ACF-Packaged Ultrathin Si-based Flexible NAND Flash Memory Do Hyun Kim, Hyeon Gyun Yoo, Daniel J. Joe, and Keon Jae Lee* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST) Daejeon, 305-701, Republic of Korea, *e-mail: keonlee@kaist.ac.kr Abstract In this paper, we demonstrate an ACF-packaged ultrathin Si-based flexible NAND flash memory by adopting a simple method, without using a conventional transfer process. By gently etching the bottom sacrificial silicon of the SOI wafer, flip-chip bonded devices were sufficiently thinned down (roughly to 1 μm) to fabricate highly flexible, fully packaged Si-based NAND flash memory, without any cracks or wrinkles. The work presented here suggests a useful methodology to realize various high-performance, fully packaged Si-based flexible LSI devices. Introduction Recently, flexible electronics have attracted increasing attention thanks to their combination of low weight, high portability, low fragility, and variable structure, offering considerable improvements over the weaknesses of conventional rigid electronics. [1] In particular, the development of electronic devices such as flexible processors and memories are considered crucial to address the increasing demands of data processing, information storage, and network communications. [2] Although many studies on flexible devices have utilized organic and polymeric materials, they typically exhibit insufficient performance due to inherent material properties, and their low degree of integration has been problematic. In this respect, silicon-based flexible large-scale integration (LSI) devices have been considered promising candidates for next-generation flexible non-volatile memory devices, due to their outstanding performance, high density capability, and compatibility with state-of-the-art semiconductor processes. [3] Although our group previously demonstrated ultrathin Si-based flexible radio frequency integrated circuits (RFICs) fabricated with 0.18 μm CMOS process, [4] significant challenges still remain in developing Si-based flexible memory devices, including how to stably realize flexible LSI on plastics. Another critical issue related to flexible and wearable electronics is the packaging process, which provides key functions such as signal interconnections, mechanical protection, and power distribution. There have been substantial efforts to utilize anisotropic conductive film (ACF) as an elastic and resilient packaging material, and our group previously demonstrated a flexible GaAs light-emitting diode (LED) interconnected by ACF to a plastic substrate. [5] However, employing ACF with thin fragile silicon chips is limited by the high pressure required by the flip-chip bonding process. As a result, ACF packaged chips have remained relatively thick, up to tens of microns, in order to withstand the pressure, which therefore limits the flexibility of Si-based electronic devices. Herein, we demonstrate an easy method for fabricating ultrathin Si-based flexible devices with the electrical interconnections necessary for practical flexible applications, by employing a flip-chip bonding technique and subsequent wafer thinning process. Also, a fully packaged ultrathin Si-based flexible NAND flash memory was successfully fabricated and analyzed. ACF-Packaged Si-based Flexible Device Fabrication Fig. 1 illustrates the fabrication method of the ACF- packaged Si-based flexible NAND flash memory. (i) A high performance NAND flash memory is constructed on the top-Si layer (400 nm) of the SOI wafer by conventional semiconductor fabrication processes. (ii) Interconnection between the fabricated device and the flexible substrate was made by flip-chip thermo- compression bonding cycle with ACF. (iii) The sacrificial bottom silicon was completely removed by aqueous alkaline potassium hydroxide (KOH) solution. In this case, the buried oxide (BOX) layer acts as the etch stop, leaving the thin, uniform device layer after the wet etch. (iv) Finally, the fully packaged ultrathin Si-based flexible NAND flash memory was fabricated with electrical interconnections. In light of previous studies on various flexible device fabrication techniques, it is important to note that this particular method has several distinct advantages. In the first place, this method utilizes a single crystalline silicon piece, which is particularly compatible with the full-chip scale complementary metal oxide semiconductor (CMOS) process. In addition, the highly thinned chip is capable of high flexibility which makes the total device resilient under various stress conditions. Moreover, this approach provides direct interconnection between the device and the outer electrode for practical use without requiring an additional wiring process. More importantly, there is no need to transfer the Si device to a flexible substrate, as demonstrated in previous works, [6] since the flip-chip bonded device is already strongly attached to the flexible printed circuit board (FPCB) substrate. Therefore, our approach enables one to fabricate ultrathin Si-based flexible devices with high yield. Fig. 2 shows a photograph of the ACF-packaged ultrathin Si-based flexible NAND flash memory wrapped on a glass rod (3.5 mm radius). The highly flexible characteristics of the developed device are presented in the inset of Fig. 2. Optical microscope (OM) images of the NAND flash active area (Fig. 3) and the electrode area (Fig. 4-5) indicate they have no cracks or wrinkles. Mechanical Properties of the Fabricated Device Fig. 6 shows a photograph of the ACF-packaged ultrathin Si-based flexible chip used for electrical measurement (left) and a magnified top-view image of the 144-series daisy-chain (right). No mechanical defects were observed in spite of the existing electrode bump, even after repetitive bending. Fig. 7 IEDM15-511 19.3.1 978-1-4673-9894-7/15/$31.00 ©2015 IEEE