] Yield requires detection of stochastic failures down to below 1e-12.^[282]

The tendency to stochastic defects is worse when the image consists of photons from different patterns, such as from a large-area pattern^[286] or from defocus over a large pupil fill.^[287][288]

Stochastic defects tend to occur where there is stochastic absorbed dose variation near the threshold dose. These are indicated by where the blue specks tend to collect.

Multiple failure modes may exist for the same population. For example, besides bridging of trenches, the lines separating the trenches may be broken.^[282] This can be attributed to stochastic resist loss,^[280] from secondary electrons.^[200][289] The randomness of the number of secondary electrons is itself a source of stochastic behavior in EUV resist images.^[206]

Some patterns are more vulnerable to stochastic defects, due to pinching between corners,^[290] sidelobes,^[291] or the gap between spots.^[292][293]

The coexistence of stochastically underexposed and overexposed defect regions leads to a loss of dose window at a certain post-etch defect level between the low-dose and high-dose patterning cliffs.^{[294][295][296][297]} Hence, the resolution benefit from shorter wavelength is lost.

The resist underlayer also plays an important role.^[282] This could be due to the secondary electrons generated by the underlayer.^[196] Secondary electrons may remove over 10 nm of resist from the exposed edge.^[200][298]

The defect level is on the order of 1K/mm².^[299] In 2020, Samsung reported that 5 nm layouts had risks for process defects and had started implementing automated check and fixing.^[300]

Photon shot noise can be linked to stochastic defects through the presence of dose-dependent blur (modeled as Gaussian).^[301][302]

Photon shot noise also leads to stochastic edge placement error,^{[303][304][305]} which can exceed 1 nm.^[306] The error can exceed 2 nm when considering the gap between two point spread functions.^[307]

Line end position affected by shot noise. The natural variation of EUV photon number can cause the position of the line end to shift.

The photon shot noise is removed to some degree by blurring factors such as secondary electrons or acids in chemically amplified resists, but when too significant the blur also reduces the image contrast at the edge.^[308] Since the blur is less than the feature size, the feature's total enclosed area can still have fluctuations.^[309] It is worse for EUV than DUV wavelengths due to lower photon density at commonly used doses, as well as smaller feature sizes being represented by smaller pixels.^[310]

Even with blur taken into account, the final distribution of chemical species is still stochastic. Higher dose helps reduce the stochasticity but leads to larger blur, which can spread the extent of the stochastic fluctuations.^[311]

Another approach to dealing with stochastic issues is to use directional etching to smooth sidewall edges.^[312]

The resolution of EUV lithography for the future faces challenges in maintaining throughput, i.e., how many wafers are processed by an EUV tool per day. These challenges arise from smaller fields, additional mirrors, and shot noise. In order to maintain throughput, the power at intermediate focus (IF) must be continually increased.

With the natural Poisson distribution due to the random arrival and absorption times of the photons,^[313][314] there is an expected natural dose (photon number) variation of at least several percent 3 sigma, making the exposure process susceptible to stochastic variations. The dose variation leads to a variation of the feature edge position, effectively becoming a blur component. Unlike the hard resolution limit imposed by diffraction, shot noise imposes a softer limit, with the main guideline being the ITRS line width roughness (LWR) spec of 8% (3s) of linewidth.^[315] Increasing the dose will reduce the shot noise,^[316] but this also requires higher source power.

A 10 nm wide, 10 nm long assist feature region, at a target non-printing dose of 15 mJ/cm², with 10% absorption, is defined by just over 100 photons, which leads to a 6s noise of 59%, corresponding to a stochastic dose range of 6 to 24 mJ/cm², which could affect the printability.

A 2017 study by Intel showed that for semi-isolated vias (whose Airy disk can be approximated by a Gaussian), the sensitivity of CD to dose was particularly strong,^[317] strong enough that a reduction of dose could nonlinearly lead to failure to print the via.

The two issues of shot noise and EUV-released electrons point out two constraining factors: 1) keeping dose high enough to reduce shot noise to tolerable levels, but also 2) avoiding too high a dose due to the increased contribution of EUV-released photoelectrons and secondary electrons to the resist exposure process, increasing the edge blur and thereby limiting the resolution. Aside from the resolution impact, higher dose also increases outgassing^[318] and limits throughput, and crosslinking^[319] occurs at very high dose levels. For chemically amplified resists, higher dose exposure also increases line edge roughness due to acid generator decomposition.^[320]

Even with higher absorption at the same dose, EUV has a larger shot noise concern than the ArF (193 nm) wavelength, mainly because it is applied to thinner resists.^[283]

As can be seen above, at the target incident dose levels, significantly fewer EUV photons are absorbed in EUV resists compared to ArF photons in ArF resists. Despite greater transparency of the resist, the incident photon flux is about 14 times larger (193/13.5) for the same energy dose per unit area. Quantum yield makes the resist response comparable for chemically amplified resists for both ArF and EUV wavelengths, but ArF resists also allow larger blur, enabling further smoothing.^[321] The resist thickness is limited by transparency as well as resist collapse^[326] and resist strip^[327] considerations.

Due to stochastic considerations, the IRDS 2022 lithography roadmap now acknowledges increasing doses for smaller feature sizes.^[328][329] However, even increasing the dose a factor of 4 may not be sufficient to eliminate stochastic defects, while at the same hurting throughput.^[293]

EUV resolution will likely be compromised by stochastic effects. Smaller pitches will be more adversely affected by photon shot noise and blur, with defects already apparent at 30 nm.^[330][331] ASML indicated 30 nm pitch would not use direct exposure but double patterning.^[332] Intel did not use EUV for 30 nm pitch.^[333]

Current stochastic defect densities have exceeded 1/cm², at 36 nm pitch; this gets worse for smaller pitches, and entails that EUV exposures should only include sufficiently larger pitches.^{[334][335][336]}

Pupil Fill Ratio

As pitch decreases, a smaller fraction of pupil (fewer illumination angle) may be used.

For pitches less than half-wavelength divided by numerical aperture, dipole illumination is necessary. This illumination fills at most a leaf-shaped area at the edge of the pupil. However, due to 3D effects in the EUV mask,^[337] smaller pitches require even smaller portions of this leaf shape. Below 20% of the pupil, the throughput and dose stability begin to suffer.^[52] Higher numerical aperture allows a higher pupil fill to be used for the same pitch, but depth of focus is significantly reduced.^[338]