Extreme ultraviolet lithography

Resource requirements

Source: Gigaphoton, Sematech Symposium Japan, September 15, 2010

The required utility resources are significantly larger for EUV compared to 193 nm immersion, even with two exposures using the latter. Hynix reported at the 2009 EUV Symposium 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 ~3x the footprint of an ArF immersion scanner, resulting in productivity loss.^[9] Additionally, to confine ion debris, a superconducting magnet may be required.^[10]

A typical EUV tool weighs 180 tons.^[11]

Summary of key features

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

The different degrees of resolution among the 0.33 NA tools are due to the different illumination options.

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.^[30] A dose error over 2% warrants tool downtime.^[30]

Comparison to other lithography light sources

While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm²,^[32] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 10¹¹ W/cm².^[33] A state-of-the-art ArF immersion lithography 120 W light source requires no more than 40 kW^[34] while EUV sources are targeted to exceed 40 kW.^[35]

The power target for EUV lithography is at least 250 W, while for other conventional lithography sources, it is much less.^[30] 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.^[30]

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.^[42] However, the layer coating inevitably absorbs EUV light, and the black border adds EUV mask processing cost.

Secondary Electrons

EUV light generates photoelectrons upon absorption by matter. These photoelectrons in turn generate secondary electrons, which slow down before engaging in chemical reactions.^[43] It should be noted that at sufficient doses 40 eV electrons are known to penetrate 180 nm thick resist leading to development.^[44] More details on secondary electrons in EUV photoresist exposure are provided below. At a dose of 160 μC/cm², corresponding to 15 mJ/cm² EUV dose assuming one electron/photon, 30 eV electrons removed 7nm of PMMA resist after standard development.^[45] For a higher 30 eV dose of 380 μC/cm², equivalent to 36 mJ/cm² at one electron/photon, 10.4 nm of PMMA resist are removed.^[46] These indicate the distances the electrons can travel in resist, regardless of direction.^[47]

Reflective optics

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)^[48] on a multilayer mask. This leads to shadowing effects resulting in asymmetry in the diffraction pattern that degrade pattern fidelity in various ways as described below.^[49]

Thick mask shadowing effects

Shadowing effect on an EUV mask. When the feature size on the mask is comparable to the thickness, the shadowing effect is more significant to the formed image, since the shadow clearance distance is not negligible compared to the feature size.

The oblique incidence in a reflective optical system leads to shadowing effects in the presence of a mask absorber. For example, one side (behind the shadow) would appear brighter than the other (within the shadow).^[50]

H-V asymmetry

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

EUV non-telecentricity. Left: Due to large multilayer reflection angle differences, one side of the illumination pupil results in more reflected light. Right: Consequently, illumination from one side will be dominant. This results in an optical path difference between diffraction orders with respect to defocus, leading to a tendency for the pattern to shift.

Most fundamentally, 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).^[51] 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.

Asymmetries in sets of parallel lines

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.^[52] 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.^[53]

An extension of the two-bar case to a grating consisting of many horizontal lines shows similar sensitivity to defocus.^[54] It is manifest in the CD difference between the top and bottom edge lines of the set of 11 horizontal lines. The table below lists the CD difference over 100 nm focus range under quasar illumination (quasar illumination will be described in the section on optimum illumination vs. pitch).

Pitch	Horizontal 11-bar Bottom-Top CD difference over 100 nm focus range (quasar)
36 nm	3.5 nm
40 nm	2.5 nm
44 nm	1.7 nm

For pitches of 40 nm or below, the linewidths are 20 nm or less, while the CD difference is at least 2.5 nm, resulting in at least 12.5% difference.

Two-bar occurrence in irregular layout. An irregular layout can include two-bar locations which are prone to asymmetric imaging.

Pattern shift from defocus (non-telecentricity)

Mask feature vertical placement

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.^[55] More significantly, mask defocus has been found to result in large overlay errors.^[56][57] In particular, for a 10nm 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.^[58] 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.^[59] 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.^[60]

Wafer defocus

10nm node metal layer pattern shift vs defocus. Different patterns in the 10nm node metal layer (24 nm hp) are shifted differently through focus, depending on orientation and slit position as well as arrangement.

