EP0050343B1 - Molybdenum coated with a noble metal and process for its production - Google Patents

Description

The invention relates to molybdenum, which is coated with a noble metal, and a method for producing such a layer sequence.

Such layer sequences are widely used in semiconductor technology as contact material, because the thermal expansion coefficient of the silicon used as the semiconductor material and the thermal expansion coefficient of the molybdenum used as the contact material are similar to each other and therefore no or only low thermal stresses occur even at changing operating temperatures. Usually, the molybdenum, which is used in the form of disks with a disk thickness of approximately 0.5 to a few mm, is coated with a noble metal, whereby a favorable electrical contact resistance is achieved. On the other hand, the noble metal protects the molybdenum contact against oxidation of the surface and against the attack of aggressive chemicals, which act on the semiconductor body in the course of further process steps during an etching treatment, the shape and properties of which develop, but must not damage the molybdenum contact during this etching treatment.

Because of the high price of the precious metals, the thinnest possible coating is desirable; however, a certain minimum layer thickness of the precious metal must not be undercut, because otherwise sufficient etch resistance is no longer guaranteed. With gold as the coating material, for example, there is sufficient etching resistance only if the layer thickness is at least 1.5 to 3 μm. With layer thicknesses of less than 1.5 µm, pores occur, which means that attacks of the base material molybdenum cannot be avoided during etching. Therefore, in practice it has not been possible to do without a thicker gold layer. In addition, thinner layer thicknesses of the coating material result in higher costs for the base material, because this has to be prepared better and more uniformly. In addition, the high mechanical sensitivity of such thin layers is disadvantageous. If, for example, blanks are to be punched from the gold-coated molybdenum sheet, which is expedient and therefore generally customary. because of this mechanical sensitivity, layer thicknesses of the noble metal of about 5 µm are required. In the case of smaller disk diameters, for example diameters of approximately 6 mm, the noble metal layer thicknesses must be up to 10 µm.

Attempts have been made to replace precious metals, such as gold, with other inexpensive metals. Almost all base metals, however, have a far too low etch resistance and are unsuitable for this reason alone. At most, chromium appears to be suitable, which is applied by vapor deposition or electroplating and shows both good adhesive strength and good resistance to etching.

A disadvantage of a coating with chromium, however, is that poor and, above all, fluctuating contact resistances are then recorded, which preclude general use. These fluctuating transition resistances of chromium are probably due to surface oxidation processes that occur randomly and irregularly and cannot be influenced in a targeted manner. In the course of the manufacturing process of a semiconductor component, for which the molybdenum wafers are used, temperatures of over 100 ° C occur during several work steps from the alloying of the diffused silicon body to the chromium-coated molybdenum wafer to the installation in a hermetically sealed housing, as a result of which the above-mentioned are evident Oxidation processes are promoted.

The poor and fluctuating contact resistances are observed particularly when the chromium is electroplated, but cannot be avoided even when the chromium is vapor-deposited. In addition, a cumbersome and considerably more expensive method must be used for a coating by vapor deposition. The advantage of chromium, which is cheaper than precious metals, is more than offset by the inefficient coating process. For the reasons mentioned above, therefore, the chromium as coating material has not found its way into practice and has not been able to replace the usual thicker precious metal layers.

The object of the invention is to reduce the high need for noble metal in the coating of molybdenum, while maintaining the favorable contact resistance, the good adhesive strength and the known other advantages of the noble metals, but also the above-mentioned disadvantages of other materials and processes, such as lack Avoid etch resistance, more effort in preparing the base material, mechanical sensitivity, fluctuating contact resistance or inefficiency.

In the case of molybdenum which is coated with a noble metal, this object is achieved according to the invention in that the noble metal in a layer thickness of 0.02 ... 1.0 µm, preferably 0.1 wm, via an intermediate layer of chromium with a layer thickness of 0.5 ... 10.0 µm, preferably 1.5 µm, is applied to the molybdenum.

