Abstract:
A method of manufacturing an integrated circuit including an analog circuit and an I 2  L circuit which both contain NPN transistors on a P-type semiconductor chip in which the method includes an N +  diffusion step for producing a flat diffusion for certain NPN transistors having smaller penetration depth than N +  regions of the other NPN transistors for connection of an emitter electrode to the said certain NPN transistors.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method of manufacturing an analog circuit integrated with an I 2  L circuit on a common semiconductor chip of the P-conductive type in the course of common technological process steps with vertical NPN switching transistors in the I 2  L circuit and with linear vertical NPN transistors in the analog circuit. 
     The manufacture of I 2  L circuits and bipolar analog circuits on a common semiconductor chip of the type mentioned above is known for example from the Valvo Reports, Volume XVIII, Book 1/2, pages 215-226. In the integrated injection logic circuit (I 2  L) lateral PNP transistors, also called injectors, are used as current sources for vertical NPN switching transistors. An I 2  L-logic element works with very small power consumption and needs little crystal surface so that high packing densities can be achieved. 
     Bipolar analog circuits contain a vertical NPN transistor which is operated, however, in contrast to the NPN switching transistor of the I 2  L circuit, in the reverse direction. Thus generally with an I 2  L circuit built up on a P-substrate, N +  regions diffused in the P-region, serving as the base, are used as collectors and the corresponding N +  regions of bipolar analog circuit are used as emitters. 
     When combining circuits on a common semiconductor chip both circuits can be manufactured in a common process. Only one further step is needed for manufacturing deep N +  regions for decoupling of adjacent I 2  L gates. 
     In I 2  L logic circuits, the binary potential conditions are below 1 V. The maximum collector voltage of their NPN switching transistors is approximately 2 to 5 V caused by a desired high operating frequency and an upward current amplification factor of approximately 4. However, in order to apply a fairly high switching power for analog circuits, supply voltages of about 30 V and more are often necessary. For this, it is known to apply epitaxial layers having a high specific resistance (1 to 3 Ω cm) and large thicknesses (10 to 15 μm) on to the substrate. However, the current amplification of the upwardly operated NPN switching transistors required for I 2  L circuits cannot be carried out without problems. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to overcome the disadvantage of the low breakdown voltage of the linear transistors occurring when combining bipolar circuits with digital circuits in I 2  L technology and to improve the properties of the combined circuit. 
     According to the invention, there is provided a method of manufacturing an integrated circuit including an analog circuit and an I 2  L circuit which both contain NPN transistors on a P-type semiconductor chip, said method comprising forming the majority of the integrated circuit by common technological methods and completing certain NPN transistors by flat diffusion of N +  regions having a smaller penetration depth than N +  regions of the other NPN transistors for connection of an emitter electrode to the associated said certain NPN transistors. 
     Further according to the invention, there is provided a method of manufacturing at least one analog circuit integrated with at least one I 2  L circuit on a common semiconductor wafer of the P-conductive type during the course of common technological process steps with vertical NPN switching transistors in the I 2  L circuit and with linear vertical NPN transistors in the analog circuit, characterised by a further process step producing N +  regions which are flat for certain NPN transistors in contrast to the penetration depths of the N +  regions of the collectors of the I 2  L circuit and which are provided in order to connect an emitter electrode respectively of a certain NPN transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail, by way of example with reference to the drawings, in which: 
     FIG. 1 shows a cross-section of a semiconductor chip manufactured according to the method in accordance with the invention and having a combined I 2  L and an analog circuit part; 
     FIG. 2 shows output characteristics for a linear NPN transistor of the analog circuit part in common base configuration and manufactured according to the method in accordance with the invention. 
     FIG. 3 shows a circuit for measuring some upward current amplification factors of an NPN-switching transistor of an I 2  L gate. 
     FIG. 4 shows upward current amplification factors of vertical NPN switching transistors of the I 2  L circuit manufactured according to the method in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Owing to the additional process step, it is now possible to substantially increase the collector base breakdown voltage and the collector emitter breakdown voltage of certain linear vertical NPN transistors of the bipolar circuit. Thus the following values were measured at an embodiment. 
     U CBo  =65-75 V 
     U Ceo  =35-40 V 
     The additional process step is carried out in an advantageous manner subsequent to the process step for producing the N +  regions 711, 712 and 713 provided as collectors of the NPN switching transistor of the I 2  L circuit. Thus it is a question of a process step to be incorporated into the standard process for manufacturing combined I 2  L circuits and bipolar circuits in a simple manner. 
