DE4026710A1 - METHOD FOR GOLD PLATING CERAMIC CIRCUITS - Google Patents

Abstract

Further gold coating of an initial gold layer on 300-3300 Angstroms thick TiW bond layers of ceramic circuits is carried out by (i) introducing the thin film circuit into an electroplating bath contg. 50 g/l. diammonium hydrogen citrate, 50 g/l ammonium sulphate, 20 g/l potassium cyanoaurate and 0.5-5 ppm Pb(II) ions (as lead acetate); (ii) setting the current density to 1-6m A/sq.cm. and electroplating until the desired gold thickness is achieved; and (iii) heat treating the ceramic circuit. USE/ADVANTAGE - The method is useful in the semi-additive or subtractive process for prodn. of thin film circuits comprising a ceramic substrate having a TaN/TiW/Au layer system on one side and a TiW/An layer system on the other side. It allows use of a TiW bond layer instead of a NiCr bond layer (to reduce underetching and allow use of higher temps. for resistance stabilisation) while reducing blister formation.

Description

To produce thin-film circuits, ceramic substrates are coated on one side with Ta _x N _y (for resistors), NiCr (or TiW) and Au (approx. 1000 Å), and on the other side with NiCr (or TiW) / Au.

Then the substrates are used in the so-called semi-technique Structured photoresist and in an acidic fine gold bath galvanically reinforced (usually 3-10 µm).

In the subtractive process, the substrates are first galvanized reinforced over the entire surface, then structured with photoresist and etched.

An acidic solution is used to galvanically strengthen the substrates Fine gold bath, such as B. is described in DE 33 41 233 A1, used.

Up to now, only NiCr has been used as an adhesive layer in production used because the following problems occur with TiW adhesive layers:

The substrates need to be the resistive layer and the Stabilize surface structure of gold, annealed will. After this aging process occur in the layer system TaN / TiW / Au bubbles (so-called blisters) in the gold layer,  Depending on the batch, up to 20 blisters in up to 80% of the substrates on each side of the substrate. Step on the TiW / Au layer system also blister, but usually less than in Layer system TaN / TiW / Au.

If there is a blister on a conductor track, the Circuit unsuitable.

For this reason, TiW has so far only been used as an adhesive layer used to a limited extent.

However, the introduction of TiW as an adhesive layer instead of NiCr desirable because the undercut compared to NiCr as an adhesive layer is much less and also to Resistance stabilization higher temperatures are applied can.

The object of the present invention is therefore a method of the type mentioned above, which makes it possible to TiW as Adhesive layer to use at the same time essential reduced blistering.

The solution to this problem is specified in claim 1.

The sub-claim describes an advantageous embodiment of the Method according to claim 1.

The method according to the invention is described in detail below described.

A cleaned and annealed substrate is in a Sputtering system using a reactive sputtering process with TaN (Resistance layer), then with a thickness of approx. 2000 Å Adhesive layer made of TiW (10% Ti, 90% W) and then with a Initial gold layer approximately 2000 Å thick. A Variant is that the TaN layer is omitted.  

Depending on the technology used for further processing, can either before or after the galvanic amplification of the Gold layer the desired structure using photolithography be applied.

To strengthen the gold layer, an acidic, watery Fine gold bath, which is composed of

50 g / l diammonium hydogen citrate,
50 g / l ammonium sulfate,
20 g / l potassium dicyanoaurate;
and,
2 ppm Pb ²⁺ (as lead acetate).

The pH of this electroplating bath is 4.5-4.9. The Temperature is kept at 60 ° C.

The substrate is switched as a cathode and a gold layer of about 5 μm thick is applied in 40 minutes with a set current density of 3 mA / cm ² . It has been found that if the lead concentration is further reduced, it is expedient to grip the substrates in holders with a conductive surface in order to obtain a uniform gold layer thickness. Subsequent to the strengthening of the gold layer, the substrate is annealed at 275 ° C.

After using the described invention Procedure, none occurred in over 90% of the treated substrates Blister more on.

With a TiW layer thickness of 2000 Å, a lead content of 2 ppm Pb ²⁺ F and current densities up to 4 mA / cm ² , the number of blisters approaches a minimum. If the current density is increased, the number of blisters increases in inverse proportion to the thickness of the TiW layer. The lowering of the lead content compared to the prior art mentioned has no influence on the current efficiency, deposition rate and gold surface structure.

Claims (2)

1. Method for further gold coating of ceramic circuits with adhesive layers made of TiW, the thickness of which is between 300 and 3300 Å (and to which an initial gold layer is applied) with the following method steps:

• a) The thin-layer circuit is placed in an electroplating bath with the composition
  50 g / l diammonium hydrogen citrate,
  50 g / l ammonium sulfate,
  20 g / l potassium dicyanoaurate,
  0.5-5.0 ppm Pb ²⁺ as lead acetate,
  brought in.
• b) The current density is set to a value between 1.0 mA / cm ² and 6 mA / cm ² .
• c) The duration of the galvanization is set depending on the desired gold layer thickness.
• d) The ceramic circuit is annealed.

2. The method according to claim 1, characterized in that the TiN layer thickness 2000 Å, the proportion of lead acetate 2 ppm and the current density equal to 4 mA / cm ^{2 is} selected.