Product category: Design and Development Software
News Release from: Comsol | Subject: Femlab
Edited by the Electronicstalk Editorial Team on 7 January 2005

Modelling makes packaging perfect

Mathematical modelling using Femlab is helping to optimise a breakthrough process for fabricating 'flip-chip' carriers

Replisaurus Technologies has developed a novel alternative to the conventional photolithographic method of depositing copper contacts on flip-chip carrier substrates. Its electrochemical pattern replication (ECPR) process not only deposits copper much faster - anywhere from 10 to a 100 times as quickly - but is also much more economical.

However, as deposition takes place inside a cavity where it is impossible to install monitoring instrumentation, Replisaurus was faced with much experimental work to optimise the technique.

Comsol's Femlab finite-element modelling software provided the solution, allowing the company to simulate hundreds of different process variations before committing to the time and expense of clean room trials.

Flip chip technology eliminates wire bonds between the silicon die and the package, and has become a cost-effective means of dealing with the packaging and thermal issues of high-density, high-power ICs.

Typically, the final wafer-processing step deposits solder beads on the chip pads, so the die package must itself have pads with positions that align with the beads.

Creating these carrier substrates with photolithography can involve almost as many manufacturing steps as when creating the IC itself.

Replisaurus, however, employs a reusable patterned master electrode as a template and provides for direct metallisation on a variety of substrates.

Compared with lithography-based metallisation, which takes as long as 120min, its ECPR process requires between 1 and 5min.

It also achieves higher precision for the plating/etching reaction, and is far more economical - primarily because it requires less capital equipment.

The process starts with two elements: a flat cathode substrate with a thin metal seed layer on which the pads and traces are to be deposited, and a master anode consisting of an electrically conducting electrode layer and a patterned insulating material.

In the pattern's gaps the operator predeposits an anode material, usually copper.

Then the operator places an electrolyte between the two layers and squeezes them together.

The sandwich goes into a pressure vessel to hold in the electrolyte, and in the presence of a voltage across the layers the metal migrates to the cathode at a deposition rate of between 1 and 4um/min.

The final step etches away the metal seed layer from the cathode, leaving an exact pattern of metal traces.

To push the limits of the process, the research team needed a deep understanding of it.

'Before we started working with Femlab, we derived our results and understanding experimentally', explains Manager of R and D Mikael Fredenberg.

'We wanted a model that would explain the theory behind the phenomena we observed'.

When he decided to build a model, Fredenberg investigated Femlab, as he had experience of this package during his studies at Sweden's Lund Institute of Technology.

Femlab's ready-made interface for electrochemical engineering offered a good starting point for the model's construction.

The initial model assumed a constant current, then Fredenberg learned how to add variations to refine the simulation.

The model determines flux with the Nernst-Planck equation in the material balance in combination with the electroneutrality condition using equations in Femlab's Chemical Engineering Module.

The Nernst-Planck equation describes mass transport of copper ions in the electrolyte that occurs due to diffusion and migration.

The diffusion rate is determined by the concentration gradient that appears when the process produces copper ions at the anode and consumes them at the cathode.

The migration defines the ion transport potential gradient causing the positively charged copper ions to move towards the more negative cathode surface.

Next, the cathode and anode boundaries give flux according to the Butler-Volmer equation as boundary conditions.

The equation describes the current density at the electrode as a function of the overpotential, which in turn is given by the difference between the electrode's surface potential (applied with an external power source) and the potential in the electrolyte closest to the electrode surface.

Mikael was able to enter these equations for the electrode kinetics directly into Femlab's graphical user interface.

'The model allows us to play with a large number of parameters such as different voltage levels, warpage or unevenness in the substrate, or electrolyte properties'.

'We can try out all sorts of ideas, no matter how wild, and get a first estimation of results, enough to let us know if they're worth pursuing'.

explains Fredenberg.

A particular modelling problem deals with moving boundaries.

As material grows on the cathode, a nonuniform current-density distribution can lead to changes in cell geometry.

Areas on the cathode with higher deposition rates grow faster and get closer to the anode, causing the current density to increase further in these areas.

Mikael's initial Femlab models didn't account for such an effect, and the Femlab support team came to his aid, showing him how to solve a moving-boundary problem using the package's implementation of the arbitrary Lagrangian Eulerian (ALE) method - a powerful tool not found in many mathematical-modelling codes.

One effect Fredenberg wanted to investigate in particular was uneven surfaces such as imperfections on the cathode.

The modelling allowed him to rule out some issues he thought might cause a non-uniform current-density distribution.

Such models are allowing Fredenberg to perform analytical research that helps to find the optimum process voltage - for best balance between deposition rate and quality.

Too low a voltage can result in a slow deposition rate whereby only a few crystallisation sites have enough energy that the copper can crystallise, leading to irregularities in plating.

Too high a voltage can result in deposits that are too porous, so fast plating can lead to poor quality.

'Femlab is helping us to explain the phenomena we've seen in the lab', Fredenberg adds.

'The information has been invaluable in debugging the process and is now helping us refine the technique for commercial operations'.

Replisaurus expects to demonstrate its ECPR process publicly later in 2005.

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