Resist process issues related to the glass transition changes in chemically amplified resist films

Microelectronic Engineering 67–68 (2003) 283–291 www.elsevier.com / locate / mee

Resist process issues related to the glass transition changes in chemically amplified resist films I. Raptis a , *, D. Niakoula a , E. Tegou a , V. Bellas a , E. Gogolides a , P. Argitis a , K.G. Papadokostaki b , A. Ioannidis c a

b

Institute of Microelectronics NCSR ‘‘ Demokritos’’ 15310 Ag. Paraskevi Attikis, Greece Institute of Physical Chemistry NCSR ‘‘ Demokritos’’ 15310 Ag. Paraskevi Attikis, Greece c ‘‘ Xenon’’, Delfon 15 Chalandri, 15233 Attikis, Greece

Abstract Optical interferometry is applied for in situ measurement of the glass transition temperature in thin resist films (T film ) spin-coated on flat reflective substrates, using a novel, low-cost, rapid methodology. Process issues, such g as film thickness and thermal processing effect on T film were explored using this methodology. In the case of g relatively thick films the calculated T film from the optical interferometry method is in good agreement with the g corresponding differential scanning calorimetry (DSC) values. The film thickness effect on T gfilm in the case of two positive chemically amplified resists (one commercial for DUV and one experimental for 157 nm lithography) is studied and discussed. In both cases, as film thickness decreases the T film increases indicating g strong surface phenomena that should be taken into account in lithographic processing. The presented methodology enabled studies on T g changes during resist processing in characteristic positive and negative tone chemically amplified resist materials allowing deeper insight in resist optimization issues.  2003 Elsevier Science B.V. All rights reserved. Keywords: Glass transition temperature; Optical interferometry; Polymers; Thin films; Lithography; Resists

1. Introduction As the dimensions of integrated circuits continue to shrink, new lithographic methods / tools are applied in the research and the fabrication of advanced devices. According to the International Technology Roadmap for Semiconductors [1], the longterm lithographic requirements for year 2010 will be 45 nm lateral dimensions using EUV and / or 157 nm lithography. Nevertheless, exposure tools, resists and masks should be available earlier. Due to the absorbance issues at 157 nm and EUV * Corresponding author. E-mail address: [email protected] (I. Raptis). 0167-9317 / 03 / $ – see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00080-7

284

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

lithography and the resolution required, resist thickness should be reduced in order to keep the aspect ratio in a reasonable range (resist thickness ,250 nm). In addition, in the case of soft lithography, which is a low cost promising technology for high throughput and high-resolution patterning in the broader area of micro- and nanotechnology, resist films used should have a thickness of several tens of nanometers [2]. Thus, in most lithographic techniques that are expected to be used in the coming years, a significant reduction of resist film thickness is anticipated. In such thin polymeric films several parameters appear totally different from the ones in thicker films. The two most important parameters that undergo significant change from their transition of relatively thick to thin films are film quality (in terms of film thickness controllability and defects density [3]) and glass transition temperature [4]. On the other hand, it is well established that the glass transition temperature must be taken into account in the design of lithographic materials and especially in the optimization of thermal processing conditions. During the last decade significant research effort has been applied to the development and application of methods for the calculation of the glass transition temperature of thin polymeric films (T film ) and understanding the parameters affecting this property. In this framework several methods g based on various principles were developed, such as: quartz crystal microbalance (QCM) [4], local thermal analysis [5], ellipsometry [6]. By applying these methods it was revealed that the T film g depends strongly on various parameters such as film thickness [6], film–substrate interactions [7], coating method [8], etc. The glass transition temperature could decrease or increase as film thickness decreases [8,9] and predictions regarding the dependence of T film to film thickness are very risky and g questionable. Thus, a complete experimental study for each material of interest would be necessary in most cases. Recently, a new methodology for the in situ measurement of T film has been introduced by our g group [10,11]. This methodology is based on the principles of optical interferometry (OPTI method) and provides in situ measurement of T film in the case of polymeric films. Optical interferometry has g been also widely applied with success in the field of micropatterning for the measurement of other parameters such as the removal and swelling rates of photoresists [12], the etching rates [13] and the study of free volume effects on the lithographic performance [14]. In the present work, the OPTI method is applied to the study of T film in resist films used in microg film and nanolithography. More specifically, the T g dependence on film thickness will be reported in the case of two positive chemically amplified (CA) resists, one commercial for DUV lithography (UVIII) and one experimental formulation under evaluation for 157-nm lithography. In addition, thermal processing effects on T film are monitored in the cases of UVIII and of an epoxy based negative CA g film resist under different post exposure bake (PEB) conditions. In the last case, the T g evolution will be correlated with the contrast curve for the same exposure dose range.