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^[61] even for design rules as loose as 100 nm pitch.^[62] 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^[58] In 2017, an actinic microscope mimicking a 0.33 NA EUV lithography system with 0.2/0.9 quasar 45 illumination showed that a 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.^[63]

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 4x projection tool by 8 α x (DOF/2) = 4 α DOF, where DOF is the depth of focus.^[64] 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.

Line tip effects

Tip-to-tip difficulty for dense line patterns. The gap between line ends is difficult to optimize simultaneously with dense lines.

Tip-to-side gap printing difficulty. The tip-to-side gap is one of the hardest features to print in a bidirectional pattern. The dipole-like illumination favors the minimum pitch but not other pitches.

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.^[65] 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.^[66][67] The rounding of the corners of the line end leads to line end shortening,^[68] and this is worse for binary masks.^[69] The use of phase-shift masks in EUV lithography has been studied but encounters difficulties from phase control in thin layers^[70] as well as the bandwidth of the EUV light itself.^[71] 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.^[72] In addition, the effectiveness of optical corrections depends on other non-optical causes such as resist blur and diffusion effects, which may also include secondary electron blur (discussed in the section on photoresist exposure).^[73] Also, larger molecular weights and sizes appear to reduce corner rounding.^[74]

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),^[65] while for 32 nm pitch horizontal lines, the tip-to-tip distance with a 14 nm nominal gap went to 31 nm with OPC.^[75] 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.^[76]

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.^[75] With OPC, these become 39 nm and 28 nm for 24 nm half-pitch and 32 nm half-pitch, respectively.^[77]

The printed space between a line tip and a perpendicular line it faces is 25-35 nm for 22 nm half-pitch lines (with a 20 nm nominally drawn gap).^[77] For a 22 nm line-space pattern with 22 nm nominal gap, conventional illumination yields a 38 nm tip-to-line distance, while quasar illumination yields a 28 nm distance.^[78] The tip-to-side gap is one of the hardest features to print in a bidirectional pattern.^[77]

Summary of EUV Line Tip and Corner Effects:^[79]

Corner Rounding	Tip to Tip	Tip to Side
~25 nm	28 nm	28 nm

Source: Semicon West 2017, IBM

The line end clearance distance of 28 nm essentially forces pitches to be at least 56 nm for EUV single exposure bi-directional patterning. 7nm node minimum metal pitch is already at 40 nm or below, while gate pitch can also be below 56 nm,^[80][81] so this is an indication that multiple patterning would be needed even for EUV at 7nm.^[82]

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).

Pattern effect on SMO. A mere widening of the power rail (top and bottom of sample pattern) leads to significant changes in the optimized pupil as well as reduced focus window, even with the use of assist features.

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.^[91][92] However, the asymmetry is reduced but not completely 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 completely eliminated by assist features; only asymmetric illumination can achieve this.^[53] 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 1x and 2x 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.^[92] 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.^[92] 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;^[93] sub-resolution assist features (SRAFs) cause the required dose to be lower, so as not to print the assist features accidentally.^[94] This results in fewer photons defining smaller features (see discussion in section on shot noise).

It is now known that the underlying mechanism for the asymmetry is different shadowing from different angles of incidence. Hence, reducing absorber thickness would be the most direct way to resolve the issue.^[95]

Source-mask optimization

Source-mask optimization (SMO) is used to reduce pattern shift for different features in a metal layer (targeted for 16 nm CD with 32 nm anchor pitch) in a single exposure, but cannot satisfy every possible case.

Pitch effect on SMO. SMO carried out targeted for one pitch (32 nm in this case) may have varying performance for other pitches. Here 36 nm has best performance, but barely exceeds the lower NILS limit of 2.0

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).^[62] 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.^[96][97] 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.

Optimum illumination vs. pitch

Degradation of image quality with pitch reduction under conventional EUV illumination. This has motivated greater consideration of other illumination modes such as QUASAR.

