A new nanocomposite resist for low and high voltage electron beam lithography

Microelectronic Engineering 70 (2003) 19–29 www.elsevier.com / locate / mee

A new nanocomposite resist for low and high voltage electron beam lithography a a, b b M. Azam Ali , Kenneth E. Gonsalves *, Ankur Agrawal , Augustin Jeyakumar , b Clifford L. Henderson a

Polymer Chemistry NanoTechnology Laboratory, Department of Chemistry and Cameron Applied Research Center, The University of North Carolina-Charlotte ( UNCC), 9201 University City Blvd., Charlotte, NC 28223, USA b School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 -0100, USA Received 16 December 2002; received in revised form 22 April 2003; accepted 12 May 2003

Abstract A novel nanocomposite photoresist was synthesized and characterized for use in both low and high voltage electron beam lithography. This resist system is shown to display the ideal combination of both enhanced etch resistance and enhanced sensitivity required to satisfy both low and high voltage patterning applications. Resist sensitivity was enhanced by the direct incorporation of a photoacid generating monomer into the resist polymer backbone while the etch resistance of the material was improved by copolymerization with a POSS containing monomer.  2003 Elsevier B.V. All rights reserved. Keywords: EB; Lithography; Resist; Nanocomposite; Photoacid generator

1. Introduction Chemically amplified resists (CARs) are a class of resists that offer improved sensitivities required for high keV electron beam patterning methods [1]. The amplification of the initial exposure reaction, i.e. acid generation events, through acid catalyzed deprotection of protected solubilizing sites on the CAR resist polymer can permit extraordinary increases in overall sensitivity. The chemically amplified resist materials discussed in this work, which directly incorporate photoacid generator functional groups into the main chain, can provide extremely sensitive resist materials through the enhanced energy transfer and sensiti*Corresponding author. E-mail address: [email protected] (K.E. Gonsalves).

zation achieved by the microstructure of such materials. In addition to high accelerating potential projection methods, there is also significant promise in various low voltage electron beam methods as well [2]. For example, massive parallel arrays of electron beam microcolumns also have the potential to solve the speed problem commonly encountered with ebeam patterning since one could in principle perform direct write patterning with tens to hundreds of beams simultaneously [3]. Likewise, novel methods such as LEEPL, in which low energy masked electron beam exposure is used, also promises to significantly increase the speed of electron beam lithography and provide industrially relevant speeds. In all of these cases, the relatively low energy nature of the electrons used in such techniques means that

0167-9317 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00363-0

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they have a shallow penetration depth in organic materials, and thus thin resist layers on the order of 100 nm in thickness or smaller will be required to make use of these exposure technologies with single layer resists. It is doubtful that conventional resist materials used currently in semiconductor fabrication processes possess sufficient etch resistance to be used at sub-100-nm layer thicknesses, and thus electron beam resists with improved etch resistance will be needed. As will be shown, the nanocomposite resist strategy explored here can aid in solving this etch resistance problem. The direct incorporation of silicon containing functional groups into the resist polymer can serve to dramatically increase the etch resistance of the resist material. The aim of our work was to produce advanced nanocomposite [4] resist materials for electron beam lithography that could provide: (1) enhanced sensitivity (i.e. ,1 mC / cm 2 at 10 keV or ,10 mC / cm 2 at 100 keV); (2) high contrast and resolution; (3) superior etch resistance; and (4) reduced proximity effects [5,6]. Here we report the initial evaluation of a novel chemically amplified nanocomposite photoresist [4,5,7,8] for both low and high voltage electron beam lithography. We have investigated the lithographic properties of this positive tone nanocomposite resist using both low and high voltage electron beam exposure and characterized these materials in terms of their coating and film forming quality, sensitivity, contrast, pattern development behavior, and etch resistance.

