A photosensitive semi-alicyclic poly(benzoxazole) with high transparency and low dielectric constant

Article abstract of DOI:10.1021/ma049063i

A photosensitive semi-alicyclic poly(benzoxazole) based on poly(o-hydroxy amide) containing adamantyl units (PAHA) and 1,3,5-tris[(2-vinyloxy)ethoxy] benzene (TVEB) as an acidolytic de-cross-linker and a photoacid generator, diphenyliodonium 9,10-dimethoxyanthracene-2-sufonate (DIAS), has been developed. PAHA with a weight-average molecular weight of 24 100 was prepared from 1,3-adamantanedicarbonyl chloride (ADG) and 4,4′- (hexafluoroisopropylidene)bis(o-aminophenol) (6FAP) in the presence of lithium chloride in N-methylpyrrolidinone (NMP) at 0°C for 12 h. By thermal treatment, PAHA was easily converted to semi-alicyclic poly(benzoxazole) (PABO) that was insoluble in organic solvents and showed high thermal stability (I _d5 = 520°C under Ns). The UV-vis spectra of PAHA and PABO indicated excellent transparency at wavelengths above 320 and 400 nm, respectively. The average refractive index of PABO was 1.523, and the dielectric constant estimated from the refractive index was 2.55 at 1 MHz. This value is significantly lower than those of conventional wholly aromatic PBOs. The photosensitive PABO precursor, PAHA containing 15 wt % TVEB and 5 wt % DIAS, showed a sensitivity of 40 mJ/cm² and a contrast of 4.0 when it was exposed to a 365 nm light (i-line) and developed with a 2.38 wt % aqueous tetramethylammonium hydroxide solution (TMAH_aq) at 25°C. A fine positive image of 10 μm line-and-space pattern was also printed in a film which was exposed to 70 mJ/cm² of i-line by contact-printing mode. The positive image in PAHA was converted to the positive image in the PABO film by the thermal treatment without pattern deformation.

Full text of DOI:10.1021/ma049063i

8256
Macromolecules 2004, 37, 8256-8261
A Photosensitive Semi-Alicyclic Poly(benzoxazole) with High
Transparency and Low Dielectric Constant
Ken -ich i F u k u k a w a , Yu ji Sh iba sa k i, a n d Mitsu r u Ued a *
Department of Organic & Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, H-120 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, J apan
Received May 13, 2004; Revised Manuscript Received August 13, 2004
ABSTRACT: A photosensitive semi-alicyclic poly(benzoxazole) based on poly(o-hydroxy amide) containing
adamantyl units (PAHA) and 1,3,5-tris[(2-vinyloxy)ethoxy]benzene (TVEB) as an acidolytic de-cross-
linker and a photoacid generator, diphenyliodonium 9,10-dimethoxyanthracene-2-sufonate (DIAS), has
been developed. PAHA with a weight-average molecular weight of 24 100 was prepared from 1,3-
adamantanedicarbonyl chloride (ADC) and 4,4′-(hexafluoroisopropylidene)bis(o-aminophenol) (6FAP) in
the presence of lithium chloride in N-methylpyrrolidinone (NMP) at 0 °C for 12 h. By thermal treatment,
PAHA was easily converted to semi-alicyclic poly(benzoxazole) (PABO) that was insoluble in organic
solvents and showed high thermal stability (Td5 ) 520 °C under N2). The UV-vis spectra of PAHA and
PABO indicated excellent transparency at wavelengths above 320 and 400 nm, respectively. The average
refractive index of PABO was 1.523, and the dielectric constant estimated from the refractive index was
2.55 at 1 MHz. This value is significantly lower than those of conventional wholly aromatic PBOs. The
photosensitive PABO precursor, PAHA containing 15 wt % TVEB and 5 wt % DIAS, showed a sensitivity
of 40 mJ /cm2 and a contrast of 4.0 when it was exposed to a 365 nm light (i-line) and developed with a
2.38 wt % aqueous tetramethylammonium hydroxide solution (TMAHaq) at 25 °C. A fine positive image
of 10 µm line-and-space pattern was also printed in a film which was exposed to 70 mJ /cm2 of i-line by
contact-printing mode. The positive image in PAHA was converted to the positive image in the PABO
film by the thermal treatment without pattern deformation.
