Polymeric Materials for Electronics Packaging and Interconnection

Chapter 6

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 15, 2018 | https://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch006

Calculated Final-State Effects of the PMDA-ODA Polyimide X-ray Photoemission Spectrum A. R. Rossi and B. D. Silverman IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY 10598

X-Ray photoemission spectroscopy has been extensively utilized in probing surfaces and interfaces involving the technologically important class of polymers: the polyimides. We have previously shown that, within the Koopmans' approximation, the calculated core level positions can yield an accurate characterization of the relative positions of the chemically inequivalent atoms observed in the photoemission spectrum (XPS) of the PMDA-ODA polyimide. Whereas the relative positions agreed well with experiment, the absolute magnitudes of the calculated XPS peak positions were tens of electron volts above the measured values. To resolve this dis crepancy, as well as to examine details involving the relative XPS peak po sitions, the carbon(C1s), nitrogen(N1s), and oxygen(O1s) ionization energies of the separate molecular units, pyromellitic diimide (PMDA) and hydroxyaniline, were calculated taking into account final state effects, and allowing for core hole relaxation. This technique is called the ΔSCF method since the ionization energy is calculated as a difference in energies between the neutral molecule and the resulting radical cation. The present calcu lations show that core-hole relaxation, treated in the ΔSCF approximation, shifts the calculated values of the photoemission peaks to within one electron volt of the observed peak positions. On the scale of relative shifts, the carbon 1s carbonyl peak was calculated within the Koopmans' approxi mation to lie 3 eV higher in binding energy relative to the main carbon 1s peak, while the present ΔSCF calculations yield a 4 eV separation between the main and carbonyl carbon C1s peaks. The observation of such an in creased separation might reflect inter-chain bonding involving the carbonyl oxygen atoms.

Ab initio molecular orbital calculations have played a central role in the analysis and inter pretation of X-ray photoemission data obtained on the PMDA-ODA polyimide surface . The repeat unit of the PMDA-ODA polyimide is shown in Figure 1 and is constructed from planar pyromellitimide (PMDA) and diphenyl ether segments. An understanding of the XPS data and its relationship to the surface chemistry prior to the deposition of any metal is cru cial with respect to the interpretation of changes in the XPS data which signify important metal-polymer chemistry that occurs upon formation of the interface. 1_4

0097-6156/89/0407-0077$06.00/0 ο 1989 American Chemical Society

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

78

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

At present, the analysis of films annealed in ultra-high vacuum has led one to infer that the surface of the PMDA-ODA polyimide approximates that expected for ideal bulk stoichiometry with, however, certain small but significant differences. This analysis has in volved an examination of the relative shifts of the chemically inequivalent species of each of the elemental groups. For example, prior work focussed attention on the small shifts of approximately one electron volt exhibited by the aromatic carbon atoms of the central benzeneringof the PMDA component. These shifts were calculated with respect to carbon Is core level positions of atoms not conjugated with electron withdrawing species. Compar isons between calculated core level positions and photoemission data in the Koopmans' ap proximation, however, always required that the calculated peak positions be shifted to higher binding energy by tens of electron volts.


5

In X-ray Photoelectron Spectroscopy (XPS) an electron is ejected from a core level Ρ

+ hv

pt + e"

•*

where Ρ is a polymer and hv is the incident radiation. The kinetic energy (KE) of the ejected photoelectrons is measured, KE = hv — BE — Φ, and information about the chemical environment (chemical shifts) from the binding energy, BE (i.e. ionization energy, IE) of electrons, can be obtained when the workfunction, Φ, is known. The usual interpretation is to relate the peaks in the observed XPS spectrum to energy differences between the doublet ion (core hole) and ground state: IE = E(Pt)

- E(P)

The SCF molecular orbitals of the neutral species, ψ, , give rise to the ground state wavefunction, Φ(Ρ) = | ψ, | , while the SCF orbitals of the positive ion for the core-hole state produce a doublet state wavefunction, Φ(Ρ ) = | ψ', | . Koopmans' Theorem assumes that there is no change in the molecular orbitals of the ground state and ionized species, φj = \p'j , and that a vertical ionization energy can be obtained by the following ex pression: 2

IE

K T

+

2

- E[ 4>f\VÏ)]

6

- Ε[Φ(Ρ)] =

-e

S C F

SCF

where e corresponds to the one-electron energy of a core orbital for the neutral molecule. When final state effects are included, calculations on both the neutral ground state and final ionized state are taken into account: I ^ASCF E* = ΕΓΦ/Pt)] - Ε[Φ(Ρ)] SCh

These are called ASCF calculations since the energy difference between two SCF calcu lations is obtained. The relaxation energy, AE , is then defined as the difference between the ionization energy derived by Koopmans' Theorem and that obtained by a ASCF calcu lation: Rel

AE

Rel

- IE

K T

-

IE

A S C F

Since it is well known that final state effects or relaxation about the core hole can induce large shifts in the photoemission spectrum , these effects have been investigated within the framework of the ASCF approximation. The results of this investigation are described in the present paper. 7

