Patent Publication Number: US-7217606-B2

Title: Method of forming vertical sub-micron CMOS transistors on (110), (111), (311), (511), and higher order surfaces of bulk, soi and thin film structures

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 09/386,313, now U.S. Pat. No. 6,483,171, entitled VERTICAL SUB-MICRON CMOS TRANSISTORS ON (110), (111), (311), (511), AND HIGHER ORDER SURFACES OF BULK SOI AND THIN FILM STRUCTURES AND METHOD OF FORMING SAME, filed on Aug. 13, 1999. The entirety of this application is hereby incorporated herein by reference. 
   This application is related to application Ser. No. 09/386,315 filed on even date herewith and entitled METHOD FOR FABRICATING CMOS TRANSISTORS HAVING MATCHING CHARACTERISTICS AND APPARATUS FORMED THEREBY, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of improving hole mobility and thus velocity saturating vertical transistors by forming the vertical transistors on crystal surfaces of higher order. Also, the present invention deals with the method of forming the vertical transistors and their use in memory cells. 
   2. Description of the Related Art 
   The nearly universal standard silicon wafer crystal orientation for VLSI (“Very Large Scale Integration”) is the (100) orientation. This orientation was chosen over the previously used (111) orientation due to its low surface state density on thermally oxidized surfaces, 9×10 10 /cm 2  versus 5×10 11 /cm 2  as shown in  FIG. 1C .  FIGS. 1A and 1B  simply illustrate conventional crystal orientations. 
   The difference between high and low surface state densities was originally a particularly important consideration for NMOS technology in that a high surface state density makes it difficult to control active and parasitic device threshold voltages as compared to low surface state densities. For the (110) surface orientation, the surface state charge density is 2×10 11  e/cm 2 , which is about double that of the (100) orientation. In present day technology, however, this would amount to less than 0.10 V offset in active device threshold voltage and is readily compensated by the surface threshold voltage ion implant, body bias, and/or potentials of the backgates in SOI (“silicon-on-insulator”) technology. 
   Even in the case of (111) surfaces, a very pessimistic estimate of the surface charge density of 5×10 11 /cm 2  would result in a threshold voltage shift of only 0.25 V in present day technology. The original benefit to NMOS device was that the electron mobility in inversion layers is greater on the (100) surface than on other low order planes. However, it was pointed out in U.S. Pat. No. 4,857,986 to Kinugawa that for modern day CMOS technology with sub-micron devices, a different set of tradeoffs exist. In such short channel devices, the NFETS operate largely in velocity saturation resulting in a source to drain current that is independent of orientation. The PFETs, on the other hand, are less likely to be in velocity saturation and thus would benefit from optimizing the choice of crystal orientation around the inversion layer hole mobility. 
   Since the inversion layer hole mobility can be twice as high on the (110) surfaces than on the (100) surfaces, as is shown in  FIG. 1C , which illustrates assorted properties of the various crystal orientations, overall circuit performance will be enhanced by basing the sub-micron CMOS technology on (110) oriented substrate wafers. 
   Both U.S. Pat. Nos. 4,857,986 to Kinugawa and 4,768,076 to Aoki et al. disclose planar bulk and SOI technologies that either use the (110) crystal wafer orientation or a recrystallized (110) surface layer. However, significant advantages have been obtained by using not just the (110) surface, which are not commonly available, but also (110), (111) and higher order surfaces on (100) and (111) orientation wafers, which are commonly available.  FIG. 2  is a graph showing 70% to 80% higher hole mobility for conduction in the &lt;100&gt; directions on (110) surfaces and for conduction on (111) surfaces than on (100) surfaces. 
   Moreover, traditional thin film transistors as shown in  FIG. 6A  have an outward crystallization structure, which causes the current to flow across grain boundaries. This traditional horizontal transistor thus has a tendency to have low carrier mobilities. 
