Members of the Brainard Research Group design, synthesize and characterize new molecules and polymers for their use in state-of-the-art technologies in use around the world.
Our projects are divided between those involving the synthesis of new compounds and those involving characterizing the functionality of these new materials in chemical systems relevant to nanotechnology.
Much of our research is focused on photoresists. Photoresists are light-sensitive layers used in nearly every step in the manufacture of integrated circuits and the building of circuit boards. Photoresists are also essential to the fabrication of micro-electro-mechanical systems (MEMs).
To fully understand these fascinating materials, the students in our group must be able to apply concepts that originate in areas such as organic chemistry, inorganic chemistry, polymer science, photochemistry, physics and material science.
We study many of the components of photoresists, including:
Organic polymers
Photoacid generators
Organometallic compounds
Acid amplifiers
Much of this research is based on first developing mechanistic understanding of how these components work (using kinetics and computer modelling) and then redesigning and synthesizing small molecules and polymers.
Ultimately, the new materials are evaluated as photoresists by exposure to 365-nm, 193-nm or 13.5-nm light. Many of our studies also involve the exposure of photoresists with electron beams with energies from 5-2000 eV.
UAlbany students developed wearable haptic devices that provide navigation assistance to the visually impaired as part of the College of Nanotechnology, Science, and Engineering’s 10-week Summer Undergraduate Research Program.
UAlbany junior Alma Kolakji is spending her summer tying to help make rocket fuel more environmentally friendly. She is one of 24 students taking part in the College of Nanotechnology, Science, and Engineering's Summer Undergraduate Research Program.
UAlbany senior Kevin Reyes has always been fascinated with engineering concepts, whether building complex lego sets or folding the perfect airplane. At UAlbany's College of Nanotechnology, Science, and Engineering, Reyes has found a home to explore the cutting edge technology of the future.
From filing a first disclosure to an issued patent, the process to take an invention from an idea to the marketplace can be long and arduous. For the first time, UAlbany recognized the faculty, staff and students who submitted a patent application as part of Research and Entrepreneurship Week.
The University at Albany’s College of Nanotechnology, Science, and Engineering (CNSE) welcomes the Capital Region community to campus for a series of conversations in celebration of NANOvember.
Extreme ultraviolet (EUV) lithography is used today to pattern the most critical layers (smallest features) in today's fastest integrated circuits using EUV (13.5 nm) light.
Starting in 1998, Dr. Brainard was the first chemist in the world to design photoresists for EUV lithography. Although this work was done before he joined the faculty of CNSE, it is included here because this early work provides a foundation upon which his academic work is built.
This RLS Triangle describes the trade-off between Resolution, line edge roughness (LER) and Sensitivity for extreme ultraviolet (EUV) photoresists.
The transition from the blue surface to the red surface represents how improvements in materials or processes can move to a better response surface.
About Our Research into Early EUV Photoresists & RLS Trade-Off
About Our Research into Early EUV Photoresists & RLS Trade-Off
The RLS Trade-off
Three of the most important properties of EUV resists are their resolution, line-edge-roughness (LER) and sensitivity. The lowest value of each property is best.
Unfortunately, these three properties are in opposition to each other such that within one resist platform, an improvement in one property leads to a degradation of another.
For this reason, the relationship between these properties is known as the RLS tradeoff. Figure 1 shows the first graphical representation of this relationship, first used in a proposal by Dr. Brainard to DARPA in 1999.
Although resist chemists had observed the nature of the RLS trade-off during the early development of EUV resists (1998-2002), it was not until 2005 that a theoretical model was developed to mathematically define the RLS trade-off.
Although Dr. Brainard was involved in creating the final mathematical model, this section will focus on two experimental studies that led to the full model.
Figure 1. Resolution, line-edge-roughness (LER) and sensitivity are in opposition to each other. The figure shows that improvement in resist design can allow resist performance to improve from the outer response surface to the inner (better) response surface.
LER versus Sensitivity
The first mathematical relationship between LER and sensitivity was discovered during a base-loading study conducted by Dr. Brainard.
Seven resists were prepared identical to EUV-2D resist except with seven levels of base such that the molar base/PAG ratios spanned 0.0 to 0.75. These seven resists were imaged using dense line patterns on EUV and DUV exposure tools.
A plot of EUV LER versus EUV E(size) and DUV LER versus DUV E(size) shows a dramatic improvement in LER with increasing levels of added base (Figure 2A).
Both curves show similar shapes: resists requiring low doses have poor LER, whereas the resists requiring high doses have good LER.
Figure 2. (A) LER (3-sigma) for seven resists after exposure at EUV and DUV. EUV: The LER of 100 nm and an average of 150- and 200-nm dense lines are plotted against E(size) for 100-nm dense lines. DUV: The LER of average 300-, 400-, and 500-nm dense lines are plotted against E(size) for 200-nm dense lines. (B) LER versus E(size)^(-1/2) for DUV and EUV exposure of the seven EUV-2D type resists with seven levels of added base.
To explain this process, the authors reasoned that LER was a result of the inherent nature of photon absorption for both EUV and DUV.
For a Poisson process such as photon absorption, the statistical variation in the number of absorbed photons is equal to the square root of the number of absorbed photons, N (equation 1).
The authors reasoned that the LER is proportional to the relative variation in dose. This result combined with equation 1 leads to the conclusion that LER is proportional to dose^(-1/2), since the number of absorbed photons is proportional to the dose (equation 2).
Replotting the data shown in Figure 2A as LER versus E(size)^(-1/2) gives curves with excellent linear fits of the DUV and EUV data (Figure 2B). These linear fits indicate that these resists are following the Poisson statistics of shot noise for exposure at both DUV and EUV.
This result is somewhat surprising because there are 18.5 times more photons/mJ in DUV than EUV, yet the LER versus E(size) relationship is defined by the statistics of shot noise in both cases.
The work of Brainard et al. led to the conclusion that the LER versus E(size) behavior at both DUV and EUV are defined by the Poisson statistics of shot noise.
EUV Quantum Yield
Dr. Brainard defined the quantum yield of photoresists as the ratio between the number of acid molecules generated during exposure and the number of photons absorbed (eqn. 3).
They determined the film quantum yield of EUV-2D so they could better understand the mathematical relationships between the numbers of photons, numbers of acid molecules, and LER. The resulting film quantum yield for EUV-2D is 2.08 (Figure 4).
The quantum yield for EUV-2D using DUV exposure was also calculated. The product of the first four columns in Figure 4 gives the number of acid molecules at E(size). The values are nearly identical at both wavelengths.
In retrospect, it makes sense that the same amount of acid is required to reach sizing when the bake steps and development are the same. Nonetheless, the implications of this result are powerful.
