Threshold Voltage Tuning of Metal-Gate MOSFETs Using an Excimer Laser

This technique enables tailored threshold voltage on a device-to-device basis for greater design flexibility.

June 1, 2010

Office of Naval Research, Arlington, Virginia

This work presents a localized method for tuning the threshold voltages (V_t) of multilayer metal-gate metal-oxide-semiconductor field-effect transistor (MOSFET) devices with a spatial area theoretically limited by the wavelength of the laser beam. This technique allows an independent means to tailor threshold voltage on a device-to-device basis that provides greater design flexibility. This maskless technique allows tailoring of thresholds by tuning the work function of the gate by intermixing titanium and titanium nitride using a laser pulse. The source and drains of the MOSFET are simultaneously annealed by the laser.

The process for tuning the threshold voltage occurs after the metal-gate stack is formed via physical vapor deposition (PVD), and the source and drain of the MOSFET have undergone ion implantation. At this point, the device is irradiated with a series of laser pulses to anneal the source and drain regions, while the majority of the laser energy impinging on the gate region is reflected off the metal gate. The substrate is preferably silicon-on-insulator (SOI) to facilitate higher temperatures near the gate interface in comparison to bulk Si.

A self-aligned process is used to fabricate metal-gate MOSFET devices on a single SOI wafer formed by Separation-by-Ion-Implantation-of-OXide (SIMOX). The multilayer metal gate of the self-aligned MOSFET was formed by PVD of 10-nm titanium (Ti), followed by 50 nm of titanium nitride (TiN) and 200-nm Al on top of a 7.5-nm gate oxide formed by dry oxidation.

After source and drain implantation, the devices were irradiated with a 308-nm XeCl excimer laser at a fluence (energy density) of 415 mJ/cm2. Each laser pulse normally occurs on a time scale of 10 to 30 ns and activates the source and drain dopants, as well as initiating thermal mixing. The TiN layer intermixes with the bottom Ti layer as a result of N diffusion at 800 °C temperatures. This intermixing causes the work function of Ti (4.33 eV) to shift towards the work function of TiN (4.55 eV), directly increasing the threshold voltage. The laser-induced thermal activation of channel dopants also contributes to the V_t increase as a smaller second-order effect for a larger number of laser pulses.

The threshold voltage was obtained using the traditional linear extrapolation method at the point of maximum slope. A typical error of ±0.01 eV is assigned to each data point to account for discrepancies in the fabrication process. The threshold voltage exhibits a linear shift of 70 mV from 0.32 to 0.39 V over 20 laser pulses, which is controlled by the laser fluence and the number of pulses. A poly-Si gate transistor was fabricated for comparison. In this situation, the majority of the laser irradiation of the poly-Si gate is absorbed in the gate.

This work was done by R.P. Lu, A.D. Ramirez, B.W. Offord, and S.D. Russell of SPAWAR Systems Center Pacific for the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. ONR-0017