Abstract:
An improved laterally diffused MOS (LDMOS) transistor architecture is provided by using a nitride cap on a gate structure and forming a spacer around the gate structure and then self-aligning a source contact and drain contact with a gate by using the same mask for source and drain dopant implantation and for silicide formation with all source and drain areas being silicided. The reduced source/drain on resistance (Rdson), shorter distance from channel to source contact, and better gate oxide integrity improves operating linearity, increases Ft and GM and reduces the drift in Idq and Rdson.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to RF power transistors, and more particularly the invention relates to a laterally diffused MOS transistor (LDMOS) and the method of manufacturing same. 
     The LDMOS transistor is used in RF applications as a power amplifier. The high frequencies require small dimensions such as gate lengths of less than a micron, thin gate oxide of less than 500 Å, a short channel for cut off frequency (F t ) greater than 10 GH 3 , reduced source-drain resistance (Rdson), and a high reverse breakdown voltage through use of a lightly doped drain extension. 
     Conventional manufacturing has limits in realizing these features. Further, the use of sputtered tungsten silicide for a gate contact results in severe shrinkage, causing lifting and reduced gate oxide integrity, and limits gate-drain hot carrier injection improvement. Further, a tungsten silicide and polysilicon gate structure is difficult to etch. Non-planarized field oxide and thick interconnect lines often result. 
     The present invention is directed to improving the manufacturing process for a LDMOS transistor and providing an improved LDMOS transistor. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, a process flow includes the use of nitride masking of the gate structure for a sidewall spacer formation, and the double use of a source and drain mask for dopant implant and for silicide contact formation. 
     In accordance with a feature of the invention, either titanium or cobalt can be used for the silicide contacts, thereby eliminating the shrinkage associated with tungsten silicide gate. 
     The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-13 are section views illustrating steps in fabricating an LDMOS transistor in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-13 are section views illustrating the fabrication of an LDMOS transistor in accordance with one embodiment of the invention. As shown in FIG. 1, the structure is formed in a silicon substrate  10  which typically comprises an epitaxial layer as the top surface. Using conventional processing (LPCVD) nitride and thermal pad oxide, a first mask and etch are employed to form a P+ doped sinker region  12  (which becomes a source conduction region), low pressure chemical vapor deposited (LPCVD) nitride and a thermal pad oxide are formed, and then a second mask and etch are employed to expose the silicon surface and grow field oxide  14 . Typically a N-type field implant is made in a recessed region and then field oxide  14  is grown. Thereafter the field oxide can be planarized using, for example, chemical mechanical planarization (CMP) which is then followed by a P+ implant  16  which will contact the channel region. 
     Next as shown in FIG. 2 the surface of the active area is stripped of the nitride/oxide layers and a thermal oxidation forms silicon oxide layer  18  which will become the gate oxide for the transistor. In FIG. 3, a polysilicon layer  20  is deposited over oxide layer  18 , and then the polysilicon layer is doped by implanting an N-dopant to form an N+ (e.g., 10 20  atoms/cc) polysilicon layer. 
     In FIG. 4, a silicon nitride layer  22  is formed by low pressure chemical vapor deposition with a thickness of about 1,000 Å and then a gate photoresist mask  24  is formed over nitride layer  22 . The exposed nitride  22 , polysilicon  20 , and oxide  18  (including the gate area) are then removed by etching. Advantageously, the polysilicon functions as an etch stop when the nitride is etched, the silicon oxide functions as an etch stop in etching the polysilicon, and the silicon substrate functions as an etch-stop in etching the silicon oxide. 
     Next as shown in FIG. 5 the gate structure comprising nitride layer  22 , polysilicon  20 , and gate oxide  18  is masked by photoresist  26 , and in FIG. 6 a P-dopant is implanted to form a lightly doped (e.g., 10 17  atoms/cc) channel region  28 . Mask  26  is then removed and N-type dopant is implanted to form lightly doped (e.g., 10 17  atoms/cc) drain (LDD)  30  on one side of the gate structure. Silicon oxide deposition forms a silicon oxide layer of approximately 2,000 Å, and then the oxide layer is etched back using preferential etchant thereby leaving sidewall spacers  32  around the gate structure. The exposed surface of substrate  10  is then oxidized (seal oxidation) with a thickness of approximately 350 Å-500 Å. This oxidation will not oxidize the top of the polysilicon gate because of the protective nitride cap  22 . Thereafter, nitride cap  22  is removed by a suitable etchant, such as phosphoric acid, with the seal oxide  34  having a remaining thickness of approximately 300 Å. An N+ source/drain implant mask  36  as shown in FIG. 7 is then employed to implant N-type dopant and forming source region  38  and N+ (e.g., 10 19-20  atoms/cc) drain region  40 . It will be noted that the source region abuts spacer  32  while the N+ drain region  40  is spaced from spacer  32  by the LDD region  30 . 
     Using photoresist mask  36 , oxide  34  is removed from over source  38  and N+ drain  40 , and then the photoresist  36  is stripped. Oxide  34  can be removed from over P+ region  16  also, using another mask and etch step. Titanium or cobalt is then sputtered over the surface in contact with P+ (e.g., greater than 10 19  atoms/cc) region  16  and source region  38  and also in contact with N+ (e.g., greater than 10 19  atoms/cc) drain region  40  as shown in FIG.  8 . The structure is heated to form a silicide contact  42  to the source, contact  44  to N+ drain  40 , and contact  46  to the gate. The remaining unreacted metal is stripped, and the silicide is then re-annealed to lower the silicide sheet resistance. Here, contact  42  extends from source region  38  to P+ region  16 , as shown. 
     Thereafter as shown in FIG. 9, an interlayer dielectric  50  (PECVD) is deposited over the surface of the structure, and after a contact mask and etch through dielectric  50 , a barrier metal is deposited. Chemical vapor deposited tungsten or tungsten compound plugs  52  contacting silicide contacts  42 ,  44 , and  46  to the source, drain, and gate, respectively, are then formed. 
     In FIG. 10 a first metal layer is sputtered on interlayer dielectric  50  and selectively etched to form contacts  54 , and thereafter a second dielectric layer  58  is deposited over metal contacts  54  and the first interlayer dielectric  50 . Finally, as shown in FIG. 11, after. planarization of the second interdielectric layer  58 , a via mask and etch is employed to expose the contacts  54  and a seed/glue metal layer  60  is formed over the surface of the second inter dielectric layer  58  and extends to contacts  54 . A second metal layer  62 , if needed, is then formed over the seed metal  60  for interconnect lines as shown in FIG.  12 . 
     Finally, as shown in FIG. 13, metal layer  62  is etched to form an interconnect structure and a passivation layer  64  of silicon nitride and silicon oxide is deposited. 
     An LDMOS transistor fabricating in accordance with the invention can have a gate length of 0.5-0.6μ with a gate oxide thickness of 350 Å-450 Å. The transistor has superior linearity due to reduced drain-source on resistance, and a higher Ft is realized due to the shorter channel length. A higher transconductance (Gm) is provided by the reduced gate oxide thickness, shorter gate, and reduced Rdson. While the invention has been described with reference to one embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.