Accepted Manuscript The effect of interface trapped charge on threshold voltage shift estimation for Gamma irradiated MOS device H. Jafari, S.A.H. Feghhi, S. Boorboor PII:
S1350-4487(14)00341-2
DOI:
10.1016/j.radmeas.2014.12.008
Reference:
RM 5342
To appear in:
Radiation Measurements
Received Date: 9 August 2014 Revised Date:
17 November 2014
Accepted Date: 15 December 2014
Please cite this article as: Jafari, H., Feghhi, S.A.H., Boorboor, S., The effect of interface trapped charge on threshold voltage shift estimation for Gamma irradiated MOS device, Radiation Measurements (2015), doi: 10.1016/j.radmeas.2014.12.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The effect of interface trapped charge on threshold voltage shift estimation for Gamma irradiated MOS device H. Jafari1) , S.A.H. Feghhi1)* , S. Boorboor1)
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1- Radiation Application Department, Shahid Beheshti University, Tehran, Iran Corresponding author: Seyed Amir Hossein Feghhi Email:
[email protected]
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Abstract
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The ionizing radiation affects the silicon-based electronic devices and leads to the build-up of trapped hole in the oxide and an increase in trap density at the Si-SiO2 interface. These defects cause degradation of device parameters such as threshold voltage shifts, sub-threshold swing and etc. Although the effect of oxide charges on the threshold voltage shift in MOS devices is dominant, accurate estimation requires for modeling of the interface trapped charge effect. In this work, the effect of interface trapped charge on threshold voltage shift in an N-channel MOS transistor device, irradiated at different total ionization doses, was estimated considering proton transport model in gate oxide. The analytical model calculations consider the time dependent buildup of trapped holes and interface trap charges. Furthermore, we used ATLAS as a numerical semiconductor simulation code which only allows modeling oxide trapped charge to calculate the variation of threshold voltage shift with TID without considering the interface trap. Computational results were compared with experimental results at several doses and gate biases. According to the experimental results, ATLAS overestimates threshold voltage shift within 30% approximately. The results based on considering proton transport mechanism in analytical model, showed significantly better agreement with experimental results for total dose level up to 2.87 krad at several different biases. Overall results indicated that considering interface trap charges modeling, substantially improves the accuracy of the threshold voltage shift estimation. Keywords: Interface trap charge, Threshold voltage shift, MOS device, TID
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1. Introduction The ionizing radiation can affect the electronic devices, such as CMOS and MOS technologies.
threshold swing and leakage current in such devices [1-4].
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Total ionizing dose (TID) leads to threshold voltage shifts, transconductance variations, sub-
The incorporation of total ionizing dose simulation capabilities into parametric analysis models is of great interest for designers of integrated circuits which are used in radiation environment
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such as space, radiology system, nuclear power plant, as the use of commercial deep-submicron technologies has greatly increased for these applications [5-8]. These models represent the
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connection between descriptions of basic TID effect mechanisms in devices and the practical art of IC design. For this purpose, the build-up of trapped hole in the oxide and increasing interface trap density at the Si-SiO2 interface must be considered. The typical model involves the hole trapping mechanisms as well as the formation of interface traps due to the release of proton by
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the two-stage hydrogen model [9-12].
MOS devices are susceptible to be damaged by ionizing radiation resulting from charge buildup in gate, field and SOI buried oxides. As ionizing radiation passes through the gate oxide, energy
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is transferred from high energy photons and charged particles to generate electron-hole pairs. The amount of energy deposited by ionizing radiation is referred to as total ionizing dose (TID)
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and is defined as the absorbed energy per unit mass of a material. Under bias electrons and holes generated in the gate oxide will transport to the anode and Si-SiO2 interface respectively. Following this process, some fractions will recombine and consequently reduce the initial density of the free charged carriers. A very short time window is available for initial recombination processes to occur, since the electron mobility is considerably more than hole mobility in SiO2 . Therefore, it is quickly removed from the oxide layer [11].
