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Spin-coated PMMA films
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Microelectronics Journal 38 (2007) 754–761 www.elsevier.com/locate/mejo
Spin-coated PMMA films N.G. Semaltianos1 School of Computing, Communications and Electronics, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Received 15 February 2007; accepted 7 April 2007 Available online 19 July 2007
Abstract Polymethylmethacrylate (PMMA) spin-coated thin films are commonly used as resist films in micro/nanofabrication processes. By using atomic force microscopy (AFM) imaging, scratching lithography and force–distance curves spectroscopy, the spin coating and post-processing conditions were determined, for obtaining films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. Featureless appear the surfaces of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 200 1C for 2 min on a hot plate, while surface defects in the form of large circular pits with diameters between 10 and 20 mm and depth of 2 nm dominate the surface morphologies of films spin coated at 7 krpm on glass substrates from a 2% solution and postbaked either at 200 1C for 2 min on a hot plate or at 170 1C for 30 min in an oven. Surface defects in the form of pinholes appear on the surfaces of films spin coated at 8 krpm on silicon substrates from a 1.25% solution (thickness of 8 nm) and postbaked at 170 1C for 60 min in an oven or left in a low vacuum chamber for a few days. The implication of the different film properties—depending on the preparation parameters—in lithographic techniques is explained and discussed in the paper. r 2007 Published by Elsevier Ltd. Keywords: Spin coating; PMMA; Atomic force microscopy
1. Introduction Polymethylmethacrylate (PMMA) is a single-copolymer (homopolymer) which is commonly used in electron beam (EBL) and scanning probe lithography (SPL) as a positive or negative resist (at high enough doses) due mainly to its highest resolution among conventional organic electron beam resists [1]. The spin-coating method is commonly used as an easy, fast and controllable method for the formation onto substrates of high-quality PMMA resist films with uniform thicknesses, although other methods which result in ultrathin films such as, for example, the Langmuir–Blodgett deposition method have also been used [2–4]. In a few lithographic applications of spin-coated PMMA films such as, for example, SPL by either voltage exposure or mechanical scratching of the resist film by the atomic force microscopy (AFM) tip, it is desirable to have a film whose surface morphology appears featureless. On E-mail address: [email protected] Present address: Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UK. 1
0026-2692/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.mejo.2007.04.019
the other hand, films whose surface morphology is dominated by surface defects may not be useful in SPL or EBL applications. In this paper, the spin coating and post-processing conditions are determined and reported, for obtaining PMMA films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. It has already been shown (by an AFM image), in the context of microelectronic device fabrication, that surface defects in the form of pinholes appear on the surfaces of PMMA films spin coated at high speeds from very dilute solutions (resulting in ultrathin films with thickness of the order of 10 nm) [4]. The exact spin coating and postprocessing conditions of the film were not mentioned in the paper. In this paper, by using AFM imaging, scratching lithography and force–distance (F–D) curves spectroscopy, it is found that featureless appears the surface morphology of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 200 1C for 2 min on a hot plate, and this is due to a non-complete removal (evaporation) of the solvent from the film because of the postbaking time of 2 min possibly being too short to dry
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films spin coated from a solution with this concentration at this spin speed. However, it is found that defects in the form of large circular pits with diameters between 10 and 20 mm and depths of 2 nm dominate the surfaces of films spin coated at 7 krpm but from a 2% solution on glass substrates and postbaked either at 200 1C for 2 min on a hot plate or at 170 1C for 30 min in an oven. Surface defects in the form of pinholes were observed to be formed on the surfaces of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 170 1C for 60 min in an oven or left in a low vacuum chamber for a few days. The negative consequences, in lithographic techniques, of the appearance of pinhole defects on the surfaces of spincoated PMMA films is discussed in the paper. 2. Experimental details The substrates were squares with dimensions of 20 20 mm2, cleaved from thermally oxidized silicon wafers (o1004N-type, thickness ¼ 381 mm, resistivity ¼ 1–10 O cm, oxide thickness ¼ 850–950 A˚) which were used as received without any further cleaning or from glass microscope slides. Before spin coating, the silicon substrates were baked at 175 1C for 1 min in an oven and the glass substrates were cleaned by sonicating them for 10 min each time, first in water with detergent (Decon Neutracon, near neutral concentrate pH ¼ 7), then in acetone and alcohol and finally dried them in an oven at 95 1C for 1 h. The original as received PMMA resist solution with a concentration of 2% in chlorobenzene (molecular weight 950 K, MicroChem Corp.) was diluted in order to obtain 1.25% and 0.50% concentrations. Spin coating was performed in the usual way by depositing the solution drop-wise onto the steady substrate ensuring that the resist covers it completely and then starts spinning at a full set speed. Special care was taken in order to avoid shaking the bottle containing the solution and creating air bubbles. Different spin-coating speeds were used in the region from 5 to 8 krpm (1 krpm ¼ 1000 rpm) for 40 s. After spin coating, the substrates were postbaked at either 170 or 200 1C and for times ranging from 2 to 60 min, either on a hot plate or in an oven. AFM was performed with a NanoRTM instrument from Pacific Nanotechnology Inc. in noncontact mode (constant amplitude) for imaging the resist film surfaces, using cantilevers from NanoWorld with tip radius of curvature p10 nm (heavily doped silicon, thickness ¼ 4 mm, width ¼ 30 mm, length ¼ 125 mm, force constant ¼ 42 N/m, resonance frequency ¼ 320 kHz) or in contact mode using longer cantilevers (force constant ¼ 0.2 N/m, resonance frequency ¼ 13 kHz). For mechanical scratching of the resist film surface in contact mode and subsequent imaging of the structures in non-contact mode, cobalt-coated tips were used (tip radius of curvature p50 nm, thickness ¼ 3 mm, width ¼ 28 mm, length ¼ 225 mm, force constant ¼ 2.8 N/m, resonance frequency ¼ 75 kHz). The SPL software of the AFM was used to ‘‘drive’’ the tip. The substrates were scanned either
755
immediately after spin coating and postbaking or after leaving them in a low vacuum chamber for sometime. Thicknesses of films were measured by scratching the resist surface with metallic tweezers and scanning at the interface. All images presented here were treated with the AFM software for image analysis (Nanorule+TM 2.0) by applying the image-levelling function in the x- and y-directions as well as adjusting contrast and brightness. 3. Results and discussion 3.1. Surface morphology of films spin coated on glass substrates Glass microscope slides are often used in lithography and micro/nanofabrication applications as inexpensive alternative to silicon substrates for initial trials toward the purpose of finding an optimized and reproducible fabrication procedure which will latter be used for the production of functional devices. Thus, films of PMMA have been examined, which were spin coated first onto glass substrates. Fig. 1(a) shows an AFM image of the surface of a glass substrate which was scanned as cleaned, without having applied previously on its surface any coating (coating-free substrate). A morphology characteristic of the surface of free from any surface coating, glass slides, consisting of nanometer-high sub-micron wide regions which appear in the form of straight lines or ridges that run parallel to the length of the slide and extend for tenths of microns, is observed in Fig. 1(a). Ridges have different widths and appear as ‘‘singlets’’, ‘‘doublets’’ or ‘‘triplets’’ which is also a characteristic of the surface morphology of glass slides which have been previously subjected for sometime to washing in a neutral detergent solution [5]. These ridges are formed from chemical inhomogeneities present in the glass melt during the manufacturing process [5–7]. The root mean square (r.m.s.) surface roughness in real space, i.e., the standard deviation of the topographical height is 19 nm. Contrary to glass substrates, uncoated silicon substrates (images not shown in the figure) are perfectly flat without any microscopic surface defects. Fig. 1(b) and (c) shows AFM images of the surfaces of the PMMA films spin coated on glass substrates at 5 and 7 krpm for 40 s from solutions with concentrations 1.25% and 2%, respectively, and postbaked at 200 1C for 2 min on a hot plate immediately after spin coating. The film spin coated at 7 krpm was scanned immediately after postbaking (image in Fig. 1(c)), while the film spin coated at 5 krpm was scanned after keeping it for 3 days in a low vacuum chamber following postbaking (image in Fig. 1(b)). An almost perfect coverage by the resist, of the characteristic surface morphology of uncoated glass is observed in the images. The r.m.s. roughnesses of the surfaces of the films are 2 and 2.3 nm, respectively. In addition to large pits with diameters around 5–12 mm, pits with small diameters of 0.5–1.5 mm and depth of 2 nm (pinholes) are also
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Fig. 1. AFM images of the surfaces of a clean glass substrate (a), and of PMMA films spin coated on glass substrates at 5 and 7 krpm for 40 s from solutions with concentrations of 1.25 (b) and 2% (c), respectively. Pits are marked on the images by dashed-line circles.
