Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2001-03-29
2002-12-10
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S292000, C257S446000
Reexamination Certificate
active
06492702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit-incorporating light receiving device including a light detection photodiode portion and a circuit element on the same substrate. More particularly, the present invention relates to a structure of the circuit-incorporating light receiving device for improving a performance of the light detection photodiode portion.
2. Description of the Related Art
Recently, optical disk apparatuses are becoming smaller and smaller while the performance thereof is becoming higher and higher. With such advances, there is an increasing need for a compact and lightweight optical pickup. To achieve such an optical pickup, a technology has been proposed in which: a function for generating a tracking beam, a function for branching light, and a function for generating an error signal are integrated onto a single hologram device; a laser diode, a split photodiode, and the like are accommodated in a single package; or the hologram device is provided on an upper surfaces of the package. Such a technology is called an optical module.
Among components included in an optical pickup apparatus is a circuit-incorporating light receiving device. In the circuit-incorporating light receiving device, circuit elements, such as a light detection photodiode portion for converting signal light to an electric signal (optoelectric transduced signal), a transistor for processing the optoelectric transduced signal, a resistor, and a capacitor, are integrated.
FIG. 5
is a cross-sectional view showing a structure of a conventional circuit-incorporating light receiving device
4000
.
The circuit-incorporating light receiving device
4000
includes a photodiode region
51
in which a light detection photodiode portion which converts signal light to an electric signal is provided, and a peripheral circuit region
52
which is used to process the optoelectric transduced signal. Specifically, in the peripheral circuit region
52
, an NPN transistor and a vertical PNP transistor are provided.
To reduce cost in fabrication of the circuit-incorporating light receiving device
4000
, the commonality of fabrication processes is increased. For both the photodiode region
51
and the peripheral circuit region
52
, a P-type substrate
53
(P), a P-type epitaxial layer
54
(P
−
), and an N-type epitaxial layer
55
(N) are successively provided in this order. In the photodiode region
51
, the P-type epitaxial layer
54
and the N-type epitaxial layer
55
, which have PN junction, form a light detection photodiode portion. In the peripheral circuit region
52
, the above-described two transistors are provided in the P-type epitaxial layer
54
and the N-type epitaxial layer
55
due to impurity diffusion.
The photosensitivity and response speed of a photodiode are generally key measures of the performance of the photodiode. The photosensitivity is determined by the sum of the number of carriers generated in a depletion layer and the number of carriers which are generated outside the depletion layer and which reach the depletion layer due to carrier diffusion when reverse bias is applied to the PN junction in light detection. The response speed is influenced to a large extent by the value of a PN junction capacitance of the light detection photodiode portion. Therefore, to enlarge the depletion layer sufficiently is effective to increase the sensitivity of the photodiode and reduce the junction capacitance to increase the response speed.
Therefore, as a first conductivity type region, the P-type substrate
53
, on a surface of which the P-type epitaxial layer
54
having a low concentration (high specific resistance) is provided, is used as described above. Alternatively, a P-type low-concentration substrate (not shown) may be used instead.
Such a structure causes a depletion layer to be easily expanded in the first conductivity type region in which light is absorbed, thereby making it possible to efficiently utilize penetrating signal light. Further, the PN junction capacitance can be reduced.
As the recording density of an optical recording medium, such as an optical disk, becomes higher and higher year after year, the wavelength of light applied to the medium is decreased. Specifically, whereas infra-red light having a wavelength of 780 nm is used for CDs, red light having a reduced wavelength of 650 nm is used for DVDs. The use of blue light having a further reduced wavelength of about 410 nm is being developed.
However, as the wavelength of signal light is reduced, the depth of silicon to which the signal light can reach (hereinafter referred to as a penetration depth) is rapidly decreased. For example, although the penetration depth of 780-nm light is as long as about 8 &mgr;m, the penetration depth of 410-nm light is less than or equal to about 0.3 &mgr;m.
There are the following problems with the structure of the photodiode region in the conventional circuit-incorporating light receiving device
4000
of FIG.
5
.
(1) The N-type epitaxial layer
55
typically needs to have a thickness of at least about 1 &mgr;m or more in order to provide a transistor in the peripheral circuit region
52
. Further, an N-type diffusion region
56
(N
+
) having a high concentration is provided in order to reduce a cathode resistance, sot hat penetrating light is mostly absorbed by the N-type epitaxial layer
55
which has substantially no depletion. Therefore, the recombination rate of carriers is high and the recombined carriers cannot contribute to an optoelectric current, so that the sensitivity cannot be enhanced. Further, the PN junction capacitance of the light detection photodiode portion is too large to achieve a high response speed.
(2) The N-type epitaxial layer
55
could be caused to be thin, not taking into account the conformity with the peripheral circuit region
52
. In this case, when the N-type epitaxial layer
55
is grown, P-type auto dope occurs due to a peripheral isolation diffusion region
57
(P
+
) or a film production apparatus. The occurrence of auto dope leads to formation of a potential peak in the vicinity of an interface between the first conductive region (the P-type substrate
53
having the low-concentration P-type epitaxial layer
54
grown thereon, or the P-type low-concentration substrate) and the N-type epitaxial layer
55
grown thereon, which deteriorates a response characteristic.
(3) To reduce the fabrication steps and improve the sensitivity to short-wavelength light, a light detection photodiode portion in which a P-type diffusion region is provided in the N-type epitaxial layer
55
may be provided. However, in this case, boron used in the P-type diffusion region is segregated on a surface thereof, so that Ns (surface concentration) is lowered. As a result, the surface recombination is increased, so that the sensitivity is lowered. Further, the photosensitivity of the light detection photodiode cannot be increased for long-wavelength light having a large penetration depth due to the structure thereof. Since the depletion layer is not so enlarged, the junction capacitance is increased and therefore the response speed is lowered.
(4) In the conventional structure of
FIG. 5
in which the light detection photodiode portion is separated by the isolation diffusion region
57
in which diffusion is carried out upward and downward, it is assumed that light is incident to the isolation diffusion region
57
. As shown in
FIGS. 6A and 6B
, the impurity concentration distribution of the isolation diffusion region
57
has a profile taken along a line A—A′ of
FIG. 5 and a
profile taken along a line B—B′ of
FIG. 5
, respectively. Generated carriers are accumulated in a valley in FIG.
6
A. Referring to
FIG. 6B
, since the middle portion has substantially no tilt, the accumulated carriers laterally move at a low speed. Therefore, the response speed cannot be improved.
According to the above-described reasons, a plurality of circuit-incorporating light receiving devices corresponding to
Fukushima Toshihiko
Hayashida Shigeki
Kubo Masaru
Jackson Jerome
Nixon & Vanderhye P.C.
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