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A fraction of the electron-hole pairs is annihilated through either columnar or geminate recombination [1].A fraction of holes that escapes initial recombination, and is called the hole fractional yield (fy), will slowly travel towards the SiO2-Si interface resulting in long-term TID
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effects. Hole fractional yield is strongly dependent on the magnitude of the oxide electric field acting upon the generated charge pairs. A higher field will tend to rapidly separate electrons and the hole and therefore, suppress recombination. Another factor that determines fractional yield is
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the mean separation between the generated electron-hole pairs, which is inversely proportional to the electronic stopping power of the ionizing radiation, and is therefore a function of the incident
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particle type and energy [4]. Holes escaping prompt recombination undergo polaron hopping transport via shallow traps in the SiO2 [9]. A fraction of these transporting holes may fall into trapping sites primarily located near heterogeneous dielectric or Si-SiO2 interfaces, thereby forming fixed trapped positive charge [13].
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The hole trapping that occurs at defect sites is generally associated with oxygen vacancies in SiO2 creating oxide-trapped charge. In conventional gate oxides, the distribution of trapped holes is normally within a few nanometers of the Si-SiO2 interface [14]. The “de-passivation” of Pb
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centers at the Si-SiO2 interface have a key role in the formation of interface traps [9,15]. As represented in the two-stage model, protons (H+) are first released within the oxide when
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irradiated by ionizing radiation and then move towards the interface where they can react with the passivated dangling bond to form interface traps. Hence, hole capture and electron compensation at a positively charged hydrogenated defect can lead to proton release. The second stage represents the interaction between passivated Pb centers (PbH) and the protons that have reached the Si-SiO2 interface. The passivated Pb centers are dangling bonds that have been passivated by hydrogen during processing. This reaction will produce a dangling bond and a
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neutral hydrogen molecule [4,16-18].The schematic of charge trapping in SiO2 and at Si-SiO2
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interface for an MOS structure is shown in Fig. 1.
Fig. 1. Schematic of basic radiation-induced processes for an MOS structure
The trapped charge will affect the threshold voltage and degrade the channel mobility.
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Neutralization of oxide trapped charge by electron tunneling from the silicon and by thermal emission can take place over long periods of time. Neutralization of interface-trapped charge is not observed at room temperature [19].
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In the present work, we investigated the impact of proton transport modeling for estimating interface trap effects on the threshold voltage shift of gamma irradiated MOS device. Although
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the gate bias affects the sensitivity of transistors as well as oxide layer thickness and oxide growth method, the results were obtained in three different gate biases. The drain current to gate voltage transfer characteristic of transistors was utilized in linear region to extract threshold voltage. The model was validated by experimental test. Furthermore, the parameterization was carried out based on the comparison with experimental data of radiation-induced degradation parameters for gamma irradiated N-channel MOS transistors belonging to CD4007 device. Moreover, In order to express the importance of proton transport in the formation of interface 4
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traps and estimation of their influences on threshold voltage shift, we used ATLAS as a
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numerical semiconductor simulation code which only allows modeling oxide trapped charge.
2. Experimental details
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Experimental data was obtained from irradiation experiments performed on N-channel MOS transistors belonging to CD4007 device at room temperature. This data allows validating the
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radiation-induced effects model against parameter such as threshold voltage.
CD4007 integrated circuit device includes three N-channel and three P-channel MOS transistors. This device is one of the most widely used components in the electronic applications. The gate oxide thickness of its N-channel transistor is 120 nm which represents relatively thick oxide layer and increases the sensitivity of transistor to TID effect [20].
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The devices under test were irradiated by a Co-60 gamma source at a dose rate of approximately 0.2 rad/s (in SiO2). The threshold voltages were measured immediately after irradiation at
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several different total doses up to 2.87 krad. The N-channel transistors were biased at three different gate voltages (Vg=0V, Vg=+3.3V and Vg=+9V). Irradiation was carried out at
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temperatures between 17-18 °C. The gate voltage was set by the HAMEG 4030 power supply and the current was measured employing a Keithley 485 picoammeter. The results were obtained by averaging the parameters which were measured for nine transistors in each dose level and bias condition.
Extrapolation in the Linear Region method (ELR) was used as an accurate and promising method to extract transistor threshold voltage in linear operating region of the drain current versus gate voltage characteristics of device [21,22]. Transistors Id-Vg characteristic was measured before 5
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and after irradiation. The voltage of drain–source electrodes was set to 10 mV for measurement in linear operating region.
