Dielectric breakdown of reactively sputtered silicon nitride

Thin Solid Films, 15 (1973) 199-205 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland

199

DIELECTRIC BREAKDOWN OF REACTIVELY SPUTTERED SILICON N I T R I D E *

W . ROTHEMUND A N D C. R. FRITZSCHE Institut y~r Angewandte Festkrrperphysik der Fraunhofer-Gesellschaft Freiburg i. Br. (Germany) (Received October 4, 1972)

Silicon nitride layers on silicon substrates were prepared by reactive sputtering of silicon in nitrogen under conditions which led to layers of maximum dielectric strength. Dielectric breakdown and its dependence on temperature, pulse width and layer thickness were investigated. It is shown that conversion from thermal to electronic breakdown can be achieved at low temperatures and short pulse widths. This enables the thickness dependence o f breakdown to be observed. F r o m this dependence the mean free time of carriers was found to be 1.1 x 10 -15 sec.

Zusammenfassung Siliziumnitrid-Schichten auf Siliziumsubstraten wurden durch reaktive Kathodenzerst~iubung von Silizium in Stickstoff hergestellt, wobei Bedingungen gew~ihlt wurden, die zu maximaler Durchschlagsfestigkeit der Schichten ffihrten. An diesen Schichten wurde der dielektrische Durchschlag und dessen Abh~ingigkeit von Temperatur, Impulsdauer und Schichtdicke untersucht. Bei niedrigen Temperaturen und kurzen Impulsen erh~ilt man einenObergang von thermischem zu elektronischem Durchschlag. Dies ermrglicht die Beobachtung der Abh~ingigkeit des Durchschlages von der Schichtdicke. Aus dieser Abh~ingigkeit berechnet sich die mittlere StoBzeit der Ladungstr~iger zu 1.1 x 10-15 sec.

INTRODUCTION

It is known that silicon nitride layers at high electric fields show a measurable conductivity which may effect the mechanism of dielectric breakdown 1. If conditions o f measurement can be found which lead to pure electronic breakdown it should be possible to determine the mean free time o f carriers from the dependence o f breakdown upon layer thickness. This dependence will set in as soon as the distance available for free acceleration o f carriers is given by the thickness rather than by the mean free path, which is expected to occur for a thickness o f the order of some 100 to some 1000 A. This range o f thickness is easily accessible if the layers are prepared by sputtering. Another reason for This paper contains parts of a thesis by W. Rothemund 9 at the University of Freiburg, Germany.

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w . ROTHEMUND, C. R. FRITZSCHE

producing the nitride by sputtering is the technological importance of procedures which do not involve the application of high temperatures. In this paper we first report briefly on the sputtering conditions which yield layers of maximum dielectric strength. Then the statistical behaviour of breakdown events and the dependence of breakdown field on temperature, pulse width and layer thickness are described and discussed. Although extended measurements of the electrical conductivity were made, they are not described here because they agree closely with the results of other authors ~-a. EXPERIMENTAL

Silicon nitride was deposited on polished 111 surfaces of 1.0+0.2 f~ cm n-type single-crystal silicon substrates by sputtering a silicon cathode in nitrogen. D.C. as well as r.f. sputtering was tried, but r.f. sputtering was chosen because we found the dielectric strength of the layers to be higher, by about 20 ~ , with this method. A higher quality, particularly a higher electrical resistivity, of layers produced by r.f. sputtering has already been reported by other authors 4"-7. It is known that sputtered silicon reacts strongly with traces of oxygen or water rather than with the ambient nitrogen 5' s, 9. We dried the bell-jar carefully and used nitrogen with less than 1 ppm oxygen and less than 2 ppm water. No Sit2 was found in the i.r. spectra of the nitride layers even in the case of d.c. sputtering, which, as Hu and Gregor 7 have found, is more sensitive to traces of oxygen and water than r.f. sputtering. However, by secondary ion mass spectroscopy (after Benninghoven 1°) oxygen was found even in the layers produced by r.f. sputtering. Up to 120 A of the nitride were removed during analysis. In this depth the oxygen content was constant, but in the outermost atomic layers it was higher by a factor of 40. No absolute values were obtained. Maguire and Augustus 1~ have recently reported that pyrolytic nitrides behave similarly. The layers used in our measurements were deposited by reactive r.f. sputtering under the following conditions. Gas: pure nitrogen without addition of other gases Pressure: 4-5 x 10 -s Torr Voltage: 850 V Target-to-substrate distance: 4 em Frequency: 2.7 MHz Deposition rate: 40 + 10 A/min Deposition rates between 15 and 180 A/min could be achieved by variation of the voltage. The rate indicated above yields layers of maximum dielectric strength. The possible reasons for this have been discussed elsewhere 9. Aluminium electrodes were evaporated onto the nitride surface for electrical measurements. In general the diameter was 0.5 ram, but larger diameters up to 2.0 mm were used to study the influence of electrode size on breakdown statistics. Contact was made to the back of the wafers with silver paint. Breakdown was preferentially measured by application of voltage pulses and observation of the voltage and current on an oscilloscope. The loading time of the sample capacitance was short compared with the pulse width down to

