Abstract:
Disclosed is a semiconductor device which has: a bipolar transistor comprising a collector region of a second conductivity type formed from the surface of a semiconductor substrate of a first conductivity type, a base region of a first conductivity type formed from the surface of the collector region, and an emitter region of a second conductivity type formed from the surface of the base region; a collector extraction region that is separated by an insulating layer and is formed in the collector region except the base region; a concave portion in the collector extraction region that is formed up to a depth where the collector region has a peak concentration in impurity distribution; and a collector extraction electrode that is connected with the collector region to extract ohmic-connecting to the bottom of the concave portion.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a semiconductor device formed combining a bipolar transistor with a complementary field-effect transistor (hereinafter referred to as ‘CMOS’) and a method for making the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    BiCMOS that a bipolar transistor and CMOS are formed on a common substrate has both the high-speed operation and high driving performance of bipolar transistor and the low consumed power of CMOS. Thus, BiCMOS is one of themost effective means to meet recent demands for low consumed power and high-speed operation.  
           [0003]    H. Suzuki et al., “Process Integration Technologies for a 0.3 μm BiCMOS SRAM with 1.5V Operation”, IEEE, Proceedings of the 1996 Bipolar/BiCMOS Circuits and Technology Meeting, pp.89-92 (hereinafter referred to as ‘first prior art’) reports a bipolar transistor structure of BiCMOS.  
           [0004]    In the first prior art (BiCMOS), the n + -type buried layer must be diffused in the horizontal and vertical directions of the wafer due to the following three processes:  
           [0005]    1) high-temperature thermal treatment in growing the epitaxial layer,  
           [0006]    2) thermal treatment in forming the device-separating oxide film, and  
           [0007]    3) thermal treatment for reducing the collector resistance.  
           [0008]    Namely, these high-temperature thermal treatments prevent the size of bipolar transistor from being reduced.  
           [0009]    Furthermore, in the first prior art, there is an essential problem that the number of fabrication steps must be increased since it needs to form the n + -type buried layer and n-type epitaxial layer which are not necessary for CMOS.  
           [0010]    In this regard, K. Ishimaru et al., “Bipolar Installed CMOS Technology without Any Process Step Increase for High Speed Cache SRAM”, Technical Digest of International Electron Devices Meeting 1995, pp.673-676 (hereinafter referred to as ‘second prior art’) gives a solution to the above problems as to the transistor size and the number of BiCMOS fabricating steps.  
           [0011]    In the second prior art, the n + -type buried layer and epitaxial layer are not formed and the collector region is formed by the ion implantation at high energy. As a result, the problem that the transistor size is prevented from being decreased because of the unnecessary expansion in impurity region due to thermal hysteresis can be solved. Also, the essential problem that the number of fabrication steps of BiCMOS is too many can be solved by having some of the steps of fabricating CMOS and bipolar transistor in common.  
           [0012]    However, in the second prior art, there occurs a new problem that the collector resistance is increased to six times, 300 Ω , compared with 50 Ω of conventional BiCMOS, as described in Table 1 in the second prior art. The collector resistance has to be compared between transistors with a same size since it depends upon the transistor size. In experimenting on a transistor with a same size as that in the first prior art, a collector resistance of 450 Ω is obtained.  
           [0013]    On the other hand, the essential problem that the number of fabrication steps of BiCMOS is too many can be also solved by the following method. A through-process of BiCMOS is, in general, designed by combining a bipolar transistor into the process of fabricating CMOS as a base process or combining CMOS into the process of fabricating a bipolar transistor as a base process. Accordingly, the essential problem can be solved by reducing the number of steps in the base process or the process for the component to be combined.  
           [0014]    A specific example of such a method is disclosed in U.S. Pat. No. 5,358,882 (hereinafter referred to as ‘third prior art’) that the number of steps in fabricating a bipolar transistor is reduced.  
           [0015]    As described above, in the first prior art, there is the problem that the size of bipolar transistor is prevented from being reduced because the n + -type buried layer must be diffused in the horizontal and vertical directions of the wafer. Also, there is the essential problem that the number of BiCMOS fabrication steps must be increased.  
           [0016]    In the second prior art, which can help solve these problems, there is the problem that the collector resistance of the bipolar transistor is increased because the collector region formed by the ion implantation must have a lowered impurity concentration.  
           [0017]    Problems caused by an increase in collector resistance will be explained in FIG.4. FIG. 4  shows a DC characteristics dependency to collector resistance in applying a voltage of 1.0V between the collector and emitter of an bipolar transistor. In FIG. 4 , full lines indicate a characteristic in case of a collector resistivity of 200  Ω , and dotted lines indicate a characteristic in case of a collector resistivity of 300 Ω . As seen from the dotted lines in FIG. 4 , in case of a collector resistivity of 300 Ω , base current (I B ) is rapidly increased, compared with corrector current (I C ), in a range of high base-to-emitter voltage (V BE &gt;1.0V). Thereby, the current-amplification factor (=I C /I B ) of the bipolar transistor is rapidly decreased. In general, this phenomenon is called ‘saturation’, and it is know that such a phenomenon affects badly the circuit operation.  
           [0018]    In the third prior art, the number of BiCMOS fabrication steps can be reduced without increasing the collector resistance. However, in the third prior art, the n + -type buried layer formed on the p-type silicon substrate must be diffused, like the first prior art, in the horizontal direction due to the high-temperature thermal treatment in growing the epitaxial layer on the n + -type buried layer. Because of this, the insulation separation width of the bipolar transistor must be increased, therefore preventing the transistor size from being reduced.  
         SUMMARY OF THE INVENTION  
         [0019]    Accordingly, it is an object of the invention to provide a semiconductor device and a method for making a semiconductor device that the number of fabrication steps can be reduced and the size of bipolar transistor can be miniaturized without increasing a collector resistance.  
           [0020]    According to the invention, a semiconductor device, comprises:  
           [0021]    a bipolar transistor comprising a collector region of a second conductivity type formed from the surface of a semiconductor substrate of a first conductivity type, a base region of a first conductivity type formed from the surface of the collector region, and an emitter region of a second conductivity type formed from the surface of the base region;  
           [0022]    a collector extraction region that is separated by an insulating layer and is formed in the collector region except the base region;  
           [0023]    a concave portion in the collector extraction region that is formed up to a depth where the collector region has a peak concentration in impurity distribution; and  
           [0024]    a collector extraction electrode that is connected with the collector region to extract ohmic-connecting to the bottom of the concave portion.  
           [0025]    According to another aspect of the invention, a semiconductor device, comprises:  
           [0026]    a bipolar transistor comprising a collector region of a second conductivity type formed from the surface of a semiconductor substrate of a first conductivity type, a base region of a first conductivity type formed from the surface of the collector region, and an emitter region of a second conductivity type formed from the surface of the base region;  
           [0027]    a collector extraction region that is separated by an insulating layer and is formed in the collector region except the base region;  
           [0028]    a collector connection region that is formed in the collector extraction region and a second conductivity type of impurity is implanted with a concentration higher than the collector region;  
           [0029]    a concave portion in the collector connection region that is formed up to a depth where the collector connection region or collector region has a peak concentration in impurity distribution; and  
           [0030]    a collector extraction electrode that is connected with the collector region to extract ohmic-connecting to the bottom of the concave portion.  
           [0031]    According to another aspect of the invention, a semiconductor device, comprises:  
           [0032]    a bipolar transistor comprising a collector region of a second conductivity type formed from the surface of a semiconductor substrate of a first conductivity type, a base region of a first conductivity type formed from the surface of the collector region, and an emitter region of a second conductivity type formed from the surface of the base region;  
           [0033]    a collector extraction region that is separated by an insulating layer and is formed in the collector region except the base region;  
           [0034]    a concave portion in the collector extraction region that is formed shallower than a depth where the collector region has a peak concentration in impurity distribution;  
           [0035]    a diffusion layer that is formed from the bottom of the concave portion up to a depth where the collector region has the peak concentration in impurity distribution and a second conductivity type of impurity is implanted; and  
           [0036]    a collector extraction electrode that is connected with the collector region to extract ohmic-connecting to the bottom of the concave portion.  
