Polyimide Hydrolysis - American Chemical Society

Chapter 4

Polyimide Hydrolysis Measurement by Fourier Transform—IR Spectroscopy Coralie A. Pryde

Downloaded by UNIV OF MINNESOTA on August 5, 2013 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch004

AT&T Bell Laboratories, Murray Hill, NJ 07974

IR spectroscopy may be used to follow two reactions occurring in polyimides exposed to high temperatures and humidities: hydrolysis of the imide linkages and hydrolysis of residual anhydride end groups. The hydrolytic susceptibilities of several polyimides were measured at 90°C/95% R.H. Polymers based on benzophenone tetracarboxylic acid dianhydride (with either oxydianiline or m-phenylene diamine) appeared to undergo rather rapid hydrolysis initially, but the reaction had essentially halted by the time the measured imide content had decreased by 5-6%. Polymers based on 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride (with p-phenylene diamine) and pyromellitic dianhydride (with oxydianiline) showed no significant imide hydrolysis. In all the polymers, the anhydride was hydrolyzed quite readily. Because of their excellent thermal stability, polyimides are considered as strong candidates for use as interlevel dielectrics in a variety of advanced packaging applications. However, the actual dielectric properties of most standard aromatic polyimides are generally considered to be only just adequate for such uses. Therefore it is important to determine whether or not degradative reactions that might occur in processing or during use could cause any significant deterioration in these dielectric properties. One such question is whether or not polyimides are susceptible to any hydrolytic reactions during long-term use. A number of reports (1_,4) have demonstrated that, during the thermal cure of polyamic acids, dissociation to give free amine and anhydride end-groups competes with closure to the imide ring. Therefore, as indicated in Scheme I, polyimides exposed to high temperatures and humidities might be expected to undergo three different hydrolysis reactions: 1) hydrolysis of anhydride remaining after cure, 2) hydrolysis of imide rings and 3) hydrolysis of those polyamic acid units which remain after cure or are formed on imide hydrolysis. Previous studies (5,6) of polyimide hydrolysis have generally been based on changes in properties dependent on molecular weight, and thus have measured only the third reaction. The first two reactions are of concern, however, because they result in the formation of polar, hygroscopic groups which would be expected to degrade the dielectric properties of the polymer. 0097-6156/89/0407-0057$06.00/0 ο 1989 American Chemical Society

In Polymeric Materials for Electronics Packaging and Interconnection; Lupinski, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

58

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

Measurements of the changes in both the imide (4) and the anhydride (_1,2) concentrations by IR spectroscopy have already been reported. This paper will demonstrate another approach to using IR to measure anhydride contents and will then present results obtained during hydrolysis of thin films of five polyimides. The structures of the polyimides are shown in Table I. The effects of changes in the curing conditions will also be discussed.


EXPERIMENTAL

The polyamic acids were prepared in these laboratories using modifications of a standard preparation (7). Benzophenone tetracarboxylic acid dianhydride ( B T D A ) , benzenetetracarboxylic acid dianhydride (pyromellitic dianhydride; P M D A ) , oxydianiline (ODA), 1,4-phenylenediamine ( P D A ) and 1,3-phenylenediamine ( M P D A ) were all obtained from Aldrich Chemical C o . The 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride ( B P D A ) was obtained from Ube Chemical Company. The polyamic acids were prepared in N-methylpyrrolidinone ( B T D A O D A , B T D A - M P D A and B T D A with a 1:1 molar ratio of M P D A and O D A ) or dimethyl acetamide ( B P D A - P D A and P M D A - O D A ) . The anhydride calibration curves were prepared by adding various amounts of the appropriate anhydride to polyamic acid solutions containing known concentrations of polymer. The films were spun onto N a C l plates or silicon wafers and cured at 2 0 0 C for 1 h. In films prepared for hydrolysis, solutions of the polymers were spun onto either A g C l discs or double-polished silicon wafers using conditions designed to give thicknesses in the 1-3 micron range. The films were dried 1 h at 100 ° C (in air), then baked 1 h at 200 ° C and 1 h at 300 ° C (under nitrogen). Some samples were given a final cure at 400 ° C . Hydrolyses were carried out at 9 0 ° C / 9 5 % R . H . by placing the samples over a saturated solution of K S 0 at 90 ± 2 C . IR spectra were recorded on a Digilab FTS-60 Spectrometer equipped with a T G S detector. A l l spectra were collected using a resolution of 4 wavenumbers. Spectra of films on A g C l were obtained using 200 scans; those of films on silicon wafers were based on 400 scans. In each case, the spectra were recorded in the absorbance mode using a background obtained from the appropriate bare substrate. Interference from water vapor was minimized by purging the sample compartment for 15-20 minutes with dry nitrogen before beginning data collection. If necessary, corrections for remaining traces of water were made water by a scaled subtraction of a spectrum of water vapor. 0

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RESULTS

Figure 1 shows the IR spectrum of a typical polyimide, B T D A - O D A : M P D A , after curing to 300 ° C on A g C l . The imide bands are seen at 1780, 1726, 1375 and 719 c m in this polymer. The residual anhydride may be identified by the small peak at 1853 c m . The peak at 1500 c m " due to one of the skeletal stretching modes of the aromatic rings has been found to provide a reliable internal standard (4). - 1

- 1

1

Anhydride Hydrolysis

Hydrolysis of anhydride may be followed by measuring the decrease in the anhydride peak near 1850 c m . Although the peak is extremely small, it can be expanded for measurement without any significant sign of interference from - 1


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59

Polyimide Hydrolysis Measurement

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Table I. Polyimide Structures Ô BTDA-ODA

