Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2001-06-07
2003-06-17
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S080000, C257S380000, C257S435000
Reexamination Certificate
active
06580095
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit-containing photodetector that includes a photodetector element and a circuit for processing a photoelectrically converted signal supplied from the photodetector element both formed on the same semiconductor substrate. The invention further relates to a method of manufacturing the circuit-containing photodetector and to an optical device using the circuit-containing photodetector.
2. Description of the Background Art
Circuit-containing photodetectors are extensively applied to various optical devices such as optical sensors, optical pickups and photocouplers.
FIG. 21
schematically shows a cross section of a conventional circuit-containing photodetector. It is noted that in the drawings attached hereto, the same reference character denotes the same or corresponding components. Referring to
FIG. 21
, on a p-type silicon substrate
1
having a specific resistance of 100 &OHgr;cm, for example, an n-type epitaxial silicon layer
2
of 5-8 &OHgr;cm in specific resistance and 2-3 &mgr;m in thickness is formed. A photodetector element
14
has a pn junction of p-type silicon substrate
1
and n-type epitaxial layer
2
. On a light-receiving surface of photodetector element
14
, an antireflection film
120
having a double layer structure of an oxide film
121
and a nitride film
122
is formed. In epitaxial layer
2
, the region having photodetector element
14
and the region having a signal processing circuit portion
15
are electrically isolated from each other by a diffused isolation layer
13
.
The signal processing circuit typically contains an npn transistor
15
, which includes an n-type buried diffusion layer
8
buried in silicon substrate
1
, n-type epitaxial layer (collector)
2
thereon, a p-type diffusion layer (base)
9
formed at the surface of the collector, an n-type diffusion layer (emitter)
6
formed at the surface of the base, and a collector compensation diffusion layer
11
extending from n-type buried diffusion layer
8
to the surface of epitaxial layer
2
. On epitaxial layer
2
, an interlayer insulating film
5
, a metal electrode interconnect
4
, a collector electrode
7
and an emitter electrode
10
are formed.
Recent years have seen increased density of optical recording, so that a shorter wavelength of signal light has been used for the optical recording. Accordingly, it is desired that the circuit-containing photodetector used for the optical recording should have a high sensitivity and a fast response to the short wavelength light. The conventional circuit-containing photodetector having the structure as shown in
FIG. 21
, however, can hardly achieve a higher sensitivity and a faster response for the reasons below.
If the signal light has a shorter wavelength, the absorption length of the light in the semiconductor becomes shorter. In silicon, for example, the absorption length is approximately 1 &mgr;m for the light having a wavelength of 0.5 &mgr;m, while it is 0.2 &mgr;m or less for a wavelength of 0.4 &mgr;m. In the semiconductor layer, the light absorbed into the depletion layer mainly contributes to the generation of photocurrent. The introduced light has to be absorbed in the vicinity of the pn junction to generate electron-hole pairs near the junction. Thus, when the signal light has a shorter wavelength, n-type epitaxial layer
2
is required to be thinner for efficient generation of the photocurrent. For example, if the light wavelength is 0.4 &mgr;m, epitaxial layer
2
should have a thickness of 1 &mgr;m or less. However, epitaxial layer
2
must be as thick as 2 to 3 &mgr;m for forming npn transistor
15
, so that photodetector element
14
has a low sensitivity to the employed short wavelength.
If a p-type impurity region having a junction depth of 1 &mgr;m or less is formed at the surface of n-type epitaxial layer
2
in photodetector element
14
, the photodetector element portion may have a high sensitivity to the short wavelength of the signal light. However, epitaxial layer
2
should have a low specific resistance (e.g. 5-8 &OHgr;cm) for forming transistor
15
, so that the photodetector element has a relatively high junction capacitance resulting in a slower response rate.
For the above reasons, the above conventional circuit-containing photodetector can hardly achieve both of a higher sensitivity and a faster response to a short wavelength light introduced into the photodetector element.
As a solution of such a problem, Japanese Patent Laying-Open No. 1-232774 discloses a method of producing a circuit-containing photodetector as mentioned below. In the method, as shown in
FIG. 20A
, an n-type epitaxial layer
2
having a specific resistance (5-8 &OHgr;cm) and a thickness suitable for a transistor is grown on an n-type silicon substrate
1
having a specific resistance (80-100 &OHgr;cm) and a thickness that are suitable for a photodetector element. At this time, an n-type high concentration buried layer
8
is also formed.
Referring to
FIG. 20B
, epitaxial layer
2
is partially etched away in the region that will provide a photodetector element, so that a trench is formed and silicon substrate
1
is partially exposed at the bottom of the trench.
Referring to
FIG. 20C
, an impurity region
3
is formed by diffusion at the bottom substrate surface of the trench, and simultaneously a base region
9
is formed by diffusion at the surface of epitaxial layer
2
in the region that will provide a transistor portion. Further, an emitter
6
is formed at the surface of base region
9
by diffusion.
In the circuit-containing photodetector as shown in
FIG. 20C
, photodetector element
14
can be formed to have both of a high sensitivity and a fast response to the short wavelength of signal light without the transistor characteristics degraded.
As mentioned above, epitaxial layer
2
should have a thickness of 2 to 3 &mgr;m for forming therein a signal processing circuit such as a transistor. Accordingly, the depth of the trench should also be 2 to 3 &mgr;m at photodetector element
14
formed in the circuit-containing photodetector as shown in FIG.
20
C.
In this case, photodetector element
14
formed at the bottom of the trench needs a metal electrode interconnect
4
that extends from an anode
3
to the upper surface of epitaxial layer
2
rising 2 to 3 &mgr;m out of the bottom of the trench.
Such a great rising in the trench, however, causes a problem as follows. When a pattern of metal electrode interconnect
4
is formed by means of photolithography, the light cannot uniformly be focused over the entire area of metal interconnect
4
. The resulting line of metal interconnect
4
may have a narrowed width which is likely to cause a defect such as breaking of the line.
SUMMARY OF THE INVENTION
The present invention reflects the above-described problems in the conventional technique. Thus, an object of the present invention is to provide a structure of a circuit-containing photodetector that can not only have a high sensitivity and a fast response to signal light of short wavelength but be manufactured in high yield, and to provide a method of manufacturing such a circuit-containing photodetector.
According to the present invention, a circuit-containing photodetector is provided that has a semiconductor substrate, a photodetector element formed on the semiconductor substrate, and a circuit for processing an electric signal from the photodetector element. The circuit-containing photodetector includes (a) a semiconductor layer which is grown on the semiconductor substrate and on which the circuit is formed, (b) a trench which is formed in the semiconductor layer and has a depth to reach the semiconductor substrate, (c) an impurity region formed at a surface of the semiconductor substrate exposed at the bottom of the trench, the impurity region constituting the photodetector element, and (d) a conductive impurity region extending from the impurity region of the photodetector element to an upper surface of the semiconductor layer
Fukushima Toshihiko
Hayashida Shigeki
Kakimoto Seizo
Morioka Tatsuya
Tani Yoshihiko
Nelms David
Nguyen Dao H.
Sharp Kabushiki Kaisha
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