An Efficient Two-Photon-Generated Photoacid Applied to Positive-Tone 3D Microfabrication

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Science  10 May 2002:
Vol. 296, Issue 5570, pp. 1106-1109
DOI: 10.1126/science.296.5570.1106


A two-photon–activatable photoacid generator, based on a bis[(diarylamino) styryl]benzene core with covalently attached sulfonium moieties, has been synthesized. The photoacid generator has both a large two-photon absorption cross section (δ = 690 × 10−50centimeter4 second per photon) and a high quantum yield for the photochemical generation of acid (φH + = 0.5). Under near-infrared laser irradiation, the molecule produces acid after two-photon excitation and initiates the polymerization of epoxides at an incident intensity that is one to two orders of magnitude lower than that needed for conventional ultraviolet-sensitive initiators. This photoacid generator was used in conjunction with a positive-tone chemically amplified resist for the fabrication of a three-dimensional (3D) microchannel structure.

The process of two-photon absorption (TPA) is currently being examined for a variety of applications, including three-dimensional (3D) microfabrication (1–4), ultra-high-density optical data storage (5), biological imaging (6), and the controlled release of biologically relevant species (7). Conventional ultraviolet (UV)–sensitive photoacid generators (PAGs), such as diaryliodonium and triarylsulfonium cations, have been used for two-photon microfabrication (8). However, the TPA cross sections, δ, of these initiators are typically very small (δ ≤ 10 × 10−50 cm4 s per photon), and as a result they exhibit low two-photon sensitivity. Resins containing these initiators can be patterned only by means of long exposure times and high excitation intensities that frequently result in damage to the structure. Higher reliability and faster write speeds would be facilitated by molecules offering a large product of δ and the photochemical quantum yield φH +. The design of highly photosensitive two-photon initiators requires (i) a chromophoric group with a large δ, such as two electron donors bridged by a π-conjugated system (a D-π-D structure) (9, 10); (ii) a functionality that can generate the initiating species with high efficiency, such as those in UV initiators; and (iii) a mechanism through which excitation of the chromophore leads to activation of the chemical functionality, such as an electron transfer process (2). Here we describe the development of a high-sensitivity two-photon PAG and the application of it in 3D microfabrication by using a chemically amplified positive-tone resist.

We targeted PAGs because they can be used to activate a wide variety of reactions, including the ring opening of epoxides and oxetanes and cleavage reactions commonly used in chemically amplified positive resists (11, 12). These resists can be particularly useful for high-fidelity 3D microfabrication because, relative to acrylate-based resins, they exhibit much lower shrinkage in both the exposure and development processes (13). Most large-δ molecules have excited states of relatively low energy because of their extended conjugation. Thus, molecules of this class typically have insufficient excited-state energy to activate direct bond cleavage after photoexcitation. We focused on the use of photoinduced electron transfer from TPA dyes to covalently linked sulfonium groups (14, 15). From the electrochemical and spectroscopic measurements of Saeva (16, 17) and our own studies (9, 10), we determined that two-photon–excited bis(diphenylamino)-substitutedbis(styryl)benzene dyes (with δ ∼ 800  × 10−50 cm4 s per photon) should have ample reducing power to transfer an electron to the S–C σ* orbital of a dimethylaryl sulfonium cation, which would cleave the S–C bond and generate acid. We note that triarylamine dialkylsulfonium salts are photosensitive in the near-UV spectrum and generate protons with a photochemical quantum efficiency of φH + ≈ 0.50 (18).

We also targeted sulfonium groups covalently linked tobis(styryl)benzene dyes as two-photon PAGs because (i) covalent attachment of the TPA donor and the sulfonium acceptor helps to ensure rapid forward electron transfer; (ii) triarylamine radical cations are known to be very stable and therefore should facilitate efficient bond cleavage (18); (iii) protonated triarylamino groups are strong acids (Ph3N+H, pK a = –5) (19), so the amine functionality should not inhibit reactions such as the ring-opening polymerization of epoxides; and (iv) non-nucleophilic anions such as PF6 , AsF6 , or SbF6 can be incorporated into these systems.

