On-Wafer Spectrofluorometric Method for Determination of Relative

However, at this loading, the dye is not a passive probe in the actual resist ..... 17 and setting ρΛ1 = ρΛ2 = 0 we obtain Equations 18 were simul...
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Anal. Chem. 2001, 73, 3472-3480

On-Wafer Spectrofluorometric Method for Determination of Relative Quantum Yields of Photoacid Generation in Chemically Amplified Resists Gilbert D. Feke,†,‡ Robert D. Grober,*,† Gerd Pohlers,§ Kathryn Moore,§,| and James F. Cameron§

Department of Applied Physics, Yale University, New Haven, Connecticut 06520-8284, and Microelectronic Materials Research and Development Laboratories, Shipley Company, Marlborough, Massachusetts 01752

Chemically amplified resists (CARs) that employ acid catalysts are widely used throughout the semiconductor industry due to the need for high throughput in the lithography process. The quantum yield of the particular photoacid generator (PAG) used to generate a given acid ultimately limits the photospeed of the CAR. Determination of quantum yields of photoacid generation is therefore an important component of resist design. We report the development of an on-wafer spectrofluorometric technique for this purpose. This technique is based on one first reported by Feke et al. (J. Vac. Sci. Technol. 2000, B18, 136-139), which involves doping the resist formulations containing the candidate PAGs with a fluorescent pH indicator dye, coating one wafer per PAG, patterning the wafers with a dose ramp, and spectroscopically imaging the wafers. The response curve of each PAG is spatially and spectrally encoded in the fluorescence images of each wafer. We investigate the efficacy of coumarin 6, a dye that was introduced as an acid sensor by Pohlers et al. (Chem. Mater. 1997, 9, 3222-3230) for this application. We further apply this technique to the determination of the quantum yield of photoacid generation of four candidate PAGs for prototype 193-nm CARs. This technique is convenient, fast, robust, and nondestructive. The success of the chemically amplified resist (CAR) scheme1-7 for production lithography is due to the decoupling of the exposure step and the resist chemistry, i.e., the creation of differential dissolution rate which defines the ultimate pattern. Specifically, * Corresponding author: (e-mail) [email protected]; (fax) (203) 4324283. † Yale University. ‡ Present address: JDS Uniphase Corp., Bloomfield, CT 06002. § Shipley Co. | Present address: Dept. of Chemical Engineering, University of Ottawa, Ottawa, Canada K1N 6N5. (1) Lamola, A. A.; Szmanda, C. R.; Thackeray, J. W. Solid State Technol. 1991, 34, 53-60. (2) Reichmanis, E.; Houlihan, F. M.; Nalamasu, O.; Neenan, T. X. Chem. Mater. 1991, 3, 394-407. (3) MacDonald, S. A.; Willson, C. G.; Fre´chet, J. M. J. Acc. Chem. Res. 1994, 27, 151-158. (4) Seeger, D. Solid State Technol. 1997, 40, 115-116, 118, 121. (5) Ito, H. J. Photopolym. Sci. Technol. 1998, 11, 379-393. (6) Ito, H. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3678, 2-12. (7) Ito, H. IBM J. Res. Dev. 2000, 44, 119-130.

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this decoupling enables photospeed enhancement by the thermally activated catalysis of solubility altering reactions in the resist. The catalyst is typically a strong Brønsted acid produced by the photolytic decomposition of a photoacid generator (PAG).8 The catalytic chain length, a measure of the number of reactions catalyzed per acid molecule, is a function of the particular acid used. The overall photospeed is a function of the catalytic chain length and the quantum yield of the PAG used to generate that acid. Because the PAG is a key component of CARs, evaluation of the quantum yields of photoacid generation of PAGs is an important component of resist design. The quantum yield of photoacid generation in a CAR is given by

Φgen ≡ [H+]/[γ]

(1)

where [H+] is the molar concentration of photogenerated acid molecules (molar hydrogen ion concentration) and [γ] is the molar concentration of absorbed photons. Similarly, the quantum yield of PAG decomposition is given by

Φdecomp ≡ [DPAG]/[γ]

(2)

where [DPAG] is the molar concentration of photolytically decomposed PAG molecules. The determination of quantum yields of photoacid generation in CARs is achieved by measuring the acid concentration generated in a resist as a function of exposure dose. The acid concentration is given by

[H+] ) ζ[DPAG] ) ζ([PAG]0 - [PAG])

(3)

ζ ≡ Φgen/Φdecomp

(4)

[PAG] ) F(E)

(5)

where

and

(8) Pappas, S. P. J. Imaging Technol. 1985, 11, 146-157. 10.1021/ac0015319 CCC: $20.00

© 2001 American Chemical Society Published on Web 06/07/2001

where F(E) is some function of exposure dose E, and

[PAG]0 ) F(0)

(6)

