Frustrated Total Internal Reflection Spectra of Diazoquinone–Novolac Photoresist Films

Radiation-induced effects in thin films of diazoquinone–novolac photoresists on silicon irradiated with high-energy electrons (~5 MeV) were investigated by frustrated total internal reflection (FTIR) IR Fourier spectroscopy. It was found that irradiation with electrons at a dose of more than 3·1015 cm–2 leads to decrease of the integral absorption at wave numbers in the region of 3700–400 cm–1. The intensity of bands due to the –O–H– and particularly aliphatic –C–H– bonds is increased most strongly during electron irradiation. At a dose of Φ = 1·1017 cm–2 the intensity of the bands due to the methylene (–CH2–) and methyl (–CH3) groups is comparable with the noise level. The intensity of the band at ~1600 cm–1, due to the vibrations of the aromatic ring, does not change at radiation doses in the range of 3·1014–1·1017 cm–2. The obtained experimental data indicate strong cross-linking of the polymeric components during irradiation with electrons. It was found that the radiation-induced changes in the FTIR spectra depend on the type of photoresist (FP9120 and S1813 G2 SP15). The experimental relationships of change in the optical characteristics of thin films of photoresists due to electrons are explained in terms of the radiation chemistry of diazoquinone–novolac resins.

The FTIR spectra of the structures of the silicon photoresist in the region of 400-4000 cm -1 were recorded at room temperature on an ALPHA IR Fourier spectrophotometer (Bruker Optik GmbH). The resolution was 2 cm -1 , and the number of scans was 24. Background correction was applied before each experiment [9]. The photoresist fi lms were not kept for more than two months before the FTIR spectra were recorded.
Results and Discussion. The FTIR spectra of the initial (unirradiated) FP9120 and S1813 G2 SP15 fi lms are similar ( Fig. 1) although they do have a number of differences. Thus, there is no band with a maximum at ~2960 cm -1 (Fig. 2a, curve 1) due to the asymmetric stretching vibrations of the methyl -CH 3 group in the region of the stretching vibrations of the -C-H bonds (the region of 2700-3100 cm -1 ) [10,11]. For the same photoresist the bands due to the skeletal vibrations of the aromatic ring (a band with a maximum at ~1600 cm -1 [12] ) and the stretching vibrations of the ArC-C bond (a band with a maximum at ~1500 cm -1 [10]) have lower intensity (Fig. 2b). The reason for these special features of the FTIR spectra of the S1813 G2 SP15 photoresist is the fact that xylene isomers (dimethylbenzene) are not used as solvents in the preparation of the photoresist. In addition, the absence of the band due to the asymmetric stretching vibrations of the -CH 3 methyl group in the spectrum of S1813 G2 SP15 indicates the absence of cresol (methylphenol) residues in this type of photoresist. Another feature of the S1813 G2 SP15 photoresist is the absence of residual formaldehyde. This is demonstrated by the extremely low (at the noise level) intensity of the band with a maximum at ~1650 cm -1 (Fig. 2b, curve 1) due to the C=O vibrations in formaldehyde [12].
Differences in the two types of photoresists are also seen in the region of 1150-1240 cm -1 , where the planar deformation vibrations of the C-O bonds of phenols are observed (Fig. 2b). The structure of the absorption band of these vibrations in the S1813 G2 SP15 photoresist differs substantially from the structure of the analogous band in FP9120. Thus, a band with one maximum at ~1170 cm -1 is observed for S1813 G2 SP15, and a band with three maxima of approximately equal intensity at 1150, 1170, and 1198 cm -1 is observed for FP9120. In addition, a band with a maximum at ~1375 cm -1 (curve 1), corresponding to the deformation planar vibrations of O-H bonds in the phenyl group, was observed in the absorption spectra of the S1813 G2 SP15 photoresist [13]. At the same time this band is absent in the spectra of the FP9120 samples, but a strong band with a maximum at ~1360 cm -1 , assigned to the C-O-C vibrations in furan, appears [12]. A band at 1100 cm -1 , the intensity of which is substantially higher for the photoresist FP9120 than for S1813 G2 SP15, also belongs to the C-O-C deformation vibrations (Fig. 2b). In addition, the spectrum of the photoresist S1813 G2 SP15 also contains a band at 1040 cm -1 usually associated with vibrations of the C-O bond in the CH 2 -C-OH fragment [12]. The experimental data show that the intensity of the bands from the phenol group is higher for the S1813 G2 SP15 photoresist.
