Pore formation during the breakdown process in anodic Ta2O5 films

PORE FORMATION DURING IN ANODIC I. MONTERO,

THE BREAKDOWN Taz05 FILMS

PROCESS

M. FERNANDEZ and J. M. ALBELLA

Institute de Fisica Estado Glide, CSIC, and Dpto Fisica Aplicada, Universidad Aut6noma Cantoblanco, Madrid 28049, Spain (Received 24 March 1986; in revisedform 30 June 1986) Abstract-The breakdown damage during the anodic oxidation of tantalum has been studied through reanodization experiments. The results have been interpreted assuming the formation of non-shorting pores oroduced as a conseauence of the breakdown orocess. On the basis of this model, the extent of the damaged area and the pore depth has been assessed.

INTRODUCTION

fluoric acids, and finally in a buffered solution of NH,F in FH. After this treatment the samples were rinsed in boiling water for 5 min. In order to avoid the spurious effect at the edges of the foil, the samples were held in a plastic holder which partially masked the surface, allowing only a circular area of the surface (S = I.18 +- 0.04 cm2) to beincontact with the electrolyte. Capacitance measurements at 100 Hz were also performed in the same electrolyte cell using a LCR meter, mod. HP4274A.

The breakdown mechanisms during the anodic oxidation of valve metals have been extensively investigated in the past[l]. However, little is known about the breakdown processes following the appearance of the first breakdown sparks. These processes are characterized by the presence of a succession of pulses in the anodization voltage, accompanied by scintillation sparks and copious oxygen evolution[2]. In a previous paper we have studied the problem in anodic Ta105 films through the measurement of the reanodization rate of samples subjected to the breakdown voltage for different times[3]. Two different processes were distinguished during the breakdown. In the first stage the breakdown is mainly characterized by a process of attack and partial healing of the oxide film with the formation of pores and microfissures. The second stage of the breakdown is dominated by the process of recrystallization which irreversibly degrades the oxide dielectric properties. In this paper we have analysed the reanodization curves after breakdown at different current densities in terms of this model. Through this analysis we have assessed the extent of the breakdown attack, ie the amount of the damaged area as well as the mean depth of the breakdown pores.

RESULTS

AND

DISCUSSION

In a first set of experiments, each sample was subjected to successive anodization steps at two different current densities, iI = 3.OOmA cm-’ and il = 0.30 mA cm-‘. A qualitative diagram of the anodization voltage-charge curve for a typical run is given in Fig. 1. Following the change in the current density there is always an abrupt increment (positive or negative) in the anodization voltage. After this initial increment the slope of the anodization curves corresponds to the value of the actual electric field, being essentially independent of the previous sequence of the anodization experiments. On the contrary, when the breakdown voltage is reached the subsequent change of current density always results in a large negative increment of the anodization voltage. As was observed in our previous work, the unitary anodization rate at this stage, given by (l/i) (d V/dt), strongly depends on the time, tg, spent by the sample under the breakdown conditionsC31. We shall examine the results considering first the reanodization experiments at voltages, V, lower than the breakdown voltage, V,, studying latterly the case of voltages higher than V,.

EXPERIMENTAL Tantalum foil of 99.96 “/, purity was anodized in a 2.0 x lo-’ M solution of phosphoric acid in water at constant current density up to the desired voltage. The temperature of the cell was kept constant at 20.0 f 0.2”C using a thermostatic bath. During the anodization, the formation voltage and current density were monitored by means of a data acquisition system based in the HP 85 microcomputer. The samples were previously degreased and chemically polished following the procedure described in Ref.[3]. Essentially this procedure consists of the successive immersion in boiling trichloroethylene, acetone, alcohol and deionized water, followed by a treatment in a solution of nitric, sulpliuric and hydro-

v < v, If Vi represents the final voltage reached at a constant current, say il, and V, the initial voltage after the change to a different current density, i,, the ratio 171

172

I.

