Abstract:
A semiconductor structure comprises a memory element, which comprises a floating gate, a control electrode, which is capacitively coupled to the floating gate, wherein a signal for controlling the memory element is applicable to the control electrode, as well as a shield, which is arranged isolated from the floating gate and covers it fully.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to semiconductor structures having a memory element, which is protected from interferences by an integrated shield.  
           [0003]    2. Description of the Related Art  
           [0004]    With an increasing packing density of electronic elements, such as transistors, which are integrated on a chip, the significance of efficient protective measures against electromagnetical or electrostatic interferences increases. If memory elements, for example, are constructed with the help of transistors, the stored amount of charge might no longer be detected correctly, if, for example either the amount of the stored amount of charge or its sign have changed significantly due to an electrical interference field. This is particularly problematic, if EEPROMs are used for permanent storage of data (EEPROM=electrically erasable programmable read only memory), since an EEPROM cell is particularly susceptible against external interference fields.  
           [0005]    Principally, an EEPROM cell is constructed similar to a MOS transistor (MOS=metal oxide semiconductor). In FIG. 1, a schematic diagram of an EEPROM cell ( 100 ) is illustrated, as it is known from the prior art. First, the EEPROM cell is characterized by a control terminal  102 , a floating gate electrode  104 , a source terminal  106 , a drain terminal  108  as well as a substrate or bulk terminal  110 . A coupling capacity  112  is effective between the control terminal  102  and the floating gate  104 , and the known MOS capacity  114  is effective between the floating gate  104  and the bulk terminal  110 .  
           [0006]    In FIG. 2, the setup of an EEPROM cell is illustrated, as it is known from the prior art. In a substrate (bulk)  116 , such a p-substrate (p bulk), an n +  source area  118  and an n +  drain area  120  are formed. A channel area  122  is formed between the n +  source area  118  and the n +  drain area  120 . A thin oxide layer  126  is formed on a surface  124  of the substrate  116 , where the floating gate  104  is formed. An oxide layer  128  is formed on the floating gate, where again a control electrode  130  (control gate=CG) is formed, which is connected to the control terminal  102 . The n +  source area  118  is connected to the source terminal  106 , and the n +  drain area  120  is connected to the drain terminal  108 . The substrate  116  is connected to the bulk terminal  110  at the surface  132  opposite to the surface  124 . The coupling capacity  112  is formed by the control electrode  130 , the floating gate  104  and the oxide layer  128  lying between them. The MOS capacity  113  is formed by the floating gate  104 , the thin oxide layer  126  and the substrate  116 . In the setup of the EEPROM illustrated in FIG. 2, this is an n-type EEPROM. The EEPROM cell can also be formed as p-type EEPROM. In this case, the substrate would be an n-substrate or an n-well would be formed in the p-substrate  116 , where a p +  source area and a p +  drain area would be formed. In this case, the n-substrate and the n-well, respectively, would be the bulk of the p-type EEPROM.  
           [0007]    The difference between the EEPROM cell and a MOS transistor is that two electrodes are provided for controlling in the EEPROM cell, the floating gate  104  (FG) and the control electrode  130  (CG). The floating gate  104  is directly opposite to the MOS channel area  122 , which is formed between the source area  118  and the drain area  120 , only separated by the thin oxide  126 , and is not connected to further parts of the circuit in an electrically conductive way, which is why it is electrically “floating”. By the other, mostly thicker, oxide  128 , the control electrode  130  is separated from the floating gate  104  and connected to the control terminal  102 . Thus, seen from the outside, the EEPROM cell is similar to a MOS transistor with source, drain, bulk and gate terminals. Particularly, in the EEPROM cell, an effective threshold voltage, U th,eff  can be defined, like in the MOS transistor. In an n-type EEPROM, which corresponds to an NMOS, the control electrode  130  has to be raised above the source potential at least by U th,eff , so that the channel formed between the drain area  128  and the source area  126  becomes conductive.  
           [0008]    By different physical mechanisms, such as by Fowler Nordheim tunneling (FN) or Hot Carrier Injection (HCI), an excess or a deficiency of net charge can be generated at the floating gate  104 . Therefore, a comparatively high voltage of a positive or negative polarity has to be applied between control terminal  102  and the bulk terminal  110 , whereby the respectively smallest information unit (bit) is stored in the form of this net charge or not, whereby, for example, a logical “1” or a logical “0” can be realized. It can be seen that after this programming the changed net charge at the floating gate  104  leads to a change of the effective threshold voltage. If the floating gate  104  is not charged, the respective effective threshold voltage is called UV level. This designation stems from the fact that an uncharged state can be achieved by irradiating the floating gate of the EEPROM cell with UV light for several minutes.  
           [0009]    If a high positive potential is applied to the control terminal  102  due to the programming, a negative net charge (electron excess) occurs at the floating gate  104 , as long as the coupling capacity  112  is larger than the MOS capacity  114 , which is always assumed in the following. The effective threshold voltage will therefore be shifted to positive values; it is therefore larger than the UV level. If the control terminal  102  is provided with a strongly negative potential against the substrate  116  in the programming, a positive net charge (electron deficiency) is obtained at the floating gate  104 , and the threshold voltage becomes smaller than the UV level.  
           [0010]    In the following, the programming is explained with an example of the Fowler Nordheim Tunnel mechanism, as it is known from the prior art.  
           [0011]    If a positive voltage ramp is applied at the control terminal  102 , first, at small voltages, a separation of the voltage into two fractional voltages occurs, namely a voltage at the coupling capacity  112  (coupling capacitor), and a second voltage between the floating gate  104  and the MOS channel and substrate  116 , respectively, which drops at the MOS capacity  114 . According to the common rules for capacitive voltage dividers, the ratio of the coupling capacity  112  to the MOS capacity  114  is inverse proportional to a ratio of the fractional voltages dropping at these two capacities. For that reason, an effort is made to make the coupling capacity larger than the MOS capacity  114 , so that a portion of the total applied voltage as high as possible drops at the thin oxide  126  of the MOS capacity  114 , and affects the Fowler Nordheim tunneling there. This is achieved constructively by making the lateral dimensions of the thin oxide  126  defined by a layout smaller than those of the oxide  126  between the control electrode  130  and the floating  104 . If, therefore, the voltage applied to the control terminal  102  is ramped up, the two fractional voltages rise as well, until finally the larger fractional voltage—namely the one at the thin oxide  126 —achieves a field strength of, for example, about 8 MV/cm to about 9 MV/cm. Then, the Fowler Nordheim tunneling begins, i.e. a small stream flows above the thin oxide  126  from the floating gate  104  to the MOS channel and the substrate  116 , respectively. The rise time of the voltage ramp should thereby be so small that those processes run more or less statically. Thus, a small Fowler Nordheim tunnel current is enough to load the coupling capacity  112  sufficiently fast, so that the potential at the floating gate  104  remains, for example, at a value of Um=8 . . . 9 MV/cm*D. Thereby, D indicates a thickness of the thin oxide  126 .  
           [0012]    If a maximum programming voltage is finally Up, a difference between Up and Um is stored at the coupling capacity  112 . This can be expressed as follows:  
           | UP|−Um=|Uc|=|Q ( FG )|* Cc    
           [0013]    Thereby, Q(FG) is the charge stored at the floating gate  104 , and Cc is the value of the coupling capacity  112 . In the above expression, for simplicity reasons, the sign was omitted, which reflects in the amount-like version of the charge stored at the floating gate  104 . The normal rise times of the voltage ramps are between 100 μs 10 ms.  
           [0014]    If, however, the rise time of the programming pulse is shorter, the small Fowler Nordheim tunnel stream is no longer sufficient to load the coupling capacity  112  sufficiently fast, so that the voltage at the thin oxide  126  rises above Um, and damages the thin oxide  126  more than absolutely necessary. This pre-damage is shown by the fact that the floating gate  104  looses the charge over time, and it comes to a data loss, which, of course, has to be avoided.  
