Polymeric Materials for Electronics Packaging and Interconnection

Chapter 3

Dynamic Fourier Transform-IR Analysis of Cure Reactions and Kinetics of Polyimides 1

2

Polymeric Materials for Electronics Packaging and Interconnection Downloaded from pubs.acs.org by CORNELL UNIV on 08/22/16. For personal use only.

Randy W. Snyder and Paul C. Painter 1

IBM Corporation, Endicott, NY 13760 Pennsylvania State University, University Park, PA 16802

2

A dynamic method for acquiring and treating infrared spectroscopic data from the imidization of a number of polyimide systems is presented. In situ FT-IR analysis of polymer reactions is preferred when doing comparitive studies on a number of polymer systems. For systems where these reactions occur at relatively high tempera tures, i t is often d i f f i c u l t to obtain good isothermal data for determining kinetic parameters. Kinetic data for several polyimide systems are shown and compared.

Polyimides are used extensively i n the electronics industry as d i e l e c t r i c and passivation layers i n e l e c t r o n i c devices. The curing behavior of these systems i s often an important consideration, e s p e c i a l l y i n situations where etching i s performed at intermediate cure l e v e l s . Infrared spectroscopy has proven to be useful for following the l e v e l of imidization of polyimides (1-4). One of the problems associated with performing k i n e t i c studies on these systems i s the high temperatures that must be used for imidization to take place. Isothermal studies are often tedious, with multiple samples being required at each time / temperature combination due to the additional curing that can occur during heating and cooling the samples (5). Such time consuming tasks are inappropriate when evaluating a number of polyimides (or other reacting systems) or evaluating l o t to l o t v a r i a t i o n s . In s i t u infrared isothermal studies are d i f f i c u l t to perform as accessories with s u f f i c i e n t heating rates and rapid s t a b i l i t y at high temperature are not r e a d i l y available. A method f o r determining k i n e t i c parameters from dynamic infrared data was developed to overcome the problems l i s t e d above (A). Through the use of constant temperature ramps, appropriate instrument software (6) (Sheen, C. W.; Snyder, R. W. Computers & Chemistry, i n press) and spreadsheet techniques the a c t i v a t i o n energy and pre-exponential factor for any reacting system can be obtained i n a few hours. When performing t h i s dynamic k i n e t i c analysis however there are some e f f e c t s which must be accounted for 0097-6156/89AM07-O049$06.00/0 ο 1989 American Chemical Society

50

POLYMERS FOR ELECTRONICS PACKAGING AND

INTERCONNECTION

in order to c o r r e c t l y interpret the data. With polyimide systems there i s a temperature e f f e c t on the i n t e n s i t y of the 1780 cm-1 (imide) band that must be examined, and a correction i n the area data must be made to r e l a t e the data to a constant temperature (7). This paper shows the k i n e t i c data f o r the imidization reaction of several polyamic acids and polyamic acid derivatives. An evaluation of the data with respect to polymer chain chemistry and acid substitution i s given.


Experimental Thin f i l m samples were prepared by spin coating 15% solutions of the polyamic acids shown i n Table I (structures are shown i n Figure 1) onto 13-mm X 2-mm NaCl disks using a spin coater running at 5000 RPM for 30 sec. The samples were cured i n an Accuspec Model 20 high temperature c e l l , which u t i l i z e s a linear temperature programmer f o r sample heating. A constant heating rate was achieved by setting the i n i t i a l temperature well below s t a r t of the reaction, the endpoint on the temperature programmer to a high temperature (normally 550°C i s s u f f i c i e n t ) and turning the programmer o f f after the reaction i s completed (approximately 250-300°C). Heating rates of 5°C/min were u t i l i z e d for the k i n e t i c experiments. The actual temperature of the sample was monitored using a K-type thermocouple, attached to the heater element i n the immediate v i c i n i t y of the sample, and a Digi-Sense Thermocouple Thermometer. Infrared spectra were obtained on an IBM Instruments FT-IR/98 spectrometer u t i l i z i n g 10 averaged scans at A cm-1 resolution and a l i q u i d nitrogen cooled MCT detector. Data c o l l e c t i o n was performed u t i l i z i n g the GC-IR software routines as described previously (6) or using the standard c o l l e c t i o n routines which can c o l l e c t four spectra per minute. Peak areas were obtained using the MAXAREA program which can determine endpoints for integration after input of the peak maximum (Sheen, C. W.; Snyder, R. W. Computers & Chemistry, in press). The band near 1780 cm-1, symmetric stretch of the imide carbonyl, i s normally used for following the progress of the imidization reaction (see Figure 2). However, i t has been shown that t h i s band i s strongly influenced by temperature (7) (see Figure 3), so the band area for each spectrum was corrected to that which would be measured at 25°C p r i o r to k i n e t i c calculations. The k i n e t i c parameters may be calculated using the equation ln(-dC/dT) - ln(C/m) = ln(A) - Ea/RT

(1)

where m i s the heating rate, C i s one minus the concentration of the absorbing species (1780 cm-1 band area), R i s the gas constant and Τ i s the current temperature of the reaction. The -dC/DT values can be determined from the slope of the lines between points i n a plot of the concentration of the reacting species versus temperature (see Figure A). The concentration, used for ln(C/m), i s then the concentration at the midpoint of the l i n e . These values are then plotted versus r e c i p r o c a l temperature to obtain Ea and A (see Figure 5). Standard deviations for Ea were determined from scatter i n the data using a 95% confidence i n t e r v a l .

