Silicon Nitride Etch

Overview

Silicon nitride (Si₃N₄) films have several important applications in advanced semiconductor devices. Included among the integrated circuit (IC) manufacturing applications of silicon nitride films are: as an alkali, water, and oxygen diffusion barrier;^1,2 as masking layers for local oxidation of silicon (LOCOS);³ as final mechanical protection layers for ICs;⁴ and as electrical insulators which have a comparatively high dielectric constant.⁵ A number of techniques have been used to form silicon nitride films. A summary of the various techniques used to prepare silicon nitride films is presented in Table 1.

Table 1. Summary of Preparation Techniques for Si₃N₄ Films,
adapted from [5].

Silicon nitride films can be etched by hydrofluoric acid (HF), buffered HF, and phosphoric acid (H₃PO₄) solutions. The rate of etching silicon nitride films depends on the exact stoichiometry of the films, the substrate temperature during deposition, and the density of the films.^1,2,5 The difference in etch rates based upon the difference in film preparation is shown in Table 2.

Table 2. Etch Rates of Silicon Nitride, adapted from [5].

The focus of this paper will be on the etching of silicon nitride with silicon dioxide used as a mask in hot phosphoric acid. It has been reported that silicon nitride is etched at a significantly higher rate than silicon dioxide when hot phosphoric acid (H₃PO₄) is used as an ethcant.² The results of the work done by van Gelder and Hauser² are presented in Table 3.

Table 3. Etch Rates of Si₃N₄ and SiO₂ in H₃PO₄ (Å/min),
adapted from [2].

From Equation 1 it can be seen that water is hydrolyzing the silicon nitride to form hydrous silica and ammonia, the ammonia remains in the solution in the form of ammonium phosphate. This stoichiometry suggests that water is an integral part of the chemistry involved in the etching of silicon nitride. It can also be seen from Equation 1 that as the nitride layer is etched hydrated silicon dioxide (H₂OSiO₂) is formed which in turn inhibits the etching of SiO₂ from the surface of the wafer. This is the result of the Le Châtelier's principle; that is, a system in equilibrium will react in such a way as to maintain equilibrium. Hence, silicon dioxide may initially be etched from the wafer surface, but as the SiO₂ concentration continues to increase over time in the etching solution the rate at which SiO₂ is etched from the wafer surface decreases.

Chemistry and Application

When using phosphoric acid as an etchant it is important to consider the effect that temperature has on acid concentration and, as a result, the etch rate and selectivity. The relationship between the boiling point temperature and phosphoric acid concentration is shown in Figure 1.

Figure 1. Boiling Point Temperature vs. Concentration, adapted from [6].

Within the semiconductor industry the typical operating temperatures for phosphoric acid range from 150 to 180C. Thus, at these boiling point temperatures the concentration of H₃PO₄ ranges from ca. 85 to 92 weight percent (wt%). The difficulty with using phosphoric acid at these elevated temperatures is that maximum concentration at which phosphoric acid is commercially available is 85 wt% which has a boiling point of ca. 154C. Since it not possible to heat a solution beyond its boiling point, it is necessary that some water be removed so that the solution becomes more concentrated thereby making it possible to heat the solution to higher temperatures. Operating at elevated temperatures is further complicated by the requirement that the acid concentration must be strictly maintained to regulate the temperature of the bath.

This requirement is the result of the Gibb's phase rule. The number of independent variables that must be arbitrarily fixed to establish the intensive state of a system, that is, the degrees of freedom of the system, is given by the Gibb's phase rule:

Where F = degrees freedom, = number of phases, and N = number of chemical species. Considering the water-phosphoric acid system for the two phases liquid and gas in equilibrium, one finds that there are two degrees of freedom. Thus, of the three variables pressure, temperature, and concentration one can only choose two; the third is fixed when the choice is made. Therefore, when operating at ambient pressure only one degree of freedom remains to be selected. Hence, for a given operating temperature the concentration is fixed. As the temperature is elevated water is evolved which in turn causes the acid concentration to increase; the temperature of the bath will increase to the boiling point of the resulting mixture.

