ABSTRACT
CONTENTS 7.1 Introduction ........................................................................................................................ 215 7.2 Measurement Techniques ................................................................................................. 218 7.3 Oxide-Trap Charge Energies............................................................................................ 219
7.3.1 Trapped-Hole Energy Distributions..................................................................... 219 7.3.2 Comparison of Radiation Response and 1=f Noise in SiO2.............................. 221 7.3.3 Defect Microstructure............................................................................................. 222 7.3.4 Energy Scale for 1=f Noise..................................................................................... 226
7.4 Dipolar Defects near the Si=SiO2 Interface .................................................................... 229 7.4.1 Switched-Bias Annealing ....................................................................................... 229 7.4.2 Capacitance-Voltage Hysteresis ........................................................................... 231 7.4.3 Stable Dipoles in the Near-Interfacial SiO2......................................................... 233
7.5 Interface Traps.................................................................................................................... 235 7.5.1 Two-Stage Interface-Trap Buildup Model........................................................... 235 7.5.2 Hydrogen Transport and Reactions..................................................................... 238
7.6 Aging and Scaling Effects................................................................................................. 242 7.6.1 Aging Effects............................................................................................................ 242 7.6.2 Scaling Effects and Emerging Materials .............................................................. 245
7.7 Summary and Conclusions .............................................................................................. 247 Acknowledgments ..................................................................................................................... 248 References.................................................................................................................................... 248
In this chapter, we discuss defects in the critical bulk and near-interfacial SiO2 regions of a metal-oxide-semiconductor (MOS) device or integrated circuit (IC). This discussion is derived mostly from experience in evaluating MOS radiation response, and therefore applies most directly to the performance and reliability of electronics in radiation environments. However, the defects that limit the radiation response of a device also can significantly affect its reliability outside a radiation environment. Hence, radiation exposure can be a very effective tool in MOS defect analysis, and the lessons learned from systems that
that limit MOS performance and long-term reliability. When a MOS device or IC is exposed to ionizing radiation, electron-hole pairs are
created in the transistor gate oxide, and in other (parasitic) insulating layers of the devices. This process is illustrated schematically in Figure 7.1. Under positive gate bias at room temperature, radiation-induced electrons rapidly transport to the gate and leave the oxide, while holes transport slowly toward the Si. A fraction of these holes is trapped near the Si=SiO2 interface, leading to a shift in the threshold voltage of the transistor [1]. During the hole transport and trapping processes, hydrogen is released within the oxide, and under suitable bias conditions may transport to the interface and react with Si dangling bonds, forming interface traps [2-5]. Interface traps shift transistor threshold voltages and degrade channel carrier mobilities. Some positive charge that is trapped near the interface can induce compensating electron traps, which are often called border traps [6-9]. Faster border traps are sometimes mistaken for interface traps in studies of MOS performance, reliability, and radiation response [7-9]. In the MOS defect literature, there are wide varieties of nomenclatures used to charac-
terize defects in materials, devices, and ICs. The different terms that are often used to describe defects that are similar or even identical in microstructure can vary with the method of characterization, the effect of the defect on the device of interest, the background of the investigator, the convention of the particular technical community, and many other factors. In Figure 7.2, a representative sampling is provided of many present and historical terms that are used to describe defects in the Si=SiO2 system. The reader will see a variety of these terms used in this chapter and book that reflects the diversity of opinion and usage in the modern literature. In Figure 7.2, the defects are grouped schematically with their physical location in the Si bulk, at the Si=SiO2 interface, in the near-interfacial SiO2, in the SiO2 bulk, or (in highly scaled devices), at the gate=SiO2 interface. For a more extensive discussion of MOS defect nomenclature, please see Refs. [6-8, and references therein]. In this chapter, we first discuss briefly in Section 7.2 the measurement techniques
that are used to distinguish the effects of MOS oxide, border, and interface-trap charge. We focus primarily on thermally stimulated current (TSC) methods to estimate MOS
Gate Oxide Sub
Oxide traps
Border traps
Interface traps
V
+
+
+
+
+
+
+
+
+ −
−
−+
oxide-trap charge densities and energy distributions, the results of which are then discussed in detail in Section 7.3. We also discuss the microstructures of defects in the nearinterfacial SiO2 that contribute to MOS oxide-trap charge and low-frequency noise. We discuss the nature of dipoles in the near-interfacial SiO2 in Section 7.4; some of these dipoles are able to exchange charge easily with the underlying Si, and others are much more stable against both bias-and temperature-dependent annealing. In Section 7.5, we briefly review the effects of hydrogen on the buildup of MOS interface-trap charge. Finally, in Section 7.6, we discuss aging and device scaling effects on the radiation response and long-term reliability of MOS devices and ICs. Experimentally observed changes in MOS radiation response and=or inferred long-term reliability with time are attributed to changes in distributions of defects and=or impurities (especially hydrogenous species) with time and=or device thermal history. We conclude that the inferred radiation response or longterm reliability of devices with mobile impurities (e.g., hydrogen) can change significantly with time. Hence, efforts to manage MOS performance, reliability, and=or radiation response via defect engineering should be viewed with caution and checked for longterm stability. The same kinds of defects are observed in highly scaled devices, e.g., with ultrathin oxides, as are observed in previous generations of devices that have been studied extensively. However, their effects on device response can differ-for example, instead of oxide-trap charge shifting the device threshold voltage in a highly scaled circuit or device, it instead may increase gate leakage current or device noise. In addition, the total dose response of many deep submicron ICs is dominated by source-to-drain or device-to-device leakage currents, rather than charge trapping in ultrathin gate oxides. In these ICs, charge trapping and defect buildup in field oxides and shallow trench isolation (STI) regions is important; these isolation oxides frequently exhibit degradation characteristics that are similar to thick gate oxides from older technologies. Hence, the
Oxide traps
Surface Interface Gate
SiO2
Si
traps/states traps/states
Hole traps Bulk traps
Fast InterfacePb defects/centers
Electron traps
Mobile ionic charge
neutral electron traps
E defects/centers Neutral centers
Surface traps/states states dangling Si bonds traps/states
Recombination centers Bulk Si defects Dopant atoms
Border traps Fixed oxide charge Switching oxide traps Anomalous positive charge Slow traps Slow states Near-interface oxide traps
MOS systems is still quite relevant and applicable to modern and future MOS circuits and devices; however, the same defects can lead to different kinds of responses, and new materials introduce new kinds of defects and impurities that can also affect MOS performance, long-term reliability, and radiation response.