Acid quantification is typically accomplished by spectrophotometric or spectrofluorometric titration of pH indicator dyes.9 Many indicators have been used for acid quantification of CARs, and many factors have influenced the choice of indicator in these studies.10-37 One factor is that the range of acid concentrations for which a given indicator is sensitive (i.e., exhibits a significant change of absorption or fluorescence with respect to acid concentration) is generally restricted to a few orders of magnitude (9) Polster, J.; Lachmann, H. Spectrometric Titrations; VCH: New York, 1989. (10) Cameron, J. F.; Fradkin, L.; Moore, K.; Pohlers, G. Proc. SPIE-Int. Soc. Opt. Eng. 2000, 3999, 190-203. (11) McKean, D. R.; Allen, R. D.; Kasai, P. H.; Schaedeli, U. P.; MacDonald, S. A. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1672, 94-103. (12) Buhr, G.; Dammel, R.; Lindley, C. R. Polym. Mater. Sci. Eng. 1989, 61, 269-277. (13) Thackeray, J. W.; Denison, M. D.; Fedynyshyn, T. H.; Kang, D.; Sinta, R. F. ACS Symp. Ser. 1995, No. 614, 110-123 (Microelectronics Technology). (14) McKean, D. R.; Schaedeli, U. P.; MacDonald, S. A. Polym. Mater. Sci. Eng. 1989, 60, 45-48. (15) McKean, D. R.; Schaedeli, U. P.; MacDonald, S. A. ACS Symp. Ser. 1989, No. 412, 27-38 (Polymers in Microlithography). (16) McKean, D. R.; Schaedeli, U. P.; MacDonald, S. A. J. Polym. Sci. 1989, A27, 3927-3935. (17) Paniez, P. J.; Demattei, D. C.; Abadie, M. J. M. Microelectron. Eng. 1992, 17, 279-282. (18) Fedynyshyn, T. H.; Thackeray, J. W.; Georger, J. H.; Denison, M. D. J. Vac. Sci. Technol. 1994, B12, 3888-3894. (19) Asakawa, K.; Ushirogouchi, T.; Nakase, M. Proc. SPIE-Int. Soc. Opt. Eng. 1995, 2438, 563-570. (20) Kihara, N.; Saito, S.; Ushirogouchi, T.; Nakase, M. J. Photopolym. Sci. Technol. 1995, 8, 561-570. (21) Cameron, J. F.; Orellana, A. J.; Rajaratnam, M. M.; Sinta, R. F. Proc. SPIEInt. Soc. Opt. Eng. 1996, 2724, 261-272. (22) Cameron, J. F.; Adams, T.; Orellana, A. J.; Rajaratnam, M. M.; Sinta, R. F. Proc. SPIE-Int. Soc. Opt. Eng. 1997, 3049, 473-484. (23) Itani, T.; Yoshino, H.; Hashimoto, S.; Yamana, M.; Samoto, N.; Kasama, K. J. Vac. Sci. Technol. 1996, B14, 4226-4228. (24) Dentinger, P. M.; Nelson, C. M.; Rhyner, S. J.; Taylor, J. W.; Fedynyshyn, T. H.; Cronin, M. F. J. Vac. Sci. Technol. 1996, B14, 4239-4245. (25) Pohlers, G.; Virdee, S.; Scaiano, J. C.; Sinta, R. F. Chem. Mater. 1996, 8, 2654-2658. (26) Pohlers, G.; Scaiano, J. C.; Sinta, R. F.; Brainard, R.; Pai, D. Chem. Mater. 1997, 9, 1353-1361. (27) Cameron, J. F.; Ablaza, S. L.; Xu, G.; Yueh, W. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3678, 785-799. (28) Eckert, A. R.; Moreau, W. M. Proc. SPIE-Int. Soc. Opt. Eng. 1997, 3049, 879-887. (29) Cameron, J. F.; Kang, D.; King, M.; Mori, J. M.; Virdee, S.; Zydowsky, T.; Sinta, R. F. Proc. 11th International Conference on Photopolymers: Principles, Processes, and Materials; Society of Plastics Engineers, Inc., Mid-Hudson Section, 1997; pp 120-139. (30) Pohlers, G.; Scaiano, J. C.; Sinta, R. F. Chem. Mater. 1997, 9, 3222-3230. (31) Cameron, J. F.; Mori, J.; Zydowsky, T. M.; Kang, D.; Sinta, R. F.; King, M.; Scaiano, J. C.; Pohlers, G.; Virdee, S.; Connolly, T. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3333, 680-691. (32) Coenjarts, C.; Cameron, J. F.; Deschamps, N.; Hambly, D.; Pohlers, G.; Scaiano, J. C.; Sinta, R.; Virdee, S.; Zampini, A. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3678, 1062-1073. (33) Okoroanyanwu, U.; Byers, J.; Cao, T.; Webber, S. E.; Willson, C. G. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3333, 747-757. (34) Bukofsky, S. J.; Feke, G. D.; Wu, Q.; Grober, R. D.; Dentinger, P. M.; Taylor, J. W. Appl. Phys. Lett. 1998, 73, 408-410. (35) Dentinger, P. M.; Lu, B.; Taylor, J. W.; Bukofsky, S. J.; Feke, G. D.; Hessman, D.; Grober, R. D. J. Vac. Sci. Technol. 1998, B16, 3767-3772. (36) Lu, B., Dentinger, P. M.; Taylor, J. W.; Feke, G. D.; Hessman, D.; Wu, Q.; Grober, R. D. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3676, 466-472. (37) Feke, G. D.; Hessman, D.; Grober, R. D.; Lu, B.; Taylor, J. W. J. Vac. Sci. Technol. 2000, B18, 136-139.

