Focussed Helium Ion Beam Exposure of Polymethylmethacrylate: Positive or Negative Tone Images, Polyenes, and Fluorescent Carbon Layers

Understanding chemical transformations in polymethylmethacrylate (PMMA) during helium ion beam exposure will help researchers involved in nanofabrication, materials synthesis or transformation, patterning polyenes, developing phantoms for radiation therapy, and realizing carbonaceous quantum emitters. The level of conjugation in PMMA can be controlled using a helium ion beam to realize patterns that are suitable for positive tone lithography, polyenes, dye‐like fluorescent materials, and polycyclic aromatic compounds with very similar properties to carbon dots. High‐resolution dose studies employing Raman scattering and atomic force microscopy (AFM) reveal the conditions under which these very different conjugated carbonaceous materials form, their spatial distribution, and dissolution characteristics in common solvents.


Introduction
Nanofabrication process flows involving electron beam (e-beam) lithography and polymethylmethacrylate (PMMA) have become commonplace in industry and academic settings. Recently, a collection of work describing helium ion (He þ ) beam lithography using PMMA resists has been published. [1][2][3][4] Exposure of PMMA films to ion beams, electron beams, X-rays, and UV light induces the formation of C═C bonds and concomitant loss of C═O bonds, resulting from main-chain scission and abstraction of the ester groups (COOCH 3 ) and hydrogen. [5,6] The COOCH 3 and H radicals evolve into gaseous methane (CH 4 ), methanol (CH 3 OH), carbon monoxide (CO), carbon dixode (CO 2 ), and hydrogen gas (H 2 ) and escape the polymer matrix. [7] These well-documented chemical changes are also accompanied by physical etching of the film due to vaporization as evidenced by thickness loss. [5,[7][8][9][10] In this connection, PMMA shows poor physical etching characteristics when using e-beam irradiation, and much better etching is achieved with X-ray exposure. [9,11] When patterning PMMA in lithography process flows, doses are carefully selected such that main-chain scission and abstraction of the ester groups take place to an extent that allows a certain developer to selectively dissolve an exposed volume. [2,6,8,9,[12][13][14][15][16] At these doses, minimal thickness loss due to vaporization occurs. A common developer solution that is used during lithography involving PMMA resist is a 1:3 ratio of methyl isobutyl ketone (MIBK) to isopropyl alcohol (IPA). In general, the ratio of MIBK:IPA can vary along with the applied dose to minimize proximity effects, alter the selectivity, and optimize patterning time when using PMMA as a positive tone resist. The number of main chain scissions as a function of dose has been shown to increase linearly for deep UV, e-beam, X-ray, and proton beam irradiation, at a rate that increases, respectively, with photon energy and particle mass. [7] Proton beams can be almost an order of magnitude more efficient at generating a main-chain scission event. [7] The dose ranges over which PMMA acts like a positive tone resist can vary significantly depending on the type of radiation being used and the thickness of the PMMA starting layer.
Negative tone behavior in MIBK:IPA solutions has been shown to occur in PMMA by irradiation with X-rays at doses in the range of 1800-2400 μC cm À2 . [12] PMMA films exposed to e-beam doses in the range of %10 4 μC cm À2 also show negative tone behavior. [17] Resistance to dissolution in acetone was reported to occur at %4.5 Â 10 5 μC cm À2 when 40 nm of PMMA was exposed to an e-beam. [18] Lithographic processes that use PMMA for negative image formation are of continued interest due to their potential to realize structures of higher resolution, as a positive tone process suffers from structures that shrink due to overdeveloping (for example). [17][18][19][20] The hardness of highly irradiated PMMA can approach 10-20 GPa for electron fluences that approach 10 15 cm À2 , implying that defined features can retain their shape. [21] According to simulations, the interaction volume for helium ion beams is much smaller than the interaction volume for ebeams, and helium ion beams become more collimated as the landing energy increases. [1,22] However, damage to the substrate can occur for high doses. For example, doses greater than %100 μC cm À2 induce subsurface dislocation in silicon (Si), and doses greater than 1000 μC cm À2 and 10 000 μC cm À2 show evidence of the onset of amorphization and bubble formation, respectively. [23,24] Both e-beam and ion beam lithographic techniques rely on the generation of low-energy secondary electrons. [1] Studies have shown that the number of secondary electrons generated by each helium ion is an order of magnitude higher than generated by electrons at the same landing energy, enabling more efficient bond scission and increased resist sensitivity. [25][26][27] Furthermore, the trajectories of the helium ions are not significantly altered, greatly reducing the number of backscattering events from the substrate into the resist layer, leading to reduced proximity effects when compared to e-beam processes. [28] Proximity effects are also reduced by the fact that helium ions generate secondary electrons with a much lower energy distribution, reducing the amount of energy that is deposited laterally into the resist layer. [29,30] Raman scattering spectroscopy is a very powerful tool to study the conjugation of a polymer network by