Process including silo-chloro passivation for etching tungsten silicide overlying polysilicon

Abstract

A method of plasma etching tungsten silicide over polysilicon particularly useful in fabricating flash memory having both a densely packed area and an open (iso) area requiring a long over etch due to microloading. Wafer biasing is decreased in the over etch. The principal etchant include NF 3 and Cl 2 . Argon is added to prevent undercutting at the dense/iso interface. Oxygen and nitrogen oxidize any exposed silicon to increase etch selectivity and straightens the etch profile. SiCl 4 as an example of a silicon and chlorine containing passivating gas may be added for additional selectivity.

1 . A method of etching a tungsten silicide layer, comprising the steps of:
 placing within a plasma reaction chamber a substrate containing the tungsten silicide layer;   flowing into the plasma reaction chamber a first gas mixture comprising an etching gas comprising a chlorine- and fluorine-containing gas and a passivating gas comprising constituents including silicon and chlorine; and   exciting the first gas mixture into a plasma.   
   
   
       2 . (canceled) 
   
   
       3 . The method of  claim 1 , wherein the passivating gas comprises silicon tetrachloride. 
   
   
       4 . The method of  claim 1 , wherein the etching gas comprises nitrogen trifluoride and chlorine gas. 
   
   
       5 . The method of  claim 1 , wherein the first gas mixture further includes a gas chosen from the group consisting of oxygen gas and nitrogen gas. 
   
   
       6 . The method of  claim 1 , wherein the exciting step includes applying RF power to an inductive coil associated with the chamber and further including supplying RF power through a capacitive coupling circuit to a pedestal electrode supporting the substrate. 
   
   
       7 . A multi-step method of etching tungsten silicide over silicon, comprising the steps of:
 placing a substrate containing a tungsten silicide layer of tungsten overlying a silicon layer onto a pedestal electrode in a plasma reaction chamber;   a first step of flowing into the plasma reaction chamber a first gas mixture comprising an etching gas comprising nitrogen trifluoride and chlorine gas, a first gas comprising oxygen gas and nitrogen gas, and argon;   exciting the first gas mixture into a plasma to etch through the tungsten silicide layer in at least a portion of the substrate while biasing the pedestal electrode with a first level of RF power;   a subsequent second step of flowing into the plasma reaction chamber a second gas mixture comprising the etching gas, the oxidizing gas, and a passivating gas comprising constituents including silicon and chlorine and exciting the second gas mixture into a plasma while biasing the pedestal electrode with a second level of RF power greater than the first level.   
   
   
       8 . The method of  claim 7 , wherein less nitrogen trifluoride is flowed into the chamber in the second step than in the first step. 
   
   
       9 . The method of  claim 7 , wherein the exciting step includes applying RF power to an inductive coil associated with the chamber. 
   
   
       10 . The method of  claim 7 , wherein the passivating gas comprises silicon tetrachloride. 
   
   
       11 . A method of etching a tungsten-containing layer overlying a silicon layer, comprising the steps of:
 placing onto a pedestal electrode in a plasma reaction chamber a substrate containing the tungsten-containing layer and the silicon layer, and by an etching mask overlying the tungsten-containing layer, wherein the silcon layer consists principally of silicon;   a first step of flowing into the chamber a first gas mixture comprising a first gas comprising chorine and fluorine and a second gas comprising oxygen gas and nitrogen gas;   biasing the pedestal electrode with a first level of RF power;   applying a second level of RF power to excite the first gas mixture into a plasma to thereby etch at least the tungsten-containing layer sufficiently that at least some of the silicon layer is exposed, wherein a ratio of the second level to the first level is between 4 and 8;   a subsequent second step of flowing into the chamber a second gas mixture comprising the etching gas having a smaller fraction of NF 3  than in the first step, the oxidizing gas having a larger fraction of nitrogen gas than in the first step, and a passivating gas comprising silicon and chlorine constituents;   biasing the pedestal electrode with a third level of RF power greater than the first level; and   applying a fourth level of RF power to excite the second gas mixture into a plasma.   
   
   
       12 . The method of  claim 11 , wherein the passivating gas comprises silicon tetrachloride. 
   
   
       13 . The method of  claim 11 , wherein the etching gas in the second step has a smaller fraction of NF 3  than in the first step and the oxidizing gas in the second step has a larger fraction of nitrogen gas than in the first step, 
   
   
       14 . The method of  claim 11 , wherein the first step further comprises flowing a noble gas selected from the group consisting of helium and argon into the chamber. 
   
   
       15 . The method of  claim 11 , wherein a ratio of the nitrogen gas to the oxygen gas in the first step is at least 4. 
   
   
       16 . The method of  claim 11 , wherein the second level of RF power is applied to an inductive coil associated with the chamber. 
   
   
       17 . The method of  claim 11 , wherein the etching gas comprises NF 3  and Cl 2 . 
   
   
       18 . The method of  claim 17 , wherein a ratio of the fourth level to the third level of RF powers is between 4 and 8. 
   
   
       19 . The method of  claim 17 , further comprising the steps of:
 a subsequent third step of flowing a third gas mixture into the chamber comprising the etching gas and the second gas, wherein a ratio of the nitrogen gas to the oxygen gas is greater than in the second step;   biasing the pedestal electrode with a fifth level of RF power less than the third level; and   applying a sixth level of RF power to excite the third gas mixture into a plasma.   
   
   
       20 . The method of  claim 17 , wherein during the second step the plasma of the second gas mixture acts on exposed portions of the silicon layer and etches any remaining portions of the tungsten-containing layer.