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spectral noise. In all the polymers discussed here, only the amine moieties contribute to the 1500 c m peak. Thus, for polymers prepared with approximately 1:1 amine:anhydride stoichiometry, the ratio of the 1850 c m to the 1500 c m band will provide a direct measure of the amount of free anhydride present in the polymer. T o determine the relationship between the size of the anhydride peak and the actual anhydride content, calibration samples were prepared which contained known amounts of anhydride added to the polyamic acid. Figure 2 shows the calibration curve for B T D A in B T D A - O D A . In this plot the data points fall in an essentially straight line. This indicates that, at these relatively high concentrations, the equilibrium formation of anhydride is strongly suppressed. A t low concentrations of added anhydride, the equilibrium dissociation will be significant and the line will curve upward, as indicated by the point obtained with no added anhydride. Similar results were obtained for the other calibration curves. The extension of the straight line obtained for B T D A - O D A (Figure 2) intercepts the y-axis at a positive value, suggesting that this particular preparation deviates from 1:1 stoichiometry in that it contains a slight excess (about one percent) of anhydride. For each calibration curve, the reciprocal of the slope of the line defined by the calibration points, i.e. the concentration divided by the IR absorption ratio, was determined. The values, R, found for this reciprocal slope in each of the polymers are given in Table II. The error limits suggested primarily reflect uncertainties in - 1

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Table II.

Anhydride Calibration Factors

Polymer

Calib. Substrate

Calib. Factor

BTDA-ODA

NaCl

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BTDA-MPDA

NaCl

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BTDA-ODA:MPDA BTDA-ODA:MPDA

NaCl Si

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BPDA-PDA BPDA-PDA

NaCl Si

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PMDA-ODA

NaCl

200 ± 70

the concentrations of anhydride in the calibration samples. Inaccuracies in weighing the components, impurities in the starting materials or deviations from 1:1 stoichiometry in the polymers could contribute to these uncertainties. In addition, however, if any anhydride is lost during heating of the films, this could significantly affect the measured absorbances. The values of R obtained on silicon are somewhat lower than those measured on N a C l . This may indicate that some anhydride is lost or destroyed when the films are heated to 200 ° C on N a C l . M u l t i p l y i n g the observed anhydride/internal standard ratio in each cured film by the calibration factor gives an estimate of the initial anhydride content. A s




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Figure 2. Calibration data for BTDA in BTDA-ODA.


62


seen in Table III, the initial anhydride contents for a given polymer are dependent on the polymer components, the substrate used (higher contents are seen on the silicon substrates) and the cure conditions (curing to 400 C significantly decreases the initial anhydride content). We would also expect that the anhydride contents would be affected by the backbone components present and by the stoichiometry of the particular preparation used. Although some of the preparations (eg. the B T D A - O D A : M P D A preparation used here) appeared to be very close to 1:1 in their stoichiometry, others deviated significantly. The preparation of B P D A - P D A used here, for instance, appeared to contain a significant excess of amine (perhaps as much as 10%). This will almost certainly lower the amount of residual anhydride observed. Figure 3 shows changes in the anhydride concentrations in typical films on A g C l and silicon during hydrolysis. Although the exact rates of hydrolysis varied with the polymer used and the cure conditions, all the films gave generally similar results. In each case the hydrolysis is quite rapid and the anhydride is generally reduced to < 2 5 % of its initial value within 100-200 h under these conditions. Essentially no anhydride was left in most of the samples after 400-600 h hydrolysis. The only exception was the sample of B P D A - P D A cured on silicon: about one-third of the initial anhydride content remained after 600 h hydrolysis.


0

Imide Hydrolysis 1

The imide peaks near 1780, 1370 and 730 c m " have all been used frequently to measure imide content. However, as was found recently, (4) both the 1780 and the 730 c m " peaks overlap anhydride absorption bands. Thus estimates of imide hydrolysis based on measurements of these bands would be considerably too large, particularly in the case of the 1780 c m " band. Therefore changes in imide concentration are best followed (2) by measuring the height of the imide peak at ~ 1370 c m " (see Figure 1) as a function of the height of the 1500 c m " internal standard. These normalized heights of this band, attributed to the C - N stretch (2) will be referred to here as the imide ratios. Readings taken on different spots on a given sample generally give imide ratios that agree well within one percent. Figure 4 shows the changes in the imide band on hydrolysis for samples of two polymers, B T D A - O D A and P M D A - O D A . The samples of B T D A - O D A underwent a rapid decrease in the imide band during the first 24-48 h of hydrolysis. The reaction then slowed markedly. Samples cured to only 300 ° C have a very slightly lower initial imide content and appear to hydrolyze somewhat more rapidly than those cured to 4 0 0 C . However, as can be seen in Table II, the percentages of imide remaining after 600 h hydrolysis, based on the final vs initial imide ratios, are the same within experimental error. The two other polymers based on B T D A showed patterns very similar to that of B T D A - O D A . The amount of imide remaining in each of these samples after prolonged hydrolysis is given in Table III. Again, the samples cured to 400 ° C appeared to hydrolyze at a slightly lower rate. The data indicate some slight differences in rates depending on the final cure temperature (samples cured to 4 0 0 C generally hydrolyze somewhat more slowly) and the substrate used (hydrolyses done on silicon were often somewhat slower than those on AgCl). Overall, however, the final degree of hydrolysis calculated for each sample does not seem to vary significantly with changes in substrate or cure conditions. The data for P M D A - O D A shown in Figure 4 indicate a much lower susceptibility to imide hydrolysis. Here, the measured hydrolysis after 400 h is only ~ 1% based on the initial ratios. The sample cured to 400 ° C appears to 1

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Polyimide Hydrolysis - American Chemical Society

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