In tr od u ction
while maintaining a high thermal stability of the
polymer. In the field of photolithographic techniques,
chemical amplification is crucial to improve both sen-
sitivity and contrast. Especially, chemical amplification
in positive working is more requested than that in
negative working due to effective elimination of dark
spots on a chip after development.
In this paper, we report the synthesis of PABO with
high transparency and low dielectric constant based on
corresponding semi-alicyclic PHA (PAHA) which is
prepared from 1,3-adamantanedicarbonyl chloride (ADC)
and 6FAP. And its application to a new PSPBO consist-
ing of PAHA, 1,3,5-tris[(2-vinyloxy)ethoxy]benzene
(TVEB) as an acidolytic decross-linker, and diphenyl-
iodonium 9,10-dimethoxyanthracene-2-sulfonate (DIAS)
as a photoacid generator in positive-tone chemically
amplification is described here as well.
Poly(benzoxazole)s (PBOs) have been used as protec-
tion and insulation layers for very large scale integra-
tion circuits (VLSI) and multichip modules for com-
puters owing to their high thermal stability and excel-
lent mechanical property.1-4 Photosensitive PBOs
(PSPBOs)5-7 have been developed to simplify the pro-
cessing steps because phenolic hydroxyl groups in a
precursor of the PBO provide adequate solubility toward
an aqueous alkaline developer. Up to now PSPBO based
on poly(o-hydroxy amide) (PHA) derived from 4,4′-
(hexafluoroisopropylidene)bis(o-aminophenol) (6FAP) and
4,4′-oxybis(benzoic acid) derivatives8,9 with a photo-
sensitive compound is widely used.
Since the lower dielectric constants are required to
increase the circuit speed,4 many researchers have
reported candidates for low dielectric materials,10-12
most of which have thermal stability but do not possess
the function of photosensitivity. Generally, PBO is
considered as relatively lower dielectric material than
polyimide due to the absence of carbonyl group in the
structure. Furthermore, high sensitivity of photopoly-
mers is an important parameter for the design of
photoresist materials. Highly sensitive resists require
the high transparency of the matrices at certain irradia-
tion wavelengths, which is desirable to maximize the
throughput of wafers in exposure tools. Alicyclic PBOs
(PABOs) are interesting materials for optoelectronics
and interlayer dielectrics. Adamantyl units will be
effective to decrease a conjugation in a main chain and
a molecular density because of their sp3 character of
carbons and bulkiness,13-15 respectively, giving PABOs
with high transparency and low dielectric constant,
Exp er im en ta l Section
Ma ter ia ls. Tetrahydrofuran (THF) was distilled from so-
dium metal and benzophenone ketyl. N-Methyl-2-pyrrolydi-
none (NMP) was dried over calcium hydride, distilled under
reduced pressure, and stored over 4 Å molecular sieves. 6FAP
was purified by recrystallization from THF. ADC was prepared
by reaction of 1,3-adamantanedicarboxylic acid with excess
amounts of thionyl chloride, followed by recrystallization from
n-hexane. A photoacid generator DIAS and an acidolytic
decross-linker TVEB were kindly donated by Toyo Gosei Co.
Ltd. and Tokyo Ohka Kogyo Co. Ltd., respectively. The other
reagents and solvents were used as received.
Syn t h esis of 5,5′-(H exa flu or oisop r op ylid en e)b is(2-
h yd r oxya d a m a n ta n eca r boa n ilid e). To a solution of lithium
chloride (0.653 g, 15.4 mmol) and 6FAP (2.56 g, 7.00 mmol) in
20 mL of NMP was added adamantane-1-carbonyl chloride
(2.92 g, 14.7 mmol). The solution mixture was stirred at 0 °C
for 3 h and then poured into water. The precipitated solid was
collected and dried in vacuo at 100 °C for 8 h. The product
* Corresponding author. E-mail mueda@u.polymer.titech.ac.jp.
