Ž .Applied Surface Science 147 1999 222–227

Limitation current in Si N rSiO stacked dielectric films3 4 2

Hiroyuki Tanaka )

LSI Production DiÕision, Oki Electric Industry, 550-1 Higashi-Asakawa, Hachioji, Tokyo 193-8550, Japan

Received 22 December 1998; accepted 24 January 1999

Abstract

The limitation current in Si N rSiO stacked dielectric films has been investigated systematically for both field3 4 2q q Ž .directions by using n and p polycrystalline Si poly-Si gate capacitors and by varying the bottom oxide thickness. It can

be explained by the combination of Poole–Frenkel current in nitride layer and Fowler–Nordheim current in bottom oxidelayer for all the cases. The trap depth for Poole–Frenkel current, the thickness of bottom oxide for Fowler–Nordheimcurrent, and the barrier heights at the interfaces are the factors to determine the limitation current. The trap depths of holesand electrons for Poole–Frenkel current in nitride layer are calculated to be 0.6 and 0.9 eV, respectively. The limitation

˚Ž .current with negative gate bias is thought to be Poole–Frenkel of holes and electrons both in nitride layer for thin 330 A˚Ž .and thick )30 A bottom oxides, respectively. On the other hand, the limitation current with positive gate bias is thought to

be Poole–Frenkel of holes in nitride layer and Fowler–Nordheim of electrons in bottom oxide layer for thin and thickbottom oxides, respectively. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 73.50.yh; 73.40.Qv

Keywords: LSI; Si N rSiO stacked dielectric films; Limitation current; Poole–Frenkel current; Fowler–Nordheim current; MNOS3 4 2

capacitor

1. Introduction

Nowadays, stacked dielectric films with Si N3 4

and SiO are widely used in LSIs, especially in2Ž .dynamic random access memories DRAMs and

flash memories. As devices are scaled down into asubmicrometer range, demand for the film qualityhas become higher-level. We reported the cause ofdegradation for time dependent dielectric breakdownŽ .TDDB characteristics, one of the main items for

) Tel.: q81-426-62-6696; Fax: q81-426-65-1960; E-mail:tanaka555@hac.oki.co.jp

w xthe film quality, in the previous paper 1 . One of theother factors for the quality is leak current in thestacked films. However, the mechanism of the leakcurrent in the stacked films is complex because ofdifferences of the current type in nitride and oxide

w xlayers, that is, Poole–Frenkel type in nitride 2–5and Fowler–Nordheim type in oxide. Several inves-tigations on the current in the stacked films havebeen described in the literature. Some of them insistthat recombination of holes from anode and electronsfrom cathode has an important role for the currentw x6–8 , but others contended that the current is due to

w xa mono-type-carrier through the stacked films 9–11 .These various explanations mainly come from the

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 99 00116-6

( )H. TanakarApplied Surface Science 147 1999 222–227 223

difference of experiences, that is, the difference ofŽthe sample thickness and the field direction positive

.or negative gate bias . There are little models whichcan explain the current in the stacked films for bothfield directions. The main purpose of the presentpaper is to clarify the limitation current in Si N r3 4

SiO stacked films for both field directions systemat-2q q Ž .ically. p and n polycrystalline silicon poly-Si ,

and bottom oxides with various types of thicknessesare prepared for the experiment.

2. Experimental

Poly-Si gate capacitors with Si N rSiO stacked3 4 2Ž . Ž .films MNOS capacitors were fabricated on Si 100

substrates. Both p- and n-type substrates were pre-pared so that the substrate is accumulated duringstress for either positive or negative gate bias. Vari-ous types of samples were prepared by varying the

˚bottom oxide thickness from 13 to 88 A. The oxide˚with 13 A was grown when the Si substrate was

inserted to the low-pressure chemical vapor deposi-Ž .tion LPCVD reactor for nitride layer, and oxides

with other thicknesses were fabricated by thermaloxidation. After bottom oxide layer formation, ni-

˚tride layer with a thickness of 89 A was deposited byconventional LPCVD with source gases of SiH Cl2 2

and NH . After the formation of the stacked films,3

poly-Si was deposited by LPCVD and phosphoruswas diffused from POCl to fabricate nq poly-Si3

gate. For some samples, BFq ion was implanted to2

the poly-Si followed by an activation annealing at9008C for 30 min in N ambient to fabricate pq

2

poly-Si gate. Finally, MNOS capacitors were formedby patterning each sample. The band diagram of theMNOS structure with nq poly-Si gate is shown inFig. 1.

