Abstract:
A semiconductor structure for high frequency operation has a substrate with a doped well formed therein and a buffer layer made of a substrate material covers the well. The buffer layer is made of an undoped material and is disposed on a top side of the well for inhibiting an outdiffusion of a dopant from the well. At least a portion of the substrate is not covered by the buffer layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/EP01/02471, filed Mar. 5, 2001, which designated the United States and was not published in English. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for manufacturing a high frequency semiconductor structure having the following method steps: preparing a substrate; forming a well on one side of the substrate; and epitaxially growing a buffer layer above the well. 
     The invention also relates to a semiconductor structure having a well that is formed in a substrate and a buffer layer made of substrate material formed above the well. 
     Issued German Patent DE 197 37 360 C1 discloses a high frequency Schottky diode and a method for manufacturing the diode. An n-type doped layer is formed in a p-type doped substrate in order to manufacture the known high frequency Schottky diode. In a subsequent step, an n-type doped epitaxial layer is applied to the well. The epitaxial layer is then patterned using photographic technology and plasma etching such that only a central region of the well continues to be covered by the epitaxial layer. This results in an elevation on the well that is surrounded by an annular depression. In subsequent method steps, inter alia, a metal layer is deposited on the elevation in order to form a Schottky contact. 
     The well is doped with arsenic and has a dopant content of 5×10 19  cm −3  to 6×10 19  cm −3 . In addition, the substrate is defined by a resistivity of more than 3 kΩcm. This effectively limits the losses of the high frequency Schottky diode. 
     In order to manufacture hand-held mobile phone devices, all of the components of the circuit arrangement should be monolithically integrated as much as possible. The operating frequencies provided are around 1 GHz and above. In this frequency range stringent requirements are made of the quality of the passive and active components used. In this frequency range, discrete circuits generally have an average quality of 100 when averaged over the various active and passive components. Thus, for example, the combination of a high quality resonator (Q=200) with average quality diodes (Q=50) still leads to an average quality in the region of 100. As should be expected, peak quality levels cannot be achieved when active and passive components are integrated, therefore an average quality of approximately 100 must be required for the individual active and passive components. In particular, it is necessary also to pay attention to the quality of the transistors. Although the loss that happens to be present is compensated by the transistor gain, increasing the transistor gain also entails an increase in the noise in the circuit. For this reason, the transistor losses should also be kept as low as possible. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a high-frequency semiconductor structure and a method for manufacturing the semiconductor structure, which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type. 
     In particular, it is an object of the invention to provide a method for manufacturing semiconductor structures that are as loss-free as possible. 
     In addition, it is an object of the invention to provide a semiconductor structure with low losses, and thus with a high quality level. 
     In the method and in the semiconductor structure, in each case a buffer layer is provided that covers the well that is formed in the substrate. Undoped material is provided for manufacturing the epitaxially applied buffer layer. A result of the autodoping is that the buffer layer contains doping atoms that evaporate from the well during the epitaxial growth of the buffer layer, and these doping atoms are introduced into the buffer layer. This results in a conductive layer. As leakage currents can flow via the conductive buffer layer, that part of the buffer layer that is not supported on the well must be removed. By removing the layer that rests directly on the substrate, the bleeder resistance is effectively increased. This is because hardly any autodoping takes place in successive epitaxy operations because of the low dopant content at the surface of the buffer layer. Accordingly, the subsequent epitaxial layers have a low conductivity. In order to limit the bleeder resistance at the edges of the well, it is therefore sufficient first to grow on a buffer layer epitaxially until no more autodoping takes place, and then to reduce the thickness of the buffer layer in the region outside the wells or remove it completely. 
     In one embodiment of the method, a high impedance substrate is used in which a well with the lowest possible impedance is formed. 
     As a result, it is possible to reduce the internal resistance of the well on one hand. On the other hand, the capacitance of the boundary area between the well and substrate is also reduced and the bleeder resistance of the substrate increased. The aforesaid measures enable the power loss of components that use the well as one of their electrodes to be reduced. 
