Abstract:
Described herein is a protective structure. The protective structure includes a semiconductor substrate, a first diode disposed at least one of in or on the semiconductor substrate and a diode arrangement disposed at least one of in or on the semiconductor substrate. The diode arrangement includes a stack of a second diode and a transient voltage suppressor (TVS) diode connected in series with the second diode. The diode arrangement is in parallel with the first diode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 14/260,301, filed on Apr. 24, 2014, now U.S. Pat. No. 8,951,879, which is a divisional of application Ser. No. 14/011,885, filed on Aug. 28, 2013, now U.S. Pat. No. 8,766,415, which is a continuation of application Ser. No. 13/566,039, filed on Aug. 3, 2012, now U.S. Pat. No. 8,531,011, which is a continuation of application Ser. No. 12/958,929, filed on Apr. 12, 2011, now U.S. Pat. No. 8,263,481, which is a continuation of application Ser. No. 12/120,401, filed on May 14, 2008, now U.S. Pat. No. 7,888,232, which claims priority to German Patent Application Serial No. DE 10 2007 024 355.5, which was filed May 24, 2007, all of which are hereby incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     Exemplary embodiments of the invention relate to a method for producing a protective structure, and the protective structure. 
     BACKGROUND OF THE INVENTION 
     Electrostatic discharges (ESD) can severely damage semiconductor components. Therefore, ESD protective concepts are realized in most semiconductor components nowadays. The ESD protective components integrated in the chip define a current path via which the ESD current can flow without causing damage. 
     In the case of high-speed data transmission lines, in particular, the requirement exists of ensuring an ESD protection up to 15 kV without the signal waveform being distorted to an excessively great extent. For this purpose, the protective element must have a particularly low capacitance. 
     Known ESD protective structures have structures that are complicated and have inaccurate producability on account of the high substrate thickness for the diode arrangement. 
     The dopant diffusion through the complete substrate thickness leads to inaccuracies and a high space requirement owing to the lateral outdiffusion that likewise takes place. The high substrate thickness brings about a high resistance, which adversely affects the performance of the protective structure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Exemplary embodiments of the invention are explained in more detail below, referring to the accompanying figures. However, the invention is not restricted to the embodiments described in concrete fashion, but rather can be modified and adapted in a suitable manner. It lies within the scope of the invention to combine individual features and feature combinations of one embodiment with features and feature combinations of another embodiment in a suitable manner in order to attain further embodiments according to the invention. 
       In the figures: 
         FIG. 1  shows a schematic cross-sectional view of a protective structure, and 
         FIG. 2  shows a schematic cross-sectional view of a further embodiment of the protective structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention specifies a cost-effective method for producing a protective structure having a high performance. 
     Embodiments of the invention generally relate to a method for producing a protective structure which involves providing a semiconductor substrate with a doping of a first conductivity type, applying a semiconductor layer with a doping of a second conductivity type at a surface of the semiconductor substrate, forming a buried layer with a doping of a second conductivity type in a first region of the semiconductor layer, wherein the buried layer is produced at the junction between the semiconductor layer and the semiconductor substrate, forming a first dopant zone with a doping of a first conductivity type in the first region of the semiconductor layer above the buried layer, forming a second dopant zone with a doping of a second conductivity type in a second region of the semiconductor layer, forming an electrical insulation between the first region and the second region of the semiconductor layer, and forming a common connection device for the first dopant zone and the second dopant zone. 
     By forming an adapted semiconductor layer, it is possible for the thickness of said semiconductor layer to be restricted to the bare essentials. A small semiconductor layer thickness, however, permits a reduction of the structure widths and lengths, in particular by virtue of shorter process times such as e.g. outdiffusions, which leads to cost savings. 
     Moreover, the resistance of a current flowing through the protective structure is lowered by a small semiconductor layer thickness, which means a low clamping voltage and therefore improves the performance of the protective structure. 
     Embodiments of the invention specifically relate to a method for producing a protective structure which involves providing a semiconductor base substrate with a doping of a first conductivity type, producing a first epitaxial layer on the semiconductor base substrate, implanting a dopant of the second conductivity type in a delimited implantation region of the first epitaxial layer, applying a second epitaxial layer with doping of a second conductivity type on the first epitaxial layer, forming insulation zones in the second epitaxial layer, such that the second epitaxial layer is subdivided into a first region and into a second region, producing a first dopant zone with a doping of a first conductivity type in the first region above the implantation region, producing a second dopant zone with a doping of a second conductivity type in the second region, outdiffusing the dopant from the implantation region in order to form a buried layer at the junction between the first epitaxial layer and the first region of the second epitaxial layer. 
     By means of the first epitaxial layer, in particular by means of the thickness thereof, it is possible to adapt to the requirements the junction between first and second conductivity type in the first epitaxial layer, especially with regard to the breakdown voltage of the PN junction. 
     The thickness of the second epitaxial layer once again determines the costs and the performance of the protective structure, which can lead to improvements through corresponding adaptation of the second epitaxial layer. 
     Before the exemplary embodiments of the present invention are explained in more detail below with reference to the figures, it is pointed out that identical elements in the figures are provided with identical or similar reference symbols, and repeated description of said elements is omitted. 