MON~ERO

et al.

Fig. 1. Qualitative variation of the anodization voltage in experiments at a constant current density, changing

alternatively the current density from a value ii to a different value i2 (ir > iz).

V,f V2 is given by

Vl _=V2

El E,

where El and E2 are the corresponding mean electric fields for the anodization current densities, i, and iz. for values of The experimental VlIV2 i, = 3.OOmAcm -2 and i2 = 0.30mAcm-2 are shown in Fig. 2 for different thicknesses of the film. The experimental data also include the case when the current density goes from iz to i, . The thickness x has been calculated using the values of El and E2 obtained in a previous work, Et = 6.8 MV cm-’ and Ez

1.120

-

1.100

___~

0 ~~~_~_~_~~____~~~~_____ 000

1.060

-

1.060

-

1.040 & ‘c 0

o

, 100

200

OXIDE

THICKNESS.

MO

LOO

300

X

Inm

1

= 6.2 MV cm-‘[4]. The horizontal straight line in this figure represents the quotient El/E, between these two values. It is evident that there is good agreement, within the + 0.5 %, between the experimental values of VI/V, and the calculated value of E1/E2.

v > v, When the anodization process is interrupted at a voltage higher than the breakdown voltage, the reanodization voltage-time curve at a lower current density presents two well-defined zones: a rapid increase of the voltage up to a certain voltage V. (zone I), followed by a slow variation of the potential up to the breakdown voltage (zone II). Figure 3 illustrates this behaviour for different samples anodized first at il = 5.00mAcm-2 up to the breakdown voltage with a breakdown time of 48 s, curve a, and then each sample reanodized at a lower current density, curves b-g. The shape of the curves of Fig. 3 can be interpreted in a simple model by assuming that the breakdown process gives rise to the formation of non-shorting pores on the oxide surface as sketched in Fig. 4. Thus, the voltage V, should correspond to the potential drop at the layer remaining under the pores, with mean thickness, x0. Within this model, the electric field across this layer, E = V,,/x,, can be calculated from the approximate law for the ionic movement of ions during the anodization: i = &exp(/IE)

(2)

where i,, and /I are constants. From this equation Fig. 2. V,/V, ratio as a function of the oxide thickness, x. The broken line represents thequotient EL/E, calculated from our previous work[4].

E = j In (i/i,,)

(3)

The breakdown process in anodic Ta,O,

Ii?

films

173

200

g

_

b

i

e

REANODlZATlW 100

CURWT

CEEISTPI, ihA

cm-3

Fig. 5. Initial rise of the reanodization voltage, V,, function of the reanodization current

as

a

reanodued area, S,, which can be calculated from the basic equation for the formation process:

0 0

L

2 TIME,

1 dV = K;E i ( dt ) elf

6

t IminI

Fig. 3. Reanodization voltage-time curves at different current densities (curves bg), of samples anodized up to the breakdown voltage for a time ra = 48 s (curve a). The current densities are (in mA ~m-~): curve a = 5.00; b = 0.17; c = 0.21; d = 0.25; e = 0.33, f = 0.42; g = 0.51.

(5)

where S = 1.18 cm2 is total area of the sample, and K = 57.0 x 10m6cm3 C-’ is the charge to mass (volume) factor conversion in the anodization process. Substitution of equation (1) in the above equation leads to the expression: 1 dV (-) = K +ln(i/i,). (6) 7 dt eI/ Thus, a plot of the experimental values of (l/i) (d V/dt) us log i should yield a straight line, whose slope is determined by the effective reanodized area Se/,. These values are represented in Fig. 6, resulting S,,, = 0.04 cm’. This means that in our experimental conditions, ie a breakdown time of 48 s, the damaged area amounts about 3.4 o/Oof the total anodized area.

Breakdown Pores.