3. Results and discussion

The relationship between current density and SiO2˚equivalent field for the samples with 13 A bottom

oxides is shown in Fig. 2. The samples are nq

poly-Sirp-type substrate with negative gate bias, pq

poly-Sirn-type substrate with positive gate bias, andnq poly-Sirn-type substrate with positive gate bias.It can be seen that pq poly-Sirn-type substratecapacitor with positive gate bias shows the high leakcurrent as same as nq poly-Sirp-type substrate ca-pacitor with negative gate bias. Only the leak currentof nq poly-Sirn-type substrate capacitor with posi-tive gate bias is low level. It indicates that thedifference between nq and pq poly-Sirn-type sub-strate is due to the difference of the barrier height ofthe anode, and is not due to the thin oxide layer with

˚13 A, which is thought to be the thickness of directtunneling for the carrier. It comes from the fact thatthe barrier height for hole injection is almost the

Ž . qsame 1.95 eV from Fig. 1 between p poly-Sirnitride layer interface with positive gate bias and

Fig. 1. Band diagram of the MNOS structure with nq poly-Si gate. Ec and Ev are conduction and valence band edge, respectively.

( )H. TanakarApplied Surface Science 147 1999 222–227224

Fig. 2. Relationship between current density, J, and SiO equiva-2˚lent field, Eox, for the samples with 13 A bottom oxides. The

samples are nq poly-Sirp-type substrate with negative gate bias,pq poly-Sirn-type substrate with positive gate bias, and nq

poly-Sirn-type substrate with positive gate bias.

p-type substraternitride layer interface with negativegate bias. This result suggests that the current innitride is limited by Poole–Frenkel of holes, notelectrons. Although it is said that the current withPoole–Frenkel type is bulk limited, it is revealedfrom this study that the electrode also works as alimitation factor in case of nitride layers.

Though the current in nitride is limited byPoole–Frenkel of holes for both electric field direc-tions as indicated above, electrons are also thought tocontribute to the current in the layer. To confirm it,

Ž .the mid gap voltage shifts Vmid during stress wereobserved. Here, Vmid is defined by the gate voltagewhen the surface potential is equal to the potentialdifference between the Fermi level and the intrinsicFermi level. Fig. 3 shows the relationship betweenVmid and stress time for positive and negative gate

q ˚bias for n poly-Si gate samples with 13 A bottomoxides. The stress was under same current condition

Ž .with initial field of 10 MVrcm SiO equivalent .2

Vmid shifted positive under positive gate bias andshifted negative under negative gate bias. It indicatesthe existence of trapped electrons and trapped holesboth near the Si substrate under positive and nega-tive gate bias, respectively, since the trapped charge

Fig. 3. Relationship between Vmid and stress time for positiveq ˚and negative gate bias for n poly-Si gate samples with 13 A

bottom oxides.

near the substrate contributes to the Vmid shift morethan the charge far from the substrate. This resultsuggests that the hole and electron current are domi-nant near the anode and cathode, respectively, andrecombined each other in nitride layer even in the

˚Ž .thin 89 A nitride films.Fig. 4 shows the relationship between current

density and SiO equivalent field for various bottom2

oxide thicknesses with negative gate bias. Here, the

Fig. 4. Relationship between current density, J, and SiO equiva-2

lent field, Eox, for various bottom oxide thicknesses with negativegate bias.

( )H. TanakarApplied Surface Science 147 1999 222–227 225

work function difference between nq poly-Si andp-type substrate is calculated to be 0.86 eV at 20.The leak current is reduced with increasing the bot-tom oxide thickness. Poole–Frenkel plot andFowler–Nordheim plot for the relationship in Fig. 4

Ž . Ž .are shown in Fig. 5 a and b , respectively. Here,the electric field is calculated by Si N equivalent3 4

Ž . Ž .Fig. 5. a Poole–Frenkel plot and b Fowler–Nordheim plot forthe relationship in Fig. 4. J, En, and Eox are current density,Si N equivalent field, and SiO equivalent field, respectively.3 4 2

Fig. 6. Relationship between current density, J, and bottom oxidethickness at 8 MVrcm of SiO equivalent field with negative gate2

bias for various measurement temperatures.

Ž .dielectric constant, e 57.0 for Poole–Frenkel plot,Ž .and by SiO equivalent e 53.9 for Fowler–2

Nordheim plot, since current transport is thought tobe Poole–Frenkel type in nitride layer, and Fowler–Nordheim type in bottom oxide layer. The linearityof Poole–Frenkel plot is higher than that ofFowler–Nordheim plot for all the samples, whichindicates that the current is limited by Poole–Frenkelin nitride layer despite the bottom oxide thickness.

Fig. 6 shows the relationship between currentdensity and bottom oxide thickness at 8 MVrcm ofSiO equivalent field with negative gate bias for2

various measurement temperatures. It can beobserved that the control current type is changed at

˚30 A. The temperature dependence indicates that thetrap depths for current transport of Poole–Frenkeltype are 0.6 eV and 0.9 eV for the thickness of

˚ ˚bottom oxide of less than 30 A and more than 30 A,respectively. From the results, the limitation currentunder negative gate bias is thought of as shown in

ŽFig. 7. When the bottom oxide is thin enough 330˚ .A , current is limited by Poole–Frenkel of holes innitride layer from the substrate, and the depth of thehole trap is 0.6 eV. The electrons injected from thegate electrode are recombined with holes in nitride

