Polyimides based on a 9-aryl-9(perfluoroalkyl)-xanthene-2,3,6,7-dianhydride or 9,9'-bis(perfluoro-alkyl)xanthene-2,3,6,7-dianhydride and benzidine derivatives

Polyimide compositions, films, and electronic devices using polyimides, based on 9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and benzidine derivatives. These polyimides offer a combination of low linear coefficient of thermal expansion, low dielectric constant, and low water absorption.

FIELD OF THE INVENTION 
This invention relates to polyimide compositions, films, and electronic 
devices using polyimides, based on 
9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 
9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and benzidine 
derivatives. 
BACKGROUND OF THE INVENTION 
Polyimides constitute a class of valuable polymers being characterized by 
thermal stability, inert character, usual insolubility in even strong 
solvents, and high T.sub.g, among others Their precursors are usually 
polyamic acids, which may take the final imidized form either by thermal 
or by chemical treatment. 
Polyimides have always found a large number of applications requiring the 
aforementioned characteristics in numerous industries, and recently their 
applications have started increasing dramatically in electronic devices, 
especially as dielectrics. With continuously escalating sophistication in 
such devices, the demands on the properties and the property control are 
becoming rather vexatious. 
Especially for the electronics industry, improvements of polyimides are 
needed in forming tough, pin-hole free coatings, having lower dielectric 
constant, lower linear coefficient of thermal expansion, and lower 
moisture absorption, among others. It is not usually possible to maximize 
all properties, since many of them are antagonistic. Thus, only a 
compromised solution has so far been achieved by at least partially 
sacrificing one or more of these properties in order to maximize a desired 
one. 
It has now been found that polyimides based on 
9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 
9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and a class of 
benzidine derivatives provide compositions which may be used to form 
dielectric films for electronic circuits characterized by high thermal 
stability, low linear coefficient of thermal expansion, low dielectric 
constant, and low water absorption. 
Japanese Patent Application Publication Kokai Hei 2-60933 (Masaki Ishisawa 
et al., Pub. Date: Mar. 1, 1990) discloses certain compositions of 
polyimides containing a benzidine derivative and derivatives containing 
fluorochains. However, one of the disadvantages of these compositions is 
the fact that the fluorochains contain adjacent carbon atoms having 
hydrogen and fluorine atoms, and are subject to premature thermal 
decomposition, due to dehydrofluorination. This publication does not 
recognize the importance of low water absorption, as this property is not 
even mentioned. Furthermore, this publication does not mention, suggest, 
or imply the combination of 
9-aryl-9-(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 
9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and benzidine 
derivatives for achieving the advantages of the present invention. 
SUMMARY OF THE INVENTION 
The instant invention is directed to polyimide compositions, films, and 
electronic devices using polyimides, based on 
9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 
9,9'-bis(perfluoroalkyl xanthene-2,3,6,7-dianhydride and benzidine 
derivatives. More particularly, it pertains to a polyimide having the 
structure: 
##STR1## 
wherein R.sup.1 is aryl Or R.sup.2, 
R.sup.2 is --CF.sub.3, 
R.sup.3 and R.sup.4 are selected from the group consisting of --C.sub.m 
F.sub.2m+1, --C.sub.p H.sub.2p+1, and --OC.sub.p H.sub.2p+1 
m is an integer 0-4, 
p is an integer 0-2, and 
q is an integer greater than 10. 
Preferably, in the above polyimide 
R.sup.3 and R.sup.4 are in 2,2'-positions, respectively, in the benzidine 
ring, and are selected from the group consisting of --C.sub.m F.sub.2m+1 
and --C.sub.p H.sub.2p+1, 
m is an integer 1-4, and 
p is an integer 1-2. 
The 2,2'- positions on the benzidine ring are preferred, because electron 
withdrawing groups, such as for example perfluorinated chains, reduce the 
reactivity of the amine groups (being in positions 4,4') of the benzidine 
derivative if they are in the 3,3' positions. They may also reduce the 
reactivity of the benzidine amino groups, due to steric effects in a 
similar manner as non-electron-withdrawing groups, such as for example 
hydrocarbon chains, may do. 
R.sup.3 and R.sup.4 may each take the form of a single chlorine atom in the 
benzidine ring, which however, is in most cases undesirable for electronic 
applications, due to potential for corrosion. 
Also, preferably, preferably both R.sup.1 and R.sup.2 are --CF.sub.3. 
Also it is preferable that R.sup.3 and R.sup.4 are --CF.sub.3, and even 
more preferable that all R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are 
--CF.sub.3. 
The instant invention also pertains to films comprising polyimides as 
defined above. 
In addition, this invention is also directed to electronic devices 
containing a conductor or semiconductor comprising: 
(a) a substrate which comprises a conductor, semiconductor, or insulator; 
and 
(b) a dielectric film in contact with the substrate, the dielectric film 
comprising a polyimide derived from a fluoroxanthene derivative and a 
benzidine ring derivative, as defined above. 
