Monday, 13 January 2014

The FD-SOI Papers at IEDM ’13

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The FD-SOI Papers at IEDM ’13

Posted by on December 16, 2013
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FD-SOI was a hot topic at this year’s IEEE International Electron Devices Meeting (IEDM) (www.ieee-iedm.org), the world’s showcase for the most important applied research breakthroughs in transistors and electronics technology.
The FD-SOI papers featured high performance, low leakage, ultra-low power (0.4V),  excellent variability, reliability and scalability down to the 10 nm node using thin SOI and thin BOX substrate. Performance boosters using high mobility materials such as thin strain Si, Ge, and III-V on-Insulator were also presented.
Brief summaries of the FD-SOI papers, culled from the Advance Program (and some of the actual papers) follow.
9.2 High Performance UTBB FDSOI Devices Featuring 20nm Gate Length for 14nm Node and Beyond (STMicroelectronics, Leti, IBM, Renesas, Soitec, GlobalFoundries)
This was the big paper reporting on ST’s flavor of high-performance FD-SOI (UTBB, which stands for ultra-thin-body-and-box) with 20nm gatelength, which target the 14nm node. In addition to excellent results, the paper demonstrated that  “…FD-SOI reliability is superior to Bulk devices.”
ST_IEDM13table1
[8] C. Auth, et al, VLSI, p.131, 2012 [9] C.-H. Jan, et al, IEDM, p.44, 2012

Specifically, the alliance reports, for the first time, on high performance UTBB FD-SOI devices with a gate length (LG) of 20nm and BOX thickness (TBOX) of 25nm, featuring dual channel FETs (Si channel NFET and compressively strained SiGe channel PFET). Competitive effective current (Ieff) reaches 630μA/μm and 670μA/μm for NFET and PFET, respectively, at off current (Ioff) of 100nA/μm and Vdd of 0.9V.
Excellent electrostatics are obtained, demonstrating the scalability of these devices to14nm and beyond. Very low AVt (1.3mV•μm) of channel SiGe (cSiGe) PFET devices is reported for the first time. BTI was improved >20% vs a comparable bulk device. The paper concludes with evidence of continued scalability to 10nm 
ST_IEDM13_Fig4
and below.
The effective current (Ieff), as a function of Ioff, is shown in Fig. 4. At Vdd=0.9V, NFET/PFET Ieff reach 630/670μA/μm at Ioff=100nA/μm, respectively. They are the best performing FDSOI CMOS devices reported so far, featuring non-strained Si channel NFET and strained SiGe channel PFET.”
7.3 Innovative ESD protections for UTBB FD-SOI Technology (STMicroelectronics, IMEP-LAHC)
ESD (electrostatic discharge) protection is often cited as a challenge in FD-SOI, and the ESD devices are typically put into a “hybrid” section of the chip, where the top silicon and insulator are etched away exposing the “bulk” silicon base wafer. In this paper, however, the ST-IMEP team presented FD-SOI ESD protection devices that achieve “remarkable performance in terms of leakage current and triggering control.” They demonstrate “ultra-low leakage current below 0.1 pA/μm and adjustable triggering (1.1V < Vt1 < 2.6V) capability. These devices rely on gate-controlled injection barriers and match the 28nm UTBB-FDSOI ESD design window by triggering before the nominal breakdown voltage of digital core MOS transistors.”

7.4 Comparison of Self-Heating Effect (SHE) in Short-Channel Bulk and Ultra-Thin BOX SOI MOSFETs: Impacts of Doped Well, Ambient Temperature, and SOI/BOX Thicknesses on SHE (Keio University, AIST)
This paper refutes those who say that the self-heating effect (SHE) is a bigger concern for SOI-based devices than bulk. The researchers investigated and compared bulk and SOI FETs including 6-nm ultra-thin (UT) BOX devices. They clarified, for the first time, that SHE is not negligible in bulk FETs, mainly due  to a decrease in the thermal conductivity of the more heavily doped well.  They found that the channel temperature of 6-nm UT BOX SOI FETs is close to that of bulk FETs at a chip temperature under operations. They then proposed a thermal-aware FD-SOI device design structure based on evaluated BOX/SOI thickness dependences of SHE. They concluded that SHEs in UTBB FETs with raised S/D and/or contact pitch scaling could be comparable to bulk FETs in deeply scaled nodes.

