Extreme ultraviolet lithography

Image formation mechanism in EUV lithography. Top: EUV multilayer and absorber (purple) constituting mask pattern for imaging a line. Bottom: EUV radiation (red) reflected from the mask pattern is absorbed in the resist (yellow) and substrate (brown), producing photoelectrons and secondary electrons (blue). These electrons increase the extent of chemical reactions in the resist. A secondary electron pattern that is random in nature is superimposed on the optical image. The unwanted secondary electron exposure results in loss of resolution, observable line edge roughness and linewidth variation.

Extreme ultraviolet lithography (also known as EUV or EUVL) is a next-generation lithography technology using an extreme ultraviolet (EUV) wavelength, currently expected to be 13.5 nm. EUV is currently being developed for high volume use by 2020.^[1][2][3][4]

Tool

EUVL tool, Lawrence Livermore National Laboratory.

The tool consists of a laser-driven Sn plasma light source, reflective optics comprising multilayer mirrors, contained within a hydrogen gas ambient. The hydrogen is used for keeping the EUV collector mirror in the source free of Sn deposition.^[5]

EUVL is a significant departure from the deep ultraviolet lithography standard. All matter absorbs EUV radiation. Hence, EUV lithography requires a vacuum. All optical elements, including the photomask, must use defect-free Mo/Si multilayers that act to reflect light by means of interlayer interference; any one of these mirrors absorb around 30% of the incident light.

Current EUVL systems contain at least two condenser multilayer mirrors, six projection multilayer mirrors and a multilayer object (mask). Since the optics already absorbs 96% of the EUV light, the ideal EUV source will need to be much brighter than its predecessors. EUV source development has focused on plasmas generated by laser or discharge pulses. The mirror responsible for collecting the light is directly exposed to the plasma and is therefore vulnerable to damage from high-energy ions^[6][7] and other debris.^[8]

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 W at intermediate focus for 100 WPH, one would require 1 MW of input power, compared to 165 kW 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]

Light source power and throughput

EUV TPT as a function of dose. The wafer throughput of an EUV tool is actually a function of exposure dose, for a fixed source power.

EUV collector degradation over time. EUV collector reflectivity degrades ~10% in a week, leading to loss of throughput.

Neutral atoms or condensed matter cannot emit EUV radiation. Ionization must precede EUV emission in matter. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV.^[12] As of 2016, the established EUV light source is a laser-pulsed Sn plasma.^[13] The ions absorb the EUV light they emit, and are easily neutralized by electrons in the plasma to lower charge states which produce light mainly at other, unusable wavelengths, which results in a much reduced efficiency of light generation for lithography at higher plasma power density.

While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm²,^[14] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 10¹¹ W/cm².^[15] A state-of-the-art ArF immersion lithography 120 W light source requires no more than 40 kW^[16] while EUV sources are targeted to exceed 40 kW.^[17]

The EUV collector has a lifetime on the order of months, over which transmission steadily decreases, over 10% in a week (30 billion 50 kHz pulses).^[18] This could be due to the accumulating Sn residue mentioned above which is not completely cleaned off.^[19][20] On the other hand, conventional immersion lithography tools for double patterning provide consistent output for up to a year.^[21]

EUV-specific optical issues

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.

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)^[22] 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.^[23]

H-V asymmetry

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).^[24] 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.^[25] 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.^[26]

An extension of the two-bar case to a grating consisting of many horizontal lines shows similar sensitivity to defocus.^[27] 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).

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.

Pattern shift from defocus (non-telecentricity)

EUV pattern shift due to defocus vs. pitch. Due to apodization of EUV diffraction orders, there is pitch-dependent pattern shift due to defocus.

24 nm line pattern shift due to wafer defocus vs. (≥48nm) pitch. The pattern shift of a 24 nm line is measured relative to a local 72 nm power line. The slit position is fixed so cross-slit variation is not included. Including all slit positions, the worst-case error in the focus window exceeds 1.5 nm.

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.

7nm node metal layer pattern shift vs. defocus. A 7nm node metal layer with 32 nm anchor pitch (16 nm target CD) also experiences pattern-dependent shifts through focus.

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.^[28] More significantly, mask defocus has been found to result in large overlay errors.^[29][30] 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.^[31] 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.^[32] With the blank thickness variation up to 80 nm being possible, ~2 nm image placement error is possible.

