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 adding the next layer of circuit features.
To do this, chipmakers use a process called chemical-mechanical planarization, or CMP, for short. CMP involves pouring a mixture of chemicals and sand (more-or-less) on a spinning disc of special sandpaper and polishing away.
At one time, the CMP process was viewed as too dirty to use for the highly-precise business of semiconductor manufacturing. CMP technology really took off in the mid-1990s, when the industry was looking 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’t easily be removed this way and so copper CMP technology was developed as a replacement. Today, every microprocessor uses copper wiring and CMP is an integral and indispensable part of any chipmaker’s tool set.
Although simple in concept, at the nano-scale CMP is a complex business. The different films that make up the wafer surface have different hardnesses, so they polish away at different rates. This could lead to “dishing”, where the soft parts are recessed below the level of the harder materials, which is not desireable. To compensate, added chemicals (the C in CMP) minimize variations in material removal rates 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 stops at the right point to avoid grinding away the critical underlying features, Applied developed a sophisticated control system that continuously measures the film thickness at multiple points across the wafer and adjusts the polishing down force several times each second to give consistent results on every wafer, every time.
The entire CMP process is completed in as few as 30 seconds, including a post-polish clean where the wafer is washed, rinsed and dried before exiting the CMP system. All this adds up to an amazing degree of flatness. If the wafer were an American Football field, the CMP system would be the ultimate mowing machine, capable of cutting every blade of grass to the same length within the width of a human hair. Now that’s flat!
Today's CMP technology is addressing even tighter demands for removal uniformity, greater productivity, and lower costs. Emerging applications, such as 3D memory structures, will demand even greater removal control, while lower costs and higher productivity will be essential if these new integration schemes are to be cost competitive.