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A quartz crystal microbalance apparatus for studying interactions of solvents with thin polymer films
Progress
in Organic
Coatings,
19 (1991)265-274
265
A quartz crystal microbalance apparatus for studying interactions of solvents with thin polymer films Gareth J. Price* and Jill M. Buley School of Chemistry,
University
of Bath, Bath BA2 7AY (U.K.)
(Received March 2, 1991; accepted April 3, 1991)
Abstract This paper describes an apparatus developed for studying the effects of solvents on thin polymer films using a quartz crystal microbalance. It is shown that both swelling and dissolution processes can be monitored over a wide range of systems. The main application was in the investigation of photoresist development and it is demonstrated that all the appropriate parameters - solvent composition, temperature, polymer molecular weight and radiation dose - on film dissolution are amenable to study in a convenient and economic manner.
Introduction There are a large number of commercial processes and techniques in current use that involve the application of thin polymer films to a substrate followed by some interaction with a solvent [ 11. These can range from examples where the film is intended to give a protective coating to the substrate and the solvent is required to have no chemical effect on the polymer, for instance in cured coatings. At the other extreme, there are instances where the film is required to dissolve away in a suitable solvent. Examples of this include areas of a lithographic plate that are dissolved away to leave an image. There are intermediate cases where it is important to know whether the components of a Clm, such as plasticisers or antioxidants, are being leached out on contact with a solvent. This paper describes an apparatus that will allow the study of all these processes in a convenient and economic manner. The major area of interest in this laboratory is in resist systems used in microlithography. Perhaps one of the areas of greatest current activity [ 21 in polymer and radiation chemistry is the development of novel materials, for photo-, electron beam and X-ray resists to allow the definition of hoer patterns and the consequent greater density of components in a microcircuit. There are two major parts of the resist process: the production of an image by irradiation followed by its development [ 31. Although dry etching and plasma development processes can be used, solvent developing is still most common. This depends on solubility differences between the irradiated and *To whom all correspondence should be addressed.
0033-0655/91/$3.50
0 1991 - Elsevier Sequoia, Lausanne
266
mm-radiated regions of the polymer film, usually due to crosslinking or chain scission to lower molecular weights; hence our interest in interactions of solvents with polymer fifms. The principle of the apparatus described is the same as that of the SOcalled ‘Quartz Crystal Microbalance’, (QCM) [ 41, which has found considerable use in analytical and electro-chemistry [5, 61. This utilises the piezoelectric properties of quartz to give a very sensitive mass detector. Certain cuts of quartz oscillate at a particular characteristic frequency when placed in a suitable alternating electric field and Sauerbrey [7] has shown that this resonant frequency changes in a linear manner depending on the mass of any material loading the crystal. Hence the mass can be related to the frequency change, AF’, by AF = - ~~~‘J~~~~~
(1)
where F, is the fundamental resonant frequency and A the active area of the uncoated crystal, ps is the density of quartz and N is the ‘frequency constant’ of the particular crystal of quartz from which the mass of coating IWP can be calculated. The original applications of these techniques were in the vapour phase where sensitivities in the sub-nanogram range have been demonstrated. Operation of the QCM under liquids is more difficult due to the damping effect of the liquid. Here the application is relatively recent [S], a somewhat more elaborate electrical circuit allowing its use over a wide range of liquids. The technique has also been applied to the study of photoresists by Hinsberg et aL. [ 9,l O] who found that it produced accurate, reproducible measurements and was applicable to a wide range of systems. Other methods for investigating solubility in resist-type materials have been reviewed by Rodriguez and coworkers [f 11 and include the so-called ‘dip-and-dry’ method f 121 where a polymer film is immersed in solvent for known lengths of time, removed and the film thickness determined using a stylus instrument. These measurements have several sources of error since dissolution does not cease immediately on removal from the solvent and this gives rise to uncertainties in the thickness measurements. Uberreiter and Asmussen [ 131 used a microscope to observe polymer pellets in a flowing solvent. Other in situ methods include the monitoring of changes in the capacitance of the system f 14f or in the ellipsometric properties of the surface [ 151. However, these have limitations as to the accuracy of the results and also in the types of system to which they can be applied. Most recent studies have involved laser interferometry [l l] where the reflection of laser light from the surface of a thin film is compared to the incident beam. The accuracy of this apparatus is high but the method is expensive and quite complex to set up. Thus, although several methods are available for these measurements, none offers all of the desirable properties for this type of work: speed, convenience, economy and applicability to a wide range of systems. This paper will demons~ate that the QCM method used in this laboratory has
267
these properties and provides an excellent method for studying dissolution and solubility in photoresist systems as well as a range of other interactions in polymer-solvent systems.