10.1021/ma049063i CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/01/2004

Macromolecules, Vol. 37, No. 22, 2004
Photosensitive Semi-Alicyclic Poly(benzoxazole)s 8257
was purified by recrystallization from mixture of toluene and
Sch em e 1. Syn th esis of
N-Ad a m a n tyl-2-h yd r oxyben za m id e Der iva tive a s a
P AHA Mod el
THF. The yield was 3.1 g (64%). Sublimation point: 348-350
1
°C. H NMR (DMSO-d6, δ, ppm): 10.32 (s, 2H, OH), 8.39 (s,
2H, NHCO), 8.11 (s, 2H, Ar), 6.87 (dd, J ) 12.9, 8.4 Hz, 4H,
Ar), 2.01 (s, 6H, CH), 1.88 (s, 12H, COC(CH2)3), 1.69 (s, 12H,
CHCH2CH). Elemental analysis: Calcd for C37H40F6N2O4: C,
64.34; H, 5.84; N, 4.06; Found: C, 64.62; H, 6.07; N, 4.07.
Syn th esis of P AHA fr om 6F AP a n d ADC. To a solution
of lithium chloride (0.0933 g, 2.20 mmol) and 6FAP (0.366 g,
1.00 mmol) in 1.5 mL of NMP under nitrogen was added ADC
(0.261 g, 1.00 mmol) at once. And the mixture was stirred at
0 °C for 12 h under a nitrogen atmosphere. The resulting
solution was poured into water. The precipitated polymer was
filtered off and washed with water several times, followed by
drying in vacuo. The yield was almost quantitative (>98%).
1H NMR (DMSO-d6, δ, ppm): 10.28 (s, 2H, OH), 8.53 (s, 2H,
NHCO), 8.08 (s, 2H, Ar), 6.94 (d, J ) 8.7 Hz, 2H, Ar), 6.87 (d,
J ) 8.7 Hz, 2H, Ar), 2.19-1.67 (m, 14H, CH2 or CH). IR (KBr,
ν, cm-1): 3424 (OH), 2915, 2857 (CH2, CH), 1654 (CdO), 1253
(CF3). Elemental analysis: Calcd for C27H24F6N2O4‚1.13H2O:
C, 56.41; H, 4.61; N, 4.87; Found: C, 56.98; H, 4.50; N, 4.30.
Sch em e 2. Syn th esis of P AHA a n d P ABO
Dissolu tion Ra te. TVEB and DIAS were added to a PAHA
solution in cyclohexanone to construct a photosensitive poly-
mer. The polymer film spin-cast from the solution (15 wt %
concentration) on a silicon wafer was prebaked at 90-140 °C
for 5 min and then exposed to a filtered super-high-pressure
mercury lamp at 365 nm (i-line), followed by postexposure
baking at the same temperature for 5 min. The exposed film
was developed with 2.38 wt % of tetramethylammonium
hydroxide (TMAHaq) solution at 25 °C to determine a dissolu-
tion rate (Å/s).
P h otosen sitivity. A 2.0 µm thick photosensitive polymer
film on a silicon wafer was exposed to i-line and then developed
with 2.38 wt % of TMAHaq at 25 °C for 60 s followed by rinsing
with water. A characteristic curve was obtained by plotting a
normalized film thickness as a function of exposure dose (mJ /
cm2).
the corresponding o-hydroxybenzamide increases in the
area of i-line because of decreasing conjugation between
2-hydroxybenzamide and the N-substituent group. Ada-
mantane is known as a robust alicyclic compound with
relatively high thermal resistance. Thus, an adamantyl
group was selected as an N-substituent (Scheme 1). The
UV-vis spectra of N-phenyl-2-hydroxybenzamide and
N-adamantyl-2-hydroxybenzamide derivatives shown in
Figure 1 clearly indicate that the N-adamantyl group
is very effective to improve the transparency of N-
substituent-o-hydroxybenzamide.
Mea su r em en ts. The infrared spectroscopy (IR) was taken
1
with Horiba FT-210 spectrophotometer. The H nuclear mag-
netic resonance (NMR) spectrum was recorded on a Bruker
GPX300 (300 MHz) spectrometer. Thermogravimetric (TG) and
differential scanning calorimetry (DSC) were performed on a
Seiko TG/DTA 6300 and DSC 6200, respectively, at a heating
rate 5 °C/min under a nitrogen stream. Ultraviolet-visible
spectroscopy (UV-vis) was performed on
a J asco V-650
spectrophotometer. Number- and weight-average molecular
weights (Mn and Mw) were determined by gel permeation
chromatography (GPC) with a Tosoh HLC-8120 GPC system
Syn th eses a n d Ch a r a cter iza tion s of P AHA a n d
P ABO. Polycondensation of 6FAP with ADC was car-
ried out in the presence of lithium chloride in NMP at
0 °C for 12 h (Scheme 2). Polycondensation proceeded
smoothly to give a desired PAHA with a weight-average
molecular weight of 24 100 and a polydispersity of 3.2.