observed on the surface of the film in Fig. 1(b). As it will be analysed in more detail in Section 3.3 (similar pinholes but with smaller diameters) are also observed on the surfaces of films which have spin coated at 8 krpm for the same time of 40 s, from a solution with the same concentration of 1.25% and postbaked at the same temperature of 200 1C and for the same time of 2 min and also not scanned immediately after their preparation but after a few days. Therefore, it can be concluded here that the 8 krpm films spin coated from the 1.25% solution still contain a considerable amount of solvent after postbaking them at 200 1C for 2 min and formation of pinholes on the surfaces of these films as well as on the surfaces of the 5 krpm films accompanies the additional removal of the solvent from the film, after leaving the films in the vacuum chamber for a few days. These conclusions will be analysed and discussed in more detail, later in the paper. The surface morphology of the film in Fig. 1(c) is dominated by circular pits with large diameters between 10 and 20 mm and depths of 2 nm, but no small diameter pinholes are distinguished on the image, as seen on the surface of the film in Fig. 1(b). This indicates that small diameter pinholes do not appear on the surfaces of films spin coated at 7 krpm for 40 s from a solution with concentration of 2% when the films are scanned immediately after their preparation. Although the formation of surface defects in the form of pinholes seems to be related to an additional removal of solvent from the films with time; however, the exact reason for the
formation of larger size pits on the surfaces of the films in Fig. 1(b) and (c) might not be related to this mechanism since such large size pits do not appear simultaneously with the smaller size pinholes on the surfaces of films which have been spin coated on silicon instead of glass substrates (as it is the case in Fig. 1) under similar spin coating and postprocessing conditions, as it will be explained in more detail in Section 3.3. The mechanism for the formation of larger size circular pits on the surfaces of the films in Fig. 1(b) and (c), which have spin coated on glass substrates, might be related to molecular forces caused by the confinement of thermal noise inside the film due to annealing the film at 200 1C, the effect of the so-called capillary instability of thin films [8,9] (dewetting phenomenon in annealed films). In this case, although the Hamaker constant A is negative (since nPMMA ¼ 1.49onglass ¼ 1.51) leading to a positive effective interfacial potential F between the film and the substrate and thus from an electromagnetic point of view the PMMA film on the glass substrates is expected to be stable; however, the film becomes unstable due to the presence of an additional surface field which arises from the geometrical confinement of thermally excited acoustic waves inside the film (due to annealing the film at 200 1C). Considering the following set of parameters for the glass substrate and for the PMMA film: [dielectric constants, eglassE6.7, ePMMAE2.6, eglassE6.7, eairE1 (20 1C, 1 MHz)], [refractive indices, nglassE1.51, nPMMAE1.49, nairE1], at T ¼ 200 1C, the Hamaker
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constant has the value: A ¼ 34.45 1022 J. Thus, for the 5 k, 1.25% film with thickness l ¼ 36.21 nm on the glass substrate (Fig. 2, Section 3.2), the van der Waals pressure which arises from the van der Waals potential because of the confinement of electromagnetic modes in the film has the value: pvdW ¼ 3.85 J/m3, while the acoustic pressure that arises from the confinement of thermal fluctuations inside the film has the value: pac ¼ 23.99 J/m3 [9]. Capillary instabilities which manifest themselves with the appearance of the larger diameter circular pits on the surfaces of the films in Fig. 1(b) and (c) take place because it is pvdWo0 and jpac =pvdW j 6. Films spin coated from a 2% solution at 7 krpm for 40 s but postbaked at 170 1C for 30 min in an oven show no difference in surface morphology with those spin coated at the same speed and time but postbaked at 200 1C for 2 min on a hot plate (Fig. 1(c)). This observation follows the prediction of a ‘‘shorter time/higher temperature’’ treatment to be equivalent to a ‘‘longer time/lower temperature’’ treatment for the relevant polymer (segment) relaxation time, according to the temperature-dependent time shift factor due to the Williams–Landel–Ferry (WLF) equation (time–temperature superposition principle) [10,11]. This indicates that the surface morphology of films spin coated from a 2% solution at 7 krpm is independent of the two different postbaking preparation conditions examined here, that of heating in an oven at 170 1C for 30 min or on a hot plate at 200 1C for 2 min. This finding has important implications in lithographic and pattern transfer procedures where PMMA spin coated films are used, since it means that the postbaking time of the films
100 Si 1.25 %, 6 krpm
90 80
resist thickness (nm)
70 60
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after spin coating can be shortened to just 2 min provided the films are postbaked on a hot plate rather than in an oven but on the other hand a bit higher temperature of 200 1C instead of 170 1C should be used. 3.2. Film thicknesses In applications in lithography of spin coated PMMA films, in addition to the film surface morphology, a knowledge of the film thickness in relation to the spin coating and post-processing conditions is also of a great importance. Thus, to establish a correlation between film surface morphology and film thickness, film thicknesses were measured next. The thicknesses of the films spin coated on glass and silicon substrates at the different spin speeds and from the solutions with the different concentrations of 2%, 1.25% and 0.50% are plotted in Fig. 2 (inset shows a representative image of an interface). A difference in film thickness of less than 1% is measured between the centre of the substrate and a region close to its edge. For the 1.25% solution on silicon substrates in Fig. 2, the thickness of the film is determined to follow here a o0.4570.06 dependence on spin speed (solid curve through the four data points), very close to the theoretically predicted one of o0.5 which is known to be valid for a Newtonian fluid in the moderate solution concentrations (10–25 g/l) regime (as it is the present case for the solutions with concentrations of 1.25% v/v ¼ 14.8 g/l and 2% v/v ¼ 23.8 g/l) [12,13]. It is interesting to note that a film as thin as 8 nm is obtained in the case of films spin coated from a 0.5% solution on silicon substrates. Such a thin film would be very useful, in, for example, SPL applications by voltage exposure of PMMA as it will allow the application of a relatively low voltage for the exposure of the film (of no more than 10 V in this case, taking into account that the voltage required to expose PMMA films with thickness of V nm is approximately equal to V Volts) as well as by providing high resolution, which is estimated to be approximately equal to the film thickness (8 nm) [14]. 3.3. Surface morphology of films spin coated on silicon substrates
50 40 30 20
glass, 2% Si, 1.25 %
10
glass, 1.25 % Si, 0.50 %
0 4
5
6 7 rotation speed (krpm)
8
9
Fig. 2. Thicknesses of the films spin coated on glass and silicon substrates at the different spin speeds and from solutions with the different concentrations of 2%, 1.25% and 0.5%.