     If during the additional process step the flat N +  diffusion is carried out partially in these regions which are subjected to the usual N +  doping and the collector regions in the I 2  L circuit are assigned to vertical NPN switching transistors and in the analog circuits emitter regions are assigned to vertical NPN transistors, then vertical NPN switching transistors are obtained in the I 2  L circuit and in the bipolar circuit linear vertical NPN transistors are obtained having a particularly high current amplification factor. 
     Referring now to the drawings, in FIG. 1 a cross-section through a semi-conductor wafer of the P-type (P-substrate 1) is shown on which can be seen an I 2  L circuit for a digital circuit part, and for an analog circuit part a linear transistor with low blocking voltage (for example U CBo  ≈30 V) of the usual type of manufacture and a linear transistor manufactured in accordance with the method according to the invention having a high blocking voltage (for example U CBo  ≈ 60 V). 
     In the so-called &#34;Standard&#34; method of manufacturing combined digital and analog circuits, essentially the following process steps are usual: 
     1. Diffusing of N +  regions 2 into a P-substrate 1 of the semiconductor chip which later form buried layers. 
     2. Growth of an epitaxial layer 3 of the N-type onto the P-substrate. 
     3. Diffusion of deep P +  regions 4 which reach down to the P-substrate 1 and separate the individual circuit parts electrically. 
     4. Diffusion of P regions 511 and 512 for injectors and base regions of the I 2  L circuits and of P-regions 52, 53 and 54 for base regions of linear transistors of the analog circuits. 
     5. Diffusion of deep N +  regions 61 for separating adjacent I 2  L circuits and for connecting their emitters normally lying at reference potential and N +  regions 62, 63 and 64 for the collectors of the linear transistors of the analog circuits. 
     6. Diffusion of N +  regions 711, 712, 713 into the base regions of the I 2  L circuits for I 2  L collectors and of N +  regions 72 for the emitters of linear transistors of the analog circuits. 
     7. Opening of the contact windows for connecting the semiconductor regions to metallic conductive tracks. 
     8. Applying the conductive tracks inclusive of contacting. 
     The step positions 4 and 5 can be exchanged in their sequence if very deep N +  regions 61, 62, 63, 64 have to be produced. 
     With this method of manufacture, vertical NPN switching transistors arise in the I 2  L circuits and linear vertical NPN transistors in the analog circuit, their emitter connections being designated E, their base connections B and their collector connections C or C 1 , C 2  and C 3 . The NPN transistor shown in FIG. 1 in the I 2  L part has three separate collectors C 1 , C 2  and C 3  in a common P-base region and is designated as a multiple collector transistor. 
     The method in accordance with the invention is characterised by a further process step which produces flat N +  regions 73 for certain NPN transistors which serve as emitters of a defined NPN transistor. By a flat N +  region is meant a N +  region 73 of the sort of penetration depth, which does not reach as deep into the base region 53 in comparison to the penetration depth of the N +  regions 711 and 713 of the collectors of the I 2  L circuit. 
     This additional process step can be carried out in an advantageous manner after the sixth process step which serves to produce the N +  regions 711, 712 713 and 72. Thus N +  doped materials, preferably phosphorous are introduced, having the desired penetration depth into the crystal. Since no further processes which substantially change the distribution of the materials follow this process the desired electrical parameters can easily be set. These defined transistors manufactured with flat emitter diffusion are characterised by a substantially higher voltage resistance. This will be explained in greater detail together with an embodiment. 
     The combined integrated circuit manufactured according to the method in accordance with the invention has the following data: 
     
         ______________________________________Conductivity of the P-substrate 1-10 Ω cmThickness of the epitaxial layer                    x.sub.7 ≈10 μmPenetration depth of the deep N.sup.+ regions  61, 62 and 63          x.sub.5 ≈7 μmPenetration depth of the injector and  base regions 511, 52 and 53                    x.sub.4 ≈3,2 μmPenetration depth of the normal N.sup.+regions 711, 713 and 72 in the base  regions 512 and 52     x.sub.2 ≈2,4 μmPenetration depth of the flat N.sup.+ region  73 in base region 53   x.sub.1 ≈2 μmSpacing between the buried N.sup.+ layer 2  and the base regions 512, 52 and 53                    x.sub.6 -x.sub.4 ≈3,94 μmImpurity concentration in the N-epitaxial  layer                  5 · 10.sup.15 cm.sup.-3Sheet resistance of the P-base regions                    150-200 Ω/□______________________________________ 
    
     The following electrical values were determined on linear transistors of the analog circuit part: 
     1. The usual linear vertical NPN transistors having a normal N +  emitter diffusion corresponding to the N +  region 72 in FIG. 1. 