Experimental A schematic diagram of the QCM apparatus is shown in Fig. 1. The circuit uses an uncoated crystal as a reference and measures the frequency difference between it and a coated, working crystal which is placed in a holder capable of rapid immersion in a stirred container of solvent thermostatted to + 0.1 “C. The resonant frequency of the working crystal was recorded in air and, when this value was stable, the crystal was immersed and the frequency change followed with time until no further change was noted. To prevent interference from other equipment and to give signals of the required stability, the apparatus was contained in an aluminium box which acted as a Faraday cage as well as containing solvent vapours, necessary for safety purposes. The frequencies were recorded on a Racal Dana 1030 frequency counter with a resolution of + 0.1 Hz and fed into an IBM compatible microcomputer, where subsequent data treatment was performed, via a Metrabyte IEEE interface card. Measurements were made at 1 s intervals, each being the average of 10 separate readings. The electrical circuit required to operate the crystals was based upon that of Bruckenstein and Shay [S], modified to function with the crystals employed here, and is shown in Fig. 2. It consists basically of two separate oscillator circuits for the reference and working crystals, the latter being tuned to its fundamental frequency using the variable capacitor. The remaining circuitry was designed to measure the difference in oscillation frequency between the two sides of the circuit and to monitor its output via an optical coupler, powered by a completely separate supply to isolate it from interferences. The isolated output was measured by the frequency counter. To test the circuit, sealed, ‘canned’ crystals were used on both sides of the circuit and under these ideal circumstances the frequency was stable to + 0.2 Hz and showed less than 2 Hz drift over the expected duration of a dissolution experiment.
Water I;=
-
Faraday Cage
Fig. 1. Schematic diagram of QCM film dissolution apparatus.
(Bb I%g. 2. Circuit diagram for the quartz crystal microbalance. Components: Xl, 74LSO4; IC2, 74LS14; IC3, 74LS74; and IC4, HCPL2601. C,, 10 nF; Cz, 20 pF; C,, 10,950 pF; C,, 50 pF; and CB, 0.1 SF. L, 10 I.LH; D, HP5082-2800; REF. and WORK. refer to the reference and working quartz crystals, respectively.
Most of the work discussed here used a PMMA polymer supplied by BDH Ltd., having a number average molecular weight of 56 000 and a polydispersity of 2.0 as measured by GPC. NMR spectroscopy showed it to be primarily atactic. To study the molecular weight dependence of dissolution, narrow (1.02-l .OS) polydispersity samples from Polymer Laboratories Ltd. covering a range from 6100 to 1 400 000 were used. The molecular weights quoted are those supplied by the manufacturers for GPC calibration purposes. The solvents used were all of reagent grade. The commercially available quartz crystals (Euroquartz Ltd., U.K.) used in this work were 0.017 mm in thickness and 12.0 mm in diameter with a gold-plated (i.e. active) area of approximately 19.6 mm’. They were prepared from ‘AT’ cut quartz giving them good temperature stability over the range of interest. The crystals were cleaned with acetone, chloroform and the solvent to be used, and then coated with a thin film (0.5-5 pm) of polymer. For this initial work both sides of the crystal were used, so that coating was performed by dipping the crystal into a l-5% solution of PMMA in chloroform, depending on the desired film thickness, and allowing the solvent to evaporate. In related work, spin coating has also been employed with equal success for applications in which the film parameters were critical. The crystals were then heated for 1 h at 160 “C to remove the remaining solvent and anneal the film.
Results
and discussion
As an introduction to the work and in order to illustrate the methods employed, poly(methy1 methacrylate), PMMA, has been studied. This polymer has been investigated by other workers and so provided a good basis for evaluating the technique; it is also in commercial use as a positive photoresist.