           [0037]    According to another aspect of the invention, a semiconductor device, comprises:  
           [0038]    a bipolar transistor comprising a collector region of a second conductivity type formed from the surface of a semiconductor substrate of a first conductivity type, a base region of a first conductivity type formed from the surface of the collector region, and an emitter region of a second conductivity type formed from the surface of the base region;  
           [0039]    a collector extraction region that is separated by an insulating layer and is formed in the collector region except the base region;  
           [0040]    a collector connection region that is formed in the collector extraction region and a second conductivity type of impurity is implanted with a concentration higher than the collector region;  
           [0041]    a concave portion in the collector connection region that is formed up to a depth where the collector connection region or collector region has a peak concentration in impurity distribution;  
           [0042]    a diffusion layer that is formed from the bottom of the concave portion up to a depth where the collector connection region or collector region has the peak concentration in impurity distribution and a second conductivity type of impurity is implanted; and  
           [0043]    a collector extraction electrode that is connected with the collector region to extract ohmic-connecting to the bottom of the concave portion.  
           [0044]    According to another aspect of the invention, a method for making a semiconductor device, comprises the steps of:  
           [0045]    forming a collector extraction region that is separated by an insulating layer in a semiconductor substrate of a first conductivity type;  
           [0046]    forming a collector region by implanting impurity of a second conductivity type into a region including the collector extraction region;  
           [0047]    forming a base region by implanting impurity of a first conductivity type into a predetermined position in the collector region;  
           [0048]    forming an emitter region by implanting impurity of a second conductivity type into a predetermined position in the base region;  
           [0049]    forming a concave portion up to a depth where the collector region has a peak concentration in impurity distribution by removing selectively the collector extraction region with using the insulating layer as a mask; and  
           [0050]    forming a collector extraction electrode that is ohmic-connected with the bottom of the concave portion.  
           [0051]    According to another aspect of the invention, a method for making a semiconductor device, comprises the steps of:  
           [0052]    forming a collector extraction region that is separated by an insulating layer in a semiconductor substrate of a first conductivity type;  
           [0053]    forming a collector region by implanting impurity of a second conductivity type into a region including the collector extraction region;  
           [0054]    forming a collector connection region by implanting impurity of a second conductivity type with a concentration higher than the collector region into the collector extraction region;  
           [0055]    forming a base region by implanting impurity of a first conductivity type into a predetermined position in the collector region;  
           [0056]    forming an emitter region by implanting impurity of a second conductivity type into a predetermined position in the base region;  
           [0057]    forming a concave portion up to a depth where the collector region or collector connection region has a peak concentration in impurity distribution by removing selectively the collector extraction region with using the insulating layer as a mask; and  
           [0058]    forming a collector extraction electrode that is ohmic-connected with the bottom of the concave portion.  
           [0059]    According to another aspect of the invention, a method for making a semiconductor device, comprises the steps of:  
           [0060]    forming a collector extraction region that is separated by an insulating layer in a semiconductor substrate of a first conductivity type;  
           [0061]    forming a collector region by implanting impurity of a second conductivity type into a region including the collector extraction region;  
           [0062]    forming a base region by implanting impurity of a first conductivity type into a predetermined position in the collector region;  
           [0063]    forming an emitter region by implanting impurity of a second conductivity type into a predetermined position in the base region;  
           [0064]    forming a concave portion with a depth shallower than that where the collector region has a peak concentration in impurity distribution by removing selectively the collector extraction region with using the insulating layer as a mask;  
           [0065]    forming a diffusion layer from the bottom of the concave portion up to the depth where the collector region has the peak concentration in impurity distribution by implanting impurity of second conductivity type; and  
           [0066]    forming a collector extraction electrode that is ohmic-connected with the bottom of the concave portion.  
           [0067]    According to another aspect of the invention, a method for making a semiconductor device, comprises the steps of:  
           [0068]    forming a collector extraction region that is separated by an insulating layer in a semiconductor substrate of a first conductivity type;  
           [0069]    forming a collector region by implanting impurity of a second conductivity type into a region including the collector extraction region;  
           [0070]    forming a collector connection region by implanting impurity of a second conductivity type with a concentration higher than the collector region into the collector extraction region;  
           [0071]    forming a base region by implanting impurity of a first conductivity type into a predetermined position in the collector region;  
           [0072]    forming an emitter region by implanting impurity of a second conductivity type into a predetermined position in the base region;  
           [0073]    forming a concave portion with a depth shallower than that where the collector region or collector extraction region has a peak concentration in impurity distribution by removing selectively the collector extraction region with using the insulating layer as a mask;  
           [0074]    forming a diffusion layer from the bottom of the concave portion up to the depth where the collector region or collector extraction region has the peak concentration in impurity distribution by implanting impurity of second conductivity type; and  
           [0075]    forming a collector extraction electrode that is ohmic-connected with the bottom of the concave portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0076]    The invention will be explained in more detail in conjunction with the appended drawings, wherein:  
         [0077]    [0077]FIGS. 1A to  1 F are cross sectional views showing the structure and fabrication method of a bipolar transistor in the first prior art,  
         [0078]    [0078]FIGS. 2A to  2 E are cross sectional views showing the structure and fabrication method of a bipolar transistor in the second prior art,  
         [0079]    [0079]FIGS.3A to  3 F are cross sectional views showing the structure and fabrication method of a bipolar transistor in the third prior art,  
         [0080]    [0080]FIG. 4 is a graph showing DC-characteristics dependency to collector resistance in applying a voltage of 1.0V between the collector and emitter of an bipolar transistor,  
         [0081]    [0081]FIGS. 5A to  5 H are cross sectional views showing the structure and fabrication method of a semiconduc tor device in a first preferred embodiment according to the invention,  
         [0082]    [0082]FIGS. 6A to  6 H are cross sectional views showing the structure and fabrication method of a semiconductor device in a second preferred embodiment according to the invention,  
         [0083]    FIG. 7  is a graph showing measurements of collector resistance when a contact plug for collector extraction is connected only at the bottom of a collector trench,  
         [0084]    [0084]FIGS. 8A to  8 K are cross sectional views showing the structure and fabrication method of a semiconductor device in a third preferred embodiment according to the invention,  
         [0085]    [0085]FIGS. 9A and 9B are partial cross sectional views illustrating problems in the third embodiment,  
         [0086]    [0086]FIGS. 10A to  10 K are cross sectional views showing the structure and fabrication method of a semiconductor device in a fourth preferred embodiment according to the invention, and  
         [0087]    [0087]FIGS. 11A to  11 C are partial cross sectional views showing alterations in the fourth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0088]    Before explaining a semiconductor device and a method for making the same in the preferred embodiments, the aforementioned conventional bipolar transistor and its fabrication method in the first to third prior arts will be explained in FIGS. 1A to  3 F.  
         [0089]    The method for making the bipolar transistor in the first prior art will be explained in FIGS. 1A to  1 F.  
         [0090]    As shown in FIG. 1A, a n + -type buried layer  1503  is first formed on a p-type silicon substrate  1501  by ion-implanting, e.g., arsenic or antimony of 10 15  to 10 16 cm −2  at 30 to 100keV into the substrate  1501 .  
