Patent Publication Number: US-8110472-B2

Title: High power and high temperature semiconductor power devices protected by non-uniform ballasted sources

Description:
This patent application is a Divisional Application and claims the Priority Date of a application Ser. No. 12/074,087 filed on Feb. 28, 2008 now U.S. Pat. No. 8,008,747 by common Inventors of this Application. The Disclosures made in the patent application Ser. No. 12/074,087 are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the vertical semiconductor power devices. More particularly, this invention relates to configurations and methods of manufacturing of vertical semiconductor power devices with transistor cells with non-uniform ballasted sources to protect the semiconductor power devices to reliably operate at high power and high temperature conditions. 
     2. Description of the Prior Art 
     Conventional technologies of manufacturing and configuring semiconductor power devices are still confronted with a technical difficulty that devices provided for reliably operating at high power and high temperature usually have high Rdson resistance. Specifically, high reliability is expected from power transistors designed for operating at high power, i.e., high voltage and current, and high temperature applications. One exemplary application is the power transistors implemented in the DC brushless motor control. On the one hand the reliability of the power transistors, such as a MOSFET device, is improved because of the positive temperature coefficient of the Rdson resistance. But in the meantime, the threshold voltage of a MOSFET device has a negative temperature coefficient at relatively low currents and in the linear mode of operation. The leads to a condition that the net voltage (Vgs−Vt), i.e., the gate to drain voltage Vgs minus the threshold voltage Vt, can increase with increasing temperature. A temperature runaway condition may occur to cause a device failure. The temperature runaway problem may occur to both the trench-gate DMOS devices as well as planar power MOSFET devices due to the underlined phenomenon of the negative temperature coefficient of the threshold voltage. 
     In order to overcome such difficulties, a technique of source ballasting is implemented to improve the thermal stability of the MOSFET devices. The source ballasting technique is similar to the emitter ballasting applied in the bipolar technology.  FIG. 1  depicts a MOSFET device implementing a source-ballasting configuration. The MOSFET device includes a source region encompassed inside a body region wherein a portion of the source region shown as S-ballast is doped with a lower concentration of source dopant. The S-ballast region constitutes a ballast resistor Rs for providing a feedback to counter the increase of the voltage due to the negative temperature coefficient of the threshold voltage. The source to drain current, i.e., Ids, increases due to the increase of the voltage when the threshold voltage is reduced with increased temperature. The Rdson thus increases to limit further increase of the current for preventing a thermal runaway problem. Significant increase of Rdson resistance often adversely affects the device performance due to a conventional configuration of uniformly implementing the source ballasting over the entire areas of the semiconductor power devices. 
     Specifically, the source ballasting techniques have been disclosed in U.S. Pat. Nos. 5,475,252, 5,763,9191, 6,268,286, 6,331,726, 6,441,410, 6,583,972, 6,587,320, and 6,927,458. The semiconductor power devices such as MOSFET devices disclosed in these patented inventions implement a technique of uniform ballasting to increase the thermal stability. However, the Rdson is greatly increased and the device performance is adversely affected due the uniform addition of the source resistance by implementing the conventional ballasting technologies. 
     Therefore, a need still exists in the art of power semiconductor device design and manufacture to provide new device configurations and methods of manufacturing the power devices such that the above discussed problems and limitations can be resolved. 
     SUMMARY OF THE PRESENT INVENTION 
     It is therefore an aspect of the present invention to provide a new and improved device configuration and method of manufacturing the semiconductor power device with source ballasting to provide internal and local feedback for minimizing the localized thermal runaway instead of using a uniform ballasting configuration such that the significant Rdson increase may be reduced and the above-discussed difficulties can be resolved. 
