Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
2001-08-06
2003-05-13
Cuneo, Kamand (Department: 2829)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S046000, C438S604000, C438S758000
Reexamination Certificate
active
06562644
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a nitride semiconductor substrate to be used as a substrate for a blue light-emitting diode or a blue semiconductor laser device or the like, a method of manufacturing the semiconductor substrate, a semiconductor device employing the nitride semiconductor substrate, and a pattern forming method for the manufacture of the semiconductor device.
Conventionally, semiconductor devices such as a blue light-emitting diode (blue LED) or a blue semiconductor laser device employing a group III nitride such as GaN (gallium nitride), InN (indium nitride), AlN (aluminum nitride), or their mixed crystals, have been in most cases formed on a sapphire substrate.
In the manufacturing process of the semiconductor devices employing the nitride semiconductor, particularly in the manufacturing process of semiconductor laser devices, registration errors on the order of 1 &mgr;m do not pose any practical problems. Accordingly, a sufficient registration accuracy can be ensured by using inexpensive exposure apparatus (costing about ten thousand yen per unit) using g-line (wavelength 436 nm) or i-line (wavelength 365 nm) of a mercury lamp, instead of the expensive KrF steppers (costing several billion yen per unit), which are used in the photolithography process for Si (silicon).
However, there was a problem that with an increasing use of a nitride semiconductor substrate as a substrate for the semiconductor device, the accuracy of the resist pattern (hereinafter referred to as a pattern accuracy) drops in the photolithography step during the formation of the semiconductor device, particularly when the pattern is formed by the exposure apparatus using the g- or i-line of the mercury lamp, thereby significantly lowering the yield of the semiconductor device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to improve the pattern accuracy in the photolithography step during the manufacture of the semiconductor device using the nitride semiconductor substrate.
To achieve this object, the present inventors analyzed the cause of deterioration in the pattern accuracy during the pattern formation by the g- or i-line when the conventional nitride semiconductor substrate is used. The analysis revealed the following facts.
FIG. 23
illustrates the exposure of the resist film formed on a conventional nitride semiconductor substrate, specifically a substrate made from GaN (hereinafter referred to as a GaN substrate).
As shown in
FIG. 23
, a resist film
2
on a GaN substrate
1
is irradiated with an exposure light beam
4
such as the i-line through a photomask
3
with an opening
3
a
. The wavelength of light that can be absorbed by the nitride semiconductor is short, such as no more than 360 nm in the case of GaN. Accordingly, when the g- or i-line is used as the exposure light beam
4
, the exposure light beam
4
that is incident on the surface of the GaN substrate
1
through the resist film
2
, i.e., an incident light beam
4
, propagates through the GaN substrate
1
without being absorbed. As a result, the incident light beam
4
splits into an emitted light
5
emitted from the back surface of the GaN substrate
1
and a reflected light
6
due to the reflection by the back surface of the GaN substrate
1
. When the back surface of the GaN substrate
1
is specular, its reflectance with respect to the incident light beam
4
, i.e., the reflectance of the interface between the GaN substrate
1
and air with respect to the incident light beam
4
, is as much as about 20%.
A region
2
a
of the resist film
2
is the region to be exposed by the incident light beam
4
. However, as the resist film
2
a
is exposed from below by the reflected light beam
6
, a region
2
b
of the resist film
2
which is not to be exposed is also exposed. This results in defects such as a peeling of the resist film
2
or a reduction in the resist pattern size, thereby preventing a correct pattern formation in the case of using the conventional nitride semiconductor substrate.
It was also found that the problem of deterioration in the pattern accuracy is pronounced when the intensity of the reflected light beam
6
is increased by a reduction in the thickness of the GaN substrate
1
which makes it easier for the incident light beam
4
to pass through the GaN substrate
1
, or when the opening width of the opening
3
a
of the photomask
3
is not more than several times the wavelength of the incident light beam
4
or exposure light beam, in which case the incident light beam
4
, after passing through the opening
3
a
, is diffracted towards the outside of the opening
3
a
, with the reflected light beam
6
being extended further outside (see FIG.
23
).
It should be noted that in the present specification, the term “reflection” means specular reflection (angle of incidence=angle of reflection), and the term “reflectance” means specular reflectance. Reflections other than the specular reflection are referred to as “diffuse reflections”. The term “substrate surface” refers to the surface on which a nitride semiconductor layer is grown during the manufacture of the semiconductor device using the nitride semiconductor substrate.
Based on the above-mentioned findings, the present invention provides a first semiconductor substrate comprising a semiconductor layer having a group III nitride as a main component, wherein a scattering portion for scattering an incident beam of light entering the semiconductor layer through one plane thereof is provided on another plane or inside of the semiconductor layer.
In the first semiconductor substrate of the invention, the scattering portion for scattering the incident beam of light entering the semiconductor layer from one plane thereof is provided at another plane or inside the semiconductor layer, the semiconductor layer forming the substrate and having a group III nitride as the main component. Accordingly, the intensity of the reflected beam of light created by the reflection of the incident light beam by the another plane can be reduced. This prevents the problem of exposing a region of the resist film that is not to be exposed by the exposure light beam entering through the one plane (hereinafter sometimes referred to as a substrate surface) and reflected by the another surface (hereinafter sometimes referred to as a substrate back surface), in a photolithography step for the manufacture of a semiconductor device using the first semiconductor substrate, i.e., a nitride semiconductor substrate. Thus the pattern accuracy in the photolithography step can be increased and therefore the manufacturing yield of the nitride semiconductor device can be improved. For example, if the first semiconductor substrate is a GaN substrate, particularly the reflection of the g- or i-line of the mercury lamp can be surely prevented, so that the pattern accuracy in the photolithography step using the g- or i-line as the exposure light beam can be significantly improved, with a resultant significant improvement in the manufacturing yield of the nitride semiconductor device.
In the first semiconductor substrate of the invention, the scattering portion preferably may comprise height irregularity formed on the another plane of the semiconductor layer, the height irregularity having a height difference of {fraction (1/10)} or more of the wavelength of the incident beam of light.
This makes the incident beam of light efficiently diffused, i.e., scattered, on the another plane, thereby reducing the reflectance of the another plane against the incident beam of light and thus surely reducing the intensity of the reflected light.
In another embodiment of the invention, the reflectance of the another plane of the semiconductor layer against the incident beam of light is preferably 13% or less, and the wavelength of the incident beam of light is preferably 365 nm (i-line) or 436 nm (g-line).
In yet another embodiment of the invention, the scattering portion is preferably provided inside the semiconductor layer and includes
Ishida Masahiro
Mannoh Masaya
Ogawa Masahiro
Yuri Masaaki
Cuneo Kamand
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Sarkar Asok Kumar
Studebaker Donald R.
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