269
The usual developing solvent is a mixture of butan-Z-one (methyl ethyl ketone, MEK) and propan-2-01 (isopropyl alcohol, IPA), and the factors involved in the dissolution process for this system have been investigated. Precise comparison of the results reported here with those of other workers is not possible, since the dissolution rate in a solvent depends on a large number of factors including the exact structure and thermal history of a particular polymer sample (as will be shown below), so that it is impossible to work on identical samples to those used for other studies quoted in the literature. In this paper, it will be shown that the main factors affecting dissolution may be measured accurately using the &CM technique developed. The experiments undertaken yield a series of frequency-time curves. From eqn. (1) and introducing the density of the polymer, 4, the frequency can be transformed into the film thickness, T,, giving
G = [(Np,) M’02d I AF
(2)
It was assumed that the density of the film was equal to that of the bulk polymer, i.e. 1.170 g cmp3. The frequency changes measured arise from two major factors; firstly due to dissolution and/or swelling of the polymer and secondly due to the effect of solvent viscosity on the crystal. This is clearly shown in Fig. 3 which shows frequency-time curves for the immersion in MEK of both a coated and an uncoated crystal. The required results were obtained simply by subtracting the effect on the bare crystal to leave the effects due to the polymer coating. As an illustration of the results obtained, Fig. 4 depicts those for the dissolution of PMMA (M,= 56 000, y= 2.0) at three different initial film thicknesses in a mixture of butan-2-one (MEK) and propan-2-01 (IPA) at 25 “C, plotted as functions of changes in both film thickness and frequency. For an initial film thickness of 0.38 pm, the total change in frequency for 50
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Fig. 4. Plots of changes in frequency and film thickness (from their initial values) versus time for PMMA dissolution in 60/40 v:v MEK/IPA solution at 25 “C.
270
complete di~olution was 11.7 kHz. Since the unce~a~ty in measuring the frequency was less than f IO Hz, film thicknesses could be measured with a resolution of approximately 0.1 nm. The results also show that, within the limits studied, the rate of dissolution as given by the slopes of the curves was independent of film thickness. Two other features are also apparent: a brief induction period before rapid dissolution started and a decrease in the dissolution rate as the last layers of polymer were removed from the crystal surface. These factors are thought to be related to the initial penetration of the solvent and swelling of the surface layer, and to the adhesion of the polymer to the substrate. To illustrate the reproducibility of the measurements, the results for four separate experiments performed on different days involving the dissolution at 25 “C in MEK of films approximately 1 pm thickness are shown in Fig. 5. The results have been normalised to the same initial thickness and are displayed as the percentage of the film remaining with development time. The resulting curves are very similar and yield an average rate of 2.422 f 0.0 17 Frn mm-‘, with the individual rates for the four runs agreeing to within 3-l%.
Effect of solvent composition
on dissolution
Most resist processing is not carried out in pure solvents but in a mixture of a thermodynamically good solvent with a moderating non-solvent, and this has been the focus of study of several workers. The results for two such systems involving MEK are discussed here. Figure 6 depicts those obtained at 25 “C for PMMA in pure MEK, pure IPA and six mixtures of varied composition. MEK is a good solvent for PMMA and leads to rapid dissolution whereas IPA is a non-solvent and displays only a small degree of swelling. As might be expected on first consideration, the mixture gave
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271
dissolution rates intermediate between those for the single components as demonstrated by Cooper et al. [ 161 using the laser interferometric technique. Many workers have assumed that the properties of mixed solvent systems would be the average of those of the single components. However, Cooper et al. [ 161 demonstrated that this is not invariably so and their findings are confirmed by the results depicted in Pig. 7. Thus although methanol is also a non-solvent for PMMA, its addition up to approximately 20 vol.% to MEK leads to an acceleration in the rate of dissolution by as much as a factor of two. Similar effects were found on addition of water. This effect has been ascribed to the small, mobile methanol molecules diffusing rapidly into the polymer structure and plasticising it so as to allow better dissolution. Effect of polymer mlecu~r weight on dissolution One of the primary parameters affecting polymer solubility and dissolution, and one that is crucial to a resist process, is the molecular weight. This is clearly shown by the results depicted in Fig. 8 for a series of narrow polydispersity PMMA standards whose number average molecular weights ranged between 6100 and 1 400 000 at 25 “C. Clearly, polymers of lower number average molecular weight dissolve at a considerably faster rate than those with higher values. Another significant point is that the polymers with number average molecular weights > 100 000 swelled prior to dissolution, the extent of swelling also increasing with increasing molecular weight. Ouano [la] has suggested a relationship between the dissolution rate, DR, and the polymer molecular weight, M, of the form: DR = aMb where a and b are constants which are characteristic for a particular system. F’igure 9 shows the results obtained in this work plotted in a double logarithmic
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Fig. 8. Dissolution rates for PMMA of varyins number average molecular weight in MEK at 500; V 67 000; X, 107 000; 25 “C. Molecular weights: I, 6100, A, 10 300; V, 22 200; 0,34 + , 330 000; + , 820 000; and 0, 1 400 000.
