The impact of heater-recess and load matching in phase change memory mushroom cells

Two-dimensional finite element simulations with rotational symmetry are used to analyze the impact of the bottom electrode recess on the reset operation of phase change memory elements with mushroom cell geometry (Ge(2)Sb(2)Te(5) (GST) film over a patterned TiN pillar). Temperature dependent materials parameters are used for GST and TiN, and the latent heat of fusion in melting of GST is included to model melting. The results of this study indicate that a lower reset current and a more favorable thermal profile may be achieved by extending the active region of GST down into the pillar, due to the heat confinement. It is shown that the current through cells with an insufficient load condition for maximum power transfer can be maintained at a level lower than that which is sufficient for reset operation for extended periods of time due to the non-linear nature of temperature dependent electrical conductivity of GST. These results suggest that if the load condition is not matched, excessive voltage levels or pulse durations would be necessary to achieve successful reset operation across cell arrays.     

Fabrication of nano-scale phase change materials using nanoimprint lithography and reactive ion etching process

Phase change random access memory (PRAM) is one of the most promising non-volatile memories due to its ability to store digital data in the form of crystalline and amorphous phases of phase change materials. As a phase change material, Ge₂Sb₂Te₅ (GST225) is usually used, due to its reversible phase transition capability with speeds of less than 100 ns between the crystalline and amorphous phases. In order to fabricate highly integrated PRAM devices, sub micron- to nanometer-sized patterning of GST225 layer must be accomplished. In this study, 70 nm-sized polymer patterns were fabricated using partial filling nanoimprint lithography (NIL) on a GST225 layer, which was deposited by RF sputtering. Then GST225 was etched using Ar/Cl₂ plasma with an ICP etcher. Etch conditions, including Cl₂ concentration, were optimized to obtain the vertical etch profile of the GST patterns.     

SiO2 doped Ge2Sb2Te5 thin films with high thermal efficiency for applications in phase change random access memory

This study examined the various physical, structural and electrical properties of SiO(2) doped Ge(2)Sb(2)Te(5) (SGST) films for phase change random access memory applications. Interestingly, SGST had a layered structure (LS) resulting from the inhomogeneous distribution of SiO(2) after annealing. The physical parameters able to affect the reset current of phase change memory (I(res)) were predicted from the Joule heating and heat conservation equations. When SiO(2) was doped into GST, thermal conductivity largely decreased by ～ 55%. The influence of SiO(2)-doping on I(res) was examined using the test phase change memory cell. I(res) was reduced by ～ 45%. An electro-thermal simulation showed that the reduced thermal conductivity contributes to the improvement of cell efficiency as well as the reduction of I(res), while the increased dynamic resistance contributes only to the latter. The formation and presence of the LS thermal conductivity in the set state test cell after repeated switching was confirmed.     

Stress reduction during phase change in Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> by capping TiN film

It has been one of the most important issues to minimize the stress reduction during phase change in GST (Ge₂Sb₂Te₅) alloy for PRAM (Phase-change Random Access Memory) applications, because the alloy has been reported to face the significant stress during the phase change. We fabricated GST/oxide/substrate as a basic structure, and then added two more structures by capping an adhesion layer (Ti) or a barrier metal (TiN) on GST layer, respectively. We report that TiN-capped structure shows about 40% stress reduction during the phase change compared with that of the basic structure. The stress reduction is considered to be due to the intrinsic compressive stress in TiN film itself.     

时效处理对Al-Zr-Sc(-Er)合金组织和性能的影响EI北大核心CSCD

通过透射电镜(TEM)和扫描透射电镜(STEM)以及硬度、电导率测试等方法系统研究了时效处理对Al-Zr-Sc三元合金以及Al-Zr-Sc-Er四元合金显微组织和性能的影响。结果表明:对于Al-Zr-Sc合金,增加Sc含量,可显著提高合金时效响应速率、峰值硬度和热稳定性;增加Zr含量,显著提高了合金硬度和热稳定性,但会降低电导率。在Al-Zr-Sc合金中添加Er,进一步提高了合金的时效响应速率,促进了Zr,Sc的脱溶析出。Er与Zr,Sc形成具有核-双壳结构的Al 3(Er,Sc,Zr)相,明显改善合金的硬度和电导率。经300℃单级时效,实验合金硬度较高,但电导率相对较低;经400℃单级时效,实验合金时效响应速率加快,电导率明显提高,但硬度显著下降;经300℃/24 h+400℃的双级时效实验合金可获得电导率、强度和热稳定性的良好匹配。     

