Initial studies are presented on the use of polysulfones as non-chemically amplified resists (non-CARs) for 193 nm
immersion lithography. Polynorbornene sulfone films on silicon wafers have been irradiated with 193 nm photons in the
absence of a photo-acid generator. Chemical contrast curves and contrast curves were obtained via spectroscopic
ellipsometry and grazing angle - attenuated total reflectance FTIR spectroscopy. Results were consistent with previously
reported mechanisms for the degradation of aliphatic polysulfones with ionizing radiation. It was shown that E0 values
could be reduced significantly by using a post exposure bake step, which propagated depolymerization of the polymer.
Initial patterning results down to 50 nm half pitch were demonstrated with EUV photons.
In immersion lithography, high index fluids are used to increase the numerical aperture (NA) of the imaging system and
decrease the minimum printable feature size. Water has been used in first generation immersion lithography at 193 nm to
reach the 45 nm node, but to reach the 38 and 32 nm nodes, fluids and resists with a higher index than water are needed.
A critical issue hindering the implementation of 193i at the 32 nm node is the availability of high refractive index (n >
1.8) and low optical absorption fluids and resists. It is critical to note that high index resists are necessary only when a
high refractive index fluid is in use. High index resist improves the depth of focus (DOF) even without high index fluids.
In this study, high refractive index nanoparticles have been synthesized and introduced into a resist matrix to increase the
overall refractive index. The strategy followed is to synthesize PGMEA-soluble nanoparticles and then disperse them
into a 193 nm resist. High index nanoparticles 1-2 nm in diameter were synthesized by a combination of hydrolysis and
sol-gel methods. A ligand exchange method was used, allowing the surface of the nanoparticles to be modified with
photoresist-friendly moieties to help them disperse uniformly in the resist matrix. The refractive index and ultraviolet
absorbance were measured to evaluate the quality of next generation immersion lithography resist materials.
Typical extreme ultraviolet (EUV) photoresist is known to outgas carbon-containing molecules, which is of particular
concern to the industry as these molecules tend to contaminate optics and diminish reflectivity. This prompted extensive
work to measure these species and the quantities that they outgas in a vacuum environment. Experiments were
performed to test whether the outgassing rate of these carbon-containing molecules is directly proportional to the rate at
which the EUV photons arrive and whether a very high power exposure will cause the same amount of outgassing as a
much lower power exposure with the dose unchanged.
For several years, SEMATECH has invested significant effort into extending 193 nm immersion lithography by
developing a set of high index materials. For high index immersion lithography (HIL) to enable 1.70NA imaging, a
high index lens element with an absorbance < 0.005/cm, a fluid with an index of ≥ 1.80, and a resist with an index >1.9
are needed. This paper reviews the success or failure of various HIL components and presents the top final material
prospects and properties in each category.
Since this abstract was submitted, the industry has decided to cease any effort in HIL, not because of fundamental
showstoppers but because of timing. This choice was made even though the only currently available technology the can
enable 32 nm and 22 nm manufacturing is double patterning. This may represent a paradigm shift for the semiconductor
industry and lithography. It may very well be that using lithography as the main driver for scaling is now past. Due to
economic forces in the industry, opportunity costs will force performance scaling using alternative technology.
This paper presents robust trilayer lithography technology for cutting-edge IC fabrication and double-patterning
applications. The goal is to reduce the thickness of a silicon hardmask so that the minimum thickness of the
photoresist is not limited by the etch budget and can be optimized for lithography performance. Successful results
of pattern etching through a 300-nm carbon layer are presented to prove that a 13.5-nm silicon hardmask is thick
enough to transfer the line pattern. Another highlight of this work is the use of a simulation tool to design the stack
so that UV light is concentrated at the bottom of the trenches. This design helps to clear the resist in the trenches
and prevent resist top loss. An experiment was designed to validate the assumption with 45-nm dense lines at
various exposure doses, using an Exitech MS-193i immersion microstepper (NA = 1.3) at the SEMATECH Resist
Test Center. Results show that such a stack design obtains very wide CD processing window and is robust for 1:3
line patterning at the diffraction limit, as well as for patterning small contact holes.
Generation-three (Gen-3) immersion lithography offers the promise of enabling the 32nm half-pitch node. For Gen-3
lithography to be successful, however, there must be major breakthroughs in materials development: The hope of
obtaining numerical aperture imaging ≥ 1.70 is dependent on a high index lens, fluid, and resist. Assuming that a fluid
and a lens will be identified, this paper focuses on a possible path to a high index resist. Simulations have shown that
the index of the resist should be ≥ 1.9 with any index higher than 1.9 leading to an increased process latitude.
