Bipolar semiconductor device having a conductive recombination layer

Abstract: A semiconductor device having a conductive recombination layer (22). The conductive recombination layer (22), comprised of doped polycrystalline material, doped polycrystalline material and tungsten silicide, or tungsten silicide, is disposed between two separate semiconductor substrates (10, 16) which are bonded together using a polished surface on the conductive recombination layer (22) as one of the bonding interfaces. The conductive recombination layer (22) recombines minority carriers and thereby increases the switching speed of the device.

Description: Background of the Invention This invention generally pertains to a bipolar semiconductor device having a conductive recombination layer. Generally, in semiconductor devices of this type, it is desirable to have a relatively high switching speed while maintaining a reduced "on" state voltage. Commonly, there is a high level of minority carriers in the voltage-blocking semiconductor region which must be removed before a semiconductor device of this type may be turned off. Removing the minority carriers takes a relatively long period of time and results in heating throughout the device. However, if no minority carriers are provided, the "on" state voltage will be extremely high due to a high resistance throughout the semiconductor region of the device. Therefore, a recombination layer which would reduce but not eliminate the minority carrier concentration in the voltage-blocking semiconductor region of a device would be highly desirable. In the past, methods of doping semiconductor devices with impurities such as gold and nickel resulted in faster minority carrier recombination during the turn off of the device thereby increasing the switching speed. However, since these impurities were present throughout the device, a large voltage drop was caused. Therefore, a recombination layer disposed between layers of varying doping concentration has been found to be highly desirable. It is known that in high voltage transistors, minority carriers move through the relatively lightly doped region of the collector quickly to reduce the voltage drop therein but do not readily combine in the more heavily substrate region of the semiconductor body. Therefore, switching speeds may be increased by increasing minority carrier recombination in or near a highly doped region of a collector. US-A-4259683 describes a semiconductor device for operation at high switching speeds. The document describes the provision of a lightly doped semiconductor layer in which it is desirable to increase the rate of carrier recombination. The document also describes a bipolar transistor having a collector behind which there is a region of peak amount of recombination. The present invention relates to an improved bipolar semiconductor device having a conductive recombination layer. Summary of the Invention According to the present invention, a semiconductor device is provided having a conductive recombination layer comprising: a collector, said collector comprising: a first doped semiconductor substrate including a first surface and a second surface; a second dope semiconductor substrate including a first surface and a second surface and a conductive re-combination layer disposed between said first and second substrates and being contiguous to said first and second substrates at their first surfaces, characterised in that the conductive recombination layer comprises tungsten silicide or a combination of tungsten silicide and doped polycrystalline material layers. The invention also provides methods of fabricating a bipolar semiconductor device as set out in claims 3 and 5. The conductive recombination layer is preferably disposed between lightly doped and highly doped regions of the collector. In a preferred process for fabricating a collector, a relatively lightly doped semiconductor substrate is provided and a relatively highly doped semiconductor substrate is provided. A conductive recombination layer is formed on a first surface of the highly doped semiconductor substrate. It is preferred that once the conductor recombination layer has been formed, it is polished, as is a first surface of the lightly doped semiconductor wafer, and these are bonded together to form the collector region of a bipolar semiconductor device. Where doped polycrystalline material is provided, this may be heavily doped polysilicon or polysilicon doped with a slow moving element such a titanium. The present invention may be incorporated into an integrated circuit. Accordingly a new and improved semiconductor device is provided with increased switching speed. The device also has a reduced voltage drop during switching. The high level of minority carriers in the collector is reduced in the vicinity of the relatively highly doped collector region. The conductive recombination layer may easily be disposed between regions of a semiconductor device or integrated circuit. Brief Description of the Drawings Figs. 1 and 2 are highly enlarged cross-sectional views of sections of a bipolar semiconductor device not forming part of the present invention having a conductive recombination layer during various stages of processing. Figs. 3 and 4 are highly enlarged cross-sectional views of sections of a bipolar semiconductor device according to the invention during various stages of processing, Fig. 5 to 6 are highly enlarged cross-sectional views of sections of an integrated circuit not forming part of the present invention having a conductive recombination layer during the various stages of processing. Description of the Preferred Embodiments Referring specifically to FIGS. 1-2, highly enlarged cross-sectional views of sections of semiconductor substrates used in providing a conductive recombination layer are shown during various stages of processing. Initially, a first semiconductor substrate, 10, is provided. First semiconductor substrate 10 includes a first surface, 12, and a second surface, 14. Also provided is a second semiconductor substrate, 16, which includes a