Ion Stratification Using Bias Pulses of Short Duration

Abstract

A plasma processing apparatus includes a plasma processing chamber configured to contain a plasma comprising a plasma sheath, ions of a first species, and ions of a second species, a substrate disposed in the plasma processing chamber, and a short pulse generator coupled to the substrate, the short pulse generator configured to generate a pulse train of negative bias pulses. Each of the negative bias pulses has a pulse duration less than 10 μs. A pulse delay between successive negative bias pulses is at least five times the pulse duration. The first species has a first mass and the second species has a second mass less than the first mass. The pulse train spatially stratifies the ions of the first species and the ions of the second species in the plasma sheath.

What is claimed is: 
     
         1 . A plasma processing apparatus comprising:
 a plasma processing chamber configured to contain a plasma comprising a plasma sheath, ions of a first species, and ions of a second species, wherein the first species has a first mass and the second species has a second mass less than the first mass;   a substrate disposed in the plasma processing chamber; and   a short pulse generator coupled to the substrate, the short pulse generator configured to generate a pulse train of negative bias pulses,   wherein each of the negative bias pulses has a pulse duration less than 10 μs,   wherein a pulse delay between successive negative bias pulses is at least five times the pulse duration, and   wherein the pulse train spatially stratifies the ions of the first species and the ions of the second species in the plasma sheath.   
     
     
         2 . The plasma processing apparatus of  claim 1 , wherein the pulse duration is less than about 250 ns. 
     
     
         3 . The plasma processing apparatus of  claim 1 , wherein the pulse delay is greater than about 10 μs. 
     
     
         4 . The plasma processing apparatus of  claim 1 , wherein the pulse train further comprises positive bias pulses immediately following each of the negative bias pulses, the pulse delay between successive negative bias pulses immediately following each of the positive bias pulse. 
     
     
         5 . A method of plasma processing comprising:
 generating plasma in a plasma processing chamber containing a less-reactive species, a more-reactive species, and a substrate comprising an etchable surface, the plasma comprising ions of the less-reactive species, and ions and radicals of the more-reactive species,   wherein the mass of the less-reactive species is less than the mass of the more-reactive species, and   wherein a reactivity of the less-reactive species towards the etchable surface is less than a reactivity of the more-reactive species towards the etchable surface;   increasing the flux and energy of the ions of the less-reactive species at the substrate relative to the flux and energy of the ions of the more-reactive species at the substrate by applying a pulse train of negative bias pulses to the substrate, each negative bias pulse having a pulse duration less than 10 μs; and   etching the etchable surface of the substrate using the radicals of the more-reactive species.   
     
     
         6 . The method of  claim 5 , wherein the less-reactive species is an inert gas and the more-reactive species is a precursor gas. 
     
     
         7 . The method of  claim 6 , wherein the inert gas is helium and the precursor gas is a fluorocarbon. 
     
     
         8 . The method of  claim 5 , wherein the less-reactive species is a first precursor gas and the more-reactive species is a second precursor gas. 
     
     
         9 . The method of  claim 8 , wherein the first precursor gas is hydrogen (H 2 ) and the second precursor gas is hydrogen chloride (HCl). 
     
     
         10 . The method of  claim 5 , wherein a pulse delay between successive negative bias pulses is at least five times the pulse duration. 
     
     
         11 . A plasma processing apparatus comprising:
 a plasma processing chamber configured to contain a plasma comprising a plasma sheath, ions of a first species, and ions of a second species, wherein the first species has a first mass and the second species has a second mass less than the first mass;   a substrate disposed in the plasma processing chamber; and   wherein the plasma processing apparatus is configured to   generate a pulse train of negative bias pulses, each having a pulse duration less than 10 μs,   wherein a pulse delay between successive negative bias pulses is at least five times the pulse duration.   
     
     
         12 . The plasma processing apparatus of  claim 11 , wherein the pulse train spatially stratifies the ions of the first species and the ions of the second species in the plasma sheath. 
     
     
         13 . The plasma processing apparatus of  claim 11 , further comprising:
 a short pulse generator coupled to the substrate and configured to generate the pulse train of negative bias pulses.   
     
     
         14 . The plasma processing apparatus of  claim 13 , wherein the short pulse generator is further configured to apply a positive bias pulse after each of the negative bias pulses and before each pulse delay. 
     
     
         15 . The plasma processing apparatus of  claim 14 , wherein each applied positive bias pulse immediately follows the corresponding negative bias pulse. 
     
     
         16 . The plasma processing apparatus of  claim 13 , wherein a leading edge of each of the negative bias pulses comprises a linear voltage slope. 
     
     
         17 . The plasma processing apparatus of  claim 13 , wherein the short pulse generator is further configured to
 after applying the pulse train for a first duration, apply no bias voltage to the substrate for a second duration greater than ten times the sum of the pulse duration and the pulse delay, and resume applying the pulse train after the second duration.   
     
     
         18 . The plasma processing apparatus of  claim 11 , further comprising:
 an upper electrode configured to generate the plasma in the plasma processing chamber, the plasma being a capacitively coupled plasma.   
     
     
         19 . The plasma processing apparatus of  claim 11 , wherein the pulse duration is less than about 250 ns. 
     
     
         20 . The plasma processing apparatus of  claim 11 , wherein the pulse delay is greater than about 10 μs.