Fabrication processes of micro/nanoelectronic devices usually involve PMMA films spin coated on silicon substrates. Fig. 3(a) and (b) shows images of the surfaces of the films spin coated on silicon substrates at 8 krpm for 40 s from 1.25% solution and postbaked on a hot plate at 200 1C for 2 min and in an oven at 170 1C for 60 min, respectively. On the surface of the film in Fig. 3(a) no pits, pinholes or any other kind of defects are observed. The surface appears featureless. On the other hand, a careful observation of the image in Fig. 3(b) reveals the appearance of pinholes on the surface of that film. In order to obtain more information about the possible mechanisms which lead to the formation of pinholes on the
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Fig. 3. AFM images of the surfaces of the PMMA films spin coated on silicon substrates at 8 krpm for 40 s from the solution with a concentration of 1.25% and postbaked on a hot plate at 200 1C for 2 min (a) and at 170 1C for 60 min in an oven (b). (c,d) AFM images of the surface of the film in (a) but after leaving the film for 1 day in a low vacuum chamber.
surfaces of the films, the film of Fig. 3(a) was left in the low vacuum chamber for 1 day and its surface was scanned again after that. Images of the film surface are presented in Fig. 3(c) and (d). The image in Fig. 3(d) (2.22 2.22 mm2) is a rescan of the same area of the film as in Fig. 3(c) (5.18 5.18 mm2) but at a higher magnification. Note that in order to be able to compare together the images of Fig. 3, all images have been rescaled identically along the z-axis (0–13 nm z-scale range). Pinholes on the surface of the film are now distinguished (images in Fig. 3(c) and (d)) as compared to the image in Fig. 3(a) where the surface of the film appeared featureless. Their size distribution is between 20 and 40 nm. The fact that pinholes do not appear on the surface of the film if the film has been postbaked for a short time of 2 min (image in Fig. 3(a)) but they appear on the surface of the film after 1 day (images in Fig. 3(c) and (d)), indicates that formation of pinholes on the film surface accompanies the additional removal (evaporation) of the solvent from the film because of keeping the film in the low vacuum chamber following postbaking. Also, this indicates that the film in Fig. 3(a) may not be in a complete dry state but it rather contains a considerable amount of solvent. This indicates that the heating time of 2 min at 200 1C may not be enough to make the glass transition temperature (Tg) of the film rise above 200 1C and thus the film does not solidify but rather stays ‘‘wet’’. It is also worth mentioning
here that, it appears that an additional characteristic of a surface which consists of pinholes, is that the morphology of such a surface is not scale invariant and therefore the surface does not exhibit a fractal self-affine behaviour [15,16]. This can be inferred from the fact that the morphology of the surface as it is seen in the image in Fig. 3(d) (which is a higher magnification of the one in Fig. 3(c)) appears to be different than the morphology which is seen in the image of Fig. 3(c). Formation of pinhole defects on the surfaces of the films which have been left in the vacuum chamber for some time could be explained by the build up of tensile stresses (s) inside the film which are larger than the yield stress (sy) of the material, when the film becomes solid by leaving it in the vacuum chamber for a few days. These are the so-called irreversible effects and take place below the Tg (‘‘assisted dewetting’’) [11]. Obtaining a resist film whose surface morphology is dominated by pinhole surface defects is undesirable in lithography applications, as the use of such a film will result in deterioration of the registration of written features including increase in the roughness of the walls of exposed trenches and thus decreasing writing resolution. On the other hand, a resist film as thin as 8 nm offers the advantages of high resolution and low voltage exposure in EBL and SPL applications, respectively.
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3.4. Mechanical scratching of the film by the AFM tip and force–distance curves
relation [17,18]: HS ¼ q
In applications of spin-coated PMMA films in lithography and especially in SPL, in addition to the surface morphology of the film, it is also very important to have information about the mechanical properties of the film such as hardness in relation to the spin coating and postprocessing conditions of the film as this will affect the choice of SPL parameters which are to be used for a reliable fabrication and pattern transferring process using the resist film [14]. Thus, in order to acquire information about hardness properties of the spin-coated PMMA films and correlate them with the corresponding surface morphology of the films as this is probed by AFM imaging, nanoscratching lithography was performed on the films by using the tip of the AFM. Fig. 4(a) shows lines scratched by using the AFM tip in contact mode under an applied load P ¼ 1000 mN (0.001 N), on the surface of the resist film spin coated at 5 krpm for 40 s from 1.25% solution of PMMA on a glass substrate and postbaked at 200 1C for 2 min on a hot plate. The scratching was performed on the film immediately after spin coating and postbaking. The lines from left to right correspond to different tip speeds of 0.5, 1 and 2 mm/s, respectively. There is an almost symmetric pilling up of ejected material on both sides of the scratched grooves except from the case of the second line, as the cross-sectional graph of Fig. 4(b) illustrates. For a plastically deformed viscoelastic material, the scratch hardness is given by the
height (nm)
300
w1
w2
w3
250
200
150 0.0 0.5 1.0 1.5
5.5 6.0 6.5 10.5 cross section (m)
11.0
11.5
Fig. 4. (a) Lines scratched on the surface of the PMMA film by using the AFM tip, with different speeds of 0.5, 1 and 2 mm/s, respectively (from left to right) and (b) cross-sections along the image in (a).