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3. ATLAS simulation
Radiation-induced degradation can be computed numerically using semiconductor device simulators. However, many simulation codes have been developed to predict radiation damage in
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semiconductor devices accurately. ATLAS is one of the physically-based device simulators, which can predict the electrical characteristics associated with specified physical structures and
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bias conditions [23]. It solves Poisson’s equation, the carrier continuity equations, and the lattice heat equation. Steady state, transient, AC-small signal and optical device simulation can be performed. Although this code calculates the rate of hole trapping in SiO2, it encounters problem for accurate evaluation of interface trap charges because it has not the ability of proton
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transporting.
The N-channel transistor of CD4007 was simulated with uniform acceptor doping of 5×1016 cm-3 effective channel width of W ~ 350 µm and gate length of L ~ 10 µm using ATLAS electronic
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simulation code. The devices have an aluminum gate contact with work function of 4.1 eV. The device response from this structure under drift and diffusion equations was obtained in ATLAS
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code. The cross-sectional view of this simulated device is shown in Fig. 2.
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Fig. 2. Cross-sectional view of simulated device in ATLAS To study the effects of total ionizing dose on voltage threshold shift, a donor-like trap as uniform distribution in oxide has been considered by TRAP statement. A donor-like trap is positively
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charged (ionized) when empty and neutral when filled (with an electron). An empty donor-type trap, which is positive, can capture an electron or emit a hole. A filled donor-type trap, which is neutral, can emit an electron or capture a hole. The donor-like traps usually lie near the valence
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band. The position of the trap is defined relative to the conduction or valence bands using
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E.LEVEL parameter. The ionized density depends upon the trap density, DENSITY, and its probability of ionization. The probability of ionization assumes that the capture cross sections are constant for all energies in a given band and follows the analysis developed by Simmons and Taylor [24].
Furthermore, the electron-hole pairs can be created in the device using BEAM statement. An option exists in the code to define the photo-generation rate. A C-INTERPRETER function
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written into a text file can be supplied to the program using the F.RADIATE parameter of the BEAM statement. The MOS parameter of the MODELS statement has been applied for simulated structure which
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configures a basic set of mobility, recombination, carrier statistics, and tunneling models. This parameter enables Shockley-Read-Hall (SRH), Fermi Statistics (FERMI), and the Lombardi
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Mobility model (CVT) for transverse field dependence.
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4. Modeling Approach
The numerical calculation was carried out based on the time dependent buildup of trapped holes and interface traps in gate oxide of CD4007 N-MOS transistors. This analytical model was developed using general equations that describe the generation, transport and trapping of holes and electron as well as the reaction of holes with hydrogenated defects (DH centers) resulting in
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the release of protons and the subsequent formation of interface traps [12]. The radiation-induced degradation on the Id-Vg characteristics of transistors has been modeled using a surface potential (ψs) based approach. In this approach, the effects of trapped hole
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density and interface trap density on surface potential are modeled through an implicit equation
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for surface potential. This equation can be solved numerically as a function of bias and the defect parameters induced by radiation [25]. The surface potential equation (SPE) based on Gauss’ theorem of electrostatics is described by equation 1[26].
(V g − Φ MS +
Qo Q I + −ψ s ) 2 = γ 2V t H ( βψ s ) C ox C ox
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(1)
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Where Vg and ФMS are gate voltage and gate-to-semiconductor work function difference respectively. V t = k bT is the thermal voltage in which kb is the Boltzmann constant, q is the q
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magnitude of the electrical charge and T is absolute temperature. The interface trap charge QI = qNI(ψs) and oxide trapped charge Qo = qNo have induced charges at Si-SiO2 interface and in oxide layer respectively. Interface trap charge will be positive for ψs –φb since filled acceptor-like interface traps contribute to negative charge. Furthermore, β = 1/Vt and γ is the body factor
γ=
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which is given by equation 2.
2q ε s N a C ox
(2)
Where εs is the semiconductor permittivity, q is the electronic charge, Na is the doping
defined as equation 3[26].
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concentration, Cox is capacitance of oxide layer per unit area. Moreover, the function of H(βψs) is
H ( βψ s ) = e − βψ s + βψ s − 1 + e − β (2φb +φn ) (e βψ s − βψ s − 1)
(3)
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Where φn is the split in the quasi-Fermi potentials and φb = Vt.ln(Na/ni) is the bulk potential in which ni is intrinsic carrier concentration. In addition, as described before, the charge yield is
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dependent on the magnitude of the local electric field in the material and can be approximated with equation 4 [11,27].