           [0015]    A bit error occurs, if, for example, high voltages at the thin oxide  126  (also referred to as gate oxide) of the MOS capacity  114  change a charge state of the floating gate  104 , whereby, for example, the sign of the stored charge changes. Often, however, it is already sufficient for a bit error when merely the amount of the charge stored at the floating gate is reduced sufficiently, without changing the sign. The reason therefore is that it is difficult in practice to apply the exact UV level as an ideal discrimination value between the positive and the negative charge at the floating gate  104  at the control electrode  102 . On the one hand, there are the discrimination value varies with temperature, on the other hand, the strong variations from batch to batch, disc to disc, chip to chip and even within a chip in dependency of a position of the memory element (memory cell) in the chip, which are common in the memory in the semiconductor technology.  
           [0016]    In practice, it can happen that an integrated circuit (IC) is subject to a high electrical field. This is particularly the case in electrically “rough” environment, such as in the use in an automobile. Therefore, it is extremely important in security relevant applications that the IC functions properly despite an adverse electrical environment, particularly that it is not damaged thereby. For a function of the IC, often those data are relevant which are stored in the EEPROM cell. As an example, imagine an integrated magnetic-field sensor, which is to detect a blocking of a wheel in an ABS system (ABS=anti blocking system), and whose calibration data are stored on chip in an EEPROM. If these calibration data are, for example, lost by an electrical field pulse, the blocking of a wheel is continuously not detected, or in another extreme case, never detected. The consequences of such damage can be dramatic.  
           [0017]    Particularly high electric fields can occur in ESD events (ESD=electrostatic discharge). Here, again, that case is to be seen as worst case, where the discharge takes place above a housing to the IC. Thereby, a tip of an ESD pistol is held directly to a surface of the IC, and, for example, charged to several kilovolt against ground. If the IC is applied to ground at the same time, an ESD flash can discharge above that part of a compound of the housing directly under the tip of the ESD pistol. If an EEPROM cell is within this area on the surface of IC, it can be damaged thereby.  
           [0018]    In the following, the orders of the influence of an ESD event to an EEPROM cell are estimated.  
           [0019]    As a model, the tip of the ESD pistol and the ESD cell is replaced by a small ball, since the capacity of a ball spark gap can be calculated analytically without too much effort as follows:  
       C   =         πɛ   0          (       2      R     +   g     )              ∑     p   =   1     ∞                     1     sinh        (     par                   sinh        (     1   +     g     2      R         )         )                                   
 
           [0020]    Thereby, R is the radius of the ba s and g the distance of their facing surfaces, ε 0  is the electrical field constant. For thin housings, particularly with magnetic-field sensors, g=0.15 mm has to be assumed. arsinh ( . . . ) is the arcus sinus hyperbolic function and p is the sum index. Depending on the radius of the ball R, the following stray capacitances result from the above expression:  
                                                                     R   C                                100   nm   5.6   aF           1   μm   56   aF       10   μm   0.59   fF       100   μm   7.4   fF       1   mm   8.7   fF                  
 
           [0021]    In practice, naturally, the EEPROM cell is very small (smaller than 1 μm), the tip of an ESD diode or the fingertip of a human being is comparatively large (larger than 1 mm), so that the above model of the ball spark gap is not very well suitable for an exact calculation of the influence of the ESD event on an EEPROM cell. Still, this model can be used for a best-case estimation. The stray capacity between the fingertip and the EEPROM cell is definitely larger than the one between two balls with 1 μm radius, since the fingertip is significantly larger than 1 μm and according to the above table, the capacity rises also with increasing radius. Even this small stray capacity of 56 aF between fingertip and floating gate  104  effects that at an assumed coupling capacity  111  Cc=20 fF already the 20/0.056=357-th part of the ESD voltage is applied to the thin oxide. Thus, at a value of the ESD voltage of, for example, 4.6 kV, 13 V are applied to the thin oxide. This is already sufficient for the FN tunneling for a thickness of 12 nm assumed in this example.  