       FIG. 1  illustrates a first embodiment of a protective structure  100  produced according to the method according to the invention. A semiconductor layer  120  is applied on a surface of a semiconductor substrate  110  provided, said substrate having a doping of a first conductivity type. The semiconductor layer  120  has a doping of a second conductivity type. The dopant concentration of the semiconductor layer  120  is kept as low as possible in order to achieve a minimal capacitive disturbing effect of the protective structure. Therefore, the dopant concentration of the semiconductor layer should not exceed 1*10 15  cm −3 . The semiconductor layer  120  is produced epitaxially, for example. In particular, the semiconductor layer  120  is produced with a thickness d1 of 2 μm≦d1&lt;20 μm. 
     The semiconductor substrate  110  has a dopant concentration of at least 1×10 18  cm −3 , while the semiconductor layer  120  is produced with a lower dopant concentration than the semiconductor substrate  110 . 
     A buried layer  140  with a doping of a second conductivity type is formed in a first region  150  of the semiconductor layer  120 . The buried layer  140  is produced at the junction  170  between the semiconductor layer  120  and the semiconductor substrate  110 . This is done for example by implantation of a dopant at the surface of the semiconductor substrate  110  and subsequent outdiffusion of the dopant into the semiconductor layer  120 . 
     As an alternative, the dopant can also be provided in the form of a deposited layer on the semiconductor substrate  120 . 
     The provision of dopant for the buried layer  140  is generally effected before the semiconductor layer  120  is produced on the semiconductor substrate  110 . In this case, the buried layer  140  is formed with a higher dopant concentration that the semiconductor layer  120 . 
     The annealing of the dopants in order to form the buried layer is effected over a time period of 1≦t≦30 minutes at a temperature T of 1000° C.≦T≦1200° C., in particular 5 minutes at 1150° C. 
     As dopant for the buried layer  140 , boron is appropriate as P-type dopant and As, P are appropriate as N-type dopant. 
     The buried layer  140  with a dopant of a second conductivity type forms together with the semiconductor substrate  110  a PN junction, that is to say a diode, in particular a TVS (transient voltage suppressor) diode. 
     Above the buried layer  140 , a first dopant zone  180  with a doping of a first conductivity type is formed in the first region  150  of the semiconductor layer  120 . For this purpose, by way of example, the dopant is implanted into the semiconductor layer  120  and subsequently activated and outdiffused. The outdiffusion takes place over a time period t2 of approximately 10 seconds at 950° C. in order to obtain a first diffusion zone  180  that is as narrow as possible. 
     Part of the semiconductor layer  120  remains between the first dopant zone  180  and the buried layer  140 . The layer sequence composed of a first dopant zone  180  with a doping of a first conductivity type, semiconductor layer  120  with a very low doping of a second conductivity type and buried layer  140  with a doping of a second conductivity type forms a PIN or NIP diode, The buried layer  140  is therefore used firstly as part of the PIN/NIP diode and secondly as part of the TVS diode. PIN/NIP diode and TVS are connected in series in this case. 
     A second dopant zone  190  with a doping of second conductivity type is formed in a second region  160  of the semiconductor layer  120 . The formation can likewise be effected by implantation of a dopant and subsequent activation and outdiffusion of the dopant. The outdiffusion time is once again set in such a way that a second dopant zone  190  that is as narrow as possible arises. This saves time, space and thus also costs. 
     The second dopant zone  190  is produced in the same way as the first dopant zone  180  at that side of the semiconductor layer  120  which is near the surface. Consequently, part of the semiconductor layer  120  also remains between second dopant zone  190  and semiconductor substrate  110 . As a result, the layer sequence of second dopant zone  190  with a doping of the second conductivity type, semiconductor layer  120  with a very low doping of a second conductivity type and semiconductor substrate  110  with a doping of the first conductivity type forms a PIN or NIP diode. 
     The first region  150  and the second region  160  are arranged alongside one another in the semiconductor layer  120 . Consequently, the series-connected diode arrangement in the first region  150  is in parallel with the PIN/NIP diode in the second region. The PIN/NIP diode in the first region  150  is biased in the opposite direction to the NIP/PIN diode in the second region  160 . 
     Electrical insulation structures  125  are formed in the semiconductor layer  120 , which insulation structures laterally delimit the first region  150  and the second region  160  and isolate them from one another. In this case, the electrical insulation  125  is formed with a depth such that it extends from the surface of the semiconductor layer  120  at least as far as the semiconductor substrate  110  through the semiconductor layer  120 . The electrical insulation  125  can be produced in a manner that is all the more rapid and saves space all the more, the smaller the layer thickness d1 chosen for the semiconductor layer  120 . 