     However, removing or thinning out the buffer layer is also advantageous for components without a well formed in the substrate. This is because the absence of a buffer layer leads to a high impedance substructure for those passive components that do not require a well in the substrate. As a result, high parallel resistances are obtained which result in low losses. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for manufacturing a high frequency semiconductor structure. The method includes steps of: preparing a substrate; forming a doped well on one side of the substrate; epitaxially growing a buffer layer above the well; manufacturing the buffer layer above the well from an undoped material; and at least partially removing a portion of the buffer layer covering the substrate. 
     In accordance with an added mode of the invention, the method includes: completely removing the portion of the buffer layer covering the substrate. 
     In accordance with an additional mode of the invention, the substrate is a high impedance substrate with a resistivity greater than 1 kΩcm. 
     In accordance with another mode of the invention, the well is doped with a concentration of more than 6×10 19  cm −3 . 
     In accordance with a further mode of the invention, after processing the buffer layer, applying layered sequences to form bipolar transistors and varactors. 
     In accordance with a further added mode of the invention, after performing the step of applying the layered sequences, performing a brief heat treatment between 750° C. and 850° C. and between 1000° C. and 1100° C. 
     In accordance with yet an added mode of the invention, the method includes: forming a well having an edge in the substrate. The substrate has a region at the edge of the well with an increased doping with respect to remaining portions of the substrate. 
     In accordance with yet an additional mode of the invention, the method includes: connecting the region at the edge of the well to a ground contact. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor structure for high frequencies. The semiconductor structure includes: a substrate having a doped well formed therein; and a buffer layer made of substrate material covering the well. The buffer layer is manufactured from an undoped material, and at least a portion of the substrate is not covered by the buffer layer. 
     In accordance with an added feature of the invention, a bipolar transistor is formed in the substrate, and the transistor has a collector formed by the well. 
     In accordance with an additional feature of the invention, there is provided, a varactor having an electrode formed by the well. 
     In accordance with another feature of the invention, there is provided, a PIN diode having an electrode formed by the well. 
     In accordance with a further feature of the invention, there is provided, a capacitor having an electrode is formed by the well. 
     In accordance with a further added feature of the invention, there is provided, a resistor manufactured from a semiconductor material; the resistor being formed on the substrate. 
     In accordance with a further additional feature of the invention, there is provided, a coil formed from conductor tracks. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for manufacturing a high frequency semiconductor structure and high frequency semiconductor structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a varactor; 
         FIG. 2  is an equivalent circuit diagram for the varactor from  FIG. 1 ; 
         FIG. 3  is a cross sectional view of a PIN diode; 
         FIG. 4  is a cross sectional view of a bipolar transistor; 
         FIG. 5  is a cross sectional view of a configuration of bipolar transistors with a high packing density; 
         FIG. 6  is a cross sectional view of a capacitor with a well formed in the substrate; 
         FIG. 7  is a cross sectional view of a resistor; 
         FIG. 8  is a cross sectional view of a coil; and 
         FIGS. 9  to  20  are cross sectional views of a semiconductor structure during the manufacturing process. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description of the exemplary embodiments represented throughout the figures, identical reference numerals identify parts that respectively correspond to one another. 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a varactor  1 . The varactor  1  has a high impedance substrate  2  with a resistivity of greater than 1 kΩcm. A low impedance well  3  (buried layer) is formed in the substrate  2 . The well  3  preferably has a resistance of less than 5 Ω/square. In order to achieve these values, the doping of the substrate  2  should be below 8×10 12  cm −3 , and the doping of the well should be above 5×10 19  cm −3 . In addition, the well  3  should have a depth of more than 10 μm. This results in layer resistances of only 3 Ω for the well  3 . 
     Above the well  3 , a buffer layer  4  is formed that has been etched back outside the well  3 . On the buffer layer  4 , an epitaxial layer  5  is arranged which, with the exception of a collector region  6  and a base region  7 , is covered by an insulating layer  8 . In the collector region  6 , a contact region  9 , which is adjoined by a collector contact layer  10 , is formed in the epitaxial layer  5 . In the base region  7 , a profiled implantation  11  is introduced in the epitaxial layer  5 . The method of doping this profiled implantation  11  corresponds to the method of doping the well  3 . In addition, a base layer  12  that has the opposite doping from the profiled implantation  11  and the well  3  is present in the epitaxial layer  5  in the base region  7 . A base contact layer  13  is arranged after the base layer  12  and the edges of the base contact layer  13  rest on the insulating layer  8 . Above the base contact layer  13 , there is a contact insulating layer  14  which is interrupted by connecting contacts  15 . A connecting contact  15  which bears against the collector contact layer  10  is also provided in the collector region  6 . The connecting contacts  15  lead through a cover layer  16  to conductor tracks  17  which are embedded in an intermediate layer dielectric  18 . 