radiation. The first studies to provide mechanistic models for abstraction, chain scission, and the formation of a vinylidene group between newly formed chain ends (end-linking model) were based on Raman spectroscopy. [6,9,12] In this case, X-ray doses which only slightly evolved the C═C peak, while maintaining the overall PMMA structure, were studied. The presence of D and G bands and luminescence have been observed in Raman spectra collected from electronirradiated PMMA as well, with the newly formed luminescence assigned to the presence of olefinic chains. [31] In these cases, the resolution in the variation of dose was not fine enough to track the physical and chemical evolution in detail. Nuclear magnetic resonance (NMR) studies have assigned proton chemical shift values observed in low-dose X-ray, UV, e-beam, and proton beam-irradiated PMMA to hydrogen atoms in vinylene and vinylidene groups. [5] The assignments were inspired by analysis of NMR spectra of various model compounds such as poly(isoprene), 2-methyl-1-butene, and 2-methyl-1-hexene. [32] However, NMR spectroscopy requires a significant amount of sample volume or surface area, preventing it from being used to analyze microscopic and nanoscopic volumes. X-ray photoelectron spectroscopy (XPS), X-ray near-edge absorption spectroscopy (XNEAS), and Fourier transform infrared (FTIR) spectroscopy [5,8,[33][34][35][36][37] have also been used to study the degree of ester group loss and formation of C═C bonds. These methods remain poor at assigning the stretching vibration associated with C═C bonds to linear or aromatic compounds, thus masking the evolution of PMMA after a significant number of C═O and C─O bonds are gone. Conversely Raman scattering can distinguish linear versus aromatic C═C bonds for example. Thus, Raman scattering spectroscopy is a powerful nondestructive method to track changes of PMMA, in nanoscale solid-state form, over all dose ranges, without any sample preparation requirements. Atomic force microscopy (AFM) allows for the residual thicknesses to be determined as well. [10] A detailed study using Raman spectroscopy and AFM of the radiolysis of PMMA by any type of radiation is lacking in the literature, which is surprising given the technological importance of PMMA radiolysis.
Here, the chemical and physical evolution of a 62 nm-thick PMMA layer on silver (Ag), exposed to 48 different doses of radiation from a helium focussed ion beam (HeFIB), then developed in a 1:3 MIBK:IPA solution, is examined via AFM, Raman spectroscopy, and Raman imaging. Ag was chosen as the substrate because of the surface-enhanced Raman scattering (SERS) effect originating from the nanoscale roughness of the film and its flat optical reflectance response. Doses between 0.95 and 1600 μC cm À2 were studied. For doses up to %3.8 μC cm À2 , the chemical evolution of the polymer is dominated by abstraction of the ester group and of hydrogen from the methyl and methylene side groups, leading to various types of vinylene and vinylidene groups in proportions that are small enough to maximize positive tone behavior. A sharp increase in dissolution resistance was seen at 38.3 μC cm À2 with a maximum resistance at 103 μC cm À2 . Here, the Raman scattering spectra are assigned to a methyl terminated polyene chain inspired by structures of polyacetylene and polyisoprene. The onset of cyclization and fluorescence is observed in the 300-800 μC cm À2 range, which is then followed by aromatization and graphitization up to 1600 μC cm À2 . AFM measurements conclusively show that increasing material loss occurs with increasing dose and follows a double-exponential trend with two characteristic doses that relate to ester loss and the onset of cyclization. Upon developing the sample in a 1:3 MIBK:IPA solution, identification of clearing doses, proximity effects, chemical identification of the solvent resistant material, and final depth profiles are readily obtained. The resistance to solvent does not explicitly require cross-linking and molecular weight increases as the formation of highly conjugated polyenes alone can render the material resistant to many solvents.

Results
The structure of PMMA is shown in Figure 1a. The repeating unit contains one ester (COOCH 3 ), one α-methyl (αCH 3 ), one methylene (CH 2 ), and one C 4 carbon. The PMMA layer was exposed to 48 different doses applied in 3 groups of 16 over 4 Â 4 μm areas as shown in Figure 1b and listed in Table 1. The Raman spectrum of an unexposed 62 nm-thick PMMA film on an Ag surface is shown in Figure 1c, for reference (the broad background which decreases with increasing Raman shift produced by the Ag surface has been removed). This spectral fingerprint is not fully understood and there are discrepancies on some band assignments in the literature. [35,38,39] Peak assignments for the Raman peaks appearing in Figure 1c are given in Table 2. The ester group is most readily characterized by the symmetric C═O stretching vibration appearing at 1736 cm À1 and the C-O-C stretch at 818 cm À1 . The evolution of the Raman scattering fingerprint of PMMA from the unexposed state shown in Figure 1c (for D = 0 μC cm À2 ) for doses ranging from 0.95 to 58.5 μC cm À2 is plotted in Figure 2a. Each Raman scattering spectrum was extracted by averaging 81 spectra over a 3 Â 3 μm area, centered within the exposed 4 Â 4 μm areas, that were acquired during the image scan. With increasing irradiation, a new peak initially centered at 1649 cm À1 associated with C═C bonds appears and grows.