By the expression "in contact with the substrate" it is meant that the film 
is required to be in contact with the conductor, or the semiconductor, or 
the insulator, or any combination thereof.

DETAILED DESCRIPTION OF THE INVENTION 
The instant invention is directed to polyimide compositions, films, and 
electronic devices using polyimides, based on 
9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 
9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and benzidine 
derivatives. 
As aforementioned, improvements of polyimides are needed in forming tough, 
pin-hole free coatings, having lower dielectric constant, lower linear 
coefficient of thermal expansion, lower moisture absorption, and high 
thermal stability, among others. It is not usually possible to maximize 
all properties, since many of them are antagonistic. Thus, only a 
compromised solution has been achieved so far, by at least partially 
sacrificing one or more of these properties in order to maximize a desired 
one. In order to reach such a compromised solution, fluorinated 
dianhydrides and/or diamines have been broadly utilized in the past. 
However, while fluorinated polyimides typically give lower moisture and 
dielectric constant, they exhibit high CTE. For example, commercial 
Pyralin.RTM. PI-2566, based on 2,2'-bis(3,4-dicarboxyphenyl) 
hexafluoropropane (6FDA) and 4,4'-diaminodiphenyl ether (4,4'-ODA) from Du 
Pont, Wilmington, Del., exhibits a linear coefficient of thermal expansion 
(CTE) of about 50 ppm between 0.degree. and 200.degree. C. It is 
understood that this linear coefficient of thermal expansion for such 
commercial Pyralin.RTM. PI-2566 is expressed in a conventional manner as 
ppm per .degree.C. i.e., this material has a linear coefficient of therml 
expansion of 50 ppm/.degree.C. between 0.degree. and 200.degree. . On the 
other hand, even highly optimized nonfluorinated polyimides, such as for 
example commercial Pyralin.RTM. PI-2611 (based on 
3,3',4,4'-biphenyltetracarboxylic dianhydride and paraphenylene diamine, 
from Du Pont, Wilmington, Del.), may have low moisture absorption and low 
linear coefficient of thermal expansion, but rather high dielectric 
constant, as shown in Example 6. 
The linear coefficient of thermal expansion is considered to be low if it 
has a value of 20 ppm, or lower. This is because the components involved, 
such as for example conductors, semiconductors, and inorganic dielectrics 
or insulators have linear coefficients of thermal expansion (CTE) in the 
range of 0-20 ppm. For example, the CTE of silicon dioxide is 0.4 ppm, the 
CTE of silicon is 3-4 ppm, the CTE of aluminum oxide is 6 ppm, and the CTE 
of copper is 17 ppm. Although it is preferable to have a perfect match of 
the CTE's of the substrate and the polyimide coating, this is impractical 
in most cases, since a variety of materials are involved within the same 
electronic device, such as for example circuitry on even a single silicon 
wafer. Depending on the particular case, CTE's closer to a narrower 
subrange within the broader 0-20 ppm range may be desired. It is 
understood that this linear coefficient of thermal expansion including 
silicon dioxide, silicon, aluminum oxide and copper is expressed in a 
conventional manner as ppm per .degree.C., i.e., the values given in this 
paragraph are based on a linear coefficient of thermal expansion of ppm 
per .degree.C. 
The water absorption in the case of polyimides at 85% R.H. is considered to 
be low, when it is less than 2.5%, preferably less than 2.0%, and even 
more preferably less than 1.5%. As a matter of fact, the lower the water 
absorption is the more preferred the polyimide is, provided there is no 
degradation of the rest of the important properties. 
The dielectric constant is considered to be low if it is lower than 2.8 
under dry conditions, and preferably lower than 2.5. A dielectric constant 
of 3.0 or over is less desirable, or it may even become unacceptable for a 
number of applications, especially as electronic circuitry becomes smaller 
and circuit patterns become finer. 
As illustrated in Examples 2 and 4, electronic devices, such as for example 
silicon wafers which may contain electronic components, such as 
conductors, semiconductors, insulators, and combinations thereof, may be 
coated with the compositions of the present invention. Other examples 
include printed circuits, hybrid circuits, and the like. The compositions 
of the present invention in the form of dielectric films are characterized 
by low linear coefficient of thermal expansion, low dielectric constant, 
low water absorption, and high thermal stability, as illustrated in 
Examples 3 and 4. 
Thus, a dielectric film made from an exemplary composition of the present 
invention, based on 9,9'-bis(trifluoromethyl)xanthene-2,3,6,7-dianhydride 
(BXDA) and 2,2'-bis(trifluoromethyl)benzidine (TFMB), was found to have a 
linear coefficient of thermal expansion (CTE) of 5-7 ppm, i.e., per 
.degree.C., water absorption 1.2% at 85% R.H., a dry dielectric constant 
of 2.4, and a decomposition temperature of 415.degree. C., in air. 
