Dose-rate sensitivity of deep sub-micro complementary metal oxide semiconductor process

Abstract

Enhancing low dose rate sensitivity (ELDRS) in bipolar device is a major problem of liner circuit radiation hardness prediction for space application. ELDRS is usually attributed to space-charge effect. A key element is the difference in transport rate between holes and protons in SiO2. Interface-trap formation at high dose rate is reduced due to positive charge buildup in the Si/SiO2 interfacial region (due to the trapping of holes and/or protons) which reduces the flow rates of subsequent holes and protons (relative to the low-dose-rate case) from the bulk of the oxide to the Si/SiO2 interface. Generally speaking, the dose rate of metal oxide semiconductor (MOS) device is time dependent when annealing of radiation-induced charge is taken into account. The degradation of MOS device induced by the low dose rate irradiation is the same as that by high dose rate when annealing of radiation-induced charge is taken into account. However, radiation response of new generation MOS device is dominated by charge buildup in shallow trench isolation (STI) rather than gate oxide as older generation device. Unlike gate oxides, which are routinely grown by thermal oxidation, field oxides are produced using a wide variety of deposition techniques. As a result, they are typically thick (100 nm), soft to ionizing radiation, and electric field is far less than that of gate oxide, which is similar to the passivation layer of bipolar device and may lead to ELDRS. Therefore, dose-rate sensitivities of n-type metal oxide semiconductor field effect transistor (NMOSFET) and static random access memory (SRAM) manufactured by 0.18 m complementary metal oxide semiconductor (CMOS) process are explored experimentally and theoretically in this paper. Radiation-induced leakages in NMOSFET and SRAM are examined each as a function of dose rate. Under the worst-case bias, the degradation of NMOSFET is more severe under the low dose rate irradiation than under the high dose rate irradiation and anneal. Moreover, radiation-induced standby current rising in SRAM is more severe under the low dose rate irradiation than under the high dose rate irradiation even when anneal is not considered. The above experimental results reveal that the dose-rate sensitivity of deep sub-micron CMOS process is not related to time-dependent effects of CMOS devices. Mathematical description of the combination between enhanced low dose-rate sensitivity and timedependent effects as applied to radiation-induced leakage in NMOSFET is developed. It has been numerically found that non time-dependent effect of deep sub-micron CMOS device arises due to the competition between enhanced low dose-rate sensitivity in bottom of STI and time-dependent effect at the top of STI. The high dose rate irradiation is overly conservative for devices used in a low dose rate environment. The test method provides an extended room temperature anneal test to allow leakage-related parameters that exceed postirradiation specifications to return to a specified range.