Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
1999-07-01
2002-03-19
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S778000
Reexamination Certificate
active
06358866
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a method of post-oxidation heating of a substrate comprising at least a SiO
2
layer, or a Si/SiO
2
or a Si/SiO
2
/poly-Si layer structure.
2. Description of the Related Art
The fabrication of insulated-gate field effect devices, like metal-oxide-semiconductor field-effect transistors (MOSFET), requires the preparation of a laterally uniform insulating silicon dioxide layer (SiO
2
) on the semiconductor (Si) substrate. In order to improve the electrical characteristics of this oxide layer in terms of the fixed charge density it is necessary to apply to the thermally grown oxide a post-oxidation heating cycle (see B. E. Deal, Journal of Electrochemical Society, Vol. 121, p. 198C, 1974). Next, during device structure fabrication on the semiconductor substrate, additional high-temperature processing heating steps are required. For example such heating steps are executed in order to diffuse-in or activate doping impurities in the semiconductor substrate or in a polycrystalline silicon (poly-Si) gate grown on the silicon oxide, in order to cure ion-implantation or electron-beam induced damage, in order to enhance the wafer bonding strength, and for other processing steps.
The post-oxidation heating steps are conventionally performed at temperatures above 900° C. in an inert, non-oxidizing ambient, such as argon as suggested in document U.S. Pat. No. 3,925,107. As known in the art, such heating treatments have a negative effect on the silicon dioxide quality. These negative effects include the disintegration of thin silicon dioxide layers, development of low-voltage dielectric breakdown, and generation of hole traps in the silicon dioxide. This degradation results in an immediate impairment of the oxide as gate insulator, or hampers the MOSFET reliability through trapping of holes generated in the oxide by radiation or through trapping of holes injected from the MOSFET channel due to hot-carrier effects. Various aspects of the degradation induced by the post-oxidation heating are broadly discussed in the literature (see e.g. G. W. Rubloff et al., Physical Review Letters, Vol. 58, p. 2379, 1987; W. L. Warren et al., Applied Physics Letters, Vol. 64, p. 3452, 1994, A. Stesmans et al., Physical Review B, Vol. 54, p. R11129, 1996). The degradation is typically explained as the interface-driven formation of volatile silicon monoxide molecules (SiO) and the oxygen depletion of the silicon dioxide. In order to chemically prevent this degradation, EP-A-0264774 suggests incorporation of oxygen-containing molecules into the ambient. This method, however, is found inapplicable to thin silicon dioxide layers (<10 nm-thick) as the oxygen pressure required to suppress degradation is found to be as high as 0.3 bar (cf. FIG. 4 in S. I. Raider, Microelectronic Engineering, Vol. 22, p. 29, 1993) which leads to undesirable additional oxidation.
Furthermore, the process heating steps performed after capping of the silicon dioxide layer with polycrystalline silicon or another material are further degrading the oxide quality. Various degradation aspects, including enhanced radiation sensitivity, interface defect density and 1/f noise in the Si/Silicon dioxide/poly-Si structures are known in the art (see e.g. J. R. Schwank et al., Applied Physics Letters, Vol. 53, 770, 1988; R. A. B. Devine et al., Journal of Applied Physics, Vol. 77, p. 175, 1995; V. V. Afanas'ev et al., Applied Physics Letters, Vol. 66, p. 1653, 1995). So far, the sole way to minimize the damage caused by thermal treatment of silicon dioxide layers was found to be the reduction of the temperature and duration of the thermal treatment. The incorporation of oxygen in the heating ambient is inapplicable for the buried silicon dioxides in a Si/SiO
2
/poly-Si structure due to the low diffusivity of the oxygen and its chemical reactivity.
In summary, the degradation of thin SiO
2
layers during high-temperature processing is known in Si Metal-Oxide-Semiconductor (MOS) device technology. Upon thermal treatment (heating) at temperatures T
an
>600° C. in non-oxidizing ambients, the initially superb insulator degrades in terms of characteristics such as the oxide integrity (low-voltage leaks), reduced dielectric breakdown strength, enhanced vulnerability to charging under hot-carrier injection or irradiation. These defects heavily impair the production yield of MOS devices, the device performance, and the reliability of the MOS devices. The origin of the thermal SiO
2
/Si degradation has been related to formation of volatile SiO molecules at the oxide/silicon interfaces. This formation is suppressed in the art by adding a small amount of an oxidant, e.g. O
2
, to the heating ambient. However, this method of chemical protection has a limited application as it causes additional oxidation undesirable in the case of thin gate oxides (d
ox
<10 nm), because the partial pressure of O
2
necessary to maintain the oxide integrity may be as high as 0.3 atm. Moreover, because of the low diffusivity of oxygen in silicon, this method cannot prevent the oxide degradation in the most widely used device structure: Si/SiO
2
/polycrystalline-Si.
SUMMARY OF THE INVENTION
The present invention aims to provide a method for a heating treatment which significantly reduces or eliminates, the electrical degradation of silicon oxide layers, for instance in Si/SiO
2
or in Si/SiO
2
/poly-Si layer structures.
The present invention aims to disclose a novel physical, rather than chemical, method of oxide protection based on performing the heat treatments in an ambient of an inert gas, preferably He. The method of the invention allows to significantly reduce the degradation of both ultrathin and polycrystalline-Si covered SiO
2
layers. Consequently, replacement of the currently used heating ambients with He is beneficial for the Si MOS device fabrication.
The present invention is related to a method for post-oxidation heating of at least one substrate comprising at least a SiO
2
layer or a SiO
2
/poly-Si layer structure, comprising the steps of: creating an inert gaseous ambient in a furnace, said ambient having a partial pressure within a predetermined range and said gaseous ambient comprising molecules having a suitable diameter for penetrating into the SiO
2
and/or poly-Si material; placing the substrate into said ambient; thereafter heating said furnace to a temperature of at least 200° C. for a predetermined period of time; cooling said furnace while maintaining said gaseous ambient in said predetermined pressure range in said furnace.
The method can further comprise the step of removing said substrate from said furnace after the step of cooling down said furnace.
The heating temperature can be comprised between 500 and 1300° C., and is preferably in-between about 550-600° C. and about 950-1200° C. Even more preferably the heating temperature is in-between about 750-800° C. and about 900-950° C.
It is an aspect of the present invention that, following the steps of the method of the invention, the degradation of both ultrathin and polycrystalline-Si covered SiO
2
layers is suppressed in the temperature range of 600-800° C. and that the degradation of both ultrathin and polycrystalline-Si covered SiO
2
layers is reduced in the temperature range of 800-950° C.
The predetermined period of time can be smaller than 1000 hours, and is preferably larger than I second and smaller than 10 minutes.
The partial pressure of said ambient is comprised between 0.05 atm and 100 atm, preferably in-between about 0.1 atm and about 5-15 atm and most preferably of about 1 atm.
The inert gaseous ambient does comprise He molecules, the He-content being larger than 99%.
In general, the efficiency of the protective action of the ambient (helium) increases with increasing ambient (helium) pressure during thermal treatment.
The substrate used in the present invention can be any suitable substrate including, but not limited to, glass, quartz, sapphire, silicon, amorphous or polycrystalline s
Afanas'ev Valery V.
Stesmans André
Elms Richard
IMEC vzw
Knobbe Martens Olson and Bear LLP
Owens Beth E.
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