The peak centered at 1736 cm À1 associated with C═O vibrations, as well as the other vibrational modes associated with the ester group (Table 2), evolve with the opposite trend (decreasing intensity with increasing dose), consistent with abstraction of the moiety. Importantly, all peaks associated with vibrational modes of the ester group display a complete loss of intensity at D % 40 μC cm À2 , except for the mode centered at 1455 cm À1 . Complete loss of intensity for the C═O groups was not observed until D > 100 μC cm À2 . Main chain methyl groups also remain in significant proportion as evidenced by continued spectral contributions appearing in the 2790-3130 cm À1 range.
Curve fitting was performed to accurately extract the positions and linewidths of the newly formed C═C mode and of the existing C═O mode for each exposure. After an appropriate linear background subtraction, each peak was fit to a pseudo-Voigt function of the form  Here, Λ is the Raman scattering intensity at a particular Raman shift ν. The peak center, full width at half maximum (FWHM), and area are denoted ν c , δ and A, respectively. The vertical offset is given by y 0 , and Δ is a profile shape factor.
For Δ = 1, the profile is purely Lorentzian and for Δ = 0 it is purely Gaussian. Figure 2b shows examples of deconvolved Raman spectra for doses of D = 3.8, 7.7, 40, and 58.5 μC cm À2 over the 1520-1900 cm À1 spectral range. The pseudo-Voigt function was selected for its versatility in fitting spectroscopic lineshapes that are otherwise poorly defined by Gaussian or Lorentzian profiles alone. In this connection, it is instructive to note that best fits were obtained at D = 0.95 and 3.8 μC cm À2 using Δ = 0 and at D = 58.5 μC cm À2 using Δ = 1. All other spectra were best fitted using a combination of Gaussian and Lorentzian line profiles to describe each peak. For D = 3.8 μC cm À2 , δ = 32.3 cm À1 and 32.1 cm À1 , and ν c = 1652 cm À1 and 1742 cm À1 for the C═C and C═O vibrational modes, respectively. Beyond D = 5.7 μC cm À2 , broadening and blueshifting of the C═C peak toward lower Raman shift values is observed. At D = 58.5 μC cm À2 , δ = 103 cm À1 and 31.3 cm À1 , and ν c = 1647 cm À1 and 1739 cm À1 , for the C═C and C═O vibrational modes, respectively.
Three Raman scattering images constructed from mapping the total integrated emission over a specific spectral range are shown in Figure 3. Each image is the combination of two 40 Â 40 μm maps of regions of PMMA that were exposed to doses ranging from 0.95 to 58.5 μC cm À2 . The Raman spectral images clearly resolve the 4 Â 4 μm regions in the resist that were irradiated with helium ions. The total areas under the spectrum over the 1570-1710, 1696-1783, and 2790-3130 cm À1 spectral ranges were mapped in real time during the measurements, corresponding to Raman shift ranges where vibrational modes associated with C═C vibrations, the symmetric C═O stretching vibration, and vibrations associated with C─H bonds in the main chain and in ester groups appear, respectively. In keeping with the literature, the Raman scattering maps demonstrate that irradiation of PMMA resist with helium ions generates more C═C 1494 v a C─H of OCH 3 [35] 1736 v s C═O [35,38,39] 2848 v s C─H of OCH 3 & αCH 3 [39] 2954 v s CH 2 and v a C─H of OCH 3 and αCH 3 [39] 3005 v a CH 2 [38,39] v s = symmetric stretching mode; v a = asymmetric stretching mode; def = deformation; tw = twist. www.advancedsciencenews.com www.aem-journal.com bonds with increasing dose, accompanied by decreasing C═O, C─H 2 , and C─H 3 bonds. Integrated peak intensities extracted from the Raman maps in Figure 3 as a function of dose for C═C, C═O, and CH 2 /CH 3 vibrations are plotted in Figure 4. The integrated intensities, I, can be modeled as first-order processes, and fit well to an exponential model Here, D is the dose, D c is the characteristic dose, and A þ B represents the integrated intensity at D = 0 μC cm À2 . The integrated intensity for vibrational modes associated with C═C, C═O, and CH 2 /CH 3 bonding evolves with characteristic doses of D C=C = 35.3 μC cm À2 , D C═O = 18.7 μC cm À2 , and D CH 2 =CH 3 = 30.1 μC cm À2 . For the C═C peak, A = 1.25 and B = À1.25, and for the C═O peak, A = 0 and B = 1. For the CH trend, A = 0.3 and B = 0.7. Loss of C═O species proceeds at a rate that is quicker than the appearance of C═C species and loss of C─H bonding.