around a particular acid concentration. Therefore, it is necessary to use an indicator that is sensitive in the range of lithographically relevant acid concentrations. Another consideration is the stability of the indicator. Indicators used for on-wafer measurements should be thermally stable in response to baking of the wafer. The solubility of the indicator is another consideration, as is the passivity of the measurement to the resist (i.e., the absorption spectrum of the indicator should be in a region of the electromagnetic spectrum for which the resist is photochemically passive, and interrogation of the indicator by the excitation light should not induce sensitization of the PAG). The optical titration techniques that are commonly used are destructive because they are performed following dissolution of the exposed resist films. The experimental protocol for acid quantification, widely used by the semiconductor industry, involves four steps.10 First, a series of wafers is coated with resists containing the test PAG. Second, a very large fraction (nearly 100%) of the surface area of each of the wafers is flood exposed with a uniform exposure dose (a different dose for each wafer). Third, the resists are extracted from the wafers into cuvettes using a solvent doped with the pH indicator. Fourth, the optical measurements (typically absorbance measurements) are performed. The indicator is calibrated by adding known amounts of acid directly to the unexposed resist in solution. This in vitro protocol is very labor intensive: typically several days are spent coating and exposing the series of wafers and performing the wet chemistry to obtain an adequate sampling of the response curve of a single PAG. Furthermore, in small exposure field systems, the flood exposure is effectively achieved by serially “quilting” together individual fields. This “quilting” itself is a time-consuming process and increasingly unwieldy as standard wafer sizes increase. Several studies have demonstrated on-wafer quantification of photoacid in exposed films.28-37 These techniques are accomplished by incorporating the indicator in the resist casting solution and coating and exposing the wafer as usual. The advantages of on-wafer techniques are that they are nondestructive, allow measurements to be performed in the same matrix as the lithography, and obviate much of the labor-intensive wet chemistry. A potential drawback is that the presence of the indicator can alter the absorbance of the resist at the lithographic wavelength as well as promote PAG decomposition through sensitization during the optical measurement. This is especially the case for spectrophotometric (i.e., absorption) detection for which very high indicator loadings are required to achieve adequate signalto-noise ratios. However, the sensitivity of fluorescence detection is inherently a great deal larger than for absorbance detection because “absorbance noise” is limited by the fluctuations in the interrogation source whereas “fluorescence noise”, by virtue of the shift of the fluorescence from the absorption spectrum, is limited by the fluctuations in the background level. Therefore, fluorescence detection permits much lower indicator concentrations, thereby minimizing the distortion of the resist system. The on-wafer studies have typically involved serial measurements (i.e., one acid concentration per measurement event).28-35 Recent reports, however, have described the extension of the onwafer technique to parallel data acquisition, which is accomplished by spectroscopic imaging of wafers exposed with a dose ramp Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

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pattern.36,37 The convenience and speed of the parallel scheme is especially relevant in the context of screening the many candidate PAGs that generate production relevant photoacids. Furthermore, parallel data acquisition is inherently more robust than serial acquisition and, in some cases, is necessary due to time dependence of environmentally sensitive resists. We herein report the application of the parallel on-wafer acid quantification technique to the evaluation of the quantum yields of photoacid generation of candidate PAGs for 193-nm CARs. A dye must have at least two prototropic forms with different spectroscopic properties (i.e., different absorption spectra, different fluorescence spectra, or both) to exhibit fluorescence contrast as a function of pH. The fluorescence contrast between the two prototropic populations is maximal if one prototropic form may be excited or detected in some spectroscopic bandwidth without interaction with the other form (i.e., in the case where some portion of either pair of spectra does not overlap). We investigate the efficacy of the commercially available laser dye coumarin 6 (C6) for this application.38-40 The absorption and fluorescence spectra of C6 have been demonstrated in prior studies to undergo a red shift upon conversion to its monocation form (C6+).30-32,41-44 By exploiting the spectroscopic properties of this dye, we obtain fluorescence images with much higher contrast than has been demonstrated in previous reports of on-wafer spectrofluorometric characterization of PAGs. EXPERIMENTAL SECTION Measurement Systems. (1) Spectroscopic Analysis of Coumarin 6 in the Chemically Amplified Resist Matrix. A Varian Cary 13 spectrophotometer is used to obtain absorption spectra. The apparatus used to obtain fluorescence spectra is described as follows. Either 457.9- or 488.0-nm excitation light from an Ar ion laser (Coherent, Palo Alto, CA) is coupled into a single-mode fiber and focused onto the wafer by imaging the fiber output. Either a 457.9 ( 5 (Chroma Technology Corp., Brattleboro, VT) or a 488.0 ( 1.5 nm (CVI, Albuquerque, NM) band-pass filter is used at the fiber output to block background emission generated in the fiber. The fluorescing spot from the resist film is confocally imaged onto the input slit of a Triplemate monochromator (Instruments S.A., Inc., Edison, NJ) through either a 470-nm longpass filter (Chroma Technology Corp.) or a 488.0 ( 5 nm notch filter (Kaiser Optical Systems, Inc., Ann Arbor, MI), each used to block the excitation light to an optical density of 6. The fluorescence is spectrally dispersed by a 600 groove/mm grating blazed at 500 nm and imaged onto a thermoelectrically cooled 1152 × 298 array 22.5 × 22.5 µm pixel charge-coupled device (CCD) camera with 16-bit resolution (Roper Scientific, Trenton, NJ). (38) Abdel-Mottaleb, M. S. A.; Loutfy, R. O.; Lapouyade, R. J. Photochem. Photobiol. 1989, A48, 87-93. (39) Abdel-Mottaleb, M. S. A.; Antonious, M. S.; Ali Abo, M. M.; Ismail, L. F. M.; El-Sayed, B. A.; Sherief, A. M. K. Proc. Indian Acad. Sci. 1992, 104, 185-196. (40) Jones, G. I.; Jackson, W. R.; Choi, C.; Bergmark, W. R. J. Phys. Chem. 1985, 89, 294-300. (41) Corrent, S.; Hahn, P.; Pohlers, G.; Connolly, T. J.; Scaiano, J. C.; Forne´s, V.; Garcı´a, H. J. Phys. Chem. 1998, B102, 5852-5858. (42) Richter, E.; Hien, S.; Sebald, M. J. Photopolym. Sci. Technol. 1999, 12, 695709. (43) Richter, E.; Hien, S.; Sebald, M. Microelectron. Eng. 2000, 53, 479-483. (44) Coenjarts, C.; Cameron, J. F.; Pohlers, G.; Scaiano, J. C.; Zampini, A. J. Appl. Polym. Sci. 2000, 78, 1897-1905.