         [0091]    Then, as shown in FIG. 1B, a n-type epitaxial layer  1504  of 0.6 to 1.5μm thick is grown on the substrate  1501 . The epitaxial layer  1504  is formed by using a known epitaxial growth technique, e.g., depositing silicon on a wafer while thermally decomposing a gas of SiC14, SiH2C12, SiH4 or the like at a high temperature of 100 to 1200° C.  
         [0092]    At this time, as seen from the comparison of FIGS. 1A and 1B, the n + -type buried layer  1503  is expanded diffusing in the horizontal and vertical directions. This is because the implanted impurity is diffused by the high temperature treatment of 1000 to 1200° C. for growing the epitaxial layer. The diffusion occurs in the horizontal and vertical directions of the wafer. It is known that the diffusing region is raised 0.2 to 0.3 μm forward the wafer surface by the diffusion.  
         [0093]    Then, as shown in FIG. 1C, device-separating oxide film  1507  of 250 to 600nm thick is formed at a temperature of 900 to 110° C. by using, e.g., a known LOCOS separation method. At this time, the n + -type buried layer  1503  is further thermally diffused in the horizontal and vertical directions, compared with that in FIG. 1B (though this is not shown clearly in FIG. 1C).  
         [0094]    Then, as shown in FIG. 1D, a collector diffusion layer  1518  is formed by ion-implanting, e.g., phosphorus, arsenic of 1×10 15  to 1×10 16 cm −2  at 50 to 100keV into a predetermined position of the n-type epitaxial layer  1504 .  
         [0095]    Then, as shown in FIG. 1E, a first p-type separation region  1502  is formed by ion-implanting, e.g., boron of 1×10 13  to 5×10 14 cm −2  at a high energy of 500keV to 1.5MeV. Further, a second p-type separation region  1505  is formed by ion-implanting boron of 1×10 13  to 5×10 14 cm −2  at 100 to 250keV.  
         [0096]    Then, as shown in FIG. 1F, a p-type base region  1509 , an emitter extraction electrode  1514 , p + -type graft base  1516  etc. are formed by using known techniques. Then, after forming interlayer insulating film  1527  on them, contact holes  1528  are formed at predetermined positions and then contact plugs  1529  are plugged thereinto. Then, metal wiring  30  is formed connecting with the contact plugs  1529 .  
         [0097]    By the process described above, BiCMOS in FIG. 1F can be obtained. Namely, the n + -type buried layer  1503  and n-type epitaxial layer  1504  are formed on the p-type silicon substrate  1501 . Also, the collector extraction can be conducted by the collector diffusion layer  1518 . Also, the first and second p-type separation regions  1502 ,  1505  allow the insulation separation from other transistors to be formed on the common substrate. Further, there are formed the p-type base region  1509  and an emitter diffusion layer  1517  for the bipolar transistor in the n + -type epitaxial layer  1504 .  
         [0098]    Meanwhile, in abipolar transistor used in BiCMOS, the collector connection is generally extracted from the device surface. Because of this, a resistivity (collector resistance) from the collector region formed in the n-type epitaxial layer  1504  under the p-type base region  1509  to the metal wiring  1530  as collector electrode is a problem to be solved.  
         [0099]    Until recently, a collector region with a high-concentration impurity introduced could not be formed by the ion implantation. Due to this, as described above, in general, the collector region is formed by using the n-type epitaxial layer  1504  and is connected through the n + -type buried layer  1503 , collector diffusion layer  1518  and contact plug  1529  to the metal wiring  1530 .  
         [0100]    In this case, to minimize the connector resistance, it is ideal that the n + -type buried layer  1503  and the collector diffusion layer  1518  are connected with each otherwhile they have high-concentration impurities. However, informing the collector diffusion layer  1518 , the peak of impurity concentration is 0.12 μm deep even by implanting phosphorus at the highest implantation energy. In this regard, the n-type epitaxial layer  1504  has to have a thickness of 0.6 μm at the minimum. Also, the rising of the n + -type buried layer  1503  forward the wafer surface due to the thermal hysteresis is about 0.3 μm at the maximum. Therefore, when the ion implantation is simply conducted, a clearance between the top of the n + -type buried layer  1503  and the bottom of the collector diffusion layer  1518  must be greater than 0.18 μm. In brief, the ideal connection between the n + -type buried layer  1503  and the collector diffusion layer  1518  cannot be obtained.  
         [0101]    To reduce the clearance of 0.18 μm to allow the collector diffusion layer  1518  to contact the n + -type buried layer  1503 , the collector diffusion layer  1518  formed by the ion implantation needs to be expanded diffusing by thermal treatment, e.g., at a temperature of 900 to 1000° C. for 30 to 60 min. It is apparent that, due to the thermal treatment, both the n + -type buried layer  1503  and the collector diffusion layer  1518  will be further expanded diffusing in the horizontal and vertical directions of the wafer.  
         [0102]    The method for making BiCMOS in the second prior art will be explained in FIGS. 2A to  2 E.  
         [0103]    As shown in FIG. 2 A, device-separating oxide film  1607  of 0.7 μm deep is first formed on a p-type silicon substrate  1601  by a known technique. Meanwhile, in the second prior art, the device-seperating oxide film  1607  is called ‘STI (shallow trench isolation’.  
         [0104]    Then, as shown in FIG. 2B, a first p-type well region  1602  is formed by ion-implanting boron of 5×10 13 cm −2  at 350keV. Also, a first n-type well region  1606  is formed by ion-implanting phosphorus of 5×10 13 cm −2  at 700keV.  
         [0105]    At this time, an insulation separation region for bipolar transistor is simultaneously formed with the first p-type well region  1602 . Also, in the second prior art, a collector region  1632  for bipolar transistor is simultaneously formed with the first n-type well region  1606 . Namely, the collector region  1632  is formed by the ion implantation.  
         [0106]    Thus, in the second prior art, the impurity concentration necessary for collector can be obtained by the ion implantation. Because of this, for example, the epitaxial layer is not necessary to form. Therefore, the number of fabrication steps can be significantly decreased, compared with that in the first prior art.  
         [0107]    Then, as shown in FIG. 2C, gate oxide film  1608  of 7nm thick is formed. Then, the gate oxide film  1608  locating at a region to form a base extraction electrode is removed. Then, p + -type and n + -type gate electrodes  1613  and the base extraction electrode  1631  with a multilayer structure that insulating film is formed on a polycide structure are formed. Then, a n-type LDD layer  1620 , a p-type LDD layer and a p-type base region  1609  are formed.  
         [0108]    Meanwhile, the impurity doping to the base extraction electrode  1631  is conducted in forming the p + -type gate electrode  1613 . Also, the p-type base region  1609  is simultaneously formed with the p-type LDD layer  1621 . Further, a p + -type graft base  1616  is formed by diffusing boron from the base extraction electrode  1631  by the thermal treatment of the fabrication process.  
         [0109]    Then, as shown in FIG. 2 D, side walls  1619  of silicon dioxide film etc. are formed on the side of the gate electrode  1613  and base extraction electrode  1631  by using a known technique. Then, by ion-implanting in self-aligned manner with using these as a mask, a n + -type source/drain  1622 , a p + -type source/drain  1623  and a collector diffusion layer  1618  are formed. Then, an emitter extraction electrode  1614  with a polycide structure is formed. Meanwhile, the collector diffusion layer  1618  is simultaneously formed with the n + -type source/drain  1622  for n-MOS.  
         [0110]    Then, as shown in FIG. 2E, an emitter diffusion layer  1617 , interlayer insulating film  1627 , contact plugs  1629  and metal wiring  1630  are formed by a known technique.  
