Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Forming nonplanar surface
Reexamination Certificate
2000-07-12
2002-10-29
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Forming nonplanar surface
C430S330000
Reexamination Certificate
active
06472127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming a photoresist pattern and, more particularly, to such method which uses a chemically amplified photoresist preferably applied to a method of manufacturing a semiconductor device such as a DRAM.
2. Description of the Prior Art
In the manufacture of semiconductor devices represented by LSIs (Large-Scale Integrated circuits), a photolithography technique is indispensable to patterning various types of thin films including an insulating film, e.g., a silicon oxide film or silicon nitride film formed on a semiconductor substrate, and a conductive film such as an aluminum alloy film or copper alloy film, into a desired shape.
In the photolithograpy technique, conventionally, a photoresist photosensitive to ultraviolet radiation is applied to a thin film to form a photoresist film, and after that ultraviolet radiation is irradiated (exposed) to the photoresist film through a mask pattern to convert a region irradiated with ultraviolet radiation to a solubilized (positive) region or to convert a region not irradiated with ultraviolet radiation to a solubilized (negative) region. Subsequently, the photoresist film is developed, and the solubilized region is partly removed with a solvent to form a resist pattern. Then, the thin film is selectively etched using the resist pattern as a mask to pattern the thin film.
As the material of the photoresist described above, a positive novolac-based photoresist is generally, conventionally used. Since the positive photoresist has a higher resolution than that of a negative photoresist, most of photoresists of the type described above are of a positive type. As an exposure light source for the photoresist, a high pressure mercury lamp is used, and ultraviolet radiation, e.g., a g line (with a wavelength of substantially 436 nm) and an i line (with a wavelength of substantially 365 nm), generated by the high pressure mercury lamp is utilized.
As the integration degree of LSIs increases, a photolithography technique capable of a finer process is required, and accordingly the exposure light source for the photoresist tends to use ultraviolet radiation with a shorter wavelength with which a high resolution can be obtained. As a result, a photolithography technique using an excimer laser which generates far-ultraviolet radiation with a shorter wavelength than that of the i line described above as the light source (for example, when KrF (krypton fluoride) is used as the laser medium, the wavelength is substantially 248 nm) has been realized.
When the novolac-based photoresist described above is exposed by the KrF excimer laser light source described above, as the novolac-based photoresist absorbs a large quantity of light, a good resist pattern is difficult to obtain. Hence, as a photoresist which can realize a photolithograpy technique capable of a finer process in combination with a light source that can obtain far-ultraviolet radiation as described above, for example, a chemically amplified photoresist as described in Japanese Examined Patent Publication No. 2-27660 has been proposed.
A chemically amplified photoresist is a photoresist to which acid catalyst reaction is applied, as described in the above reference, and is roughly comprised of a base resin, e.g., polyhydroxystyrene (PHS), which becomes insoluble to alkali when protection groups are coupled to its predetermined portion and soluble to alkali when protection groups are free from its predetermined portion, an optical acid generating agent which generates hydrogen ions (acid) upon irradiation with light, a very small amount of additive for performance adjustment, and an organic solvent for spinner coating.
This chemically amplified photoresist is applied to a semiconductor substrate, dried, and solidified, and far-ultraviolet radiation emitted from an excimer laser as a light source irradiates a photoresist film on the obtained semiconductor substrate. Then, the optical acid generating agent generates hydrogen ions serving as the trigger species of chemical amplification. The hydrogen ions substitute the protection groups coupled to the base resin during a post exposure bake (PEB) process performed after exposure, so that the protection groups are eliminated. The photoresist which is insoluble to alkali is thus changed to be soluble to alkali. Also, since hydrogen ions are generated subsidiarily during this process, a chain reaction for eliminating the protection groups from the base resin progresses. This reaction is called an acid catalyst sensitization reaction. This acid catalyst sensitization reaction increases the solubility selectivity of the photoresist, so that highly photosensitive characteristics can be realized. Therefore, after exposure, if this photoresist is developed with an alkali developer, a desired very fine resist pattern can be obtained.
FIG. 1
is a process view showing a conventional method of forming a photoresist pattern using the chemically amplified photoresist described above in the order of steps. This photoresist pattern forming method will be described in the order of steps with reference to FIG.
1
.
As shown in step A of
FIG. 1
, the surface of a semiconductor substrate having a desired thin film, where a photoresist pattern is to be formed, is subjected to a hydrophobic process, so that the adhesion of the photoresist is increased. As shown in step B, for example, a positive photoresist made of a chemically sensitizable type photoresist, containing a base resin formed of polymer compounds having a terpolymer structure as shown in formula (1) and an optical acid generating agent with a structure as shown in formula (2), is applied to the semiconductor substrate in accordance with spin coating, thereby forming a photoresist film.
Subsequently, as shown in step C of
FIG. 1
, the photoresist film is prebaked to remove the solvent from it. The semiconductor substrate is cooled to room temperature, as shown in step D, and the photoresist film is irradiated with far-ultraviolet radiation from, e.g., a KrF excimer laser, through a mask pattern drawn with a desired pattern, to expose it, as shown in step E. The photoresist film is then subjected to a PEB (Post Exposure Bake) process to promote elimination reaction (acid catalyst sensitization reaction) of eliminating the protection groups from the photoresist film, as shown in step F.
The semiconductor substrate is then cooled again to room temperature, as shown in step G of
FIG. 1
, and the photoresist film is developed with an alkali developer to form a resist pattern, as shown in step H. The photoresist film that forms the resist pattern is post-baked to remove the water content produced by development, as shown in step I.
Subsequently, the thin film on the semiconductor substrate is selectively etched by using the resist pattern described above as a mask to pattern the thin film.
When a photoresist pattern is to be formed by using a chemically amplified photoresist, in recent semiconductor devices, a focal depth S must be set to about 0.7 &mgr;m or more so that the manufacturing yield is increased. The focal depth S depends on a blocking level C of the employed photoresist. The blocking level C is determined by a ratio of numbers x, y, and z of repeating units in the formula (1) described above, and is expressed as:
Blocking level
C
=((
x+y
)/(
x+y+z
))×100(%)
FIG. 4
is a graph for explaining the relationship between the focal depth S (axis of ordinate) and the blocking level C (axis of abscissa). In the example shown in
FIG. 4
, a photoresist pattern having a contact hole with a diameter of 0.2 &mgr;m is formed. As is apparent from
FIG. 4
, the focal depth S increases linearly proportionally where the blocking level C falls within a range of about 42%, but tends to saturate gradually when the blocking level C exceeds about 42%. In order to obtain the focal depth S of 0.7 &mgr;m or more as described above, the blocking level C must be increased to about 40% or more.
In
Duda Kathleen
McGinn & Gibb PLLC
NEC Corporation
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