Liu, B., Dong, Z. L., Hua, Y., Fu, C., Li, X., Tan, P. K., & Zhao, Y. (2018). Electron‑beamradiation induced degradation of silicon nitride and its impact to semiconductor failureanalysis by TEM. AIP Advances, 8(11), 115327‑. doi:10.1063/1.5051813
Electron-beam radiation induced degradation of siliconnitride and its impact to semiconductor failureanalysis by TEM
Binghai Liu,1 Zhi Li Dong,2,a Younan Hua,1 Chao Fu,1 Xiaomin Li,1Pik Kee Tan,3 and Yuzhe Zhao31Wintech Nano-Technology Service Pte Ltd, 117684, Singapore2School of Materials Science & Engineering, College of Engineering,Nangyang Technologic University, 639798, Singapore3Department of Product, Test and Failure Analysis, Globalfoundries Singapore Pte Ltd,738406, Singapore
(Received 13 August 2018; accepted 15 November 2018; published online 28 November 2018)
Since the early semiconductor development, silicon nitride (SixNy) by chemical vapour deposi-tion (CVD) techniques has been widely used for various process layers and for various purposes insemiconductor devices. Such silicon nitride materials, in amorphous state essentially, has intrinsicimpermeability to most impurities, which qualifies its primary use as a passivation layer, especially asa diffusion barrier to moisture and sodium.1 With its high dielectric constant of 6∼9, silicon nitride isan ideal dielectric material for capacitor devices such as metal-insulator-metal (MIM) capacitors. Byutilization of the large difference in etch rate of SiO2 and SixNy, SixNy can be used as etch hard maskfor oxide dielectric etch processes. By tuning the deposition processes and film composition, SixNy
can be used as tensile or compressive stress linear to enhance the carrier mobility of N-channel or P-channel MOSFET devices.2,3 Furthermore, silicon nitride has important applications in non-volatilememory (NVM) devices either in the floating gate-based or nitride-based charge-trapping memorydevices, as shown in Fig. 1.
aCorresponding author: [email protected], Tel: (+65)6790 6727, School of Materials Science & Engineering, College ofEngineering, Nangyang Technologic University, 639798 Singapore.
In both floating-gate based and nitride-based NVM devices, the nitride process layer by CVD isthe indispensable structure. In a typical silicon-oxide-nitride-oxide-silicon (SONOS) NVM device,nitride in the oxide-nitride-oxide (ONO) layers is used for charge storage with its intrinsic charg-ing trapping characteristics. While in a typical floating-gate based NVM device, ONO is used asthe inter-poly dielectrics for blocking any charge leakage during the programming and erase oper-ations. With the high dielectric constant of nitride, ONO helps to ensure reliable charge retentionin the floating gates and to provide good capacitive coupling of the control gate to the floating gateso as to lower the voltages needed for read, program and erase operations.4 Therefore, the ONOdielectric layer plays a crucial role in affecting the performance and the data retention reliabilityof NVM devices under the harsh programming and erasing conditions. With the continuous scalingdown of NVM devices, the thickness of ONO dielectric films is also reduced to maintain enoughstorage capacitance. As a consequence, the degradation of the ONO breakage-down characteristicsis becoming a critical problem for NVM memory devices. In terms of failure analysis of NVMdevices, many failure cases are associated with breakdown or degradation of ONO dielectric lay-ers, in which the physical characterization of ONO structure is required in order to understand ifthere is any abnormality associated with the ONO film stacks. Various studies have shown thattime-dependent dielectric breakdown (TDDB) characteristics of ONO dielectric film is stronglydependent on the quality and morphology of nitride film stacks, such as film roughness, thicknessuniformity.5–7
For the material characterization and physical failure analysis of the nanostructured ONOstructures in NVM devices, TEM is widely employed with its high spatial resolution and variousmicroanalysis techniques. However, as it is well known, for failure analysis by TEM, electron-beamradiation damage is always a concern, posing great challenges to those electron-beam sensitivematerials such as low k dielectric materials.8 Although silicon nitride is not so sensitive to electronradiation as low k dielectrics, the radiation damage to silicon nitride by high-dose electron beam hasbeen reported, such as radiation-induced hole drilling,9 the changes in luminescence behaviour10 andthe nature of chemical bonding.11