It means that: (1) the number and distribution of acid molecules is a key indicator of LER; and (2) the LER/sensitivity behavior of both EUV and DUV is defined by the Poisson statistics of shot noise: LER proportional to dose^(-1/2):
Wavelength
Esize (mJ/cm2)
Number of Photons in 1 mJ/cm2 x 1013
Absorption of 125 nm
Quantum Efficiency
Number of Acids Generated at Esize x 1013
EUV
6.7
6.7
0.41
2.08
38.2
DUV
9.7
125
0.10
0.33
40.6
Figure 4. Film quantum yield comparisons using EUV and EUV light for the tool test resist, EUV-2D.
Maximum Quantum Yields of Chemically Amplified EUV Resists
Although the first quantum yield was measured by a team of industrial researchers, the first measurement of maximum quantum yields was first done by members of the Brainard and Denbeaux groups at CNSE.
This academic research team determined quantum yields for resists containing three different photoacid generators (PAGs) over a range of [PAG].
Ultimately, the quantum yields of these resists were shown to plateau at high concentrations of PAG, with iodonium PAGs producing more acid than sulfonium or nonionic PAGs at all concentrations (Figure 5A).
Shortly thereafter, a team at Osaka University did very similar experiments with similar but not identical polymers and PAGs and obtained very similar results (Figure 5B).
Both studies obtained maximum quantum yields of 4.9-5.9 acids per absorbed photon for resists formulated with high levels of iodonium PAGs. This maximum quantum yield has been used by our group to help elucidate the number of secondary electrons generated during the exposure of photoresists to EUV photons.
Figure 5. Film quantum yields for an iodonium, sulfonium, and non-ionic PAG measured by two groups using absorbance measurements of protonated Coumarin-6. (A) Results from CNSE. (B) Results from Osaka University.
References
R. Brainard, C. Henderson, J. Cobb, Jonathan, V. Rao, J. Mackevich, U. Okoroanyanwu, S. Gunn, J. Chambers, S. Connolly, "Comparison of the lithographic properties of positive resists upon exposure to deep- and extreme-ultraviolet radiation", J. Vac. Sci. Tech., B17(6), 3384-3389 (1999).
C. Szmanda, R. Brainard, J. Mackevich, A. Awaji, T. Tanaka, Y. Yamada, J. Bohland, S. Tedesco, B. Dal'Zotto, W. Bruenger, M. Torkler, W. Fallmann, H. Loeschner, R. Kaesmaier, P. Nealey, A. Pawloski, "Measuring acid generation efficiency in chemically amplified resists with all three beams", J. Vac. Sci. Tech., B17(6), 3356-3361 (1999).
V. Rao, J. Cobb, C. Henderson, U. Okoroanyanwu, D. Bozman, P. Mangat, R. Brainard, J. Mackevich, "Ultrathin photoresists for EUV lithography", Proceed. SPIE, 3676, 615-626 (1999).
R. Brainard, “Multiple Anion Nonvolatile Acetals (MANA) proposal to DARPA (1999).
R. Brainard, G. Barclay, E. Anderson, L. Ocola, "Resists for next generation lithography", Microelectron. Eng. 61-62, 707-715 (2002).
J. Cobb, R. Brainard, D. O'Connell, P. Dentinger, "EUV lithography: patterning to the end of the road", Mater. Res. Soc. Symp. P., 705, 91-100 (2002).
R. Brainard, J. Cobb, C. Cutler, "Current status of EUV photoresists", J. Photopoly. Sci. Tech., 16(3), 401-410 (2003).
S. Robertson, P. Naulleau, D. O'Connell, K. McDonald, T. Delano, K. Goldberg, S. Hansen, K. Brown, R. Brainard, “Calibration of EUV-2D photoresist simulation parameters for accurate predictive modeling", Proceed. SPIE, 5037, 900-909 (2003).
C. Cutler, J. Mackevich, J. Li, D. O'Connell, G. Cardinale, R. Brainard, "Effect of polymer molecular weight on AFM polymer aggregate size and LER of EUV resists", Proceed. SPIE, 5037, 406-417 (2003).
R. L. Brainard, P. Trefonas, J. H. Lammers, et al., “Shot noise, LER, and quantum efficiency of EUV photoresists,” Proc. SPIE 5374, 74–85 (2004).
A. Eckert, C. Seiler, R. Brainard, "Resist formulation effects on contrast and top-loss as measured by 3D-SEM metrology", Proceed. SPIE, 5374, 460-467, (2004).
M. Chandhok, H. Cao, Y. Wang, E. Gullikson, R. Brainard, S. Robertson, "Techniques for directly measuring the absorbance of photoresists at EUV wavelengths", Proceed. SPIE, 5374, 861-868 (2004).
T. Köhler, R. Brainard, P. Naulleau, D. Steenwinckel, J. Lammers, K. Goldberg, J. Mackevich, P. Trefonas, "Performance of EUV Photoresists on the ALS Micro Exposure Tool", Proceed. SPIE, 5753, 754-764 (2005).
D. Van Steenwinckel, J. Lammers, T. Koehler, R. Brainard, P. Trefonas, "Resist Effects at Small Pitches", J. Vac. Sci. Tech., B24(1), 316-320 (2006).
G. M. Gallatin, P. Naulleau, and R. Brainard, “Fundamental limits to EUV photoresist,” Proc. SPIE 6519, 651911/1–651911/10 (2007).
E. Hassanein, C. Higgins, P. Naulleau, R. Matyi, G. Gallatin, G. Denbeaux, A. Antohe, J. Thackeray, K. Spear, C. Szmanda, C. Anderson, D. Niakoula, M. Malloy, A. Khurshid, C. Montgomery, E. Piscani, A. Rudack, J. Byers, A. Ma, K. Dean, R. Brainard, "Film quantum yields of EUV and ultra-high PAG photoresists", Proceed. SPIE, 6921, 69211I/1-69211I/13 (2008) doi: 10.1117/12.774009].
C. Higgins, A. Antohe, G. Denbeaux, S. Kruger, J. Georger, and R. L. Brainard, “RLS tradeoff vs. quantum yield of high PAG EUV resists,” Proc. SPIE 7271, 727147 (2009) [doi: 10.1117/12.814307].
A. Narasimhan, L. Wisehart, S. Grzeskowiak, L. E. Ocola, G. Denbeaux, R. L. Brainard, “What We Don’t Know About EUV Exposure Mechanisms”, J. Photopolym. Sci. Technol. 30(1), 113-120 (2017).
Acid Amplifiers
Acid amplifiers are organic molecules that react with acids to create more acid. Although our group did not invent these types of molecules, we specifically designed acid amplifiers that could be used in EUV photoresists.
About Our Research into Acid Amplifiers
About Our Research into Acid Amplifiers
Acid amplifiers (AAs) are molecules that decompose in the presence of catalytic acid to generate more acid.
From 1998 to 2002, there were 26 AAs reported in the literature. For the most part, the AAs presented in this early work almost always generated weak sulfonic acids, typically, toluene sulfonic acid. Only two AAs generated fluorinated sulfonic acids.