first surface, 18, and a second surface, 20. In this example, first semiconductor substrate 10 and second semiconductor substrate 16 are silicon semiconductor substrates. However, it should be understood that semiconductor substrates made of other material may also be used. Also in this example, first semiconductor substrate 10 is relatively lightly doped and has an N conductivity type while second semiconductor substrate 16 is relatively highly doped having an N+ conductivity type. It will be understood by those skilled in the art that semiconductor substrates 10 and 16 may have varying dopant concentrations as well as different conductivity types. It is desirable that first surface 18 and second surface 20 of second semiconductor substrate 16 be parallel. Additionally, it is also desirable that first surface 12 and second surface 14 of first semiconductor substrate 10 be parallel. This is so that semiconductor substrates 10 and 16 may be lapped and polished during or after the fabrication process of the semiconductor device to meet size requirements and remain parallel while doing so. Once first surface 18 and second surface 20 of second semiconductor substrate 16 have been made parallel, a conductive recombination layer 22, is formed on first surface 18 of second semiconductor substrate 16. It is understood that conventional semiconductor process techniques are utilized to form recombination layer 22 on substrate 16. For example, the conductive recombination layer 22 may comprise a doped polycrystalline material such as polysilicon. The polysilicon material conductive recombination layer 22 is deposited and then a phosphorous dopant is diffused to make the polysilicon highly conductive. In this example, conductive recombination layer 22 has an N++ conductivity type but those skilled in the art will understand that other conductivity types may be used. While the phosphorous dopant is being diffused throughout conductive recombination layer 22, a certain amount will diffuse into second semiconductor substrate 16 in the area of its first surface 18 thereby creating an N++ conductivity type layer, 19, in substrate 16. Additionally, the polysilicon of conductive recombination layer 22 may be doped with a slow moving element which produces a highly active recombination center. An ideal dopant of this type is titanium although others may be used. It should be understood that conductive recombination layer 22 must be sufficiently conductive to avoid excessive voltage drop by the current passing through it. FIG. 3 shows conductive recombination layer 22 formed according to one embodiment of the invention. Following making first surface 18 and second surface 20 of second semiconductor substrate 16 parallel, a first doped polysilicon layer, 24, is formed on first surface 18 of second semiconductor substrate 16. Next, a tungsten silicide layer, 26, is formed on first doped polysilicon layer 24. This is followed by a second doped polysilicon layer, 28, being formed on tungsten silicide layer 26. Although layers 24 and 28 are comprised of polysilicon in this example, other polycrystalline materials well known by those in the art may also be used. In another embodiment of the present invention, conductive recombination layer 22 may be comprised solely of tungsten silicide. It should be understood by those skilled in the art that conductive recombination layer 22 does not have to be formed on first surface 18 of second semiconductor substrate 16 and may be formed by other methods as long as it is disposed between first surfaces 12 and 18 of semiconductor substrates 10 and 16. Once conductive recombination layer 22 has been formed, it is polished as is first surface 12 of first semiconductor substrate 10. This is so that a strong and uniform bond will be formed between them. After the polishing has occurred, first surface 12 of first semiconductor substrate 10 is bonded to conductive recombination layer 22. Typically, the bonding occurs at a high temperature in an oxidizing atmosphere. It should be understood that exact alignment of substrates 10 and 16 is not required in the present invention. Each of the materials used to make the various conductive recombination layers 22 which were described above, that is, the individual layers comprising conductive recombination layer 22 are firmly bonded together. Once first semiconductor substrate 10 has been bonded to second semiconductor substrate 16 with conductive recombination layer 22 disposed therebetween, the remainder of the semiconductor device may be formed using standard techniques well known in the art. First semiconductor substrate 10 may be thinned after being bonded to second semiconductor substrate 16 in a uniform manner by lapping and polishing second surface 14, using second surface 20 as a reference, since surfaces 12, 14, 18 and 20 are all parallel after bonding. FIG. 4 shows a base, 30, which has been formed in first semiconductor substrate 10. Additionally, an emitter, 32, has been formed in base 30. The remainder of first semiconductor substrate 10 as well as second semiconductor substrate 16 and conductive recombination layer 22 serve as the collector, 34, for the bipolar semiconductor device. FIGS. 5-6 show highly enlarged cross-sectional views of sections of an integrated circuit having a conductive recombination layer during various stages of processing. Initially, a first semiconductor substrate, 40, and a second semiconductor substrate, 42, are provided. In this example, first semiconductor substrate 40 has an N conductivity type while second semiconductor substrate 42 has an N+ conductivity type. Both semiconductor substrates are comprised of silicon. It should be understood, however, that other semiconductor materials and conductivity types may be employed. The first surface, 44, and second surface, 46, of first semiconductor substrate 40 are parallel to each other as are the first surface, 48, and second surface, 50, of second semiconductor substrate 42. Again, this allows size requirements to be met following lapping and/or polishing of the substrates while the