Abstract:
A photodetector is integrated on a single semiconductor chip with bipolar transistors including a high speed poly-emitter vertical NPN transistor. The photodetector includes a silicon nitride layer serving as an anti-reflective film. The silicon nitride layer and oxide layers on opposite sides thereof insulate edges of a polysilicon emitter from the underlying transistor regions, minimizing the parasitic capacitance between the NPN transistor&#39;s emitter and achieving a high frequency response. The method of manufacture is compatible with existing BiCMOS process technology, the silicon nitride layer of the anti-reflective film being formed over the photodetector as well as regions of the chip that include the vertical NPN transistor and other circuit elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of U.S. patent application Ser. No. 09/838,909 filed Apr. 20, 2001, now U.S. Pat. No. 6,559,488, which is a Continuation-in-Part of U.S. application Ser. No. 09/677,268 filed Oct. 2, 2000, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to semiconductor photodetectors, and more particularly to an integrated circuit device that includes a photodetector and a process for its fabrication. 
     Semiconductor photodetectors are well known in the art. U.S. Pat. Nos. 4,670,765 and 5,177,581 disclose examples. Often such photodetectors are integrated with other circuit elements on the same semiconductor chip. U.S. Pat. Nos. 5,049,733 and 5,105,090 disclose examples. Another example that is compatible with a more advanced fabrication process is disclosed in U.S. Pat. No. 5,994,162. These five patents are incorporated by reference herein as background technology. 
     Semiconductor manufacturers have developed complex process technologies that permit fabrication of high circuit densities on a single silicon chip. Many such technologies have the flexibility of forming either bipolar transistors or field-effect transistors (FETs), or more typically, both types of transistors on the same chip. When both complementary forms of metal-oxide-semiconductor FETs (both N-channel and P-channel MOSFETs) are formed with bipolar transistors on the same chip, the generic process technology is referred to as BiCMOS. Advanced BiCMOS processes provide IC devices that operate at high frequencies suitable for high performance electronic products. The incorporation of a photodetector onto a single semiconductor chip may involve modification of an existing process technology that is compatible with the end-use application. It would be desirable, therefore, to facilitate such a process modification in a way that does not significantly change the structures and functions of basic circuit elements (transistors and capacitors), while minimizing any increase in the number and complexity of process operations. 
     SUMMARY OF THE INVENTION 
     In accordance with a principal object of the invention, a photodetector is integrated with high speed bipolar transistors and other semiconductor elements on a single chip, using advanced BiCMOS process technology. The photodetector comprises a thin, light-transmissive layer disposed above a diode having a PN junction lying generally parallel to the light-transmissive layer. The diode is physically isolated from other circuit elements on the same chip, and is electrically interconnected with the chip circuitry using conductive interconnects. The material that forms the light-transmissive layer extends laterally over regions of the chip that include these other circuit elements, which primarily include different types of bipolar transistors. The fabrication process accommodates inclusion of the photodetector structure with a minimum of photolithographic operations by incorporating the laterally extending portions of the light-transmissive layer into the structures of various transistors on the chip without degrading their characteristics. 
     The novel features believed characteristic of the invention are set forth in the appended claims. The nature of the invention, however, as well as its essential features and advantages, may be understood more fully upon consideration of an illustrative embodiment, when read in conjunction with the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic vertical cross-section (not necessarily through a single plane) of a portion of a device embodying the present invention, showing part of a semiconductor chip broken away at its left and right edges from the complete chip; 
     FIG. 2A is an enlarged left-hand portion of FIG. 1 that includes a poly-emitter vertical NPN transistor; 
     FIG. 2B is an enlarged center portion of FIG. 1 that includes a vertical PNP transistor; 
     FIG. 2C is an enlarged right-hand portion of FIG. 1 that includes a photodetector; and 
     FIG. 3 is a greatly enlarged portion of FIG. 2C showing additional details of a light-transmissive layer that defines part of the disclosed photodetector. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, FIG. 2A, FIG.  2 B and FIG. 2C, a portion of a semiconductor chip is illustrated and designated generally by reference numeral  10 . For clarity, some of the numerals used in FIGS. 2A,  2 B and  2 C are not included in FIG.  1 . FIG. 1 shows two of many possible transistors that can be fabricated on the chip  10  together with the photodetector integrated thereon. The portion labeled “poly-emitter vertical NPN transistor” is shown in the enlarged view of FIG. 2A, the portion labeled “vertical PNP transistor” is shown in the enlarged view of FIG. 2B, and the portion labeled “photodetector” is shown in the enlarged view of FIG.  2 C. 
     The chip  10  is fabricated on a substrate  12 , which preferably is lightly doped P-type silicon having a resistivity of 10 to 20 ohm-cm. Various buried layers are formed in the substrate  12  using conventional processing techniques, including N −  buried layer  14 , P +  buried layers  16 , and N +  buried layers  18 . 
     A lightly doped N-type epitaxial layer  20  is formed on the substrate  12  using a conventional epitaxial deposition process. Various conventional ion implantation operations are then performed to selectively dope regions within the epitaxial layer  20 . These doped regions include N− field implants  22  (some of which are labeled in FIGS. 2A,  2 B and  2 C), P −  wells  24   a ,  24   b , and  24   c , and P field implants  26 . 
     Oxide isolation regions are then formed, preferably using a patterned nitride layer (not shown) to selectively grow thermal oxide to a thickness of about 6000 Å. This produces a patterned field oxide layer  28 . Further ion implantation operations are then performed to produce N +  sinkers  30 , N +  contact regions  32 , P +  contact regions  34 , P −  base implant  36  (FIG.  2 A), and N −  base implant  38  (FIG.  2 B). Preferably, a clean-up sequence follows that includes formation of a new, extremely thin, thermal oxide layer (not shown) in the active areas, preferably to a thickness of about 65 Å. 
