Effect of Ohmic Contacts on Polysilicon Memory Effect
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E5.6.1
Effect of ohmic contacts on polysilicon memory effect
S.B. Herner, C. Jahn, and D. Kidwell Matrix Semiconductor, Santa Clara, CA ABSTRACT
Polysilicon memory switching is demonstrated in vertical n-i-p diodes with TiN contacts and feature size 150 nm. An increase of more than three orders of magnitude forward current at +2 V is achieved after the application of a +8 V programming pulse. The programming pulse electro thermally melts the polysilicon through Joule heating, and the quenched diodes have higher forward current. By changing one of the contacts from TiN to TiSi2, polysilicon memory switching is eliminated, with high forward current in the diode before and after a programming pulse. The TiSi2 contact seeds the crystallization of polysilicon with fewer defects during fabrication, compared to crystallization with TiN-only contacts, as shown by transmission electron microscopy. The mechanisms for increased forward current induced by either programming the TiN-only contacted diodes or making one contact TiSi2 are discussed. INTRODUCTION
One method of storing data is based on sensing a difference between current in two memory cells. The ability to change the resistance of the cell is the ability to store data. It has been shown that the resistance of polycrystalline silicon, or polysilicon, can be changed by the application of biasing voltage. This effect has been referred to as polysilicon memory switching. Switching has been demonstrated in polysilicon resistors [1-3], pn junctions [4], and p-i-n amorphous Si diodes [5-7]. The devices have small currents at small biasing voltages (known as the read voltage). After the application of a large pulse bias voltage (known as the programming pulse), where the voltage is much larger than the read voltage, the current at the read voltage has been shown to increase up to six orders of magnitude. The change in resistance has been attributed to the formation of a low resistivity filament through higher resistivity polysilicon or amorphous Si material [1,8,7]. Electro thermal heating at a critical threshold voltage causes all or a portion of the material to melt. It is speculated that when the melted material is quenched, the resulting filament has single crystal-like properties. Previous work on polysilicon memory switching used devices with feature sizes greater than 2 µm. Polysilicon memory switching in vertical n-i-p diodes with TiN contacts is demonstrated for application as a one time programmable (OTP) memory cell with feature size 150 nm. An increase of more than three orders of magnitude in the forward current of the diodes at +2 V is achieved after the application of +8 V programming pulse across the diode. A high forward current can be achieved by changing the contact material to the polysilicon diode. The mechanism for increased forward current by replacing one of the TiN contacts with TiSi2 is shown, and the mechanism for increased forward current by voltage pulsing TiN-only contacted diodes is thought to be related.
E5.6.2
TiN p+ Si
W i-Si W
(a)
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