DIELECTRIC BREAKDOWN OF REACTIVELY SPUTTERED S i 3 N 4

201

pulses of 1.5 ~sec. The voltage pulse height was raised at a rate o f 4 V/sec until the current pulse indicated breakdown. We observed the definitive shortening breakdown as well as the first breakdown event, which in general is self-healing. Only the first events were evaluated. This required the investigation of a large number o f electrodes and evaluation by statistical methods. The details will be explained in the next section. D.C. measurements o f breakdown are difficult because o f the relatively high conductivity of the nitride. Conditions similar to d.c. were achieved by applying a saw-tooth voltage with an increment o f 300 V/sec. RESULTS

Figure 1 shows the breakdown field distribution of 72 electrodes, each 0.5 mm in diameter, on a layer of 1700/~ thickness. The experiment was repeated o

t

.Q

o ~6 .tD E Z Field strength (MV/cm) Fig. 1. Statistical distribution of breakdown events at 20 °C. Pulse measurements. Pulse width 16 Ilsec, repetition time 800 ~sec. Layer thickness 1700 A.

with other diameters and it was seen that in the distributions, like that in Fig. 1, the peak at about 5 x 10 6 V / c m increased with increasing electrode size while the peak between 8 and 8.7 x 10 6 V / c m decreased. This behaviour suggests that the peak at lower field strength is caused by local defects which are possibly small crystalline grains in the otherwise amorphous layer. Indeed, such grains have been observed in an electron microscope and electron diffraction showed them to be crystalline 9. The high field peak in Fig. 1 represents the dielectric strength of the defect-free layer. Breakdowns occurring within a range of _ 1.67 x l06 V/cm around this peak were used to calculate the average values plotted in Figs. 2 4 . Figure 2 shows the temperature dependence o f breakdown. The square root of the breakdown field is plotted v e r s u s temperature. In this plot pulse measurements show a linear increase of breakdown field with decreasing temperature between about 380 ° and 210 °K, whereas at lower temperatures there is only slight temperature dependence. The knee at about 200 °K was reproduced in three independent sets o f measurements. For saw-tooth measurements the dielectric strength increased distinctly even below 200 °K.

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w . ROTHEMUND, C. R. FRITZSCHE

V~B( M vl/~c m 1/2) 3.25

Pulse -

measurement 50)4 sec, 5 m s e c

275

2.25

I

2C)0

3C)0 400 (*K) Fig. 2. Dependence of breakdown upon temperature. Layer thickness 2000 ,~. I(50

Temperature

The pulse width dependence of breakdown at 300 °K is shown in Fig. 3. A sharp decay of electric strength is observed between 4 and 15 Ixsec. For long pulses there is little difference between pulse and saw-tooth measurements. For short pulses the breakdown at 300 °K approaches the long pulse and sawtooth values measured at 90 °K. With pulse measurements at low temperature we were able to observe clearly the dependence of breakdown upon layer thickness, as shown in Fig. 4. Within the limits of experimental error the reciprocal value of the breakdown field increases linearly with the logarithm of thickness. Mean

1°I

field

breakdown

strength

(MV/cm)

Breakdown

field

strength,

pulse-

measurement,5Ojusec,gO*K

Breakdown

field

strength, saw- tooth-measurement, 3000K

9"

<> I

1

. . . .

i

10

i . . . . i 100 1000 Pulse width ( ~ s e c )

. . . .