The optimum illumination, taking into account both exposure dose and focus windows, is a strong function of pitch in the range between 32 nm and 48 nm (relevant to 7nm and 10nm foundry nodes), which is where most of the work on EUV application has been focused. For pitches larger than 44 nm, the illumination pupil shape is preferably conventional, which is a circular disc, possibly including a central obscuration to provide an annular appearance.^[75] For pitches in the range 44 nm down to 34 nm, the optimum shape is no longer conventional or annular but more shaped like the "quasar" (Quadrupole-shaped annular)^[107] source, i.e., an arc within each quadrant of the pupil.^[75] For pitches of 32 nm and below, the optimum illumination becomes more dipole like, i.e., concentrated toward the top and bottom or the left and right ends of the pupil.^[65] When source-mask optimization is performed, the resulting shape will resemble the closest of the standard set (conventional, annular, quasar, dipole). For pitches less than 41 nm, the central portion of the pupil must be excluded for a tool with NA=0.33, as 13.5 nm EUV light going through that portion would only contribute the zeroth diffraction order (unscattered light), effectively adding flare.^[108]

Pitch-dependent focus windows

Best focus position vs. pitch. The best focus position varies strongly as a function of pitch.

In the 48-64 nm pitch range, the best focus position shifts roughly linearly as a function of pitch, by as much as 10-20 nm.^[109] For the 34-48 nm pitch range, the best focus position shifts roughly linearly in the opposite direction as a function of pitch. This can be correlated with the phase difference between the zero and first diffraction orders.^[110] Assist features, if they can fit within the pitch, were found not to reduce this tendency much, for a range of intermediate pitches,^[111] or even worsened it for the case of 18-27 nm and quasar illumination.^[112] 50 nm contact holes on 100 nm and 150 pitches had best focus positions separated by roughly 25 nm; smaller features are expected to be worse.^[113] Contact holes in the 48-100 nm pitch range showed a 37 nm best focus range.^[114] The best focus position vs. pitch is also dependent on resist.^[115] Critical layers often contain lines at one minimum pitch of one polarity, e.g., darkfield trenches, in one orientation, e.g., vertical, mixed with spaces of the other polarity of the other orientation. This often magnifies the best focus differences, and challenges the tip-to-tip and tip-to-line imaging.^[116]

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),^{[119][120][121][122][123][124]} whereas this resolution enhancement is not available for EUV.^[125][126] In particular, 3D mask effects including scattering at the absorber edges distort the desired phase profile.^[126] 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.^[127] Without absorbers, near-field distortion also occurs at an etched multilayer sidewall due to the oblique incidence illumination;^[128] some light traverses only a limited number of bilayers near the sidewall.^[50] Additionally, the different polarizations (TE and TM) have different phase shifts.^[50]

Impact of photoelectron and secondary electron travel on resolution

Resist loss from 80 eV EUV photoelectrons. The 80 eV photoelectron is expected to incur ~7.5 nm resist loss, which would make it difficult to control resist dimensions to within ~15 nm.

Resist loss from 30 eV decelerated EUV photoelectrons. 30 eV electrons (from the slowed down photoelectrons) result in significant resist loss as well as crosslinking at higher doses in PMMA.

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.^[130] This can be compared with the image contrast degradation reported for sub-40 nm pitches later in 2015.^[39]

Depth of focus is affected by EUV photoelectron emission from underlying layer.

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.^[131] 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.^[132] 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.^[38] On the other hand, for doses >500 uc/cm², the resist begins to thicken due to crosslinking.^[132]

The degree of photoelectron emission from the layer underlying the EUV photoresist has been shown to affect the depth of focus.^[133] Unfortunately, hardmask layers tend to increase photoelectron emission, degrading the depth of focus.

