Abstract:
A vertical thin film transistor formed in a single grain of polysilicon having few or no grain boundaries for use in memory, logic and display applications. The transistor is formed from a thin film of polysilicon having large columnar grains, in which source and drain regions have been formed. The large grain size and columnar grain orientation of the thin film are provided by recrystallizing a thin amorphous silicon film, or by specialized deposition of the thin film. Use of a thin film permits the transistor to be formed on an insulating substrate such as glass, quartz, or inexpensive silicon rather than a semiconductor chip, thereby significantly decreasing device cost.

Description:
This application is a divisional of application Ser. No. 09/229,891 filed on Jan. 14, 1999 now U.S. Pat. No. 6,049,106, which is hereby incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an improved thin film metal-oxide-semiconductor field effect transistor (MOSFET) structure, and in particular to an improved thin film MOSFET formed in a single grain of polysilicon, and to a process for its formation. 
     BACKGROUND OF THE INVENTION 
     As circuit density continues to increase, there is a corresponding drive to produce ever smaller field effect transistors (FETs). Field effect transistors have typically been formed by providing active areas within a bulk substrate material or within a complementary conductivity type well formed within a bulk substrate. A technique finding greater application in achieving reduced transistor size is to form FETs with thin films, commonly referred to as “thin film transistor” (TFT) technology. 
     In the fabrication of TFTs, a thin film of material (typically polysilicon) with a substantially constant thickness is provided on an insulating substrate instead of a semiconductor chip. Source and drain regions are formed by ion implantation or diffusion, and gate insulators and gates are also formed, thus providing a FET having active and channel regions formed entirely within a thin film as opposed to a bulk substrate. The use of insulating substrates such as silicon dioxide, glass, or quartz decreases the cost of the completed device while offering benefits such as reduced bulk capacitance and increased operating speed. Because of these and other advantages, TFTs are especially desirable for use in memory and logic applications, particularly SRAMs, and in liquid crystal displays. 
     It is desirable in TFTs to use a film that is as thin as possible so that the channel region provides maximized desired on/off characteristics for the transistors. This thinness adversely affects source/drain region conductance, however, because the diminished volume of material creates undesirable elevated V cc  source/drain resistance. Another disadvantage of TFTs is poor electrical performance because of defects in the polysilicon film such as grain boundaries. The effects of grain boundary defects, which include unwanted energy levels in the forbidden band, alteration of etching properties, and changes in electrical properties such as the value of the source/drain current and threshold voltage, are magnified in TFTs because of their small size. 
     Grain size and grain boundary consistency has a significant effect on the electrical current flow characteristics of thin films. Current resistance occurs as electrons cross grain boundaries, especially boundaries perpendicular to the direction of current flow. The larger and more numerous the grain boundaries, the higher the resistance. Typical TFTs have multiple grain boundaries within them because the channel length of the devices is much larger than the film thickness, which is usually approximately the same as the grain size. 
     There is needed, therefore, a thin film MOSFET with a minimum of grain boundaries, such that the MOSFET is typically formed in a single grain of polysilicon. A simple method of fabricating a thin film MOSFET with a minimum of grain boundaries is also needed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a vertical thin film MOSFET having improved electrical functioning due to the formation of the transistor in a single grain of polysilicon. Also provided is a method for its formation, in which the transistor is formed from a thin film of polysilicon having large columnar grains. The large grain size and columnar grain orientation of the thin film are provided by recrystallizing a thin amorphous silicon film, or by specialized deposition of the thin film. Ion implantation and annealing are then performed to form source and drain regions in the thin film, which is patterned to form individual thin film transistors. Because of the large grain size and columnar orientation, the FETs have no grain boundaries within them, thereby maximizing conductance and threshold voltage reliability. 
     Additional advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the thin film transistor of the present invention. 
     FIG. 2 is a cross-sectional view of a substrate undergoing the process of a preferred embodiment. 
     FIG. 3 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  2 . 
     FIG. 4 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  3 . 
     FIG. 5 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  4 . 
     FIG. 6 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  5 . 
     FIG. 7 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  6 . 
     FIG. 8 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  7 . 
     FIG. 9 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  8 . 
     FIG. 10 shows the substrate of FIG. 2 at a processing step, subsequent to that shown in FIG.  9 . 
     FIG. 11 shows the substrate of FIG. 2 at a processing step subsequent to that shown in FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
     The term “substrate” is to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, glass, quartz, silicon dioxide, other silicon-containing foundations, and other insulating structures. The term “recrystallization” as used herein refers to the nucleation and growth of new grains within a preexisting crystalline matrix that has been made amorphous, or to grain enlargement of preexisting grains. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Referring now to the drawings, where like elements are designated by like reference numerals, an embodiment of the thin film transistor  20  of the present invention is shown in FIG.  1 . The thin film transistor  20  is formed vertically on a quartz substrate  22 , and comprises two gates  24 ,  26  formed on either side of a vertical device stack  28 . The gates  24 ,  26  may be formed of polysilicon, tungsten silicide, or other suitable conductive material, and have a width (horizontal thickness) within the range of 50 to 200 nm, preferably about 100 nm. A gate oxide region  30   b  approximately 100 Angstroms thick is formed between the gates  24 ,  26  and the device stack  28 . 