With the invention it is achieved that the need for precious metal can be reduced to an amount that is almost two orders of magnitude less, the proportion of the cost of the precious metal in the entire process now being of minor importance. At the same time, the layer sequence has excellent etching resistance despite the lower proportion of noble metal compared to the known etching solution mixtures of nitric acid, hydrofluoric acid, acetic acid and optionally phosphoric acid and shows surprisingly good and uniform contact resistances. Another advantage of the chrome-gold layer sequence compared to pure precious metal coatings, for example gold coatings, is achieved by their better wear resistance. Since, at the same time, the known advantages of galvanic processes can be exploited in the production of the layer sequence and complex vapor deposition processes can be avoided, the use of the layer sequence according to the invention is extremely economical and suitable for precious metals such as gold, silver, platinum, palladium, rhodium, ruthenium.

It can be assumed that the excellent properties of the layer sequence according to the invention are based on a diffusion of the noble metal into the chromium surface, which is thereby stabilized against undesired oxidation processes. By reducing the electron work function, the contact resistance is also reduced accordingly.

• a) Vorentfettung des Molybdäns in einem organischen Lösungsmittel
• b) Spülen in Wasser
• c) Reinigungsätzen des Molybdäns
• d) Spülen in Wasser
• e) Wiederholen des Reinigungsätzens gemäß c)
• f) Spülen in Wasser
• g) Behandeln in einem alkalischen Entfettungsbad
• h) Spülen in Wasser
• i) Aktivierung der Molybdänoberfläche
• j) Galvanisches Beschichten des Chroms
• k) Spülen in Wasser
• I) Kathodische Aktivierung der Chromschicht
• m) Spülen in Wasser
• n) Galvanisches Beschichten des Edelmetalls
• o) Spülen in Wasser
• p) Tempern der aufgebrachten Beschichtungen unter einem Wasserstoff-Schutzgas

Another part of the invention is seen in a method with which the layer sequence of molybdenum, chromium and noble metal is advantageously produced. This process is characterized by the sequence of the following - partly known - process steps:

• a) Pre-degreasing the molybdenum in an organic solvent
• b) Rinse in water
• c) cleaning etching of the molybdenum
• d) rinsing in water
• e) repeating the cleaning etching according to c)
• f) rinsing in water
• g) Treat in an alkaline degreasing bath
• h) Rinse in water
• i) Activation of the molybdenum surface
• j) Electroplating the chrome
• k) Rinse in water
• I) Cathodic activation of the chrome layer
• m) rinsing in water
• n) Electroplating the precious metal
• o) Rinse in water
• p) annealing the applied coatings under a hydrogen protective gas

It has proven advantageous to carry out process step c) in a solution of 1 part by volume of concentrated sulfuric acid, 1 part by volume of concentrated nitric acid and 3 parts by volume of concentrated phosphoric acid at room temperature for a period of 15 and the step g) at a current density of about 20 A / allow dm ^{2 to run} at room temperature for a period of 30 s. It is also expedient in process step i) to activate the molybdenum surface in concentrated hydrochloric acid at room temperature for a period of about 15 s; according to j) the molybdenum with chromium in a bath of 400 g Cr ₂ 0 ₃ and 4 g H ₂ S0 ₄ per liter at a current density of 15 A / dm ² at a temperature of 50 ± 5 ° C for a period of 8 min to coat; 1) the chromium layer in 1 ... 15% sulfuric acid, preferably 8% sulfuric acid, at a current density of 2 ... 40 A / dm ² , preferably 20 A / dm ² , at room temperature for a period of 2. .. 60 s, preferably 20 s, to be activated cathodically; according to n) to coat the chrome layer with gold in a weakly acidic to neutral gold bath at a cover current density of about 3 A / dm ² at a temperature of 50 ± 5 ° C. for a period of 10 s; according to n) to coat the chrome layer with silver in a cyanide pre-silver bath at a cover current density of about 2 A / dm ² at a temperature of 20 ± 3 ° C for a period of 20 s; according to n) to coat the chrome layer with platinum in an acid bath at a cover current density of about 3 A / dm ² at a temperature of 50 ± 5 ° C. for a period of 15 s; according to n) to coat the chromium layer with palladium in a neutral bath at a cover current density of about 3 A / dm ² at a temperature of 45 ± 5 ° C. for a period of 15 s; according to n) coating the chromium layer with rhodium in an acid bath at a cover current density of about 3 A / dm ² at a temperature of 50 ± 5 ° C. for a period of 25 s; according to n) coating the chromium layer with ruthenium in an acid bath at a cover current density of about 3 A / dm ² at a temperature of 70 ± 5 ° C. for a period of 20 s; according to p) the applied coatings at a temperature of 500 ... 1 100 ° C, preferably for gold, platinum, palladium, rhodium and ruthenium at a temperature of about 750 ° C, preferably for silver at a temperature of about 950 ° C to anneal for a period of 10 min.

