Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-04-30
2002-08-20
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S290000, C257S316000, C257S431000, C365S185320
Reexamination Certificate
active
06437398
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a one-time UV-programmable non-volatile semiconductor memory comprising a number of MOS transistors, which are arranged in a matrix of rows and columns and which serve as memory cells, which MOS transistors include source, drain and channel regions, which are formed in a surface zone of a semiconductor substrate and which adjoin a surface of said semiconductor substrate, which MOS transistors further include floating gates and control gates, which are formed in a layer structure extending on the surface, which layer structure is provided with windows allowing UV radiation to penetrate almost to the floating gates.
Between the channel region and the floating gate, a potential barrier is formed by a layer of a gate oxide, between the floating gate and the control gate a potential barrier is formed by a dielectric which is composed of, for example, a layer of silicon nitride enclosed between two layers of silicon oxide. By exposing charge carriers, which are present near these potential barriers, to UV radiation, these charge carriers can be excited and attain such a high energy level that they are capable of passing these potential barriers. If no electric voltage is applied between the substrate and the control gate during such a radiation process, then any charge present on the floating gate will flow away to the substrate or the control gate. The floating gate will then be discharged. If an electric voltage is applied between the substrate and the control gate during such a radiation process, then electrically charging the floating gate will be possible. By charging the floating gates of a part of the transistors, the memory is programmed. The transistors with a charge on the floating gates exhibit a different threshold voltage than the transistors without a charge on the floating gates. If a voltage that ranges between these threshold voltages is applied to the control gates via word lines, then electric current can flow through transistors having a low threshold voltage, but not through transistors having a high threshold voltage. This is checked by applying a suitable voltage between the source and drain regions via bit lines.
After the pattern of charges has been provided on the floating gates, the semiconductor memory thus programmed is subjected to a “final assembly” process, i.e. provided with a customary airtight envelope of a black synthetic resin with pins or strips for external contact. The information stored in the semiconductor memory in the form of charges can be read many times, but it cannot be changed. Such semiconductor memories, also referred to as OTP-ROMs (One-Time Programmable Read Only Memory) can also be used in computers for storing, for example, computer programs, printer fonts or games, but they can also be used, for example, in smart cards.
DE-A-29 12 859 discloses a semiconductor memory of the type mentioned in the opening paragraph, in which, at the location of the floating gates, the UV radiation-transmitting windows are provided in the control gates. The charge carriers present near the dielectric between the floating gate and the control gate, can be excited by exposure to radiation and, as a result thereof, attain such a high energy level that they can pass the potential barrier formed by this dielectric.
The semiconductor memory can be programmed by first providing all floating gates with a charge. In this process, all transistors are simultaneously irradiated, while a voltage is applied between the substrate and the control gates. Subsequently, a part of the floating gates is electrically discharged in accordance with a pattern to be programmed, whereby use is made of, for example, the Fowler-Nordheim tunnel effect. The semiconductor memory can alternatively be programmed by first discharging all floating gates as described above and, subsequently, individually and successively exposing a part of the memory cells, in accordance with the pattern to be programmed, by means of, for example, an UV laser beam, while said voltage is applied between the substrate and the control gates.
In order to program the known semiconductor memory, suitable voltages must be applied between the substrate and the control gates of the individual transistors to electrically discharge the floating gates. For this purpose, electronic circuits must be provided on the semiconductor substrate which, after programming, are no longer necessary, and bond pads must be provided on the semiconductor substrate to enable these circuits to be externally contacted during programming. In addition, in order to externally contact these circuits during programming, expensive equipment is necessary which is provided with contact pins which can be pressed onto said bond pads.
Since the UV radiation-transmitting windows of the known semiconductor memory are formed in the control gates, the memory cells of this semiconductor memory must be comparatively large. For example, when use is made of a “0.5 &mgr;m technology”, the manufacture of windows having a length and a width of 0.5 &mgr;m requires the control gates and the underlying floating gates to have a length and a width of at least 1 &mgr;m in practice. The known semiconductor memory thus takes up a comparatively large substrate surface area.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a one-time UV-programmable non-volatile semiconductor memory comprising a number of MOS transistors, which are arranged in a matrix of rows and columns and which serve as memory cells, which semiconductor memory, unlike the above-described prior-art memory, can be programmed without the formation of additional electronic circuits on the semiconductor substrate, without providing bond pads on the semiconductor substrate to externally contact the circuits, and without expensive equipment for externally contacting the circuits.
To achieve this, the semiconductor memory mentioned in the opening paragraph is characterized in accordance with the invention in that the semiconductor memory is provided with means for generating, by means of UV radiation, an electric voltage necessary during programming, between the substrate and the control gates. As a result, the semiconductor memory can be readily programmed in two steps. During the first step, the whole surface of the semiconductor memory is exposed to UV radiation, so that, simultaneously, a voltage is generated between the substrate and the control gates, and charge carriers near the floating gates are excited. The floating gates of all transistors are thus provided with a charge. During the second step, the means for generating an electric voltage between the substrate and the control gates by means of UV radiation are not irradiated and, hence, without a voltage between the substrate and the control gates, a part of the memory cells are individually irradiated in accordance with the pattern to be programmed. The charge on the floating gates of these irradiated transistors is thus removed again. In this manner, the semiconductor memory is programmed without an external voltage being required. The radiation process can be carried out, for example, by successively exposing the memory cells to radiation originating from an UV laser beam. Alternatively, use can be made of an optical projector, such as a so-called “stepper” which is customarily used in the semiconductor technology, which optical projector is used to image a mask which corresponds to the pattern to be programmed.
The radiation process during the second step can be carried out by means of much simpler radiation equipment if for the patterned irradiation of a part of the memory cells a mask is formed on the surface, during this second step, which is provided with UV-transmitting windows only at the location of these memory cells to be programmed, after which the entire surface is irradiated by, for example, an UV lamp. Consequently, the means for generating an electric voltage between the substrate and the control gates are shielded by the mask during t
Ho Tu-Tu
Koninklijke Philips Electronics , N.V.
Nelms David
Waxler Aaron
LandOfFree
One-time UV-programmable non-volatile semiconductor memory... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with One-time UV-programmable non-volatile semiconductor memory..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and One-time UV-programmable non-volatile semiconductor memory... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2972119