Abstract:
A process for fabricating a transistor may include forming source and drain regions in a substrate, and forming a floating gate having electrically conductive nanoparticles able to accumulate electrical charge. The process may include deoxidizing part of the floating gate located on the source side, and oxidizing the space resulting from the prior deoxidation so as to form an insulating layer on the source side.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a transistor, an electrically programmable memory, and to an electronic device, and also to a process for making the transistor and the memory. 
     BACKGROUND OF THE INVENTION 
     Some uses of electrically programmable memories are highly constrained in terms of available space, as in the case for cards incorporating microprocessors, also called smart cards, or any integrated-circuit device on a semiconductor wafer. In addition, in contactless smart card applications, for example, little power may be available because such smart cards generally do not have an internal power supply and are powered remotely by an external reader. In this sort of application, low power consumption may be a second constraint. 
     In these applications, it is common to use flash memories, which have the advantage of a very simple and compact structure, providing a large storage capacity with a small footprint. However, programming these flash memories may require substantial voltages, which induce large leakage currents and overall high power consumption. Thus, these memories may fail to meet some requirements. 
     Actual flash memories are produced using metal oxide semiconductor (MOS) transistors. Programming of such transistors takes place in the presence of a high drain-source current and makes use of a mechanism for injecting electrical charge into the floating gate called “hot carrier injection.” In contrast, the memory is erased by a tunneling effect (Fowler-Nordheim tunneling) by applying, to the transistors, voltages that extract the charge trapped in the floating gate, with a negligible flow of current between the drain and the source. 
     The typical hot-carrier-injection transistor has low injection efficiency, and may require a high programming current of about 50 μA per transistor. This property limits the number of transistors that can be programmed simultaneously in a memory, this number generally being equal to 8, 16, or 32 transistors. In low-consumption semiconductor products, the number of transistors that can be simultaneously programmed is furthermore limited by power-consumption constraints. Currently available approaches do not satisfactorily address these problems. 
       FIG. 1  is a cross-sectional view of a typical structure of a hot-electron-injection MOS transistor  10  comprising nanocrystals. This transistor  10  comprises a p-type substrate  1 , n-type source (S) and drain (D) regions ( 2  and  3 , respectively), a floating gate  8  made of a dielectric  11  in which electrically conductive nanoparticles  12  have been incorporated, for example, silicon or germanium nanocrystals, and a control gate  4 . Each nanoparticle may act as an independent floating gate and is able to trap electrons. In the following, the expression “floating gate” will be used to denote the layer comprising all the nanocrystals of such a transistor. The control gate  4  is generally made of polysilicon (polycrystalline silicon). The control gate  4  and the source  2  and drain  3  regions are provided with electrical contacts, shown schematically. The floating gate  8  is electrically insulated from the substrate  1  and the control gate  4  by the dielectric  11 . The conductive particles  12  of the floating gate  8  may accumulate electrical charge that modifies the threshold voltage of the transistor. 
     The transistor  10  is programmed by applying a positive voltage VD to the drain region  3 , a positive voltage VCG to the control gate  4 , a zero voltage VS (ground or GND) to the source region  2 , and a zero or negative voltage to the substrate, these voltages being chosen so as to place the transistor into a saturated operating mode. The gate voltage VCG generates a vertical electric field EV. The electric field EV causes an inversion zone  5  to appear in the substrate  1 , forming an n-type conductive channel in which electrons can flow and which has a pinch-off zone  6  neighboring the drain region  3 . The drain-source potential difference generates a current IDS, between the drain region  3  and the source region  2 , corresponding to a stream of electrons flowing in the opposite direction between the source region  2  and the drain region  3 . At the pinch-off zone  6 , the electrons have a high kinetic energy, thereby generating hot carriers, some of which overcome the potential barrier of the dielectric material  11  and penetrate into the nanocrystal zones  12  of the floating gate  8  where they remain trapped locally. The substrate region  1  lying between the pinch-off zone  6  and the drain region  3  is thus called the injection zone  7 . 
