Extreme ultraviolet lithography

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Extreme ultraviolet lithography (also known as EUV or EUVL) is a next-generation lithography technology using the 13.5 nm wavelength. EUV is a significant departure from the deep ultraviolet lithography used today. All matter absorbs EUV radiation. Hence, EUV lithography needs to take place in a vacuum. All the optical elements, including the photomask, must make use of defect-free Mo/Si multilayers which act to reflect light by means of interlayer interference; any one of these mirrors will absorb around 30% of the incident light. This limitation can be avoided in maskless interference lithography systems. However, the latter tools are restricted to producing periodic patterns only.

EUV tools

The pre-production EUV systems being built to date are expected to contain at least two condenser multilayer mirrors, six projection multilayer mirrors, and a multilayer object (mask). Since the optics already absorbs 96% of the available EUV light, the ideal EUV source will need to be sufficiently bright. 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 the high-energy ions and other debris. This damage associated with the high-energy process of generating EUV radiation has precluded the successful implementation of practical EUV light sources for lithography.

The wafer throughput of an EUV exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high vacuum, the throughput is limited mainly by the transfer of wafers into and out of the tool chamber, to a few wafers per hour.

Another aspect of the pre-production EUV tools is the off-axis illumination (at an angle of 6 degrees) on a multilayer mask. The resulting asymmetry in the diffraction pattern causes shadowing effects which degrade the pattern fidelity.

EUV's shorter wavelength also increases flare, resulting in increased line width roughness.

EUV absorption in matter

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. These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside photoresist before initiating the desired chemical reaction. A contributing factor for this rather large distance is the fact that polymers have significant amounts of free volume. In a recent actual EUV print test, it was found that 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor.

The response of matter to EUV radiation can be captured in the following equations:

Point of absorption:

EUV photon energy = 92 eV = Electron binding energy + kinetic energy of the emitted photoelectron

Within 3 mean free paths of photoelectron (1-2 nm):

Kinetic energy of photoelectron = Electron binding energy + kinetic energy of secondary electron + remaining kinetic energy of photoelectron

Within 3 mean free paths of secondary electron (~30 nm):

Kinetic energy of Nth (final generation) secondary electron ~ 0-5 eV = Chemical dissociation energy + heating

where the electron binding energy is typically 7-9 eV for organic materials and 4-5 eV for metals. The photoelectron subsequently causes the emission of secondary electrons through the process of impact ionization. Sometimes, an Auger transition is also possible, resulting in the emission of two electrons with the absorption of a single photon.

Strictly speaking, photoelectrons, Auger electrons and secondary electrons are all accompanied by positively charged holes (ions which can be neutralized by pulling electrons from nearby molecules) in order to preserve charge neutrality. An electron-hole pair is often referred to as an exciton. For highly energetic electrons, the electron-hole separation can be quite large and the binding energy is correspondingly low, but at lower energy, the electron and hole can be closer to each other. As the name implies, an exciton is an excited state; only when it disappears as the electron and hole recombine, can stable chemical reaction products form.

EUV photoresist images often require resist thicknesses roughly equal to the pitch. This is not only due to EUV absorption causing less light to reach the bottom of the resist but also to forward scattering from the secondary electrons (similar to low-energy electron beam lithography).

Since the photon absorption depth exceeds the electron escape depth, as the released electrons eventually slow down,they dissipate their energy ultimately as heat.

EUV Damage

Like other forms of ionizing radiation, EUV and EUV-generated electrons are a likely source of device damage. Damage may result from oxide desorption or trapped charge following ionization. Damage may also occur through indefinite positive charging by the Malter effect. If free electrons cannot return to neutralize the net positive charge, positive ion desorption is the only way to restore neutrality. However, desorption essentially means the photoresist is degraded during exposure, and furthermore, the desorbed atoms contaminate the optics. EUV damage has already been documented in the CCD radiation aging of the Extreme UV Imaging Telescope (EIT).

Radiation damage is a well-known issue that has been studied in the process of plasma processing damage. A recent study at the University of Wisconsin Synchrotron indicated that wavelengths below 200 nm are capable of measurable surface charging. EUV radiation showed positive charging centimeters beyond the borders of exposure while VUV radiation showed positive charging within the borders of exposure.

Studies using EUV femtosecond pulses at the FLASH synchrotron beam facility indicated thermal melting-induced damage thresholds below 100 mJ/cm².