In this work, in order to understand the electron-beam sensitivity of nanostructured siliconnitride thin films in semiconductor devices, we present detailed electron-beam radiation studyon a typical ONO structure in a sub-65nm NVM device by in-situ TEM. As it is well known,for sub-65nm CMOS processes, the control of thermal budget during front-end-of-line processesis crucial to achieve desirable device performance, for which the thin film deposition normallyutilizes low-temperature CVD processes, including the nitride thin film process layers in theembedded NVM devices in this study. It was reported that thin-film materials deposited by low-temperature CVD processes possess different microstructure and properties compared with theircounterparts deposited at high temperatures, such as film density, refractive index, etch-rate, thechemical bonding formation and etc.12,13 All these apply to nitride thin film materials deposited bylow-temperature CVD processes, which in turn will impact their sensitivity to electron-beam radi-ation. Our results clearly indicated the relatively high electron-beam sensitivity of silicon nitride
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deposited by low-temperature CVD processes. High-dose electron radiation resulted in not only thedegradation of nitride layer with changes in both composition and microstructure, but also the sig-nificant O and N diffusion and segregation across the ONO structure. The work may give usefulinformation about the electron-beam sensitivity of silicon nitride materials, and the importance ofelectron-dose control during the TEM failure analysis of semiconductor devices with nitride processlayers.
II. EXPERIMENTAL PROCEDURES
The nanostructured ONO process layers in a sub-65nm embedded-NVM device (floating-gate)were used for the electron-beam radiation experimental study in this work. A die was cut froma short-looped wafer and was deprocessed to the contact process layer by mechanical polishingand reactive ion etch techniques. The thin TEM lamella was prepared by using FEI Helios 450Sfocus ion beam (FIB) at 30 kV by following standard TEM sample preparation procedures. TEManalysis was carried out by using JEOL JEM 2100F TEM (200 kV ) with a Gatan Quantum imagingfilter. Electron-beam radiation experiments were performed under different electron beam currents(2.11 nA and 10.31 nA) at 200 kV with a beam illumination area of around 300 nm in diameter.In-situ TEM study under constant electron dose rates was conducted by recording energy-filtered TEMmicrographs at 1∼2 mins interval at the same magnification and with the same electron illuminationarea.
III. RESULTS AND DISCUSSION
A. Electron-beam radiation induced microstructure change in ONO structure
Figure 2 showed time-dependent microstructure evolution of process layers in a typical floating-gate NVM cell under the electron-beam radiation with electron beam current of 2.11 nA at 200 kV.As shown in Fig. 2, electron radiation for 11.3 mins led to apparent microstructure changes in nitrideprocess layers, i.e. the thinning-down of nitride layer in ONO dielectric layers (marked by the yellowrectangle in Fig. 2) and the interface fusing between different nitride spacer layers (marked by bluerectangle in Fig. 2). Further electron radiation for around 18.3 mins resulted in the further thinning-down of the nitride thin layer in the ONO structure, and the formation of the white-contrastedbubble-like defects in the thick nitride spacer layer as indicated by the red arrows in Fig. 2(c).After electron radiation for 45 mins, the ONO dielectric layers showed the diffused contrast withbarely visible nitride layer, and the white-contrasted bubble-like defects became larger and formeda continuous porous layer in the middle of the nitride spacer layer as indicated by the red arrows inFig. 2(f). Fig. 3 showed the TEM micrographs of another NVM cell irradiated under a larger electronbeam current of 10.31 nA. Apparently, much faster degradation of nitride process layers was observedunder such a larger electron dose. A short beam illumination for just 2.7 mins directly resulted inthe contrast blurring of nitride layer in ONO dielectric layers and thinning-down of nitride layer, asindicated by Fig. 3(b). Bubble-like defects started to form after beam illumination for just around7 mins. Meanwhile the nitride layer in ONO further thinned down and the contrast in betweenONO process layers became further weakening, as indicated Fig. 3(c). After electron radiation for11.8 mins, the ONO layers were rarely distinguishable, and more and more bubble-like defects formedwith their size continuously increased in the middle of the nitride spacer, as shown in Fig. 3(d). Fig. 3(e)showed more clearly the layer of bubble-like defects formed along the middle of nitride spacer afterelectron radiation for 12.5 mins.
