TFT and reliability evaluation method thereof
AssigneeMitsubishi Denki Kabushiki Kaisha
InventorShigenobu Maeda
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Abstract
In a method of evaluating the reliability of a thin film transistor (TFT), time coefficient β, voltage coefficient d and temperature coefficient φ0 are experimentally produced from −BT stress tests, and the life of a TFT under −BT stress conditions is evaluated using the following expression: τ = t 0 ( Δ V th τ Δ V th0 ) β exp β q φ 0 kT exp ( - β qd [LeftBracketingBar] V G [RightBracketingBar] 2 kTt OX ) ( 8 ) where τ represents the life time of the TFT, ΔVthτ the tolerant threshold voltage shift amount of the TFT, t0 (1/ΔVth0) constant, q elementary electric charge, k Boltzmann constant, T temperature, VG gate voltage, and tOX the thickness of the gate oxide film.
Claims5
A method of determining an optimized gate insulating film thickness tox−opt of a TFT having a channel layer of a polycrystalline thin film and a gate insulating film of a silicon oxide film and used at an absolute temperature T during operation, comprising the steps of:
- determining voltage coefficient d using the equation Δ V th ∝ exp qd [LeftBracketingBar] V G [RightBracketingBar] 2 kTt OX
- and
- determining tox−opt=qd VGburn-in/2kT using the determined voltage coefficient d.
A machine-readable medium storing a machine executable software procedure for determining an optimized gate insulating film thickness tox−opt of a TFT having a channel layer of a polycrystalline thin film and a gate insulating film of a silicon oxide film and used at an absolute temperature T during operation, the machine executable software procedure comprising the steps of:
- determining voltage coefficient d using the equation Δ V th ∝ exp qd [LeftBracketingBar] V G [RightBracketingBar] 2 kTt OX
- and
- determining tox−opt=qd VGburn-in/2kT using the determined voltage coefficient d.
A machine-readable medium storing a machine executable software procedure for evaluating the reliability of a TFT in a −BT stress state, the TFT having a channel layer of a silicon thin film and a gate insulating film of a silicon oxide, and in the −BT stress state a gate is supplied with arbitrary constant voltage VG and held at an arbitrary constant temperature T based on the following expressions:
- ΔVth∝tα (i)
- Δ V th ∝ exp qd [LeftBracketingBar] V G [RightBracketingBar] 2 kTt ox ( ii ) Δ V th ∝ exp { - q kT ( ∅ 0 - d [LeftBracketingBar] V G [RightBracketingBar] 2 t ox ) } ( iii ) Δ V th = Δ V tho ( t t 0 ) α exp { - q kT ( ∅ 0 - d [LeftBracketingBar] V G [RightBracketingBar] 2 t ox ) } ( iv ) τ = t 0 ( Δ V th τ Δ V th 0 ) β exp β q ∅ 0 kT exp ( - β qd [LeftBracketingBar] V G [RightBracketingBar] 2 kTt ox ) ( v )
- where ΔVth represents the threshold voltage shift amount of the TFT, t time, α time coefficient, q elementary electric charge, d voltage coefficient, k Boltzman constant, tOX the thickness of the gate oxide film Φ0 temperature coefficient, and ΔVthτ the tolerant threshold voltage shift amount of the TFT, and β=1/α, the machine executable software procedure comprising the steps of:
- determining the time coefficient α in expression (i) based on the relation between a threshold voltage shift amount ΔVth obtained from at least one −BT stress test and time t;
- determining coefficient d in expression (ii) based on the relation between a threshold voltage shift amount ΔVth obtained from at least two −BT stress tests and different gate voltages VG applied in the tests;
- determining the temperature coefficient Φ0 in expression (iii) based on the relation between a threshold voltage shift amount ΔVth obtained from at least two −BT stress tests and different temperatures T used in the tests;
- and
- determining the following constant of proportion in expression (iv) obtained from the relation between expressions (i) and (ii), using the determined time coefficient α, voltage coefficient d and temperature coefficient Φ0, Δ V th 0 ( 1 t 0 ) α = C 2
- thereby obtaining the life of the TFT from expression (v) obtained by transforming expression (iv) using the determined constant of proportion C2 and the tolerant threshold voltage shift amount ΔVthτ of the TFT.
Dependencies are inferred from the claim text.