Abstract:
A field effect transistor (FET) device structure and method for forming FETs for scaled semiconductor devices. Specifically, FinFET devices are fabricated from bulk semiconductor wafers, as opposed to silicon-on-insulator (SOI) or separation by implantation of oxygen (SIMOX) wafers, in a highly uniform and reproducible manner. The method facilitates formation of FinFET devices from readily-available bulk semiconductor substrates with improved and reproducible fin height control while providing isolation between source and drain regions of the FinFET device.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to FET and MOSFET transistors, and more particularly the invention relates to field effect transistors having channel regions extending vertically from a supporting substrate between horizontally disposed source and drain regions.  
       BACKGROUND ART  
       [0002]     Metal-oxide-semiconductor field effect transistor (MOSFET) technology is a dominant electronic device technology in use today. Performance enhancement between generations of devices is generally achieved by reducing an overall size of the device, resulting in an enhancement in device speed. This size reduction is generally referred to as device scaling. As MOSFETs are scaled to channel lengths below about 100 nm, conventional MOSFETs suffer from several problems. In particular, interactions between the source and drain of the MOSFET degrade an ability of the gate to control whether the device is on or off. The degradation in control ability phenomenon is called a short-channel effect (SCE). Silicon-on-insulator (SOI) MOSFETs are formed with an insulator (usually, but not limited to, silicon dioxide or sapphire) below an active region of the device, unlike conventional bulk MOSFETs, which are formed directly on silicon substrates, and hence have silicon below all active regions. SOI is generally considered advantageous as it reduces unwanted coupling between the source and the drain of the MOSFET through the region below the channel. Other techniques, such as separation by implantation of oxygen (SIMOX) functions similarly to SOI. The reduction in coupling in SOI and SIMOX is often achieved by ensuring that all the silicon in the MOSFET channel region can be either inverted or depleted by the gate (called a fully depleted MOSFET). As device size is scaled, however, ensuring a fully depleted channel region becomes increasingly difficult, since the distance between the source and drain is reduced. The reduced distance results in an increased interaction with the channel thus reducing gate control and increasing short channel effects.  
         [0003]     A double-gate MOSFET structure places a second gate in the device, such that there is a gate on either side of the channel. The double-gate allows gate control of the channel from both sides. Additionally, when the device is turned off using both gates, the off-state transistor current is reduced. An extension of the double-gate concept is a surround-gate or wraparound-gate concept, where the gate is placed such that it completely or almost-completely surrounds the channel, providing improved channel control. These surround-gate and wraparound-gate concepts are also formed on SOI or SIMOX and are referred to as FinFET devices due to the silicon-etched fin produced above the oxide/insulator level. Such a FinFET device is presented in U.S. Pat. No. 6,413,802, entitled “FinFET Transistor Structures Having a Double Gate Channel Extending Vertically from a Substrate and Methods of Manufacture,” issued to Hu et al.  
         [0004]     Additional significant unsolved problems remain with the aforementioned electronic devices. For example, if a FET device, especially a CMOS device, is used as nonvolatile memory, a body contact needs to be made to the top layer of silicon, thus increasing the area of the device. In addition to the increased area required, the body contact problem has prevented the semiconductor industry from using FinFET devices for nonvolatile memory due to current processing complications in making the contact. To create the contact, an additional via needs to be formed followed by a typically multi-part fill process using tungsten and tantalum. On a typical 300 mm wafer currently used today, each of the tens or hundreds of millions of vias must fully clear and each metal contact must be fully formed to prevent degradation in device performance. U.S. Pat. No. 6,642,090 to Fried et al. entitled “Fin FET Devices from Bulk Semiconductor and Method for Forming Same” defines a process for forming a FinFET device on a semiconductor wafer by forming a fin from the semiconductor wafer wherein the fin includes a fin sidewall; exposing areas of the semiconductor wafer adjacent to the fin; damaging at least a portion of the semiconductor substrate areas adjacent to the fin; and oxidizing the semiconductor substrate such that silicon dioxide is formed in the damaged portion of the semiconductor substrate to a greater thickness than is formed on the fin sidewall. However, the Fried et al. method requires careful control of a silicon etch depth in the bulk wafer, a “damaging process” in areas of silicon surrounding the fin, oxidation of the damaged areas, and a fin oxidation process to thin the fin to a required level. Each of these processes require tremendous process control and are likely non-manufacturable in a production environment.  
         [0005]     Therefore, what is needed is a method of forming a FinFET device in a bulk semiconducting material by a process that is reproducible and fully adaptable to high-volume semiconductor fabrication processes.  