     For example p-type conductive silicon with the crystal orientation &lt;100&gt; is used as the substrate  2 . As already mentioned, the doping of the silicon should be less than 8×10 12  cm −3 . The well  3  is formed, for example, by doping the substrate  2  with arsenic with a concentration of more than 5×10 19  cm −3 . The buffer layer  4 , the epitaxial layer  5 , and the base layer  12  are also manufactured from silicon in this case, and the base layer  12  is n-type doped using boron. The base contact layer  13  and the collector contact layer  10  are expediently fabricated from n-type doped polysilicon in the exemplary embodiment illustrated in FIG.  1 . The insulating layer  8  and the contact insulating layer  14  are oxide layers, for example, SiO 2 . The cover layer  16  can be fabricated from boron-containing phosphorus glass (BPSG). The intermediate layer dielectric  18  is finally an IMOX layer. The connecting contacts  15  may be fabricated from a tungsten alloy. AlSiCu is provided for the conductor tracks  17 . 
     The concentration of the doping atoms in the base layer  12  is 10 20  cm −3 . This is followed by the profiled implantation  11  with a concentration of doping atoms that drops from 10 18  cm −3  to 10 16  cm −3 . The base contact layer  13  and the collector contact layer  10  are each doped with a concentration of 10 21  cm −3 . 
       FIG. 2  shows an equivalent circuit diagram for the tuning diode or the varactor  1  shown in FIG.  1 . The equivalent circuit diagram has a base terminal  19  which corresponds to the terminal  15  in the base region  7 . The base terminal  19  leads to an ideal diode  20  which is connected in series with an ohmic resistor  21 . A capacitance  22  is connected in series with a further ohmic resistor  23 , and in parallel with the ideal diode  20 . The resistors  21  and  23  represent the losses in the connecting contacts  15 , the base contact layer  13 , the base layer  12  and the profiled implantation  11 . A further series resistor  24  leads to a collector terminal  25  that corresponds to the connecting contact  15  in the collector contact region  9 . The resistance  24  illustrates the losses in the well  3 . 
     The capacitance  22  represents the capacitance for the pn-type junction along the boundary area between the base layer  12  and the profiled implantation  11 . In addition, there is a further pn-type junction between the well  3  and the substrate  2 . The capacitance of this pn-type junction is represented in the equivalent circuit diagram in  FIG. 2  by a boundary area capacitance  26 . The bleeder resistance through the substrate  2  to an alloy island conducted to ground is illustrated in  FIG. 2  by a substrate resistor  27  that leads to a ground terminal  28 . 
     If a channel stop implantation has been introduced in the region of the epitaxial layer  5 , there is a conductive layer between the substrate  2  and the channel stop implantation. In addition, the autodoped buffer layer  4  forms a conductive layer. The resistance of the two conductive layers is combined in  FIG. 2  in an edge layer resistor  29 . The fact that both conductive layers act via a coupling capacitance is allowed for in  FIG. 2  by an edge layer capacitance  30 . 
     In order to keep the losses of the varactor  1  as small as possible, it is necessary to keep the well resistance  24  as low as possible. This is achieved by the high doping of the well  3 . 
     Moreover, it is necessary to limit, as much as possible, the losses generated in the conductive edge layers. In the exemplary embodiment illustrated in  FIG. 1 , there is therefore no channel stop implantation provided in the epitaxial layer  5 . The channel stop implantation can be dispensed with as the parasitic bipolar transistor between the wells  3  is too slow to play a role technically at the relevant high frequencies. The only important factor is that this parasitic bipolar transistor cannot assume a state that leads to problems. Model calculations have shown that when there is a clear spacing of 20 μm, the base region of the parasitic bipolar transistor, which is located between the wells  3 , acts as a high impedance resistance region that exhibits capacitive behavior at the relevant high frequencies. 
     In addition, the conductive, autodoped buffer layer  4  has been removed outside the region of the well  3  before the epitaxial layer was grown on. 
