(Invited) Sub-40mV Sigma VTH Igzo nFETs in 300mm Fab
Authors: Mitard Jerome , Kljucar Luka , Rassoul Nouredine , Dekkers Harold F. W. , van Setten Michiel , Chasin Adrian , Pourtois Geoffrey , Belmonte Attilio , Delhougne Romain , Donadio Gabriele Luca , Goux Ludovic , Nag Manoj , Wilson Chris , Tokei Zsolt , Borniquel Jose Ignacio del agua , Steudel Soeren , Kar Gouri Sankar 
Published: December 2020

In this work, IGZO device integration is reported leveraging our 300mm-fab facilities. Our objective is mainly to gain insights into the process and material elements which drive the control of the performance parameters of IGZO nFETs.

To control the final doping of the IGZO channel, it is well reported in literature that a final oxygen anneal can be applied to passivate the oxygen vacancies which are formed during the device fabrication. This technique is also used in our 300mm flow. As it might be expected, in front gate IGZO nFETs, the passivation efficiency is limited by the presence of the top metal gate stack. Therefore, it seems important to limit the formation of the oxygen vacancies during the deposition of the gate dielectrics. Three oxides were studied: SiO2, Al2O3 and HfO2 on top of the IGZO channel. After oxygen annealing, only PECVD-SiO2 shows a large recovery while for Al2O3 and HfO2, it remains low. This result challenges the implementation of conventional water-based high-K materials in gate first IGZO integration.

Contrary to the use of undoped IGZO in the channel for IOFF control, contacts can rely on maximizing the oxygen vacancies to increase the dopant concentration in the S/D regions. When not done locally, it could be an extra source of variability. This doping is made through oxygen scavenging from IGZO by a thin metal contact barrier. Low specific contact resistivity down to 1x107 Ohm.cm2 is demonstrated when Ti is thinner than 5nm. With thicker barrier, the formation of TiO2 and specific alloys is taking place at the IGZO/Contact barrier interface. This is both confirmed by ab initio simulations and by advanced physical characterization techniques.

Regarding the channel, X-ray diffraction techniques are extensively here since this is a nondestructive in-fab technique that provides detailed information about the crystallographic structure of the IGZO material. We show a typical spectrum where clear peaks/humps are seen and attributed to: 1. amorphous IGZO, 2. CAAC-IGZO and 3. a previously not reported, to the best of our knowledge, phase called here s-IGZO. The s-phase is only formed under certain conditions of power, temperature and oxygen flow during material deposition.

Thick IGZO (>12nm) back-gated nFETs with active layers submitted to final O2 anneal are used to study the different phases of IGZO and their electrical impact on device parameters. In this configuration, the carrier transport preferentially occurs in the bottom half of the IGZO channel while the top half (SiO2/IGZO interface) mostly drives the electrostatic control of BG transistors. The amorphous IGZO has much reduced spread in VTH-ON and higher ID,LIN (~mobility) than that of CAAC-IGZO. Reliability tests have also been carried out to compare the two phases and the results show the existence of two competing PBTI degradation mechanisms for CAAC-IGZO. It is important to report the degradation over time and different VG stress because a cut line at a specific value could have shown no BTI degradation of CAAC-IGZO and then attributing an unfair benefit of this phase over the a-IGZO phase. Combining these findings to some other results where the sheet resistance of IGZO under hydrogen exposure depends on the phases, we can reasonably conclude that CAAC owns different doping levels in comparison to a-IGZO.

Based on the previous learning about n-type dopant location in IGZO, we show that the performance of transistors keeps increasing when the channel length is reduced and scales with the channel width. A measure of the VTH-ON variation has been performed at the 300mm wafer scale from long to ~120nm LCH and down to 200nm WCH dimensions. The figure in attachement shows the Id-Vg curves of >100 Back Gated IGZO-nFETs with no failed devices detected. The standard variation of the VTH-ON across LCH and WCH is often less than 40mV with a minimum of 20mV.

In conclusion, we have demonstrated in this study scaled IGZO nFETs with excellent VTH_ON control using an industry compatible 300mm process flow. This has been achieved thanks to a careful mapping of n-type doping in the three dimensions of the IGZO channel. While semi-crystalline IGZO seems to be more robust against hydrogen than that of amorphous-IGZO, a new IGZO phase is found to help boost ION at short channel. The demonstration of back-gate IGZO nFETs with low variability and relatively high drive will benefit to the top-gate architecture development. This will provide new opportunities for the IGZO-based devices like an “on-the-fly” VTH setting for the performance control of advanced applications.

Figure 1