         [0111]    By the process described above, as shown in FIG. 2E, the collector region  1632  as well as the first n-type well region  1606  for CMOS is formed on the p-type silicon substrate  1601  by ion implantation. Also, the collector extraction can be conducted by the collector diffusion layer  1618  formed simultaneously with the n + -type source/drain  1622  for n-MOS. Also, the first p-type well region  1602  allows the insulation separation from other transistors to be formed on the common substrate. Further, there are formed the p-type base region  1609  and emitter diffusion layer  1617  for the bipolar transistor in the collector region  1632  formed on the p-type silicon substrate  1601 .  
         [0112]    In the second prior art, the n + -type buried layer  1503  and n-type epitaxial layer  1504  in FIG. 1F are not employed.  
         [0113]    The process of fabricating a bipolar transistor in the third prior art will be explained in FIGS. 3A to  3 F.  
         [0114]    At first, as shown in FIG. 3A, a n + -type buried layer  1703  is formed on a p-type silicon substrate  1701  by ion-implanting arsenic or antimony using oxide film as a mask. Then, a n-type epitaxial layer  1704  of 0.2 to 0.5μm is formed.  
         [0115]    Then, for example, silicon thermal oxidation film, silicon nitride film and TEOS silicon dioxide film of 50nm, 150nm and 600nm, respectively, are formed as a hard mask for forming a trench, and then an aperture to surround the bipolar transistor is formed by photolithography. Then, as shown in FIG. 3B, the trench  1715  that reaches the p-type silicon substrate  1701  penetrating through the n + -type buried layer  1703  is formed by digging 4.4 μm deep from the surface of the n-type epitaxial layer  1704  by known anisotropic etching.  
         [0116]    Then, after growing first oxide film  1733  of 50 to 100nm on the side wall and the bottom of the trench  1715 , a channel stopper region  1734  is formed in the p-type silicon substrate  1701  under the bottom of the trench  1715 . Then, a filler  1735  of oxide film doped or non-doped by CVD is filled into the inside of the trench  1715 , and then the filler  1735  is removed up to the surface of the n-type epitaxial layer  1704  by a known etching technique while leaving the silicon thermal oxidation film of 50nm. Then, second oxide film  1737  of 100 nm is formed by thermal oxidation to cap the trench.  
         [0117]    Then, as shown in FIG. 3C, device-separating oxide film (TEOS-SiO 2  film)  1707  of 200 to 500 nm is formed by CVD using a gas of Si(OC 2 H 5 ) 4  at 730° C. Then, an active transistor region including emitter, base and collector, a collector contact  1726  and an on-substrate contact  1740  are positioned by photolithography using photoresist, and then the device-separating oxide film  1707  is selectively removed by a known etching technique to expose the surface of the n-type epitaxial layer.  
         [0118]    Then, as shown in FIG. 3D, p + -type polysilicon film  1738  and third oxide film  1739  are grown. Then, a base extraction electrode  1731  is formed by etching simultaneously the third oxide film  1739  and p + -type polysilicon film  1738  by a known anisotropic etching. Then, a p-type base region (active base region)  1709  is formed by ion-implanting boron or BF 2  of 5×10 13 cm −2  at 15keV while using photoresist as a mask. Further, side walls  1719  of oxide film are formed.  
         [0119]    Then, as shown in FIG. 3E, an emitter extraction electrode  1714  is formed by growing n + -type-doped polysilicon and then etching selectively by anisotropic etching using photoresist as a mask. Then, a collector trench  1724  is formed by further over-etching the on-substrate contact  1740  until reaching the n + -type buried layer  1703 . At this time, a collector trench  1724  is simultaneously formed etching the collector contact  1726 .  
         [0120]    Then, as shown in FIG. 3F, interlayer insulating film  1727  composed of oxide film (TEOS-SiO 2  film) of 50nm and BPSG (boronphosphorus-silicate-glass) of 800nm is formed. Then, an emitter diffusion layer  1717  is formed by, for example, RTA (rapid thermal annealing) at 1050° C. for 5 to 15sec. or furnace annealing at 900° C. for 20 to 30min. Then, contact holes are formed and metal wiring  1730  is formed.  
         [0121]    As described above, in the third prior art, when the pattern of the emitter extraction electrode  1714  is formed by etching, the collector trench  1724  is also formed by etching the surface of the n-type epitaxial layer  1704 . Also, at the bottom of the collector trench  1724 , the n + -type buried layer  1703  and metal wiring  1730  are directly connected.  
         [0122]    Thus, the collector diffusion layer  1518  for collector extraction in the first prior art is not necessary to form. As a result, the number of fabrication steps can be reduced. Also, in the third prior art, the collector resistance is not increased.  
         [0123]    Next, a semiconductor device and a method for making the same in the first preferred embodiment will be explained in FIGS. 5A to  5 H.  
         [0124]    At first, referring to FIG. 5H, the structure of a bipolar transistor in the semiconductor device in the first embodiment will be explained below.  
         [0125]    In the bipolar transistor, a n-well region  106  and a collector region  106   a  are formed on a p-type silicon substrate  101 . A collector contact  126  and ap-type base region  109  are separated from each other by device-separating oxide film  107  formed on the surface of the collector region  106   a . Also, in the region of the collector contact  126 , the bottom of a collector trench (concave portion)  124  is formed to abut on a portion with peak impurity concentration in the collector region  106   a.    
         [0126]    Also, side walls  119  are formed on the side of the collector trench  124 . A n + -type diffusion layer  128  to be formed in implanting n + -type source/drain for CMOS is formed at part of the bottom of the collector trench  124  where the side wall  119  is not left. Also, a silicide layer  125   a  is formed on the surface of n + -type diffusion layer  128 . It is connected through barrier metal (not shown) and a contact plug  129  or not through the contact plug to metal wiring  130 .  
         [0127]    Meanwhile, the bottom of the collector trench  124  may be formed a little lower than the portion with peak impurity concentration in the collector region  106   a  so that a portion with peak impurity concentration in the n + -type diffusion layer  128  can abut on the portion with peak impurity concentration in the collector region  106   a.    
         [0128]    Next, the method for making the semiconductor device in the first embodiment will be explained in FIGS. 5A to  5 H.  
         [0129]    At first, as shown in FIG. 5A, device-separating oxide film  107  and first oxide film  133  are formed on the p-type silicon substrate  101  by using known LOCOS or STI etc.  
         [0130]    Then, as shown in FIG. 5B, a p-well region  102  to form n-MOS is formed by implanting, e.g., boron of 5×10 13 cm −2  at 350keV. Also, a n-well region  106  to form p-MOS and a collector region  106   a  to form the collector of the bipolar transistor are formed by implanting phosphorus of 5×10 13 cm −2  at 700keV.  
         [0131]    Then, as shown in FIG. 5C, gate oxide film  108  of 5 to 10nm is formed on the p-type silicon substrate  101 . Then, a p-type base region  109  is formed by implanting, e.g., boron or BF 2  of 1×10 13  to 5×10 14 cm −2  at 10 to 50keV. Also, after forming an emitter contact  110  and a collector contact  126 , first polysilicon film  112  of 150 to 400nm is grown.  
         [0132]    Then, as shown in FIG. 5D, gate electrodes  113  and an emitter extraction electrode  114  are formed by known anisotropic etching using a pattern, such as photoresist, as a mask. Then, the collector trench  124  is formed by further etching it using the photoresist pattern and gate oxide film  108  as a mask. Then, the mask of photoresist is removed. Meanwhile, these etchings may be conducted sequentially on same conditions or separately in several steps.  
         [0133]    Then, as shown in FIG. 5E, a n-type LDD layer  120  and a p-type LDD  121  are formed. Then, after forming oxide film for side wall, the side walls  119  are formed on the side of the gate electrode  113 , emitter extraction electrode  114  and collector trench  124  by known anisotropic etching.  
