Swelling and dissolution rate measurements of polymer thin films in supercritical carbon dioxide

J. of Supercritical Fluids 31 (2004) 323–328

Swelling and dissolution rate measurements of polymer thin films in supercritical carbon dioxide Victor Q. Pham a , Nagesh Rao b , Christopher K. Ober b,∗ b

a School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA Department of Materials Science and Engineering, Cornell University, Bard Hall, Ithaca, NY 14853, USA

Received in revised form 2 December 2003; accepted 22 December 2003

Abstract The first interferometric dissolution rate monitor (DRM) to probe polymer swelling and dissolution in supercritical carbon dioxide (scCO2 ) is reported. Refractive index measurements of scCO2 as a function of pressure agree with literature and calculated values using the Lorenz–Lorentz equation, even for short equilibration times. The reflected intensity plot with <2 min of pressure and thermal equilibration time shows ±10−3 deviation from calculation. Maximum swollen film thickness for a scCO2 soluble random copolymer of tetrahydropyranyl methacrylate (35 mol%) and 1H,1H-perfluorooctyl methacrylate (65 mol%) is estimated for pressures 128–197 bar. Dissolution rates show anomalous thickness dependence that can be explained in terms of polymer/surface interactions. It was found that complete film removal requires more than 1000 s below and less than 90 s above ∼191 bar. © 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Photoresist dissolution; Interferometry; Thin film swelling; Dissolution measurement

1. Introduction Industrial and academic researchers have shown convincingly that carbon dioxide can be used as a processing and cleaning solvent in microelectronics fabrication [1]. In addition to significant environmental benefits—reduction of water consumption and supplanting hazardous processing chemicals with an abundant, nontoxic, nonflammable, recyclable source—the special physical and chemical properties of carbon dioxide in the supercritical state can enhance processing performance in photolithographic and cleaning stages of chip production. Supercritical carbon dioxide (scCO2 ) has no surface tension, low viscosity, and possesses considerably higher solvent strength than a gas but with more favorable transport properties than liquids [2]. Consequently, scCO2 can reach, dissolve, and remove unwanted residual materials in small, deep crevices inaccessible to aqueous solvents due to high capillary forces. In sub-micron structures, the absence of surface tension serves to eliminate pattern collapse and stiction that commonly result when developing with ∗ Corresponding author. Tel.: +1-607-255-8417; fax: +1-607-255-6575. E-mail address: [email protected] (C.K. Ober).

0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2003.12.004

aqueous solvents [3–5]. Most importantly, scCO2 dissolves fluorine-containing polymers that may be critically important in future photoresist materials such as those used in deep ultraviolet (DUV) 193 and 157 nm lithography. Several groups have previously reported a high fluorine content model 157 nm resist that was shown to be imageable at 193 nm wavelength and by E-beam direct-write patterning [6]. The copolymer poly(tetrahydropyranyl methacrylate 1H,1H-perfluorooctyl methacrylate) [THPMA-F7MA] was processed in scCO2 to yield line—space features as small as 100 nm [7–9]. Processing conditions, however, were chosen largely by trial and error. Simple adjustments in temperature and pressure determine the density of the supercritical fluid, and hence its solvating power. It is expected that if dissolution rates and behavior can be understood and controlled, higher quality features with increased resolution can be achieved by choosing proper processing conditions. While equilibrium thermodynamic studies of fluorinated polymer solubility in scCO2 exist in large numbers [10–15], techniques to examine the dissolution and swelling process of polymer thin films have not been documented. Unlike aqueous development, pressurization and equilibration of CO2 takes place simultaneously with swelling and dissolution to make development a highly transient process. Established methods to measure polymer dissolution rates

V.Q. Pham et al. / J. of Supercritical Fluids 31 (2004) 323–328

such as the quartz crystal microbalance, ellipsometry, or optical microscopy [16–21], remain prohibitively difficult to incorporate in high-pressure scCO2 processing equipment for various reasons. Swelling of polymer films in scCO2 has been investigated in situ for poly(dimethyl siloxane) [17], but reports on dissolution rate measurements are not available. To study thin film dissolution in scCO2 , we have designed a dissolution rate monitor (DRM) based on the principles of interferometry. Results demonstrate that with the simple equipment setup described here important information such as dissolution rate and swelling can be obtained. In this report, we will discuss the capabilities and limitations of this technique, show representative plots for polymer swelling and dissolution, and compare experimental findings with theoretical calculations.

