In Focus: Chemical-Mechanical Planarization
Chipmaking is an amazing thing. I’m constantly impressed by ourindustry’s ability to fabricate trillions of nanoscale features on asingle silicon wafer with such precision and repeatability.
Some of the processes we use seem suitably high-tech, employing lasers, electron beams and depositing films atom by atom. Then, on the otherhand, there are processes that are, at least in principle, positivelyprimitive in comparison.
At various points in making a microchip, the surface of the wafer has to be made perfectly flat, or planarized. This is done either to remove excess material, or to create a perfectly flat foundation for addingthe next layer of circuit features.
To do this, chipmakers use a process called chemical-mechanicalplanarization, or CMP, for short. CMP involves pouring a mixture ofchemicals and sand (more-or-less) on a spinning disc of specialsandpaper and polishing away.
Let’s remember here that semiconductor fabs go to great lengths tokeep wafers free of contaminants. Cleanrooms have elaborate filtrationsystems to keep the air purity a million times higher than outside andworkers are enveloped in bunny suits and masks to trap organic debris.
So, on the face of it, CMP seems to be a very odd thing to do in a cleanroom.
At one time, the CMP process was indeed viewed as too dirty to usefor the highly-precise business of semiconductor manufacturing. CMPtechnology really took off in the mid-1990s, when the industry waslooking for ways to increase chip performance by replacing aluminum with speedier copper circuits.
Aluminum interconnects were made by depositing a layer of metal and then etching away the unwanted parts using reactive gases. Copper, however, can’teasily be removed this way and so copper CMP technology was developed as a replacement. Today, every microprocessor uses copper wiring and CMPis an integral and indispensible part of any chipmaker’s toolkit.
Applied’s role was to take CMP technology from the lab and providethe industry with sophisticated systems suitable for high-volumemanufacturing. There are now over 3,000 Applied CMP systems in the field distributed across the fabs of virtually every chipmaker on the planet.
Although simple in concept, at the nanoscale level CMP is a complexbusiness. You can’t just use force to remove material from a wafer. Thedifferent films that make up the wafer surface have differenthardnesses, so they polish away at different rates. This could lead to“dishing”, where the soft parts are recessed below the level of theharder materials – which clearly is not a good thing. To compensate, weadd chemicals (the C in CMP) to minimize variations in material removalrates between harder and softer materials.
To make sure material is removed evenly across an entire 300mm wafer, regardless of the incoming film uniformity and that the process stopsat the right point to avoid grinding away the critical underlyingfeatures, Applied developed a sophisticated control system for its Reflexion® CMP systems. The control system continuously measures the film thickness at multiple points across the wafer and adjusts the polishing downforce severaltimes each second to give consistent results on every wafer, every time.
The entire process is completed in as few as 30 seconds, including apost-polish clean where the wafer is washed, rinsed and dried. Thefinished wafer then exits the Reflexion system, probably cleaner thanwhen it went in.
All this adds up to a mind-boggling level of flatness. If the waferwas an American Football field, the Reflexion system would be theultimate mowing machine, capable of cutting every blade of grass to thesame length within the width of a human hair. Now that’s flat!
Today, Applied unveiled a new version of its flagship Reflexion GT™ CMP system designed to planarize tungsten, which is used to connecttransistors and memory cells together in advanced logic and DRAM chips.You can read more how the new system will help chipmakers fabricatetheir latest and greatest devices on the Applied Materials website here.
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