         [0134]    Then, as shown in FIG. 5F, a n + -type source/drain region  122  for n-MOS is formed by ion-implanting impurity of phosphorus, arsenic or the like. Also, the n + -type diffusion layer  128  is formed at the bottom of the collector trench  124 . Then, a p + -type source/drain region  123  for p-MOS and a p + -type graft base  116  are formed by ion-implanting impurity of boron, BF 2  or the like.  
         [0135]    Meanwhile, the impurity introduction into the emitter extraction electrode  114  may be conducted simultaneously with the ion implantation of impurity of phosphorus, arsenic or the like to form the n + -type source/drain region  122  for n-MOS. Alternatively, it may be conducted by another step of implanting impurity of phosphorus, arsenic or the like.  
         [0136]    Also, it is desired that these ion implantations be conducted through thin oxide film  141  of about 5 to 20nm to be further formed on, e.g., the exposed surface of the p-type silicon substrate  101  so as to prevent a crystal defect at the end of the side wall  119 .  
         [0137]    Then, as shown in FIG. 5G, a silicide layer  125  or  125   a  is formed by siliciding the surface of the gate electrode  113 , emitter extraction electrode  114 , n + -type diffusion layer  128  at the bottom of the collector trench  124 , n + -type source/drain region  122 , p + -type source/drain region  123  and p + -type graft base  116  by known method using metal of titanium, cobalt, nickel or the like.  
         [0138]    Then, as shown in FIG. 5H, interlayer insulating film  127  composed of, e.g., oxide film (TEOS-SiO 2  film) of 50nm and BPSG (boron-phosphorus-silicate-glass) of 800nm like the third prior art is formed. In addition, contacts are formed at predetermined positions of the interlayer insulating film  127 , and then the contact plug  129  are formed through barrier metal (not shown). Then, the metal wiring  130  to contact the contact plug  129  is formed on the interlayer insulating film  127 . Though, in FIG. 5H, the contact width for the collector trench  124  is shown to be smaller than the width of the collector trench  124 , it may be equal to or greater than that of the collector trench  124 .  
         [0139]    As described above, in the first embodiment, the n + -type buried layer and epitaxial layer are not formed and the collector region is formed by the ion implantation at high energy. As a result, the problem that the transistor size is prevented from being decreased because of the unnecessary expansion in impurity region due to thermal hysteresis can be solved.  
         [0140]    Also, in the first embodiment, the collector resistance is not increased because the collector region  106   a  and the contact plug  129  connected to the metal wiring  130  are directly connected at the bottom of the collector trench  124 .  
         [0141]    Further, the collector trench  124  is formed by etching with using the photoresist pattern and device-separating oxide film  107  as a mask, subsequently after forming the gate electrode  113  and emitter extraction electrode  114 . Therefore, the number of fabrication steps is not increased. Namely, in the first embodiment, the essential problem that the number of fabrication steps of BiCMOS is too many can be solved by having some of the steps of fabricating CMOS and bipolar transistor in common.  
         [0142]    However, in the first embodiment, as shown in FIG. 5D, the collector trench  124  is formed by etching with using the photoresist pattern and gate oxide film as a mask. Because of this, it will be difficult to use the gate oxide film as a mask when the thickness of the gate oxide film is reduced to miniaturize the transistor. In this regard, a method for making a semiconductor device in the second embodiment will give a solution.  
         [0143]    The method for making a semiconductor device in the second preferred embodiment will be explained below.  
         [0144]    At first, as shown in FIG. 6A, device-separating oxide film  207  and first oxide film  233  are formed on the p-type silicon substrate  201  by using known LOCOS or STI etc.  
         [0145]    Then, as shown in FIG.6B, a p-well region  202  to form n-MOS is formed by implanting, e.g., boron of 5×10 — cm −2  at 350keV. Also, a n-well region  206  to form p-MOS and a collector region  206   a  to form the collector of the bipolar transistor are formed by implanting phosphorus of 5×10 13 cm −2  at 700keV.  
         [0146]    Then, as shown in FIG. 6C, gate oxide film  208  of 5 to 10nm is formed on the p-type silicon substrate  101 . Then, first polysilicon film  212  of 150 to 400nm is formed.  
         [0147]    Then, as shown in FIG. 6D, gate electrodes  213  are formed by removing selectively the first polysilicon film  212  by known anisotropic etching with using a photoresist pattern, and then the photoresist pattern is removed.  
         [0148]    Then, as shown in FIG. 6E, a p-type base region  209  is formed by implanting, e.g., boron or BF 2  of 1×10 13  to 5×10 14 cm −2  at 10 to 50keV. Then, a n-type LDD layer  220 , a p-type LDD layer  221  are formed. Then, second oxide film  237  is grown.  
         [0149]    Then, as shown in FIG. 6F, an emitter contact  210  and a collector contact  226  are formed by etching selectively the second oxide film  237  with using a photoresist mask pattern. Then, second polysilicon film  242  of 150 to 400nm is grown.  
         [0150]    Meanwhile, the second polysilicon film  242  may be grown non-doping or doping with impurity of, e.g., phosphorus or arsenic, of 1×10 18  and 1×10 21 cm −2.    
         [0151]    Then, as shown in FIG. 6G, an emitter extraction electrode  214  is formed by removing selectively the polysilicon film  242  by known anisotropic etching with using a photoresist mask pattern. Then, a collector trench  224  is formed by further etching with using the photoresist pattern and second oxide film  237  as a mask. Then, the photoresist pattern is removed. These etchings may be conducted sequentially on same conditions or separately in several steps.  
         [0152]    Then, after growing third oxide film, as shown in FIG. 6H, side walls  239  with one layer of the third oxide film or side walls with two layers of the second oxide film  237  and third oxide film are formed on the side of the gate electrode  213 , emitter extraction electrode  214  and collector trench  224  by known anisotropic etching.  
         [0153]    Then, as shown in FIG. 6I, a n + -type source/drain region  222  for n-MOS is formed by ion-implanting impurity of phosphorus, arsenic or the like. Also, a n + -type diffusion layer  228  is formed at the bottom of the collector trench  224 . Then, a p + -type source/drain region  223  for p-MOS and a p + -type graft base  216  are formed by ion-implanting impurity of boron, BF 2  or the like.  
         [0154]    Meanwhile, the impurity introduction into the emitter extraction electrode  214  when the second polysilicon film  242  (FIG. 6F) to form the emitter extraction electrode  214  is grown non-doping may be conducted simultaneously with the ion implantation of impurity of phosphorus, arsenic or the like to form the n + -type source/drain region  222  for n-MOS. Alternatively, it may be conducted by another step of implanting impurity of phosphorus, arsenic or the like.  
         [0155]    Also, it is desired that these ion implantations be conducted through thin oxide film  241  of about 5 to 20nm to be further formed on, e.g., the exposed surface of the p-type silicon substrate  201  so as to prevent a crystal defect at the end of the side wall  219 .  
         [0156]    Then, as shown in FIG. 6J, a silicide layer  225  is formed by siliciding the surface of the gate electrode  213 , emitter extraction electrode  214 , n + -type diffusion layer  228  at the bottom of the collector trench  224 , n + -type source/drain region  222 , p + -type source/drain region  223  and p + -type graft base  216  by known method using metal of titanium, cobalt, nickel or the like.  
         [0157]    Then, as shown in FIG. 6K, interlaler insulating film  227  composed of, e.g., oxide film (TEOS-Sio 2  film) of 50nm and BPSG (boron-phosphorus-silicate-glass) of 800nm like the third prior art is formed. In addition, contacts are formed at predetermined positions of the interlayer insulating film  227 , and then a contact plug  229  are formed through barrier metal (not shown) . Then, metal wiring  230  to contact the contact plug  229  is formed on the interlayer insulating film  227 . Though, in FIG. 6K, the contact width for the collector trench  224  is shown to be smaller than the width of the collector trench  224 , it may be equal to or greater than that of the collector trench  224 .  
