Abstract:
This invention is forming the DMOS channel after CMOS active layer before gate poly layer to make the modular DMOS process step easily adding into the sub-micron CMOS or BiCMOS process. And DMOS source is formed by implant which is separated by a spacer self-aligned to the window for DMOS body. By this method, the performance of CMOS and bipolar devices formed original CMOS or BiCMOS process keeps no change. The product design kit, such as standard cell library of CMOS and BiCMOS, can be used continuously with no change.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a fabrication method for semiconductor, particularly to a method for integrating DMOS into a sub-micron CMOS process or a sub-micron BiCOMS process. 
   BACKGROUND OF THE INVENTION 
   DMOS is the abbreviation of “Double Diffused Metal Oxide Semiconductor”, which can achieve a very high working frequency and a very high operational speed, wherein two dopants of opposite conductivity types are diffused through an identical window to form self-aligned sub-micron channels. 
   According to DMOS structures, DMOS can be classified into LDMOS (lateral DMOS) and VMOS (vertical DMOS). LDMOS has three electrodes all extending from the upper surface thereof and is suitable to integrate with other elements. In a LDMOS, the source and the body are formed via a self-aligned diffusion; however, the gate layer and the drain are respectively formed via separated diffusion processes so that the input capacitor and the feedback capacitor can be reduced, and the short-channel effect can also be relieved. In a VDMOS, an N −  epitaxial layer is grown from an N +  silicon substrate; after flowing through the channels, electrons flow vertically to exit from the substrate; therefore, the drain electrode extends from the bottom of the chip, and there are only the source electrode and the gate electrode on the top surface of the chip; such a structure can promote the integration level but will limit the usage. In comparison with common MOS transistors, the structure of LDMOS has two features: firstly, P-type and N-type dopants are sequentially diffused through an identical window of an oxide layer to form a very short channel; and secondly, a lightly doped N −  drift zone is formed between the channel zone and the drain zone with the doping concentration of the N −  drift zone less than that of the channel zone. The N −  drift zone sustains most of the applied leakage voltage and increases the punchthrough voltage; therefore, LDMOS can combine the advantages of a high punchthrough voltage and a short channel. 
   DMOS is a double diffused MOS and its channel length is defined by the two dopants of opposite conductivity types diffused from same window which is formed by single mask to get the channel length very well controlled. In general, the gate poly is used as the window. Since the dopant diffusion needs high temperature drive while sub-micron CMOS (Complementary Metal Oxide Semiconductor) can not afford this thermal cycle, it is difficulty to integrated DMOS in sub-micron CMOS process forming a CDMOS process or sub-micron BiCMOS process forming a Bi-CDMOS process. 
   U.S. Pat. Nos. 5,491,105 and 6,022,778 have patented their method to solve above problem. U.S. Pat. No. 5,491,105 forms the DMOS body and source by implant two dopants with different diffusion rate through same mask window and then high temperature drive them to define the DMOS channel length before the CMOS active layer. U.S. Pat. No. 6,022,778 forms the DMOS body after gate poly layer by large angle implant and low temperature anneal. To mask the high energy large angle implant the polyside gate material must be used. 
   U.S. Pat. No. 5,491,105 is really a simple and cost effective method, but it is hard to form PDMOS due to diffusion rates of boron and arsenic are difference while phosphorous and boron are almost same. U.S. Pat. No. 6,022,778 is also a very good method, but its cost is relatively high due to the polyside gate and large angle implant equipment. 
   SUMMARY OF THE INVENTION 
   Consequently, for solving the abovementioned problems, the present invention proposes a modular method to integrate DMOS (LDMOS and VDMOS) into sub-micron CMOS or BiCMOS process. By this method, the performance of CMOS and bipolar devices formed original CMOS or BiCMOS process keeps no change. So, the product design kit, such as standard cell library of CMOS and BiCMOS, can be used continuously. 
   The fabrication method of the present invention essentially comprises the following steps: burying buried layers needed by DMOS into a semiconductor substrate; forming a TUB structure needed by HV DMOS in the succeeding process, and forming a well structure needed by LV DMOS and CMOS in the succeeding process; utilizing a silicon nitride layer and a field oxidation to define required active regions, and utilizing a thermal oxidation process to form a required field oxide layer, wherein the process from forming the well structure to forming the field oxide layer may adopt the process originally used by CMOS; sequentially forming a first polysilicon layer and a silicon nitride layer; utilizing a lithographic process to form a pattern required by DMOS bodies; performing a body ion implant process and a drive-in process to form the required DMOS bodies; utilizing PECVD to form a SiO 2  film; utilizing a dry-etching process to selectively remove the SiO 2  film on the silicon nitride layer and to cut given-size openings at the central regions of the SiO 2  films above the DMOS bodies; utilizing those openings to perform a DMOS source implant process to form required DMOS sources; and utilizing a capacitor mask and a lithographic process to selectively etch the silicon nitride layer and the first polysilicon layer so that the silicon nitride layers will be the dielectric layers of capacitors and the first polysilicon layers will be the lower electrodes of the capacitors. 