E f y (E ) ≈ E + E0
(4)
Where E is the local field vector and E0 is threshold field constant (= 5.5×105 V/cm) [13].
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Qo and QI are obtained by solving the continuity equations for the mobile species (i.e., electron, hole and proton) at every time step while updating the surface potential. The hole transport in SiO2 is a dispersive process characterized by charge hopping between shallow defect sites in the
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oxide. However in most cases, carrier transport in SiO2 can be approximated with coupled continuity and drift-diffusion equations using effective mobilities obtained experimentally [1].
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The hole flux vector in a 1-D MOS system can be introduced as f p = f p ,x where f
p ,x
is real
∂f ∂p = − p + G + N TS σ p f p ∂t ∂x
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valued scalar. Therefore, equation 5 represents the general form of the 1-D hole continuity
(5)
where p, NTS and σp are the hole concentration (1/cm3) , the density of hole trapping sites (1/cm3) and the hole capture cross section for hole trapping sites (cm2) respectively. In addition, .
is the hole generation rate (1/cm3s) in which D is the dose rate (rad/s), go is the
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ɺ f G = Dg o y
generation conversion factor with units of (#ehp/cm3 rad), and fy is the yield function for holes [28]. By assuming steady state condition, the hole flux is achieved by integrating equation 5.
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Similarly, the electron flux term is approximated by electron continuity equation. Moreover, the
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electrostatic potential is obtained by solving Poisson’s equation. The calculation process for oxide trapped charges (electron-hole generation, recombination and hole transport) is approximately similar to corresponding to interface trap charges. In this work, the focus is on interface trap formation process. The first key reaction for the formation of interface traps occurs between transporting holes and DH centers leading to the release of a proton (H+). For the analytical model of interface trap density, the DH centers are assumed to be uniformly distributed inside the oxide. This reaction is 10
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coupled with proton transport and can be represented by the proton continuity equation represented by equation 6 [12].
∂t
=−
∂f H +
∂x
+ N DH σ DH f p
(6)
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∂n H +
Where NDH is the concentration of DH centers (1/cm3), σDH is the capture cross section for holes
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at DH centers (cm2) , nH+ is the proton concentration (1/cm3) and fp is the hole flux. The proton flux fH+ can be obtained by integrating equation 6 assuming steady state condition and ∆NI over
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discrete time intervals (∆t) is also obtained by equation 7.
.
∆N I = D g o f y ∆t ( N SiH − N I ) N DH σ DH σ i
.
x2 2
(7)
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Where NSiH, σi , go , fy , D and x are the density of passivated dangling bonds at the interface, the capture cross section for protons at the passivated dangling bonds, generation constant, the hole yield , radiation dose rate and the drift length for protons, respectively. The reaction between
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transporting protons and hydrogen-passivated dangling bonds at the Si-SiO2 interface is
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described as equation 8 [28].
SiH + H + → Si + + H 2
(8)
The variation of interface trap charge is obtained by iteratively solving equation (7) at discrete time steps while updating the surface potential and the oxide electric field. The Id-Vg characteristics are modeled analytically using an adapted form of the charge-sheet model (CSM) which includes the effects of No and NI through calculations of surface potential using the SPE
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[29]. Since the CSM makes the assumption that the inversion layer is of infinitesimal thickness, drain current (Id) can be expressed as equation 9 [22,30].
dψ s − µW dQ in −V t (Q in ) L dy dy
(9)
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Id ≅
Where µ is the electron mobility, y represents the lateral direction in the channel (i.e., from
[30].
Q in = −C ox (V g − Φ MS − ψ s − γ ψ s −V t )
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source to drain) and Qin is the inversion charge per unit area which is represented by equation 10
(10)
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Since the threshold voltage is obtained by extrapolation in linear operating region, a line was fitted to the maximum point of transconductance curve based on equation 9. The model parameters were obtained by fitting the experimentally achieved device characteristics. The analytical model parameters used for these calculations are summarized in
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table 1.
