Abstract:
Embodiments of the present invention include a method for manufacturing a transistor comprising forming a gate conductor above a semiconductor substrate; forming a lightly doped implant region within the substrate, wherein the lightly doped implant region is substantially on the source side of the transistor; and forming a counter doping implant region within the substrate, wherein the counter-doping implant region is substantially on the drain side and wherein the counter-doping reduces the net channel impurity concentration on the drain side.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the manufacture of an integrated circuit. More specifically, embodiments of the present invention relate to the formation of an n-channel and/or p-channel asymmetrical transistor using an arsenic drain side implantation to reduce DIBL. 
     RELATED ART 
     Moore&#39;s Law states that the number of semiconductor devices, (e.g., transistors), per unit area will double every 18–24 months. While other factors such as design improvements contribute to the rapid growth, one of the fundamental drivers of this inexorable density increase is the ever-shrinking minimum feature size of semiconductors. For example, a common minimum feature size of modern semiconductors is 0.15 microns. 
     A modern integrated circuit, IC, for example a flash memory device, may have millions to hundreds of millions of devices made up of complex, multi-layered structures that are fabricated through hundreds of processing steps. Those structures, for example a gate stack, are formed by repeated deposition and patterning of thin films on a silicon substrate, also known as a wafer. 
     As channel length grows shorter, threshold voltage, the voltage required to turn on a transistor, begins to decrease and leakage current increases. These effects are commonly referred to in the semiconductor arts as the “short channel effects” (SCE). An increase in leakage current is particularly onerous in flash memory devices as flash has found wide acceptance in very low power applications, for example mobile phones, due to the ability of flash to retain information without applied power. Increases in leakage current may have a significant deleterious effect on total power consumption of the flash device and the product using the flash device. 
     The distance between source and drain implants is often referred to as the physical channel length. However, after implantation and subsequent diffusion of the source and drains, distance between the source and drains regions becomes less than the physical channel length and is often referred to as the effective channel length (Leff). In VLSI designs, as the physical channel becomes small, so must the Leff. 
     Generally speaking, SCE impacts device operation by, inter alia, reducing device threshold voltages and increasing sub-threshold currents. As Leff becomes quite small, the depletion regions associated with the source and drain areas may extend toward one another and substantially occupy the channel area. Hence, some of the channel will be partially depleted without any influence of gate voltage. As a result, less gate bias is required to invert the channel of a transistor having a short Leff. Somewhat related to threshold voltage lowering is the concept of subthreshold current flow. Even when the gate voltage is below the threshold amount, current between the source and drain nonetheless exist. 
     Two primary causes of increased sub-threshold current are punchthrough and drain-induced barrier lowering (DIBL). Punchthrough results from widening of the drain depletion region when a reverse-bias voltage is applied across the drain-well diode. The electric field of the drain may eventually penetrate to the source area, thereby reducing the potential energy barrier of the source-to-body junction. Punchthrough current is therefore associated within the substrate bulk material, well below the substrate surface. Contrary to punchthrough current, DIBL induced current occurs mostly at the substrate surface. Application of a drain voltage can cause the surface potential to be lowered, resulting in a lowered potential energy barrier at the surface and causing the sub-threshold current in the channel near the silicon—silicon dioxide interface to be increased. One method in which to control SCE is to increase the dopant concentration within the body of the device. Unfortunately, increasing dopant within the body deleteriously increases potential gradients in the ensuing device. For example, increasing dopant causes a hotter junction and lowers the breakdown voltage. 
     For example, Prior Art  FIG. 1A  depicts a conventional flash memory cell  10  during a pocket implant. The gate stack formed includes a floating gate  12  and a control gate  14 . Included are the source  16  and drain  18 . The channel  17  is between the source  16  and the drain  18 . The drain is masked by photoresist  19 . 
     The drain is masked by photoresist to ensure that little of the dopant provided by a pocket implant reaches the portion of the channel  17  near the drain  18 . Typically, high DIBL results in current leakage during high voltage programming. By grading the concentration of dopant (typically Boron) between the source and drain, DIBL can be reduced, without raising the threshold voltage. 
     Prior Art  FIG. 1B  illustrates the memory cell  10  after an implant, subsequent thermal anneal and after removing photoresist  19 . A thermal anneal is used to drive the source side implant across the channel. As a result, the concentration of dopant is graded across the channel length. The highest concentration is near the source and then the concentration tapers off towards the drain side. As a result, the memory cell  10  may have fewer short channel effects and DIBL can be reduced. 
     Although this method is effective for relatively large channel widths, as feature sizes become smaller, the concentration of dopant becomes practically flat. As a result, DIBL is not reduced to a level previously achieved. 
     Prior Art  FIG. 2  illustrates a graph  20  depicting channel dopant concentration  22  with respect to the length of the channel  24 . Channel length  28  illustrates a relatively large channel length and as a result of the relatively large channel length, the dopant concentration  22  is graded across the channel length. As mentioned above, with a relatively large channel length, a graded dopant concentration can be achieved using the above-mentioned method. As feature size continues to decrease, the channel length becomes smaller. Channel length  29  depicts a typical feature size that can be currently manufactured. As a result of the small channel length, the dopant concentration is virtually flat across the channel, thus resulting in poor reduction of DIBL. 
     A properly designed transistor that overcomes the above problems must therefore be applicable to an n-channel transistor. That transistor must be one that is readily fabricated within existing process technologies. 
     SUMMARY OF THE INVENTION 
     Accordingly, what is needed is a structure and method for reducing DIBL without complicating the manufacture process. The structure and method for reducing DIBL should utilize established semiconductor manufacturing equipment. In addition, the structure and method for reducing DIBL should facilitate in maintaining precise critical dimensions for small-scale semiconductor manufacturing. 
     A structure and method for reducing DIBL in a transistor is presented. Embodiments of the present invention include a method for manufacturing a transistor comprising forming a gate conductor above a semiconductor substrate, forming a lightly doped implant region within the substrate, wherein the lightly doped implant region is substantially on the source side of the transistor; and forming a counter doping implant region within the substrate, wherein the counter doping implant region is substantially on the drain side and wherein the counter doping reduces the net impurity concentration on the drain side 
     Embodiments of the present invention also include a structure for reducing DIBL in a semiconductor comprising a gate conductor formed above a semiconductor substrate, a lightly doped implant region formed within the substrate, wherein the lightly doped implant region is substantially on the source side of the transistor; and a counter doping implant region formed within the substrate, wherein the counter doping implant region is substantially on the drain side and wherein the counter doping reduces the net impurity concentration on the drain side. 
     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 embodiments, which are illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       Prior Art  FIG. 1A  is an illustration of a conventional processing approach used to reduce DIBL illustrating a source implant. 
       Prior Art  FIG. 1B  is an illustration of a conventional processing approach used to reduce DIBL illustrating a source side boron implant (SSBI). 
       Prior Art  FIG. 2  is a graph showing concentrations of dopant across a channel length for 2 different channel lengths. 
         FIG. 3  is a graph illustrating the voltages applied to an exemplary semiconductor device during read and program modes in accordance with an embodiment of the present invention. 
         FIG. 4  is a graph illustrating the effectiveness of an exemplary semiconductor to reduce DIBL in accordance with an embodiment of the present invention. 
         FIG. 5  is a flow diagram of an exemplary semiconductor manufacture process is implemented to reduce DIBL in accordance with an embodiment of the present invention. 
         FIG. 6A  is an illustration of an exemplary semiconductor device illustrating a gate stack in accordance with an embodiment of the present invention. 
         FIG. 6B  is an illustration of an exemplary semiconductor device illustrating a source side boron implant in accordance with an embodiment of the present invention. 
         FIG. 6C  is an illustration of an exemplary semiconductor device illustrating a source implant in accordance with an embodiment of the present invention. 
         FIG. 6D  is an illustration of an exemplary semiconductor device illustrating a drain side implant in accordance with an embodiment of the present invention. 
         FIG. 6E  is an illustration of an exemplary semiconductor device illustrating a drain side counter doping implant in accordance with an embodiment of the present invention. 