In 2007, we wrote a proposal to Intel to develop Acid Amplifiers that could be used as additives to chemically amplified resists (CAR), most specifically for use in EUV photoresists.
The idea was to use AAs to amplify the amount of acid generated by photoacid generators (PAGs) during exposure and make progress against the RLS tradeoff. These AAs were specifically designed to create very strong fluorinated acids since they give the best lithographic performance in chemically amplified resists.
Our AAs are composed of three parts: a body, a trigger, and an acid precursor (Figure 1). In the presence of catalytic acid, the trigger is “pulled,” generating an intermediate that has a weak bond between the body and the acid precursor. The acid is released from the body and can then catalyze further decomposition of the AA.
Figure 1. Generic description of acid amplifiers (AAs) developed by our group. Conceptually, our AAs are composed of a body, a trigger and an acid precursor.
A possible mechanism for catalyzed and uncatalyzed decomposition of acid amplifier 3HB is shown in Figure 2.
When the decomposition is autocatalytic, the trigger is first protonated and then cleaved (T) to produce an allylic sulfonic ester, which then decomposes thermally to yield a second double bond and a sulfonic acid (A).
The sulfonic acid produced by the AA should be strong enough to cause autocatalytic decomposition. In the absence of acid, the sulfonic ester can still decompose to generate a sulfonic acid via an uncatalyzed mechanism (U).
The strategy for designing new acid amplifiers is to maximize the rate of the acid-catalyzed decomposition while suppressing the rate of the uncatalyzed decomposition.
Figure 2. Proposed mechanism for catalyzed and uncatalyzed AA-3HB; R represents p-C6H4CF3.
Lithographic Studies of Acid Amplifiers
As a proof-of-concept study, a control open-source resist (OS1) was prepared with and without 70 mM of added acid amplifier.
The addition of AA 6AB improves resolution, LER, and sensitivity versus the control resist (Figure 3A). A 2D Z-parameter plot shows how the addition of two AAs can improve Z from 74 x 10–8 mJ·nm3 to 54 x 10–8 mJ·nm3 (for 3HB) and 25 x 10–8 mJ·nm3 (for 6AB) (Figure 3B).
This proved that AAs did in fact improve the lithographic performance of EUV photoresists.
To study the importance of the acid strength on the performance of AAs, our group prepared and evaluated four acid amplifiers with the same body, but two types of triggers and two types of acid precursors.
We found that AAs that produce fluorinated acids (3AB and 3HB) yield resists with higher sensitivity, better LER, and a better Z-parameter relative to AAs that produce nonfluorinated toluene sulfonic acid (Figure 4).
These results show fairly clear evidence that AAs that produce strong fluorinated acids give the best combination of sensitivity, LER, and exposure latitude.
Figure 3. (A) Comparison of EUV L/S images (Berkeley MET) of the OS1 control resist + 70 mM 6AB, SB = 90 °C / 60 s, PEB = 90 °C / 90 s. (B) Z-parameter plot for a resist without AA and with 70 mM of 3HB or 6AB, SB = 90 °C / 60 s, PEB = 90 °C / 90 s (SB is softbake; PEB is post-exposure bake).
Figure 4. Comparison of EUV images (Berkeley MET) using 50-nm lines and spaces using the OS1 control resist + 70 mM concentrations of AAs: 3HB, 3MB, 2HB, and 2MB.
In another study, we systematically varied the chemical structure of 12 AAs and studied their reaction kinetics by using the Z-parameter to compare the lithographic performance of EUV resists prepared with and without 70 mM concentration of AA (Figure 5).
Seven of the AAs were capable of showing improved lithographic performance (lower Z-parameter) versus the control resist. The Z-parameter improved three-fold with the addition of 3MA.
For the most part, resists prepared with perfluorobenzenesulfonate esters (3HC, 3MC or 2MC) gave poor lithographic performance versus the control.
We demonstrated that these AAs have the poorest thermal stabilities and explained that the poor lithographic performance of these highly fluorinated sulfonates was due to their poor thermal stability in the resist.
Figure 5. One control resist without AA and twelve resists with 70 mM AA exposed to EUV light. The sensitivity Esize, LER, and Z-parameter of these resists are reported for 50-nm equal lines and spaces or at best resolution. Z-parameters were not found for the AA’s outlined in red.
References
S. Kruger, S. Revuru, C. Higgins, S. Gibbons, D. A. Freedman, W. Yueh, T. R. Younkin, and R. L. Brainard, “Fluorinated acid amplifiers for EUV lithography,” J. Am. Chem. Soc.131(29), 9862–9863 (2009).
S. A. Kruger, C. Higgins, B. Cardineau, T. R. Younkin, and R. L. Brainard, “Catalytic and autocatalytic mechanisms of acid amplifiers for use in EUV photoresists,” Chem. Mat.22(19), 5609–5616 (2010).
S. A. Kruger, “Fluorinated acid amplifiers for extreme ultraviolet lithography,” Ph.D. Thesis, University at Albany, Albany, New York, 2011.
S. Kruger, K. Hosoi, B. Cardineau, K. Miyauchi, and R. Brainard, “Stable, fluorinated acid amplifiers for use in EUV lithography,” Proc. SPIE 8325, 832514 (2012) [doi: 10.1117/12.917015].
K. Hosoi, B. Cardineau, S. Kruger, K. Miyauchi, and R. Brainard, “Fluorine-stabilized acid amplifiers for use in EUV lithography,” J. Photopolym. Sci. Technol. 25, 575–581 (2012).
G. M. Gallatin, P. P. Naulleau and R. L. Brainard, "Modeling the effects of acid amplifiers on photoresist stochastics," Proc. SPIE 8322, 83221C/83221-83221C/83229 (2012).
S. A. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, and R. L. Brainard, “Can acid amplifiers help beat the resolution, line edge roughness and sensitivity trade-off?” Jap. J. App. Phys. 49(4), 041602 (2010).
R. Brainard, S. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, Y. Wang, and T. Younkin, “Kinetics, chemical modeling and lithography of novel acid amplifiers for use in EUV photoresists,” J. Photopolym. Sci. Technol. 22, 43–50 (2009).
K. Hosoi, B. Cardineau, W. Earley, S. Kruger, K. Miyauchi and R. Brainard; "Synthesis of stable acid amplifiers that produce strong highly-fluorinated polymer-bound acid," Proc. SPIE 8325, 83251S/83251-83251S/83257 (2012).
S. Kruger, C. Higgins, G. Gallatin and R. L. Brainard; "Lithography and chemical modeling of acid amplifiers for use in EUV photoresists," J. Photopolym. Sci. Technol.24 (2), 143-152 (2011).
R. Brainard, C. Higgins, S. Kruger, S. Revuru, B. Cardineau, S. Gibbons, D. Freedman, H. Solak, Y. Wang, T. Younkin, "Lithographic evaluation and chemical modeling of acid amplifiers used in EUV photoresists", Proceed. SPIE, 7273, 72733Q/1-72733Q/10 (2009).
C. D. Higgins, C. R. Szmanda, A. Antohe, G. Denbeaux, J. Georger, and R. L. Brainard, “Resolution, line-edge roughness, sensitivity tradeoff, and quantum yield of high photo acid generator resists for extreme ultraviolet lithography,” Jpn. J. Appl. Phys. 50(3), 036504 (2011).
Organic Molecules & Polymers
These projects explore how mechanistic hypotheses can be explored using chemical synthesis, modelling and kinetics can be used to make polymers with improved chemical properties.
The Brainard group has a large body of work where we use specific mechanisms rooted in organic chemistry in order to create materials to improve lithography. Acid Amplifiers is our largest body of work in this area, and in the following section we show two more similar ideas:
Double-Deprotected Chemically Amplified Photoresists (DD-CAMP) was designed to use higher-order reaction kinetics to improve photoresist imaging quality.
Chain Scission Polymers for EUV Lithography were investigated to find a way of improving resolution capability of resists with the improving resolution of the optical image.
Both of these platforms were developed starting from a chemical mechanism and a lithographic principle and applying them in conjunction.
About Our Research into Double-Deprotected Chemically Amplified Photoresists (DD-CAMP)
About Our Research into Double-Deprotected Chemically Amplified Photoresists (DD-CAMP)
Double-Deprotected Chemically Amplified Photoresists (DD-CAMP) were developed to utilize higher-order reaction kinetics to create photoresists with improved lithographic performance.
DD-CAMP uses an acid-body-trigger design similar to what we have used previously for acid amplifiers, in which two acid-catalyzed reactions are required to completely remove the blocking group (Figure 1A and B).
For DD-CAMP, the removal of the blocking group yields a polymer-bound carboxylic acid which provides the solubility switch in positive-tone chemically amplified photoresists. For acid amplifiers, the removal of the blocking group yields a sulfonic acid that can catalyze reactions in chemically amplified photoresists.
The primary hypothesis behind DD-CAMP is that two sequential acid-catalyzed reactions (Figure 1A and B) will create an overall reaction (ester deprotection) that will be higher than first order in acid (e.g. k is proportional to [H+]^x, where x is greater than 1) if:
The two reaction steps occur at roughly the same rate, and
The bake time (PEB) is optimized.
Figure 1. (A) Basic design and mechanism of DD-CAMP photoresists. (B) A specific example of a DD-CAMP blocking group. (C) A mathematical expression describing the proposed kinetics of acid-catalyzed decomposition of DD-CAMP blocking groups.
DD-CAMP polymers were prepared for use in both 193-nm and EUV lithography. Figure 2 shows seven monomers that were prepared and evaluated for use in 193-nm lithography and Figure 3 shows seventeen monomers that were prepared and evaluated for use in EUV lithography.
Figures 4 and 5 show line-width-roughness (LWR) as a function of post-exposure bake (PEB) time for resists prepared from DD-CAMP and control polymers. As predicted, the LWR performance is an improvement over the control resists and a function of PEB time for the DD-CAMP blocking groups.
Figure 2. Seven of the ten monomers designed, synthesized and lithographically evaluated using 193-nm lithography. "E" represents the polymer-bound carboxylate that is deprotected when the DD-CAMP blocking group is removed.
Figure 3. Seventeen of the 22 monomers designed, synthesized and lithographically evaluated using EUV lithography. "E" represents the polymer-bound carboxylate that is deprotected when the DD-CAMP blocking group is removed.
Figure 4. Line-Width-Roughness (LWR) versus time of the post-exposure bake (PEB) for 193-nm DD-CAMP and control polymers.
Figure 5. Line-Width-Roughness (LWR) versus time of the post-exposure bake (PEB) for 193-nm DD-CAMP and control polymers.
References
D. Soucie W. Earley, K. Hosoi, A. Takahashi, T. Aoki, B. Cardineau, K. Miyauch, R. Brainard; “Higher-Order Lithography: Double-Deprotected Chemically Amplified Photoresists (DD-CAMP)”, J. Photopolym. Sci. Technol. 30(3), 351-360 (2017).
W. Earley, D. Soucie, K, Hosoi, A. Takahashi, T. Aoki, B. Cardineau, K. Miyauchi, J. Chun, M. O'Sullivan, R. L. Brainard; "Double-deprotected chemically amplified photoresists (DD-CAMP): higher-order lithography”; Proc. SPIE, 9779, Adv. Pat. Mat. Proc., 101460H (2017).
S. Kruger, S. Revuru, C. Higgins, S. Gibbons, D. A. Freedman, W. Yueh, T. R. Younkin, and R. L. Brainard, submitted to J. Am. Chem. Soc. (2009).
Brainard, R.; Kruger, S.; Higgins, C.; Revuru, S.; Gibbons, S.; Freedman, D.; Wang, Y.; Younkin, T. Kinetics, chemical modeling and lithography of novel acid amplifiers for use in EUV photoresists. J. Photopolym. Sci. Technol. 2009a, 22, 43-50.
S. A. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, and R. L. Brainard, “Can acid amplifiers help beat the resolution, line edge roughness and sensitivity trade-off?” Jap. J. App. Phys., 2010, 49(4), p. 041602/1-041602/5.
S. A. Kruger, C. Higgins, B. Cardineau, T. R. Younkin, and R. L. Brainard, “Catalytic and Autocatalytic Mechanisms of Acid Amplifiers for Use in EUV Photoresists,” Chem. Mat., 2010, 22 (19), p. 5609-5616.
R. L. Brainard, S. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, Y. Wang, T. Younkin, J. Photopoly. Sci. Tech., 2009, 22, 43-50.
About Our Research into Chain-Scission Polymers for EUV Lithography
About Our Research into Chain-Scission Polymers for EUV Lithography
Our group has explored the use of chain-scission polymers in an effort to simultaneously improve resolution, line-edge-roughness and sensitivity.
Traditional resist polymers have an acid-sensitive pendant group that is cleaved during photolithography, allowing dissolution of the polymer in alkaline aqueous developer.
Chain-scission polymers are polymers that undergo backbone cleavage during the photolithographic process. Cleaving the main chain converts the polymer from a single high molecular-weight chain to many low molecular-weight segments. These low molecular-weight segments dissolve faster than the starting polymer thereby producing positive-tone imaging.
Our group explored two approaches for incorporating acid-cleavable polyester groups into the polymer backbone. The first approach utilized diacids chlorides (Figure 1A); the second utilized alpha-chloroesters (Figure 1B).
Figure 1. (A) Synthetic scheme for the first set of chain scission polymers by diacyl chloride condensation with diols. (B) Example synthesis for the second set of chain scission polymers by condensation of alpha-chloroesters and diphenols.
