טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBen-David Eran
SubjectA New Instrument for Tensile Testing of Thin Free
Standing Films at High Strain Rates
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Doron Shilo
Professor David Elata
Professor Daniel Rittel
Full Thesis textFull thesis text - English Version


Abstract

Thin free standing films (TFSF) are commonly employed in micro and nano electro mechanical systems (MEMS & NEMS). TFSF may experience various mechanical constraints. The design of more reliable and sophisticated devices relies on the knowledge, understanding and ability to control the mechanical properties of TFSF.

TFSF often exhibit a mechanical behavior which is different from that of bulk specimens (referred to as "size effect") when their dimensions become comparable to the characteristic length-scales that govern the mechanical behavior.

Mechanical properties of TFSF are strongly dependent on the fabrication process and on the substrate. To predict the mechanical properties of TFSF, specimens from the exact same fabrication procedure need to be tested first.

Tensile test is a method for the exploration of mechanical behavior of TFSF, which benefits from two major advantages compared to all other tests:

1.      It is not influenced by the substrate.

2.      Stress and strain states are uniform during the test which simplifies the extraction of the mechanical properties of the specimen.

Another important topic is the rate dependant mechanical behavior and fatigue properties of TFSF.  In particular, the behavior of µm-scale specimens under high strain rates is crucial in some MEMS applications in which the device is exposed to shock environment, either during fabrication, deployment, or operation.

The objective of this research was to develop a new instrument for testing TFSF under pure tension, at different rates.

The experimental setup includes a silicon micro-device in which an aluminum tensile specimen is embedded. Silicon springs and a rigid frame protect the specimen during its handling. The micro device was mounted between a piezo-electric linear stage and a custom made piezo-electric force sensor. A novel method, based on a commercial optical encoder and a custom-made aluminum grating, was developed to measure displacements. Experimental tests demonstrate the ability of the encoder to measure displacements with a resolution of 25 nm and sampling rate of 1 MHz, under a variety of displacement rate functions.

Due to technical problems with the micro-device fabrication, no intact specimens remained after the fabrication rounds. Therefore, the capabilities of the developed instrument were tested using aluminum bonding wires. All the setup components were tested and the capability of the testing procedure to extract of the net force and displacement of the specimen was verified. The measured stress-strain curve for the bonding wire was plotted.