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Table. 1. Analytical model parameters for N-channel MOS transistor of CD4007 parameter value Units 14 NDH 1.5×10 cm-3 -10 σDH 4×10 cm2 10 NSiH 5×10 cm-3 σi 7×10-11 cm2 12 go 8.1×10 #ehp/cm3-rad
5. Results and discussion The MOS transistors were irradiated with total dose levels up to 2.87 krad by Co-60 gamma source in three different bias conditions. The drain current versus gate voltage characteristic for N-channel transistor of CD4007 before irradiation is shown in Fig. 3a. As illustrated, the results
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associated with CSM model, experimental data and ATLAS simulations are compared. In addition, the first derivatives of Id-Vg curves were calculated to obtain the transconductance at
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each gate voltage (Fig. 3b).
Fig. 3. a) Drain current and b) transconductance vs. gate voltage characteristics for N-channel MOS transistors derived from experiment, ATLAS simulation and analytical model The threshold voltages were extracted according to the described method (ELR) for N-channel transistors biased at Vg= 0 V, Vg= +3.3 V and Vg= +9 V during irradiation. 13
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The source of peak variations around 1.9 V can be described by differences in modeling of effective mobility. In a MOS transistor, electrons in the inversion layer flow near the semiconductor oxide interface. The electric field component, perpendicular to the direction of
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current flow, tends to accelerate the inversion layer electrons toward the interface subjected them to additional scattering. Now there is Coulomb scattering not only due to ionized impurity atoms, but also due to interface trapped charges, and to charges trapped within the oxide. The Coulomb
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scattering is the one of the mechanisms related to the electrically charged ionized impurity atoms which determines the value of the mobility. Additional scattering occurs due to surface
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roughness which tends to lower the mobility of electrons in the inversion layer to values smaller than the bulk mobility. Another scattering mechanism corresponds to the energy of lattice vibrations which is referred to as phonon scattering [22]. Owing to these mechanisms, the modeling of effective mobility becomes complicated. Therefore, the different ways of defining
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and estimating the electrons mobility in our model and ATLAS simulation lead to the peak variation in transconductance curve.
Fig. 4 shows a comparison between Id-Vg characteristics obtained using ATLAS device
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simulation and experimental data for several ionizing dose levels up to 2.87 krad (SiO2) under 9 V gate bias. The threshold voltage shifts (∆VTh) at different bias conditions during irradiation are
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listed in Table 2.
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Fig. 4. Comparison between Id-Vg characteristics obtained from ATLAS device simulation and experimental data for several ionizing dose levels under 9 V gate bias Table. 2. The threshold voltage shifts for CD4007 N-channel transistors at different bias conditions during irradiation Vg = 0V 6.98×101 1.36×102 2.54×102 4.67×102
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Vg = 9V 1.01×102 1.91×102 3.94×102 7.92×102
Experimental -∆VTh (mV) Vg = 3V 9.98×101 1.88×102 3.98×102 7.71×102
Vg = 9V 1.16×102 2.75×102 4.84×102 9.41×102
ATLAS simulation -∆VTh (mV) Vg = 3V Vg = 0V 1.31×102 8.96×101 2.85×102 1.67×102 2 4.80×10 2.66×102 2 8.87×10 5.42×102
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Total ionizing dose (rad) 359 718 1436 2872
It can be noticed that the negative threshold voltage shift increases with ionizing dose. As
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discussed earlier, both charges of oxide and interface contribute to the total threshold voltage shift. The maximum relative difference between the experimental and simulation results was about 30%. This difference is due to the fact that ATLAS does not include the variation of interface trap charges in calculation of threshold voltage shift. An analytical calculation of defect densities was executed for modeling the impact of radiation induced defects on the surface potential of each elementary transistor. As described before, the
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interface and the oxide trapped charges were obtained at every time step while updating the surface potential. The calculated surface potentials were inserted to the closed form expression for diffusion and drift current (equation 9) to compute the drain current. Therefore the degraded
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drain current versus gate voltage can be analytically modeled for various levels of radiation exposure. Fig. 5 reveals the response characteristic curves which are obtained from the analytical
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model followed with the experimental data before and after irradiation at gate bias of 9 V.
Fig. 5. Id-Vg characteristics for several ionizing dose levels at gate bias of 9 V for gamma irradiated N-channel MOS transistors derived from analytical model and experimental data
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As shown in Fig. 5, there is a reasonable agreement between experimental and analytical model prediction of Id-Vg characteristics. Fig. 6 shows a comparison between the experimental data and analytical calculations of threshold voltage shift with and without considering the interface trap charges in irradiated transistors at different gate biases.