2. Experimental The principle of operation of the OPTI method is based on the monitoring of interference (Is ) signal as a function of temperature (during the heating or cooling) of a polymeric film which is coated on a Si wafer. A detailed explanation of the OPTI apparatus (Fig. 1a) and principle of operation are published elsewhere [11]. In the present study, since thermal processing at high temperature

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

285

Fig. 1. (a) Schematic representation of the principle of operation of the OPTI methodology. It consists of a hot plate with a digital temperature controller (acc.60.5 8C), a laser beam source (650 nm) properly mounted and levelled on the hot plate, a properly mounted detector, and a commercial data acquisition card with 12-bit resolution. (b) Interference signal analysis.

significantly affects the properties (crosslinking or deprotection) of the resist materials examined, all measurements were obtained during heating. In Fig. 1b, a typical measurement (interferogram) with the OPTI method is illustrated in the case of a 450-nm thick poly(methylmethacrylate) film. In the horizontal axis the temperature is plotted while in the vertical axes the interference signal (Is ) (left axis) and first derivative of Is (dIs / dT ) are plotted. It is known that at glass transition, significant change of the specific volume rate occurs, causing a corresponding change of film thickness. Therefore, Is changes due to the change in the optical path film difference between beams A and B (Fig. 1a). The T g value according to this methodology corresponds to the point where the slope of the dIs / dT plot changes. All experimental T gfilm data presented in this work are the mean values from measurements that were performed at least 2–3 times each, in order to minimize any possible errors. In all cases the deviation from the mean value was 61.5 8C. The resist films were spin coated on 3-inch Si wafers from solutions with appropriate concentrations. Prior to spinning, silicon wafers were immersed in a solution of sulfuric acid–hydrogen peroxide (80:20, v / v) for 30 min and washed with distilled water to remove any organic material from the surface. The film thickness measurements were carried out after the post apply bake (PAB) step, on a calibrated Ambios XP-2 profilometer using low stylus force (0.01 mg). For the DUV exposures a broadband Hg–Xe lamp (Oriel) was used.

3. Results and discussion In the present work, the OPTI method is applied for the study of T film dependence on film thickness g and on T film monitoring under thermal processing (PAB, PEB). g

286

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

3.1. Resist film thickness effect on T film g The resist materials that will be used for 157 nm and EUV lithography should fulfill the strict absorbance requirements necessary for high resolution patterning methods. In the case of 157 nm lithography new polymeric materials rich in C–F or Si–O bonds which are totally different from the ones used in 248 and 193 nm, are under evaluation. On the other hand, for the EUVL applications, the appropriate polymeric materials are expected to be similar in chemical composition to the ones used for 248-nm lithography. In both cases the resist film thickness is expected to be lower than 250 nm. Given this film thickness reduction, certain significant physicochemical properties such as film homogeneity and thermal properties are expected to be different from those in the case of thicker films. For this reason resist material design and process optimization should include careful examination of the relevant issues. In the current work, the T film dependence on resist film thickness g will be studied for selected resist materials that are considered as candidates for future thin film lithographic applications i.e. the well known commercial positive tone CA resist (UVIII) for DUV lithography from Shipley [15] and an experimental silsesquioxane positive CA resist for 157 nm lithography [16]. In Fig. 2a a typical interferogram for a 650-nm thick UVIII film is presented. The resist film was prepared by spin coating (5000 rpm) and a subsequent PAB under the standard conditions (135 8C for 2 min on a hot plate). Following the methodology explained in the previous section, the T film was g calculated to be 142 8C. This value is in good agreement with the T g value obtained from the differential scanning calorimetry (DSC) (138 8C) and thermomechanical analysis (139 8C) [17] measurements. For the DSC measurements, the required sample quantity is several milligrams and the casting solvent must be removed. In order to fulfill these requirements the following process was adopted: the resist was spun on a 4-inch Si wafer, then a PAB step at a suitable temperature for a long time on a hot plate followed, and finally the resist film was scraped from the wafer. For compatibility reasons, the PAB step was performed at the same conditions used for the OPTI measurement. In Fig. 2b the T film of UVIII for a wide film thickness range (45–650 nm) is illustrated. The PAB g