   Thus, the present invention seeks to achieve more uniform current flow as well as improve surface area utilization of the crystal surface. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to mitigate the problems of the prior art by forming vertical transistors on higher order crystal orientations. The thus-formed vertical transistors have side gates, and may have back side gates, rather than a gate formed on top of the transistor, i.e. on the opposite side of the transistor from the substrate. The side gates and the back side gates facilitate current flow in the transistor. 
   The basic premise of the present invention is to increase the hole mobility in the transistors by forming the transistors on surfaces with higher order crystal orientations. By higher order, we mean surfaces which include the (110), (111), (311), and (511) surfaces as well as higher order surfaces. Such an increase in the hole mobility makes it more likely that the transistors, both NMOS and PMOS, are velocity saturated, thus making the transistors function better. Furthermore, the thus formed transistors are biased to operate in veolcity saturation. Moreover, this improvement in hole mobility is a direct result of forming the transistors on higher order crystal orientations. 
   The operation of the transistor is further enhanced by forming the transistors vertically, rather than horizontally. The vertical transistors allow for a shorter channel length, thus improving transistor operation. The thus formed transistors may be utilized in digital logic circuits, for example, in memory cells, such as DRAMs or SRAMs, among others. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and characteristics of the present invention will become apparent to one skilled in the art to which the present invention pertains by studying the following detailed description in conjunction with the appended drawings. In the drawings: 
       FIGS. 1A and 1B  are illustrations showing assorted crystal surfaces and directions; 
       FIG. 1C  is a table listing assorted properties of the surfaces pictured in  FIG. 1 ; 
       FIG. 2  is a graph depicting hole mobility on various surfaces as a function of voltage; 
       FIG. 3  illustrates an example of a vertical transistor according to a first embodiment of the present invention; 
       FIGS. 4A–4C  illustrate embodiments of the present invention; 
       FIG. 5  illustrates a crystal and various planes therein; 
       FIG. 6A  depicts the recrystallization of thin films a prior art horizontal transistor; 
       FIG. 6B  depicts vertical transistors according to the present invention; and 
       FIG. 7  shows a schematic diagram of a processing system including a DRAM or an SRAM, or both, having CMOS transistors fabricated according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
   Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings. The present invention describes the use of vertical device structures to achieve the advantages noted above as well as further advantages of the present invention. 
     FIG. 3  illustrates an example of a vertical device  10 , used in logic or memory applications. Vertical device  10  is formed on the sidewalls of intersecting trenches. As shown in  FIG. 3 , device  10  includes, from top to bottom, a drain  12 , a conduction area  14 , and a source  16 , all of which are formed on a substrate or insulator  18 . Gate  20  is formed on the side of the device  10 . In the pictured example, the upper drain surface  12 - a  is formed on a (100) crystal orientation. As shown by the arrow, conduction occurs in the &lt;100&gt; direction, from the source to the drain. The sidewalls  10 - a  of the device  10  are in this example formed on a (110) crystal orientation. Such devices as that pictured at  10  in  FIG. 3  have the features of SOI devices, and may be fabricated on conventional bulk wafers via a conventional process. Similar vertical devices have been proposed for electrically programmable devices and SRAMs. In the process sequence for each embodiment, the first mask defines a set of parallel stripes for etching trenches to define section areas upon which the devices will be built. 
   As shown in  FIG. 4A , if the starting wafer is a (100) orientation, for example, a p-type orientation wafer  40 , and the first mask is oriented along the &lt;110&gt; direction, the trenches  46 ,  48 , which are formed prior to the n-p-n structure is built vertically, will be the (110) planes. The process used, if followed to completion, will result in vertical NFET and PFET devices having channels lying in the (110) plane with a current flow in the &lt;100&gt; direction. The vertical transistors each include a lower doped section  42 ,  44 , which is either n+ or p+, with the lower doped sections  42 ,  44  being formed in trenches  46 ,  48  respectively. According to the data in  FIG. 1C , the surface hole mobility of the room temperature PFET thus formed is 70% greater than that of the conventional planar device on the (100) plane. Also, the resultant device current is correspondingly higher. Because circuit performance is limited by PFET current, a corresponding performance improvement will occur. The enhanced performance will not be as great as if the device were formed such that channel conduction is in the &lt;110&gt; direction parallel to the wafer surfaces, but the vertical transistors are easier to fabricate and occupy less surface area and thus have a higher density of transistors. Vertical (110) surfaces  50  perpendicular to the wafer surface can also be formed by etching the (111) surface orientation wafers as illustrated in  FIG. 5 , the (110) and (111) surfaces being at 90 degrees to one another. 
