1364 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 3, JUNE 2002 High-Energy Ion Irradiation Effects on Thin Oxide p-Channel MOSFETs A. Candelori, Student Member, IEEE, D. Contarato, N. Bacchetta, D. Bisello, Member, IEEE, G. Hall, E. Noah, M. Raymond, and J. Wyss Abstract—P-channel MOSFETs of a commercial 0.25 m CMOS technology have been irradiated by high linear energy transfer (LET) iodine (I) and low LET silicon (Si) ions up to 300 Mrad(Si) and 500 Mrad(Si), respectively. Threshold voltage vari- ations ( ) up to 0.46 V and 0.44 V have been measured at the highest I and Si doses. Both oxide positive trapped charge ( ) and interface states ( ) contribute to with a ratio for high (low) LET ions. After 40 days at room temperature, most of the positive charge is recom- bined by electron tunneling from the oxide interfaces, while only a small amount (6%–16%) of interface states is annealed. A huge noise increase (higher for I ions) is observed after irradiation. Finally, radiation induced soft breakdown (radiation-induced leakage current) conduction through the gate oxide is generated by high (low) LET ions for V( 3.4 V). Index Terms—Ion radiation effects, MOSFETs, noise. I. INTRODUCTION T HE continuous scaling down of the contemporary CMOS technologies and the consequent decrease in the gate oxide thickness have shown their “intrinsic” radiation hardness. Oxide trapped charge and threshold voltage variations are drastically reduced by thinning the gate oxide thickness [1], suggesting their possible application in radiation environments. For instance, a commercial 0.25 m CMOS process will be used for the APV-25 readout electronics in the tracker of the compact muon solenoid (CMS) experiment at CERN, Switzerland, where doses up to 10 Mrad(Si) are expected [2]. The radiation tolerance of this technology has been tested up to 100 Mrad(SiO ) by 10 keV X-rays [3], [4]. Nevertheless, new reliability problems for radiation hardness appear by thinning the gate oxide thickness. The scientific com- munity has widely studied some of these issues focusing the at- tention on parasitic conduction through the gate oxide. Radia- tion exposure to ionizing radiation and, in particular, to high-en- ergy ions can activate radiation induced leakage current (RILC) [5]–[7], radiation induced soft breakdown (RSB) [7], [8] and radiation induced hard breakdown (RHB) [9]–[11]. The RILC, Manuscript received September 10, 2001; revised June 15, 2002. This work was supported by the Istituto Nazionale di Fisica Nucleare. A. Candelori, D. Contarato, N. Bacchetta, and D. Bisello are with the Istituto Nazionale di Fisica Nucleare, Sezione di Padova and Dipartimento di Fisica, Università di Padova, I-35100 Padova, Italy (e-mail: candelori@pd.infn.it; con- tarato@pd.infn.it; bacchetta@pd.infn.it; bisello@pd.infn.it). G. Hall, E. Noah, and M. Raymond are with Blackett Laboratory, Impe- rial College, London SW72BW, UK (e-mail: g.hall@ic.ac.uk; e.noah@ic.ac.uk; m.raymond@ic.ac.uk). J. Wyss is with the Dipartimento di Ingegneria, Università di Cassino, I-03043, Cassino (FR), Italy (e-mail: wyss@pd.infn.it). Publisher Item Identifier S 0018-9499(02)05828-8. TABLE I MOSFET AND PROCESS CHARACTERISTICS RSB, and RHB activation strongly depends on oxide thickness, bias condition during irradiation, linear energy transfer (LET) of incident radiation, and obviously on radiation dose. All these studies [5]–[11] were performed on MOS capacitors (which are the simplest test structures to investigate the conduction through gate oxides) for a wide range of radiation doses between 100 rad(Si) up to more than 100 Mrad(Si). On the other hand, less attention has been devoted to the study of how microscopic defects in the oxide and at the SiO /Si in- terface induced by high-energy ions at high doses can affect the MOSFET reliability from the macroscopic point of view of the device operation. In this contribution we address some of these issues by investigating radiation effects on the static and noise spectral density characteristics of p-channel MOSFETs from a commercial 0.25 m CMOS technology, induced by high-en- ergy Si and I ions featuring low (9.27 MeV cm /mg) and high (61.8 MeV cm /mg) LET values, respectively. High radiation doses from 10 Mrad(Si) up to more than a few 100 Mrad(Si) were delivered in order to: 1) point out possible satura- tion effects in such thin gate oxide (5.5 nm) devices and 2) allow the comparison with previous published data for MOSFETs of 0.25- m CMOS technologies irradiated by 10 keV X-rays and 8-MeV electrons [2]–[4]. After the description of devices, irradiations and experi- mental conditions (Section II), we will present the subthreshold characteristics (Section III), the threshold voltage variations ( ) and the contributions from oxide charge and interface states to both 20 min and 40 days after irradiation (Sec- tion IV). Finally, radiation effects on device transconductance (Section V), excess current through the gate oxide (Section VI), and noise spectral density properties (Section VII) will be discussed. II. DEVICES,IRRADIATIONS, AND EXPERIMENTAL CONDITIONS Tested devices are p-channel MOSFETs from a commercial 0.25- m CMOS technology. The typical threshold voltage of the devices before irradiation is V. The main de- vice characteristics are reported in Table I. Large channel width ( m) MOSFETs are typical for the preamplifier 0018-9499/02$17.00 © 2002 IEEE