Abstract:
A semiconductor device includes a semiconductor layer provided above a pair of bipolar transistors formed in a surface region of a semiconductor body. Schottky barrier diodes and resistors are formed in the semiconductor layer. The pair of bipolar transistors, the Schottky barrier diodes and the resistors are electrically connected to constitute a bipolar memory. Since the Schottky barrier diodes and the resistors can be formed above the bipolar transistors, an area required for the memory cell can be made greatly small and the occurrence of an hindrance caused by α particles is minimal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device, and more specifically to a bipolar memory which has a high α-particle immunity and which is to be fabricated with a high integration density. 
     In the conventional bipolar memory as disclosed by JP-A-61-104655, all of Schottky barrier diodes SBD1 and SBD2, high resistive polycrystalline silicon layer, high resistive impurity-doped layer, and transistors TR1 and TR2 are arranged in a plane to form a flip-flop type of static bipolar memory cell circuit as shown in FIG. 2. However, the plane-like arrangement of constituent elements of the memory has a problem that an area required for the memory cell cannot be made small, thereby rendering high integration difficult. 
     U.S. Pat. No. 4,636,833 has proposed a bipolar memory in which the required area is reduced by superimposing an MOS capacitor on a Schottky barrier diode. In order to further improve the integration density of the bipolar memory, it is desired to further reduce the required area. 
     A semiconductor memory is subjected to an hindrance in that an erroneous storage may result from the presence of external α particles. This hindrance is generally called a soft error. Since the soft error increases as the integration density of the memory becomes higher, the prevention of the soft error is a very important problem for a semiconductor memory having a high integration density. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above-mentioned problems in the prior art and to provide a bipolar memory which has a high α-particle immunity and fabricated with a very high integration density. 
     To that end, a polycrystalline or monocrystalline semiconductor layer is formed on a semiconductor body and the semiconductor layer is used for forming a Schottky barrier diode and a resistor. The Schottky barrier diode and the resistor are formed above a bipolar transistor formed in the semiconductor body, thereby simultaneously realizing the reduction of an area required for the memory and the prevention of a soft error caused by α particles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a main part of a semiconductor device according to an embodiment of the present invention; 
     FIG. 2 is a circuit diagram of the semiconductor device shown in FIG. 1; 
     FIGS. 3 to 8 are cross sections for explaining steps of a process of fabricating the semiconductor device having the structure shown in FIG. 1; 
     FIG. 9 is a cross section of a semiconductor device according to another embodiment of the present invention; and 
     FIGS. 10 and 11 are a cross section and a circuit diagram for explaining a further embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A memory cell area can be remarkably reduced by providing a stacked structure in which a semiconductor layer is formed on an emitter (or collector) of a bipolar transistor, and a Schottky diode and other semiconductor elements are formed in the semiconductor layer. Since the bipolar transistor can be operated in a reverse direction, a variation in a collector potential due to external noises is small. Since the collector can be disposed on the outermost surface side of a semiconductor body, the α-particle immunity can be remarkably improved. 
     The semiconductor layer may be made of polycrystalline silicon or monocrystalline silicon. The monocrystalline silicon layer can be formed by forming a film of amorphous silicon on the emitter (or collector) and mono-crystallizing the amorphous silicon film by means of a solid-phase epitaxial technique. The polycrystalline silicon layer can be formed by any well-known technique. 
     An embodiment of the present invention will now be explained by use of FIGS. 1 and 3 to 8. 
     Referring to FIG. 3, a desired portion of a p-type silicon substrate or body 1 is doped with an impurity by a well-known thermal diffusion or ion-implantation technique to provide an n +  -type buried layer 2. Thereafter, an n -  -type epitaxial layer 3 is grown by means of a well-known vapor-phase epitaxial technique (see FIG. 4). Next, as shown in FIG. 5, unnecessary portions of the n -  epitaxial layer 3 are removed by use of a conventional photo-etching process to form a raised region of the monocrystalline n -  epitaxial layer 3, and a silicon dioxide layer 10 and a polycrystalline silicon layer 17 are provided on the both sides of the raised region. As shown in FIG. 6, a part of the polycrystalline silicon layer 17 is oxidized to provide a silicon dioxide layer 9, and thereafter a p-type layer 5, a p +  -type layer 6 serving as a graft base, and an n +  -type layer 4 are formed in the above-mentioned raised region to provide an emitter 4, a base 5 and a collector 3. An insulating film 8 (made of, for example, SiO.sub. 2) is formed on the surface of the semiconductor body. Openings are provided in portions of the insulating film 8 which correspond to at least the n +  -type layer 4. 
     Then, a polycrystalline silicon layer is deposited on the surface of the semiconductor body, and desired portions of the polycrystalline silicon layer are doped with impurities by means of an ion-implantation technique, thereby providing an n-type polycrystalline silicon region 12 and an n -  -type polycrystalline silicon region 13 as shown in FIG. 7. In place of the polycrystalline silicon layer, an amorphous silicon layer may be deposited on the surface of the semiconductor body. In that case, it is possible to monocrystallize the amorphous silicon layer by carrying out a heat treatment, at temperatures not higher than 800° C., where a solid-phase epitaxial growth takes place starting from an interface between the amorphous silicon layer and the underlying monocrystalline silicon layer. The characteristics of a Schottky barrier diode or the like formed in the monocrystalline silicon layer are superior to those formed in the polycrystalline silicon layer in that a leakage current is small. 
