AVS 47th International Symposium
    Semiconductors Wednesday Sessions
       Session SC+EL+SS-WeM

Paper SC+EL+SS-WeM4
Etching of BPSG Films Using Anhydrous HF

Wednesday, October 4, 2000, 9:20 am, Room 306

Session: Passivation and Etching of Semiconductors
Presenter: A. Muscat, University of Arizona
Authors: A. Thorsness, University of Arizona
G. Montano, University of Arizona
A. Muscat, University of Arizona
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The absorption of water within BPSG films and its affect on the rate and the product distribution on the surface during etching with anhydrous HF (AHF) has been studied with transmission FTIR and ellipsometry. Gas phase HF chemistries are currently used to selectively etch doped oxide films relative to undoped films for both DRAM and MEMs applications, but often require a post-process water rinse. BPSG films containing 3.5%B/4.0%P were deposited to a nominal thickness of 5K Å on 8" Si wafers containing a 2K Å TEOS buffer layer. Sets of 25 wafers were annealed after deposition to four different temperatures: as deposited (400°C), 500°C, 750°C, and 900°C. The films were etched at atmospheric pressure and at temperatures from 25 to 75°C in a commercial gas phase oxide etching tool (FSI Excalibur ISR) using AHF in a nitrogen carrier. Water uptake by the films did not saturate but increased at a steady rate as a function of annealing temperature during storage in a cleanroom. Absorbed water reacted with P=O producing P-OH groups. The induction time to start etching decreased with water absorption and loss of P=O. Based on relative bond strengths AHF attacks the P-O bonds first opening up the silicon dioxide lattice and producing both a P-bearing acid and product water. The product water H-bonds to the P-bearing acid creating a thin liquid film on the wafer surface. The liquid film supports etching of the stronger Si-O-Si and B-O-Si bonds and solvates etching products. The water H-bonded to the P-bearing acid lowers the activation barrier to breaking Si-O bonds by forming a (SiO)HOH complex. The weakened Si-O bond is more amenable to attack by a polar HF molecule. Metaphosphoric acid, pyrophosphoric acid, boric acid, boron trifluoride, and water were present in the liquid films after etching. The relative amount of these products was a strong function of the annealing temperature.