Photoacid Generating Polymers for Surface Modification Resists - ACS

Chapter 23

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Masamitsu Shirai , Mitsuho Masuda , Masahiro Tsunooka , Masayuki Endo , and Takahiro Matsuo 3

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Department of Applied Chemistry, College of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Association of Super-Advanced Electronics Technologies, Yokohama Research Center, 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa 244, Japan Semiconductor Research Center, Matsushita Electric Industrial Company, Ltd., 3-15 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan

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Terpolymers of methyl methacrylate, 1,2,3,4-tetrahydro-1 -naphthylideneamino p-styrenesulfonate, and an amide-containing monomer or tetrahydrofurfuryl methacrylate were prepared. These polymers can be used as oxygen plasma-developable surface modification resists. When the irradiated polymer films were exposed to the vapor of alkoxysilanes at 30 °C, polysiloxane networks were formed at the film surface. The polysiloxane formation rate was enhanced by incorporation of amide or ether unit into polymers. Amide units were more effective than ether units. The film surface modified with polysiloxanes showed a good resistance to the etching with an oxygen plasma. Using the surface modification resist system, negative-tone images were generated by an ArF excimer laser lithography. Photolithographic processes using surface modification techniques have been reported by several groups. The predominant approach involves the post-exposure silylation of organic polymer films (7,2) and the selective deposition of Si containing materials on the exposed or unexposed film surface (3). Previously we reported plasma-developable photoresists based on the photoinduced acid-catalyzed polysiloxane formation at the irradiated polymer surface by a chemical vapor deposition ( C V D ) method (4-6). The methodology of the system is as follows: Upon irradiation with U V light the surface of the polymers having photoacid generating units becomes hydrophilic because of the formation of acids. Water sorption from the atmosphere occurs at the surface. When the irradiated film is exposed to the vapor of alkoxysilanes, polysiloxane networks are formed at the surface of the polymer films (Figure 1). N o polysiloxane networks are formed in unirradiated areas because the photochemically formed acids are necessary for the polysiloxane formation by hydrolysis and subsequent condensation of alkoxysilanes. This system gives a negative tone image by oxygen reactive ion etching (O2 R I E ) .

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© 1 9 9 8 American Chemical Society Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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No Reaction \

\

Hydrolysis

Si(OR) ) (5^0) H*

H+

/\f

\

•(Si(OH) )(ROg)

4

4

(Si(OR)JHA) Unexposed Area

H+ Exposed Area

Substrate

11 1

I

-0-SI-O-Si-O-

Polycondensation

?1 H+

Figure 1. surface.

H+

H*

H+

H*

Selective deposition of polysiloxanes at the irradiated polymer

Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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In the previous work we obtained by a K r F (248 nm) excimer laser lithography 0.25 line and space images based on the surface modification process using a copolymer of methyl methacrylate and 1,2,3,4-tetrahydro-l-naphthylideneamino pstyrenesulfonate (NIS) as photoacid generating units (7). The present system has an advantage to allow imaging at both 248 and 193 nm wavelengths, without the need for introducing changes in the chemistry involved in the image forming process. However, the copolymer showed relatively low sensitivity toward A r F (193 nm) excimer laser exposure. T o obtain a highly sensitive resist system, enhanced polysiloxane formation at the exposed film surface is required. The polysiloxane formation rate at the irradiated surface is reported to be proportional to the number of acid units generated photochemically and to the number of water molecules adsorbed (6). In this study we synthesized polymers containing both N I S units and amide or ether units. W e also studied the surface modification of the polymer films by a chemical vapor deposition ( C V D ) method using alkoxysilane vapor at 30 °C. The incorporation of amide groups into polymers enhanced the sorption of water and thus increased the polysiloxane formation rate at the irradiated film surface. Pattern fabrication based on the present system was also investigated using an A r F excimer laser lithography. Experimental Materials. Synthesis of 1,2,3,4-tetrahydro-l-naphthylideneamino pstyrenesulfonate (NIS) was reported in detail elsewhere (6). Methyltriethoxysilane (MTEOS), tetramethoxysilane (TMOS), tetraethoxysilane (TEOS) and methyltrimethoxysilane ( M T M O S ) were of reagent grade and used without further purification. Methyl methacrylate ( M M A ) , N,N-dimethylacrylamide ( D M A A ) , acryloylmorpholine ( A M O ) , and tetrahydrofurfuryl methacrylate ( T H F M A ) were distilled before use. P r e p a r a t i o n o f P o l y m e r s . Terpolymers ( 2 a - c and 4 ) and copolymers ( 1 , 3 , 5 , and 6 ) were prepared by the conventional radical copolymerization of corresponding monomers with 2,2'-azobis(isobutyronitrile) ( A I B N ) as an initiator at 55 °C. The concentrations of total monomer and A I B N in benzene or N , N dimethylformamide were usually 4.5 and 1.6X10-2 mol/L, respectively. The sample solution was degassed under vacuum by repeating freeze-thaw cycles before polymerization. The content of the NIS units in the copolymers was determined by measuring the absorbances at 254 nm in CH2CI2. The molar extinction coefficient of the NIS units in the polymers was estimated to be equal to that of the model compound, 1,2,3,4-tetrahydro-l-naphthylideneamino p-toluenesulfonate, e being 15,300 L / m o l c m at 254 nm at room temperature. The composition of the terpolymers was determined by measuring absorbance at 254 nm in CH2CI2 and from 1 H - N M R spectra. Polymerization conditions and characteristics of the polymers are shown in Table I. Structures of the polymers used i n this study are shown in Scheme I.