Computational Details 8

All calculations were carried out with the MELD series of programs. In particular, the RHFSCF program was specifically modified to enable the calculation of core hole states without variational collapse . The PMDA and oxyaniline molecular units, Figure 2, were treated separately, largely because of molecular size and program limitations. These partie9


Calculated Final-State Effects


ROSSI & SILVERMAN


80

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION ular molecular fragments were chosen for investigation since the calculated results obtained enabled us to reconstruct the XPS spectrum expected for the PMDA-ODA polymer. In ad dition, the calculations for each fragment were performed in reduced symmetry, i.e., the core holes were localized at single atomic sites, (C for PMDA and C for hydroxyaniline), since previous calculations have shown that lower energies are obtained for localized hole states. s

t

10

11

The basis set employed in the present calculations included the Huzinaga (9s,5p) for second row atoms and (4s) for hydrogen contracted to [3s,2p] and [2s], respectively following the scheme of Dunning and Hay. The structural parameters for the PMDA and hydroxyaniline fragments were obtained from the X-ray study of 4,4'-bis(phthalimide) diphenyl ether , as well as from standard bond lengths and angles. For the hydroxyaniline fragment, all phenyl groups were chosen as idealized structures with all bond angles equal to 120° and C-C and C-H bond distances equal to 1.39 A and 1.10 A, respectively. 12


13

In order to compare the experimental and calculated core ionization energies, it is convenient to have a naming convention. This convention was discussed previously , but a brief review will be given here. The symbols Ar (arene) and Ph (phenyl) denote the benzeneringson the PMDA and hydroxyaniline fragments, respectively. Using this convention, Cls (Ar-X) re presents a Is ionization from a carbon atom situated in the benzene ring of the PMDA fragment where X = H, C=0 indicating carbon bonded to either hydrogen or a carbonyl group. Thus, Cls(Ar-H) represents ionization from either of the two carbon atoms located in the central benzene of the PMDA fragment and connected to hydrogen atoms, while Cls(C-C=0) involves ionization from the remaining four carbon atoms of the central benzene which are bonded to C=0 groups. Cls(C=0) represents the carbon Is ionization energy for any of the carbonyl carbon atoms located in the PMDA fragment. Since the splitting is small, Cls (Ph-H) indicates ionization from any of the carbon atoms of the benzene located in the hydroxyaniline molecular fragment and bonded to hydrogen atoms. The remaining carbon ionizations for the hydroxyaniline fragment are given by Cls(Ph-N) and Cls(Ph-O), respectively. The Nls and Ο Is notations for both fragments then become obvious. 6

Results and Discussion Computed XPS Spectrum One problem in reconstructing the PMDA-ODA XPS photoemission spectrum from calcu lations on separate fragments arises because of differences in ionization energies of the PMDA fragment relative to the oxydianiline molecular unit. In other words, the electronic environment of the complete PMDA-ODA repeat unit changes when separate units are cal culated. For example, previous calculations on a neutral PMDA-ODA molecule have yielded relative core level splittings to be ~ 0.5 eV greater than those obtained from calculations on the individual PMDA and oxydianiline units. This is presumably due to charge transferred from the ODA to PMDA segment. To correct such relative shifts in an approximate manner, the two calculated Ols ionization energies have been adjusted to correspond to the exper imental splitting. Comparison of Theory and Experiment The calculated photoemission spectra for the PMDA-ODA repeat unit (obtained by adding the calculated results for the PMDA and hydroxyaniline units) for both the Koopmans' (dotted line) and ASCF (solid line) methods are compared to the experimental spectrum (triangles) in Figure 3. For comparison with experiment, each of the Is ionization energies has been assigned equal intensity, Gaussian broadened by ~ 1 eV and then summed to yield a calculated energy distribution curve. The Koopmans' derived Is ionization energies are tens of electron volts higher, relative to both the ASCF and experimental results. Examination of Figure 3 clearly shows that the effect of core-hole relaxation is responsible for, essentially, the major difference between the Koopmans' result and experiment. The carbon, nitrogen, and oxygen calculated ASCF photoemission peaks are now within 1.0 eV of the experimental


6.

ROSSI & SILVERMAN

81


data. Even though the Koopmans' result is shifted significantly in absolute magnitude with respect to experiment, its' utility in interpreting photoemission data is generally recognized, since within each elemental grouping of photoemission levels, relaxation of inequivalent at oms yields comparable relaxation energies.


