Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1998-06-12
2002-02-19
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C438S906000, C134S001300, C134S003000
Reexamination Certificate
active
06348157
ABSTRACT:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF RELATED ART
1. Technical Field
The present invention relates to a cleaning method, and more particularly relates to a cleaning method which is capable of an ultrahigh level of cleaning using far fewer processes than conventional methods and without conducting heating operations.
2. Background Art
The semiconductor devices which are formed on semiconductor substrates have become more highly integrated and smaller, reaching the submicron level. In order to achieve this increase in integration, it is necessary to maintain the surface of the substrate in an ultraclean state. In other words, it is necessary to remove organic materials, metals, various types of particles and oxides (oxide films) from the surface of the substrate. It is for this reason that cleaning of the substrate is conducted.
In order to obtain a clean substrate surface, cleaning methods have been developed, all processes of which take place at room temperature, and in which chemical vapors are not generated.
In other words, a cleaning method is known (Japanese Patent Application, First Publication No. HEI 8-306655), which comprises a first process, in which cleaning is conducted by means of pure water containing ozone, a second process, in which, while applying vibrations having a frequency of 500 kHz or more, cleaning is conducted using a cleaning liquid containing HF, H
2
O
2
and/or O
3
, H
2
O, and surfactant, a third process, in which cleaning is conducted by means of pure water, and a fourth process in which oxide films are removed.
In this method, the cleaning liquid containing HF, H
2
O
2
and/or O
3
, H
2
O, and surfactant, which is employed in the second process, is a cleaning liquid which has very good cleaning properties, and the reactivity thereof is extremely high. This high degree of reactivity is extremely effective with respect to the substrate to be cleaned; however, because it is necessary to provide the members comprising the device with resistance to this highly active cleaning liquid, it is necessary to use materials for the device which have corrosion resistance. For this reason, a carbon layer is formed on top of a nickel fluoride layer in, for examples the inner surfaces of the cleaning liquid storage unit such as a metallic vessel (Japanese Patent Application, First Publication No. HEI 8-306655).
Furthermore, chemicals having good removal effects with respect to organic materials, metals, various types of particles, and oxides (oxide films) are employed as the solutions used in the cleaning of the semiconductor. However, in the cleaning process using pure water or ultrapure water which serves to conduct rinsing after cleaning with chemicals, there are no removal effects with respect to the organic materials, metals, various types of particles, or oxides (oxide films). On the contrary, there are reports of formation of oxide films as a result of the oxygen which is dissolved in the pure water or ultrapure water. Such growth of oxide films interferes with epitaxial growth after the oxide film removal process using, as an example, hydrofluoric acid. As a method for solving such problems, pure water or ultrapure water, termed deoxygenated water (deaerated water), in which the oxygen content in the pure water or ultrapure water is reduced to a level of a few ppb, is employed. Furthermore, it is known that the n
+
silicon surface (indicating n type silicon in which the doping level is in excess of 1×10
19
/cm
3
) experiences rapid oxidation. n
+
silicon is extremely important as a contact material for the formation of metallic electrodes in silicon type elements. It is desirable that the n
+
silicon surface remain unoxidized to the greatest extent possible, so that the contact resistance between the metal and the semiconductor does not increase. However, it is extremely difficult to suppress the growth of oxide films on n
+
silicon surfaces simply by employing deaerated water.
In the wet cleaning process of the silicon substrate, the surface of the substrate has no oxide film thereon after cleaning using a cleaning liquid containing hydrofluoric acid. However, substrates in such a state are especially subject to the deposition of various types of particles. In particular, in cases in which there is only a rinse process using pure water or ultrapure water after cleaning using a cleaning liquid containing hydrofluoric acid, the pure water or ultrapure water themselves do not have the effect of removing particles, so that this leads to defects in crystallization arising from such particles in later processes, such as film formation processes and the like.
After cleaning the silicon substrate using chemicals containing hydrofluoric acid in the wet cleaning process, the silicon atoms at the outermost surface of the substrate bond with hydrogen atoms, so that the outermost surface of the substrate adopts a structure in which termination with hydrogen atoms is observed. This hydrogen-terminated silicon surface is known to be extremely chemically stable. However, since not all silicon atoms are bonded to hydrogen atoms, the presence of silicon atoms in an unbonded state at the surface and the presence of silicon atoms which are bonded with fluorine molecules, has been observed. Such silicon atoms are chemically extremely unstable and are likely oxidation sites.
OBJECT AND SUMMARY OF THE INVENTION
The present invention has as an object thereof to provide a cleaning method, which is capable of room temperature processing without conducting heating, uses little chemicals and water, and does not require the use of special devices or materials.
The present invention also has as an object thereof to provide a cleaning method which, in the chemical cleaning process or the rinse process employing pure water or ultrapure water in the semiconductor wet cleaning process, restricts the formation of surface oxide films, removes, and prevents the deposition of, particles, and aids in the hydrogen termination of silicon atoms.
The cleaning method of the present invention comprises: a first process, in which cleaning is conducted using pure water containing ozone, a second process, in which, while applying vibration having a frequency within a range of 500 kHz-3 MHz, cleaning is conducted using a cleaning liquid containing HF, H
2
O, and surfactant, a third process, in which cleaning is conducted using pure water containing ozone, a fourth process, in which oxide films are removed, and a fifth process, in which cleaning is conducted using pure water.
In the cleaning method of the present invention, after cleaning the target material with chemicals, rinsing is conducted using pure water or ultrapure water containing hydrogen gas in an amount of 0.5 ppm or more, and containing dissolved oxygen gas in an amount of 100 ppb or less.
In the present invention, the first cleaning which is conducted employs pure water containing ozone (first process). In this first process, the majority of the metals and organic materials are removed. By conducting this first process, it is possible to minimize variations in the surface roughness after all cleaning processes.
After the first process, it is possible to begin the cleaning of the second process employing chemicals containing HF, H
2
O, and surfactant, without conducting ultrapure water cleaning. In other words, it is possible to omit one ultrapure water cleaning process ozone-containing ultrapure water remains on the substrate surface after the first process, but if the substrate proceeds to the second process with this water remaining on the surface thereof, there are no undesirable effects. It is preferable for the ozone concentration at this time to be within a range of 2 ppm-10 ppm. At amounts of less than 2 ppm, the removal of metals is insufficient, while when the amount is in excess of 10 ppm, the roughness of the substrate surface increases dramatically.
The second process involves cleaning using chemicals containing HF, H
2
O and surfactant, while applying vibration of 500 kHz or more; partic
II Toshihiro
Kawada Kazuhiko
Nakamori Mitsunori
Nitta Takahisa
Ohmi Tadahiro
Knuth Randall J.
Lawrence Frank M.
Ohmi Tadahiro
Simmons David A.
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