Langmuir-Blodgett films of acetalized poly(vinyl alcohol)s

Thin Solid Films, 161 (1988) 305-313 LANGMUIR--BLODGETT AND RELATED FILMS

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L A N G M U I R - B L O D G E T T FILMS O F A C E T A L I Z E D POLY(VINYL ALCOHOL)S KIYOSHI OGUCHI* AND TOMOKOYODEN Research and Development Corporation of Japan, c/o Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102 (Japan)

YOZOU KOSAKA, MASAYOSHIWATANABE,KOHEISANUI AND NAOYAOGATA Faculty of Science and Technology, Sophia UniversiO,, 7-I Kioieho, Chiyoda-ku, Tokyo 102 (Japan)

(ReceivedJuly 16, 1987;revisedNovember30, 1987;acceptedJanuary 4, 1988)

Monolayers and multilayers of amphiphilic polymers consisting of acetalized poly(vinyl alcohol) (PVA) having various linear aliphatic side chains were investigated and their electron beam (EB) exposure characteristics were measured. It was found that monolayers of acetalized PVA having long alkyl side chains were stable on water and could be deposited onto both hydrophobic and hydrophilic substrates with a deposition ratio of 1.0. The resulting multilayers were Y type. The wettability of the multilayers changed according as there was an odd or an even number of layers. The thickness per layer increased with the length of the alkyl side chain. These results suggested that Langmuir-Blodgett (LB) films ofacetalized PVA having long alkyl side chains were rather well-ordered with the side chains directed normal to the main chains. The acetalized PVA prepared by the LB method is adaptable as a high resolution negative-type EB resist.

I. INTRODUCTION The Langmuir-Blodgett (LB) method is one of the important techniques for producing very thin and uniform films that can be controlled to molecular dimensions l, 2. Many kinds of amphiphilic molecules such as long-chain fatty acids are being investigated. Recently, applications of LB films have been expected in many fields and discussed especially in the electronics field. However, the resulting LB films have poor thermal and mechanical stability. This is a disadvantage with respect to practical applications. In order to make stable multilayers, the use of polymers has been proposed and two ideas have been proposed. One method is to form monolayers or multilayers having polymerizable groups and then to bring about polymerization by some appropriate treatment such as heating, UV or electron beam (EB) irradiation 3 a. The other method is to form monolayers using suitable amphiphilic polymers and to deposit them onto substrates by the LB method. Tredgold and Winter 9 were the first * Present address: Central Research Institute, Dai Nippon Printing Co. Ltd., 1-1 Ichigaya-Kagacho1chome, Shinjuku-ku,Tokyo 162,Japan. 0040-6090/88/$3.50

© ElsevierSequoia/Printedin The Netherlands

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to describe the transfer of LB monolayers of preformed polymers. They reported the development of electronic devices using the LB films, but analysis of the orientation of polymers in the LB films was not undertaken. Mumby et al. x° described the orientation of the LB films from preformed amphiphilic comb-like polymers using IR spectroscopy, but multilayers consisting of more than 7 layers could not be produced. It has previously been reported 11 that poly(vinyl octal) (OA PVA) spread on the air-water interface could be transferred onto various substrates by Y-type deposition and that the single-layer thickness of LB films of OA PVA was constant from 1 monolayer to 100 layers. This paper deals with the possibility of creating oriented LB films from synthetic polymers consisting of acetalized poly(vinyl alcohol) (PVA) having various long aliphatic side chains, and their EB exposure properties are also examined. 2.

E X P E R I M E N T A L DETAILS

2.1. Preparation of Langmuir-Blodgett films

Acetalized derivatives of PVA were synthesized from the corresponding aldehyde using hydrochloric acid as a catalyst 12'~3. The structure of acetalized PVAs are shown in Fig. 1. A typical synthesis was as follows. 1.0 g of PVA having a degree of polymerization of 2000 and 8.0 g of octyl aldehyde were dissolved in 16 ml of benzene and 4 ml of ethanol, and two drops of hydrochloric acid (35~o aqueous) added. The reaction was carried out with stirring at 40 °C for 15 h. The solution was poured into excess methanol containing an equimolar amount of sodium hydroxide and acetalized polymer was separated by filtration. It was further purified by reprecipitation from chloroform into methanol and finally dried under vacuum. The other acetalized PVAs based on acetaldyhyde (AA), butylaldehyde (BuA), decylaldehyde (DA), dodecylaldehyde(DDA), tridecylaldehyde (TrDA) and tetradecylaldehyde (TeDA) were also synthesized in the same way, -(-CHTCH~- . CH~CH~-~rgCHO

OH

H÷ ~ -~CHTCH zCH2"~ ' CH~CH2CH~ x

.

