09-05-2012, 02:51 PM
Extreme ultraviolet lithography
Extreme ultraviolet lithography.docx (Size: 76.65 KB / Downloads: 41)
EUVL light source
Neutral atoms or condensed matter cannot emit EUV radiation. For matter to emit it, Ionization must take place first. EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV.[1] Such electrons are more tightly bound than typical valence electrons. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV.[2][3] The Xe or Sn plasma sources for EUV lithography are either discharge-produced or laser-produced. Power output exceeding 100 W is a requirement for sufficient throughput. While state-of-the-art 193 nm excimer lasers offer intensities of 200 W/cm2,[4] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 1011 W/cm2.[5] This indicates the enormous energy burden imposed by switching from generating 193 nm light (laser output approaching 100 W)[6] to generating EUV light (required laser or equivalent power source output exceeding 10 kW).[7]
A further characteristic of the plasma-based EUV sources under development is that they are not even partially coherent,[8] unlike the KrF and ArF excimer lasers used for current optical lithography. Further power reduction (energy loss) is expected in converting incoherent sources (emitting in all possible directions at many independent wavelengths) to partially coherent (emitting in a limited range of directions within a narrow band of wavelengths) sources by filtering (unwanted wavelengths and directions). On the other hand, coherent light poses a risk of monochromatic reflection interference and mismatch of multilayer reflectance bandwidth.[9]
As of 2008, the development tools had a throughput of 4 wafers per hour with a 120 W source.[10] For a 100 WPH requirement, therefore, a 3 kW source would be needed, which is not available in the foreseeable future. However, EUV photon count is determined by the number of electrons generated per photon which are collected by a photodiode; since this is essentially the highly variable secondary yield of the initial photoelectron, the dose measurement will be impacted by high variability. In fact, data by Gullikson et al.[11] indicated ~10% natural variation of the photocurrent responsivity. More recent data for silicon photodiodes remain consistent with this assessment.[12] Calibration of the EUV dosimeter is a nontrivial unsolved issue.[13] The secondary electron number variability is the well-known root cause of noise in avalanche photodiodes.[14]
If other problems are solved well enough to justify the investment, free electron lasers may provide the required light quality.
EUVL optics
EUVL 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.
The pre-production EUVL systems built to date 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[15][16] and other debris.[17] 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 EUVL exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high vacuum, the throughput is limited (aside from the source power) by the transfer of wafers into and out of the tool chamber, to a few wafers per hour.[18]
Another aspect of the pre-production EUVL tools is the off-axis illumination (at an angle of 6 degrees)[19] on a multilayer mask. The resulting asymmetry in the diffraction pattern causes shadowing effects which degrade the pattern fidelity.[20]
EUVL's shorter wavelength also increases flare, resulting in less than perfect image quality and increased line width roughness.[21]
Heating per feature volume (e.g., 20 nm cube) is higher per EUV photon compared to a DUV photon, due to higher absorption in resist. In addition, EUV lithography results in more heating due to the vacuum environment, in contrast to the water cooling environment of immersion lithography.
Heating is also a particularly serious issue for multilayer mirrors used, because EUV is absorbed within a thin distance from the surface. The heating density is higher. As a result, water cooling is expected to be used for the high heating load; however, the resulting vibration is a concern.[22]
A recent study by NIST and Rutgers University found that multilayer optics contamination was highly affected by the resonant structure of the EUV mirror influencing the photoelectron generation and secondary electron yield.[23]
Since EUV is highly absorbed by all materials, even EUV optical components inside the lithography tool are susceptible to damage, mainly manifest as observable ablation.[24] Such damage is a new concern specific to EUV lithography, as conventional optical lithography systems use mainly transmissive components and electron beam lithography systems do not put any component in the way of electrons, although these electrons end up depositing energy in the exposed sample substrate.
EUV exposure of photoresist
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.[25] It has been estimated that about 4 secondary electrons on average are generated for every EUV photon, although the generation volume is not definite.[26] These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside photoresist (see below) before initiating the desired chemical reaction. This is very similar to the photoelectron migration for the latent image formation in silver halide photographic films. A contributing factor for this rather large distance is the fact that polymers have significant amounts of free volume.[27] In a recent actual EUV print test,[28] it was found 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor.