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^[33] even for design rules as loose as 100 nm pitch.^[34] In particular, 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^[31]

Line tip effects

Tip-to-tip (T2T) distance vs. half-pitch. The smallest published T2T is plotted vs. half pitch. EUV OPC is applied where possible to reduce the tip-to-tip distance. The dotted line indicates the half-pitch.

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.^[35] 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.^[36][37] The rounding of the corners of the line end leads to line end shortening,^[38] and this is worse for binary masks.^[39] The use of phase-shift masks in EUV lithography has been studied but encounters difficulties from phase control in thin layers^[40] as well as the bandwidth of the EUV light itself.^[41] 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.^[42] 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).^[43] Also, larger molecular weights and sizes appear to reduce corner rounding.^[44]

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.

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),^[35] while for 32 nm pitch horizontal lines, the tip-to-tip distance with a 14 nm nominal gap went to 31 nm with OPC.^[45] 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.^[46]

Tip-to-tip under optimized illumination. For 36 nm pitch vertical lines under optimized illumination, the tip-to-tip gap cannot be reduced as aggressively.

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.^[45] With OPC, these become 39 nm and 28 nm for 24 nm half-pitch and 32 nm half-pitch, respectively.^[47]

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).^[47] 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.^[48] The tip-to-side gap is one of the hardest features to print in a bidirectional pattern.^[49]

Summary of EUV Line Tip and Corner Effects:^[50]

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,^[51][52] so this is an indication that multiple patterning would be needed even for EUV at 7nm.^[53]

Slit position dependence

Difficulty of EUV OPC for tilted patterns. Tilted patterns are more difficult to correct in EUV, due to pupil rotation and shadowing as well as edge conversion to stair shape.

The direction of illumination is also highly dependent on slit position. Hence identical die patterns on different halves of the slit would get different OPC. This renders them uninspectable by die-to-die comparison, as they are no longer truly identical dies.

The slit position dependence is particularly difficult for the tilted patterns encountered in DRAM.^[54] Besides the more complicated effects due to shadowing and pupil rotation, tilted edges are converted to stair shape, which may be distorted by OPC.

Enhancements for EUV Patterning

Assist features

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.^[55][56] 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.^[57] 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.^[56] 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.^[56] 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, due to fewer photons defining smaller features (see discussion below).

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.^[58]

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).^[34] 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.^[59][60] 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

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, 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.^[45] 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)^[61] source, i.e., an arc within each quadrant of the pupil.^[45] 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.^[35] 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.^[62]

EUV standard illumination pupil shapes. The optimized illumination shape will generally vary with pitch. The largest pitches use conventional or annular illumination, tighter pitches use quasar illumination, while the tightest pitches use dipole and c-quad.

Annular illumination best focus vs. pitch. Annular illumination could result in loss of best focus across pitch.

Outlook for Advanced Nodes

Best focus position decreases significantly with pitch for EUV quasar illumination, for pitches greater than 48 nm.

Best focus position increases significantly with pitch for EUV quasar illumination, for pitches smaller than 48 nm.

For the foundry "5nm" node, the minimum metal pitch for horizontal lines is taken to be around 32 nm,^[63] for which dipole-like illumination is preferred, but the minimum metal pitch for vertical lines (parallel to the gates) is taken to be around 40 nm,^[63] for which quasar-like illumination is preferred. Moreover, for the foundry "7nm" node, the minimum metal pitch for horizontal lines is taken to be around 40 nm,^[63] for which quasar-like illumination is expected, while the minimum metal pitch for vertical lines is taken to be around 50 nm,^[63] for which conventional or annular illumination is preferred. For the quasar illumination, the best focus position varies strongly as a function of pitch, particularly 36-40 nm vs. 48-60 nm, as well as over the 48-72 nm range.^[64] For these nodes, it is impossible to have a single EUV exposure illumination setting that fits both metal line directions at different respective pitches. Unidirectional metal layers are expected for these nodes anyway.^[63] The line tip-to-tip gap in this case is expected to be kept small enough by using cut exposures in a multiple patterning scenario.^[35]

Photoresist exposure

When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.^[65] 10 mJ/cm² EUV photon dose results in the generation of 109 uC/cm² dose of photoelectrons. The more highly absorbing resist removes more light in the top of the resist, leaving less for the bottom of the resist. The larger absorption leads to larger, more significant differences between the absorbed doses at the top and the bottom of the resist.

In other words, the less absorbing the resist, the more vertically uniform the absorption. Conventionally, photoresists are made as transparent as possible to strive for this vertical uniformity, which enables straighter resist profiles. On the other hand, for EUV, this conflicts with the goal of increasing absorption for more sensitivity at current EUV power levels. Shot noise is another concern, to be explained further below.