     A local interconnect layer  32  of highly doped polysilicon is formed between the device stacks  28  and the substrate  22 . For an n-channel thin film transistor  20 , the local interconnect layer  32  is doped to a first conductivity level, e.g., n++. The vertical device stack  28  comprises a lower source region  34 , a central channel or body region  36 , and an upper drain region  38 . For an exemplary transistor  20  of approximately 1 micron (10,000 Angstroms) in height, the source region  34  is approximately 1000 to 2000 Angstroms in height, the body or channel region  36  is approximately 6000 to 8000 Angstroms in height, and the drain region  38  is approximately 1000 to 2000 Angstroms in height. Preferably, for a 1 micron tall transistor, the source region  34  is approximately 1500 Angstroms in height, the body or channel region  36  is approximately 7000 Angstroms in height, and the drain region  38  is approximately 1500 Angstroms in height. 
     The source region  34  adjoins the local interconnect layer  32 , and is made of polysilicon doped to a second conductivity level, e.g., n++. The channel or body region  36  is fabricated of single crystal silicon having a grain size of approximately 1 micron (10,000 Angstroms), so that a 1 micron tall transistor has its entire channel or body region  36  formed in a single grain of silicon. The drain region  38  overlies the upper surface of the channel or body region  36 , and comprises polysilicon doped to a second conductivity level, e.g., n+ for an n-channel transistor. If a p-channel device were desired, the doping types and levels of these elements would be adjusted as is known in the art. Conventional wiring  40  connects the drain regions  38  of each thin film transistor  20  in a device array to other transistors in the array and to peripheral circuitry on the substrate  22 . 
     The thin film transistor  20  is a MOSFET (metal-oxide-semiconductor FET) device having four contacts to other electrical cells or devices. First, the source  34  is in contact with the local interconnect layer  32 , which connects the transistor  20  to other local devices. Second, the drain  38  of the transistor  20  is connected to conventional wiring  40  so that the transistor  20  is connected to other transistors and peripheral circuitry located on the substrate  22 . The third and fourth contacts are made by a first gate  24  and a second gate  26 , which are formed of conductive material such as doped polysilicon to gate the transistor  20  and to electrically connect all of the transistors of a given row in an array. 
     The thin film transistor  20  will typically be an enhancement-mode device, in which a voltage (the threshold voltage V T ) must be applied before the transistor will conduct. The exemplary n-channel transistor described herein operates when a positive gate voltage is applied, forming a depletion layer in the channel region  36  of the thin film transistor  20 . When the voltage is increased to the threshold voltage V T , the semiconductor becomes inverted at the surface and electrical current flows through the depletion layer between the source  34  and the drain  38 . A p-channel transistor would operate similarly when a negative gate voltage is applied. Alternatively, the thin film transistor  20  may be a depletion-mode device, in which conduction takes place at a zero gate bias. 
     The large grain size (approximately 1 micron or 10,000 Angstroms) of the polysilicon material used to fabricate the thin film transistor  20  of the present invention enables the transistor to be formed in a single grain of silicon, thereby almost eliminating grain boundary defects. Even if a grain boundary is present in the transistor, the columnar orientation of the crystal grains results in a grain boundary that is parallel to electron flow in the transistor. Parallel grain boundaries do not directly impede current flow as perpendicular boundaries do. 
     The thin film transistor  20  is manufactured through a process described as following, and illustrated by FIGS. 2 through 11. For exemplary purposes, dimensions are suggested which are suitable for a transistor of 1 micron in height, and it should be understood that dimensions should be scaled accordingly for other transistor heights. First, a substrate  22 , which may be any of the types of substrate described above, is selected as the base for the thin film transistor  20 . For exemplary purposes, the substrate  22  will be described as a quartz substrate, and the following process should be modified as appropriate and as known in the art if a non-silicon-based substrate is used. 
     As shown in FIG. 2, the first step in the process is to form a local interconnect layer  32  of highly doped polysilicon on the substrate  22 . This layer may be formed by means such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or epitaxial growth, and the method used will be adjusted as is known in the art to compensate for the type of substrate  22  used. The local interconnect layer  32  is formed of polysilicon doped to a first conductivity level, e.g., n++ for an n-channel transistor, and p++ for a p-channel transistor, and should be approximately 1000 to 3000 Angstroms thick. Next, an oxide layer  30  is formed on the local interconnect layer  32  by thermal oxidation, CVD, or other suitable means. This oxide layer  30   a  should be approximately 500 to 5000 Angstroms in thickness. 
     A photoresist  50  and mask (not shown) are then applied over the oxide layer  30   a , and photolithographic techniques are used to produce a patterned mask as depicted in FIG. 3, which is used for defining a trench on the oxide surface for each transistor to be formed. A directional etching process such as wet etching using an acid such as nitric and/or hydrofluoric acid, or dry etching methods such as plasma etching or reactive ion etching (RIE) is used to etch through the oxide layer  30   a  to expose the local interconnect layer  32 , forming a set of trenches  48 , as shown in FIG.  4 . The photoresist and mask are then stripped. 