2.6

1.3

1.3

1.1

1.1

2c

3

4

5

6

1.5

0.8

1.8

3.2

1.6

1.8

2.4

DMAA (g)

4.0

2.1

AMO (g)

2.9

THFMA (g)

3

3.5

4

3.5

5

2.5

3

7

Time (h)

Polymerization

33

32

46

38

30

34

52

40

(%)

Conversion

16.5

5.5

2.3

76

85

20

84

44

-e) 2.6

39

37

0

X

0

0

60

0

8

37

59

83

y

24

15

20

16

48

24

4

17

z

b

Composition (mol %))

4.1

3.8

5.6

XI0-4

Mn

g

g

c )

78

-d)

-d)

-d)

-d)

-d)

106

96

CC)

T

a) [Total monomer]=4.5 mol/L, [AIBN]=1.6X10-2 mol/L. N,N-Dimethylformamide (2a-c, 3, 4 and 5) and benzene (1 and 6) were used as a solvent, b) See Scheme I. c) Glass transition temperature from DSC. d) No distinct T was observed, e) Not measured.

1.3

2b

2.4

3.7

1.3

0.31

1

MMA (g)

Monomer in Feed

NIS (g)

2a

Polymerb)

Table I. Polymerization Conditions and Polymer Properties*)

Scheme I. Structures and photochemical reaction of polymers


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Water Sorption. A laboratory-constructed piezoelectric apparatus (quartz crystal microbalance) was used to measure water sorption in the polymer films. The AT-cut quartz crystal with gold electrodes (Hokuto Electronics) had a resonance frequency of 10.000 M H z . W i t h this crystal, a frequency shift of 1 H z corresponded to a mass change of 0.58 ng. The frequency change is linearly related to the mass sorbed on the quartz plate (5,9). Polymers were deposited onto the quartz crystal (0.8 c m diameter) by casting from chloroform solution. The area coated with the polymer film was usually 0.13 cm2. The quartz crystal was placed in the middle of the sealed glass vessel which had a quartz window for U V irradiation. 2 M K N O 3 aqueous solution was placed at the bottom of the vessel to control its humidity (RH=95%) at 25 °C. Irradiation of polymer films on the quartz crystal through the quartz window of the vessel was carried out with 254-nm light using a 5-W low-pressure H g lamp (Toshiba L P - 1 I B ) . The intensity of the incident light determined with a chemical actinometer (potassium ferrioxalate) (10) was 0.1 mJ/cm2 sec at 254 nm. D e p o s i t i o n o f P o l y s i l o x a n e . The polymer films (8.8 X 22 mm) were prepared on silicon wafers by spin-casting from diglyme solutions and drying under vacuum at room temperature. After exposure with 254-nm light using a low-pressure H g lamp, the silicon wafer coated with polymer film was placed at the center of a 500 m L glass vessel which had gas-inlet and -outlet valves. Fifty m L of water was placed at the bottom of the vessel to adjust the relative humidity in the vessel and equilibrated for 10 min prior to introduction of the vapor of alkoxysilanes. During the polysiloxane network formation nitrogen gas (50 mL/min) flowed through a bubbler which contained liquid alkoxysilanes. The bubbler and reaction vessel were placed in a thermostatic oven at 30 °C. The amounts of polysiloxanes formed at the near surface of the polymer films were determined from the difference between the FT-IR absorbance at 1121 cm-i of the sample plate before and after exposure to the vapor of alkoxysilanes. E t c h i n g w i t h O x y g e n P l a s m a . Oxygen plasma etching was carried out at room temperature using a laboratory-constructed apparatus where the oxygen plasma was generated using two parallel electrodes and R F power supplies. The typical etching conditions were as follows: 2 0 W power (13.56 M H z ) , power density of 1.0 W / c m , 1 2 5 mTorr, and oxygen flow of 1 seem. 2