~..6

~H -

i

C~H#o CHj n=0 (AA-F:'VA) 2(BuA-PVA) 6(OA-PVA) 8(DA-PVA)

n=10(DDA-PVA) 11(TrDA-PVA) 12(TeDA-PVA)

Fig. 1. S t r u c t u r e s o f a c e t a l i z e d P V A s .

The acetalized PVAs were dissolved in benzene at about 0.01wt.~o and were spread on the surface of distilled water using a Lauda film balance. The obtained monolayer on water was deposited onto various substrates by means of the LB method. 2.2. Electron beam exposure

EB exposure characteristics of acetalized PVA were evaluated as EB resists. The films of acetalized PVA prepared by the LB method were exposed using an EB

LB FILMS OF ACETALIZED POLY(VINYL ALCOHOL)S

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resist evaluation apparatus (ERE-301, Elionix Co.) at an accelerating voltage of 10 kV. Several different lines with widths between 0.5 lam and 5.0 lam were exposed to determine the resolution. The exposed resists were developed in a solution of chlorobenzene for 60 s and rinsed with isopropyl alcohol for 30 s. After development and post-baking, wet etching was performed. A film prepared by conventional spin coating was also investigated in order to compare it with the LB film of the same polymer. 3. RESULTS AND DISCUSSION

3.1. Monolayers of acetalizedpoly(vinyl alcohol) The properties of acetalized PVAs are summarized in Table I. The acetalized group content X was about 60-80 mol.~. The glass transition temperatures T~ decreased with increasing length of the methylene side chain, but the decomposition temperatures Td were almost the same. TABLE I PROPERTIESOF ACETAL1ZEDPOLY(VINYLALCOHOL)S

Polymer

AA PVA BuA PVA OA PVA DA PVA D D A PVA TrDA PVA TeDA PVA

Elemental analyses (%) C

H

61.55 66.20 71.48 73.07 74.30 75.19 75.58

8,97 10,03 11,37 11.74 12,05 12.25 12.41

X~

Mn(x 104) b Mw(x lOa)bMw/M, b Ts(°C) ¢

Td(°C) d

0.61 0.72 0.75 0.74 0.73 0.78 0.76

9.96 8.14 13.37 13.33 9.72 11.75 15.68

340 317 337 325 350 337 347

23.24 22.14 29.26 29.98 26.17 35.47 32.24

2.33 2.72 2.19 2.25 2.69 3.02 2.06

116.0 60.0 25.0 21.5 13.0 3.0 4.8

a Determined by elemental analyses. b Measured by gel permeation chromatography based on polystyrene standards. c Measured by differential scanning calorimetry. d Measured by thermogravimetric analysis.

Figure 2 shows the surface pressure-area curves of acetalized PVA on distilled water. Each curve exhibits a shoulder. With increasing length of the side chains the pressure a t t h e shoulders increased and the slope of the curves became steeper. It was found that the monolayer of acetalized PVA was more stable on water when the length of the alkyl side chain was longer than when it was short. Figure 3 shows the effect of the subphase temperature on the surface pressurearea curve of OA PVA. The pressure at the shoulder increased with decreasing temperature of the subphase, but the curves under the shoulder were temperature independent. In order to examine the plateau region, a compression-expansion recycle test was performed. Figure 4 shows the compression-expansion curves of OA PVA. When the polymer was compressed from a to b and then expanded again, no hysteresis occurred and the recompression curve went from a to b to c to d. Next, when the polymer was compressed from a to c and then expanded, hysteresis was

K. OGUCHIet al.

308

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6O

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Fig. 2. Surface pressure area curves of acetalized PVA on distilled water. Fig. 3. Effect ofsubphase temperature on surface pressure-area curves ofOA PVA.

observed (c-f-a), but the recompression curve was the same as before. In contrast, in the case of the compression from a to d, the expansion curve was from d to e to a, and the recompression was from a t o g to c to d, which is different. It was found from these results that collapse did not occur at a pressure in the plateau region. The plateau region might be due to a conformational change, owing to the balance between the hydrophilic and hydrophobic parts of the polymer. The limiting area per repeat unit was determined from the surface pressurearea curves in Fig. 2 as extrapolated area at zero pressure for the condensed region below the shoulder. In order to eliminate the influence of the different acetal group content on limiting areas, limiting areas per acetal unit were estimated by assuming that the limiting area og PVA is 0.12 nm 2 unit 1 (ref. 14). The calculation method was as follows: area per acetal unit = {A -0.12(1 - X ) } / X where A is the area per unit determined from Fig. 2 and X the mole fraction of acetal group content. Figure 5 shows the effect of the side chain length on the limiting area. It was found that the limiting area of acetalized PVA tended to a constant area with increasing length of the methylene side chain. The constant value of about 0.32 nm 2 per acetal unit is in rather reasonable agreement with the molecular model of acetalized PVA, assuming that the main chain lies flat on the water and the side chain is above the main chain.

