Abstract:
A method of fabricating a T-gate HEMT with a club extension comprising the steps of: providing a substrate; providing a bi-layer resist on the substrate; exposing an area of the bi-layer resist to electron beam lithography where the area corresponds to a T-gate opening; exposing an area of the bi-layer resist to electron beam lithography where the area corresponds to the shape of the club extension wherein the area corresponding to the club extension is approximately 1 micron to an ohmic source side of a T-gate and approximately 0.5 microns forward from a front of the T-gate; developing out the bi-layer resist in the exposed area that corresponds to the T-gate opening; developing out the bi-layer resist in the exposed area that corresponds to the club extension; and forming the T-gate and club extension through a metallization process.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to a T-gate High Electron Mobility Transistor and, more particularly, to a club extension to a T-gate High Electron Mobility Transistor. 
       BACKGROUND 
       [0002]    As demands on wireless and other electronic devices evolve there is an increased need for electronic devices that can provide higher performance at high frequency. One way of meeting these requirements is to create devices using T-gates. The T-gate is a gate conductor structure for a semiconductor device, such as a Gallium Nitride High Electron Mobility Transistor (GaN HEMT). For high performance such as a high operating frequency and a high transconductance, the stem of the T-gate is narrow. For high switching speeds the wings (or top) of the T-gate are wide. The result is a gate conductor structure that provides the high performance and high frequency demanded in electronic devices such as high performance commercial communications and military systems. 
         [0003]    The demand for higher performance conductor structures leads to a more demanding semiconductor fabrication process. Particularly in the area of fabricating T-gates using bi-layer resists, there cannot be any spurious material extending from a T-gate to a source or drain ohmic contact. Electron beam exposure and development may cause stress cracks in a bi-layer resist. Fabricating a T-gate using a cracked resist may lead to spurious material extending from these cracks. Such spurious material may cause the T-gate to short to an ohmic contact. Even if the spurious material does not cause the T-gate to short, the spurious material may cause electrical breakdown of HEMT devices. 
         [0004]    Therefore, there is a need in the art for an improved method and system for fabricating T-gates such that electron beam exposure and development does not cause stress cracks in a resist, and spurious material does not extend from a T-gate to a source or drain ohmic contact. 
       SUMMARY 
       [0005]    One embodiment of a method and system is a method of fabricating a T-gate HEMT with a club extension comprising the steps of: providing a substrate; providing a bi-layer resist on the substrate; exposing an area of the bi-layer resist to electron beam lithography where the area corresponds to a T-gate opening; exposing an area of the bi-layer resist to electron beam lithography where the area corresponds to the shape of the club extension wherein the area corresponding to the club extension is approximately 1 micron to an ohmic source side of a T-gate and approximately 0.03 to 0.5 microns forward from a front of the T-gate; developing out the bi-layer resist in the exposed area that corresponds to the T-gate opening; developing out the bi-layer resist in the exposed area that corresponds to the club extension; and forming the T-gate and club extension through a metallization process. 
         [0006]    Another embodiment of the method and system encompasses a system. The system may comprise: a T-gate HEMT; a club extension positioned on an ohmic source side of a proximate front of the T-gate and approximately 0.03 to 0.5 microns forward from a front of the T-gate; and wherein the club extension is metallically affixed to the T-gate and the T-gate is affixed to a substrate. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The features of the embodiments of the present method and apparatus are set forth with particularity in the appended claims. These embodiments may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
           [0008]      FIG. 1   a  is a front view of a T-gate,  FIG. 1   b  is a side view of the T-gate; 
           [0009]      FIG. 2  is a front view of a bi-layer resist on a substrate; 
           [0010]      FIGS. 3   a - d  are overhead views of areas of bi-layer resists that are exposed to electron beam lithography in order to form T-gates with a club extension; 
           [0011]      FIG. 4   a  and  FIG. 4   b  are front views of bi-layer resists on substrates after electron beam lithography exposure and development; 
           [0012]      FIG. 5   a  and  FIG. 5   b  are front views of T-gates and club extensions on substrates after metallization and before lift-off; 
           [0013]      FIG. 6   a  and  FIG. 6   b  are front views of T-gates and club extensions after metallization and lift-off; and 
           [0014]      FIGS. 7   a - d  are overhead views of T-gates with a club extension after lift-off. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Embodiments of the present method and system fabricate a T-gate HEMT without spurious metal extending from the T-gate to an ohmic contact. 