3.

SNYDER & PAINTER

51

FT—IR Analysis of Cure Reactions

ΟΗ

Η α ΟH I II -N-C HO-C C-OH PMDR/ODR II II ο Ο HΟ • Η I II -N-C c HO-C G r t O C-OH BTDR/QDR

α


C-OH

II

II

ο

ο

ΗΟ I II -N-C c r u HO-C

II

Ο BPDR/POR

QΗ BTDR/ORPFP

C-OH

ΟΗ

Η • -

BPDR/ODR

II

ο

C-O CHgCHgO-C U"+HN-CCH 3 Ο CH-,-CH-O-C-C-CH^ II I * Ο CH.. 3

TORHY PHOTQNEECE Figure 1.

ΟΗ

ΗΟ

Z

Z

2

9

a

C-OCHCHg Q PNDR/OOR ETHYL ESTER 2

Polyamic acid structures and t h e i r derivatives.

52

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

1.05

1.00

α

0.95

lu

0.90


α

0.85

O

PMDR/ODR

Δ BTDR/ODR 0.80

0.75

•

BPDR/GDR

*

BPDR/PDR

+ BTDR/DRPFP

0.70

0.85

50

IÛÛ

150

200

250

TEMPERATURE

300

350

400

450

CO

Figure 3. Plot showing the e f f e c t of temperature on the 1780 cm band i n several polyimides. The slope of the l i n e appears to be affected by the structure of the amine. 1.0

swag:

V

0.9

PoI yam i c Rc i d Ethyl Ester

0.8 •ΰ, BTDR/ODR

0.7F 0.6 r 0.5F Ξ

0.4 Δ

0-3 j- To r a y \ Photoneece Δ 0.21% 0.1

%

Χ.

0.0

so

110

130

150

170 190 210 230 TEMPERATURE C O

250

270

290

310

Figure 4. Plot of imide concentration versus temperature f o r Toray Photoneece, BTDA/ODA polyamic acid, and an ethyl ester of PMDA/ODA.

3. SNYDER & PAINTER

53

FT-IR Analysis of Cure Reactions

A recent re-evaluation of the data has shown that a better f i t can be obtained using the equation f o r 2nd order reaction k i n e t i c s : 2

ln(-dC/dT) - ln(C /m) = ln(A) - Ea/RT


(2) (This f i t can be seen i n Figure 6.) Only small differences are observed between calculated parameters using the two equations. We have no chemical reason f o r assuming 2nd order k i n e t i c s f o r the imidization reaction, so we chose to calculate a l l k i n e t i c parameters shown i n t h i s paper using the Equation 1. Work i s i n progress to address the descrepancy i n the f i t of the data. Results and Discussion Activation energies f o r the polyimide systems tested are shown i n Table I, and the calculated reaction rates f o r imidization are included i n Table I I . There are some differences i n the k i n e t i c Table I. Imidization Kinetic Parameters

POLYIMIDE

ln(A) (min-1)

Ea (Kcal/mol)

PMDA/ODA BTDA/ODA BPDA/ODA BPDA/PDA BTDA/DAPFP

28 22 16 14 21

±2 ±2 ±1 ±1 ±1

29.5 22.8 15.3 13.8 22.1

Toray Photoneece PMDA/ODA Ethyl Ester

26 ± 1 42 ± 2

29.7 40.3

Toray Photoneece i s a t e r t i a r y amine s a l t of BTDA/ODA polyamic acid.

Table I I . Reaction Rate Constants f o r Polyimides (min-1)

POLYAMIC ACID PMDA/ODA BTDA/ODA BPDA/ODA BPDA/PDA BTDA/DAPFP

150°C 2.2 3.4 2.4 5.7 5.6

X χ X X X

200°C

250'>C

lO'l

7 5 X 10'

ίο"; 10"^ 10"^ 10

5 1 3 7

Toray Photoneece 2.9 X lO'l PMDA/ODA Ethyl Ester 6.3 X 10

1

4 8 3 8

χ io": X 10~! X 10"! X 10

7 7 1 2 X 10

?

12 5 0 1 6

9 1 9 4 6

108 0 0 9

54



Δ

c

-4 H

"S,

-6 -8 0.0019

0.0021

0.0023 1/Τ

0.0025

0.0027

Figure 5. Plot of k i n e t i c data for the curing of PMDA/ODA polyimide using Eq. 1. The slope of the l i n e y i e l d s Ea and ln(A) i s obtained from the intercept.