       SUMMARY  
       [0006]     A field effect transistor (FET) device structure and method for forming FETs for scaled semiconductor devices is presented. Specifically, FinFET devices described are fabricated from bulk semiconductor wafers, as opposed to silicon-on-insulator (SOI) or separation by implantation of oxygen (SIMOX) wafers, in a highly uniform and reproducible manner. The method facilitates formation of FinFET devices from readily-available bulk semiconductor substrates with improved and reproducible fin height control while providing isolation between source and drain regions of the FinFET device.  
         [0007]     An exemplary FET device structure includes a fin fabricated from a bulk semiconducting material, for example, bulk silicon; a gate region comprised of a semiconducting material, with the gate region overlying a first portion of the fin and being doped with a majority carrier of a first type; a drain region formed on a second portion of the fin, the drain region being located on a first side of the gate region, the drain region being doped with a majority carrier of a second type; and a source region formed on a third portion of the fin, the source region being distal to the drain region and located on a second side of the gate region, the source region being doped with the majority carrier of the second type.  
         [0008]     An exemplary method for producing the FinFET structure includes forming a fin in a bulk semiconducting material. The bulk semiconducting material may be, for example, silicon. A first dielectric layer is formed on the silicon substrate prior to forming the fin. The first dielectric layer is comprised of a first type of dielectric material. A second dielectric layer is then formed over the substrate, also prior to forming the fin where the second dielectric layer is comprised of a second type of dielectric material. An additional dielectric material is formed on sidewalls of the fin after the fin has been formed and a space on either side of the fin is filled with a non-conducting material. An uppermost portion of the non-conducting material is etched-back such that a given height of the fin is exposed above the etched-back portion of the non-conducting material. A thin gate oxide is formed over the fin and a semiconducting gate region is subsequently formed over the thin oxide. The semiconducting gate region is doped with a first type of majority carrier. Portions of the fin not covered by the semiconducting gate region may be doped with a dopant having a second type of majority carrier. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1A-1K  show various cross-sectional stages in an exemplary process flow for producing a MOS transistor with surrounded gate. 
     
    
     DETAILED DESCRIPTION  
       [0010]     With reference to  FIG. 1A , a substrate  101 A has a thin silicon dioxide layer  103 A, a thicker silicon nitride layer  105 A, and a patterned photoresist mask layer  107 . In a specific exemplary embodiment, the substrate  101 A is be a silicon wafer. However, a skilled artisan will recognize that other semiconductor materials may be used instead of silicon for the substrate  101 A. Other semiconductor materials include, for example, elemental semiconductors such as germanium, compound semiconductors such as group III-V, and II-VI materials, and semiconducting alloys. If elemental semiconductors other than silicon, or compound semiconductors are employed, an atomic layer deposition (ALD) process may be employed for producing thin, high quality oxide layers.  
         [0011]     The silicon dioxide layer  103 A is a pad oxide to prevent thermally-induced stresses from developing between particular dissimilar materials, such as between silicon and the silicon nitride layer  105 A. The silicon dioxide layer  103 A may be thermally grown or deposited. The silicon nitride layer  105 A is then formed over the silicon dioxide layer  103 A by, for example, chemical vapor deposition (CVD). In a specific exemplary embodiment, the silicon dioxide layer  105 A is between 50 Å and 100 Å while the silicon nitride layer  105 A is between 400 Å and 1000 Å. The patterned photoresist mask layer  107  may be repeated a number of times and disposed laterally over a surface of the substrate  101 A to fabricate multiple surrounded-gate devices. For clarity, only one such device will be shown and described herein.  
         [0012]      FIG. 1B  indicates a fin area  102  being fabricated from an etched substrate  101 B. To form the fin area  102 , the photoresist mask layer  107  defines an area for which underlying areas will not be etched. These layers (i.e., the silicon nitride layer  105 A and silicon dioxide layer  103 A) are etched in accordance with methods well-known in the semiconductor arts. For example, depending upon a chemical composition of a given layer, etching may be accomplished through various wet etch (e.g., in hydrofluoric acid, such as contained in a standard buffered oxide etch, or orthophosphoric acid) or dry-etch techniques (e.g., reactive-ion etch (RIE)). Once an etched silicon nitride layer  105 B and an etched silicon dioxide layer  103 B are formed, the underlying substrate  101 A is etched, defining the etched substrate  101 B. For example, dry-etch techniques, such as a reactive ion etch (RIE), may be employed.  
         [0013]     A sidewall slope of the fin area  102  may be controlled through a choice of the chemistry used in a dry-etch recipe and/or through a choice of the substrate  101 A if a monocrystalline semiconductor is used. A skilled artisan will recognize that a sidewall of the fin area  102  may be fabricated in any appropriate crystallographic plane. The appropriate plane may be selected to enhance device characteristics such as, for example, electron mobility.  