3.2. Multilayers of acetalized poly(vinyl alcohol) Table II shows the conditions for depositing acetalized PVA onto various

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LB FILMS OF ACETALIZEDPOLY(VINYL ALCOHOL)S

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Fig. 4. Compression-expansion-compression curves ofOA PVA. Compression

Expansion

Recompression

a-b a-~c a-~c~l

b-a c-f-a d--e-a

a-b~;~l a-~c~l a-g-c~l

Fig. 5. Effectof number of methylenegroups on limiting area. TABLE II DEPOSITION CONDITIONS OF ACETALIZED POLY(VINYL ALCOHOL)S a

Polymer

Subphase temperature (°C)

Surface pressure (mN m - l)

Substrate

Deposition ratio

AA PVA BuA PVA OA PVA DDA PVA TeDA PVA OA PVA OA PVA OA PVA OA PVA

11.8 11.0 10.5 10.8 10.6 13.5 10.5 10.5 10.8

10 25 30 32 36 30 30 30 30

Si Si Si Si Si Cr AI Ge Glass

-0.98 0.99 0.98 1.00 0.94 1.00 1.01 0.94

aSubphase, distilled water; deposition speed, 1,25-2.50cm min ~. substrates. The substrates were cleaned by a p p r o p r i a t e methods. Silicon a n d g e r m a n i u m were cleaned with hydrofluoric acid a n d c h r o m i u m was cleaned with a mixture of h y d r o g e n peroxide a n d sulphuric acid, which p r o d u c e d h y d r o p h o b i c a n d hydrophilic surfaces respectively. A l u m i n i u m a n d glass substrates were cleaned simply with water in a n ultrasonic bath. W i t h the exception o f A A PVA, m o n o l a y e r s of acetalized P V A were easily deposited o n t o various substrates. The resulting multilayers were Y-type layers. Figure 6 shows the effect of the surface pressure of deposition o n the film thickness of O A PVA, D D A PVA a n d T e D A P V A o n silicon

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at 60 layers. The film thickness was measured by ellipsometry. The thickness of multilayers was thicker with increasing surface pressure. This result and the surface pressure-area curves in Fig. 2 may signify that the packing between the side chains is improved with increasing surface pressure. The lower film thickness at 10 m N m - 1 is due to the lack of packing and the low deposition ratio. Figure 7 shows the relationship between the number of layers and the thickness of the multilayers as measured by ellipsometry. It was clear from Fig. 7 that each case gave a linear relationship, showing the thickness of the multilayers to be proportional to the number of layers. The thickness of the multilayers increased with increasing length of the methylene side chain for a given number of layers. This result was also found to be the case when a Talystep mechanical stylus was used. Figure 8 shows the effect of the number of methylene groups in the side chain on the thickness per layer calculated from the results shown in Fig. 7. The thickness per layer increased dramatically with the length of the side chain.

150

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Fig. 6. Relationship between film thickness and surface pressure of deposition: O, T e D A PVA; 0, D D A

PVA; A, OA PVA. Fig. 7. Relationship between the number of layers and the thickness of the multilayers as measured by ellipsometry: O, TeDA PVA; 0, DDA PVA; A, OA PVA; A, BuA PVA. The slope between n = 10 and n = 12 gave 0.14 nm per methylene unit. This value was close to the value for long-chain fatty acids (0.145 nm per methylene unit) determined by Blodgett x. The thickness per layer may increase linearly with the number of the methylene units in the side chain when n is larger than 10. When the LB film from the acetalized PVA is a Y-type layer, it is expected that the surface wettability would be alternate according as the numbers of LB were odd or even. Figure 9 shows the wettability of the LB film of OA PVA on an aluminium substrate as a function of the number N of layers. The wettability was estimated by measuring the contact angle of water at 1 min after dropping distilled water on the

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FILMS OF ACETALIZED POLY(VINYL ALCOHOL)S