Lithographic Evaluation. Resists were spin-coated on S i substrate and baked at 90 °C for 90 sec. Diglyme was used as a solvent to make resist solutions. Exposure was carried out using a prototype A r F excimer laser exposure tool (NA=0.55). Results and Discussion The polymers 1, 2 a - c , 3, 4 and 6 were soluble in organic solvents such as diglyme, tetrahydrofuran, and dichloromethane. The polymer 5 was insoluble in diglyme.


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Glass transition temperatures ( T ) of 1, 2a and 6 were observed to be 96, 106 and 78 °C, respectively. N o distinct T was observed for the polymers other than 1, 2a and 6. Polymers in CH2CI2 solutions showed an absorption peak at about 200 nm and shoulder peaks at about 235 and 255 nm. The absorption peak at about 200 nm was due to the NIS units and amide units. The shoulder peaks were due to the NIS units. It has been reported that upon irradiation at 254 nm the cleavage of - 0 - N = bonds in the NIS units and subsequent abstraction of hydrogen atoms from residual solvent in the polymer film and/or from polymer molecules lead to the formation of pstyrenesulfonic acid units, tetralone, and tetralone azine (see Scheme I) (77). Change in the absorption spectrum of the 2a film upon irradiation at 254 nm is shown in Figure 2. The absorbances at 200 and 254 nm decreased with irradiation time, and an isosbestic point was observed at 230 nm. In the present system photolytic decomposition of the NIS units was complete after irradiation of 207 mJ/cm2 The quantum yield for the photolysis of the NIS units incorporated into poly(methyl methacrylate) was about 0.3 for 254-nm irradiation in air. A photolysis degree of the NIS units of 1, 2a, 2b and 3 is plotted as a function of exposure dose in Figure 3. The photolysis rate was not strongly dependent on either structure of the amide units or the NIS unit fraction of the polymers. g

g

In the presence of water and strong acids, the hydrolysis and subsequent polycondensation reactions of methyltriethoxysilane ( M T E O S ) and its homologues lead to the formation of polysiloxane networks, which is well known as the sol-gel process for the silica glass formation (72). When irradiated polymer films bearing the NIS units were exposed to the vapor of M T E O S at 30 °C, polysiloxane networks were tormed in the near surface region or the t i l m , which was contirmed by r I -IK analysis (Figure 4). The sample films irradiated and subsequently exposed to the M T E O S vapor showed new peaks at 3500 (Si-OH), 1272 ( S i - C H ) ,1000-1200 ( S i O-Si), and 790 (S1-CH3) c m - i . The presence of the peak due to S i - O H suggested the incomplete polycondensation reaction of the silanol moieties. N o polysiloxane networks were formed in the unirradiated areas, because the p-styrenesulfonic acid units formed photochemically are essentially important for the acid-catalyzed hydrolysis of M T E O S molecules. Figure 5 shows the effect of incorporation of hydrophilic units such as amide or 3

etner units into tne pnotoacid generating polymers on tne polysiloxane iormation rate at the irradiated film surface. NIS unit fractions of 1, 2b, 3 and 6 were 0.17, 0.24, 0.16 and 0.24, respectively. The amounts of acids photochemically formed were adjusted to be 4.5 m o l % by controlling the irradiation time. The amounts of polysiloxanes tormed at tne rum surrace were measured oy tne ennanced aosoroance at 1121 cm-i due to Si-O-Si. The amounts of polysiloxane networks increased with C V D treatment time for all the polymers. The polysiloxane formation rate decreased in the order 3 > 2b > 6 >1 and the rate for 3 was about 18 times higher than that for 1. O n the other hand, the rate for 6 was about twice as that for 1. Thus the incorporation of amide units in polymers effectively enhanced the polysiloxane


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Figure 3.

Relationship between photolysis degree of NIS units and

exposure dose. Polymer: (O)l;

(•)

2 a ; (O) 2 b ; ( A ) 3.


