Short-Time Hydrogen Passivation of Poly-Si CMOS Thin film Transistors by High Dose Rate Plasma Ion Implantation
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Mat. Res. Soc. Symp. Proc. Vol. 396 ©1996 Materials Research Society
(LPCVD) and crystallized at 600 'C. After the definition of silicon island regions, a 85 nm-thick gate oxide was deposited by LPCVD and then annealed at 950 'C in an 02 ambient, resulting in a final gate-dielectric thickness of 100 rm. A 350 m-thick poly-Si gate layer was then deposited and patterned. Self-aligned source/drain regions were formed by phosphorus or boron ion implantation for n-channel or p-channel devices, respectively. Afterwards, a 700 nm-thick passivation layer of LPCVD oxide was deposited and a dopant activation anneal was performed at 600 'C. Device fabrication was completed with conventional contact hole formation and AlCu deposition and etch processes, and the TFTs were sintered at 450 'C for 30 minutes in forming gas. A schematic cross-sectional view of the TFT structure is shown in Fig. 1. PMOS TFT
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Fig. 1 Schematic cross-sectional view of the CMOS TFT structure. A microwave multipolar bucket (MMPvIB) plasma system was used for the PII hydrogenation experiment. It consists of a chamber with a multipolar magnetic confinement structure, microwave source, vacuum and gas handling, temperature control, and high voltage pulse generator [6]. Typical equilibrium hydrogen plasma parameters for this experiment are: plasma density ni 2.5x1010 /cm 3, electron temperature T, t 4.5 eV, and plasma potential Vp A,20 V. PII hydrogenation process conditions were as follows: base pressure 10-6 Torr, working pressure 50 mTorr, hydrogen gas flow rate -16 sccm, microwave power 730 W, pulse voltage 6 kV, pulse repetition rate 12.5 kHz, pulse width 5 jisec, and sample temperature 350 ± 20 °C. TFT characteristics were measured with a HP 5155A semiconductor parameter analyzer. RESULTS AND DISCUSSION Figs. 2 (a) and 2 (b) show typical Id,-Vg characteristics for n- and p-channel devices with width to length ratios W/L = 50 jim/50 gim before and after PII hydrogenation. Threshold voltage (VT), leakage current (IL), subthreshold slope (S), and mobility (j.ie) are dramatically improved after a 5 minute PII hydrogenation treatment. A comparison of n-channel and p-channel poly-Si TFT parameters before and after hydrogen PII is shown in Table I. Compared to the hydrogenation results of a 8 hour conventional parallel plate plasma treatment, 80 minute ECR microwave plasma treatment, and 30 minute 4 kHz pulse repetition frequency (prf) PII treatment [5], we obtain similar device improvements in a fraction of the process time. Fig. 3 shows the evolution of TFT parameter improvement versus PII process time for nchannel device. Device parameter improvement saturates in 5 minutes.
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