DE3833441A1 - Process for metallising aluminium oxide substrates - Google Patents

Abstract

To metallise aluminium oxide substrates, the substrate is first etched using a mixture of phosphoric acid/sulphuric acid, the proportions by weight being from 1:5 to 5:1, at elevated temperature. The substrate is then rinsed, dipped into an activation bath containing a noble metal or copper, rinsed again and then chemically metallised in a metallisation bath.

Description

The invention relates to a method for metallizing Alumina substrates, starting with the substrate with a mixture of phosphoric acid and sulfuric acid etched on the surface and then on the etched surface deposits a metal without current.

Aluminum oxide substrates are usually after sintering so smooth that it's hard to put layers of metal on them to deposit firmly. One has therefore tried the Roughen substrate surfaces by etching.

According to DE-OS 34 21 988 used for the etching of Alumina Substrates Melting Alkali Metal Compounds, especially alkaline compounds such as Alkali hydroxide or alkali carbonate. Working with However, alkaline melting is dangerous and uncomfortable. Man can instead the substrates to be treated in aqueous Dip alkaline solutions and then in the oven heat up until a thin, rough surface Film has formed a melt (US-PS 36 90 921). These Variant is time consuming and is not well suited for Automation. Another disadvantage is that the substrates when Removing from the oven can be subjected to severe thermal stress.

According to DE-OS 33 45 353, the surface of a ceramic substrate, for. B. from Al ₂ O ₃ , in a medium containing hydrofluoric acid, possibly in combination with an upstream treatment by sodium hydroxide. A disadvantage of using hydrofluoric acid is its toxicity and aggressiveness towards skin surfaces. According to our own observations, substrates with over 96% Al ₂ O ₃ can no longer be sufficiently roughened by fluoride etching solutions at room temperature and consequently metal layers adhering to them cannot be obtained.

According to US Pat. No. 3,296,012, 85% phosphoric acid is used for the etching of aluminum oxide substrates. The substrate with porous surface thus obtained is rinsed, optionally activated with tin / palladium ions and rinsed again. Finally, copper, e.g. B. from alkaline copper tartrate solutions. This process provides sufficient adhesive strength values for alumina ceramics with a purity below 80% Al ₂ O ₃ . In contrast, it is less suitable for substrates with a higher content of Al ₂ O ₃ .

According to Japanese Patent Application Sho-61-1 51 081 for etching mixtures of aluminum oxide Phosphoric acid and an inorganic acid (nitric acid, Hydrochloric acid and fluoric acid). These acids are volatile and therefore in the applied Treatment temperatures of 330-390 ° C in the liquid Phase practically no longer exists. You are less supposed to Etching process better than that with dehydration ongoing formation of highly condensed phosphates suppress and so the lifespan of the phosphoric acid baths extend. This method has the disadvantage that because of the volatile components at the specified temperatures Pressure vessels are required.

The object was therefore to find a method with which aluminum oxide substrates over 85% Al ₂ O ₃ , in particular over 90%, in particular over 95%, preferably over 99% Al ₂ O _{3 can} still be sufficiently etched. In particular, the etching should proceed in such a way that the subsequently applied metal layers showed high adhesive strength and optimal structurability. Good adhesive strength is important for later processing, e.g. B. for soldering power leads (lead frames) or housing covers using soft or hard solder.

These disadvantages are indicated by the claim 1 Process according to the invention eliminated.

The surface rinsed after the etching can be dried or wet processed further.

After the method according to the invention the metals silver, gold, Palladium, platinum, nickel, cobalt and especially copper separate. The etching temperature is 200-300 ° C, preferably 200-280 ° C, especially 200-250 ° C. At Temperatures above 250 ° C can be too strong Coming out of sulfuric acid.

At temperatures of 220 ° C., the etching temperature is preferably at least 20 minutes, in particular at least 45 minutes. It has been shown that the adhesive strength of a copper layer applied to the etched surface increases with increasing etching duration, namely according to the mathematical relationship H = a + b · log t . H means the adhesive strength and t the duration of the etching; a and b are constants that depend on the substrate, temperature and bath composition. It is favorable for good adhesive strength values if the etching solutions contain small amounts of water, in particular 5-10% by weight. This also coincides with the fact that used solutions attack more slowly over a longer period of time and (with a constant exposure time) lead to lower adhesive strength values.