Charging and electron trapping

Due to the production of secondary electrons of various energies, the charge of the resist may locally fluctuate.^[134] An EUV exposure with less blur leads to more pronounced charge differences at the feature edge,^[135] which can lead to larger electric fields.^[136] Such large electric fields have been observed to lead to dielectric breakdown.^[134] The trapping of secondary electrons leads to a reduction of secondary electrons emitted from the film;^[134] however, the trap sites may themselves be depleted, resulting in effectively extended secondary electron blur for larger doses.^[137] Electron trapping is predicted to occur as part of polaronic behavior,^[138][139] 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.^[139]

DUV sensitivity

It should be noted that EUV resists are also exposable by wavelengths longer than EUV, particular VUV and DUV wavelengths in the 150–250 nm range.^[140]

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. Organic photoresists outgas hydrocarbons^[141] while metal oxide photoresists outgas water and oxygen^[142] and metal (in a hydrogen ambient); the last is uncleanable.^[26] The carbon contamination is known to affect multilayer reflectivity^[143] while the oxygen is particularly harmful for the ruthenium capping layers on the EUV multilayer optics.^[144]

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.^[150] 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 damage

Multiple EUV pulses at less than 10 mJ/cm² could accumulate damage to a Ru-capped Mo/Si multilayer mirror optic element.^[157] 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. Currently, the pellicle is not yet guaranteed to withstand 250 W power necessary for high volume manufacturing; the specification is 40 W.^[158]

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.^[159]

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.^[160] 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.^[161] SiNx pellicle membranes also failed at 82 W equivalent EUV source power levels.^[162] At target 250 W levels, the pellicle is expected to reach 686 degrees Celsius,^[163] 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^[164] and also too easily etched in the hydrogen cleaning plasma expected to be deployed in EUV scanners.^[165] Hydrogen plasmas can also etch silicon as well.^[166][167] A coating helps improve hydrogen resistance, but this reduces transmission and/or emissivity, and may also affect mechanical stability (e.g., bulging).^[168] The current lack of any suitable pellicle material, aggravated by the use of hydrogen plasma cleaning in the EUV scanner,^[169][170] presents an obstacle to volume production.^[171]

Hydrogen bulging defects

Hydrogen used in recent EUV systems can penetrate into the EUV mask layers. Once trapped, bulge defects were produced.^[172] These defects arise after a sufficient number of EUV mask exposures in the hydrogen environment.

Reduced fields

Reduction of field size by demagnification. Increasing the demagnification from 4X to 8X in one dimension would split the original full imaging field into two parts to preserve the same die area (26 mm × 33 mm).

Field stitching. Stitching together exposure fields is a concern where critical features cross a field boundary (red dotted line).

Preparation of an anamorphic lens with an NA between 0.5 and 0.6 is underway as of 2016. The demagnification will be 8X in one dimension and 4X in the other, and the angle of reflection will increase.^[173]

Higher demagnification will increase the mask size or reduce the size of the printed field. Reduced field size would divide full-size chip patterns (normally taking up 26 mm × 33 mm) among two or more conventional 6-inch EUV masks. Large (approaching or exceeding 500 mm²) chips, typically used for GPUs^[174] or servers,^[175] would have to be stitched together from two or more sub-patterns from different masks.^[176] Without field stitching, die size would be limited. With field stitching, features that cross field boundaries would have alignment errors, and the extra time required to change masks would reduce the throughput of the EUV system.^[177]

Shot noise: the statistical resolution limit

Shot noise causing significant CD variations. A set of holes patterned at 64 nm pitch shows significant effects of shot noise with an EUV dose of just over 10 mJ/cm². A larger dose would result in the same range of variation over a larger sample size.

With the natural Poisson distribution due to the random arrival and absorption times of the photons,^[178] 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.^[179] Increasing the dose will reduce the shot noise, 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,^[180] strong enough that a reduction of dose could nonlinearly lead to failure to print the via.

Via printing failure from noise-induced dose reduction. Shot noise-induced dose reduction could in extreme cases lead to via printing failure (CD->0).

Minimum dose to restrain shot noise for shrinking areas:

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^[181] and limits throughput, and crosslinking^[182] occurs at very high dose levels. For chemically amplified resists, higher dose exposure also increases line edge roughness due to acid generator decomposition.^[183]

As mentioned earlier, a more absorbing resist actually leads to less vertical dose uniformity. This also means shot noise is worse toward the bottom of a highly absorbing EUV resist layer.

Even with higher absorption, EUV has a larger shot noise concern than the ArF (193 nm) wavelength, mainly because it is applied to smaller dimensions and current dose targets are lower due to currently available source power levels.

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. The resist thickness is limited by transparency as well as resist collapse^[188] and resist strip^[189] considerations.