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Figure 1. PAG structures.

(2) On-Wafer Acid Quantification. The wafer spectrofluormeter apparatus is similar to that described in ref 37. The wafer is illuminated by a fiber-optic bundle style halogen lamp (DolanJenner Industries, Lawrence, MA). Either a 457.9 ( 5 or a 514.5 ( 1.5 nm (CVI) band-pass filter is placed at the fiber-optic bundle output. The angle of incidence onto the wafer is ∼30° with respect to the surface normal. The wafer is imaged onto a liquid nitrogencooled 512 × 512 array 24 × 24 µm pixel CCD camera with 16-bit resolution (Roper Scientific) using a f3.9 80-200-mm zoom lens (Sakar Optical Co., New York, NY). Either a 490 ( 20 nm bandpass filter (Chroma Technology Corp.) in series with a 470- or a 550-nm long-pass filter (CVI) in series with a 514.5 ( 5 nm notch filter (Kaiser Optical Systems, Inc.) is used for collection and suppression of the small amount of scattered excitation light from surface contaminants. Because the optical axis of the collection beampath is normal to the wafer, the wafer surface specularly reflects the excitation light away from the collection optics (i.e., dark-field imaging). Materials. C6 was purchased from Aldrich (Milwaukee, WI) and recrystallized from CH3OH/CH2CL2. Rhodamine B base (RB) was purchased from Aldrich and purified as described in ref 30. Trifluoromethanesulfonic acid (TFA) was purchased from Aldrich. The PAGs triphenylsulfonium triflate (TPSOTf), di(1-naphthyl)phenylsulfonium triflate (DNPSOTf), di[(4-tert-butyl)phenyl]iodonium triflate (DTBPIOTf), and N-(trifluoromethanesulfonyloxy)5-norbornene-2,3-dicarboximide (MDT) were synthesized at the Shipley Co. The structures of these PAGs are shown in Figure 1. Each of these PAGs generates TFA. Sample Preparation. (1) Spectroscopic Analysis of Coumarin 6 in the Chemically Amplified Resist Matrix. C6 was doped at 1 wt % (vs solids content) into two otherwise photoinert 193-nm polymer formulations, A and B, for the purpose of measuring its absorption and fluorescence spectra. (This high dye loading was necessary to provide a sufficient signal-to-noise ratio for absorption spectroscopy. However, at this loading, the dye is not a passive probe in the actual resist but instead was found to cause significant PAG sensitization. The dye concentration used for acid quantification was lower by a factor of 20 and was observed to not cause any significant PAG sensitization while providing a sufficient signal-to-noise ratio for fluorescence detection.) TFA was