In our first approach, we reacted diacyl chlorides with diols to produce polyesters (Figure 1A). This approach allowed us to incorporate protected phenolic groups within the diols that could later be converted to phenols within the polymer chain. These phenols help us tune the baseline solubility of the polymers.
Using this approach, we were able to print 36-nm lines using a sizing dose of 22.5 mJ/cm^2 with one polymer (Figure 2A). With a different polymer, high sensitivity was achieved, where the resist was able to modulate down to 60-nm lines at 6.9 mJ/cm^2.
We found that the inclusion of the phenol unit was vital, as the polymers with no phenolic units were 2X less sensitive and resolved only 100-nm lines. While this was a great start, the diacyl chloride reagents were difficult to keep pure, leading to inconsistent results. These issues led us to explore a second approach.
Figure 2. Lithographic results of phenol-containing chain scission polyesters. (A) One polymer resolved 36-nm lines at 22.5 mJ/cm2 dose while another (B) modulated down to 60-nm at 6.9 mJ/cm^2.
In our second approach, alpha-chloroesters and diphenols were reacted to form condensation polymers (Figure 1B). In these polymers, the cleavable group was incorporated into the alpha-chloroester leading to polymers which had both esters and ethers in the main chain.
Using this approach, one polymer produced a resist that showed modulation down to 14-nm, however, this resist suffered from a large amount of serpentine deformation and bridging, so the actual resolution was only 40-nm (Figure 3).
We attribute both of these patterning issues to low glass transition temperatures (Tg) of our polymers due to the ether linkages in the polymer backbone. Polyethers are commonly known to have much lower glass transition temperatures (Tg) than polyesters and our modelling predicted that our polymers have a very low Tg of 59 degrees C.
Figure 3. Polymer composition (A) and imaging results (B) of the best performing generation 2 polymer, showing resolution down to 40-nm lines and modulation down to 14-nm.
References
D. Soucie W. Earley, K. Hosoi, A. Takahashi, T. Aoki, B. Cardineau, K. Miyauch, R. Brainard; “Higher-Order Lithography: Double-Deprotected Chemically Amplified Photoresists (DD-CAMP)”, J. Photopolym. Sci. Technol. 30(3), 351-360 (2017).
W. Earley, D. Soucie, K, Hosoi, A. Takahashi, T. Aoki, B. Cardineau, K. Miyauchi, J. Chun, M. O'Sullivan, R. L. Brainard; "Double-deprotected chemically amplified photoresists (DD-CAMP): higher-order lithography”; Proc. SPIE, 9779, Adv. Pat. Mat. Proc., 101460H (2017).
S. Kruger, S. Revuru, C. Higgins, S. Gibbons, D. A. Freedman, W. Yueh, T. R. Younkin, and R. L. Brainard, submitted to J. Am. Chem. Soc. (2009).
Brainard, R.; Kruger, S.; Higgins, C.; Revuru, S.; Gibbons, S.; Freedman, D.; Wang, Y.; Younkin, T. Kinetics, chemical modeling and lithography of novel acid amplifiers for use in EUV photoresists. J. Photopolym. Sci. Technol. 2009a, 22, 43-50.
S. A. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, and R. L. Brainard, “Can acid amplifiers help beat the resolution, line edge roughness and sensitivity trade-off?” Jap. J. App. Phys., 2010, 49(4), p. 041602/1-041602/5.
S. A. Kruger, C. Higgins, B. Cardineau, T. R. Younkin, and R. L. Brainard, “Catalytic and Autocatalytic Mechanisms of Acid Amplifiers for Use in EUV Photoresists,” Chem. Mat., 2010, 22 (19), p. 5609-5616.
R. L. Brainard, S. Kruger, C. Higgins, S. Revuru, S. Gibbons, D. Freedman, Y. Wang, T. Younkin, J. Photopoly. Sci. Tech., 2009, 22, 43-50.
B. Cardineau, P. Garczynski, W. Earley and R. L. Brainard; "Chain-scission polyethers for EUV lithography", J. Photopolym. Sci. Technol. 26 (5), 665-671 (2013).
B. Cardineau, S. Kruger, W. Earley, C. Higgins, S. Revuru, J. Georger, R. Brainard, "Chain-scission polyesters for EUV lithography", J. Photopoly. Sci. Tech., 23(5), 665-671, (2010).
EUV Mechanisms
This research primarily focuses on the physics of the exposure of photoresists by EUV light.
About Our Research into EUV Mechanisms
About Our Research into EUV Mechanisms
The physics of EUV exposure mechanisms
In EUV lithography, photons can interact strongly with any resist components, generating electrons through photo-ionization.
An initial photoelectron with an energy range of about 75 to 82 eV may cause further ionization in the resist, generating additional electrons that contribute to the chemical reactions during exposure.
In this section, we describe the physics of the interactions of these electrons as they interact with molecules from their initial creation with energies of about 80 eV until they decay to energies of about 2 eV.
There are four underlying elementary interactions between these about 2 to 80 eV photo- and secondary electrons (Figure 1):
Photoionization: A photon is absorbed by an atom and liberates an electron with enough kinetic energy to interact further. The ionized resist component may yield further electrons or charged ion fragments through electronic and atomic relaxation processes.
Electron ionization: A ballistic electron scatters off an atom and produces another electron such that the total kinetic energy of the two electrons equals the energy of the incident electron less the binding energy of the “daughter” electron.
Plasmon generation: An incident electron scatters off an atom, losing energy and causing a coherent displacement wave in the bound electrons. It is currently unclear if plasmons can be an additional source of electron–hole pairs.
Elastic scattering: The trajectory of an incident electron is altered by the Coulombic potential of an atom with no associated energy loss. In CARs, ionization and plasmon generation of PAG or polymer may directly produce acid.
Figure 1. A schematic drawing of the four elementary physical electron–matter interactions described above.
The chemistry of EUV exposure mechanisms
Researchers have proposed that the dominant chemical mechanisms involved in EUV CARs include:
Electron trapping (or dissociative electron attachment)
Hole-initiated chemistry
Internal excitation (or dissociative electron excitation), as illustrated in Figure 2
In electron trapping, a low-energy electron (perhaps 0 to 5 eV) may be trapped by a PAG molecule, thereby occupying an antibonding orbital in the PAG. This leads to a change in the electronic structure of the PAG, causing the molecule to fall apart.
Holes left in ionized atomic species within the resist may also contribute to resist chemistry. A hole within a polymer may lead to a disproportionation reaction with other polymer side-chains, ultimately producing acid on its own.
Additionally, electrons of higher energy (10 to 80 eV) in resists may deposit energy by exciting electrons from bonding to antibonding states in PAG molecules. This excited PAG molecule may then dissociate to produce acid.
Electrons also interact with polymers directly. Enomoto et al. exposed different polymers to increasing doses of e-beam (90 keV) and analyzed the resulting resist through gel permeation chromatography.
They observed that PHS-type chemically amplified photoresists exhibited crosslinking, while poly-2-methyladamantane-2-methacrylate underwent chain scission upon exposure, similar to PMMA.
High-dose e-beam exposure of polystyrene was also posited by Narasimhan et al. to cause a high degree of backbone conjugation that ultimately may lead to e-beam-induced fluorescence.
Fedynyshyn et al. have demonstrated the relationship between resist polymer composition and clearing dose E0, and calculated chain scission and crosslinking reaction quantum yields in polymers due to EUV exposure.
Figure 2. Chemical mechanisms of EUV exposure.
References
J. Torok, R. Del Re, H. Herbol, S. Das, I. Bocharova, A. Paolucci, L. E. Ocola, C. Ventrice, Jr., E. Lifshin, G. Denbeaux, and R. L. Brainard, “Secondary electrons in EUV lithography,” J. Photopolym. Sci. Tech. 26(5), 625–634 (2013).
R. Brainard, E. Hassanein, J. Li, et al., “Photons, electrons, and acid yields in EUV photoresists: a progress report,” Proc. SPIE 6923, 692325 (2008) [doi: 10.1117/12.773869].
Narasimhan, S. Grzeskowiak, B. Srivats, H. Herbol, L. Wisehart, J. Schad, C. Kelly, W. Earley, L. E. Ocola, M. Neisser, G. Denbeaux, and R. L. Brainard, “Studying thickness loss in extreme ultraviolet resists due to electron beam exposure using experiment and modeling” J. Micro/Nanolith. MEMS MOEMS 14(4), 043502 (2015). [doi: 10.1117/1.JMM.14.4.043502].
J. Thackeray, J. Cameron, V. Jain, P. LaBeaume, S. Coley, O. Ongayi, M. Wagner, A. Rachford, and J. Biafore, “Progress in resolution, sensitivity, and critical dimensional uniformity of EUV chemically amplified resists,” Proc. SPIE 8682, 868213 (2013) [doi: 10.1117/12.2011565].
A. Narasimhan, S. Grzeskowiak, J. Ostrander, J. Schad, E. Rebeyev, M. Neisser, L. E. Ocola, G. Denbeaux, and R. L. Brainard, “Studying electron-PAG interactions using electron-induced fluorescence,” Proc. SPIE 9779, 97790F (2016) [doi: 10.1117/12.2219850].
S. Enomoto, A. Oshima, and S. Tagawa, “Reaction mechanisms of various chemically amplified EUV and EB resist,” Proc. SPIE 8679, 86792C (2013) [doi: 10.1117/12.2011634].
S. Ptasińska, D. Gschliesser, P. Bartl, I. Janik, P. Scheier, and S. Denifl, “Dissociative electron attachment to triflates,” J. Chem. Phys. 135(21), 214309 (2011).
D. L. Goldfarb, A. Afzali-Ardakani, and M. Glodde, “Acid generation efficiency: EUV photons versus photoelectrons,” Proc. SPIE 9779, 97790A (2016) [doi: 10.1117/12.2218457].
S. Tarutani, H. Tsubaki, H. Tamaoki, H. Takahashi, and T. Itou, “Study on approaches for improvement of EUV-resist sensitivity,” Proc. SPIE 7639, 763909 (2010) [doi: 10.1117/12.846031].
T. H. Fedynyshyn, R. B. Goodman, and J. Roberts, “Polymer matrix effects on acid generation,” Proc. SPIE 6923, 692319 (2008) [doi: 10.1117/12.7716692].
T. H. Fedynyshyn, R. B. Goodman, A. Cabral, C. Tarrio, and T. B. Lucatorto, “Polymer photochemistry at the EUV wavelength,” Proc. SPIE 7639, 76390A (2010) [doi: 10.1117/12.845997].
R. L. Brainard, P. Trefonas, J. H. Lammers, et al., “Shot noise, LER, and quantum efficiency of EUV photoresists,” Proc. SPIE 5374, 74–85 (2004).
R. L. Brainard, G. G. Barclay, E. H. Anderson, and L. E. Ocola, “Resists for next generation lithography,” Microelec. Eng. 61-62, 707–715 (2002).
R. L. Brainard, C. Henderson, J. Cobb, et al., “Comparison of the lithographic properties of positive resists upon exposure to deep- and extreme-ultraviolet radiation,” J. Vac. Sci. Tech. B 17(6), 3384–3389 (1999).
S. P. Pappas, B. C. Pappas, L. R. Gatechair, and W. Schnabel, “Photoinitiation of cationic polymerization. II. Laser flash photolysis of diphenyliodonium salts,” J. Polymer Sci. 22(1), 69–76 (1984).
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J. F. Cameron, N. Chan, K. Moore, G. Pohlers, “Comparison of acid-generating efficiencies in 248 and 193-nm photoresists,” Proc. SPIE 4345, 106–118 (2001).
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T. Kozawa and S. Tagawa, “Basic aspects of acid generation processes in chemically amplified resists for electron beam lithography,” Proc. SPIE 5753, 361–367 (2005); T. Kozawa and S. Tagawa, “Basic aspects of acid generation processes in chemically amplified electron beam resist,” J. Photopoly. Sci. Tech. 18(4), 471–474 (2005).
S. Tagawa, S. Nagahara, T. Iwamoto, et al., “Radiation and photochemistry of onium salt acid generators in chemically amplified resists,” Proc. SPIE 3999, 204–213 (2000).
A. Nakano, K. Okamoto, Y. Yamamoto, et al., “Deprotonation mechanism of poly(4-hydroxystyrene) and its derivative,” Proc. SPIE 5753, 1034–1039 (2005).
T. Kozawa, A. Saeki, and S. Tagawa, “Modeling and simulation of chemically amplified electron beam, x-ray, and EUV resist processes,” J. Vac. Sci. Tech. B 22(6), 3489–3492 (2004).
H. Yamamoto, T. Kozawa, A. Nakano, et al., “Dependence of acid generation efficiency on the protection ratio of hydroxyl groups in chemically amplified electron beam, x-ray and EUV resists,” J. Vac. Sci. Tech. B 22(6), 3522–3524 (2004).
T. Kozawa, A. Saeki, A. Nakano, Y. Yoshida, and S. Tagawa, “Relation between spatial resolution and reaction mechanism of chemically amplified resists for electron beam lithography,” J. Vac. Sci. Tech. B 21(6), 3149–3152 (2003).
E. Hassanein, C. Higgins, P. Naulleau, R. Matyi, G. Gallatin, G. Denbeaux, A. Antohe, J. Thackeray, K. Spear, C. Szmanda, and C. N. Anderson, “Film quantum yields of EUV and ultra-high PAG photoresists” Proc. SPIE 6921, 69211I (2008) doi: 10.1117/12.774009].