2. Experimental

2.1. Materials The components of the nanocomposite photoresist (nanoRT-3b) used in this work were: tert-butyl methacrylate (t-BMA), methyl methacrylate (MMA), methacrylic acid (MAA) purchased from Aldrich Chemicals. Monomers t-BMA and MMA were distilled under vacuum for the removal of inhibitors. Polyhedral oligosilsesquioxane methacrylate (POSS) was received from Hybrid-Plastic, Inc, CA, USA and used as received and a monomer containing a photoacid generator (PAG) was synthesized in our laboratory [8]. NanoRT-3b was synthesized through

a free radical polymerization process using N,Nazobisisobutyronitrile (AIBN) as the initiator. This polymerization process was similar as described by us earlier [4,8]. Silicon k100l substrates were obtained from Nova Electronic Materials and were cleaned using a standard RCA procedure before use. Hexamethyldisilizane (HMDS) was purchased from Aldrich Chemical and used as received as a substrate surface priming agent to improve resist adhesion. Propylene glycol monomethyl ether (PGME) was also purchased from Aldrich Chemical Co. and used as received as a film casting solvent.

2.2. Sample preparation NanoRT-3b resist solution (5% solution in PGME) was spin coated on 3-inch HMDS primed silicon wafers using a CEE Model 100CB spin coat and bake system. Resist coated wafers were postapply baked (PAB) at 120 8C for 120 s and the film thickness was measured using the ellipsometry method described below. Typical resist film thicknesses used were approximately 140 nm for low voltage experiments and 340 nm for some high keV (20 keV) experiments. Smaller pieces (1 cm31 cm) were diced from the 3-inch wafer for use as samples for electron beam exposure.

2.3. Film thickness measurements Resist film thicknesses were measured using a V-VASE variable angle spectroscopic ellipsometer made by J.A. Woollam Inc. The ellipsometry parameters, C and D, were measured for each sample over the wavelength range from 400 to 1100 nm at incident angles of 65, 70, and 758. This C and D data were fit using the WVASE32 analysis software from J.A. Woollam Inc. A Cauchy layer optical model was used to represent the resist film layer during all ellipsometry data analysis. Both refractive index and film thickness data were calculated for each sample.

2.4. Resist contrast curve generation and fine line patterning One of the main objectives of this resist evaluation study was to measure the sensitivity (S), contrast (gm ), and resolution of the NanoRT-3b using elec-

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tron beam lithography. Electron beam exposures were carried out using a JEOL 5910 SEM with the exposure pattern being generated using the Nabity Pattern Generator Software (NPGS). At 20 kV the beam spot size is on the order of 12 nm. For contrast and sensitivity studies, the exposed patterns consisted of 434 dose matrices of 16 pads (each pad was 10 mm310 mm in dimension). Each pad in the matrix was exposed to a different exposure dose and subsequently used to generate conventional contrast curves for the material. For high resolution pattern tests, isolated space arrays were imaged using varying doses. Exposures for contrast and sensitivity experiments were performed using electron beam accelerating potentials of 2 and 20 keV using a beam current of 5 pA. These same exposure conditions were used for the high-resolution electron beam imaging of the isolated space structures. Post-exposure bakes (PEB) were performed at 120 8C for 120 s immediately (less than 10 s transfer time) after samples were removed from the SEM chamber. After PEB, the exposed patterns were developed in 0.26 N TMAH developer (AZ 300 MIF from Clariant) at a temperature of 71 8F for 60 s in puddle mode. After exposure and development, all contrast curve dose matrix samples were analyzed by measurement of the pattern surface relief profiles using a Dimension 3100 AFM from Digital Instruments, Santa Barbara. All scans were taken in tapping mode with a standard silicon tip. High resolution patterns such as the isolated space structures were analyzed using the JEOL 5910 SEM.

