X-ray Inspection in Semiconductor Industry

How is X-ray inspection used in semiconductor manufacturing?

Technology is Key

With the ever present pressure to produce more efficient devices with more power, the sizes of the structures and electrical connections in the production of chips have become smaller and smaller. In addition the shear number of these connections in a given unit area has also increased in a spectacular way. At the heart of all X-ray inspection, whether it is manual or fully automated metrology, is the imaging chain. This can be thought about in basic terms by the 2 main elements, the source and the detector. Over the decades these 2 fundamental items have had to evolve enormously to keep up with the ever growing demands of the semiconductor industry.

All X-ray sources are designed to balance 3 things. Power, resolution and lifetime. The most advanced today being the Sealed Transmissive Tube fig 1. This combines high resolution, long lifetimes at high power while producing a consistent source of X-rays. This stability, flexibility and quality could not be more important to the inspection of such small but ever so important structures.

Fundamental to this design is the crystal filament which produces a very fine, efficient electron beam. This material is able to do this far below it’s critical temperature meaning much longer life than with a Tungsten filament. When this beam hits the target material the X-rays are produced. This design allows the device under test to be placed very close to the source of the X-rays giving the highest possible magnification. Magnification being defined by the ratio of the distance from source to object and source to detector.


Inspection strategy should always be a zero defect strategy

The further into the manufacturing process, the more time and expense has been invested into  a single device. That can start adding up very, very quickly into 10s of 1000s of dollars.

Identifying defects early is essential, particularly as more and more devices are being used in sensitive applications such as aerospace, autonomous driving vehicles, medical devices etc. Failure or breakdown in the real world is simply not an option. If the entertainment system breaks in a car, it’s inconvenient, but if the ABS system fails, lives depend on it. In this context, X-ray has the task to make sure that everyone is safe.

There are a number of ways this can go wrong. If the electronic connections aren’t strong, they can become susceptible to early life failures through for example, thermal cycling. Let's look at how processors are usually used. They get really hot, they get turned off, and then they're cold, then hot, then cold and so on. Components and bonds can then start cracking and breaking, if they don’t have a really good connection or are made of low quality materials.

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