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Fig. 6. The model calculation of threshold voltage shift with and without considering interface trap charges in comparison with the experimental data for several ionizing doses
It can be observed that the analytical model accurately predicts the voltage threshold shift of N-
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channel MOS transistors of CD4007 by considering the proton transport in calculations. The relative difference between experimental and model results at 9V gate bias and total dose of 2.87krad is 6.2%. However, without taking the interface trap charges into account, the
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differences between negative threshold voltage shift under gate biases of 9 V, 3 V and 0 at total
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dose level of 2.87 krad are 74 mV, 62 mV and 165 mV respectively. The individual contribution of oxide and interface charges to the threshold voltage shift for experimental data can be identified by using the charge separation technique [26]. In Fig. 7, the voltage shifts resulting from oxide trapped charges (∆VTh_O) and interface trapped charges at SiSiO2 interface (∆VTh_I) for several dose levels at 9 V gate bias are demonstrated. Moreover, these contributions were calculated by analytical model and compared with experimental data.
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Fig. 7. The Contribution of oxide and interface charges to threshold voltage shift of gamma irradiated N-channel MOS transistors for several doses at 9 V gate bias
As plotted in Fig. 7, the interface charges shift the threshold voltage towards positive voltages,
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while the oxide charges shift that towards more negative voltages. This interface negative charge responsible for reduction of the threshold voltage shift associated with positive charge in the gate oxide leading to roll over effect in threshold voltage. The threshold voltage shift for an irradiated
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transistor with ionizing dose of 2.87 krad was found to be −0.78 V for which the shift due to the
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oxide and interface charges is −0.86 V and 0.07 V respectively. As described earlier, the developed model is able to follow holes and protons until they are trapped in oxide or hydrogen containing defects at Si-SiO2 interface. Therefore, the changes in oxide charge density (∆NO) and interface charge density (∆NI) for irradiated N-channel MOS transistors of CD4007 were calculated. In addition, these charge densities were obtained from ∆VTh_O and ∆VTh_I for experimental data using equations 8, 9 [31]. Fig. 8 shows the variation of ∆NO and ∆NI in several dose levels for irradiated transistors.
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(8)
q ∆V Th _OC ox
q
(9)
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∆N O =
∆V Th _ I C ox
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∆N I =
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Fig. 8. The variation of oxide and interface charge density of N-channel MOS transistors (CD4007) for several ionizing doses
The ∆NI and ∆NO for 2.87 krad gamma irradiated N-channel MOS transistors were found to be 3.20×1010 cm−2 and 3.25×1011 cm−2 respectively, by model calculation. These estimations reveal the average relative difference of 8% with experimental data for interface charge density and 1.5% for oxide charge density in the same conditions. The calculated values of oxide and interface charge density for various doses of gamma rays are summarized in Table 3.
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Table. 3. The model results of threshold voltage shifts and trapped charge densities of gamma irradiated N-channel MOS transistors for several ionizing dose levels under 9 V gate bias ∆VTh(mV) -1.03×102 -2.10×102 -4.04×102 -7.87×102
∆VTh_O(mV) -1.17×102 -2.36×102 -4.51×102 -8.61×102
∆NO(cm-2) 3.85×1010 7.68×1010 1.53×1011 3.25×1011
∆VTh_I(mV) 1.39×101 2.62×101 4.65×101 7.43×101
∆NI(cm-2) 6.03×109 1.13×1010 2.00×1010 3.20×1010
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Total Ionizing Dose (rad) 359 718 1436 2872
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6. Conclusions
The exposure of silicon-based electronic devices to radiation leads to the build-up of trapped
degradation of device parameters.
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holes in the oxide and increasing the interface trap density at the Si-SiO2 interface causing
The estimation of interface trapped charges involves in the transport of protons and modeling the reaction between them and hydrogen-passivated dangling bonds at the Si-SiO2 interface. Since ATLAS numerical semiconductor simulation code is not able to follow the protons generating
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the interface trap charges, the difference between experimental and ATLAS simulation results in threshold voltage shift for N-channel MOS transistors of CD4007 reaches 30%. An analytical model based on general equations describing the generation, transport and trapping
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of holes and protons was employed in this study. This model accounts for contribution of
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interface trapped charges in total threshold voltage shift for irradiated transistors in several doses at different gate biases.
