次级电子平衡对器件辐照试验吸收剂量的影响研究

Absorbed dose is the most important physical quantity in the study of radiation effect. The variation law of the electronic device parameters with the absorbed dose is the basis for the study of radiation effect. At present, irradiation tests are performed by applying electrical bias to the electronic device placed on a circuit board for irradiation. There was great difference in geometry size of different type device. For a thinner device, the thicknesses for packaging material above sensitive layer and substrate under sensitive layer are thinner. In this situation there maybe no secondary charged particle equilibrium in the sensitive area of a thinner device. Actual absorbed dose could be smaller than irradiated dose measured by the dosimeter at the same position, thus it will result in underestimation to radiation damage of electronic devices. By the studies of dosimetric technique and analysis method of high spatial resolution, combined with the measurement data of radiation effects of electronic devices , this project will explore the relationship between the absorbed doses of the device sensitive area and the structure, geometry size, packaging materials of the device, socket as well as the material composition and thickness of bias circuit board during irradiation tests of typical electronic devices. Meanwhile, through analyzing whether or not the device sensitive area is in secondary charged particle equilibrium condition and the absorbed doses of the device sensitive area is identical to the measurement result of dosimeter, this project will establish an irradiation method which meet secondary charged particle equilibrium condition, so that ensure the accuracy of irradiation dose of electronic devices. This study will provide accurate reference data for research of irradiation effect and verification of radiation hardening of the electronic device. This will promote the development of dosimetry of radiation effect.

吸收剂量是辐射效应研究最重要的量，器件参数随吸收剂量的变化规律是效应研究的依据。目前辐照试验都采用元器件放置在辐照电路板上加电辐照的方法。不同类型器件尺寸差异较大，对几何尺寸较薄的器件，辐射敏感区上面的封装材料和下方的基底层都较薄。这种情况下由于元器件敏感区不满足次级电子平衡条件，敏感区实际接受的吸收剂量，可能小于剂量计预先测量的辐照剂量，这会造成对器件辐射损伤的过低估计。本项目通过研究高空间分辨率的剂量测量技术和分析方法，结合器件效应测量数据，考查典型电子元器件辐照试验中，器件敏感区吸收剂量与器件结构、尺寸、封装材料、插座、偏置电路板的材质、厚度等的关系。分析元器件敏感区是否处于次级电子平衡状况，元器件敏感区吸收剂量是否与剂量计测量结果相同。进而建立满足次级电子平衡的辐照方法，保证元器件辐照剂量的准确性。为电子元器件辐射效应研究、辐射加固验证提供准确的基准数据，推动辐射效应剂量学的发展。

电子器件总剂量效应试验通常是用Co-60伽马源进行的，封装比较薄的器件敏感区实际辐照剂量会小于标称辐照剂量，这是由于伽马射线是通过光电效应、康普顿效应和电子对效应，转换为电子，再在器件敏感层电离产生电子空穴对，因此在器件敏感层之前需要有一定厚度的累积层才能使器件实际吸收剂量和标称辐照剂量相一致。项目通过研究微区高分辨率剂量测量方法，建立了器件各层结构中剂量分布的蒙特卡罗分析方法，并结合器件辐射敏感参数测量方法，发现对CMOS 反相器等典型CMOS器件，器件敏感层吸收剂量与标称辐照剂量的最大偏差达到了47%。 对双极晶体管，最大偏差达到了72%。超出了标准规定的10%的偏差范围，会对器件抗辐射水平的评估造成影响。. 通过施加补偿材料的对比试验，器件辐射敏感参数的比较也证明了施加补偿材料的器件，参数退化大于没有补偿材料的器件。同时前补偿材料的作用大于后补偿材料。器件参数退化在不同的辐照节点是不同的，辐照剂量越大，参数退化的差异越小。这表明辐射效应和辐照剂量的关系不是线性的，随着辐照剂量的增加，辐射损伤趋于饱和。. 通过试验研究和蒙特卡罗分析，选用能量在光子能量1.25MeV 时，质能吸收系数大于硅的聚乙烯等材料作为补偿材料，可以显著减小器件敏感层吸收剂量的偏差，满足试验标准的要求。并对补偿材料的厚度、尺度及其与器件的距离等进行了研究，形成了补偿方法。研究成果在电离总剂量效应试验单位得到了应用。