2.5. Plasma etch studies Etch rate studies using unexposed NanoRT3b resist samples were performed using a PlasmaTherm reactive ion etching (RIE) system. Two different etch gas chemistries, one used for silicon dioxide etching and one used for organic film etching and resist removal, were used in the study. First, a gas mixture for silicon dioxide etching was used to compare the etch rates and selectivities of a series of resist polymers with respect to a silicon dioxide film. In this study, a total of four different resist polymers were used: the NanoRT3b material, a commercial electronic grade novolac resin (Mw 5 22,000), a commercial electronic grade poly(hydrox-

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ystyrene) resin (Mw 511,800, also referred to as PHOST), and poly(methyl methacrylate) (Mw 5 950,000, also referred to as PMMA). Novolac resins are used as the matrix for conventional DNQnovolac I-line photoresists, while PHOST resins are one of the common matrix materials for deep-UV resists used with 248-nm exposure tools. PMMA is of course a classic high resolution electron beam resist material. Films of the various resist polymers were spin coated onto silicon k100l wafers and soft baked at 90 8C for 2 min to remove residual casting solvent. The thicknesses of the coated films were measured using the variable angle spectroscopic ellipsometer. The samples were then etched for various periods of time (1, 2, 4, 8, and 12 min) in the PlasmaTherm RIE system using a gas mixture consisting of CHF 3 and O 2 flowing at 22.5 and 2.5 SCCM, respectively. The RIE chamber pressure and chuck power were maintained at 50 mTorr and 150 W. After each etching period, the samples were removed from the etch chamber and the remaining film thicknesses were measured by spectroscopic ellipsometry. For selectivity and etch rate comparison purposes, a silicon dioxide film sample was also prepared etched at the same time as the various polymer samples. The thickness versus etch time data for the polymer films and the silicon dioxide sample exhibited a linear decrease in film thickness as a function of etch time, and thus each data set was fit using a linear least squares method to obtain the etch rate of the samples. The ratio of the etch rate of the various polymers with respect to the etch rate of the silicon dioxide sample was also computed as a measure of the selectivity of the materials using this oxide etch. In addition to the silicon dioxide etch study, the RIE etch behavior of the NanoRT3b material using an organic etch or ‘ashing plasma’ gas chemistry was also studied. Novolac film samples were also processed in this etch gas chemistry for comparison purposes. The specific plasma conditions used to study the etching behavior of were: (1) gas flow rates of 40 sccm O 2 and 16 sccm Ar; (2) chamber pressure of 100 mTorr; and (3) total power of 200 W. This type of plasma etch can be used to etch organic films and is commonly used to simply ‘ash’ and remove photoresist films. The initial thickness of the NanoRT3b resist and novolac films prior to

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etching was obtained using detailed spectroscopic ellipsometry as described previously. Film samples of each material were then etched for 10-, 20-, 30-, 40-, 50-, and 60-s periods. A portion of each sample was protected from the etch environment by covering one section of the sample with a glass coverslide. After each sample was etched, variable angle spectroscopic ellipsometry and profilometry (Tencor AlphaStep) measurements were used to calculate the etch depth. This thickness versus etch time data were then fit to obtain etch rates for the samples. AFM scans were also performed on the sample to monitor the film surface roughness as a function of etch time. Finally, X-ray photoelectron spectroscopy (XPS) was performed on each sample to determine the elemental composition of the etched film surface.

2.6. Results and discussion 2.6.1. Preparation of NanoRT-3 b NanoRT-3b was prepared using a polymerization process previously reported [4,8]. Specifically, the polymerization behavior between the photo-acid generator (PAG) monomer and methacrylate monomers was investigated previously. The objective of this copolymerization was to prepare a novel chemically amplified nanocomposite photoresist. tert-Butyl methacrylate (t-BMA) was used as a protecting group for NanoRT-3b. Carboxylic acid monomer (i.e. MAA) was used as an adhesion promoter in this polymer resin. Polyhedral oligosilsesquioxane methacrylate (POSS) was incorporated to achieve a higher plasma etch resistance as required for conventional pattern transfer methods [7]. It has been previously demonstrated that incorporation of POSS into methacrylate based CA resists could improve their plasma etch resistance [4–7]. However, the most important molecular parameter for low voltage electron beam (LVEB) exposure is the incorporation of PAGs within the molecular architecture of the resist as depicted in Scheme 1 [4–8]. This is necessary in order to significantly enhance the sensitivity of the resist. Note that PAG also could function as a dissolution promoter due to its solubility in bases. The microstructure of the resist is shown in Scheme 2. Solubility or compatibility between the photoacid generator (PAG) and the polymer matrix in a CA

