WO2004068918A2

WO2004068918A2 - Method for producing thin silver layers

Info

Publication number: WO2004068918A2
Application number: PCT/IB2004/000230
Authority: WO
Inventors: James Anthony Cairns; Graham James Berry; Gary John Callon; Robert Dermot Smith; Barry Gowans
Original assignee: Aktina Limited
Priority date: 2003-01-31
Filing date: 2004-01-29
Publication date: 2004-08-12
Also published as: WO2004068918A3

Abstract

The present invention relates to a method for producing thin metal layers on printed circuit boards, in particular, silver coated printed circuit boards. The method of producing a thin film silver coated multilayer structure comprises the steps of providing a polyimide coated dielectric substrate; coating the substrate with a solution containing zirconium propionate and palladium acetate; heating the substrate to dry the coating; and immersing in electroless silver plating solution until a layer of silver of desired thickness is deposited. The invention provides maximum benefit from conductor surface treatment of pcb's to minimize skin effect losses.

Description

METHOD FOR PRODUCING THIN SILVER LAYERS

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method for producing thin metal layers on printed circuit boards. More particularly, the present invention concerns thin film silver layers providing maximum benefit from conductor surface treatment of pcb's to minimize skin effect losses. Background of the Invention

Data rate of signals transmitted over PCB in electronic equipment, such as computers, network switches and routers, servers or supercomputers is reaching level at which dissipation in the PCBs could be as big as 10 times over single transmission line. This limits data rate and/or distance of the channel. There are two major components which cause losses. They are dielectric losses in the insulator and losses caused by limited conductivity of the metal used to transmit signals. At frequencies above 1GHz and distances up to 80" both components are in same order of magnitude.

An extensive work is carried out to make insulators with lower dielectric losses for such high speed interconnect systems. In particular, very good results are achieved with PTFE based (Teflon) materials which reduce dielectric losses and make skin effect losses dominating.

Moreover, skin effect losses are amplified when conductor surface roughness, i.e. average variations in the metal surface height, exceeds the depth of the skin layer. This effect causes losses to be growing much faster with higher frequencies than it will be with flat surface.

One of the known approaches to reduce this effect is additional surface treatment to reduce roughness from typical 15μm down to 5μm and less. For example, US 5,215,645 describes a process for preparing a low profile copper foil for printed circuit boards by using a special electrodeposition bath solution comprising a surface roughness decreasing agent.

Other known approach to solve this problem is to provide extra process steps where core is additionally flattened and then metal of the conductor is built having bottom layer also flat. However, this method might not be suitable for low cost applications due to more complicated processing involved. A double treated copper foil is mentioned also in "Surface Roughness" by Howard Johnson, in Electronic Design News, pages 30-32, December 6, 2001.

A process of preparing a PCB's package having good electrical characteristics at frequencies in excess of 1 GHz is disclosed in US 5,268,064. To counteract skin effect losses, the process avoids oxidising step so that the PCB package obtained comprises a double sided double treated copper clad laminate used as an inner layer in which propagation speeds are not worsened by oxide. However, the known method of preparing PCB package requires some additional steps that makes the process more complex.

Other multilayer PCB packages are known comprising single and double sided copper clad laminates that have only single treated copper foil. However, typically, they have significantly lower characteristics.

DETAILED DESCRIPTION OF THE INVENTION

Thus, there is a need in providing a method of preparing thin metal film coated pcb materials having excellent conductivity combined with reduced signal losses, using conventional materials and relatively inexpensive surface treatment techniques. Attempts have been made by the inventors of the present invention to deposit a thin silver layer directly by evaporation onto the SU-8 or other polyimide coated pcb, such as FR4, however the obtained silver layer exhibited extremely poor adhesion and came off very easily when 'Scotch' tape was applied or indeed when subjected to gentle abrasion.

Surprisingly, it was found that small amounts of ceramic precursors applied onto pcb surface prior to metal coating result in the increased adhesion of the metal layer and smooth low dissipation silver coatings on pcb surface. Thus, according to the invention, a multilayer printed circuit board is provided comprising a plurality of planar layers made of dielectric material and a thin metal film secured to these dielectric material layers, wherein a thin porous ceramic layer is incorporated between the metal layer and dielectric material. Further, conductive traces can be formed in the thin metal film by using conventional methods, such as photolithography and wet etching.

The surface of the core layer can be coated preliminary with SU-8 or another polyimide to produce a very flat surface, such as flat to within 15nm over short distances. According to the invention, a method of producing a thin film silver coated multilayer structure comprises the steps of:

- providing a polyimide coated dielectric substrate;

- coating the substrate with a solution containing zirconium propionate and palladium acetate; - heating the substrate to dry the coating;

- immersing in electroless silver plating solution until a layer of silver of desired thickness is deposited.

In another aspect, a low dissipation silver plated pcb structure is provided comprising: - a polyimide coated dielectric substrate;

- an intermediate ceramic zirconia-palladium layer at least partially protruding into the polyimide coated substrate; and

- a thin silver layer deposited onto the ceramic layer to a thickness of from 50 nm to 350 nm.

EXAMPLE !