272
form; the plot is clearly not linear as would be predicted by the above relationship, and hence the linear function may only be valid over restricted ranges of molecular weight. Eflect of temperature on dissolution The solvent temperature is also of great importance to the dissolution process, with dissolution being faster at higher temperatures. Figure 10 illustrates this for the PMMA standard of number average molecular weight 107 000 and a polydispersity of 1.03 in MEK over the temperature range 16-45 “C. The swelling of the polymer also appeared to be temperaturedependent, being negligible above 30 “C. When the results are plotted in the Arrhenius manner, a linear relationship is obtained as shown in Fig 11 which is of the form DR= 2.04 x 10’ exp[ -5239.5/Z’] where DR is the dissolution rate in pm mm-’ and T the absolute temperature. This leads to an apparent activation energy for the dissolution process of 43.5 kJ mol-‘. Application to photoresist systems To illustrate potential applications in photoactive systems, crystals were coated with PMMA, exposed to UV irradiation from a medium-pressure mercury lamp and their dissolution rates measured. PMMA is a positive photoresist which undergoes chain scission on irradiation so that its rate of dissolution should be faster in irradiated samples. This is demonstrated by the results depicted in Fig. 12 where increasing the irradiation dose also increased the dissolution rate, with is a linear relationship existing between the dissolution rate and the irradiation time when plotted in a semi-logarithmic manner. These results clearly indicate that the methods employed in this 120.0
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work are suitable for application to resist-type materials under conditions very similar to those employed commercially. Conclusion
The work described in this paper demonstratesthat the methods employed for studying dissolution processes are potentially extremely useful for application to thin film polymer systems such as photoresists. The results produced are comparable to those of other workers and demonstrate that a range of effects of f~d~ental importance to the dissolution process in polymers may be studied. There are few, if any, restrictions on the types of polymer that can be studied as long as the crystals can be coated. Although the results described here cover a temperature range close to ambient, i.e. as most often encountered in commercial processes, there is no reason why wider ranges should not be used. In other work to be described [ 171, a wide range of solvents, both organic and aqueous, have been studied and the apparatus shown to be very flexible in this regard also. Small crystals have been used which may be conveniently introduced into irradiation systems under conditions very close to those used for commercial resist development. Perhaps the major advantage of the system is that results may be obtained economically and very quickly with little set-up time. Although the measurements described in #is paper are concerned with dissolution, other processes are equally open to study using the QCM apparatus. Other work in progress includes the monitoring of leaching from polymers and the measurement of the etch and solvent resistance of cured coatings. Acknowledgements
The authors are grateful to Mr M. Harriman of the School of Physics at the University of Bath for technical assistance with the construction of
274
the &CM circuit. They also gratefully acknowledge the financial support of the Science and Engineering Research Council for this work.
References 1 FLR. Meyers and J. S. Long (eds.), Treatise cm Coatirqs, Marcel Dekker, New 2 E. Reichmanis and L. F. Thompson, Churn. l&v., 89 (1989) 1273. 3 Introduction to ~~~~~~~thog~~~h~, L. F. Thompson and C. G. W&ran (eds.), Ser. No. 219, Am. Chem. Sot., Washington, D.C., 1983. 4 G. Gmibault, in C. Lu and A. W. Czanderna (eds.), Methods and Phewmenu New York, 1984. 5 J. J. McCallum, Analyst @kmck@, II4 (1989) 1173. 6 M. R. Deakin and D. A. Buttry, Anal. C&em., 61 (1989) 1147. 7 G. Sauerbrey, Z. Phys., 15.5 (1959) 206. 8 S. Bruckenstein and M. Shay, Electrochim. Acta, 30 (1985) 1295. 9 W. D. Hinsberg, C. G. Willson and K. K. Kanazawa, J. Ebctrochem. Sot., 1448. 10 W. D. Hinsberg and K. K. Kanazawa, Rev. Sci. In&rum., 60 (1989) 489. II F. Rodriguez, P. D. Krasicky and R. 3. Groele, Solid State Techno,?., (1985) 12 A. C. Ouano, Polym. Eng. SC&, 18 (1978) 306. 13 K. Ueberreiter and F. Asmussen, ~~~o~L, C%xm., 43 (1961) 324. 14 W. Oldham, Opt. Eqz., 18 (1979) 59. 15 J. S. Papanu, D. W. Hess, A. T. Be1 and D. S. Soane, J. ELectrochem. Sot., 1195. 16 W. J. Cooper, P. D. Krasicky and F. Rodriguez, J. Appl. P&m. 1’7 G. J. Price and J. M. Buley, manuscript in preparation.
York, 1976. ACS Swp. 7, Elsevier,
X33 (1986)
125.
136 (1989)
Sci., 31 (1985)
65.
in Organic
Coatings,
19 (1991)265-274
265
A quartz crystal microbalance apparatus for studying interactions of solvents with thin polymer films Gareth J. Price* and Jill M. Buley School of Chemistry,
University
of Bath, Bath BA2 7AY (U.K.)