1

-8 0.0019

' 0.0021

1

0.0023

1

0.0025

1

0.0027

t/T

Figure 6. Plot of k i n e t i c data for the curing of PMDA/ODA polyimide using Eq. 2. The slope of the l i n e y i e l d s Ea and ln(A) i s obtained from the intercept.


3.

SNYDER & PAINTER

FT—IR Analysis of Cure Reactions

55

parameters for imidization of the d i f f e r e n t polyimides that appear to be dependent on the structure of the dianhydride used to form the polyimide. A change from PMDA/ODA to BPDA/ODA gives i n a large decrease i n the a c t i v a t i o n energy. The e f f e c t of t h i s decrease i s not obvious i n the calculated reaction rate constants u n t i l higher temperatures (> 200°C) are achieved, so i t appears that i t i s related to the f l e x i b i l i t y of the polyimide structure. Most of the v a r i a b i l i t y i s observed when changing dianydrides i n the structures, with very l i t t l e change observed when the dianhydride remains constant. Further work w i l l have to be performed to more firmly e s t a b l i s h t h i s c o r r e l a t i o n , but i t seems plausible that such an e f f e c t should occur as the f l e x i b i l i t y of the chain could a f f e c t the a b i l i t y of the reacting species to interact. The differences between the curing of the polyamic acids and t h e i r t e r t i a r y amine s a l t s or esters i s even larger. The Toray Photoneece photosensitive polyimide (which i s a t e r t i a r y amine s a l t of BTDA/ODA polyamic acid) begins reacting at much lower temperatures and the reaction i s completed faster during the temperature ramp (see Table 2 and Figure A). This e f f e c t i s consistent with work of Kruez et a l . (1_) on t e r t i a r y amine s a l t s of PMDA/ODA polyamic acid. The e f f e c t of e s t e r i f y i n g the polyamic acid on the cure rate i s opposite that of the amine s a l t . The imidization reaction requires a much higher temperature than the polyamic acid, from which i t was derived, before i t w i l l begin. However, once the temperature i s reached where the reaction can begin, i t does not require a the large increase i n temperature, as with the polyamic acid, to give a r e l a t i v e l y high reaction rate constant (see Table I I ) . The slope of the curve i n Figure A for the ester i s very high and t h i s results i n the high calculated activation energy shown i n Table I. This may be the result of forcing the equilibrium of the reaction more heavily towards the imide species by having a leaving group (ethanol) that i s less l i k e l y to react with the imide than the water given o f f by the polyamic acid. Both the amine s a l t and ester results are consistent with the mechanism for imidization proposed by Kruez et a l . (1_) where i t i s speculated that an intermediate species contains a carboxylate ion. In the case of the amine s a l t , the ion i s already present and not much energy i s required to cause ring closure. While with the ester i t would be much more d i f f i c u l t to form the carboxylate ion intermediate and therefore the beginning of the reaction would be somewhat slower. CONCLUSIONS The dynamic k i n e t i c s FT-IR method i s an e f f e c t i v e means of studying high temperature curing reactions i n polymers. Both the acid group substitution and polymer backbone appear to have strong influences on the rate of imidization i n polyimides. The dianhydride used to make the polymer appears to have a greater e f f e c t on the k i n e t i c s than does the diamine.

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Literature Cited 1. Kreuz, J. Α.; Endrey, A. L.; Gay, F. P.; Sroog, C. E. J. Polym. Sci.: Part A-1 1966, 4, 2607. 2. Denixov, V. M.; Kol'tsov, A. I.; Mikhailova, Ν. V.; Nikitin, V. N.; Bessonov, M. I.; Glukhov, Ν. Α.; Shcherbakova, L. M. Polymer Sci. USSR, 1976, 18, 1780. 3. Koton, M. M.; Meleshko, T. K.; Kudryavtsev, V. V.; Nechayev, P. P.; Kamxolkina, Ye. V.; Bogorad, Ν. N. Polymer Sci. USSR 1982, 24, 791. 4. Snyder, R. W.; Sheen, C. W. Appl. Spectrosc. 1988, 42, 655. 5. Snyder, R. W.; Sheen, C. W.; Painter, P. C. In The Proceedings of the Symposium on Polymeric Materials for Electronic Packaging and High Technology Applications, J.R. Susko, R.W. Snyder and R.A. Susko, Eds., The Electrochemical Society, Vol. 88-17, 1988, 71. 6. Snyder, R. W.; Sheen, C. W. Appl. Spectrosc. 1988, 42, 296. 7. Snyder, R. W.; Sheen, C. W.; Painter, P. C. Appl. Spectrosc. 1988, 42, 503. RECEIVED January 24, 1989

Polymeric Materials for Electronics Packaging and Interconnection

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