The observed features of the S1813 G2 SP15 photoresist are most likely caused by the following reasons. Theformation of intra-and intermolecular hydrogen bonds through the phenolic hydroxyl group is observed in phenolformaldehyde resins [3]. The presence of such bonds leads to a shift of the frequency of the vibrations of the C-O in the phenol group and to the appearance of several bands in the region of 1150-1230 cm -1 . Cross-linking of the resin molecules through the oxygen atom of the phenolic group with the formation of C-O-C fragments can be observed under certain conditions during the synthesis of the polymeric composite of the photoresist. They occur in the FP9120 photoresist and are poorly defi ned in S1813 G2 SP15.
Irradiation with electrons leads to decrease of the integral (background) absorption in both photoresists ( Fig. 1). We note that the radiation-induced changes in the FTIR spectra are more pronounced in FP9120 than in S1813 G2 SP15. The decrease of the background absorption is stronger in FP9120 (Fig. 1). The changes in the intensity of the FTIR bands in FP9120 were observed even at dose Φ ~ 3·10 15 cm -2 (Fig 3), whereas they are only observed at Φ ≥ 3·10 16 cm -2 in S1813 G2 SP15.
The intensity of the bands from vibrations of O-H and particularly C-H bonds decrease most strongly during electron irradiation (Fig. 2a). With a dose of Φ = 1·10 17 cm -2 the bands due to methylene (-CH 2 -) and methyl (-CH 3 ) groups (2750-3100 cm -1 ) in Fig. 2a are not visible in the spectra of FP9120, while in S1813 G2 SP15 their intensity is at the noise level (curve 3). The intensity of the band in the region of 3100-3700 cm -1 , due to the stretching vibrations of the O-H bonds, decreases substantially (by 3-5 times) during irradiation (curves 3 and 4). It should be noted that during irradiation there is a strong decrease in the intensity of the band at 1375 cm -1 caused by in-plane deformation vibrations of the O-H bonds in the C-O-H phenol fragment and mostly observed for the S1813 G2 SP15 photoresist (Fig. 4). This band disappears almost completely in the FTIR spectrum of S1813 G2 SP15 at Φ ≥ 3·10 16 cm -2 . This also affects the bands in the region of 630-900 cm -1 corresponding to out-of-plane vibrations of the C-H bonds in aromatic compounds [13]. They are not observed in the FTIR spectra at Φ > 1·10 16 cm -1 (Fig. 4).
Absorption bands not associated with hydrogen are more resistant to electron irradiation. This applies particularly to the band at ~1600 cm -1 , corresponding to skeletal vibrations of the aromatic ring [10]. Its intensity decreases weakly even at   Fig. 3. The FTIR spectra of fi lms of FP9120 photoresist irradiated with electrons at doses of 0 (1), 3·10 15 (2), 3·10 16 (3), and 10 17 cm -1 (4). Fig. 4. The FTIR spectra of the initial fi lm of S1813 G2 SP15 photoresist (1) and of the fi lm irradiated with electrons at a dose of 1·10 17 cm -1 (2). a dose of 1·10 17 cm -2 (Fig. 4). The intensity of the band with a maximum at ~1430 cm -1 , due to vibrations of a benzene ring connected to a CH 2 bridge, decreases quite strongly [11,13]. The reason for this is the superimposition of a weak band for the deformation vibrations of the C-H bonds in formaldehyde [12].