     By way of example, the electrical insulation  125  is produced by implantation and outdiffusion of a dopant of a first conductivity type. Since the outdiffusion is intended to be effected through the entire semiconductor layer  120 , long diffusion times are necessary in the case of a large semiconductor layer thickness d1. In this case, the dopant will also out diffuse laterally, which leads to a significant widening of the structures and the space requirement therefore rises. Long production times and a large space requirement are critical factors for higher costs, however. Therefore, the layer thickness d1 of the semiconductor layer should be kept as small as possible. On the other hand, the parasitic capacitance of the PIN/NIP diodes is increased by a small layer thickness, which leads to higher disturbing influences such as e.g. distortions of the signals in the application of the protective structure. Consequently, the thickness d1 of the semiconductor layer must be chosen in such a way as to achieve a compromise between capacitance and production costs. This compromise currently lies at a layer thickness d1≅8 μm. For the outdiffusion of the dopant through the semiconductor layer  120 , diffusion times of 60 minutes at a diffusion temperature of 1150° are necessary for this. 
     As an alternative, the electrical insulation  125  can also be produced by producing a trench through the semiconductor layer  120 . Said trench can in particular also be at least partly filled with electrically insulating material. In this case, too, the dimension of the trench is influenced by the layer thickness d1 of the semiconductor layer  120 . In order to produce a deep trench, the width of the trench has to be increased. Consequently, a thick semiconductor layer also affects the width of the trench, which leads to a higher space requirement for the insulation and therefore likewise increases the costs. 
     A common connection device  135  is formed for the first dopant zone  180  and the second dopant zone  190 . This is done for example by producing an insulation layer  145 , such as an oxide layer, for example, on the semiconductor layer  120 . In said insulation layer  145 , openings are formed above the first dopant zone  180  and above the second dopant zone  190 . The electrically conductive connection structure  135 , such as a metal layer or a polysilicon layer, for example, is then produced on the insulation layer  145  and in the openings. The connection device  145  therefore makes contact with the first dopant zone  180  and the second dopant zone  190 . 
       FIG. 1  schematically outlines a circuit in order to illustrate the functioning of the protective structure. If a negative voltage is present for example at the input node I/O then the current will flow away to ground GND via the PIN diode of the second region  160 . By contrast, if a positive voltage is present at the I/O, then the PIN diode of the second region  160  is reverse-biased and the current is conducted through the forward-biased PIN diode in the first region  150 . However, a current will flow only when the voltage of the initially turned-off TVS diode exceeds the threshold voltage. As soon as said voltage is exceeded, the TVS diode starts to conduct and the current can flow away via the first region  180  to the ground connection GND at the rear side of the semiconductor substrate  110 . Consequently, the circuit in which the protective structure  100  is integrated is protected against over voltages that are greater than the threshold voltage of the TVS diode. 
       FIG. 2  shows a further embodiment of a protective structure according to the invention. 
     The protective structure  200  has developments of the protective structure  100  from  FIG. 1 . Thus, by way of example, the semiconductor substrate  110  is composed of a semiconductor base substrate  210  and an intrinsic layer  220  produced thereon. 
     The intrinsic layer  220  is produced epitaxially, for example, and has no or only a very low doping of an arbitrary conduction type. 
     By contrast, the semiconductor base substrate  210  has a very high doping with a dopant of a first conductivity type. Subsequent thermal steps cause the doping to out diffuse from the semiconductor base substrate  210  into the intrinsic layer  220 . 
     In this case, the intrinsic layer  220  acts as a buffer layer for the outdiffusion from the semiconductor base substrate  210 . The thicker the intrinsic layer  220 , the further the dopant out diffuses from the semiconductor base substrate  210  to the pn junction. As a result, the junction takes place at lower, flatter doping profiles, which leads to an increase in the breakdown voltage. Consequently, the breakdown voltage of the TVS diode can be set by the thickness of the intrinsic layer  220 . 
     Moreover, the electrical insulation is produced in a two-part process in the protective structure  200  as a development of the protective structure  100 . In this case, firstly a dopant is implanted into the semiconductor substrate  110 . After the semiconductor layer  120  has been produced, dopant is once again implanted, but now into the semiconductor layer  120 . The dopant is subsequently outdiffused, which leads to the formation of two dopant regions  125 ′ and  125 ″ lying one above another, which together produce the electrical insulation  125 . As a result, it is possible to produce for example deeper electrical insulation  125  right into the semiconductor substrate  110 . 
     A further development of the method according to the invention is that at least one connecting zone  230  is formed between the connection device  135  and the buried layer  140 . Consequently, the buried layer can be allocated a bias voltage, for example. For this purpose, the connecting zone  230  is formed for example with a dopant of a second conductivity type and with a dopant concentration k v &gt;1×10 17  cm −3 . Moreover, the connecting zone can be formed in two-part form with two zones  230 ′ and  230 ″ lying one above another. 
     Moreover, the protective structure  200  has by comparison with the protective structure  100  the development that a metallic layer  240  is produced at the rear side of the semiconductor substrate  110 , which metallic layer can be used as a substrate connection. 
     However, the substrate connection can also be led to the surface via the electrical insulation  125  (p-type sinker), whereby for example a WLP package also becomes possible. 
     All the developments in the protective structure  200  can be made individually or in combination, which respectively leads to further embodiments of the invention. 
     As already explained with regard to  FIG. 1 ,  FIG. 2  once again shows the schematic circuit diagram with the diodes forming the protective structure.