     Now referring to FIG. 3, an important feature of the invention will be described. First, an oxide layer  40   a  is deposited, preferably to a thickness of about 350 Å. This deposition is preferably performed at about 670° C. using TEOS (tetra-ethyl-ortho-silicate) as the source material in accordance with well-known process techniques. Next, a silicon nitride (Si 3 N 4 ) layer  40   b  is deposited in a conventional manner to a preferred thickness of about 500 Å. It will be appreciated that this layer will serve as an anti-reflective film in the completed device. Next, an additional TEOS deposition is performed to form oxide layer  40   c  to a preferred thickness of about 650 Å. 
     Layers  40   a ,  40   b  and  40   c  are not shown separately in FIGS. 1,  2 A,  2 B and  2 C because they are too thin to illustrate without great distortion. Where all three layers are present, they are referred to herein as a composite insulating layer and are designated collectively by reference numeral  40 . In FIG. 3, it is shown that oxide layer  40   c  terminates just to the right of the edge of the field oxide layer  28 , so that only layers  40   a  and  40   b  extend out over P −  well  24   c . Layers  40   a  and  40   b  are collectively referred to herein as light-transmissive layer  41 , which is an important structural feature of the photodetector element, the operation of which is described below. 
     Referring again to FIG. 2A, after composite insulating layer  40  has been formed, an opening is dry cut therethrough over base region  36 . This is followed by a phosphorus implantation preferably at a dose of 2.0×10 12  atoms/cm 2  at an energy of 240 KeV. This forms selectively implanted collector (SIC) region  42 . The SIC region contributes to the speed of the poly-emitter vertical NPN transistor, which has a frequency response preferably greater than 9 gigahertz. 
     Then, after a clean-up operation, a deposition of polycrystalline silicon (more simply referred to as polysilicon) is performed. The polysilicon layer is implanted with arsenic. This ion implantation procedure not only dopes the polysilicon but also dopes a portion of the underlying silicon producing emitter region  44 . The polysilicon layer is then selectively etched to leave poly-emitter  46  in place over the emitter region  44 . It will be appreciated that the resulting structural features also contribute to high speed transistor response. 
     A glass deposition follows to form BPSG layer  48  using well-known process steps. Contact openings are then made by selective etching followed by metal deposition and patterning to form base, emitter, collector, and source contacts (labeled B, E, C and S in FIGS.  2 A and  2 B), and a photodetector contact (labeled P in FIG.  2 C). 
     Referring again to FIG. 1, an interlevel dielectric (ILD) layer  50  is formed using conventional oxide deposition and spin-on-glass (SOG) planarization techniques. This produces a relatively flat surface atop ILD layer  50 . A second metal deposition and patterning sequence is performed to form metal screen plate  52 . This is followed by a conventional oxide passivation deposition to produce PSG layer  54 . Then, an optional polyimide layer  56  is formed atop the device, which can serve to reduce the stress on the chip  10  during the subsequent packaging operation. 
     Referring again to FIG. 2C, an opening or window  58  is formed down to light-transmissive layer  41  by a sequence of selective etching steps, removing portions of layers  56 ,  54 ,  50 ,  48 . Also, as shown specifically in FIG. 3, oxide layer  40   c  is removed to expose nitride layer  40   b , which is the top layer of light-transmissive layer  41 . Preferably, the window  58  is rectangular in top view, having length to width ratio of 1.5, which improves the optical sensitivity. 
     The chip  10  is then packaged in a protective housing by bonding the substrate  12  to an electrode (not shown) and encapsulating the chip  10  in an IC package (not shown), which will include a transparent resin portion (not shown) over the window  58 . The chip  10  may include many transistors and other IC elements (capacitors and resistors) of which only two transistors are shown, together with the integrated photodetector. In a modified form of the invention in which the chip  10  includes MOS-gated elements, the composite insulating layer  40  can be used as an interlevel dielectric between first and second level polysilicon layers to form a switch-mode capacitor. 
     Those skilled in the art will understand without further elaboration how the illustrated structures function. The poly-emitter vertical NPN transistor (FIG. 2A) and the vertical PNP transistor (FIG. 2B) each operate in a well-known manner. For the most basic IC device with an integrated photodetector according to the invention, only bipolar transistors may need to be fabricated with only slight modification to an existing BiCMOS process technology. 
     The photodetector (FIG. 2C) also operates in a well-known manner. P-type region  24   c  and the underlying portion of the N-type epitaxial layer  20  form a PN junction or diode  60  that produces current when energized by photons passing through the light-transmissive layer. Light generates electron-hole pairs inside the space-charge region of the PN junction  60 . As a result, positive current flows from photodetector contact P through regions  32 ,  30 ,  18  and  20  to the PN junction  60 . In effect, the metal contact P serves as the current drain for the diode  60 . 
     In accordance with an important feature of the invention, the composite insulating layer  40  forms part of the transistor structures as well as contributing its lower two layers  40   a  and  40   b  (FIG. 3) to the photodetector. The overall thickness and dielectric properties of the composite insulating layer  40  are chosen to provide a suitable parasitic capacitance between the edges or shoulder portions of the poly-emitter  46  and the base region  36  of the NPN transistor (FIG.  2 A). The thicknesses of layers  40   a  and  40   b  of light-transmissive layer  41  (FIGS. 2C and 3) are also selected so that light-transmissive layer  41  is anti-reflective to light wavelengths in the 650 to 750 nanometer range. The use of silicon nitride as an anti-reflective film of a photodetector is known in the art as disclosed, for example, in U.S. Pat. No. 5,177,581. The present invention achieves a substantial improvement in performance over such prior art devices. 
     Although a preferred embodiment of the invention has been described in detail, it is to be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.