by Laura Peters, contributing editor

October 27, 2010 – The theme of the upcoming International Electron Devices Meeting (IEDM, 12/6-8 in San Francisco, CA) is finding better ways to generate, transmit, use and save energy. One method of reducing power consumption is by optimizing devices for low power and multi-threshold voltage (V_t) operation. At the conference, a group of researchers will demonstrate low-V_t nMOS and pMOS workfunction-adjusted devices manufactured using TiN/TaAlN metal gates in ultrathin fully-depleted silicon-on-insulator (FDSOI) substrates. The researchers, from CEA-Leti Minatec (Grenoble, France), STMicroelectronics (Crolles, France), AIXTRON AG (Herzogenrath, Germany) and Soitec (Bernin, France), used ground planes under the SOI to expand the V_t capability and body bias boosts for power management. Transistor and SRAM bit cell testing showed neither the gate engineering nor the substrate introduced threshold voltage variability.

The group used 300mm ultrathin body and buried oxide substrates (SOI = 6nm, BOX = 10nm) with indium (p) or arsenic (n) implants to form the ground planes. For the low-V_t pMOS device, which is the most difficult to engineer because of flat-band voltage roll-off, various CVD TaAlN stacks were optimized for Al concentration to obtain tuned workfunction above midgap with good mobility and reliability. The TiN metal gate was tuned at 80mV below the midgap for low-V_t nMOS.

Figure 1. CMOS configurations to achieve four threshold voltages with two metal workfunctions and two ground plane types. (Source: CEA-Leti)

The multi-V_t scheme provides two V_tn and two V_tp devices in four CMOS device configurations (Figure 1) using ground planes of the same type (V_t = 0.32-0.6V). Low V_ts are achieved for TiN nMOS and TaAlN/TaN pMOS and vice-versa for high-V_ts (Figure 2). The pMOS devices performed at I_on = 500μA/μm and I_eff = 245μA/μm at I_off = 2nA/μm and V_dd = 0.9V. The researchers point out that the architecture is compatible with power management techniques such as reverse and forward back biasing for each threshold voltage configuration.
This work follows on from CEA-Leti’s announcement earlier this month that it has made available its 20nm FDSOI process to the design community, including high-k/metal gate (HKMG) processes.

Figure 2. Multiple threshold voltages result from the combinations shown in Figure 1. (Source: CEA-Leti)