Degradation of ultra-thin oxides with tungsten gates under high voltage: wear-out and breakdown transient

Abstract

An analysis of the dynamics of degradation of ultra-thin gate S i 0 2 films under accelerated high voltage stress, from the growth of defect concentration up to the final phase of the oxide breakdorm, was performed on MOS samples with Tungsten 8s gate material. INTRODUGnONMetal gates are considered a promising approach to increase transistor tranmduotance and to reduce gate resistance [I].Tungsten in particular, being a midgap work-function material, is considered particularly useful in the case of short channel fully depleted SO1 CMOS circuits. To introduce W, in addition to major technological challenges to be solved, concerns related to the reliability of the MOS stacks have to be taken into consideration. Moreover, the study of the reliability of metal gates with Si02 is in itself particularly interesting for the basic understanding of the dielectric breakdown (BD) phenomenon. In fact, according to the physical models most accepted and most adherent to the experimental observations concerning oxide degradation under high field stresses (anode hole injection and hydrogen release model [2]), the anode material plays a major role in the oxide w e a r a t . &e would expect that by changing the anode, both anode hole injection and hydrogen mlea~e should substantially vary. So oxide wear-out kinetics should depend on the gate material in stresses where the anode is the gate.Moreover, experimental evidences suggest that with poly-Si gates also the BD event in itself, i.e. the degradation of the initial BD spot leading to the final transistor failure (hard breakdown), has at low voltage the same physical mechanism that produces the oxide degradation 131. So this means that, again, the anode material should play a major role in the BD build-up rate kinetics. In addition, at high voltage there is a transition in the BD mode in which the failure becomes extremely fast [4]. The measurement of the voltage threshold at about 4 V has indicated a number of possible physical mechanisms involving a threshold energy of about 4 eV and responsible forthe BD mnaway.In order to check such aspects we have investigated the behaviour of two different anode materials: tungsten and poly-Si gates (both p+ and n+). The latter is used as reference. In this report we show the main results. EXPERIMENTAL Most of the work here reported refers to samples consisting in nand p M O S capacitors With gate material either tungsten or poly-Si (used BS control), oxide thickness ranging from 2 to 4 nm, and with Q78M4315-XA41$M.W@ZXl4 IEEE 122(100) oriented Si substrates. The analyzed combination of gate materials and substrates are therefore n+ poly-Si / p s i , W / p-Si, p+ poly-Si /"-Si, and W /"-Si.The evolution of the oxide degradation was investigated by monitoring both the growth of defect concentration up to the oxide breakdo-and the evolution of the initial BD spot toward hard BD. All MOS capacitor samples were stressed in accumulation with consecutives pulses at constant voltage (CVS) of duration ranging from IO" s up to IO" s. After each puls...

Abrupt breakdown in dielectric/metal gate stacks: a potential reliability limitation?

“…We have studied different material combinations and thicknesses and show that large is a generally observed phenomenon with metal gates. These observations are consistent with earlier work on SiO with tungsten gate by Palumbo et al [3].…”

Physical mechanism of progressive breakdown in gate oxides

“…1,3,13 It is a noisy and progressive process well in agreement with those reported in the literature for the cases of HfO 2 3,9,15 and SiO 2 . 1-3, 16 The duration of the progressive increase of the current shows a strong voltage dependence and reaches current levels of the order of 1-10 lA, where the gate current jumps abruptly to very high levels in times of the order of microseconds, i.e., limited by the bandwidth of the equipment (see Figure 1). It is worth to note that the current level where the gate current jumps abruptly is quite similar for all cases with different CVS voltages.…”

“…14 The initial works on poly-Si/SiO x N y /Si gate stacks were later extended to the area of MOS stacks with metal gates and gate oxides such as SiO 2 , SiO x N y , and/or high k dielectrics on Si substrates. 3,15,16 On large area capacitor samples with tungsten metal gates on SiO 2 dielectrics during CVS (constant voltage stress) experiments, it was found that the PBD phase is observed only when the current level through the BD spot during the CVS is below 1-10 lA, 15-17 barely distinguishable from the background gate leakage current. When such a level of BD current is reached, a fast BD runaway takes place and the BD current suddenly jumps to the mA level.…”

Failure Defects Observed in Post-Breakdown High-κ/Metal Gate Stack Mosfet

“…High degradation rate in the gate leakage current is very common in breakdown (BD) of tungsten-gate/SiON metal-oxidesemiconductor (MOS) capacitors [7], and in high-κ/metal gate stacks metal-oxide-semiconductor field effect transistors (MOSFETs) [8]. Kauerauf et al [8] found that the metal gate electrode plays an important role for the abrupt large gate leakage current increase upon breakdown of high-κ/metal gate stacks.…”

“…The fast transient in metal gate of the order of dI g /dt >10 4 A/s [7] reduces the time taken to reach a gate leakage current value of 20-50µA, and hence it has strong implication on the post-breakdown device/circuit reliability. But the exact mechanism responsible for fast breakdown transient in metal gate stacks, and the physical nature of microstructral defects induced by high-κ breakdown in metal gate MOSFET remain unclear.…”