The structure of the polymer was identified as the
equipped with polystyrene gel columns (TSK GELs; GMHHR
M, and GMHHR-L) at 40 °C in DMF (containing 0.01 M LiBr)
at a flow rate of 1.0 mL/min, calibrated with polystyrene
standards. The film thickness on silicon wafers was measured
by a Veeco Instrument Dektak3 surface profiler. The field
emission scanning electron microscope (SEM) was taken with
a Hitachi S-800 scanning electron microscope with 15 kV
accelerating voltage for imaging. Refractive indices of PABO
-
1
corresponding PAHA by H NMR and IR spectroscopy.
films formed on quartz substrates were measured at
a
wavelength of 1.320 µm at room temperature with a Metricon
model PC-2000 prism coupler. Using linearly polarized laser
with parallel (TE: transverse electric) and perpendicular
(TM: transverse magnetic) polarization to the film plane, the
in-plane (nTE) and out-of-plane (nTM) refractive indices and the
film thickness of the samples were determined. The dielectric
constant (ꢀ) at 1.0 MHz frequency was calculated from the
equation as follows: ꢀ ) 1.10 nAV2, where nAV is average
refractive index (i.e., nAV ) (2nTE + nTM)/3).
Resu lts a n d Discu ssion
Since i-line is used as exposure source in PSPBOs,
the transparency of PAHA in the region of i-line is
important to improve the sensitivity. When N-substitu-
ent groups of o-hydroxybenzamide are replaced from
aryl derivatives to alicyclic ones, the transparency of
F igu r e 1. Transparencies of (a) alicyclic compound (adaman-
tyl) and (b) aromatic compound (phenyl) as N-substituent
groups of o-hydroxybenzamide in THF at 1.0 µM. Inset shows
chemical structures of (a) and (b).

8258 Fukukawa et al.
Macromolecules, Vol. 37, No. 22, 2004
F igu r e 2. 1H NMR spectrum of PAHA in DMSO-d6 at 40 °C.
F igu r e 5. TG and DSC curve of PABO. The heating rate was
5 °C/min under nitrogen.
Ta ble 1. Solu bility of P oly(o-h yd r oxya m id e) (P AHA) a n d
P olyben zoxa zole (P ABO)a
solubility
solubility
F igu r e 3. IR spectra of (a) PAHA and (b) PABO on a silicon
wafer.
solvent
acetone
chloroform
m-cresol
PAHA PABO
solvent
HMPAd
n-hexane
methanol
2-methoxy-
ethanol
PAHA PABO
(
-
-
-
-
-
+
-
-
-
-
-
+
(
cyclohexanone
++
++
diethyl ether
DIGLYMb
DMFc
-
++
++
-
-
-
-
-
2-propanol
PGMEAe
sulfuric acid
1,1,2,2-tetra-
chloroethane
THFf
-
-
-
-
(
-
++
-
1,4-dioxane
ethyl acetate
ethyl lactate
-
-
-
-
++
-
-
-
toluene
a
Symbols: ++, soluble at room temperature; +, soluble by
b
heating; (, partially soluble or swelling; -, insoluble. Diethylene
glycol dimethyl ether. c N,N-Dimethylformamide. Hexamethyl-
d
phosphoramide. e Propylene glycol-1-monomethyl ether 2-acetate.