         FIG. 7A  is a close up view of an exemplary semiconductor device illustrating a graded concentration of a source side boron implant (SSBI) in accordance with an embodiment of the present invention. 
         FIG. 7B  is a graph showing an alternative view of the graded concentration of a source side boron implant in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     The present invention provides a method and structure for reducing DIBL and short channel effects without significantly complicating processing. The method and structure provide a flash memory cell having a source and a drain. To provide the flash memory cell, a source side channel implant is first provided. In one embodiment, the source side channel implant is boron. After the source side implant is completed, an implant is done on the drain side to counter dope the boron implant on the drain side. Typically, in manufacture of a flash memory cell, there are numerous thermal anneals in subsequent steps that cause the boron to diffuse across the channel. By implanting a material (such as arsenic) that does not diffuse easily in the substrate, the net impurity concentration on the drain side is reduced, thus reducing DIBL without raisiong the gate threshold voltage. 
     In one embodiment of the present invention, the drain side implant is arsenic and in another embodiment, the drain side implant is phosphorus. Typically, the present invention is incorporated into the manufacture of a semiconductor device after a gate stack is formed and before a spacer is formed. 
       FIG. 3  is a graph  30  illustrating the difference in voltage applied to a flash memory cell during read and program activity. Typically the voltage applied to the drain during reading  34  is approximately 0.5 volts and the voltage applied to the drain during programming  32  is approximately 5.5 volts. As stated above, DIBL is the lowering of a threshold barrier voltage for the conduction of current across a transistor. DIBL becomes a concern typically when higher voltages are used during programming. When reading or programming a flash memory, voltage is applied to all devices along a bit line. If DIBL causes the threshold voltage to drop enough, current leaks across memory cells that are not selected (no gate bias) causing deleterious effects during programming. 
     As a result of the high voltage applied during programming, the threshold voltage difference between reading and programming causes current leakage in unselected transistors. Optimally, the difference in threshold voltage between the read bias and the program bias would be zero, but a small voltage such as 0.4 volts would be acceptable. 
       FIG. 4  is a graph  40  showing the difference in gate threshold voltages between read and program in a flash memory cell. The graph  40  plots the log of drain current  42  against the gate voltage  41 . Ideally, the difference in threshold voltage between read and program would be zero. Data plot  48  and  49  illustrate an example of an ideal situation wherein the difference in gate threshold voltage is zero. For example, for data plot  48  (corresponding to read) and  49  (corresponding to program) have a gate threshold voltage of 1.7 volts (the two plots overlap and appear as one data plot). In this ideal case, wherein the difference in threshold is zero, DIBL is non-existent. This example wherein DILB is zero is for illustrative purposes to show ideal conditions. 
     When using a high voltage for programming, the difference between the read and program threshold voltage can be substantial. For example, data plot  44  corresponds to the behavior of a transistor during high voltage programming when conventional channel doping is used to prevent DIBL. The threshold voltage for data plot  44  (program) is approximately 1 volt, wherein the threshold voltage for data plot  48  (read) is 1.7 volts. The difference in threshold voltage (DIBL) between read and program is approximately 0.7 volts. With DIBL close to 0.7 volts, leakage across unselected transistors is substantial and the leakage causes deleterious effects during programming and in some cases makes programming impossible. 
     By using a counter dopant to reduce the net impurity concentration near the drain in accordance with embodiments of the present invention, DIBL can be reduced to an acceptable level. For example, data plot  46  (program) illustrates how a transistor would behave when treated with a drain side counter doping in accordance with embodiments of the present invention. As opposed to data plot  44  with a threshold voltage of 1 volt, data plot  56  has a threshold voltage of 1.4 volts. When compared to the read threshold voltage of 1.7 volts, the difference in gate threshold voltage (DIBL) between read and program is only 0.3 volts. By using a drain side counter doping to control the diffusion of a source implant, DIBL is reduced substantially. 
     For purposes of clarity, process  500  of  FIG. 5  will be described in conjunction with the structure  600  of  FIGS. 6A–6E  which illustrate structure  600  as it undergoes process  500  in accordance with an embodiment of the present invention. 
