1.5　Surface Modification of Polyimide Films

Aromatic polyimide films have been extensively used in many high-tech fields such as electrical insulation， microelectronics， military aircraft and spacecraft， etc.， due to their attractive thermal， mechanical， and electrical properties. These important applications usually require the polyimide films to have a modified surface to improve its adhesion to other materials such as metals， oxides， or other polymers. Plasma treatment is the common accepted technology for this purpose. Plasma are collections of charged particles， most commonly occurring in the gaseous state， with stars， fluorescent lights， and neon signs among the more familiar examples. Plasma technology has been applied in the microelectronics industry for stripping of organic etching masks and removal of organic surface contaminants since the late 1960s， of which oxygen plasma has been used to simulate the degradation of polyimide films used as a protective material on satellite systems and space stations by AO encountered at orbital velocities in LEO ［95，96］.

The components of gaseous plasma that can interact with polymer surfaces are electrons， reactive-neutral atoms and molecules， photons including those in the highly energetic VUV region， and energetic positive ions. The region between a plasma and an adjacent solid surface is known as a space charge sheath. There is a voltage drop across this sheath that accelerates positive ions toward the solid surface. These ions can have a major effect on chemical reactions occurring on the substrate surfaces and can impact enough energy to a solid surface to eject particles （sputtering）. Whereas the goal of plasma etching is removal of materials， plasma surface modification is intended only to alter the nature of the film surface without changing the bulk properties. It is often desirable to improve upon the surface properties （wettability and adhesion to subsequently deposited or laminated materials） of polyimide films. Plasma technology has been used to enhance the wettability of many polymer film surfaces ［97-99］. Depending on the plasma system configuration and selection of gases used for treatment， material removal and modification can occur simultaneously， proceeding by chemical reactions， by physical means， or by a combination of these two mechanisms.

As mentioned above， polyimide films are often used as dielectric materials in the fabrication of thin-film electronic packages such as TAB and FPCs. FPCs are now in common use as low-cost IC chip carriers. One strategy for reducing cost is to fabricate these carriers in a roll format. Polyimide films are commonly the preferred dielectric for this application because of their high thermal stability and good mechanical properties. Kapton H and Upilex R and S films are the commonly used polyimide films， of which Upilex S film has low moisture regain and a different thermal coefficient of expansion than Kapton H.

The plasma reactor configurations have obvious effects on the surface modification of polyimide films. Oxygen microwave （remote） and DC plasmas are employed to treat the polyimide film surface. Compared with the untreated polyimide films， microwave plasma treatment reduced practical adhesion levels for Kapton H and Uplix S， while DC-glow treatment produced enhanced adhesion about triple the values of the untreated films. Analysis of polymer surface and characterization of the plasma environment are required to understand the dependence of the resulting adhesion on the plasma system configuration. There are many factors that can affect the adhesion strength including thickness of the film being peeled， width of the peeled line， peel rate， ductility of the metals， and peel technique.

To illustrate the effects of modification by the reactive-neutral oxygen atoms， the surface chemistry of Kapton H， Upilex S， and Upilex R films treated downstream from oxygen microwave plasmas has been extensively investigated. Deionized （DI） water contact angles on Kapton H films were measured as a function of time of treatment downstream from oxygen plasmas at 30， 60， and 120 W （O atom concentrations increase in that order）. For short treatments （less than 15 s）， no discernible dependence on O-atom concentration was observed， indicating that initially a reaction with a relatively high rate constant occurs， that reduces the DI water contact angle from 72 degrees to about 45 degrees. With longer treatments， the effect of variation in O-atom concentrations becomes apparent， indicating the occurrence of at least one other reaction with a lower rate constant. Eventually， at each power， the surface comes to the same steady-state contact angle. In each case， after samples were subjected to a DI water rinse， subsequently measured contact angles increased to roughly 48 degrees， similar to the value obtained following the initial， fast reaction.

Upilex S film shows similar surface modification with Kapton H. Although the surface compositions are similar， 1.0% of silicon is detected in the commercially available Upilex S film. The silicon incorporation could be intentional （e.g.， as part of an additive to reduce frictional drag） or unintentional （as a contaminant， e.g.， from silicon rubber rollers used in the curing process）. The degree of surface modification depended not only on treatment time， but also on the number density of oxygen atoms in the plasma ［100］. Several gas additives， including nitrogen， can be used to increase the AO concentrations in the plasma.

Ion beam treatment can also be used to promote polyimide film adhesion. Several ion beam techniques can be used， such as surface treatment of polyimide film prior to deposition， simultaneous beam irradiation and metal deposition， and treatment following deposition. Ion beam treatment is not a form of plasma modification， but bombardment by energetic ion beams can be similar to that occurring at the surface residing in the plasma. There are several examples of improved adhesion for metal films vapor deposition onto polyimide film surface pretreated by irradiation with argon （and oxygen） ion beams. The degree of enhancement in etching or modification due to ion bombardment depends on the dose of ions incident onto the surface， such as the ion density， energy， and duration of exposure. The energy of ions bombarding the surface in plasma systems typically does not exceed a few hundred eV. The ion penetration depth at this energy is of the order of a few tenths of a nanometer， but the ions can extend several nanometers into the polyimide film due to the effect of the energy transfer through the solid. The surface properties of polyimides subjected to interactions with energetic ions are altered with respect to chemical composition and polymer backbone structures.

Energetic ions at 200 eV were used to modify the surface of both Kapton H and Upilex S films prior to sequential metallization with chromium and copper. With argon ions， Kapton H film gave 90 degrees peel strengths that were about twice the values measured for untreated polyimide films. On the other hand， very little adhesion improvement was obtained for Upilex S film with the same treatment. Kapton H film treated with argon ions and subsequently by oxygen ions， or using only oxygen ions showed 90 degrees peel values enhanced by a factor of 2 relative to the untreated film. The latter treatments resulted in a factor of four increase in peel values for Upilex S film. Hence， chemical structure of the polyimide films plays an important role in the surface modification and resulting improvements in adhesion.

A low-energy （500 eV） Ar+ ion beam was used to modify the surface of Kapton H film. Adhesion of thin gold film overlayers to the treated Kapton H film increased by an order of magnitude. Adhesion of copper to the treated Kapton H film was increased by a factor of 5. Table 1.18 summarizes the improvements in surface adhesion of polyimide films to the multilayer of vacuum-deposited metals plus plated copper using various plasma and beam techniques. The surface-treating technique by reactive neutrals + ions + electrons + photons give a surface-actived Kapton H film using O₂ DC glow which has 90 degrees peel strength to the multilayer of Cr （20 nm） plus Cu （>10 µm） of 490-657 N·m−1， and Upilex S film using O₂ sputter etching with 90degrees peel strength to the multilayer of Ta （50nm） plus Cu （>8 µm） of 784-882 N·m−1. In addition， the surface treatment by ions + electrons + photons also gives high adhesion surfaces both for Kapton H and Upilex S films. The 90 degrees peel strength of the Kapton H film treated using Ar+ sputter etching to the multilayer of E-beam evaporated Cr （20nm） + plated Cu （>8 µm） was 882N·m−1， comparable to that of the treated Upilex S film. Experimental results showed that the ions surface treatment yielded Upilex S film with better adhesion to copper layer than Kapton H film， implying that the chemical structure of polyimide films have obvious effects on the polymer surface modifications. In comparison， the reactive neutrals treating technique does not show apparent adhesion enhancement for Kapton H film.

TABLE1.18　The Surface Adhesion of Polyimide Films Treated by Various Plasma and Beam Techniques to the Vacuum-Deposited Metals Plus Copper Layer