Photodetector with built-in circuit

Details Photodetector with built-in circuit Photodetector with built-in circuit

2001-08-06

2004-03-16

Zarabian, Amir (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S290000

Reexamination Certificate

active

06707081

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photodetector with a built-in circuit, in which a photodiode for converting incident light into an electric signal and an integrated circuit for processing the converted signal are provided on the same silicon substrate, and to a method for producing such a photodetector.
2. Description of the Related Art
Photodetectors with a built-in circuit are used in a wide range of applications such as, particularly, optical pickups and optical space transmission. In optical pickups, photodetectors with a built-in circuit are used to detect a focus error signal for adjusting a focal position of semiconductor laser light on a disk or a radial error signal for adjusting a focal position of semiconductor laser light to a pit in the disk (i.e., tracking). In recent years, there has been an increasing demand for an improvement in speed and sensitivity of photodetectors with a built-in circuit.
FIG. 16
shows a conventional photodetector with a built-in circuit disclosed in Japanese Laid-Open Publication No. 10-107243. A photodetector with a built-in circuit
600
includes a P-type semiconductor substrate
603
, a photodiode
601
, and an integrated circuit
602
. The photodiode
601
includes a P-type buried isolation diffusion layer
102
A, an N-type buried diffusion layer
103
A, an N-type epitaxial layer
104
, a P-type isolation diffusion layer
105
A, a P-type diffusion layer
107
, a silicon thermal oxide film
111
, and a silicon nitride film
112
. The integrated circuit
602
includes a P-type buried isolation diffusion layer
102
B, an N-type buried diffusion layer
103
B, the N-type epitaxial layer
104
, a P-type isolation diffusion layer
105
B, a collector compensation diffusion layer
106
, an NPN transistor external base diffusion layer
109
, an NPN transistor internal base diffusion layer
108
, an NPN transistor emitter diffusion layer
110
, the silicon thermal oxide film
111
, the silicon nitride film
112
, a first-layer conductor
113
, an interlayer insulation film
114
, a second-layer conductor
115
, and a silicon nitride film
116
.
In order to speed up the photodiode
601
, it is necessary to reduce a diffusion current components and a CR time constant component both of which have slow response. The diffusion current component is reduced by providing the N-type buried diffusion layer
103
A and the P-type diffusion layer
107
in the neighborhood of an isolation portion (i.e., the P-type buried isolation diffusion layer
102
A and the P-type isolation diffusion layer
105
A, respectively). The CR time constant component is reduced by decreasing the capacitance C
PD
of the photodiode
601
. Therefore, the N-type buried diffusion layer
103
A and the P-type diffusion layer
107
are each designed to have as small a size as possible but at which practical use is still allowable. The P-type diffusion layer
107
is provided in a region which is irradiated with a laser beam reflected from a disk (not shown) when reading a signal from the disk.
An antireflection film is provided on a light receiving surface of the photodiode
601
so as to improve sensitivity of the photodiode
601
. The silicon thermal oxide film
111
and the silicon nitride film
112
form a laminated layer which serves as the antireflection film.
The silicon thermal oxide film
111
needs to be provided on the light receiving surface of the photodiode
601
. This is because a junction surface of the P-type diffusion layer
107
and the N-type epitaxial layer
104
reaches the light receiving surface of the photodiode
601
, so that there occurs a leakage current on the light receiving surface.
The silicon thermal oxide film
111
and the silicon nitride film
112
are provided in such a manner as to have a low reflectance with respect to laser wavelengths used for CD-ROMs and DVD-ROMs (i.e., 780 nm and 650 nm, respectively).
In the integrated circuit
602
, device isolation is achieved by diffusion isolation. The NPN transistor external base diffusion layer
109
and the NPN transistor internal base diffusion layer
108
are formed by implantation of boron (B+) ions. The NPN transistor emitter diffusion layer
110
is formed by implantation of arsenic (As+) ions. The thus-constructed NPN transistor has a maximum frequency (fTmax) of 3 GHz, and a response of as high as 60 MHz for a photodetector with a built-in circuit.
However, there is a demand for even higher-speed photodetectors with a built-in circuit. To meet the demand, production methods of Poly-Si emitters, Poly-Si bases, emitters having a double polysilicon structure, and the like have been developed.
Among transistors having such structures, heterojunction bipolar transistors (hereinafter referred to as “HBT”) which employ a heterojunction such as Si/SiGe have drawn attention in recent years. In the HBT, an emitter-base junction is formed between two substances having different bandgaps (e.g., Si and SiGe). The HBT allows a barrier height against holes injected from a base into an emitter to be higher than that against electrons injected from the emitter into the base, so that carrier density in the base region can be increased without decreasing the efficiency of injection from the base into the emitter. Accordingly, the HBT allows base resistance, which is increased due to miniaturization, to be decreased, thereby speeding up the transistor.
In an attempt to obtain a high-speed photodetector with a built-in circuit, the photodiode
601
shown in FIG.
16
and the integrated circuit
602
sped up by employing the HBT may be provided on the same P-type semiconductor substrate
603
. In this case, however, there arises the following problem.
For the photodiode
601
of
FIG. 16
in which a PN junction of the P-type diffusion layer
107
and the N-type epitaxial layer
104
are formed, if a film deposited by CVD or the like is provided as an antireflection film on the light receiving surface of the photodiode
601
, leakage current is increased on the surface of the photodiode
601
. To avoid this, the silicon thermal oxide film
111
as an antireflection film is required.
As explained above, diffusion layers
108
and
109
in Prior Art
FIG. 16
are formed by implantation of boron (B+) ions (they are not made of SiGe). However, in order to explain certain embodiments of the instant invention, if SiGe layers were to be provided as the NPN transistor external base diffusion layer
109
and the NPN transistor internal base diffusion layer
108
of the integrated circuit
602
(HBT), such layers may have a strain caused by lattice mismatch due to a difference in a lattice constant between Si and Ge. Accordingly, when the SiGe layers are formed at a high temperature, dislocation occurs at an interface between the Si layer and the SiGe layer, thereby increasing a generation recombination current.
In the case where the silicon thermal oxide film
111
, which serves as the antireflection layer, is formed after the formation of the SiGe layer, strain energy held in the strain caused by the lattice mismatch of the SiGe layer is released. This leads to lattice relaxation which triggers the occurrence of dislocation, so that junction leakage characteristics of the NPN transistor of the integrated circuit
602
are deteriorated. Moreover, the composition of the SiGe layer is changed such that the resultant SiGe layer does not have desired characteristics (e.g., bandgap).
After the NPN transistor external base diffusion layer
109
and the NPN transistor internal base diffusion layer
108
are formed, the NPN transistor emitter diffusion layer
110
, the first-layer conductor
113
, and the second-layer conductor
115
are formed. Typically, dry etching is used to etch Poly-Si when forming the NPN transistor emitter diffusion layer
110
. Dry etching is also used to etch AlSi which is usually used as a material for the first-layer conductor
113
and the second-layer conductor
115
. The silicon nitride film
116
is etched with a gas. For example,

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