Total Dose Hardening Of SIMOX SOI Materials

Silicon on insulator (SOI) is a new type of silicon-based material. Compared to traditional bulk silicon devices and circuits, SOI devices and circuits have a higher speed, lower power ,higher integration level and no latch-up effect. Particularly, they have a larger resistance to high dose rate transient upset effect and effectively reduce short channel effect when bulk silicon devices scale down. Therefore,SOI technology is praised as a silicon integrated circuit technology in the 21st century. Separated-by-implanted-oxygen (SIMOX) method is one of the mainstream SOI technologies in the present that oxygen ions are implanted into silicon and subsequently annealed to form buried insulator, SiO2, through ion implantation technique.The advantages of SOI technology come from the full dielectric isolation that separates devices and circuits from substrate. However, the buried oxide makes hardening SOI devices to total dose irradiation complex, for the buried oxide of SIMOX SOI wafers has a large number of hole traps which can capture radiation-induced holes in ionizing radiation environment. This effect will cause the properties of SOI devices and circuits degraded, even invalid. Therefore, the total dose radiation hardness of SOI devices and circuits is declined. In order to improve the radiation tolerance of SOI devices and circuits, nitrogen ions were implanted into the buried oxide of SIMOX SOI materials through ion-implantation technique. Finally, the SOI devices and circuits are hardened by way of hardening the materials.Commercial SIMOX SOI wafers were adopted in this work, and the thickness of their buried oxide and top silicon is 150nm, 190nm, respectively. In order to study the effect of anneal time on the radiation hardness of the buried oxide of SIMOX SOI materials, nitrogen ions were implanted into the buried oxide with a dose of 1016cm-2, and an optimized implantation energy 90keV was adopted. the subsequent annealing was performed at 1100℃for 0.0, 0.5, 1.0, 1.5, 2.5h, respectively. In order to characterize the radiation effects of the buried oxide, the capacitors of PBS (polysilicon-Buried oxide-semiconductor) were fabricated on the SIMOX materials after the top silicon layers were removed by reactive-ion etching, and were irradiated with Co-60γ-ray source at a dose of 100rad (Si)/s. High frequency voltage- capacitance(C-V) was used to characterize the irradiation response of the buried oxide after the tested capacitors were irradiated total doses of 1×105, 3×105, 5×105 and 7×105 rad(Si), the results suggest that the improvement of the radiation resistance of the wafers can be achieved through a shorter time annealing (0.5h) after nitrogen implantation. The Secondary ions mass spectrometry (SIMS) results show that nitrogen ions accumulate at the interface of buried oxide and Si. Therefore, the improvement of radiation hardness is possibly attributed to the two factors: on the one hand, When the strained Si-Si bonds near the interface combine with implanted nitrogen, the release of the interface stress will happen, suppressing the formation of the E'centers; on the other hand, the electron traps related to nitrogen will also offset the increase of radiation-induced positive charges in BOX by trapping electrons. Moreover, for the 1.0 and 1.5h annealing samples, both total dose responses were unusual. After 300krad (Si) irradiation, the C-V shifts reached a maximum, respectively, and then positive shift with increasing total dose. Analysis suggests that the hole traps in the buried oxide of the two samples have much larger capture cross section, when these hole traps occupied by radiation induced holes, after relaxation, they may be turn to capture electrons for the action of the Coulomb attraction.In additional, the experimental results show that the positive charge density of the nitrogen-implanted buried oxide is obviously increased, compared with the control sample without nitrogen implantation. In order to investigate the mechanism that positive charge increase, simulate nitrogen ions implanted into the SOI materials. According to the simulating results, a lot of vacancies were introduced into the buried oxide during the implantation, but they aren't the main factors that cause positive charge increase in the buried oxide. The Fourier transform infrared (FTIR) spectroscopy of the samples indicates that implantation induced defects can be basically eliminated after annealing 0.5 h. So the increase of the positive charge density of the nitrogen implanted buried oxide is ascribed to the accumulation of implanted nitrogen near the interface of buried oxide and silicon, which caused the break of weak Si-Si bonds.The C-V technique was used to characterize the above irradiated PBS capacitors after self-annealing more than for half a year. The C-V shifts of nitrogen implanted and annealed samples were decreased more or less that in accordance with normal behavior. However, the C-V shift of the nitrogen implanted sample without anneal shows abnormal negative shift. It is concluded that radiation induced holes diffuse in the buried oxide that cause the charge redistribution.The above irradiated PBS capacitors were reradiated to study the total dose irradiation response after self-annealing. Experimental results found that the irradiation response of different capacitor samples appear a large difference after different irradiated dose. Even if the C-V shifts of the same sample weren't monotonic change after different radiation dose. So the radiation resistance of capacitors can't be judged simply by the direction and degree of C-V shifts, which involves the distribution of captured charge and the electrical field of captured charge in the buried oxide, and the influence of radiation induced interface traps and other factors. In-depth of understanding this mechanism, still need further study.