Abstract:
Disclosed is a semiconductor structure and manufacturing process for making an integrated FET and photodetector optical receiver on a semiconductor substrate. A FET is formed by forming at least one p region in a p-well of the substrate and forming at least one n region in the p-well of the substrate. A p-i-n photodetector is formed in the substrate by forming at least one p region in an absorption region of the substrate when forming the at least one p region in the p well of the FET and forming at least one n region in the absorption region of the substrate when forming the at least one n region in the p-well of the FET.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming an integrated MOSFET and p-i-n optical receiver. 
     2. Description of the Related Art 
     Optoelectronic integrated circuits (OEIC), which may often consist if a p-i-n (PIN) photodector and field effect transistor (FET), are commonly used in high speed digital optical communication systems. One goal in designing an integrated photodetector and transistor, is to design a structure that optimizes the manufacturing process of the two elements, photodetector and transistor. Another goal is to optimize bandwidth in the photodetector and, at the same time, the responsiveness of the photodetector to light. In photodector design, a trade-off is made between capacitance and the speed or bandwidth of transmissions in the photodetector. Another goal is to operate at low supply voltages while maintaining a high bandwidth. Thus, there is a need in the art for a structure and manufacturing process for an improved monolithic lateral integrated transistor and photodetector that maintains a relatively high bandwidth, high responsivity of the photodetector to light, and that is capable of operating at low supply voltages. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     To overcome the limitations in the prior art described above, preferred embodiments disclose a semiconductor structure and manufacturing process for making an integrated field effect transistor (FET) and photodetector optical receiver on a semiconductor substrate. The FET is formed by forming at least one p region in a p-well of the substrate and forming at least one n region in the p-well of the substrate. A p-i-n photodetector is formed in the substrate by forming at least one p region in an absorption region of the substrate when forming the at least one p region in the p well of the FET and forming at least one n region in the absorption region of the substrate when forming the at least one n region in the p-well of the FET. 
     Preferred embodiments provide an improved technique for producing a monolithically integrated optical receiver in which the photodetector and preamplifier circuit share a common substrate. Monolithic integration is advantageous over hybrid integration, in which the photo-detector and amplifier circuits are fabricated separately and then interconnected as the cost of manufacturing is less, the yield is higher, and the performance is improved due to a reduction in packaging parasitics. Further, preferred embodiments allow the production of an integrated amplifier and optical receiver that can be used in high speed communication and, at the same time, is capable of operation at low supply voltages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIGS. 1A through 1I show cross-sectional views of successive process steps for making an integrated p-i-n photodetector and metal semiconductor field effect transistor (MESFET) in accordance with an embodiment of the invention; and 
     FIGS. 2A and 2B show top-down perspective views of channels formed in the p-i-n photodetector in accordance with preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments of the present invention. In the drawings, depicted elements are not necessarily drawn to scale and like or similar elements may be designated by the same reference numeral throughout the several views. Further, it is understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the present invention. 
     In FIG. 1A, a photoresist  2  is deposited as a continuous layer on an undoped gallium arsenide semi-insulating (GaAs—SI) substrate  4 . The photoresist is selectively irradiated using a photo lithographic system, such as a step and repeat optical projection system to remove an opening  6  in the photoresist  2  using a mask. A P-region is implanted into the substrate  4  to form the p-well  8 , in which the MESFET portion of the monolithic MESFET and PIN photodetector is formed. 
     In FIG. 1B, a field oxide (FOX)  10  is deposited in non-active regions of the substrate  4 , thereby exposing the active regions  8  and  12  for the FET and the p-i-n photodetector. 
     In FIG. 1C, a photoresist layer  16  is formed on the substrate to cover the inactive and active regions. The photoresist  16  is irradiated and portions of the irradiated photoresist  16  are removed using a mask and etchings to form openings  18  and  20  for p+ implants  22  and  24  in the FET and p-i-n regions of the substrate  4 . The active region  12  includes an absorption region  26  where the dopings to the p-i-n photodetector occur. Zinc (Zn) or Beryllium (Be) may be used for the p+ dopings at this step. 
     In FIG. 1D, a photoresist layer  30  is formed on the substrate  4 , which is then irradiated and etched using a mask to expose a section of the absorption region  26  of the p-i-n photo-conductor region. A low dose of n−- 32 , shown by the downward arrows, is implanted in the absorption region  26  to convert the absorption region  26  to a low dose n−-region. 