759
4P 6P ffi 2 pw pw2
(1)
(assuming q ¼ 1.5 for polymers [19]), where w ¼ 2h tan a is the so-called recovered width of the scratch (the internal width of the groove without taking into account the externally pilled up material), h is the depth of the scratch and 2a is the opening angle of the tip. From the crosssectional graphs of Fig. 4(b), the widths of the scratch lines are measured to be: w1 ¼ 151710 nm, w2 ¼ 154715 nm and w3 ¼ 145710 nm which yield for the scratch hardnesses the values HS1 ¼ 84711 MPa, HS2 ¼ 81714 MPa and HS3 ¼ 91712 MPa, respectively. The scratch hardness of 3 mm solid sheets of PMMA measured under the application of 1 N load for scratching velocities in the range of 0.1–2 mm/s is 125 MPa (assuming q ¼ 1 in Eq. (1)), almost independent of the contact strain (scratch hardness map for PMMA, p. 142 of Ref. [18]) and a decreasing scratch hardness for increasing load was reported in Ref. [20]. However, recent studies of the scratch hardness of PMMA yield values of 590–690 MPa for scratch velocities in the range of 200–2000 mm/s [19] (assuming q ¼ 1 in Eq. (1)) at a load of 1 N but the ratio of P/w2 was found not to be affected much from the change in scratch speed. Also the scratch hardness was found to increase for loads from 0.5 to 1 N but it was showing a decreasing trend for higher loads up to 10 N [19]. Since a value in the range of 81–91 MPa for the scratch hardness of spin-coated PMMA films was measured here by the application of a load which is much less than the loads which were used in the literature for the measurement of the scratch hardness of solid sheets of PMMA, following Ref. [20], it can be concluded that the scratch hardness of the spin-coated PMMA film of Fig. 4 which would correspond to the application of 1 N load is smaller than 81–91 MPa. This in turn indicates that the film in Fig. 4 is not in the solid state but it contains indeed an amount of solvent. This fact correlated with its surface morphology as it is seen in the image of Fig. 4 where a pinholes free surface is observed as compared to the film surface shown in the image of Fig. 1(b), confirms once again that the additional removal of solvent from the film (evaporation of solvent) results in the formation of pinholes on the film surface. Finally, in order to obtain information about adhesive forces of the spin-coated PMMA films, F–D curves were measured on the films by using the AFM. Typical F–D curves measured on the films are shown in Fig. 5, where the horizontal scale corresponds to the vertical extension of the piezo-tube. The continuous line was measured on the surface of a clean silicon substrate without any coating. The dashed and dashed–dotted lines were measured on the films spin coated at 8 krpm and 5 krpm for 40 s from 1.25% solution and postbaked on a hot plate at 200 1C for 2 min, respectively. The F–D curves were measured on the 8 krpm film after keeping it for 7 days in the vacuum chamber (this provides with the case of a film which has not been
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10 PMMA, 8krmps PMMA, 5 krpms Si uncoated
8
force (nN)
6 4 2 0 adhesion force
-2 -4 -3000
-2000
-1000 0 1000 distance (nm)
2000
3000
Fig. 5. Force–distance curves measured on the film spin coated at 8 krpm (dashed line) and on the film spin coated at 5 krpm (dashed–dotted line). Solid line corresponds to curves measured on the surface of an uncoated silicon substrate.
characterized immediately after its preparation but after keeping it instead in the vacuum chamber for sometime, similarly to the case of the film in Fig. 3(c)) while for the 5 krpm film were measured immediately after its preparation (this provides with the case of a film which has been characterized immediately after its preparation similarly to the case of the film in Fig. 4). Approaching and retracting curves for the silicon substrate have almost the same slope but they do not coincide due to scanner hysteresis and the absence of a snap-out force in the retracting curve indicates that there is no water layer contamination on its surface under the environmental conditions (humidity and temperature) in the laboratory and also no mechanical deformation of the substrate by the tip [21]. For the resist films, approaching and retracting curves during contact of cantilever with the film have different slopes indicating that there is plastic deformation (indentation) of the film by the tip. The snap-out forces correspond to the adhesive forces of the film. Curves with the same overall characteristics, as the ones presented in Fig. 5, have been measured on films spin coated at other speeds. From the graph of Fig. 5, the adhesion forces are measured equal to: F8k ¼ 1.33 nN and F5k ¼ 4.24 nN. Since it is F5k4F8k, this indicates that the 8k film contains less amount of solvent, i.e. it is dryer than the 5k film. This result provides with one more confirmation that solvent evaporation from the film, results in the formation of pinholes on its surface. 4. Conclusions By using AFM both as an imaging and as an indentation tool, PMMA films spin coated from solutions with concentrations and at spin speeds in the range of the ones commonly used in lithographic applications have been
investigated with the purpose of determining the spin coating and post-processing conditions, for obtaining films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. It is found that pinholes are formed on the surfaces of films which have been spin coated from solution with a concentration of 1.25% at either 5 or at 8 krpm on glass or silicon substrates, as a result of the additional removal (evaporation) of the solvent from the film because of either leaving the film in a low vacuum chamber for a few days or postbaking it for 60 min at 170 1C in an oven. This means that films spin coated and post-treated under the above conditions will always show pinhole defects on their surfaces. Large pits with diameters in the range of 10–20 mm and depth of 2 nm, dominate the surfaces of films spin coated from solution with concentration of 2% at 7 krpm on glass substrates, while featureless appear the surfaces of films spin coated from a 1.25% solution at 8 krpm on silicon substrates. As a thinner film is obtained at higher spin speeds than at lower ones, this finding indicates that films spin coated on silicon substrates at higher speeds and from solutions with lower concentrations are superior in terms of their surface morphology as compared to films spin coated on glass substrates and from solutions with higher concentrations. It is also found that no difference in the surface morphology is observed between films spin coated from a 2% solution at 7 krpm for 40 s but postbaked at 170 1C for 30 min in an oven, and films spin coated at the same speed and for the same time but postbaked at 200 1C for 2 min on a hot plate. Finally, it seems that a good compromise between film dryness and formation of low density of pinholes for films spin coated on silicon substrates at 8 krpm from 1.25% solution is a postbaking time of 60 min at 170 1C in an oven or less than 60 min at 200 1C on a hot plate. The results and findings presented here have important implications in the use of spin-coated PMMA films in lithographic applications.