Fig. 2. (a) Interferogram of a UVIII film; (b) T film dependence on UVIII film thickness. g

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

287

conditions were 135 8C, 2 min on a hot plate. From this plot it is obvious that T film increases slightly g as film thickness decreases. The DT film increase for a 45-nm thick film is 10 8C from the T g bulk value. g This DT film is relatively small compared with the values for other polymers like poly(methyl g methacrylate) PMMA, and poly(hydroxyethyl methacrylate) PHEMA [10] and is in accordance with values already published in the literature [4] for the same material. Thus, for optimum lithographic performance, in the case of UVIII films, T PAB has to be increased. Then an experimental resist based on an acrylate copolymer containing polyhedral oligomeric silsesqioxane (POSS), which is under evaluation for 157-nm lithography, was examined. This resist will be referred in the following as POSS resist. In Fig. 3a, a typical interferogram of a 60 nm thick POSS resist film is presented. The calculated T film is 138 8C while the T g resist from DSC measurement g is 113 8C. This significant deviation is attributed to the film thickness. Nevertheless such resist film thicknesses will be standard in the coming years for the 157-nm and EUV lithography. In Fig. 3b the T film evolution for the POSS resist in the 50–500 nm thickness range is presented. g From this plot it is obvious that at 130-nm film thickness, a small increase of T gfilm from the T g resist occurs. This increase becomes significantly stronger as film thickness decreases more. In both cases the observed T film dependence on film thickness provides strong evidence for g interfacial phenomena that will probably affect the lithographic performance and should be taken into account in process optimization. In the context of this work we tried to fit these changes in a simple formula: Œ]

T film (d) 5 T bulk (1 1 e f d ) g g bulk

(1)

where T g defines the saturation level with increasing the film thickness and f a fitting parameter. In all cases T g bulk is the T g resist value. When film thickness diminishes to zero (d a` 0 nm) the film bulk film bulk T g – .2T g , while for large values of d (thick films) the T g converges to T g value since parameter f has negative values. By applying this formula to the data of Fig. 3b a satisfactory fitting was obtained with T g bulk 5111 8C and f 5 20.23. In the case of UVIII (Fig. 2b) the parameters found to be T g bulk 5140 8C and f 5 20.3 with limited fitting accuracy. Further work including the examination of other resist materials is in progress.

Fig. 3. (a) Interferogram of a thin POSS film; (b) T film dependence on POSS film thickness. g