Raman Scattering Spectra for Applied Doses in the Range 100-1600 μC cm À2
A corresponding Raman spectrum for each dose ranging from 100 to 1600 μC cm À2 was extracted (81 spectra averaged over a 3 Â 3 μm area centered within the 4 Â 4 μm exposed area) and are plotted in Figure 5a. At D = 302 μC cm À2 , a broad background emerges centered %2000 cm À1 , and continues to increase. The D and G bands centered at 1366 and 1591 cm À1 begin to form at D = 802 μC cm À2 . Beyond D = 1200 μC cm À2 , a significant increase in D and G band intensities is observed, suggesting that pyrolysis is taking place. Figure 5b shows Raman images mapping the total integrated intensity and integrated intensity of the D and G bands over the 250-3250 cm À1 and 1100-1780 cm À1 ranges, respectively, for regions in the PMMA layer exposed to doses of 100-1600 μC cm À2 . A very different spectral evolution was observed in this dose range, beginning first with the onset of fluorescence and masking of Raman scattering, followed by pyrolysis and re-emerging Raman scattering revealing D and G bands commonly observed in graphite-like carbon. Hence, the choices for the ranges over which the integrated intensity was mapped. Figure 5c shows the extracted trends for the integrated intensities of the total emission, fluorescence, and D and G bands. The Raman scattering maps in Figure 5b, and the extracted fluorescence and D and G band areas in Figure 5c, clearly show that the fluorescence emission begins to increase at D = 300 μC cm À2 , and that an increase in spectral area corresponding to D and G peaks in the 1100-1780 cm À1 range occurs after D = 802 μC cm À2 . The fluorescence signal is very uniform over the entire exposed region up to D = 1100-1200 μC cm À2 , whereas D and G band scattering shows spatial variation. A photograph of the emission from the material formed at D = 1600 μC cm À2 , captured through a camera attached to microscope eyepiece, is shown in Figure 5d. A bright orange hue is observed when this material is pumped by the 10 mW 532 nm laser. Upon prolonged exposure to laser light (on the minutes timescale), the broad fluorescence background would slowly decrease in intensity. However, the Raman bands would remain unmodified, suggesting that quenching of  www.advancedsciencenews.com www.aem-journal.com the emission process occurs in air without any significant change to the material chemistry.

Development (Dissolution) and AFM Scans
AFM scans of all exposed regions for the entire dose range obtained immediately after exposure are shown in Figure 6a. AFM scans of the same exposed regions after development in 1:3 MIBK:IPA solution for 20 min are shown in Figure 6b. The amount of material that was removed by each process step (etch depth) is plotted in Figure 6c. The thickness loss due to PMMA vaporization during helium ion exposure as a function of dose was extracted from the AFM scans and is plotted in Figure 6c as red circles. Empirically, PMMA vaporization follows a double exponential profile, and the solid red line in Figure 6c represents a fit to the data. Two characteristic doses at D c1 = 30.6 μC cm À2 and D c2 = 707 μC cm À2 are identified. These values correspond to the characteristic dose that was calculated from Raman scattering measurements for C═C bond evolution and C─H bond loss below 58.5 μC cm À2 (Figure 4), and for the dose range where the onset of graphitization and the emergence of D and G bands occurs (Figure 5), respectively. The extracted dissolution depth is also plotted in Figure 6c as black squares, and the total cleared thickness (taken as the sum of the vaporization thickness loss and dissolution depth) is plotted in blue triangles. The scans reveal that at D = 3.8, 5.7 and 7.7 μC cm À2 , complete dissolution in MIBK-IPA of the remaining material takes place. These doses optimize the positive tone behavior of PMMA. The onset of resistance to the developer solution begins at D = 9.6 μC cm À2 and increases steadily up to D = 38.2 μC cm À2 after which a sharp jump in resistance to dissolution is observed. At D = 103 μC cm À2 , the minimum amount of material is dissolved, so this dose optimizes the negative tone behavior of PMMA. The identified doses are only valid for the specific PMMA thickness, structure dimensions, and the beam current used in this study. In general, one would have to perform Figure 5. a) Raman scattering spectra for doses in the range D = 100-1600 μC cm À2 . The onset of fluorescence is seen at D = 300 μC cm À2 after which characteristic scattering from D and G bands centered at 1395 and 1595 cm À1 is easily resolved. b) Raman mapping the total integrated emission and integrated intensity of D and G bands. c) Extracted trends of the total integrated emission, fluorescence, and D and G band intensities versus dose. d) Photograph of the emission from the material formed at D = 1600 μC cm À2 when excited with 532 nm laser light.
www.advancedsciencenews.com www.aem-journal.com a dose gradient and dissolution study to identify patterning parameters which optimize negative and positive tone behavior. Selected Raman spectra taken after dissolution of exposed PMMA, followed by an IPA rinse, are shown in Figure 7a for doses D = 0.95, 1.9, 3.8, 45.5, 103, 802, and 1600 μC cm À2 . For D = 1.9 μC cm À2 , the featureless spectrum suggests that no material remains in this region. However, the corresponding AFM measurement of Figure 6b confirms that %10 nm of material remains. About 5 nm of material remains at D = 1600 μC cm À2 and yet a well-resolved Raman spectrum is recorded, suggesting that the different carbon bonding environments evolving within the polymeric network have different scattering cross sections. Thus, detection, identification, and quantification of material that remains after irradiation and dissolution using only Raman scattering become challenging. For all cases where Raman scattering was detectable, it was observed that the signals bear strong resemblance to those acquired after irradiation. The 1:3 MIBK:IPA developer dissolves material in all cases as shown in Figure 6c, and the remaining material, if any, (Figure 7a) has the same spectroscopic signature as the material that formed after irradiation (Figure 2 and 5).