(Received March 2, 1991; accepted April 3, 1991)
Abstract This paper describes an apparatus developed for studying the effects of solvents on thin polymer films using a quartz crystal microbalance. It is shown that both swelling and dissolution processes can be monitored over a wide range of systems. The main application was in the investigation of photoresist development and it is demonstrated that all the appropriate parameters - solvent composition, temperature, polymer molecular weight and radiation dose - on film dissolution are amenable to study in a convenient and economic manner.
Introduction There are a large number of commercial processes and techniques in current use that involve the application of thin polymer films to a substrate followed by some interaction with a solvent [ 11. These can range from examples where the film is intended to give a protective coating to the substrate and the solvent is required to have no chemical effect on the polymer, for instance in cured coatings. At the other extreme, there are instances where the film is required to dissolve away in a suitable solvent. Examples of this include areas of a lithographic plate that are dissolved away to leave an image. There are intermediate cases where it is important to know whether the components of a Clm, such as plasticisers or antioxidants, are being leached out on contact with a solvent. This paper describes an apparatus that will allow the study of all these processes in a convenient and economic manner. The major area of interest in this laboratory is in resist systems used in microlithography. Perhaps one of the areas of greatest current activity [ 21 in polymer and radiation chemistry is the development of novel materials, for photo-, electron beam and X-ray resists to allow the definition of hoer patterns and the consequent greater density of components in a microcircuit. There are two major parts of the resist process: the production of an image by irradiation followed by its development [ 31. Although dry etching and plasma development processes can be used, solvent developing is still most common. This depends on solubility differences between the irradiated and *To whom all correspondence should be addressed.
0033-0655/91/$3.50
0 1991 - Elsevier Sequoia, Lausanne
266
mm-radiated regions of the polymer film, usually due to crosslinking or chain scission to lower molecular weights; hence our interest in interactions of solvents with polymer fifms. The principle of the apparatus described is the same as that of the SOcalled ‘Quartz Crystal Microbalance’, (QCM) [ 41, which has found considerable use in analytical and electro-chemistry [5, 61. This utilises the piezoelectric properties of quartz to give a very sensitive mass detector. Certain cuts of quartz oscillate at a particular characteristic frequency when placed in a suitable alternating electric field and Sauerbrey [7] has shown that this resonant frequency changes in a linear manner depending on the mass of any material loading the crystal. Hence the mass can be related to the frequency change, AF’, by AF = - ~~~‘J~~~~~
(1)
where F, is the fundamental resonant frequency and A the active area of the uncoated crystal, ps is the density of quartz and N is the ‘frequency constant’ of the particular crystal of quartz from which the mass of coating IWP can be calculated. The original applications of these techniques were in the vapour phase where sensitivities in the sub-nanogram range have been demonstrated. Operation of the QCM under liquids is more difficult due to the damping effect of the liquid. Here the application is relatively recent [S], a somewhat more elaborate electrical circuit allowing its use over a wide range of liquids. The technique has also been applied to the study of photoresists by Hinsberg et aL. [ 9,l O] who found that it produced accurate, reproducible measurements and was applicable to a wide range of systems. Other methods for investigating solubility in resist-type materials have been reviewed by Rodriguez and coworkers [f 11 and include the so-called ‘dip-and-dry’ method f 121 where a polymer film is immersed in solvent for known lengths of time, removed and the film thickness determined using a stylus instrument. These measurements have several sources of error since dissolution does not cease immediately on removal from the solvent and this gives rise to uncertainties in the thickness measurements. Uberreiter and Asmussen [ 131 used a microscope to observe polymer pellets in a flowing solvent. Other in situ methods include the monitoring of changes in the capacitance of the system f 14f or in the ellipsometric properties of the surface [ 151. However, these have limitations as to the accuracy of the results and also in the types of system to which they can be applied. Most recent studies have involved laser interferometry [l l] where the reflection of laser light from the surface of a thin film is compared to the incident beam. The accuracy of this apparatus is high but the method is expensive and quite complex to set up. Thus, although several methods are available for these measurements, none offers all of the desirable properties for this type of work: speed, convenience, economy and applicability to a wide range of systems. This paper will demons~ate that the QCM method used in this laboratory has
267
these properties and provides an excellent method for studying dissolution and solubility in photoresist systems as well as a range of other interactions in polymer-solvent systems.