added to A until the formulation was observed by the naked eye to change color from green to orange. Hence, A contained C6+ whereas B contained C6. Each formulation was spin coated to a thickness of ∼0.7 µm on a 1-in. quartz wafer and received a postapplication bake (PAB) at 120 °C for 60 s. The amount of acid added to A was so large that any acid loss that may have occurred during PAB did not significantly affect the spectroscopic measurements. (2) In Vitro Acid Quantification. The sample preparation for an in vitro acid quantification experiment is similar to that described in ref 10. Two 193-nm resist formulations were prepared, one containing TPSOTf and the other containing DNPSOTf. These formulations were coated to a thickness of ∼0.4 µm onto two series of 8-in. bare silicon wafers which received PABs at 120 °C for 60 s. The wafers were flood exposed with various doses at 248 nm using a GCA XLS 7800 stepper. (3) On-Wafer Acid Quantification. Four 193-nm resist formulations were prepared, each containing a different PAG (TPSOTf, DNPSOTf, DTBPIOTf, MDT) and doped with 0.05 wt % (vs solids content) C6. The PAG loadings in the TPSOTf and DNPSOTf formulations were the same as for the in vitro acid quantification experiment. The PAG loadings in all four formulations were such that the films had equal absorbance at 193 nm. For an experiment involving calibration of the dye response, the TPSOTf and DNPSOTf formulations were spin coated to a thickness of ∼0.4 µm onto two 8-in. bare silicon wafers which received PABs at 120 °C for 60 s. Each of the four wafers was exposed with a 10 × 10 array of 7.5 × 7.5 mm squares at 248 nm using the same stepper as for in vitro acid quantification. The array consisted of two interleaved subarrays, each consisting of 50 squares. One subarray consisted of a linear dose ramp from 0.1 to 100 mJ/cm2. The other subarray consisted of exposures at a constant dose of 75.5 mJ/cm2, which is one of the doses in the ramp. For an experiment involving acid quantification using 193nm exposures, each of the TPSOTf, DNPSOTf, DTBPIOTf, and MDT formulations was spin coated to a thickness of ∼0.4 µm onto three 6-in. bare silicon wafers. To test reproducibility, a second TPSOTf formulation was prepared and spin coated onto an additional set of three wafers. Each of the 15 wafers received PABs at 120 °C for 60 s and was exposed with a 10 × 10 array of 1.5 × 1.5 mm squares at 193 nm using an ISI microstepper. Two of the wafers in each set of three were exposed with an array consisting of a linear dose ramp from 1 to 100 mJ/cm2. The third wafer in each set was exposed with an array consisting of exposures at a constant dose of 75 mJ/cm2, which is one of the doses in the ramp. Preliminary experiments had shown a dependence of the fluorescence measurements on the time delay after the exposure. However, we found that cooling the wafers in dry ice suspended this time dependence. Therefore, the wafers were placed in separate containers, sealed in plastic bags, and packed in dry ice for transport from the Shipley Co. to Yale University. Procedures. (1) In Vitro Acid Quantification. The procedure for in vitro acid quantification is similar to that described in ref 10. The resists are extracted from the wafers using an RB dye solution, and the amount of acid generated for each exposure dose is determined by measuring the absorbance of each resist extract and using a calibration curve. Acid concentrations in the resist

Figure 2. Coumarin 6 (C6) protonation equilibrium.

film are determined by dividing the amount of acid generated by the volume of exposed resist. (2) On-Wafer Acid Quantification. The procedure for wafer spectrofluorometry involves acquiring fluorescence images of each wafer. The image integration times are between 100 and 300 s in order to use a substantial fraction of the dynamic range of the camera (with the given excitation intensity). Images are taken immediately after unpacking each of the wafers. RESULTS AND DISCUSSION Spectroscopic Analysis of Coumarin 6 in the Chemically Amplified Resist Matrix. The neutral-monocation protonation equilibrium of C6 is shown in Figure 2. On-wafer spectrofluorometric titration involves exciting and imaging the dye-doped resist using spectroscopic filters. Because the positions and shapes of the absorption and fluorescence spectra generally depend on the chemical composition of the host matrix, it was necessary to perform absorption and fluorescence spectroscopy of the C6 and C6+ forms of the dye in the 193-nm CAR matrix in order to choose the appropriate filters for on-wafer acid quantification. The absorption spectra are shown in Figure 3. The red shift from C6 to C6+ is so large that little overlap exists between the two spectra. Fluorescence spectra were obtained from the identical samples as the absorption spectra. For A, the Ar ion laser was tuned to its 488.0-nm line (a wavelength that would be significantly absorbed by the dye yet would not overlap with the expected fluorescence spectrum), a 488.0 ( 1.5 nm band-pass filter was used at the fiber output, and a 488.0 ( 5 nm notch filter was used to reject the excitation light. For B, the Ar ion laser was tuned to its 457.9-nm line (also a wavelength that would be significantly absorbed by the dye yet would not overlap with the expected fluorescence spectrum, as well as the shortest possible wavelength from this laser), a 457.9 ( 5 nm band-pass filter was used at the fiber output, and a 470-nm long-pass filter was used to reject the excitation light. The fluorescence spectra are also shown in Figure 3. Although less dramatic than between the absorption spectra, the red shift from C6 to C6+ is so large that the overlap between the two spectra is sufficiently small. In Vitro Acid Quantification. Acid quantification by titration of a pH indicator dye requires calibration of the dye response Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

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Table 1. Comparison of Results for Different PAGs and Different Exposure Wavelengths exposure (nm) 248 248 193 193 193 193 193 a

Figure 3. Absorption (indicated by left-pointing arrows) and fluorescence (indicated by right-pointing arrows) spectra of C6 (neutral) and C6+ (monocation) in the 193-nm resist matrix.

PAG

[H+]∞ (10-3 mol/cm3) E1/2 (mJ/cm2) Φgen,rel

TPSOTf 6.02 × 10-2 DNPSOTf 6.88 × 10-2 TPSOTf (a) (8.03-9.46) × 10-2 TPSOTf (b) (9.31-11.19) × 10-2 DNPSOTf (10.98-13.55) × 10-2 DTBPIOTf (9.30-11.18) × 10-2 MDT (14.94-19.68) × 10-2

13.0 49.1 28.6-32.1 30.9-35.4 101.9-119.9 78.6-90.0 109.0-136.9

n/aa n/a 1 1.073 0.384 0.422 0.489

n/a, not applicable.

exposure dose and E1/2 is the exposure dose at which [H+] ) [H+]∞/2. Fits of eq 7 to the data are also shown in Figure 4 and the best fit parameters are given in Table 1. Using the physical interpretations

