At various stages 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 removes and planarizes excess material on the wafer’s front surface by applying precise downforce across the backside of the wafer and pressing the front surface against a rotating pad of special material that also contains a mixture of chemicals and abrasives.
Although simple in concept, at the nano-scale CMP is a highly complex process. The different films that make up the wafer surface have different hardnesses, so they polish away at different rates. This could lead to undesirable effects such as “dishing” and “erosion,” where the soft parts are recessed below the level of the harder materials. To compensate, specially added chemicals and pad materials minimize variations in material removal rates. To ensure that the right amount of material remains evenly across an entire 300mm wafer, regardless of the incoming film uniformity, the process must apply varying amounts of downforce across the wafer’s backside during material removal, while stopping at the right point to avoid polishing away critical underlying features. Applied has 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 60 seconds, including a post-polish clean during which the wafer is washed, rinsed, and dried before exiting the CMP system. Today's CMP technology is addressing even tighter demands for removal uniformity, greater productivity, and lower costs. Advanced applications, such as 3D logic, memory, and advanced packaging structures, demand even greater removal control, while lower costs and higher productivity are essential for these new integration schemes to be cost viable.