Method for fabricating a p-channel field-effect transistor on a semiconductor substrate
View Patent ↗A p-channel field-effect transistor is formed on a semiconductor substrate. The transistor has an n-doped gate electrode, a buried channel, a p-doped source and a p-doped drain. The transistor is fabricated by a procedure in which, after an implantation for defining an n-type well, an oxidation is performed to form a gate-oxide layer and n-doped polysilicon is subsequently deposited. The latter is doped with boron or boron fluoride particles either in situ or by a dedicated implantation step. In a thermal process, the boron acceptors penetrate through the oxide layer into the substrate of the n-type well, where they form a p-doped zone, which serves for counter doping and sets the threshold voltage. This results in a steep profile that permits a shallow buried channel. The control of the number particles penetrating through the oxide layer is achieved by nitriding the oxide layer in an N 2 O atmosphere.
1. A method for fabricating a p-channel field-effect transistor, which comprises the steps of:
providing a semiconductor substrate;
doping the semiconductor substrate with donors by performing a first implantation for forming an n-doped well;
carrying out a thermal oxidation for forming a thin oxide layer on a surface of the semiconductor substrate;
nitriding the oxide layer in an N 2 O atmosphere to control a permeability of the oxide layer with respect to one of boron and boron fluoride particles penetrating through the oxide layer;
depositing a first layer formed from n-doped polysilicon above the oxide layer;
p-doping the first layer with particles selected from the group consisting of boron particles and boron fluoride particles, a p-type dopant concentration of the particles being lower than an n-type dopant concentration of the first layer;
carrying out a lithographic projection step and an etching step for removing the first layer in a first and a second region and for forming an n-doped gate electrode in a third region located between the first and second regions on the semiconductor substrate;
doping the semiconductor substrate with acceptors by performing a second implantation for forming a p-doped source region in the first region and a p-doped drain region in the second region in the semiconductor substrate; and
exposing the semiconductor substrate to a temperature elevated to such an extent that a number of the particles migrate from the first layer through the oxide layer into the semiconductor substrate for forming a p-doped zone within the n-doped well for defining a threshold voltage of the field-effect transistor.
2. The method according to claim 1 , which comprises carrying out the p-doping of the first layer with the particles during the step of depositing the first layer.
3. The method according to claim 1 , which comprises carrying out the p-doping of the first layer after the step of depositing the first layer by performing a third implantation process.
4. The method according to claim 1 , wherein no p-doping of the semiconductor substrate with the particles in order to form a doping profile within the n-doped well is carried out after the first implantation and before the depositing of the first layer.
5. The method according to claim 1 , which comprises after the p-doping of the first layer with the particles, performing the steps of:
depositing a second layer containing an electrically conductive material onto the first layer; and
depositing a third layer containing a nitride onto the second layer.
6. The method according to claim 1 , which comprises before performing the first implantation, forming a sacrificial oxide layer and removing the sacrificial oxide layer after the first implantation is performed.
7. The method according to claim 3 , which comprises carrying out the third implantation with a dose of 10 13 to 10 15 particles per square centimeter.
8. The method according to claim 7 , which comprises carrying out the third implantation process with an energy of 2.5 to 10 keV.
9. The method according to claim 1 , which comprises:
forming the n-doped polysilicon of the first layer to have a dopant concentration of 10 19 to 10 20 particles per cubic centimeter; and
carrying out the p-doping of the n-doped polysilicon with the particles such that the first layer has a dopant concentration of 10 17 to 10 18 particles per cubic centimeter.