     These measures ensure a low edge layer resistance  29  and a negligibly small edge layer capacitance  30  so that the edge layer losses can be ignored. 
     In the modified exemplary embodiment (not illustrated) of the varactor  1 , the buffer layer  4  is applied, together with the epitaxial layer  5 , in an epitaxy operation and is subsequently etched back outside the region of the well  3  to the substrate  2 . However, this is possible only if the resulting step of approximately 1 μm can be tolerated in the following processes. 
     However, if value is placed on having the flattest possible structure, it is advantageous to first grow on the buffer layer  4  with a thickness of 0.15 μm, and then to etch it back by 0.2 μm outside the region of the well  3 . In this way, the autodoped buffer layer is completely removed with a high degree of certainty so that there is no conductive layer produced along the boundary area between the substrate and the insulating layer lying above it. 
     Even with other types of components having a semiconductor structure, the losses can be limited by providing a buffer layer  4  and subsequently etching back the buffer layer  4  outside the region of the wells  3 . 
     For example, a PIN diode  31  is illustrated in FIG.  3 . In this PIN diode  31 , the intrinsic region is formed by an epitaxial layer with a large thickness, which is etched in a mesa form in the base region  7 . 
     Furthermore, a bipolar transistor  32  with low collector losses is illustrated in cross section in FIG.  4 . The bipolar transistor  32  also has the buffer layer  4  which is removed outside the well  3 . A collector deep implantation  33  is introduced in the base region  7  in the epitaxial layer  5  lying above the buffer layer  4 . The collector deep implantation  33  is adjoined by the base layer  12  with a base zone  34 . The base zone  34  surrounds an emitter zone  35  which is adjoined by an emitter contact layer  36 . To the side, the emitter contact layer  36  is bounded by spacer elements  37 . The base contact layer  13  and the emitter contact layer  36  are connected to conductor tracks  17  via connecting contacts  15 . 
     As already stated in connection with the varactor illustrated in  FIG. 1 , the collector losses are also kept low in the PIN diode  31  and the bipolar transistor  32  by interrupting the buffer layer  4 . 
     For this purpose, it is, however, also necessary to have a sufficiently clear space between the wells  3  of the components, as otherwise the parasitic bipolar transistor between the wells  3  gives rise to losses. 
     An exemplary embodiment with which high packing densities are possible is illustrated in  FIG. 5  using the example of a modified bipolar transistor  38  and adjacent wells  3 . In this exemplary embodiment, the wells  3  are surrounded by boundary regions  39  with an increased, opposite doping. For example, the substrate  2  is p-type doped, and the n-type doped wells  3  are surrounded by a p-type doped boundary region  39  with an increased concentration. As a result, the boundary area capacitance  26  (see  FIG. 2 ) is reduced. In addition, a packing density such as that in conventional components is possible because the wells  3  are effectively electrically isolated by the boundary regions  39 . 
     In order to ensure the electrical isolation of the wells  3  in every operating state, an isolating region  40  is also provided, which is connected via a connecting contact  15  to a conductor track  17  which is at ground potential. This ensures that the parasitic bipolar transistor between the wells  3  is not opened. 
     In  FIG. 6 , finally a capacitor  41  is illustrated whose anode  42  is formed by the well  3  and an electrode implantation  43  in the epitaxial layer  5 . An intermediate dielectric  44 , on which a cathode  45  of the capacitor  41  is mounted, is arranged after the electrode implantation  43 . 
     Here too, losses are avoided by providing a buffer layer which is restricted in its extent by the region of the well  3 . 
     The removal of the autodoped buffer layer  4  outside the region of the wells  3  is also advantageous for passive components which do not require a well  3  in the substrate  2 . 
     For example, an ohmic resistor  46  is illustrated in FIG.  7 . The resistor  46  is formed by a resistance layer  47  on the insulating layer  8 . The resistance layer  47  is composed, for example, of a silicon layer with low boron doping. 
     As there is no autodoped buffer layer  4  along the boundary area between the substrate  2  and the insulating layer  8 , there are also no charge carriers along this boundary area. Therefore, the capacitive coupling of the resistance layer  47  to the substrate  2  does not lead to parasitic losses. 