         [0016]    T-gates are typically formed on a substrate that is covered with a resist. The resist may be a bi-layer resist. Electron Beam Lithography (EBL) is a technique used to form fine patterns used in integrated circuits. The patterns are typically formed in the resist. The resist may be an electron sensitive polymer that forms a coating on the substrate. The resist is exposed to an electron beam and the resist is chemically treated to form a pattern in the resist. The pattern formed may comprise an area where a T-gate and club extension is ultimately created. Resting on the substrate may be ohmic contacts. Ohmic contacts serve the purpose of carrying electrical current into and out of the semiconductor. 
         [0017]    Turning to  FIG. 1 , a typical T-gate  110  is shown. As shown in  FIG. 1   a , the T-gate  110  may have a stem  115  and wings  120 . The wings  120  of the T-gate  110  may be wider than the stem  115  of the T-gate  110 . A part of the T-gate that sits above the stem may be considered a top of the T-gate. Although in  FIG. 1   a  the T-gate  110  is shown with a top that comes to an approximate apex, the top of the T-gate  110  may form an apex, or the top of the T-gate  110  may form an irregular shape. 
         [0018]    Herein, a “front” view of the T-gate  110  provides the viewer with the widest view of the wings  120  of the T-gate  110 . Thus the view of the T-gate  110  as seen in  FIG. 1   a  is a front view.  FIG. 1   b  illustrates a side-view  122  of  FIG. 1   a . As seen from a side  122 , the T-gate  110  may appear as two rectangles  124 ,  126  stacked on top of each other. A lower rectangle  124  may be a side-view  122  of the stem  115  of the T-gate  110 . An upper rectangle  126  may be a view of the wing  120  of the T-gate  110 . The T-gate  110  may also have a length  128 . 
         [0019]    The T-gate  110  may have a first end  130  and a second end  132 . Either end  130 ,  132  of the T-gate  110  may be referred to as a front or a back. For example, the first end  130  may be referred to as a front end  130  of the T-gate  110 , and the second end  132  may be referred to as the back end  132  of the T-gate. It is equally true that the second end  132  may be referred to as a front end  132 , and the first end  130  may be referred to as a back end  132 . Each end  130 ,  132  may have a position that is forward from that end  130 ,  132 . The forward position from a front end may a direction that is perpendicularly away from the end  130 ,  132  of the T-gate  110 . Thus, if the first end  130  were a front end, the forward  134  direction would be perpendicularly away from the first end  130 . On the other hand, if the second end  132  were a front end, the forward  136  direction would be perpendicularly away from the second end  132 . 
         [0020]    Turning to  FIG. 2  that depicts a structure  200  that may be used to form the T-gate  110 . The structure  200  may consist of a bi-layer resist  210  resting on a substrate  220 . Resting on the substrate  220  may be ohmic contacts  240 ,  250 . As seen in  FIG. 2  a left side ohmic contact  240  may be a source ohmic contact. A right side ohmic contact  250  may be drain ohmic contact. Among other materials, the substrate  220  may be comprised of Gallium Nitride (GaN), Silicon Carbide (SiC), SiN, Sapphire, or any III V substrate. The bi-layer resist  210  may be comprised of two layers of materials. A bottom layer  260  of the resist  210  may be comprised of a polymethyl methacrylate (PMMA). A top layer  270  of the resist  210  may be a copolymer of methacrylic acid and methyl methacrylate. 
         [0021]    The top of the resist  210  may be exposed to an electron beam  280 . This is the electron beam lithography (EBL) process. The electron beam  280  may form a pattern in the resist  210 . The pattern in the resist  210  may correspond to an opening where the T-gate  110  may reside. Thus the pattern may approximate a rectangle. Another pattern in the resist  210  may also correspond to a club extension. The T-gate  110  and club extension patterns may be formed using two or more passes of the electron beam  280  or by using one pass of the electron beam  280 . 
         [0022]      FIG. 3  is an overhead view of the structure illustrated in  FIG. 2 .  FIG. 3  illustrates an area of the resist  210  that may be exposed to the electron beam  280  to form an opening that may contain the T-gate  110  and an opening that may contain a club extension.  FIG. 3  is broken down into four separate figures. In each figure, a large rectangle  300  illustrates a view from above the structure  200 . The area exposed to the electron beam  280  that may correspond to an opening where the T-gate  110  may rest is shown by a smaller rectangle  310 . An area exposed to the electron beam  280  that may correspond to an opening where a club extension may be formed is shown by one of four example shapes  320 ,  330 ,  340 ,  350 . 
         [0023]    Turning to  FIG. 3   a , an approximately circular shape  320  illustrates the area exposed to the electron beam  280  that may correspond to an approximately circular shaped club extension. Although the shape  320  shown in  FIG. 3  is a circle, shapes that are not perfectly circular may be exposed to form an area where an approximately circular club extension may reside. For example, the side of the example circular shape  320  may be jagged, irregular, or misshapen. A misshapen circular shape may approximate an oval or ellipse. The example shape  320  has an approximate diameter of 0.4 microns to 2 microns. There is a gap  355  between the approximately circular shape  320  and a side of the exposed area  310 . The gap  355  may be approximately greater or equal to 0.1 microns. The approximately circular shape  320  may rest a distance  360  forward from a front  365  of the exposed area  310 . The distance  360  may be approximately 0.3 to 0.5 microns from the center of the approximately circular shape  320 . 
         [0024]      FIG. 3   b  illustrates an approximately square shape  330  exposed to the electron beam  280  that may correspond to an approximately square shaped club extension. Although the shape shown in  FIG. 3   b  is a square, shapes that do not form a perfect square may be exposed to the electron beam  280  to form an area where an approximately square club extension may reside. For example, the sides of the approximately square shape  330  do not have to be of equal length. The angles that form the approximately square shape  330  do not have to be 90 degrees. Sides of the approximately square shape  330  may be irregular, curved or jagged. To form an approximately square club extension, a side of the approximately square shape  330  may be approximately 0.4 microns to approximately 2 microns long. There is a gap  370  between a side of the approximately square shape  330  and the exposed area  310 . The gap  370  may be approximately 0.1 micron or more. The approximately square shape  330  may rest a distance  375  slightly forward from a front  365  of the exposed area  310 . The distance  375  may be approximately 0.3 to 0.5 microns from the center of the approximate square shape  330 . 
         [0025]      FIG. 3   c  illustrates an example approximately parallelogram shape  340  exposed to the electron beam  280  that may correspond to an approximately parallelogram shaped club extension. Although the area shown in  FIG. 3   c  is a parallelogram, areas that do not form a perfect parallelogram may be exposed to form an area that may contain an approximate parallelogram shaped club extension. For example, the sides of the approximately parallelogram shape  340  may be curved or jagged. Furthermore, the opposite angles of the approximate parallelogram shape  340  may be incongruent or the opposite sides may be unparallel. A height  394  of the parallelogram shape  340  may be approximately 0.4 microns to 2 microns. A width  396  of the parallelogram shape  340  may be approximately 0.4 microns to approximately 2 microns. A longer side  398  of the shape  340  may be parallel to the exposed area  365 . There is a gap  380  between a side of the parallelogram shape  340  and the exposed area  310 . The gap  380  may be approximately equal to or greater than 0.1 micron. The approximate parallelogram  340  may rest a distance  385  slightly forward a front  365  of the exposed area  310 . The distance  385  may be approximately 0.3 to 0.5 microns from the center of the approximate parallelogram  340 . 
         [0026]      FIG. 3   d  illustrates an approximate polygon shape  350  exposed to the electron beam  280  that may correspond to an approximately polygon shaped club extension. Herein a polygon is a figure that has at least two sides that forms an enclosure. Although the area shown in  FIG. 3   d  is a polygon, areas that do not form a perfect polygon may be exposed to EBL to form an area where an approximately polygon club extension may reside. For example, the sides of the polygon shape  350  may be curved or jagged. The approximate polygon shape  350  may have an approximate diameter of 0.4 microns to 2 microns. Although a polygon does not have a radius per se, an approximate radius of a polygon may be calculated by taking the average distance from an approximate center of the polygon to each vertex. Alternatively, an approximate radius of a polygon may be calculated by taking an average distance of a plurality of distances between an approximate center of the polygon and an edge of the polygon. Measuring a circumference of the polygon and dividing the circumference by twice pi may also provide an approximate radius of a polygon. There is a gap  390  between the approximate polygon shape  350  and exposed area  310 . The gap may be approximately greater than or equal to 0.1 microns. The approximate polygon shape  350  may rest a distance  392  slightly forward from a front  365  of the exposed area  310 . The distance may be approximately 0.3 to 0.5 microns from the center of the approximate polygon  350 . 
         [0027]    After the resist  210  is exposed to the electron beam  280 , the resist  210  may be developed, or developed out. Developing the resist  210  may entail immersing the resist  210  in a solution comprised of a methyl isobutyl ketone or a combination of methyl isobutyl ketone and isopropanol. After immersion, resist  210  that was exposed to the electron beam  280  is developed out. Developing out the resist may entail removing parts of the resist that were exposed to the electron beam  280 . The result is an opening in the resist where the T-gate  110  and the club extension may sit. The resist  210  may develop stress cracks in the process of electron beam  280  exposure and development. Developing an area of the resist  210  where a club extension may sit may alleviate stress cracks formed during electron beam  280  exposure and development. 
         [0028]    The combination of the size of the area exposed and ebeam conditions on the ebeam  280  may affect the final three dimensional club shape obtained in the resist profile. By modifying the ebeam conditions on the ebeam  280  and the area exposed, some resist may remain  260  and the upper portion of the resist may be developed out  270 . Modifying ebeam conditions on the ebeam  280  and area exposed may result in the development of the resist  270 ,  260  (or  210 ) to the substrate  220 . For example, the type of ebeam conditions used on the ebeam  280  and the area exposed may result in exposure through the resist  210  to the substrate  220 . The area of resist  210  exposed to ebeam  280  may be developed out to the substrate  220 . On the other hand, if a different area of the resist  210  is exposed and the ebeam conditions is re-modified on the ebeam  280 , the area of the resist exposed to the ebeam  280  may not be developed out fully to the substrate  220 . In this case, there may be resist  220  remaining under the exposed area after the exposed area is developed out. 
         [0029]    An example of the developed resist  210  is illustrated in  FIG. 4 .  FIG. 4   a  illustrates the bi-layer resist  210  developed such that a portion of the bi-layer resist  405  remains on the substrate  220  under the area of the resist  210  that was exposed to create a space for a club extension. After the resist  210  is developed, there is an opening where the T-gate stem may rest  415 . There is also an area where the wings of the T-gate  420  and an area where the club extension may reside  425 . In this particular case, a portion of the lower layer of the bi-layer resist  405  remains on the substrate  220  after the resist  210  is developed. In this example, the beam conditions of the ebeam  280  used on an exposed area results in the lower layer  260  of the bi-layer resist  210  remaining. In other examples, by using other types of electron beam conditions, the developed area may extend partly through the upper layer  270  of the resist  210 . Alternatively, modifying electron beam conditions on the ebeam  280  may result in the upper layer  270  of the resist  210  being completely exposed and partly exposed through the lower layer  260  of the resist  210 . The electron beam  280 , depending on the conditions used, may expose the resist  210  anywhere between a portion of the top layer  270  of the bi-layer resist  210  to a depth through both layers  260 ,  270  of the resist  210  to the substrate  220 . 
         [0030]      FIG. 4   b  illustrates the bi-layer resist  210  developed such that all the resist  210  is removed in the area where the club extension may reside  440 . There is an area where the T-gate stem may reside  430 . There is an area where the wings of the T-gate may reside  435 . There is also an area where the club extension may reside  440 . In this particular example, the exposed area corresponding to the club extension  440  may be large. The resist  210  is completely removed where the club extension may reside  440  depending on condition used on the ebeam  280 . After the resist is developed, it is possible that small portions of resist  437  may remain between the space for the T-gate stem  430  and the location the club extension may reside  440 . 
         [0031]    After the resist  210  is developed, a T-gate and club extension may be formed using a metallization process. During the metallization process electrically conductive material such as gold, titanium, nickel or tantalum is used to form the T-gate and club extension. After the T-gate and club extension are formed, any resist  210  remaining on the substrate  220  is removed during a lift-off process. After the resist  210  has been lifted off, the T-gate and club extension may remain on the substrate  220 . 
         [0032]    Turning to  FIG. 5 , a T-gate  505 ,  535  and club extension  510 ,  520  are shown after metallization and before lift-off.  FIG. 5   a , illustrates a club extension  510  that does not extend to the substrate  220 . In this example, resist  415  remains under the club extension  510 . The club extension  510  rests on the resist  210  on a side of the T-gate  505  nearest the source ohmic contact  240 . The club extension  510  may be affixed to a T-gate wing  515 . 
         [0033]      FIG. 5   b  illustrates a club extension  520  that extends to the substrate  220 . In this example, the club extension  520  extends to the substrate  220 . As discussed, a small portion of resist  437  may remain between a base  540  of the club extension and the stem of the T-gate  547 . It is also possible that after development no resist  210  remains between the club extension  520  and the T-gate stem  537 . The base of the club extension  540  may rest on the substrate  220  on a side of the T-gate  535  nearest the source ohmic contact  240 . The club extension  520  may be affixed to the T-gate  535  at a T-gate wing  545 . 
         [0034]    Turning to  FIG. 6  that illustrates the T-gates  505 ,  535  and club extensions  510 ,  520  of  FIG. 5  after the resist  210  has been lifted off. In  FIG. 6   a  the club extension  510  is affixed to the wing  515  of the T-gate  505 . There may be a space  605  between a bottom  610  of the club extension  510  and the substrate  220 . The size of the space  605  between the bottom  610  of the club extension  510  and the substrate  220  may vary depending on the results of electron beam  280  exposure. In other words, the depth  612  of the club  510  may vary depending on the results of electron beam  280  exposure. The club  510  also extends a distance  614  from the T-gate  505 . The distance  614  may vary depending on the results of electron beam  280  exposure. 
         [0035]      FIG. 6   b  is an illustration of the T-gate  535  and club extension  520  of  FIG. 5   b  after the resist  220  has been lifted off. The base  540  of the club extension  520  is affixed to the substrate  220 . The club extension  520  is also affixed to a wing  545  of the T-gate  535 . There may be a gap  615  between the base  540  of the club extension  520  and the stem  620  of the T-gate  535 . The size of the gap  615  may vary depending on the results of electron beam  280  exposure. 
         [0036]    Turning to  FIG. 7  that is an overhead view of club extensions  705 ,  710 ,  715 ,  720  and T-gates  722 ,  724 ,  726 ,  728  after development and lift off. The club extensions  705 ,  710 ,  715 ,  720  and T-gates  722 ,  724 ,  726 ,  728  illustrated in  FIG. 7  correlate to the example shapes depicted in  FIG. 3 . Thus  FIG. 7   a  is an example illustration of a club extension  705  that may be created from the exposed shape  320  as shown in  FIG. 3   a .  FIG. 7   b  is an example illustration of a club extension  710  that may be created from the exposed shape  330  as shown in  FIG. 3   b .  FIG. 7   c  is an example illustration of a club extension  715  that may be created from the exposed shape  340  as shown in  FIG. 3   b .  FIG. 7   d  is an example illustration of a club extension  720  that may be created from the exposed shape  350  as shown in  FIG. 3   d.    
         [0037]    Depending on conditions used during electron beam exposure, the example club extensions  705 ,  710 ,  715 ,  720  shown in  FIG. 7  may be larger than the shape  320 ,  330 ,  340 ,  350  exposed to create the club extension  705 ,  710 ,  715 ,  720 . The club extensions  705 ,  710 ,  715 ,  720  depicted in  FIG. 7  are affixed to the T-gate  722 ,  724 ,  726 ,  728 . The club extensions  705 ,  710 ,  715 ,  720  may be affixed to a side  730 ,  732 ,  734 ,  736  of the T-gate  722 ,  724 ,  726 ,  728  as well as a front  738 ,  740 ,  742 ,  744  of the T-gate  722 ,  724 ,  726 ,  728 . Although the edges extending from the club extension  705 ,  710 ,  715 ,  720  to the T-gate  722 ,  724 ,  726 ,  728  are depicted using straight lines, in practice the edges that extend from the club extension  705 ,  710 ,  715 ,  720  to the T-gate  722 ,  724 ,  726 ,  728  may be jagged, curved, or some other non-linear shape. 
         [0038]    The present method and apparatus are not limited to the particular details of the depicted embodiments and other modifications and applications are contemplated. Certain other changes may be made in the above-described embodiments without departing from the true spirit and scope of the present method and apparatus herein involved. It is intended, therefore, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.