Abstract:
Methods for forming a vertically movable gate field effect transistor (VMGFET) on a silicon-on-insulator (SOI) wafer are described. The methods include providing a process of making VMGFET devices without critical alignment of masks between sequential etch and diffusion steps. The oxide layer of the SOI wafer is used for a self-limiting etch stop layer and for a sacrificial layer to form an insulating layer between a gate electrode and a substrate. The proper location of the gate electrode with respect to the source and drain junctions is insured by using a silicon gate structure as a mask layer for the diffusion process for defining the source and drain junctions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to semiconductor devices. More particularly, the invention relates to suspended gate transistors. 
     2. Description of the Relevant Art 
     Suspended gate field effect transistors (FETs) are semiconductor devices that have a suspended, movable gate. If the gate is movable in the vertical direction, the device is referred to as vertically movable gate field effect transistors (VMGFETs). A VMGFET includes a suspended gate electrode, insulator and semiconductor composite structure. In current VMGFETs, metal or poly-silicon has been used as a gate material. The use of metal or polysilicon requires additional deposition and etching processes for the formation of the VMGFET. It is desirable to have more efficient and cost effective methods of making VMGFETs. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a semiconductor includes: a semiconductor substrate; source and drain regions disposed in the semiconductor substrate; a gate electrode, positioned between the source and drain regions; and an insulator disposed between the gate electrode and the semiconductor substrate, wherein the insulator comprises an air layer and a gate oxide layer, wherein the gate electrode is vertically movable within the air layer of the insulator. The conductivity type of the gate electrode is the opposite to that of the semiconductor substrate. The conductivity type of the gate electrode is the same conductivity type as the source and drain regions. In an embodiment, the gate electrode is composed of a single crystalline semiconductor material that has been highly doped to be electrically conductive. 
     A method of making a semiconductor device includes: forming a sacrificial layer on a semiconductor substrate; forming a gate electrode on the sacrificial layer; removing the sacrificial layer over the source and drain regions; simultaneously implanting ions into the source region, the drain region and the gate electrode; and removing the sacrificial layer, wherein removal of the sacrificial layer creates an air layer between the gate electrode and the semiconductor substrate, wherein the gate electrode is vertically movable within the air layer. An oxide layer may be formed on the semiconductor substrate after the sacrificial layer is removed. 
     In an embodiment, forming the gate electrode comprises: forming a device layer on the sacrificial layer, wherein the device layer comprises a single crystalline semiconductor material that has been doped to be electrically conductive; and patterning the device layer to form the gate electrode. 
     In an embodiment, removing the sacrificial layer over the source and drain regions comprises: forming a photoresist layer on the sacrificial layer; etching the photoresist layer such that the photoresist is removed from the source and drain regions and the gate electrode; and etching the portion of the exposed sacrificial layer. 
     The ion source used to implant the source, drain and gate electrode has an opposite conductivity than the conductivity of the semiconductor substrate. During implantation, the gate electrode masks the channel from implantation when the source and drain regions are implanted with ions. 
     The VMGFET described herein may be used as part of a microelectromechanical system. VMGFETs are particularly useful for microelectromechanical resonators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of embodiments and upon reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view partly in section showing a VMGFET structure; and 
         FIG. 2  is a schematic sequential representation of a process of forming a VMGFET structure. 
     
    
    
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood the present invention is not limited to particular devices or methods, which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include singular and plural referents unless the content clearly dictates otherwise. Furthermore, the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not in a mandatory sense (i.e., must). The term “include,” and derivations thereof, mean “including, but not limited to.” The term “coupled” means directly or indirectly connected. 
     In an embodiment, the gate electrode of a VMGFET is formed with a highly doped silicon gate using a device layer of a SOI wafer, which is widely used in the wafer manufacturing process. The described method reduces the fabrication process and time, allowing manufacturers to develop efficient and cost-effective production methods. The process relies on the use of a highly doped silicon device layer of a SOI wafer as a gate material and a mildly doped handle layer of a SOI wafer as a substrate. The conductivity type of the gate layer is the opposite to that of the substrate. The oxide layer between the device layer and handle layer works as a sacrificial layer to release gate structure from the substrate. 
     The gate structure is patterned before forming the source and drain regions. The gate structure, therefore, may be used as a masking structure, hence, the method reduces a processing step and insures the alignment of channel and gate structure. The gate electrode provides a diffusion mask layer for forming source and drain regions without a critical alignment since the gate electrode overlies the channel area of the FET. While diffusing or ion implanting, the gate structure does not need to be protected or covered with other material, resulting in a reduced fabrication processes (deposition and etching). The gate electrode and the source/drain regions are opened for the diffusion process so that they are doped at the same time. The doping process makes the gate electrode more conductive. The oxide layer under the patterned gate structure absorbs and blocks diffusion of the impurities, which is etched completely before the oxidation process for the gate oxide layer. 