Densification of La0.6Sr0.4Co0.2Fe0.8O3 ceramic by flash sintering at temperature less than 100 °C

The effect of DC electric field on sintering and electrical conductivity of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF), considered as highly promising cathode material for solid oxide fuel cell, is investigated in the present work. It is shown that sintering can be carried out at (furnace) temperature <100 °C under electric field ranging from 7.5 to 12.5 V/cm; such extraordinary effect is associated with the high electrical conductivity of LSCF through a peculiar mechanism. Microstructural analysis suggests similar morphology and enhanced grain growth compared to traditional sintering; with the proper choice of processing parameters (electric field and current density) during flash sintering, homogeneous porous microstructure for cathodic application can be obtained in very short time. The role of electric field and specimen temperature in flash sintering is analyzed for the understanding of observed outstanding event. The conductivity is found to be a coupled response of electric field and temperature; 2–3 V/cm and 15–25 °C are sufficient for dense LSCF specimen to stimulate the electric field effect on sintering. Electric field controls the conductivity in the same way as temperature does suggesting that under flash effect conductivity is increased by usual mechanism. On the same basis, flash sintering is proposed to be accelerated by the “polaron hopping” phenomenon.     

Thermal conductivity of helium in the eange 400–1500°K as determined by a molybdenum measurement cell

The apparatus described uses a heated wire (filament) to heat a molybdenum measurement cell by passage of electric current. The thermal conductivity of helium is measured in the range 400–1500°K with maximum experimental error of ±4%.     

四通换向阀中采用新型材料的节能效果

为了降低四通换向阀传热损失导致的热泵空调系统的性能损失,提出采用低导热系数的阀座材料替代原阀座材料的方案,并结合实验和系统仿真分析了该方案的实际节能效果。研究结果表明:现有典型四通换向阀的传热损失不可忽略;考虑到结构设计和加工工艺等影响因素,采用低导热系数阀座材料是降低四通阀传热损失较为可行的方案;阀座材料导热系数由常见的110 W/(m.K)降为60 W/(m.K)后,四通阀传热损失减小21%,热泵系统COP提高0.4%。     

硅锥场发射阴极电热状态的数值模拟

用有限差分法求解相互耦合的电流连续方程和导热方程 ,对硅锥阴极的电热状态进行了数值模拟。模拟中采用了与温度、电场相关电导率模型以及和温度相关的热导率模型。模拟的结果表明 ,锥体顶端的电位变化比较突出 ,在较小的电流下其内部电场即可以达到临界场强 ,使载流子漂移速度开始饱和 ,并表现出饱和的发射特性。在尖锥顶端内部电场达到临界场强直至发生碰撞电离时 ,温升不显著     

电热作用对碳纤维树脂基复合材料力学性能的影响

采用复合材料电热实验平台,测试碳纤维树脂基复合材料（Carbon Fiber Reinforced Polymer,CFRP）电热作用下温度场变化规律,同时从单丝拉伸断裂界面剪切强度、短梁剪切性能变化和剪切断口等多方面揭示电热作用对CFRP力学性能的影响机制。结果表明:电热作用会使CFRP整体温度迅速升高,在约4 min时达到稳态温度,随着电流强度的增大,CFRP层板表面温度越高,当电流强度为8 A（0.44 A/mm2）时,CFRP的表面温度达到151℃;单丝拉伸和短梁剪切界面强度都随着电流强度增加呈现先增加后降低的趋势;小电流时,电热作用产生较少的焦耳热,优化界面性能,提高界面剪切强度,大电流时,电热作用产生的焦耳热过大,对界面产生烧蚀等不可逆损伤,降低了界面结合性能。      

Electromagnetic control of convective heat transfer during electron beam evaporation: model experiments

The present paper aims to demonstrate that melt-flow during electron beam evaporation can be effectively controlled by using external magnetic fields to considerably reduce the convective heat transfer. We discuss the various effects of a static magnetic field, a static field combined with an applied electrical current, and a rotating magnetic field. We perform model experiments using GaInSn in eutectic composition as a test liquid. The liquid metal is heated locally at its free surface by an electric resistance heater. The results of the measurements are compared to prediction of numerical simulations.     

