WO2002083980A1

WO2002083980A1 - Electroless plating

Info

Publication number: WO2002083980A1
Application number: PCT/IE2002/000044
Authority: WO
Inventors: James F. Rohan; John Barrett; Patricia A. Murphy; Jane Boardman; Elaine Geary; Alan Mathewson
Original assignee: University College Cork
Current assignee: University College Cork
Priority date: 2001-04-12
Filing date: 2002-04-11
Publication date: 2002-10-24
Anticipated expiration: 2003-10-12
Also published as: IE20020264A1

Abstract

An A1 bond pad (1) of a semiconductor metallisation is coated with Ni (4) in a simple process. The substrate is etched to remove oxide, and is subsequently rinsed in a non-aqueous liquid. With the non-aqueous liquid (acetone) still on the surface the substrate is placed in an electroless bath having a salt complex containing metal ions as cations and a reducing agent as anions. The bath temperature is higher than boiling point of the non-aqueous rinse solution so that there is immediate evaporation.

Description

"Electroless Plating"

INTRODUCTION

Field of the Invention

The invention relates to deposition of a metal such as nickel (Ni) on a semiconductor metallisation substrate such as aluminium (Al) or copper (Cu) or alloys of these, and barrier layer materials such as titanium nitride (TiN), titanium tungsten (TiW) or tantalum nitride (TaN) .

Prior Art Discussion

Such metals are very commonly used in the electronics industry. For example Al or Cu may be used for bond pads on a wafer, which are also partly covered by a polyimide passivation layer, or the switch contact of a microelectromechanical device. In this example, there is a need to selectively coat the Al with Ni. In another example, the metallisation may comprise Al or Cu coated with TiN, TiW or TaN to provide a coating which is protective, anti-reflective, and a diffusion barrier.

To coat such substrates with Ni there is a need to deal with an oxide on the surface. For example, A1₂0₃ forms on Al, TiO on TiN or TiW and Ta₂O_s on TaN.

One existing method is zincation treatment referred to in United States Patent Specification Numbers US5437887 and US4840820. This treatment involves immersing the sample in a series of basic and acidic baths to replace the aluminium ions on the surface with zinc. The substrate is then plated in an electroless Ni plating solution. A problem with this approach is that a significant layer of aluminium is removed and thus it is not suitable for thin aluminium films. Another disadvantage is that the method exposes the substrate to ionic species without which it would not otherwise come into contact.

Another existing approach is to immerse the substrate in a bath containing metal salts and acid or base. This removes the surface oxide layer and deposits a thin metal layer such as Ni, Cu, Cr, or Pd. The substrate is then plated in an electroless Ni plating solution. This method is referred to in United States Patent Specification number US4670312. A problem with this method is that the surfaces other than the Al or TiN (such as the bond pads) are activated to some extent. For example, surrounding dielectric surfaces may be activated.

Therefore, it is an object of the invention to provide an electroless plating method which has the following improvements over existing technologies :-

- is simpler, requiring fewer steps, and/ or - has improved selectivity, and/ or

- does not expose the substrate to potentially harmful foreign species.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method for deposition of a material onto a semiconductor metallisation substrate, the method comprising the steps of:

cleaning the substrate, and

plating the substrate with the material in an electroless bath,

wherein the substrate is cleaned by etching until oxide is removed and the surface is activated and subsequently rinsing the substrate in a non-aqueous liquid to prevent re-oxidation, and

electroless plating is initiated while some non-aqueous solvent remains on the substrate surface and the operating temperature of the electroless bath is higher than the boiling point of the non-aqueous liquid.

In one embodiment, the electroless both comprises a salt complex containing metal ions as cations and a reducing agent as anions.

In another embodiment, the complex comprises Ni ions and a hypophosphite reducing agent.

In a further embodiment, the electroless bath further comprises a lubricant.

In one embodiment, the lubricant is in particulate form.

In another embodiment, the lubricant comprises Teflon™.

In a further embodiment, the electroless bath comprises a hardening component.

In one embodiment, the hardening component comprises silicon carbide in particulate form.

In another embodiment, the etching is performed in an aqueous basic solution.

In a further embodiment, the etching solution comprises NH₄OH, H₂0₂₎ and H₂0.

In one embodiment, the relative composition of NELjOH : H₂0₂ : H₂0 is 1 : 1: 20. In another embodiment, the non-aqueous rinse liquid comprises acetone.