288

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

3.2. T film monitoring during resist processing g In the lithographic applications, it is necessary to know in depth the properties of the resist used, in order to process it under the optimum conditions for highest possible lithographic performance in terms of sensitivity, resolution and process latitude. Typical important resist thermal parameters affecting the lithographic performance are the glass transition and degradation temperatures. Usually, PAB is performed at a temperature higher than the T film in order to reduce as much as possible the g residual solvent quantity and free volume in the polymer matrix and relax the resist film. At the same time if T PAB is higher than the degradation temperature then the resist film will become unusable. In addition, T film plays an important role in soft lithography where the softening of a very thin resist film g is necessary for the imprint of the mask. On the other hand, in the case of CA resists, T g controls the kinetics of chemical reactions taking place during PEB and which are responsible for the solubility change of the exposed areas [18]. In the current work, the T film changes due to thermal processing are monitored for the UVIII resist g and for an epoxy based negative CA resist for DUV lithography. In the case of UVIII, it is not certain under the processing conditions used, that the total quantity of solvent has been removed from the resist film (e.g. the boiling point of ethyl-lactate is 154 8C). The solvent remaining in the resist film, acts as a plasticizer lowering the resulting T film . Thus a T gfilm g film film study on the T PAB is required in order to evaluate the apparent T g and the T g of a completely film dried film which corresponds to the actual T g of the resist. In Fig. 4 UVIII’s T g dependence on T PAB film is presented. In this plot, the apparent T g increases as T PAB increases and saturates for T PAB . 160 8C. The saturated T film is 160 8C. Similar behavior has been observed for similar resist systems g [19]. The presented results so far are for unprocessed positive CA systems. Additionally, it is very helpful to know the T film of a resist film after processing (exposure and PEB). In Fig. 5a and b the g

Fig. 4. T film dependence on PAB temperature in the case of UVIII resist. g

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

289

Fig. 5. Processing effect on T film for UVIII. Processing conditions were PAB: 135 8C, 2 min, PEB 135 8C, 2 min. g

interferograms of two UVIII films after PEB, for two different exposure doses are presented. In the case of higher exposure dose (Fig. 5b) (corresponds to the lithographically useful dose for complete resist removal after development) the T film is 152 8C, while in the case of lower dose (Fig. 5a) (partial g film film development) T g is 147 8C. In the case of unexposed film (Fig. 2a) it was found that T g 5142 8C. Therefore, the deprotection reaction progress is accompanied by an increase of the T gfilm , which suggests that T gfilm changes could be used for monitoring of reaction progress. On the other hand, probably more interesting is the case of negative tone resists based on acid catalysed crosslinking, where the reaction kinetics is affected by the crosslink network formation. The processing effect was studied also in the case of a simple negative tone CA resist based on a commercially available epoxy polymer (Epicote 164) after fractionation and a suitable photoacid generator for DUV lithography [20]. The epoxy polymers, crosslink easily in the presence of a suitable acid, forming an almost insoluble matrix. Thus, an insoluble crosslinked network is formed [21,22]. The density of this matrix depends strongly on the PEB conditions and the exposure dose for a given PAG concentration. In Fig. 6a, the interferogram in the case of a 500-nm unexposed resist film (PAB: 110 8C, 4 min on a hot plate) is presented. The T film value evaluated from this interferogram is 45 8C, which is in very g good agreement with the value from the DSC measurements. For a processed film (0.1 mJ / cm 2 , PEB: 90 8C, 4 min on a hot plate) of the same thickness however the T film was evaluated to be 51 8C (Fig. g 6b). It must be noted that this processing is insufficient (crosslinking density is below the threshold value) and after development the resist film is totally removed. In Fig. 7a and b the T film of processed EPR films versus the exposure dose are presented for two g PEB conditions (a) 110 8C, 4 min and (b) 90 8C, 4 min on a hot plate). It is obvious that T film g increases with exposure dose as was expected. In the same figures, the contrast curve results are also presented. Interestingly, the maximum measured T film in both cases coincides with the PEB g temperature as one would expect for chemical kinetic reasons. The contrast value is strongly affected as well. Thus, the higher PEB temperature results in higher contrast due to the fact that this temperature (110 8C) remains well above the T film at least until reaction proceeds to a point which g

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

290

Fig. 6. Interferograms for the experimental epoxy based resist. (a) Unexposed (b) 0.1 mJ / cm 2 .

corresponds to higher than 0.95 normalized remaining thickness. Further investigations on the influence of epoxy material parameters such as: Mw and initial T g on resist contrast, are in progress. 4. Conclusions Glass transition temperature represents a significant parameter for the optimum lithographic processing of a resist film. In the current work, the effect of several processing parameters on the glass transition temperature in the case of thin CA resist films was presented using an optical interferometry method. In the case of relatively thick films, the calculated T film from the OPTI method is in very g good agreement with the corresponding DSC values (DT , 3 8C). This value is considered to be

Fig. 7. Contrast curve (exposure at 248 nm) and T film evolution for 500 nm EPR resist. The T gfilm increases as exposure dose g increases and reaches a plateau for high exposure doses (a) PEB conditions were 90 8C, 4 min on a hot plate; (b) PEB conditions were 110 8C, 4 min on a hot plate.