         [0158]    Next, referring to FIG. 6K, the structure of the bipolar transistor in the semiconductor device in the second embodiment will be explained below.  
         [0159]    In the bipolar transistor, the collector region  206   a  is formed on the p-type silicon substrate  201 . The collector contact  226  and the p-type base region  209  are separated from each other by the device-separating oxide film  207  formed on the surface of the collector region  206   a . Also, in the region of the collector contact  226 , the bottom of the collector trench (concave portion)  224  is formed to abut on a portion with peak impurity concentration in the collector region  206   a.    
         [0160]    Also, the side walls  219  are formed on the side of the collector trench  224 . The n + -type diffusion layer  228  to be formed in implanting the n + -type source/drain  222  for CMOS is formed at part of the bottom of the collector trench  224  where the side wall  219  is not left. Also, the silicide layer  225  is formed on the surface of the n + -type diffusion layer  228 . It is connected through barrier metal (not shown) to the contact plug  229 . The contact plug  229  is connected to the metal wiring  230  formed on the interlayer insulating film  227 .  
         [0161]    Meanwhile, the bottom of the collector trench  224  may be formed a little lower than the portion with peak impurity concentration in the collector region  206   a  so that a portion with peak impurity concentration in the n + -type diffusion layer  228  can abut on the portion with peak impurity concentration in the collector region  206   a.    
         [0162]    As described above, in the second embodiment, the n + -type buried layer and epitaxial layer are not formed and the collector region is formed by the ion implantation at high energy. As a result, the problem that the transistor size is prevented from being decreased because of the unnecessary expansion in impurity region due to thermal hysteresis can be solved.  
         [0163]    Also, in the second embodiment, the collector resistance is not increased because the collector region  206   a  and the contact plug  229  connected to the metal wiring  230  are directly connected at the bottom of the collector trench  224 .  
         [0164]    Further, the collector trench  224  is formed by etching with using the photoresist pattern and device-separating oxide film  207  as a mask, subsequently after forming the emitter extraction electrode  214 . Therefore, the number of fabrication steps is not increased. Namely, in the second embodiment, the essential problem that the number of fabrication steps of BiCMOS is too many can be solved by having some of the steps of fabricating CMOS and bipolar transistor in common.  
         [0165]    As described above, in the first and second embodiments, an increase in the collector resistance can be suppressed by forming the trench at the collector contact.  
         [0166]    Further, in the first and second embodiments, the collector resistance can be significantly reduced by designing the trench depth at the collector trench to abut on the portion with peak impurity concentration in the collector region. This is proved by the experimental result in FIG. 7 that the collector resistance is minimized at a position where the depth of collector trench reaches the portion with peak impurity concentration in the collector region.  
         [0167]    [0167]FIG. 7 shows the measurements of collector resistance when the contact plug for collector extraction is connected only at the bottom of the collector trench according to the first and second embodiments. A bipolar transistor used in this experiment has a peak of impurity concentration at a depth of about 0.9 μm.  
         [0168]    In the first and second embodiments, the collector resistance can be reduced to 250 Ω , compared with a collector resistance of 450  Ω in case of no collector trench. When the collector resistance is 250 Ω , the base-to-emitter voltage of transistor characteristics can be improved from 1.1V to 1.2V, i.e., by about 20%, compared with the collector resistance of 300 Ω . Therefore, no deterioration in base current characteristic can be observed.  
         [0169]    On the other hand, in the first and second embodiments, the n-type well region for p-MOS and the collector region for bipolar transistor are formed by the ion implantation conducted commonly as shown in FIGS. 5H and 6K, respectively. However, it is difficult to optimize simultaneously the conditions of these formations.  
         [0170]    First, to suppress the short-channel effect of p-MOS transistor, it is desired that a region with a high n-type impurity concentration be formed at a position of about 0.2 μm deep equal to the junction depth of source/drain region. Further, forp-MOS, the junction depth of source/drain region will be reduced with the miniaturization.  
         [0171]    For the bipolar transistor, it is desired that the collector-to-base or emitter-to-collector breakdown voltage be higher than the power-source voltage, the collector-to-base capacity be reduced as much as possible, and the collector resistance be reduced as much as possible. Thus, it is required that the impurity concentration around the surface of the p-type silicon substrate be lowered as much as possible and a region with a high n-type impurity concentration be formed at a position of about 0.6 to 1.0 μm deep.  
         [0172]    As understood from these, when the transistor size is further reduced, it will be difficult to optimize simultaneously the conditions of these formations.  
         [0173]    In this regard, a method for making a semiconductor device in the third embodiment will give a solution.  
         [0174]    The method for making a semiconductor device in the third preferred embodiment will be explained below.  
         [0175]    At first, as shown in FIG. 8A, device-separating oxide film  307  and first oxide film  333  are formed on a p-type silicon substrate  301  by using known LOCOS or STI etc.  
         [0176]    Then, as shown in FIG. 8B, a p-well region  302  to form n-MOS is formed by implanting, e.g., boron of 5×10 13 cm −2  at 350keV. Also, a n-well region  306  to form p-MOS and a collector connection region  306   a  to form the collector of the bipolar transistor are formed by implanting, e.g., phosphorus of 1×10 13 l to  5×10 13 cm −2  at high energy of 300 to 800keV and further phosphorus of 1×10 12  to 1×10 13 cm −2  at 50 to 150keV.  
         [0177]    Then, as shown in FIG. 8C, gate oxide film  308  of 5 to 10nm thick is formed on the p-type silicon substrate  301 . Then, a collector region  311  forbipolar transistor is formed by implanting selectively phosphorus of 1×10 13  to 1×10cm −2  at high energy of 700keV to 1.5MeV with using a photoresist mask pattern. Further, a p-type base region  309  is formed by implanting, e.g., boron or BF2 of 1×10 13  to 5×10 4 cm −2  at 10 to 50keV with using the same photoresist mask pattern. Then, first polysilicon film  312  of 150 to 400nm is grown thereon.  
         [0178]    Meanwhile, in the third embodiment, the collector region  311  and the p-type base region  309  are formed after forming the gate oxide film  308 . However, they may be formed before forming the gate oxide film  308 .  
         [0179]    Then, as shown in FIG. 8D, gate electrodes  313  are formed by removing selectively the first polysilicon film  312  by known anisotropic etching with using a photoresist mask pattern, and then the photoresist pattern is removed.  
         [0180]    Then, as shown in FIG. 8E, a n-type LDD layer  320 , a p-type LDD layer  321  are formed. Then, second oxide film  337  is grown.  
         [0181]    Meanwhile, in the third embodiment, the collector region  311  and p-type base region  309  are formed at the step in FIG. 8C. However, they may be formed before growing the second oxide film  337 .  
         [0182]    Then, as shown in FIG. 8F, an emitter contact  310  and a collector contact  326  are formed by etching selectively the second oxide film  337  wlth using a photoresist mask pattern. Then, second polysilicon film  342  of 150 to 400nm is grown.  
         [0183]    Meanwhile, the second polysilicon film  342  may be grown non-doping or doping with impurity of, e.g., phosphorus or arsenic, of 1×10 18  to 1×10 21 cm −2.    
         [0184]    Then, as shown in FIG. 8G, an emitter extraction electrode  314  is formed by removing selectively the polysilicon film  342  by known anisotropic etching with using a photoresist mask pattern. Then, a collector trench  324  is formed by further etching with using the photoresist pattern and second oxide film  337  as a mask. Then, the photoresist pattern is removed. These etchings may be conducted sequentially on same conditions or separately in several steps.  