In the region of the stretching vibrations of the multiple bonds the intensity of the band at ~1700 cm -1 decreases during irradiation with simultaneous broadening and a shift of ~30 cm -1 into the high-energy region of the spectrum. As known [14,15], the band at ~1700 cm -1 corresponds to the stretching vibrations of the C=O bonds in aromatic compounds. With change of the nearest environment the position of the maximum of this band can be shifted both into the low-energy and into the high-energy regions. Thus, during conjugation with C=C bonds the maximum of the band for these vibrations is shifted into the low-energy region [10]. According to [16], structures for which the maximum of the absorption band must be in the region of 1645-1690 cm -1 [10], depending on the number of conjugated bonds, are formed during implantation of the FP9120 photoresist. In addition, in aromatic esters the maximum of the band for the stretching vibrations of the C=O bonds is shifted into the high-energy region at 1715-1736 cm -1 [10]. The formation of aromatic esters during irradiation with electrons is probably the main reason for shift of the maximum of this band. Change of the nearest environment of the C=O bonds arising as a result of irradiation leads to broadening of the band at ~1700 cm -1 as a result of the multifaceted action of the abovementioned factors. The band with a maximum at ~1650 cm -1 , observed for the FP9120 photoresist and due to the stretching vibrations of the C=O bonds in formaldehyde CH 2 =O, also remains stable during irradiation with electrons (Fig. 2b).
The broad structured absorption band with several maxima in the region of 1100-1230 cm -1 after irradiation of the photoresist becomes smooth, maxima do not appear, and its intensity decreases (Fig. 4). This band is due, in particular, to the stretching vibrations of the C-O bonds in the phenoxyl group (C-OH) [9]. Such behavior of the FTIR bands in the region of 1050-1230 cm -1 may indicate the formation of ester linkages with OH groups of phenolic type. Such cross linking was observed earlier during implantation of Sb + in fi lms of photoresist FP9120, leading to increase of their microhardness [17]. Earlier [18] hardening of FP9120 fi lms was observed during high-energy irradiation with γ quanta. A similar effect can also occur in the oxidation of phenols to quinone methines, as a result of which the C-OH group is converted into C=O.
The intensity of the band with a maximum at ~1500 cm -1 , due to the stretching vibrations of aromatic C-C bonds [10], decreases appreciably during irradiation with electrons at doses of >1·10 16 cm -2 , and the decrease is greater in S1813 G2 SP15 (Figs. 2b and 4). This may be due to splitting of substituents from the aromatic ring or to their signifi cant modifi cation during irradiation of the polymer with electrons.
The changes in fi lms of photoresists discovered by FTIR IR spectrometry are caused by the following set of radiationchemical processes. Absorption of the energy of electron irradiation occurs uniformly throughout the photoresist fi lm unlike ion implantation, where thermalization of ions is observed exclusively in a surface layer 60-150 nm deep [8,9,11,16]. Ionization and electron excitation of the macromolecules occur as a result of interaction of the high-energy electrons with the phenolformaldehyde resin. Subsequent decay of the excited particles and ion-molecular processes will lead to the formation of predominantly oxygen-centered radicals of the phenoxyl type (I) and also carbon-centered radicals of the methylene (II) and benzyl (III) types [9]. The latter radicals are only formed when there are arylmethyl fragments in the polymer as, for example, in the FP9120 photoresist.
