Extending storage dielectric scaling limit by reoxidizing nitrided NO dielectric for trench DRAM

Conventional nitride/oxide (NO)-based storage dielectric has been demonstrated to possess the capability to extend its employment in trench dynamic random access memory (DRAM) by additional NH/sub 3/ nitridation and in situ N/sub 2/O reoxidation. Through this technique, cell capacitance could be enhanced by 12.2% as compared with NO dielectric while preserving tunneling current below 1 fA/cell. Even with NH/sub 3/ nitridation and consequent well-known introduction of electron traps, the great improvement in tunneling leakage and reliability are exhibited after N/sub 2/O treatment. With prominent electrical properties, this technique proves its eligibility for next-generation DRAM before the maturity of introduction of high-/spl kappa/ material into production.     

Localization and physical analysis of dielectric weaknesses for state-of-the-art deep trench based DRAM products

Stressed deep trench capacitors of Dynamic Random Access Memories (DRAM) were analyzed regarding the localization within a test array. A preparation method to find the position within the failing trench and to give information for process improvements is reported. Differences between the dielectrics nitride/oxide and aluminumoxide were seen. The investigations were done mostly for trench geometries of a 110nm technology. One first preparation was also successful for a 90nm technology with enlarged trench surfaces by hemispherical silicon grains ( HSG ).     

DRAM macros for ASIC chips

DRAM macros in 4-Mb (0.8-μm) and 16-Mb (0.5-μm) DRAM process technology generations have been developed for CMOS ASIC applications. The macros use the same area efficient one transistor trench cells as 4-Mb (SPT cell) and 16-R Mb (MINT cell) DRAM products. It is shown that the trench cells with capacitor plates by the grounded substrate are ideal structures as embedded DRAM's. The trench cells built entirely under the silicon surface allow cost effective DRAM and CMOS logic merged process technologies. In the 0.8-μm rule, the DRAM macro has a 32-K×9-b configuration in a silicon area of 1.7×5.0 mm² . It achieves a 27-ns access and a 50-ns cycle times. The other DRAM macro in the 0.5-μm technology is organized in 64 K×18 b. It has a macro area of 2.1×4.9 mm and demonstrated a 23-ns access and a 40-ns cycle times. Small densities and multiple bit data configurations provide a flexibility to ASIC designs and a wide variety of application capabilities. Multiple uses of the DRAM macros bring significant performance leverages to ASIC chips because of the wide data bus and the fast access and cycle times. A data rate more than 1.3 Gb/s is possible by a single chip. Some examples of actual DRAM macro embedded ASIC chips are shown     

Oxide-Nitride Storage Dielectric Formation in a Single-Furnace Process for Trench DRAM

A simplified and integrated technique has been proposed to form an oxide/nitride storage dielectric in a single-furnace process by low-pressure oxidation and nitride film deposition with an extra$hboxN_2hboxO$treatment for the trench dynamic random access memory (DRAM). Compared to the conventional nitride/oxide dielectric, this newly developed dielectric enjoys cell-capacitance-enhancement factor as high as 12.5% without degrading the leakage current and electron-trapping property. From the reliability test, the qualification for the DRAM application is also proven by the dielectric lifetime longer than 10-years. Most importantly, this technique can reduce the production cycle time without an additional equipment investment, which is essential in the cost-competitive DRAM arena.     

A 400-MHz random-cycle dual-port interleaved DRAM (D/sup 2/RAM) with standard CMOS Process

This paper describes a standard CMOS process based on embedded DRAM macro with dual-port interleaved DRAM architecture (D/sup 2/RAM), which is suitable for the leading edge CMOS LSIs with both high-speed and large-scale memories on a chip. This macro exploits three key technologies: fully sense-signal-loss compensating technology based on the whole detailed noise element breakdowns, the novel striped trench capacitor (STC) cell, and the write-before-sensing (WBS) circuit by decoded write-bus. A 400-MHz random cycle access has been verified for D/sup 2/RAM fabricated by a 0.15-/spl mu/m standard CMOS process.     

2.4F2 memory cell technology with stacked-surroundinggate transistor (S-SGT) DRAM

This paper proposes 2.4F² memory cell technology with stacked-surrounding gate transistor (S-SGT) DRAM. One unit of the S-SGT DRAM is formed by stacking several SGT-type cells in series vertically. The SGT-type cell itself arranges gate, source, drain and plate on a silicon pillar vertically. Both gate and plate electrode surround the silicon pillar. Subsequently applied trench etching and sidewall spacer formation during S-SGT DRAM formation causes a step-like silicon pillar structure. Due to these steps, gate, plate and diffusion layer in one S-SGT DRAM unit are fabricated vertically by a self-aligned process. The cell size dependence of the self-aligned-type S-SGT DRAM was analyzed with regard to the above step widths and the number of cells in one unit. As a result, the cell design for minimizing the cell size of this device has been formulated. By using the proposed cell design, it is demonstrated by process simulation that the S-SGT DRAM in 0.5 μm design rule can achieve a cell size of 2.4F², which is half of the cell size of a conventional SGT DRAM cell (4.8F²). Therefore, the S-SGT DRAM is a promising candidate for future ultra high density DRAMs     