The compound BSB-S2 (Fig. 1) was synthesized as shown in fig. S1 and was characterized by spectroscopic and analytical methods (20). Solutions of BSB-S2 in acetonitrile become acidic after irradiation into the lowest energy absorption band [wavelength of maximum absorption (λmax) = 392 nm, extinction coefficient at λmax (ɛ 392) = 5.5 × 104 M−1 cm−1]. The quantum efficiency for proton generation was determined to be φH + = 0.50 ± 0.05 (21). This value is comparable to or greater than that of most commercial UV-sensitive PAGs (22). It is also consistent with the excited state of the bis[(diarylamino)styryl]benzene core being quenched efficiently by transfer of an electron to the dimethylsulfonium group, as evidenced by the low fluorescence quantum efficiency, φf = 0.013 ± 0.001. The acid photogenerated by BSB-S2 efficiently initiates the polymerization of epoxides (23).

Figure 1

Structures of BSB-S2 and the commercially available PAGs used in this study: DPI-DMAS, diphenyliodonium 9,10-dimethoxyanthracenesulfonate; ITX, isopropylthioxanthone; CD1012, [4-[(2-hydroxytetradecyl)oxy]phenyl]phenyliodonium hexafluoroantimonate (Sartomer); and TPS, triphenylsulfonium hexafluoroantimonate.

The two-photon excitation (TPE) spectrum of Fig. 2A shows that BSB-S2 exhibits strong TPA from 705 to 850 nm (δ > 100 × 10−50cm4 s per photon) that appears to peak near 710 nm (δ = 690 × 10−50 cm4 s per photon) and is consistent with measurements obtained for similarbis[(diarylamino)styryl]benzenes (10). The TPE and acid yield efficiency spectra (Fig. 2A) exhibit similar features and the acid yield at 745 nm increases quadratically with excitation power (Fig. 2B), as expected for a photochemical process activated by TPA.

Figure 2

(A) One-photon absorption (solid line), TPE (□), and relative two-photon acid yield efficiency (○) spectra of BSB-S2 in acetonitrile. The TPE and acid yield spectra are plotted versus half of the excitation wavelength. The one-photon absorption spectrum was recorded using a 2.2 × 10−5 M solution. For the TPE and acid yield spectra, 4.0 × 10−4 M solutions were irradiated in 1-cm path-length fluorescence cells using focused (f= 75 mm) 80-fs pulses from a CW mode-locked Ti:sapphire laser (82 MHz, 8.5-mm-diameter spot size at lens). The TPE spectrum was recorded using the two-photon fluorescence method (9, 29), with fluorescein in water (1.54 × 10−5 M, pH = 11) as a reference. The up-converted fluorescence was detected at 520 nm, and its intensity was proportional to the square of the excitation power, as expected for TPA. The concentration of the photogenerated acid, [H+], was determined spectrophotometrically after a 30-min exposure. The relative acid yield efficiency at each wavelength was calculated by dividing [H+] by the ratio of the emission counts and the TPE action cross section of the fluorescein reference (<F(t)>/φfδ)ref(to account for the temporal and spatial dependence of the excitation intensity as a function of wavelength) (29) and normalizing to δ of BSB-S2 at 705 nm. For both experiments, the concentration of BSB-S2 decreased by less than 2.5% after exposure. (B) Plot of log[H+] against log[excitation power] at 745 nm. The smooth curve is the best fit of a line to the data and has a slope of 2.3, which indicates that the acid yield increases quadratically with the excitation intensity. {/ANNT;3168n;;109296n;60379n}

There are few reports of two-photon–initiated cationic polymerization of epoxides (8, 24), and each of these involves the use of UV-sensitive PAGs that have very low two-photon sensitivity. We compared the epoxide polymerization sensitivity of BSB-S2 under near-infrared excitation with four widely used PAGs (Fig. 1): CD1012, CD1012/ITX (1:1.6 molar ratio), TPS, and DPI-DMAS (20). The two-photon sensitivity of BSB-S2 is nearly one order of magnitude greater than that of the best-performing conventional initiator, CD1012/ITX, and more than two orders of magnitude greater than that of TPS, a commercial initiator commonly used in industry. Indeed, we found that BSB-S2 is a high-sensitivity initiator for cationic polymerization of various epoxide monomers under diverse two-photon excitation conditions utilizing nanosecond or femtosecond pulses (25). It is also possible to fabricate freestanding microstructures using solid resin films of BSB-S2 in the cross-linkable epoxide Epon SU-8 (26).

