Charge generation in thin SiO2 polysilicon-gate MOS capacitors

0038-l101/87$3.00+ 0.00 Copyright 0 1987Pergamon Journals Ltd

Vol. 30, No. 8, pp. 829-834, 1987 Printed in Great Britain. All rights reserved

Solid-State Electronics

CHARGE GENERATION IN THIN SiO, POLYSILICON-GATE MOS CAPACITORS P. FAZAN, M. DUTOIT, C. MARTINT and M. ILEGEMS Institute for Microelectronics, (Received

Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland 17 July 1986; in revised form

18 November

1986)

AbstractXharge generation in the bulk of thin oxides (l(t-33 nm) during high-field (Fowler-Nordheim) stress is investigated on polysilicon-gate MOS capacitors using constant current injection and high frequency C-V measurements. Our data show that initially positive charges are generated in the bulk of the oxide, followed by negative ones. The mechanisms that govern the creation of these different charges are investigated. A qualitative physical model is presented. We propose that positive charges are created by electron detrapping. Negative charges are due to electron trapping at preexistent and possibly, newly created, trap sites in the region of the oxide where the injected electrons have low kinetic energy. This model helps us understand the wear-out and breakdown of thin oxides.

INTRODUCTION MOS (metal oxide semiconductor) very large scale integrated (VLSI) circuits require devices with very thin gate and tunnel oxides. The problems associated with charge flow (Fowler-Nordheim or hot electron injection) through such oxides are therefore of primary concern. During injection, charges are generated in the oxide and at its interfaces and eventually lead to dielectric breakdown. This represents an important source of failures. In order to minimize these problems, a good understanding of the physical phenomena involved in charge generation and wearout of thin oxides is necessary. In this work, we consider only polysilicon-gate devices because of their greater relevance to VLSI circuits. Most work to date on charge trapping in SiOZ has been performed on Al gate MOS capacitors (Ref.[l] and references cited therein). Yet, Al and polysilicon gate capacitors show quite different behavior. In the former, a large positive charge near the Si/SiOZ interface masks bulk charge generation. This is not the case in the latter. Some studies on polysilicon-gate capacitors show that three types of charges are generated in the oxide during Fowler-Nordheim (F-N) injection (bulk negative charges, bulk positive charges, interface charges)[2-4]. However, the position and the mechanisms producing these charges are still controversial[2-171. For example, the generation of positive charges is sometimes attributed to hole trapping. Electron-hole pairs are assumed to be created by impact ionisation in the SiO,[4,12] or by electrons leaving the oxide[3]. Other explanations deal with electron detrapping[9,14,15]. The position of these charges is

TPresent address: EM-Microelectronic Marin, Switzerland.

Marin,

2074

different depending on the explanation adopted for their generation. Note that this positive charge is located within the bulk of the oxide and is different from that observed in Al-gate devices[l,l4]. Likewise, the generation of negative charges is often assumed to consist of two processes: trapping of electrons in existing traps and the creation of new electron traps. The creation of new traps has been attributed to the breaking of strained or weak bonds by electron impact in the presence of a high electric field[8,10,13,17]. For some authors, the centroid of these charges is near the cathode[5,6,9,13]. For others, negative charges are uniformly distributed across the oxide[2,4,7] or trapped at both interfaces[ 11,161. In this paper, we study the charges generated in the bulk of the oxide using high frequency C-V measurements and constant current injection from the gate of the substrate. Measurements on samples with different oxide thicknesses (d,,) on n and p type substrates are presented. They can be explained with a qualitative model in which the type of charges generated depends on the kinetic energy of the injected electrons. According to these investigations, bulk positive charges are created some distance away from the injecting electrode (cathode) by impact ionisation of filled electron traps (detrapping). Negative bulk charges, resulting from two different mechanisms, electron trapping and, possibly, trap generation, are located near the cathode. In this region, conduction electrons have low kinetic energy. Interface charges are not studied in detail here but could easily be included in this model[l8]. The proposed model, which considers only the effects of injected electrons, explains the generation and the position of the charges created during F-N injection in a simple and plausible manner. It can also describe effects due to hot electron injection, and allows us to understand the phenomena responsible for oxide wear-out leading to dielectric breakdown.

829

830

P.

