DE1489019B2 - METHOD OF MANUFACTURING THIN FILM CAPACITORS - Google Patents

Description

After the layer 4 has been applied to the plate 2, the dielectric layer 6 is applied, to be precise by placing the metal layer 4 as an anode in an anodization electrolyte bath is immersed, with the metal layer one with respect to another in the Electrolyte immersed electrode positive bias is applied. This bias is maintained until the anodization forming voltage approaches a constant value. this occurs as shown in FIG. 2 shows a when the anodizing current (dashed line in FIG. 2) is in its Curve flattens out, which occurs about ten to twenty minutes after the start of the process. to At this point the polarity of the voltage is briefly for a duration of about thirty seconds conversely to a value which corresponds to about half the voltage initially applied, whereby the metal layer becomes the cathode. The magnitude of the reverse voltage applied and the duration Their effectiveness can exceed the specified values, but are harmful effects possible if the reverse voltage is the original value of the applied voltage approaches and is effective for more than two minutes.

Then the polarity of the voltage is reversed again and the normal positive bias applied and the process continued until the oxide layer 6 reaches a thickness in the range of about 200 Å. After completion of the anodization, a counter electrode 8 made of a 'suitable metal, such as B. gold, tin, zinc or aluminum, in a thickness of several thousand Å by vapor deposition applied to the oxide layer 6 in a vacuum.

When reversing the polarity in the anodizing process, it is desirable to use a ceramic substrate to be used with a rough surface, i.e. with a roughness of between 1.0 and 2.5 μ, im In contrast to the previously used substrates with a surface roughness of about 0.15 μ. In the Table I below shows the yield of capacitors built on ceramic substrates are compared with different voltages with and without the application of the reversal technique described. From this compilation it can be seen that the yield when the polarity reversal is used is increased significantly, but that reached below a voltage of 15 volts Effect decreases at higher voltages.

Table I. Table II

Tantalum

Tantalum oxide.

niobium

Niobium Oxide ..

bismuth
Bismuth oxide

titanium

Titanium oxide ..

16.6
8/7

8.6
4.8

9.8
5.6

4.5
4.26

MoIe-

kular-

weight

180.9
441.8

92.9
265.8

209.0
466.0

47.9
79.9

Mole volume

11.4 50.8

10.8 55.3

21.2 83.2

10.6 18.8

degree of

Mismatch

2.2 2.55 2.0 1.8

tension yield yield volt with polarity reversal without reversing the polarity 8th 80% 30% 12th 80 «/ ο 250/0 16 450/0 12% 20th 20% 15%. 25th 20 ο /. 00/0 30th 10% 0%

The following is a hypothesis for the causes of the process especially in the case of thin film capacitors achievable results. When converting difficult-to-melt metal into an oxide, a certain lattice mismatch occurs, as shown in Table II. This lattice mismatch is of no concern while a completely smooth metal layer is oxidized, since the oxide grows uniformly both into and out of the metal, in a way that that the ratio of the oxide strength to the strength of the metal that was consumed by the oxidation, is equal to the ratio of the molar volume. However, this does not apply to a point on the rough surface more to. As a result of the greater electric field strength in the vicinity of the rough spot, this spot anodizes faster than the other smooth surface areas. The oxide therefore tends to smooth itself out instead of following the original roughness profile.

Complete smoothing occurs at about 60 volts of the formation voltage. As a result, it will be everywhere where there was a rough spot in the original layer, there was more material than before, in pressed a given volume. This results in a considerable amount of stress in the oxide layer this can only be canceled by cracking. The tendency for cracks to form is a function of the macroscopic one Structure of the ceramic surface and identified as the source of the short circuits.

The tension in the oxide layer is greatly reduced when the layer thickness is reduced to a depth of is limited to about 200 Å or less, which corresponds to a formation voltage of corresponds to about 15 volts. When anodizing layers at low voltages to achieve of the desired strength, the performance is very low due to the passivation at various points the surface of the refractory metal. There is no oxidation at these passivated areas instead, which then leads to the short circuits occurring in the finished capacitor.

By briefly reversing the polarity of the applied voltage when the formation voltage approaches a constant value, the passivated areas can be activated so that there the growth of the oxide layer is enabled. When the polarity is reversed, continuous ones are created Layers that are free from short circuits.

To manufacture the capacitor, a ceramic substrate, e.g. B. aluminum oxide, cleaned, by dipping it in a solution of a boiling detergent, rinsing it in water, re-in boiled a 15% solution of hydrogen peroxide, rinsed again in water, in isopropyl alcohol washed and then steam degreased in isopropyl alcohol.

Then a difficult-to-melt metal, e.g. B. tantalum, applied to the substrate surface, both

for example by cathode sputtering. The thin layers are applied in an argon glow discharge, for example 3000 volts at 1 mA / cm ² at a rate of about 10E / s for a period of about 15 minutes. Such layers have specific sheet resistances of about 4 ohm / cm ² .

The substrate is then immersed in an anodizing bath in such a way that the layer serves as the anode of the circuit. The anodization takes place in a suitable electrolyte, which does not dissolve the metal oxide layer when it is formed. A 1% aqueous solution of ammonium tetraborate is particularly suitable for this purpose, but another 1% solution can also be used, such as sodium sulphate, sulfuric acid, phosphoric acid, sodium citrate or citric acid. During the anodization, the temperature of the electrolyte is kept approximately at room temperature, the current density is between 0.1 and 1 mA / cm ^{2 of} the anode area, until the formation voltage is reached.