Three 40 Â 40 μm Raman scattering maps of the integrated intensity of the CH 3 /CH 2 peaks for all doses after dissolution, over the 2790-3130 cm À1 spectral range, are stitched together and shown in Figure 7b. The CH 2 /CH 3 signals represent the strongest Raman peaks in unexposed PMMA (Figure 1c) and are an excellent proxy to spatially resolve where exposed PMMA has been affected by the developer solution. The Raman map clearly shows that the regions affected by the developer become larger with increasing dose. The widening of structures is due to lateral deposition of energy resulting in unintentional exposure over a larger area than desired and is commonly referred to as a proximity effect. It is instructive to note that proximity effects in PMMA are consistently lower when using a helium ion beam versus an electron beam. [1,2] The relative lengths and widths of the dissolved regions were extracted by averaging line cuts across each square from the CH 3 /CH 2 Raman scattering maps and the AFM maps to generate 2D integrated intensity and height profiles, respectively. The integrated Raman Intensity was normalized to the signal  In all cases where some material remains, the spectral features are consistent with those that appeared prior to dissolution (Figure 4 and 6). b) Raman scattering map of the total area appearing under peaks associated with C─H vibrations in the 3000 cm À1 range. c) Change in width relative to that measured for D = 3.8 μC cm À2 versus dose.
www.advancedsciencenews.com www.aem-journal.com appearing in unexposed regions. Figure 7c plots the calculated increase in width, Δw, of the dissolved regions versus dose, extracted from Figure 6b and 7b, relative to the width measured for D = 3.8 μC cm À2 (optimal dose for positive tone behavior). Raman spectroscopy and AFM measurements produce similar Δw versus D trends that are readily fit to a single exponential profile of the form of Equation (2), with characteristic doses of D c = 268 and 323 μC cm À2 , respectively. Widths of dissolved regions estimated using CH 2 and CH 3 spectral features were consistently greater than the measurement via AFM.

Discussion
The presently accepted mechanistic model for radiolysis of PMMA is the abstraction of an ester group and of hydrogen from the α-methyl group, leading to either main chain scission and C═C bond formation at the chain ends, or chain ends recombining to form a vinylene or vinylidene group. [5,6,9,12] In this connection, the length of the backbone might not play a role in determining the dissolution rate of the polymer. The dissolution rate of the polymer could, instead, depend on the number of ester groups that are removed, and thus the degree to which the backbone is conjugated. Figure 8 sketches the various double bonds that will appear after abstraction of the ester group and of hydrogen from the pendant methyl group. All these double bonds will yield a new peak centered at %1640-1650 cm À1 in Raman scattering spectra due to symmetric C═C stretch vibrations.
The PMMA used in this study is atactic, and the average molecular weight of an individual polymer chain is %950 kDa, which corresponds to %9448 monomers. The set of vibrational modes related to the ester and methyl groups are known to decrease in intensity as photolysis and radiolysis occur, and the C═C scattering peak appears. [6,9,36] The focussed He ion beam used in this study has the same effect. This is readily observed in the Raman scattering maps and spectral signature of the C═C, C═O, and CH 3 peaks in Figure 2 and 3. The molecular weight of the ester group is 59 Da and represents about 60% of the chain weight. Reduction of molecular weight forms the basis for UV, electron beam, and, more recently, He ion beam lithography. [1][2][3][4] A developer solution, for example, 1:3 MIBK: IPA, will selectively dissolve regions in the PMMA matrix where molecular weight has been reduced, allowing for subsequent structure definition via deposition and lift-off steps. Consistent with continued abstraction of the ester, the C═O mode at 1736 cm À1 decreases and maintains its lineshape (δ % 31-32 cm À1 ) as the dose increases to 58.5 μC cm À2 .
