Precise Slurry Requirements Increase CMP Process Windows but Necessitate greater dependence on Inline Metrology
Chemical Mechanical Planarization or Polishing (CMP) is the method of using chemical slurry formulations with mechanical polishing-pad-action to flatten conductive or dielectric materials on silicon wafers in semiconductor manufacturing.
The end goal is to achieve a flat enough surface aspect ratio as required by photolithography steps, where layers of integrated circuits are layered step by step.
The process uses abrasive and corrosive chemical slurries which are applied to a polishing pad. These slurries are commonly a colloid, which is a mixture where one substance of microscopically dispersed insoluble particles is suspended throughout another substance. Generally, the polishing pad is of a greater diameter than the wafer it is tasked with interfacing.
During the CMP process, the pad and wafer are pressed together by a dynamic polishing head. The polishing head is then rotated within different axes. This action with the chemical slurries removes material and levels topography producing a wafer that is planarized.
CMP can be used to planarize within the required depth of a photolithography, or be selective in its removal of material based on its position alone. Typical removal depth requirements are measured down to the angstrom level (a unit of length used in measuring wavelengths of light, equal to 10−10 meter, or 0.1 nanometer).
The primary objective in CMP is to achieve a precise removal rate in a controlled manner and achieve this precision without creating common CMP process-related defects, or physical impacts, that disrupt yield overall.
There are many known limitations in the CMP process including stress cracking, delaminating at weak interfaces, and corrosive attacks from slurry chemicals, to name a few. Or, as in the oxide polishing process; a lack of end points. This results in blind polishing, making it hard to determine when the desired amount of material has been removed, and making it more operator dependent.
Specific CMP slurry compositions and chemical additives are used to help target these removal processes effectively and help reduce the aforementioned dependencies.
Coupled with post clean chemistries, precision in physical attributes and makeup allow the CMP process to be more effective in achieving desired endpoints.
In blending and formulation, many of these chemicals have specific attributes to do their specific targeting jobs. This equates to a specific metrology set to achieve reliable CMP slurry particle sizes, distribution, and chemical concentrations.
At DFS we match these metrology sets in the processes we design to produce and supply the CMP and post CMP chemical formulations with ensured precision. In essence, this ensures that what the CMP tool receives is as correct as it can be in composition, enabling a detailed picture of what's in the solutions, as well their overall health.