     Referring to  FIG. 1 , a substrate  101  acts as a silicon handle layer of a silicon-on-insulator (SOI) wafer. The gate electrode  103  is formed from a highly doped silicon device layer of a SOI wafer, which is the opposite conductivity type semiconductor material to the substrate  101 . Two regions  106   s  are diffused for the source and the drain of field effect transistor having the opposite conductivity type formed in a semiconductor substrate  101 . Overlying the silicon substrate  101 , containing diffused regions  106   s , is a layer of silicon dioxide  107 . The source electrode at  108 , the drain electrode at  109  and the gate electrode contact at  110  are also shown. The VMGFET includes a suspended gate electrode  103 , composed of a single crystalline semiconductor material that has been doped to be electrically conductive. An insulator is disposed between gate electrode  103  and a substrate  101  which includes air layer  115  and oxide layer  107 . Gate electrode  103  is vertically movable within air layer  115  of the insulator. 
     A specific sequence of steps for forming VMGFET structure in  FIG. 1  is shown in  FIG. 2 . A silicon-on-insulator (SOI) wafer is used to form the VMGFET. Initially (Step  1 ) the silicon-on-insulator wafer includes a semiconductor substrate  101 , which serves as the handle layer. A sacrificial layer  102  (e.g., a silicon oxide layer) is formed on the semiconductor substrate. Sacrificial layer  102  has a thickness of the order of less than 1 μm. Finally, a highly doped silicon layer ( 103 ) is formed on the sacrificial layer  102 . The conductivity type of handle layer  101  of the SOI wafer is the opposite conductivity type of the material of the device layer  103 . For example, for an n-channel case, the gate material incorporates an n-type doped device layer while a p-type doped handle layer is used, and for a p-channel case, a p-type doped device layer for a gate material and an n-type doped handle layer are applied. The handle layer  101  is used for a substrate of field effect transistor embedding source and drain regions. The sacrificial layer  102  of the SOI wafer is used to release a gate structure from the substrate and to limit the distance between the gate structure and the substrate to a maximum of 1 μm. The thickness of the device layer determines the flexibility of a resonant gate structure. So, the thickness of the device layer can be chosen depending on the application and its specifications. 
     In Step  2 , a photoresist  104  is applied to the device layer  103  (e.g., by spin-coating). Photoresist layer  104  is patterned using a UV-lithography process. The patterned photoresist  104  acts as a mask layer for the next silicon etching step. 
     In Step  3 , the exposed device layer  103  is etched to create the gate. Device layer  103  may be etched using a reactive ion etching process. The use of a SOI wafer proves beneficial. During the etching process sacrificial layer  102  limits etching to the silicon device layer  103 , protecting the semiconductor substrate  101  from etching. Thus, sacrificial layer  102  also acts as a self-limiting etch stop layer. After the etching process is completed, the photoresist  104  is removed. 
     In Step  4 , a photoresist layer  105  is applied and patterned using a UV-lithography process. Photoresist layer  105  is patterned to define the source/drain regions. The exposed part of sacrificial layer  102  is etched to expose the source/drain regions. 
     In Step  5 , the exposed areas, which includes the gate electrode, the source region and the drain region are diffused using an ion implantation technique or diffusion technique. In this process, the ion/diffusion source necessary to form the opposite conductivity type of substrate  101  is selected and implanted to the gate electrode. The gate electrode provides a diffusion mask layer when forming the source and drain regions, without a critical alignment step, since the gate electrode overlies the channel area of the FET. The sacrificial layer  102  also prevents impurity diffusion into the FET channel by absorbing the impurities that diffused through the gate electrode. The gate electrode does not need to be protected, since adding additional impurities into the gate electrode during source/drain formation, makes the gate electrode more conductive. 
     In Step  6 , the sacrificial layer  102  underneath the gate electrode is completely etched away after removing photoresist  105 . The removal of sacrificial layer  102  forms an air gap underneath gate electrode  103 , allowing the gate electrode to be suspended and free to move in a vertical direction. 
     In Step  7 , gate oxide layer  107  is grown on the entire silicon substrate  101  to prevent surface current leakage during use and to inhibit natural oxidation. 
     In this method, device layer  103  is used to form a movable gate, and oxide layer  102  as a sacrificial layer to release the gate electrode from the semiconductor substrate. The semiconductor substrate  101  is used as the handle layer and includes the source and drain. 
     A major advantage of using the device layer of a SOI wafer as a gate electrode is that it minimizes the internal stress of the gate structure. Other materials used to form a gate electrode in a VMGFET, such as metal or poly-silicon, induce the deformation of the gate structure due to internal stresses, such as tensile stress or compressive stress, resulting from the deposition process. This stress limits the shape of the gate structure. The single crystalline device layer of the SOI wafer, however, ideally lacks any internal stress. Therefore, using a single crystalline gate device layer yields greater design flexibility. In addition, SOI wafers are known to offer lateral and vertical isolation of integrated components and also known to provide devices with superior electrical properties. 
     Currently, the SOI wafer is also applied in forming Microelectromechanical Systems (MEMS) to fabricate resonant structures using an oxide layer as a sacrificial layer. Hence, the use of a SOI wafer to form VMGFET is beneficial to monolithic integration with electronics: both the VMGFET and the electronic circuitry can be fabricated on the same chip. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.