           [0022]    Due to the above-described problem it is thus necessary to shield the floating gate  104  appropriately. This can, for example, be realized by covering the floating gate  104  fully with the control electrode  130 , so that the control electrode  130  electrically shields the floating gate  104 . This shield effect can be insufficient, when the control electrode  130  is not low-resistively connected to the substrate  116  of the EEPROM cell. Assumed that a person who is electrostatically charged touches the IC. The stray capacity between the fingertip of the person and the control electrode  130  raises-the potential of the control electrode  130 , when the control electrode  130  is not brought to a defined potential by other circuit parts. This case occurs particularly when the IC is not supplied with a voltage, because then the inner nodes of an MOS circuit have a high impedance, since no sufficient potentials are present at the gates of the belonging MOS transistors in order to switch those to being conductive. The coupling capacity  112  formed by the floating gate  104 , the oxide layer  128  and the control electrode  130  influences then subsequently the potential of the floating gate  104 , so that its charge state might change.  
         SUMMARY OF THE INVENTION  
         [0023]    It is the object of the present invention to provide a semiconductor structure with an efficient shield of a floating gate arranged in a semiconductor structure.  
           [0024]    In accordance with a first aspect, the present invention provides a semiconductor structure having a memory element with a floating gate, a control electrode, which is capacitively coupled to the floating gate, wherein a signal for controlling the memory element is applicable to the control electrode, and a shield, which is arranged isolated from the floating gate and covers it fully.  
           [0025]    The present invention is based on the knowledge that both the floating gate and the control electrode can be formed during a semiconductor manufacturing process such that the floating gate is fully covered with a further conductive layer and can thus be shielded.  
           [0026]    Preferably, both the floating gate and the control electrode are covered with a conductive layer, which is preferably electrically isolated from the floating gate and the control electrode, wherein the conductive layer is low-resistively connected, preferably with the bulk of the EEPROM cell (e. g. p-substrate of the MOS transistor or ah n-sink of a PMOS transistor). Since the bulk of the EEPROM is mostly also connected to the system ground (e. g. 0 V), the conductive layer is thus also connected to the system ground.  
           [0027]    The conductive layer can be realized by inserting an additional conductive layer during the semiconductor manufacturing process, which covers the appropriately arranged floating gate. Mostly, the floating gate and the control electrode overlap to some extend anyway, so that the conductive layer (shield) merely has to lie above both electrodes.  
           [0028]    If the wiring levels and the floating gate and the control electrode are arranged in a predetermined positional relationship to each other, the formation of an additional conductive layer can be omitted during the semiconductor manufacturing process, since the shield can be realized by a wiring level. Since the wiring levels have a low impedance, they also have a good shielding effect, even at transient events (such as the actual ESD impact, “flash”), due to the low penetration depths. This is another reason why the shield should be connected to the bulk of an EEPROM with an impedance as low as possible.  
           [0029]    Since there is a constantly increasing number of wiring levels (supply levels) in modern semiconductor technologies, it should be considered which of the wiring levels is ideally suited for a realization of the shield. This problem will be discussed below.  
           [0030]    If the control electrode is formed such that it lies below the floating gate (see FIG. 7), a remarkable stray capacity arises between the floating gate (particularly between the part  104   b  much larger than  104   a,    104   c ), and the wiring levels lying above it, which form the inventive shield. However, this is disadvantageous in programming the EEPROM, since this stray capacity decreases the efficiency of the coupling capacity, i.e. the shield is applied to ground and prevents with its stray capacity against the floating gate the floating gate from following a programming pulse at the control electrode. Thus, it is to be preferred to use an upper wiring level for the shield, i.e. a wiring level whose vertical distance to the floating gate is as large as possible. The distance between the used wiring level and the floating gate should, however, still be minuscully small against a housing thickness over this part of the IC, so that the shield effect is sufficiently good. Otherwise, the electric field lines surround the shield and form again a stray capacity between, for example, a fingertip and an ESD pistol, respectively, and the floating gate.  