. . . .

i

Fig. 3. Dependence of breakdown upon pulse width at 20 °C. Pulse width to repetition time 1:50. Layer thickness 1700 A.

DIELECTRIC BREAKDOWN OF REACTIVELY SPUTTERED S i 3 N 4

.10 "e

203

.12

-11

-10 15~, 27. 5 x 10"a ¢mV4

~[

.9

J

4

In d do

-1'2

Fig. 4. Reciprocal of breakdown field ment 50 lasec, 1:100.

-1'1 vs.

,4-

logarithm of layer thickness at - 1 8 5 °C. Pulse measure-

DISCUSSION

F r o m Figs. 2 and 3 we conclude that at room temperature and with long pulse or saw-tooth measurements the breakdown is caused by thermal instabilities, while electronic breakdown can be achieved at low temperatures as well as by short pulses. The argument is as follows. Assuming a Poole-Frenkel mechanism of conductivity Sze 1 has shown the breakdown field FB to be related to the absolute temperature T in the case of thermal instability by FB x /2 = ct( (a -- C T )

(1)

where ~ and ~b are constants, while C still depends weakly upon FB and T. In the same way one finds that eqn. (1) also holds with the assumption o f Schottky emission, except that the constants are different. Thus, even if the commonly assumed Poole-Frenkel mechanism was overlapped by Schottky emission, we could base our considerations on this equation. Furthermore, Sze has shown that the pure electronic breakdown in amorphous materials becomes temperature independent at low temperatures. Figure 2 indicates that at high temperatures eqn. (1) is fulfilled. The existence of pulse dependence at pulse widths of some microseconds supports the assumption of a thermal breakdown mechanism at high temperatures with long pulses. With decreasing temperature the field required for thermal breakdown will at a certain point exceed the field for electronic breakdown, since the latter becomes constant. The breakdown follows the mechanism which needs the lower field. Thus, on the low temperature side o f the knee in Fig. 2 we can assume that electronic breakdown occurs. Furthermore, thermal breakdown needs fields higher than those given by eqn. (1) if pulse measurements are made with pulse widths shorter than the thermal relaxation time o f the sample. Therefore, the thermal breakdown field can exceed the

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w . ROTHEMUND, C. R. FRITZSCHE

electronic breakdown field even at temperatures higher than 200 °K provided that the pulses are sufficiently short. This explains Fig. 3. Electronic breakdown is caused by impact ionization. If, as a consequence of a small laye r thickness d, a substantial number of carriers can cross the layer without impact, then

FB = (2mI)l/2 {qzln(lz(2qdi)!/2) } -1

(2)

where m is the effective mass, I the ionization energy, q the electronic charge,/z the mobility and z the mean free time. This equation results from a comparison of transit time with the m e a n time of acceleration needed for impact ionization (see ref. 12). According to the plot o f Fig. 4 we m a y write eqn. (2) as

1/Fa = A In(d/do) + B

(2a)

A = qz(m°/m)l/2 (2moi)1/2

(2b)

with

B = A In

(m/mo)l-7~2moi)l/2

(2c)

where do is the unit length and mo the free electron mass, which is introduced because m is unknown. A and B are determined experimentally as shown in Fig. 4. F o r I we m a y take the value of 4 eV given by H u and Gregor ?. Then, from eqns. (2b) and (2c) we get z = 1.1 x 10- 1s (m/mo)l/2 p=0.2

(molto) 1/2

[sec] [cm2/V sec]