Because of delocalization in the aromatic system the radicals (I-III) exhibit thermodynamic stability and are mostly consumed in recombination reactions that lead to the formation of new C-C or C-O bonds (in reaction 1, for example) between the polymer chains. With somewhat lower probability the radicals (I-III) will enter into disproportionation reaction (2), as a result of which the quinonemethine fragments are formed: (1) Special features of the radiation chemistry of aromatic compounds are high radiation-chemical yields of particles in an electron-excited state and, as a consequence, their important role in the formation of the fi nal molecular products of radiolysis. The accumulation of quinonemethine structures during irradiation of phenolformaldehyde resins can then also occur in molecular processes involving the electron-excited states of macromolecules and, in particular, as a result of dehydration (3) and dehydrogenation (4) reactions: (2) Thus, the decrease in the intensity of the IR absorption bands in the region of ~2800-3000 cm -1 , corresponding to the vibrations of C-H bonds in methylene (-CH 2 -) and methyl (-CH 3 ) groups, during irradiation of DQN photoresists arises as a result of the realization of biradical recombination (1) and disproportionation (2) reactions and also of the molecular processes of decay of the particles (3) and (4). Realization of reactions (2)-(4) also leads to the formation of quinonemethine structures, which shows up as decrease of the signal from phenolic -OH groups in the region of ~3100-3600 and also as increase of the intensity of absorption for wave numbers characteristic of >C=O groups conjugated with multiple carboncarbon bonds. It is also necessary to note that the decrease in the amount of phenolic OH groups in the DQN photoresist during electron irradiation can also arise as a result of reactions involving the photosensitive component of the composite. Transfer of electron excitation from the macromolecules of phenolformaldehyde resin to the ortho-naphthoquinone diazide leads to its deazotization [9] by reaction (5) with the formation of the highly reactive ketene (IV): This compound is capable not only of adding water present in the DQN photoresist with the formation of indene-3carboxylic acid but also of converting the phenols into esters according to reaction (6): (5) We will now consider the band found for the photoresist FP9120 with a maximum at ~1650 cm -1 due to the stretching vibrations of the C=O bonds in formaldehyde CH 2 =O. Together with phenol the CH 2 =O is used in the synthesis of the phenolformaldehyde resin and could remain entirely in the polymer as a chemical impurity. However, formaldehyde readily accepts hydrogen atoms and electrons in reaction (7) and like other low-molecular aldehydes can add carbon-centered radicals [19]. The CH 2 =O must therefore quickly disappear from the phenolformaldehyde resin under the conditions of irradiation with electrons, being converted predominantly into new hydroxymethyl groups according, for example, to reaction (8).
The fact that the concentration of formaldehyde changes little from the absorbed dose indicates the realization of processes that lead to its formation in the course of irradiation of the FP9120 photoresist fi lm with electrons. The most likely mechanism of elimination of formaldehyde may be β-fragmentation of the oxygen-centered radical (V) in reaction (9): (9) Simultaneous realization of reactions leading to consumption (7,8) and formation (9) of CH 2 =O leads to the appearance of constancy in its concentration in the FP9120 photoresist fi lm under the conditions of irradiation.
Thus, the lower radiation stability of the FP9120 photoresist compared with S1813 G2 SP15 is due to the presence of methyl groups in the composition of the polymer base of FP9120 and also to the larger content of hydroxymethyl fragments, and this must be taken into account during ion implantation of monocrystalline silicon using photoresist masks.
Conclusions. Analysis of the frustrated total internal refl ection spectra indicates that DQN photoresists FP9120 and S1813 G2 SP15 are almost identical in composition. The lower content of cresol fragments and residual formaldehyde must be included among the special features of the S1813 G2 SP15 photoresist. A decrease of integral absorption in the region of 3700-400 cm -1 is observed when the diazoquinone-novolac photoresists S1813 G2 SP15 and FP9120 are irradiated with electrons at doses of >3·10 15 cm -2 . The radiation-induced changes in the FTIR spectra are more pronounced in the FP9120 photoresist compared with S1813 G2 SP15. The intensity of the bands due to vibrations of the hydroxyl (-O-H), methyl (-CH 3 ), and methylene (-CH 2 -) groups are reduced most strongly during irradiation. The intensity of the band at ~1600 cm -1 , due to the stretching vibrations of the aromatic ring, remains practically unchanged right up to the highest electron dose Φ =1·10 17 cm -1 . The formation of cross links between the macromolecules of the phenolformaldehyde resin with the formation of new C-O-C fragments was observed in the FP9120 photoresist. It was shown that the concentration of formaldehyde in the FP9120 photoresist as a result of superimposition of the formation and consumption processes does not change with increase of the absorbed dose. An explanation for the obtained experimental relationships with regard to the characteristics of the radiation chemistry of diazoquinone-novolac resins is presented.