Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2000-11-07
2002-04-30
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S461000, C257S465000, C257S544000
Reexamination Certificate
active
06380603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, e.g., a photosensitive device with internal circuitry that includes on the same substrate both a photosensitive device for converting incident light into an electrical signal and an integrated circuit portion for processing the electrical signal which is output from the photosensitive device, and in particular to a high-performance semiconductor device incorporating a photosensitive device having an enhanced response speed; and a method for producing the same.
2. Description of the Related Art
Photosensitive devices with internal circuitry, which are composed of semiconductor devices, are employed for optical pickups, for example. In recent years, optical pickups employed in CD-ROM, CD-R/RW, or DVD-ROM drives have been increasing in operation speed, and there has been a demand for higher-performance (i.e., higher sensitivity and lower noise) photosensitive device having internal circuitry.
For example, obtaining a photosensitive device with internal circuitry having a high response speed requires at least a photodiode having rapid photoelectric conversion characteristics. In an attempt to enhance the photoelectric conversion characteristics of a photodiode, a semiconductor device shown in
FIG. 9
has been proposed (Japanese Laid-Open Patent Publication No. 10-209411), for example. The semiconductor device shown in
FIG. 9
includes bipolar transistors and a photodiode structural portion on a P-type semiconductor substrate
501
. The photodiode structural portion includes a photodiode of a cathode-common type and a photodiode of an anode-common type. The following description will be focused on the photodiode of an anode-common type shown in FIG.
9
.
A high concentration P-type embedded diffusion layer
502
, an ultra-low concentration P-type epitaxial layer
503
, and an N-type epitaxial layer
507
are laminated in this order on a P-type semiconductor substrate
501
. In the P-type epitaxial layer
503
, a P-type separation diffusion layer
504
is provided which extends from the surface of the P-type epitaxial layer
503
into the P-type embedded diffusion layer
502
. A P-type embedded diffusion layer
506
is provided in the N-type epitaxial layer
507
so as to overlie the P-type separation diffusion layer
504
. In accordance with this structure, split photodiodes of an anode-common type (i.e., which share the same anode portion in common), each having a rectangular shape, can be formed as shown in
FIGS. 10A
to
10
C, for example. Herein, the P-type separation diffusion layer
504
and the P-type separation diffusion layer
506
together compose a partitioning portion which splits the four rectangular photodiode regions from one another. In order to improve the frequency characteristics of the photodiodes, it is necessary to reduce the junction capacitance and serial resistance in the first place.
In accordance with the above-described structure, the junction capacitance of concern exists between the P-type epitaxial layer
503
and the N-type epitaxial layer
507
. Since the P-type epitaxial layer
503
has an ultra-low concentration, it is possible to ensure a sufficient expanse of a depletion layer from the N-type epitaxial layer
507
to the P-type epitaxial layer
503
when a reverse voltage is applied. As a result, the junction capacitance between the P-type epitaxial layer
503
and the N-type epitaxial layer
507
can be reduced. The serial resistance of concern is determined by the serial resistance of the high concentration P-type embedded diffusion layer
502
and the P-type embedded diffusion layer
504
. Since both layers
502
and
504
have a high concentration and a small resistivity, their serial resistance can also be made small. The frequency characteristics of the split photodiodes can be improved in this manner.
An optical pickup is operative to track the data carried on a disk which is rotating at a fast rate, so as to read out a reproduction signal therefrom, while acquiring servo signals which are provided for facilitating the accurate reading of the data from the disk. The servo signals include a focus error signal (FES) for positioning the focal point of laser light emitted from a semiconductor laser on the disk and a radial error signal (RES) for positioning the focal point of the laser light on a certain pit (or track) on the disk. The latter positioning control is often referred to as “tracking”. A number of methods exist for detecting such servo signals. As an example of a method for detecting an FES, an astigmatic method will be described below.
In order to detect an FES by the astigmatic method, it is necessary to employ four split photodiodes which have respectively different light-receiving regions.
FIGS. 10A
to
10
C illustrate how a beam spot may appear on a photosensitive device in accordance with the astigmatic method.
FIG. 10A
shows the case where a focal point of laser light emitted from a semiconductor laser is on the surface of a disk, in which case the beam spot has a truly circular shape.
FIGS. 10B and 10C
show the cases where a focal point of laser light emitted from a semiconductor laser is in front of or behind the surface of a disk, respectively, in which cases the beam spot has an elliptical shape. The different shapes of light beams are ascribable to the use of a cylindrical lens which exerts a lens effect on only light which is polarized in a certain direction. An FES can be obtained by applying the respective output signals Sa, Sb, Sc, and Sd from the four split photodiodes PDa, PDb, PDc, and PDd (as shown in
FIGS. 10A
to
10
C) to the following equation:
FES=(Sa+Sd)−(Sb+Sc),
where a difference between a sum of the output signals from one pair of diagonally-disposed photodiodes and a sum of the output signals from the other pair of diagonally-disposed photodiodes is derived. Different calculation results of this equation correspond to different beam spot convergence states as follows:
FIG. 10A
(where the focal point of laser light is on the disk surface): FES=0
FIG. 10B
(where the focal point of laser light is in front of the disk surface): FES>0
FIG. 10C
(where the focal point of laser light is behind the disk): FES<0
Accordingly, by performing a feedback control so that the value of FES equals zero, the beam spot will be properly placed on the disk surface.
The state shown in
FIG. 10A
is also a state where a reproduction signal RF is constantly being read based on proper servo control. Hence, the reproduction signal RF can be calculated by taking a sum of the output signals Sa, Sb, Sc, and Sd from the four split photodiodes PDa, PDb, PDc, and PDd, as shown by the following equation:
RF=Sa+Sb+Sc+Sd.
In view of the aforementioned manner of using an optical pickup, it can be seen that enhancing the performance of an optical pickup in speed, sensitivity, and noise level can only be achieved through enhancing its performance in the state where laser light is incident on the partitioning portion (i.e., the P-type separation diffusion layer
504
and the P-type separation diffusion layer
506
) as well as the light-sensitive portions of the split photodiodes. However, the response of the split photodiodes of an anode-common type as shown in
FIG. 9
generally deteriorates when laser light is incident on the partitioning portion for the following reasons.
Laser light of a wavelength of 780 nm or 650 nm (which are typically employed for an optical pickup) would intrude into the partitioning portion of an optical pickup to a depth of about 9 &mgr;m or about 3.5 &mgr;m, respectively. The “depth” as used herein is defined as a depth at which the light intensity is reduced to 1/e that of the incident light (where e represents the base of a natural logarithm). Therefore, when laser light is incident on the partitioning portion, a large portion of photocarriers are generated in the high concentration P-type separation diffusion layer
504
or
Fukushima Toshihiko
Hosokawa Makoto
Kubo Masaru
Ohkubo Isamu
Takimoto Takahiro
Ngo Ngan V.
Nixon & Vanderhye,. P.C.
Sharp Kabushiki Kaisha
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