Thermal properties of poly(neopentylmethacrylate) thin films deposited via solventless, radical initiated chemical vapor deposition Vijay Jain Bharamaiah Jeevendrakumar a , Bruce A. Altemus b , Adam J. Gildea b , Magnus Bergkvist a, ⁎ a College of Nanoscale Science and Engineering, University at Albany, 257 Fuller Road, Albany, NY 12203, USA b U.S. Technology Development Center, Tokyo Electron U.S. Holdings, Inc., 2400 Grove Boulevard, Austin, TX 78741, USA abstract article info Article history: Received 11 February 2013 Received in revised form 4 June 2013 Accepted 12 June 2013 Available online 27 June 2013 Keywords: Initiated chemical vapor deposition Spectroscopic ellipsometry Glass transition temperature Thermal degradation Gel-permeation chromatography This paper investigates the thermal properties of thin films of poly(neopentylmethacrylate) (PnPMA) depos- ited via solventless, radical initiated chemical vapor deposition (iCVD). The effects of monomer to initiator molar ratio (M/I) on deposition kinetics, thermal properties and composition of the film were investigated. The molecular weight and conversion of PnPMA were observed to increase with increasing initiator concen- tration. Thermal properties of the film stabilized when annealed to 150 °C which was attributed to removal of short-chain molecules acting as “plasticizers”. Gel-permeation chromatography (GPC) studies and non-linear regression analysis of GPC data confirmed these results. M/I had no significant effect on the thermal stability of iCVD PnPMA and we hypothesize that this behavior is primarily due to weak bond linkages formed during polymer chain termination. The activation energy for the final thermal degradation stage of iCVD PnPMA was similar to that of anionically polymerized PnPMA, indicating that the iCVD polymer at this point was primarily composed of stable polymers that degrade through random chain scission. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Initiated chemical vapor deposition (iCVD) is a solventless poly- merization technique capable of depositing thin organic films in a conformal manner on a variety of substrates [1–3]. Unlike plasma enhanced CVD (PECVD) and parylene CVD, the relatively mild reac- tion conditions employed in iCVD process enable higher retention of chemical functionality of deposited polymer [4]. Also, results show that iCVD outperforms PECVD in realizing conformal deposition on microstructured surfaces. Thus iCVD of polymers with various chem- ical groups including amines, fluorophenyls, epoxies and hydroxyls has been demonstrated on various substrate surfaces [4–7]. Com- bined, these traits make iCVD attractive for depositing polymer thin films for applications like biocompatibility coatings, chemical and biological sensors, selective membranes and drug delivery [8–11]. Although iCVD polymerization involves similar reaction steps as free-radical bulk polymerization, there are some fundamental differ- ences between the two processes [12]. Compared to bulk polymeriza- tion, which is mostly a batch process, iCVD is a surface driven, bottom-up, continuous process [13]. The iCVD polymerization initia- tion reaction happens at the interface between surface adsorbed monomers and initiator radicals arriving from gas phase. The simulta- neous polymerization and deposition steps may lead to monomer molecules getting trapped in the matrix of the polymer and signifi- cantly alter their physical properties [14]. Monomer inclusion has previously been reported by Bakker et al., and is also observed here using gel permeation chromatography (GPC) studies [15]. Typically, iCVD films are deposited at lower temperatures (10–40 °C); however they are frequently exposed to higher operating temperatures and it is important to characterize the behavior of these films over a wide temperature range [16–19]. These factors prompt us to investigate the effects of deposition conditions and thermal treatments on prop- erties and stability of iCVD films. It is well established that the physical properties of polymers such as glass transition temperature (T g ), coefficient of thermal expansion (α) and elastic modulus (E) are a function of film thickness, molecu- lar weight and composition [20]. Among these T g is the most impor- tant and widely studied property since the trend of T g variation with film thickness and molecular weight can be used to predict the behavior of most other polymer properties [21]. In this work we investigate the thermal properties and thermal stability of iCVD poly(neopentylmethacrylate) (PnPMA) thin films deposited at differ- ent monomer to initiator molar ratio (M/I) and study the effects of film composition and thermal anneal on film properties. Initiator con- centration can influence the molecular weight, polydispersity, degree of conversion and rate of polymerization reaction and offer means to tune the properties of the polymer film. PnPMA is chosen as a model polymer since it is a linear, non-crosslinked polymer enabling GPC studies to determine their molecular weight and polydispersity. All iCVD PnPMA films were deposited on silicon wafers with a native oxide to provide a consistent substrate surface. A variety of techniques including thermogravimetric analysis (TGA), thermal analysis-Fourier transform infrared spectroscopy (TA-FTIR) and thermal volatilization Thin Solid Films 542 (2013) 81–86 ⁎ Corresponding author. Tel.: +1 607 342 3729; fax: +1 518 437 8603. E-mail address: mbergkvist@albany.edu (M. Bergkvist). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.06.056 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf