Abstract:
A process is set forth for providing a self-aligned, vertical bipolar transistor. A controlled technique is provided for providing the base and emitter features of the transistor with appropriate dimensions and properties to be useful in high frequency microwave applications. A microwave transistor is provided by this technique.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 08/579,703 filed Dec. 28, 1995, now abandoned. 
    
    
     FIELD OF INVENTION 
     The present invention is directed to a technique for manufacturing a self-aligned vertical bipolar transistor on an SOI device. More particularly, the present invention forms such device in a manner to closely control the device dimensions and form a highly conductive base structure of a multiple conductive stack for use in high frequency devices for microwave applications. 
     BACKGROUND OF THE INVENTION 
     In communication systems operating at less than 10 Ghz, silicon bipolar transistors can be used. The making of devices for microwave applications has been expensive such as for the making of bipolar MMIC devices using conventional bulk silicon technology. This problem occurs because of the process complexity associated with buried collectors and isolation of the parts. 
     The making of such devices with SOI technology reduces the complexity of the process with the added advantage of low capacitances. But while a lateral bipolar transistor can be made for use at microwave frequencies, high power has not been achieved in such devices. 
     SUMMARY OF THE INVENTION 
     In the present invention, a self-aligned vertical bipolar transistor is provided in which a high density, high power, microwave frequency device is made using silicon technology. 
     This device is made according to the present invention by the steps of forming an SOI wafer of n and n+ layers of silicon on an insulating layer on a substrate, implanting an n+ collector through the n layer of silicon into the n+ layer of silicon, depositing a multiple layer of four different materials at opposite sides of an active region on the n layer of silicon, forming a p type base within the active region, forming a p+ contact layer to the base below the multiple layer, forming an n+ polysilicon emitter contact on said p type base within said active region, depositing a dielectric layer over the already formed structure, and providing conductive contacts to the collector and the emitter through the dielectric layer. 
     The formation of the device according to the present invention provides the four different materials of the multiple layer from a conductive poly silicon layer, a barrier layer, a silicide layer and a layer of a low temperature oxide (LTO). 
     Further, according to the present invention, the multiple layer is used as a mask to form the base region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The features and technique of the present invention is shown by way of example in the attached drawing figures in which distances and sizes have been changed in order to more clearly show the features of the present invention, where: 
     FIG. 1 is a schematic illustration of the semiconductor device made according to the present invention, and 
     FIGS. 2,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11  and  12  show a sequence of process steps in forming the device of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The self aligned vertical bipolar transistor on an SOI device is shown in FIG.  1 . In this figure an SOI is formed by an isolated, buried oxide insulating layer, such as SiO 2 , disposed on a substrate. Various substrate materials are known for substrates in SOI devices, such as sapphire or silicon, but a preferred material for the present invention is silicon for the substrate. On the buried oxide layer  2  is a double layer of silicon, first an n+ type conductivity layer  3  with an n type conductivity layer  4  disposed thereon. 
     Various oxide portions  5  are disposed on the SOI structure and an n+ type conductivity portion providing the collector  6  is disposed between two of the oxide portions  5 . High diffusivity phosphorus may be used to form the collector  6  by sinking n+ dopant through the n conductivity layer  4  to the n+ type layer  3 . 
     Between one of the oxide portions  5  at the collector  6  and a second oxide portion  5 , a p type base layer  9  is formed between two base contact portions  8 . Over this base  9  is provided an emitter  10  in contact with the base  9  between two sides of the multiple layer  7 . An insulating layer  11  of a low temperature oxide (LTO) is provided over the device structure and openings are provided for an emitter contact  12  and a collector contact  13 . Both of these contacts are of a conductivity material, such as the metal, Al. The base contacts  8  may run through the structure to the ends where electrical connection is made to the base. 
     The multiple layers  7  at sides of the base and emitter are provided of a first layer  16  of a p+ poly, covered with TiN  17 , a silicide  18  and a LTO portion  19 . This multiple layer  7  provides a self-aligning stack to form the base and emitter parts. 
     This composite structure of a self-aligned vertical bipolar transistor is made according to the steps illustrated in FIGS. 2-12. Thus, in FIG. 2 the SOI structure is formed of the silicon substrate  1 , the buried oxide layer  2  of SiO 2  and the n+/n layers  3  and  4 . The formation of this structure may be the n/n+ silicon film as the starting material. This combined film is formed over the SiO 2  isolation insulator  2  which is formed on the silicon substrate  1 . The bonded SOI wafer is made with the n epitaxial layer  3  doped with n+ dopant and a follow up oxidation. This formation is made upside down and then bonded to the silicon substrate. The n+ layer  3  under the n layer  4  has a high conductivity in order to be used as a low loss collector. 
     Subsequently, the oxide portions  5  are formed by a LOCOS technique in which about 500A thick pad oxide is first grown and an about 1000A nitride is deposited by a LPCVD method. While the active area is covered with a photoresist, the nitride and oxide pad are etched away by reactive ion etching (RIE). The nitride is used as an oxidation mask during oxidation to selectively grow the oxide portions  5  on the surface of the SOI wafer. The nitride and oxide pads are then removed leaving the arrangement shown in FIG.  3 . 
     Then, a thin screen oxide layer  15  is grown to about 500A thickness, and a n+ collector sink is implanted, as seen in FIG.  4 . High diffusivity phosphorus may be used to be quickly driven into the n+ collector layer  6 . Thereafter, the screen oxide  15  oil the active area is removed by etching, as may be seen in FIG.  5 . This may be done either by a selective lithographic process or by merely washing away the oxide. In either case, the etching is closely controlled to prevent too much reduction of the LOCOS oxide portions  5 . 
     To provide a self-aligning arrangement a stack of layers of four different type of materials is deposited over the structure. This stack of layers is patterned and anisotopically etched to form the arrangement seen in FIG.  6 . In this stack a layer of p+ poly  16 , a barrier layer  17  of TiN, a layer  18  of a silicide and a layer  19  of LTO are sequentially deposited to make the composite layer  7 . The p+ poly layer  16  and the LTO layer  19  may be deposited by standard CVD, while the silicide layer  18  and TiN barrier layer  17  may be either sputtered onto the deposition surface or reactively made. The TiN barrier layer  17  may be easily made by a reactive sputtering of Ti in a nitrogen atmosphere, or by a thermal reaction of a Ti film with a diluted ammonia gas, for example. 
     The stack of layers is then patterned and etched to form the arrangement shown in FIG.  6 . The different layers can each be anisotropically etched by changing the RIE chemistry for each layer. In practice it is not too difficult because the top oxide layer  19  can be used as an etching mask once it is first patterned and etched. While the total thickness of the composite layer  7  may be about 1 micron, care must be taken to provide the top layer  19  of oxide to be thick enough to maintain dielectric isolation from the subsequently formed n+ poly emitter  10 . The appropriate etching produces the self-aligned stack of layers providing the composite layer  7  adjacent to the active area. 
     A non-selective oxidation produces an oxide in the exposed active area. This oxidation is a screen oxide  21  for assuring that the later formed p+ base  9  is isolated from the subsequently formed n+ emitter  10 . Since the oxidation rate of the poly is much faster than that of the silicon oxidation rate, the lateral oxide growth along the p+ poly layer  16  provides a thickness more than twice the thickness of the screen oxide  21 . Therefore, as seen in FIG. 7, the p base  9  is implanted using the stacked layers  7  as a self-aligned mask, and the p base is initially separated from the p+ poly layer  16  to ensure that the p+ contact layer  8  is free of electrical contact with the n+ emitter  10 , eliminating any possibility of shorting between the p+ contact layer  8  and the n+ emitter  10 . Alternatively, however, if the separation is too large, then a long drive-in of dopant is needed to make an interdiffusion between the p base  9  and the p+ contact layer  8 . 
     Therefore, as seen in FIG. 8, the p base  9  is driven in at the same time that the heavily doped p+ poly  16  produces the p+ contact  8  below the base. The lateral diffusion of the p+ poly layer  16  can be suppressed because of the lateral oxidation of the poly. During the high temperature process, the TiN barrier layer  17  reduces thermal interaction between the p+ poly layer  16  and the silicide layer  18 . 
     Next, as seen in FIG. 9, oxide spacers  14  are formed at the steps of the stacked composite layer  7  and the oxide. Such oxide spacers can be formed by deposition of LTO material followed by an anisotropic RIE etching. The width of the spacers is determined by the width of the LTO thickness. During formation of the oxide spacers  14 , over etching should be minimized in order to not reduce the thickness of the top layer  19  of LTO on the stacked, composite layer  7 . 
     Then, a cleaning of the active area surface over the base  9  is followed by the formation of the n+ emitter  10 , as seen in FIG.  10 . The heavily doped n+ emitter  10  is deposited and patterned by a conventional lithographic process. Either arsenic or phosphorus may be used as a dopant for the emitter, depending on the desire depth of the emitter junction. For the same drive in conditions, phosphorus provides a deeper emitter junction. 
     As may be seen in FIG. 11, a thick interlevel dielectric LTO layer  11  is deposited over the structure to provide a metalization level. This layer  11  may be formed to be planar if necessary. Then, as shown in FIG. 12, openings  22  and  23  are formed in the layer  11  by conventional lithography to provide emitter and collector contacts. A uniform oxide layer thickness of the layer  11  makes the contact openings easy to form. The large emitter size further provides a good frame for the contact hole. 
     Finally, as seen in FIG. 1, metallization into the respective contact holes  22  and  23  provides ohmic contact to the emitter and collector. The metallization may be of aluminum which may also be alloyed with appropriate materials for electrical contacts. The base contacts  8  of the base  9  may run through the device to metal contacts at the end of the SOI device. Also, device isolation can be provided by trench etching through the thin SOI film.