         [0014]     After producing the fin area  102 , the photoresist mask layer  107  is removed ( FIG. 1C ) and a gate oxide layer  109  is thermally grown ( FIG. 1D ) after an appropriate pre-oxidation clean. In other exemplary embodiments, the gate oxide layer may be conformally deposited by CVD. Thinner layers of gate oxide (e.g., 10 Å to 30 Å) may be deposited by techniques such as atomic layer deposition (ALD), or other advanced technology.  
         [0015]     In  FIG. 1E , a dielectric fill layer  111 A is deposited over the gate oxide layer  109 . The dielectric fill process may be similar to shallow-trench isolation (STI) type fills known in the art, for example, a high density plasma (HDP) oxide fill. The dielectric fill layer  111 A may be comprised of any insulative material such as silicon dioxide, or any of a number of other appropriate materials deposited or otherwise formed over the gate oxide layer  109 . The dielectric fill layer  111 A may be brought to a level roughly coplanar with an uppermost part of the etched silicon nitride layer  105 B through, for example, chemical mechanical planarization (CMP).  
         [0016]     The dielectric fill layer  111 A is then etched, producing an etched dielectric fill layer  111 B ( FIG. 1F ). A high-selectivity etchant prevents the etched silicon nitride layer  105 B from being substantially etched away during the dielectric fill layer  111 A etch. Further, if the gate oxide layer  109  is comprised of thermally grown silicon dioxide, the gate oxide layer  109  will etch more slowly than an HDP layer used for the dielectric fill layer  111 A (i.e., an etch rate of thermal oxide is lower than HDP oxide as HDP is less dense). If an exposed portion of the gate oxide layer  109  (i.e., the portion above the etched dielectric fill layer  111 B) is not etched completely, it will be removed prior to a final gate oxidation step described infra.  
         [0017]     With reference to  FIG. 1G , the etched silicon nitride layer  105 B, the etched silicon dioxide layer  103 B, and exposed portions of the gate oxide layer  109  are substantially etched away using techniques known to a skilled artisan. An exposed section of the fin area  102  defines an active region of the FET device. The active region has a given height, H, length, L 1 , and width, W. In a specific exemplary embodiment, the active region height, H, is approximately 100 nm to 500 nm. The width of the active region, W, is approximately 100 nm or less and the length, L 1 , is dependent on a number of factors such as device design rules and gate widths.  
         [0018]     A thermal oxidation, ALD, or high-k oxide deposition process forms a final thin gate oxide  113 A ( FIG. 1H ) over the active region. In a specific exemplary embodiment, the final thin gate oxide is grown or deposited to a thickness of approximately 10 Å to 30 Å. A polysilicon layer  115 A is then conformally deposited ( FIG. 1H ). The polysilicon layer  115 A will form a gate region, described infra. A patterned second photoresist layer  117  is formed and patterned to define the gate; the gate having a width commensurate with a length, L 2 , of the patterned second photoresist layer  117 .  FIG. 1I  is a plan view of the FET device and thus provides clarity in understanding a layout of the device after the second photoresist layer  117  is added. The hidden section is an uppermost portion of the channel active region of  FIG. 1G .  
         [0019]     With reference to  FIG. 1J , a selective etch process (either wet-etch or dry-etch) is used to fully define a gate region  115 B of the device. Generally, a high selectivity to either silicon or silicon dioxide is accomplished using an appropriate chemistry. Doped areas are added (e.g., by diffusion or implantation) to define a drain contact region  119  and a source contact region  121  (or LDD structure) of the device by well-established practice. The patterned second photoresist layer  117  is then removed.  FIG. 1K  provides a plan view-showing source, gate, and drain contact areas of a completed FinFET device fabricated according to exemplary embodiments described herein. Subsequent steps for completion of the FinFET device follow conventional CMOS fabrication processes.  
         [0020]     In the foregoing specification, the present invention has been described with reference to specific embodiments thereof. It will, however, be evident to a skilled artisan that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, skilled artisans will appreciate that other types of semiconducting and insulating materials other than those listed may be employed. Additional particular process fabrication and deposition techniques, such as low pressure chemical vapor deposition (LPCVD), ultra-high vacuum CVD (UHCVD), and low pressure tetra-ethoxysilane (LPTEOS) may be readily employed for various layers and still be within the scope of the present invention. Although the exemplary embodiments describe a second gate oxide being formed, a single gate oxide may also be used provided that subsequent process steps do not etch through the gate oxide. The substrate may also be comprised of a non-semiconducting material, for example, a quartz pellicle with a deposited and doped polysilicon layer. (In this embodiment, the doped polysilicon layer becomes the bulk semiconductor.) Additionally, although the exemplary embodiments are described in terms of MOS integrated circuit devices, a person of ordinary skill in the art will recognize that other fabrication techniques, such as bipolar or BiCMOS techniques, may readily be employed as well. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.