Creation of a high index resist from conventional chemistry has been shown to be unrealistic. The answer may be to
introduce a high index, polarizable material into a resist that is inert relative to the polymer behavior, but will this too
degrade the performance of the overall system? The specific approach is to add very high index (~2.9) nanoparticles
to an existing resist system. These nanoparticles have a low absorbance; consequently the imaging of conventional
193nm resists does not degrade. Further, the nanoparticles are on the order of 3nm in diameter, thus minimizing any
impact on line edge roughness (LER).
The practical extendibility of immersion lithography to the 32nm and 22nm nodes is being supported on immersion
microsteppers installed at SEMATECH in Albany, New York. As the industry pushes the limits of water-based
immersion technologies, research has continued into developing alternative materials to extend optical lithography for
upcoming device generations. High index materials have been the primary focus of investigation, including optical lens
materials such as lutetium aluminum garnet (LuAG with n=2.14) and barium lithium fluoride (BaLiF3 with n=1.64),
high index fluids (Gen 2 and Gen 3 with n>=1.64), and resists. On a parallel and potentially complementary path,
double patterning and double exposure technologies have been proposed. For high index materials research, the
Amphibian XIS has demonstrated imaging at 1.50NA (32nm half-pitch) with high index fluids. A prism module is also
available to enable imaging with potential BaLiF3 and LuAG prisms. The Exitech MS193i has demonstrated
performance and imaging capability at 38nm hp with k1=0.256 at 1.30NA. Modifications at the mask plane now
provide a double exposure capability, offering an imaging platform to investigate experimental classes of nonlinear
materials and enabling double exposure imaging below k1eff=0.25. In this paper, we will discuss recent developments in
these research areas supported by the toolset at SEMATECH.
The impact of bottom reflection on critical dimension (CD) processing window is intensively investigated with a
simulation using a full diffraction model (FDM) in which the effective reflectivity is calculated from standing wave
amplitude. Most importantly, the optical phase shift of the reflection is used as a design criterion and was found to be
the primary factor that affects the UV distribution, and, hence, has a strong impact on exposure latitude and depth of
focus. Foot exposure (FE) is introduced as a new metric to characterize the phase shift. Some single-layer and dual-layer
bottom anti-reflective coating (BARC) designs were implemented with an Exitech MS-193i immersion micro-stepper
(NA=1.3) for 45-nm dense lines at the Resist Test Center (RTC) at International SEMATECH, Albany, New
York. The experimental results show that FE is closely related to the CD processing window. In contrast to
conventional BARC usage, a small amount of substrate reflection with a controlled optical phase shift dramatically
improves CD processing window.
Base titration methods are used to determine C-parameters for three industrial EUV photoresist platforms (EUV-
2D, MET-2D, XP5496) and twenty academic EUV photoresist platforms. X-ray reflectometry is used to measure the
density of these resists, and leads to the determination of absorbance and film quantum yields (FQY). Ultrahigh levels
of PAG show divergent mechanisms for production of photoacids beyond PAG concentrations of 0.35 moles/liter. The
FQY of sulfonium PAGs level off, whereas resists prepared with iodonium PAG show FQYs that increase beyond PAG
concentrations of 0.35 moles/liter, reaching record highs of 8-13 acids generated/EUV photons absorbed.
For many years, lithographic resolution has been the main obstacle for keeping the pace of transistor densification to
meet Moore's Law. For the 45 nm node and beyond, new lithography techniques are being considered, including
immersion ArF lithography (iArF) and extreme ultraviolet (EUV) lithography. As in the past, these techniques will use
new types of photoresists with the capability to print 45 nm node (and beyond) feature widths and pitches.
In a previous paper ("SEM Metrology for Advanced Lithographies," Proc SPIE, v6518, 65182B, 2007), we compared
the effects of several types of resists, ranging from deep ultraviolet (DUV) (248 nm) through ArF (193 nm) and iArF to
extreme UV (EUV, 13.5 nm). iArF resists were examined, and the results from the available resist sample showed a
tendency to shrink in the same manner as the ArF resist but at a lower magnitude.
This paper focuses on variations of iArF resists (different chemical formulations and different lithographic sensitivities)
and examine new developments in iArF resists during the last year. We characterize the resist electron beam induced
shrinkage behavior under scanning electron microscopy (SEM) and evaluate the shrinkage magnitude on mature resists
as well as R&D resists. We conclude with findings on the readiness of SEM metrology for these challenges.
The practical extendibility of immersion lithography to the 45nm half-pitch is being investigated on a 1.30NA immersion projection microstepper installed at SEMATECH North in Albany, New York. Preliminary implementation of various aperture designs and polarization configurations have been used to demonstrate imaging beyond the 90nm pitch. Optical proximity correction (OPC) and other resolution enhancement technique (RET) strategies coupled with resist stack optimization of dual-layer bottom anti-reflective coating (BARC) systems offer a growing platform of materials and illumination configurations for the 45nm node. In this demonstration of a RET strategy, linear-polarized light is selectively rotated at the coherence aperture to simultaneously image sub-90nm pitch features along the x and y axes within the same field. Scanning electron microscope (SEM) images demonstrate the capability of the immersion micro-exposure tool (iMET) to support dual-orientation imaging with resolution down to the 84nm pitch.