14

The experimental Cls spectrum (connected triangles) along with the calculated (Koopmans'-dashed, ASCF-solid) broadened Cls spectra are given in Figure 4. The ex panded scale in Figure 4 highlights the differences between the experimental spectrum of PMDA-ODA and the ASCF Cls values. It should be noted, however, that there is excellent agreement between the magnitude of the calculated and observed Cls spectrum. The Koopman's result on this scale is truly shown to be outside of the range of experiment. On a finer scale, there are differences between the ASCF and experimental values. The exper imental peak at ~ 288 eV corresponds to the Cls(C=0) level and is approximately 1 eV lower binding energy that the calculated ASCF result. This difference is expected to arise from two factors. First, interchain coupling between carbonyl oxygen atoms and adjacent groups will decrease the carbon-oxygen bond order involving the carbonyl group, resulting in a shift of the carbonyl Cls core level to lower binding energy. Second, it is expected that the incomplete basis set describing the Is orbital on the carbon atom can result in incomplete relaxation of the C(C=0) core hole. There is closer agreement between the main peak oc curring at 285 eV and the ASCF results. The main peak, centered at 285 eV, has contibutions from the Cls(Ph-H) levels for the low-energy side, while the central and highenergy portions are derived from the Cls(Ar-H), Cls(Ar-C=0), and Cls(C-N) levels of the PMDA fragment. The absence of a double hump for the main peak of the calculated spec trum is primarily the result of values chosen to simulate the experimental broadening. 20 C1S

solid - ASCF dash - Koopmans triangles - experiment

15h

p '-Ω

10h

< "c: c75 c cus

shN1S

0 -600

-500

-400

-300

-200

-100

Electron Volts (eV)

Figure 3.

The calculated photoemission spectra for the PMDA-ODA repeat unit for both Koopmans' (dotted line) and ASCF (solid line) and the experimental spectrum (triangles).



82


-305

-300

-295

-290

-285

-280

-275

Electron Volts (eV)

Figure 4.

The experimental Cls spectrum (connected triangles) and calculated (Koopmans'-dashed, ASCF-solid) broadened Cls spectra.


6.

83


ROSSI & SILVERMAN


Charge Shifts and Screening To illustrate the screening process taking place upon core hole ionization, the electron pop ulation shifts between the neutral and ionized species will be examined. An electron density difference plot for the σ valence electrons is given in Figure 5 for the Cls(C=0) ionization process. Only σ electrons are shown in the plot since the electron density differences have been plotted in the nodal plane of the π molecular PMDA fragment. This plot illustrates that valence σ electron density moves from immediate regions surrounding the core hole (C(Ph), 0(C=0), and N) to the C(C=0) atom. This increased electron density from surrounding atoms is the result of core hole relaxation which is driven by essentially, Coulomb forces. A similar shift of charge is also observed for the π system, but the amount of charge shifted is less than for the σ electrons. For the calculated core holes on the other atoms, the trend in neighboring atoms supplying electrons, as for case the of Cls(C=0), is maintained.

I

II

I

I

I

I

I

I

I

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I

I

I

I

I

I

'

t?H3

:

• H4 I

Figure 5.

I

I

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'

'

I

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I

σ electron density difference plot for Cls(C=0) state of the PMDA segment. The dashed contour lines indicate removal of electronic charge relative to the neutral species. The solid lines specify increased electron charge with respect to the neutral PMDA fragment.


84


Summary


The agreement between the magnitudes of the core hole ionization energies calculated by the ASCF method and obtained from experiment is excellent. Differences, however, still remain when comparing experiment and theory to values within 1 eV. These differences are expected to be due to shortcomings in treating the actual polymeric environment, as well as to inherent limitations of the computational procedure. The Koopmans' results yield, as expected, very good relative splittings, but absolute ionization energies that are far from the experimental values.

Literature Cited 1.

Hahn, P. O.; Rubloff, G . W.; Ho, P. S. J. Vac. Sci. Technol., 1984, A2, 756.

2.

Silverman, P. N.; Sanda, P. N.; Ho, P. S.; Rossi, A. R. J. Polym. Sci., Pol. Chem. Ed., 1985, 23, 2857.

3.

Silverman, B. D.; Bartha, J. W.; Clabes, J. G . ; Ho, P. S.; Rossi, A. R. J. Polym. Sci., Pol. Chem. Ed., 1986, 24, 3325.

4.

Rossi, A. R.; Sanda, P. N . ; Silverman, B. D.; Ho, P. S. Organometallics, 1987, 6, 580.

5.

Haight, R.; Silverman, B. D.; White, R. C.; Ho, P. S.; Rossi, A. R. Mat. Res. Soc. Symp., 1988, 108, 233.

6.

Koopmans T. Physica, 1934, 1, 104.

7.

Ford, P. C.; Hillier, I. H . J. Chem. Phys. 1984, 80, 5664 and references therein.

8.

Professor E . R. Davidson and co-workers, Department of Chemistry, Indiana University, Bloomington, Indiana 47405.

9.

Hsu, H . ; Davidson, E . R.; Pitzer, R. M . J. Chem. Phys. 1976, 65, 609.

10.

Bagus, P. S.; Schaefer III, H . F. J. Chem. Phys., 1972, 56, 224.

11.

Huzinaga, S. J. Chem Phys. 1965, 42, 1293.

12. Dunning, Jr., T. H . ; Hay, P. J. in Modem Theoretical Chemistry.; Schaefer, III, H . F . , Ed.; Plenum Press: New York, 1976; Vol. 3, Chap. 1. 13.

Takahashi, N.; Yoon, D. Y.; Parrish, W. Macromolecules, 1984, 17, 2853.

14. Haight, R.; White, R. C.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol., 1988, A 6, 2188. R E C E I V E D January 24, 1989


Polymeric Materials for Electronics Packaging and Interconnection

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