[H+]∞ ) ζ[PAG]0

(8)

E1/2 ) 1/R

(9)

and

where R is the photolysis rate constant, eq 7 can be derived from the differential equation for second-order exposure kinetics, Figure 4. Acid generation curves as measured by the in vitro technique. The lines are fits of eq 7 to the data.

against known acid concentrations. However, it is not ideal to calibrate the dye by adding acid directly to resist solutions because of acid loss during PAB. Therefore, the response of C6 in the 193-nm CAR matrix was calibrated by measuring fluorescence as a function of dose using our on-wafer technique and measuring absorbance (of a different dye) as a function of dose using the traditional in vitro technique. The in vitro technique is straightforward to calibrate against known acid concentrations.10 However, because the “quilting” method used to achieve the effective flood exposure was not practical with our 193-nm stepper, the 193-nm CARs had to be exposed with a 248-nm stepper for both the onwafer and in vitro experiments to provide the calibration. The result of the calibration is then used for the on-wafer experiments using 193-nm exposures. Plots of acid concentration versus dose for the TPSOTf and DNPSOTf formulations obtained from the in vitro technique are shown in Figure 4. Acid concentrations are reported in units of 10-3 mol/cm3 because these units are equivalent to moles per liter for H2O (the units used for calculations of pH). These acid generation curves were found to be described by

[H+] ) [H+]∞(E/(E + E1/2))

(7)

where [H+]∞ is the acid concentration extrapolated to infinite 3476

Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

d[PAG] [PAG]2 ) -R dE [PAG]0

(10)

Equation 10 is surprising because first-order exposure kinetics are generally assumed for the photolytic decomposition of PAGs. However, second-order behavior can result from a special case of the general photoresist model of Dill et al.45 when first-order exposure kinetics are coupled with an optical absorption in the film which increases with increasing PAG conversion. Equation 10 has the solution

[PAG] ) F(E) ) [PAG]0(1/(RE + 1))

(11)

Substitution of eqs 3, 8, and 9 into eq 11 yields eq 7. However, although eq 7 fits well in the available range of exposure dose (from 0 to 100 mJ/cm2), F(E) for E > 100 mJ/cm2 may in fact be significantly different from eq 11. Specifically, the photoproducts of the PAG decomposition may influence the exposure kinetics, and their influence increases with their concentration (i.e., with exposure dose). Hence, the true value of the acid concentration in the limit of infinite exposure dose may not be accurately determined by the fits of eq 7 to the available data. Therefore, [H+]∞ and E1/2 should be interpreted only as parameters that describe the behavior of the acid generation curves within the given, lithographically relevant exposure range. (45) Dill, F. H.; Hornberger, W. P.; Hauge, P. S.; Shaw, J. M. IEEE Trans. Electron. Dev. 1975, ED-22, 445-452.

Figure 5. Λ1 and Λ2 images of 8-in. wafers coated with formulations containing TPSOTf and exposed at 248 nm with an array consisting of two interleaved subarrays. The dose ramp subarray is exposed in a raster pattern beginning at the lower left and ending at the upper left. The first and last few doses in the dose ramp subarray are labeled (in units of mJ/cm2). The dose ramp subarray is interleaved with the constant dose (75.5 mJ/cm2) subarray in a “checkerboard” fashion.

On-Wafer Acid Quantification. The photospeed of resists is typically determined using wafers exposed with a dose ramp pattern. The exposure fields of the pattern are typically millimeters in size and are distributed over the entire surface area of the wafer. Due to convenience, the same pattern used for the determination of photospeed was used for the spectrofluorometric determination of quantum yields of photoacid generation. In general, the fluorescence measurements will depend on the filter excitation and collection wavelengths. We will therefore use the notation Λ ≡ {λexcitation, λcollection} to represent the set of filters used for a particular measurement. For detection of C6, a 457.9 ( 5 nm filter was used for excitation, and a 490 ( 20 nm filter was used in series with a 470-nm long-pass filter for collection and suppression of the small amount of scattered excitation light from surface contaminants. As can be seen in Figure 3, the C6+ population is very weakly excited by light at 457.9 nm and is very weakly fluorescent between 470 and 510 nm. This choice of filters therefore provides detection of C6 with very good contrast. We therefore define Λ1 ≡ {457.9 ( 5 nm, 490 ( 20 nm}. For detection of C6+, a 514.5 ( 1.5 nm filter was used for excitation, and a 550nm long-pass filter was used in series with a 514.5 ( 5 nm notch filter for collection and suppression of the small amount of scattered excitation light from surface contaminants. As can be seen in Figure 3, although the C6 population does fluoresce above 550 nm, it does not absorb at 514.5 nm. This choice of filters therefore provides detection of C6+ with very good contrast. We therefore define Λ2 ≡ {514.5 ( 1.5 nm, 550 + nm}. Calibration Using 248-nm Exposures. Λ1 and Λ2 images of the 248-nm exposed TPSOTf wafers are shown in Figure 5. The fluorescence in the Λ1 image is seen to decrease with increasing dose. This is because the fraction of C6 is decreasing. The fluorescence in the Λ2 image is seen to increase with increasing dose. This is because the fraction of C6+ is increasing. The fluorescence from each of the squares in the dose ramp and constant dose subarrays is measured. The fluorescence