Scheme 1. Molecular architecture of nanocomposite resist.

resist is of fundamental importance for several reasons. PAGs play an important role in a CA resist system. Presently several classes of PAGs have been used in CA resists, including ionic PAGs, such as sulfonium and ionium salts, and non-ionic PAGs [9]. These PAGs are almost exclusively used in their monomeric forms. Small molecule PAGs generally have limited solubility or compatibility with the polymer matrix [10], which results in a number of problems such as phase separation in the cast film and non-uniform acid distribution and migration during temperature fluctuations such as during the post exposure bake (PEB). All of these problems can lead to degradation of the lithographic performance of a resist material. To alleviate these problems, this work explores the direct incorporation of photoacid generating groups directly into the resist polymer chain rather than simple addition and mixing of monomeric PAGs into the resist polymers. We hypothesize, based on our promising preliminary results [4,8], that a combination of mechanisms that are enabled by this direct incorporation of PAG units into the polymer make it possible to produce CA resist materials with significantly enhanced sensitivities. First, direct incorporation of the PAG into the polymer chain makes it possible to increase PAG loadings (PAGs$10 wt.%) in the resist to levels that are not possible in conventional resists without encountering the phase separation and solubility problems observed with small molecule PAG additives. Also, the direct incorporation of the PAG into the polymer ensures a more uniform distribution of PAG in the final resist polymer film and this in turn results in a more homogenous acid distribution within the film after exposure. This homogeneity makes the deblocking reactions in the matrix more efficient and less prone to be affected by the detailed

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Scheme 2. Microstructure of the nanocomposite resist (NanoRT-3b).

microstructure of the polymeric resist. We have confirmed this by determining that the acid generation [8] during exposure is independent of the polymer film microstructure when the PAG is chemically bonded to the polymer resist backbone. Finally, because the PAG unit is covalently attached to the resist polymer chain, energy transfer from the polymer to the PAG is more efficient. Since simulation [11] has shown that it is not uncommon for approximately 90% of the deposited electron beam energy to be transferred to the matrix polymer, any mechanism that leads to better energy transfer between the polymer and PAG should dramatically increase the sensitivity of the resist. We believe that a combination of these factors is responsible for the extremely high sensitivities of the resist materials explored in this work. The 1 H-NMR spectrum in Fig. 1 for the NanoRT3b material was used to estimate the polymeric composition for NanoRT-3b. From the NMR spectrum (d 1.4), we inferred that the protecting ratio (i.e. t-BMA) of NanoRT-3b is approximately 47 wt.%. The absorptions between d 7.5 and d 8.5 were used to estimate that the PAG loading in this polymer is approximately 5.0 wt.%. POSS composition was estimated independently by thermogravimetric analysis (TGA). For this experiment, 10 mg of NanoRT-3b was placed in a TGA pan and heated at a rate of 20 8C / min from room temperature to 800 8C in pure O 2 with a gas flow rate of 25 ml / min, and maintained at 800 8C for 30 min. TGA curves were recorded. Weight loss of

the sample over this period was used to calculate the POSS content in the sample, assuming that the residue remaining after burning the polymeric sample in oxygen was pure silicon oxide.

2.6.2. Lithographic evaluation As mentioned earlier in the Experimental section, the main objectives of the lithographic evaluation study of NanoRT-3b were to measure the sensitivity (S) and contrast (gm ) of the material and to estimate the resolution or minimum feature size that can be printed on NanoRT-3b using electron beam lithography. S and gm were measured by generating conventional contrast curves [i.e. normalized remaining thickness (NRT) vs. log(dose)] for the NanoRT3b resist (see Figs. 2–4). The gm was measured by using a linear fit to the contrast curve for normalized remaining thickness values between 0.2 and 0.8. The slope of this linear fit can be assigned as the contrast of the material by analogy to the conventional definition for the resist contrast as shown in Eq. (1) [12].

