TANTALUM THIN FILM CAPACITORS WITH VARIOUS TYPES OF COUNTERELECTRODES

Capacitors have been prepared from N2-doped, triode sputtered Tantalum films by conventional anodization and 
photolitographic techniques.

The possibility of matching very closely the temperature characteristics of thin film Ta capacitors and resistors, makes them a very attractive proposition in high precision, high stability RC circuits.In recent years, capacitor properties have been improved by N2 doping the Ta base film 1,2 ,a at various atomic percentages, giving better yield, temperature and life stability, and lower dissipation factors when compared to first generation capacitors based on pure -Ta films. 4 ,s Moreover, N2-doped Ta capacitors can withstand exposure to high tempera- tures, such as those occurring when R and C are obtained on the same substrate: also, thermal treat- ment can be included in the fabrication process to improve capacitor properties. 4 ,6,7most exclusively, NiCr-Au has been used as the counterelectrode metallization, although sputtered NiCr-Pd with subsequent selective gold plating has been used for the counterelectrodes of critical RC active Filters. 8 the present paper the characteristics of nitrogen doped capacitors with Ta based triode sputtered top electrodes (TaO x Ny-NiCr-Pd-Au and TaOxNy-Ti-Pd-Au) are compared to those of NiCr-Au standard capacitors.The excellent properties of the N2-doped capacitors are not negatively affected by the presence of the Ta-oxinitride layer at the dielectric interface.

PREPARATION OF CAPACITORS AND TEST EQUIPMENT
The Tantalum base electrode films were deposited by triode sputtering from a 60 mm diameter Ta target in a stainless steel, oil diffusion pumped chamber; the sputtering system has been described in more detail in a previous paper, 9 the only difference being the substrate holder, which in the present experiments was of the planetary type, bearing 30 2" x 2" square substrates.
Both Corning Glass 7059 and glazed alumina were used and prior to deposition of a-Ta an etch-stop layer of 3000 A Tantalum pentoxide was reactively deposited onto the substrates in an Ar and O2 atmosphere.Pure nitrogen was then released in the chamber at flow rates ranging from 0 to 1.6 sccm and, after some minutes of presputtering, nitrogen doped Ta was deposited for 40 minutes at a deposition rate of 110 A/min.Thickness, sheet resistance, TCR and thermoelectric power were measured on test samples.Other substrates were processed into capacitors by anodization to 180 V or 230 V in 0.01% wt.
aqueous solution of citric acid at room temperature.After reaching the forming voltage, a soak time of hour was allowed for all samples.Photoresist was used to delineate the dielectric pattern during the anodization process.
On each substrate, four arrays of 18 capacitors, each 0.1 cm of active area, were patterned by photolitho techniques and some samples were thermally treated at 250,300,350C in air.For voltage step-stress testing and accelerated life tests, leads were soldered on after separating the individual circuits.
Capacitance (C) and dissipation factor (tan 6) were measured with a Hewlett-Packard 4270A automatic bridge at 1 kHz and TCC calculated from values measured at 15 C and 85 C. Some measurements to establish accurately the dependence on temperature and frequency of C and tan 6 were performed with an ESI manual bridge.
For dc insulation resistance measurements, a G.R.

Effect of Nitrogen Doping
The electrical properties of triode sputtered, 4400 A thick Ta films are summarized in Table I.With no intentional nitrogen doping, the values of p 202 #ohm.cm and TCR -130 ppm/C can be taken as for a fl-Ta crystalline structure.Nitrogen doping at increasing rates first causes the resistivity to fall (65 #ohm.cm at 0.6 sccm N) and the TCR to rise sharply (+760 ppm/C at 0.6 sccm N ).These values are typically referred to a bcc structure.
Between 0.6 and 1.6 sccm N, TCR steadily decreases to + 160 ppm/ C, where a corresponding resistivity of 133 #ohm.cm has been measured.
The nitrogen atomic content has not been determined by direct analysis.Because of differences in deposition systems and parameters only a rough estimate is possible from the literature, 6 ,1 o based on TCR and resistivity.The dielectric constant of anodic oxide grown on these films can also be taken into account.
About 15 at % of N has been estimated for Ta  The effect of doped Ta composition on capacitor properties has been analyzed for capacitors anodized to 230 V with sputtered NiCr counterelectrodes.
An almost linear decrease of capacitance density has been found with increasing nitrogen in the sputtering atmosphere; this is shown in Figure where the normalized capacitance has been plotted against N2 flow rate.Dissipation factors and TCC are virtually insensitive to nitrogen doping of base electrodes over the concentration range examined: 2.2 x 10 -3 and 235 ppm/C are typical figures, the only exception being nondoped samples which exhibited higher values for both parameters of 3.5 x 10-3 and 560 ppm/C respectively.This is most probably due to temperature rise during sputtering the counterelectrode, as confirmed in a separate experiment where vacuum evaporation was used instead of sputtering.Different capacitor properties begin to emerge under thermal treatment, as shown in Figures 2 and 3 and the effects of nitrogen content become more  pronounced as heating time and temperature are increased.
At 250C for hour, heat treated capacitors show uniform reductions of tan 6 and TCC to 1.4 x 10-3 and 150 ppm/ C, regardless of N2 concentration, and no variations occur after 5 hours.At 3 50 C, minimum TCC and tan 6 occur between 0.8 and 1.0 sccm flow rate, and both parameters increase with heat treatment time.After 5 hours, capacitors fabricated from low-doped bcc Ta (0.6 sccm N2 ) are heavily degraded (TCC 1100 ppm/C tan 6 10 -2), while those sputtered at 0.8 to 1.2 sccm N2 still have a TCC lower than   between 23 and 85C of capacitors as in Figure 2.
200 ppm/C and a tan 6 of about 0.0022, showing that they are very resistant to thermal treatment.The temperature dependence of the dissipation factor has been examined by measurements at 25 and 85 C. In this case also the dependence on nitrogen content becomes evident at the highest stabilization temperature: a minimum value of TCDF x 10 -s/o C has been observed for capacitors treated at 250 C, while h at 350C is sufficient to shift this parameter up to 3 x 10 -s/C (Figure 4).