     The electrical charge trapped in the nanocrystals of the floating gate  8  modifies the threshold voltage of the transistor. This threshold voltage may then be measured by a read amplifier that delivers as output a 0 or 1 logic value depending on whether the transistor is in the programmed state, i.e. electrons are present in the floating gate, or not (the unprogrammed state being called the erased state). 
     As indicated above, such a transistor structure  10  has a low injection efficiency, i.e. the ratio of the current IDS that flows during programming to the current that is injected into the nanocrystals, is low (the amount of electrical charge trapped being equal to the current injected multiplied by the injection time). 
     This low efficiency is a result of the vertical electric field EV being greatly attenuated near the injection zone  7 . The field strength is high near the source region  2 , but is gradually attenuated as it approaches the drain region  3  because it is neutralized by the drain voltage VD, thereby also causing pinch-off  6  of the channel. 
     Production of such a transistor generally comprises first producing an insulating layer, generally a silicon oxide layer, then depositing conductive nanoparticles, generally silicon nanocrystals, and then depositing a dielectric layer on the nanocrystals, which may be an oxide or an oxide/nitride/oxide trilayer. Next, a step of depositing and etching polycrystalline silicon allows the control gate  4  of the transistor to be formed. This gate then acts as a mask for forming the drain and source regions of the transistor. 
     SUMMARY OF THE INVENTION 
     An object may include providing a transistor that has a small footprint and low power consumption. 
     An aspect is directed to a process for fabricating a transistor that may comprise forming source and drain regions in a substrate and forming a floating gate comprising electrically conductive nanoparticles able to accumulate electrical charge. The process may comprise deoxidizing part of the floating gate located on the source side, and oxidizing the space resulting from the prior deoxidation so as to form an insulating layer on the source side. The deoxidizing step may comprise a wet etching method of the buffered oxide etch (BOE) type. 
     Another aspect is directed to a process for fabricating a programmable memory using the process for fabricating a transistor such as described above to fabricate a number of memory cells. The memory may be a flash memory, and the process may comprise a step of forming a self-aligned source by etching with the aid of a mask, this mask being likewise used for the deoxidizing step on the source side between a number of gates of a number of transistors and the substrate. 
     Another aspect is directed to a transistor comprising source and drain regions in a substrate, and at least one floating gate comprising electrically conductive nanoparticles able to accumulate electrical charge and a control gate. The space between the control gate and the substrate may comprise a first zone on the drain side, comprising electrically conductive nanoparticles able to accumulate electrical charge, and a second zone juxtaposed on the side of the source region comprising a single insulating layer. The second zone on the side of the source region may not comprise nanoparticles. 
     The first zone may have a smaller thickness than that of the second zone. The source may comprise a part located under the floating gate, possibly forming a junction with a channel region located between the drain region and the source region, and the second zone may extend beyond this junction. 
     The second zone may have a length of between 25% and 75% of the total gate length. The drain and source regions may be regions doped n-type or p-type, and the substrate may be a p-type or an n-type well. The transistor may be a hot-carrier-injection MOS transistor. 
     Another aspect is directed to a non-volatile electrically programmable memory that may comprise memory cells connected to word lines and bit lines, and a number of cells comprising at least one transistor such as described above. The memory may be of the flash or EEPROM type. 
     Another aspect is directed to an electronic device comprising at least one non-volatile electrically programmable memory as described above. The electronic device may be a contact or contactless smart card. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These subjects, features and advantages of the present invention will be described in detail, in the following non-limiting description of a particular implementation, with regard to the appended figures in which: 
         FIG. 1  is a schematic cross-sectional view of a transistor, according to the prior art. 
         FIGS. 2 to 4  are schematic diagrams of a method for fabricating a transistor, according to one embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a transistor, according to an embodiment of the present invention. 
         FIG. 6  is a schematic diagram of an enlarged view of part of the transistor, according to an embodiment of the present invention. 