F S DG

D0 gm 5 log ]] D100

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(1)

Here D0 is the minimum dose at which no resist film remains after development and D100 is the maximum dose that can be used while retaining 100% of the original film after development. The sensitivity (S) value reported has been defined as the electron beam exposure dose required to completely develop the exposed resist film [13–15]. The mea-

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Fig. 1. 1 H-NMR spectrum of nanocomposite resist NanoRT-3b. The characteristic peaks are: t-BMA, 1.42–1.49 ppm t-butyl protons; MMA, |3.60 ppm, methyl ester protons; POSS, |3.80–3.90 ppm, 2-methylene protons adjacent to –OOCR); MA, |10.00 ppm, acid proton; PAG, 7.50 –8.50 ppm (phenyl proton and methylene and methyl protons) were observed.

sured contrast (gm ) and sensitivity (S) for NanoRT3b were found to be 5.4 and 2.0 mC / cm 2 , respectively for low keV (2 keV) exposures on a 140-nm-thick resist film (see Fig. 2). The lower portion of the contrast curve (NRT less than 0.3) for the 2-keV exposures indicates the potential presence of some form of residue of reduced solubility at the bottom of the resist film. Further investigation into the nature of this slower dissolving material is underway. For 20-keV exposures using the same film thickness of approximately 140 nm, the gm was reduced to 1.34 and the S value increased to 6.1 mC / cm 2 (Fig. 3). The decreased sensitivity (or increased S value) is expected in this case since less energy is deposited

into extremely thin resist films (thinner than the penetration depth of the electrons) as the accelerating potential is increased since more electrons deposit their energy into the substrate as opposed to the resist film. Thus, for the same nominal exposure dose, less energy is deposited in the resist at higher accelerating potentials. The contrast of the material can also be degraded through similar means as the accelerating potential is increased. Hence, thicker films of approximately 340 nm were also used for generating contrast curves for 20-keV exposures. It can be seen from Fig. 4 that gm increases in this case up to approximately 8.8 and the S value is reduced to 1.5 mC / cm 2 . Hence the resist sensitivity actually

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Fig. 2. Contrast curve for NanoRT-3b at 2 keV, 5 pA exposure for 2 a 140-nm-thick film. [Sensitivity52.0 mC / cm , contrast (gm )5 5.4].

increases (i.e. S value decreases) as the film thickness is increased from 140 to 340 nm for a 20-keV exposure. Therefore, it can be seen that for a highly

Fig. 3. Contrast curve for NanoRT-3b at 20 keV, 5 pA exposure for a 140-nm-thick film. [Sensitivity56.1 mC / cm 2 , contrast (gm )51.34].

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Fig. 4. Contrast curve for NanoRT-3b at 20 keV, 5 pA exposure for a 340-nm-thick film. [Sensitivity51.5 mC / cm 2 , contrast (gm )58.8].

sensitive resist like NanoRT-3b, the resist contrast and sensitivity are greatly influenced by the film thickness. Thus, it is obvious that a resist film thickness optimization should be performed when adapting an electron beam resist material for exposure at a specific accelerating potential. All the contrast curve experiments indicate that NanoRT-3b is a high contrast, high sensitivity e-beam resist for both high keV and low keV exposures. For comparison purposes, the sensitivity and contrast of poly(methyl methacrylate), referred to as PMMA, when used as an electron beam resist is on the order of 50 mC / cm 2 at 10 keV. Also, ZEP7000A exhibits a sensitivity of 8 mC / cm 2 and a contrast of 1.55 at 10 keV. High-resolution imaging was performed at both low and high electron beam accelerating potentials of 2 and 20 keV, respectively. Electron beam exposures were performed on 130-nm and 392-nm-thick films for 2- and 20-keV electron beam accelerating potentials, respectively. Isolated space patterns were printed for various doses. Fig. 5 shows a top down SEM micrograph of a 500-nm isolated space printed at a dose of 4 mC / cm 2 using a 2-keV accelerating potential. Features significantly smaller than this

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Fig. 5. Top down SEM micrograph of a 500-nm isolated space printed at 2 keV accelerating potential at a dose of 4 mC / cm 2 where the beam spot size was 52 nm.

