There are many stumbling blocks on the road to the 22 nm technology node.
The lithography-related specifications of the 2003 International Roadmap
for Semiconductors (ITRS) for this node calls for a DRAM half-pitch
of 22 nm, printed gate length of 13 nm, resist thickness of 40-80 nm,
line-edge (LER) roughness (3-sigma) of 1 nm, and critical dimension (CD)
control (3-sigma) of 1 nm. These specifications call for near-atomic-scale
resolution – something that is impossible with the current resist design
concepts and imaging mechanism. These stumbling blocks are intimately
tied to the nature of resist chemical amplification imaging mechanism,
thin film confinement effects, and polymer molecular properties. Acting
either separately or in concert, these stumbling blocks are resolution-limiting,
with disastrous consequences such as poor CD control, LER, and pattern
collapse. How these three resist properties and issues limit resolution
is discussed in more detail in the following sections.
Resolution Limits Due to Chemical Amplification
While the chemical amplification concept has served the semiconductor
industry very well since its invention in the early 1980s[2-4] in terms
of high sensitivity and resolution, it is becoming apparent that by 2016,
when the 22 nm technology node is expected to be in production, the very
attributes of this concept that made resists based on it become the workhorses
of the industry for the last 25 years will unfortunately become resolutionlimiting
due to uncontrollable diffusion, image spreading, or resist blur. A growing
body of experimental evidence suggests that chemically amplified resists
have an intrinsic bias that limits resolution,[5-12] which given aggressive
scaling of gate length, critical dimension control will present a very
difficult challenge at the 22 nm node.
Empirical results suggest that chemical amplification starts to limit
resolution in high activation energy resist systems like those based on
tert-butyloxycarbonyl-protected (t- BOC) resist systems as the pitch approaches
80 nm.[5-7] For instance, the blur at full width half maximum (FWHM) of
state-of-theart DUV 248 nm and 193 nm environmentally stable, chemically
amplified photoresists are 100 nm and 60 nm, respectively. State-of-the-art
high-activation energy resist blur (at FWHM) is roughly 50-60 nm, while
that of a state-of-the-art low-activation energy resist is less than 20
nm. Currently, FWHM blur as much as 50-90 nm have been observed at the
180 nm pitch. Lithographic patterning at the 22 nm node will require blur
that should not exceed 15 nm, in order to capture the high-resolution
aerial image. Blur value less than 15 nm may be difficult to achieve at
high throughput, especially with weak EUV sources. Resist blur increases
with resist sensitivity (amplification factor), and this will inevitably
limit throughput at high resolution. Changing exposure wavelength does
not address any of these issues.
The design concept of chemical amplification is based on generation of
a chemically stable catalytic species – commonly referred to as a photoacid
and designated as a proton H+, as illustrated in Figure 1 for a resist
system comprising a copolymer: poly(4-hydroxystyrene- co-4-t butyloxycarbonyloxystyrene)
– of the photoresist film that are irradiated. Rather than using exposure
energy to directly cause a solubility switch, chemically amplified resists
use exposure energy only to generate a catalytic species. The photogenerated
catalyst then initiates a chain reaction or promotes a cascade of solubility-switching
reactions in the exposed regions of the photoresist. The apparent quantum
efficiency for the solubility switching reaction in such a system is the
product of the quantum efficiency for catalyst generation and the catalytic
chain length. Catalytic chain lengths in the many hundreds are common,
so in effect the quantum efficiency can be greater than one. One photochemical
conversion can cause several chemical reactions, and thus the exposure
can be said to have been “chemically amplified.” In this manner, lithographic
imaging can be accomplished with very low exposure doses, saving time
and money in manufacturing.[11-12]
Figure 1. Catalytic deprotection reaction of
a photoresist system based on copolymer: poly(4-hydroxystyrene-co-4-t-butyloxycarbonyloxystyrene).
While chemically amplified resists have many advantages, they have one
major limitation: It is possible for the catalyst generated in exposed
regions to diffuse into unexposed regions, causing blurring of the latent
image (Figure 2). The migration of the photoacid effectively results in
a bias between the distribution of exposure energy and the final distribution
of exposure photoproducts. This bias depends upon the photoresist chemistry
and the processing conditions, but is largely independent of the exposure
conditions and therefore cannot be eliminated by improving the exposure
Figure 2. Exposure process of a chemically
amplified resist, illustrating photoacid diffusion in the presence
of a non-diffusing quencher.
It is, however, possible to reduce catalyst migration by increasing the
size of the catalyst or by reducing the temperature of the post exposure
bake (PEB) step. Another approach for mitigating the problem of feature
dimension bias due to migration of acid catalyst is to incorporate base
additives into the photoresist formulation. These approaches for reducing
acid catalyst migration also generally reduce the catalytic efficiency
of each photoproduct and therefore increase the total dose of exposure
energy that is required to pattern the film. Increases in required exposure
dosage effectively reduce the throughput of the expensive exposure tools
and can seriously reduce the profitability of the manufacturing process.