However, the C═C peak line shape is only stable to doses up to D = 5.7 μC cm À2 and then rapidly broadens by 69 cm À1 with increasing dose (Figure 2b), suggesting that isolated C═C bonds form only for doses <5-6 μC cm À2 and minimal disorder of the polymer chains occurs. Furthermore, the thickness loss due to vaporization for doses <5-6 μC cm À2 (red curve in Figure 6c) remains below 5 nm which is consistent with very little ester loss and disorder. The AFM and Raman scattering analyses of these exposed regions, shown in Figure 6 and 7, reveal that all material is dissolved by the 1:3 MIBK:IPA solution and washing steps as soon as the dose reaches 3.8 μC cm À2 . This represents the irradiation level that optimizes a positive tone helium ion beam lithography (HIBL) process in a 62 nm-thick PMMA layer. It is instructive to note that Raman scattering measurements alone cannot detect the clearing dose as evidenced by the featureless spectrum in Figure 7a for D = 1.9 μC cm À2 and the corresponding AFM scan in Figure 6b which shows that %10 nm of material remains. In this connection, under the current measurement conditions, Raman scattering is unable to detect PMMA at these thicknesses. The minimal material thickness loss due to vaporization, combined with strong dissolution characteristics, and a C═C peak with a linewidth %30-35 cm À1 are indicative of an exposed PMMA network that can be modeled with the chain fragments sketched in Figure 8. Within this small dose window,  www.advancedsciencenews.com www.aem-journal.com PMMA behaves as a positive-tone scission-based resist with minimal proximity effects. Cross-linking increases molecular weight and is not a salient process for doses <5.7 μC cm À2 . Beyond 5.7 μC cm À2 up to 58.5 μC cm À2 , increasing intensity and broadening of the C═C peak, as shown in Figure 2b, is indicative of a larger distribution of C═C bonding environments and disorder as more conjugation takes place. As the exposure time is increased, the rate of thickness loss due to vaporization also increases (Figure 6a,c), and the corresponding Raman scattering spectra still show vibrational modes associated with the COOCH 3 group. At D = 40 μC cm À2 , a sharp increase in resistance to the developer solution is observed (blue trace in Figure 6c), which is referred to as negative tone behavior in resists. At D = 58.5 μC cm À2 , about 25 nm of material is removed by the helium ion beam, and peaks associated with C═O vibrations are barely resolvable in the %35 nm of material that remains. Upon dissolution in 1:3 MIBK:IPA, only an additional 10 nm is dissolved and 25 nm remains. Furthermore, quantitative analyses of spectral features have suggested that removal of a COOCH 3 group and abstraction of hydrogen from the methyl group generates one C═C bond. [4] In this study, the scattering maps, and the extracted evolution of the C = C and C = O peaks in Figure 4, show that the ratio of C = C spectral area that increases versus the C = O spectral area that is lost is 1.14, which is in very good agreement with the literature. [5] If a 1:1 ratio is assumed, the ratio of the scattering cross sections would be %1.14. At D = 40 μC cm À2 about 90% of the ester groups have been abstracted, so nine double bonds (C═C) now exist for every ester group that remains. The extracted integrated area of the ester group C═O mode in Figure 4 suggests that only 10-20% of the ester groups need to be abstracted for effective dissolution in the developer (1-2 C═C bonds for every ten ester groups). Thus, increasing conjugation of the polymer chains greatly increases resistance to the solvent.
It appears that various authors have attributed the decreased dissolution rate caused by increasing the dose to an increase in molecular weight. This conclusion was drawn by connecting the observations to studies that have shown that PMMA chains of lower molecular weight dissolve at faster rates. [14] This assumption (of increasing molecular weight with dose) does not necessarily hold true for a radiolytic product that forms around the reversion dose and implies that C═C bonds generated around the reversion dose preferentially form cross-links over end-links. Increased conjugation and changes to the line shape of C═C does not imply PMMA chains with increased molecular weight. Crosslinking is different than chain recombination or end-linking. Cross-linking is when carbon backbones of two different chains become grafted together by two ends of a third, and thus, the endlinked structures in Figure 8 are not the same as cross-linked structures. It is unclear why a cross-linking process would dominate at a particular dose, and mechanistic models justifying that cross-linking would result in C═C bond formation is lacking in the literature. Although end-links can increase the molecular weight, on average, recombination will not generate chains that are longer than the original chain length. Furthermore, as shown in Figure 6b,c, significant dissolution of the resist persists up to doses of D = 36.2 μC cm À2 , but a rapid transition to developer resistance occurs at D = 40 μC cm À2 . This small change in dose increases the number of abstracted ester groups from 85% to 90%, and thus it is quite unlikely that all newly formed C═C bonds within this small dose window suddenly form C═C bonds that all increase molecular weight. Recall that the ester groups make up %60% of the weight of the PMMA chains. Also, consider that for D = 1.9 μC cm À2 , only 10% of the ester groups vanished and dissolution resistance exists due to the abundance of high molecular weight chains (Figure 6b,c, D = 1.9 μC cm À2 ). Thus, if cross-linking was responsible for increasing the molecular weight, rendering the PMMA resistant to the developer, then a %50% increase in average molecular weight would be required. A single cross-link will significantly increase the molecular weight; however, the sudden onset of such behavior is very unlikely.