I. Raptis et al. / Microelectronic Engineering 67–68 (2003) 283–291

291

small, taking into account the experimental character of the apparatus used and the different principle of operation. The film thickness effect on T film in the case of two positive tone CA resists (one for g DUV and one for 157-nm lithography) was studied and it was revealed that as film thickness decreases the T film increases. In addition, the existence of solvent in the resist film lowers the apparent g film T g . In the case of an epoxy based negative CA resist, the effect of exposure dose and PEB conditions on T film was studied and it was revealed that increase of exposure dose leads to T film g g increase that never exceeds the applied PEB temperature. In conclusion, the OPTI method proves to be a very helpful tool for the process optimization of lithographic performance of photoresist films with application in next generation lithographic schemes (157 nm, EUV and soft lithography). This method could also be used to complement data obtained by other methods.

Acknowledgements This research work has been funded by the NATO SfP 973718 project and EU-SOARING project.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

http: / / public.itrs.net / Files / 2001ITRS / Litho.pdf S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85. U. Okoroanyanwu, J. Vac. Sci. Technol. B18 (2000) 3381. J.A. Forest, C. Svanberg, K. Revesz, M. Rodahl, L.M. Torell, B. Kasemo, Phys. Rev. E58 (1998) R1226. D.S. Fryer, P.F. Nealey, J.J. de Pablo, J. Vac. Sci. Technol. B18 (2000) 3376. J.L. Keddie, R.A.L. Jones, R.A. Cory, Europhys. Lett. 27 (1994) 59. J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens, J.R. Dutcher, Phys. Rev. Lett 77 (1996) 2002. Y.-K. See, J. Cha, T. Chang, M. Ree, Langmuir 16 (2000) 2351. J.K. Kim, J. Jang, W.-C. Zin, Langmuir 17 (2001) 2703. I. Raptis, C.D. Diakoumakos, Microelectron. Eng. 61–62 (2002) 829. C.D. Diakoumakos, I. Raptis, Polymer 44 (2003) 251. I. Raptis, D. Velesiotis, M. Vasilopoulou, P. Argitis, Microelectron. Eng. 53 (2000) 489. E. Steinsland, T. Finstad, A. Hanneborg, Sens. Actuat. A 86 (2000) 73. I. Raptis, Microelectron. Eng. 57–58 (2001) 525. D. Macintyre, S. Thoms, Microelectron. Eng. 35 (1997) 213. V. Bellas, E. Tegou, I. Raptis, E. Gogolides, P. Argitis, H. Iatrou, N. Hadjichristidis, E. Sarantopoulou, A.C. Cefalas, J. Vac. Sci. Technol. B 20 (2002) 2902. E. Tegou, E. Gogolides, M. Hatzakis, Microelectron. Eng. 35 (1997) 141. B.S. Fryer, S. Bollepali, J.J. dePablo, P.F. Nealey, J. Vac. Sci. Technol. B17 (1999) 3351. P.J. Paniez, L. Pain, J. Photopolymer Sci. Technol. 8 (1995) 643. P. Argitis, S. Boyatzis, I. Raptis, N. Glezos, M. Hatzakis, in: ACS Sym. Ser, Vol. 706, 1998, Chapter 26. M. Hatzakis, K. Stewart, J. Shaw, S. Rishton, J. Electrochem. Soc. 138 (1991) 1076. K.Y. Lee, N. LaBianca, S.A. Rishton, S. Zolghamain, J.D. Gelorme, J. Shaw, T.H.P. Chang, J. Vac. Sci. Technol. B13 (1995) 3012.