F igu r e 4. TG trace of PAHA. The heating rate was 5 °C/min
under nitrogen.
f Tetrahydrofuran.
range, the weight loss of 7.5% is in good agreement with
the value calculated from the elimination of water due
to cyclization (6.5%). The IR spectrum of thermally
treated PAHA exhibits a characteristic benzoxazole ring
absorption at 1620 cm-1, and the peaks due to the amide
(1654 cm-1) and hydroxyl (around 3400 cm-1) groups
of PAHA completely disappear (Figure 3b). Thus, PAHA
is readily converted to the corresponding PABO by
thermal treatment at 350 °C under nitrogen for 1 h. A
5% weight loss temperature (Td5) of PABO is 518 °C
(Figure 5), which indicates a high thermal stability of
PABO. The DSC curve of PABO in Figure 5 shows a
glass transition temperature, Tg, at 302 °C whereas it
is enough for microelectronics application. Table 1 shows
the results of qualitative solubility both for PAHA and
1
The H NMR spectrum of PAHA is presented in Figure
2 with a good assignment of each resonance. The peaks
due to the hydroxyl, amide, and aromatic protons are
observed at 10.28 (broad), 8.53 (singlet), and 8.08-6.84
ppm (singlet and quartet), respectively, while adamantyl
protons appeared at 2.19-1.67 ppm. The IR spectrum
shows characteristic O-H and amide absorptions at
3400 and 1654 cm-1, and no peak corresponding to the
ester unit is observed as shown in Figure 3a. These
results clearly indicate that the addition of lithium
chloride is effective in promoting the chemoselective
amidation in this system.16
The trace of TG for PAHA is shown in Figure 4. A
rapid weight loss was observed at 250-350 °C. In this

Macromolecules, Vol. 37, No. 22, 2004
Photosensitive Semi-Alicyclic Poly(benzoxazole)s 8259
F igu r e 6. UV-vis spectra of (a) PAHA film, (b) PABO film,
and (c) PHA on a quartz plate. The absorbance of each film is
set by film thickness, µm-1
.
PABO. The latter was prepared by casting the solution
of PAHA on a silicon wafer and then cured up to 350
°C for 1 h under nitrogen. PAHA exhibits excellent
solubility in various solvents compared to that of the
corresponding PABO.
F igu r e 7. Effect of TVEB and DIAS loadings on the dissolu-
tion rate in 2.38 wt % TMAHaq. Each film was prebaked at
120 °C for 5 min, exposed to i-line of 100 mJ /cm2, and
postexposure baked at 120 °C for 5 min.
The transmission UV-vis spectra (film on quartz) of
PAHA and PHA which was prepared from 6FAP and
4,4′-oxybis(benzoic acid) derivatives are shown in Figure
6a,c. PAHA and PHA films show cutoff at 320 and 370
nm, respectively. Therefore, PAHA is very transparent
in the UV-vis region, especially at 365 nm (i-line),
compared to that of PHA. A cutoff wavelength is defined
here as the point where the transmittance becomes
below 1% in the spectrum. In addition, the PABO film
shows relative high transparency although the small
absorption is extended to 400 nm (Figure 6b).
Dielectr ic Con sta n t of P ABO. The average refrac-
tive indices (nAV) of PABO was determined as 1.523. A
dielectric constant (ꢀ) of the material at optical frequen-
cies can be estimated from the refractive index (n)
according to Maxwell’s equation, ꢀ ) n2. The ꢀ around
1 MHz has been evaluated as ꢀ ) 1.10nAV2, including
an additional contribution of approximately 10% from
the infrared absorption. The nAV of 1.523 can be
translated into the dielectric constant of 2.55. This value
is considerably lower than the optically estimated ꢀ of
the other wholly aromatic PBOs even which contains
hexafluoroisopropylidene groups (ꢀ ) around 2.8)17,18
due to the introduction of an alicyclic bulky unit,
adamantyl, in the polymer structure.
Lith ogr a p h ic Eva lu a tion of P AHA, TVEB, a n d
DIAS. As no absorption in the UV-vis spectrum of
PAHA was observed above 320 nm, DIAS having an
strong absorption in the range 300-420 nm was selected
as a photoacid generator.19 On the other hand, TVEB
was selected as an acidolytic de-cross-linker for positive-
tone photoresists because it is useful for a thermal cross-
linking and the following acidolytic de-cross-linking20
(Scheme 3).
The vinyl ether groups of TVEB react with the
hydroxy group of phenol at high temperature to form
cross-linked networks with acetal structures, and then,
these cross-linked junctions are cleaved acid-catalyti-
cally by a photogenerated acid.
