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以直接式法拉第杯为例,当0.5~9.0 MeV的Au离子轰击靶盘(圆槽中仅放有铜片)时,利用中国科学院应用物理研究所研制的89型束流积分仪测量了抑制极板、杯筒和靶盘上的电流 I 2, I 3和 I 4。测试时,抑制极板和杯筒上一直加有–300 V电压,因此测到的电流是由次级正离子引起的。测试结果如 图4所示,为了便于比较,进行了归一处理,将靶盘上的电流 I 4设为100%。可见,对于较低能量的Au离子轰击铜片,在杯筒上收集到的次级正离子引起的电流与靶盘上的电流大小相当,并且随束流能量升高而降低。此外,在相同实验设置下测试了比Au离子轻很多的C离子的情况,在1~4 MeV能量范围内, I 3/ I 4的比例在3%左右。
为了检验上述两种法拉第杯的实际使用效果,分别利用几种常用能量的Au离子在10 mm×10 mm的单晶Si片上进行了注入实验。实验后利用RBS方法对注入剂量进行了测量 [ 21- 22] 。入射束流为He 2+离子,能量采用3.0或3.8 MeV,使注入的Au离子信号峰与Si衬底信号分开,避免重叠,便于数据分析。束流沿样品法线入射,在散射角为160°方向用钝化注入平面硅(Passivated Implanted Planar Silicon (PIPS))探测器探测散射的He粒子,探测器的分辨率约为20 keV。在靶盘上加+300 V电压抑制次级电子, 4He 2+流强约10 nA,死时间小于1%,入射离子的总电荷量为8.5 μC。为了得到精确的实验结果,利用SIMNRA7.01软件对各实验谱进行了模拟分析 [ 23] ,其中电子阻止本领数据采用了KKKNS参数,其精度为2% [ 24- 25] 。典型模拟谱如 图5所示,模拟谱与实验谱符合得很好,模拟结果准确可靠。在每块样品的上下左右四个位置各做了测试,其平均剂量作为最终测量剂量。 表1列出了直接式法拉第杯与间接式法拉第杯应用于离子注入实验的测试结果。测量剂量的误差主要来自Au峰的统计误差 (约为1.4%,对于注入剂量为5.0×10 15 cm –2)、散射角偏差(约为1.22%)以及阻止本领(精度约为2%)。可见,测量剂量与期望剂量的误差在4%以内,效果很好,满足注入实验的要求。
表 1 对于直接式与间接式法拉第杯结构,RBS测量注入剂量与期望注入剂量的比较
能量/MeV 离子 直接式法拉第杯 间接式法拉第杯 期望剂量/(10 15 cm –2) 测量剂量/(10 15 cm –2) 误差/% 期望剂量/(10 15 cm –2) 测量剂量/(10 15 cm –2) 误差/% 0.5 Au + 1.5 1.55(3) 3.3 — — — 1.0 Au + 5.2 5.36(7) 3.1 — — — 3.0 Au 2+ 5.0 5.09(7) 1.8 3.0 2.97(5) –1.0 6.0 Au 3+ 3.0 3.11(5) 3.7 3.5 3.50(6) 0 由于室温注入系统的辐照面积较大,为了检验注入的均匀性,利用3 MeV的Au 2+在20 mm×20 mm的Si片样品上进行了注入实验,期望剂量为4.0×10 15 cm –2。为了尽量减小管道中残余气体对注入均匀性的影响,实验过程中靶室真空约1.1×10 –4 Pa。注入实验结束后,在Si片样品上的9个位置进行RBS测试分析,测量各位置的注入剂量。结果如 图6所示,9个位置的测量剂量相近,平均值为(3.99±0.04)×10 15 cm –2,与期望剂量相差0.25%,相对标准偏差(注入剂量的均匀性)为2%,满足注入实验的要求。
Ion Implantation/Irradiation System of 1.7 MV Tandem Accelerator at Peking University
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摘要: 北京大学1.7 MV串列静电加速器运行至今已有三十多年。该加速器配备有高频电荷交换负离子源和铯溅射负离子源,能够引出从H到 Au之间 的大部分元素的离子。离子能量可被加速至几百keV到若干MeV,主要开展离子注入/辐照实验和卢瑟福背散射(RBS)和沟道分析等离子束分析工作。基于辐照实验需求,建立了高温辐照系统,温度最高可达950 ℃。为了实现更加精确的离子注入,设计了直接式与间接式两种法拉第杯结构,使束流扫描面积精确控制,并且在测量束流强度时除了抑制次级电子,还考虑到了次级正离子的影响。利用不同能量的Au离子在单晶Si片上进行了注入实验,通过RBS分析显示测量剂量与期望剂量误差在4%以内,此外,注入均匀性的测试表明注入剂量的相对标准偏差为2%。Abstract: The 1.7 MV tandem accelerator at Peking University has been running for more than 30 years. The accelerator is equipped with a Radio Frequency(RF) charge exchange negative ion source and a cesium sputtering negative ion source, which can produce most of the ions from H to Au. It can accelerate the ions to energies from several hundreds of keV to several MeV. The accelerator is used for ion implantation and irradiation as well as for ion beam analysis, such as Rutherford Backscattering Spectroscopy(RBS) and channeling. Based on the experimental requirements, a high temperature irradiation system was established, with the highest temperature of 950 ℃. In order to achieve more accurate ion implantation, two Faraday cup structures, direct type and indirect type, are designed. The scanning area of the beam is controlled accurately. These designs can not only suppress the secondary electrons, the influence of secondary positive ions is also considered when measuring the beam intensity. The ion implantation experiments of Au ions with different energy on single crystal silicon were carried out. RBS analysis shows that the error between the measured fluence and the expected fluence is within 4%. In addition, the uniformity measurement shows that the relative standard deviation of the implant fluence is 2%.
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表 1 对于直接式与间接式法拉第杯结构,RBS测量注入剂量与期望注入剂量的比较
能量/MeV 离子 直接式法拉第杯 间接式法拉第杯 期望剂量/(10 15 cm –2) 测量剂量/(10 15 cm –2) 误差/% 期望剂量/(10 15 cm –2) 测量剂量/(10 15 cm –2) 误差/% 0.5 Au + 1.5 1.55(3) 3.3 — — — 1.0 Au + 5.2 5.36(7) 3.1 — — — 3.0 Au 2+ 5.0 5.09(7) 1.8 3.0 2.97(5) –1.0 6.0 Au 3+ 3.0 3.11(5) 3.7 3.5 3.50(6) 0 -
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