         [0185]    Then, after growing third oxide film  339 , as shown in FIG. 8H, side walls  339  of the third oxide film are formed on the side of the gate electrode  313 , emitter extraction electrode  314  and collector trench  324  by known anisotropic etching. Meanwhile, the side wall formed on the side of the gate electrode  313  is of two layers of the second oxide film  337  and third oxide film  339 .  
         [0186]    Then, as shown in FIG. 8I, a n + -type source/drain region  322  for n-MOS is formed by ion-implanting impurity of phosphorus, arsenic or the like. Also, a n + -type diffusion layer  328  is formed at the bottom of the collector trench  324 . Then, a p + -type source/drain region  323  for p-MOS and a p + -type graft base  316  are formed by ion-implanting impurity of boron, BF 2  or the like.  
         [0187]    Meanwhile, the impurity introduction into the emitter extraction electrode  314  when the second polysilicon film  342  to form the emitter extraction electrode  314  is grown non-doping may be conducted simultaneously with the ion implantation of impurity to form the n + -type source/drain region  322  for n-MOS. Alternatively, it may be conducted by another step of implanting impurity of phosphorus, arsenic or the like.  
         [0188]    Also, it is desired that these ion implantations be conducted through thin oxide film  341  of about 5 to 20nm to be further formed on, e.g., the exposed surface of the p-type silicon substrate  301  so as to prevent a crystal defect at the end of the side wall  319 .  
         [0189]    Then, as shown in FIG. 8J, a silicide layer  325  is formed by siliciding the surface of the gate electrode  313 , emitter extraction electrode  314 , n + -type diffusion layer  328  at the bottom of the collector trench  324 , n + -type source/drain region  322 , p + -type source/drain region  323  and p + -type graft base  316  by known method using metal of titanium, cobalt, nickel or the like.  
         [0190]    Then, as shown in FIG. 8K, interlayer insulating film  327  composed of, e.g., oxide film (TEOS-SiO 2  film) of 50nm and BPSG (boron-phosphorus-silicate-glass) of 800nm like the third prior art is formed. In addition, contacts are formed at predetermined positions of the interlayer insulating film  327 , and then a contact plug  329  are formed through barrier metal (not shown). Then, metal wiring  330  to contact the contact plug  329  is formed on the interlayer insulating film  327 . Though, in FIG. 8K, the contact width for the collector trench  324  is shown to be smaller than the width of the collector trench  324 , it may be equal to or greater than that of the collector trench  324 .  
         [0191]    Next, referring to FIG. 8K, the structure of the bipolar transistor in the semiconductor device in the third embodiment will be explained below.  
         [0192]    In the bipolar transistor, the collector region  311  is formed on the p-type silicon substrate  301  and the collector connection region  306   a  is formed only at the collector contact  326  in the collector region  311 . The collector contact  326  and the p-type base region  309  are separated from each other by the device-separating oxide film  307  formed on the surface of the collector region  311 . Also, in the region of the collector contact  326 , the bottom of the collector trench  324  is formed to abut on a portion with peak impurity concentration in the collector region  311 .  
         [0193]    Also, the side walls  319  are formed on the side of the collector trench  324 . The n + -type diffusion layer  328  to be formed in implanting the n + -type source/drain  322  for CMOS is formed at part of the bottom of the collector trench  324  where the side wall  319  is not left. Also, the silicide layer  325  is formed on the surface of the n + -type diffusion layer  328 . It is connected through barrier metal (not shown) to the contact plug  329 . The contact plug  329  is connected to the metal wiring  330  formed on the interlayer insulating film  327 .  
         [0194]    Meanwhile, the bottom of the collector trench  324  may be formed a little lower than the portion with peak impurity concentration in the collector region  311  so that a portion with peak impurity concentration in the n + -type diffusion layer  328  can abut on the portion with peak impurity concentration in the collect or region  311 .  
         [0195]    As described above, in the third embodiment, the n + -type buried layer and epitaxial layer are not formed and the collector region is formed by the ion implantation at high energy. As a result, the problem that the transistor size is prevented from being decreased because of the unnecessary expansion in impurity region due to thermal hysteresis can be solved.  
         [0196]    Also, the collector trench  324  is formed by etching with using the photoresist pattern and second oxide film  337  as a mask, subsequently after forming the emitter extraction electrode  314 . Therefore, the number of fabrication steps is not increased. Namely, in the third embodiment, the essential problem that the number of fabrication steps of BiCMOS is too many can be solved by having some of the steps of fabricating CMOS and bipolar transistor in common.  
         [0197]    Also, in the third embodiment, the collector resistance is not increased because the collector region  311  and the contact plug  329  connected to the metal wiring  330  are directly connected at the bottom of the collector trench  324 .  
         [0198]    In addition, in the third embodiment, the impurity concentration can be increased by forming the collector connection region  306   a  and collector region  311  at the collector contact  326 . Thereby, the collector resistance can be further reduced to 200  Ω . Meanwhile, when a collector resistance of 200 Ω is given, the characteristic shown by the full lines in FIG. 4 can be obtained. In this case, no deterioration in base current characteristic can be observed.  
         [0199]    In the first to third embodiments, as shown in FIGS. 5H, 6K and  8 K, the silicide layer exists only at a part of the bottom of the collector trench because the side wall is formed on the sides of the collector trench. If the etching to form the contact hole is insufficiently conducted, then the following problems may occur.  
         [0200]    The formation of contact hole before forming the contact plug is generally conducted deeper for the collector trench buried with the interlayer insulating film. However, when the contact hole is insufficiently formed, a remainder of interlayer insulating film  327   a  shown in FIG. 9A occurs.  
         [0201]    Also, when the positioning in forming the contact hole is inaccurately conducted as shown in FIG. 9B, the contact area at the bottom of the contact plug  329  may be varied. Thereby, the collector resistance may be increased or dispersed. Meanwhile, numerals used in FIGS. 9A and 9B are identical with those in FIGS. 8A to  8 K.  
         [0202]    In this regard, a method for making a semiconductor device in the fourth embodiment will give a solution.  
         [0203]    The method for making a semiconductor device in the fourth preferred embodiment will be explained below.  
         [0204]    At first, as shown in FIG. 10A, device-separating oxide film  407  and first oxide film  433  are formed on a p-type silicon substrate  401  by using known LOCOS or STI etc.  
         [0205]    Then, as shown in FIG. 10B, a p-well region  402  to form n-MOS is formed by implanting, e.g., boron of 5×10 — cm −2  at 350keV. Also, a n-well region  406  to form p-MOS and a collector connection region  406   a  to form the collector of the bipolar transistor are formed by implanting, e.g., phosphorus of 1×10 13  to 5×10 13 cm −2  at high energy of 300 to 800keV and further phosphorus of 1×10 12  to 1×10 13 cm −2  at 50 to 150keV.  
         [0206]    Then, as shown in FIG. 10C, gate oxide film  408  of 5 to 10nm thick is formed on the p-type silicon substrate  401 . Then, a collector region  411  for bipolar transistor is formed by implanting selectively phosphorus of 1×10 13  to 1×10 14 cm −2  at high energy of 700keV to 1.5MeV with using a photoresist mask pattern. Further, a p-type base region  409  is formed by implanting, e.g., boron or BF2 of 1×10 13  to 5×10 14 cm −2  at 10 to 50keV with using the same photoresist mask pattern. Then, after removing the photoresist pattern, first polysilicon film  412  of 150 to 400nm is grown thereon.  
         [0207]    Meanwhile, in the fourth embodiment, the collector region  411  and the p-type base region  409  are formed after forming the gate oxide film  408 . However, they may be formed before forming the gate oxide film  408 .  
         [0208]    Then, as shown in FIG. 10D, gate electrodes  413  are formed by removing selectively the first polysilicon film  412  by known anisotropic etching with using a photoresist mask pattern, and then the photoresist pattern is removed.  
