Journal of Hazardous Materials 169 (2009) 153–157 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Efficient stripping of photoresist on metallized wafers by a pause flow of supercritical fluid Mu-Rong Chao a , Jian-Lian Chen b,∗ a Department of Occupational Safety and Health, Chung Shan Medical University, No. 110, Sec. 1, Chien-Kuo N Road, Taichung 402, Taiwan b School of Pharmacy, China Medical University, No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan article info Article history: Received 21 June 2008 Received in revised form 18 March 2009 Accepted 19 March 2009 Available online 27 March 2009 Keywords: Extraction Factorial design Pause flow Photoresist Stripping Supercritical fluid abstract Utilization of supercritical fluids (SCFs) is studied here on the premises of a saving of hazardous organic solvents and of the specification for stripping the photoresist (PR) on metallization layers, which is one of the integrated circuit processing modules. By using factorial experimental designs with five factors and four level ranges, this research focuses on determining an optimized recipe with high stripping effi- ciency and to determine the stripping mechanism. In the case of PR on an aluminum layer, the initial use of the pulse flow mode could increase the extraction ratio remarkably when compared to the con- ventional continuous flow mode. Based on the limitation of a total volume of 30 mL purging SCF-CO 2 for economical considerations, the optimum conditions can be summarized as follows: 120 ◦ C, oven tem- perature; 350 atm, CO 2 pressure; 0.2mL of ethylacetate spiking to SCF-CO 2 ; 2.0 min, static equilibrium time; and five cycles of dynamic flow pausing. A recovery of 94.6% (n = 3, RSD = 6.5%) was obtained, while the diffusion of stripped PR from substrate matrix prevailed over the dissolution of binding PR into the SCF medium. In the case of copper, the optimum parameters in a pause flow mode were 140 ◦ C, oven temperature; 500 atm, CO 2 pressure; 0.75 mL, ethylacetate spiking volume; 5.0 min, static time; and six cycles of flow pausing. These extreme parameters still did not produce an SCF environment suitable for diffusion or dissolution mass transfer, and thus a recovery of 76.2% (n = 3, RSD = 7.5%) was only obtained. Removing PR coated on a Cu layer was harder than that on an Al layer. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Microelectronics manufacturing is the largest industry in the world and has continued to keep pace with Moore’s law of expo- nential progress for decades [1,2]. In a typical chip-fabrication plant, production of a 2 g microchip can consume 32 kg of water, 700 g of ultra-pure gases, 1.6 kg of fossil fuels and 72 g of chemicals [3]. Besides the considerations of the environment, safety and health, any alternative technologies are not just “greener” but provide valid technical advantages that may allow innovative component designs. Supercritical fluid (SCF) technology is not surprisingly the prime candidate for the identification of global challenges that will be met in 2015 and was outlined in the 2003 International Technol- ogy Roadmap for Semiconductors [4]. Some applications of SCF in integrated circuit (IC) manufac- turing operations, including the processing of photoresists (PR), wafer cleaning and etching chemistries, the deposition of metals and dielectric constant films, and chemical mechanical planariza- ∗ Corresponding author. Tel.: +886 4 22053366; fax: +886 4 22031075. E-mail addresses: mrchao@csmu.edu.tw (M.-R. Chao), cjl@mail.cmu.edu.tw (J.-L. Chen). tion, have been reviewed [5–8]. PR stripping and cleaning is the first application of SCF in IC processing. A group in Los Alamos National Laboratory developed a series of SCF cleaning processes and col- laborated with equipment makers to produce a commercial device [9–13]. They showed that SCF removal of PR minimizes the use of hazardous solvents and eliminates rinsing and drying steps. More- over, SCF cleaning allows production of features of less than 100 nm due to the low surface tension and gas-like viscosity of supercritical CO 2 . Supercritical CO 2 is the solvent of choice because it is non-flammable, environmentally benign and exhibits convenient critical properties (T c = 304.3 K, P c = 7.38 MPa). The high compress- ibility of the CO 2 -SCF medium allows the solubility and diffusivity to be widely varied with the pressure control. In addition, the tem- perature factor also affects the SCF density and the detachment of adhesive PR from wafer substrate. Furthermore, the addition of suitable cosolvents to the CO 2 -SCF adjusts the polarity so that it is compatible to that of PR and thus facilitates the dissolving PR into SCF medium. All described parameters need to be coordinately optimized so that PR molecules can freely escape from the binding matrices and then blend smoothly with the modified SCF. To meet these needs, the factorial design [14,15] and multilinear regression [16,17] are formal optimization methods, which are certainly supe- 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.03.092