C. Higgins, A. Antohe, G. Denbeaux, S. Kruger, J. Georger, and R. L. Brainard, “RLS tradeoff vs. quantum yield of high PAG EUV resists,” Proc. SPIE 7271, 727147 (2009) [doi: 10.1117/12.814307].
R. Hirose, T. Kozawa, S. Tagawa, T. Kai, and T. Shimokawa, “Dependence of acid generation efficiency on molecular structures of acid generators upon exposure to extreme ultraviolet radiation,” Appl. Phys. Express 1(2), 027004 (2008).
Narasimhan, S. Grzeskowiak, C. Ackerman, T. Flynn, G. Denbeaux, R. L. Brainard; "Mechanisms of EUV exposure: electrons and holes”; Proc. SPIE, 10143, EUVL, 101430W (2017).
Molecular Organometallic Resists for EUV (MORE)
These projects explore EUV photoresists that are composed of compounds that contain elements in the periodic table that strongly absorb EUV (13.5 nm) light. A total of five chemical platforms have been invented and developed by students in our group.
About Our Research into MORE
About Our Research into MORE
For decades chemists in the microelectronics industry designed resists with two accepted goals:
To minimize the absorption of the light used to pattern them
To minimize the amount of contaminant metals below 20 ppm
However, in 2011, two discoveries reversed these accepted goals:
A paper by Jim Thackeray demonstrated that EUV absorption by resists should increase dramatically.
Researchers at Oregon State University and Cornell demonstrated that the relatively-transparent hafnium-oxide films could be excellent photoresists.
Our group responded to these two discoveries by proposing to develop resists containing metals that strongly absorb EUV light (Figure 1).
Figure 1. Periodic Table of Elements colored by EUV Optical density of the element. Our group has focused on designing, synthesizing and lithographically evaluating compounds containing the metals within the oval that strongly absorb EUV (13.5 nm) light.
Since this time, our group has successfully invented EUV photoresists composed of amorphous thin-films of compounds containing cobalt, tin, platinum, palladium, bismuth and antimony in our project called Molecular Organometallic Resists for EUV (MORE).
Described here are five organometallic platforms containing tin, palladium and antimony.
Multinuclear Tin Clusters
One of the first MORE platforms was composed of tin-oxo clusters of the formula [(RSn)12O14(OH)6]X2; where R equals phenyl, butyl and allyl and X equals carboxylate or halide.
These tin clusters were spin-coated into amorphous thin films and exposed to EUV light. These EUV photoresists were able to resolve 18-nm dense line patterns; albeit with poor sensitivities (150-525 mJ/cm2; Figure 2).
Figure 2. (A) Structure of the semi-spherical tin oxo-cluster shown with tosylate anions. (B) 18-nm dense lines printed using the dichloride complex.
One study compared the relative sensitivities of complexes prepared with tin bonds to phenyl, butyl and benzyl (Figure 3).
Figure 3. (A) 50-nm dense lines prepared with tin clusters in which R groups were allyl, butyl and phenyl (X = Cl-). (B) A plot of sizing dose vs. C-H bond energy.
A plot of photospeed vs. R-H bond strength of the corresponding organic ligand showed that ligand with the weakest R-H bond gave the fastest photospeed.
One of the conclusions from this work was that cleavage of the Sn-C bonds is an important step in the EUV photomechanism.
Mononuclear Tin Compounds with Excellent LER
Del Re and coworkers developed a series of organometallic tin dicarboxylates [R2Sn(O2CRʹ)2] that print dense L/S patterns with very low LER.
Although the sensitivity of many of these compounds is poor (Esize = 50–600 mJ/cm2), these compounds are capable of good resolution and excellent LER when exposed using interference lithography at PSI.
In particular, dibenzyltindibenzoate (Figure 4A) prints 50- and 35-nm dense lines with LWR of 1.7 nm, and dibenzyltin dipivalate resolves 16-nm half-pitch dense lines with 2.1-nm LER and resolves 22-nm half-pitch dense lines with 1.4-nm LER (Figure 4B).
Figure 4. EUV imaging capabilities of (A) dibenzyltindibenzoate and (B) dibenzyltindipivalate.
Interestingly, these resists seem to show better sensitivity with lower molecular weights, perhaps indicating that higher metal content is important for achieving better sensitivity.
We also see a linear trend when we compare molecular weights with Emax within a particular compound family (Figure 5).
Figure 5. Strong correlations exist between sensitivity and molecular weight for R2Sn(O2CRʹ)2 complexes when the hydrocarbon moiety (butyl, phenyl, or benzyl) bonded to tin is the same.
Positive-Tone Palladium Oxalates
Sortland and coworkers developed the first mononuclear positive-tone organometallic resists for EUVL (Figure 6A).
Positive-tone photoresists are attractive as they allow for dark-field masks in contact hole manufacturing. Dark-field masks are preferred as they minimize flare and are easier to correct for mask defects than bright-field masks.
Researchers found that bis-phosphine palladium or platinum oxalates give resists that produce positive-tone images when developed in nonpolar solvents (Figure 6B).
Figure 6. (A) E0 for different platinum and palladium-phosphine oxalates when exposed using the open-frame technique at PSI. (B) Contrast curve of one palladium oxalate resist.
Comparison with solution-phase photochemistry in the literature and exposure of crystals to i-line light by the authors led the authors to propose the exposure mechanism shown in Figure 7, in which metal oxalates create CO2 as well as zero-valent palladium-phosphine complexes and palladium metal.
Figure 7. Proposed photoreaction for (dppm)Pd(C2O4) and (Ph2EtP)2PdC2O4. The photoreaction is consistent with all experimental data.
This photoreaction is consistent with all experimental data, and similar results are found in the literature.
The L4Pd complex is more soluble than the starting material, explaining the positive-tone behavior of this resist system. Unfortunately, palladium metal is insoluble, and the authors were unable to get complete clearing in the exposed regions.
Olefin-Containing Antimony Complexes
Passarelli and coworkers have presented a series of high-speed, negative-tone resists of the general form R3Sb(O2CRʹ)2 in which the R and/or R' group contains an olefin (acrylate, methacrylate, styrene carboxylate, para-vinylphenyl).
Members of our group explored the systematic variations of these complexes by preparing and lithographically evaluating more than 30 compounds.
One resist, tri(phenyl)antimony diacrylate (JP-20), prints 35-nm lines with modulation down to 16 nm, with sensitivities of 5.6 and 9.2 mJ/cm2 when developed in hexanes or water, respectively (Figure 8). This resist exhibits negative-tone imaging in both developers.
Figure 8. Lithographic performance of (A) Ph3Sb(O2CCH=CH2)2 (JP-20). (B) High-contrast and high-sensitivity are achieved. (C) JP-20 can be developed in either hexane or (d) water. Both developers produce negative-tone imaging.