     FIG. 5 shows the next step in the process, in which a thin amorphous silicon film  52  is deposited on the oxide layer  30   a  and in the trenches  48 . The amorphous silicon film  52  should be approximately 1 micron thick, and is formed by CVD at low temperatures, e.g., below 580 degrees Celsius, or by other suitable means. 
     Referring now to FIG. 6, the amorphous silicon film  52  is now transformed by a recrystallization process into a large grain silicon film  52 . A preferred recrystallization method involves irradiation with a laser, such as an ArF or XeCl excimer laser, to accomplish crystal growth. Crystals grow preferentially out of the recessed areas of the amorphous silicon film  52  (the filled trenches  48 ) because the rate of cooling is faster in the trench  48  than over the oxide layer  30   a  outside the trench  48 . The differential cooling rate is due to the trench  48  having a lower thermal resistance to the polysilicon below the oxide layer  30 , creating lateral temperature gradients and lateral grain growth during the solidification process. The temperature gradient also causes the melt front to move outward from the trench  48 , making a band several microns wide that consists primarily of columnar grains. 
     Alternatively, a large grain silicon film  52  may be formed by specialized CVD techniques, such as low pressure CVD, on the structure of FIG.  4 . If the CVD pressure is low enough, the transition temperature of the silicon crystallization is lowered, and the surface silicon migration rate is enhanced. Careful manipulation of process parameters may be achieved such that the crystal growth rate greatly exceeds the deposition rate, resulting in columnar grain structure of the deposited polysilicon. Other methods of forming the large grain silicon film  52  on the structure of FIG. 4 include depositing the polysilicon film by vapor-phase growth on a pre-formed silicon nucleus, which results in a columnar crystalline orientation normal to the surface of the substrate. The particular method used to obtain the large grain size and columnar crystal orientation may be chosen depending on the electrical characteristics desired in the finished device, as is understood by those skilled in the art. 
     As shown in FIG. 7, a doped polysilicon layer  54  is now formed in the large grain silicon film  52  by ion implantation. Dopant ions are ion implanted into the upper portion of the large grain silicon film  52  in a concentration sufficient to form a doped layer  54  of a second conductivity level, e.g., n+ for an n-channel device. An annealing step is then performed at approximately 900 degrees Celsius to redistribute and activate the implanted dopant. The anneal also causes dopant ions to outdiffuse from the local interconnect layer  32  into lower portions of the large grain silicon film  52 , forming source regions  34  of a first conductivity level, e.g., n++ for an n-channel device. 
     Next, a photoresist  56  and mask (not shown) are then applied over the doped layer  54 , as depicted by FIG. 8, and photolithographic techniques are used to define a device stack  28  from the large grain silicon film  52  for each transistor to be formed. A directional etching process such as wet etching using an acid such as nitric and/or hydrofluoric acid, or dry etching methods such as plasma etching or reactive ion etching (RIE) is used to etch through the large grain silicon film  52 , yielding a device stack  28 , as shown in FIG.  9 . The device stack  28  comprises a source region  34  on the local interconnect layer  32 , a channel or body region  36  on the source region  34 , and a drain region  38  formed on the channel region  36  from the doped layer  54 . The photoresist and mask are then stripped, removing layer  56  from the structure of FIG.  9 . 
     Thermal oxidation is then performed to create a gate oxide layer  30   b  on the sides of the device stacks  28 , as shown in FIG.  10 . The device stacks  28  are thermally oxidized by a suitable process known in the art, such as by heating the substrate  22  in a standard silicon processing furnace at a temperature of approximately 900 to 1100 degrees Celsius in a wet ambient. 
     FIG. 11 depicts the last step of the process of forming the thin film transistor, in which polysilicon or other conductive material is deposited by CVD or other suitable means on opposite sides of the device stacks  28  to form first and second gates  24 ,  26 . Deposition of the polysilicon or other conductive material is followed by directional etching such as RIE to shape the gates  24 ,  26  and to remove any excess polysilicon or other conductive material from horizontal surfaces of the oxide layer  30   a.    
     The thin film transistor  20  is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect gate lines and other connections. For example, the thin film transistor  20  may be covered with a passivation layer of, e.g., silicon dioxide, BSG, PSG, or BPSG, which is CMP planarized and etched to provide contact holes, which may then be metallized to provide contacts to the transistor gates and interconnect layer  32 . Metallization layers and dielectric layers may be applied in conventional fashion to interconnect the individual transistors into a functioning circuit. 
     As can be seen by the embodiments described herein, the present invention encompasses thin film transistors formed in a single grain of polysilicon, thereby reducing the number of grain boundaries found in the device channel. As may be readily appreciated by persons skilled in the art, this lack of grain boundaries provides improved electrical functioning for the transistor. 
     The process sequence described and illustrated above provides for the formation of minimum dimension devices of one carrier type. It follows that, devices of either carrier type may be made on the same substrate by substitution of the appropriately doped materials and addition of appropriate masks. In addition, implanted source/drain devices may be fabricated on the same substrate by process integration with common process steps. 
     The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.