     Specifically, it is an aspect of the present invention to provide a new and improved device configuration and method of manufacturing the semiconductor power device by varying the amount of ballasting over the device areas by taking advantage of the thermal dissipation and profile of the structure to overall increase of the Rdson. Specifically, the semiconductor power device has a lower temperature near the edges and higher temperature near the center of die and near the bond pads when the larger amount of current passing through. Corresponding to these temperature distributions and electric operation characteristics in different areas of the device, the source ballasting is selective distributed with variation to effectively prevent the thermal runaway problems while minimizing the increase of the resistance. 
     Briefly in a preferred embodiment this invention discloses a semiconductor power device disposed on a semiconductor substrate. The semiconductor power device includes a plurality of transistor cells distributed over different areas having varying amount of ballasting resistances depending on a local thermal dissipation in each of the different areas. In an exemplary embodiment, the transistor cells having a lower ballasting resistance are disposed near a peripheral area and the transistor cells having a higher ballasting resistance are disposed near a bond pad area. In another exemplary embodiment, the transistor cells having a highest ballasting resistance are disposed in an area around a wire-bonding pad, the transistor cells having a lower resistance are disposed underneath the wire-bonding pad connected to bonding wires for dissipating therefrom and the transistor cells having a lowest ballasting resistance are disposed in an areas away from the bonding pad. 
     Furthermore, this invention discloses a method of manufacturing a semiconductor power device on a semiconductor substrate by forming a plurality of transistor cells on the semiconductor substrate. The method further includes a step of manufacturing the transistor cells with different ballasting resistance for transistor cells distributed over different areas depending on a local thermal dissipation from each of the different areas. In an exemplary embodiment, the method includes a step to manufacture the transistor cells with a lower resistance in a peripheral area of the semiconductor substrate and higher ballasting resistances near a bond pad of the semiconductor substrate. In another exemplary embodiment, the step of manufacturing the transistor cells with varying ballasting resistances further includes a step of manufacturing the transistor cells with a highest ballasting resistance in an area around a wire-bonding area and a lower ballasting resistance under a bonding pad with bonding wires conducting heat therefrom and a lowest ballasting resistance in a peripheral area away from the bonding pad. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view for showing a source ballasting structure of a MOSFET device. 
         FIGS. 2A ,  2 B,  3 A,  3 B,  4 ,  5 A and  5 B are top views of the semiconductor power device of this invention with non-uniform ballasting configuration depending on the location of different regions in the device. 
         FIG. 6  is a cross sectional view for showing a MOSFET with remote metal to the N+ source contacts with smaller source contacts for increasing the space of the source contacts over the source diffusion regions to increase of source ballasting effects. 
         FIGS. 7A ,  7 B and  7 C are cross sectional views showing possible implementations of source ballasting areas Rs 1 , Rs 2 , and Rs 3 . 
         FIGS. 8A and 8B  are cross sectional views for showing a MOSFET with a resistive layer between the source and the N+ silicon to increase the source ballasting effects. 
     
    
    
     DETAILED DESCRIPTION OF THE METHOD 
     Referring to  FIGS. 2A and 2B  for two alternate top views of a semiconductor power device  100 - 1  and  100 - 2  respectively implemented with source ballasting configuration of this invention. In  FIG. 2A , the entire surface of the device is divided into several areas include a gate bond pad area near the lower-left corner of the surface. The device further includes three different ballasting areas, i.e., Rs 1 , Rs 2 , and Rs 3 . The Rs 3  area is an area designated as location for source wire bonding with area Rs 2  surrounding the area Rs 3  and a peripheral area Rs 1  near the edges of the device surface. According to the actual inspection of device failures caused by thermal runaway problems, the damaged silicon usually occurs in five to fifteen seconds with a current of one to twenty amperes at a temperature of 180 to 200 degrees Celsius. The damages are observed mostly near or between the bond wires while the regions outside the wire bonding areas and under the bond wires rarely have any problems of overheating damages. For this reason, the ballasting resistors implemented with reduced source doping are varyingly distributed by making the resistances in these different areas having different ballasting effects. Specifically, in  FIGS. 2A and 2B , a non-uniform ballasting source doping is implemented with Rs 3 &gt;Rs 2 &gt;Rs 1 . The highest ballasting effect is localized for an area with highest current and highest probability of thermal damages. In  FIG. 2A , the surrounding areas Rs 2  and the peripheral area Rs 1  are implemented with reduced ballasting dopant profile thus the increase of the Rdson resistance can be reduced while the most vulnerable regions near the source wires, e.g., area Rs 3 , are protected. In  FIG. 2B , the thermal dissipation effect of bonding wires is considered and the area directly under bond wires is expected to be cooler than the direct surrounding area therefore the bond wire area has a lower ballasting than its direct surrounding area. 
