In 1964, Hughes first mentioned the effect of high-energy γ-rays on the MOSFET structure, and then in 1988, this type of PMOS detector was named as RADFET[
Journal of Semiconductors, Volume. 46, Issue 8, 082302(2025)
A semiconductor radiation dosimeter fabricated in 8-inch process
The radiation-sensitive field effect transistors (RADFET) radiation dosimeter is a type of radiation detector based on the total dose effects of the p-channel metal?oxide?semiconductor (PMOS) transistor. The RADFET chip was fabricated in United Microelectronics Center 8-inch process with a six-layer photomask. The chip including two identical PMOS transistors, occupies a size of 610 μm × 610 μm. Each PMOS has a W/L ratio of 300 μm/50 μm, and a 400 nm thick gate oxide, which is formed by a dry-wet-dry oxygen process. The wet oxygen-formed gate oxide with more traps can capture more holes during irradiation, thus significantly changing the PMOS threshold voltage. Pre-irradiation measurement results from ten test chips show that the initial average voltage of the PMOS is 1.961 V with a dispersion of 5.7%. The irradiation experiment is conducted in a cobalt source facility with a dose rate of 50 rad(Si)/s. During irradiation, a constant current source circuit of 10 μA was connected to monitoring the shift in threshold voltage under different total dose. When the total dose is 100 krad(Si), the shift in threshold voltage was approximately 1.37 V, which demonstrates that an excellent radiation function was achieved.
Introduction
In 1964, Hughes first mentioned the effect of high-energy γ-rays on the MOSFET structure, and then in 1988, this type of PMOS detector was named as RADFET[
However, up till now, no domestically-produced RADFET products were fabricated in an 8-inch process. Therefore, based on CUMEC 8-inch 90 nm pilot line, the RADFET research is carried out to achieve domestic chip production.
Structure of radiation detectors
As shown in
Figure 1.(Color online) The structure of PMOS transistor.
The general formula for the threshold voltage of the PMOS transistor is
where Φms is the work function difference between the gate and the silicon substrate, and COX is the capacitance per unit area of the gate oxide, and ΦFn is the substrate Fermi potential, and QA(−2ΦFn) is the charge density of ionized donors in the depletion region at strong inversion, and Q0 is the effective interface charge density, determined by the oxide charge, closely related to radiation-induced charge and radiation-induced interface states, and is the primary cause of radiation-induced threshold voltage shift. The oxide charge distribution in the MOS structure is illustrated in
Figure 2.(Color online) Schematic diagram of charge distribution in MOS gate oxide.
When a PMOS device is exposed to γ-rays, the radiation energy creates electron−hole pairs in the gate oxide layer. When a positive voltage is applied to the gate, due to the much higher mobility of electrons than that of holes in the oxide layer, electrons are quickly removed by the electric field, while holes move slowly within the oxide layer. These holes are trapped by interface traps and traps within the oxide layer, becoming interface trap charges and oxide trap charges, respectively. This results in an increase in the effective interface charge density Q0 of the PMOS, which in turn increases the threshold voltage. Because Q0 is proportional to the total dose and the threshold voltage variation (∆Vt) is proportional to Q0, ∆Vt is proportional to the total dose. As a consequence, the intensity of radiation can be estimated by measuring the shift in threshold voltage of the RADFET before and after irradiation.
Design of radiation detectors
According to the datasheet from a foreign company, the RADFET utilizing a gate oxide thickness of 400 nm is used to detect a total dose of 1 rad to 100 krad with the maximum sensitivity of 55 mV/100rad, which has been widely used in various detection field. Therefore, a gate oxide thickness of 400 nm and a W/L ratio of 300 µm/50 µm are chosen to design domestic RADFET chips for detecting a total dose of 1 rad to100 krad. The main process flow of RADFET fabrication is shown in
Figure 3.The main process flow of RADFET fabrication.
(a) Both N-type and P-type doping are used sequentially to form the body region and the source/drain regions of the PMOS. After each implantation step, thermal oxidation is performed to increase the junction depth.