           [0031]    If the floating gate is formed such that it is below the control electrode (see FIG. 8), then it is optimal in this case to use a lower wiring level lying above the floating gate and above the control electrode, i.e. a wiring level whose vertical distance to the floating gate is as low as possible, for the shield, since the stray capacity of the lower wiring level to the control electrode, which is respectively large in this case, does not matter. The reason therefore is that the control electrode is controlled, for example, by a voltage source in programming, which can charge this stray capacity against ground without difficulty such that an appropriate programming voltage arises. If, however, the stray capacity between  104   a  and the shield already significantly reduces the coupling efficiency of the coupling capacity to the floating gate, it can even be advisable in that case, to form the shield in a higher wiring level (such as  136   b,    136   c ). Thereby, the coupling efficiency is defined as follows:  
           0 &lt;Um/Up&lt; 1.  
           [0032]    To minimize the above-mentioned surrounding of the field lines around the shield, the shield should not only cover the floating gate and the control electrode flush, but should protrude for several micrometers. Generally, the shield should protrude so far as a normal distance of the shield to the floating gate of the EEPROM cell (which means in a vertical direction to the chip).  
           [0033]    It is an object of the present invention that by inserting a conductive layer during the semiconductor manufacturing process, the floating gate is shielded against external interferences independent of a spatial extension of the control electrode, so that less bit errors are caused due to electrostatical interferences.  
           [0034]    It is another object of the present invention that a good shield of the floating gate can be achieved without increasing the costs of the semiconductor manufacturing process, by arranging the floating gate and the overlaying wiring levels already during the manufacturing process such that the floating gate is shielded by one or by several wiring levels.  
           [0035]    Although above an electrostatical shield was discussed primarily, it is also necessary to keep transient (i.e. electromagnetical) fields away from the floating gate by a shield, which is also achieved by the inventive shield. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    Preferred embodiments of the present invention will be discussed in more detail below with reference to the accompanying drawings. They show:  
         [0037]    [0037]FIG. 1 a schematic diagram of an EEPROM cell;  
         [0038]    [0038]FIG. 2 a setup of an EEPROM cell;  
         [0039]    [0039]FIG. 3 a schematic diagram of a first embodiment of an EEPROM cell according to the present invention;  
         [0040]    [0040]FIG. 4 a setup of a semiconductor structure according to the embodiment illustrated in FIG. 3;  
         [0041]    [0041]FIG. 5 a schematic diagram of a further embodiment of an EEPROM cell according to the present invention;  
         [0042]    [0042]FIG. 6 a setup of a semiconductor structure according to the embodiment illustrated in FIG. 5;  
         [0043]    [0043]FIG. 7 a further embodiment of a semiconductor structure according to the present invention; and  
         [0044]    [0044]FIG. 8 a further embodiment of a semiconductor structure according to the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    In the following description of the preferred embodiments, elements, which have already been explained with reference to FIG. 1 and  2  are provided with the same reference numbers. There will be no further description of these elements. Further, the same elements in the Figures are provided with the same reference numbers.  
         [0046]    [0046]FIG. 3 shows a schematic diagram of the inventive semiconductor structure. An EEPROM cell  100  is shown, as it has already been described with reference to FIG. 1. As can be seen, the bulk terminal  110  is connected to a potential  134 , e. g. ground. According to the invention, a shield  136  is provided, which is also connected to the potential  132  and shields the floating gate  104 , as it is schematically illustrated.  
         [0047]    [0047]FIG. 4 shows an embodiment of a setup of a semiconductor structure according to the embodiment illustrated in FIG. 3. The semiconductor structure (n type EEPROM) comprises, similar to FIG. 2, a substrate (p bulk)  116 . The n +  source area  118  and the n +  drain area  120  are formed in the substrate  116 . The drain and source terminals illustrated in FIG. 2 are not illustrated in FIG. 4 for simplicity. Further, a first p +  area  138  and a second p +  area  140  are formed in a substrate  116 . A thin oxide layer  126  is formed on the substrate  116 , which extends on the surface of the substrate  116  between the n +  source area  118  and the n +  drain area  120 .  
