Degradation of Pt-Al₂O₃-Ge Metal Oxide Semiconductor Structures due to Pt-Al₂O₃ Induced Reactions

Abstract

In this work, Pt-alumina-Ge metal oxide semiconductor structures are investigated. An important finding is that the Pt metal layer reacts with the underlying aluminum of the atomic layer deposited amorphous alumina, forming a crystalline Al-Pt alloy between the Pt and the remaining alumina layer. This effect is attributed to the alumina surface destabilization due to the short exposure to photolithography developer. Subsequent annealing at 350 °C in inert gas ambient does not impose significant changes to the stack, while forming gas annealing at the same temperature seems to further promote the Pt-Al reaction. One of the consequences of the Al-Pt alloy formation is the reduction of the alumina layer thickness. In turn, the insulating properties of the Pt-Al 2 O 3 -Ge MOS stacks are degraded, manifested by enhanced density of interface traps near the valence band edge and by the observation of dc conduction mechanism in conductance characteristics, observable at low temperatures. The level of this degradation depends on the extent of the Pt-Al reaction, which is enhanced by the hydrogen annealing, yielding structures characterized by high leakage currents. These findings may add additional knowledge to the understanding of the Ge-based MOS metallization processes, as well as to the subsequent post-metallization annealing recipes.

Post-metallization annealing and photolithography effects in p-type Ge/Al2O3/Al MOS structures

“…In any case, the effect is of moderate intensity as compared to the formation of Pt/Al 2 O 3 /Ge MOS capacitors under the same experimental conditions, 17 where a thick Pt-Al alloy is formed even at the as-deposited state consuming about 1.5 nm of the destabilized Al 2 O 3 film. The main reason is that Pt-Al alloying takes place at very low temperatures (∼50 °C), 25 which are, most probably, exceeded during a sputtering deposition process.…”

“…This comparison suggests that the influence of the photolithographic step does not result in observable thickness variations of the Al 2 O 3 dielectric layer in contradiction to the case of Pt depositions. 17 Figures 2a-2b show HRTEM micrographs of the sample annealed in FG for 20 min in an area under the aluminum gate electrode. The d-spacing of the fringes in the aluminum grain is 0.236 nm which is very close to the (111) d-spacing of Al (nominal value 0.2338 nm).…”

“…However, the TEM study does not reveal the existence of an interfacial layer between Al and Al 2 O 3 as in the case of the Pt/ Al 2 O 3 combination. 17 To get more information regarding the Al/Al 2 O 3 interface, EDS measurements have been also performed in both samples. Figure 3a shows the depth distribution across a line of the Al, O, Ge, elements in the as-grown sample.…”

“…29 In our case, we had used f D values of 0.2 as in our previous studies. 15,17 The interface traps density distribution within the bandgap can be estimated from the relation: where E is the energy of the traps, u th is the thermal velocity of carriers, N v is the density of valence band states, k is the Boltzmann constant, T is the absolute temperature and σ is the capture-cross section of the traps. 29 Since the last parameter is unknown, we had used a value of 10 −15 cm 2 , which is characteristic for a large majority of traps.…”

“…In our previous work, 17 we had reported on the properties of photolithographically formed Pt/Al 2 O 3 /Ge MOS capacitors and the role of post metallization annealing in both forming gas and inert (N 2 ) ambient. These previous findings revealed that the atomic layer deposited Al 2 O 3 is attacked chemically by the photoresist developer, which contains tetramethylammonium hydroxide (TMAH).…”