Organic and Large Area Electronics (OLAE) are composed of thin film layer stacks, whose combined functionality exhibit the required properties to make a possible large variety of electrical devices, ranging from temperature sensors to AMOLED displays. These thin films are typically in the thickness of a few nanometers up to a few microns.
The thin form factor provides the inherent property of flexibility when fabrication is done on flexible substrates. The flexural rigidity (D) or bending stiffness, i.e. the amount of force required to bend, is given by:
where E is the Young’s modulus, h the thickness of the material and v the Poisson’s ratio.
For a stack of different materials and layers of different thicknesses, it can be seen that the flexural rigidity is much more strongly determined by the thickness of a layer compared to the Young’s modulus of the material. This is one of the reasons why, in flexible electronics, inorganic brittle layers such as oxides or nitrides, having a high Young’s modulus, are typically very thin to achieve high flexibility.
Commonly used low-cost plastic substrates are PET or PEN, where PEN is slightly more expensive and has higher temperature stability. Both materials have a Young’s modulus higher than 1GPa. To provide stability, while remaining flexible, substrate thickness is typically 100-200 µm. For these types of devices, the flexural rigidity is dominated by the substrate. Since many of the very thin layers and layer stacks can be prone to damage by touch, a thicker encapsulation or coating is often provided on top of the device for mechanical protection, which in turn has a significant contribution to the bending stiffness of the stack.
Figure 1. IGZO-TFT stack fabricated with brittle inorganic layers as insulator and dielectric
When working on devices based on a driving and/or readout by a thin film transistor (TFT) backplane, e.g. organic-based imagers in the MADRAS project, other requirements have to be taken into account. State-of-the-art backplanes in flat panel display fabrication are based on either amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS) or oxide-based, which is in our case at TNO-Holst centre TFTs based on indium gallium zinc oxide (IGZO).
The process temperatures required for the thin film stack in these types of devices typically exceed the maximum temperatures PET and PEN can withstand. For these reasons, a different and much more expensive polymer substrate is used: polyimide (PI), which can be stable at> 350 °C. To limit the costs of the PI, typically very thin substrates are used, i.e. in our case around 15 µm in thickness. As we have seen that the flexural rigidity or bending stiffness scales with the thickness to the power 3, the bending stiffness is reduced in the device stack to such a low level that freestanding devices on PI cannot be used in most applications without any form of mechanical damage.
Figure 2. Example of a conical defect upon handling of a 15 um PI foil
One directional bending is not problematic and can be done to very low bending radii of only a few mm. However, the most common damage occurring when handling such freestanding devices are conical defects -sharp point defects occurring at the end of larger out-of-plane deformations of the foil- which easily occur for larger substrates. These conical defects are the same types that cause local creases when handling large sheets of paper or aluminium foil. At such a defect location, the local bending radius is very small and strains in the layers are above the failure strain, leading to local damage in many of the layers in the stack.
Hence, to work with thin films on polyimide, safe debonding methods have been developed to debond the foils from the glass carrier and to be laminated on a more rigid substrate. Many everyday objects have curved plastic surfaces made at large quantities by injection moulding and/or thermoforming. Such hard plastic surfaces can provide even much longer durability of the thin film electronics when these are properly protected by such a hard outer layer.
A possible method to integrate thin film devices on polyimide onto these curved surfaces can be done by lamination. However, logically this is extremely difficult to achieve defect-free and this would furthermore require a specialised lamination tool for each curved object. Instead, a better option is the development achieved within the framework of MADRAS: to apply these thin substrates with electronics on flat surfaces and thermoform these in the required shape, so-called in-mould electronics. This allows for much more freedom in prototyping, better scalability, and lower associated costs. Obviously, all layers in the device stack have to survive mechanical deformations, as well as thermal and pressure conditions during the process.
The in-mould biometric sensor under development in MADRAS consists of an organic photodiode (OPD) frontplane on top of an IGZO-TFT backplane. To finalise the device and protect it mechanically, as well as from degradation by moisture in the ambient, a thin film encapsulation is applied. This device will be thermoformed into the desired shape and over-moulded for its integration into the body of a scooter.
As a first step, TNO-Holst Centre has focused on the OPD frontplane with the thin film encapsulation containing thin and brittle silicon nitride layers for excellent moisture blocking properties. As it is at the exterior of the total stack, the thin film encapsulation is most sensitive to mechanical deformation.
An experiment has been set up where an array of 16×16 OPDs (256 optical sensor elements) on PI were connected to a readout IC and a (low resolution) optoelectronic readout could be performed. The samples on glass//PI were bonded to a polycarbonate (PC) substrate by using a layer of thermoplastic polyurethane. Next, the PI was debonded from the glass carrier and the thermoforming process optimisation for temperature and pressure compatibility with the encapsulated OPD was performed. Finally, the PC substrate with the device was thermoformed to the required curved shape at the location of the OPD array.
Figure 3. Thermoforming process of OPD array on PI, with an interlayer of TPU to PC
After process optimisations, where the introduction of a TPU interlayer was a crucial aspect, the stack has survived the thermoforming process and a permanently curved OPD array has been obtained with high mechanical stability. For clarity, the flex with commercial readout IC connected to the array on PI was kept outside the thermoformed area. The extension of the PC substrate covering the connection location where the flex is bonded to the fanout electrodes on the PI substrate also improves the mechanical stability of this connection.
Figure 4. Curved in-moulded OPD array to PC with a transparent sheet with print placed on top. Inset: readout of the 16×16 array depicting clearly the original input image
To show the functionality of the imager array after thermoforming, a transparency with a low-resolution pixelated sheep has been printed, matching the sensor resolution. This has been placed on top of the curved imaging array. The readout image has clearly demonstrated good readout by the imager after thermoforming. No damage has been observed to the OPD functionality, nor to the thin film encapsulation stack. Within MADRAS the next steps will focus on the thermoforming and over-moulding of a complete stack on PI, including a newly developed OPD stack and materials as well as a TFT backplane.
About the author
Hylke Akkerman, Program Manager and Senior Scientist Thin Film Imagers and Wearable Optical Sensor Arrays
- PhD in Applied Physics at the University of Groningen,
The Netherlands - Postdoctoral Fellow at Stanford University, CA – USA
- 18 years of experience in molecular and organic electronics
- 10 years at Holst Centre /TNO on the topics of thin film encapsulation, thin film imagers, and wearable optical sensor arrays.
TNO is an independent applied research institution with a staff of ±3000 people and an annual turnover exceeding €400M€. TNO maintains close contacts with universities and basic research institutions in order to translate up-to-date knowledge and insights into practical applications. Clients include government, large companies and SME’s.
