M.S. Kim, G.A. Torres Sevilla, M.M. Hussain
Purdue University,
United States
Keywords: CMOS, bare die, integration, packaging, flexible electronics, polymer encapsulation
Summary:
Flexible electronics have a large application spectrum including wearables, sensors, healthcare and robotic skins. For such a system to be effective, it is crucial to render most of its components fully flexible while maintaining high performance. Traditional packaged complementary metal oxide semiconductors (CMOS) chips are rigid and offer high performance. However, they lack the physical flexibility highly coveted in flexible electronics. On the other hand, systems based on flexible transistors possess excellent physical conformity. However, many efforts are still being made to scale them and improve their performance. Thus, we present a monolithically integrated system featuring commercially available system-on-chip (SoC) – a mixed signal microcontroller and an analog temperature sensor – whose form factors were transformed to become fully flexible. We then methodically characterize its performance under various mechanical conditions to demonstrate its performance, flexibility and mechanical reliability. The system was fabricated using the standard cleanroom process and consists of a soft polymeric encapsulation, metal interconnects, commercially available silicon (Si) based SoC bare dies and a polymeric packaging layer. Polyimide was chosen as the substrate due to its resistance to heat and chemicals. A thin layer of metal was deposited and patterned using optical lithography to make the metal interconnects. Two bare dies were then flip-chip bonded: a microcontroller (MSP430G2252-DIE from Texas Instruments) and an analog temperature sensor (SiS60A from Silicon Supplies). After bonding, the bare dies were still rigid. By using the back-etch method, where the Si substrates were etched from the back side by deep reactive ion etching (DRIE), we reduced their thickness and hence achieved flexibility. In the Bosch DRIE process, a combination of sulfur hexafluoride (SF6) and oxygen (O2) was used for the etching cycle, and octafluorocyclobutane (C4F8) was used during the polymer deposition cycle. The Aluminum oxide (Al2O3) hard mask was used since Al2O3 has an extremely high selectivity in the Bosch DRIE process owing to the formation of nonvolatile aluminum fluoride (AlFx) during the C4F8 deposition cycle. The device was then packaged with polydimethylsiloxane (PDMS) to protect and provide mechanical support to the system. To quantify the microcontroller’s performance, clock frequency and power consumption were measured with an oscilloscope and a source measure unit since they are critical parameters in digital logic. Also, the temperature sensor’s accuracy was assessed by testing it against an independent thermocouple. Under zero mechanical stress, the flexible system showed no deterioration in clock frequency, power consumption and temperature-sensing accuracy. Furthermore, to investigate the flexibility and mechanical reliability of the system, we demonstrate its performance under mechanical stress, such as bending at bending radii of 50 mm, 20 mm, 10 mm and 5 mm and twisting by 90°, and after 500, 1000, 5000, 10000 and 15000 bending cycles at a bending radius of 5 mm. In summary, we show the applicability of flexible bare die integration for flexible electronic systems and its ability to possess both high performance and physical conformity.