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Mikro-Elektro-Mechanisches System

 

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. 2a: Basic version
Fig. 2b: Enhanced version
Fig. 2c: Improved version



Fig. 2a: Basic version
Fig. 2b: Enhanced version
Fig. 2c: Improved version



Fig. 2a: Basic version
Fig. 2b: Enhanced version
Fig. 2c: Improved version



Fig. 2d: Prototype of the MEMS force actuator

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. 3a: Fee standing thin film without supporting
Fig. 3b:Free standing thin film with folded spring supporting
Fig. 3a: Fee standing thin film without 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. 4:Coupling of a specimen and a nano-force actuator
Fig. 5: Typical measurement curve
Fig. 4:Coupling of a specimen and a nano-force actuator
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