Thermomechanical behavior of single crystal silicon microbeams under bending at elevated temperatures

Understanding the deformation mechanisms of materials at high temperature at the micro/nanoscale became important with the increasing miniaturization of electronic and mechanical devices. Silicon is the most commonly used material in these devices. Bulk silicon is brittle at room temperature, but is ductile at 550 ºC - a phenomenon known as brittle-to-ductile transition (BDT). Recently, there has been a speculation in the literature that BDT is size dependent - smaller the size, lower is the BDT temperature. This led to a controversy: some studies report ductility at room temperature, others reported brittle behavior. This thesis challenges the notion of a precise BDT temperature in single crystal silicon, unlike Curie temperature in ferromagnetic materials when phase change occurs. We hypothesize that brittleness or ductility in silicon is the outcome of a competition between cleavage or plastic deformation due to presence of flaws and nucleation of dislocation. At any temperature, dislocations may nucleate at high enough stress, τn, if flaw induced fracture can be avoided, and as long as τn < τc, the cleavage stress of silicon. However, τn is temperature dependent. It increases with decrease in temperature. On the other hand, flaw probability decreases with sample size (increased flaw tolerance) or the size of the high stress zone. This gives access to high stress in small samples and hence an apparent size dependence of BDT temperature. We tested the hypothesis by carrying out experiments with single crystal silicon micro beams under near pure bending which limits the high stress region to a small volume of the material near the surface and the support. We developed a novel method using microfabrication to carry out these experiments. The method involves a silicon MEMS stage that allows to test silicon samples under bending at high temperature (up to 450°C) in-situ in SEM. The stage performance was first tested by measuring the elastic modulus of SCS micro-beams oriented along [0 1 1] and by comparing the measured value with that in the literature. We found that the strength of silicon increases compared to the uniaxial tension test results. Second, specimens with thicknesses 2-5 µm exhibited BDT at 400 ºC, much less than the bulk value of 550ºC. Third, we studied stress relaxation behavior in silicon after initiation of ductility at 400 ºC. Here, silicon deforms with time while stress decreases. We also revealed the detailed mechanism of stress relaxation using a combined SEM, TEM and AFM analysis. We find, at peak stress of about 3.2 GPa, multiple dislocation sites nucleate simultaneously. Dislocations begin to initiate from these sites with time mediating plastic deformation and stress relaxation. We developed a simple mechanistic model to correlate dislocations activities, nucleation and glide, with changes in load during stress relaxation. The model takes into consideration the effect of stress gradient and predicts the plastic zone size along the beam thickness.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2019

Author dc:creator

• Elhebeary, Mohamed Mohamed Rashad Ibrahim

Contributors dc:contributor

• Saif, Taher
• Sehitoglu, Huseyin
• Krogstad, Jessica
• Tawfick, Sameh

• Silicon, stress relaxation, plasticity, bending, MEMS, insitu SEM

Statement dc:rights

• Copyright 2019 by Mohamed Elhebeary

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/105928

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/105928