Method to reduce polysilicon depletion in MOS transistors
First Claim
1. A method of forming a polysilicon electrode with reduced dopant depletion comprising:
- providing a substrate;
forming a dielectric layer over said substrate;
forming a polysilicon layer over said dielectric layer;
in-situ amorphizing upper portion of said polysilicon layer by implanting ions into said polysilicon layer to form an amorphous (α
-Si) layer over said polysilicon layer;
then doping said polysilicon layer by implanting ions; and
performing, laser irradiation of selected wavelength and fluence to melt and regrow said α
-Si layer while at the same time driving said ions deeply into said polysilicon layer adjacent said underlying dielectric layer.
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Accused Products
Abstract
A method is provided to reduce poly depletion in MOS transistors. Conventionally, after a polysilicon electrode has been doped, an anneal step is usually performed to activate the dopants. However, the anneal step may be insufficient to drive the implanted impurities down the entire depth of the polysilicon electrode. Consequently, a portion of the polysilicon gate nearest to the gate oxide will be depleted of dopants. This poly depletion will have a detrimental effect on the control of the threshold voltage, and hence on the performance of the device. It is disclosed in the present invention a method of forming polysilicon gates where dopant depletion at the interface near the gate oxide layer is alleviated substantially by using laser annealing; however, by first pre-amorphizing the polycrystalline silicon prior to ion (implantation to a desired depth such that during laser annealing the dopants will diffuse uniformly to a melt depth. In this manner, poly depletion effect is greatly reduced and hence performance of the device improved. The disclosed method is applicable to both n+ doped polysilicon gates (NMOS) and p+ doped polysilicon gates (PMOS).
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Citations
25 Claims
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1. A method of forming a polysilicon electrode with reduced dopant depletion comprising:
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providing a substrate;
forming a dielectric layer over said substrate;
forming a polysilicon layer over said dielectric layer;
in-situ amorphizing upper portion of said polysilicon layer by implanting ions into said polysilicon layer to form an amorphous (α
-Si) layer over said polysilicon layer;
thendoping said polysilicon layer by implanting ions; and
performing, laser irradiation of selected wavelength and fluence to melt and regrow said α
-Si layer while at the same time driving said ions deeply into said polysilicon layer adjacent said underlying dielectric layer.- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A method of reducing polysilicon depletion in polysilicon electrodes in MOS transistors comprising the steps of:
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providing a substrate;
forming a pad oxide layer over said substrate;
forming a nitride layer over said pad oxide layer;
patterning said nitride layer to form a trench over said pad oxide layer;
removing said pad oxide layer and exposing said substrate at the bottom of said trench;
optionally forming and removing a sacrificial layer;
forming a gate oxide layer over said substrate at the bottom of said trench;
forming a polysilicon layer over said substrate, including within said trench;
performing chemical mechanical polishing (CMP) to remove excess polysilicon over said trench;
selective etching said polysilicon layer in said trench to a desired thickness;
in-situ amorphizing upper portion of said polysilicon layer by implanting ions into said polysilicon layer to form an amorphous (α
-Si) layer over said polysilicon layer;
doping said polysilicon layer by implanting ions;
performing laser irradiation of selected wavelength and fluence to melt and regrow said α
-Si layer while at the same time driving said ions deeply into said polysilicon electrode reaching but adjacent said underlying oxide layer; and
continuing the completion of said MOS transistor by a depositing another material into said trench followed by CMP to form a contact. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25)
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Specification