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[17] B.J. Briscoe, P.D. Evans, E. Pelillo, S.K. Sinha, Wear 200 (1996) 137. [18] B.J. Briscoe, P.D. Evans, S.K. Biswas, S.K. Sinha, Tribol. Int. 29 (1996) 93. [19] S.K. Sinha, D.B.J. Lim, Wear 260 (2006) 751. [20] B.J. Briscoe, E. Pelillo, F. Ragazzi, S.K. Sinha, Polymer 39 (1998) 2161. [21] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, New York, 1992.
Microelectronics Journal 38 (2007) 754–761 www.elsevier.com/locate/mejo
Spin-coated PMMA films N.G. Semaltianos1 School of Computing, Communications and Electronics, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Received 15 February 2007; accepted 7 April 2007 Available online 19 July 2007
Abstract Polymethylmethacrylate (PMMA) spin-coated thin films are commonly used as resist films in micro/nanofabrication processes. By using atomic force microscopy (AFM) imaging, scratching lithography and force–distance curves spectroscopy, the spin coating and post-processing conditions were determined, for obtaining films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. Featureless appear the surfaces of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 200 1C for 2 min on a hot plate, while surface defects in the form of large circular pits with diameters between 10 and 20 mm and depth of 2 nm dominate the surface morphologies of films spin coated at 7 krpm on glass substrates from a 2% solution and postbaked either at 200 1C for 2 min on a hot plate or at 170 1C for 30 min in an oven. Surface defects in the form of pinholes appear on the surfaces of films spin coated at 8 krpm on silicon substrates from a 1.25% solution (thickness of 8 nm) and postbaked at 170 1C for 60 min in an oven or left in a low vacuum chamber for a few days. The implication of the different film properties—depending on the preparation parameters—in lithographic techniques is explained and discussed in the paper. r 2007 Published by Elsevier Ltd. Keywords: Spin coating; PMMA; Atomic force microscopy
1. Introduction Polymethylmethacrylate (PMMA) is a single-copolymer (homopolymer) which is commonly used in electron beam (EBL) and scanning probe lithography (SPL) as a positive or negative resist (at high enough doses) due mainly to its highest resolution among conventional organic electron beam resists [1]. The spin-coating method is commonly used as an easy, fast and controllable method for the formation onto substrates of high-quality PMMA resist films with uniform thicknesses, although other methods which result in ultrathin films such as, for example, the Langmuir–Blodgett deposition method have also been used [2–4]. In a few lithographic applications of spin-coated PMMA films such as, for example, SPL by either voltage exposure or mechanical scratching of the resist film by the atomic force microscopy (AFM) tip, it is desirable to have a film whose surface morphology appears featureless. On E-mail address: [email protected] Present address: Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UK. 1
0026-2692/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.mejo.2007.04.019
the other hand, films whose surface morphology is dominated by surface defects may not be useful in SPL or EBL applications. In this paper, the spin coating and post-processing conditions are determined and reported, for obtaining PMMA films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. It has already been shown (by an AFM image), in the context of microelectronic device fabrication, that surface defects in the form of pinholes appear on the surfaces of PMMA films spin coated at high speeds from very dilute solutions (resulting in ultrathin films with thickness of the order of 10 nm) [4]. The exact spin coating and postprocessing conditions of the film were not mentioned in the paper. In this paper, by using AFM imaging, scratching lithography and force–distance (F–D) curves spectroscopy, it is found that featureless appears the surface morphology of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 200 1C for 2 min on a hot plate, and this is due to a non-complete removal (evaporation) of the solvent from the film because of the postbaking time of 2 min possibly being too short to dry
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films spin coated from a solution with this concentration at this spin speed. However, it is found that defects in the form of large circular pits with diameters between 10 and 20 mm and depths of 2 nm dominate the surfaces of films spin coated at 7 krpm but from a 2% solution on glass substrates and postbaked either at 200 1C for 2 min on a hot plate or at 170 1C for 30 min in an oven. Surface defects in the form of pinholes were observed to be formed on the surfaces of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 170 1C for 60 min in an oven or left in a low vacuum chamber for a few days. The negative consequences, in lithographic techniques, of the appearance of pinhole defects on the surfaces of spincoated PMMA films is discussed in the paper. 2. Experimental details The substrates were squares with dimensions of 20 20 mm2, cleaved from thermally oxidized silicon wafers (o1004N-type, thickness ¼ 381 mm, resistivity ¼ 1–10 O cm, oxide thickness ¼ 850–950 A˚) which were used as received without any further cleaning or from glass microscope slides. Before spin coating, the silicon substrates were baked at 175 1C for 1 min in an oven and the glass substrates were cleaned by sonicating them for 10 min each time, first in water with detergent (Decon Neutracon, near neutral concentrate pH ¼ 7), then in acetone and alcohol and finally dried them in an oven at 95 1C for 1 h. The original as received PMMA resist solution with a concentration of 2% in chlorobenzene (molecular weight 950 K, MicroChem Corp.) was diluted in order to obtain 1.25% and 0.50% concentrations. Spin coating was performed in the usual way by depositing the solution drop-wise onto the steady substrate ensuring that the resist covers it completely and then starts spinning at a full set speed. Special care was taken in order to avoid shaking the bottle containing the solution and creating air bubbles. Different spin-coating speeds were used in the region from 5 to 8 krpm (1 krpm ¼ 1000 rpm) for 40 s. After spin coating, the substrates were postbaked at either 170 or 200 1C and for times ranging from 2 to 60 min, either on a hot plate or in an oven. AFM was performed with a NanoRTM instrument from Pacific Nanotechnology Inc. in noncontact mode (constant amplitude) for imaging the resist film surfaces, using cantilevers from NanoWorld with tip radius of curvature p10 nm (heavily doped silicon, thickness ¼ 4 mm, width ¼ 30 mm, length ¼ 125 mm, force constant ¼ 42 N/m, resonance frequency ¼ 320 kHz) or in contact mode using longer cantilevers (force constant ¼ 0.2 N/m, resonance frequency ¼ 13 kHz). For mechanical scratching of the resist film surface in contact mode and subsequent imaging of the structures in non-contact mode, cobalt-coated tips were used (tip radius of curvature p50 nm, thickness ¼ 3 mm, width ¼ 28 mm, length ¼ 225 mm, force constant ¼ 2.8 N/m, resonance frequency ¼ 75 kHz). The SPL software of the AFM was used to ‘‘drive’’ the tip. The substrates were scanned either
755
immediately after spin coating and postbaking or after leaving them in a low vacuum chamber for sometime. Thicknesses of films were measured by scratching the resist surface with metallic tweezers and scanning at the interface. All images presented here were treated with the AFM software for image analysis (Nanorule+TM 2.0) by applying the image-levelling function in the x- and y-directions as well as adjusting contrast and brightness. 3. Results and discussion 3.1. Surface morphology of films spin coated on glass substrates Glass microscope slides are often used in lithography and micro/nanofabrication applications as inexpensive alternative to silicon substrates for initial trials toward the purpose of finding an optimized and reproducible fabrication procedure which will latter be used for the production of functional devices. Thus, films of PMMA have been examined, which were spin coated first onto glass substrates. Fig. 1(a) shows an AFM image of the surface of a glass substrate which was scanned as cleaned, without having applied previously on its surface any coating (coating-free substrate). A morphology characteristic of the surface of free from any surface coating, glass slides, consisting of nanometer-high sub-micron wide regions which appear in the form of straight lines or ridges that run parallel to the length of the slide and extend for tenths of microns, is observed in Fig. 1(a). Ridges have different widths and appear as ‘‘singlets’’, ‘‘doublets’’ or ‘‘triplets’’ which is also a characteristic of the surface morphology of glass slides which have been previously subjected for sometime to washing in a neutral detergent solution [5]. These ridges are formed from chemical inhomogeneities present in the glass melt during the manufacturing process [5–7]. The root mean square (r.m.s.) surface roughness in real space, i.e., the standard deviation of the topographical height is 19 nm. Contrary to glass substrates, uncoated silicon substrates (images not shown in the figure) are perfectly flat without any microscopic surface defects. Fig. 1(b) and (c) shows AFM images of the surfaces of the PMMA films spin coated on glass substrates at 5 and 7 krpm for 40 s from solutions with concentrations 1.25% and 2%, respectively, and postbaked at 200 1C for 2 min on a hot plate immediately after spin coating. The film spin coated at 7 krpm was scanned immediately after postbaking (image in Fig. 1(c)), while the film spin coated at 5 krpm was scanned after keeping it for 3 days in a low vacuum chamber following postbaking (image in Fig. 1(b)). An almost perfect coverage by the resist, of the characteristic surface morphology of uncoated glass is observed in the images. The r.m.s. roughnesses of the surfaces of the films are 2 and 2.3 nm, respectively. In addition to large pits with diameters around 5–12 mm, pits with small diameters of 0.5–1.5 mm and depth of 2 nm (pinholes) are also