The fundamental equation governing the temperature dependence of diffusion
as a rate process is that given by Gladstone, Laidler, and Eyring (1941)
in Equation 1, where e is the exponential constant, ë is the length of
the diffusive jump, k is the Boltzmann’s constant, h is Planck’s constant,
T is the absolute temperature, .S–: is the entropy of activation of diffusion,
and R is the universal gas constant.
Photoacid diffusion in chemically amplified resists is temperature-dependent.
Within normal processing conditions, the higher the PEB temperature, the
faster the rate of the photoacid diffusion is and the longer the diffusion
length is. Although the PEB temperature can be used to modulate photoacid
diffusion and consequently resist sensitivity, it nevertheless involves
a tradeoff with LER and CD uniformity. While highactivation energy resists
based on t-butyl ester and butoxycarbonyl protecting groups typically
need high PEB temperature to be deprotected, low-activation energy resists
based on acetal and ketal protecting groups typically need low PEB temperature,
oftentimes room temperature while within the exposure chamber, to be deprotected.
Because of their high PEB temperatures, the high-activation energy resists
have much higher blur values and lower LER than their low-activation energy
counterparts, which suffer more from high LER and outgassing. So in principle,
chemical amplification concept is extendible to the 22 nm node with low
activation energy, low blur, poor LER, and bad outgassing resists of the
acetal and ketal platforms. High-activation energy resists in their current
form are not extendible to this node because of their high blur.
Reduction of Resist Contrast Due to Diffusion – Linear Theory
The photoresist polymer experiences the average acid concentration c
during PEB, leading to deprotection. Useful for line/space structures
is the well-known gaussian response of the diffusion equation, to the
initial concentration c (x,y,t = 0) = ä (x), where x and y are spatial
coordinates, t is time, and ä the Dirac impulse. The average concentration
field is then given by Equation 2, with D denoting the diffusion coefficient,
and illustrated in Figure 3.
Figure 3. Shape of the average acid concentration
due to diffusion, the term between brackets in equation 2.
Applying the superposition principle, one can calculate the response
to an initially sinusoidal modulation of the photoacid concentration with
pitch p, the Modulation Transfer Function (MTF) as MTFdiff = p2(1– exp(–4ð2
Dt/p2)/4ð2 Dt), shown in Figure 4. For example, suppose that one wants
to know which diffusion length is allowed if one accepts a relative 30
percent reduction of the resist image due to diffusion, then the diffusion
length Ld= 2 Dt may be as large as 0.2 times the pitch.
Figure 4. Modulation Transfer Function due
to diffusion MTFdiff vs. pitch (p)/diffusion length (Ld)
In the continuum theory, one can shrink the pitch as long as one can
shrink the diffusion length. A pitch of 45 nm requires a diffusion length
of about 7 nm or less. The blur value (FWHM) is about 2.35 times this
value. Even by tuning the size of the anion, the PEB temperature, and
the coating thickness, blur value of this magnitude is going to be nearly
impossible to achieve with high-activation energy resist systems, but
seems achievable with low-activation energy resist systems.
Scaling of LER With Diffusion Length
Shot noise will be a significant contributor to LER at the 22 nm node.
Poisson statistics stipulate that the relative variation of the number
of absorbed photons N in a volume V scales as 1/ N. For a given number
density, the larger the volume, which is the diffusion sphere, and is
proportional to Ld 3, the relative variation scales as (Ld)-3/2. We conjecture
that it is by increasing the diffusion length that one obtains lower LER
numbers for a given dose at large pitches. However, if the diffusion
length becomes so large, or the pitch so small, that blurring due to diffusion
becomes significant, then this will limit the ultimate resolution. Figure
5 shows the scaling relationship between LER scaling factor as a function
of the ratio of diffusion length and pitch. Combining the Poisson statistics
of the absorbed photons with the MTF results in the following observation:
Increasing the diffusion length up to 0.2 times the pitch will reduce
the LER, while beyond that length one has practically reached a plateau,
such that there is no more gain in LER to be made, only further loss in
exposure latitude. Preliminary indications are that taking quencher
statistics, or quantum yield into account does not significantly change
the picture described by linear theory.
Figure 5. Line edge roughness scaling factor
versus Ld/p ( Scaling factor: ((pLd)-3/2 /MTF(Ld/p), and asymptote
(dashed) (pLd)-3/2) ).
When Does Chemical Amplification Cease?