Through-the-gate-implanted ultrathin gate oxide MOSFET's withcorner parasitics-free shallow-trench-isolation

We have realized direct-tunneling (DT) gate oxide (1.6 nm) NMOS and PMOS transistors by means of through-the-gate-implantation in a corner parasitics-free shallow-trench-isolation CMOS technology. In order to take full advantage of in situ cluster-tool processing and to preserve initial wafer-surface quality, the essential part of the MOS gate is fabricated prior to device isolation and through-the-gate-implantation is utilized for well- and channel-doping. In addition, a fully-reinforced-gate-oxide-perimeter is provided and trench corner parasitics are eliminated by the advanced process architecture without increasing process complexity. Fully functional direct-tunneling oxide MOSFET's with excellent electrical characteristics confirm the feasibility of this novel approach     

TLM: a trench leakage monitor for a four megabit SPT DRAMtechnology

The authors discuss a single trench capacitor macro-array structure used for trench dynamic random access memory (DRAM) device design and characterization, and as a manufacturing test vehicle. A nonaddressable array of trench-capacitor DRAM cells is used for quantification of trench DRAM leakage parameters, storage node parasitic device characterization, and silicon defects. Used with an addressable functional monitor, it is found to be a valuable semiconductor process development vehicle to achieve functionality and cell retention yield for a 4-Mb CMOS DRAM technology     

A unique dual-poly gate technology for 1.2-V mobile DRAM with simple in situ n/sup +/-doped polysilicon

Highly manufacturable sub-100-nm 1.2-V mobile dynamic random access memory (DRAM) having full functionality and excellent reliability have been successfully developed. A unique and simple DRAM technology with dual-gate CMOSFET was realized using plasma-nitrided thin gate oxide and p/sup +/ poly gate formed by BF/sub 2/ ion implanted compensation of in situ phosphorus (n/sup +/) doped amorphous silicon gate. Using this technology, boron penetration into the channel, gate poly depletion, and dopant interdiffusion between n/sup +/- and p/sup +/-doped WSi/sub x/-polycide gates were successfully suppressed. In addition, a negatively biased word line scheme and a storage capacitor with laminated high-/spl kappa/ Al/sub 2/O/sub 3/ and HfO/sub 2/ dielectrics were also developed to achieve mobile DRAM operating at 1.2 V with excellent performance and reliability.     

Trench transistor DRAM cell

A new one-transistor DRAM cell with both the transistor and the capacitor fabricated on the trench sidewalls is described. With the signal stored on the polysilicon node surrounded by oxide, the cell is expected to have a high alpha particle immunity. The cell occupies only 9 µm²using 1-µm design rules. This cell size is sufficiently small to enable a 4-Mbit DRAM of reasonable chip size with these design rules, and possesses further scalability for 16-Mbit DRAM's.     

A Submicrometer Megabit DRAM Process Technology Using Trench Capacitors

This paper describes guidelines for developing a 1-4-Mbit DRAM process, and device/process technologies for fabricating an experimental 1-Mbit DRAM. A single transistor cell combined with a trench capacitor and on-chip ECC technologies has the potential to realize a cell size of 10 /spl mu/m/sup 2/ without degrading soft error immunity. A depletion trench capacitor, submicrometer n-well CMOS process, Mo-poly gate, and sub-micrometer pattern formation technologies are developed, and an experimental 1-Mbit DRAM with a cell size of 20 /spl mu/m/sup 2/ is successfully developed by using these technologies.     

Gate-drain charge analysis for switching in power trench MOSFETs

For the switching performance of low-voltage (LV) power MOSFETs, the gate-drain charge density (Q/sub gd/) is an important parameter. The so-called figure-of-merit, which is defined as the product of the specific on-resistance (R/sub ds,on/) and Q/sub gd/ is commonly used for quantifying the switching performance for a specified off-state breakdown voltage (BV/sub ds/). In this paper, we analyzed the switching behavior in power trench MOSFETs by using experiments and simulations, focusing on the charge density Q/sub gd/. The results of this analysis can be used for further optimization of these devices. The results show that the Q/sub d/ can be split into three charge contributions: accumulation, depletion, and inversion charge. It is shown that the inversion charge is located mainly underneath the trench bottom. The accumulation and depletion charge contribute each about 45% in conventional LV trench MOSFETs and can be reduced by using a thick bottom oxide in a shallow trench gate just extending in the drift region. Further, we derived an analytical model for calculating the Q/sub gd/, that takes into account the geometry dependence.     

Performance-augmented storage capacitor by reduced resistance of polysilicon electrode for trench DRAM

The influence of process condition on the resistivity of the arsenic-doped polysilicon electrode for the cell capacitors in trench DRAM was investigated. Rather than the arsine flow, it is found that the thickness of the undoped amorphous silicon plays a vital role in determining the overall resistivity. When the total polysilicon thickness is kept unchanged, moderately increasing the thickness of the lower two undoped amorphous silicon would engender a reduced resistivity while moving the arsenic peak location away from the polysilicon/storage dielectric interface, which is critical in leakage current suppression. With these improved properties, less bit line coupling (BLC) loss from the characterization on DRAM product is observed. The result of this study is quite important for trench DRAM manufacturer to enhance product yield before the technology of metal electrode is mature.     