Let us instead assume at the outset that newly formed C═C bonds within chain segments (end-links), and grafted C─C segments (cross-links), occur with equal probability. We assume that ester removal dominates molecular weight reduction over molecular weight increases from cross-linking, and that end-linking alone cannot increase molecular weight on average. The alternate explanation to the dissolution resistance is that as more conjugation occurs, and all ester groups are abstracted, polyene and polydiene copolymer backbones form in the irradiated matrix. Polyenes are extremely resistant to solvents. Figure 9 presents models of cis and trans configurations of polyacetylene and polyisoprene, which are two well-studied conjugated polymer systems. An entire field of study has been devoted to functionalization of polyenes to render them more soluble. [40] The Raman scattering spectra of polyacetylene and polyisoprene have been intensively studied. [41][42][43][44][45][46] The spectrum for polyacetylene consists of two strong bands centered at 1080 and 1474 cm À1 corresponding to in-plane symmetric C─C and C═C vibrations. [41] The fingerprint for polyisoprene is more complicated. [42] In this material, the C═C band is centered at 1640 cm À1 , and vibrational modes associated with symmetric and asymmetric vibrations in the 2800-3200 cm À1 region are readily detected. Methyl CH 3  www.advancedsciencenews.com www.aem-journal.com and 1370 cm À1 , and vinyl modes appear in the 1300-1330 cm À1 range. Additional modes corresponding to C─C backbone vibrations appear at 1070-1080 cm À1 . Raman scattering spectra for PMMA exposed at D = 103 μC cm À2 and for unexposed PMMA are shown in Figure 10a. At D = 103 μC cm À2 , the highest resistance to the developer solution was observed. Based on the peak assignments and structures of polyacetylene and polyisoprene, and for the PMMA used in this study, we can assign peaks in the recorded Raman spectrum from this material and propose some possible structures. [41][42][43][44][45][46] The peak at %1640 cm À1 is assigned to C═C stretching, and the newly formed peak structure at 895 cm À1 is assigned to a CH mode. Vinyl end groups are seen in the recorded spectrum of PMMA centered at 1335 cm À1 and are assigned to =CH 2 chain end groups ( Figure 8). This peak is also observed in the modified material and has a higher relative intensity to other features when compared to unexposed PMMA, so increased proportions of =CH 2 groups exist after exposure. The scattering peaks at 1005 and 1455 cm À1 are assigned O-CH 3 vibrations and CH twist modes (see Table 2). Weak scattering due to C─O stretching centered at 1120 and 1167 cm À1 , C─C skeletal vibrations centered at 1246 cm À1 , and a very small shoulder centered at 1736 cm À1 from C═O bonds also appear. A significant spectral contribution also remains in the 2800-3200 cm À1 region. As with all polymeric systems, this spectral range remains a convolution of CH 3 , CH 2 , and CH modes. Polyisoprene consists of only C─H bonds and has a very weak contribution in this range. Hence, the weak presence of C─O and C═O, and noticeable scattering in the %3000 cm À1 range, suggests that a significant number of CH 2 and CH 3 groups are present, but comparatively fewer COOCH 3 groups remain. Furthermore, the C═C peak has a linewidth of δ = 103 cm À1 , suggesting that a highly disordered network is emerging. At low doses this peak is only 31-32 cm À1 wide (Figure 2b). A similar spectrum has been published in connection with e-beam-irradiated PMMA yielding fluorescence and was attributed to the presence of olefinic chains. [31] However, a significant fluorescence signal in polymeric compounds can also be associated with the presence of rings.
For D = 302 μC cm À2 , we also observed the onset of a broad fluorescence background that continued to increase up to D = 1600 μC cm À2 . The extracted integrated area of the fluorescence background signals in Figure 5c readily shows this in addition to the spectra plotted in Figure 5a. A selected spectrum for D = 302 μC cm À2 plotted in Figure 10b highlights the earliest onset of the fluorescence background. At this point the C═C mode is still resolved but has shifted to %1590-1600 cm À1 , and the disappearance of the CH 2 /CH 3 peaks in the 3000 cm À1 region is also obvious. The shift of the C═C mode is consistent with increasing backbone length from interchain bonding or cyclization to form fiveand six-membered rings. [45] Cyclization is consistent with the onset of fluorescence, as fluorophores are readily assigned to cyclized structures. [47] If photoluminescent transitions only required highly conjugated chains, then the broad background should have manifested itself steadily as abstraction of COOCH 3 groups proceeded to form C═C bonds. As the dose is increased to 802 μC cm À2 , in addition to a strong fluorescence background, well-resolved D and G bands characteristic of graphite-like carbon is seen. The Raman spectrum for D = 1600 μC cm À2 is reproduced in Figure 10b to highlight the final product that has formed. In this case, the onset of cyclization and graphitization leads to a highly luminescent material that shares remarkably similar properties to carbon dots and polymer dots. Here, the formation of this product was closely tracked via Raman scattering, allowing for a full mechanistic model that accounts for the various types of conjugated materials that are formed as a function of dose.
Hypothetical models of newly formed interlinked poly-ene/ diene chains that represent the D = 100, 300, and 1600 μC cm À2 materials are shown in Figure 11. For D = 100 μC cm À2 , structures contain ene-and diene-like moieties along the backbone and are functionalized with vinyl, methyl, and ester groups. The proportion of ester groups relative to the number of C═C and =CH 2 bond groups approaches zero at reversion doses ( Figure 2, 3 and 4). These systems are highly conjugated due to the well-accepted mechanism involving a 1:1 ratio of COOCH 3 þ H abstraction to C═C double bond formation, and would share the dissolution characteristics of polyacetylene and polyisoprene─specifically, these types of compounds are difficult to dissolve. [5] At D = 300 μC cm À2 , cyclization and formation of fluorophores begin, along with the onset of a broad fluorescence background, and reduces the number of CH 2 , CH 3 , and CH bonds. Both effects are seen in the corresponding experimental measurement in Figure 5a and Figure 10b. Thus, Figure 10. a) Raman scattering spectrum for D = 103 μC cm À2 and for unexposed PMMA. C═C and CH 3 vibrational modes are seen in addition to weak scattering from vinyl and CH groups. Comparison with the spectrum of unexposed PMMA reveals that little to no oxygen remains. b) Spectra for D = 302 and 1600 μC cm À2 showing the onset of the broad fluorescence background and eventual graphitization of material.
www.advancedsciencenews.com www.aem-journal.com models presented for D = 300 μC cm À2 bear similar resemblance to dyes. [47] As cyclization proceeds, the scattering spectrum evolves into a fingerprint characteristic of carbon blacks, [48] pyrolysis products, and graphitic oxides. [49][50][51][52][53][54] In this connection, the final residue products that form are polycyclic structures that are terminated by mostly hydrogen and methyl groups. As a significant proportion of the ester groups were removed, oxygen containing groups will not occur in proportions that are larger than a few percent. However, the existence of lactones, ketones, COOH, COOCH 3 , and OH groups in proportions <1% would still be consistent with a defective graphitic oxide type of structure and the corresponding spectral characteristics observed in this study.

