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高k氧化物材料及其MOS器件的重离子辐照效应

Radiation Effects of High-k Oxides and HfO2 Based MOS Device Induced by Heavy Ions

  • 摘要: 高介电常数材料(高k材料)是器件特征尺寸缩小至45 nm及以下时栅介质材料的必然选择。常见的高k材料以HfO2、ZrO2、TiO2、Al2O3等金属氧化物材料为主,开展这些材料的重离子辐照效应研究,对于评估新型纳米器件在空间辐射环境下的长期可靠性有重要意义。低能重离子在材料中沉积能量的主要方式为核能损,而快重离子则以电子能损为主。本工作较系统地总结和讨论了这种差异导致的低能和快重离子在这些材料中辐照效应的异同之处。同时,在实际应用场景中可能涉及这些材料不同的晶态、晶粒尺寸和晶型。因此,本文还对比汇总了不同晶态,如单晶、纳米晶、非晶,甚至晶型的高k材料对重离子辐照效应的响应。具体地,快重离子引起块体ZrO2由单斜相向四方相转变的辐照注量较低能重离子低约两至三个数量级,而重离子辐照纳米晶ZrO2则可引起非晶化;基于在金红石型TiO2中快重离子潜径迹的系统实验研究,首次发现了金红石型TiO2中快重离子潜径迹的精细结构和径迹形貌随离子穿过长度(或样品厚度)的演化过程,提出了快重离子潜径迹形成新机理,即快重离子热峰效应导致的材料熔融相外流和自身强再结晶能力的共同作用是此类径迹形貌演化的关键。本文据此质疑了此前关于块体UO2中未发现裂变径迹的理论解释,同时兰州重离子加速器上开展的辐照实验结果支持了本研究的机理预测,表明了新机理具有较好的普适性。最后,以HfO2基MOS器件为例,探讨了低能和快重离子引起器件宏观电学特性退化与材料微观结构损伤的内在联系。

     

    Abstract: High-k dielectric materials (high-k materials) have become the inevitable choice for gate dielectric layers when device feature sizes shrink to 45 nm and below. Common high-k materials primarily include metal oxides such as HfO2, ZrO2, TiO2 and Al2O3. Research on the heavy ion irradiation effects of these materials is of significant importance for evaluating the long-term reliability of novel nanodevices in space radiation environments. Low-energy heavy ions primarily deposit energy in materials through nuclear energy loss, while swift heavy ions mainly cause electronic energy loss. This paper systematically summarizes and discusses the differences in irradiation effects caused by this fundamental distinction between low-energy and swift heavy ions in these materials. Furthermore, practical applications may involve different crystalline states, grain sizes, and crystal phases of these materials. Therefore, this study also comparatively summarizes the responses of high-k materials with varying crystalline states (single crystal, nanocrystalline, amorphous) and even different crystal phases to heavy ion irradiation. Specifically, swift heavy ions can induce phase transformation from monoclinic to tetragonal in bulk ZrO2 at relatively low ion fluences, which is usually two to three orders of magnitude lower than required by low-energy heavy ions. For nanocrystalline ZrO2, heavy ions irradiation could cause amorphization. By systematic experimental studies on latent tracks in rutile TiO2 induced by swift heavy ions, we first observed the fine structure of latent tracks and their morphological evolution along ion penetration depth (or sample thickness). A new mechanism for swift heavy ion track formation is proposed: the combined effect of material melt flow caused by thermal spikes and strong recrystallization capability is the key factor governing such track morphology evolution. This finding challenges the previous theoretical explanation for the absence of fission tracks in bulk UO2. The irradiation experiments performed at the Heavy Ion Research Facility in Lanzhou support our predictions, demonstrating the good universality of the new mechanism. Finally, taking HfO2-based MOS devices as an example, we explore the intrinsic relationship between macroscopic electrical characteristic degradation caused by low-energy/swift heavy ions and microscopic material structural damage.

     

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