         FIG. 7  is a diagram of the operation of a transistor, according to an embodiment of the present invention compared with a transistor, according to the prior art. 
         FIG. 8  is a schematic diagram of a flash memory comprising a transistor, according to an embodiment of the present invention. 
         FIG. 9  illustrates the electrical symbol of a transistor, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a semiconductor substrate  21  on which a number of transistors have been fabricated, distributed in rows and columns so as to form a flash memory according to the architecture described below with reference to  FIG. 8 . Each transistor is therefore advantageously produced in an integrated circuit (IC) on a semiconductor chip. Naturally, the process described below applies to any number of transistors, even a single transistor, and any distribution or organization on an integrated-circuit wafer. It may also be applied to any other type of transistor comprising nanocrystals. 
     In the implemented process, source  22  and drain  23  regions, a floating gate  25  comprising electrically conductive nanoparticles  32 , and a control gate  24  have been fabricated for each transistor using a typical method. The floating gate  25 , comprising nanoparticles  32  (for example, nanocrystals), placed between the control gate  24  and the substrate  21  is then modified at the end of the fabrication process. 
       FIG. 2  shows an optional step of implantation into the source region. This implantation, which is optional, is carried out at an angle so as to locally over dope the channel. Next, a second step of deoxidizing part of the layer comprising the floating gate  25  next to the source  22  is implemented, which removes part of this layer. This step employs a wet etching method, for example, based on the known BOE method, a mask  30 , possibly made of a protective resist, having been previously deposited so as to protect the other zones of the transistor, in particular the drain. The result of this deoxidizing step is shown in  FIG. 3 , an empty zone  28  being located between the control gate  24  and the substrate  21  and/or the source region  22  on the source  22  side, the empty zone extending beyond the junction  29 , between the source  22  and the channel  26  intermediate between the source  22  and the drain  23 . 
     It should be noted that this step therefore requires a mask  30 . However, in the context of a process for fabricating a flash memory, comprising a step of forming a self-aligned source (SAS), the mask used for etching of the source lines may be used for this deoxidizing step: there is then no need to add an additional mask to implement the solution according to the embodiment. Next, the resist forming the mask  30  is removed, and an oxidation step (of the polyoxidation type) is carried out, which allows the space  28  to be reoxidized by forming a single insulating layer  31 , which extends continuously from the floating gate  25  comprising nanocrystals. However, this added insulating layer  31  does not comprise nanocrystals. This figure shows the final result obtained by these steps of the fabrication process. 
     This fabrication process allows the nanocrystals  32  intended to trap electrical charge to be distributed near the drain  23  and far from the source  22 , this is an advantageous configuration that increases the performance of the transistor. This is because it removes the nanocrystals that are not useful to the operation of the transistor in its electrical programming function. 
     Moreover, the process used also makes it possible to obtain the desired form of the layer  31  separating the control gate  24  from the substrate  21 , in a precise manner, with a precision greater than 2 nanometers. It thus allows fabrication of a transistor having a shorter total length L2, which depends on the technology chosen, and a control gate  24  length L1 that may be 40 nanometers or less. The process thus allows transistors having a much shorter total length to be fabricated while preserving a power consumption equivalent to that of typical transistors, or it makes it possible to obtain a transistor of equivalent size but with reduced power consumption. The user will therefore choose the best compromise between power consumption and transistor footprint depending on the particular application envisioned. 
       FIG. 5  shows the hot-electron-injection MOS transistor  20  obtained by the fabrication process described above. This transistor comprises a p-type substrate  21 , n-type source S and drain D regions ( 22  and  23 , respectively), and a control gate  24 . The space lying between this control gate  24  and the substrate  21  comprises a first zone z1 on the drain  23  side, forming a floating gate  25  comprising conductive nanoparticles (not shown), and a second zone z2 on the source  22  side forming a nanoparticle-free insulating layer  31 . It should be noted that this process makes it possible to completely fill the space extending from the upper surface of the substrate  21  to the lower surface of the control gate  24  with a single layer  31  of insulating material, on the source side. The control gate  24  is electrically insulated from the substrate  21  by the insulators of the intermediate layers  25 ,  31  of the two juxtaposed zones z1, z2. These two zones may have substantially the same length i.e. about ½ L1. In another embodiment, the length of the zone z2 is between 25% and 75% of the total gate length L1, i.e. the total length of the zones z1, z2 and z3. 
