Organic films and a process for producing fine pattern using...

Details Organic films and a process for producing fine pattern using... Organic films and a process for producing fine pattern using...

1998-09-30

2001-05-01

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S780000

Reexamination Certificate

active

06225239

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for manufacturing a monomolecular film and an organic built-up film that are two-dimensionally regularly arranged on both an oxidized and a bare III-V-group compound semiconductor substrate, and to a method for producing a fine pattern using the organic films.
DESCRIPTION OF THE PRIOR ART
Langmuir Blodgett films (hereafter referred to as “LB films”) and self-assembled monolayers (hereafter referred to as “SA films”) are the well known types of organic monomolecular films or organic built-up films (Abraham Ulman, “An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly,” Academic Press 1991). Of the two, the LB film is formed by developing amphiphilic molecules having both hydrophilic and hydrophobic atomic groups, on the water surface as a monomolecular film (an “L film”), transferring the film to a solid substrate, and accumulating a number of films thereon. The LB film is named after Langmuir and Blodgett, who developed the process. Since this process uses the difference in hydrophobic and hydrophilic properties between the film and the substrate to transfer the film spread on the water surface to the substrate, the crystallinity of the film is determined when it is spread and compressed. Thus, the crystallinity of the film does not depend on the type of substrate used, and the film can be formed on any substrate. Due to the nature of the LB film, however, the interaction between the substrate and the monomolecular film is very weak and the acid and alkali resistance and durability of the film are insufficient for the construction of complex devices. In addition, to produce the LB film, an exclusive apparatus is required for developing amphiphilic molecules on the surface of a solution, compressing the molecules on the surface, and transferring them to the substrate.
The SA film is formed when functional groups at the terminals of molecules selectively chemically adsorb atoms constituting a substrate. Thus, due to the nature of the adsorption mechanism, this film is formed in such a way that only a monomolecular film is self-assembled. Since the film can be formed by simply immersing the substrate in a solvent containing the molecules, this process does not require a large complicated apparatus, and it is also essentially possible to form a built-up film by selecting the type of the terminal group outside the formed SA film. These molecular films form a two-dimensional molecular set due to the Van der Waals force, and this process can be used to create an array in which molecules are regularly packed, that is, a two-dimensional crystal. This feature can be used to construct various electronic and optical devices.
The SA film does not have the disadvantages of the LB film, but due to the use of chemical adsorption between the functional groups and the substrate, limited combinations of functional groups and substrates can be formed. The monomolecular films are conventionally formed on a substrate of silicon oxide, aluminum oxide, silver oxide, mica, gold, copper, or GaAs. In the case of a GaAs substrate, an SA film can be obtained only by subjecting the substrate to hydrochloric processing in such a way that it is As-terminated, applying a molten liquid of organic molecules containing SH groups to the surface of the substrate in a nitrogen atmosphere, and allowing the substrate to stand at 100° C. for five hours. Thus, although the SA film has a larger number of advantages than the LB film, there are very few examples of III-V-group compound semiconductor SA films or their multilayer films.
In addition, techniques for forming a multilayer film that inherits the large number of advantages of the SA film are very important because they can provide characteristics that cannot be obtained through creation of more complex monomolecular films, and because the multilayer is expected to amplify the film's functions.
That is, the SA film has several preferable characteristics. For example, SA systems are thermodinamically stable because of its film formation process. Thus, systems can be obtained that allow a highly defectless and uniform SA film to be self-assembled. Moreover, a damaged SA film can be self-repaired when returned to an appropriate environment. Another advantage of the SA film is that due to easy preparation the film can be formed inexpensively.
Although the SA molecules can be bound to the surface of the above substrate, binding to a host substance depends on the chemical composition of the SA molecule. The SA molecule can be divided into two groups: a head and an end group. An example of an SA molecule is CH
3
(CH
2
)
17
SH, and in this case, S—H is a head group and CH
3
(CH
2
)
17
is an end group with respect to, for example, a GaAs substrate.
The head group has a strong affinity for particular substances, so it is immediately bound to a host substance during chemical adsorption. The head group is chemically bound to the substrate, but the adsorbed molecules probably still have a certain level of mobility. The end group is divided into two subgroups: an alkyl and a tail group. The alkyl group or an alkyl derivative group constitutes a main part of the SA molecule. The length of this molecular main part can be adjusted easily by varying the number of units in an alkyl chain (C
n
H
2n+1
). The tail group is used to terminate the end of the molecule. A methyl group (CH
3
) is often used for a simple SA molecule. The same SA molecule, however, provides different head and end groups depending on the host substance.
The SA film is expected to have applications to a passivation layer, a resist material for lithography, or a functional element in a future electronic device. An example technique for applying SA film to an electronic device is the use of SA film as a photo resist, as described in Jpn. J. Appl. Phys. Vol. 32 (1993) pp. 5829 to 5839. The SA film process enables a defect free film as thin as a monomolecular film to be produced, in order to reduce the wavelength while improving the focal depth, making this film effective as a photo resist because the patterns used for electronic devices are expected to become finer in the future.
Despite the several advantages of the SA film and its expected applications to various fields including electronic devices, application techniques have not yet been sufficiently developed. For example, the application of the SA film to photo resist relies on the conventional exposure process, so finer patterns mostly contribute to an exposure apparatus; and reduction of the pattern's line width is limited. On the other hand, as the integration of electronic devices is improved, the industry desires to establish techniques for creating finer patterns, including existing lithography techniques. In parallel with the development of devices having fine patterns, electronic physical properties of fine lines on the nanometer scale, for example, 1 to 20 nm must be clarified before such devices can be designed. Such thin lines involve notable quantum-mechanic phenomena and their physical properties cannot be directly assumed based on the physical properties of thicker lines.
Thus, a technique for producing lines on the nanometer scale is required, but existing techniques cannot produce such lines.
SUMMARY OF THE INVENTION
It is an object of this invention to enable an excellent and firm organic monomolecular film with few defects to be produced by allowing film-producing molecules to be selectively chemically adsorbed onto an oxidized III-V-group compound semiconductor substrate, in contrast to the conventional techniques.
It is another object of this invention to provide a process for using chemical adsorption to further accumulate different types of organic molecules on the above monomolecular film, in order to control the film thickness in terms of the molecular length, thereby enabling the fabrication of an excellent organic film that is flat and dense at an atomic level and that has no pin holes.
It is yet another object of this inve

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