     The high impedance substrate  2  can also be advantageous when manufacturing coils  48  in the semiconductor structure.  FIG. 8  shows an embodiment of such a coil  48 . The conductor tracks  17 , which lead through the intermediate layer dielectric  18  via connecting contacts  50  to a first turn  51  and a second turn  52 , are arranged in a first metal plane  49 . The first turn  51  and the second turn  52  are embedded in a layer dielectric  53 . The plane in which the first turn  51  and the second turn  52  are located is also referred to as the second metal plane  54 . 
     It has become apparent that inductances with a high quality can be manufactured on the high impedance substrate with frequencies of up to 1 GHz if the metalization layer thickness in the second metal plane  54  can be kept to approximately twice the depth of the skin, that is to say approximately 4 μm. Inductances with qualities of over  50  at a frequency of 0.8 GHz can be implemented with coil diameters of less than 500 μm. Even higher quality levels can be achieved if coils with larger coil diameters are used. However, the parallel capacitances then increase. This is because, on the one hand, the capacitance between the turns increases, and on the other hand, the capacitance between the turns and the substrate rises. In addition, losses occur as a result of the line resistance of the turns  51  and  52  and the parallel resistance of the substrate  2 . This parallel resistance is due to autodoping of the buffer layer  4 , which is for example, still present if the buffer layer  4  is not removed. 
     A high impedance substrate  2  is of particular significance for the coils  48  as it is not possible to prevent losses by using a substrate that is a good conductor. In particular, the resistivity of silicon with optimum values of 3 Ωcm is not sufficient to avoid large losses. 
     It should be noted that a metal that is a good conductor has to be used as the corresponding ground electrode for the coils  48 . In the case of the high impedance substrate  2 , this is the rear metalization of the substrate  2 . 
     Manufacturing the components described in  FIGS. 1  to  8  will be described below by way of example with reference to  FIGS. 9  to  20 . 
       FIG. 9  is a cross sectional view of the substrate  2 . Here the substrate  2  is p-type conductive silicon with a resistivity of 1 kΩcm and a crystal orientation in the &lt;100&gt; direction. An oxide layer  55  with a thickness of 1 μm is oxidized onto the substrate  2 . The oxide layer  55  is then patterned in such a way that it has windows at the points provided for the wells  3 . If appropriate, the oxide layer  55  can also have windows for the isolating regions  40 . If appropriate, the boundary regions  39  of the substrate  2  are formed by implanting boron with a concentration of 2×10 13  cm −2  and a subsequent diffusion process at 1170° C. for 500 minutes. This is followed by the implantation of arsenic with a concentration of 2.2×10 16  cm −2  with an ion energy of 100 keV. The arsenic diffuses into the substrate  2  to a depth of 10 μm in a subsequent diffusion process at 1170° C. having a duration of 1000 minutes. 
     The result of these method steps is illustrated in  FIG. 9  (the boundary regions  39  have not been illustrated). 
     The oxide layer  55  is then removed by etching, and the buffer layer  4  is grown on epitaxially to a depth of 0.3 μm of undoped material. This results in a structure having the cross section illustrated in FIG.  10 . 
     In further method steps, a photoresist layer  56  is applied to the buffer layer  4 . The photoresist layer  56  is then patterned such that in a subsequent etching process, the buffer layer  4  outside the region of the wells  3  is removed. The buffer layer  4  is expediently etched back into the substrate  2 , which ensures the complete removal of the buffer layer  4 , so that a structure having the cross section shown in  FIG. 11  is obtained. 
     This is followed by growing on the epitaxial layer  5  including patterning the epitaxial layer  5  in order, for example, to form a mesa structure in the region of the PIN diode  31 . The epitaxial layer  5  is also etched back in the region of the varactor  1 , the capacitor  41 , and the bipolar transistor  32 , as illustrated in FIG.  12 . 
     This is followed by a local oxidation of the epitaxial layer  5 , which as illustrated in  FIG. 13 , gives rise to an insulating layer  8  that is up to 850 nm thick. 
     This is followed by the implantation of phosphorus and its deep diffusion to form the collector contact regions  9 , and in particular, the electrode implantation  43  of the capacitor  41 . This results in the state shown in FIG.  14 . 
