Performance Analysis of Si-Based Ultra-Shallow Junction Photodiodes for UV Radiation Detection

This thesis presents a performance investigation of newly-developed ultra-shallow junction photodiodes (PureB-diodes) for ultraviolet (UV) radiation detection. The photodiodes are fabricated by pure boron chemical vapor deposition (PureB CVD) technology, which can provide nanometer-thin boron capping layer and nanometer-deep defect-free p+n junction. Due to its unique features, the PureB-diode technology is considered to be a promising solution for developing advanced UV photodetectors. Chapter 1 presents the motivation and the objectives of the research work. In view of one of the most demanding UV-related industrial applications: deep-UV (DUV) and extreme-UV (EUV) lithography; the major challenges for designing high-performance UV photodetectors are defined. In Chapter 2, the results of a comparative study of the existing ultraviolet (UV) photodetectors are presented, based on the performance requirements defined in Chapter 1. The subsequent analyses are restricted to silicon-based devices, due to their superior characteristics: high sensitivity, simplicity, low costs, and IC-compatibility. The optical performance of state-of-the-art Si-based UV photodiode is reviewed, and their advantages and drawbacks are summarized in comparison with a proposed ideal structure of Si-based UV photodiode. Owing to the nanometer-deep junction and the nanometer-thin ? -boron capping layer, PureB-diodes have an ultra-thin front absorption window which can greatly reduce the unwanted photon loss, consequently achieve a near-theoretical responsivity at 13.5- nm wavelength in the EUV spectral range. This unique feature indicates that PureB-diodes can potentially provide better sensitivity than any other commercially available Si-based photodetector over the full UV spectral range. Therefore, photodiodes fabricated by the PureB CVD technology was chosen as the main candidate to design high-sensitivity, high-stability photodiodes for UV radiation detection. In Chapter 3, for proper interpretation and analysis of the reported optical experimental data in this thesis, the fabrication process and the device structure of PureB-diodes are introduced. Due to the limited attenuation length (penetration depth) of DUV/VUV photons in silicon, the surface conditions of Si-based photodiodes play an important role in their optical performance. The surface structure and properties of PureB-diodes are discussed in detail. In Chapter 4, a superior DUV/VUV optical performance of PureB-diodes has been presented. A doping-profile-induced surface charge-collection effect is validated as an additional important factor for achieving the high DUV/VUV sensitivity, especially when the junction depth is significantly increased by extra thermal treatment. It has been demonstrated that, along with the ultra-shallow junction, an optimized surface-layer doping profile plus a damage-free doping technique could be an alternative way to reduce the equivalent thickness of the pre-absorption layer on top of the effective charge-collection zone of the photodiode. The observed doping-profile-enhanced effect of surface charge-collection offers significant opportunities for designing a high-sensitivity photodiode for the case that an “ultra-shallow junction” is difficult to apply; for instance, when a low surface sheet resistance is the primary goal. At the same time, this doping-profile-enhanced effect of surface charge-collection also enhances the compatibility of PureB CVD technology with standard Si processing, and becomes one of the inherent advantages of these silicon-based photodiodes when it comes to fabricating fully-integrated UV smart-sensor systems. In Chapter 5, two potential negative effects related to a high surface sheet resistance of PureB-diodes have been studied: unpredictable response times and high series resistance. With respect to the response time, it has been demonstrated experimentally that, when PureB-diodes have a high surface sheet resistance, as well as a large active area (in the order of mm2), their response time is only slightly affected by variations of the size and location of the incident light spot. In most of the practical applications, this influence is negligible. Yet, for achieving a shorter response time, it makes sense to reduce the series resistance. This can be achieved, for instance, by improving the structure of the PureB-diodes either by creating an extended anode with the help of an aluminum grid, or by extra thermal annealing. Although PureB-diodes with an aluminum grid have a higher responsivity for the same series resistance value, the local non-uniformity of the sensitivity limits the applications of this method. Alternatively, a low series resistance can also be achieved with a deeper junction of the PureB-diodes. Due to the high efficiency of collecting charge at the surface, PureB-diodes with a deeper junction still maintain a good VUV/EUV responsivity and an excellent uniformity. In Chapter 6, the performance stability of PureB-diodes is evaluated. The experimental results prove that the p+n junction formed by PureB CVD technology is stable under DUV/VUV and EUV radiation: after prolonged exposures, no performance degradation caused by the damage to the junction itself has been observed. Nevertheless, a surface oxidation, which can be positively charged by the photoelectron emission effect under UV radiation, was found to be the main reason for the small responsivity degradation that was measured after receiving some VUV/DUV and EUV radiation. This charging-induced responsivity loss can be avoided by forming a thicker boron layer or adding a conductive capping layer. The former can eliminate the harmful surface oxidation on PureB-diodes; the latter is helpful in minimizing the charge-accumulation on the diode surface. Moreover, PureB-diodes have also demonstrated good electrical performance stability under intensive EUV radiation when an extra nitride passivation layer is applied to protect the Si-SiO2 interface along the diode perimeter. The still observed small dark current increase can be further scaled down by reducing the reverse bias voltage to the mV range in future applications. In Chapter 7, the durability of PureB-diodes in detrimental environments (under aggressive cleaning) is evaluated. Experimental results prove that PureB-diodes have excellent robustness to hydrogen radical (H*) cleaning treatment: the nanometer-thin ? -boron layer and the underlying boron-doped ultra-shallow p+n junction do not degrade during cleaning with aggressive H*. In particular, no responsivity drop has been observed after continuous H* exposure. However, H*-induced defects created at the Si-SiO2 interface with only SiO2 surface passivation outside the photodiode sensitive area, nearby the diode perimeter, increase the dark current at certain biasing conditions. This effect is almost eliminated by either a low-temperature bake (~ 200 oC) or by including a thin Si3N4 layer in the passivation layer stack to prevent the hydrogen from reaching the Si-SiO2 interface. In Chapter 8, a short overview of the UV photodiode design strategies is offered and a summary of the major contributions of this research is presented. Next, the ongoing work and potential extensions are shortly discussed.