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.

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.^[66] This can be compared with the image contrast degradation reported for sub-40 nm pitches later in 2015.^[67]

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.^[68] 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.^[69] 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.^[70] On the other hand, for doses >500 uc/cm², the resist begins to thicken due to crosslinking.^[69]

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.^[71]

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^[72] while metal oxide photoresists outgas water and oxygen^[73] and metal (in a hydrogen ambient); the last is uncleanable.^[20] The carbon contamination is known to affect multilayer reflectivity^[74] while the oxygen is particularly harmful for the ruthenium capping layers on the EUV multilayer optics.^[75]

Contamination effects

One well-known issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions.^[76] Also, hydrogen gas in the tool chambers interacts with tin in the light source or resist to form SnH4 which reaches the coatings of the EUV optical surfaces, leaving Sn which is subsequently unremovable.^[19][20] Hydrogen also reacts with metal-containing compounds to reduce them to metal,^[77] and/or diffuses through to the multilayer, eventually causing blistering.^[78] Hydrogen also reacts with resists to etch^[79] or decompose^[80] them.

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.^[81] 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).

EUV Mask Defects

EUV mask defect printability. Defects with atomic-scale heights can affect dimensions printed by EUV even though buried by many layers. Source: Lawrence Berkeley National Laboratory and Intel.

EUV defect printability vs. pitch. The printability (here 10% CD) of a defect of a given height and width varies with pitch. Note that even the surface roughness on the multilayer here can have noticeable impact.

Reducing defects on extreme ultraviolet (EUV) masks is currently one of the most critical issues to be addressed for commercialization of EUV lithography.^[82] Defects can be buried underneath or within the multilayer stack^[83] or be on top of the multilayer stack. Mesas or protrusions form on the sputtering targets used for multilayer deposition, which may fall off as particles during the multilayer deposition.^[84] In fact, defects of atomic scale height (0.3–0.5 nm) with 100 nm FWHM can still be printable by exhibiting 10% CD impact.^[85] IBM and Toppan reported at Photomask Japan 2015 that smaller defects, e.g., 50 nm size, can have 10% CD impact even with 0.6 nm height, yet remain undetectable.^[86]

Furthermore, the edge of a phase defect will further reduce reflectivity by more than 10% if its deviation from flatness exceeds 3 degrees, due to the deviation from the target angle of incidence of 84 degrees with respect to the surface. Even if the defect height is shallow, the edge still deforms the overlying multilayer, producing an extended region where the multilayer is sloped. The more abrupt the deformation, the narrower the defect edge extension, the greater the loss in reflectivity.

EUV mask defect repair is also more complicated due to the across-slit illumination variation mentioned above. Due to the varying shadowing sensitivity across the slit, the repair deposition height must be controlled very carefully, being different at different positions across the EUV mask illumination slit.^[87]

Multilayer damage

Multiple EUV pulses at less than 10 mJ/cm² could accumulate damage to a Ru-capped Mo/Si multilayer mirror optic element.^[88] 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.^[89]

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.^[90]

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.^[91] 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.^[92] SiNx pellicle membranes also failed at 82 W equivalent EUV source power levels.^[93] 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^[94] and also too easily etched in the hydrogen cleaning plasma expected to be deployed in EUV scanners.^[95] Hydrogen plasmas can also etch silicon as well.^[96][97] A coating helps improve hydrogen resistance, but this reduces transmission and/or emissivity, and may also affect mechanical stability (e.g., bulging).^[98] The current lack of any suitable pellicle material, aggravated by the use of hydrogen plasma cleaning in the EUV scanner,^[99][100] presents an obstacle to volume production.^[101]

Throughput-scaling limits

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.

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.^[102]

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^[103] or servers,^[104] would have to be stitched together from two or more sub-patterns from different masks.^[105] 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.^[106]

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 times of the photons, 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.^[107] 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,^[108] 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^[109] and limits throughput, and crosslinking^[110] occurs at very high dose levels. For chemically amplified resists, higher dose exposure also increases line edge roughness due to acid generator decomposition.^[111]

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 same 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^[113] and resist strip^[114] considerations.

Uptime and productivity

Current throughput at customer site is 1,200 wafers per day with 80% availability,^[115] while conventional tools produce 5,000 wafers per day with 95% availability.^[116] As of 2017, the cost of a 7 nm process with 3 metal layers patterned by single EUV exposure is still 20% higher than the current 10 nm non-EUV multipatterned process.^[117] Hence, multiple patterning with immersion lithography has been deployed for volume manufacturing, while deployment of EUV is expected in 2018–2020.

Deployment History

The deployment of EUVL for volume manufacturing has been delayed for a decade,^[118][119] though the forecasts for deployment had timelines of 2–5 years. Deployment was targeted in 2007 (5 years after the forecast was made in 2002),^[118] in 2009 (5 years after the forecast), in 2012–2013 (3–4 years), in 2013–2015 (2–4 years),^[120][121] in 2016–2017 (2–3 years),^[122] and in 2018–2020 (2–4 years after the forecasts).^[123][124] However, deployment could be delayed further.^[125]

Shipments of the NXE:3350 system began at the end of 2015, with claimed throughput of 1,250 wafers/day or 65 wafers per hour (WPH) assuming 80% uptime.^[126][127] By comparison, the 300-unit installed base of NXT 193-nm immersion systems had 96% availability and 275 WPH in 2015.^[128][129]

Twenty EUV units were shipped in 2010–2016, short of the number that would be required for volume manufacturing. By comparison, ASML shipped over 60 NXT 193-nm immersion systems in 2016, and forecasts that 48 EUV units will be shipped in 2019.^[134][135] Six NXE:3100 units were shipped in 2010–2011.^[136] Eight NXE:3300B units were shipped in 2013Q3–2015Q1,^[129] fewer than the forecast 11 units.^[137] Two NXE:3350B units were shipped in late 2015,^[128] compared to a forecast six units.^[129] Four units were shipped in 2016, compared to a forecast six or seven units from the start of the year.^[138]

As of 2016, 12 units were forecast to ship in 2017,^[138] and 24 units in 2018.^[134] However, the shipment forecast for 2017 was halved at the beginning of the year to six or seven units.^[139] The NXE:3350B is planned to be discontinued by 2017, to be replaced by the NXE:3400B. At the time of shipping of the first NXE:3400B,^[140] eight NXE:3300B and six NXE:3350B systems were up and working in the field.^[141]

EUV and multiple patterning

EUV multiple patterning for multiple SMOs. Different patterns in the same layer could require different optimized illuminations, therefore requiring different exposures.

In Intel's complementary lithography scheme at 20 nm half-pitch, EUV would be used only in a second line-cutting exposure after a first 193 nm line-printing exposure.^[142]

Multiple exposures would also be expected where two or more patterns in the same layer, e.g., different pitches or widths, must use different optimized source pupil shapes.^{[143][144][145][146]}

Single patterning extension: Anamorphic High-NA

A return to extended generations of single exposure patterning would be possible with higher numerical aperture (NA) tools. An NA of 0.45 using 13.5 nm wavelength could require retuning of a few percent.^[147] Increasing demagnification could avoid this retuning, but the reduced field size severely affects large patterns (one die per 26 mm × 33 mm field) such as the many-core multi-billion transistor 14 nm Xeon chips.^[148] by requiring field stitching.

In 2015, ASML disclosed details of its anamorphic next-generation EUV (13.5 nm wavelength) scanner, with an NA of 0.55. The demagnification is increased from 4x to 8x only in one direction (in the plane of incidence). ^[149] However, the 0.55 NA has a much smaller depth of focus than immersion lithography.^[150] Also, an anamorphic 0.52 NA tool has been found to exhibit too much CD and placement variability for 5 nm node single exposure and multi-patterning cutting.^[151]

Depth of focus^[152] being reduced by increasing NA is also a concern,^[153] especially in comparison with multipatterning exposures using 193 nm immersion lithography:

The first high-NA tools are expected by 2020 at earliest.^[154]

Beyond EUV wavelength

A much shorter wavelength (~6.7 nm) would be beyond EUV, and is often referred to as BEUV (Beyond Extreme UltraViolet).^[155] A shorter wavelength would have worse shot noise effects without ensuring sufficient dose.^[156]

References

Further reading

• Banqiu Wu and Ajay Kumar (May 2009). Extreme Ultraviolet Lithography. McGraw-Hill Professional, Inc. ISBN 0-07-154918-8. 
• Banqiu Wu and Ajay Kumar (2009). "Extreme Ultraviolet Lithography: Towards the Next Generation of Integrated Circuits". Optics & Photonics Focus. 7 (4). 

Related links

• EUV presents economic challenges
• Industry mulls 6.7-nm wavelength EUV