The assumption that shot noise of the absorbed photons is a dominant
factor for LER implicitly assumes that there is a large chemical amplification
factor, such that the deprotection profile of the photopolymer is governed
by continuum theory, and fluctuations are due to the statistics of the
photon-absorption process only. We present an argument for why chemical
amplification will cease to be possible around the 22 nm node.
Suppose that the amplification factor is approaching unity, and that
50 percent of the photoacid generator (PAG) (volume ~0.71nm3/PAG) is converted
into acid, which then deprotects 50 percent of the protecting groups of
the polymer (for t- BOC systems, volume ~0.22nm3/t-BOC group). This typical
unit cell size, consisting of at least two PAGs and two protecting groups,
will be of the order of 1.8nm3. To store the information of the location
of the line edge, one needs at least 1,000 unit cells, which correspond
to a cubic pixel of (10 nm)3 or a rectangular slab at the edge of 1 nm
thickness, 60 nm height, and 30 nm length. Poisson statistics stipulate
that the 3-sigma variation for an average number of 1,000 is 10 percent.
So given this pixel size, the 3-sigma fluctuation on the location of the
line edge is 1 nm. Now, the pixel size is set by the acid diffusion length,
10 nm in this example, which should be smaller than 0.2 times the pitch,
from the argument presented earlier. Thus, one arrives at a crude estimate
that for about 50 nm pitch one can no longer have chemical amplification
if one wants to keep LER of the resist under control. Adding quencher
does not dramatically change this picture.
Resolution Limits Due to Confinement Effects in Resists
Resist technologies that will enable the extreme ultraviolet lithography
(EUV) that will be used to pattern the 22 nm technology node will need
to boost CD control with appropriate LER of resist features to levels
that seem unrealizable today. Given the significant attenuation of EUV
radiation in organic materials, ultrathin resist (UTR) (thickness ¡Ü 100
nm) imaging appears to be the only viable option for patterning at the
22 nm technology node. The 2003 ITRS specifications call for a resist
thickness of 40-80 nm for the 22 nm technology node. However, the stability
of UTR films presents very difficult challenges due primarily to interfacial
and confinement effects. In addition, UTR imaging will require materials
(resists and underlayers) with superb etch selectivity and etch processes
that lead to negligible roughness transfer between layer stacks. Practical
considerations often impose the choice of a resist and a substrate that
are not fully compatible, resulting in films that are unstable or metastable
with finite relaxation time. While thick films may be stable or metastable
due to gravity,[16,17] for thin films intermolecular and surface forces
dominate. UTR films are susceptible to both spontaneous thin-film
instabilities due to Londonvan der Waals interactions, and defects caused
by substrate imperfections (topographical features like pits or mounds).[19,20]
Because UTR films have higher surfaceto- volume ratio than their thicker
counterparts, they are highly interfacial. Each macromolecular component
of a UTR system is close to an interface, either the solid substrate or
the free surface. Consequently, in UTR films, the material properties
may be quite different from those of the bulk. In particular, the glass
transition temperature (Tg) may be depressed or elevated by as much as
40ºC relative to the bulk values, depending upon the film thickness and
the chemical nature of the solid substrate upon which the film is deposited.
This may have consequences on the viscoelastic response of the film during
subsequent thermal annealing. The interfacial properties of such polymerinorganic
interfaces oftentimes determine their lithographic performance and consequently
the performance of devices fabricated with them.
Experimental results suggest that material properties of UTR films can
differ in significant ways from their bulk counterparts. For example,
physical properties such as the degree of crystallinity and diffusion
coefficient can not only be different in the ultrathin film state,
but these properties become increasingly sensitive to film thickness,
substrate surface energy, and local ordering into non-homogeneous and
structured phases. Of particular interest, because of its influence on
the viscoelastic behavior of the spin-coated film, is the effect of film
thickness on Tg (see Figure 6). A variety of experimental methods including
spectroscopic ellipsometry,[22- 24] X-ray reflectometry, positron
annihilation lifetime spectroscopy, and Brillouin scattering have
shown that Tg depends on film thickness and on the chemical nature of
the polymer-substrate interaction. For non-interacting substrates, e.g.,
hydrophobic polymer on a hydrophobic substrate, the Tg value can be depressed
by as much as 40ºC. By contrast, for moderately strongly interacting
substrates, e.g., an oxide surface interacting with a polymer capable
of hydrogen bonding, the Tg value can be elevated by a comparable
Figure 6. Glass transition temperature of resist polymer, and resist
polymer with photoacid generator (from Shipley Co.) as a function
of film thickness.[23,24]
Practical consequences of Tg modification in polymer films include significant
changes of dissolution, diffusional and etching characteristics, mechanical
creep behavior, and adhesion. For example, dissolution rate measurements
of spincoated films of poly (3-methyl-4-hydroxy styrene), a common matrix
resin in chemically amplified resists used in deep UV lithography, showed
dramatic reduction in the diffusion coefficient of the developing base
solution in the region close to the native silicon oxide surface of the
solid substrate. This was attributed to enhancement of the hydrogen-bonding
network due to chain orientation effects. Figure 7 shows a plot of effective
diffusion coefficient of perfluorooctane sulfonate photoacid as a function
of film thickness of partially protected poly (4-t-butyloxycarbonyloxstyrene).[
14] The profile shows asymptotic behavior at ~600A, below which diffusion
slows remarkably, probably due to interfacial and confinement effects.