It is advantageous if after etching the substrate surface the ceramic is treated with solutions that increase the adsorption favor from metal germs. For this are on sale  "Conditioner solutions" available. With that the Process reliability increased and later the share was imperfect metallized substrates reduced.

The substrate will then be catalyzed. Baths which contain ions of metals with a low oxidation potential, for example of copper or precious metals such as silver, are particularly suitable for this. Solutions containing Pd ⁺⁺ ions are preferred. "Catalyst" solutions that contain a colloidal noble metal (eg colloidal palladium) and preferably a protective colloid can also be used. When using colloidal solutions, the surrounding protective colloid is then oxidatively removed by an "activator". Silver and copper solutions also have a catalytic effect.

High concentrations of Pd ²⁺ can have a favorable effect on the adhesive strength of the metallization layers.

If the catalyst bath contains Pd ⁺⁺ ions, a reducing agent is necessary for their reduction. The reducing agent can already be added to the bath containing noble metal ions. You can also immerse the etched substrate first in a bath with precious metal ions ("activator") and then in a bath of the reducing agent ("reductor"). In both cases, germs of the noble metal are formed on the surface of the substrate, which allow the later deposition of the metal from the metallization bath.

Before etching, the surface of the substrate must be thorough getting cleaned. All organic assignments are included to remove e.g. B. by degreasing with trichloroethane or treatment with inorganic acids.

The metallization baths that can be used are known to the person skilled in the art known and commercially available. A copper containing bath  is described in US-PS 34 21 955. The currentless Deposition of nickel and copper is described in the Textbook "Practical Electroplating, Eugen G. Lenze Verlag, Saulgau / Württ., 1970).

Within the specified range of phosphoric acid / sulfuric acid, it makes sense to increase the proportion of sulfuric acid with increasing substrate alumina content. In the case of substrates with a content of 95 to 98.5% Al ₂ O ₃ , etching baths with a composition of phosphoric acid / sulfuric acid of 1: 1 to 3.5: 1 have proven successful. In the case of substrates with a purity of at least 99%, preferably 99.4 to 99.8% Al ₂ O ₃ , etching baths with a weight ratio of phosphoric acid / sulfuric acid of 0.8: 1 to 1.2: 1 have proven particularly useful.

The adhesive strength of copper layers on a post deposited the etched substrate is higher than with a pure etching Phosphoric acid.

While it is always when etching the substrates with phosphoric acid There are areas that are completely unetched and not copper-plated this effect occurs with mixtures with phosphoric acid / Only a minor amount of sulfuric acid. The yield on strongly metallized substrates is therefore clear higher.

The comparison of the surface morphology of the pure phosphoric acid etching with the mixture etching of phosphoric acid and sulfuric acid shows that the pure phosphoric acid exerts such a strong grain boundary attack that, for. T. Grains lie loosely on the surface, ie without an adhesive bond to the ceramic matrix. On the other hand, the mixture etching achieves a strongly jagged three-dimensional network, which offers very good anchoring points for the metal layers to be deposited.

The invention is illustrated by the following examples explains:

example 1

An alumina substrate (purity 99.6% Al ₂ O ₃ ) was cleaned with trichloroethane and dried at 160 ° C. The dried substrate was immersed in a hot etching solution of 50% by weight phosphoric acid (85%) and 50% sulfuric acid (97%) at 220 ° C. for 20 minutes. The substrate was then rinsed intensively with warm and cold water and neutralized in a solution containing sodium carbonate. It was rinsed with water with simultaneous treatment with ultrasound in order to remove any loose particles from the substrate surface.

After drying in the drying cabinet for 30 minutes The substrate was placed in a 200 ° C for 10 minutes commercially available activator solution (activator neoganth, Manufacturer: Schering AG) immersed. This solution was alkaline and contained a palladium salt. After rinsing with deionized Water became a commercially available substrate Reductor solution (Neoganth 406 reductor, manufacturer: Schering AG) immersed. After rinsing again, it became Substrate immersed in a electroless copper plating bath, the copper sulfate, formaldehyde, ethylenediaminetetraacetic acid contained as complexing agents, sodium hydroxide solution and stabilizers (Metalyt Cu 40, Blasberg) and between 10 and 60 minutes metallized. The substrates treated in this way showed Adhesion strength values above 15 MPa in the point pull test.

Example 2

Example 1 was repeated, but the temperature was Etching reduced to 200 ° C. The value of the adhesive strength degraded.  

Example 3

Example 1 was at an etching temperature of 175 ° C repeated. It turned out that the separated Copper layer did not adhere to the substrate.

Example 4

Example 1 was repeated with a 96% substrate Aluminum oxide and an etching solution of 3 parts phosphoric acid and 1 part sulfuric acid. Even with this treatment Adhesive strength values of over 15 MPa in the point pull test reached.

Example 5

Example 1 was repeated with a commercially available Catalyst solution containing colloidal palladium. The average adhesive strength value was slightly lower than in Example 1.

Example 6

Example 1 was repeated, but became a copper containing catalyst solution used. The adhesive strength values were over 15 MPa.

Example 7

The substrate was treated as in Example 1, but was the drying step after the etching by treatment during 6 Minutes with a commercially available conditioner solution (Blasolit-MSH, Blasberg, Solingen) replaced. These Solution contained a wetting agent. The ionic Pd catalyst of Example 1 was replaced by a catalyst solution  with colloidal palladium ("catalyst solution K 125", company Blasberg). The determined adhesive strength values were somewhat higher than in example 1.

The adhesive strength values were determined using a so-called Deduction test determined. Because the storage of the substrates has an influence on the adhesive strength, the Tensile tests one week after the metallization of the Rehearsals performed.

For this purpose, the contact pins, which have a diameter of 2.5 mm and are soldered onto the copper substrates, are removed in a tractor at a constant removal speed of 0.25 mm / sec. The force required (measured in N) divided by the peel area (4.9 mm ² ) gives the adhesive strength in N / mm ² (= MPa).

Example 8

Substrates (99.6% Al ₂ O ₃ ) were etched with H ₂ SO ₄ / H ₃ PO ₄ (1: 1) at 200 ° C for 60 minutes. Activation by "Ronamet-Ronacat Activator", manufacturer: LeaRonal GmbH (7534 Birkenfeld 2). Catalysis using "Ronamet-Catayst M" (manufacturer: LeaRonal GmbH), containing colloidal copper.

After removal of excess catalyst, a Thick copper bath ("Ronamet-Ronadep 100", manufacturer LeaRonal GmbH) copper (approx. 10 µm) deposited.

The point deduction test gave adhesive strengths of on average 19.3 MPa (mean, standard deviation 0.4 MPa).  

Comparative Example 9

Example 8 was repeated but was etched with pure phosphoric acid (87% by weight) at 220 ° C. The average adhesive strength was 12 MPa (standard deviation 2.5 MPa). This shows that, despite the higher temperature, the pure phosphoric acid was inferior to the H ₃ PO ₄ / H ₂ SO ₄ mixture.

Example 10

Example 8 was repeated with reduced etching times. At Etching times of 30 minutes reduced the adhesive strength 16.5 MPa, with etching times from only 10 min to 13.8 MPa.

Claims (10)

1. A method for metallizing alumina substrates, wherein one

• a) etching the substrate with a mixture of phosphoric acid and an inorganic acid at elevated temperature,
• b) rinsing the substrate,
• c) if necessary, drying the substrate,
• d) immersing the substrate in a bath containing precious metal or copper,
• e) rinsing the substrate and
• f) chemically metallized in a metallization bath,

characterized in that the etching in step a) is carried out with a mixture of phosphoric acid / sulfuric acid, the weight fractions being 1: 5 to 5: 1. 2. The method according to claim 1, characterized in that one in step d) the substrate one after the other Bath containing precious metal ions and a bath for Immersion reduction of precious metal ions. 3. The method according to claim 1, characterized in that one in step d) the substrate one after the other colloidal precious metal and a protective colloid containing bath and a bath to destroy the Protective colloids immersed. 4. The method according to claim 2, characterized in that the bath containing noble metal ions contains Pd ²⁺ ions. 5. The method according to claim 3, characterized in that the bath contains colloidal palladium.   6. The method according to claim 1, characterized in that the substrate has a purity of 95 to 98.5% of Al ₂ O ₃ and the etching bath has a phosphoric acid / sulfuric acid composition of 1: 1 to 3.5: 1. 7. The method according to claim 1, characterized in that the substrate has a purity of at least 99% Al ₂ O ₃ and the weight ratio phosphoric acid / sulfuric acid 0.8: 1 to 2: 1, in particular 0.8: 1 to 1 in the etching bath , 2: 1 is. 8. The method according to claim 1, characterized in that one etches at temperatures from 200 to 280 ° C. 9. The method according to claim 1, characterized in that in step f) a chemical copper plating bath is used becomes.