Patent Publication Number: US-2007108517-A1

Title: LDMOS with independently biased source

Description:
BACKGROUND  
      The present invention relates generally to semiconductor devices, and more particularly to a structure for allowing a bias in the base region and for limiting vertical punch-through in power metal-oxide semiconductor devices.  
      A conventional lateral double-diffused metal-oxide-semiconductor (LDMOS) device, typically a power MOS, can provide reduced “on state” resistance in a circuit. Therefore, such a device is very suitable to be used in the output stage of any power management circuits because of its inherently lower RC time delay.  
      A conventional LDMOS structure typically includes a very narrow channel length, which in turn in decided by a smaller P-type base region inside a P-well, with the smaller P-type base region defined by the location of a self-aligned gate of the device, one or more field oxide implants and other resurf structures. To prevent parasitic bipolar junction transistor (BJT) from turning on, and to prevent from having a body effect, the P-type base region pick-up and the source pick-up are typically butted contacts.  
      Some power management applications require the source side to be loaded. In other words, the source may need to have a bias within device operation rage. Unfortunately, in conventional designs, the P-well which encloses the P-type base region is typically connected to a P-type substrate, thereby essentially shorting the P-type base region to the P-type substrate. Since the P-type substrate is typically grounded, it is impossible to provide the source, which is connected to the P-type base region, with any other bias.  
      Other power management applications require more protection from punch-through. Unfortunately, in conventional designs, the N-well depth is limited by the thermal budget in processing. Since the N-well depth is typically limited, vertical punch-through between the P-type base region and the P-type substrate occurs rather frequently and easily. This punch-through may disable completely the LDMOS device and further may stall one or more circuit operations.  
      Therefore, desirable in the art of LDMOS device designs are improved structures for allowing a bias in the base region and for limiting vertical punch-through in power metal-oxide semiconductor devices.  
     SUMMARY  
      In view of the foregoing, the following provides an improved structure for allowing a bias in the base region and for limiting vertical punch-through in power metal-oxide semiconductor devices.  
      In one embodiment, a lateral double-diffused metal-oxide-semiconductor device of a NPN configuration comprises a semiconductor substrate of a first type and a first well of the first type that extends downward from the surface of the semiconductor substrate. A second well of a second type is enclosed subjacently by the semiconductor substrate and laterally by the first well. The device drain is formed in the second well. A dielectric is formed over the second well, and a conductor formed over the dielectric to form a device gate. A base region of the first type is formed in the second well and separated from the first well by a predetermined separation. The device source is disposed in the base region, and preferably electrically coupled to a first contact of the base region for ensuring that the parasitic equivalent is not turned on. For the NPN configuration, the first type is P-type and the second type is N-type. In an exemplary embodiment, an N+ buried layer may be advantageously formed between the second well and the semiconductor substrate for preventing or at least limiting punch-through. The invention also provides for the formation and biasing of the LDMOS.  
      The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a conventional LDMOS.  
       FIG. 2  illustrates a LDMOS in accordance with the first embodiment of the present invention.  
       FIG. 3  illustrates a LDMOS with a resurf structure in accordance with the second embodiment of the present invention. 
    
    
     DESCRIPTION  
      The following provides a detailed description of an improved structure for allowing a bias in the base region and for limiting vertical punch-through in power metal-oxide semiconductor devices.  
       FIG. 1  illustrates a conventional lateral double-diffused metal-oxide-semiconductor (LDMOS)  100 . The conventional LDMOS  100  may provide reduced “on state” resistance in a circuit, including a high voltage application, but it is typically not amenable to be deployed in a circuit that requires the insertion of a circuit load between the source and the electrical ground. As shown, the LDMOS  100  has both the source and the drain on the same active surface of a semiconductor substrate  102 , which is of P-type. A P-well  104  extends downward from the active surface of the semiconductor substrate  102 . A N-well  106  is enclosed and isolated by the semiconductor substrate  102 , and the P-well  104 . Surface structures are further electrically isolated by a ring of field oxides (FOXs)  108 . A base region  110 , which is of P-type, is diffused into the inner edge of the P-well  104 , and is extended slightly into the N-well  106 . A gate oxide  112  and a gate conductor  114  are constructed on top of partially the N-well  106  and the base region  110  to bridge from within the base region  110  to the channel length within the N-well  106 . It is understood that the gate conductor  114  is typically polycrystalline silicon (poly). A N+ source  116  is diffused into the base region  110  and adjacent to the end of the gate conductor  114 . On the opposite side of the N+ source  116  is a P+ contact  118 , which is diffused to make electrical contact to the base region  110 . A N+ drain  120  is diffused into the N-well  106  and adjacent to the opposite end of the gate conductor  114 . Later in the process, the N+ source  116  and the P+ contact  118  are shorted together by a metal pattern, not shown. In an exemplary embodiment, a butted contact may be filled with metal and used to couple the N+ source  116  and the P+ contact  118 . As shown, the base drive of the parasitic transistor is held to zero by the electrical short between the N+ source  116 , which can be alternatively seen as the parasitic bipolar emitter, and the base region  110 . This insures that the parasitic NPN transistor cannot become active. It is understood that the prevented parasitic lateral bipolar NPN transistor is composed of the N+ source  116  as its bipolar emitter, the base region  110  as its bipolar base, and the N+ drain  120  as its bipolar collector.  
      It should be understood that the “+” designation signifies a high level of dopant impurity concentration as known in the art. For example a P+ region is a P-type region with a high concentration of holes and areas designated P+ will generally have a higher dopant concentration than those simply designated as P-type.  
      The N+ source  116  is shorted to the P+ contact  118  for contact to the base region  110 . It is understood that the base region  110  is contiguous with the P-well  104 , which is contiguous with the semiconductor substrate  102 . In other words, since the base region  110  cannot be electrically separated from the semiconductor substrate  102 , it is nearly impossible to form any circuit that utilizes a source load, or any non-zero source bias.  
      The present invention offers an improved structure that allows the insertion of a load between the N+ source  116  and ground.  
       FIG. 2  illustrates a LDMOS  200  in accordance with the first embodiment of the present invention. As shown, the LDMOS  200  has both its source and its drain on the same active surface of a semiconductor substrate  202 , which is of P-type. A P-well  204  extends downward from the active surface of the semiconductor substrate  202 . A N-well  206  is enclosed and isolated by the semiconductor substrate  202  and the P-well  204 . In an exemplary embodiment, N-well  206  is laterally surrounded by P-well  204  and subjacently enclosed by semiconductor substrate  202 . Surface structures are further electrically isolated by a ring of FOXs  208 .  
      A base region  210 , which is P-type, is diffused into the N-well  206 , with a predetermined lateral separation from the P-well  204 . The base region  210  may be formed using a self-aligned gate and one or more field oxide implants. As shown, the base region  210  is physically and electrically separated from the semiconductor substrate  202 , thereby allowing it to be biased differently from the semiconductor substrate  202 . Now, it is possible to insert, or apply, an electrical load between the base region  210  and the electrical ground, which is connected to the semiconductor substrate  202 . As such, the source (N+ source  216 ) of the LDMOS  200  can be biased independently from the grounded substrate.  
      A gate oxide  212  and a gate conductor  214  are constructed on the active surface of the semiconductor substrate  202  to bridge from within the base region  210  to a MOS channel length within the N-well  206 . It is understood that the gate conductor  214  is typically poly and forms a gate electrode. A N+ source  216  is diffused into the base region  210 , adjacent to the end of the gate conductor  214 . P+ contact  218  is disposed on the opposite side of the N+ source  216 , from the gate conductor  214 . P+ contact  218  is diffused to make electrical contact to the base region  210 . A N+ drain  220  is diffused into the N-well  206 , adjacent to the opposite end of the gate conductor  214 . The N+ source  216  and the P+ contact  218  are advantageously shorted together by a patterned metal wire, not shown. In an exemplary embodiment, a butted contact may be filled with metal and used to couple the N+ source  216  and the P+ contact  218 . Conventional methods may be used to form the butted contact. In this embodiment, the base drive of a parasitic NPN transistor is held to zero by the electrical short between the N+ source  216 , which can be seen as the parasitic bipolar emitter, and the base region  210 . This insures that the parasitic NPN transistor cannot become active. The parasitic lateral bipolar NPN transistor, that is now prevented, is composed of the N+ source  216  as its bipolar emitter, the base region  210  as its bipolar base, and the N+ drain  220  as its bipolar collector. Unless otherwise noted, the doped wells an other impurity areas may be formed by conventional implantation and or diffusion methods.  
      The present invention obviates a potential hazard with the new structure. The reasonable depth of N-well  206  is limited and governed by the thermal budget of the total process. In addition to the problem of the parasitic lateral bipolar transistor addressed by the invention, there is the potential problem of vertical punch-through from the base region  210  to the semiconductor substrate  202 . In normal operation, voltage reverse biases the junction between the base region  210  and the N-well  206 . That depletion region can easily extend to the junction between the N-well  206  and the semiconductor substrate  202 . That is punch-through, which allows too much current flow and is likely to cause latch-up, a condition in which current cannot be turned off by normal functional means. The present invention prevents this condition by optionally inserting a N+ buried layer  222  between the N-well  206  and the substrate  202 . If the depletion region extends to the N+ buried layer  222 , the growth of the depletion region is severely slowed since the required accumulation of compensating current carriers is swift from the heavily doped N+ buried layer  222 . Therefore, the depletion region expansion no longer reaches the semiconductor substrate  202 , thereby preventing or at least limiting punch-through.  
       FIG. 3  illustrates a LDMOS  300  with a resurf structure in accordance with the second embodiment of the present invention. The LDMOS  300  is essentially the same as the LDMOS  200 , except that an additional FOX  302  pattern is present between the N+ drain  220  and the gate conductor  214  is added. The FOX  302 , to be seen as a lateral separator, provides an additional distance over which a high voltage depletion width is spaced.  
      It is understood that all isolations, shown as local or field oxide, can be shallow trench isolations (STI). These structures can be merged into other process technologies, such as low voltage mixed-mode processes, low voltage logic, and high voltage processes. In various exemplary embodiments, the drain may be multi-diffused, i.e., formed using multiple diffusion operations.  
      The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.  
      Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.