We proposed that these olefin complexes undergo photolysis and solubility changes through mechanisms that involve free-radical polymerization of double bonds (Figure 9). This mechanism is supported by three additional studies:
Ph3Sb(O2CCH=CH2)2 is about 15 times faster than Ph3Sb(O2CCH3)2.
Ph3Sb(O2CCH=CH2)2 and (Cyclohexyl)3Sb(O2CCH=CH2)2 have exactly the same photospeed, suggesting that M-C bond cleavage is not involved in the mechanism.
The sensitivity of the resist directly correlates to the concentration of olefins in the resist (discussed below).
We tested the hypothesis that the greatest contributor to the sensitivity of molecules of the type R3Sb(O2CRʹ)2 is the concentration of polymerizable olefins in the resist.
We reasoned that in order for the propagation of free-radical polymerization to proceed, the olefins must be in contact with other olefins, and that higher concentrations of olefins would lead to higher turnover numbers.
Larger turnover number should lead to greater sensitivity because more molecules are reacted per initiation event, and because higher-molecular-weight molecules have lower solubility.
To test this hypothesis, we defined a parameter called polymerizable olefin loading (POL) as the ratio of olefins/atoms (excluding hydrogen). A semi-log plot of Emax versus POL is shown for fifteen resist materials in Figure 10.
Figure 10. Plot of Emax vs. Polymerizable Olefin Loading (POL). A linear trend is demonstrated between Log Emax vs. POL for multiple antimony organometallic carboxylates.
There is a linear relationship between LogEmax versus POL, whereas POL increases, Emax decreases (the resist becomes more sensitive). We think that this correlation is further evidence in support of the imaging mechanism shown in Figure 9.
Figure 9. Proposed mechanism of an antimony organometallic carboxylate resist system. The authors propose that good sensitivity is achieved through one photochemical homolysis event leading to a cascade of subsequent solubility-changing polymerization reactions.
Non-Olefin Antimony Complexes
Two mechanistic studies were conducted using complexes of the type R3SbX2 which did not contain olefins — as an effort to study the photochemical reactions occurring independent of the olefinic ligands.
For Mechanistic Study 1, we synthesized a series of (C6H5)3Sb(O2CR′)2 complexes in which the R′ group was selected to provide a range of R′–H bond energies so that we could determine how these bond energies affect the rates of outgassing (Figure 11).
Figure 11. A series of (C6H5)3Sb(O2CR′)2 photoresists with R′ groups of increasing ability to stabilize a radical.
Upon exposure to EUV light, these complexes produce CO2, benzene, and phenol. CO2 is created in the largest amounts and its production increases by 320X between R′ = phenyl to R' = benzyl (Figure 12).
Figure 12. A trend is observed for (A) CO2 outgassing and (B) benzene and phenol outgassing. In which, the energy required for decarboxylation decreases in conjunction with increased outgassing per unit dose for a series of carboxylate ligands.
The degree of benzene and phenol outgassing likewise increased with bond energy of the R′ group. However, the change in magnitude from least stable to most stable was much less with the benzene signal increasing by a factor of fourteen and the phenol signal by a factor of seven.
Decarboxylation produces CO2 and an R′ radical neither of which are a constituent of benzene or phenol. However, the reactivities of benzene and phenol correlate with decarboxylation activation energy of the carboxylate ligand.
This correlation suggests the initial bond cleavage upon EUV photon interaction is decarboxylation of a carboxylate ligand. Subsequent reduction of the antimony center may result in the creation of benzene and/or phenol by additional reaction pathways.
Mechanistic Study 2
Benzene and phenol have been identified as two of the most abundant photoproducts outgassed upon EUV exposure of (C6H5)3Sb(O2CR′)2 photoresists.
Unlike CO2, the generation of benzene and phenol do not solely depend upon the decomposition of a single ligand.
The creation of benzene requires the abstraction of a phenyl ligand and a hydrogen. Phenol requires an even greater level of complexity as a phenyl ligand, a source of oxygen and a source of hydrogen must come together to create the alcohol.
To identify the source of these hydrogens, partially deuterated forms of JP-18 were synthesized in which every hydrogen composing the acetate ligands was substituted for deuterium, (C6H5)3Sb(O2CCD3)2 (d6-JP-18).
Direct comparison of d6-JP-18 and d0-JP-18 outgassing mass spectra would identify the abstraction of a hydrogen from the acetate ligand by a shift in a peak by one atomic mass unit.
Side-by-side outgassing experiments were performed of d0-JP-18 and d6-JP-18. The data obtained for the appropriate mass ranges were compared to identify any shifts in mass denoting the abstraction of a deuterium versus hydrogen to create benzene and/or phenol.
The data was plotted as relative outgassing in which the largest peak is normalized to one and the surrounding peaks were scaled accordingly.
No significant amu shift in benzene outgassing was observed between the d0-JP-18 and d6-JP-18 mass spectra upon EUV exposure (Figure 13A). With a minimum of 95% isotopic purity, we can deduce that hydrogen is not abstracted at the acetate ligand to produce benzene.
The mass spectra of d0-JP-18 and d6-JP-18 in the mass range of phenol’s parent peak were compared (Figure 13B).
Figure 13. (A) Benzene outgassing of d6-JP-18 demonstrates minimal hydrogen abstracted from the carboxylate. (B) Greater than ninety-five percent of phenol outgassed during exposure of d6-JP-18 abstracts hydrogen from the carboxylate.
A distinct shift of one amu is observed between d0-JP-18 and d6-JP-18 for the outgassing of phenol. With a minimum of 95% isotopic purity, we concluded that a hydrogen is abstracted from the acetate ligand to generate phenol during EUV exposure.
Summary
Since 2011, two new directions in the development of EUV photoresists has been explored — increased EUV absorption and metal-containing resists.
Our group is one of only a handful of academic research groups in the world working on the development of these fascinating materials. We were one of the first groups to develop EUV resists containing main-group metals.
This remains an active area of research in our group.
References
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L. Li, S. Chakrabarty, J. Jiang, B. Zhang, C. Ober, and E. P. Giannelis, “Solubility studies of inorganic-organic hybrid nanoparticle photoresists with different surface functional groups,” Nanoscale 8, 1338–1343 (2016).
Y. Ekinci, M. Vockenhuber, L. Wang, and N. Mojarad, “Evaluation of EUV resist performance with interference lithography towards 11 nm half-pitch and beyond,” Proc. SPIE 8679, 867910 (2013) [doi: 10.1117/12.2011533].
B. Cardineau, R. Del Re, M. Marnell, H. Al-Mashat, M. Vockenhuber, Y. Ekinci, C. Sarma, D. A. Freedman, and R. L. Brainard, “Photolithographic properties of tin-oxo clusters using extreme ultraviolet light (13.5 nm),” Microelectron. Eng. 127, 44–50 (2014).
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