MEMS-Kammantriebsaktoren
With thin films and coatings being now widely applied in microsystems and microelectro-mechanical systems (MEMS) for protective or functional aims, determination of the mechanical properties of such thin layers gains more and more importance. As a rule, most of the knowledge in characterising bulk material behaviour fails to describe the material response of thin layers. Therefore a variety of methods has been proposed to investigate the mechanical properties of small volume of material. The here applied micro-tensile testing method measures the stress-strain (σ-ε) response of micro-scale materials directly, and is also applicable for dynamic testing.
A nano-tensile testing system based on MEMS techniques was developed (see Fig.1). The key points of the proposed research consists of (1) a high resolution nano-force generator based on a MEMS actuator, (2) a free-standing thin-film specimen for nano-tensile testing, (3) in-plane strain measurement on basis of capacitive sensing or SPM technique, (4) investigation of the mechanical properties of single-/multi layer materials, and its relationship to geometrical dimensions and technological factors.
Fig.1 Scheme of PTB´s nano-tensile testing system
MEMS nano-force actuator
The nano-force actuator is based on the principle of electrostatic lateral comb-drive, which features: (1) the electrostatic force is proportional to U2 (U is the applied electrostatic voltage), (2) the electrostatic force is independent from the displacement of the moveable part, (3) the positioning and sensing can be accomplished simultaneously on basis of the same electrostatic structure.
Three configurations of the nanoforce actuator are designed as shown in Fig. 2
Fig. 2b: Enhanced version
Fig. 2c: Improved version
Fig. 2b: Enhanced version
Fig. 2c: Improved version
Fig. 2b: Enhanced version
Fig. 2c: Improved version
Mechanical properties of the MEMS nano-force actuators:
| Type of actuator | Spring constant (N/m) | Maximum output force (mN @ 50 V driving voltage) | Resonance frequency (kHz) |
| Basic version | 12.4 | 0.55 | 1.69 |
| Enhanced version | 31.6 | 0.74 | 2.14 |
| Improved version | 31.6 | 1.33 | 2.53 |
Specimen for tensile testing
The basic configuration of the free-standing specimen is shown in Fig. 3, in which two types of specimens are designed. One has a totally free-standing movable end, and the other uses folded springs to support its movable end/holder.
Fig. 3b:Free standing thin film with folded spring supporting
Fig. 3b:Free standing thin film with folded spring supporting
Fig. 3 Basic configuration of the designed free-standing specimen
System realization
The structures were realized by Bonding-DRIE technology (TU Chemnitz, Germany).
The force actuator and the thin film can be coupled together mechanically or fabricated in one chip as integrated version. Fig. 4 shows the coupling of a specimen and a nano-force actuator and Fig. 5 shows the typical measurement curve for a 200 nm thickness, 1.5 µm width, 180 µm length Aluminium thin film. In this system, the in-plane displacement of the force actuator is realized by the capacitive sensing method.
Fig. 5: Typical measurement curve
Fig. 5: Typical measurement curve
Other applications of the MEMS actuator: stiffness calibration of AFM cantilevers
Besides the application in nano-tensile testing, the nano-force actuator (with the capacitive sensing system) has some other potential applications, such as cantilever calibration, nanoindentation and nanoforce artefact. Fig. 6 shows typical cantilever calibration curves, in which (a) and (b) are for cantilevers with stiffness about 1.6 N/m and 46 N/m, respectively. Fig. 7 shows a ball-shaped indenter mounted on the indenter holder of the main shaft of the MEMS
Fig. 6 Calibration curves of two AFM cantilevers using a MEMS actuator. Left: Result for kCantilever = 1.6 N/m < kActor and on the right hand: kCantilever = 46 N/m > kActor
Fig. 7: The special designed indenter holder and the mounted spherical indenter