         [0048]    The floating gate is formed by a first floating gate electrode  104   a,  a second floating gate electrode  104   b  and a connection electrode  104   c  between the first and the second floating gate electrode. The second floating gate electrode  104   b  is arranged laterally shifted with regard to the first floating gate electrode  104   a.  The control electrode  130  is disposed opposite to the second floating gate electrode, with a lesser vertical distance to the surface of the substrate  116 . The oxide layer  128  is formed between the second floating gate electrode  104   b  and the control electrode  130 . Thus, as can be seen in FIG. 4, the floating gate is formed above the control electrode  130  with reference to the surface of the substrate  116 . The first floating gate electrode  104   a,  the second floating gate electrode  104   b,  the connection electrode  104   c,  the control electrode  130  and the oxide layers  126 ,  128  are formed in a first portion  142  above the substrate  142 .  
         [0049]    The shield  136 , which is, for example, formed as a metallizing layer, is disposed in a second portion  144  above the first portion  142 . The shield  136  is arranged such that it fully covers all portions  104   a,    104   b,    104   c  of the floating gate, and preferably protrudes on the side, respectively. The shield  136  is further connected to the p +  area  138 , which is formed in the substrate  116 , via a connection  146 , so that the potential applied to the substrate  116  is applicable above the bulk terminal  110 , which is connected to the area  140  and designed towards the top, to the shield  136  so that it is accessible from the top, as it is illustrated in FIG. 3. Thereby, the shield  136  keeps an electromagnetical and an electrostatical influence away from the sensible floating gate, wherein the conductive bulk (substrate  116 ) provides a shielding effect from below. The control electrode  130  is connected to the control terminal  102  via a connection  148 , which extends through the second portion  144 .  
         [0050]    In the embodiment illustrated in FIG. 4, the shield  136  is formed continuously. For the case illustrated in FIG. 4 that the floating gate has a plurality of portions  104   a,    104   b,    104   c,  the shield  136  can be formed according to the invention, such that it also comprises a plurality of conductive areas, which mainly lie on the same potential and cover the plurality of portions of the floating gate.  
         [0051]    The EEPROM cell can be realized in a single well CMOS process with an analog option by a standard NMOS transistor and, for example, a standard poly-poly capacitor, when the gate of the NMOS is connected to one of the two electrodes of the poly-poly capacitor to the floating gate. According to the invention, the floating gate is arranged such that it is fully covered by the shield  136  (the low impedance layer) towards the top. Above that, this layer is conductively connected to the bulk of the NMOS (for example directly realized in a substrate), which is formed by a substrate of the IC. Thereby, the stored charge at the floating gate is protected from ESD and other voltage events as well as from electromagnetical fields, since the floating gate is electrostatically shielded towards the top by the shield  136  and towards the bottom by the likewise conductive substrate  136 , such as a p-substrate, which is mostly supported in its shielding effect by a still better conductive supply frame lying below it. Therefore, the shield  136  (the screen) should cover all parts of the floating gate and protrude sufficiently on the sides. Since the p-substrate  116  is normally put on reference potential, this requirement is identical with putting the shield  136  also put on the reference potential.  
         [0052]    Analogous considerations hold true for an EEPROM cell, consisting of a PMOS transistor by using an n-substrate (exchange of n- and p-doping) and a p-substrate with n-well, respectively.  
         [0053]    [0053]FIG. 5 shows a schematic diagram of a further embodiment of an EEPROM cell according to the present invention. Different to the embodiment illustrated in FIG. 3, the EEPROM cell illustrated in FIG. 5 has a terminal  150 . The bulk terminal  110  is connected to the terminal  150 . Above that, the shield  136  is connected to the terminal  150 , so that the shield  136  is directly connected to the bulk terminal  110 . Above the terminal  150 , a potential is applicable both to the bulk terminal  110  and to the shield  136 . This embodiment is thus advantageous for CMOS transistors with their own well, which is not identical to the substrate.  