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Fig. 1. AFM images of the surfaces of a clean glass substrate (a), and of PMMA films spin coated on glass substrates at 5 and 7 krpm for 40 s from solutions with concentrations of 1.25 (b) and 2% (c), respectively. Pits are marked on the images by dashed-line circles.
observed on the surface of the film in Fig. 1(b). As it will be analysed in more detail in Section 3.3 (similar pinholes but with smaller diameters) are also observed on the surfaces of films which have spin coated at 8 krpm for the same time of 40 s, from a solution with the same concentration of 1.25% and postbaked at the same temperature of 200 1C and for the same time of 2 min and also not scanned immediately after their preparation but after a few days. Therefore, it can be concluded here that the 8 krpm films spin coated from the 1.25% solution still contain a considerable amount of solvent after postbaking them at 200 1C for 2 min and formation of pinholes on the surfaces of these films as well as on the surfaces of the 5 krpm films accompanies the additional removal of the solvent from the film, after leaving the films in the vacuum chamber for a few days. These conclusions will be analysed and discussed in more detail, later in the paper. The surface morphology of the film in Fig. 1(c) is dominated by circular pits with large diameters between 10 and 20 mm and depths of 2 nm, but no small diameter pinholes are distinguished on the image, as seen on the surface of the film in Fig. 1(b). This indicates that small diameter pinholes do not appear on the surfaces of films spin coated at 7 krpm for 40 s from a solution with concentration of 2% when the films are scanned immediately after their preparation. Although the formation of surface defects in the form of pinholes seems to be related to an additional removal of solvent from the films with time; however, the exact reason for the
formation of larger size pits on the surfaces of the films in Fig. 1(b) and (c) might not be related to this mechanism since such large size pits do not appear simultaneously with the smaller size pinholes on the surfaces of films which have been spin coated on silicon instead of glass substrates (as it is the case in Fig. 1) under similar spin coating and postprocessing conditions, as it will be explained in more detail in Section 3.3. The mechanism for the formation of larger size circular pits on the surfaces of the films in Fig. 1(b) and (c), which have spin coated on glass substrates, might be related to molecular forces caused by the confinement of thermal noise inside the film due to annealing the film at 200 1C, the effect of the so-called capillary instability of thin films [8,9] (dewetting phenomenon in annealed films). In this case, although the Hamaker constant A is negative (since nPMMA ¼ 1.49onglass ¼ 1.51) leading to a positive effective interfacial potential F between the film and the substrate and thus from an electromagnetic point of view the PMMA film on the glass substrates is expected to be stable; however, the film becomes unstable due to the presence of an additional surface field which arises from the geometrical confinement of thermally excited acoustic waves inside the film (due to annealing the film at 200 1C). Considering the following set of parameters for the glass substrate and for the PMMA film: [dielectric constants, eglassE6.7, ePMMAE2.6, eglassE6.7, eairE1 (20 1C, 1 MHz)], [refractive indices, nglassE1.51, nPMMAE1.49, nairE1], at T ¼ 200 1C, the Hamaker
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constant has the value: A ¼ 34.45 1022 J. Thus, for the 5 k, 1.25% film with thickness l ¼ 36.21 nm on the glass substrate (Fig. 2, Section 3.2), the van der Waals pressure which arises from the van der Waals potential because of the confinement of electromagnetic modes in the film has the value: pvdW ¼ 3.85 J/m3, while the acoustic pressure that arises from the confinement of thermal fluctuations inside the film has the value: pac ¼ 23.99 J/m3 [9]. Capillary instabilities which manifest themselves with the appearance of the larger diameter circular pits on the surfaces of the films in Fig. 1(b) and (c) take place because it is pvdWo0 and jpac =pvdW j 6. Films spin coated from a 2% solution at 7 krpm for 40 s but postbaked at 170 1C for 30 min in an oven show no difference in surface morphology with those spin coated at the same speed and time but postbaked at 200 1C for 2 min on a hot plate (Fig. 1(c)). This observation follows the prediction of a ‘‘shorter time/higher temperature’’ treatment to be equivalent to a ‘‘longer time/lower temperature’’ treatment for the relevant polymer (segment) relaxation time, according to the temperature-dependent time shift factor due to the Williams–Landel–Ferry (WLF) equation (time–temperature superposition principle) [10,11]. This indicates that the surface morphology of films spin coated from a 2% solution at 7 krpm is independent of the two different postbaking preparation conditions examined here, that of heating in an oven at 170 1C for 30 min or on a hot plate at 200 1C for 2 min. This finding has important implications in lithographic and pattern transfer procedures where PMMA spin coated films are used, since it means that the postbaking time of the films
100 Si 1.25 %, 6 krpm
90 80
resist thickness (nm)
70 60
757
after spin coating can be shortened to just 2 min provided the films are postbaked on a hot plate rather than in an oven but on the other hand a bit higher temperature of 200 1C instead of 170 1C should be used. 3.2. Film thicknesses In applications in lithography of spin coated PMMA films, in addition to the film surface morphology, a knowledge of the film thickness in relation to the spin coating and post-processing conditions is also of a great importance. Thus, to establish a correlation between film surface morphology and film thickness, film thicknesses were measured next. The thicknesses of the films spin coated on glass and silicon substrates at the different spin speeds and from the solutions with the different concentrations of 2%, 1.25% and 0.50% are plotted in Fig. 2 (inset shows a representative image of an interface). A difference in film thickness of less than 1% is measured between the centre of the substrate and a region close to its edge. For the 1.25% solution on silicon substrates in Fig. 2, the thickness of the film is determined to follow here a o0.4570.06 dependence on spin speed (solid curve through the four data points), very close to the theoretically predicted one of o0.5 which is known to be valid for a Newtonian fluid in the moderate solution concentrations (10–25 g/l) regime (as it is the present case for the solutions with concentrations of 1.25% v/v ¼ 14.8 g/l and 2% v/v ¼ 23.8 g/l) [12,13]. It is interesting to note that a film as thin as 8 nm is obtained in the case of films spin coated from a 0.5% solution on silicon substrates. Such a thin film would be very useful, in, for example, SPL applications by voltage exposure of PMMA as it will allow the application of a relatively low voltage for the exposure of the film (of no more than 10 V in this case, taking into account that the voltage required to expose PMMA films with thickness of V nm is approximately equal to V Volts) as well as by providing high resolution, which is estimated to be approximately equal to the film thickness (8 nm) [14]. 3.3. Surface morphology of films spin coated on silicon substrates
50 40 30 20
glass, 2% Si, 1.25 %
10
glass, 1.25 % Si, 0.50 %
0 4
5
6 7 rotation speed (krpm)
8
9
Fig. 2. Thicknesses of the films spin coated on glass and silicon substrates at the different spin speeds and from solutions with the different concentrations of 2%, 1.25% and 0.5%.