In a further embodiment, the non-aqueous rinse liquid comprises propanol-2.ol.

In one embodiment, the substrate comprises Al deposited onto a semiconductor at a semiconductor temperature of less than 150°C.

In another embodiment, the substrate comprises an alloy of Al with Ti or Cu or Si.

In a further embodiment, the substrate comprises Cu.

In one embodiment, the substrate comprises a protective layer over the metallisation.

In another embodiment, the protective layer comprises TiN, TiW, TaN, TiSiN, or TaSiN.

In one embodiment, the substrate comprises bond pads, and the method applies Ni to the bond pads.

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which: -

Fig. 1 is a diagrammatic cross-sectional representation of a silicon wafer with Al bond pads before and after Ni plating, Fig. 2 is an equivalent representation in which the Al is initially coated with TiN; and

Fig. 3 is a flow diagram illustrating a plating method for the example of Fig.

1.

Description of the Embodiments

Referring to Fig. 1, in one embodiment Ni is coated on an Al bond pad 1 on a silicon wafer 2. A polyimide passivation layer 3 also overlies the wafer 2. Aluminium deposited on semiconductors as the integrated circuit metallisation is generally evaporated or sputtered. In some cases it has been found to be beneficial to heat the substrate during the deposition process to achieve for example better step coverage. It has been found in the present invention that low temperature (less than 150°C) deposited aluminium is more readily etched by the etchant described and kept active by the non-aqueous solvent step in the process until immersion in the nickel plating bath.

The process involves an initial etching step in an aqueous basic solution consisting of 1:1:20 NH₄OH:H₂0₂:H₂0 for 10 - 60 seconds. This removes the surface A1/A1₂0₃ present and exposes a clean fresh Al surface for electroless nickel deposition.

The sample is then immersed in an acetone solution for 30 s. This step removes excess etch solution from the surface but does not cause the oxidation of the fresh Al, TiN, TiW or TaN surface. The wafer sample is removed from the acetone and immediately immersed into a nickel plating bath. Residual acetone solvent remaining on the sample surface during the transfer of the sample from acetone to the plating bath protects the Al surface from oxidation at this point. The acetone remaining on the wafer surface evaporates immediately on immersion into the nickel bath. The operating temperature of the plating bath is substantially higher (85°C to 95°C) than the boiling point of acetone (56°C). Nickel is deposited selectively on the Al bond pads and not on the polyimide passivation layer. The Nickel is formed into bumps 4 on the bond pads. In more detail, the plating bath consists of:- Nickel sulphate in the range 10-30g/l Ammonium sulphate in the range 20-50g/l Calcium succinate in the range l-5g/l Lactic acid in the range 15-35g/l Nickel hyposphosphite 10-30g/l

The pH of the bath is adjusted to between 4.0-5.0 by the addition of ammonium hydroxide or sulphuric acid.

A similar method may be carried out for Al bond pads having a protective TiN, TiW or TaN layer 5, as shown in Fig. 2.

In electroless or autocatalytic deposition the plating occurs at the active substrate through the reaction of the substrate with the reducing agent, in this case the hypophosphite ions and the metal ions. In the present invention the plating bath is particularly active because the concentration of nickel ions in solution is higher than in commercially available electroless nickel baths through its incorporation in the reducing agent complex in addition to the nickel sulphate source. The present bath enables a high rate selective electroless deposition of nickel on the substrate immediately on immersion in the plating bath through evaporation of the protective non-aqueous solvent at the elevated operating temperature of the bath and the high nickel ion concentration together with low additive /stabiliser concentration in the bath.

As illustrated in Fig. 3, the plating method of the invention comprises generally the following three simple steps:- (a) Etching the sample in an aqueous solution to remove any oxides present and to activate the substrate surface to plating.

(b) Rinsing the sample in a bath of non-aqueous solvent. This step removes excess etch solution and protects the surface of the sample from re- oxidation in aqueous solution or in the air. The sample is immediately removed from the etch solution to the solvent rinse and from the solvent rinse to the plating solution.

(c) Immersing the sample in an electroless metal plating bath affords nickel plating on the Al and TiN, TiW or TaN surface and not on any dielectric surfaces present on the sample. Using a proprietary electroless nickel plating bath, rates of 20μm/h have been obtained.

