What's the difference between lapping, polishing, and grinding substrates used in the fabrication of RF/microwave integrated circuits (ICs)?

Posted on Tuesday, August 21, 2018

Lapping, polishing, and grinding (aka diamond sawing) are three processes by which material is removed from a substrate to produce a desired dimension, surface or edge finish, flatness, parallelism, and/or shape as key preparation process before beginning fabrication of a RF/microwave IC or semiconductor. The process of lapping, polishing, and grinding substrates can be applied to a wide range of materials and applications, ranging from metals, ceramics (including alumina and AlN), glasses, optics, semiconductors, and ceramics. Lapping and polishing techniques are beneficial due to the precision and control with which thin layers of material can be removed to produce desired circuit preparation outcomes.

Lapping is performed on ceramic and metal substrates when a smooth, flat, unpolished surface is desired. Lapping processes are used to produce dimensionally perfect finished substrate blanks or "sheets". In this process, a lapping plate rotates at a low speed (<80 rpm) and a mid-range abrasive particle (5-20µm) is typically introduced. There are two types of lapping: free abrasive lapping and fixed abrasive lapping. Free abrasive lapping is when abrasive slurry is applied directly to a lapping plate. This is the most accurate method for producing repeatable substrates and has the reputation for minimal rework when in expert hands. Free abrasive lapping is accurate because of the rigid lapping surface which can be tailored to suit a particular material. Fixed abrasive lapping is when an abrasive particle is first bonded to a substrate, via lapping films or papers. Abrasive lapping films have various particles bonded to a thin, uniform polyester substrate, and are known for producing the flattest surfaces. 

Polishing is a more subtle process of removal which produces an even smoother finish, often after lapping. Polishing is also typically done at very low speeds using either polishing cloths, abrasive films, or specially designed plates. Polishing with a cloth or lapping plate requires the use of free abrasive, and has a very low risk of damage when performed properly. Knowing the plate material and cloth material combination is critical however, as the chemical properties of substrates that are very important to the IC fabrication process, can be thrown off-balance. The ability to select a proper polishing cloth is therefore vitally important. And the properties of the cloth depend on the application. If flatness is of primary concern, short nap cloths (such as Nylon) are used to maintain flatness. When the final surface finish is of primary concern, longer napped cloths (such as Rayon and Silk) are used. Many cloth materials today combine the best of both worlds, allowing flatness and surface finish balance to provide maximum results in short periods of time. Polyurethane pads are commonly used for final polishing processes and aid in producing superior flatness and surface finish. Polishing with abrasive films also produces excellent results.

In general, polishing is employed to:

Enable thinner resistor layers for higher value resistors in smaller areas

Allow for denser circuit designs with better tolerances and consistency

Facilitate the design of circuits with a high degree of integration, such as in bandpass filters, circulators, Lange couplers, and Wilkinson power dividers

Achieve more intricate metallization patterns with finer pitch and thinner metal layers, such as needed to create high-performance spiral inductors, high density interconnects, and intricate resistors. 

Grinding, or diamond sawing, can be defined as the rapid cutting or removal of substrate material to create a suitable size or shape, or to remove bumps and imperfections on the adhesion surface, or the material's edge. A diamond grinding wheel spins at high speed and a coarse, bonded abrasive is introduced. Grinding is a quick and relatively easy procedure, but can cause damage when not administered with expert hands, especially when thin or otherwise fragile substrates are being processed. Grinding or sawing is applied to everything from alumina substrates to hard metals such as high carbon steels. For delicate materials, the grinding process must be a balanced blend of removing material whole doing no damage to the critical subsurface. This is another area of expertise for Accumet.

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