Another dielectric film made from a different exemplary composition of the 
present invention, based on 
9-phenyl-9(trifluoromethyl)xanthene-2,3,6,7-dianhydride (PXDA) and 
2,2'-bis(trifluoromethyl)benzidine (TFMB), was found to have a linear 
coefficient of thermal expansion (CTE) of 6-20 ppm, i.e., per .degree.C., 
water absorption 1.55% at 85% R.H., a dry dielectric constant of 2.7, and 
a decomposition temperature of 419.degree. C., in air. 
The variability in the linear coefficient of thermal expansion is due to 
the fact that the type of application (such as for example the angular 
speed of the spin-coater) of the film influences the molecular 
orientation, and consequently the linear coefficient of thermal expansion 
within certain limits. 
Examples of preferred solvents, which may be used in the practice of the 
present invention are polar organic solvents, such as sulfoxide type 
solvents including dimethylsulfoxide, diethylsulfoxide, and the like, 
formamide type solvents including N,N-dimethylformamide, 
N,N-diethylformamide, and the like, acetamide type solvents including 
N,N-dimethylacetamide, N,N-diethylacetamide, and the like, pyrrolidone 
type solvents including N-methyl-2-pyrrolidone, N-cyclohexyl, 
2-pyrrolidone, 1,3-dimethyl-2-imidozolidione, N-vinyl-2-pyrrolidone, and 
the like, phenolic solvents including phenol, o-, m-, p-cresol, xylenol, 
halogenated phenol, catechol, and the like, hexamethylphosphoramide, and a 
number of lactones including .gamma.-butyrolactones. These solvents may be 
used alone or as a mixture. Partial use of aromatic hydrocarbons such as 
xylene, toluene, and the like, is also possible, and sometimes desirable, 
when for example removal of water as an azeotrope is needed. 
BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride 
BXDA: 9,9'-bis(trifluoromethyl)xanthene-2,3,6,7-dianhydride 
CHP: N-cyclohexyl-2-pyrrolidone 
CTE: Linear Coefficient of Thermal Expansion 
DMAC: Dimethylacetamide 
DMB: 2,2'-dimethylbenzidine 
DMSO: Dimethylsulfoxide 
DSC: Differential Scanning Calorimetry 
6FDA: 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane 
GPa: Gigapascal 
GPC: Gel Permeation Chromatography 
mmole: Millimole 
MPa: Megapascal 
NMP: N-methyl-2-pyrrolidone 
ODA: 4,4'-Diaminodiphenyl ether 
PPD: Paraphenylenediamine 
ppm: Parts per million 
PXDA: 9-phenyl-9'-trifluoromethylxanthene-2,3,6,7-dianhydride 
R.H.: Relative Humidity 
Tg: Glass transition temperature 
TMA: Thermomechanical analysis 
All parts and percentages are given by weight unless otherwise stated. 
EXAMPLE 1 
Synthesis of Polyimide based on BXDA and TFMB 
Into a 100 ml reaction kettle fitted with a nitrogen inlet and outlet, and 
a mechanical stirrer were charged 4.7090 g (10.2766 mmol) of 
9,9'-bis(trifluoromethyl)xanthene-2,3,6,7-dianhydride (BXDA) and 3.2910 g 
(10.2766 mmol) of 2,2'-bis(trifluoromethyl)benzidine (TFMB). Shortly 
thereafter, 42 ml of NMP were added and stirring was begun. BXDA dissolved 
slowly into the reaction mixture and the temperature was maintained at 
room temperature overnight (ca. 20 hrs). Afterwards, the polymer solution 
was slowly pressure filtered through a 1 micron filter to yield a clear 
yellow solution. 
EXAMPLE 2 
Coating of silicon wafer with polyimide based on BXDA and TFMB 
Part of the clear solution of Example 1 was spin coated onto a 5" silicon 
wafer. After spin coating, the wafer was immediately placed in an air oven 
at 135.degree. C. for 30 min., then placed into a nitrogen oven and heated 
to 200.degree. C. for 30 min and 350.degree. C. for 1 hr. The resulting 
polyimide dielectric film was coherent and adhered well to the wafer. 
EXAMPLE 3 
Evaluation of the dielectric film of Example 2 
In order to determine the properties of the dielectric film, the oxide 
layer of the silicon wafer was etched in aqueous HF. It yielded a free 
standing polyimide film which was pale yellow in color and creasable. The 
thickness of the film was 10.3 micrometers, and it gave the following 
mechanical properties when tested on an Instron Model 4501 per ASTM D 882 
-83 (Method A): Tensile Strength=200 MPa, Tensile Elongation at Break=6%, 
and Young'ss Modulus=6.1 GPa. The linear coefficient of thermal expansion 
(CTE) of the film measured by TMA (10.degree. C./min, 
0.degree.-200.degree. C.) was found to be 5 ppm i.e., per .degree.C., and 
7 ppm i.e., per .degree.C., by two separate measurements indicating the 
low CTE capability of this structure. 