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surface. The wettability changed according to whether there was an odd or an even number of layers. The wettability of the surface prepared by spin coating lay between the values for even and odd numbers N, when N was larger than 6. This indicates that in the case of an even number of layers the surface is hydrophilic, and in the case of an odd number of layers, the surface is hydrophobic. From the results mentioned above, it was found that LB films ofacetalized PVA having long alkyl side chains are rather well ordered with the side chain directed normal to the main chain. 3.3 Characteristics of electron beam exposure Figure 10 shows the sensitivity curve of acetalized PVA prepared by conventional spin coating. It was found that acetalized PVAs were easily crosslinked by EB exposure and that the sensitivity was dependent on the length of the methylene units, though the degree of polymerization of the starting PVA was the same. The cross-linking reaction of acetalized PVA may occur at the acetal groups 15. Figure 11 shows the exposure characteristics of OA PVA prepared by the LB method and by spin coating. It was found that the sensitivity curve changed depending on the film thickness and the sensitivity of OA PVA prepared by the LB method was lower than that of the same polymer prepared by spin coating. Figure 12 shows the relationship between electron sensitivity and the number of methylene groups in the side chains. At almost the same thickness, the sensitivities of films prepared by the LB and spin coating methods were different with an increasing number of methylene units. This difference of the sensitivity may be ascribed to the morphology of the film. In the case of the LB method, the multilayer cross-linked by EB exposure may consist of two-dimensional networks rather than threedimensional networks owing to the oriented structure with the side chain directed normal to the main chain, and this tendency may become marked when the number of methylene units increases.

K. OGUCHI et al.

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Fig. 10. Exposurecharacteristicsofacetalized PVAs(development, chlorobenzene, 60s): O , T e D A PVA (film thickness, 630 nm); @, OA PVA (film thickness, 660 nm); A, BuA PVA (film thickness, 610 nm). Fig. 11. Exposure characteristics of OA PVA prepared by the LB method and spin coating (development, chlorobenzene, 60 s): O, @, spin coating films; ,~, A , D, LB films. Film thicknesses: O, 660 nm; @, 120 n m ; A, 150 n m ; A , 100 n m ; E], 62 nm.

~ 1 0 -6 E u

@

I0 s

5

0

Number

10 15 of methytene g r o u p s

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~

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?~

~r~x.~

~

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Fig. 13. Scanning electron micrograph of etched patterns of chromium using OA PVA prepared by the LB method.

LB FILMS OF ACETALIZED POLY(VINYL ALCOHOL)S

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Figure 13 shows a scanning electron micrograph of etched patterns of chromium using OA PVA prepared by the LB method. Submicron patterns could be easily obtained. From a thick film of the same polymer prepared by spin coating it was difficult to make fine patterns with submicron resolution. In this study, 0.5 p,m patterns could be obtained by the LB method when the beam diameter was 0.5 jam. The higher resolution patterns may be obtained by changing the beam diameter. As very thin, defect-free and uniform films could be obtained by the LB method in comparison with the spin coating method, the resolution of the acetalized PVA was higher in LB films than in films of the same polymer prepared by spin coating. Thus the LB method is useful in achieving high resolution in microlithography. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

K.B. Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. K.B. Blodgett amd I. Langmuir, Phys. Rev., 51 (1937) 1964. V. Enkelmann and J. B. Lando, J. Polym. Sci., 62 (1977) 509. G.L. Larkis, Jr., C. W. Burkhart, E. D. Thompson, J. B. Lando and E. Ortiz, Thin Solid Films, 99 (1983) 277. K. Fukuda, Y. ShibasakiandH. Nakahara, ThinSolidFilms, 99(1983)87. K. Fukuda and T. Shibasaki, Thin Solid Films, 68 (1980) 55. G. Lieser, B. TiekeandG. Wegner, ThinSolidFilms, 68(1980)77. A. Barraud, C. Rosilio and A. Ruaudel-Teixier, Thin Solid Films, 68 (1980) 91. R.H. TredgoldandC. S. Winter, ThinSolidFilms, 99(1983)81. S.J. Mumby, J. D. Swalen and J. F. Rabolt, Macromolecules, 19 (1986) 1054. M. Watanabe, Y. Kosaka, K. Sanui, N. Ogata, K. Oguchi and T. Yoden, Macromolecules, 20 (1987) 452. H. Noma and T. Koh, Kobunshi Kagaku, 4 (1943) 123. H. Noma, T. Koh and T. Tsuneta, Kobunshi Kagaku, 6 (1949) 439. D.J. Crisp, J. ColloidSci., l (1946)49. N. Ogata, K. Sanui, K. Oguchi, T. NakadaandY. Takahashi, J. AppI. Polym. Sci.,28(1983)2433.