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Flexible Hybrid Electronics (FHE)

Integrated circuits on a flexible substrate
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Description

Flexible hybrid electronics, or FHE, is a method of creating flexible, stretchable, or conformable devices with electronic capabilities. They combine both printed and advanced CMOS-based components on a flexible, usually plastic polymer film, substrate.

Printed chips are used for applications from monitoring temperature and vibration in industrial operations to flexible sensors that can be applied on the skin, and they are being tested for applications where rigid chips don’t fit or where they are difficult to replace.

Many printed electronics rely on existing printing technologies. Inkjet is the most prevalent for low-volume applications, where it has been proven as a low-cost but relatively slow option. Alongside of that are technologies like offset and gravure printing, which have gained in popularity due to the ability to print precise layers in high volume. Gravure technology, which uses images etched into plates, dates back to 1800s, when it was used to make high-quality art prints. Offset printing has been used for newspapers for even longer. There also is some research into using existing chip lithography equipment for creating more complex structures on a flexible substrate.

Key to creating effective printed electronics is the chemistry and stability of the inks and how they are diced, bonded to a substrate, and calibrated for accuracy and reliability. As with all semiconductors, the emphasis is on repeatability, consistency and economies of scale.

FHE, in particular, refers to devices where not all components need to be flexible. Certain chips are thin and small enough that they can be manufactured with metal oxide lithography placed on the flexible substrate, rather than printed.

Transferring silicon circuits to a flexible substrate without damage can be a challenge. Transfer techniques fall into three main categories. The first category, brute force methods, use mechanical and/or chemical polishing to thin the semiconductor. The second, epitaxial layer liftoff, depends on the selective lateral removal of an embedded sacrificial layer, for example by etching.

The third method is mechanical exfoliation via substrate cracking, also known as spalling. Controlled spalling first deposits a metastable film with high fracture toughness and intrinsic tensile stress onto the substrate. For example, pre-tensioned nickel can be deposited on top of extremely thin silicon-on-insulator (ETSOI) devices. Flexible tape is then used to pull the nickel layer until the underlying substrate failed, cracking about 20 microns below the buried oxide.

FHE is an emerging area, with most projects funded by government, research institutes, or VCs. Techniques and standardization efforts are in flux.

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