I

4 Fig. 4. Qualitative. diagram of the breakdown attack of a film of thickness x, showing the presence of non-shorting pores.

zoo0

/

-

I

/ I la

and Vo = 3 In (i/Q B

(4)

Figure 5 gives a plot of the values of V,, as a function of the reanodization current density in a log scale. Using the value j? = 6.81 x lo-* V- ’ cm[4] the thickness x,, = 2300 A is obtained from the slope of the straight line of Fig. 5. This thickness represents approximately half the whole thickness of the film. In effect, using 15.8 A V-’ as the Angstrom per volt ratio for our experimentalconditions[4], the estimated total thickness at the breakdown voltage, V, ‘Y 325 V, is x = 4810 A. In the above picture, the second part of the curves of Fig. 3 (zone II) should be associated with the filling up of the breakdown pores through the formation of new oxide layers in them. The unitary reanodization rate must then be inversely proportional to the effective

!’ I’ 1000

I

: P *I I I’

0’ 1.0

0-1

REANODIZATION

CURRENT

CENSITY

hAcni21

Fig. 6. Unitary reanodization rate as a function of the reanodization current.

174

I.

MONTERO

The presence of an oxide layer at the bottom of the pores which remains unaffected in the early stages of the breakdown process is also confirmed by the I-V characteristics of the samples subjected to breakdown. Figure 7, curve a, shows the variation of the leakage current density as a function of the applied voltage for a sample held at the breakdown time during 48 s. The measurements were taken after 20 min of polarization at each voltage in order to allow the current to decay until it reaches a stationary value. As depicted in Fig. 7, the final current exhibits an exponential increase with the applied voltage with a break in the slope at about 160 V. The first part of the curve, up to 160 V is consistent with equation (4), the slope factor x0//I being nearly coincident to that obtained in Fig. 5. Once the applied voltage reaches a critical value, high enough to initiate an appreciable ionic movement, the pores start to fill up and new oxide layers start to form, the applied electric field remaining practically constant. This stage corresponds to the second part of the I-V characteristic, for values higher than 160 V. Capacitance measurements, performed after every polarization step, also support this view. As can be observed in Fig. 7, curve b, for applied voltages lower than 160 V the capacitance of the sample remains essentially constant, while in the high voltage range the capacitance varies inversely with the applied voltage. At this point, one can speculate about the formation of non-shorting pores during the early stages of the breakdown process. According to the currently accepted theories, the breakdown is initiated by the avalanche electronic currents developed across the oxide during the anodization[5]. Thermal effects associated with these highly localized electronic currents are supposed to be the direct cause of the oxide breakdown at some definite spots[6]. In this regard, it is very well known that the nature of the anodic films is duplex, composed of an inner layer of pure stoichiometric oxide and the outermost doped with electrolyte species[7]. For Ta205, the thickness of each layer is about half the whole oxide thickness, which nearly coincides with mean thickness x0 calculated in this work. These two layers present different physicochemica1 properties, in particular the resistance to thermal recrystallization[8]. As a consequence, the breakdown could occur when the avalanche electronic current reaches the critical size to partially recrystallize the outer layer at the breakdown sites, giving rise to the development of pores and microfissures only across this layer. Although at first sight this interpretation seems to contradict previous observations on electron beam induced crystallization, there is no apparent reason to think that thermal crystallization of anodic oxides should follow the same mechanism[9].

et al.

1

oc

1ci6 10' lo-* 0

100 POLARIZATION

200 MLTAE,

300 ” h’oltd

Fig. 7. Leakage current (curve a) and capacitance (curve b) after 20 min of polarization as a function of the polarization voltage.

According to our previous paper[3], the breakdown phenomena can now be envisaged as an alternative sequence of pore formation across the outer layer and pore filling with new oxide, the number of pores increasing with the breakdown time. As the breakdown progresses, the recrystallized areas become more apparent giving rise to the so-called “grey oxide”, which irreversibly degrades the oxide dielectric characteristics.

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77, 313 (1981).