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Studies on a plasma soft X-ray source for microlithography using a chemically-amplified resist
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Type
Thesis
Author
Wong, Darren Jon Sien
Supervisor
Tan, Augustine Tuck Lee
Abstract
This thesis describes studies done on a high performance repetitive 3 kJ dense plasma focus (DPF) device as a soft X-ray (SXR) source for microlithographic applications using the SU-8 chemically-amplified resist (CAR).
Various SXR yield optimisation studies with argon and neon as filling gases were performed under different operating conditions of charging voltage, filling pressure, anode length, anode shape and insulator sleeve length. The SXR yield was computed based on the absorption filtering method using PIN diodes. The optimum SXR yield in neon (λ ∼1 nm) was found to be ∼140 J/shot, which corresponded to a relatively good wall plug efficiency of 5.6 %. Operation in argon showed that optimized SXR (λ ∼0.4 nm) yield was up to 1.3 J/shot.
A refinement to the code based on the Lee’s plasma model was made to take into account the tapering of the anode in the axial phase and to include the energy loss due to recombination radiation in the radiative phase. Our improved code was calibrated for the NX2, a 3 kJ plasma focus device, operated in neon for a pressure range of 4 – 7 mbar on a tapered copper anode. The resulting theoretical X-ray yield predictions are compared against experimental data, showing good agreement.
A cost effective, compact, eight-channel PIN diode soft X-ray spectrometric system was developed, tested and calibrated. It can discriminate between X-ray photons in the energy ranges of 0.7-1.6, 1.6-2.8, 2.5-5.0, 5.0-7.9 and 7.9-8.5 keV. The data acquisition system includes sampling circuits, an analogue switch, an A/D converter, a microcontroller, a host computer and the related software. It is tailored to monitor the SXR dose under repetitive operation, at adjustable preset trigger frequencies. Our results show that the dynamics involved in single-shot machines may not always be transferable by simple scaling when applied to machines operating under repetitive firing. Studies under repetitive operation with copper anode and Al-insert illustrate how the relative amounts of these elements enhance or suppress the X-ray yield for the different energy regions.
SU-8 resist was characterised for X-ray lithographic applications by studying the cross-linking process under different conditions of resist thickness, exposure dose and post-exposure bake (PEB) conditions. The infrared absorption peaks at 862, 914, 972, and 1128 cm-1 were found to be useful indicators for the completion of crosslinking in the resist. Results of the experiments showed that the cross-linking of SU-8 was optimised at an exposure dose of 800 mJ/cm2 for resist thickness of up to 15 µm, with the PEB temperature set at 95oC and PEB time of 3 min. Work with 25 µm resists showed that the optimum processing conditions for exposure dose of 2500 mJ/cm2 consisted of an intermediate PEB at 65°C for 5 min, with the PEB temperature ramped up to 95°C over 1.5 min and then followed by a final PEB at 95°C for 5 min.
Our work on the optimisation of X-ray yield, characterization of resist, modelling of the plasma and the construction of the X-ray spectrometric system culminated towards application for microlithography. Test structures with an excellent aspect ratio of up to 20:1 on 25 µm thick SU-8 resist were successfully obtained, possibly the best hitherto reported, based on a plasma focus X-ray source.
Various SXR yield optimisation studies with argon and neon as filling gases were performed under different operating conditions of charging voltage, filling pressure, anode length, anode shape and insulator sleeve length. The SXR yield was computed based on the absorption filtering method using PIN diodes. The optimum SXR yield in neon (λ ∼1 nm) was found to be ∼140 J/shot, which corresponded to a relatively good wall plug efficiency of 5.6 %. Operation in argon showed that optimized SXR (λ ∼0.4 nm) yield was up to 1.3 J/shot.
A refinement to the code based on the Lee’s plasma model was made to take into account the tapering of the anode in the axial phase and to include the energy loss due to recombination radiation in the radiative phase. Our improved code was calibrated for the NX2, a 3 kJ plasma focus device, operated in neon for a pressure range of 4 – 7 mbar on a tapered copper anode. The resulting theoretical X-ray yield predictions are compared against experimental data, showing good agreement.
A cost effective, compact, eight-channel PIN diode soft X-ray spectrometric system was developed, tested and calibrated. It can discriminate between X-ray photons in the energy ranges of 0.7-1.6, 1.6-2.8, 2.5-5.0, 5.0-7.9 and 7.9-8.5 keV. The data acquisition system includes sampling circuits, an analogue switch, an A/D converter, a microcontroller, a host computer and the related software. It is tailored to monitor the SXR dose under repetitive operation, at adjustable preset trigger frequencies. Our results show that the dynamics involved in single-shot machines may not always be transferable by simple scaling when applied to machines operating under repetitive firing. Studies under repetitive operation with copper anode and Al-insert illustrate how the relative amounts of these elements enhance or suppress the X-ray yield for the different energy regions.
SU-8 resist was characterised for X-ray lithographic applications by studying the cross-linking process under different conditions of resist thickness, exposure dose and post-exposure bake (PEB) conditions. The infrared absorption peaks at 862, 914, 972, and 1128 cm-1 were found to be useful indicators for the completion of crosslinking in the resist. Results of the experiments showed that the cross-linking of SU-8 was optimised at an exposure dose of 800 mJ/cm2 for resist thickness of up to 15 µm, with the PEB temperature set at 95oC and PEB time of 3 min. Work with 25 µm resists showed that the optimum processing conditions for exposure dose of 2500 mJ/cm2 consisted of an intermediate PEB at 65°C for 5 min, with the PEB temperature ramped up to 95°C over 1.5 min and then followed by a final PEB at 95°C for 5 min.
Our work on the optimisation of X-ray yield, characterization of resist, modelling of the plasma and the construction of the X-ray spectrometric system culminated towards application for microlithography. Test structures with an excellent aspect ratio of up to 20:1 on 25 µm thick SU-8 resist were successfully obtained, possibly the best hitherto reported, based on a plasma focus X-ray source.
Date Issued
2005
Call Number
QC718.5.D38 Won
Date Submitted
2005