Polyimide film surfaces, cured

Polyimide surface modification by a wet chemical process is described. Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) and poly(bisphenyl dianhydride-para-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution. These modified surfaces are further treated with aqueous HC1 solution to protonate the ionic molecules. Modified surfaces are identified with X-ray photoelectron spectroscopy (XPS), external reflectance infrared (ER IR) spectroscopy, gravimetric analysis, contact angle and thickness measurement. Initial reaction with KOH transforms the polyimide surface to a potassium polyamate surface. The reaction of the polyamate surface with HC1 yields a polyamic acid surface. Upon curing the modified surface, the starting polyimide surface is produced. The depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the KOH reaction temperature and the reaction time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. Relationship between surface structure and adhesion is discussed. 

ESCA was found to be useful in studying the effects of different methods of curing, the adhesion promoter and the deposition of copper on polyamic acid-polyimide surfaces. The results show different C Is spectral features for hot plate vs. oven cured films but identical stoichiometries. The silane treated amic acid surface showed an additional N Is component at 402.0 eV due to NH . Heat treatment of this film resulted in the formation of a silicon-polyimide. Grazing angle ESCA results showed that the silane diffused into the sub-surface of the amic-acid film. Analysis of cured polyimide films after the etch (FeCl ) removal of a... 

Cobalt(II) chloride was dissolved in poly(amide acid)/ N,N-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bul)c of the film. Two properties of these films, surface resistivity and bullc thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. 

Polyimides. In situ co-deposition of metal salts such as Co(II), or LiCl into a polyimide precursor from 3,3, 4,4 -tetracarboxybenzophenone dianhydride and 4,4 -diaminobenzophenone with subsequent thermal curing offers surface-conductive polyimide film [43]. By similar procedures, Taylor et al. prepared a series of polyimides modified with Pd, Pt, Ag, Au, Cu, Sn, Ti and magnetic-Fe [44, 45, 46]. 

Polyimide-Polyimide Adhesion. To study polyimide-polyimide adhesion (10). a thin layer (200 A) of gold was sputter-coated onto one side (20% of the total area) of the polyimide sample to initiate peel (polyimide has a poor adhesion to gold). The surface of the polyimide in the exposed area was modified to the polyamic acid surface. PMDA-ODA polyamic acid in NMP solvent was spin-coated to the surface-modified (to polyamic acid) polyimide film, and subsequently cured at 400 °C under nitrogen. Thickness of the adherate layer (peel layer) after curing is approximately 20 um and the width of the peel layers is 5 mm. The peel strengths were measured by 90° peel of the top polyimide layer. The failure occurs at the interface between... 

In figure 7 we show Infrared-Reflection Absorption data for a thin polyamic acid film (a), a thin polyimide film (b) (after curing the PAA film of spectra a) and a thick (d > 10 nm) polyimide film (c) on a polycrystalline silver surface. Transmission and reflection infrared absorption spectra of thick (d > 1000 nm) spun-on polyamic acid and polyimide films have been reported previously [22-24], Salem et al. [2] presented in their pioneering paper the infrared transmission spectra for vapor deposited polyamic acid and polyimide. Except for the relative intensities between the individual absorption bands, our results for a thick polyimide film (Fig. 7c) are identical to those for spun-on and vapor deposited films, showing the unresolved symmetric and asymmetric (strong) vc Q stretches between 1700-1800 cm 1, the v (l,4-CgH4) ring... 

The surface chemical structure of several thin polyimide films formed by curing of polyamic acid resins was studied using X-ray photoelectron spectroscopy (ESCA or XPS). The surface modifications of one of the polymer systems after exposure to KOH, after exposure to temperature and humidity, after exposure to boiling water, and after exposure to O2 and 02/CF plasmas were also evaluated. The results showed imide bond formation for all cured polyimide systems. It was found that (a) K on the surface of the polyamic acid alters the "normal" imidization process, (b) cured polyimide surfaces are not invarient after T H and boiling water exposures, and (c) extensive modifications of cured polyimide surfaces occur after exposures to plasma environments. Very complex surfaces for these polymer films were illustrated by the C Is, 0 Is, N Is and F Is line characteristics. 

Highly reflective, surface-metallized, flexible polyimide films containing silver are accessible by the incorporation of [Ag(hfac)] into poly(amic acid) solutions followed by thermal curing to 300 "C . The silvered films are thermally stable and maintain mechanical properties similar to those of the parent polyamide. 

Highly anisotropic copper-containing polyimide films have been produced through the homogeneous incorporation of bis(trifluoroacetyl-acetonato)copper(II), Cu(TFA), with the polyimide precursor followed by thermal cyclodehydration. By judious choice of polyimide precursor, doping level, and curing atmosphere two specific types of surface structure have been obtained. Bi- and tri-layered. 

Fig. 8. Transmission electron micrograph of ultramicrotomed cross section of the surface region of a wet air-cured CuCTFA) modified BTDA-ODA polyimide film.

Table 1 contains the elemental compositional results obtained by ESCA for Cu coated polyimide films before and after T H exposure. As shown, the metal diffuses into the polymer matrix after the T H exposure. The same result was obtained for Ni. The diffusion of Cu into PI-5878 surfaces detected in our ESCA results correlates with the 4X increase in peel strength (after 500 hours at T H - 85 C 80%) measured (4) for thick Cu films deposited onto cured PI-5878... 

It is necessary to prebake the PI film to 200°C to improve its resistance towards negative photoresist with a commercial stripper. After baking, remove the photoresist with a commercial stripper which is usually composed of phenol, strong mineral acids and solvents. 11) Neutralization and rinse. 12) final cure. Typical schedules are 30 min. at 350°C or 15 min. at 400°C. 10)PIasma,chemically etching) or physically (roughening) treat the polyimide surface to improve adhesion for next level metal. 

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