Claims (15)

1. Molybdenum coated with precious metal, characterised thereby, that the precious metal is applied onto the molybdenum in a layer thickness of 0.02 to 1.0 microns, preferably 0.1 microns, over an intermediate layer of chrome with a layer thickness of 0.5 to 10.0 microns, preferably 1.5 microns.

2. Molybdenum coated with precious metal according to claim 1, characterised thereby, that gold, silver, platinum, palladium, rhodium or ruthenium is used as precious metal.

3. Process for the electro-plating of molybdenum with a layer sequence according to claim 1 or 2 characterised by the sequence of the following, partially known process steps :

a) Preliminary degreasing of the molybdenum in an organic solvent,

b) rinsing in water,

c) etching clean of the molybdenum,

d) rinsing in water,

e) repeating the etching clean according to c),

f) rinsing in water,

g) treating in an alkaline degreasing bath,

h) rinsing in water,

i) activation of the molybdenum surface,

j) electro-plating with chrome,

k) rinsing in water,

I) cathodic activation of the chrome layer,

m) rinsing in water,

n) electro-plating with the precious metal,

o) rinsing in water and

p) heating of the applied coatings under a protective hydrogen gas.

4. Process according to claim 3, characterised thereby, that etching according to c) is done at room temperature for a duration of 15 seconds in a solution of 1 part by volume of concentrated sulfuric acid, 1 part by volume of concentrated nitric acid and 3 parts by volume of concentrated phosphoric acid.

5. Process according to claim 3, characterised thereby, that treating according to g) is done at room temperature for a duration of 30 seconds at a current density of about 20 amps per square decimetre.

6. Process according to claim 3, characterised thereby, that the molybdenum surface is activated according to i) in concentrated hydrochloric acid at room temperature for a duration of about 15 seconds.

7. Process according to claim 3, characterised thereby, that the molybdenum is coated according to j) with chrome in a bath of 400 grams of Cr0₃ and 4 grams of H₂S0₄ per litre at a current density of 15 amps per square decimetre at a temperature of 50 ± 5 °C for a duration of 8 minutes.

8. Process according to claim 3, characterised thereby, that the chrome layer is cathodically activated according to 1) in 1 to 15 % sulfuric acid, preferably 8 % sulfuric acid, at a current density of 2 to 40 amps per square decimetre, preferably 20 amps per square decimetre, at room temperature for a duration of 2 to 60 seconds, preferably 20 seconds.

9. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with gold in a slightly acidic to neutral usual gold bath at a surface current density of about 3 amps per square decimetre at a temperature of 50 ± 5 °C for a duration of 10 seconds.

10. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with silver in a cyanidic preliminary silver bath at a surface current density of about 2 amps per square decimetre at a temperature of 20 ± 3 °C for a duration of 20 seconds.

11. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with platinum in an acidic bath at a surface current density of about 3 amps per square decimetre at a temperature of 50 ± 5 °C for a duration of 15 seconds.

12. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with palladium in a neutral bath at a surface current density of about 3 amps per square decimetre at a temperature of 45 ± 5 °C for a duration of 15 seconds.

13. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with rhodium in an acidic bath at a surface current density of about 3 amps per square decimetre at a temperature of 50 ± 5 °C for a duration of 25 seconds.

14. Process according to claim 3, characterised thereby, that the chrome layer is coated according to n) with ruthenium in an acidic bath at a surface current density of about 3 amps per square decimetre at a temperature of 70 ± 5 °C for a duration of 20 seconds.

15. Process according to claim 3, characterised thereby, that the applied coatings are heated according to p) for a duration of 10 minutes at a temperature of 500 to 1 100 °C, preferably at a temperature of about 750 °C in the case of gold, platinum, palladium, rhodium and ruthenium and preferably at a temperature of about 950 °C in the case of silver.