Breaking point

A technique that measures fracture resistance of dielectric material could help avoid unnecessary costs in the development of integrated circuits. Siobhan Wagner reports.

A new method for testing the toughness of dielectric material could lead to more reliable electronic devices.

The technique, developed by researchers at the National Institute of Standards and Technology (NIST) in the US, measures a dielectric material’s resistance to fracture using equipment that most microelectronic manufacturers will already own.

The performance of dielectric material is becoming increasingly important as electronic devices, and subsequently their integrated circuits, shrink in size.

Integrated circuits are tiny electric circuits made up of layers of resistors, transistors and capacitors used to perform electronic functions in a device. These layers must be insulated using thin films of dielectric material to prevent electrical interference.

As circuits decrease in size, space becomes increasingly cramped and designers are being forced to make insulating films more porous with nanoscale voids. Unfortunately, this also makes the films more fragile.

Robert Cook, NIST researcher, who worked with dielectric films for IBM two decades ago, said the fragility of the material has been a problem chip manufacturers have been trying to grapple with for a long time.

‘These glassy, brittle materials would fracture and this was a major problem for both integrating them into chips for manufacturing purposes and predicting their reliability during operation of the devices,’ he said.

Elasticity and plasticity, the forces needed to bend a dielectric material either temporarily or permanently, have been measured for about 20 years using nanoindentation. The process works by pressing a diamond tip onto a material and observing how much pressure it takes to deform it.

Cook and his colleague, Dylan Morris, at NIST claim they have used a similar method to accurately determine toughness for the first time. The new NIST technique uses a slightly modified nanoindentation probe.

‘We still use a diamond, but it is much pointier,’ said Cook. ‘To be semi-quantitative, most of the diamonds used for elastic and plastic assessments have angles the order of 140º. The probe that we use for toughness assessments has an angle of about 80º or 90º.’

The probe is pressed onto the dielectric film and subsequently creates tiny nanometre-sized cracks. The researchers then observe the shape and size of these cracks with an electron microscope.

‘We have to interpret exactly what the size and shape of those cracks mean,’ added Morris.

Cook and Morris developed a fracture mechanics model that takes into account variables such as indentation force, film thickness, film stress, and the elastic properties of the film and its silicon substrate.

‘The model is based on fracture and contact mechanics, and the analysis can be done with inexpensive scientific analysis software,’ said Morris.

The resulting information tells microelectronic manufacturers the force needed to actually break the material and how thick a material needs to be to avoid spontaneous fracture.

Morris said the fracture mechanics model is free for any manufacturer who wants to use it and they are happy to speak to anyone about how it works. He hopes the prevalence of the model will improve the reliability and manufacturability of integrated circuits.

‘We’ve giving suppliers and manufacturers a measurement tool to evaluate fracture properties when fracture was in fact a mechanical reliability problem they were having,’ he said. ‘It will let developers and also manufacturers make speedy informed choices about the materials they’re using, before actually trying to build a very expensive integrated circuit and then seeing whether or not it fails.’