Experimental A schematic diagram of the QCM apparatus is shown in Fig. 1. The circuit uses an uncoated crystal as a reference and measures the frequency difference between it and a coated, working crystal which is placed in a holder capable of rapid immersion in a stirred container of solvent thermostatted to + 0.1 “C. The resonant frequency of the working crystal was recorded in air and, when this value was stable, the crystal was immersed and the frequency change followed with time until no further change was noted. To prevent interference from other equipment and to give signals of the required stability, the apparatus was contained in an aluminium box which acted as a Faraday cage as well as containing solvent vapours, necessary for safety purposes. The frequencies were recorded on a Racal Dana 1030 frequency counter with a resolution of + 0.1 Hz and fed into an IBM compatible microcomputer, where subsequent data treatment was performed, via a Metrabyte IEEE interface card. Measurements were made at 1 s intervals, each being the average of 10 separate readings. The electrical circuit required to operate the crystals was based upon that of Bruckenstein and Shay [S], modified to function with the crystals employed here, and is shown in Fig. 2. It consists basically of two separate oscillator circuits for the reference and working crystals, the latter being tuned to its fundamental frequency using the variable capacitor. The remaining circuitry was designed to measure the difference in oscillation frequency between the two sides of the circuit and to monitor its output via an optical coupler, powered by a completely separate supply to isolate it from interferences. The isolated output was measured by the frequency counter. To test the circuit, sealed, ‘canned’ crystals were used on both sides of the circuit and under these ideal circumstances the frequency was stable to + 0.2 Hz and showed less than 2 Hz drift over the expected duration of a dissolution experiment.
Water I;=
-
Faraday Cage
Fig. 1. Schematic diagram of QCM film dissolution apparatus.
(Bb I%g. 2. Circuit diagram for the quartz crystal microbalance. Components: Xl, 74LSO4; IC2, 74LS14; IC3, 74LS74; and IC4, HCPL2601. C,, 10 nF; Cz, 20 pF; C,, 10,950 pF; C,, 50 pF; and CB, 0.1 SF. L, 10 I.LH; D, HP5082-2800; REF. and WORK. refer to the reference and working quartz crystals, respectively.
Most of the work discussed here used a PMMA polymer supplied by BDH Ltd., having a number average molecular weight of 56 000 and a polydispersity of 2.0 as measured by GPC. NMR spectroscopy showed it to be primarily atactic. To study the molecular weight dependence of dissolution, narrow (1.02-l .OS) polydispersity samples from Polymer Laboratories Ltd. covering a range from 6100 to 1 400 000 were used. The molecular weights quoted are those supplied by the manufacturers for GPC calibration purposes. The solvents used were all of reagent grade. The commercially available quartz crystals (Euroquartz Ltd., U.K.) used in this work were 0.017 mm in thickness and 12.0 mm in diameter with a gold-plated (i.e. active) area of approximately 19.6 mm’. They were prepared from ‘AT’ cut quartz giving them good temperature stability over the range of interest. The crystals were cleaned with acetone, chloroform and the solvent to be used, and then coated with a thin film (0.5-5 pm) of polymer. For this initial work both sides of the crystal were used, so that coating was performed by dipping the crystal into a l-5% solution of PMMA in chloroform, depending on the desired film thickness, and allowing the solvent to evaporate. In related work, spin coating has also been employed with equal success for applications in which the film parameters were critical. The crystals were then heated for 1 h at 160 “C to remove the remaining solvent and anneal the film.
Results
and discussion
As an introduction to the work and in order to illustrate the methods employed, poly(methy1 methacrylate), PMMA, has been studied. This polymer has been investigated by other workers and so provided a good basis for evaluating the technique; it is also in commercial use as a positive photoresist.
269
The usual developing solvent is a mixture of butan-Z-one (methyl ethyl ketone, MEK) and propan-2-01 (isopropyl alcohol, IPA), and the factors involved in the dissolution process for this system have been investigated. Precise comparison of the results reported here with those of other workers is not possible, since the dissolution rate in a solvent depends on a large number of factors including the exact structure and thermal history of a particular polymer sample (as will be shown below), so that it is impossible to work on identical samples to those used for other studies quoted in the literature. In this paper, it will be shown that the main factors affecting dissolution may be measured accurately using the &CM technique developed. The experiments undertaken yield a series of frequency-time curves. From eqn. (1) and introducing the density of the polymer, 4, the frequency can be transformed into the film thickness, T,, giving
G = [(Np,) M’02d I AF
(2)
It was assumed that the density of the film was equal to that of the bulk polymer, i.e. 1.170 g cmp3. The frequency changes measured arise from two major factors; firstly due to dissolution and/or swelling of the polymer and secondly due to the effect of solvent viscosity on the crystal. This is clearly shown in Fig. 3 which shows frequency-time curves for the immersion in MEK of both a coated and an uncoated crystal. The required results were obtained simply by subtracting the effect on the bare crystal to leave the effects due to the polymer coating. As an illustration of the results obtained, Fig. 4 depicts those for the dissolution of PMMA (M,= 56 000, y= 2.0) at three different initial film thicknesses in a mixture of butan-2-one (MEK) and propan-2-01 (IPA) at 25 “C, plotted as functions of changes in both film thickness and frequency. For an initial film thickness of 0.38 pm, the total change in frequency for 50
12.0
z 2 5 %
40
% 5
8 2
3 3 9.0
0.3
;
6.0
0.2
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coated with PMMA.
