Method for stressing a thin pattern and transistor fabrication method incorporating said method
A method for stressing a pattern having a pattern surface, in a layer of semiconductive material that can be silicon on the surface of a stack of layers generated on the surface of a substrate, said stack comprising at least one stress layer of alloy Si x Ge y with x and y being molar fractions, and a buried layer of silicon oxide, comprises: etching at the periphery of a surface of dimensions greater than said pattern surface, of the buried layer of silicon oxide and layer of alloy Si x Ge y over a part of the depth of said layer of alloy; the buried layer of silicon oxide being situated between said layer of semiconductive material and said stress layer of alloy Si x Ge y . In a transistor structure, etching at the periphery of said surface obtains a pattern thus defined having dimensions greater than the area of interest situated under the gate of the transistor.
1. A method for stressing a pattern having a pattern surface with pattern dimensions, in a layer of semiconductive material on the surface of a stack of layers generated on the surface of a substrate, said stack comprising at least one stress layer of alloy Si x Ge y with x and y being molar fractions, and a buried layer of silicon oxide, the buried layer of silicon oxide being situated between said layer of semiconductive material and said stress layer of alloy Si x Ge y , the method comprising:
determining a first function t SiGe =f(W), wherein t SiGe is an optimum thickness of the layer of alloy Si x Ge y as a function of a pattern dimension (W), thereby obtaining a maximum stress (S0 ZZ ) in said pattern, said optimum thickness being independent of the molar fraction y;
determining a second function tc SiGe =g(y) wherein tc SiGe is a critical thickness of the layer of alloy Si x Ge y beyond which dislocations appear in said stress layer for a given molar fraction y;
determining the molar fraction y by equality of the thicknesses t SiGe =tc SiGe , thereby defining, for a given pattern dimension (W), an optimum pair of values (t SiGe , y);
fabricating the stack including the stress layer of alloy Si x Ge y using the optimum pair of values (t SiGe , y); and
etching at the periphery of a surface of dimensions greater than or equal to those of said pattern surface dimensions, of the buried layer of silicon oxide and of the stress layer of alloy Si x Ge y over at least a part of the depth of said layer of alloy.
2. The method for stressing a pattern in a layer of semiconductive material according to claim 1 , in which the alloy Si x Ge y is doped.
3. The method for stressing a pattern in a layer of semiconductive material according to claim 1 , in which the thickness of the layer of alloy Si x Ge y is substantially equal to the critical thickness beyond which dislocations are likely to appear in said layer of alloy Si x Ge y .
4. The method for stressing a pattern in a layer of semiconductive material according to claim 1 , in which the etching is performed also in a part of said substrate over a non-zero etching depth.
5. A transistor fabrication method, comprising the method for stressing a pattern according to claim 1 , and further comprising etching of an active area of which a dimension (LZA) in the direction of the current in the plane of the layers is at least greater than or equal to the sum of the dimensions of a source area, of a gate area and of a drain area.
6. The transistor fabrication method according to claim 5 , further comprising:
a step of deposition of a layer of silicon nitride, after the step of etching of the active area;
a step of thick oxide layer deposition;
production of oxide insulation blocks on either side of the source, gate and drain areas.
7. The method for stressing a pattern in a layer of semiconductive material according to claim 1 , in which the molar fraction y is equal to 1−x.
8. The method for stressing a pattern in a layer of semiconductive material according to claim 7 , in which the alloy Si x Ge y is doped.
9. The method for stressing a pattern in a layer of semiconductive material according to claim 7 , in which the thickness of the layer of alloy Si x Ge y is substantially equal to the critical thickness beyond which dislocations are likely to appear in said layer of alloy Si x Ge y .
10. The method for stressing a pattern in a layer of semiconductive material according to claim 7 , in which the etching is performed also in a part of said substrate over a non-zero etching depth.
11. A transistor fabrication method, comprising the method for stressing a pattern according to claim 7 , and further comprising etching of an active area of which a dimension (LZA) in the direction of the current in the plane of the layers is at least greater than or equal to the sum of the dimensions of a source area, of a gate area and of a drain area.
12. The transistor fabrication method according to claim 11 , comprising:
a step of deposition of a layer of silicon nitride, after the step of etching of the active area;
a step of thick oxide layer deposition;
production of oxide insulation blocks on either side of the source, gate and drain areas.
13. The method for stressing a pattern in a layer of semiconductive material according to claim 1 , in which the semiconductive material is silicon.