Patent Document

FIELD OF THE INVENTION 
   The present invention generally relates to the field of BiCMOS technology and, more specifically, relates to a method of producing complementary SiGe bipolar transistors. 
   BACKGROUND OF THE INVENTION 
   In a conventional process of producing complementary SiGe bipolar transistors, collector and base zones of the two conductivity types are formed in adjacent areas on a support wafer. An emitter interface oxide layer is then grown simultaneously for both transistors, and the emitters are patterned thereon. The presence of an interfacial oxide reduces the base current and increases the transistor gain. The interface oxide, however, creates a higher barrier for hole than for electron tunneling so that the NPN gain increases much more than the PNP gain. An increase in gain reduces the transistor breakdown voltage. A thickness of the interface oxide layer sufficient to improve the PNP gain causes the NPN breakdown voltage to drop below acceptable values. This can be corrected by increasing the NPN base dose, but at the cost of reducing transistor speed. An increase in base dose not only increases the Gummel number, it also reduces electron mobility, increases emitter-base capacitance, and slightly increases the base width. The net result of an increased base dose is a drop in the transient frequency (fT) by about 4 GHz. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of producing complementary SiGe bipolar transistors wherein a loss in speed is avoided. 
   In accordance with the invention, the method of producing complementary SiGe bipolar transistors comprises the following steps: 
   a support wafer is provided; 
   a first collector zone is formed on the support wafer in epitaxial silicon of a first conductivity type; 
   a second collector zone is formed on the support wafer adjacent the first collector zone in epitaxial silicon of a second conductivity type; 
   a first base layer is formed over the first collector zone from crystalline SiGe; 
   a second base layer is formed over the second collector zone from crystalline SiGe; 
   a continuous insulating layer is formed over the base layers; 
   the first base layer is selectively exposed; 
   a first emitter interface oxide layer optimized for the first conductivity type is deposited on the exposed first base layer; 
   a first emitter structure is patterned over the first interface oxide layer and the emitter structure is covered with a protective layer; 
   the second base layer is selectively exposed; 
   a second emitter interface oxide layer optimized for the second conductivity type is deposited on the exposed second base layer; and 
   a second emitter structure is patterned over the second interface oxide layer. 
   By separately forming the interface oxide layers for the NPN and the PNP emitters, each of these interface oxide layers can be optimized for the respective conductivity type. As a result, a loss in speed and a deterioration of the breakdown voltage are avoided. 
   Further advantages and features of the invention will become apparent from the following detailed description with reference to the appending drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a sectional view across PNP and NPN collector and base structures on a support wafer in a condition prior to forming the NPN emitter; 
       FIG. 2  is a sectional view illustrating a subsequent step of selective NPN collector ion implantation; 
       FIG. 3  is a sectional view illustrating subsequent steps of NPN emitter surface preparation, interface oxide deposition, emitter polysilicon deposition, screen oxide deposition and patterning; 
       FIG. 4  is a sectional view illustrating a subsequent step of nitride deposition in preparation for the PNP emitter; 
       FIG. 5  is a sectional view illustrating a subsequent step of selective PNP collector ion implantation; and 
       FIG. 6  is a sectional view illustrating subsequent steps of PNP surface preparation, emitter polysilicon deposition, screen oxide deposition and patterning. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   With reference to  FIG. 1 , there is illustrated in a side view a section through a wafer  10  consisting substantially of silicon. Deposited on the wafer  10  are a few structures serving as the starting point for the method in accordance with the invention as detailed in the following. The wafer  10  comprises, running parallel to the top face, a buried layer  12  consisting of silicon dioxide, for example, and used for electrically insulating the overlying layers. Over the buried layer  12  the wafer  10  is provided with two regions, each of which is intended to form a collector zone of a bipolar transistor. In the following, the method in accordance with the invention is described for the case that in the left-hand region, termed PNP in  FIG. 1 , a bipolar PNP transistor is to be formed whilst in the right-hand region termed NPN in  FIG. 1 , a bipolar NPN transistor is to be formed. 
   In the NPN region the collector of an NPN transistor is formed. This zone is termed in the following NPN collector zone  14 . The NPN collector zone  14  consists of an epitaxial crystalline layer of silicon. Interposed between the NPN collector region  14  and the buried layer  12  is a so-called buried n-layer  15 . The buried n-layer  15  consists of silicon doped with a high concentration of n-type dopant (for example arsenic or phosphorous) to make available a low impedance contact from a contact terminal  13  to the NPN collector region  14 . 
   In the PNP region the collector of a PNP transistor is formed. This collector zone is termed PNP collector zone  16  in the following. The PNP collector zone  16  for the PNP transistor also consists of epitaxial crystalline silicon. Interposed between the PNP collector region  16  and the buried layer  12  is a so-called buried p-layer  17 . The buried p-layer  17  consists of silicon doped with a high concentration of p-type dopant (for example boron) to make available a low impedance contact from a contact terminal  19  to the PNP collector region  16 . 
   Thereafter, a first base layer  18  is formed over the NPN collector zone  14  from crystalline SiGe. Likewise, a second base layer  20  is formed over the PNP collector zone  16  from crystalline SiGe. 
   After the base layers  18 ,  20  have been thus formed, a thin oxide film  26  is grown, covering the surface of the wafer in the current state including the base layers  18 ,  20 . A continuous thin insulating layer  28  of tetraethylorthosilicate (TEOS) and a continuous thin nitride layer  30  are deposited over the oxide film  26 . 
   With reference to  FIG. 2 , a photoresist layer  32  is deposited and a structure is patterned therein, allowing to etch an emitter window  34  into the insulating layer, selectively exposing the first base layer  18 . The NPN collector zone  14  is selectively subjected to an ion implantation through the emitter window  34 , as indicated at  36 . 
   Turning now to  FIG. 3 , a first emitter interface oxide layer (IFO)  38 , optimized for the NPN conductivity type, is grown on the exposed first base layer  18 . Emitter polysilicon  40  is then deposited, covered by a thin screen oxide as a protective layer  42 . The emitter polysilicon  40  is blanket implanted with arsenic without a mask. Then the protective layer  42  and the emitter polysilicon  40  are patterned by etching, stopping the etching on the oxide, leaving only the NPN emitter  44  as an island. It is estimated that upon patterning of the emitter  44  a small fraction of the insulating layer  28  is removed. A thin nitride film is deposited to compensate for this removal and to protect the NPN emitter sidewalls during subsequent etches. 
   In a similar way, a PNP emitter is created. Specifically, with reference to  FIG. 4 , a second nitride film  50  is deposited, covering the entire surface of the wafer. Thereafter, a photoresist layer  52  ( FIG. 5 ) is deposited and an emitter window  54  is opened penetrating the nitride film  50 , the oxide film  26  and the insulating layer  28 , selectively exposing the second base layer  20 . The PNP  10  collector zone  16  is selectively subjected to an ion implantation through the emitter window  54 , as indicated at  56 . 
   After removal of the photoresist layer  52 , as seen in  FIG. 6 , a second emitter interface oxide layer (IFO)  58 , optimized for the PNP transistor, is grown, on the exposed second base layer  20 . Emitter polysilicon  60  is deposited and covered by a thin screen oxide as a protective layer  62 . The emitter polysilicon  60  is blanket implanted with boron without a mask. Then the protective layer  62  and the emitter polysilicon  60  are patterned by etching, stopping the etching on the underlying oxide  28 , leaving only the PNP emitter  64  as an island ( FIG. 6 ). 
   If any polysilicon stringers around the NPN emitter  44  are present, they can be removed by over-etching, leaving nitride spacers  66  as also shown in  FIG. 6 . The presence of the nitride spacers  66  reduces the aspect ratio for PNP polysilicon emitter etch and stringer removal and allows stringer removal by over-etching without attacking the NPN emitter sidewalls.

Technology Category: 5