Abstract:
A low noise (1/f) junction field effect transistor (JFET) is disclosed, wherein multiple implants push a conduction path of the transistor away from the surface of a layer upon which the transistor is formed. In this manner, current flow in the conduction path is less likely to be disturbed by defects that may exist at the surface of the layer, thereby mitigating (1/f) noise.

Description:
[0001]    This is a division of U.S. application Ser. No. 11/733,816, filed Apr. 11, 2007, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The disclosure herein relates generally to semiconductor processing, and more particularly to fashioning a low noise junction field effect transistor (JFET). It can be appreciated that different electronic devices may have different requirements depending upon a particular device&#39;s application. For example, high performance precision analog applications may require very low noise, simple yet precise component matching, high speed and long term matching stability. In addition to demanding low component noise and precise component matching, precision analog products also require that operations of critical components be reliable and not modulated by other undesired sources such as overlying conducting metal buses. It would, therefore, be desirable to fashion transistors that operate with low noise, good matching and high disturbance immunity characteristics. 
         [0003]    It can also be appreciated that transistors are basic building blocks of semiconductor circuitry and electronic devices. Accordingly, the type of transistor used depends upon the applications and the characteristics of the transistor. For example, junction field effect transistors (JFETs) generally exhibit very low 1/f noise and high input impedance. Complementary metal oxide semiconductor (CMOS) transistors, on the other hand, operate with a relatively higher level of noise and have a high impedance or low input current. Bipolar transistors, in contrast, accommodate good matching and, low noise, but exhibit a low impedance or a high input current. Given the desire for low noise in high performance precision analog applications and the propensity for JFET transistors to operate with low noise, it would thus be desirable to produce a JFET in a cost effective manner that allows the JFET to operate with even lower noise so that the JFET can be implemented in a high performance precision analog application. 
       SUMMARY 
       [0004]    The following presents a summary to provide a basic understanding of one or more aspects of the disclosure herein. This summary is not an extensive overview. It is intended neither to identify key or critical elements nor to delineate scope of the disclosure herein. Rather, its primary purpose is merely to present one or more aspects in a simplified form as a prelude to a more detailed description that is presented later. 
         [0005]    Fashioning a low noise (1/f) junction field effect transistor (JFET) is disclosed, where multiple implants are performed to push a conduction path of the transistor away from the surface of a layer upon which the transistor is formed. In this manner, current flow in the conduction path is less likely to be disturbed by defects that may exist at the surface of the layer, thereby mitigating (1/f) noise. 
         [0006]    To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth certain illustrative aspects. Other aspects, advantages and/or features may, however, become apparent from the following detailed description when considered in conjunction with the annexed drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a flow diagram illustrating an exemplary methodology for forming a low noise junction field effect transistor (JFET). 
           [0008]      FIGS. 2-10  are cross-sectional views of a semiconductor substrate wherein an exemplary low noise JFET is formed. 
           [0009]      FIG. 11  is a top view of a semiconductor substrate wherein an exemplary low noise JFET is formed, where a cross section taken along lines  10 - 10  in  FIG. 11  may correspond to the device illustrated in  FIG. 10 . 
           [0010]      FIG. 12  is a top view of a semiconductor substrate wherein an exemplary low noise JFET is formed, where a cross section taken along lines  13 - 13  of  FIG. 12  may correspond to the device illustrated in  FIG. 13 . 
           [0011]      FIG. 13  is a cross-sectional view of a semiconductor substrate wherein an exemplary low noise JFET is formed. 
           [0012]      FIG. 14  is a cross-sectional view of a semiconductor substrate wherein an exemplary low noise JFET is formed. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The description herein is made with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to facilitate understanding. It may be evident, however, to one skilled in the art, that one or more aspects described herein may be practiced with a lesser degree of these specific details. In other instances, known structures and devices are shown in block diagram form to facilitate understanding. 
         [0014]    An exemplary methodology  100  for forming a low noise (1/f) junction field effect transistor (JFET) is illustrated in  FIG. 1 , and  FIGS. 2-14  are views of a semiconductor substrate  200  wherein such a method is implemented. While the method  100  is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the disclosure herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. 
         [0015]    At the outset, a buried layer or a back or bottom gate (BG) region  202  having a first electrical conductivity type (e.g., n or p type) is formed in the semiconductor substrate  200  at  102  ( FIG. 2 ). The BG  202  can be formed, for example, by forming and patterning a first resist  204  over the substrate  200  and performing a first (selective) implantation  206  of one or more dopants into the substrate  200 . It will be appreciated that substrate as referred to herein may comprise any type of semiconductor body (e.g., silicon, SiGe, SOI) such as a semiconductor wafer or one or more die on a wafer, as well as any other type of semiconductor and/or epitaxial layers formed thereon and/or otherwise associated therewith. It will also be appreciated that the patterning of the first resist  204  (as with all masking and/or patterning mentioned herein) can be performed in any suitable manner, such as with lithographic techniques, for example, where lithography broadly refers to processes for transferring one or more patterns between various media. In lithography, a light sensitive resist coating (e.g.,  204 ) is formed over one or more layers to which a pattern is to be transferred. The resist coating is then patterned by exposing it to one or more types of radiation or light which (selectively) passes through an intervening lithography mask containing the pattern. The light causes exposed or unexposed portions of the resist coating to become more or less soluble, depending on the type of resist used. A developer is then used to remove the more soluble areas leaving the patterned resist. The patterned resist can then serve as a mask for the underlying layer or layers which can be selectively treated (e.g., doped). 
         [0016]    At  104 , an epitaxial or EPI layer  208  having a second electrical conductivity type (e.g., p or n type) is formed (e.g., grown) over the surface of the substrate  200  ( FIG. 3 ). At  106 , first well regions  212  having the first electrical conductivity type are (selectively) formed within the EPI layer  208  down to the BG  202  ( FIG. 3 ). The first well regions  212  may be formed, for example, with a second patterned resist  214  and a second implantation  216 . The first well regions  212  are typically formed to have a dopant concentration that is lower than the dopant concentration of the BG  202 . 
         [0017]    Second well regions  218  having the second electrical conductivity type are then (selectively) formed within the EPI layer  208  adjacent to the first well regions  212  at  108  ( FIG. 4 ). The second well regions  218  may be formed, for example, with a third patterned resist  220  and a third implantation  222 . The second well regions  218  are typically formed to have a dopant concentration that is greater than the dopant concentration of the EPI layer  208 . 
         [0018]    At  110 , isolation regions  226  are formed in the EPI layer  208  to electrically isolate different active areas from one another ( FIG. 5 ). The isolation regions  226  can comprise LOCOS (local oxidation of silicon) FOX (field oxide) regions and/or STI (shallow trench isolation) regions, for example. To implement LOCOS isolation regions, for example, a hardmask (e.g., of Si 3 N 4 ) (not shown) may be formed over the EPI layer  208  and patterned so that underlying areas where the isolation regions are to be formed are exposed. Then, one or more dopants (e.g., boron) can be implanted into these exposed areas to form channel stops that mitigate the formation of inversion layers which could create undesirable conductive channels between adjacent devices. A relatively thick (e.g., around 500 nm) oxide pad (e.g., SiO 2 ) is then grown in the exposed areas, where the hardmask inhibits such growth in locations other than these areas. Due to its thickness, the oxide is formed relatively quickly by a wet growth process to keep the fabrication process timely. The hardmask is then removed, leaving the LOCOS or FOX isolation regions  226 . 
         [0019]    A first threshold voltage (Vt) region  230  having the first electrical conductivity type is formed in the EPI layer  208  at  112  ( FIG. 6 ). The first Vt region  230  may be formed, for example, with a fourth patterned resist  232  and a fourth implantation  234 . It can be appreciated that the implanted dopants constituting region  230  are self-aligned with the isolation regions  226 . Accordingly, if the resist  232  happens to be pulled back, the first Vt region  230  will still be positioned as desired because of the self-alignment with the isolation regions  226 . The first Vt region  230  is typically formed to have a dopant concentration that is greater than the dopant concentration of the first well regions  212 . In one example, the first Vt region  230  is formed to a depth of between about 0.5 um and about 1.5 um, for example. 
         [0020]    Also, as will be appreciated, forming the low (1/f) noise transistor as described herein can be done as part of/concurrently with a CMOS fabrication process, thus allowing the JFET to be to be produced in a cost effective manner. That is, acts necessary to form the JFET can be “borrowed” from the CMOS flow. In this manner, the low noise JFET can be fabricated as part of a CMOS process flow without having to modify the CMOS flow (or otherwise develop a special flow for the JFET). Accordingly, region  230  is referred to as a (first) threshold voltage (Vt) region because the (fourth) implantation  234  used to form it may correspond to a threshold voltage (Vt) implantation that is performed as part of a CMOS fabrication process. 
         [0021]    A second threshold voltage (Vt) region  240  having the second electrical conductivity type is then formed over the first threshold voltage region  230  at  114  ( FIG. 7 ). The second Vt region  240  may be formed, for example, with a fifth patterned resist  242  and a fifth implantation  244 . As with region  230 , the implanted dopants constituting region  240  are self-aligned with the isolation regions  226 , thus being somewhat independent of the patterning of the fifth resist  242 . The second Vt region  240  is typically formed to have a dopant concentration that is greater than the dopant concentration of the second well regions  218 . In any event, since the second Vt region  240  is formed over/within the first Vt region  230 , the second Vt region  240  has a concentration of second electrical conductivity type (e.g., p or n type) dopants that is sufficient to overpower/overcome the concentration of first electrical conductivity type (e.g., n or p type) dopants of the first Vt region  230  in this area of the EPI layer  208 . In one example, the second Vt region  240  is formed to a depth of between about 0.1 um and about 0.3 um, for example. 
         [0022]    Similar to region  230 , region  240  is referred to as a (second) threshold voltage (Vt) region because the (fifth) implantation  244  used to form it may also correspond to a threshold voltage (Vt) implantation that is performed as part of a CMOS fabrication process. Moreover, since the second Vt region  240  is formed over/within the first Vt region  230 , and more particularly has the same dimensions as the first Vt region  230 , the fifth patterned resist  242  used to form the second Vt region  240  may correspond to the fourth patterned resist  232  used to form the first Vt region  230 . That is, the fifth patterned resist  242  and the fourth patterned resist  232  may, in fact, comprise the same resist. It can be appreciated that utilizing a single patterned resist/mask to form both the first and second Vt regions  230 ,  240  streamlines the fabrication process, at least, by reducing the number of patterning activities that have to be performed. 
         [0023]    At  116 , source and drain regions  250 ,  252  having the first electrical conductivity type are formed in a spaced apart manner over/within the first and second Vt regions  230 ,  240  ( FIG. 8 ). The source and drain regions  250 ,  252  may be formed, for example, with a sixth patterned resist  254  and a sixth implantation  256 . The source and drain regions  250 ,  252  are again self-aligned with at least some of the isolation regions  226 , thus tolerating some variation in the patterning of the sixth resist  254 . The source and drain regions  250 ,  252  are typically formed to have a dopant concentration that is greater than the dopant concentration of the first well regions  212 . In any event, since the source and drain regions  250 ,  252  are formed over/within the first and second Vt regions  230 ,  240 , the source and drain regions  250 ,  252  have a concentration of first electrical conductivity type (e.g., n or p type) dopants that is sufficient to overpower/overcome the concentration of second electrical conductivity type (e.g., p or n type) dopants of the second Vt region  240  as well as the concentration of first electrical conductivity type (e.g., n or p type) dopants of the first Vt region  230  in this area of the EPI layer  208 . In one example, the source and drain regions  250 ,  252  are formed to a depth of between about 0.1 um and about 0.35 um, for example. Forming the source and drain regions to a depth that is greater than the depth of the second Vt region  240  allows the source and drain regions  250 ,  252  to electrically couple to the first Vt region  230 . 
         [0024]    At  118 , a source or drain region  260  having the second electrical conductivity type is formed in a second well region  218  between the first well regions  212  ( FIG. 9 ). The source or drain region  260  may be formed, for example, with a seventh patterned resist  264  and a seventh implantation  266 . The source or drain region  260  is again self-aligned with at least some of the isolation regions  226 , thus tolerating some variation in the patterning of the seventh resist  264 . The source or drain region  260  is typically formed to have a dopant concentration that is greater than the dopant concentration of the second well region  218 . It will be appreciated that the source or drain region  260  serves as a mechanism for contacting the second well region  218 . In one example, the source or drain region  260  is formed to a depth of between about 0.1 um and about 0.3 um, for example. 
         [0025]      FIG. 10  illustrates the structure of  FIG. 9  with the resist  264  removed (e.g., stripped).  FIG. 11  is a top view of a the structure of  FIG. 10 , where a cross section taken along lines  10 - 10  in  FIG. 11  corresponds to the device illustrated in  FIG. 10 . Isolation regions  226  are not depicted in  FIG. 11  (or  FIG. 12 ) for simplicity. It can be appreciated that the entire structure is isolated (e.g., by the buried layer  202  on the bottom and the first well regions  212  on all sides). It can also be appreciated that the first and second Vt regions  230 ,  240  extend (into and out of the page in  FIG. 10 ) more than the source and drain regions  250 ,  252 . That is, the source and drain regions  250 ,  252  have a length  251  that is less than a length  253  of the first and second Vt regions  230 ,  240  so that the source and drain regions  250 ,  252  do not overlie all of the first and second Vt regions  230 ,  240 . This allows the second Vt region  240  to electrically couple to the second well region  218 . 
         [0026]    The ability of the second Vt region  240  to connect to the second well region  218  can be further appreciated in  FIGS. 12 and 13 .  FIG. 12  generally corresponds to the top view presented in  FIG. 11 , but rotated by about ninety degrees.  FIG. 13  corresponds to a cross-sectional view of  FIG. 12  taken along lines  13 - 13 . It can be seen that since the second Vt region  240  extends past the drain region  252  (and source region  250 ) this second Vt region  240  which has the second electrical conductivity type can couple to the second well  218  which also has the second electrical conductivity type. It will also be appreciated that the dopants implanted to form the source or drain region  260  can also be implanted into regions  280  to facilitate this coupling. More particularly, the mask  264  utilized to form the source or drain region  260  ( FIG. 9 ) can be patterned so that the dopants implanted by the seventh implantation  266  are also implanted into regions  280 . It will be appreciated that regions  280  can overlap regions  230  and  240  a little, but should not get too close to the source or drain regions  250 ,  252  to mitigate leakage. Establishing the regions  280  also mitigates current flow near the surface of the EPI layer  208  at the edges of regions  230  and  240 , where such surface current flow is undesirable as it may promote 1/f noise. 
         [0027]    The methodology then advances to further back end processing at  120  and ends thereafter. By way of example, one or more conductive and/or dielectric layers can be formed and/or patterned during back end processing.  FIG. 14  depicts the structure of  FIG. 10 , for example, with a high quality thin (gate) oxide  282  over  230 ,  240 ,  250 ,  252  and  260 . Similarly, a thicker layer of oxide based material  284  is then formed over everything and conductive contacts  286  are formed therethrough down to  250 ,  252  and  260 . 
         [0028]    It will be appreciated that the first and second Vt regions  230 ,  240  allow current to conduct between the source and drain regions  250 ,  252  (which have the first electrical conductivity type) through the first Vt region  230  (which also has the first electrical conductivity type). More particularly, since the second Vt region  240  has the second electrical conductivity type, carriers are significantly less prone to traverse this area, and are instead much more willing to travel through the first Vt region  230 . In this manner, current flow is effectively pushed down away from the surface of the EPI layer  208 . This is advantageous because defects (e.g., dangling bonds, scratches and/or other irregularities, etc.) are more likely to occur at the surface of the EPI layer  208  than at other locations, and such defects can interrupt current flow, thus leading to 1/f noise. As such, forcing the current down to where defects are less likely to occur effectively mitigates 1/f noise by promoting more regular, uninterrupted and/or uniform current flow. 
         [0029]    As alluded to above, fashioning a JFET as described herein can be readily integrated into a standard CMOS fabrication process. For example, the first through seventh patterned resists and corresponding implantations mentioned herein can be borrowed from or implemented with existing masking and implantation actions that are part of a standard CMOS fabrication process. For example, the source and drain implantations performed at  256  and  266  may correspond to implantations performed to form source and drain regions in CMOS transistors. Similarly, regions  230 ,  240  are referred to as Vt regions herein because they may be formed when threshold voltage (Vt) implantations are performed in a CMOS fabrication process. Nevertheless, regions  230 ,  240  may be formed at any suitable time (e.g., with implantations other than threshold voltage implantations). Additionally, the illustrated ordering of acts or events disclosed herein can be varied. Fashioning a JFET as part of a standard CMOS fabrication process thus allows the JFET to be produced in a cost effective manner, which satisfies an ongoing desire in semiconductor fabrication. 
         [0030]    While reference is made throughout this document to exemplary structures in discussing aspects of methodologies described herein (e.g., those structures presented in  FIGS. 2-14  while discussing the methodology set forth in  FIG. 1 ), those methodologies are not to be limited by the corresponding structures presented. Rather, the methodologies (and structures) are to be considered independent of one another and able to stand alone and be practiced without regard to any of the particular aspects depicted in the Figs. By way of further example, circular or race-track types structures can be fashioned in accordance with the methodology disclosed herein. Additionally, layers described herein, can be formed in any suitable manner, such as with spin on, sputtering, growth and/or deposition techniques, etc. 
         [0031]    Also, equivalent alterations and/or modifications may occur to those skilled in the art based upon a reading and/or understanding of the specification and annexed drawings. The disclosure herein includes all such modifications and alterations and is generally not intended to be limited thereby. In addition, while a particular feature or aspect may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features and/or aspects of other implementations as may be desired. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, and/or variants thereof are used herein, such terms are intended to be inclusive in meaning—like “comprising.” Also, “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features, layers and/or elements depicted herein are illustrated with particular dimensions and/or orientations relative to one another for purposes of simplicity and ease of understanding, and that the actual dimensions and/or orientations may differ substantially from that illustrated herein. Further, some regions that are illustrated as having distinct or abrupt edges may not be so precisely delineated, but may instead blend slightly with other regions. This is particularly true of doped or implanted regions that may diffuse with other regions, particularly at abutting edges.