The photoablation properties of two soluble polyimides DMDB/6FDA and OT/6FDA with thicknesses of over 300 mum, synthesized by the polycondensation of a hexafluoropropyl group contained in a ...dianhydride with two kinds of diamines, are investigated using a 248 nm krypton fluoride (KrF) laser. The incorporation of the hexafluoropropyl group into the chemical structure gives these two polyimides higher etching rates than Kapton (a commercial polyimide film which is difficult to dissolve). The etching rates of synthesized polyimides are about 0.1-0.5 mum/pulse over a fluence range of 0.25-2.25 J cm-2. The photothermal mechanism for DMDB/6FDA contributes about 19% of etching depth at a laser fluence of 0.82 J cm-2. Moreover, the number of laser pulses seriously affects the taper angle of microstructures, especially at low fluence. Near-vertical side-wall structures can be built at high fluence (~2 J cm-2). Fresnel patterns with a thickness of 300 mum and a linewidth of 10 mum were fabricated, with an attainable aspect ratio of around 30. After photoablation, the complementary metallic microstructures were also fabricated by a sequential electroplating procedure. Then, those two new polyimides could be dissolved easily in most common solvents (such as THF, DMSO, NMP and DMF). These results indicate that these two soluble polyimides are highly suitable for use in the KrF laser LIGA process.
The reliability of MANOS devices with an oxide buffer layer (MAONOS) in between SiN trapping layer and high-K Al 2 O 3 top dielectric is extensively studied. We conclude that the primary function of ...high-K Al 2 O 3 is to suppress the gate electron injection during erase instead of increasing the P/E speed. As a result, inserting a buffer oxide only changes EOT but does not change the P/E mechanisms. On the other hand, the buffer oxide can greatly improve data retention by suppressing leakage through Al 2 O 3 . However, owing to the slow erase performances with a thick bottom oxide, both MANOS and MAONOS erase slowly and very high erase voltages must be used. Also, both MANOS and MAONOS devices show very fast endurance degradation below P/E<10, which is inherent due to electron de-trapping mechanism. Moreover, the large erase voltage also causes severe degradation of tunnel oxide after many P/E cycling. To get both speed and reliability performances, it is necessary to introduce bandgap engineered tunneling barrier (BE-MANOS) to solve the fundamental problems of MANOS.
In NAND flash, devices are normally erased to negative Vt and then programmed to positive Vt. In this work we introduce a novel depletion-mode (normally on) buried-channel, junction-free n-channel ...NAND flash device. The buried-channel NAND flash shifts the P/E Vt ranges below those for the conventional surface-channel device, and is more suitable for the NAND Flash memory design. Due to the lower initial Vt, the device shows faster erase speed and higher immunity to read disturb. Furthermore, the buried-channel device has significantly improved cycling endurance, because the buried channel is insensitive to the interface state (Dit) generation during program/erase stressing. A lightly doped shallow n-type channel serves both as the buried bit line and as the source/drain of the junction-free structure. The short channel effects are overcome by using FinFET. The buried-channel NAND flash uses a simple program-inhibit method by directly raising the buried bit line potential without introducing a deep depletion in the conventional self-boosting method. A successful sub-30 nm buried-channel FinFET BE-SONOS NAND flash with MLC is demonstrated.
The erase characteristics and mechanism of metal- Al sub(2)O sub(3)-nitride-oxide-s ilicon (MANOS) devices are extensively studied. We use transient analysis to transform the erase curve (V sub(FB) - ...time) into a J-E curve (J = transient current, E = field in the tunnel oxide) in order to understand the underlying physics. The measured erase current of MANOS is three orders of magnitude higher than that can be theoretically provided by substrate hole current. In addition, the erase current is very sensitive to the Al sub(2)O sub(3) processing condition - also inconsistent with substrate hole injection model. Thus, we propose that MANOS erase occurs through an electron detrapping mechanism. We have further carried out a refill test and its results support the detrapping model. Our results suggest that the interfacial layer between Al sub(2)O sub(3) and nitride is a key process that dominates the erase mechanism of MANOS.