   Succeeding to the abovementioned process, the standard CMOS process follows. The following processes are sequentially performed, including: forming a gate dielectric layer, modulating CMOS Vt, forming gate electrodes on of a second polysilicon layer, forming electrode structures of capacitors, and forming CMOS drain electrodes and source electrodes via an LDD (Lightly Doped Drain) process and the related structure. Then, a multilayer-wiring process is performed, and next, a protective layer is formed on the chip, and next, the pattern of contact windows are defined on the protective layer, and the protective layer is selectively etched to form the contact window, and lastly, a metallic layer is deposited and patterned on the protective layer. Thus, a CDMOS structure, which integrates DMOS into a sub-micron CMOS process, is completed. 
   In the present invention, the TUB structure and the well structure are simultaneously either N-type or P-type, and the DMOS body is also either N-type or P-type, but the conductivity type of the DMOS body is different from that of the TUB and well structures. The dose of the body ion implant depends on the punchthrough voltage and the threshold voltage required by DMOS. 
   This invention provides a solution by forming the DMOS channel after CMOS active layer while before gate poly layer to make the modular DMOS process step easily adding into the sub-micron CMOS or BiCMOS process. The advantages of this method are: 
   1. DMOS body is formed by high energy implant only for low-voltage DMOS and high energy/low energy implant for high voltage DMOS to control the punchthrough voltage and threshold voltage separately. 
   2. DMOS source is formed by implant which is separated by a spacer self-aligned to the window for DMOS body. 
   3. A low temperature anneal is used to active the body implant and source implant. So, the DMOS channel is defined by the spacer width and following temperature cycle. 
   4. Since the DMOS body and source is formed before gate oxidation and CMOS Vt implant and gate poly, there is no influence to the sub-micron CMOS device performance. So, the DMOS and CMOS devices can be optimized separately. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  to  FIG. 20  are sectional views schematically showing the fabrication process of NDMOS. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The technical contents of the present invention will be described below in detail via the exemplification of a NDMOS process and the attached drawings. 
   Refer to from  FIG. 1  to  FIG. 20  section views schematically showing the fabrication process of NDMOS. As shown in  FIG. 1 , a semiconductor substrate  10  (such as a P-substrate) is firstly provided, and an initial oxide layer  11  is formed on the surface of the substrate  10 . Next, as shown in  FIG. 2 , via a lithographic process, the oxide layer  11  is selectively etched to form a shield mask of a required pattern, wherein some areas of the semiconductor substrate  10  are exposed, and N-type buried layers will be formed on the exposed areas of the substrate  10  in the following process. 
   Next, as shown in  FIG. 3 , N-type ions, such as Sb ions or As ions, are implanted into the exposed areas of the substrate  10 , and then, as shown in  FIG. 4 , a drive-in process is performed to form N-type buried layers  12 . 
   Next, as shown in  FIG. 5 , the oxide layer  11 , which has been polluted by the N-type ions, is removed; then, a P-type epitaxial layer  13  is formed on the surface; then, an oxide (SiO 2 ) layer  14  is formed on the surface of the P-type epitaxial layer  13 ; and then, a silicon nitride (Si 3 N 4 ) layer  15  is deposited on the surface of the oxide (SiO 2 ) layer  14 . Next, as shown in  FIG. 6 , via a lithographic process, the silicon nitride (Si 3 N 4 ) layer  15  is selectively etched to form a shield mask of a NTUB pattern, which is required in the succeeding process and exposes the oxide (SiO 2 ) layer  14  within the areas above NTUB regions, and then, ions are implanted into the exposed areas via a NTUB implant process. Next, as shown in  FIG. 7 , a NTUB drive-in process is performed to form required NTUB  16 . 
   Refer to  FIG. 8  to  FIG. 11  further. As shown in  FIG. 8 , a photoresist layer  41  is formed on the surfaces of the oxide (SiO 2 ) layer  14  and the silicon nitride (Si 3 N 4 ) layer  15 ; then, via a lithographic process, the photoresist layer  41  is processed to form a pattern of Nwell, which is required in the succeeding process and exposes Nwell-defining areas of the oxide (SiO 2 ) layer  14 ; and then, an Nwell implant process is performed. 
   Next, as shown in  FIG. 9 , the photoresist layer  41  is removed; then, a thermal oxidation process is performed to grow local oxide layers  141  from the exposed oxide layers  14  of the active regions; after the thermal oxidation process, the silicon nitride (Si 3 N 4 ) layer  15  is removed, and thus, the regions between those local oxide layers  141  can be used in the succeeding self-alignment PWell process. Next, as shown in  FIG. 10 , ions are implanted into the regions between those local oxide layers  141  via a Pwell implant process. Next, as shown in  FIG. 11 , a well drive-in process is performed to form required NTUB  16 , NWells  17  and PWells  18 . 
