Frontiers in Transparent Electronics in 2023

The electronics industry almost exclusively relies on planar devices built on organic substrates, which are obviously transparent. Today, with a wealth of new materials available, electronics systems designers have material options that enable transparency in terms of substrates, active sensors or devices, and even the conductors used to route circuits. Transparent electronic materials are an important element of optically active devices, but the range of applications does not end there.

In addition, when we refer to transparent electronics, we aren’t just interested in optoelectronics. Transparency in electronics allows mounting and printing a huge range of circuitry on surfaces without blocking light from reaching the user, so it enables different product experiences than traditional approaches to electronics. Not all devices are candidates for transparency, but we can find a huge range of applications in automotive, mil-aero, and even medical devices.

Materials Enable Transparent Electronics

The current set of advances in transparent electronics are reliant on a new class of transparent or translucent materials. Device designers can mix and match a range of functions and materials, ranging from transparent circuits on opaque materials to a complete set of transparent materials with embedded active devices. The electronic materials industry has managed to produce and commercialize a huge range of transparent systems, such as:

• Transparent substrates that can support transparent conductors or semiconductors
• Transparent conductors like that can provide the main circuit functionality and interconnects
• Transparent semiconductors that can be used to build active devices embedded on opaque or transparent substrates
• Fully transparent devices that combine the above material sets, typically as planar devices

The current class of low-cost deposition methods are electrochemical or solution/suspension-based, and these enable simple fabrication of transparent components and systems. 3D printing is another option for depositing these material systems without the need to fabricate deposition  masks, use seed layers, or use etching to produce thin films for these devices.

Transparent Substrates

There is a broad range of standard transparent materials that can be used as substrates for transparent electronics. Transparent substrates include glass, fused quartz (for UV transparency), polyethylene (PET), sapphire, and ceramic borosilicate glass. Ceramics are a very attractive option for any transparent electronics that may experience significant heat flux as they can have very high thermal conductivity, and this will prevent the device from reaching an excessively high temperature.

FTO-coated glass slabs are one of the most common substrates used for transparent electronics.

Applications of these materials range broadly from resistive heaters to large-format touch screens and displays. RF circuits operating into the high GHz are also possible, but these work best when the conductor material offers the lowest possible sheet resistance.

Transparent Conductors

The most common transparent conductors are dope tin oxide films, which are commonly patterned on glass or plastic substrates. Glass slides coated with tin oxide films can be purchased commercially. In addition, other films can be deposited and patterned (etched), or deposited through masks, to define conductors for transparent circuits:

• Transparent conductive polymers
• Metal nanostructured films
• Indium-doped tin oxide (ITO)
• Fluorine-doped tin oxide (FTO)
• Aluminum-doped zinc oxide (AZO)

Other wide bandgap metal oxide films, particularly zinc oxide films, can also be used as transparent conductors or semiconductors, depending on the bandgap shift induced by the presence of any dopants.

There are two specifications that are important for transparent conductors. The main specification that determines the usefulness of a transparent conductor on a planar substrate is its sheet resistance, sometimes known as square resistance, measured in Ohms per square (OPS). This quantity defines the resistance measured across a sheet of material with any area, and it can be easily defined based on the film thickness and conductivity.

The other quantity is visible light transmittance (VLT), which can be roughly defined by comparing the total power of light transmitted to incident light throughout the visible range. Ideal transparent electronic devices will have well above 90% VLT.

These specifications for transparent conductors are shown in the table below.

Lower values of VLT may create a hazy appearance that can be seen by the human eye, particularly at oblique incidence. For very small patterned circuits, a transparent conductor film with low VLT can create Moire fringes that can be seen very clearly, and which could be distracting to the user of the device. There is also a tradeoff between sheet resistance and VLT; sheet resistance can be reduced if the film is made thicker, but this also reduces VLT. When low VLT is required, metal nanostructures are the best path forward.

Transparent Electronics Fabrication Processes

The two most common processes used for transparent electronics fabrication are etching of conductive thin films, or deposition of patterned films. Deposition is an additive process that can typically be performed in solution, while etching is a subtractive encompassing plasma etching or solution etching. Additive deposition typically requires a conductive or semiconductive seed layer (such as ITO), which then later must be etched after the primary conductive or semiconducting film is deposited.

The subtractive process for fabricating a patterned transparent circuit is shown below; this process is suitable with wet etching chemistry or with plasma etching. This first requires uniform deposition of the conductor on a transparent or opaque substrate. For multilayer planar devices, a similar etch process could be used with the right wet etching chemistry such that lower-level layers are not removed during the etching process.

Additive fabrication is shown below, where we have selective deposition of a seed layer and layer deposition of conductive traces. In the case where the seed layer is also conductive, it will need to be selectively patterned or subsequently etched so that it does not leave unintended connections between the patterned circuit conductors. This is typically the case when the circuit is being fabricated using electrodeposition, where the substrate participates in an electrochemical reaction and is regulated with a potentiostat.

The other additive process for fabricating transparent electronic circuits is to use a printing process, such as a 3D printer but operating only on the planar substrate. Printing processes are already used for deposition of thick metal conductors, such as on rigid substrates or on flexible substrates like PET. The most common is inkjet printing from conductive ink suspensions, which are then dried to leave behind solid metal conductors.

Deposition of materials for transparent conductors is also possible from suspensions, but this requires deposition of metal oxide or carbon nanotubes. Controlled deposition and curing of metal oxide nanoparticles can be difficult in some cases when attempting to reach a particular conductivity target because the surface conductivity of metal oxides is highly dependent on surface stoichiometry, namely due to defect states associated with the content of oxygen vacancies. Therefore, low-temperature annealing is often used to remove vacancies and increase conductance.

Carbon nanotubes are an attractive emerging technology for use in printing that can be a viable alternative to metal oxide deposition on transparent substrates. They offer higher VLT and lower sheet resistance in the cured conductive film. This level of conductivity is more desirable in some of the application areas listed above.

The current state-of-the-art in flex PCB fabrication on PET can be improved with more advanced transparent conductors.

Assembly is another matter as it depends what exactly is being assembled into a transparent system. Typically, transparent electronics are not meant to be designed like circuit boards that have components mounted and soldered onto exposed conductors.

The reason for this is that not all transparent conductors are capable of supporting solder. Therefore, placement of standard electronic components on a transparent substrate would require plating of through-holes or SMD pads for mounting components. The other reason is that these components block light, obviously defeating the purpose of transparent electronics.

Tooling for Transparent Electronics Fabrication

Standard CAM software can be used to create tooling for mask-based deposition and patterning of transparent conductor circuits and transparent semiconductors in planar processes. A vector file format like Gerbers is appropriate for defining circuitry in CAM software and creating deposition masks and photolithography masks for circuit patterning and etching. This means these circuits can be designed in a standard PCB design application and exported for as a deliverables package, just as you would with a standard circuit board.

If a printing process will be used, your software may require conversion of Gerbers to STL files so that layer-by-layer printing instructions can be programmed into your printing equipment. This will likely require an MCAD application or a vendor tool to accomplish this task. Make sure to check with your vendor’s capabilities before pursuing printing processes for transparent electronics.

Whenever you want to model the functionality of transparent electronics and create physical layouts that are manufacturable, use the complete set of system analysis tools from Cadence to evaluate systems functionality. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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