18 Jan 2017
Recent Advances in TEM using an In Situ Heating to replicate Service Conditions
Loughborough Materials Characterisation Centre (LMCC) is continually reviewing new technology to ensure the centre continues to provide facilities which can help produce world class microstructural characterisation of materials. In this month’s news we shall be reviewing the DENSsolutions Wildfire In-Situ Heating TEM holder we have been testing.
LMCC has a high resolution transmission electron microscope (FEI Tecnai F20 FEGSTEM). It proves extremely valuable in delivering atomic resolution imaging, structural identification and chemical analysis for materials characterisation. However, the historical limitation of samples being introduced into a high vacuum and static environment for electron microscopes means difficult to replicate real-world environmental conditions (heating, gas, liquid, etc.). Instead of using the conventional route to produce an “environmental TEM”, the in-situ holders modified the conventional sample holders to get closer to real in-service conditions. DENSsolutions utilizes MEMS devices, called Nano-Chips, to control the sample’s environmental surroundings.
A Wildfire D6 heating holder was kindly loaned to LMCC. The in-situ investigation for bulk materials was enabled by focussed ion beam sample preparation. Site specific TEM samples are produced and transferred onto a heating chip using our in-house FEI Nanolab FIB. It allows for heating up to 1300°C with heating and quenching rate 200°C/millisecond. The microstructure of the material can be studied in real time as it is being heated or cooled, even thermal cycles can be simulated in a fast rate. This new development allows us to study dislocations and precipitation/dissolution of second phases as a function of temperature to name a couple of examples. Staff within LMCC feel this development will be a significant benefit to the capability already available within the centre and shall benefit researchers at Loughborough University and external clients.
ZnAl is one of the alternatives of Pb free high temperature solders. Semiconductor or power die attachment requires the use of high temperature solders in order to maintain the integrity of the joint between the die and the lead frame at board level assembly. The solder was heated from room temperature up to 300°C in the TEM, the analysis shows live growth of intermetallic compound between the ZnAl solder and the under bump metallization layer, as well as the strain induced during the heating as evidenced by the dislocations evolution. The conventional ex-situ characterisation would only have chance to record the microstructures of the starting point and the final heat-treatment. All the evolution stages would have been missed, hence the conclusions would be hypothesis.
A video showing the live growth of intermetallic compound between the ZnAl solder and the under bump metallization layer, when a ZnAl solder was heated from room temperature up to 300°C in a TEM.
In addition to the developments to carry out in-situ (cyclic) heating experiments at the nano scale within the TEM, LMCC now has the capability to carry out mechanical testing at the micron level within our focussed ion beam. In the next month’s news we shall report on the results obtained from micro bend tests on a ceramic substrate one of our research students has been carrying out in the FIB.