       FIG. 5  is a flow diagram of an exemplary process  500  wherein counter doping is implanted on the drain side of a semiconductor device to reduce DIBL. Typically, a drain side counter-doping would be implanted after a gate stack is formed. Detailed processing steps of forming and cutting a gate stack are eliminated from process  500  for clarity. In addition, intermediate processing steps such as rapid thermal anneals (RTAs) and spacer formations are not included in process  500  for clarity. While many processing steps may be provided in-between the processing steps of the present invention, the additional steps have very little bearing on the details of process  500  of the present invention. 
     Process  500  of  FIG. 5  starts with step  501  to form the gate stack  604  above substrate  602  as illustrated in  FIG. 6A . After the gate stack is formed, in step  503 , a source side boron implant (SSBI)  606  of  FIG. 6B  is provided using conventional processing steps used in the art. Typically, the SSBI is a vertical implant because physical space limitations prevent an angled implant on the source. Many times, sources are in very close in proximity to each other and combined with a relatively tall gate stack an angled implant is not feasible. Dosage of the SSBI is approximately 1.5×10 14  p/cm 2  (particles per square centimeter). The length of the SSBI  608  of  FIG. 6B  is exaggerated for illustrative purposes. 
     Next in step  505 , a source implant  608  of  FIG. 6C  is provided to form the source. Typically, the source doping is an n-type dopant. After the source implant is provided, in step  507 , a drain implant  610  of  FIG. 6D  is provided to form the drain. Typically, the drain implant is also an n-type dopant. Then in step  509 , a drain side counter doping  612  of  FIG. 6E  is provided to redude the net channel doping near the drain. In one embodiment of the present invention, a drain side counter-doping  612  is arsenic. In another embodiment of the present invention, a drain side counter-doping  612  is phosphorous. Dosage of the drain side counter-doping is around 1×10 14  p/cm 2 . When providing a drain side counter-doping late in the processing steps, typically an angled implant is done to implant underneath the gate stack as far as possible. In one embodiment of the present invention, an angled counter-doping implant is provided at an angle  614  of  FIG. 6E  within approximately 30 degrees of perpendicular to the surface of the semiconductor. If an angled implant is not feasible, a vertical implant is done and then subsequent thermal cycles are provided to drive the counter-doping across the channel length. 
     The present invention, a drain side counter-doping to reduce DIBL, can be implemented between many different processing steps of the manufacture of a transistor. Process  500  illustrates an “late” approach wherein the counter-doping is provided by an angled implant. Alternatively, in the case wherein an angled implant is infeasible, an “early” approach is used wherein a vertical implant is used in conjunction with a thermal cycle to drive the counter-doping across the channel length. Process  500  of the present invention can be applied after forming a gate stack but before forming a spacer. 
       FIG. 7A  is a close up illustration of semiconductor  700  in accordance with an embodiment of the present invention. Semiconductor  700  comprises a gate stack  704  formed above a substrate  702 . A source  708  has been formed on one side and a drain  710  has been formed on the opposite side. A SSBI  706  has been formed on the source side and a counter doping  712  has been formed on the drain side.  FIG. 8A  illustrates a graded concentration of doping (SSBI  706 ) on the source side because of the presence of the counter-doping  712  on the drain side. 
       FIG. 7B  is a graph  800  illustrating the concentration of doping across the channel length in accordance with an embodiment of the present invention. Graph  800  plots the concentration of SSBI  802  against the channel length  804  of a semiconductor. The concentration of the SSBI is greatest on the source side  708  and then slopes across the length of the channel towards the drain  710 . 
     A beneficial consequence of implanting a drain side counter-doping is that the concentration if the SSBI is graded, thus reducing DIBL to a reasonable level even when feature size is small. The presence of arsenic (or phosphorous) on the drain side of a transistor allows the formation of a steeper graded concentration of the net doping on the source side of the transistor. A non-uniform concentration of the SSBI at the source reduces DIBL without increasing the gate threshold voltage. 
     Embodiments of the present invention, a structure and method for reducing DIBL have been described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the following claims. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. The embodiments were chosen and described in order to best explain the principles of the invention and it&#39;s practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.