Grinding, lapping, and polishing are machining techniques that refine a substrate to exact dimensions and tolerances for optimal circuit metallization during both thick and thin film technology procedures. Because the “as-fired”, or as delivered, condition of ceramic, fused silica, or titanate substrates are most often imperfect (wavy, pitted, bumpy, varied in thickness), one or more of these processes is employed to create the exact parallelism, camber, thickness, and surface finish needed. Designers working in high power, and/or high frequency microwave applications, for example, require their ceramic substrates to be consistently reliable and repeatable to meet the quality standards of their scrutinizing instrumentation, test and measurement, commercial communications, military radar, and aerospace customers. To do so, an optimal base substrate is key. In this first blog in this series our primary focus will be on several justifications for lapping and/or polishing the ceramic substrates for your microelectronic circuits.
Some of these considerations include achieving consistency from part to part, as well as meeting tolerance requirements for specific yield levels needed in thick or thin film device fabrication. For instance, the camber or flatness tolerance of a substrate may impact the resolution of the trace tolerance during the transfer of the photomask onto a substrate. Additionally, for high power and high frequency applications, the substrate itself becomes a significant contributor to mechanical, thermal, and electrical behavior.
For example, inductors and capacitors are highly influenced by the dielectric permittivity and magnetic permeability properties of a substrate. Moreover, the exact electrical performance of these components depends upon the metallization developed on the substrate. Hence, the capacitance and inductance of an inductor or capacitor can be influenced by the parallelism, thickness tolerance, and flatness of the substrate. As these components are critical factors in filters, power dividers, circulators, and impedance matching circuitry, the tolerances and behavior of the devices are directly related to the substrate condition.
The metallization step in thin or thick film can also be largely dictated by the surface condition of a substrate. For example, variations in the surface condition of a substrate can cause variations in trace thickness. This directly affects the resistance of the trace which can lead to poor thermal performance in high power applications. A better surface finish can also enable tighter metallization tolerances, leading to more densely patterned resistors, finer pitch spiral inductors, and generally higher density circuits.
Lapping, and to a greater extent polishing, reduces the peaks and troughs of an as-fired substrate's surface, and provides and the thinnest metallization possible. In the case of thin film resistors, the ability to produce thinner metallization allows for much higher resistances. When plates of material, especially larger boards, are manufactured, there are often variations from board to board. These variations could be in terms of waviness (or camber), non-parallel top and bottom surfaces, and porosity of the surface pits, voids, and scratches (See detail A & B). Lapping will address all these potential characteristics of as-fired material.