Fig. 4. Plots of changes in frequency and film thickness (from their initial values) versus time for PMMA dissolution in 60/40 v:v MEK/IPA solution at 25 “C.
270
complete di~olution was 11.7 kHz. Since the unce~a~ty in measuring the frequency was less than f IO Hz, film thicknesses could be measured with a resolution of approximately 0.1 nm. The results also show that, within the limits studied, the rate of dissolution as given by the slopes of the curves was independent of film thickness. Two other features are also apparent: a brief induction period before rapid dissolution started and a decrease in the dissolution rate as the last layers of polymer were removed from the crystal surface. These factors are thought to be related to the initial penetration of the solvent and swelling of the surface layer, and to the adhesion of the polymer to the substrate. To illustrate the reproducibility of the measurements, the results for four separate experiments performed on different days involving the dissolution at 25 “C in MEK of films approximately 1 pm thickness are shown in Fig. 5. The results have been normalised to the same initial thickness and are displayed as the percentage of the film remaining with development time. The resulting curves are very similar and yield an average rate of 2.422 f 0.0 17 Frn mm-‘, with the individual rates for the four runs agreeing to within 3-l%.
Effect of solvent composition
on dissolution
Most resist processing is not carried out in pure solvents but in a mixture of a thermodynamically good solvent with a moderating non-solvent, and this has been the focus of study of several workers. The results for two such systems involving MEK are discussed here. Figure 6 depicts those obtained at 25 “C for PMMA in pure MEK, pure IPA and six mixtures of varied composition. MEK is a good solvent for PMMA and leads to rapid dissolution whereas IPA is a non-solvent and displays only a small degree of swelling. As might be expected on first consideration, the mixture gave
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l?ig. 6. Dissolution of PMMA in mixed MEKAPA solvents at 25 “C. IPA content (vol.%): 0; tl, 20; ~3, 40; 0, 50; t, 60; V, 70; 4, SO; and /?I,,100.
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271
dissolution rates intermediate between those for the single components as demonstrated by Cooper et al. [ 161 using the laser interferometric technique. Many workers have assumed that the properties of mixed solvent systems would be the average of those of the single components. However, Cooper et al. [ 161 demonstrated that this is not invariably so and their findings are confirmed by the results depicted in Pig. 7. Thus although methanol is also a non-solvent for PMMA, its addition up to approximately 20 vol.% to MEK leads to an acceleration in the rate of dissolution by as much as a factor of two. Similar effects were found on addition of water. This effect has been ascribed to the small, mobile methanol molecules diffusing rapidly into the polymer structure and plasticising it so as to allow better dissolution. Effect of polymer mlecu~r weight on dissolution One of the primary parameters affecting polymer solubility and dissolution, and one that is crucial to a resist process, is the molecular weight. This is clearly shown by the results depicted in Fig. 8 for a series of narrow polydispersity PMMA standards whose number average molecular weights ranged between 6100 and 1 400 000 at 25 “C. Clearly, polymers of lower number average molecular weight dissolve at a considerably faster rate than those with higher values. Another significant point is that the polymers with number average molecular weights > 100 000 swelled prior to dissolution, the extent of swelling also increasing with increasing molecular weight. Ouano [la] has suggested a relationship between the dissolution rate, DR, and the polymer molecular weight, M, of the form: DR = aMb where a and b are constants which are characteristic for a particular system. F’igure 9 shows the results obtained in this work plotted in a double logarithmic
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Fig. 8. Dissolution rates for PMMA of varyins number average molecular weight in MEK at 500; V 67 000; X, 107 000; 25 “C. Molecular weights: I, 6100, A, 10 300; V, 22 200; 0,34 + , 330 000; + , 820 000; and 0, 1 400 000.