2. Background He–Ne laser interferometry at 628.32 nm has been widely used to probe dissolution phenomena in microlithography. This technique is well understood and has been treated extensively in introductory textbooks on microelectronic processing [22]. Here, a brief summary outlines the important theories pertinent to laser interferometry. The reader is referred to the work of Krasicky and co-workers for a more detailed treatment [23–28]. The measurable quantity in laser interferometry is the intensity of the reflected light when a laser source is shone on a reflective silicon surface coated with semi-transparent photoresist films. Practical film thickness ranges from 0.5 to 1.5 ␮m. During dissolution, the path length difference between the beams reflected from solvent-polymer and polymer-silicon interface produce periodic intensity oscillations at the plane of the detector due to constructive and destructive interference. If the angle of the incident beam with respect to the surface normal of the substrate is <30◦ , and the laser beam is unpolarized, simplified calculations assuming normal incidence have been shown to yield highly accurate results. Since the angle of incidence of the laser beam is constant at ∼3◦ in our setup, such an assumption is safely made. The thickness D that corresponds to a period of oscillation is given by λ D=
. 2 2 n2 − n21 sin2 θ1

Intensity (au)

324

7 6 5 4 3 2 1 0

0

2 4 Time (seconds)

6

Fig. 1. Example interferometry intensity vs. time plot for a perfectly dissolving film with one optically distinct moving boundary.

3. Experimental 3.1. Sample preparations Photoresist random copolymers of poly(tetrahydropyranyl methacrylate-co-1H,1H-perfluorooctyl methacrylate), Mn = 14k, 65% fluorinated methacrylate, were synthesized by free radical polymerization with monomers purchased from Polysciences and Aldrich. Polymers were dissolved in ␣,␣,␣-trifluorotoluene (TFT) with approximately 25 weight percent with respect to solvent. The polymer solution was then spincoated onto a clean silicon wafer at 3000 rpm. Post-apply bake (PAB) treatment was performed on a vacuum-suction hotplate at 90 ◦ C for 60 s to remove excessive solvent. Film thicknesses were measured with a Tencor AlphaStep 200 profilometer at the Cornell Nanofabrication Facility (CNF). 3.2. Instruments An scCO2 extraction system consisting of a high-pressure pump, a digital temperature control oven, and two stainless steel processing vessels connected in series was purchased from Applied Separations, Inc. A schematic of the equipment is provided in Fig. 2. A 7/8 in. thick, 1/2 in. diameter quartz glass window was custom installed into a processing vessel for passage of the incident and reflected laser beams. A 632.8 nm wavelength unpolarized laser source is mounted on top of the oven with the beam directed through a hole Detector

Computer

Laser

(1)

where λ is the wavelength of the incident beam and n1 and n2 are refractive indices of solvent and polymer, respectively. The reflectance, or the ratio of intensities of the reflected and incident beams, reaches a maximum when the thickness of the film, d, is an integral multiple of the period thickness D, including d = 0. Thus, a constant maximum intensity following periods of oscillation indicates that the film has completely dissolved. Fig. 1 illustrates an expected DRM plot for an ideally dissolving film.

pressure pump

P2

preheat coil

CO2 cyl.

Vessel #1

P1 T1

T2

Vessel #2

Heating oven

Fig. 2. Schematic of processing equipment. P1, P2, T1, T2 are pressure and temperature detectors, respectively.

4. Results and discussion 4.1. Density and refractive index Researchers have developed interferometry methods with varying degrees of complexity and accuracy to measure refractive indices of fluids. Sophisticated instruments using differential-interferometry and infrared spectrometry are capable of measuring fluid refractive index to within ±10−5 [29–33]. Simpler methods such as the laser displacement technique have been developed to yield still very small errors of ±10−3 [34]. The techniques mentioned rely on high precision instrumentation, carefully controlled process parameters, and well-equilibrated fluid systems. Because photoresist developing times range from seconds to a few minutes in microlithography, processing with scCO2 is a transient process especially since CO2 gas requires time to equilibrate after pressurization. It is important to verify that our optical instruments produce results within acceptable variability under normal development conditions. Calibration of the instrument is done by measuring the reflectance of the beam incident on a clean silicon wafer surface in the presence CO2 fluid at various pressures. A sample plot is shown in Fig. 3. Initially under atmospheric conditions, the intensity of the beam is constant and depends on the source power, transmittance of the quartz glass window, and reflectivity of the silicon surface. When

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

325

100

200

300

400

Time (seconds)

Fig. 3. Intensity plot of laser beam reflecting from a plain silicon wafer. Severe scattering occurs at ∼50 s as chamber is pressurized. Constant signal is restored in approximately 15 s.