FA~AN

EXPERIMENTAL

Silicon gate MOS capacitors were manufactured at the Swiss Center for Electronics and Microtechnics (CSEM) on 3 inch diameter, (100) oriented n and p type wafers with a resistivity of 5-7&m. Openings were wet etched in a 680 nm thick field oxide. Thin gate oxides were grown in dry 0, at 900°C to l&33 nm. Polysilicon was then deposited by LPCVD at 620°C and doped at 1000°C using POCl,. Gates were defined by plasma etching. A 320 nm thick layer of LTO (pure and P-doped SiO,) was used for insulation. Contact holes were wet-etched. Magnetron sputtered Al/l% Si was deposited at room temperature. The gates were contacted over the field

oxide to avoid the deleterious effect of Al/polysilicon interdiffusion[ 191. A post metallisation anneal in forming gas at 430°C for 60min completed the process. For this study we used square capacitors with a thin oxide area of 2.56 x 10-4cm2. The experimental setup used to test and stress the devices consisted of an HP 4275A multifrequency LCR meter for high frequency C-V measurements, and an HP 4145A semiconductor parameter analyzer for constant current injections, both controlled by an HP 9836 computer. Most F-N injections were performed at a constant current level of lo-‘A/cm3, which corresponds to an average electric field in the oxide of about 10 MV/cm. This current density is comparable to those encountered in EEPROMs. High frequency C-V measurements are alternated with F-N injections to determine the flatband voltage V,, and its shift AVFB during injection. The shift A VFNin the voltage V,, necessary to drive a constant current through the capacitor is determined on a separate identical test capacitor. Measurements show that the interruptions of injection necessary to record the C-V curves only negligibly affect the charges generated in the oxide. Also, C-V measurements performed after a given charge Qinj has crossed the oxide yield the same flatband voltage whether Q,,, is injected in one pulse only or in several pulses alternated with C-V measurements. All measurements made in inversion were performed under the illumination provided by a microscope light to speed up the formation of an inversion layer.

et al.

grown on n type Si and for positive gate injection, Fig. 2 shows that VFN does not saturate up to breakdown, which occurs between 30 and 40 C/cm2. At high current densities, the slope of the VFNvs Q, curve increases again before breakdown. A similar behavior is observed for the opposite substrate type and gate polarity and for other oxide thicknesses. I

I

pas.

0.5

gate

n type

I

I

J = 10e3 A/cm’

f

Si

> f

0.0

f al P ;

-0.5

z

- 1.0

-I

0.5

I

I

neg.

gate

n type

I

I

J D 10-3A/cm2

SI

5 -

0.0

E yi 0 P 5 -“.5 > t - 1.0

33 I

I

1o-4

I

10-3 Injected

10-z charge

1

I

lo-’ (C/cm’

1 )

@I

RESULTS

The shift AV,, of the voltage VFN necessary to maintain a constant current through the oxide is shown in Fig. 1 as a function of the injected charge density Qinj for different oxide thicknesses d,,, for both n and p type substrates and for injections of both polarities. The substrate type has almost no, the polarity, little effect on AVFN. All curves go through a minimum (turn-around point) for an injected charge density around 1O-2 C/cm2, independent of oxide thickness. This turn-around was already mentioned in the literature[2,3,6]. For a 27 nm thick oxide

10-4

10-a Injected

10-z charge

lo-’ ( C/cm2

1 )

(0) Fig.

1.

Fowler-Nordheim voltage vs injected charge shift of the absolute value of the density for different oxide thicknesses: (a) positive gate, (b) negative gate, (c) effect of gate polarity and substrate type.

Charge generation in thin SiO, polysilicon-gate MOS capacitors I PCS.

n

I

To derive equations (2) and (4), we assumed that x, and x- are larger than the tunneling distance at the cathode. Indeed, the effect of possible charges within the tunneling distance is not well known[ 1,221 and is neglected here for the sake of simplicity. These relations would not be valid for very thin oxides, i.e. thinner than about 5 nm. For a positive bias on the gate, A Vsm = A ViB (Figs la and 3a). From eqns (1) and (2), we conclude that

I

gate

type

831

Si

pas. gate doxt

27 nm

n type

J - 10-3A/cm2 Si Pf

0

10

20

Injected

charge

30 ( C/cm2

40 d,,

)

- 9.8 nm

Fig. 2. Fowler-Nordheim voltage up to breakdown for two current densities. The flatband voltage shift deduced from high frequency C-V curves is presented in Fig. 3 for the same conditions. The occurrence of a minimum at a fixed position independent of d,, is again observed. Our measurements are not affected by substrate type but strongly depend on gate polarity. For positive gate injection, the turn-around point is reached for Q, ‘v 1O-3-lO-2 C/cm* compared to l-3 C/cm* for negative gate injection. It is interesting to note that for avalanche electron injection in similar, annealed, polysilicon-gate capacitors VFB steadily increases, without turn-around[20,21]. This behavior of polysilicon-gate capacitors is different from that of Al-gate ones, as shown in Ref.[20].

