Process for oxide fabrication using oxidation steps below and above a threshold temperature

Abstract

A process for forming an oxide layer includes forming a first oxide portion over a substrate at a temperature below a threshold temperature. A second oxide portion is formed under the first oxide portion at a temperature above the threshold temperature. The substrate is illustratively oxidizable silicon and the threshold temperature is the viscoelastic temperature of silicon dioxide.

What is claimed is:  
     
         1 . A process for fabricating an oxide, the process comprising: 
 (a) exposing said substrate to a first oxidizing ambient, wherein exposing said substrate to a first oxidizing ambient includes increasing from an initial temperature to a first temperature below a threshold temperature at a first ramp rate, increasing from said first temperature to a second temperature below said threshold temperature at a second ramp rate, and growing at least a portion of said oxide;    (b) exposing said substrate to a second oxidizing ambient, wherein exposing said substrate to a second oxidizing ambient includes increasing from said second temperature to a third temperature at a third ramp rate, and increasing from said third temperature to a temperature above said threshold temperature at a fourth ramp rate; and    (c) cooling said substrate to a temperature below said threshold temperature, wherein said oxide has a thickness of 40 Å or less.    
     
     
         2 . The process as recited in  claim 1 , wherein said first temperature below said threshold temperature is in the range of 750° C. to 850° C. and said first ramp rate is approximately 50° C.-125° C. per minute.  
     
     
         3 . The process as recited in  claim 1 , wherein said second temperature below said threshold temperature is approximately 800° C.-900° C. and said second ramp rate is approximately 10° C.-25° C. per minute.  
     
     
         4 . The process as recited in  claim 1 , wherein step (b) further comprises: 
 increasing from said second temperature to said third temperature at a ramp rate of approximately 5-15° C./minute in an ambient oxygen concentration of approximately 0%-5%;    increasing from said third temperature to said temperature above said threshold temperature at a ramp rate of 5-10° C./minute in an ambient oxygen concentration of approximately 0%-5%; and    growing at least a portion of the oxide in an oxygen ambient concentration of about 25% or less.    
     
     
         5 . The process as recited in  claim 1 , wherein step (c) further comprises: 
 reducing from said temperature above said threshold temperature to approximately 800° C. to 900° C. at a rate of about 2° C./min-5° C./min; and    reducing said temperature of approximately 800° C. to 900° C. to a boat pull temperature at a rate of about 35° C./min-65° C./min, wherein said oxide portion formed in step (a) is a first oxide portion and acts as a stress sink to a second oxide portion formed in step (b) during at least a portion of said cooling.    
     
     
         6 . The process as recited in  claim 1 , wherein said substrate is oxidizable silicon and said threshold temperature is the viscoelastic temperature of silicon dioxide.  
     
     
         7 . The process as recited in  claim 1 , wherein said substrate is oxidizable.  
     
     
         8 . The process as recited in  claim 1 , wherein said substrate is chosen from the group consisting essentially of monocrystalline silicon, polycrystalline silicon and silicon islands in a silicon on insulation (SOI) substrate.  
     
     
         9 . The process as recited in  claim 1 , wherein said threshold temperature is the viscoelastic temperature of SiO 2 .  
     
     
         10 . The process as recited in  claim 5 , wherein said threshold temperature is the viscoelastic.