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100

(b) YV

Si-CH U

Si-CH

3

3

Si-O-Si

0 4600

I

I

3000

1000

2000

400

-1

Wavenumber (cm ) Figure 4. FT-IR spectra of the irradiated 2 b film (a) before and (b) after C V D treatment with M T E O S vapor at 30 °C for 20 min. Exposure dose: 166 mJ/cm . 2

CVD treatment time (min)

Figure 5.

Effect of hydrophilic units in polymers on the polysiloxane

formation at the irradiated film surface.

C V D treatment was done with

M T E O S vapor at 30 °C. Polymer: ( A ) 1; ( • ) 2 b ; (O) 3 ; ( V ) 6 .


315

formation rate. This may be due to the enhanced water sorption ability of the polymers because the amide units are generally hydrophilic. It was reported that the amounts of water sorbed onto the irradiated surface critically affected the hydrolysis rate of M T E O S molecules (6). The enhanced water sorption ability of the amide-containing polymers was confirmed by measuring the amounts of water adsorbed using a quartz crystal microbalance. Figure 6 shows the relationship between the irradiation time and water sorbed into the films of 1, 2b, and 3 at relative humidity of 95%. The weight of polymer films cast on the quartz crystal was 2500 ng. If the polymer is assumed to have a density of 1 g/cm3, the film thickness is 192 nm. Water sorption began the moment that the polymer film was irradiated with 254-nm light. It increased with the irradiation time and reached a constant value after irradiation. The water sorption ability of the polymers decreased in the order 3 > 2b > 1, suggesting that the amide units of the polymer enhanced the water sorption ability. The water sorption ability of 3 was about 5 times higher than that of 1. It was found that the water sorbed onto the irradiated film surface was removed when the sample was placed under a dry nitrogen atmosphere. Figure 7 shows the effect of acid generating unit fraction on the polysiloxane formation rate. The polysiloxane formation rate decreased i n the order 2c > 2b > 2a, i f the photolysis degrees of the N I S units in the polymers were same (4.5 mol%). The higher the concentration of the photoinduced acids at the surface, the larger the polysiloxane formation rate. The polysiloxane formation rate is also dependent on the structure of alkoxysilanes used. Although in the present system alkoxysilanes such as M T M O S , T M O S , M T E O S and T E O S could be used, the polysiloxane formation rate decreased in the order M T M O S > T M O S > M T E O S > T E O S . It was reported that the polysiloxane formation rate was determined by both hydrolytic reactivity of alkoxysilanes and vapor pressure of alkoxysilanes (boiling point of alkoxysilanes) (6). A s shown in Figure 8, the polysiloxane formation rate at the irradiated surface of 2b and 4 films was almost the same. In this experiment, the photolysis degree of the NIS units of 2b and 4 films was adjusted to be 4.5 m o l % . Since the morpholine unit fraction of 4 was about half of the N,N-dimethylacrylamide ( D M A A ) units of 2b, the morpholine units seem more effective for the enhancement of the water sorption ability. This was confirmed by the experiments on the water sorption of these polymers using a quartz crystal microbalance. The polymer 5 was not checked because the solubility of 5 in diglyme was too low to make sample films by spin-casting. Thus, polymer with high content of morpholine units has a disadvantage in terms of solubility in organic solvents. Figure 9 shows the effect of surface modification of 2b film on the oxygen plasma etching. The etching rate of 2b film without modification was 0.08 //m/min under the present etching conditions. It was almost the same as that observed for poly(methyl methacrylate) film. The sample film was irradiated at 254 nm (10 mJ/cm2) and subsequently exposed to the vapor of M T E O S for 5 min at 30 °C. The etch rate of the modified film was lower than 1/40 compared to that of the unmodified


316

500 Standing

Irradiation 400

B

300

o CO 2 CO

200

5

100 QOOOOOOOOOO

10

20

15

Time (min)

Figure 6.

Relationship between irradiation time and water sorption into

polymer fdms.

Polymer weight: 2500 ng; relative humidity: 95%.

Polymer: ( A ) 1; ( • ) 2 b ; (O)

Figure 7.

3.

Effect of acid generating unit fraction of the polymers on the

polysiloxane formation at the exposed areas. C V D treatment was done with M T E O S vapor at 30 °C. Polymer: ( A ) 2 a ; ( • ) 2 b ; (O)

2c.


317 0.3

/ E o

/ 0.2

/

/

0

15

20

• / / -Q

Photoacid Generating Polymers for Surface Modification Resists - ACS

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