Series Resistance Optimization of High-Sensitivity Si-based VUV Photodiodes Abstract— Recently, silicon ultrashallow p + n photodiodes, fabricated by a pure boron deposition technology (B-layer diodes), were evaluated for detection in the Vacuum Ultra-Violet (VUV) spectral range from 115 nm to 215 nm wavelengths, where the attenuation length in silicon is only a few nanometers. Superior sensitivity in the order of 0.1 A/W in the whole VUV spectral range was reported [1]. Next to the sensitivity, another important parameter of any photodetector is the response time, which is directly related to its series resistance. In this work a study of the relation between the sensitivity and the series resistance of the B-diodes is presented, supported by simulation results and optical/electrical experimental results. Moreover, practical methods for designing a high sensitivity VUV photodiodes while keeping a relatively low series resistance, are proposed. The experimental results demonstrate that by modifying the diode structure, the series resistance can be effectively reduced. At the same time, the B-layer diodes still maintain a high VUV sensitivity. Keywords- Vacuum Ultra-Violet; photodiode; ultrashallow junctions; series resistance; responsivity; time constant. I. INTRODUCTION At present the demand for radiation detectors in the VUV spectral range (wavelengths below 200 nm) is noticeably increasing. For example, the ever decreasing feature size of the projected patterns on silicon wafers has led to the development of 193 nm-wavelength lithography [2]. The metrology and the dose control of the VUV pulsed sources require the use of fast and sensitive VUV radiation detectors. Therefore, the development and the fabrication of high-performance detectors for VUV radiation are important for the future of nanoelectronics manufacturing. Due to the availability of a low-cost and well-developed technology, Si-based photodiodes are good candidates for radiation detection applications. However, the extremely small penetration depth of the VUV radiation in silicon, as shown in Figure 1, implies that the depletion zone of the photodiode, where the photo-generated charge is collected, should be very close to the device surface [3]. Therefore, for Si-based photodiodes, an ultrashallow junction would be the most straightforward means of achieving a high sensitivity in the VUV spectral range. As a drawback, the attractively ultrashallow junctions fabricated in B-layer diodes, have a p + boron doping of the top 1-10 nm of the Si surface that represents a large sheet resistance R sheet of ~ 10 kȍ/sq or more, depending on the exact processing scheme. At the same time, covering the photosensitive surface with an extra conductive layer will give undesirable absorption of the incident radiation. Instead, the metal electrode is placed at the edge of the active area, to form what is often referred to as a “ring” electrode, as indicated in Figure 2. As a result, the photocurrent, which removes the photo-generated charge from the depletion zone, travels through the thin top-layer with a relatively large R sheet . For large diodes, the R sheet of the B-layer diode is dominated by the resistance of this thin top-layer. The series resistance (R s ) and junction capacitance (C j ) of the diode form a time constant IJ= R s C j (1) (shown in equation 1) which poses a lower limit to the response time, i.e., the operational speed, of the photodiode [4]. This work is supported by The Dutch Technology Foundation STW, project number: 10024. L. Shi, L. K. Nanver, A. Šakiü, Delft University of Technology Delft, the Netherlands l.shi@tudelft.nl S. Nihtianov ASML Netherlands B.V. Veldhoven, the Netherland Figure 1. Penetration depth vs. incident radiation wavelength in Si. Figure 2. Schematic of a Si-based p + n photodiode with a ring electrode and an ultrashallow junction T. Kneževiü University of Zagreb Zagreb, Croatia A. Gottwald, U. Kroth Physikalisch-Technische Bundesanstalt (PTB) Berlin, Germany 978-1-4244-7935-1/11/$26.00 ©2011 IEEE