Phase transition characteristics of Bi/Sn doped Ge2Sb2Te5thin film for PRAM application

The Bi and Sn were doped to Ge₂Sb₂Te₅ (GST) to investigate and modify the phase transition characteristics. The Bi/Sn doped GST thin film was prepared by RF magnetron co-sputtering and its crystal structure, sheet resistance, and phase transition kinetics were analyzed. By the doping of Bi/Sn, the crystallization temperature or stable phase was changed slightly compared with GST. For the PRAM application, the optimum doping concentration was Bi 5.9 and Sn 17.7 at.%, and its minimum time for crystallization was shorten more than 30% compared with GST. The sheet resistance difference between amorphous and crystalline state was higher than 10⁴ Ω/□.     

Performances of thermoelectric cooler integrated with microchannel heat sinks

In this study, experimental and theoretical studies on thermoelectric cooler (TEC) performance for cooling a refrigerated object (water in a tank) were performed. Microchannel heat sinks fabricated with etched silicon wafers were employed on the TEC hot side to dissipate heat. The measurements show that the temperature of the refrigerated object decreased with time. A theoretical model based on a lumped system was established to predict the transient behavior of the variation in temperature for the refrigerated object with time. The theoretical predicted temperature variation was in good agreement with the measured data. The relationship among the heat sink thermal resistances, TEC electric current input and minimum refrigerated objected temperature was examined based on the theoretical model. The calculated minimum temperatures were showed for the several cases of heat sink thermal resistance on the TEC hot side and electric current input. The minimum temperature can be obtained by increasing the electrical current input and decreasing the heat sink thermal resistance.     

Automated electric valve for electrokinetic separation in a networked microfluidic chip

This paper describes an automated electric valve system designed to reduce dispersion and sample loss into a side channel when an electrokinetically mobilized concentration zone passes a T-junction in a networked microfluidic chip. One way to reduce dispersion is to control current streamlines since charged species are driven along them in the absence of electroosmotic flow. Computer simulations demonstrate that dispersion and sample loss can be reduced by applying a constant additional electric field in the side channel to straighten current streamlines in linear electrokinetic flow (zone electrophoresis). This additional electric field was provided by a pair of platinum microelectrodes integrated into the chip in the vicinity of the T-junction. Both simulations and experiments of this electric valve with constant valve voltages were shown to provide unsatisfactory valve performance during nonlinear electrophoresis (isotachophoresis). On the basis of these results, however, an automated electric valve system was developed with improved valve performance. Experiments conducted with this system showed decreased dispersion and increased reproducibility as protein zones isotachophoretically passed the T-junction. Simulations of the automated electric valve offer further support that the desired shape of current streamlines was maintained at the T-junction during isotachophoresis. Valve performance was evaluated at different valve currents based on statistical variance due to dispersion. With the automated control system, two integrated microelectrodes provide an effective way to manipulate current streamlines, thus acting as an electric valve for charged species in electrokinetic separations.     

Scaling Effect on Silicon Nitride Memristor with Highly Doped Si Substrate

A feasible approach is reported to reduce the switching current and increase the nonlinearity in a complementary metal–oxide–semiconductor (CMOS)‐compatible Ti/SiN_x/p⁺‐Si memristor by simply reducing the cell size down to sub‐100 nm. Even though the switching voltages gradually increase with decreasing device size, the reset current is reduced because of the reduced current overshoot effect. The scaled devices (sub‐100 nm) exhibit gradual reset switching driven by the electric field, whereas that of the large devices (≥1 µm) is driven by Joule heating. For the scaled cell (60 nm), the current levels are tunable by adjusting the reset stop voltage for multilevel cells. It is revealed that the nonlinearity in the low‐resistance state is attributed to Fowler–Nordheim tunneling dominating in the high‐voltage regime (≥1 V) for the scaled cells. The experimental findings demonstrate that the scaled metal–nitride–silicon memristor device paves the way to realize CMOS‐compatible high‐density crosspoint array applications.     

A simple simulation method for designing fibrous insulation materials

Conductive heat in a fibrous material travels through both the air (interstitial fluid) and the fibers (solid phase). The numerical simulations reported in this paper are devised to study the effective thermal conductivity of fibrous media with different microstructural parameters. Simulations were conducted in 3-D fibrous geometries resembling the microstructure of a fibrous material. Assuming that the heat transfer through the interstitial fluid is independent of the geometrical parameters of the solid phase (for when the porosity is held constant), the energy equation was solved only for the solid structures, and the resulting values were used to predict the effective thermal conductivity of the whole media. This treatment allows us to drastically reduce the computational cost of such simulations. The results indicate that heat conduction through the solid fibrous structure increases by increasing the material’s solid volume fraction, fiber diameter, and fibers’ through-plane orientations. The in-plane orientation of the fibers, on the other hand, did not show any significant influence on the material’s conductivity. It was also shown that the microstructural parameters of fibrous insulations have negligible influence on the material’s performance if the conductivity of the solid phase is close to that of the interstitial fluid.     