the chamber is pressurized, as shown in Fig. 3 at t ∼ 50 s, the sudden influx of compressed carbon dioxide results in phase inhomogeneity that scatters light. Set point pressure and optical transparency follow after 10–15 s, depending on whether the fluid is allowed to flow out of the processing chamber at a controlled rate or remain stagnant, longer times for the former (as shown). A constant intensity signal, an indication that the system has regained optical transparency of a homogeneous fluid, is recorded along with the pressure after 3 min. The procedure is then repeated for different temperature and pressure conditions. Data points are shown in Fig. 4. Experimental data points are compared to reference curves calculated from literature CO2 isotherms [35]. Density as a function of pressure is converted to refractive index n by the classic Lorenz–Lorentz equation,  2rρ + 1 n= (2) 1 − rρ by iterating r, the constant specific refraction of CO2 . Intensity of the reflected beam is calculated by    2 n1 − n3 2 4nw n1 R = I0 e , (3) n1 + n3 (nw + n1 )2 0.9 Th_35C

Intensity (a.u.)

drilled in the oven wall and into the processing vessel at a 3◦ angle. Samples are horizontally secured inside the processing vessel. The reflected beams are collected by an intensity sensitive silicon photosensor with 1 cm active area diameter. Analog signals from the detector pass through a Vernier LabProTM data collection interface and are recorded with a personal computer using Vernier LoggerProTM software. The supercritical fluid extraction vessels, with internal volumes of 12.8 cm3 , are situated inside a digitally controlled heating oven. With 1 in. thick stainless steel walls, the heavy processing vessels are considered heat baths and are thus allowed to thermally equilibrate at least for an hour before processing. Heating coils along with the preprocessing vessel (#1) increase the distance of travel of the incoming fluid and with added valves allow better control to minimize equilibration time inside the main processing vessel (#2). Thermocouples, as shown in Fig. 2, record fluid and heat bath temperatures. SFC grade liquid CO2 is drawn via a dip tube into a Haskell pressure pump. Pressurization of the processing vessel generally takes ∼10 s in the 140–310 bar range. Initially the temperature inside the processing vessel decreases by ∼10 ◦ C when the set point pressure is reached. As temperature gradually returns to set point in approximately 3 min, pressure increases by a maximum of 3 bar. Set point temperature and average pressures are given in the report.

Intensity (a.u.)

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DRM_35C

0.85

Th_45C DRM_45C

0.8 0.75 0.7

0

100

200

300

400

Pressure (bar)

Fig. 4. Reflected intensity as a function of pressure. Circles and diamonds are experimental data. Lines are calculated from NIST fluid tables and the Lorenz–Lorentz equation.

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where e is the transmission efficiency of the 7/8 in. quartz glass window and nw its refractive index. The first term in parentheses involves contributions from refractive indices of CO2 , n1 , and the silicon substrate, n3 . The second term involves transmission of the incident and reflected beams through the quartz glass window [36–38]. Fig. 4 shows best-fit curves for e = 0.685 and r = .151 cm3 g−1 . This value of r for CO2 agrees with literature exactly [39]. Calculation and experimental data compare reasonably well, especially in the high-pressure regions (>120 bar) where dissolution experiments are generally conducted. In these regions, errors are on the order of 10−3 . 4.2. Swelling and dissolution THPMA-F7MA photoresist films were prepared by spincoating at 3000 rpm a 1:4 weight ratio polymer:solvent (trifluorotoluene) solution onto a clean silicon wafer. Post-apply bake involved heating for 60 s at 90 ◦ C to remove residual solvent and increase film uniformity. Film thickness was measured at multiple locations across the wafer and averaged to be 1.05±0.007 ␮m. Dissolution plots were obtained at 45 ◦ C and 128, 176, 181, 191, 197 bar with temperature and pressure fluctuations described in the experimental section. Plots of 181 and 191 bar are shown in Fig. 5 as each is representative of the behaviors observed either below and above ∼191 bar where the scCO2 density is high enough to completely remove the polymer film from the silicon substrate. To quantify the extent of film removal and swelling, profilometry measurements (Tencor P10) of the samples were made after processing. Table 1 summarizes the results. The refractive index of polymer films, rfilm were calculated from experimental plots using the equation 2 2 rfilm = r23 + 2fr12 r23 (1 − r23 ) cos φ,

(4)

Table 1 Measured and calculated film thicknesses Pressure (bar)

tswell

Atmosphere 128 176 181 191 197

– 1.72 2.08 2.22 – –

a

(␮m)

tfinal (␮m) 1.052 0.746 0.106 0.116 0 ± 0.01 0 ± 0.01

Data for five samples processed at 45 ◦ C. Maximum swelled thickness values (tswell ) are calculated from DRM plots. a Calculated from DRM plots.