20.3 Gate-Last Integration on Planar FDSOI MOSFET: Impact of Mechanical Boosters and Channel Orientations  (Leti, ST)
This paper presents the industry’s first “gate last” (GL) results for FD-SOI, with ultra-thin silicon body (3-5nm) and BOX (25nm).  The team successfully fabricated transistors down to the 15nm gate length, with metal-last on high-k first (TiN/HfSiON). They thoroughly characterized the gate stack (reliability, work-function tuning on Equivalent Oxide Thickness EOT=0.85nm) and transport (hole mobility, Raccess) for different surface and channel orientations. They report excellent Ion, p=1020μA/μm at Ioff, p=100nA/μm at Vdd=0.9V supply voltage for <110> pMOS channel on (001) surface with in-situ boron doped SiGe Raised Source and Drain (RSD) and compressive CESL. They cite the high efficiency of the strain transfer into the ultra-thin channel (-1.5%), as evidenced by physical strain measurements by dark field holography.

12.4 UTSOI2: A Complete Physical Compact Model for UTBB and Independent Double Gate MOSFETs (ST, Leti)
Compact models of transistors and other elementary devices are used to predict the behavior of a design. As such, they are embedded in simulations like SPICE that designers run before actual manufacturing. In this paper, ST and Leti researchers presented a complete physical compact model called UTSOI2, which is dedicated to Ultra-Thin Body and Box FD-SOI technology, and is able to describe accurately independent double gate operation for sub-20nm nodes. It meets standard Quality and Robustness tests for circuit design applications.
12.5 Mobility in High-K Metal Gate UTBB-FDSOI Devices: From NEGF to TCAD Perspectives (Invited) (ST, Leti, U. Udine, Synopsys, Laboratoire Hubert Curien & Institut d’Optique, IBM)
This paper reviews important theoretical and experimental aspects of both electrostatics and channel mobility in High-K Metal Gate UTBB-FDSOI MOSFETs. With an eye toward optimization, the team presents a simulation chain, including advanced quantum solvers, and semi-empirical Technology Computer Assisted Design (TCAD) tools.

33.2 Suppression of Die-to-Die Delay Variability of Silicon on Thin Buried Oxide (SOTB) CMOS Circuits by Balanced P/N Drivability Control with Back-Bias for Ultralow-Voltage (0.4 V) Operation (LEAP, U. Tokyo)
SOTB is what Hitachi calls its flavor of FD-SOI.  The researchers point out that small-variability transistors like SOTB are effective for reducing the operation voltage (Vdd). This paper proposes the balanced n/p drivability for reducing the die-to-die delay variation by back bias for various circuits. Excellent delay variability reduction by this n/p balanced control is demonstrated at ultra-low Vdd of 0.4 V.

2.8: Co-Integration of InGaAs n- and SiGe p-MOSFETs into Digital CMOS Circuits Using Hybrid Dual-Channel ETXOI Substrate (IBM)
ETSOI is IBM’s flavor of FD-SOI, and this paper is about FD-SOI devices using high mobility material for boosting performance. The presenters “demonstrate for the first time on the same wafer and on the same device level a dense co-integration of co-planar nano-scaled SiGe p-FETs and InGaAs n-FETs UTBB FETs. This result is based on hybrid substrates containing extremely-thin SiGe and InGaAs layers on insulators (ETXOI) using double bonding.” They showed a) that it could be done; b) it’s viable hybrid high-mobility dual-channel CMOS; c) it still supports back-biasing for Vt tuning.

5.2 Surface Roughness Limited Mobility Modeling in Ultra-Thin SOI and Quantum Well III-V MOSFETs  (DIEGM – U. Udine)
As with the IBM paper (2.8) above, this paper is about FD-SOI devices using high mobility material for boosting performance. The abstract explains, “This paper presents a new model for surface roughness mobility accounting for the wave-function oxide penetration and can naturally deal with Hetero-Structure. Calibration with experiments in Si MOSFETs results in a r.m.s. value of the SR spectrum in close agreement with AFM and TEM measurements.” The simulated μSR in III-V UTB MOSFETs shows a weaker degradation at small channel thickness (Tw) than predicted by the T6w law observed in UTB Si MOSFETs.
Please stay tuned for a subsequent ASN post that will cover the meeting’s SOI-FinFET, RF-SOI and advanced device papers.  (The papers themselves are typically available through the IEEE Xplore Digital Libary within a few months of the conference.)

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