         [0209]    Then, as shown in FIG. 10E, a n-type LDD layer  420 , a p-type LDD layer  421  are formed. Then, second oxide film  437  is grown.  
         [0210]    Meanwhile, in the fourth embodiment, the collector region  411  and p-type base region  409  are formed at the step in FIG. 10C. However, they may be formed before growing the second oxide film  437 .  
         [0211]    Then, as shown in FIG. 10F, an emitter contact  410  and a collector contact  426  are formed by etching selectively the second oxide film  437  with using a photoresist mask pattern. Then, second polysilicon film  442  of 150 to 400nm is grown thereon.  
         [0212]    Meanwhile, the second polysilicon film  442  may be grown non-doping or doping with impurity of, e.g., phosphorus or arsenic, of 1×10 18  to 1×10 21 cm −2.    
         [0213]    Then, as shown in FIG. 10G, an emitter extraction electrode  414  is formed by removing selectively the second polysilicon film  442  by known anisotropic etching with using a photoresist mask pattern. Then, a collector trench  424  is formed by further etching with using the photoresist pattern and second oxide film  437  as a mask. Then, the photoresist pattern is removed. These etchings may be conducted sequentially on same conditions or separately in several steps.  
         [0214]    Then, as shown in FIG. 10H, side walls  419  composed of third oxide film  339  are formed on the side of the gate electrode  413  by known anisotropic etching. Meanwhile, as seen from FIG. 10H, the third oxide film  439  is removed from the base region while remaining a part thereof under the emitter extraction electrode  414 .  
         [0215]    Then, as shown in FIG. 10I, a n + -type source/drain region  422  for n-MOS is formed by ion-implanting impurity of phosphorus, arsenic or the like. Also, a n + -type diffusion layer  428  is formed at the bottom of the collector trench  424 . Then, a p + -type source/drain region  423  for p-MOS and a p + -type graft base  416  are formed by ion-implanting impurity of boron, BF 2  or the like.  
         [0216]    Meanwhile, these ion implantations maybe, as described earlier, conducted through thin oxide film of about 5 to 20nm so as to prevent a crystal defect at the end of the side wall  419  or conducted with remaining the third oxide film  439  of about 5 to 10nm when the third oxide film  439  is anisotropic-etched to form the side wall. In the fourth embodiment, the latter method is employed.  
         [0217]    Meanwhile, the impurity introduction into the emitter extraction electrode  414  when the second polysilicon film  442  to form the emitter extraction electrode  414  is grown non-doping may be conducted simultaneously with the ion implantation of impurity to form the n + -type source/drain region  422  for n-MOS. Alternatively, it may be conducted by another step of implanting impurity of phosphorus, arsenic or the like.  
         [0218]    Then, as shown in FIG. 10J, a silicide layer  425  is formed by siliciding the surface of the gate electrode  413 , emitter extraction electrode  414 , the bottom and sides of the collector trench  424 , n + -type source/drain region  422 , p + -type source/drain region  423  and p + -type graft base  416  by known method using metal of titanium, cobalt, nickel or the like.  
         [0219]    Then, as shown in FIG. 10K, interlayer insulating film  427  composed of, e.g., oxide film (TEOS-SiO 2  film) of 50nm and BPSG (boron-phosphorus-silicate-glass) of 800nm like the third prior art is formed. In addition, contacts are formed at predetermined positions of the interlayer insulating film  427 , and then a contact plug  429  are formed through barrier metal (not shown) . Then, metal wiring  430  to contact the contact plug  429  is formed on the interlayer insulating film  427 .  
         [0220]    Next, referring to FIG. 10K, the structure of the bipolar transistor in the semiconductor device in the fourth embodiment will be explained below.  
         [0221]    In the bipolar transistor, the collector region  411  is formed on the p-type silicon substrate  401  and the collector connection region  406   a  is formed only at the collector contact  426  in the collector region  411 . The collector contact  426  and the p-type base region  409  are separated from each other by the device-separating oxide film  407  formed on the surface of the collector region  411 . Also, in the region of the collector contact  426 , the bottom of the collector trench  424  is formed to abut on a portion with peak impurity concentration in the collector region  411 .  
         [0222]    Also, the impurity region to be formed in implanting the n + -type source/drain for CMOS is formed at the bottom of the collector trench  424 . Also, the silicide layer  425  is formed on the sides and bottom surface of the collector trench  424 . It is connected through barrier metal (not shown) to the contact plug  429 . The contact plug  429  is connected to the metal wiring  430  formed on the interlayer insulating film  427 .  
         [0223]    Meanwhile, the bottom of the collector trench  424  may be formed a little lower than the portion with peak impurity concentration in the collector region  411  so that a portion with peak impurity concentration in the n + -type diffusion layer  428  can abut on the portion with peak impurity concentration in the collector region  411 .  
         [0224]    As described above, in the fourth embodiment, the n + -type buried layer and epitaxial layer are not formed and the collector region is formed by the ion implantation at high energy. As a result, the problem that the transistor size is prevented from being decreased because of the unnecessary expansion in impurity region due to thermal hysteresis can be solved.  
         [0225]    Also, in the fourth embodiment, the collector resistance is not increased because the collector region  411  and the contact plug  429  connected to the metal wiring  430  are directly connected at the bottom of the collector trench  424 .  
         [0226]    Also, the collector trench  424  is formed by etching with using the photoresist pattern and device-separating oxide film  407  as a mask, subsequently after forming the gate electrode  413  and emitter extraction electrode  414 . Therefore, the number of fabrication steps is not increased. Namely, in the fourth embodiment, the essential problem that the number of fabrication steps of BiCMOS is too many can be solved by having some of the steps of fabricating CMOS and bipolar transistor in common.  
         [0227]    Furthermore, in the fourth embodiment, as seen from FIGS. 10J and 10K, the side wall  419  is not formed on the sides of the collector trench  424 . Therefore, the silicide layer can be formed on the entire bottom surface and side of the collector trench  424 , thereby reducing the collector resistance.  
         [0228]    Moreover, in the fourth embodiment, the problems illustrated in FIGS. 9A and 9B can be solved.  
         [0229]    Namely, an increase or dispersion in collector resistance can be suppressed because the silicide layer formed on the entire bottom surface and side of the collector trench  424  has a very low layer resistance of about 2 Ω /□, even when the contact plug  329  does not reach the bottom of the collector trench  324  due to the remainder  327   a  of interlayer insulating film to be left in forming the contact hole as shown in FIG. 9A or the contact plug  329  contacts the side of the collector trench due to the inaccurate positioning of the contact hole as shown in FIG. 9B.  
         [0230]    Next, alterations in the fourth embodiment will be explained in FIGS. 11A to  11 C. FIGS. 11A to  11 C shows enlarged parts, particularly around the collector trench  424 , corresponding to those in FIG. 10J in the fourth embodiment. Numerals used in FIGS. 11A to  11 C are identical with those in FIGS. 10A to  10 K.  
         [0231]    [0231]FIG. 11A shows the alteration that the collector connection region  406   a  is formed deeper than the collector region  411 .  
         [0232]    [0232]FIG. 11B shows the alteration that the collector connection region  406   a  is formed deeper than the collector region  411  and further the collector trench  424  is also formed deeper than the collector region  411 .  
         [0233]    [0233]FIG. 11C shows the alteration that the collector connection region  406   a  is formed shallower than the collector region  411  and further the collector trench  424  is formed deeper than the collector region  406   a.    
         [0234]    Meanwhile, by the alterations in FIGS. 11A to  11 C, it will be easily understood that the collector trench  424  can be so disposed that the bottom of the collector trench  424  can contact directly or indirectly through the n + -type diffusion layer  428  the impurity peak of the collector connection region  406   a  or collector region  411 .  
         [0235]    Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth.