Influence of the substrate surface texture on the photon-sensitivity stability of CsI thin film photocathodes M.A. Nitti a,Ã , A. Tinti a , A. Valentini a , E. Nappi a , P. Acquafredda b , E. Fanizza c , C. Ingrosso c , B.R. Pistillo c , E. Sardella d a INFN—Sezione di Bari—Via Amendola 173, 70126 Bari, Italy b Geomineralogical Department of the University of Bari, Via Orabona 4, 70126 Bari, Italy c CNR IPCF Sez. Bari c/o Dipartimento di Chimica, Universit‘a di Bari, Italy d Institute of Inorganic Methodologies and Plasmas (IMIP-CNR), Via Orabona 4, 70126 Bari, Italy article info Available online 2 June 2009 Keywords: Photocathodes Nanolithography Morphology of films Substrate texture abstract A study on the influence of the substrate morphology on the photoemission properties of caesium iodide (CsI) thin film photocathodes, in the range 150–200 nm, has been performed. Various types of conductive substrates, patterned by colloidal lithography, have been compared to the standard printed circuit board (PCB), used for the ALICE experiment at CERN [M.A. Nitti, et al., Nucl. Instr. and Meth. A 523 (2004) 323.]. A correlation between the substrate surface texture and the photoemission stability of the films has been demonstrated. The combination of colloidal lithography and plasma etching, or physical evaporation, allows to create on substrates arrays of nanostructures whose shape and pitch can be controlled by changing some parameters during the patterning process. In order to be comparable with the CsI photoelectron escape length and to preserve the substrate morphology in the film, a layer of 20 nm has been deposited on all the samples. Scanning electron microscopy (SEM) investigations of the colloidal lithography patterned (CLP) substrates have been performed. Atomic force microscopy (AFM) topographic images of the CsI thin film evaporated on PCB and CLP substrates have also been acquired and compared, showing a clear difference in the surface texture. An ageing test, consisting of an air exposure with a relative humidity of about 45% for 24 h, resulted in a higher quantum efficiency stability of textured CsI thin film photocathodes evaporated on nanostructured substrates with respect to those grown on standard PCB ones. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Caesium iodide (CsI) is widely used in several nuclear physics experiments as photoemissive material for the ultraviolet (UV) thin film photocathode (PC) preparation. However, the main drawback of this material is its high hygroscopicity [1]. Many parameters have been studied in order to enhance the CsI PC stability in air, attracting great attention in the scientific world. Results concerning 24 h air exposure of CsI thin film PCs evaporated on various conductive materials with surfaces nanos- tructured by colloidal lithography, other than on a standard printed circuit board (PCB) substrate, are presented in this paper. This ageing test evidences a correlation existing between the substrate surface texture and the photoemission stability of the CsI films. 2. Experimental details 2.1. Substrates preparation The patterning of n-type conductive silicon (n-Si) and stainless steel (SS) substrates was performed by colloidal lithography. A physical nanomask was obtained by exploiting the spontaneous self-assembly of polystyrene-sulphated nanoparticles (PS-np, 500750 nm in diameter) on these substrates. Colloidal suspension of PS-np was spin-coated for this purpose [2–5]. The pattern of the colloidal template was transferred into the surface either by plasma etching processes (Fig. 1a) or by metal thermal evaporation (Fig. 1b). Etching was carried out by feeding plasma with hexafluoropropylene oxide (HFPO)/oxygen mixture on a set of n-Si samples. Thermal evaporation allowed the growth of a 50-nm-thick nickel/gold layer for a SS sample. In case of Si etching, four different operative conditions were investigated by changing the radio frequency (RF) power, the treatment time and then the energy [6] as reported in Table 1 . Finally, the PS-np debris was lifted-off by ultrasonic bath. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.05.169 Ã Corresponding author. Tel.: +39 080 544 3252; fax: +39 080 544 2434. E-mail address: mariangela.nitti@ba.infn.it (M.A. Nitti). Nuclear Instruments and Methods in Physics Research A 610 (2009) 234–237