Extreme Ultraviolet Lithography (EUVL) is one of the patterning technologies for the 22 nm node and below. Generally,
EUVL used a reflectivity type mask consist of absorber layer on a mask blank substrate coated with Mo/Si multilayer.
Especially, reflectivity from EUV mask multilayer could be one of the important factors to make EUV process to be
ready for 22 nm node. In spite of the developed technologies, the reported experimental reflectivity (60-66 %) is much
less than the theoretical reflectivity (73 %) from the perfect EUV mask multilayer because of the Mo/Si rough
boundaries and multilayer top surface roughness. The surface roughness that occurs in deposition of multilayer makes
the reflectivity loss. It seems that it might be difficult to reach the ideal reflectivity and 22 nm node process has to live up
with the imperfect reflectivity.
In this study, we focused on the influence of the surface roughness on the Mo/Si multilayer for 22 nm node. First we
studied the reflectivity loss for the multilayer surface roughness. The magnitudes of short, medium, and long range
roughness are compared in terms of the amplitude and phase non-uniformity because even 1 nm roughness can make
huge difference in EUV. The aerial image and process latitude with surface roughness are studied and the possibility of
22 nm node patterning with surface roughness will be reported.
It will directly affects pellicle degradation, at the irradiated part of the pellicle, and make a sloped
pellicle surface and will act like a prism before change of phase or transmittance occurs, because the
energies of C, F and O single bondings composing the ArF pellicle film is quite smaller than the
energy of 193 nm ArF. Thus, outgoing light has information of smaller space than mask size. In order
to offer some tip to find the appearance of pellicle thinning caused defect, several types of pattern
deformation caused by pellicle degradation is studied.
The lithography industry has been working to extend 193 nm immersion with double patterning and
complex computational lithographic techniques for 32 nm and below. Also extreme ultraviolet
lithography (EUV) are used to make the 22 nm half-pitch and below. However, technical challenges
remain to be addressed, as well as the high cost of the manufacturing tool. There was a report that a
new wavelength, 172 or 175 nm, can be used for next generation lithography system. 172 nm
lithography, although, has higher absorbance than 193 nm, it has much higher transmission than 157
nm in high refractive index liquid. Compared with 193 nm immersion lithography that has the
resolution limit of 35.7 nm by using maximum numerical aperture (NA) of 1.35, 172 nm immersion
lithography can be used for possible resolution limit of 27.4 nm by using maximum NA of 1.57. In
this paper, we evaluated the 172 nm immersion lithography using commercial lithography simulation
for 28 nm node by single exposure. We also checked the patterning possibility of 22 and 16 nm node
by using 172 nm and double patterning because a totally new wavelength should show the possible
extension to multiple generations.
Contact hole (CH) patterning, specially for sub-50 nm node, is one of the most difficult technique in optical
lithography. Resist reflow process (RRP) can be used to obtain smaller CH. RRP is a simple technique that the
resist, after the develop process, is baked above the glass transition temperature (Tg). Heating causes the resist flowing, and we can obtain smaller dimension of CHs. However, RRP is unmanageable method because CH
offset caused by pattern position in random array CH. So we tried OPC to find uniform CD for every CH, and
we could obtain the uniform CD for every CH after RRP. However, we still have CH position shift problem.
Because of a difference in an amount of resist that flow into the hole in random array during the reflow process,
position shift occurs. This position shift makes overlay error, and it may exceed the overlay error limit suggested
by ITRS roadmap. In this work, we try to find not only uniform CD size of each CH, but also optimum
condition for correcting CH position shift by using home-made simulation. Moreover, we confirmed the
tendency of CH position shift by e-beam lithography experiment. Consequently, we confirmed that CH moved
to receding direction from each other, and obtained sub-50nm CHs in random array by considering the position
shift through the simulation and experiment.
Extreme ultraviolet lithography (EUVL) is believed to be possible patterning technology which can make 22 nm
and below. EUV uses a reflective mask so that the mask is shined with the oblique incident light. Thus, the study of
incident angle effect is very important. Currently, 6 degree oblique incidence is main stream, but 5 degree incident angle
is also studied for 0.25 NA. Incident angles larger than 6 degree are also considered for larger NA. This incident angle
will affect many things, eventually to the line width. Shadow effect also strongly depends on the incident angle. This
shadow effect in the EUVL mask is an important factor that decreases the contrast of the aerial image and causes a
directional problem, thus it will make line width variation. The off-axis illumination (OAI) will be used with
conventional on-axis illumination to make much smaller patterns. This OAI will split the main beam and change the
incident angle. We found that if the incident angle increased with higher degree of coherence, the aerial image went
worse. The CD difference between the horizontal and the vertical pattern is also dependent on the degree of coherence
even though it is small.
According to ITRS road map, it will be achieved 22 nm half pitch until 2016. However, it is hard to make although EUV, high index immersion. We have positive strategy for 22 nm half pitch with immersion and double patterning and RRP. We can make 22 nm half-pitch with hard mask by using RRP that can shrink trench pattern and double patterning that can get over resolution limitation. Immersion technology can make 44 nm half pitch in NA 1.35. When the developed resist profile can be reflow, so line is increased and space is decreased. It can be 22 nm trench pattern with 66 nm width by using RRP. Hence, we can obtain 66 nm line and 22nm space pattern by etching. And then, we can obtain 22 nm half pitch after doing double patterning. We tried to evaluate this strategy by commercial and home-made simulator.
Mask defect is one of the biggest problems in Extreme Ultraviolet Lithography (EUV) technology. EUV mask must be free of small defects, requiring development of new inspection tools and low defect fabrication processes. So, we studied the influences of the defects on the mask for 22 nm line and space pattern. First, we changed the light quality caused by the various wavelength shift, incident angle, and the defect material with different refractive index. Second, we changed the defect size from 20 nm to 16 nm because 18 nm defect is assumed to a critical defect size for 22 nm node. Third, we also changed the defect positions; on top of the absorber, on the valley of the absorber, and at the sides of the absorber. Finally, we simulated the influence for the different shaped defect. A square pillar defect shows very different behavior compared to the more realistic round shaped defect. Defect of higher refractive index gives little influence, while defect of lower refractive index gives larger influence. A more realistic elliptical shaped defect gives less influence compared to square shaped defect. All the defect and EUV parameters will influence to the printability of the defect, but more study is needed to judge whether a certain defect can influence the printed pattern.
We applied the immersion lithography to get 32 nm node pattern with 1.55 NA, without using double exposure /
double patterning. A chromeless phase shift mask is compared with an attenuated phase shift mask to make 32 nm
dense 1:1 line and space pattern. We compared the aerial image, normalized image log slope, exposure latitude,
and depth of focus for each mask type in order to see the effect of the post exposure bake and acid diffusion length.
The process window shrinks fast if the diffusion length is larger than 10 nm for both mask types. However, up to
20 nm diffusion length, 32 nm can be processible if the exposure latitude of 5% is used in production.