Figure 4 showed the radiation time dependence of the nitride layer thickness in ONO dielectriclayers. As seen from it, under the electron radiation with the beam currents of both 2.11nA and10.31nA, the nitride thinning-down speed showed approximate linear dependence on the electron-beam radiation time. Such a linear time dependence suggested that the nitride thinning-down is adiffusion-controlled process.14,15 The fitting of the experiment data revealed that the nitride thinning-down speeds were 0.7nm/min and 2.6 nm/min at 2.11 nA and 10.31 nA respectively. Considering thethickness of the TEM lamella was around 100 nm, diffusion rates of silicon nitride materials under
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FIG. 2. TEM micrographs of a typical floating-gate NVM cell after electron beam radiation for different period of time withbeam current of 2.11nA at 200 kV : (a) ∼0 min; (b) 11.3 mins; (c) 18.3 mins; (d) 23.3 mins; (e) 31.8 mins; (f) 45.4 mins.
electron radiation were around 4.2 x 10-11 cm2/sec and 1.6 x 10-10 cm2/sec under the beam currentsof 2.11 nA and 10.31 nA respectively. W. Orellana and et al16 had a detailed study on the diffusion-reaction mechanism of nitriding species in SiO2 materials, and reported that, for N diffusion in SiO2,the activation energy and D0 were around 0.6 eV and 8.6x10-4 cm2/sec respectively. Therefore, thediffusion rates of N species in SiO2 are around 2.8 x 10-14 cm2/sec, at room temperature (298 K),4.3 x 10-11 cm2/sec at 570 K and 1.6 x 10-10 cm2/sec at 640 K. In view of this, the electron-radiationinduced nitride diffusion rates at 2.11 nA and 10.31 nA corresponded to those at elevated temperaturesof 570K and 640K respectively if such nitride diffusion was a thermally driven process. We willfurther discuss the underlying diffusion mechanism associated with the electron-radiation effectslater.
Such fast electron-radiation induced nitride degradation or diffusion may give the artificial phys-ical signatures of ONO dielectric process layers. Without proper electron dose control during TEManalysis, the abnormal microstructure of ONO process layers could be resulted from high-dose elec-tron radiation, rather than the manufacturing processes. Therefore, to get authentic TEM failureanalysis results, electron dose control is necessary in order to minimize electron radiation damage tonitride materials as shown in Fig. 2 and Fig. 3.
B. The mechanism of electron-beam radiation damage to silicon nitride
As it is well recognized, the electron-beam radiation damage involves complex physical andchemical processes during the interaction between high energetic electron beam and materials.17–19
Generally there are three major radiation damage mechanisms, i.e. (i) elastic electron scattering
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FIG. 3. TEM micrographs of a typical floating-gate NVM cell after electron beam radiation for different period of time withbeam current of 10.31 nA at 200 kV : (a) ∼0 min; (b) 2.7 mins; (c) 7.5 mins; (d) 11.8 mins; (e) 12.5 mins.
induced knock-on damage which directly leads to atom displacement by highly energetic electrons,and inelastic electron scattering induced ionization damage which includes (ii) thermal damage and(iii) radiolysis damage. In the following, we will discuss if these three types of radiation damagecould occur to silicon nitride as observed in this study.