Leakage mechanisms in the trench transistor DRAM cell

The authors discuss a band-to-band tunneling mechanism in the trench transistor cell (TTC), which is used in Texas Instruments' 4-Mbit DRAM. This effect should be operative in the class of trench cells in which the charge is stored inside the trench and the substrate forms a capacitor plate. This effect does not compromise the functionality of the cell; in fact, it has the potential of improving the long-term reliability of the cell by preventing electrical overstress of the trench capacitor oxide     

A novel lightly doped drain polysilicon thin-film transistor withoxide sidewall spacer formed by one-step selective liquid phasedeposition

We have proposed and successfully demonstrated a novel process for fabricating lightly doped drain (LDD) polycrystalline silicon thin-film transistors (TFT's). The oxide sidewall spacer in the new process is formed by a simple one-step selective liquid phase deposition (LPD) oxide performed at 23°C. Devices fabricated with the new process exhibit a lower leakage current and a better ON/OFF current ratio than non-LDD control devices. Since the apparatus used for LPD oxide deposition is simple and inexpensive, the new process appears to be quite promising for future high-performance poly-Si TFT fabrication     

DRAM plate electrode bias optimization for reducing leakage current in UV-O/sub 3/ and O/sub 2/ annealed CVD deposited Ta/sub 2/O/sub 5/ dielectric films

A new plate biasing scheme is described which allowed the use of 65% higher supply voltage without increasing the leakage current for the UV-O/sub 3/ and O/sub 2/ annealed chemical-vapor-deposited tantalum pentaoxide dielectric film capacitors in stacked DRAM cells. Dielectric leakage was reduced by biasing the capacitor plate electrode to a voltage lower than the conventionally used value of V/sub cc//2. Ta/sub 2/O/sub 5/ films with 3.9 nm effective gate oxide, 8.5 fF//spl mu/m/sup 2/ capacitance and <0.3 /spl mu/A/cm/sup 2/ leakage at 100/spl deg/C and 3.3 V supply are demonstrated.<>     

适用于深沟槽结构观测的TEM样品制备技术

对于深沟槽DRAM电容这类纵向深度深(超过5μm)但是平面尺寸又很小(小于0.2μm×0.2μm)的结构来说,传统的TEM制样方法,无法满足其细微结构全面观测的需求,此外传统的方法制样也比较费时,成功率也比较低。介绍了一种FIB横向切割技术,适用于对这类结构的观测。它与传统FIB制样方法的主要区别在于,切割方向由纵向切割改为横向切割。用这种方法制备的TEM样品,可以完整地观测同一个深沟槽DRAM电容结构的所有细微结构。制样过程比较简单、速度快、成功率高。以一个实例分析、比较了传统制样方法和新的制样方法,突显了FIB横向切割技术的优点。     

1.5-V single work-function W/WN/n/sup +/-poly gate CMOS device design with 110-nm buried-channel PMOS for 90-nm vertical-cell DRAM

This letter reports on 1.5-V single work-function W/WN/n/sup +/-poly gate CMOS transistors for high-performance stand-alone dynamic random access memory (DRAM) and low-cost low-leakage embedded DRAM applications. At V/sub dd/ Of 1.5-V and 25/spl deg/C, drive currents of 634 /spl mu/A//spl mu/m for 90-nm L/sub gate/ NMOS and 208 /spl mu/A-/spl mu/m for 110-nm L/sub gate/ buried-channel PMOS are achieved at 25 pA//spl mu/m off-state leakage. Device performance of this single work function technology is comparable to published low leakage 1.5-V dual work-function technologies and 25% better than previously reported 1.8-V single work-function technology. Data illustrating hot-carrier immunity of these devices under high electric fields is also presented. Scalability of single work-function CMOS device design for the 90-nm DRAM generation is demonstrated.     

Anomalous junction leakage current induced by STI dislocations andits impact on dynamic random access memory devices

As the density of dynamic random access memory (DRAM) increases up to giga-bit regime, one of the important problems is the control of the process-induced defects and damage. Although the shallow trench isolation (STI) is widely used for deep submicron devices, it has a great possibility of generating STI dislocations due to its inherently large mechanical stress and damage. When STI dislocations are located within the depletion region of pn junction, anomalous junction leakage current could flow. This junction leakage current degrades the memory cell data retention time and the standby current of DRAM. We resolved the problems from STI dislocations as follows; the crystal defects and the mechanical stress were reduced by optimizing the implantation condition and the densification temperature of trench filled high-density plasma (HDP) oxide, respectively. In addition, the residual mechanical stress before source/drain implantation was relieved through rapid thermal nitridation (RTN). By using these methods, STI dislocations were successfully clamped outside the depletion region of pn junction