Initial distribution of reactive species after EUV absorption. Molecules are excited and ionized within a few nanometers from the absorption point, and electrons are thermalized within 20 nanometers from the absorption point. The inset picture shows the multispur effect, where several electron-ion pairs generated by the EUV photon may interact with one another.
In particular, for photoresists utilizing chemical amplification for higher throughput:[29][30]
e- + acid generator -> anion -> dissociated anion products
This reaction, also known as "electron attachment" or "dissociative electron attachment" is most likely to occur after the electron has essentially slowed to a halt, since it is easiest to capture at that point. The cross-section for electron attachment is inversely proportional to electron energy at high energies, but approaches a maximum limiting value at zero energy.[31] On the other hand, it is already known that the mean free path at the lowest energies (few to several eV or less, where dissociative attachment is significant) is well over 10 nm,[32][33] thus limiting the ability to consistently achieve resolution at this scale. In addition, electrons with energies < 20 eV are capable of desorbing hydrogen and fluorine anions from the resist,[34] leading to potential damage to the EUV optical system.[35]
EUV photoresist images often require resist thicknesses roughly equal to the pitch.[36] 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). Conversely, thinner resist transmits a larger fraction of incident light allowing damage to underlying films, yet requires more dosage to achieve the same level of absorption.
Since the photon absorption depth exceeds the electron escape depth, as the released electrons eventually slow down, they dissipate their energy ultimately as heat.
An EUV dose of 1 mJ/cm2 generates an equivalent photoelectron dose of 10.9 μC/cm2. Current demonstration doses exceed 10 mJ/cm2, or equivalently, 109 μC/cm2 photoelectron dose.
The use of higher doses and/or reduced resist thicknesses to produce smaller features only results in increased irradiation of the layer underneath the photoresist. This adds another significant source of photoelectrons and secondary electrons which effectively reduce the image contrast. In addition, there is increased possibility of ionizing radiation damage to the layers below.
The extent of secondary electron and photoelectrons in blurring the resolution is dependent on factors such as dose, surface contamination, temperature, etc.
EUVL 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[37] 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.[38] A third issue is etching of the resist by oxygen,[39] 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.[40]
Extreme ultraviolet lithography.docx (Size: 76.65 KB / Downloads: 41)
EUVL light source
Neutral atoms or condensed matter cannot emit EUV radiation. For matter to emit it, Ionization must take place first. EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV.[1] Such electrons are more tightly bound than typical valence electrons. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV.[2][3] The Xe or Sn plasma sources for EUV lithography are either discharge-produced or laser-produced. Power output exceeding 100 W is a requirement for sufficient throughput. While state-of-the-art 193 nm excimer lasers offer intensities of 200 W/cm2,[4] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 1011 W/cm2.[5] This indicates the enormous energy burden imposed by switching from generating 193 nm light (laser output approaching 100 W)[6] to generating EUV light (required laser or equivalent power source output exceeding 10 kW).[7]
A further characteristic of the plasma-based EUV sources under development is that they are not even partially coherent,[8] unlike the KrF and ArF excimer lasers used for current optical lithography. Further power reduction (energy loss) is expected in converting incoherent sources (emitting in all possible directions at many independent wavelengths) to partially coherent (emitting in a limited range of directions within a narrow band of wavelengths) sources by filtering (unwanted wavelengths and directions). On the other hand, coherent light poses a risk of monochromatic reflection interference and mismatch of multilayer reflectance bandwidth.[9]
As of 2008, the development tools had a throughput of 4 wafers per hour with a 120 W source.[10] For a 100 WPH requirement, therefore, a 3 kW source would be needed, which is not available in the foreseeable future. However, EUV photon count is determined by the number of electrons generated per photon which are collected by a photodiode; since this is essentially the highly variable secondary yield of the initial photoelectron, the dose measurement will be impacted by high variability. In fact, data by Gullikson et al.[11] indicated ~10% natural variation of the photocurrent responsivity. More recent data for silicon photodiodes remain consistent with this assessment.[12] Calibration of the EUV dosimeter is a nontrivial unsolved issue.[13] The secondary electron number variability is the well-known root cause of noise in avalanche photodiodes.[14]
If other problems are solved well enough to justify the investment, free electron lasers may provide the required light quality.