In this study two organic soluble polyimides were successfully synthesized via polycondensation of a hexafluoropropyl group contained dianhydride with two kinds of diamines. These two polyimides ...(DMDB/6FDA, OT/6FDA) can be dissolved in most aprotic solvents (for example, NMP, DMAc) and many other common solvents (for example, THF, DMSO). The soluble polyimides also show good thermal stability and high absorption at the KrF excimer laser wavelength (λ = 248 nm). (The absorption coefficients of DMDB/6FDA and OT/6FDA were 1.26×105 cm–1 and 1.49×105 cm–1, respectively.) The photoablation properties of the polyimides were studied in detail. Due to incorporating the hexafluoropropyl group into the chemical structure, these two polyimides have higher etching rates than Kapton (a commercial PI film). The effects of exposure parameters on linewidth and taper‐angle of polyimides with different chemical structure were also investigated here. Under the proper control of exposure conditions, good precision patterns with near‐vertical side wall can be produced. The 300 mm thick comb‐shape and Fresnel patterns with 10 mm linewidth have been made. Moreover, the complementary metallic microstructures have also been realized in the following electroforming procedure. The results showed that these two soluble polyimides were very suitable for KrF Laser LIGA process.
The behavior of WO
X
resistive random access memory (ReRAM) is a strong function of the top electrode material, which controls the conduction mechanism and the forming process. When using a top ...electrode with low work function, the current conduction is limited by space charges. On the other hand, the mechanism becomes thermionic emission for devices with a high work function top electrode. These (thermionic) devices are also found to have higher initial resistance, reduced forming current, and larger resistance window. Based on these insights and considering the compatibility to complementary metal–oxide–semiconductor (CMOS) process, we proposed to use Ni as the top electrode for high performance WO
X
ReRAM devices. The new Ni/WO
X
/W device can be switched at a low current density less than 8×10
5
A/cm
2
, with RESET/SET resistance ratio greater than 100, and extremely good data retention of more than 300 years at 85 °C.
A bandgap engineered SONOS with greatly improved reliability properties is proposed. This concept is demonstrated by a multilayer structure of O1/N1/O2/N2/O3, where the ultra-thin "O1/N1/O2" serves ...as a non-trapping tunneling dielectric, N2 the high-trapping-rate charge storage layer, and O3 the blocking oxide. The ultra-thin "O1/N1/O2" provides a "modulated tunneling barrier" - it suppresses direct tunneling at low electric field during retention, while it allows efficient hole tunneling erase at high electric field due to the band offset. Therefore, this BE-SONOS offers fast hole tunneling erase, while it is immune to the retention problem of the conventional SONOS. With a N + -poly gate, we achieve self-convergent erased Vt ~3 V, suitable for NOR flash application. On the other hand, by using a P + -poly gate, a depletion mode device (Vt < 0) is obtained, and a very large memory window (> 6 V) is achieved, ideal for MLC-NAND application. Excellent performance and reliability for both applications are demonstrated. Furthermore, with this simple structure and no new materials BE-SONOS is readily manufacturable
A critical review of charge-trapping NAND flash devices Hang-Ting Lue; Sheng-Chih Lai; Tzu-Hsuan Hsu ...
2008 9th International Conference on Solid-State and Integrated-Circuit Technology,
2008-Oct.
Conference Proceeding
This paper carefully analyzes various charge-trapping NAND Flash devices including SONOS, MANOS, BE-SONOS, BE-MANOS, and BE-MAONOS. The erase mechanisms using electron de-trapping or hole injection, ...and the role of the high-k top dielectric (Al 2 O 3 ) are critically examined. In addition to the intrinsic charge-trapping properties, the STI edge geometry in the NAND array also plays a crucial role in determining the programming/erasing and reliability characteristics. Erase saturation and incremental-step-pulse programming (ISPP) characteristics are strongly affected by the STI edge effects. Our analysis of recent progress provides a clear understanding to charge-trapping NAND devices and serves as a guideline for future development.
