An Introduction to Mechanical Testers for the Microelectromechanical Systems (MEMS)

Today, the microelectromechanical systems (MEMS) industry requires mechanical testers that are capable of applying controlled high-resolution forces. Moreover, these mechanical testers need to have a wide range of travel in the case of ­flexible devices sensitive to force. The Nanovea Mechanical Tester showcases its high resolution and large travel distance capabilities by conducting ­at-punch compression tests on soft and fl­exible samples, at very low loads and displacement ranges exceeding 1 mm.

Importance of Testing Soft, Flexible Materials

A microelectromechanical system is an apt example of very soft and fl­exible samples. They are used in routine, daily commercial products such as printers, mobile phones, as well as cars [1]. The use of MEMS also includes special functions, like biosensors [2] and energy harvesting [3]. To find use in applications, MEMS are required to be able to repeatedly and reversibly transition between their original configuration and a compressed configuration [4].

Thus, compression testing needs to be conducted to understand the manner in which the structures will react to mechanical forces. Moreover, compression testing finds application in the testing and tuning of various MEMS configurations. It can also be used to test upper and lower force limits for these samples.

The Nanovea Mechanical Tester Nano Module is ideal for testing the soft and ­flexible samples because of its ability to collect data at very low loads in an accurate manner. Moreover, it can also travel over 1 mm of distance. What’s more, owing to its independent load and depth sensors, large indenter displacement has no impact on the readings taken by the load sensor.

Thus, when compared to other nanoindentation systems, this is a unique system with the ability to carry out low-load testing over a range of more than 1 mm of indenter travel. When compared to a nanoscale indentation system, a reasonable travel distance is typically below 250 µm.

Equipment Featured

NANOVEA CB500

Load Control, Multi-Module, & Load Resolution 0.004microN

  • Broad Application Use
  • Full range of testing modes including hardness, scratch, and wear
  • Compact, Modern Design
  • Motorized Stages (X,Y,Z) with Lateral accuracy of 0.1 um

Measurement Objectives

Through this case study, Nanovea conducted an experiment using compression testing on two uniquely di­fferent flexible, spring-like samples. The system’s ability to conduct compression at very low loads and record large displacement while accurately obtaining data at low loads was thus showcased, while demonstrating how this can be applied to the MEMS industry.

Due to stringent privacy policies, the samples and their origin shall remain unnamed in this study.

Measurement Parameters

Representative schematic describing sample tested. Top-down view of a _exible sample (left) and side view with indenter (right). Not drawn to scale.

Figure 1: Representative schematic describing sample tested. Top-down view of a flexible sample (left) and side view with indenter (right). Not drawn to scale.

Table 1: Test parameters for tribology testing on pistons

Test Parameter Sample A Sample B
Maximum Force (mN) 0.075 8.00
Loading Rate (V/min) 1.00 15.00
Unloading Rate (V/min) 1.00 15.00
Indenter Type Flat Flat
Indenter Radius (μm) 50 50

Note: When the indenter is in the air, the loading rate of 1 V/min is proportional to approximately 100 µm of displacement.

Compression Test Results

The response of the sample to mechanical forces can be observed through the load vs depth curves. Sample A, however, only displays linear elastic deformation within the test parameters listed previously. For a load vs. depth curve at 75 µN, Figure 2 is a great example of the stability that can be achieved. Thus, it would be easy to perceive any signi­ficant mechanical response from the sample, owing to the load and stability of depth sensors.

On the other hand, Sample B displays a mechanical response that differs from Sample A. What begins to appear is fracture-like behavior in the graph, past 750 µm of depth. This can be observed with the sharp reductions in load at 850 and 975 µm of depth. Nevertheless, despite traveling at a high loading rate for more than 1 mm over a range of 8 mN, the highly sensitive load and depth sensors equip the user with the sleek load vs depth curves below.

Stiffness was calculated from the unloading portion of the load as opposed to depth curves. In other words, stiffness represents the amount of force necessary to deform the sample. To calculate this stiffness, a pseudo Poisson’s ratio of 0.3 was used – because the actual ratio of the material is not known. Here, Sample B proved to be stiffer than Sample A.

Table 2: Results from compression testing on soft, flexible samples.

Max Depth (μm) Unloading Stiffness (mN/nm x 106)
Sample A 29.20 ± 0.71 2.552 ± 0.319
Sample B 1084.33 ± 18.49 7.470 ± 0.204

Load vs Depth curve for Sample A at 75 μN.

Figure 2: Load vs Depth curve for Sample A at 75 μN.

Load vs Depth curve for Sample B at 8 mN.

Figure 3: Load vs Depth curve for Sample B at 8 mN.

Conclusion

Using the Nanovea Mechanical Tester’s Nano Module, two very di­fferent flexible samples were tested under compression. Both tests were conducted at extremely low loads (1 mm). Using the Nano Module, nano-scaled compression testing proved the module’s ability to test very soft and flexible samples. Further, additional testing for this study could potentially address the effect of repeated cyclical loading on the elastic recovery aspect of the spring-like samples via the Nanovea Mechanical Tester’s multi-loading option.

References

[1] “Introduction and Application Areas for MEMS.” EEHerald, 1 Mar. 2017, www.eeherald.com/section/design-guide/mems_application_introduction.html.

[2] Louizos, Louizos-Alexandros; Athanasopoulos, Panagiotis G.; Varty, Kevin (2012). "Microelectromechanical Systems and Nanotechnology. A Platform for the Next Stent Technological Era". Vasc Endovascular Surg. 46 (8): 605–609. doi:10.1177/1538574412462637. PMID 23047818.

[3] Hajati, Arman; Sang-Gook Kim (2011). "Ultra-wide bandwidth piezoelectric energy harvesting". Applied Physics Letters. 99 (8): 083105. doi:10.1063/1.3629551.

[4] Fu, Haoran, et al. "Morphable 3D mesostructures and microelectronic devices by multistable buckling mechanics." Nature materials 17.3 (2018): 268.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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