Figure 7. Effective diffusion coefficient of
perfluorooctane sulfonate photoacid as a function of film thickness
in partially protected poly(4-t-butyloxycarbonyloxstyrene). (Adapted
from reference 14.)
Resist additives such as leveling agents, plasticizers, bases, PAGs,
and the like can segregate and partition themselves at different layers
of the film, driven by interfacial effects, which are magnified as the
film thickness decreases.[31,32] As a result, UTR films tend to have higher
LER than their thicker counterparts. They also tend to have higher susceptibility
to poisoning effects and unexposed film thickness loss than their thick
resist counterparts.[31,32] It is widely appreciated that understanding
and controlling the causes of defects associated with UTR films are major
challenges for the near-term development of UTR process technology.
Resolution Limits Due to Resist Polymer Molecular Properties
A 1 nm CD and LER tolerance of resist features as specified in the 2003
ITRS is smaller than the dimensions of the polymer molecules in today’s
resists, given a typical carboncarbon bond length of 0.13-0.15 nm.
A printed gate length of 13 nm is only a small multiple of the radius
of gyration, rg (rg ~0.6×MW0.5, where MW is molecular weight) of typical
amorphous polymers used in resists today. Therefore, the ITRS specifications
call for near-atomic-scale resolution, which is realistically untenable
with the present resist architectures. Resolution will in effect be limited,
and LER will be negatively impacted, at the 22 nm node as the length scale
of resist polymer material properties (polymerization length distribution,
polymer end-to-end distance, and radius of gyration, etc.) approach the
printed gate length in resists.
Resist Outlook for the 22 nm Technology Node
Image blur caused by acid diffusion into unexposed areas of resists can
only be limited, but not eliminated, if high sensitivity due to chemical
amplification is to be maintained. This is because the mechanisms responsible
for high sensitivity in chemically amplified resists are intimately tied
to the processes that lead to image blurring. This makes it extremely
difficult, if not impossible, to have a highly sensitive, highresolution
chemically amplified resist based on the conventional resist design concepts.
Therefore, novel resist design concepts, incorporating different polymer
architectures and photoacid generators that can achieve high sensitivity,
should be explored. Such alternative resist design concepts could incorporate
photo-catalysts with welldefined limited lifetimes, whose diffusion length
at nominal processing temperature could be targeted to match the resolution
requirements of the 22 nm design rule. This will ensure that at the barest
minimum, the acid can only diffuse to a manageable distance that should
fall within the geometry of the 22 nm design rule.
Low-activation energy acetal and ketalbased chemically amplified resists
could be extendible to the 22 nm technology node, if ways are found to
mitigate their outgassing and LER problems. It is of course possible for
exposure tools to be designed such that outgassed species from resists
are prevented from reaching the optical elements. It is also possible
to smoothen the walls of patterned resist features using standard post-lithographic
processing techniques like UV-hardening, electron beam irradiation, thermal
flow, and surfactant rinse. Also, non-chemically amplified resists are,
in principle, extendible to the 22 nm technology node, with highexposure
doses and low throughput, if only the exposure tools can support that.
Conventional resist technology will be resolution limited at the 22 nm
design rule. Chemical amplification chemistry runs into “diffusional limits”
with high-activation energy resist systems that are not extendible to
this node, while low-activation energy ones may be extendible this node,
but at the cost of significant outgassing and poor LER. Non-chemically
amplified resists are also extendible to this node, but at the cost of
throughput and high exposure doses. For this node, photoresists will no
longer be chemically amplified, at least as we know them today, for the
following reason: Starting from molecular dimensions, we derive a typical
minimum pixel size of (10 nm)3 in order to contain at least 1,000 sensitizers
and switchable groups. With the pitch being at least five times the maximum
diffusion length of 10 nm, the minimum pitch for chemically amplified
resists is 50 nm. So for the 32 nm half-pitch node there will be hardly
any chemical amplification; for the 22 nm there will be no chemical amplification
at all. Thin film instabilities, confinement effects as manifested in
the degradation of thermophysical properties, will become significant
at the 40-80 nm specified resist thickness.
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