Fabrication processes of micro/nanoelectronic devices usually involve PMMA films spin coated on silicon substrates. Fig. 3(a) and (b) shows images of the surfaces of the films spin coated on silicon substrates at 8 krpm for 40 s from 1.25% solution and postbaked on a hot plate at 200 1C for 2 min and in an oven at 170 1C for 60 min, respectively. On the surface of the film in Fig. 3(a) no pits, pinholes or any other kind of defects are observed. The surface appears featureless. On the other hand, a careful observation of the image in Fig. 3(b) reveals the appearance of pinholes on the surface of that film. In order to obtain more information about the possible mechanisms which lead to the formation of pinholes on the
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Fig. 3. AFM images of the surfaces of the PMMA films spin coated on silicon substrates at 8 krpm for 40 s from the solution with a concentration of 1.25% and postbaked on a hot plate at 200 1C for 2 min (a) and at 170 1C for 60 min in an oven (b). (c,d) AFM images of the surface of the film in (a) but after leaving the film for 1 day in a low vacuum chamber.
surfaces of the films, the film of Fig. 3(a) was left in the low vacuum chamber for 1 day and its surface was scanned again after that. Images of the film surface are presented in Fig. 3(c) and (d). The image in Fig. 3(d) (2.22 2.22 mm2) is a rescan of the same area of the film as in Fig. 3(c) (5.18 5.18 mm2) but at a higher magnification. Note that in order to be able to compare together the images of Fig. 3, all images have been rescaled identically along the z-axis (0–13 nm z-scale range). Pinholes on the surface of the film are now distinguished (images in Fig. 3(c) and (d)) as compared to the image in Fig. 3(a) where the surface of the film appeared featureless. Their size distribution is between 20 and 40 nm. The fact that pinholes do not appear on the surface of the film if the film has been postbaked for a short time of 2 min (image in Fig. 3(a)) but they appear on the surface of the film after 1 day (images in Fig. 3(c) and (d)), indicates that formation of pinholes on the film surface accompanies the additional removal (evaporation) of the solvent from the film because of keeping the film in the low vacuum chamber following postbaking. Also, this indicates that the film in Fig. 3(a) may not be in a complete dry state but it rather contains a considerable amount of solvent. This indicates that the heating time of 2 min at 200 1C may not be enough to make the glass transition temperature (Tg) of the film rise above 200 1C and thus the film does not solidify but rather stays ‘‘wet’’. It is also worth mentioning
here that, it appears that an additional characteristic of a surface which consists of pinholes, is that the morphology of such a surface is not scale invariant and therefore the surface does not exhibit a fractal self-affine behaviour [15,16]. This can be inferred from the fact that the morphology of the surface as it is seen in the image in Fig. 3(d) (which is a higher magnification of the one in Fig. 3(c)) appears to be different than the morphology which is seen in the image of Fig. 3(c). Formation of pinhole defects on the surfaces of the films which have been left in the vacuum chamber for some time could be explained by the build up of tensile stresses (s) inside the film which are larger than the yield stress (sy) of the material, when the film becomes solid by leaving it in the vacuum chamber for a few days. These are the so-called irreversible effects and take place below the Tg (‘‘assisted dewetting’’) [11]. Obtaining a resist film whose surface morphology is dominated by pinhole surface defects is undesirable in lithography applications, as the use of such a film will result in deterioration of the registration of written features including increase in the roughness of the walls of exposed trenches and thus decreasing writing resolution. On the other hand, a resist film as thin as 8 nm offers the advantages of high resolution and low voltage exposure in EBL and SPL applications, respectively.
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3.4. Mechanical scratching of the film by the AFM tip and force–distance curves
relation [17,18]: HS ¼ q
In applications of spin-coated PMMA films in lithography and especially in SPL, in addition to the surface morphology of the film, it is also very important to have information about the mechanical properties of the film such as hardness in relation to the spin coating and postprocessing conditions of the film as this will affect the choice of SPL parameters which are to be used for a reliable fabrication and pattern transferring process using the resist film [14]. Thus, in order to acquire information about hardness properties of the spin-coated PMMA films and correlate them with the corresponding surface morphology of the films as this is probed by AFM imaging, nanoscratching lithography was performed on the films by using the tip of the AFM. Fig. 4(a) shows lines scratched by using the AFM tip in contact mode under an applied load P ¼ 1000 mN (0.001 N), on the surface of the resist film spin coated at 5 krpm for 40 s from 1.25% solution of PMMA on a glass substrate and postbaked at 200 1C for 2 min on a hot plate. The scratching was performed on the film immediately after spin coating and postbaking. The lines from left to right correspond to different tip speeds of 0.5, 1 and 2 mm/s, respectively. There is an almost symmetric pilling up of ejected material on both sides of the scratched grooves except from the case of the second line, as the cross-sectional graph of Fig. 4(b) illustrates. For a plastically deformed viscoelastic material, the scratch hardness is given by the
height (nm)
300
w1
w2
w3
250
200
150 0.0 0.5 1.0 1.5
5.5 6.0 6.5 10.5 cross section (m)
11.0
11.5
Fig. 4. (a) Lines scratched on the surface of the PMMA film by using the AFM tip, with different speeds of 0.5, 1 and 2 mm/s, respectively (from left to right) and (b) cross-sections along the image in (a).