The oxide layer is formed anodically on the tantalum layer with a constant current density, while the voltage on the growing oxide is increased to the desired formation voltage, according to FIG. 2, at an initial formation ^{current density of about 1 mA / cm 2 of} the anode area, the formation current levels off to a value of about 1 μΑ / cm ^{2 of} the anode area, while the formation voltage rises to its desired value of about 15 volts. The formation process is continued at this constant voltage, as illustrated by the decrease in current loss down to the μΑ range. When the formation voltage approaches or reaches this constant value and when the formation current begins to flatten out asymptotically, the polarity of the applied voltage is reversed for a period of about 30 seconds so that the metal layer is now the cathode. This activates the areas that are resistant to oxide formation. The polarity is then reversed again at a voltage of around 7.5 volts, and the process is continued until the oxide layer is around 200 Å.

Then a counter electrode made of electrically conductive metal with a thickness of about 1000 Å applied by placing aluminum through a suitable mask on the surface of the dielectric Tantalum oxide is evaporated.

1 sheet of drawings

Claims (5)

1 2 fusible metal as a dielectric layer. to form a substrate if the substrate had an extremely smooth surface, namely with a degree of roughness of 0.15 μπι or 1. Process for the production of thin film 5 underneath. Such a slight roughness is only capacitors in which a ceramic substrate with glass substrates or glazed substrates can be achieved thin layer of either a metal bar. On substrates of higher roughness, e.g. B. aluminum heights Melting point or from bismuth applied to genium oxide, steatite, magnesium or beryl is brought, then the substrate with the capacitors, however, very often had a short metal layer as an anode for applying an io terminal points and could therefore be in the Oxide layer immersed in an electrolyte bath and practice does not prevail. For exploration and besodann on the anodized part of the metal coping with this intolerance are numerous Layer an electrically conductive metal membrane has been attempted. is steamed, thereby marked- With the invention, a method has now been genet, that the surface of the ceramic substrate 15 found, by means of which even at a relatively high level Has roughness that is greater than 1 μπι, surface roughness of the substrate under more complete and that during the formation, after avoidance of short-circuit points, a thin-film-resistant value of the anodization formation chip capacitor with the required characteristic values is achieved the polarity of the formation current is adjustable for. The method of the invention is a time from about half a minute to up to 20, characterized in that the surface of the at least two minutes at up to about half the height of the ceramic substrate has a roughness that reduced voltage is reversed and then greater than 1 μΐη, and that in the formation after Afterwards, the anodization is polarized back to the original preload again, reaching a fixed value will. Formation voltage the polarity of the formation 2. The method as claimed in claim 1, characterized in that it lasts for a period of about half a minute draws that as a difficult-to-melt metal up to a maximum of two minutes at up to about half Tantalum, niobium, titanium is used. The amount of reduced voltage vice versa and 3. The method according to claims 1 and 2, then back to the original bias characterized in that the application of the is reversed. The reversal of the polarity of formation sputtering, vapor deposition in a vacuum or GaI current in an electrolyte bath is known, e.g. B. vanization takes place. by the USA patent specification 1 534 709, however, 4. The method of claim 1, marked thereby delt it there for a while the effect of the records in that as electrolyte an L ° / _o solution bath continuously performed reversal of ammonium tetraborate, sodium sulphate, 35 Furthermore, even when in the USA.-Patent sulfuric acid, phosphoric acid, sodium citrate or writing 2 901412 described device for citric acid is used. Forming aluminum surfaces using 5. The method according to claim 1, characterized in that two successively charged electrolyte baths are identified, that for the conductive metal film gold, container a polarity change of the forming voltage an-tin, Zinc or aluminum is used. 40 turns. From a polarization change of this kind . . . . does not make use of the invention. With the method according to the invention, it is possible to manufacture thin-film capacitors with improved feed to establish reliability and performance. Into the- 45 special it has a favorable effect on the manufacturing costs that here substrates with a relatively rough surface, namely in the above range over 1 μπι can be used. For some time now, in the course of the microminiatu- Explained through the drawings, the invented ization of electrical circuits surface conformation 50 method according to the following in greater detail capacitors with hard-to-melt thin metal letters. films made. In the method for manufacturing Fig. 1 shows a thin film capacitor in such capacitors will, as in the German section and Auslegeschrift 1 149 113 described on a ceramic F i g. 2 is a diagram showing the course substrate a thin layer of a metal high 55 of voltage and current during the formation melting point applied, then the sub-process in the electrolyte bath. strat with the metal layer as the anode for application. The thin-film capacitor according to FIG. 1 is an oxide layer immersed in an electrolyte bath and applied to a ceramic plate 2, the top surface of which then on the anodized part of the metal layer surface 3 has a roughness 5 between 1.0 and 2.5 μ an electrically conductive metal film is evaporated. Da- 60 has. Difficult-to-melt metal 4 is by means of a reduction in current losses is one of the well-known metal coating processes low resonance factor, a cheaper dielectric with a thickness between 0.25 and 1.2 μ on the Efficiency and a high capacitor capacity ceramic surface 3 applied, z. B. by Kathodenpro Area unit achievable. atomization, vapor deposition in a vacuum or electroplating. The limits for improving such condensation. During the production of the layer 4, a The type of substrate depends on the type of catalyst, metal with a particularly high melting point is preferred, that is used for these circuits. It is ins- like z. B. tantalum, niobium, titanium, or it becomes bismuth special previously not been possible, heavily used.