The analytical model results showed good agreement with experimental data for irradiated Nchannel MOS transistors by Co-60 gamma source (with the dose rate of 0.2 rad/s in SiO2) in several total doses and different biases. The results indicated that considering interface trapped charges significantly improves the accuracy of the threshold voltage shift estimation. Generally,
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Because of the predictive nature of these devices, a proper parameterization of the model allows
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extrapolating the calculations to higher doses for high level dose applications.
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[1] T. R. Oldham, “Analysis of damage in MOS devices in several radiation environments,” IEEE Transactions on Nuclear Science, vol. NS-31, no. 6, pp. 1236–1241, 1984.
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[2] A. P. G. Prakash, S. C. Ke, and K. Siddappa, “High-energy radiation effects on subthreshold characteristics, transconductance and mobility of n-channel MOSFETs,” Semiconductor Science and Technology, vol. 18, no. 12, pp. 1037–1042, 2003. [3] J. O. Attia, et al., "Effects of TID on transistor parameters of dc-dc converters," in Radiation and Its Effects on Components and Systems, 2007. RADECS 2007. 9th European Conference on, pp. 1-4, 2007. [4] T. P. Ma, P. V. Dressendorfer, Ionizing Radiation Effects in MOS Devices and Circuits, John Wiley & Sons, New York, NY, USA, 1989.
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[5] A.I. Chumakov, et al., “IC Space Radiation Effects Experimental Simulation and Estimation Methods,” Radiation Measurements, vol. 30. pp. 547-552, 1999 [6] H. J. Barnaby, et al., “Modeling ionizing radiation effects in solid state materials and CMOS devices,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 56, pp. 1870–1883, 2009.
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[8] J. E. Gover , and T. A. Fischer, “Radiation-hardened microelectronics for accelerators,” IEEE Transactions on Nuclear Science, vol. 35, no. 1, pp. 160–165, 1987. [9] J. R. Schwank, et al., "Radiation Effects in MOS Oxides," IEEE Transactions on Nuclear Science, vol. 55, 2008. [10] M. Murat, A. Akkerman, and J. Barak, "Spatial distribution of electron-hole pairs induced by electrons and protons in SiO2," IEEE Transactions on Nuclear Science, vol. 51, pp. 3211- 3218, 2004. [11] M. Murat, A. Akkerman, and J. Barak, "Charge Yield and Related Phenomena Induced by Ionizing Radiation in SiO2 Layers," IEEE Transactions on Nuclear Science, vol. 53, pp. 19731980, 2006. 21
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List of Figure Captions:
Fig. 1. Schematic of basic radiation-induced processes for an MOS structure Fig. 2. Cross-sectional view of simulated device in ATLAS
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Fig. 3. a) Drain current and b) transconductance vs. gate voltage characteristics for N-channel MOS transistors derived from experiment, ATLAS simulation and analytical model Fig. 4. Comparison between Id-Vg characteristics obtained from ATLAS device simulation and experimental data for several ionizing dose levels under 9 V gate bias
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Fig. 5. Id-Vg characteristics for several ionizing dose levels at gate bias of 9 V for gamma irradiated N-channel MOS transistors derived from analytical model and experimental data.
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Fig. 6. The model calculation of threshold voltage shift with and without considering interface trap charges in comparison with the experimental data for several ionizing doses Fig. 7. The Contribution of oxide and interface charges to threshold voltage shift of gamma irradiated N-channel MOS transistors for several doses at 9 V gate bias
List of Table Captions:
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Fig. 8. The variation of oxide and interface charge density of N-channel MOS transistors (CD4007) for several ionizing doses
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Table. 1. Analytical model parameters for N-channel MOS transistor of CD4007
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Table. 2. The threshold voltage shifts for CD4007 N-channel transistors at different bias conditions during irradiation Table. 3. The model results of threshold voltage shifts and trapped charge densities of gamma irradiated N-channel MOS transistors for several ionizing dose levels under 9 V gate bias
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Variation of interface trap charge due to TID affects on thereshold voltage shift. Time dependent buildup of trapped holes and interface trap charges were calculated. An analytical model considering proton transport have been used. ATLAS simulations codewas used to investigated the model preformance. Computational results were validated via comparison with theexperimental data.
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