纳米TiO2/液体硅橡胶直流电缆附件绝缘复合材料的介电性能

高压直流电缆附件在电力系统运行中,由于复合绝缘电导率不匹配极易导致电场畸变引发绝缘故障。针对这一问题,采用直接共混法制备了不同掺杂浓度的纳米TiO₂/液体硅橡胶（LSR）复合材料,并对其微观形貌和介电性能进行了测试研究。结果表明:纳米TiO₂粒子在LSR基体中分散较均匀,随着TiO₂掺杂含量的增加,纳米TiO₂/LSR复合材料试样的相对介电常数和介质损耗因数增大。当纳米TiO₂粒子添加量为4wt%时,纳米TiO₂/LSR复合材料的电导率与电缆主绝缘交联聚乙烯（XLPE）的电导率近似相等,且随着电场强度的增大,两者的电导率变化趋势也基本一致。电声脉冲法（PEA）测量结果表明,添加4wt% TiO₂的纳米TiO₂/LSR复合材料内积聚的空间电荷最少。纳米TiO₂粒子的掺杂,提高了TiO₂/LSR复合材料电缆附件绝缘电导率对电场强度的响应依赖特性,使其能与XLPE绝缘电导率较好地匹配,同时一定程度地抑制了空间电荷的积累,有助于直流电缆附件内复合绝缘电场的均匀分布。      

影响Mo-Cu合金电热性能的因素

主要叙述了适用封接用Mo-Cu导热导电性能的测定方法，论述了Cu和Ni含量对钼铜合金热电性能的影响：机械活化处理及热处理等因素对钼铜合金热电性能的影响。认为Ni等少量杂质元素的加入，Mo-Cu合金材料的热电性能降低；机械活化处理使Mo-Cu合金的热导和导电性能下降；Cu含量的加入和适当的热处理使Mo-Cu合金的热电性能提高。     

Sensitivity of gold nano-conductors to voids,substitutions, and electric field: ab initio results

Gold nanowires are good candidates for nano-electronics devices. A previous study has shown that the beryllium-terminated BeO (0001) surface may be a useful platform for supporting gold nano-conductors, since it preserves the nano wire configuration and does not restrict its conductivity. Here, we used ab initio simulations to determine the sensitivity of potential gold nano-conductors to the presence of point defects, O₂ substitutions and to an applied perpendicular electric field, as in field effect transistors. We found that the presence of the point defects causes only small changes in the atomic bond lengths of the NW, does not alter the NW configuration, but may affect the overall conductivity. Single or double voids on the same channel reduce the conductance by 28 % at most, but when the voids arrange in a way that only one channel remains for conductance, it reduces by factor of two to ≈1 G ₀ (G ₀ = 2e ²/h). The presence of a single O₂ molecule as a substitution reduces the electron availability in the neighboring Au atoms, in most cases reducing the conductance. The perpendicular electric field, which is typical for field effect transistors, affects the electron density distribution, shifts and changes the conductance spectra profile, but does not decrease the conductivity.     

Multiharmonics Method Characterizing In-Plane Thermal Conductivity and TBR of Semiconductor Nanofilm on Substrate: Theoretical Analysis

The in-plane thermal conductivity of semiconductor nanofilm is difficult to be tested due to suspension problem. The thermal boundary resistance (TBR) plays a key role in semiconductor nanoscale structures and nanoscale thermal experiments. By applying alternating current and direct current currents simultaneously on the semiconducting nanofilm on highly insulated substrate, multiharmonics including \(1\upomega \), \(2\upomega \) and \(3\upomega \) signals originating from the self-heating of nanofilm are measured. The thermal boundary resistance is introduced into the heat diffusion equation in in-plane direction. The expression of temperature oscillation and theoretical analysis of heat transport process show that the in-plane thermal conductivity and TBR can be decoupled from the multiharmonics in frequency domain. Thermal analysis justifies the multiharmonics method according to the effect of in-plane thermal conductivity, TBR between nanofilm and insulated substrate, resistance coefficient of semiconductor nanofilm on temperature oscillation at low frequency. Results show the multiharmonic method sensitivity variations depending on the TBR, the in-plane thermal conductivity, and the electric current frequency.