Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2001-09-05
2002-12-31
Chaudhari, Chandra (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C257S464000
Reexamination Certificate
active
06500691
ABSTRACT:
The invention relates to an image sensor comprising a semiconductor body which is provided, at a surface, with electrodes, each electrode being combined with the semiconductor body and an intermediate dielectric so as to form a MOS capacitor, which electrodes have a portion which is thinner than a surrounding zone, a photosensitive region in the semiconductor body being situated below each electrode, which photosensitive region is capable of absorbing electromagnetic radiation and converting said electromagnetic radiation to electric charge.
The invention also relates to a method of manufacturing an image sensor comprising a semiconductor body which is provided, at a surface, with electrodes, each electrode being combined with the semiconductor body and an intermediate dielectric so as to form a MOS capacitor, second electrodes having a portion which is thinner than a surrounding zone, a photosensitive region in the semiconductor body being situated below each electrode, which photosensitive region is capable of absorbing electromagnetic radiation and converting said radiation to electric charge, first electrodes being formed from a first layer of polysilicon, and insulation being provided between the first electrodes and the second electrodes.
A method of manufacturing such an image sensor is known from U.S. Pat. No. 5,210,049. In the known method, an image sensor is manufactured, which image sensor comprises, inter alia, a matrix of pixels and a CCD shift register. A pixel comprises a photocapacitor, a transfer gate and an overflow gate. Below each electrode of the photocapacitor there is a photosensitive region in the semiconductor body, which photosensitive region absorbs electromagnetic radiation and converts it to electric charge. Said electric charge can be displaced via the transfer gate and read via a shift register. If too much charge is converted in a pixel, a part of the charge is removed via the overflow gate. To increase the light sensitivity of the photocapacitor, the light-receiving electrode is locally made thinner. The absorption of short-wave electromagnetic radiation, in particular the blue light of the visible spectrum, is substantially reduced thereby.
In the known method, a conductive region is formed in a semiconductor body. dielectric is provided on the semiconductor body. On the dielectric, a first layer of conductive polycrystalline Si is applied above the conductive region, from which the transfer gate and the overflow gate are formed. After the application of insulating material, a second layer of conductive polycrystalline Si is applied above the conductive region, from which the electrodes of the photocapacitors and the electrodes of the shift register are simultaneously formed. The uppermost electrode of the photocapacitor is locally reduced in thickness to a value at which the absorption and losses caused by interference of incident light are reduced, so that the amount of light reaching the conductive region and the region of the semiconductor below the conductive region increases. The uppermost electrode of the photocapacitor is locally reduced in thickness by means of a pattern in a resist layer and by etching the second conductive polysilicon layer in the apertures of the resist pattern.
A drawback of the known image sensor resides in that the photocapacitors, the transfer gates and the overflow transfer gates take up a comparatively large Si semiconductor surface. The photosensitive part formed by the photocapacitors is only a small part of the overall surface of the image sensor. The sensitivity of the image sensor to, in particular, short-wave electromagnetic radiation is small.
An additional disadvantage resides in that the thickness of the thin portion of the electrodes of the photocapacitors is difficult to control. As a result of the topography of the first transfer gate and the overflow gate, on top of which the second polysilicon layer is deposited, the step coverage depends substantially upon the space between the transfer gate and the overflow gate and the thickness of the first polysilicon layer. The second polysilicon layer is a very thick layer having a thickness of several microns. The thickness of the thick layer can be locally reduced to 50 nm by subjecting it to an etching operation, however, such an etching operation is poorly reproducible and leads to the introduction of a large spread. As the thickness of the thin polysilicon is not uniform, the sensitivity of the pixels varies substantially.
It is an object of the invention to provide an image sensor of the type described in the opening paragraph, which image sensor has a greater sensitivity to electromagnetic radiation, in particular short-wave electromagnetic radiation.
A further object of the invention is to provide a method of manufacturing an image sensor of the type described in the opening paragraph, which image sensor has a greater sensitivity, can be manufactured more readily and is more reliable.
In the device in accordance with the invention, this object is achieved in that the MOS capacitors are arranged next to each other in a matrix array, the electrodes in a row being interconnected and electrically contacting each other, and the electrodes in a column being separated only by electrically insulating material.
As the MOS capacitors are interconnected in a row and, in the column direction are very closely spaced, substantially the entire photosensitive surface is covered with electrodes. The electrodes comprise a comparatively large thinner portion in order to absorb more electromagnetic radiation in the photosensitive regions, which electromagnetic radiation is converted to electric charge. The photosensitivity to, in particular, short-wave electromagnetic radiation is improved substantially by increasing the photosensitive surface. By means of the electrode of a MOS capacitor, the charge is collected below the electrode. A larger photosensitive surface does not only increase the sensitivity but also the charge-storage capacity of a pixel. By virtue thereof, the signal-to-noise ratio of the image sensor is improved, as a result of which, ultimately, the image can be sharper and brighter.
In order to be able to sufficiently rapidly read the charge below each electrode of the MOS capacitor using a clock signal of, for example, 1 MHz, the delay caused by the RC time may not become excessively long. The interconnected electrodes in a row electrically contact each other and determine the resistance. By providing the electrodes with thicker portions around the thinner portions, the resistance is reduced substantially. It is very favorable that, by means of said thick portions of the electrodes, it becomes possible to just reach the clock rate, and the remaining surface of the electrodes is very thin in order to allow as much electromagnetic radiation as possible to pass to the photosensitive regions.
Advantageously, the locally thinner portion of each electrode is centered in the relevant electrode so as to preclude, to the extent possible, reflections of light at the edges between different media and any differences in thickness at the edges of the polysilicon electrodes. In addition, thicker edge portions of the electrodes are very favorable because, in general, the current densities that can be attained along edges are larger than in the center, leading to a reduction of the resistance of the electrodes. In addition, it is advantageous if as much as possible of the electromagnetic radiation lands on the photosensitive regions, i.e. the so-called channels for the charge transport. The photosensitive regions are bounded in the horizontal directions by zones of a different doping type. Depletion regions develop between the two doping regions. The zones and the depletion regions at the edges of the electrodes are less suitable for converting electromagnetic radiation to electric charge. Therefore, it is favorable for the locally thinner portion of each electrode to be centered.
Favorably, the locally thinner portion of each electrode covers at least 25% of each
Klaassens Wilco
Kreider Gregory Lee
Peek Hermanus Leonardus
Chaudhari Chandra
Gathman Laurie E.
Koninklijke Philips Electronics
Thompson Craig
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