surfaces remain parallel. Following making surfaces 48 and 50 of second semiconductor substrate 42 parallel to each other, first surface 48 is polished and a phosphorous dopant is diffused into it giving an N++ layer, 49. This gives second semiconductor substrate 42 a highly doped N++ conductivity type at its first surface 48. A plurality of trenches, 52, are etched into first semiconductor substrate 40 at its second surface 46. Standard etching methods are used to do this. Following the etching, first semiconductor substrate 40 is oxidized thereby forming an oxide layer, 54. Next, oxide layer 54 is removed in the power device area, 56, of first semiconductor substrate 40. This establishes a means for contact to power device area 56. A polysilicon layer 58, is then formed, here by deposition, on second surface 46 of first semiconductor substrate 40. The trenches 52 are overfilled by polysilicon layer 58 which is then polished to leave a coating of polysilicon layer 58 on the entire second surface 46 of first semiconductor substrate 40. Before polishing polysilicon layer 58, a phosphorous dopant is diffused into polysilicon layer 58, converting it to N++, and forming and N++ layer, 59, on area 56. Now, the polished polysilicon layer 58 on second surface 46 of first semiconductor substrate 40 is bonded to polished first surface 48 of second semiconductor substrate 42. This bonding also occurs at a high temperature and in an oxidizing atmosphere. This bonding forms a conductive recombination layer, 60, comprising layers 49, 58, and 59. These layers may be utilized as part of the device to be fabricated in an area, 61. The device may be similar to that shown in FIG. 4. Once the bonding has occurred, first surface 44 of first semiconductor substrate is polished or ground so that the plurality of trenches are exposed. The thicknesses of all regions such as 61, 62, 63, and 64 will be the same because surfaces 44, 46, 48 and 50 were all parallel prior to grinding or polishing. This completes the isolation for regions of the integrated circuit which require isolation. The remainder of the integrated circuit may be formed by using standard techniques which are well known in the art. It should be understood that this is extremely useful for an integrated circuit having a bipolar power device but may also be an effective way of processing to be used in other types of integrated circuits which incorporate MOS devices.

Claims: A semiconductor device having a conductive recombination layer (22) comprising: a collector, said collector comprising: a first doped semiconductor substrate (10) including a first surface (12) and a second surface (14); a second doped semiconductor substrate (16) including a first surface (18) and a second surface (20); and a conductive recombination layer (22) disposed between said first (10) and second (16) substrates and further being contiguous to said first (10) and second (16) substrates at their first surface (12, 18) characterised in that said conductive recombination layer (22) comprises tungsten silicide or a combination of tungsten silicide and doped polycrystalline material layers. The device of claim 1 wherein the recombination layer (22) comprises: a first doped polycrystalline layer (24); a second doped polycrystalline layer (28); and a tungsten silicide layer (26) disposed between said first (24) and second (28) doped polycrystalline layers. A method of fabricating a bipolar semiconductor device having a conductive recombination layer (22) comprising the steps of: providing a first doped semiconductor substrate (10) having a first surface (12) and a second surface (14); providing a second doped semiconductor substrate (16) having a first surface (18) and a second surface (20); forming a conductive recombination layer (22) on said first surface (12, 18) of one of said first (10) and second (16) semiconductor substrates; and bonding said first surface (12, 18) of the remaining one of said first (10) and second (16) semiconductor substrates to said conductive recombination layer (22) characterised in that the step of forming the conductive recombination layer comprises forming a layer (22) of tungsten silicide or a combination of tungsten silicide and doped polycrystalline material layers. The method of claim 3 wherein the step of forming a conductive recombination layer (22) comprises the steps of: forming a first doped polycrystalline layer (24); forming a tungsten silicide layer (26) on said first doped polycrystalline layer (24); and forming a second doped polycrystalline layer (28) on said tungsten silicide layer (26). A method of fabricating a bipolar semiconductor device having a conductive recombination layer (22) comprising the steps of: providing a first doped semiconductor substrate (10) having a polished first surface (12) and a second surface (14); providing a second doped semiconductor substrate (16) having a first surface (18) and a second surface (20) which are parallel to each other, said second semiconductor substrate (16) further having a greater dopant concentration than said first substrate (10); the invention being characterised by: forming a conductive recombination layer (22) on said first surface (18) of said second semiconductor substrate (16) wherein said conductive recombination layer (22) comprises tungsten silicide or a combination of tungsten silicide and doped polycrystalline material layers. polishing said conductive recombination layer (22); and bonding said polished conductive recombination layer (22) to said polished first surface (12) of said first semiconductor substrate (10). The method of claim 9 wherein the recombination layer (22) is doped by diffusing a phosphorous dopant, said dopant being diffused to also give the second semiconductor substrate (16) a doping concentration which is greater at its first surface (18) than at its second surface (20).

Inventors: CLARK LOWELL EUGENE, LESK ISRAEL ARNOLD, CLARK, LOWELL EUGENE, LESK, ISRAEL ARNOLD
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1988
Main classification: H01L29
IPC codes: H01L21, H01L29, H01L27