Nickel electroplating

Nickel electroplating
 
1.  Introduction
 
Nickel electroplating is not an existing technology in Twente and research to develop it is done here parallel to the design and development of a microvalve for a flow controller. At the end of the project we should be able to know whether it is feasible to use nickel plating for flow controllers. Nickel is chosen as the material to make the valve because of several reasons. Electrodeposited nickel can be strong, tough and resistant to corrosion, erosion and wear. Its mechanical properties can be varied at will between wide limits by changing plating conditions, by alloying with other elements, and by incorporating particles and fibers within the electrodeposited nickel matrix. Our plating system consists of a power section and an electroplating section with an electroplating tank of eight liters purchased from ECSI Inc.

The schema of our nickel-electroplating cell is shown in Fig.1 where the cathode is the wafer to plate with a conducting layer and the photoresist structure on it. When the power supply is turned on, the positive ions in the solution are attracted to the negatively biased cathode. The nickel ions that reach the cathode, gain electrons and are deposited on the surface of the cathode forming a layer. Simultaneously, another reaction that depends on the nickel solution used to plate, occurs at the anode, to produce ions and electrons for the power supply.

Fig 1: Scheme of the electrochemical plating of Ni

For electroforming, solutions based upon nickel sulfamate are preferred because the internal stress in the nickel deposits is very low. Our conventional sulfamate bath contains 300 g/l of nickel sulfamate together with nickel chloride and boric acid. The solution is agitated during plating and at a temperature of 55˚C. Boric acid buffers pH variations and nickel chloride is used in our case to improve conductivity. Because an insoluble anode is used, sulfamate nickel is the source of nickel serving as well as conductive electrolyte for carrying nickel ions from the anode to the cathode for deposition. The pH of the solution is kept between 4 and 4.5 at the plating temperature by adding nickel carbonate.

In order to make micro-electromechanical systems it is very important to have films with low stress values and low roughness. Both properties depend on operating variables such as temperature and current density.

2.  Bath optimization for MEMS devices

The two most common types of stress in thin films are thermal and intrinsic stress. Due to the difference in thermal-mechanical properties between the film and substrate thermal stress is usually unavoidable. Intrinsic stress is generated during film formation and is strongly dependent on process conditions. A common way to measure stress in a film is by measuring how much a substrate bends after the film is deposited, i.e. the wafer curvature. The amount of bending is dependent on the internal mechanical properties of the film and substrate (Young’s modulus and Poisson’s ratio) the thickness, and the stresses in the film and substrate. By measuring the amount of bending and knowing the film thickness and mechanical constraints, one can extract the total stress in the film, σtotal , using Stoney’s relation (Fig.2).

  

Fig. 2 Wafer curvature

Here is Ys Young’s modulus of the substrate, vs is Poisson’s ration for the substrate, ts is the substrate thickness and tf is the film thickness. The deflection δ at the center of a surface scan over a distance D is a measure for the residual stress. In practice, the deflection or curvature is measured before and after the film is deposited and then related to the stress.

The substrates used in our experiments were 4” silicon wafers with a conducting layer on top. This conducting layer was made evaporating a 10 nm thick Cr layer and a 100 nm thick Au layer.. After evaporation the wafer is scanned in two perpendicular directions with a surface profilometer in order to get the deflection of the wafer prior to nickel deposition. The nickel film is then electroplated and scanned again. The deflection difference is then related to the stress with Stoney’s equation. This experiment was repeated for current densities (J) from 2 to 7 A/dm2 in a bath with a pH between 4 and 4.5 at 40ºC, 45ºC, 55ºC and 60ºC.

The stress values obtained were always below 60 MPa, a normal value found in literature for a sulfamate bath. From the results it was concluded that for our nickel sulfamate bath and plating on chromium-gold seed layers with pH value between 4 and 4.5, the best results are observed for a plating current density between 4-5 A/dm2 and a bath temperature of 55°C.

3.  Future

Continuous control of the process parameters and of the electrolyte is needed to obtain optimum results. On the background of our research lies the nickel plating bath itself. The use of a soluble anode to avoid fluctuations on the pH of the solution is highly recommended and this would mean optimizing again our plating conditions.

Research with nickel plating will be done in order to meet the requirements of the design of the microvalve for a flow controller. Discussions about the design of the microvalve in nickel technology are very important. The idea behind all this is to acquire such a control of the bath and such knowledge of the possibilities qua plating that a quick and successful result will be reached the moment that the design is completed.