were difficult to obtain using the low keV exposure due to electron beam spot focusing problems encountered with the SEM during these experiments. Further work on characterizing the ultimate resolution and the feature size as compared to spot size for low voltage electron beam exposure in these systems is currently in progress. Similarly, Fig. 6 shows a 130-nm isolated space pattern that was resolved using a 20-keV accelerating potential with a dose of 20 mC / cm 2 . Detailed investigations of the line edge roughness in these resist systems are also currently in progress. In addition to electron beam exposure, initial exposure tests using X-ray (XRL) sources indicated that the sensitivity of the nanocomposite resist (NanoRT-3b) is 50–100 mJ / cm 2 . This resist sensitivity for X-ray is quite good compared with conventional X-ray resists such as TDUR-N908 (150 mJ / cm 2 ) and UVII-HS (190 mJ / cm 2 ) [13]. Similarly, exposure to extreme ultra violet lithography (EUVL) using 13.2-nm radiation showed an imaging dose on the order of 1.0–1.8 mJ / cm 2 [16,17] and high-resolution patterns approximately 130 nm in size were obtained using simple EUV interferometric exposures. These results, combined with the excellent contrast and sensitivity results obtained in this

Fig. 6. Top down SEM micrograph of a 200-nm isolated space printed at 20 keV accelerating potential at a dose of 20 mC / cm 2 where the beam spot size was 20 nm.

work for electron beam lithography, indicate that the nanocomposite resist design has the potential to deliver extremely high sensitive chemically amplified resist materials that can be used with a variety of advanced exposure methods including electron beam, X-ray, and extreme ultraviolet lithography.

2.6.3. Results of etch studies Table 1 shows the etch rates and selectivities of the various resist polymers using the halogenated silicon dioxide etch. As can be seen, the NanoRT3b material, which is an acrylate polymer, performs worse in terms of etch resistance using the halogenated plasma as compared to the novolac and PHOST aromatic polymers. It is also observed that Table 1 Etch rates and selectivities with respect to silicon dioxide for various resist polymers as measured using a PlasmaTherm RIE with a CHF 3 and O 2 flow of 22.5 and 2.5 SCCM, respectively, chamber pressure of 50 mTorr, and 150 W chuck power Material

Etch rate (nm / min)

Selectivity

Silicon dioxide Novolac PHOST NanoRT3b PMMA

12.1 30.4 36.9 80.8 83.6

– 0.40 0.33 0.15 0.14

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the NanoRT3b material performs only slightly better in terms of etch resistance than conventional PMMA materials, although the POSS loading used in the materials reported in this work are extremely low. It is expected that the etch resistance in halogenated plasmas could be further improved through the incorporation of higher levels of the POSS monomer in the polymer. Further studies are currently in progress to elucidate the relationship between POSS loading and other resist properties including etch resistance. The results of etching the NanoRT3b material in the essentially oxygen plasma are also quite interesting. During the initial 40 s of the oxygen etch, the NanoRT-3b film etched with an almost constant ˚ / s (Fig. 7). However, for the average rate of 38 A next 20 s, the etch rate dropped to an average of only ˚ / s. Upon visual inspection of the film, a glassy 12 A capping layer appeared to form on the film, which was suspected to serve as an etch stop layer to the ashing O 2 /Ar plasma. AFM analysis revealed that the surface roughness of the NanoRT-3b film increases slowly as a function of etch time (Fig. 8). In comparison with the data obtained for the NanoRT3b films, novolac etched at an average rate of 47

Fig. 7. Film thickness removed during etch versus etch time for NanoRT-3b sample using 40:16 sccm O 2 /Ar at 100 mTorr and 200 W total power.

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Fig. 8. RMS surface roughness versus plasma etch time for NanoRT-3b and novolac film samples using 40:16 sccm O 2 /Ar at 100 mTorr and 200 W total power. Obtaining surface roughness values for the novolac samples beyond 20-s etch times was difficult due to large surface roughness and topography, and as such the reported values are most likely underestimating the true roughness for the novolac sampled for etch times beyond 20 s.

