Vertical MOSFET with dual work function materials
First Claim
1. A DRAM cell comprising a vertical MOSFET transistor on a substrate having an N doped region forming a buried plate, said vertical transistor being provided with an N+ type doped drain, a P type doped well above said buried plate, and an N+ type doped region forming the transistor source;
- a deep trench formed within said substrate, said deep trench having a collar separating said drain from said buried plate, said deep trench having an insolated first region filled with polysilicon and a second region on top of said insulated first region forming said gate, wherein said gate is formed by dual work function materials.
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Accused Products
Abstract
A vertical pass transistor used in a DRAM cell for maintaining a low total leakage current and providing adequate drive current is described together with a method of fabricating such a device. The transistor gate is engineered in lieu of the channel. The vertical pass transistor for the DRAM cell incorporates two gate materials having different work functions. The gate material near the storage node is n-type doped polysilicon. The gate material near the bit line diffusion is made of silicide or metal having a higher work function than the n-polysilicon. The novel device structure shows several advantages: the channel doping is reduced while maintaining a high Vt and a low sub-threshold leakage current; the carrier mobility improves with the reduced channel doping; the body effect of the device is reduced which improves the write back current; and the sub-threshold swing is reduced because of the low channel doping.
37 Citations
15 Claims
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1. A DRAM cell comprising
a vertical MOSFET transistor on a substrate having an N doped region forming a buried plate, said vertical transistor being provided with an N+ type doped drain, a P type doped well above said buried plate, and an N+ type doped region forming the transistor source; a deep trench formed within said substrate, said deep trench having a collar separating said drain from said buried plate, said deep trench having an insolated first region filled with polysilicon and a second region on top of said insulated first region forming said gate, wherein said gate is formed by dual work function materials. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A DRAM cell provided with a vertical MOSFET transistor on a substrate having an N doped region forming a buried plate, said vertical transistor being provided with an N+ type doped drain, a P type doped well above said buried plate, and an N+ type doped region forming the transistor source, the DRAM cell comprising:
a deep trench formed within said substrate, said deep trench provided with a collar to separate said drain from said buried plate, said deep trench having an insolated first region filled with polysilicon and a second region on top of said insulated first region forming said gate, wherein said gate is formed by dual work function materials.
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8. The method of fabricating a DRAM cell comprising the steps of:
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forming a deep trench capacitor within a substrate, said deep trench being partially filled with polysilicon and topped by a trench top oxide;
forming a vertical transistor by diffusing a drain adjoining to the outer surface of said deep trench and contiguous to said trench top oxide;
forming said vertical transistor gate oxide on the walls of said deep trench;
filling the upper portion of said deep trench with a first gate material to a first height of said deep trench, and a second gate material on top of said first gate material, said second gate material only partially filling said deep trench to a second height of said deep trench, said first and second heights being less than the total depth of said deep trench;
forming spacers on the exposed walls of said deep trench;
filling the remainder of said deep trench contacting said uppermost gate material with conductive material, said uppermost gate material being surrounded by said spacers; and
successively implanting into said substrate a buried plate, a well, and said transistor source. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15)
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Specification