摘要
随着CMOS集成电路按照摩尔定律的快速发展,器件的特征尺寸已进入了纳米量级,传统的SiO2栅介质厚度减小会导致很高的隧穿电流,已不能用于纳米CMOS器件的制备,需要采用具有较高介电常数的(高k)栅介质薄膜替代SiO2。目前,高k栅介质材料(HfSiON)已经用于45 nm和32 nm技术节点集成电路,然而仍不满足下一代集成电路的要求,需要寻找新的高k栅介质材料,作为具有较大应用前景的高k材料HfO2,由于其结晶温度较低,抑制氧扩散的能力较差,需要对其进行改良,Gd2O3具有良好的热稳定性以及较大的带隙宽度,且能有效地抑制氧的扩散,本文采用Gd2O3掺杂HfO2的方法对其进行改良。
1.采用磁控溅射方法,在不同的掺杂功率下制备了多种Gd203掺杂HfO2(GDH)高k栅介质薄膜,研究了掺杂功率与GDH薄膜电学性能的关系,掺杂功率为20 W时,电学性能最优,主要原因是Gd的电负性比Hf的大,对O离子的吸附能力强,能有效地抑制氧的扩散,减少了氧空位的缺陷密度,从而降低了漏电流密度,并且使C-V曲线向左漂移现象减弱。研究了N2中快速热处理工艺(RTP)对GDH-20(掺杂功率为20W)栅介质薄膜电学性能的影响,GDH-20最佳的快速热处理温度为700℃,C-V曲线的回滞窗口宽度会随着RTP温度的升高逐渐减小,并且积累区到耗尽区的过渡变得平缓。高的RTP温度使GDH-20/Si的界面得到更进一步钝化,减少了界面态密度,降低了界面处的负电荷积累,此外,N在高温下更易与GDH-20中的氧空位结合,降低了氧空位密度,使得C-V曲线发生正向偏移。
2.分析了GDH-20薄膜的成分及结构,Gd203在GDH-20薄膜中的掺杂量为10mol%,并且在GDH-20/Si界面处产生了硅酸盐,HRTEM分析表明GDH-20薄膜结构为非晶,并且与纯HfO2相比,界面层厚度明显减小,说明通过Gd203的掺杂能有效降低氧在GDH-20栅介质中的扩散。
3.研究了GDH-20薄膜的击穿特性,GDH-20的TDDB的软击穿是由于栅介质中的电荷积累造成的,栅压越大,击穿电量反而越小,软击穿后GDH-20栅介质的介电性能仍然良好。与施加在GDH-20上脉冲方波电压的测试结果相比,同等大小的栅压下,脉冲电压发生击穿的时间与电荷量均比直流栅压的大,这是由于氧化层的损伤自修复和脉冲应力分散造成的。GDH-20经受硬击穿后,栅介质漏电流显著上升,电容急剧下降,栅介质的介电特性变差,这主要是由正电荷的积累造成的。此外,研究了工作温度对GDH-20薄膜击穿性能的影响,GDH-20薄膜在100℃以下均能保持良好的介电性能,说明其具有较强的抗热击穿能力。
综上可见,在Si(001)衬底上生长的非晶GDH栅介质可以作为高k栅介质应用的候选材料。
With the rapid development of Complementary Metal-Oxide-Semiconductor (CMOS) integrated circuit according to Moore's law, the device feature size will be reduced into the nanometer scale, decreasing of thickness of SiO2 gate causes dielectric high leakage current. It is necessary to find new gate materials with high dielectric constant to replace SiO2. The high-k gate dielectric material (HfSiON) has been used for 45 nm and 32 nm technology nodes, but still cannot meet the requirements of next-generation integrated circuits, it needs to find a new high-k gate dielectric material. HfO2, as a good promising high k material, can not be directly used for integrated circuits due to its low crystallization temperature and poor ability of preventing the oxygen diffusion. We selected Gd2O3 to dope in HfO2 for improving the properties of pure HfO2 gate dielectric due to its good thermalstability, larger band gap and effective prevention of oxygen diffusion.
1. We prepared Gd2O3 doped HfO2 (GDH) high-k gate dielectric films under a variety of doping power by magnetron co-sputtering. The relationship between different doping power of Gd2O3 and the electrical properties of the GDH films was studied. The best electrical properties appeared when the doping power was 20 W. The relationship between the rapid thermal process (RTP) in N2 and the electrical properties of GDH-20 (doping power of 20W) gate dielectric films has been studied. The best RTP temperature was 700℃for GDH-20 films. With the RTP temperature increasing, the width of hysteresis windows of C-V curves was gradually decreased and the depletion region from the accumulation to the transition became smooth. GDH-20/Si interface has been further passivated under high RTP temperature, and the interface state density and the accumulation of negative charges were reduced. In addition, oxygen vacancies can be combined by N easily in high RTP temperatures, leading to a reduction of the oxygen vacancy density.
2. We have analyzed the composition and structure of GDH-20 film. The doping content of Gd2O3 in GDH-20 film was 10mol%, there was silicate generated at the GDH-20/Si interface. The amorphous GDH-20 film was revealed by HRTEM, and the interface layer thickness was significantly reduced compared with pure HfO2, indicating the effective prevention of oxygen diffusion in the GDH gate dielectric by doping Gd2O3
3. Breakdown properties of GDH-20 high k dielectrics have been studied. GDH-20 gate dielectric maintained good dielectric properties after SBD (soft breakdown). QBD (the charges when breakdown) reduced with the gate voltage increasing. The tBD (time of breakdown) and QBD of breakdown under square wave pulse voltage for GDH-20 dielectric were larger than the case of DC voltage breakdown, due to the self-reparation of GDH-20 film and the dispersion of the pulse stress. The leakage current density increased significantly, while the capacitance density decreased sharply after hard breakdown (HBD). The dielectric properties of GDH-20 gate dielectric deteriorated because of the accumulation of positive charges. In addition, we have studied the effects of the working temperature on GDH-20 film breakdown charactors. GDH-20 thin films maintained good dielectric properties when the work temperature was 100℃
In summary, the amorphous GDH high-k gate dielectrics grown on Si (001) substrates can be used as candidate for high-k gate dielectric applications.
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