Ž Ž ..layer Fig. 7 a . When the bottom oxide is thick˚Ž .)30 A , holes from the substrate is blocked by the

bottom oxide. As a result, the current is limited byPoole–Frenkel of electrons in nitride layer from thegate electrode, and the depth of the electron trap is

Ž Ž ..0.9 eV Fig. 7 b . The electrons which are injected

( )H. TanakarApplied Surface Science 147 1999 222–227226

Ž .Fig. 7. Limitation current under negative gate bias with a; top˚ ˚Ž . Ž . Ž .thin 330 A bottom oxide and b; bottom thick )30 A

bottom oxide. Electrons and holes are denoted by e and h,respectively.

from the gate electrode and flowed in into the nitridelayer to the Si N rSiO interface, are thought to3 4 2

flow in the bottom oxide by Fowler–Nordheim tun-neling. Since the barrier height between Si N and3 4

SiO is 1.05 eV for electrons, Poole–Frenkel current2

of electrons in the nitride is thought to flow harderthan Fowler–Nordheim current of electrons in thebottom oxide. Therefore, the Poole–Frenkel currentof electrons in nitride layer controls in case of thickbottom oxide with negative gate bias. The difference

Žof the trap depth for holes and electrons 0.9 and 0.6.eV is thought to affect the limitation current in

nitride monolayer as described earlier.Fig. 8 shows the relationship between current

density and SiO equivalent field for various bottom2

oxide thicknesses with positive gate bias. The cha-˚racteristic of 95 A-thick oxide monolayer is also

plotted in the figure with dotted line. It can be seenthat the leak current of the stacked film changesfrom Poole–Frenkel type to Fowler–Nordheim typewith increasing the bottom oxide thickness, and al-most agrees with that of oxide monolayer. From theresults, the limitation current under positive gate biasis thought of as shown in Fig. 9. When the bottom

Fig. 8. Relationship between current density, J, and SiO equiva-2

lent field, Eox, for various bottom oxide thicknesses with positive˚gate bias. Dotted line indicates the characteristic of 95 A-thick

oxide monolayer.

oxide is thin, the current is limited by Poole–Frenkelof holes in nitride layer from the gate electrode. The

Ž .Fig. 9. Limitation current under positive gate bias with a; topŽ .thin bottom oxide and b; bottom thick bottom oxide. Electrons

and holes are denoted by e and h, respectively.

( )H. TanakarApplied Surface Science 147 1999 222–227 227

electrons injected from the substrate is recombinedŽ Ž ..with holes in nitride layer Fig. 9 a . When the

bottom oxide is thick, current is limited by Fowler–Nordheim of electrons in bottom oxide layer fromthe substrate as shown in Fig. 8, since the barrierheight of the cathode for electron injection is highŽ .3.1 eV . If the electrons flow in nitride layer byPoole–Frenkel, the current must be controlled byPoole–Frenkel of electrons and the leak currentshould be lower like in the case of negative gate biaswith thick bottom oxides. Therefore, the electrons,which are injected from the substrate and flowed ininto the bottom oxide by Fowler–Nordheim, arethought to recombine in nitride layer with holes

Ž Ž ..injected from the gate electrode Fig. 9 b . Since theholes which are injected from the anode and do notrecombine with electrons are thought to be blockedby the thick bottom oxide, they do not take part inthe limitation current.

The results indicated above prove that the limita-tion current in the Si N rSiO stacked dielectric3 4 2

films can be explained by the combination ofPoole–Frenkel current in nitride layer and Fowler–Nordheim current in bottom oxide layer for all thecases. The trap depth for Poole–Frenkel current, thethickness of bottom oxide for Fowler–Nordheim cur-rent, and the barrier heights at the interfaces are thefactors to determine the limitation current.

4. Conclusion

The limitation current in Si N rSiO stacked3 4 2

dielectric films has been studied systematically forboth field directions by using nq and pq poly-Sigate capacitors and by varying the bottom oxidethickness. For negative gate bias, the limitation cur-rent is as follows. When the bottom oxide is thin

˚Ž .enough 330 A , current is limited by Poole–Fren-kel of holes in nitride layer from the substrate, and

the depth of the hole trap is 0.6 eV. The electronsinjected from the gate electrode are recombined withholes in nitride layer. When the bottom oxide is thick

˚Ž .)30 A , holes from the substrate are blocked bythe bottom oxide. As a result, the current is limitedby Poole–Frenkel of electrons in nitride layer fromthe gate electrode, and the depth of the electron trapis 0.9 eV. On the other hand, the limitation currentfor positive gate bias is as follows. When the bottomoxide is thin, the current is limited by Poole–Frenkelof holes in nitride layer from the gate electrode, andthe electrons injected from the substrate are recom-bined with holes in nitride layer. When the bottomoxide is thick, current is limited by Fowler–Nordheimof electrons in bottom oxide layer from the substrate.

Acknowledgements

The author would like to thank Dr. N. Hirashitafor valuable discussions.

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