- 1.0

I

I

10-4

10-a

33

I

I

I

-1

10-z

lo-’

1

10’

Injected

charge

neg.gate n type

)

( C/cm2

J = 10e3A/cm2

‘I

Si d,,x = 9.8 nm

’

0.0

-

f uI-2.o 0

-

P = 0

DISCUSSION

The occurrence of a turn-around point for AVFN and AV,, implies that positive bulk charges are generated for low Qinj followed by negative bulk charges at higher Q,. The latter become dominant above lo-* C/cm*. The shifts of VFN and VFB can be written in terms of these charge densities and their locations in the following way[2]: Av;, = - (Q; x”+ - Qs- x” + qNkd,,,)lt,,

(1)

AL’;, = - (Q;x:

(2)

- QT_x~)/t,,

‘-4.0 0 : P % ii - 6.0

-

-

Injected

charge

( C/cm*

)

@)

for injection from the substrate (positive gate), and AV$, = - (QB, x”+ - Q5 x8_ + qNid,,)/t,, A Vh = -

(3)

IQ%(4, - x”c) - Q!. Wx- x!. )I/~,, (4)

for injection from the gate (negative gate). In these expressions, Q, and Q_ are the changes of the positive, respective negative bulk oxide charge densities per unit area, x, and x_, the distance between their respective centroids and the gate (Fig. 4). qN,, is the change in the interface charge. It is not possible to deduce absolute densities or the charge state of the traps from these equations. Only changes can be monitored.

10-a

10-s

10-Z

Injected

charge

10-l

1

10’

( C/cm*)

@I

Fig. 3. Flatband voltage shift versus injected charge density for different oxide thicknesses: (a) positive gate, (b) negative gate, (c) effect of gate polarity and substrate type.

P. FAZAN et al.

832

I

I

Qin,

= 5x10-*C/cm2

dox -

22

I

I

nm

Fig. 4. Schematic representation of the different charges and their locations as used in equations (lH4). in this case the variation

in interface charge can be neglected at least up to 1 C/cm’. As eqns (lH4) show, AV,, is most sensitive to charges located near the cathode, while A V,, mainly reflects charges near the bulk Si/Si02 interface. For negative gate injection at low Qinj (QK and NP;N 0), eqns (3) and (4) yield: x!+ = &,I(1 + AVFNIAVM

(5)

Qg, = ~oxA%lxP,

(6)

I

I

I ,

X+= do,

/’ I’

Oxide

thickness

( nm )

(b) 8

I

Qinj

I

I

2X10-’

ClCm2

f-

Oxode

thickness

( nm )

(4

Fig. 5. Positive charge distribution vs oxide thickness: (a) charge centroid, (b) charge density with injected charge density as a parameter.

I

I

1

1

10-e

10-S

10-a

10-S

Current

density

( A/cm*

)

Fig. 6. Positive charge density versus current density at a fixed injected charge density.

These quantities are plotted in Fig. 5 as a function of oxide thickness for different Q,“j. Different values of Q, give nearly the same xP, . The maximum error on x”+ and Q$ is estimated to be about 10%. The observed non-linearity of Q% with do, and the fact that x”+ is different from d,,/2 indicates that positive charges are not uniformly distributed throughout the oxide. The distribution of positive charges does not seem to move during injection. x”+ is also independent of current density J for a given Q,. For large do,, xc tends towards a constant value[l4]. The density of positive charges Q$ for constant Qinj increases with J (Fig. 6), which implies that their rate of creation depends on the applied field. This is an indication that generation depends on the kinetic energy of electrons in the oxide. It is not possible to ascertain from our data whether Q$ saturates at large Ql,,, or simply increases at lower rate than Q8 beyond the turn-around point. If Q$ does reach a limit, it certainly happens above 1 C/cm2 (Fig. 3b). The similarity between the behavior of AVF, for positive and negative gate biases (Fig. 1) implies that the densities and locations of bulk oxide charges are nearly symmetrical with respect to the direction of current flow. Thus we may conclude that (d,, - x”+) N x; and (do, - xB) N xt . This is confirmed by the initial variations of AV”,, and AVf, up to about lo-’ C/cm2. The fact that the turn-around on AV,, occurs for different injected charge densities depending on the polarity of gate bias can be explained if the bulk negative charges are located near the cathode. Indeed, for negative gate injection, electrons are trapped near the gate, far away from the Si/SiOr interface. They consequently do not affect VFB until their density is very high. For positive gate injection, electrons are trapped close to the Si/Si02 interface, and the turn-around on VFB appears for a lower injected charge density. Since VFN does not saturate, two processes are generally considered for the generation of bulk negative charges: (1) the filling of preexistent electron traps; (2) the generation of new electron traps. For

833

Charge generation in thin SiO, polysilicon-gate MOS capacitors

injection from the substrate, described by the following neglecting positive charges the distribution of negative during injection: A V& =

-XL

{

these processes can be equation[ 131 derived by and by assuming that charges does not move

Si substrate ( Gate )

