The use of Metal Additions to Phosphoric Acid to Etch Polysilicon in Poly Buffered Locos Processes
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Aqueous phosphoric acid is commonly used to remove the nitride layer after the field oxidation and the polysilicon layer is removed by wet and/or dry etching. Either approach must not remove the underlying oxide and attack the silicon substrate. We have found that metal additions to the hot phosphoric acid used to etch the silicon nitride will produce acceptable etch rates on the polysilicon layer while retaining a low oxide etch rate. Beyond the good etch selectivity this process offers, it allows for the removal of the nitride and polysilicon layers in one step. EXPERIMENTAL The etching experiments were conducted mostly at temperatures ranging from 163 'C to 165 0C in a Pyrex glass jar containing 1250 ml. The H3P0 4 solutions were kept under magnetic stirring and boiling. Water was occasionally added to maintain the boiling of the mixture within the temperature range and, thus, the composition of the etchant. Polysilicon control wafers, having a nominal thickness of 4100 angstroms, were used for the etch rate measurement. The polysilicon thickness was measured with a Nanospec instrument. Thermal oxide controls had a nominal film thickness of 600 angstroms and the oxide thickness was measured using an ellipsometer. Metals residual on the wafers after etching and after various chemical cleans were detected by TXRF. 459 Mat. Res. Soc. Symp. Proc. Vol. 477 0 1997 Materials Research Society
RESULTS The etching parameters and etch rates determined on the polysilicon and the thermal oxide are summarized in Tables I and II. Table I
Temp (C) 163-167
Po•y Si Etch Rate in H 3 PO4 Conc. (ppm) Additives
163-166
-
162-165 163-167 162-167 162-168 163-165 163-166 163-170
HNO 3 Zn+2 Ni÷2 As+ 3 Cu+ 2 Fe÷3 Sb+3
Etch Rate (A/min.) 5.7 4.1
10000 50 50 50 60 150 50
18 6.4 8.9 12 20.4 24.9 41.2
Essentially, all the additives were found to increase the etch rate of polysilicon in H 3 PO 4 . For the Cu÷2 ion, a minimum concentration of 10 ppm is needed to be effective, and a similar minimum concentration is expected to apply to the Fe÷3 ion also. As shown in Table II, the oxide etch rate was determined to be very low in all the mixtures used. Table II
Temp (0 C) 163-165 163-165 162-165 163-165 1) 70% HNO 3
Thermal Oxide Etch Rate in Conc. Additives (ppm) HNO 3 1) 1000 Cu+ 2 60 Fe+3 150
H 3 P0 4
Etch Rate (A/min.) 0.25 1.0 0.8 0.5
FOX Thickness After PBL Wet Etch (Lot - 25497) The etching of oxide is dependent on its solubility in H3PO 4 , and the dissolution rate is very slow and hard to measure precisely. Table III shows the TXRF results on wafers etched with Cu or Fe additions to the phosphoric acid as compared to a solution without metal additions. In each case, the metals added to the etch bath were detected indicating the need for a clean to remove them. Also in Table III are the results of various cleans on samples etched in a bath with Fe additions. The SC-2 and sulfuric acid-hydrogen peroxide cleans were seen to be very effective in removing the Fe.
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TABLE III
TXRF RESULTS USING COLD WATER RINSING
CI
K
C
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