Thermal stability was evaluated by TGA (Thermo Gravimetric Analysis) in air 
at a temperature increase rate of 15.degree. C./min. Initial weight loss 
was observed at 415.degree. C. 
The dielectric constant on the dried film was 2.4 at 1 MHz. 
The clear solution of Example 1 was also used to spin coat a layer onto a 
quartz crystal. After treating the coated quartz crystal as described in 
Example 2, a dielectric film having a thickness of approximately 3 
micrometers resulted. The moisture absorption of this film was measured on 
a quartz crystal microbalance and found to be 1.2% at 85% R.H. 
EXAMPLE 4 
Synthesis, use, and evaluation of Polyimide based on PXDA and TFMB 
Similar to the procedure given in Example 1, a polymer was prepared from 
4.7429 g (10.1709 mmol) 
9-phenyl-9'-trifluoromethylxanthene-2,3,6,7-dianhydride (PXDA) and 3.2571 
g (10.1709 mmol) 2,2'-bis(trifluoromethyl) benzidine (TFMB) in 32 ml NMP 
(20% solids). After dissolution, the reaction was allowed to proceed 
overnight at room temperature (ca. 16 hrs). An extremely thick poly(amic 
acid) solution resulted which was stepwise diluted with 10 ml NMP (16% 
solids), then 10 ml NMP (13% solids), then 7 ml NMP (.about.12% solids), 
then 6 ml NMP (11% solids), and finally 7 ml NMP (10% solids). Ample 
stirring time was given between dilutions to allow for homogenization. The 
dilution procedure was performed over a two day period. Afterwards, the 
solution was slowly pressure filtered through a 5 micron filter (1 micron 
filtration was found to be laboriously slow for this sample) and then spin 
coated onto silicon wafers and cured as given in Example 2. The resulting 
8.1 micron pale yellow film had the following tensile properties: Tensile 
Strength=197 MPa, Tensile Elongation at Break=8%, and Young's Modulus=5.0 
GPa. The linear coefficient of thermal expansion of the film was found to 
be 6 ppm and 20 ppm by two separate measurements indicating the low CTE 
potential for this structure. Likewise, dielectric constant and moisture 
absorption (measured as in Example 3) were found to be 2.7 and 1.55%, 
respectively. 
Thermal stability was evaluated by TGA (Thermo Gravimetric Analysis) in air 
at a temperature increase rate of 15.degree. C./min. Initial weight loss 
was observed at 419.degree. C. 
EXAMPLE 5 
Synthesis, use, and evaluation of Polyimide based on BXDA and DMB 
Into a 100 ml reaction kettle fitted with a nitrogen inlet and outlet, and 
a mechanical stirrer were charged 2.5329 g (11.9309 mmol) of 
2,2'-dimethylbenzidine (DMB). The DMB was allowed to dissolve and then 
5.4671 g (11.9309 mmol) of BXDA were added as a slurry in 25 ml NMP. 
Residual BXDA was carefully washed into the reactor with 11 ml NMP. The 
BXDA gradually dissolved into the reaction mixture and the temperature was 
maintained at room temperature overnight (ca. 17 hrs). Afterwards, the 
polymer solution was slowly pressure filtered through a 1 micron filter 
and then spin coated onto 5" silicon wafers according to the procedure in 
Example 1. The resulting 9.4 micron polyimide film was light in color and 
gave the following mechanical properties when tested as in Example 3. 
Tensile Strength=340 MPa, Tensile Elongation at Break =14%, and Young's 
Modulus=7.5 GPa. The linear coefficient of thermal expansion (CTE) of the 
film measured by TMA (10.degree. C./min, 0.degree.-200.degree. C.) was 
found to be -3 ppm i.e., .degree.C., indicating the low CTE capability of 
this structure. The dielectric constant on the dried film at 1 MHz was 
2.6. The moisture absorption of a 3 micron film spin coated onto a quartz 
crystal and measured on a quartz crystal microbalance at room temperature 
was 2.3% at 85% relative humidity. 
EXAMPLE 6 
Comparative Example 
Similar methods were used to prepare and analyze films based on commercial 
DuPont Pyralin.RTM. PI-2611 (based on 3,3',4,4'-biphenyltetracarboxylic 
dianhydride (BPDA) and paraphenylenediamine (PPD). The CTE of this sample 
was found to be 4 ppm i.e., per .degree.C., moisture absorption 1.4% but 
the dielectric constant was 3.1.