Method for forming contact holes of a semiconductor device

A method for forming contact holes, capable of achieving an increased tolerance in design rule for formation of contact holes by: forming an insulating film over a semiconductor substrate; coating a positive photoresist film over the insulating film; primarily exposing the photoresist film to a light using a first exposure mask having windows adapted to allow portions of the insulating film corresponding to a part of contact holes to be exposed to the light, the part of contact holes having contact holes arranged diagonally to each other; secondarily exposing the photoresist film to the light using a second exposure mask having windows arranged diagonally to each other and not overlapped with those of the first exposure mask; removing the light-exposed portions of the photoresist film to form a photoresist film pattern for exposing portions of the insulating film respectively corresponding to the contact holes; and forming the contact holes using the photoresist film pattern as a mask. Since an approximation effect of contact holes is reduced by virtue of the increase tolerance in design rule for formation of contact holes, it is possible to obtain a more reduced space between adjacent contacts and thereby to form contact holes having a more increased dimension. Accordingly, the process yield can be increased.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for forming contact holes of a 
semiconductor device, and more particularly to a method for forming 
contact holes of a semiconductor device, capable of achieving the 
formation of contact holes having a critical dimension and yet exhibiting 
an increased tolerance in design rule. 
2. Description of the Prior Art 
The recent tendency in fabricating highly integrated semiconductor devices 
is greatly affected by the development of techniques for forming patterns 
having a critical dimension. Photoresist film patterns formed by a 
photolithography process are widely used as masks for carrying out an 
etching process or an ion implantation process in the fabrication of 
semiconductor devices. In this regard, it is required to achieve a 
patterning of photoresist film patterns having a critical dimension, a 
stable performance of process steps, a clean removal of the photoresist 
film patterns after completion of the process steps, and an easy 
re-working for forming a new photoresist film pattern after the removal of 
an inaccurately formed photoresist film pattern. 
Generally, a photoresist film pattern is formed by uniformly coating a 
photoresist solution consisting of a photoresist agent and a resin solved 
in a solvent in certain amounts over a semiconductor substrate by use of a 
spin coating process to form a photoresist film, and then selectively 
irradiating a light onto the photoresist film through a light exposure 
mask provided with light shield film patterns made of chromium by use of a 
step and repeat exposure device (hereinafter referred to as "stepper") 
adapted to repeatedly carry out an alignment and a light exposure, thereby 
exposing to the light a portion of the photoresist film except for its 
portion to form the photoresist film pattern. In this case, the light 
shield film patterns of the light exposure mask have a dimension larger 
than that corresponding to the resolution of the stepper. 
Thereafter, the light-exposed portion of the photoresist film is removed 
using a weak alkali development solution containing tetra methyl ammonium 
hydroxide as its base component. 
However, this general technique has difficulty in forming a fine pattern 
having a critical dimension not larger than a predetermined dimension, for 
example, a pattern having a pattern space not more than 0.4 .mu.m due to 
various limitations on the accuracy of the light exposure device and 
wavelength of light. 
In other words, the resolution R of the stepper used to fabricate 
conventional semiconductor devices is in reverse proportion to the 
numerical aperture of the light exposure device and in proportion to the 
wavelength of a light source. As a result, the resolution is limited 
because there are limitations on reducing the wavelength of light and 
increasing the numerical aperture. 
For example, G-line, i-line and excimez laser steppers having wavelengths 
of 436, 365 and 248 nm have resolutions only capable of forming patterns 
of about 0.7, 0.5 and 0.3 .mu.m, respectively. 
As a semiconductor device has a higher integration degree, its cell size is 
reduced. This also results in a limited design rule for each thin film. In 
the case of forming contact holes, the design rule is limited due to the 
reduced unit cell size and the degraded process capability. Such a limited 
design rule results in reductions in various design margins such as the 
contact size, the space between adjacent contacts and the overlap of each 
contact with a lower layer disposed beneath the contact. 
Where contact holes are defined using a positive photoresist film for 
forming the contact holes having a critical dimension of a semiconductor 
device, an exposure mask 10 shown in FIG. 1 is used. 
As shown in FIG. 1, the exposure mask 10 has a light shield film 12 formed 
over a quartz substrate (not shown). The exposure mask 10 also has windows 
14 each formed by removing a portion of the light shield film 12 
corresponding to each contact hole to be formed. 
Since each contact hole has a reduced dimension due to a high integration 
of a semiconductor device, the exposure mask 10 has a reduced dimension a 
of each window 14. In this case, the space b between adjacent windows 14 
corresponding to the space between adjacent contact holes is also reduced. 
As the dimension a approximates to an optical limitation by the light 
exposure device, light beams passing through the exposure mask 10 
interfere with one another if the value of b/a is less than 1. In this 
case, an interference light is generated between adjacent regions where 
contact holes are defined, as shown in FIG. 2. Due to this interference 
light, a photoresist film pattern for forming the contact holes is damaged 
or inaccurately formed. In a severe case, no contact hole is formed. 
Consequently, the process yield is degraded. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to eliminate the 
above-mentioned problems encountered in the prior art and, thus to provide 
a method for forming contact holes, capable of achieving an increased 
design rule and thereby forming contact holes having a critical dimension 
by coating a positive photoresist film over a semiconductor structure to 
be formed with a plurality of contact holes, successively exposing the 
photoresist film to a light using two exposure masks each having windows 
arranged diagonally to each other and arranged orthogonally to those of 
the other mask, thereby forming a photoresist film pattern for exposing 
portions of an insulating film on the semiconductor structure respectively 
corresponding to the contact holes. 
In accordance with the present invention, this object can be accomplished 
by providing a method for forming a plurality of contact holes in a 
semiconductor device, comprising the steps of: 
forming an insulating film over a semiconductor substrate; 
coating a photoresist film over the insulating film; 
primarily exposing the photoresist film to a light using a first exposure 
mask having windows arranged such that portions of the insulating film 
respectively corresponding to those of the contact holes arranged 
diagonally to each other are exposed to the light; 
secondarily exposing the photoresist film to the light using a second 
exposure mask having windows arranged such that portions of the contact 
holes respectively corresponding to the remaining ones of the contact 
holes are arranged diagonally to each other; forming a photoresist film 
pattern for exposing the light-exposed portions of the insulating film 
respectively corresponding to the contact holes; and removing the 
light-exposed portions of the insulating film, thereby forming the contact 
holes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 5A to 5C are sectional views respectively illustrating a method for 
forming contact holes in accordance with the present invention. In 
accordance with the present invention, the formation of contact holes is 
carried out using exposure masks respectively shown in FIGS. 3A and 3B. 
In accordance with the method of the present invention, first, an 
insulating film 32 comprised of an oxide film, a nitride film or a 
spin-on-glass film is formed over a semiconductor substrate 30, as shown 
in FIG. 5A. Over the insulating film 32, a positive photoresist film 34 is 
coated. Thereafter, the photoresist film 34 is subjected to a primary 
light exposure using a first exposure mask 20 shown in FIG. 3A, thereby 
forming two photoresist regions 36 arranged diagonally to each other. In 
order to form only two contact holes arranged diagonally to each other, 
the first exposure mask 20 has two windows 24 formed by corresponding 
portions of a light shield film 22 therefor. Accordingly, the first 
exposure mask 20 has an increased space d between adjacent windows 24, as 
compared to the spaces a and b in the case of FIG. 1 (FIG. 5A). 
Subsequently, the photoresist film 34 is subjected to a secondary light 
exposure using a second exposure mask 21 shown in FIG. 3B, thereby forming 
two photoresist regions 37 arranged diagonally to each other and arranged 
orthogonally to the photoresist regions 36 already formed. The second 
exposure mask 21 has two windows 25 formed by corresponding portions of a 
light shield film 23 thereof and arranged such that they are not 
overlapped with the windows 22 of the first exposure mask 20. Similar to 
the first exposure mask 20, the second exposure mask 21 has an increased 
space d between adjacent windows 25. Since the first and second exposure 
masks 20 and 21 have windows 24 and 25 having a sufficiently large space 
d, no interference occurs among light beams passing through the windows 24 
and 25. As a result, an accurate light exposure is achieved, as shown in 
FIG. 5B. 
Thereafter, portions of the photoresist film 34 respectively corresponding 
to the light-exposed photoresist regions 36 and 37 are removed, thereby 
forming a pattern of the photoresist film 34 for exposing portions of the 
insulating film 32 respectively corresponding to contact holes to be 
formed, as shown in FIG. 5C. As a result, contact holes 38 are formed. 
After the formation of contact holes 38, the photoresist film pattern is 
removed. Since the contact holes can be formed without any effect caused 
by the space between adjacent contact holes, it is possible to realize the 
design rule defining a contact hole space of not more than 0.3 .mu.m. 
Accordingly, the method of the present invention can be applied to the 
fabrication of dynamic random access memories of 256 Mega grade. 
Although the illustrated embodiment has been described as using the 
positive photoresist film to have a pattern corresponding to a 
non-exposure region, the present invention may be applied to the case 
using a negative photoresist film to have a pattern corresponding to an 
exposure region. In this case, the pattern having a critical dimension can 
be obtained by shifting patterns of the exposure masks. 
As apparent from the above description, the present invention provides a 
method for forming contact holes, capable of achieving an increased 
tolerance in design rule for formation of contact holes by: forming an 
insulating film over a semiconductor substrate; coating a positive 
photoresist film over the insulating film; primarily exposing the 
photoresist film to a light using a first exposure mask having windows 
adapted to allow portions of the insulating film corresponding to a part 
of contact holes to be exposed to the light, the part of contact holes 
having contact holes arranged diagonally to each other; secondarily 
exposing the photoresist film to the light using a second exposure mask 
having windows arranged diagonally to each other and not overlapped with 
those of the first exposure mask; removing the light-exposed portions of 
the photoresist film to form a photoresist film pattern for exposing 
portions of the insulating film respectively corresponding to the contact 
holes; and forming the contact holes using the photoresist film pattern as 
a mask. Since the approximation effect of contact holes is reduced by 
virtue of the increased tolerance in design rule for formation of contact 
holes, it is possible to obtain a more reduced space between adjacent 
contacts and thereby to form contact holes having a more increased 
dimension. Accordingly, the process yield can be increased. 
Although the preferred embodiments of the invention have been disclosed for 
illustrative purposes, those skilled in the art will appreciate that 
various modifications, additions and substitutions are possible, without 
departing from the scope and spirit of the invention as disclosed in the 
accompanying claims.