Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-06-02
2001-08-07
Everhart, Caridad (Department: 2825)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S775000, C438S795000, C438S066000, C427S534000, C427S579000
Reexamination Certificate
active
06271054
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for significantly reducing, if not eliminating, the dark current effects in a charge couple device (CCD). More particularly, the present invention involves a combination of a hydrogen anneal followed by depositing a hydrogen-rich barrier layer of silicon nitride by rapid thermal chemical vapor deposition (RTCVD). In addition, the present invention relates to charge couple devices obtained according to the process of the present invention. The process of the present invention makes it possible to achieve active storage area dark currents of less than 250 picoamps/cm
2
on full frame, progressive scan CCD image sensors incorporating vertical anti-blooming device structures.
BACKGROUND OF INVENTION
A charge couple device (CCD) functions as an active matrix to absorb light and convert it to electrons. Such are used for example as the electronic film in digital still cameras(DSC).
Desirably, a charge couple device should absorb as much light as possible while, at the same time, minimizing leakage between pixels as much as possible.
A particularly perplexing problem of charge couple devices is referred to as dark current. Dark current is the undesired (noise/leakage) current which flows when no light is presented to the CCD array. Excessive dark current destroys the dynamic range of the CCD array when there is insufficient ability to distinguish between light and dark conditions.
The problem concerning dark current defects is especially critical in consumer digital still picture applications where cameras must function in both bright (short shutter speed) and low (long shutter speed) light conditions. This is typically not a factor in commercial (i.e. professional) in a studio setting.
It is known that hydrogen present in the CCD device will reduce dark current by passivating dangling bonds. However, while hydrogen could be used to solve the problem, keeping the hydrogen in the device is problematic since hydrogen diffuses very readily.
Attempts to prevent hydrogen diffusion have involved certain nitride films. For example, work on hot-electron effect mitigation with deuterium has shown that a barrier nitride layer limits loss of deuterium (D-2) from CMOS devices. Further, for solving this problem in CMOS devices, deposited nitride films containing some amount of hydrogen, trapped during the deposition process have been suggested. This hydrogen, in addition to any hydrogen annealed in before a barrier nitride deposition, acts as a hydrogen reservoir.
However, barrier nitride films suggested in the prior art are from plasma enhanced chemical vapor deposition (PECVD) or furnace low pressure chemical vapor deposition (LPCVD). In the case of a CCD device, neither of these process solutions is workable. PECVD films are too high in stress with the addition of sufficient hydrogen. While they prevent hydrogen diffusion which would limit dark current, their higher stress levels create dark current via a stress mechanism.
In addition, an attempt with a relatively thick (e.g. 1750 angstroms) hydrogen enriched PECVD reduced dark current, but the thickness of the film actually destroyed the necessary light transmissivity of the array. On the other hand, use of a relatively thin (e.g. 300 angstroms) hydrogen enriched PECVD nitride film had undesirable stress characteristics but adequate light transmissivity.
Furthermore, LPCVD films by comparison are very low in hydrogen content and also create integration problems due to the long heat cycle. Therefore, they are unable to provide a sufficient hydrogen reservoir. Additionally, the use of LPCVD barrier nitride adds a substantial thermal cycle which can change dopant profiles and allow for dopant deactivation.
Accordingly, a need exists to address the dark current problem of charge couple devices.
SUMMARY OF INVENTION
The present invention overcomes problems of the prior art and provides for trapping hydrogen on the surface of the cell to minimize leakage from pixel to pixel and still permit the absorption of sufficient light. The present invention provides for at least significantly reducing the dark current effects in a charge couple device. In fact, the present invention makes it possible to achieve active storage area dark currents of less than 250 picoamps/cm
2
on full frame, progressive scan CCD image sensors incorporating vertical anti-blooming device structures.
More particularly, the process of the present invention relates to reducing dark current defects in a charge couple device which comprises:
a) subjecting the charge couple device to an anneal in hydrogen at a temperature of about 850° C. to about 950° C. for at least 20 minutes; and
b) then depositing a barrier layer of silicon nitride by RTCVD to a thickness of about 250 to about 350 angstroms.
The present invention also relates to a charge couple device obtained by the above process.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.
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P.A. Tiner, et al., Extending the use of NO dielectrics for DRAM by ultrathin silicon nitride RTCVD, Rapid Thermal and Integrated Processing VI. Symp., pp. 387-92. (Abstract), Apr. 1997.
Ballantine Arne W.
Dunbar, III George A.
Hart, III James V.
Johnson Donna K.
MacDougall Glenn C.
Connolly Bove Lodge & Hutz
Everhart Caridad
International Business Machines - Corporation
Sabo William D.
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