FIG. 4. The dependence of nitride layer thickness in ONO on the electron-beam illumination time under two different electronbeam current at 200kV (nitride thinning speed: 0.7nm/min at 2.11nA; 2.6nm/min at 10.31nA).
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1. The possibility of electron beam radiation induced elastic knock-on damage
Under the elastic collision model, the maximum knock-on energy (Emax) transferred to N and Siatoms can be estimated by Equation 1.20,21
Emax =4MiMaE0
(Mi + Ma)2, (1)
where Emax is the maximum energy transferred from electrons to a target atom, M i the mass of theincident electron, and Ma the mass of the target atom in the material, and E0 the energy of the incidentelectrons.
Based on Equation (1), the voltage dependent Emax was plotted in Fig. 5. At 200 kV, Emax are15.63 eV and 31.41 eV for Si and N atoms respectively, as shown in Fig. 5. It was reported that thethreshold displacement energies of Si and N in silicon nitride were around 11∼13 eV and 21∼27 eVfor Si and N atoms respectively.7 Therefore, highly energetic electron radiation at 200 kV can leadto the direct displacement of Si and N atoms from silicon nitride materials with the elastic head-oncollision by energetic electrons.2. The possibility of electron radiation induced thermal damage
For the inelastic electron scattering induced ionization damage, it involves significant energytransfer from the highly energetic incident electrons to the target atoms. The most of as-transferredenergy could be ended up as thermal heat within the specimen, leading to the local temperatureincrease, and thus thermal damage in the beam illuminated area.
Based on the model by Fisher and Jencic et al.,22,23 the electron beam radiation inducedtemperature rise in SixNy film can be estimated by Equation (2),
∆T =Iπκe
(∆Ed
)ln
br0
, (2)
where k is the thermal conductivity of the target materials, e the electron charge, b the radius ofthe heat sink, r0 beam radius, I the electron beam current and ∆E/d may be approximated to be thestopping power for electron, i.e. dE/dx, which can be calculated using Equation (3).
dEdx= 2πNar2
e meC2ρZA
1
β2
lnτ2(τ + 2)
2(J/me
C2)2
+ F(τ)
(3)
where Na is Avogadro’s number, re the electron radius, me the electron mass, c the light velocity invacuum, ρ the density of absorbing material, Z the average atomic number, A the average atomicmass of absorber, β = v/c where v is the electron velocity, τ= E/mec2 where E is the electron energy,J the mean electron excitation energy of the absorber, and F(τ) can be calculated by Equation (4)
F(τ)= 1 − β2 +τ2
8 − (2re + 1)ln2
(τ + 1)2. (4)
FIG. 5. The dependent of Emax , the maximum energy transferred to Si and N atoms on the acceleration voltage of electronbeam.
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In this work, with beam illumination diameter of 300 nm and Cu grid of 3 mm in diameter, wehave r0=150 nm, and b =1.5 mm. By assuming silicon nitride thin film has stoichiometric Si3N4
composition, it has density of ρ = 3.17 × 103 kg/m2, thermal conductivity of κ = 5 W ∙ k−1m−1,24
average atomic mass of A = 0.14 kg/mol, and the mean electron excitation of J = 117.2 eV.25 It shouldbe noted that, for amorphous silicon nitride thin film materials, their thermal conductivity was reportedto be in the range of 0.3∼10 W ∙ k−1m−1, dependent on various factors such as stoichiometry of Si andN, and CVD deposition processes.24 In this calculation, we used a medium value of κ = 5 W ∙ k−1m−1
for the estimation of possible temperature increment by electron radiation.Based on Equation (2)–(4), the dependence of temperature increment in Si3N4 on the electron
beam current was plotted in Fig. 6. The local temperature rise was only 2.0 ◦C and 9.8 ◦C when electronradiation was conducted with the beam current of 2.11nA and 10.31 nA respectively. Apparently, sucha low temperature rise definitely could not lead to any thermal damage to silicon nitride thin filmprocess layers. The results are in line with the studies reported for the different material systemssuch as CoFe,14 C,19 CoFeB,26 in which it was generally accepted that electron radiation inducedtemperature increment could be ignored.