         [0054]    [0054]FIG. 6 showed a setup—similar to FIG. 4—of a semiconductor structure according to the embodiment illustrated in FIG. 5. Different to FIG. 4, an n-well  152  is formed in a substrate  116 , wherein again a p-well  154  is formed, wherein the source area  118  and the drain area  120  as well as the first p +  area  138  are formed. Further, an n +  area  156  is formed in the n-well  152 , above which a potential, e.g.  3  Volt, is applicable to the n-well  152 . Above the first p +  area  134 , a potential is applicable to the p-well and thus also to the shield  136 , as it is schematically shown by the terminal  150 . By the fact that the shield  136  and the p-well terminal  150  are on the same potential, it is achieved that the floating gate  104  is shielded both by the shield  136  as well as by the p-well  154 .  
         [0055]    The significant difference between the embodiment illustrated in FIG. 3 and FIG. 5 is that in the embodiment according to FIG. 5, the common potential of p-well  154  and shield  136  is freely available, while in the embodiment according to FIG. 3, the common potential is identical to ground.  
         [0056]    By the fact that the p-well  154  covers the floating gate formed of portions  104   a,    104   b,    104   c  from below, it is achieved that the floating gate is arranged between the shield  136  and the p-well  154 , whereby a better shield is achieved from below.  
         [0057]    According to the invention, the floating gate  104 , which is accessible towards the top, is preferably fully covered by a low-impedance layer (shield  136 ), and this layer is connected to the substrate  116  and a p-well  154 , respectively. Thereby, the stored charge at the floating gate  104  is protected from ESD and other high voltage events, since the floating gate  104  is shielded towards the top by the shield  136 , and towards the bottom by the likewise conductive substrate and the p-well, respectively. Therefore, the shield  136  should preferably cover all parts of the floating gate  104  and protrude sufficiently on the sides. Preferably, in the case of a p-well, it is extended such that all parts of the floating gate  104  are covered seen from below.  
         [0058]    In FIG. 7, another embodiment of the semiconductor structure is illustrated according to the present invention. Different to the embodiment illustrated in FIG. 4, the semiconductor structure illustrated in FIG. 7 has a plurality of metallizing levels or supply levels  136   a  to  136   c  in the second portion  144 . It should be noted here, that an arbitrary number of supply levels could be provided.  
         [0059]    In the embodiment illustrated in FIG. 7, the floating gatt (portions  104   a,    104   b,    104   c ) is disposed over the control electrode  130 , i.e. the vertical distance of the control electrode  130  to the substrate  116  is smaller than the distance of the second floating gate electrode  104   a  to the substrate  116 . For such an arrangement of the floating gate  104  it is advantageous to use a supply level or wiring level for the shield, whose distance to the floating gate is highest, since the stray capacity between floating gate and shield should be kept small. In the embodiment illustrated in FIG. 7, the supply level  136   c  is chosen as shield, which is connected to the substrate  116  like in FIG. 4.  
         [0060]    The supply levels  136   a,    136   b,    136   c  are several elements out of which the shield can be formed. If, for example, the first supply level  136   a  is arranged such that it is not optimally accessible to establish a connection to the substrate  116 , the second supply level  136   b,  for example, can be used as shield element. If the extension of the supply level is not sufficient to cover the floating gate, several supply levels or portions of several supply levels can be interconnected to obtain a sufficient coverage.  
         [0061]    [0061]FIG. 8 shows a further embodiment of a semiconductor structure according to the present invention. Similar to FIG. 7, the floating gate is here also formed by the first floating gate electrode  104   a,  the second floating gate electrode  104   b  and the connection electrode  104   c  between the first and the second floating gate electrode. The second floating gate electrode  104   b  is arranged laterally shifted with regard to the first floating gate electrode  104   a.  The control electrode  130  is arranged opposite to the second floating gate electrode, but with a higher vertical distance to the surface of the substrate  116 . Thus, the floating gate is formed below the control electrode with reference to the surface of the substrate  116 .  
         [0062]    In this case, however, different to FIG. 7, the first supply level  136   c,  whose distance to the control electrode  130  is the smallest, is connected to the substrate  116 .  
         [0063]    Here, it should be noted that the embodiments discussed above with reference to an n-type EEPROM are analogously valid for a p-type EEPROM, wherein then the source and the drain areas are p-doped and formed either in an n-substrate or in an n-well in a p-substrate.  
         [0064]    While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.