EUV Defects

EUVL faces specific defect issues analogous to those being encountered by immersion lithography. Whereas the immersion-specific defects are due to unoptimized contact between the water and the photoresist, EUV-related defects are attributed to the inherently ionizing energy of EUV radiation. The first issue is positive charging, due to ejection of photoelectrons freed from the top resist surface by the EUV radiation. This could lead to electrostatic discharge or particle contamination as well as the device damage mentioned above. A second issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions. A third issue is etching of the resist by oxygen, argon or other ambient gases, which have been dissociated by the EUV radiation or the electrons generated by EUV. Ambient gases in the lithography chamber may be used for purging and contamination reduction. These gases are ionized by EUV radiation, leading to plasma generation in the vicinity of exposed surfaces, resulting in damage to the multilayer optics and inadvertent exposure of the sample.

Of course mask defects are also a known source of defects for EUVL. The mask defects comprise multilayer defects and defects buried under the multilayer as well as pattern defects. The buried defects are particularly insidious, and even 10 nm defects may be considered risky.

Unexpected Resolution Limits

Given that EUV is a significant reduction in wavelength compared to current lithography wavelengths, one would expect significantly better resolution. However, the resolution is ultimately determined by the interaction volume in the image recording medium, i.e., the photoresist. As noted above, the low energy electrons released by EUV could blur the original EUV image. In addition, there are statistical effects, especially for feature areas less than 1500 square nanometers. The required dose sensitivity of 5 mJ/cm² implies only several thousand EUV photons or so accumulate in such a small area. With the natural Poisson distribution due to the random arrival times of the photons, there is an expected natural dose variation of at least a few percent 3 sigma, making the exposure process fundamentally uncontrollable for features less than about 40 nm. Increasing the dose will reduce the shot noise, but will also increase the flare dose and generate more free electrons. The free electrons will spread out before slowing to a stop. Since the free electron density is lower than the initial photon density, the shot noise is always effectively larger than expected from just considering the EUV dose.

EUV Demonstrations

In 1996, a collaboration between Sandia National Laboratories, University of California at Berkeley, and Lucent Technologies, produced NMOS transistors with gate lengths from 75 nm to 180 nm. The gate lengths were defined by EUV lithography. The device saturation current at 130 nm gate length was ~0.2 mA/um. A 100 nm gate device showed subthreshold swing of 90 mV/decade and saturated transconductance of 250 mS/mm. A commercial NMOS at the same design rule patterned by then-state-of-the-art DUV lithography showed 0.94 mA/um saturation current and 860 mS/mm saturated transconductance. The subthreshold swing in this case was less than 90 mV/decade.

In 2008, a collaboration including IBM and AMD, based at the College of Nanoscale Science and Engineering (CNSE) in Albany, New York, used EUV lithography to pattern the first metal layer of a 45 nm node test chip. No specific details on device performance were given. However, the lithographic performance details given still indicated much to be desired:

CD uniformity: 6.6%
Overlay: 17.9 nm x, 15.6 nm y, possibly correctable to 6.7 nm x, 5.9 nm y
Power: 1 W at wafer (>200 W required for high volume), with a dose of 3.75 mJ/sq. cm.
Defects: 1/sq. cm.

Apparently, the CNSE EUV tool suffered from a well-known 16% flare problem. Flare effects may be difficult to separate from the secondary electron effects discussed earlier.

EUV Development: Forever Delayed?

EUV has been the subject of ongoing research and development by many groups. The predicted optical resolution capability has been demonstrated. However, optical resolution is not the limiting factor for EUV. Given that it is still under development in key areas such as light source, photoresists, and defect inspection, and that other areas such as EUV interaction with matter require further study, it is unlikely to be implemented in manufacturing in time to displace 193 nm immersion lithography. Instead, EUV will continue to compete against other next-generation lithography techniques, including high-index immersion lithography, nanoimprint lithography and maskless lithography.

The difficulties of EUV stem from the dramatically higher energy of the EUV photon (92 eV for EUV light vs. 6.4 eV for 193 nm light), which underlies the difficulty of damage-free generation and control of EUV light and confining the energy absorption within materials. It is also fundamentally impossible for EUV with low resolution enhancement and single patterning to compete with the larger depth of focus from the more established approach of using the 193 nm wavelength with strong resolution enhancement and double patterning. There is a growing realization that the resolution capabilities of the EUV wavelength are being countered by the effects of electrons released after absorption. Flare, already a consideration even for 193 nm lithography, continues to increase with the use of EUV, which becomes a significant line roughness issue for smaller lines. Finally, the throughput of EUV lithography is ultimately limited by the time to transfer wafers into and out of vacuum.

References

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Last updated on Saturday March 08, 2008 at 13:30:18 PST (GMT -0800)
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