759
4P 6P ffi 2 pw pw2
(1)
(assuming q ¼ 1.5 for polymers [19]), where w ¼ 2h tan a is the so-called recovered width of the scratch (the internal width of the groove without taking into account the externally pilled up material), h is the depth of the scratch and 2a is the opening angle of the tip. From the crosssectional graphs of Fig. 4(b), the widths of the scratch lines are measured to be: w1 ¼ 151710 nm, w2 ¼ 154715 nm and w3 ¼ 145710 nm which yield for the scratch hardnesses the values HS1 ¼ 84711 MPa, HS2 ¼ 81714 MPa and HS3 ¼ 91712 MPa, respectively. The scratch hardness of 3 mm solid sheets of PMMA measured under the application of 1 N load for scratching velocities in the range of 0.1–2 mm/s is 125 MPa (assuming q ¼ 1 in Eq. (1)), almost independent of the contact strain (scratch hardness map for PMMA, p. 142 of Ref. [18]) and a decreasing scratch hardness for increasing load was reported in Ref. [20]. However, recent studies of the scratch hardness of PMMA yield values of 590–690 MPa for scratch velocities in the range of 200–2000 mm/s [19] (assuming q ¼ 1 in Eq. (1)) at a load of 1 N but the ratio of P/w2 was found not to be affected much from the change in scratch speed. Also the scratch hardness was found to increase for loads from 0.5 to 1 N but it was showing a decreasing trend for higher loads up to 10 N [19]. Since a value in the range of 81–91 MPa for the scratch hardness of spin-coated PMMA films was measured here by the application of a load which is much less than the loads which were used in the literature for the measurement of the scratch hardness of solid sheets of PMMA, following Ref. [20], it can be concluded that the scratch hardness of the spin-coated PMMA film of Fig. 4 which would correspond to the application of 1 N load is smaller than 81–91 MPa. This in turn indicates that the film in Fig. 4 is not in the solid state but it contains indeed an amount of solvent. This fact correlated with its surface morphology as it is seen in the image of Fig. 4 where a pinholes free surface is observed as compared to the film surface shown in the image of Fig. 1(b), confirms once again that the additional removal of solvent from the film (evaporation of solvent) results in the formation of pinholes on the film surface. Finally, in order to obtain information about adhesive forces of the spin-coated PMMA films, F–D curves were measured on the films by using the AFM. Typical F–D curves measured on the films are shown in Fig. 5, where the horizontal scale corresponds to the vertical extension of the piezo-tube. The continuous line was measured on the surface of a clean silicon substrate without any coating. The dashed and dashed–dotted lines were measured on the films spin coated at 8 krpm and 5 krpm for 40 s from 1.25% solution and postbaked on a hot plate at 200 1C for 2 min, respectively. The F–D curves were measured on the 8 krpm film after keeping it for 7 days in the vacuum chamber (this provides with the case of a film which has not been
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10 PMMA, 8krmps PMMA, 5 krpms Si uncoated
8
force (nN)
6 4 2 0 adhesion force
-2 -4 -3000
-2000
-1000 0 1000 distance (nm)
2000
3000
Fig. 5. Force–distance curves measured on the film spin coated at 8 krpm (dashed line) and on the film spin coated at 5 krpm (dashed–dotted line). Solid line corresponds to curves measured on the surface of an uncoated silicon substrate.
characterized immediately after its preparation but after keeping it instead in the vacuum chamber for sometime, similarly to the case of the film in Fig. 3(c)) while for the 5 krpm film were measured immediately after its preparation (this provides with the case of a film which has been characterized immediately after its preparation similarly to the case of the film in Fig. 4). Approaching and retracting curves for the silicon substrate have almost the same slope but they do not coincide due to scanner hysteresis and the absence of a snap-out force in the retracting curve indicates that there is no water layer contamination on its surface under the environmental conditions (humidity and temperature) in the laboratory and also no mechanical deformation of the substrate by the tip [21]. For the resist films, approaching and retracting curves during contact of cantilever with the film have different slopes indicating that there is plastic deformation (indentation) of the film by the tip. The snap-out forces correspond to the adhesive forces of the film. Curves with the same overall characteristics, as the ones presented in Fig. 5, have been measured on films spin coated at other speeds. From the graph of Fig. 5, the adhesion forces are measured equal to: F8k ¼ 1.33 nN and F5k ¼ 4.24 nN. Since it is F5k4F8k, this indicates that the 8k film contains less amount of solvent, i.e. it is dryer than the 5k film. This result provides with one more confirmation that solvent evaporation from the film, results in the formation of pinholes on its surface. 4. Conclusions By using AFM both as an imaging and as an indentation tool, PMMA films spin coated from solutions with concentrations and at spin speeds in the range of the ones commonly used in lithographic applications have been
investigated with the purpose of determining the spin coating and post-processing conditions, for obtaining films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. It is found that pinholes are formed on the surfaces of films which have been spin coated from solution with a concentration of 1.25% at either 5 or at 8 krpm on glass or silicon substrates, as a result of the additional removal (evaporation) of the solvent from the film because of either leaving the film in a low vacuum chamber for a few days or postbaking it for 60 min at 170 1C in an oven. This means that films spin coated and post-treated under the above conditions will always show pinhole defects on their surfaces. Large pits with diameters in the range of 10–20 mm and depth of 2 nm, dominate the surfaces of films spin coated from solution with concentration of 2% at 7 krpm on glass substrates, while featureless appear the surfaces of films spin coated from a 1.25% solution at 8 krpm on silicon substrates. As a thinner film is obtained at higher spin speeds than at lower ones, this finding indicates that films spin coated on silicon substrates at higher speeds and from solutions with lower concentrations are superior in terms of their surface morphology as compared to films spin coated on glass substrates and from solutions with higher concentrations. It is also found that no difference in the surface morphology is observed between films spin coated from a 2% solution at 7 krpm for 40 s but postbaked at 170 1C for 30 min in an oven, and films spin coated at the same speed and for the same time but postbaked at 200 1C for 2 min on a hot plate. Finally, it seems that a good compromise between film dryness and formation of low density of pinholes for films spin coated on silicon substrates at 8 krpm from 1.25% solution is a postbaking time of 60 min at 170 1C in an oven or less than 60 min at 200 1C on a hot plate. The results and findings presented here have important implications in the use of spin-coated PMMA films in lithographic applications.
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