The photosensitive polymer system consisting of
PAHA, TVEB, and DIAS in cyclohexanone was formu-
Sch em e 3. Mech a n ism for Cr oss-Lin k in g a n d Acid olytic De-Cr oss-Lin k in g of P AHA w ith TVEB

8260 Fukukawa et al.
Macromolecules, Vol. 37, No. 22, 2004
To clarify a difference of dissolution behaviors be-
tween the exposed and unexposed area, the effect of
TVEB and DIAS loadings on the dissolution rate in 2.38
wt % TMAHaq was investigated (Figure 7), where each
film was prebaked at 120 °C for 5 min, exposed to 100
mJ /cm2 of i-line, and then postexposure baked at 120
°C for 5 min. The difference of the dissolution rates
between the exposed and unexposed area reaches 320
times when 15 wt % of TVEB and 5 wt % of DIAS were
loaded.
Next, the effect of postexposure bake (PEB) temper-
ature on the dissolution rate in 2.38 wt % TMAHaq was
studied. The results are shown in Figure 8. By changing
the PEB temperature from 90 to 140 °C, an adequate
dissolution contrast (320 times) was obtained at 120 °C.
When the PEB temperature was set above 120 °C, the
film surface became rough probably due to a decomposi-
tion of DIAS.
On the basis of the findings described above, a resist
system was formulated as 85 wt % of PAHA, 15 wt %
of TVEB, and 5 wt % of DIAS. A sensitivity curve for
the 2.0 µm thick photopolymer film shown in Figure 9
indicated a sensitivity of 40 mJ /cm2 and a contrast of
4.0.
F igu r e 8. Effect of postexposure bake (PEB) temperature on
the dissolution rate in 2.38 wt % TMAHaq. PAHA:TVEB:DIAS
(wt %) ) 80:15:5. Both prebake and PEB time were set for 5
min at each desired temperature. When PEB was subjected
at 130 or 140 °C, the remained unexposed surface showed
rough significantly (marked with /).
Figure 10 shows scanning electron micrographs (SEMs)
of the contact printed positive image after exposure to
70 mJ /cm2 of i-line through a photomask and the
following annealed to convert into PABO. The resist was
capable of resolving 10 µm featured patterns where a
2.3 µm thickness film was used (Figure 10a). The
printed film was converted to the PABO film by heating
at elevated temperature up to 350 °C for 1 h under
nitrogen (Figure 10b). The formation of the PABO was
confirmed by the IR spectrum, where the characteristic
oxazole ring absorption appeared at 1616 cm-1 and the
absorptions due to the hydroxyl and amide carbonyl
groups disappeared at 3300 and 1650 cm-1 in PAHA,
respectively.
F igu r e 9. Photosensitive curve of the PABO resist film
(thickness: 2.0 µm) consisting of PAHA (85 wt %), TVEB (15
wt %), and DIAS (5 wt %). Condition was fixed as follows:
prebaked temperature, 120 °C for 5 min; PEB temperature,
120 °C for 5 min; development, 2.38 wt % TMAHaq for 60 s at
20 °C.
Con clu sion
PAHA containing adamantyl units in the main chain
was prepared from ADC and 6FAP in the presence of
lithium chloride in NMP. Thermal treatment of PAHA
easily produced PABO. A Td5 of the PABO was 518 °C
under nitrogen. The optically estimated dielectric con-
stant is 2.55, which is lower than that of conventional
aromatic PBOs. Since PAHA has a high transparency
in the UV-vis region, PAHA containing 15 wt % TVEB
lated. The film spin-cast on a silicon wafer was prep-
baked, exposed to a filtered super-high-pressure mer-
cury lamp (365 nm), and developed with 2.38 wt %
TMAHaq. The dissolution rate was calculated by mea-
suring the change of the film thickness after the
development.
F igu r e 10. SEM images of (a) PAHA film (thickness: 2.3 µm) and (b) PABO film (thickness: 1.9 µm) after curing at 350 °C
under nitrogen. A scale bar indicates 30 µm in each micrograph.

Macromolecules, Vol. 37, No. 22, 2004
Photosensitive Semi-Alicyclic Poly(benzoxazole)s 8261
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printing mode was converted to the corresponding
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