     Next, a silicon dioxide layer 7 is disposed on the surface of the semiconductor device and openings are provided in predetermined portions of the silicon dioxide layer 7 (see FIG. 8). Then, n +  -type layer 14 and a silicide layer 15 are formed through impurity diffusion and ion-implantation via the openings and the subsequent heat treatment. Without forming the silicide layer 15, a metal electrode may be deposited directly on the n-type polycrystalline silicon layer 12 to form a Schottky barrier junction, thereby obtaining a desired diode characteristic. Thereafter, an electrode or wiring layer 16 of Al is formed to complete a semiconductor device having a structure shown in FIG. 1. In FIG. 1, a Schottky barrier diode SBD1 or SBD2 is formed by the silicide layer 15 and the n-type polycrystalline silicon region 12, a resistor HR1 or HR2 having a high value resistance is formed by the n +  -type layer 14 and the n -  -type polycrystalline silicon region 13, a transistor TR1 or TR2 is formed by the n +  -type layer 4, the p-type layer 5 and the n -  -type layer 3. The Schottky barrier diodes SBD1 and SBD2, the high-value resistors HR1 and HR2, and the transistor TR1 and TR2 are interconnected by the Al wiring layer 16, the n-type polycrystalline silicon layer 12, etc. to provide a bipolar memory shown in FIG. 2. A resistor R1 or R2 shown in FIG. 2 has a value determined by the sheet resistance of the n-type polycrystalline silicon region 12 beneath the Schottky diode. 
     FIG. 9 shows another embodiment of the present invention. In this embodiment, the n +  -type layer 4 of the transistor is formed by thermal diffusion using an n +  -type polycrystalline silicon layer 19 as a diffusion source. For that purpose, an additional silicon dioxide layer 7&#39; is provided. Reference numeral 18 designates a silicon dioxide layer similar to the silicon dioxide layer 7 of FIG. 1. 
     According to a further embodiment of the present invention, a clamp diode D as shown in FIG. 11 can be added to a part of the circuit by providing three regions A, B and C in the polycrystalline silicon layer as shown in FIG. 10. The region A including a p-type layer 21 and the n -  -type polycrystalline silicon layer 13 is used for providing the clamp diode D, the region B including the silicide layer 15 and the n-type polycrystalline silicon layer 12 is used for providing a Schottky barrier diode SBD and a resistor R, and the region C including the n +  -type layer 14 and the n -  -type polycrystalline silicon layer 13 is used for providing a high-value resistor HR. In FIG. 10, reference numeral 7&#39; designates a silicon dioxide layer, and numeral 20 generally designates the semiconductor body which has the same structure as that shown in FIG. 1 or 9 inclusive of the transistor TR (only the n +  -type region 4 of which is shown in FIG. 10). 
     According to the present invention, since the Schottky barrier diode, the clamp diode, the high-value resistor and the low-value resistor can be all formed just above the transistor, as shown by the embodiments of FIGS. 1, 9 and 10, an area required for the bipolar memory cell can be greatly reduced. 
     The present invention is not limited to the disclosed embodiments. For example, all the conductivity types of the shown regions or layers can be inverted. 
     According to the present invention hereinabove explained using the embodiments, it is possible to reduce the memory cell area of a bipolar memory to 200 μm 2  which is about two-fifths of the conventional memory cell area (500 μm 2 ). As a result, a high integration not smaller than 64Kbits bipolar SRAM is possible. Also, the α-particle immunity of the memory cell according to the present invention can be improved by two or more orders in comparison with that of the conventional memory cell so that the occurrence of a soft error is remarkably reduced. Further, a delay time due to the wiring can be remarkably decreased owing to the available high integration. For example, there can be realized a 64Kb bipolar memory having an address access time not slower than 1 nano-second which is about one-fifth of the conventional bipolar memory. 
     In the foregoing embodiments, a vertical bipolar transistor is used in which the graft base 6 having a high impurity concentration is provided on the side of the intrinsic base 5 and the polycrystalline silicon layer 17 having a low resistivity is used as a drawing electrode. Though the use of such a type of transistor is most preferable in the present invention, a different type of vertical bipolar transistor can be used in the present invention. For example, a vertical bipolar transistor can be used in which a semiconductor layer is connected to an upper marginal or end portion of the transistor, as is shown in FIGS. 6(c) and 7 of JP-A-59-139678. It is also possible to use a well-known lateral bipolar transistor. In the lateral bipolar transistor, however, since a collector is formed at a relatively deep portion of a semiconductor body, the improvement of the immunity against α particles cannot be expected, though the reduction in an area required is possible by virtue of the present invention.