       FIGS. 3A and 3B  are two alternate top views for showing another exemplary embodiment of this invention.  FIGS. 3A and 3B  show the semiconductor power device  100 - 3  and  110 - 4  have designated source wire locations depicted as area Rs 3  with adjacent areas depicted as Rs 2  and surrounding peripheral areas depicted as Rs 1 . Again, the ballasting resistors are implemented in the transistor cells in these areas such that varyingly distribution of ballasting resistances is structured with Rs 3 &gt;Rs 2 &gt;Rs 1 .  FIG. 3A  shows the wire bonding area has the highest ballasting. In case the bond wires provide good thermal dissipation effect, the ballasting in bond wire region can be reduced as shown in  FIG. 3B .  FIG. 4  is a top view of another exemplary semiconductor power device  100 - 5  that has multiple areas as bond wire locations depicted as area Rs 2  with surrounding areas depicted as Rs 3  and the peripheral areas depicted as Rs 1 . A non-uniform source ballasting doping profile is implemented to produce a device with varyingly distribution of ballasting resistances in these different areas with Rs 3 &gt;Rs 2 &gt;Rs 1 . Again the thermal cooling effect of bond wires enable the reduction of ballasting in bond wire areas.  FIGS. 5A and 5B  show two top views corresponding to the devices illustrated in  FIGS. 4 and 3B  respectively. The only difference is that the devices  100 - 6  and  100 - 7  shown in  FIGS. 5A and 5B  have non-uniform ballasting profiles with Rs 3 &gt;Rs 2 &gt;Rs 1  and Rs 0 =0. The ballasting is limited to a center area and the source dopant concentrations for the transistor cells in the area Rs 0  is kept the same as a transistor cell without source ballasting reduction such that the increase of Rdson resistance can be further reduced. 
     The exemplary embodiments shown in  FIGS. 2A to 5B  are only examples for illustrating some of possible applications. The variable ballasting doping profiles can be implemented in many different ways including but not limited to gradually increasing the ballasting from the edge of the die to the bond pad regions. A highest ballasting may be formed directly around the bond pads. The heat generated in the areas directly under the bond pads is expected to conduct through the bond wires; therefore, the areas directly under the bond pads may be implemented with reduced ballasting. In a power device that has multiple wire bonding areas, a highest ballasting may be implemented under and around the bonding areas with lowest ballasting on the peripheral and outside edges of the device and medium ballasting between the bond pads. Another configuration may be implemented with a source doping profile to generate highest ballasting resistance between the bond pads taking advantage of the fact that the part of the heat direct under the bond pads is removed by conduction through the bonding wires. This is confirmed by the observation that actual damages are found mostly between the bond pads and not directly under the bond pads. The combination of these implementation options may be further combined with a doping profile that the transistor cells on the peripheral edges are manufactured without source doping reduction thus providing zero ballasting on the peripheral and areas away from the bonding pads thus further reducing the resistance increase due to the implementation of the ballasting doping configurations. 
     The variable and non-uniform ballasting configuration for protecting the semiconductor power devices as discussed above may be applied to vertical DMOS devices including but not limited to devices with planar gate, trench gate, shielded gate and various trench gate structures. The ballasting source dopant variation processes as discussed above may further be implemented for high power lateral DMOS structure even though the power density for this type of devices is generally lower due to larger cell pitch of the LDMOS structures compared to the vertical DMOS structures. This technique may also be applied to other types of power semiconductor devices such as Insulated Gate Bipolar Transistors (IGBT). 
     The non-uniform ballasting configuration for the semiconductor power devices may be implemented with various methods. A remote metal to the N+ silicon contact may be utilized that has smaller source contacts thus increasing the space of the source contacts over the source diffusion regions and resulting in increase of source ballasting effects as shown in  FIG. 6 . 
     Another configuration of the device to implement the ballasting protection is to provide higher resistance diffusion between the source contact and the source edge of the gate regions. This is realized by applying a lightly doped drain (LDD) type of concept by using a mask or spacer to separate the highly doped N++ source diffusion regions from the lower doped N− regions that reaches the gate for a NMOS device. 
       FIGS. 7A-7C  show a possible implementation of the variable source ballasting by separating the highly doped N++ source diffusion regions from the gate region with lower doped N− Source Ballast regions.  FIG. 6A ,  FIG. 6B  and  FIG. 6C  show the source ballasting areas Rs 3 , Rs 2 , and Rs 1 , respectively. In  FIG. 6A , the N− Source Ballast region is relatively large, so the ballast resistance for Rs 3  is fairly large. In  FIG. 6B , the N− Source Ballast region is relatively small, so the ballast resistance of Rs 2  is less than that of Rs 3 . In  FIG. 6C , there is no N− Source Ballast region at all, so the resistance of Rs 1  is even less than that of Rs 2 . 
     The N− Source Ballast regions can be formed by first performing a light N− implant, using the gate for self-alignment. Later a mask or spacer oxide around the gate region can allow a highly doped N++ source implant to be performed while preserving the lightly doped N− Source Ballast regions near the gate region. 
     Alternately, the source-ballasting configuration may be implemented by the use of a resistive layer to contact the source by adding a resistive layer between the source contact and the N+ silicon source.  FIG. 8A  shows one embodiment where the resistive layer is formed by a lightly doped N− diffused layer.  FIG. 8B  shows another embodiment where the resistive layer is formed by deposition of a Polysilicon or thin-film resistive layer in contact with the source at one end, and sits on an oxide layer at the other end, with a metal contact to it (not shown). The length of resistive layer between the contact metal and the source connection point controls the source ballasting. The characteristics of the resistive layer may also be selectively adjusted in order to control the source ballasting. For manufacturing a device with non-uniform ballasting structure, the resistive layer may be omitted in some regions where no ballasting protection is required. The resistive layer as shown in  FIG. 8B  can be deposited after the source and contact formation, like thin-film resistors in IC processes. A combination of these different alternative implementations may be applied depending on the specific application of each of these semiconductor power devices. 
     In summary, this invention discloses a method of manufacturing a semiconductor power device on a semiconductor substrate. The method includes a step of manufacturing a plurality of transistor cells over different areas on the semiconductor substrate having varying amount of ballasting resistances. In an exemplary embodiment, the step of manufacturing the transistor cells having varying amount of ballasting resistances further comprising a step of varying the ballasting resistances depending on a local thermal dissipation in different areas on the semiconductor substrate. Furthermore, this invention also discloses a semiconductor power device disposed on a semiconductor substrate. The semiconductor power device includes a plurality of transistor cells over different areas on the semiconductor substrate having varying amount of ballasting resistances. 
     The semiconductor power devices with the new and improved ballasting protection can achieve the reduction of the total Rdson because the source ballasting resistors are implemented as function of the thermal dissipation requirements to localized the ballasting functions. Compared to the global uniform distribution of ballasting configurations applied in the conventional technologies, the power device of this invention has significantly reduced Rdson resistance. Furthermore, the localized ballasting protection provides improved reliability when the device is operated under high stress with high current, high current and high temperature with reduce Rdson voltage. Localized and target ballasting protection of the power devices disclosed in this invention therefore achieve improved device performance characteristics, reliably protections, extended operational life, and reduced total operational costs. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.