3. The possibility of electron radiation induced radiolysis damage
The radiation-induced radiolysis damage also arises from significant energy transfer from inci-dent electrons due to inelastic electron scattering processes. Radiolysis damage is also called asradiolysis decomposition, which can lead to diverse physical and chemical changes in materials,depending on the composition and chemical bonding nature of the materials. It may occur in bothorganic and inorganic compounds, resulting in chemical bond dissociation, atomic displacement,loss of crystallinity, mass loss (sublimation of materials), enhanced material diffusion and phasetransformation.14,17–19
D. G. Howitt and et al had detailed study on the hole-drilling process in silicon nitride thin filmmaterial by highly focused electron beam radiation with TEM at different acceleration voltages.7
Their results indicated that hole-drilling in silicon nitride by electron beam radiation was primarilyascribed to direct atomic displacement due to electron knock-on damage. We assume similar keyrole of knock-on damage by highly energetic electron beam at 200 kV. Nevertheless, the inelasticionization induced radiolysis may be also contributed to the radiation damage to silicon nitride thinfilm in terms of the formation of interesting bubble-like defects, and N and O diffusion behaviour asobserved during radiation experiments. This will be further discussed in the following.
4. Electro-beam radiation enhanced material diffusion in the ONO structure
Based on above discussion, we excluded the possibility of electron radiation induced thermaldamage, and confirmed elastic knock-on damage may be the key contributor to radiation damageto silicon nitride process layers in NVM cells. However, as radiation experiment was performed inTEM mode in which electron radiation was essentially in parallel beam illumination mode. Therefore,atomic displacement by knock-on damage should be of isotropic nature and thus would occur globally
FIG. 6. The dependence of temperature rise in Si3N4 on the electron beam current.
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across the all nitride process layers in the beam illuminated area. Therefore, direct displacementknock-on damage cannot explain the continuous thinning-down of the nitride layer in ONO and theappearance of bubble-like defects in the middle of nitride spacer layers in Fig. 2 and Fig. 3.
As far as electron radiation induced fast diffusion of silicon nitride is concerned, the drivingforce was unlikely due to electron-beam radiation local temperature rise. As mentioned above, thehighest temperature was only around 9.8 ◦C with electron radiation under 10.31 nA. Therefore,thermal effects unlikely induced such a high diffusion rate of silicon nitride. We suspected that suchnitride diffusion-induced thinning-down and the formation of bubble-like defects arose from REDeffect under high-dose electron beam radiation, which was verified by the results shown in Fig. 7.At the beginning of electron radiation, ONO dielectric layers were well distinguished, and EELSmapping showed well defined oxide-nitride-oxide interfaces, as shown in Fig. 7(a). After electronradiation with the beam current of 10.31 nA for around 7.5mins, EELS mapping and line scan profilein Fig. 7(b) revealed the diffusion of N from nitride layer and segregated at the interfaces in betweenoxide and the ploy-Si control gate, and oxide and poly-Si floating gate. With electron radiation for11.8 mins, severe diffusion of both N and O occurred for which the ONO structure was completelydamaged. The nitride layer and two oxide layers were completely mixed each other, as shown by the
FIG. 7. Scanning TEM micrographs and EELS mapping analysis of one NVM cell at (a) ∼0min beam radiation; (b) around7.5min beam radiation; (c) 11.8min beam radiation under electron beam current of 10.31nA at 200kV.