     In FIG. 1E, photoresist layer  40  is formed on the substrate  4 , which is then irradiated and irradiated portions etched away using a mask to expose openings in the active regions  12  and  14  to receive the n− implant, leaving the p+ regions  22  and  24  covered. The n− doping forms n-type implants  42  and  44  in the upper area of the p-well and the absorption region, respectively. This n− doping increases the threshold of the FET device. substitute the paragraph on page 4, lines 9-17 with the following paragraph: 
     In FIG. 1F, metal gates  50  are deposited in the FET region  14 . In certain embodiments, metal gates are not placed in the p-i-n region  12  in order to optimize the responsiveness of the p-i-n photodetector, because the use of metal gates can reduce the responsiveness of the p-i-n photodetector by reflecting light away from the photodetector sensor. In alternative embodiments, metal gates may be deposited above the implanted n− and p+ regions of the p-i-n photodetector to reduce resistance and increase the bandwidth in the p-i-n. The deposition of the metal gates  50  above the n region  42  produces two n regions or fingers  42   a, b  in the FET region  14 , as shown in FIG.  1 G. 
     In FIG. 1G, photoresist  60  is deposited, irradiated, and etched away using a mask to expose the n region in the p-i-n region  12  and the n regions  42   a, b  in the FET region  14 . A lightly doped drain (LDD)  62  is then implanted over the n− region  42   a, b  in the FET region  14  as shown as LDD regions  46  and  48 . This LDD doping improves the performance of the FET. The LDD doping has little effect on the n− region  44  in the p-i-n region  12 . 
     In FIG. 1I, photoresist  80  is applied to the p-i-n region  12  and portions are irradiated and etched away using a mask to expose the n− region  44  as well as the regions  42   a, b  in the FET region  14 . A heavy n+ source/drain region  82 , shown by the downward arrows, is implanted into the n− finger  44  of the p-i-n region to convert the n− regions  44  to n+ regions. The heavy n+ source/drain is implanted over the n− region  42   a, b  and LDD regions  46 ,  48  of the n regions in the FET region  14  into the region  84  and  86 , respectively. 
     FIG. 2 a  illustrates illustrate a top-down view of the n+  24  and p+  44  regions in the p-i-n photodetector region  12  in which interconnected n+  100  and p+  102  type regions are formed having vertical finger regions interconnected by a horizontal bar. After the process in FIG. 1I, FIG. 2 b  illustrates how the horizontal bar region of the region  100  and  102  formations may be metalized by depositing a metal  104  and  106  on the horizontal bars. In alternative embodiments, to maximize the responsivity of the p-i-n photodetector, the regions  100  and  102  would not be metalized because the metal in the p-i-n region  14  could deflect light and thereby degrade responsivity. However, adding the metalized regions in the regions  100  and  102  reduces resistance and may improve the bandwidth of the photodetector. 
     The end-result of the above described process is a monolithic lateral integrated MESFET and p-i-n photodetector grown on a gallium arsenide (GaAs) substrate. The preferred embodiment semiconductor process described herein forms a monolithic integrated optical receiver in which the photodetector and preamplifier circuit share a common substrate. In preferred embodiments, the p-i-n photodetector is formed by alternately implanting n+ and p+ doped regions into an undoped GaAs substrate. The process for producing the integrated receiver structure described herein is compatible with existing GaAs MESFET transistor fabrication processes and does not require additional processing procedures. The monolithic integrated transimpedence MESFET amplifier and p-i-n photodetector may be formed using other planar MESFET or HEMT transistor processes. 
     The preferred embodiments are particularly suited for fabricating n and p MOSFETs integrated with a p-i-n photodector. Although only a single pair of N region and P region devices has been shown for purposes of illustration, it is understood that in actual practice many devices are fabricated on a single semiconductor wafer as widely practiced in the art. Accordingly, the invention is well suited for use in integrated circuit chip, as well as an electronic system including a microprocessor, a memory and a system bus. 
     Moreover, preferred embodiments allow the production of an integrated amplifier and optical receiver that can be used in high speed communication and is capable of operation at low supply voltages. For instance, the integrated amplifier and optical receiver of the preferred embodiments is capable of operating at supply voltages lower than 5 V and at bit-rates greater than 2.5 gigabits per second. 
     In alternative embodiments, certain of the n dopings, such as the LDD doping in FIG.  1 G and n−-doping in FIG. 1E may not be performed in lieu of a single n+ heavy source/drain doping of FIG.  1 I. 
     Those skilled in the art will readily implement the steps necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only and can be varied to achieve the desired structure as well as modifications which are within the scope of the invention. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.