(b) Since thick gate oxide layers grow slowly through only dry thermal oxidation and cannot continue to grow later on, and even due to insufficient density, dry thermal oxidation methods cannot be used. Moreover, the quality of SiO2 oxide layers grown by wet oxidation cannot be guaranteed. Therefore, a combination of dry-wet-dry oxidation methods is adopted.
(c) Threshold voltage adjustment implantation with boron ions is conducted after the formation of the PMOS gate oxide layer. On one hand, defects and traps in the gate oxide layer will significantly increase, enhancing the ability to capture holes and thus improving device sensitivity. On the other hand, it allows for precise control of the device threshold voltage.
(d) Finally, the gate electrode formation with an aluminum gate process instead of a polysilicon gate process is considered for the following two main reasons. Firstly, the conductivity of polysilicon gates is not as good as that of aluminum gates. Secondly, the resistance of polysilicon gate can be reduced by increasing its thickness, but the doping concentration at bottom of polysilicon is relatively low without high thermal diffusion, which increases threshold voltage as a result of the polysilicon bottom depletion.
Process simulation
Based on the above process flow, the PMOS was simulated using Sentaurus process simulation tools, resulting in the device structure shown in
Figure 4.(Color online) Process simulation structure.
Layout design
The layout of the RADFET chip with a 610 µm × 610 µm size, includes two identical PMOS devices with a W/L ratio of 300 µm/50 µm, as illustrated in
Figure 5.(Color online) RADFET chip layout.
Measurement results
The RADFET chip measurement is divided into pre-irradiation and post-irradiation tests. The former uses WAT (Wafer Acceptance Test) to monitor parameters, such as the PMOS threshold voltage and BVDS. The latter involves irradiation test for the RADFET dies packed in a DIP-14 package, which monitors the shift in threshold voltage before and after irradiation.
Pre-irradiation measurement
The transfer and the output characteristics of the PMOS are shown in
Figure 6.(Color online) Pre-irradiation measurement data. (a) The transfer characteristics curve, (b) the output characteristics curve with different gate to source voltage (VGS).
A map test was conducted to check the uniformity of RADFET across the entire 8-inch wafer, as illustrated in
Figure 7.(Color online) Map data. (a) The drain to source breakdown voltage (BVDS), (b) the threshold voltage (Vtlin_cal), (c) the leakage current (ioff). The map data exhibit very uniform distribution.
Post-irradiation measurement
The irradiation experiment is conducted using a cobalt source with a dose rate of 50 rad(Si)/s. Current RC_I is forced into the RADFET, connected in RC configuration, as shown in
Figure 8.The reader circuit (RC) configuration.
Figure 9.(Color online) The dependence of threshold voltage variation on irradiation dose.
Figure 10.(Color online) The characteristics curves of the RADFETs before and after irradiation at 100 krad(Si). (a) The current RC_I change curve, (b) the transfer characteristics curve of RADFET.
Conclusion
In summary, this paper established the RADFET fabrication process flow, and completed process simulation and device electrical parameter simulation. The simulated results keep competitive with those of the existing products. Based on the process flow and device structural parameters, the RADFET chip layout design was completed. Wafer measurement results show that the pre-irradiation electrical parameters are consistent with TCAD simulation values, and map test results indicate good uniformity across the wafer. Post-irradiation measurement results demonstrate good consistency of RADFETs, achieving a fantastic radiation detection function.
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Jun Huang, Bojin Pan, Hang bao, Qiuyue Huo, Renxiong Li, Qi Ding, Yutuo Guo, Yu Wang, Kunqin He, Yaxin Liu, Ziyi Zeng, Ning Ning, Lulu Peng. A semiconductor radiation dosimeter fabricated in 8-inch process[J]. Journal of Semiconductors, 2025, 46(8): 082302
Category: Research Articles
Received: Dec. 19, 2024
Accepted: --
Published Online: Aug. 27, 2025
The Author Email: Lulu Peng (LLPeng)