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Sheela Sowariraj (promotion date: 9 June 2005)

Full chip modelling of IC’s under CDM stress

Promotion date: 9 June 2005

Sheela Sowarira

We tackled the problem of static electricity in the housed chips. Static charge accumulation results in voltage difference. We can feel the presence of static charge only when the voltage drop is above 3000V. The static discharge happens in a very short time span (fraction of a millisecond). ICs (chips) encounter static charge during different stages of their production while being handled or transported. A static charge creating a voltage drop of 1000V giving rise to a static current of 1 ampere for a short duration of few ns can already damage the IC. The package material of the IC being dielectric accumulates the static charge. When such an IC, the static charge source being the IC itself, touches a grounded surface, static discharge occurs. This is known as Charged Device Model type of electrostatic discharge.

What was your thesis about?

We tackled the problem of static electricity in the housed chips. Static charge accumulation results in voltage difference. We can feel the presence of static charge only when the voltage drop is above 3000V. The static discharge happens in a very short time span (fraction of a millisecond). ICs (chips) encounter static charge during different stages of their production while being handled or transported. A static charge creating a voltage drop of 1000V giving rise to a static current of 1 ampere for a short duration of few ns can already damage the IC. The package material of the IC being dielectric accumulates the static charge. When such an IC, the static charge source being the IC itself, touches a grounded surface, static discharge occurs. This is known as Charged Device Model type of electrostatic discharge.

The question was how do charges flow within the chip and how does the flow of static charge damage the IC. Can we model it and how can ICs be made robust against CDM stress? We studied what exactly happens in the chip when it is subjected to CDM stress. The presence of static charge itself is not the cause for damage. Grounding of charged IC causes current to flow through the IC. It is the voltage drop which is caused by this current flow that causes the damage. The amount of current that flows depends on the amount of charge stored in the package capacitance. Package capacitance is a composite of several capacitors formed by the conducting layers within the IC with the ground with package material as the dielectric.

To protect the ICs from ESD damage, each pin of the IC is clamped to the supply lines. Also the power lines are clamped to each other. But this is not sufficient for CDM protection.

And what did you discover?

Among all the CDM sources, the capacitance formed by the metal plate on which the die is placed is the largest and it is not wrong to consider this capacitor as the CDM source. As a result CDM stress can cause voltage drop not only across the circuit but also across the circuit and the substrate. The role of the capacitance formed by this major CDM current source namely the metal plate on which the die is attached was not studied before.

And after having found out the cause, do you have suggestions for the remedy?

CDM current pulses are of large amplitude (~10A) and very short rise time (fraction of ns).

So the protection devices which are designed to carry large currents should have very fast turn-on time. So protection devices with gate-length lesser than 1mm should be used to carry CDM current. As the source is IC itself, the discharge path is highly distributed. CDM damage is from voltage overshoots beyond a certain threshold. To protect the ICs against CDM stress, the substrate and source nodes of the transistors should be well shorted and the bus line resistance should be kept low.

You were financed by Philips, how did you get into contact?

Philips contacted the group. How I got here was bit weird in a sense. I was doing my Masters in Physics back home in Madras, and there were officials from a Dutch recruiting company offering research positions because they did not have enough PhD’s. After a month a got a letter from the group. I think now the situation has changed, there are even quite a few Dutch students going abroad to do their PhD.

What did you know about the Netherlands at that time?

Not much. Den Haag, International Court of Justice, and about the dykes. We were also wondering if Holland and the Netherlands were two different countries.

What made you decide to come?

The universities in India are run by government organisations. The interaction between the research institutes and the industries is very less. So funding is low. As a result it takes a very long time (few months to few years) to get or replace equipments needed for the lab. Hence more of the research work in India is based on theoretical work. Also as the scholarship amount is not a large amount, most of the bright students prefer an industrial job rather than to pursuing research.

Did you get a shock when you arrived here?

I am trying to remember….. I did know somebody here already and he instructed me what to buy etc. But I missed my flight in Frankfurt, was hungry by that time, did not find anybody willing to change some money and I did not want to leave the gate, and then the person that was supposed to meet me just dumped me on the train. When I arrived, I was too dizzy to experience anything. But the next morning, after a good sleep, I saw snow. For the first time in my life! It was amazing and then I suddenly realized that I was in a very different country far away from my home.

What did you like best in the Netherlands?

The way people keep time. It is very punctual. And I think Dutch people are also quite tolerant. If they get angry, they do not show it in the face.

And what didn’t you like?

The strict adherence to the rules. I once wanted to catch a bus at 6:40 hrs (first bus from the university). I was waiting at the bus stop where the bus does not stop during that time. There was no one inside the bus except for the driver. The bus driver slowed down the bus and showed me signs that I was in the wrong bus stop and then went on. And there I was, waiting in the cold for another half hour for the next one to come but this time in the correct one.