Appl. Phys. A 68, 103–105 (1999) Applied Physics A Materials Science & Processing  Springer-Verlag 1999 Rapid communication A study of defects in ultra-thin transparent coatings on polymers A.S. da Silva Sobrinho 1 , G. Czeremuszkin 2 , M. Latrèche 2 , M.R. Wertheimer 1, ∗ 1 Groupe des Couches Minces (GCM) and Department of Engineering Physics, ´ Ecole Polytechnique, Montr´ eal, QC H3C 3A7, Canada 2 Polyplasma Inc., 3744 Jean Brillant, Montr´ eal, QC H3T 1P1, Canada Received: 1 September 1998/Accepted: 6 October 1998 Abstract. We describe new techniques, based mainly on re- active ion etching (RIE) in oxygen plasma, to render visible micrometer- or sub-micrometer-sized defects in transparent barrier films on transparent polymers. These techniques can be used to characterize and better understand the origins of defects in these coatings on a microscopic scale, as well as for mapping and counting defect density on a macroscopic scale (tens of cm 2 or more). PACS: 52.75.R; 81.15.R Transparent barriers against gas and vapor permeation, de- posited by plasma enhanced chemical vapor deposition (PECVD), for example, are the object of increasing interest in the packaging, pharmaceutical, optical, and electronics indus- tries. Such barriers can decrease permeation through flexible films of commercial or optical grade polymer substrates, or through rigid walls of plastic containers by several orders of magnitude. Typically, coatings obtained by PECVD, such as silicon dioxide (SiO 2 ) or nitride (SiN), provide an excellent barrier even when they are extremely thin. We have shown [1] that O 2 and H 2 O permeation decreases as much as a thousand-fold with increasing coating thickness, d , to a certain asymptotic minimum value, when the thickness exceeds d ≃ 70 nm. The residual permeation is attributed to the presence of micro- scopic defects in the coating [1, 2], which may result from dust particles on the substrate surface, from geometric shad- owing and stress buildup during film growth at sites of high surface roughness, or from other causes. So-called antiblock particles, which are incorporated into commercial polymer films to prevent adhesion between adjacent layers on a roll, constitute an important example of localized roughness, to be discussed further below. The detection of micrometer- or sub-micrometer defects in an opaque coating, for example in the case of aluminized ∗ Corresponding author. (Fax: +1-514/340-3218, E-mail: mwertheimer@mail.polymtl.ca) polyester (PET), can quite readily be done by optical mi- croscopy using transmitted light [3]. However, the detection of defects in an ultra-thin transparent layer on a transparent substrate evidently presents a sizable challenge. In this communication, we describe various techniques we have developed to render visible such defects in trans- parent films, and to characterize them; important aspects of this include reactive ion etching in oxygen plasma, followed by optical and/or electron microscopies. When larger surface areas (tens of cm 2 or more) are to be analyzed, the modi- fied method consists of etching, followed by decoration, for example using iodine vapor. This latter procedure can even render defects visible to the unaided eye, so that pinholes, cracks and scratches can readily be distinguished, and their number density quantified. 1 Experimental methodology Exposure to atomic oxygen (AO), the main reactive species in O 2 plasma, causes rapid etching of most organic poly- mers [4], while for inorganic materials like ceramics, or sili- con compounds deposited by PECVD, the etch rate tends to be immeasurably small, on account of the chemical inertness of these materials toward AO. For these reasons, silica coat- ings can also be used to protect the outer surface of polymeric materials employed in the construction of spacecraft [5]; it is now well-known that hyperthermal AO is responsible for the observed erosive degradation of polymers during low Earth orbital (LEO) spaceflight [6]. As just pointed out, a thin inorganic coating is resistant to etching by AO while the polymeric substrate is not, but AO can attack the latter through even the smallest of breaches or defects in the former: Modified versions of this principle are used deliberately in modern microfabrication, for example in ULSI microelectronics, to create sub-μ m-sized patterns in organic photoresist materials [4, 7]. In 1988, Banks and Rut- ledge [8] published their observations of “undercutting” ero- sion of DuPont Kapton  polyimide (PI) film, caused by AO penetration through μ m-sized defects in a transparent oxide coating on the PI; undercutting is illustrated schematically