Abstract:
The invention includes a laterally diffused metal oxide semiconductor transistor comprising a gate electrode and comprising tapered oxide self aligned to the gate electrode.

Description:
This Application is a Divisional of prior application Ser. No. 09/641,086 filed on Aug. 17, 2000, now U.S. Pat. No. 6,506,641, to Charles W. Pearce. The above-listed Application is commonly assigned with the present invention and is incorporated herein by reference as if reproduced herein in its entirety under Rule 1.53(b). 
    
    
     FIELD OF THE INVENTION 
     This invention relates to laterally diffused metal oxide semiconductor (LDMOS) transistors. 
     BACKGROUND OF THE INVENTION 
     Lateral Diffused Metal Oxide Semiconductor devices (LDMOS) are employed in various power control and amplifier applications. In the latter use, the operation is often constrained in frequency by the gate to drain overlap capacitance as well as reliability issues that relate to the electric field in the vicinity of the drain and its effect on “hot carrier aging.” Past attempts to minimize these problems deviate from using a gate oxide of uniform thickness by placing one edge of the gate over a region of thicker oxide (thicker than gate oxide) as shown in prior art FIGS. 1 and 2. Presently, this thicker oxide is grown first and the gate is then aligned to the thicker oxide. This results in uncertain device characteristics due to the vagaries of the alignment and shape of the transition region between the thick and thin oxide regions. 
     There are several disadvantages to using a uniform gate oxide thickness and the prior art structures of FIGS.  1  &amp;  2 : 
     A. With a uniform gate oxide, the gate to drain (N-region) overlap capacitance is large, which limits the frequency response of the LDMOS. 
     B. The abrupt transition from the gate-to-N-region oxide transition is significant, which results in a high electric field and, thus, undesirable hot carrier generation and possibly low breakdown voltage. 
     The gate oxide thickness significantly determines the threshold voltage of the device and the maximum gate voltage, so increasing the gate oxide thickness to improve (A) and (B) above result in a high threshold voltage, V TH , and a low transconductance. 
     SUMMARY OF THE INVENTION 
     The invention includes a laterally diffused metal oxide semiconductor transistor comprising a gate electrode and comprising tapered oxide self aligned to the gate electrode and a method of making the transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice in the semiconductor industry, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a cross-sectional view of a prior art LDMOS showing an edge of the gate electrode over a gate oxide containing a step; 
     FIG. 2 is a cross-sectional view of a prior art LDMOS showing an edge of the gate electrode over two gate oxide thicknesses t 1  and t 2  where t 2 ≦2t 1  (typically). 
     FIG. 3 is a cross-sectional view of an LDMOS showing the edge of the gate electrode over a tapered oxide, for one embodiment of the invention; and 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention includes a selective oxidation that self-aligns a thick, tapered oxide region to a gate electrode edge. In one embodiment, a nitride layer is placed over the entire wafer and an opening is patterned and subsequently etched into the nitride. One side of the opening is placed over the gate electrode and the other is placed some distance W away. The wafer is then oxidized to grow a thicker, tapered (or graded) oxide under the gate electrode. The nitride layer is then removed conventionally, such as with a phosphoric acid etch. The thicker, tapered oxide is the result of oxidation of both the polysilicon gate and the substrate silicon. 
     Thus, to optimize the characteristics of the RF-LDMOS of the present invention, an asymmetric graded (or tapered) gate oxide is desired such that the oxide thickness is greater on the drain side of the gate than in the channel region next to the source. 
     Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1 is a cross-sectional view of a prior art LDMOS device  1 . Device  1  deviates-from using a gate oxide  8  of uniform thickness by placing one edge of the gate  10 , of gate width  4 , over a region of thicker oxide (thicker than gate oxide), the field oxide  6 . Also shown are the source  5 , the drain  7 , N +  regions  9  and  11 , P region  13  and an N-well drift region  19  in a substrate  23 . 
     Note that gate width  4  is typically greater than 1 to 1.5 microns and the channel region is large because of the misalignment tolerance between the gate electrode  10  and the transition region between the thin gate oxide  2  and the thick field oxide  6 . Also, the gate-drain overlap capacitance is large because of the deleterious existence of a region of gate oxide  20  above the N-well drift region  19  near drain  7  and the region of gate electrode  10  above the thick field oxide  6 , thus limiting the high frequency performance of the device. This compromises the speed of the device so that it operates at less than 1 GHz. 
     FIG. 2 is a cross-sectional view of yet another prior art LDMOS device  2 . In device  2 , a gate oxide  25  is provided that has a step in thickness, t 1    27  and t2  29 . 
     The gate-drain overlap capacitance is large in device  2  because t 2  of oxide  27  is approximately equal to 2t 1  of oxide  29 , again limiting the high frequency performance of the device to less than 1 GHz. 
     In contrast to the prior art of FIGS. 1 and 2, FIG. 3 shows an exemplary device  3  having a tapered oxide  31  as part of the gate oxide  8 , distinct from the field oxide  6 , for one embodiment of the invention. The taper oxide  31  is grown after the gate electrode  10  is deposited, the gate electrode  10  is preferably made of doped polysilicon. The tapered oxide  31  is formed by depositing a mask  33 , such as silicon nitride, and patterning an opening in the mask adjacent to the gate electrode  10 . The mask  33  is patterned such that one side of the opening is placed over the gate electrode  10  and the other side of the opening is placed some distance away from the gate electrode  10 . In a preferred embodiment, the polysilicon gate electrode  10  is aligned to the existing field (LOCOS or isolation) oxide  6 . 
     The tapered (or graded) oxide  31  is then grown by thermal oxidation or oxygen implant, for example. In a preferred embodiment, the mask  33  is then removed. The tapered oxide  31  is distinct from the field oxide  6  and is less than about 500 nm thick at its thickest point. 200 to 250 nm is preferred. The field oxide  6  of the prior art would typically be greater than 500 nm at its thickest point. 
     The structure of FIG. 3 can typically have a gate width  35  less than 1 micron. The structure can easily be realized with a gate width  35  equal to 0.3 micron and even less. The device  3  has an f T  (unity gain frequency) above 10 GHz. 
     The LDMOS structure of FIG. 3 reduces the disadvantages of the prior art without negatively affecting the threshold voltage or transconductance of the LDMOS. Note that the gate width  35  could be adjusted to control the electric field at high drain-source voltage near the N-N.sup.+transition  37 , which maybe the region where breakdown occurs. In a preferred embodiment, a device of the present invention will also comprise an N lightly dope drain (LDD)  39 . Note that FIG. 3 shows oxide  31  (silicon dioxide) on the side of the polysilicon gate, which does not occur when the field oxide is grown before gate electrode deposition, as in the prior art. What results is a transistor capable of operating in the region of 1 to 10 GHz. 
     An alternative embodiment (not shown) is to deposit a gate electrode, deposit a hardmask, such as silicon nitride, and pattern the hardmask to provide an opening in the mask adjacent to the gate electrode such that one side of the opening is placed over the gate electrode and the other side of the opening is placed some distance away from the gate electrode. Tapered oxide is then grown simultaneously with field oxide growth. Alternatively, the tapered oxide may itself be used to perform the function, device isolation, of the field oxide. Although the invention has been described with reference to illustrative embodiments, it is not limited to those embodiments. Rather, the appended claims should be construed to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the true spirit and scope of the present invention.