Getting Ready For 15nm

By David Lammers
The trends towards vertical transistors, non-silicon channel materials, and resistive RAMs promise to hold center stage at the 2010 IEEE International Electron Devices Meeting (IEDM), set to begin Dec. 6 in San Francisco, Calif. (www.ieee-iedm.org)

Taiwan Semiconductor Manufacturing Co. (TSMC, Hsinchu, Taiwan) will present a 22/20nm technology platform based on a FinFET architecture. The TSMC paper describes a full CMOS technology, complete with silicon germanium stressors, high-k/metal gate, and dual-epitaxy technology. TSMC said it demonstrated a 0.1µm2 SRAM cell, which operated at a 0.45V operating voltage (Vmin) with a 90 mV noise margin.

While TSMC is expected to shift from today’s planar transistors to the vertical FinFET devices at the 14nm generation in the 2015 time frame, the IEDM 22/20nm paper demonstrates that the world’s leading foundry has the FinFET manufacturing challenges well in hand. TSMC used 193nm immersion lithography to achieve NMOS and PMOS drive currents of 1200/1100 µA/µm respectively, at off-currents of 100 nA/µm.

Fig. 1: TSMC will unveil a complete FinFET-based 22/20nm CMOS logic technology at IEDM 2010. Electron microscope images show a cross-section of the vertical fin’s sidewall.

While creating 20nm gate-length vertical transistors is “demanding,” due to parasitic capacitances and other challenges, an abstract of the TSMC paper said the FinFET architecture allows continued scaling with good electrostatic control of the channel. To accomplish its scaling goals, TSMC turned a series of process technology knobs, including embedded SiGe to strain the PMOS channel, stress memorization techniques in the NMOS devices, an optimized contact edge stop layer (CESL), dual work functions, and both epitaxial silicon and boron-doped e-SiGe in the source and drain regions. Compared with planar transistors, the TSMC paper will describe much (100x) improved leakage from the source and drain regions, critical for low-power mobile systems.

Intel and IQE Inc. researchers will describe their latest advances with a FinFET architecture based on an InGaAs quantum well technology. At the 2009 IEDM, Intel described a surface-channel InGaAs FinFET. The quantum well InGaAs FinFET features fins, which are 35nm-wide and smaller, 5nm gate-to-drain and gate-to-source separations, and a high-k gate dielectric.

Intel and its research partner have been developing quantum-well compound devices as successors to silicon CMOS. The paper to be presented at the 2010 IEDM takes the InGaAs technology from a planar to a FinFET architecture, which delivers much-improved control of the channel compared with the planar devices described at the previous meetings. Also, the paper describes a high-k dielectric with a Tox of 20.5 Angstroms and good interface properties.

An InGaAs MOSFET will be presented by a team led by the University of Tokyo. The device features a 3.5nm channel, the smallest such device to be described thus far. The dual-gate device was created on a silicon substrate using wafer bonding.

Memories taking resistive turn
On the memory front, researchers from Intel and Micron Technology have developed a 25nm multi-level cell (MLC) NAND memory technology, with a cell size of 0.0028 µm2 – the smallest transistor now in production. An air gap was introduced between word lines to control the word line-to-word line capacitance and cell-to-cell interference.

The MLC device uses only 30 to 40 electrons per level, which requires advancements in the insulating tunnel oxide and the inter-poly dielectric in order to confine the charges. The cell has an asymmetric design, with a word line half pitch of 24.5nm and a 28.5nm half pitch in the bit line direction, allowing for insertion of the control gate between the floating gates. The technology is used for 64-Gbit NAND memories.

The authors will describe how the Intel-Micron team dealt with dopant fluctuations, structural bending, and other challenges presented at such small dimensions.

Fig. 2: Researchers from Intel and Micron Technology will describe the 25nm 64Gbit multi-level cell (MLC) NAND technology. The image shows the select gate and contacts in the bit line direction.

Resistive RAMs (RRAMs), which use a voltage to alter the resistive state of metal-based compounds, have emerged as a path to higher-density non-volatile memories once NAND flash scaling reaches its limit. A functional transition-metal-oxide resistive memory (TMO-RRAM) developed at the National Nano Device Laboratories in Taiwan has a record 9nm half-pitch, with a programming current of less than 1 µA, which compares with about 20 mA for phase-change memories. The researchers controlled the device’s resistivity by changing the chemical composition of the tungsten-oxide layer. They postulate that the memory’s change in resistance is due to the controlled movement of oxygen ions, with a monotonically varying ratio of oxygen and tungsten atoms.

The Taiwan laboratory’s research team includes Chinming Hu, a professor at the University of California, Berkeley. In an abstract of the paper, they said the “unexpectedly low” 1 µA current required to set and reset the RRAM cell makes it a promising candidate for low-power non-volatile memories.

The reported progress with exploratory RRAMs comes amid concerns about power consumption with the phase-change RAMs (PC-RAMs), which use heat to change the resistive state of a chalcogenide material. At IEDM, a team from the IBM/Macronix PCRAM Joint Project will describe a previously unknown failure mechanism for phase-change memories, apparently related to electromigration stemming from the polarity of the operating current.

At the high current densities required to change the state of the chalcogenide material, the researchers found that hole-induced electromigration occurs when current polarity is reversed. The paper claims that the phenomenon causes voids at the interface between the phase-change material and the bottom electrodes, limiting their cycling endurance by four orders of magnitude. The team also will discuss countermeasures to deal with the effect.

IBM researchers also will describe their latest-generation SOI-based embedded DRAM (eDRAM), enhanced with a high-k/metal gate technology. Big Blue claims eDRAM delivers several advantages over SRAM for large on-chip caches, including higher density, better soft error rates, and lower power consumption. The performance rivals SRAM speeds, with the SOI eDRAM delivering a sub-1.5ns latency and 2ns cycle time.

The 32nm eDRAM uses a deep trench capacitor with 25 percent higher capacitance and much less resistance than conventional memory stacks based on SiON/poly gate stacks. IBM said it use of a high-k/metal gate technology to reduce leakage and control the threshold voltage of 40 mV. IBM created a 32 Mbit array from cells measuring 0.39 µm2. The eDRAM is 3-4x smaller than a comparable SRAM, enabling a much-higher density on-chip cache, the abstract of the paper said.