EUVL optics
EUVL 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.
The pre-production EUVL systems built to date 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[15][16] and other debris.[17] 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 EUVL exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high vacuum, the throughput is limited (aside from the source power) by the transfer of wafers into and out of the tool chamber, to a few wafers per hour.[18]
Another aspect of the pre-production EUVL tools is the off-axis illumination (at an angle of 6 degrees)[19] on a multilayer mask. The resulting asymmetry in the diffraction pattern causes shadowing effects which degrade the pattern fidelity.[20]
EUVL's shorter wavelength also increases flare, resulting in less than perfect image quality and increased line width roughness.[21]
Heating per feature volume (e.g., 20 nm cube) is higher per EUV photon compared to a DUV photon, due to higher absorption in resist. In addition, EUV lithography results in more heating due to the vacuum environment, in contrast to the water cooling environment of immersion lithography.
Heating is also a particularly serious issue for multilayer mirrors used, because EUV is absorbed within a thin distance from the surface. The heating density is higher. As a result, water cooling is expected to be used for the high heating load; however, the resulting vibration is a concern.[22]
A recent study by NIST and Rutgers University found that multilayer optics contamination was highly affected by the resonant structure of the EUV mirror influencing the photoelectron generation and secondary electron yield.[23]
Since EUV is highly absorbed by all materials, even EUV optical components inside the lithography tool are susceptible to damage, mainly manifest as observable ablation.[24] Such damage is a new concern specific to EUV lithography, as conventional optical lithography systems use mainly transmissive components and electron beam lithography systems do not put any component in the way of electrons, although these electrons end up depositing energy in the exposed sample substrate.
EUV exposure of photoresist
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.[25] It has been estimated that about 4 secondary electrons on average are generated for every EUV photon, although the generation volume is not definite.[26] These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside photoresist (see below) before initiating the desired chemical reaction. This is very similar to the photoelectron migration for the latent image formation in silver halide photographic films. A contributing factor for this rather large distance is the fact that polymers have significant amounts of free volume.[27] In a recent actual EUV print test,[28] it was found 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor.
Initial distribution of reactive species after EUV absorption. Molecules are excited and ionized within a few nanometers from the absorption point, and electrons are thermalized within 20 nanometers from the absorption point. The inset picture shows the multispur effect, where several electron-ion pairs generated by the EUV photon may interact with one another.
In particular, for photoresists utilizing chemical amplification for higher throughput:[29][30]
e- + acid generator -> anion -> dissociated anion products
This reaction, also known as "electron attachment" or "dissociative electron attachment" is most likely to occur after the electron has essentially slowed to a halt, since it is easiest to capture at that point. The cross-section for electron attachment is inversely proportional to electron energy at high energies, but approaches a maximum limiting value at zero energy.[31] On the other hand, it is already known that the mean free path at the lowest energies (few to several eV or less, where dissociative attachment is significant) is well over 10 nm,[32][33] thus limiting the ability to consistently achieve resolution at this scale. In addition, electrons with energies < 20 eV are capable of desorbing hydrogen and fluorine anions from the resist,[34] leading to potential damage to the EUV optical system.[35]
EUV photoresist images often require resist thicknesses roughly equal to the pitch.[36] 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). Conversely, thinner resist transmits a larger fraction of incident light allowing damage to underlying films, yet requires more dosage to achieve the same level of absorption.
Since the photon absorption depth exceeds the electron escape depth, as the released electrons eventually slow down, they dissipate their energy ultimately as heat.
An EUV dose of 1 mJ/cm2 generates an equivalent photoelectron dose of 10.9 μC/cm2. Current demonstration doses exceed 10 mJ/cm2, or equivalently, 109 μC/cm2 photoelectron dose.
The use of higher doses and/or reduced resist thicknesses to produce smaller features only results in increased irradiation of the layer underneath the photoresist. This adds another significant source of photoelectrons and secondary electrons which effectively reduce the image contrast. In addition, there is increased possibility of ionizing radiation damage to the layers below.
The extent of secondary electron and photoelectrons in blurring the resolution is dependent on factors such as dose, surface contamination, temperature, etc.
EUVL 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[37] 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.[38] A third issue is etching of the resist by oxygen,[39] 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.[40]