The etching step is important. It both removes the oxides and activates the substrate surface for electroless plating. This avoids the need for a series of basic and acidic baths as in zincation and avoids the need for exposure to metal salts as in the second existing method. The surface is both cleaned of oxides and activated for electroless plating because a fresh aluminium surface is exposed upon immersion in the etch solution. Upon removal from the etch solution re-oxidisation of the aluminium surface is minimised by direct immersion of the sample into the solvent rinse and from this bath into the electroless nickel plating bath. A pure aluminium surface is active to electroless nickel plating.

The electroless nickel plating bath used in this process is particularly active. This bath contains less stabilising agents than standard commercial solutions, and thus will afford nickel deposition on substrates that standard commercial solutions cannot deposit as a result of their lower activity. The selectivity of the nickel plating obtained using this bath is excellent. The process of the invention justifies the cost of the electroless bath complex because of the benefits achieved.

An application of the selective deposition process of the invention is in CMOS compatible micromechanical switches where the IC metallisation substrate is plated with a switch contact material. To facilitate a longer lifetime switch the contact material is preferably a non stick and/or hard material. The hardness of the deposited Ni-P may be increased by annealing at up to 400°C. Alternatively lubricity and hardness may be introduced to electroless nickel deposits by the codeposition as a composite of organic materials such as PTFE or inorganic materials such as SiC, diamond or graphite.

The non-aqueous solvent may alternatively be any organic solvent with a boiling point that is less than the operating temperature on the electroless plating bath and which does not react adversely with the etch solution to form a film on the sample surface.

It will be appreciated that the invention provides a plating method which is very simple because it comprises only three steps, and these steps are relatively simple. As a comparison, zincation typically requires six steps prior to electroless plating. This advantage has been achieved using materials which are compatible with the current technologies.

Another advantage is that the substrate is not exposed to small mobile ions or additional metal ions such as fluoride ions.

A further advantage is that the method provides excellent selectivity because plating only occurs on the substrate and not on other materials such as dielectric materials. The invention is not limited to the embodiments described but may be varied in construction and detail. For example, the method may include the additional step of annealing after plating to improve adhesion of the plating.

Claims

1. A method for deposition of a material onto a semiconductor metallisation substrate, the method comprising the steps of:

cleaning the substrate, and

plating the substrate with the material in an electroless bath,

wherein

the substrate is cleaned by etching until oxide is removed and the surface is activated and subsequently rinsing the substrate in a non- aqueous liquid to prevent re-oxidation, and

2. A method as claimed in claim 1, wherein the electroless both comprises a salt complex containing metal ions as cations and a reducing agent as anions.

3. A method as claimed in claim 2, wherein the complex comprises Ni ions and a hypophosphite reducing agent.

4. A method as claimed in any preceding claim, wherein the electroless bath further comprises a lubricant.

5. A method as claimed in claim 4, wherein the lubricant is in particulate form.

6. A method as claimed in claim 5, wherein the lubricant comprises Teflon™.

7. A method as claimed in any preceding claim, wherein the electroless bath comprises a hardening component.

8. A method as claimed in claim 7, wherein the hardening component comprises silicon carbide in particulate form.

9. A method as claimed in any preceding claim, wherein the etching is performed in an aqueous basic solution.

10. A method as claimed in claim 9, wherein the etching solution comprises NH₄OH, H₂0₂, and H₂0.

11. A method as claimed in claim 10, wherein the relative composition of NH₄OH : H₂0₂ : H₂0 is 1 : 1: 20.

12. A method as claimed in any preceding claim, wherein the non-aqueous rinse liquid comprises acetone.

13. A method as claimed in any of claims 1 to 12, wherein the non-aqueous rinse liquid comprises propanol-2.ol.

14. A method as claimed in any preceding claim, wherein the substrate comprises Al deposited onto a semiconductor at a semiconductor temperature of less than 150°C.

15. A method as claimed in claim 14, wherein the substrate comprises an alloy of Al with Ti or Cu or Si.

16. A method as claimed in any preceding claim, wherein the substrate comprises Cu.

17. A method as claimed in any preceding claim, wherein the substrate comprises a protective layer over the metallisation.

18. A method as claimed in claim 17, wherein the protective layer comprises TiN, TiW, TaN, TiSiN, or TaSiN.

19. A method as claimed in any preceding claim, wherein the substrate comprises bond pads, and the method applies Ni to the bond pads.

20. A deposition method substantially as described with reference to the drawings.

21. A semiconductor metallisation substrate whenever plated by a method as claimed in any preceding claim.