measurements from the constant dose subarray are in fact not constant but are subject to nonuniformity of the resist thickness, the illumination, and the collection efficiency across the CCD chip. However, this nonuniformity only affects the low spatial frequencies of the images. Therefore, a two-dimensional polynomial function is fit to the fluorescence measurements from the constant dose subarray. The best fit function is then used as a normalization for the fluorescence measurements from the dose ramp subarray. The normalizing function itself is scaled by the fluorescence measured from the 75.5 mJ/cm2 element in the dose ramp. Λ1 and Λ2 plots of fluorescence versus dose (at 248 nm) for both the TPSOTf and DNPSOTf wafers are shown in Figure 6. Although optical titration techniques are well understood (see ref 9 for example), a review of the method of data analysis is appropriate here. For a single-step titration, the fluorescence intensity FΛ measured from an ensemble of molecules is given by

[C6] [C6+] + F FΛ ) FC6,Λ +,Λ C6 [C6] + [C6+] [C6] + [C6+]

(12)

where [C6] ([C6+]) is the molar concentration of C6 (C6+) and FC6,Λ (FC6+,Λ) is the fluorescence measured from an equivalent ensemble with [C6+] ([C6]) equal to zero. The fluorescence measurements are in general affected by an additive background bΛ due to leakage of residual laboratory light into the wafer spectrofluorometer, so that eq 12 must be rewritten as

[C6] [C6+] FΛ ) FC6,Λ + FC6+,Λ + bΛ + [C6] + [C6 ] [C6] + [C6+] (13) [C6] and [C6+] vary with acid concentration according to Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

3477

Ka ) [H+][C6]/[C6+]

(14)

where Ka is the mixed dissociation constant. We define

FΛ1 ≡ FC6+,Λ1/FC6,Λ1, FΛ2 ≡ FC6,Λ2/FC6+,Λ2

(15)

where, in general, 0 < FΛ1 < 1 and 0 < FΛ2 < 1. We also define

βΛ1 ≡ bΛ1/FC6,Λ1, βΛ2 ≡ bΛ2/FC6+,Λ2

(16)

We estimate βΛ1 and βΛ2 to be 0.05 ( 0.025. By combining eqs 13-16, we obtain the Λ1 and Λ2 titration curves: Figure 6. Λ1 and Λ2 fluorescence vs dose (at 248 nm) data for the TPSOTf and DNPSOTf wafers. The lines are the fit of eqs 18 to the data.

[H+](FΛ1 + βΛ1) + Ka(1 + βΛ1) FΛ1 ) FC6,Λ1 , [H+] + Ka [H+](1 + βΛ2) + Ka(FΛ2 + βΛ2)

FΛ2 ) FC6+,Λ2

[H+] + Ka

(17)

Each curve is a function of four parameters. We measured FΛ1 and FΛ2 in a different apparatus (specifically, a microscope) and found these parameters to be virtually zero. This is not surprising given the careful choice of Λ1 and Λ2 Combining eqs 7 and 17 and setting FΛ1 ) FΛ2 ) 0 we obtain F Λ1 )

(

)(

)

[H+]∞βΛ1 + Ka(1 + βΛ1) Ka(1 + βΛ1)E1/2 + FC6,Λ1 E FC6,Λ1 + [H ]∞ + Ka [H+]∞ + Ka KaE1/2 E+ [H+]∞ + Ka

(

FΛ2 )

(

+

)

)(

)

[H ]∞(1 + βΛ2) + KaβΛ2 KaβΛ2E1/2 E FC6+,Λ2 + F +,Λ C6 2 [H+]∞ + Ka [H+]∞ + Ka KaE1/2 E+ [H+]∞ + Ka

(

)

(18) Equations 18 were simultaneously fit to the Λ1 and Λ2 fluorescence vs dose (at 248 nm) data for TPSOTf and DNPSOTf using the results for [H+]∞ and E1/2 found from the in vitro experiments. FC6,Λ1, FC6+,Λ2, βΛ1, βΛ2, and Ka were free parameters. The fit curves are also shown in Figure 6. The best fit value for Ka was (3.57 ( 0.16) × 10-2 (10-3 mol/cm3). This value agrees reasonably well with the previously reported value of 2.51 × 10-2 (mol/L) in 50: 50 CH3OH/H2O.41 A complete agreement of the two values would be surprising, however, since any given indicator’s value of Ka depends strongly on the solvent. For example, pKa generally decreases by 0.5-1.5 units from a water solution to a 50:50 water/ alcohol mixture.46 (46) Sober, H. A., Ed. Handbook of Biochemistry; The Chemical Rubber Co.: Cleveland, OH, 1968.

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Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

Figure 7. Data and fit curves of Figure 6 plotted as functions of acid concentration ([H+]). The fit curves are titration curves with Ka indicated by the vertical line.

