Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-06-04
2001-03-20
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S363020, C250S370110
Reexamination Certificate
active
06204506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a back-illuminated photodetector such as a back-illuminated charge coupled device (CCD) or back-illuminated active pixel sensor (APS) for detecting a radiation having a large absorption coefficient such as ultraviolet ray, &ggr;-ray, or charged particle beam, and a method of fabricating the same.
2. Related Background Art
As a photodetector for detecting the energy distribution of an electromagnetic wave or light containing charged particles as an image, a CCD is known. The full frame transfer (FFT) CCD is mainly used for measurement purposes.
A general CCD based on the FFT has no storage portions, and individual imaging portions can have a large size. Because of the large photosensitive area, this device prefers for weak light level. However, incident light must reach the imaging portion through a polysilicon electrode or protective PSG film formed on the surface of the device. Since such an element formed on the surface absorbs input light having a large absorption coefficient, e.g., light with a wavelength of 400 nm or less, the light does not reach the imaging portion, so high sensitivity cannot be expected for light in such a short wavelength region.
To detect light with a short wavelength, a back-illuminated CCD can be used as it has an imaging portion with a substrate as thin as about 10 to 30 &mgr;m and illuminates from the opposite side of the electrode formation surface. In this back-illuminated CCD, even light having a short wavelength reaches the imaging portion without being largely absorbed, so high sensitivity can be obtained even in a short wavelength band of about 200 nm. The device can also be applied to, e.g., X-ray imaging for medical use.
In the X-ray imaging, an image is difficult to reduce and project on the imaging portion of the CCD, unlike the normal visible light image sensing, and a CCD having an imaging surface with almost the same size as that of the object must be prepared. A CCD to be used for chest X-ray radiography must be a large CCD or a buttable CCDS. However, the back-illuminated CCD with a thin imaging portion is fragile and is difficult to increase its size. For the firm CCD, a thick frame is fitted on the imaging portion. However, even a device of this type can hardly have a large size, and the frame around the imaging portion is dead space.
Techniques for solving this problem are disclosed in Japanese Patent Laid-Open Nos. 53-114361 (to be referred to as “prior art 1” hereinafter), 62-30373 (to be referred to as “prior art 2” hereinafter), 6-268183 (to be referred to as “prior art 3” hereinafter), and 6-291291 (to be referred to as “prior art 4” hereinafter).
In prior art 1, a CCD is bonded on a support substrate via boro-silicate glass (BSG) or boro-phospho-silicate glass (BPSG) softened by heating, and then, a thin imaging portion is formed. This prior art is characterized in that V-shaped grooves are formed in the bonding surfaces of the CCD and the support substrate in advance.
In prior art 2, boro-silicate glass is deposited thick on that surface of a CCD or a substrate having a CCD, which is opposite to the illuminated surface, and is fired to form a reinforcing member.
In prior art 3, a sapphire substrate is anodically bonded on the illuminated surface of a CCD. In prior art 4, a protective insulating film is formed on the device surface side with a CCD and planarized, and then, a glass substrate is anodically bonded on the protective insulating film.
SUMMARY OF THE INVENTION
However, these prior arts have the following problems.
In prior art 1, air is confined in the V-shaped grooves upon bonding. This air expands in the fabrication process to form a void. The silicon substrate may deform to degrade the planarity of the illuminated surface, resulting in a poor uniformity in sensitivity. Sometimes, a chemical solution or the like enters the device through the V-shaped grooves in the process to locally degrade the device, resulting in a poor uniformity in sensitivity.
In prior art 2, when the adhesion strength between the glass layer and the silicon substrate of the CCD is increased, the glass layer may warp. Consequently, the CCD warps and it causes defocus in photographing, resulting in a poor uniformity in image.
In prior art 3, the sapphire substrate anodically bonded on the imaging surface absorbs short-wavelength light, an X-ray, an electron beam, or the like, resulting in a decrease in sensitivity for such short-wavelength light.
Prior art 4 is out of the problem of light absorption by the reinforcing plate because the surface opposite to the illuminated surface is reinforced. However, electrostatic charges are locally generated on the glass substrate as a dielectric, resulting in a poor uniformity in image.
For these reasons, conventionally, a large back-illuminated photodetector for short-wavelength light with a good uniformity in image is difficult to fabricate.
The present invention has been made in consideration of the above problems, and has as its object to provide a back-illuminated photodetector having a sufficient strength and high sensitivity for short-wavelength light, which has a good uniformity in image, and a method of fabricating the same.
In order to solve the above problems, according to an aspect of the present invention, there is provided a back-illuminated photodetector having an imaging portion for outputting charges in response to incidence of an electromagnetic wave or charged particles on a first surface of a thin semiconductor essentially consisting of silicon, the electromagnetic wave or charged particles being transmitted through the thin semiconductor, comprising (1) an insulating film essentially consisting of one of inorganic and organic materials and formed on a surface covering the imaging portion on the first surface, (2) an oxidizable conductive film essentially consisting of one of a semiconductor and a metal and deposited on the insulating film, and (3) a glass substrate anodically bonded on an entire surface of the oxidizable conductive film.
According to this arrangement, the insulating film, the oxidizable conductive film, and the glass substrate are stacked on the imaging portion of the thin semiconductor. The insulating film electrically insulates the conductive film from the imaging portion. The conductive film unifies electrostatic charges generated in the glass substrate. The interface of the conductive film is oxidized by oxygen atoms in the glass substrate to form covalent bonds, so the glass substrate and the conductive film are firmly bonded.
The present invention can particularly suitably be used for an image sensing device that is a one- or two-dimensional array of a plurality of pixels for individually detecting the incident electromagnetic wave or charged particles. With this arrangement, a large image sensing device with a good uniformity in image can be obtained.
A connection terminal region where a connection terminal for connecting the imaging portion to an external circuit is formed is preferably exposed at an end portion on a second surface side opposite to the first surface. With this arrangement, extraction of a detection signal is facilitated.
The connection terminal region is preferably arranged along one or adjacent two sides of the second surface. With this arrangement, when a plurality of devices are to be arranged, the terminal is prevented from being arranged between the devices, so a larger sensing area can be easily obtained.
The imaging portion may comprise a charge coupled device. With this device, an optical image can be easily converted into an image signal.
The insulating film may consist of one of inorganic glass and polyimide resin. With this material, the insulating properties and heat resistance of the insulating film become high.
The conductive film may consist of one of aluminum and polysilicon doped with a conductive impurity. With this arrangement, a conductive film having high electrical conductivity and heat resistance can be obtained.
The thermal expan
Akahori Hiroshi
Muramatsu Masaharu
Hamamatsu Photonics K.K.
Hannaher Constantine
Israel Andrew
Pillsbury Madison & Sutro LLP
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