Physical vapor deposition (PVD) is a critical technology for fabricating flexible electronics, enabling the deposition of high-performance thin films on polymer substrates without compromising their mechanical integrity. Unlike solution-based methods, PVD offers precise control over film composition, thickness, and uniformity, making it indispensable for applications requiring high conductivity, optical transparency, or semiconductor functionality. Key challenges include achieving low-temperature deposition, managing thin-film stress, and ensuring strong adhesion to flexible substrates.

Low-temperature PVD techniques are essential for flexible electronics because most polymer substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI), degrade or deform at temperatures above 200°C. Magnetron sputtering, a widely used PVD method, can operate at near-room temperature by optimizing plasma power, pressure, and target-to-substrate distance. For instance, indium tin oxide (ITO) films sputtered at 50°C exhibit sheet resistances below 50 ohms per square with transmittance exceeding 85% in the visible spectrum. Similarly, thermal evaporation can deposit metals like aluminum or silver at temperatures below 100°C, making it suitable for flexible conductive electrodes.

Stress management in PVD films is crucial to prevent delamination or cracking when the substrate undergoes bending or stretching. Intrinsic stress arises from atomic peening during sputtering or thermal expansion mismatches between the film and substrate. Compressive stress often dominates in sputtered films due to energetic particle bombardment, while evaporated films tend to develop tensile stress. Strategies to mitigate stress include:
- Using intermediate buffer layers (e.g., chromium or titanium) to improve adhesion and redistribute strain.
- Adjusting deposition parameters such as argon pressure in sputtering to reduce compressive stress.
- Employing multilayer structures where alternating materials balance overall stress.

For example, a bilayer of silicon nitride and silicon oxide deposited by reactive sputtering can achieve near-neutral stress, enhancing mechanical durability in flexible thin-film transistors (TFTs).

Adhesion promotion is another critical factor, as weak interfacial bonding leads to film failure under mechanical stress. Plasma pretreatment of polymer surfaces increases surface energy by introducing polar functional groups, improving wettability for subsequent PVD layers. Oxygen or argon plasma treatment of PET, for instance, raises its surface energy from 40 mN/m to over 70 mN/m, significantly enhancing metal adhesion. Additionally, thin adhesion layers (e.g., 5–10 nm of titanium or nickel) are commonly deposited prior to the main functional layer to ensure robust bonding.

Flexible displays benefit from PVD-deposited transparent conductive oxides (TCOs) and metal grids. Sputtered ITO remains a standard despite the emergence of alternatives like aluminum-doped zinc oxide (AZO), which offers better flexibility and cost advantages. For foldable OLEDs, PVD is used to deposit thin encapsulation layers such as aluminum oxide, which provide moisture and oxygen barriers without compromising flexibility.

Wearable sensors leverage PVD for strain-insensitive metallic traces and active semiconductor layers. Gold or platinum films deposited by evaporation or sputtering serve as stable electrodes for electrophysiological sensors, while sputtered zinc oxide or indium gallium zinc oxide (IGZO) enable high-mobility channels in flexible TFTs. A notable example is a sputtered IGZO-based pressure sensor array with a mobility exceeding 10 cm²/Vs, capable of conforming to skin surfaces.

Thin-film transistors for flexible electronics rely on PVD for gate dielectrics, semiconductors, and electrodes. Sputtered IGZO is a leading semiconductor due to its high mobility (>20 cm²/Vs) and low processing temperature. Dielectrics like silicon oxide or aluminum oxide, deposited by reactive sputtering, ensure low leakage currents even at thicknesses below 100 nm. For gate electrodes, PVD techniques enable the deposition of ultrathin (<50 nm) metal films with minimal resistivity, critical for high-frequency operation.

Emerging trends include hybrid PVD-ALD approaches for conformal coatings on 3D flexible structures and the use of sacrificial layers to enable transfer-printing of PVD films onto elastomeric substrates. Challenges remain in scaling PVD processes for roll-to-roll manufacturing, where uniform deposition on moving webs requires precise control of plasma dynamics and substrate handling.

In summary, PVD techniques provide a versatile toolkit for flexible electronics, balancing performance and mechanical reliability. Advances in low-temperature processes, stress engineering, and adhesion science continue to expand their applications in displays, wearables, and TFTs, solidifying PVD’s role in next-generation flexible devices.