Gate SiO,

( Si substrate

)

RQi, + qN, X [ 1 - exp( -

CQinj /q)l}/Eox. (7)

Here, both preexistent and newly generated traps are described by a common (constant) charge centroid XL. o Is the capture cross-section of preexistent traps of area1 density N,, R is the generation rate of new traps. These parameters are obtained by fitting equation (7) to experimental results. For lack of an independent way to determine x”_, we will take (do,-- X-Y)= 3 nm. For the case of Fig. 2, N, N 1012cm-2, Q 2: 10-19cm2 and R u 7.7 x 10m9at the lower current density (J = lo-’ A/cm’). N, and Q are not significantly affected by variations in gate polarity, J, do, or substrate type. R depends on these parameters in a complex manner: it is larger for a positive gate bias, increases with J, tends to increase for thinner oxides, and does not depend on substrate type. Whereas enhanced trap generation for higher applied fields, hence larger currents, is expected, it is somewhat surprising that R should vary with do, at constant J. Indeed, the electric field at the cathode is the same in all cases. This can only be explained by assuming that the quality of the oxide films depends on their thickness[ 131.The variation of A VFNwith Qi,,, for large J (Fig. 2) cannot be described by equation (7), which shows the need for a more complete model.

MODEL

The above-mentioned mechanisms can be explained by a physical model based only on the effect of conduction electrons. Anode hole injection, as observed in Al-gate devices[l] is not deemed important in our case, except possibly for the generation of interface charge during negative gate injections. Let us consider an electron tunneling into the conduction band of the Si02 during F-N injection (Fig. 7). This electron will first cross a region with zero kinetic energy. It is then accelerated by the oxide field. After some distance, it reaches its limiting velocity through collisions and continues through the remainder of the oxide with a constant kinetic energy. The mean kinetic energy of electrons in thin films of Si02 was recently measured experimentally by different techniques[23] and found to amount to 4-5 eV for an applied field of 10 MV/cm, which is well below the threshold for band-to-band impact ionisation (9 eV). This energy, however, is sufficient to detrap electrons in defect states within the gap of Si02 by impact ionization. Following Nissan-Cohen et af.[14,15,17] we therefore attribute the generation of negative and positive bulk charges to electron trapping and detrapping.

\\ i \,J do,

Fig. 7. Schematic band diagram of a MOS capacitor before (solid lines) and after (dotted lines) charge generation due to Fowler-Nordheim injection from the substrate or from the gate.

In the region of the oxide where conduction electrons have low kinetic energy, electrons are trapped by existing and perhaps newly created defects, which produces a net negative charge. Further from the cathode, where electrons have high kinetic energy, impact ionization of filled electron traps can occur, thereby creating positive charges. Since the kinetic energy of conduction electrons depends on the applied field, positive charge generation will increase with injected current, as illustrated by Fig. 6. As a result of these two processes, bulk negative charges appear near the cathode and positive charges some distance away from the cathode. We assume that filled electron traps at the origin of positive charge generation are introduced during the oxide fabrication process, as suggested by NissanCohen et a1.[15]. Their number being finite, the density of positive charges will reach a limiting value, contrary to what happens for negative charges. It thus seems improbable that positive charges alone be responsible for dielectric breakdown. As a result of charge trapping, the electric field in the oxide is no longer uniform but reaches a maximum in the central region (Fig. 7). Experimental results suggest that breakdown occurs when the field in this region reaches a critical value[5,24]. Based on the model presented in this paper, we are able to correctly explain the dependence of breakdown charge on current density and temperature. These results are discussed elsewhere[25]. For injections at low applied fields (avalanche electron injection), conduction electrons in the oxide do not reach large kinetic energies. They are thus unabled to generate bulk positive charges, but are trapped uniformly throughout the oxide. This has indeed been confirmed experimentally[20,21].

P. FAZAN et al.

834 CONCLUSION

Constant current injection and C-V measurements allow us to estimate the localization of charges generated in thin SiOZ polysilicon-gate MOS capacitors and to shed some light on the mechanisms involved. Accordingly, it appears that two different types of bulk charges must be invoked to explain our experimental results: positive charges at some distance from the cathode and negative charges near the cathode. A very simple physical model explaining the generation of these different charges and their distribution across the Si02 film is presented. Charge generation appears to be closely related to the kinetic energy of conduction electrons in the oxide. This model is consistent with the differences observed in the build-up of bulk charges during F-N and hot electron injection. It allows us to understand the phenomena responsible for oxide degradation leading to dielectric breakdown. Acknowledgements-This work was supported by the Swiss National Foundation for Scientific Research under the NF 13 program. The authors would like to thank Dr J. M. Moret for providing samples and the reviewers for valuable suggestions. REFERENCES

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