CMP Process Flow: Chemical Mechanical Processing for Electronics

October 21, 2020 OrCAD PCB Solutions

Key Takeaways

  • CMP has many similar names, but they all describe the same fabrication process that makes electronics manufacturing more precise.

  • There are many different applications of CMP in the semiconductor industry, resulting in different CMP process flows.

  • Although CMP is essential to fabricating many electronics, there are some potential drawbacks of the technique that can be problematic.

CMP Process Flow

The chemical mechanical planarization (CMP) process.

The chemical mechanical planarization (CMP) process.

Proper fit and sizing between components, as well as proper interactions between materials, are vital to creating robust electronics projects. As semiconductors become more complex, it becomes necessary to combine materials and create fits with very low tolerances between components. Chemical mechanical processing, or CMP, is a vital step in the fabrication process for semiconductors and other electronics. CMP combines chemical and mechanical processes to ensure that each component’s fit, tolerance, and combination of materials work well together. 

What is Chemical Mechanical Processing (CMP)?

Chemical mechanical processing (CMP) is a process that ensures regions of semiconductor components are level, or planarized. The final word of its name can vary, and the process can be described as planarization or polishing, among others. Precursors to this technology were developed in the 1950s, but it wasn’t until the 1980s and 1990s that semiconductors contained enough layers where CMP became vital. 

Interactions between layers often cause uneven surfaces that need to be corrected, and even small imperfections can cause problems with current flow, heat dissipation, or component durability. CMP combines physical grinding and chemical etching to smooth the surfaces of semiconductors. Without this two-pronged approach, physical grinding alone would cause too much damage for a component to be usable, while chemical etching alone could not planarize a surface well enough. The friction generated by these processes can produce a lot of heat, so careful temperature control is necessary.

CMP includes precise, leveling surfaces within a tolerance of 0.2 microns, or less than one-thousandth of a millimeter. The size of polishing particles, the shape and size of the pads which hold polishing particles, the material from which a component is made, and the chemicals used for etching can all affect the precision of CMP. This is not an exhaustive list of factors that can influence CMP, therefore, there are many different strategies, polishing materials, and chemical reagents that can be effective for a given project. It is important that a designer or machinist choose the proper type of CMP for their component because the wrong type can render a component unusable.

What Are the Applications of CMP?

As the size of electronics and their components decrease, removal of debris and proper fit between components becomes more and more important. CMP for integrated circuits is an area of ongoing research in the electronics community, as demand for smaller and sleeker electronics increases. 

The main application of CMP is during the manufacturing of integrated circuits. CMP can smooth surfaces and create inlaid metal structures for the circuits. CMP works equally well for a single circuit as it does for multiple circuits on a single chip. Eliminating rough edges on chips and integrated circuits allows more components to be placed in less space, leading to more compact and higher performing electronics.

CMP also has applications in optics, where its precision capabilities can shape lenses and physical data stores read by laser scanning. Glass cell phone screens are another common place to apply CMP, since tiny imperfections in the glass can cause unwanted reflections or a lack of clarity. Some hard drives are fabricated from a glass-based substance, so CMP is vital to shaping those drives. These hard drives are not the same as optical drives, which read data with a laser from grooves in physical media. Instead, these drives use a laser to heat up the glass-like substrate for writing. For this writing to be accurate, the substrate’s surface must first be essentially flawless, and only CMP can currently provide that precision.

CMP done over a 2 micron scale.

CMP can be precise to the micron.

What Are the Potential Side Effects of CMP?

CMP has the potential to cause major damage to a component. The combined mechanical and chemical processes put significant stress on components, and that potential for strain increases when many components are in close proximity. CMP can cause defects in a component or in neighboring components if it is not carefully controlled. Gaps can open within internal component layers, which cause electrical weak spots, and those weak spots can greatly increase a component’s potential for failure. Although it’s usually possible to detect these spots with proper CMP control and testing, some may slip through the cracks.

The processes used for CMP can sometimes remove multiple materials from a component. If a component contains more than one material that CMP can impact, engineers must be aware of a condition called dishing. Dishing happens when the CMP removes multiple materials at once from a component, and those materials are removed at different rates. The difference in removal rate causes the exposed surface of the component to increase and often leads to planarization failure.

The chemicals used in CMP are not specially formulated to only act on a specific material—they have the potential to interact with a component as much as the material they are attempting to remove. Therefore, the chemical processing of CMP can cause corrosion of a component. This can sometimes be remedied, but components that require close tolerances are usually ruined if a component becomes corroded. Copper corrosion is a particularly persistent problem for the semiconductor industry since copper does not often etch well with other methods.

A clean room for CMP and semiconductor fabrication.

A clean room for CMP and semiconductor fabrication.

The  PCB Design and Analysis overview page at Cadence Design Systems provides tools ideal for managing your CMP process flow, including the Allegro PCB Designer. Learn what's new in Allegro PCB Design so your CMP processes stay efficient and up-to-date. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

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