Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
1998-12-18
2001-09-11
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S756000, C438S757000
Reexamination Certificate
active
06287983
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to integrated circuit fabrication methods, and particularly to wet etching methods for selectively etching silicon nitride.
BACKGROUND: SELECTIVE NITRIDE ETCHING
One of the very useful basic steps in integrated circuit processing is the ability to remove silicon nitride selectively with respect to silicon dioxide. Conventionally, a hot phosphoric acid bath is used to etch silicon nitride. However, a basic problem is that the nitride/oxide selectivity changes during the course of an etching cycle. (When the bath is new, it will achieve a nitride-to-oxide selectivity of only about 5 to 1. As the bath continues to etch nitride, the selectivity will improve by a factor of 20 or more.)
This presents problems in process controllability. To compensate for such loading effects, it may be necessary to continually adjust the chemistry of the bath, or to precondition a bath before it is used, or to discard a bath early.
One proposed approach to this problem was a four-step procedure, for silicon nitride strip in phosphoric acid, described in U.S. Pat. No. 5,188,701 to Yoshio, which is hereby incorporated by reference. In this method, a complex procedures was used to control the concentration and temperature of phosphoric acid. The complexity of this method is a significant drawback.
V. D. Wohlheiter used a mixed solution of phosphoric acid and sulfuric acid to etch silicon nitride and effectively passivate the exposed n+ silicon surface. See Wohlheiter, “Silicon Nitride Ledge Removal Techniques for Integrated Circuit Devices,” 122 J. Electrochem. Soc. 1736 (1975), which is hereby incorporated by reference. However, that method requires a significant shift in the chemistry, which is undesirable.
NITRIDE ETCHING WITH SILICATE ION PRE-LOADING
The present application discloses a new method for etching silicon nitride selectively with respect to silicon oxides. According to this method, a source of silicate ions is added to the hot phosphoric acid bath immediately, as a way to directly precondition it before wafers are etched. Preferably the silicate ion source is a liquid such as “TEOS” (tetraethoxysilane). It has been found that the oxide etching rate is determined by the concentration of silicic acid, and that the concentration of silicic acid has essentially no effect on the nitride etching rate. Thus, by preloading the bath with silicic acid, the nitride-to-oxide selectivity is very good, even at the start of etching with a new bath, or after fresh chemicals are added. Preferably, the loading of the bath is accomplished by injecting liquid TEOS directly into the hot phosphoric acid bath below its surface. The temperature of the bath is actually above the flashpoint of TEOS in air, but the injected TEOS reacts rapidly with the acid bath to form silicic acid. In one example, injection of 125 cc of TEOS into a forty liter bath of hot phosphoric acid has been found to reduce the silicon dioxide etching rate to approximately zero.
Advantages of the disclosed inventions include: better control over wet etching of integrated circuits having silicon nitrides and silicon oxides on their surfaces; more efficient use of chemicals; the ability to use undensified oxides; and consistently achievable high nitride:oxide selectivity.
REFERENCES:
patent: 3607480 (1971-09-01), Harrap et al.
patent: 3635774 (1972-01-01), Ohta
patent: 4092211 (1978-05-01), Morris
patent: 4269654 (1981-05-01), Deckert et al.
patent: 5188701 (1993-02-01), Hirano
patent: 5472562 (1995-12-01), Ziger
patent: 5918134 (1999-06-01), Gardner et al.
Wolheiter, V. D., “Silicon nitride Ledge Removal Techniques for Integrated Circuit Devices”, J. Electrochemical Society: Solid-State Science and technology, pp. 1736-1738, Dec. 1975.
Jan Der'E
Kirkpatrick Brian K.
Parrill Thomas M.
Brady III W. James
Deo Duy-Vu
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Utech Benjamin L.
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