   The surface mobility of holes is 80% higher on the (111) surfaces than on the (100) surfaces. For deep sub-micron, e.g., sized at about 0.5 microns or less, CMOS technology, (111) surfaces can also be used as illustrated in  FIG. 4B , as can higher order surfaces such as (311), (511) and other higher order surfaces. Previously, (111) and higher order surfaces have not been widely utilized in CMOS technology, since the higher surface state charge density is detrimental to micron sized dimensional technology, but is irrelevant in deep sub-micron technology. Higher order surfaces can be used for deep sub-micron CMOS technology by etching (110) surface oriented wafers and forming vertical sidewalls. A wide variety of orientations can be formed depending upon the orientation of the mask used for etching. The thus formed transistors have substantially matching characteristics, such as the switching characteristics of the transistors and the current characteristics of the transistors. 
   If the mask used for etching is oriented in one of the equivalent &lt;112&gt; directions, then a vertical etch will produce (111) surfaces.  FIG. 5  illustrates perpendicular (111) and (110) planes. However, as shown below, the dot product of two vectors (a, b, c) and (x, y, z) is ax+by+cz and if this sum is zero then the planes are perpendicular. It is well known that the (01  ι ) is equivalent to the (110) plane and thus, by taking the dot product of (311) and (01  ι ), (511) and (01  ι ), and (111) and (01  ι ), which is also known to be equivalent to the (110) plane, then the results show that the (311) and (511) planes are perpendicular to the (011) plane. The results also show that the (111) plane is perpendicular to the (110) plane in that all turn out to be have a zero sum vector dot product. 
   Other higher order surfaces such as the (311) surface and the (511) surface can be produced by rotation of the etch mask. As shown in  FIG. 4B , these planes, i.e. the (311) and (511) surfaces, can be perpendicular to the (110) surface. The characteristics of the (311) surface have been previously investigated for use in MOS technology, and the (511) surfaces have some unique properties and can have a very low surface state density. Transistors can be formed in bulk along these sidewalls as shown in  FIGS. 4A and 4B , as well as in SOI technology as shown in  FIG. 4C . 
     FIG. 6B  illustrates a vertical transistor formed in thin film transistor technology and the current flow therein. The vertical transistor has a front gate  70  and may have a back gate  72 . The front gate  70  and the back gate  72  allow the current to flow vertically from source to drain rather than across grain boundaries as in the horizontal transistors of  FIG. 6A . A body contact may be used rather than a back gate. 
     FIG. 7  illustrates a processor-based system including a CPU  100 , input/out devices  110 , either SRAM unit  120  or DRAM unit  130 , either of which functions as a memory device, and a storage device such as a RAM  140 . The DRAM  130  or the SRAM  120  include peripheral circuits having NMOS and PMOS transistors fabricated in accordance with the method of the present invention. The NMOS DRAM or SRAM transistors are as pictured in  FIGS. 4A and 4C , as are the PMOS transistors, which differ from the NMOS transistors by switching the n type semiconductor material for p type semiconductor material. The transistors of the DRAM or SRAM are formed on the (111), (311), (511) or higher order crystal surfaces of the silicon substrate. The RAM  140  is connected to the NMOS/PMOS DRAM or SRAM transistor pair. 
   While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, embodiments and substitution of equivalents all fall within the scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description, but instead is limited by the scope of the appended claims.