where r12 and r23 are the Fresnel reflection coefficients for the polymer/substrate and solvent/polymer interfaces, respectively. The phase angle φ (=φ2 + φt ) has contributions from the glassy polymer film and solvent (φ2 ), and the transition layer if one is present at the polymer/solvent interface. The refractive index of CO2 is taken from the 45 ◦ C data points used in Fig. 4. The horizontal dotted lines in Fig. 5 show constant peak maxima and minima obtained theoretically with f = 1. In the presence of CO2 , calculated film refractive indices were reduced to 1.24 ± 0.005 from 1.4 of the original film. If present, the transition layer (or gel layer) shifts reflectance oscillations by the phase angle φt and reduces peak amplitudes by the factor f. Krasicky and co-workers have shown that the presence of the transition layer produces an offset between the maximum oscillation amplitude and the final intensity signal from a bare substrate [23–28]. The relative offset ψ, defined as absolute offset normalized by peak-to-peak amplitude, is used to calculate the factor f (=1/(1 + 2ψ)) and to estimate the thickness of the transition layer by making a simplifying assumption that the concentration profile is linear in this region. Raptis et al. have also shown that when dissolution involves swelling,

Fig. 5. Representative dissolution rate monitor plots at 181 bar (a), and 191 bar (b). The dotted lines show constant peak maxima and minima calculated by inserting appropriate refractive index values for the polymer films in SCCO2 .

V.Q. Pham et al. / J. of Supercritical Fluids 31 (2004) 323–328 140

191 bar

Rate (nm/s)

120

181 bar

100

176 bar

80

128 bar

60 40 20 0 0

1

2

3

327

proximately 191 bar for THPMA-F7MA at 45 ◦ C. At pressures ≥191 bar, complete film dissolution is observed, but the diffuse tail in intensity plots and profilmetry measurements of ±10 nm substrate surface roughness after processing indicate good polymer/substrate adhesion as residual polymer remains. Surface treatment or the use of cosolvent to enhance CO2 /polymer interaction may lead to complete removal. These topics are currently of great interest to academia and industry alike and are under study.

Thickness (microns)

Fig. 6. Calculated dissolution rates vs. thickness. Trend lines are drawn to aid the eye.

oscillation amplitudes fluctuate irregularly and apparently by large amounts [40]. Our DRM plots show that except for small fluctuations, reflectance amplitudes remain constant throughout the development process, even when processed above 191 bar where film dissolution goes to completion. Yet all evidence, the most obvious of which is the presence of more oscillation peaks than can be accounted for with the original film thickness, point to the fact that the polymer films swell significantly. A likely explanation is that unlike aqueous development, high density scCO2 with gas-like diffusivity quickly penetrates the thin and highly CO2 -phillic polymer film to cause uniform swelling. Nearly maximum CO2 uptake throughout the film is achieved in seconds during the period when the laser beam is severely scattered by the incoming fluid. This initial scattering of light remains the limitation of our DRM technique. Estimated maximum thicknesses of swelled samples are listed in Table 1. With the exclusion of 191 and 197 bar data where film dissolution occurred too quickly, swelled thicknesses were calculated by multiplying the number of discernable oscillation periods by corresponding layer thicknesses D from Eq. (1) and added to the final film thicknesses measured by profilometry. Swelling effects were calculated to be 63, 98, and 111% for 128, 197, and 181 bar, respectively. These results are not surprising since swelling up to as much as 112% was documented for thin PDMS films at 120 bar scCO2 [16]. Dissolution rates as a function film thickness for the above plots are shown in Fig. 6. Rates are averaged per oscillation period, or half-period time intervals when peaks are asymmetric. As in aqueous development of photoresists, films remaining on the surface after 1000 s (∼17 min) are deemed practically insoluble. At all pressures, dissolution rates decrease gradually with film thickness. This effect can be explained by considering the strength of interaction between polar groups of the polymeric photoresist with the SiO2 surface layer. It is also very possible that some deprotection of the resist occurs in the interface region due to interaction with the substrate. A partially deprotected polymer is sparingly soluble in scCO2 . The strength of polymer/substrate interaction can be overcome above a threshold density, ap-

5. Conclusion A simple and effective method to understand swelling and dissolution behavior of polymers in supercritical CO2 was demonstrated with the use of interferometry. Because swelling and dissolution occur during pressurization and thermal equilibration, detailed knowledge of this highly transient process is limited in the initial seconds due to the lack of optical transparency. Despite some uncertainties in the estimation of maximum swelled thicknesses, film refractive index was calculated as a function of pressure and dissolution rate as a function of film thickness. Anomalous behavior was observed at all pressures as dissolution rates decrease monotonically with film thickness. This is explained in terms of polymer/substrate strength of interaction that can be overcome above a specific threshold supercritical CO2 density. It is our expectation that this interferometry technique will be a useful tool in the area of photoresist development and surface cleaning with supercritical CO2 .

Acknowledgements The authors thank the NSF/SRC ERC for Environmentally Benign Semiconductor Manufacturing Center for financial support of this work and the Cornell Nanofabrication Facility and the Cornell Center for Materials Research for technical support.

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