     The drain and the source are separated by a space forming a channel  26  in which electrons flow in the programming phase of the transistor, in accordance with the principle described above. The nanoparticles may be incorporated into the electrical material filling the space between the control gate and the substrate or be incorporated in a different dielectric. In another embodiment, the transistor could have an n-type substrate and p-type source and drain regions. 
     As may be seen in  FIG. 6 , the space between the substrate  21  and the control gate  24  varies in thickness in this embodiment. More precisely, it has a larger thickness e2 in the second zone z2 on the source  22  side, where the insulator  31  added in the oxidation step is located, and a smaller thickness e1 in the first zone z1 on the drain  23  side, where the nanoparticles are located in the dielectric. As a variant, these two thicknesses could be the same or e2 could be smaller than e1. By way of example, the nanoparticles can be incorporated in a dielectric layer of thickness e1 lying between 8 and 60 nanometers, or advantageously between 10 and 25 nanometers. 
     The geometry shown in  FIG. 6  has the effect of obtaining a vertical electric field that varies at the channel  26  during a transistor-programming operation, the electric field in particular increasing towards the drain since such an electric field is inversely proportional to the distance between the control gate  24  and the substrate  21 . Thus, the end result of this effect is to increase the coupling factor of the transistor and reduce its power consumption. In addition, the absence of nanoparticles on the source side promotes electron trapping in the most advantageous zone. 
       FIG. 7  shows two curves  35 ,  36  illustrating the program window of two MOS transistors, of the typical approach, and according to an embodiment of the invention, respectively, and two curves  33 ,  34  illustrating the current Id that flows for the same transistors as a function of the applied drain voltage Vd. These curves show that for the same program window, the power consumption of the transistor according to the embodiment is clearly lower than that of the typical transistor, about more than 20% less, even close to 50% less. 
     Naturally, the method described above may be used to fabricate any type of transistor comprising an insulating layer, a gate comprising nanocrystals and source and drain zones. The transistor described above, and the fabrication process described above, may allow production of an advantageous electrically programmable memory, since it has a small footprint and requires little current for its programming. 
     In particular,  FIG. 8  shows a segment of a NOR-type flash memory according to one implementation. It comprises n×m cells Cij organized in rows and columns, each cell being located at the intersection of a word line WLi and a bit line BLj. Each cell Cij comprises a transistor with a floating gate comprising nanocrystals, such as described above, having its gate G connected to the word line WLi and its drain D connected to the bit line BLj. All the cells have their source S connected to the same source line SL. 
     The architecture described above makes it possible to implement an advantageous method for programming the memory segment. Specifically, if it is desired to program the cells thereof, it is proposed to set the potential VD of its drain D, connected to a bit line, to 3.8 V, or more generally to between 3.5 and 4.5 V, the potential of its source VS to 0 V, and the potential of its well (or substrate) VB to a negative or zero value, for example, −0.5 V, or more generally to between 0 and −1.5 V, and the control gate voltage VG to between 8 and 9 V, inclusively.  FIG. 9  shows a transistor and its various terminals and potentials. Naturally, the aforementioned numerical values are given by way of example. The technical effect of reduced power consumption compared to typical transistors would be obtained with any other values. 
     The fact that power consumption is reduced while the memory density remains very high, for a flash memory such as described above, may enable an advantageous implementation within any integrated circuit on a semiconductor chip, and in any portable device such as a smart card, and in particular in contactless devices such as contactless smart cards. Furthermore, the transistor described above may also be used to form other types of electrically programmable memory, such as EEPROM, in integrated circuits on semiconductor chips.