Investigation of the Surface Defects in N-Channel MOS Transistors Under Long-Term Low-Dose-Rate Irradiation

Gamma-radiation is commonly used to study surface defects in MOS transistors. Early experiments show two stages of surface-defect formation in a MOS structure under low-intensity gamma irradiation (Popov & Vin, 2014; Popov, 2016). On the first stage the defect formation take place on interface Si-SiO2 from the oxide side. This process is described by an exponential dependence (Rashkeev et al., 2002). In the second stage “additional” surface defects are formed from the Si side. Radiation defects of silicon migrated to interface Si-SiO2 from the semiconductor. The goal of this paper is investigation of surface-defect formation in a MOS transistor using the changing of surface electron mobility.


Introduction
Gamma-radiation is commonly used to study surface defects in MOS transistors.Early experiments show two stages of surface-defect formation in a MOS structure under low-intensity gamma irradiation (Popov & Vin, 2014;Popov, 2016).On the first stage the defect formation take place on interface Si-SiO 2 from the oxide side.This process is described by an exponential dependence (Rashkeev et al., 2002).In the second stage "additional" surface defects are formed from the Si side.Radiation defects of silicon migrated to interface Si-SiO 2 from the semiconductor.
The goal of this paper is investigation of surface-defect formation in a MOS transistor using the changing of surface electron mobility.

Description of Experiment
MOS transistors with an n channel in CMOS integrated circuits CD4069UBCN were used in the experimental study.The current-voltage characteristics of the transistors were measured with an Agilent Technologies B1500A Analyzer of semiconductor devices.
The samples were irradiated with gamma photons from a Cs 137 source at dose rate of 1.0, 0.1 and 0.01 rad(Si)/s in the passive mode (with all pins of the integrated circuit short circuited during irradiation).In experiment 3 microcircuits in which characteristics of 3 transistors were measured were used.
The density of surface defects was determined by a measuring of slope of transconductance k of the transistors (Emel'anov, Zhukov, Loshkarev, & Meshurov, 1995).Average arithmetic values were calculated.As parameters of structure MOS transistor (W and L -width and length of the channel, and also gate capacitence per unit area C ox ) were unknown a relative changes of a slope of transconductance were analyzed where k (0) and k (D) -values of a of slope of transconductance, and µ s (0) and µ s (D) -values of surface mobility of carriers before and after an dose D irradiation, W and L -width and length of channel MOS transistor, C ox -gate capacitence per unit area.
From relative change of the slope of transconductance (1) resulted relative value of surface mobility µ s (0) / µ s (0) in channel of MOS transistor was determined.In Sexton and Schwank (1985) the model in which change of surface mobility µ s connected with surface defects density N it is offered.This model looks like where α ≈ 7•10 -13 cm 2 -parameter of model, ΔN it -change of surface defect density.

Experimental Results
Results of definition of relative change of surface mobility of electron are presented in Figure 1.As it is possible to see, it was observed practically identical dose dependences of surface mobility for different dose rate radiation.
Especially it is visible on an initial site on dependence [µ s (D)/µ s (0)] from D.  The received results of experiment allowed to estimate change of surface defect density on interface Si-SiO 2 .Using model ( 2), dependences of surface defect density on time of an irradiation which are showen in Figure 3 have been obtained.For first stage the exponential dependence (Rashkeev et al., 2002). (3) found and are listed in Table 1.In (3) it is ΔN it.s-saturation of density of surface defects and β -the parameter of model.
Subtracting from values of density of the surface defects received in experiment, the values designed with use (3), we receive density of "additional" surface defects  * .Change of these surface defects is shown on Figure 4.As it is possible to see, there is some "threshold" value of time during which there is a migration of defects from silicon to interface Si-SiO 2 .

Conclusion
Low-intensity gamma irradiation МОS transistor at low ionizing-radiation dose rate P allows to observe two stages surface-defect formation in structure Si-SiO 2 .At the first stage there is a formation of surface-defect formation at participation holes and the protons formed interface states from the side oxide.This process is well described by an exponential dependence, and the physical model of this process is stated, for example, in Rashkeev et al. (2002).At the second stage defects migrate on the part of silicon that increases their density ΔN it on interface Si-SiO 2 by side Si."Threshold" time of the beginning of the second stage is observed.The qualitative model of this process is described in (Popov, 2016).
Thus, both at Russian CMOS IC (Popov, 2016), and at the USA microcircuits, observed two stages surface-defect formation.At use dose dependences of surface-defect density ΔNit(D) these processes are difficultly distinct.

Figure 1 .
Figure 1.Change of relative surface mobility vs dose at dose rates 1.0, 0.1 and 0.01 rad(Si)/s

Figure 2 .
Figure 2. Change of relative surface mobility vs irradiation time at dose rates 1.0, 0.1 and 0.01 rad(Si)/s

Figure 3 .
Figure 3.The change of density of surface defect vs irradiation time at dose rates 1.0, 0.1 and 0.01 rad(Si)/s

Table 1 .
Parameters of exponential dependence in the first stage