A8 / s in a linear fashion over the full 60-s etch period. The surface roughness values obtained using the AFM for the novolac films are also shown in comparison to the NanoRT-3b data in Fig. 8. During the initial 20 s of the etching, the novolac film surface appeared relatively smooth with slight increases in the surface roughness. However, beyond 20 s of etch time, AFM scans of the novolac surface revealed extreme roughness with both significant short and long wavelength roughness features. In many cases, accurate determination of the surface roughness for the novolac films was not possible using the AFM due to extreme non-uniformities in the topography of the film surface. The extreme surface roughness observed under these etch conditions may be due to the relatively high argon content which enhances the sputtering action of the plasma. Nonetheless, under the conditions used, it is observed that the NanoRT-3b film displays comparable or lower surface roughness values than a standard novolac film and behaves in a much more predictable fashion in terms of its etch behavior. This oxygen etch study does point out the possibility that

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the NanoRT3b resist could be used as an organic film etch mask due to its ability to form this glassy etch resistant layer once exposed to the oxygen plasma. XPS analysis of the NanoRT-3b film surface showed increasing concentrations of both silicon and oxygen and decreasing carbon content as compared to the original Nano-RT 3b film surface composition as a function of oxygen plasma etch time (Table 2). This is most likely due to the ability to form volatile carbon containing etch products while the rather stable silicon oxide components in the resist material remain in the film, thus resulting in the enrichment of the film in silicon content. The elemental composition of the cited ‘stoichiometric’ film is based on the nominal stoichiometric ratio of the different elements in the molecular formula for the Nano-RT 3b material (C 73 H 117 O 24 Si 8 S 2 F 3 ).

2.7. Conclusions The lithographic properties of a novel nanocomposite electron beam resist material, which directly incorporates both a photoacid generator and a silicon containing etch resistance group into the polymer main chain have been quantified in this work. It was found that this resist material design can be tuned to possess high lithographic sensitivities (|1 mC / cm 2 ) and high contrasts (.5) for both low and high voltage electron beam exposure. One key requirement in obtaining maximum performance from these and other electron beam resists is that the resist film

thickness must be appropriately matched to the penetration depth of the electrons available at the accelerating potential used during the electron beam exposure. It was observed that in halogenated plasmas the acrylate nanocomposite resist has significantly lower etch resistance than aromatic resist polymers such as novolac, but that it performs marginally better than PMMA. The nanocomposite material used in this work has a relatively low POSS content, and it is believed that increasing the POSS content in the polymer can help to improve the halogenated plasma etch resistance of the material. Under oxygen plasma etch conditions, the nanocomposite resist design displayed etch resistance superior to that of conventional novolac resins. Measurement of the surface roughness of this new resist material under oxygen plasma environments also showed that the film roughness increased approximately linearly with etch time, while novolac samples were observed to roughen more significantly, especially at longer etch times. XPS studies revealed that a silicon rich, glassy capping layer is formed on the surface of the nanocomposite resist during oxygen plasma etching. The formation of this oxygen etch resistant layer may make it possible to use such a nanocomposite resist as an etch mask for organic films. Overall, the flexibility in the design of the nanocomposite resist offers numerous potential advantages for electron beam resist material design and current studies are focused on further elucidating the relationships between polymer structure and lithographic performance.

Table 2 X-Ray photoelectron spectroscopy (XPS) study of atomic composition (at.%) of NanoRT3b film surface as a function of oxygen plasma etching time [only significant atomic components (.1%) are shown] Etch time (s)

C (using C1s)

O (using O1s)

Si (using Si2p)

F (using F1s)

0 (stoichiometric) 0 (actual) 10 20 30 40 50 60

66.4 61.11 46.84 39.53 36.58 29.79 21.71 11.46

21.8 35.95 43.49 47.66 51 54.39 59.13 65.36

7.3 1.12 7.07 9.08 10.22 12.56 15.82 21.36

2.7 1.83 2.6 2.81 1.92 2.46 2.2 1.81

CA resists with PAG and inorganic units in the chain.

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Acknowledgements C.L.H. would like to acknowledge the College of Sciences at the Georgia Institute of Technology for funding of the SEM lithography facilities used in this work. K.E.G. acknowledges NanoResist Technologies Inc., Charlotte.

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