272
form; the plot is clearly not linear as would be predicted by the above relationship, and hence the linear function may only be valid over restricted ranges of molecular weight. Eflect of temperature on dissolution The solvent temperature is also of great importance to the dissolution process, with dissolution being faster at higher temperatures. Figure 10 illustrates this for the PMMA standard of number average molecular weight 107 000 and a polydispersity of 1.03 in MEK over the temperature range 16-45 “C. The swelling of the polymer also appeared to be temperaturedependent, being negligible above 30 “C. When the results are plotted in the Arrhenius manner, a linear relationship is obtained as shown in Fig 11 which is of the form DR= 2.04 x 10’ exp[ -5239.5/Z’] where DR is the dissolution rate in pm mm-’ and T the absolute temperature. This leads to an apparent activation energy for the dissolution process of 43.5 kJ mol-‘. Application to photoresist systems To illustrate potential applications in photoactive systems, crystals were coated with PMMA, exposed to UV irradiation from a medium-pressure mercury lamp and their dissolution rates measured. PMMA is a positive photoresist which undergoes chain scission on irradiation so that its rate of dissolution should be faster in irradiated samples. This is demonstrated by the results depicted in Fig. 12 where increasing the irradiation dose also increased the dissolution rate, with is a linear relationship existing between the dissolution rate and the irradiation time when plotted in a semi-logarithmic manner. These results clearly indicate that the methods employed in this 120.0
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9. Influence of polymer
0.0
50.0
100.0
150.0
200.0
Time (s)
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“C. Fig. 10. Dissolution curves for PMMA (M,= Temp. (“C): n , 45.0; A, 40.0; V, 35.1; f,
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work are suitable for application to resist-type materials under conditions very similar to those employed commercially. Conclusion
The work described in this paper demonstratesthat the methods employed for studying dissolution processes are potentially extremely useful for application to thin film polymer systems such as photoresists. The results produced are comparable to those of other workers and demonstrate that a range of effects of f~d~ental importance to the dissolution process in polymers may be studied. There are few, if any, restrictions on the types of polymer that can be studied as long as the crystals can be coated. Although the results described here cover a temperature range close to ambient, i.e. as most often encountered in commercial processes, there is no reason why wider ranges should not be used. In other work to be described [ 171, a wide range of solvents, both organic and aqueous, have been studied and the apparatus shown to be very flexible in this regard also. Small crystals have been used which may be conveniently introduced into irradiation systems under conditions very close to those used for commercial resist development. Perhaps the major advantage of the system is that results may be obtained economically and very quickly with little set-up time. Although the measurements described in #is paper are concerned with dissolution, other processes are equally open to study using the QCM apparatus. Other work in progress includes the monitoring of leaching from polymers and the measurement of the etch and solvent resistance of cured coatings. Acknowledgements
The authors are grateful to Mr M. Harriman of the School of Physics at the University of Bath for technical assistance with the construction of
274
the &CM circuit. They also gratefully acknowledge the financial support of the Science and Engineering Research Council for this work.
References 1 FLR. Meyers and J. S. Long (eds.), Treatise cm Coatirqs, Marcel Dekker, New 2 E. Reichmanis and L. F. Thompson, Churn. l&v., 89 (1989) 1273. 3 Introduction to ~~~~~~~thog~~~h~, L. F. Thompson and C. G. W&ran (eds.), Ser. No. 219, Am. Chem. Sot., Washington, D.C., 1983. 4 G. Gmibault, in C. Lu and A. W. Czanderna (eds.), Methods and Phewmenu New York, 1984. 5 J. J. McCallum, Analyst @kmck@, II4 (1989) 1173. 6 M. R. Deakin and D. A. Buttry, Anal. C&em., 61 (1989) 1147. 7 G. Sauerbrey, Z. Phys., 15.5 (1959) 206. 8 S. Bruckenstein and M. Shay, Electrochim. Acta, 30 (1985) 1295. 9 W. D. Hinsberg, C. G. Willson and K. K. Kanazawa, J. Ebctrochem. Sot., 1448. 10 W. D. Hinsberg and K. K. Kanazawa, Rev. Sci. In&rum., 60 (1989) 489. II F. Rodriguez, P. D. Krasicky and R. 3. Groele, Solid State Techno,?., (1985) 12 A. C. Ouano, Polym. Eng. SC&, 18 (1978) 306. 13 K. Ueberreiter and F. Asmussen, ~~~o~L, C%xm., 43 (1961) 324. 14 W. Oldham, Opt. Eqz., 18 (1979) 59. 15 J. S. Papanu, D. W. Hess, A. T. Be1 and D. S. Soane, J. ELectrochem. Sot., 1195. 16 W. J. Cooper, P. D. Krasicky and F. Rodriguez, J. Appl. P&m. 1’7 G. J. Price and J. M. Buley, manuscript in preparation.
York, 1976. ACS Swp. 7, Elsevier,
X33 (1986)
125.
136 (1989)
Sci., 31 (1985)
65.