Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1999-01-14
2001-07-10
Chaudhuri, Olik (Department: 2823)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S151000, C438S164000, C257S223000, C257S291000
Reexamination Certificate
active
06258636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to solid state image sensors, and more particularly, to active pixel sensor (APS) technology.
2. Description of Related Art
Active pixel sensors and charge coupled devices (CCD) are solid state photosensitive devices which are commonly constructed as an array of photosensitive cells, each cell in the array corresponding to a pixel. A typical application for CCD or APS image sensing arrays is in a digital camera or other type of image sensor.
One advantage to APS devices over CCD devices is that APS technology is more compatible with metal oxide semiconductor (MOS) technology. This allows the support electronics needed to read signals from the APS array, and to process those signals, to be constructed on the same chip and at the same time as the APS array itself. This can significantly reduce the total cost of an APS technology based imaging device.
A basic prior art APS device comprises a reverse-biased photosensitive region of semiconductor material that absorbs incident electromagnetic radiation and produces hole-electron pairs. The electrons generated by the incoming light are collected and held in the photosensitive region by the action of a pin diode formed between a pinning layer at the incident surface of the device and the semiconductor material in the photosensitive region.
Incoming electromagnetic radiation first passes through the pinning layer and then into the photosensitive region. Holes generated when the incoming electromagnetic radiation is absorbed are collected and removed from the photosensitive region by the pinning layer and the photodiode formed between the reverse-biased photosensitive region and the substrate. The pinning layer also serves to isolate the stored electrons from the semiconductor surface, which is known to provide significantly more sites for recombination than the silicon bulk.
Electrons generated when the incoming electromagnetic radiation is absorbed in the photosensitive region remain trapped in the photosensitive region until a transfer device removes them. The transfer device is typically a polysilicon gate and an adjacent semiconductor region. The polysilicon gate can be triggered by the application of a potential source to allow current flow between the photosensitive region and the adjacent semiconductor region. The number of electrons trapped in the photosensitive region relates to the intensity of the absorbed electromagnetic radiation and to the duration of exposure of the APS device to the incoming radiation.
Thus, the current flow which occurs when the transfer device is activated determines the brightness at the pixel corresponding to the APS device. With multiple APS devices in an array, each one corresponding to a single pixel, a multiple pixel image can be built up by scanning the APS array and activating the transfer device for each cell to determine the brightness of the image at each pixel.
One difficulty with APS designs is that the amount of charge that can be collected and held in the pin diode is limited by the total reverse potential of the diode. If the APS cell is strongly overexposed, the diode electron charge collected will exceed this limit and forward bias the pin diode. Excess electrons will then spill out and disturb adjacent APS cells. When excess electrons from one cell spill over into adjacent cells, causing those cells to also appear to be strongly illuminated, the disturbance is referred to as “blooming”. The excess electrons from overexposed APS pixels can also interfere with the proper operation of other devices on the substrate.
To solve the blooming problem, anti-blooming gates have sometimes been used, however this increases cost and device complexity.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a photosensitive device including an APS array having excellent isolation between adjacent APS devices in the array.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a photosensitive device including:
a substrate;
an insulating layer formed on the substrate;
a semiconductor layer formed on the insulating layer;
an insulating barrier extending through the semiconductor layer to the insulating layer, the insulating barrier dividing the semiconductor layer into a plurality of cells of semiconductor material; and
a plurality of photosensitive active pixel sensors constructed in corresponding ones of the plurality of cells in the semiconductor layer.
The active pixel sensors may be constructed in the cells of semiconductor material in any conventional manner, producing an active pixel sensor having a body, a pinning layer, and a photosensitive region formed below the pinning layer. In most APS applications, it is desirable to connect the body portion and the pinning layer to each other and to ground. In conventional designs, it is relatively easy to make these connections. However in the design of the present invention, the isolated nature of each cell creates difficulties in making this connection.
Accordingly, the present invention also is directed to various constructions of the device which incorporate an integrated connector between the pinning layer and the body, and to methods of making the device, particularly methods which construct this connector from semiconductor material during the method. In one embodiment of the photosensitive device, the semiconductor connector is a semiconductor plug in the form of a vertical column extending from the pinning layer to the body of the active pixel sensor. In this first embodiment, the body and pinning layer of each APS pixel is individually connected by its corresponding plug.
In a second embodiment of the photosensitive device, the semiconductor connector is a trench filled with semiconductor material, the trench extending along one side of the cell containing the active pixel sensor. The trench may extend along only one cell, connecting the body and pinning layer of only that cell, or it may extend along multiple adjacent cells, interconnecting all of the corresponding pinning layers and body portions.
Regardless of whether the filled trench design or the individual vertical plug design is used to make the body to pinning layer connection, it is generally desirable to connect these elements to ground and to the corresponding elements of the other pixel cells. This may be achieved simply by individually connecting wires to each cell when wiring is formed on the surface of the photosensitive device. However, it may also be accomplished by extending the semiconductor connector vertically downward to a depth sufficient to penetrate the insulating layer and reach the substrate, which is made electrically conductive. In this the substrate is usually constructed of semiconductor material.
The present invention relates to both the photosensitive device and the method of making the photosensitive device. The preferred method of making the photosensitive device includes the steps of:
providing a substrate;
depositing an insulating layer on the substrate;
forming a semiconductor layer on the insulating layer;
etching a plurality of trenches into the semiconductor layer, the trenches extending through the semiconductor layer to the insulating layer, the trenches dividing the semiconductor layer into a plurality of isolated cells of semiconductor material;
depositing an insulating material into the trenches to form an insulating barrier extending through the semiconductor layer to the insulating layer, the insulating barrier electrically isolating the semiconductor material in each cell from the semiconductor material in other cells; and
constructing an active pixel sensor in the semiconductor material of each cell, each active pixel sensor
Johnson Jeffrey B.
Wong Hon-Sum P.
Chaudhuri Olik
Coleman William David
DeLio & Peterson LLC
International Business Machines - Corporation
Peterson Peter W.
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