     U CBo  ≈30 V (blocking voltage between C and B with an open E 
     U CEO  ≈15 V (blocking voltage between C and E with open B) 
     B d  ≈400 (downward current amplifications factor I C  /I B  with I C  =1 mA 
     f T  ≈400 MHz (transit frequency with I C  =1 mA) 
     2. Linear vertical NPN transistors having a flat N +  emitter diffusion corresponding to N +  region 73 and manufactured according to the method in accordance with the invention in FIG. 1 under the same conditions show the following values: 
     U CBo  ≈65-75 V 
     U CEo  ≈35-40 V 
     B d  ≈15 . . . 30 
     f T  ≈100 MHz 
     The breakdown voltage of the transistors manufactured in accordance with the method according to the invention is consequently more than twice as high as that of the linear transistors according to the usual method of manufacture. As a result, in an advantageous manner, gas discharge displays can be triggered with the voltage resistant transistors for example. Since these display units only require a small amount of current and are triggered quasi-statically, the low downward current amplification factor B d  and the low transit frequency f T  are unimportant. 
     FIG. 2 shows the output characteristics I C  =f (U CE ) of a voltage resistant transistor in common base configuration and manufactured according to the method in accordance with the invention. The base current was increased in steps of 20 μA. Normally the collector current I C  is approximately equal to the emitter current I E . The increase in the collector current with an increase in voltage U CE  between the collector and the emitter of the transistor is reversible and not caused by enlarging the collector junction up to the emitter (punch-through effect). 
     In further refinement of the method in accordance with the invention, the flat N +  diffusion is partially carried out in those regions which are subjected to the usual N +  doping, and which are the I 2  L circuit collector regions assigned to vertical NPN switching transistors and the emitter regions in the analog circuits assigned to the linear vertical NPN transistors. 
     This type of embodiment in the I 2  L circuit is shown in FIG. 1. Here the flat emitter diffusion was also used on a region which is assigned to the collector region 712 of the collector C 2 . By means of this additional flat diffusion, which takes place at the same time, i.e. with the same masking step as the flat diffusion for the emitter region 73, the penetration depth of the collector region into the base region 512 of the I 2  L circuit is greater and vertical switching transistors are formed having a particularly high upward current amplification factor B a . 
     A measuring circuit for the I 2  L circuit shown in FIG. 1 for measuring the upward current amplification factor B a  is shown in FIG. 3. The designations correspond to those of FIG. 1. As with measurements on I 2  L circuits, the injector is connected to the emitter of the vertical NPN switching transistor lying at reference potential by a switch S1 and the collector currents, for example I C1 , I C2  of the collectors C 1 , C 2  which are at reference potential via an operating voltage source U o  =0.5 V are brought into a relationship to the base current I B  impressed into the base B. FIG. 4 shows the upward current amplificaton factors determined depending on the base current I B   
     B a10  =I C1  /I B  for the open switch S1 
     B a20  =I C2  /I B  for the open switch S1 
     B a1  =I C1  /I B  for the closed switch S1 
     B a2  =I C2  /I B  for the closed switch S1 
     With a closed switch (characteristics B a1 , B a2 ) the so-called effective upward current amplification factor is measured. It is a measure of noise margin of the circuit. It is determined in part by the spacing base-injector and can be influenced by the dimensions of masks, (layout). 
     The NPN switching transistor arising because of the additional process step in accordance with the method according to the invention, having the collector region 712, has an approximately twice as large a current amplification factor (B a20  ≈20) in the embodiment as the remaining NPN switching transistors of the I 2  L circuit. The fall off in the current amplification factors towards smaller base currents is only small in an advantageous manner. This is above all a result of the small spacing of the P-injector region 511 from the P-base region 512 and thus owing to the small base width (≈1.8 μm) of the lateral PNP transistor formed by these regions and the N-epitaxial layer here between. 
     The additional flat emitter diffusion in the certain transistors can be used advantageously on linear transistors of the analog circuit which are subjected to the usual N +  doping. Then, as shown on the right-hand side in FIG. 1, linear transistors are formed having N +  penetration depths of their emitter region 74 as in the vertical NPN switching transistor with the collector region 712 i.e. with N +  penetration depths of &gt; 2.4 μm. These linear transistors of the analog circuit, manufactured with additional emitter diffusion also show a substantially higher downward current amplification factor B d  relative to linear transistors of conventional manufacture and for certain applications where it is not a question of high blocking voltage of the relevant transistor, can be very desireable. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptions.