Thermal Analysis of Thin Films

By Sarah Ackermann, Laboratory Services Manager

Thin films are layers of material ranging from fractions of a nanometer to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of thin film deposition consists of the application of a thin layer of material – such as a metal, semiconductor, or insulator – onto a “substrate” or onto previously deposited layers. Thermal Analysis of Thin Films is crucial for understanding the thermal properties of thin films, which have a wide range of applications in various industries, including electronics, optics, and materials science. Read on for an overview of the common thermal analysis methods used in the field of thin films. 

What is Thermal Analysis?

Thermal analysis is a family of techniques which study the variation of material properties as a function of temperature and the responses of materials to temperatures. These techniques are crucial tools for researchers and engineers working with thin film materials. It allows them to understand the thermal behavior of materials and their suitability for various applications. In this article, we will review some of the most commonly used methods in thermal analysis of thin films, including:

• Differential scanning calorimetry (DSC) including Dynamic DSC (DDSC)
• Thermogravimetric analysis (TGA)
• Simultaneous Thermal Analysis (STA)
• Dynamic Mechanical Analysis (DMA),
• Thermomechanical Analysis (TMA) and
• Thermal Conductivity Testing

Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) is a technique that measures the heat absorbed or released by a sample as a function of temperature. The technique is widely used in the thermal analysis of thin films to determine specific heat capacity, phase transitions, and reactions.

DSC places a reference pan on the reference side of the sensor plate, a sample pan on the sample side of the plate, and measures the difference in heat flux between the sample and the reference sides to determine the heat flux associated with the sample behavior while the sample is heated or cooled according to a pre-determined thermal profile.

DSC can be paired with a sample camera to observe sample behavior. Together, this allows for unambiguous quantification of even complicated phase behavior.

Dynamic DSC (DDSC) is a variant of DSC that applies a sinusoidal modulation to a linear temperature program. This temperature-modulation allows the sample’s response to be separated into in-phase (thermal) and out-of-phase (kinetic) components. DDSC can separate overlapping events that would be indistinguishable in conventional DSC.

In the image above, the separation of reversible and irreversible components shows the presence of a small endotherm at 76.8°C which not visible in the total heat flow signal – it also reveals a complex chemical transition in the range of 225°C-275°C which overlaps with a reversible endotherm and is not visible on the total heat flow.

This allows confident assignment of transitions and chemical events, as well as measurement of heat capacity during chemical and physical transitions. This data becomes key design inputs for engineers and designers, ensuring safe and efficient thermal management and process designs.

Thermogravimetric Analysis and Simultaneous Thermal Analysis (STA)

Thermogravimetric analysis (TGA) measures the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere. The measurements are used primarily to determine the thermal stability of a material and its compositional properties. The technique can analyze materials that exhibit weight loss or gain due to decomposition, oxidation, or dehydration.

Simultaneous Thermal Analysis pairs the powerful heat flow measurements possible with DSC with the sensitive mass change measurements possible on a TGA.

In the video above, PET is heated and decomposed, illustrating the use of TGA – coupled with sample observation and DTA – to understand the thermal stability of a material.

Dynamic Mechanical Analysis

Dynamic Mechanical Analysis (DMA) is a technique that studies the viscoelastic behavior of materials by applying a periodic stress and analyzing the material’s response. It is particularly useful in studying the mechanical behavior of thin films under different thermal conditions. It can measure changes in Viscoelastic Properties as a function of Temperature, Force, Time, Humidity and Frequency.

An important step in any DMA test is to determine how the material should be deformed.  Sometimes this is chosen to replicate the loading condition or meet a specific standard, while other times can be determined out of necessity.

For thin films, samples are typically loaded in tension.  An important mechanical loading consideration is to determine whether a static preload will be used in your experiment.

• No static preload: The thin samples can buckle during the upstroke.
• Static preload:  Samples can ‘relax’ or expand/contract during temperature tests.

One way to eliminate the issue of static loading (or lack of it), can be to switch the deformation mode.  For instance, Metravib’s ‘shear for films’ holder, which doesn’t require a static load to prevent buckling, seen below:

DMA can accurately measure the glass transition of a film and its secondary transitions with a higher sensitivity than any other method for measuring subtle changes not always apparent on other techniques.

The plot above shows the change in the tan delta and storage modulus of a thin film material during a temperature sweep at a steady frequency of 10 Hz.

DMA can also be used to measure long term cyclic and static effects of loading through fatigue and creep/relaxation tests. The plot above shows a static creep test on a thin film. The displacement is plotted in red and force is plotted in blue. The creep of the sample can be seen increasing non-linearly with time until the force is removed, after which the material relaxes gradually.

Thermomechanical Analysis

Thermomechanical Analysis (TMA) measures the dimensional changes of a material as a function of temperature. It can provide thermal expansion coefficients, glass transition temperatures, and softening points. TMA is especially beneficial for films that exhibit significant dimensional changes with temperature.

TMA enables engineers and designers to understand and engineer for thermal stresses, which is key to avoid delamination, deformation and other device failure modes. The figure above explains why this is important: If the thin film material and what it is placed on have dissimilar thermal expansion behavior, then either the substrate or the film will experience thermal stress. This can cause quality issues, like delamination, reduced service life, or deformation – and to manage these issues, engineering and material solutions are advised.

The plot above shows the TMA behavior of a polyimide thin film material. Two crystal phase transition points, at 51.4°C and 66.9°C are obvious from analysis of the TMA plot. Understanding these crystal transitions, and the overall thermal expansion behavior of the film with temperature, is key to ensuring good long-term thermal performance of laminated structures.

Thermal Conductivity

Thermal conductivity measures a material’s ability to conduct heat. It is a critical parameter in designing electronics, solar cells, and other devices. Researchers and engineers can use thermal conductivity measurements to evaluate the thermal performance of materials, optimize the design of devices using thin films, and troubleshoot performance issues and failure modes. Thermal conductivity testing includes many methods, each of which is suited to different sample thermal conductivity ranges and geometries. The two most commonly used for thin materials include the Flex TPS Thin Films method and the Slab method. Details on the methods and their strengths and weaknesses can be found in our Method Selection Guide.

In the image above, a thin piece of aluminum is analyzed using the Flex TPS accessory with the TPS Slab test method. For more details on the performance of the TPS slab utility int he testing of aluminum foil, check out our application note here.

The image above shows the principle of the TPS Thin Films utility in testing a thin film of polyimide. This utility is well suited to determining the thermal conductivity of thermally insulative thin film materials. For more information on thermal conductivity testing of thin-film materials, you can watch our webinar video below:

Conclusion

In conclusion, thermal analysis is a powerful tool for researchers and engineers working with thin film materials. The thermal properties of thin films are critical for their effective utilization in various applications. The thermal analysis techniques discussed in this paper – DSC, DDSC, TGA, STA, TMA, DMA, and Thermal Conductivity Testing – offer valuable insights into these properties, including specific heat capacity, thermal stability, dimensional changes with temperature, and heat-transfer properties. As the field of thin films continues to advance, these techniques will undoubtedly remain integral to research and development.

Next Steps

If you have thin films you need to understand better, we can help! Click the link here to make an appointment with one of our thermal analysis experts to discuss testing solutions, or you can learn more about our thermal analysis equipment here.

Acknowledgements

The author thanks Travis Parkman for assistance in drafting the DMA section, and Rigaku and Metravib for use of image assets and video.