New Energy-Dependent Soft X-Rav Damage In MOS Devices

New Energy- Dependent Soft X -Ray Damage in MOS DevicesTung -Yi Chan, Henry Gaw, Daniel SeligsonIntel Corporation, SC2 -253065 Bowers AvenueSanta Clara, CA 95051Lawrence Pan, Paul L. King, and Piero PianettaStanford Synchrotron Radiation LaboratorySLAC /SSRL Bin 69, P.O. Box 4349Stanford, CA 94305AbstractAn energy- dependent soft x- ray- induced device damage has been discovered in MOSdevices fabricated using standard CMOS process. MOS devices were irradiated bymonochromatic x -rays in energy range just above and below the silicon K -edge (1.84keV). Photons below the K -edge is found to create more damage in the oxide andoxide /silicon interface than photons above the K -edge. This energy- dependent damageeffect is believed to be due to charge traps generated during device fabrication. Itis found that data for both n- and p -type devices lie along a universal curve ifnormalized threshold voltage shifts are plotted against absorbed dose in the oxide.The threshold voltage shift saturates when the absorbed dose in the oxide exceeds1.4X105 mJ /cm3, corresponding to 6 Mrad in the oxide.Using isochronal anneals, the trapped charge damage is found to recover with anactivation energy of 0.38 eV. A discrete radiation - induced damage state appears inthe low frequency C -V curve in a temperature range from 175 0C to 325 °C.I. IntroductionX -ray lithography is likely to displace optical lithography for semiconductordevice fabrication with dimensions < 0.4 um. Since x -ray photons have energies muchhigher than the band gap of silicon dioxide (about 9 eV), oxide damage caused by x-ray irradiation may be a serious concern in the application of x -ray lithography. Theeffects of x -rays and other forms of ionizing radiation on MOS structures have beenextensively studied for years [1 -4]. However, little work has been done to examinethe effect of varying the incident x -ray energy [5].In this paper, synchrotron x -ray sources were used to study the effect of x-rayenergy on radiation - induced device damage. X -ray photons studied have energy justabove and just below the silicon K -edge (1.84 keV). An energy- dependent soft