Therefore, a constant temperature can be maintained by regulating the acid concentration. This can be accomplished by using a reflux system or by replacing the water at the same rate at which it is evolved. The former method is difficult to maintain in a production environment. Thus, the latter method is commonly used in the semiconductor industry. This method provides stable temperature and acid concentration; two important considerations for production.

It is of paramount importance that the temperature be controlled by the acid concentration and not by proportioning the heat source. If the temperature is controlled by proportioning the heat source water could be evolved, yet the temperature would not increase due to the proportioning of the heaters. This would result in an ever increasing acid concentration which would be detrimental to the consistency of the etch rate from one lot to the next. The effects of varying acid concentration on the etch rate of silicon nitride are illustrated in Figure 2.

Figure 2. Etch Rates of Silicon, Silicon Dioxide, and Silicon Nitride in Hot
Phosphoric Acid, adapted from [2]*.

*In the work conducted by van Gelder and Hauser the Si₃N₄ film was prepared by the use of a pyrolytic process with SiH₄ and NH₃ at 880C, the SiO₂ film was prepared by reacting SiCl₄ with O₂ and H₂ at 880C.

Here the solid line labeled Si₃N₄ represents the etch rate at the boiling point of the solution, whereas the dashed line represents the etch rate of concentrated phosphoric acid at a given temperature. The results presented in Figure 2 show that within the temperature range of 150 to 180C the silicon nitride etch rate of concentrated H₃PO₄ is lower than the etch rate of a more dilute acid concentration.. For example, a solution which boils at 160C (ca. 87 wt% H₃PO₄--see Figure 1) produces a nitride etch rate of ca. 55 Å/min, at the same temperature a 94.5 wt% phosphoric acid etches nitride at a rate of ca. 25 Å/min. This implies that for a given temperature a solution which has a higher water content will result in a higher nitride etch rate.

However, it is important to remember that while a more dilute acid concentration results in a higher nitride etch rate there is a limit as to how much water can be added at a given temperature. That is, since it is not possible to heat a solution above its boiling point one is limited to the boiling point concentration for that specific temperature. Thus, at 160C the maximum sustainable water concentration is 13 wt%, which corresponds to a H₃PO₄ concentration of 87 wt%. If the solution is diluted such that the H₃PO₄ concentration is lowered to 86 wt% the maximum temperature of the solution would be ca. 156C which would result in a nitride etch rate of ca. 50 Å/min. This etch rate is 9 % lower than the etch rate of the more concentrated 87 wt% solution at the higher temperature of 160C.

References

1.	Kern, W., and Schnable, G. L., in The Chemistry of the Semiconductor Industry, Moss, S. J., and Ledwith, A. (eds.), Blackie, New York (1987).
2.	van Gelder, W., and Hauser, V. E., "The Etching of Silicon Nitride in Phosphoric Acid with Silicon Dioxide as a Mask," J. Electrochem. Soc., 114 (8), 869 (1967).
3.	Loewenstein, L. M., Tipton, C. M., "Chemical Etching of Thermally Oxidized Silicon Nitride: Comparison of Wet and Dry Etching Methods," J. Electrochem. Soc., 138 (5), 1389 (1991).
4.	Gray, D. C., Butterbaugh, J. W., Hiatt, F. C., Lawing, A. S., and Sawin, H., "Photochemical Dry Stripping of Silicon Nitride Films," J. Electrochem. Soc. 142 (11), 3919 (1995).
5.	Morosanu, C. -E., "The Preparation, Characterization and Applications of Silicon Nitride Thin Films," Thin Solid Films, 65, 171 (1980).
6.	Brown, E. H., Whitt, C. D., "Vapor Pressure of Phosphoric Acids," Ind. Eng. Chem., 44, 615 (1952).