A preliminary Quantitative Structure Property Relationship (QSPR) model for predicting the refractive index of small
molecules and polymers at 193 nm is presented. Although at this stage the model is only semiquantitative we have found
it useful for screening databases of commercially-available compounds for high refractive index targets to include in our
program of synthesis of high refractive index resist polymers. These resists are targeted for use in 2nd and 3rd generation
193 nm immersion lithography. Using this methodology a range of targets were identified and synthesized via free
radical polymerization. Novel resist polymers were also synthesized via Michael addition polymerization. Preliminary
dose to clear experiments identified a number of promising candidates for incorporation into high refractive index resist
materials. Furthermore, we have demonstrated imaging of a high index resist using water-based 193 nm immersion
lithography.
Projection and interference imaging modalities for application to IC microlithography were compared at the 90 nm imaging node. The basis for comparison included simulated two-dimensional image in resist, simulated resist linesize, as well as experimental resist linesize response through a wide range of dose and focus values. Using resist CD as the main response (both in simulation and experimental comparisons), the two imaging modes were found nearly equivalent, as long as a suitable Focus-Modulation conversion is used. A Focus-Modulation lookup table was generated for the 45 nm imaging node, and experimental resist response was measured using an interferometric tool. A process window was constructed to match a hypothetical projection tool, with an estimated error of prediction of 0.6 nm. A demodulated interferometric imaging technique was determined to be a viable method for experimental measurement of process window data. As long as accurate assumptions can be made about the optical performance of such projection tools, the response of photoresist to the delivered image can be studied experimentally using the demodulated interferometric imaging approach.
An approach to measurement of resist CD response to image modulation and dose is presented. An empirical model with just three terms is used to describe this response, allowing for direct calculation of photoresist modulation curves. A thresholded latent image response model has been tested to describe CD response for both 90 nm and 45 nm geometry. An assumption of a linear optical image to photoresist latent image correlation is shown as adequate for the 90 nm case, while the 45 nm case demonstrates significant non-linear behavior. This failure indicates the inadequacy of a "resist blur" as a complete descriptive function and uncovers the need for an additional spread function in OPE-style resist models.
Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of very large scale integrated optics (VLSIO), with over 50,000 high speed logic gates per square centimeter, is a new class of ultra high confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid-infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently inter-connected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low cost three dimensional packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components are presented.
The Fourier Transform Infrared (FTIR) absorption spectrum for the range of 500 to 4000 cm-1 wavenumbers was measured for several Ge films deposited on GaAs using ultra high vacuum E-beam deposition at various substrate temperatures ranging from room temperature (RT) to 500 degree(s)C. Using transmission electron microscopy, we show that Ge films deposited at room temperature and 100 degree(s)C on a (100) GaAs surface that did not have the oxides removed are amorphous while those deposited at 100 degree(s)C with the oxide removed are crystalline, but are highly defective. Secondary ion mass spectroscopy (SIMS) measurements show that the amorphous films at RT contain more than two orders of magnitude more oxygen than the films deposited at 100 degree(s)C or a single crystal film deposited at 400 degree(s)C. The oxygen-18 diffusion studies definitively show that the excess oxygen in the amorphous films percolates in from the atmosphere. SIMS studies further reveal that thermally removing the GaAs substrate surface oxide or depositing a Au film on top of the Ge film has little effect on the incorporation of oxygen.
A high speed modulator at low voltage is created in the mid-infrared at 10 micrometers wavelengths by using field-induced absorption on otherwise forbidden intersubband transitions. The physical effects could scale to 1.5 micrometers wavelength light. This modulator is packaged into a unique 350 micrometers long ultra high confinement (UHC) waveguide for low capacitance and high speed. The modulator quantum wells are at the interface of a 2.1 micrometers thick by 3.75 micrometers wide UHC Ge waveguide and the GaAs substrate. The quantum wells have a 17% power coupling to the evanescent fields of the Ge waveguide. A connector to the UHC waveguides, with dimensions much smaller than a free space wavelength, has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate and non-uniform grating couplers. Fields are applied across the modulator quantum wells via an ohmic contact to the side of the Ge waveguide on the top of the QWs. The ground is on the other side of the waveguide and lower towards the substrate. The 7 micrometers wide mesa supporting the quantum wells on the bottom of the Ge waveguide is slightly wider to accommodate a gold electrode.
Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of Very Large Scale Integrated Optics, with over 50,000 high speed logic gates per square centimeter, is a new class of Ultra High Confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid- infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently interconnected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low-cost 3D packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components will be presented.
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