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O and N EELS maps in Fig. 7(c). The results indicated that high-dose electron beam radiation ledto not only the knock-on damage but also fast diffusion of O and N atoms. Such atomic diffusionbehaviour well explained the linear time-dependent thinning-down of nitride in ONO as shown inFig. 3 and Fig. 4, which can be ascribed to the so-called radiation-enhanced diffusion (RED) effects,as widely reported in other materials systems.14,27,28
According to the RED theory, the enhanced diffusivity of impurities or host atoms in crystallineand amorphous materials is directly related to the concentration of active point defects (vacanciesand interstitials) generated by electron and ion beam radiation. As discussed above, electron beamradiation at 200 kV not only displaced Si and N atoms by knock-on effects but also induced strongionization effects for which the rapture of chemical bonding in between Si and N occurred. With suchboth knock-on and ionization damage, lots of point defects were generated, which acted as activediffusion carriers, to promote the diffusion of Si, N and O atoms, leading to continuous thinning-downof nitride layer in the ONO structure.
As far as the formation of bubble-like defects in the middle of nitride spacer is concerned,it was associated with the mass loss induced by electron radiation. With Scanning TEM (STEM)imaging at a high magnification, it was found there was additional layer formed along the nitridespacer after electron radiation for 11.8mins, overlapping with the oxide layer outside of the nitridespacer, as indicated by the red arrows in Fig. 8(b). EELS mapping and spectroscopy analysis (in thearea marked by yellow-dotted square) revealed that this newly formed layer was N-rich (Fig. 8(c)),which implied the diffusion of N atoms from the nitride spacer with electron radiation. In addition,it is interesting noted that the bubble-like defect region formed in the middle of the spacer was Ndeficient, indicating the mass loss of N element after electron radiation for 11.8mins, as shown inFig. 9.
Above results indicated that high-dose electron radiation at 200kV led to not only atomic diffusionbut also the mass loss of N element. Such phenomena might be attributed to the relatively largethickness of the nitride spacer. On the one hand, electron radiation induced RED effects promotedthe atomic diffusion of N atoms to the both sides of the nitride spacer. On the other hand, for thoseN atoms in the middle of nitride spacer knocked out by electron radiation, they could not timelydiffuse out due to large thickness of nitride spacer. Instead they condensed together and diffusedto the surface and finally evaporated out, leading to the N loss, which in turn led to formation ofbubble-like defect. Such mass loss process is similar to halogen loss in the halide materials with theformation of double halogen ions (H-centers) by electron radiation.17–19
FIG. 8. Scanning TEM micrographs of nitride spacer of one NVM cell (a) at ∼0min beam radiation; (b) at around 11.8 minbeam radiation; (c) EELS mapping in the nitride spacer region after electron radiation for 11.8 min with electron beam currentof 10.31nA at 200kV.
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FIG. 9. EELS mapping in the nitride spacer region after electron radiation for 11.8 min with electron beam current of 10.31nAat 200kV.
IV. CONCLUSION
In this work, we studied the impacts pf electron-beam radiation on silicon nitride thin filmprocess layers in a typical NVM cell structure. The results indicated that high-dose electron radiationcan result in fast radiation damage to silicon nitride thin film process layers with the modificationof both microstructure and chemical composition of silicon nitride. The electron radiation-inducedcontinuous thinning-down of silicon nitride process layers and the formation of bubble-like defectivemicrostructure were ascribed to the atomic displacement and dissociation of Si-N bonds as well asRED effects with the formation bountiful of defects as diffusion carriers by both radiation-inducedknock-on or ionization damage mechanisms.
In terms of TEM failure analysis of semiconductor devices with nitride thin-film process layers,we need to be cautious about the processes of physical failure analysis by TEM and the interpre-tation of the failure analysis results related to silicon nitride. Otherwise, electron radiation inducedmicrostructure and phase changes could give some misleading results related to some critical semi-conductor structures such as ONO as discussed above. To get authentic FA results associated withsilicon nitride materials, we should take corresponding measures by using low-dose TEM technique.This is crucial for catching the authentic physical signatures of nitride process layers that are reallycorrelated with the issues arising from the drift of semiconductor manufacturing processes by whichwe can understand the root cause of the electrical and reliability failure of the semiconductor devices.
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