   Refer to  FIG. 12  to  FIG. 14  further. As shown in  FIG. 12 , the oxide (SiO 2 ) layer  14  and the local oxide layers  141  are stripped away; then, an oxide layer  19  is formed, and a silicon nitride (Si 3 N 4 ) layer  20  is formed on the surface of the oxide layer  19 ; and then, via a photographic process, the silicon nitride (Si 3 N 4 ) layer  20  and the oxide layer  19  are selectively etched to form the pattern of active regions, which is required in the succeeding process. Next, as shown in  FIG. 13 , a photoresist layer  42  is formed on the abovementioned pattern of active regions; then, via a lithographic process, some areas of the photoresist layer  42  above the PWells  18  are removed to partially expose the PWells  18  and form a PFLD (field) pattern; and then, a PFLD implant process is performed. Next, as shown in  FIG. 14 , the photoresist layer  42  is removed; then, a thermal oxidation process is performed to form required field oxide layers  22  and PFLD  21 ; after the thermal oxidation process, the silicon nitride (Si 3 N 4 ) layer  20  and the oxide layer  19  are removed. 
   Refer to  FIG. 15  and  FIG. 16  further. As shown in  FIG. 15 , a sacrificed oxide layer  221 , a first polysilicon layer  23  and a silicon nitride layer  24  are sequentially formed. Next, as shown in  FIG. 16 , a photoresist layer  43  is formed on the surface of the silicon nitride layer  24 ; then, a lithographic process is used to selectively remove the photoresist layer  43 , the first polysilicon layer  23  and the silicon nitride layer  24  in order to partially expose the NWell  17  and the NWells  17  and form a required PBODY pattern; and then, a PBODY B+ implant process and a drive-in process are performed to form required PBODY  25 , wherein DMOS body implant can use high energy implant only for low-voltage DMOS and high energy/low energy implant for high voltage DMOS to control the punchthrough voltage and threshold voltage separately. 
   Refer to  FIG. 17  and  FIG. 18  further. As shown in  FIG. 17 , a PECVD process is used to form an oxide (SiO 2 ) film  44 ; then, a dry-etching process is performed to selectively remove the oxide (SiO 2 ) films  44  above the silicon nitride layers  24  and simultaneously form given-size openings on the central regions of the oxide (SiO 2 ) films  44  above the PBODY  25 ; then, ions are implanted into those openings via a DMOS source implant process to form required DMOS sources  26 . The dose and the number of said body ion implants depend on the desired punch-through voltage and threshold voltage. Further, the dose of the DMOS source implant may be more than ten times the dose of the body ion implant. Next, as shown in  FIG. 18 , the photoresist layer  43  is removed, a wet-etching process is performed to remove all the oxide (SiO 2 ) films  44 ; and then, a moderate annealing and a moderate oxidation are performed. 
   Next, as shown in  FIG. 19 , via a capacitor mask and a lithographic process, the silicon nitride layer  24  and the first polysilicon layer  23  are selectively etched so that the silicon nitride layers can function as  24  dielectric layers of capacitors, and the first polysilicon layers  23  can function lower electrodes of the capacitors. 
   Next, as shown in  FIG. 20 , succeeding to the abovementioned processes, the standard CMOS process follows, and the following processes are sequentially performed, including: forming a second polysilicon layer  27  as upper electrodes of the capacitors and gate electrodes  31 , and forming CMOS drain electrodes  32  and source electrodes  33  via an LDD (Lightly Doped Drain) process and the related structure. Then, a protective layer  34  is formed on the entire chip; next, the pattern of contact windows is defined on the protective layer  34 , and the protective layer  34  is selectively etched to form the contact windows, and lastly, a metallic layer  35  is deposited and patterned on the protective layer  34 . Thus, a structure, which includes: a high-voltage LDMOS, a middle-voltage LDMOS and a low-voltage LDMOS, is completed. 
   This invention provides a solution by forming the DMOS channel after CMOS active layer while before gate poly layer to make the modular DMOS process step easily adding into the sub-micron CMOS or BiCMOS process. The advantages of this method are: 
   1. DMOS body is formed by high energy implant only for low-voltage DMOS and high energy/low energy implant for high voltage DMOS to control the punchthrough voltage and threshold voltage separately. 
   2. DMOS source is formed by implant which is separated by a spacer self-aligned to the window for DMOS body. 
   3. A low temperature anneal is used to active the body implant and source implant. So, the DMOS channel is defined by the spacer width and following temperature cycle. 
   4. Since the DMOS body and source is formed before gate oxidation and CMOS Vt implant and gate poly, there is no influence to the sub-micron CMOS device performance. Thus, the DMOS and CMOS devices can be optimized separately. 
   5. It is easy to form NDMOS or PDMOS. 
   However, the above description is only a better practice example for the current invention, which is not used to limit the practice scope of the invention. All equivalent changes and modifications based on the claimed items of this invention are in the scope of the present invention.