A square of FR4 laminate circuit board material, approximately 5cm by 5cm was spin coated with Microchem Inc. 2015 SU-8 photoresist, by spinning at lOOOrpm for 50 seconds. The substrate was then heated on a hot plate to 95°C for 10 minutes and the SU-8 layer was then exposed to ultraviolet light of wavelength 365nm for 20 seconds. The substrate was then baked on a hot plate for 10 minutes at 95°C and then heated in an oven at 150°C for 30 minutes. Figure 1 schematically illustrates the SU-8 coated FR4.

Figures 2a and 2b show a comparison between the surfaces of the uncoated FR4 and the SU-8 coated FR4 and it can be seen that the SU-8 coating yields a much smoother surface.

The substrate was immersed in dichloromethane for 2 minutes, rinsed with isopropanol and dried in a jet of nitrogen. A solution containing 1g of zirconium propionate and 50mg of palladium acetate, dissolved in 10ml of dichloromethane, was then spin coated onto the SU-8 layer at 3500rpm for 20 seconds. The substrate was then heated to 170°C for 1 hour in a furnace. The substrate was immersed in a 5% solution of tin chloride and then thoroughly rinsed with deionised water. The substrate was then immersed in an electroless silver plating solution as supplied by Peacock Laboratories Inc. Philadelphia, USA until a layer of silver had been deposited. This solution consisted of: A: HE-300 silver solution B: HE-300 activator solution C: HE-300 reducer solution

In a typical procedure, component solution A was diluted with water in the ratio 1A to 30 water by volume. Component solution B was diluted 1 B to 30 water by volume. Component solution C was diluted 1C to 100 water by volume. The final mixture contained equivolumes of these diluted component solutions. After silver plating, the substrate was rinsed in deionised water and dried in a jet of nitrogen. Figure 3 illustrates schematically the silver coated substrate.

It was then found that the silver exhibited excellent adhesion to the substrate. This was demonstrated by applying a layer of 'Scotch' tape to the silver and attempting to peel it off. It was found that the silver remained on the substrate and that none had been transferred to the tape. It was also demonstrated that the silver layer could be patterned by photolithography and wet etching, employing a solution comprising: 10ml hydrogen peroxide, 10ml ammonium hydroxide and 40ml of methanol. The photolithography process involved spin coating a layer of Microposit S1813 photoresist onto the silver and then heating the substrate to 90°C for 10 minutes on a hotplate. The layer of photoresist was then exposed to ultraviolet light, of wavelength 365nm, through an appropriate photomask and subsequently developed in Microposit MF319 developer. The substrate was then baked on a hotplate at 120°C for 20 minutes prior to immersion in the silver etching solution. The remaining photoresist was then removed by rinsing in acetone to yield the desired, patterned layer of silver on the substrate.

The thickness of the silver was measured as being approximately 100nm and after patterning, the adhesion was still excellent. Additional samples were made by the same process and it was found that the thickness of the silver was dependent on the concentration of the electroless silver solution and the length of time that the sample was plated for.

Figure 4 illustrates a patterned layer of silver on the SU-8 coated FR4. Example 2.

A solution containing 1.5g of zirconium propionate dissolved in ethanol can be spin coated onto the surface of the SU-8 coated FR4, or onto polyester sheet and then thermally cured at 150°C for 20 minutes on a hotplate. The SU-8 coated FR4 is initially immersed in acetone for 2 minutes, followed by a rinse in isopropanol prior to coating with the zirconium propionate in ethanol solution. These substrates can then be subjected to the electroless silver deposition process to produce a well adhered silver layer on the substrates. The silver can be patterned by photolithography and wet etching with a solution containing; methanol, ammonium hydroxide and hydrogen peroxide. Electrical resistivity measurements have been made on these silver layers and a value of 5.56 micro ohm centimetres has been obtained for a film which is 160nm in thickness.

Claims

1. A method of producing a thin film silver coated multilayer structure comprising the steps of: - providing a polymer coated dielectric substrate that withstand at least

150°C;

- coating the substrate with a coating solution containing zirconium propionate;

- heating the substrate to dry the coating; and - immersing in electroless silver plating solution until a layer of silver of desired thickness is deposited.

2. A method according to claim 1 , wherein the coated substrate is heat dried at temperatures from 150 to 200°C.

3. A method according to claim 1 , wherein the dielectric substrate is made of flexible plastic.

4. A method according to claim 1 , wherein the dielectric substrate is made of transparent plastic, e.g. such as used for producing displays.

5. A method according to claim 1 , wherein zirconium propionate is dissolved in dichloromethane.

6. A method according to claim 1 , wherein zirconium propionate is dissolved in ethanol.

7. A method according to claim 1 , wherein the coating solution further contains palladium acetate.

8. A method according to claims 2 or 3, wherein the silver layer is further patterned by photolithography and wet etching.

9. A low dissipation silver plated pcb structure comprising:

- a polyimide coated dielectric substrate;

- an intermediate ceramic zirconia-palladium layer at least partially protruding into the polyimide coated substrate; and - a thin silver layer deposited onto the ceramic layer to a thickness of from 50 nm to 350 nm. 5. A silver coated pcb structure according to claim 4, wherein the silver layer is patterned by photolithography and wet etching.