Little is known from the literature about the m e a n free time o f carriers in insulators at high fields. F o r anodically grown S i O 2 "[ has been determined f r o m the decay o f the anodization current at constant voltage la. To get a further value for comparison we evaluated published data 14 on the dependence o f breakdown on thickness of S i O 2 using eqns. (2a) and (2b) with I = 8.0 eV. The results are summarized in Table I. Since all z values in this table agree in order o f magnitude the application o f eqn. (2b) seems to be reasonable. Our determination o f / t has to be considered as a very rough estimation since A and B enter/z exponentially and are subject to experimental error. It is interesting to compare the values given above with the relation

p = q z*/m

(3)

By the introduction of two different free times z and z* we take into account that z serves to calculate the time necessary to reach the ionization energy while z* serves to calculate the transit time. In the latter case only the velocity component perpendicular to the layer surface is important and, in contrast to the kinetic energy, after impact this component is not independent o f the velocity before

205

DIELECTRIC BREAKDOWN OF REACTIVELY SPUTTERED S i 3 N 4 TABLE I FREE TIME "C OF

MEAN

SOME I N S U L A T I N G LAYERS

Material

Preparation

• (see)

Temperature (°K)

Type o f measurement

SiaN 4 SiO2 SiO 2

Reactive sputtering Thermal oxidation Anodization

1.1 × 10 - t ~ 4.1 x 10- z5 1.2 × 10-15

88 272 298

Breakdown vs. thickness Breakdown vs. thickness Current decay

impact. Furthermore z and T* may depend upon the electron energy and different methods of averaging are needed because o f the different meaning of z in eqn. (2) and z* in eqn. (3). By eqn. (3) z = ~* yields/~ = 1.9 cm2/V sec. We think that the value of/~ given above is too small. This could also be concluded from the unreasonably short mean free path resulting from F p z, but our z is obviously related to the fast electrons. With the velocity Vl corresponding to the impact ionization energy we get z v~= 13 A which is 8 times the length o f the S i - N bond. Our results can be summarized as follows. By reactive sputtering silicon nitride films can be prepared which show a dielectric strength of 9 x 10 6 V / c m at 2 lasec pulse width. The mechanism o f dielectric breakdown is thermal at high temperatures and electronic at low temperatures and low pulse measurements. We were able to find the expected dependence o f breakdown field upon layer thickness. A mean free time o f 1.1 × 10-15 sec was derived from this dependence. ACKNOWLEDGEMENTS

Thanks are expressed to W. Haydl for suggestions concerning the manuscript, and to A. Benninghoven and S. Storp, 1. Physikalisches Institut der Universit~it K61n, who carried out the analysis of the silicon nitride layers by secondary ion mass spectroscopy. REFERENCES 1 S . M . Sze, J. Appl. Phys., 38 (1967) 2951. 2 S . M . Hu, D. R. Kerr and L. V. Gregor, Appl. Phys. Letters, 10 (1967) 97. 3 L . F . Cordes, Appl. Phys. Letters, 11 (1967) 383. 4 A . R . Janus and G. A. Shirn, J. Electrochem. Soc., 113 (1966) 212 C. 5 R . I . Frank and W. L. Moberg, J. Electrochem. Soc., 117 (1970) 524. 6 S . M . Hu, J. Electrochem. Soc., 113 (1966) 693. 7 S . M . H u and L. V. Gregor, J. Electrochem. Soc., 114 (1967) 826. 8 E . A . Taft, J. Electrochem. Soc., 118 (1971) 1341. 9 W. R o t h e m u n d , Preparation of silicon nitride layers by the reaction of silicon with energetic nitrogen ions and properties o f such layers in MIS structures (in German), Thesis, Freiburg, 1971. 10 A. Benninghoven, Z. angew. Physik, 27 (1969) 51. 11 H . G . Maguire and P. D. Augustus, J. Electrochem. Soc., 119 (1972) 791. 12 W. Franz. Dielectric breakdown (in German), Handbook of Physics, Vol. 17, Springer, BerlinG6ttingen-Heidelberg, 1956. p. 155. 13 C . R . Fritzsche, J. Phys. Chem. Solids, 30 (1969) 1885. 14 C. R. Fritzsche, Z. angew. Physik, 24 (1967) 48.