     In further method steps, the intermediate dielectric  44  of the capacitor  41  and the profiled implantation  11  of the varactor  1  are formed. This is then followed by the deposition of a 200 nm thick polysilicon layer  57  into which boron is implanted in the region of the resistor  46  in order to form the resistance layer  47  in the region of the resistor  46 . Otherwise, the polysilicon layer  57  is weakly doped with boron in order to prepare the base layer  12 , and the cathode  45  in the region of the capacitor  41 . 
     According to  FIG. 16 , an oxide layer with a thickness of 300 nm is then deposited on the polysilicon layer  57  in order to produce the contact insulating layer  14 . The polysilicon layer  57  is patterned with the oxide layer lying above it, in one etching process. Here, an emitter contact hole  58  that extends at least through the base layer  12  is etched out in the region of the bipolar transistor  38 . In the remaining part of the epitaxial layer  5 , the collector deep implantation  33  is formed by the implantation of phosphorus. This is followed by implanting boron to make the base zone  34 . After these method steps are completed, the cross section illustrated in  FIG. 16  is obtained. 
     In  FIG. 17  the method has already progressed further. First, the spacer elements  37  have been formed in the emitter contact hole  58 . This is expediently carried out by conformal deposition and subsequent anisotropic etching. In addition, the collector contact layers  10  and the emitter contact layers  37  have been formed by depositing polysilicon and subsequent patterning. 
     Then, as shown in  FIG. 18 , the cover layer  16  of boron-containing phosphorus glass is deposited over the entire area with a thickness of up to 1200 nm. In the subsequent flowing at 800° C., the boron atoms diffuse out of the base contact layer  13  into the epitaxial layer underneath and form the base layers  12 . In the bipolar transistor  32 , in particular, the emitter zone  35  is manufactured by the diffusion of the phosphorus atoms in the emitter contact layer  36 . 
     It is particularly advantageous if the flowing is performed at two points where the temperature is held, on the one hand between 750 and 850° C. and on the other hand between 1000 and 1100° C. As the temperature sensitivity of the parameters of the bipolar transistor  38  is particularly high at low temperatures, and the temperature sensitivity of the parameters of the PIN diode  31  and of the varactor  1  are particularly high at high temperatures, the doping profiles in the varactor  1  and in the bipolar transistor  38  can be respectively optimized by the flowing at different temperatures. 
     Then, holes for the connecting contacts  15  are etched into the cover layer  16  and are filled with tungsten so that the cross section shown in  FIG. 18  is finally obtained. 
     According to  FIG. 19 , a layer made of AlSiCu is then sputtered onto the cover layer  16  and is patterned, which produces the conductor tracks  17 . A 2 μm thick oxide layer is deposited on the conductor tracks  17  as the intermediate layer dielectric  18 . 
     According to  FIG. 20 , contact holes for the connecting contacts  50  are etched into this intermediate layer dielectric  18 . This is then followed by vapor deposition using TiAu. In addition, up to 7 μm of Au is deposited galvanically in order to form conductor tracks  59  of the second metal plane  54 . If appropriate, the turns  51  and  52  of the coil  48  are also manufactured here. 
     Finally, it is noted that the method described with reference to  FIGS. 9  to  20  markedly reduces the edge losses due to conductive, autodoped epitaxial layers. The method makes use of the fact that at the start of the epitaxial growth, the doping atoms vaporize out of the wells  3  and are introduced into the epitaxial layer. However, as the layer thickness of the epitaxial layer increases, this effect decreases rapidly. Therefore, it is sufficient to deposit the buffer layer in the region between the wells  3  in order to prevent the formation of conductive layers between the wells  3  and to allow the substrate  2  and the epitaxial layer  5  to assume a high impedance in this region. As there are only a few doping atoms at the surface of the buffer layer  4 , the well  3  is, as it were, sealed for subsequent epitaxy processes by the buffer layer  4 . This leads to a situation in which the epitaxial layer  5  that is arranged afterwards hardly contains any doping atoms and exhibits a good insulating effect. 
     However, it is also noted that it is possible for an intermediate layer whose thickness corresponds to the buffer layer  4  and to the epitaxial layer  5  to be applied epitaxially to the substrate  2  first, and for the intermediate layer then to be etched back in the region between the wells  3  as far as the substrate  2  in order to remove the particularly highly autodoped regions of the intermediate layer in the region between the wells  3 . This modification of the method is particularly suitable if steps of up to 1 μm are to be tolerated in the following method steps.