BS IEC 62047-31:2019 pdf free download – Semiconductor devices – Micro- electromechanical devices
1 Scope
This part of IEC 62047 specifies a four-point bending test method for measuring interfacial adhesion energy of the weakest interface in the layered micro-electromechanical systems (MEMS) based on the concept of fracture mechanics. In a variety of MEMS devices, there are many layered material interfaces, and their adhesion energies are critical to the reliability of the MEMS devices. The four-point bending test utilizes a pure bending moment applied to a test piece of layered MEMS device, and the interfacial adhesion energy is measured from the critical bending moment for the steady state cracking in the weakest interface. This test method applies to MEMS devices with thin film layers deposited on semiconductor substrates. The total thickness of the thin film layers should be 100 times less than the thickness of a supporting substrate (typically a silicon wafer piece).
4.1 General
The test piece for the layered MEMS materials shall be prepared using the same fabrication process that applies to actual MEMS devices. Machining of the test piece shall be performed to prevent formation of unintended cracks or flaws and delamination in the test piece.
4.2 Shape of a test piece
The overall shape of a test piece is shown in Figure 1. Because the evaluation of the energy release rate relies on several simplifying assumptions, the geometric shape of the test piece should be designed as follows: the thickness of the test piece should be 50 times less than the length and width of the test piece, and the length should be 10 times larger than the width. The total thickness of the layered materials should be 100 times less than the thickness of a supporting substrate. A pre-crack or notch shown in Figure 1 is machined using conventional ways like a diamond saw, laser ablation, or chemical etching. This pre-crack initiates cracking in the supporting substrate under bending, and after that the cracking leads to the introduction of an interface crack between two adjacent layers of the weakest interface in the layered materials.
4.3 Measurement of dimensions
To analyze the test results, the test piece dimensions shall be accurately measured because the dimensions are used to determine the mechanical properties of test materials. Spacing between the pins (L), width (b)t and thickness (h) should be measured with an error of less than ±5 %• Information on thickness measurement can be found in IEC 62047-2:2006, Annex C and in IEC 62047-3:2006, Clause 6.
5.1 Test principle
The test is performed by applying a pure bending moment to a test piece with a layered material interface as shown in Figure 1. The test consists of two steps: the first is the introduction of an interface crack between two layered materials with the weakest interface. By gradually increasing the four-point bending load applied to the test piece, a crack is initiated from a machined pre-crack. The initiated crack extends in a direction of thickness and then becomes an interface crack when it touches the weakest interface. When the interface crack is formed, the bending load is relaxed. The second step is the extension of the interface crack. By reloading the bending load, the energy release rate at the tip of the interface crack increases, and the interface crack starts to extend. The energy release rate of an interface crack is independent of the crack length when the test piece is under pure bending. For this test method, it is unnecessary to measure the length of the interface crack.