Sung Han

Induced Surface Reactions and Chemical States: A Kiloelectronvolt Ion Irradiation on Simple Linear Chain Structure Polymers in an O 2 Environment

Argon ions of a kiloelectronvolt energy were irradiated on saturated simple‐chain polymers, such as polyethylene , poly(vinylidene fluoride) , and poly(tetrafluoroethylene) in an oxygen environment. Irradiating ions have induced chemical changes in polymeric chains, which includes substitution reactions for environment gas species resulting in the formation of radicals, carbonization, and bond scission but, exceeding the optimum ion dose, a carbonized phase was formed on the polymer surface. The relationship between the surface chemical reaction and the formation of hydrophilic polar groups was investigated with a view of the compositional change on the polymer surface and the kinetic energy of ions. From experimental results, identification of newly formed bonds and chemical changes depending on the depositing energy of the incident ions is represented.

Hydrophilic surface formation on materials and its applications

Abstract A new surface modification technique, ion assisted reaction (IAR) has been developed at the Korea Institute of Science and Technology (KIST) ion beam laboratory for modifying the surfaces of polymers. IAR, in which a kiloelectron volt ion beam is irradiated on the surface of a polymer in a reactive gases environment, has been developed for improving wettability of materials and enhancing adhesion to other materials. The contact angles of water drops with modified polymers were reduced more by Ar + ion irradiation with a flowing oxygen gas environment than without flowing oxygen gas. The change in contact angles for the modified polymers was explained by a two-step chemical reaction between the polymer matrix, energetic ions and oxygen gas. X-Ray photoelectron spectrometry (XPS) showed that hydrophilic groups were formed on the surface of polymers by chemical reaction between the unstable chains induced by ion irradiation and the oxygen gas, and the hydrophilic groups were identified as –(CO)–, –(CO)– and –(CO)O– bonds. The enhanced adhesion between metal and modified polymers was explained by the formation of electron acceptor groups in polymer and electron donors in metal.

Surface reaction on polyvinylidenefluoride (PVDF) irradiated by low energy ion beam in reactive gas environment

Polyvinylidenefluoride (PVDF) was irradiated by a keV Ar+ ion in O2 environment for improving adhesion between PVDF and Pt, and reaction between PVDF and the ion beam has been investigated by X-ray photoelectron spectroscopy (XPS). The adhesion test between Pt and the modified PVDF was carried out by boiling test, in which the specimens were kept in boiling water for 4 h. Two failure modes (buckling up due to weak adhesion and crack formation due to strong adhesion) of Pt films have been observed in the system. Contact angle of PVDF was reduced to 31 from 75° by the irradiation of 1 × 1015 Ar+ ions/cm2 with oxygen flow rate of 8 sccm. The surface of the irradiated PVDF became more rough as ion dose increased. The improved adhesion mechanism and identification of newly formed chemical species have been confirmed by Carbon 1s and Fluorine 1s X-ray photoelectron core-level spectra. The main reaction occurred at the irradiated PVDF surface is an ion-beam-induced oxidation accompanied with preferential sputtering of fluorine. Newly formed chemical species at interface are regarded as ester and carboxyl groups. Adhesion of the Pt–PVDF interface was improved by ion irradiation in O2 environment. This improvement is originated from the presence of carbon—oxygen bonds on the irradiated PVDF surface. Comparison of failure modes on the irradiated PVDF at various conditions after the boiling test shows that adhesion of Pt film is largely affected by the product of ion-assisted reaction.

Surface Modification by Ion Assisted Reaction

Surface characteristics of polymers determine their interfacial properties and technological applications. There have been many attempts to modify the surface of polymers to improve wettability, dye printing and adhesion to other materials. Plasma technology, high energy ion beam irradiation, corona discharge and other techniques have been used. However, rough surface and/or surface damage such as bond scission, carbonization, crosslinking, etc. are produced by the above methods. A coating of a surfactant was found to be relatively successful in enhancing the wettability of polymers, but a lifetime of the surfactant is too short for practical use. Therefore, new surface modification method is demanded to get the polymer surfaces free of the surface damage and having good wettability with a long lifetime. Koh et al 1–3) successfully modified hydrophobic surface of polymers into hydrophilic one by the combination of a low energy ion beam and reactive gas environment, and they named this surface modification method “Ion Assisted Reaction (IAR)”. They also reported that the activated polymer surfaces irradiated by energetic ions induce a chemical reaction with reactive gas, and new formed bonds such as carboxyl, carbonyl, hydroxyl, and ester radicals improve wettability and adhesion to other materials.

Adhesion improvement between Au films and glass by 1 keV Ar+ ion irradiation

Au films with a thickness of around 1600 A were deposited onto glass substrates at room temperature by an ion beam sputtering method, using a 5 cm cold hollow cathode ion gun at a pressure of 4–8×10−6 Torr. Irradiation of the Au/glass samples was carried out at pressure of 7×10−6 Torr. For the sputter deposition, the Ar+ ion energy was 1 keV and the current density at the surface of the substrate was 15 μA cm−2. Effects of 1 keV Ar+ ion doses (Id) ranging from 1×1016 to 2×1017 Ar+cm−2 on the properties such as crystallinity, surface roughness, and adhesion of the films have been investigated. Rutherford backscattering spectrometry showed that the thickness of Au films and sputtering yield were reduced by increase of Id and rms surface roughness of the films increased from 16 to 118 A, as the ion dose was changed from 0 to 2×1017 Ar+ cm−2. Adhesion of Au films on the glass irradiated at Id=2×1017 Ar+ cm−2 was nine times greater than that of untreated Au films, as determined by a scratch test.

Cu films by partially ionized beam deposition for ultra large scale integration metallization

Highly (111) oriented Cu films with a thickness around 1800 A were prepared on Si(100) at room temperature by partially ionized beam deposition (PIBD) at pressure of 8 × 10 -7 -1 × 10 -6 Torr. Effects of acceleration voltage ( Va ) between 0 and 4 kV on such properties as crystallinity, surface roughness, resistivity, etc. of the films have been investigated. The Cu films deposited by PIBD had only (111) and (200) planes, and the relative intensity ratio, I (111)/ I (200) of the Cu films increased from 6.8 at V a = 0 kV to 37 at V a = 4 kV. There was no indication of impurities in the system from Auger electron spectroscopy (AES) analyses. A large increase in grain size of the films occurred with V a up to V a = 1 kV, but little increase occurred with V a > 1 kV. Surface roughness of the Cu films decreased with V a , and resistivity showed the same trends as that of the surface roughness. In the Cu films by PIBD, it is considered that changes of resistivity are mainly due to a surface scattering rather than a grain boundary scattering. The via holes, dimensions of which are 0.5 μm in diameter and 1.5 μm in depth, in the Cu films made at V a = 4 kV were completely filled without voids. Interface adhesion of the Cu film on Si(100) deposited at V a = 3 kV was five times greater than that of Cu film deposited at V a = 0 kV, as determined by a scratch test.