Pulsed plasma with low wafer temperature for ultra thin layer etches
Ultrathin material layers are plasma etched with an etch system configured for cryogenic cooling of a substrate to reduce the diffusion coefficients of foreign and intrinsic stop layer atoms (e.g., of the bombarded crystal lattice), and further configured for plasma pulsing to reduce the energy of the impinging ions with cryogenic wafer temperatures. Substrate temperatures of −50° C. or more are employed to reduce the susceptibility of a stop layer material to damage associated with ion impact. Ion energy is reduced to below the threshold where stop layer lattice atoms are displaced or ions are implanted into the bulk lattice. In embodiments, a plasma of an etchant gas having ion energies less than 10 eV are achieved through plasma pulsing, which when directed at the low temperature substrate may controllably etch ultra-thin material layers.
1. A method for plasma etching a workpiece, comprising:
cooling a substrate to between −20° C. and −50° C.;
etching a target material layer disposed on a stop layer disposed over the substrate by exposing the target material layer to a plasma of an etchant gas, and wherein the plasma is energized by RF power that is pulsed to reduce the average energy of ionic species impinging on the substrate to below 5 eV.
2. The method of claim 1 , wherein the RF power is delivered by only an inductively coupled source and the RF power is pulsed at a frequency of between 1 Hz and 200 kHz and a duty cycle between 20% and 90%.
3. The method of claim 1 , further comprising biasing the sample with negative charge during an on phase of a plasma power duty cycle, and applying no bias during an off phase of the plasma power duty cycle.
4. The method of claim 1 , wherein cooling the substrate further comprises transporting liquid nitrogen (LN 2 ) to a surface of a chuck upon which the substrate is disposed.
5. The method of claim 1 , wherein the material layer is graphene and the material layer thickness etched is between 2 nm and 20 nm.
6. The method of claim 1 , wherein the stop layer is graphene and the material layer is a dielectric material having a wider bandgap than the graphene.
7. A method for plasma etching a thin film stack comprising a graphene layer, the method comprising:
loading a substrate including the thin film stack into a process chamber;
cooling the substrate to between −20° C. and −50° C.;
introducing a hydrocarbon-free etchant gas to the process chamber;
generating a plasma of the etchant gas, wherein the plasma is energized by an RF power that is pulsed to reduce the average energy of ionic species impinging on the substrate to below 5 eV.
8. The method of claim 7 , wherein introducing the hydrocarbon-free etchant gas comprises pulsing a flow of HBr or Cl into the chamber.
9. The method of claim 7 , wherein generating the plasma of the etchant gas further comprises pulsing an inductively coupled source at a frequency of between 1 Hz and 200 kHz and a duty cycle between 20% and 90%.
10. The method of claim 9 , wherein the graphene layer thickness is between 2 nm and 20 nm and wherein the thin film stack further comprises a dielectric material having a wider bandgap than that of the graphene layer.