The material removal has to be controlled. It is necessary to gently remove each layer of material and to be able to stop the process precisely at the failure or feature of interest. If a defect is removed from the sample due to excessive grinding, it may persist in production and lead to large losses of money.
Parallel material removal or parallel lapping is usually done manually, enabling the operator to continuously examine the surface. Each time the operator must confirm the layer of interest is not yet exposed and that the surface is still parallel.This takes time and an operator with great skill.
Automating parallel material removal provides the operator with control. The risk of passing the area of interest is reduced and time saved. This Sum-Note will focus on automated parallel material removal with Buehler’s MPC™ 3000 Micro-Precise Backside Grinding System. It is designed for both front-side and back side material removal on bare chips or packaged components.The versatility of this system enables a variety of electronics applications to be tackled.
The first step is to section the specimen to a suitable working size. For electronic materials, two common methods of sectioning are cleaving and precision sectioning.
Cleaving is a simple and effective way of removing a die from the parent wafer for cross sectional analysis. The tools required are a sharp carbide or diamond scribing tool and a means of initiating the break. The distance of the plane of cleaving, from the plane to be examined, must take the sample preparation procedure into consideration. For example, if the die is to be prepared using only fine abrasive techniques; the plane of cleaving should be as near to the plane of interest as possible.
A precision saw, such as the IsoMet® 5000, will produce cuts at a precise location with minimum deformation. This is accomplished with relatively thin blades, which have diamond or cubic boron nitride abrasive bonded to the periphery of the blade. The cutting process is controlled by a combination of the blade speed and feed rate. Typically low concentration diamond blades, Buehler IsoMet® 10LC or 5LC wafering blades, are recommended for electronic materials.
Cross-section of a 4GB SD Drive, approximately at 100x. Taking measurements of a cross-sectioned package can determine the exact location to backside grind.
For sectioning thin materials, the Precision Table accessory (11-2694-160) is ideal. It will readily accommodate wafers up to 4″ (100mm). When using this accessory, you must first mount the wafer to a support as indicated below.
The Sample Mounting Unit supplied with the MPC™ 3000 adjusts for any differences in parallelism between the outer surface and layer of interest. For example, the exposed surface of the silicon device is not always parallel with the opposite surface of the package, meaning the silicon provides only a reference surface.
Using special consumables, it is possible to remove material (layers) with a precision of 0.4µm. Surface areas of 20x20mm or less can be processed with minimum edge rounding. In addition, LSI (Large Scale Integration) packages can be backside lapped to expose the interconnects.
To continue grinding, apply the appropriate consumables (see Table 1) to the platen surface and repeat the steps 6-12. Change the angle of the sample surface 90 degrees after every grinding or polishing operation. After reaching the target, select the polishing pad and suspension based on the materials in the sample. Some examples are listed In Table 2. Polish at 100 – 150 RPM until the desired result is achieved.
Buehler. (2019, July 15). Semi Automated Parallel Lapping of Electronics for Microscopic Examination by Buehler. AZoM. Retrieved on February 25, 2020 from https://www.azom.com/article.aspx?ArticleID=5218.
Buehler. "Semi Automated Parallel Lapping of Electronics for Microscopic Examination by Buehler". AZoM. 25 February 2020. .
Buehler. "Semi Automated Parallel Lapping of Electronics for Microscopic Examination by Buehler". AZoM. https://www.azom.com/article.aspx?ArticleID=5218. (accessed February 25, 2020).
Buehler. 2019. Semi Automated Parallel Lapping of Electronics for Microscopic Examination by Buehler. AZoM, viewed 25 February 2020, https://www.azom.com/article.aspx?ArticleID=5218.
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