The epoxide-based resins are “negative-tone” systems, in which the exposed regions of the resin become insoluble in a post-exposure development solvent. The polymer structure is then a replica of the exposure pattern. In contrast, positive-tone resists are solid-state systems for which the exposed regions become soluble in a developer. The final structure is then the complement of the exposure pattern. The use of a positive-tone resist in 3D microfabrication is desirable for a number of applications. For example, it should be simpler and more efficient to construct microfluidic devices by excavating a positive-tone material, rather than building up the walls of each microchannel in the structure using a negative-tone resist. In addition, positive-tone resists should help minimize the shrinkage and distortion typically encountered in the photoprocessing of acrylates and other negative-tone resists.

Accordingly, we have developed a solid-state, acid-sensitive, positive-tone, chemically amplified resist, designed for 3D microfabrication by two-photon laser excitation using BSB-S2 as the PAG. The resist (Fig. 3) is a random copolymer of tetrahydropyranyl methacrylate (THPMA) and methyl methacrylate (MMA). In the presence of acid, an ester cleavage reaction converts the THP-ester groups to carboxylic acid groups and renders the exposed material soluble in aqueous base developer. The MMA groups provide strength and optical clarity. The carboxylic acid groups generated after exposure and development provide a chemically active site for surface functionalization. This feature is especially useful for the fabrication of microstructures for microfluidic and bioanalytical applications.

Figure 3

A 3D microchannel structure fabricated by two-photon exposure of BSB-S2 in THPMA-MMA. A 50-μm-thick film of 4 wt % BSB-S2 in THPMA-MMA was exposed in the pattern of the target structure at 745 nm with tightly focused (1.4 N.A.) 80-fs pulses from a Ti:sapphire laser (82-MHz repetition rate) at an average power of 40 μW and a linear scan speed of 50 μm/s. After irradiation, the film was baked for 1 min at 90°C. The target structure was then obtained by dissolving the exposed resin in aqueous 0.26 M tetramethylammonium hydroxide. (A) Target structure consisting of two rectangular cavities (width, 100 μm; length, 20 μm; depth, 20 μm) with a sloped side wall that are connected by 12 channels (length, 50 μm; cross section, 4 μm by 4 μm) lying 10 μm below the surface and spaced apart by 8 μm (center to center). (B) Scanning electron micrograph of the final structure, viewed normal to the substrate. (C toE) Two-photon fluorescence images of the final structure (viewed normal to the substrate) (C) at the surface of the film, (D) 10 μm below the surface, and (E) 19 μm below the surface. (F) Two-photon fluorescence cross-sectional image of the buried channels. The scale bar in (B) through (F) corresponds to 20 μm.

To illustrate the use of BSB-S2 and THPMA-MMA for positive-tone microfabrication, we produced the microchannel structure shown in Fig. 3 using a one-step exposure. The scanning electron micrograph (Fig. 3B) shows that the openings of the channels lie below the film surface and are open to the larger rectangular cavities that extend to the film surface at either end. The two-photon fluorescence images (Fig. 3, C through F), taken after removing the exposed material, reveal that the channels are open and make a continuous connection between the cavities. We have also shown that these channels can be back-filled with liquids, including liquid crystals. This demonstration illustrates that the BSB-S2/THPMA-MMA material system can be used to pattern complex 3D structures and suggests that microfluidic structures and even microoptical structures, such as gratings, waveguides, and photonic lattices, can be fabricated readily. The low irradiation power used to pattern the structure also demonstrates the high two-photon sensitivity of this microfabrication material system.

  • * To whom correspondence should be addressed. E-mail: cober{at} (C.K.O.); jwperry{at}; smarder{at} (S.R.M.).


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