Conclusion
A 62 nm-thick PMMA film on Ag was exposed to 48 different doses from a helium ion beam over 4 Â 4 μm areas and examined via AFM and Raman spectroscopy. Different types of material form due to continued abstraction of ester groups and the formation of C═C bonds. Dissolution of the exposed resist in 1:3 MIBK:IPA reveals that positive tone behavior is optimized for doses <5.4 μC cm À2 . A slight resistance to dissolution is observed at 40 μC cm À2 , beyond which a very sharp resistance to the developer is observed. At D = 100 μC cm À2 , the exposed regions exhibit a strong negative tone behavior, and the corresponding Raman spectrum suggests this material is highly conjugated, consisting mostly of C═C and C─H bonds, and emits no fluorescence. For higher doses in the 200-1600 μC cm À2 range, the onset of cyclization forming dye-like materials and eventually aromatic structures exhibiting strong D and G bands are observed. Doses which generate Raman scattering spectra in exposed PMMA that show very little C═O peak intensity, no fluorescence, and have a spectrum resembling that of simple polyenes such as polyisoprene or polyacetylene could be of interest to form and pattern conductive polymer systems. Doses that generate high levels of fluorescence from exposed PMMA are of interest to the spatial definition of carbon-based quantum emitters. Doses  Figure 11. Models of exposed PMMA inspired by the structures of polyisoprene and polyacetylene for exposure doses of D = 100, 300, and 1600 μC cm À2 . Continued exposure results in continued loss of ester groups and thus the continual formation of new C═C bonds. Cyclization and fluorophore centers begin to form beyond 300 μC cm À2 . Finally, complete graphitization and aromatization of the material is observed in addition to strong PL at doses beyond 800 μC cm À2 . The Raman scattering spectrum at 1600 μC cm À2 is consistent with a graphitic oxide as sketched.
www.advancedsciencenews.com www.aem-journal.com that alter the solubility of PMMA to yield positive or negative tone images are of strong interest for lithography processes. More generally, Raman scattering is a powerful spectroscopic technique that can identify subtle changes in polymeric structures in connection with aromatic and conjugated chain C═C vibrations, the onset of fluorescence, and graphitization in nanoscale sample volumes.

Experimental Section
PMMA Layer on Ag: Exposure and Developing: A Si substrate was first cleaned in ultrasonic baths of acetone and IPA, followed by drying with nitrogen gas. Ag pellets (99.99% purity) were thermally evaporated (Angstrom Engineering NEXDEP physical vapor deposition system) to grow a 200 nm-thick film, at a rate of 5 Å s À1 , onto an e-beam evaporated 3 nm-thick chromium (Cr) adhesion layer, on the Si substrate. A 62 nmthick layer of PMMA resist (950 k mol wt, Kayaku Advanced Materials) was spin-coated onto the Ag layer (Laurell WS-650-23NPP spin coater), and then baked at 180°C for 1 h.
Patterning of the resist was carried out using a helium ion beam lithography process described elsewhere. [2] Briefly, structures were patterned into the PMMA resist using a Zeiss Orion NanoFab Helium Ion Microscope (HIM). The layout and exposure were controlled using the Fibics Nanopatterning and Visualization Engine (NPVE). A beam current of 1.5 pA at a landing energy of 25 kV was used to deliver a dot-dose exposure to 4 Â 4 μm areas with an x = y step size of 4 nm, and various dwell times ranging from 0.1 to 170 μs to apply doses ranging from 0.95 to 1600 μC cm À2 (Table 1). Samples were then developed in a 1:3 mixture of methyl isobutyl ketone (MIBK) and isopropanol (IPA) for 2 min at 20°C, followed by a 30 s IPA bath.
Raman Scattering and AFM Measurements: A WiTec a300 Raman microscope equipped with a 10 mW 532 nm diode laser source operating in the backscattering configuration was used to collect Raman spectra. A 100 Â 0.9 NA objective was used to collect Raman scattered light which is fiber-coupled to a CCD camera cooled to À70°C. A 600 groove mm À1 grating blazed at 500 nm was used maximize resolution over the spectral range studied. Raman imaging was performed by acquiring 120 Â 120 spectra over a 40 Â 40 μm area at an integration time 0.5 s per spectrum. The intensity for various vibrational modes at each pixel is plotted as a color map to form images of specific chemical changes that are occurring. Processing of individual spectra and generation of Raman maps was done in the Origin 2022 and WiTec project FOUR software environments, respectively.