Effect of Counterelectrode
Additional substrates were collectively processed by sputtering 4000 of a-Ta at a nitrogen flow rate of 1.2 sccm and subsequent anodization at 180 V.These were then split into three groups and capacitors were fabricated with three different types of top electrode, NiCr-Au, Ta-oxinitride-NiCr-Pd-Au, and Ta-oxinitride-Ti-Pd-Auo The Ta-oxinitride layer, about 300 A thick, has resistivity 330/ohm.cm and TCR-150 ppm/ C. The cumulative distributions of capacitance at 23C and kHz are shown in Figure 5.
The dissipation factor at room temperature is not affected by the structure of counterelectrode employed; mean values of 1.4, 1.6, 1.5 x 10 -3 were measured on samples of the three types.
Leakage current is an important parameter to evaluate Ta capacitor quality; 50 V dc was applied with the lower electrode positive and insulation resistance was measured after 30 seconds.NiC, I-Au initial 3h, 300C 1.5 h, 3500C TaOxNcTi-Pd-Au initial  No capacitor with a NiCr counterelectrode exceeded a limiting value of 13 nA except about 4% which broke down under the test voltage.The median value is 0.22 nA.
Leakage currents of capacitors with Ta oxinitride based counterelectrodes are generally higher and more dispersed.No differences were found between Ta-NiCr-Pd-Au and Ta-Ti-Pd-Au, both having a median value of 0.5 nA and this is valid for all electrical parameters considered in this paper.
The results of leakage current measurements are shown in Table II.
The effects of high temperature treatment on capacitor properties can be summarized as follows.
Very small negative capacitance variations, less than -0.15%, are observed for Ta based counter- electrode capacitors which received heat treatment at 300C, and even after hour at 350C this type of capacitor exhibited a median capacitance shift of 0.28%, mean deviation of 0.02%.A larger zC/C has been measured on NiCr-Au capacitors: 1%, 1.1% and 1.5% for h at 300C, 3 h at 300C and h at 350 C respectively.tan  NiCr-Au initial Dissipation factors at room temperature and TCC are consistent at 1.6 x 10 -a and 145 to 150 ppm/C respectively after hour at 300C and small variations occur after prolonged treatment. NiCr-Aapacitors show a larger dependence of tan 6 on Leakage Current (nA) .os ..s .s .Leakage Current (nA) .05 :1 .5 .5 . 1' 5 FIGURE 9 Cumulative distribution of leakage current for capacitors with Ta oxinitride NiCr-Pd-Au and Ta oxinitridc Ti-Pd-Au counterelectrode.
temperature when heat treated at 350 C, as shown in Figures 6 and 7.
The leakage current of Ta counterelectrode capacitors is somewhat degraded with high tempera- ture treatment, in contrast to NiCr capacitors whose insulation resistance is practically unchanged even after 5 hours at 350 C.This situation is shown in Figures 8 and 9 where the cumulative distributions of 1.c.for treated and untreated samples are plotted.

Voltage
Step-stress Tests For preliminary evaluation of capacitor reliability, 12 minute voltage step-stress tests at 85C were carried out on capacitors with the three types of counterelectrode, before and after thermal stabilization at 300 and 350C.An important point was to establish whether the leakage current degradation, induced by the thermal treatment on Ta counterelectrode capacitors, yielded lower break- down voltages and thus potential low reliability.
All groups tested exhibit an average break-down voltage of between 90 and 100 V, regardless of the counterelectrode and stabilization treatment;note that unheated NiCr capacitors show the lowest mean breakdown voltage of about 90 V.
These values are comparable to previously published results 3 ,6 for nitrogen doped, 230 V Ta film capacitors, if the dielectric thickness is taken into account.D.c. bias accelerated life tests at 85C are in progress.
Results after 2000 hours with 50 V applied show 8 to 13% failures on 300C heat treated capacitors, related more to the fabrication lot than to the counterelectrode structure.Step stress failures at 85 C of Ta based counterelectrode capacitors. 4. CONCLUSION Capacitors exhibiting excellent a.c. and d.c.electrical properties have been obtained from triode sputtered thin films of nitrogen doped Tantalum.
A Ta oxinitride layer sputtered onto the dielectric and left as a part of the counterelectrode structure does not degrade the capacitor characteristics or their repeatability.
Both NiCr-Au and Ta based counterelectrode capacitors can withstand temperatures as high as 350C, but optimum results have been obtained by heat treating for 3 hours at 300C; the temperature stability of the dissipation factor is the parameter most affected by high temperatures.On the basis of these results, the fabrication process of high stability RC circuits can be simplified, in that the capacitors counterelectrodes, resistors and conductors can be obtained from a single multilayer.

FIGURE
FIGURE

FIGURE 4
FIGURE 4 Temperature coefficient of dissipation factor

FIGURE 6
FIGURE 6 Temperature dependence of capacitance for treated and untreated capacitors.

FIGURE 7
FIGURE 7  Temperature dependence of dissipation factor. . FIGURE

TABLE
Electrical properties of N -doped Ta films