About Microelectronics

From a materialographic point of view, microelectronics can be divided into three types of samples:

Silicon wafers
Integrated Circuits (IC) and components
Printed Circuit Boards (PCBs)

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Silicon Wafers

The performance of the semiconducting silicon is closely linked to the materials properties, in terms of microstructure and chemical composition.

Thin slices of the cylindrical silicon ingot are materialographically prepared for analysis by, typically, IR-microscopy, and FTIR- Spectroscopy.

Wafer fabrication includes many repeated processes to produce complete integrated electronic circuits on the wafer substrate surface and, subsequently, sectioning into individual wafer dice.

Inspection of tiny parallel or cross sections of the wafer in its non-encapsulated form is done after accurate materialographic polishing. Details in the integrated circuit are studied in a light or electron microscope, depending on the scale and the type of analysis.

Silicon Wafers

The performance of the semiconducting silicon is closely linked to the materials properties, in terms of microstructure and chemical composition.

Thin slices of the cylindrical silicon ingot are materialographically prepared for analysis by, typically, IR-microscopy, and FTIR- Spectroscopy.

Wafer fabrication includes many repeated processes to produce complete integrated electronic circuits on the wafer substrate surface and, subsequently, sectioning into individual wafer dice. Inspection of tiny parallel or cross sections of the wafer in its non-encapsulated form is done after accurate materialographic polishing. Details in the integrated circuit are studied in a light or electron microscope, depending on the scale and the type of analysis.

Integrated Circuits (ICs) and components

From the individual wafer dice as base material, packaging takes place.

Packaging methods include different interconnections and coating technologies, all aiming for extreme compactness.

Materialographic cross sections of these tiny and highly complex components are used in the development, design, production spot checks, and failure analysis. The objective of the examination is to look at cracks, voids, solder balls, conducting, and isolating layers, connections, etc. A particular area inside a package is often the subject of the examination, and materialographic applications therefore include identifying and revealing this target.

Discrete components like capacitors, resistors, etc. are also subject to materialographic examination to analyze for geometric and microstructural imperfections.

Printed circuit boards

Printed circuit boards (PCBs) consist of a base sheet of epoxy/fiberglass or ceramics, plated metallic layers of copper, and plated holes, also referred to as "vias."

Sample preparation of circuit board materials is performed to help locate defects in the substrate material, which carries an electronic assembly.
According to leading industry standards, the quality of a PCB-plated through-hole has to be inspected materialographically. For this purpose, a test coupon is produced and prepared so that the center of the plated through-holes can be inspected by a microscope.

Additionally, connections, coating coherence, and thicknesses are studied in, primarily, cross sections. Accurate materialographic processes are also used for PCBs as very specific areas usually define the purpose of the examination.

How to do materialographic preparation of microelectronics, controlled material removal (CMR), and target preparation

Materialography on microelectronics is challenging in principally three ways:

1. Miniature dimensions require specialized equipment and accessories suitable for handling small samples. The requirements to the accuracy in the materialographic processes, like cutting and grinding, are also more pronounced for samples defined by dimensions in the µm range.

2. Complex material compositions are common in microelectronics where soft metals, ceramics, and composites are often closely packed. This makes the choice of preparation methods and parameters a compromise, carefully selected to meet the specific requirements.

3. Controlled and accurate preparation is needed when small targets are the subjects for the examination. Solutions with high mechanical precision, optical measuring units, and mechanical stops are the modern automated or optimized solutions of the more basic grind-and-look technique.

Cutting and Sampling

Depending on what kind of sample needs to be investigated, the cutting can be done on various precision cutoff machines.

A mobile phone, or a board mounted with components, can easily be cross-sectioned on a manual or automatic medium-sized machine.
For sectioning individual, small or fragile components, which require higher precision, a precision cutoff machine is recommended.
An electroplated diamond wheel for cutting plastics or a resin bonded diamond wheel is recommended.

In any case, the cut should be placed far enough from the actual area that is to be observed, to avoid possible direct damage to it. The remaining material can then be carefully ground away after sectioning. The more carefully this initial step is carried out, the less likely it is for cracks to be introduced in ceramic, chips, and glass, or cause delamination of layers or solder spots.

For extraction of PCB coupons, dedicated sampling equipment is brought into use. Automation and optical measurement techniques enable high precision drilling and routing of pinpointed coupons. For sensitive or small samples, it is advisable to impregnate the sample before cutting.

Mounting

Due to their composite and fragile nature, microelectronic components are not suited for hot compression mounting, and are therefore always cold mounted.

Cold mounting resins, with low curing temperatures, are recommended to avoid the influence of heat on solders and polymers. For small or brittle samples, e.g. silicon wafers, a low shrinkage resin is preferred in order to minimize the risk of cracks.

Mounting methods differ depending on the analytical method used.

For regular mounts, the optical microscope and transparent epoxy resins are used.
If voids and holes have to be filled, vacuum impregnation is recommended. Mixing a fluorescent dye with the epoxy provides an excellent contrast of voids and cracks in the optical microscope.
For very small vias, a transparent resin with a low viscosity that flows easily into the holes is recommended.

For dedicated systems, components may be mounted directly in the special sample support used for via inspection or target preparation, for example.

Grinding and Polishing

For manual and semiautomatic grinding and polishing of microelectronics, conventional equipment can be used. Aligning the sample and controlling the material removal is secured by special sample holders as a more accurate alternative to the manual grind-and-look method.

For automatically controlled surface preparation, cross and parallel sections of mounted and unmounted samples can be ground and polished to visible and hidden targets in dedicated equipment.
Laser measurements can ensure an accuracy of ± 5.0 µm and automatically recalculated removal rate during the preparation process.
Alignment and measuring can either be video-based for samples with a visible target or X-ray-based for samples with a hidden target.

Parallel and cross sections of thin and brittle silicon wafers, arrays of solder balls of lead or tin, PCBs with copper-plated vias on brittle ceramic or ductile polymeric substrate, and cross sections of ICs with silicon, ceramic, gold, copper, aluminum and tin within a few hundred microns are examples of material combinations to consider, in the choice of grinding and polishing methods.

Requirements for removal rate, planeness, relief, edge retention, and smearing often determine the choice of grinding and polishing surfaces and suspensions. Around 25 dedicated methods are found in the e-Metalog for electronic components, covering a broad selection of materials combinations and preparation requirements.