The 248-nm exposed fluorescence data for TPSOTf and DNPSOTf and fits of eqs 18 are plotted as a function of [H+] in Figure 7. Each data set is fit by its own titration curve with individual values of FC6,Λ1, FC6+,Λ2, βΛ1, and βΛ2 and a shared Ka. This Ka is used below in the analysis of the 193-nm exposure data, for which [H+]∞ and E1/2 are unknown (i.e., free parameters). Fortunately, the Ka of C6 in the 193-nm resist matrix lies within the range of acid concentrations that can be photogenerated, so that the fluorescence data is predominantly from the portion of the titration curves with highest slope (highest sensitivity). Acid Quantification Using 193-nm Exposures. Λ1 and Λ2 fluorescence versus dose (at 193 nm) curves for each PAG are shown in Figure 8, where each data point in each curve was normalized by the fluorescence from the data point at the corresponding position on the constant dose wafer of the corresponding PAG. The data sets from the two TPSOTf formulations are labeled “TPSOTf (a)” and “TPSOTf (b)”. We rewrite eqs 18 as

FΛ1 )

where

EP1 + P2 EP4 + P5 , FΛ2 ) E + P3 E + P3

(19)

[H+]∞βΛ1 + Ka(1 + βΛ1) P1 ≡ FC6,Λ1 [H+]∞ + Ka Ka(1 + βΛ1)E1/2 P2 ≡ FC6,Λ1 [H+]∞ + Ka P3 ≡

KaE1/2 [H+]∞ + Ka

[H+]∞(1 + βΛ2) + KaβΛ2

P4 ≡ FC6+,Λ2

[H+]∞ + Ka

KaβΛ2E1/2 P5 ≡ FC6+,Λ2 + [H ]∞ + Ka

(20)

Equations 19 are simultaneously fit to each pair of curves and the fit curves are also shown in Figure 8. [H+]∞ and E1/2 are obtained from eqs 20 using

[H+]∞ )

(

Ka

( ( ) ) 1-

and

E1/2 )

)

P2 -1 P1P3 βΛ1 P2

(

P2 P1

(21)

1 + βΛ1 P1P3

1 P2 -1 P1P3

(

1 - βΛ1

))

(22)

[H+]∞ and E1/2 can therefore be determined with an accuracy limited by βΛ1. Equations 21 and 22 implicitly assume that the acid generation curves are described by eq 7. Values for [H+]∞ and E1/2 for the 193-nm exposures of each of the PAGs are also given in Table 1 (the lesser values are obtained using βΛ1 ) 0.025; the greater using βΛ1 ) 0.075). The values of E1/2 are larger for the 193-nm exposures of the TPSOTf and DNPSOTf wafers than the 248-nm exposures. Substitution of eqs 21 and 22 into eq 7 yields the acid generation curves. Plots of the acid generation curves for each of the PAGs are shown in Figure 9. The relative quantum yield of photoacid generation of each PAG is given by

Φgen,i [H+]i/[γ]i Φgen,rel ≡ ) + Φgen,ref [H ] /[γ] ref

(23)

depend on E. Therefore, instead of defining Φgen,rel as a ratio of acid concentrations, we redefine Φgen,rel as the ratio of the derivative of the acid generation curves at E ) 0.

( ) ( ) ( ) ( ) ( ) ( )

[H+]∞ d[H+]i | E1/2 i dE E)0 Φgen,rel ≡ ) ) + d[H ]ref [H+]∞ | dE E)0 E1/2 ref Ka(1 + βΛ1)

P1 1 P3 P2

P1 1 Ka(1 + βΛ1) P3 P2

ref

where i indexes each PAG and ref denotes a reference PAG. Because the films were designed to have equal absorbance at 193 nm (i.e., [γ]i ) [γ]ref), we may rewrite eq 23 as

φgen,rel ≡ [H+]/[H+]ref

Figure 8. Λ1 and Λ2 fluorescence vs dose (at 193 nm) data for each of the four PAGs. Two sets of wafers were coated with two TPSOTf formulations. The lines are fits of eqs 19 to the data.

(24)

However, because [H+] is given by eq 7, the result of eq 24 will

i

)

ref

P1 1 P3 P2

P1 1 P3 P2

i

(25)

ref

The definition of Φgen,rel given by eq 25 is valid because

[H+] ≈ ([H+]∞/E1/2)E

(26)

for E ≈ 0. Calculation of Φgen,rel at the E ) 0 limit is necessary Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

3479

independent of βΛ1 We will choose TPSOTf as the reference PAG. Φgen,rel for the 193-nm exposures of each of the PAGs is also given in Table 1.

CONCLUSIONS We have demonstrated a fast, convenient, and nondestructive fluorescence technique for quantitative determination of quantum yields of photoacid generation. We have applied this technique to the characterization of four candidate PAGs for prototype 193nm photoresists. Measurements of acid generation curves are robust to within a small systematic error. Elimination of this error could in principle be accomplished by performing the fluorescence measurements in a system with a better controlled collection beam path (such as a microscope) and, hence, not affected by laboratory fluctuations. This would require that the dose array be restricted to a much smaller field of view (∼1 mm2). Relative measures of quantum yields of photoacid generation, however, are immune to this systematic error.

ACKNOWLEDGMENT This work was supported by the Semiconductor Research Corp., SRC Grant 98-LJ-438, and by the David and Lucile Packard Foundation. Figure 9. Bands bounded by the acid generation curves obtained from eq 7 with minimum and maximum values of βΛ1.

because eq 26 is insensitive to the finite PAG concentration. Equation 25 provides a robust measurement of Φgen,rel since it is

3480 Analytical Chemistry, Vol. 73, No. 14, July 15, 2001

Received for review December 28, 2000. Accepted April 27, 2001. AC0015319