Ultra-thin and flexible silicon chips are pivotal for high-performance conformal electronics. Backside grinding reduces chip thickness below 50 μm, enabling mass production of ultra-thin flexible chips in a low-cost way, yet the resulting defects and residual stress on the backside may propagate to the frontside circuits degrading the electric performance, especially for the critical-thickness devices. Here, commercial pulse-width modulation (PWM) chip wafers (involving tens of thousands of dies) based on bipolar transistors have been thinned to the thickness of ~ 20 μm by mechanical grinding with different parameters. Experimental results reveal a thickness-dependent bifurcated failure mechanism: Short-circuit current decays progressively with thickness reduction, while leakage current exhibits a catastrophic surge below the critical thickness (~ 18 μm). This work reveals a dual degradation mechanism in ultrathin ICs: Mechanical grinding not only amplifies substrate parasitic coupling via geometric thinning but also generates stress fields that induces dislocation rearrangement-aggregation cascades, ultimately dictating electrical failure modes. Chemical mechanical polishing (CMP) and reactive ion etching (RIE) have been deployed to inhibit leakage current surges by removing grinding-induced damaged layers and relieving interfacial residual stress, which collectively validate the stress-defect interaction as the governing mechanism of electrical failure in ultra-thin chips (UTCs). Hopefully, this study can throw light on the impact of mechanical grinding thinning on the electrical performance of ultra-thin chips paving the way to the wide applications of high-performance flexible electronics in the future.

Wearable devices possess excellent flexibility and can conform to irregular surfaces, extensively changing human healthcare fields. Ultrasonic technology, with its extensive penetration depth, nondestructive nature, and versatile functionalities, has been widely applied in the diagnosis and treatment of various diseases. However, traditional ultrasound devices are often bulky and rigid, significantly limiting their further development in the biomedical field. Wearable and flexible ultrasound devices combine the advantages of wearable electronics and ultrasound technology, providing real‐time, continuous, and nondestructive strategies for biomedical applications. Wearable ultrasound devices can seamlessly conform to human skin or organ surfaces, substantially enhancing working performance, durability, and comfort. Here, we review recent advancements in developing wearable ultrasound devices for biomedical applications, including materials, structural design, and applications in biomedical fields. We provide an overview of wearable ultrasound devices utilized for hemodynamics monitoring, deep‐tissue energy transmission, and closed‐loop therapy. Finally, we discuss existing challenges and future trends in developing wearable ultrasound devices.