Abstract:
A process of producing controllable thicknesses of silicon-on-insulator (SOI) for fully-depleted double-gate applications is provided. The process comprises depositing an oxide layer on a silicon wafer, depositing a nitride layer of a controlled thickness on the oxide layer, etching the nitride layer to open a first trench of controlled thickness, opening a second trench down to the silicon substrate, growing epitaxial silicon using epitaxial lateral overgrowth (ELO) to fill the second trench and grow sideways to fill the first trench, perform planarization of ELO silicon using the nitride layer as a chemical-mechanical polishing (CMP) stop layer, and fabricating a SOI device in the first trench.

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor process and, more particularly, relates to a silicon-on-insulator device and a process of producing controllable thickness for a silicon layer of a silicon-on-insulator (SOI) device. 
     BACKGROUND 
     Silicon-on-insulator (SOI) substrates have recently become desirable for many technologies, including extreme scaling of metal-oxide semiconductor (MOS) and complementary metal-oxide semiconductor (CMOS) devices, advanced MOS junction-type field-effect transistors (MOSFETs), and quantum wires and dots. This is primarily because SOI fabrication processes result in increased packing densities, improved performances, better device isolations and reduced extrinsic parasitic elements, particularly those of the source and drain as well as leakage currents and thus significantly speeding up circuit operations. 
     As the name implies, SOI substrates generally comprise a thin layer of silicon on top of an insulator, wherein circuit components are formed in and on the thin layer of silicon. The insulator can be silicon oxide (SO 2 ), sapphire, or any appropriate material. For example, a sapphire substrate may be used as an insulator for target radio-frequency (RF) applications. In contrast, a bulk silicon wafer with an oxide layer as the substrate may be used as an insulator for target digital logic applications. In both cases, the insulator may serve to reduce junction capacitance between the heavily-doped devices and the lightly-doped bulk substrate which may translate to less power consumption and greater circuit speed. 
     SOI substrate may be fully-deleted in which a depletion region of an entire channel is fully active, or may be partially-depleted in which the depletion region of the channel is not fully active. In both partially or fully depleted devices, the silicon layer may be thinner than the depletion region. However, fully-depleted SOI substrates have rarely been used in the fabrication of SOI MOSFETs, for example. This is because the uniform thickness control of the silicon layer of a SOI substrate on which the channel is formed for fully-depleted SOI substrates has been exceedingly difficult. Variation in the thickness of the silicon layer can severely impact the threshold voltage of each transistor on the SOI substrate. Consequently, partially-depleted (PD) SOI substrates have widely been used instead. 
     Currently, there are several techniques available for the fabrication of SOI substrates. An established technique for fabricating SOI substrates is known as “separation by implantation of oxygen” (SIMOX), where oxygen is implanted below the silicon surface and the substrate is annealed to provide a buried silicon oxide layer with a silicon overlayer. The implantation time can be intensive and cost prohibitive. Moreover, the SOI substrate may be exposed to high surface damage and contamination. A second technique is known as “bond-and-etch-back” SOI (BESOI), where an oxidized wafer is first diffusion-bonded to an unoxidized wafer, and the backside of the oxidized wafer is then grinded, polished, and etched to the desired device layer. The BESOI approach may be free from the implant damage inherent in the SIMOX approach. However, time consuming sequence of grinding, polishing, and etching may be required. Another technique is known as the hydrogen implantation and separation approach (also called Smart-Cut®), where hydrogen is implanted into silicon with a thermally grown oxide to form embrittlement of the silicon substrate underneath the oxide layer. The implanted wafer may then be bonded with another silicon wafer with an oxide overlayer. The bonded wafer may be “cut” across the wafer at the peak location of the hydrogen implant by appropriate annealing. These fabrication techniques may not be suitable for fabricating fully-depleted SOI substrates, since the uniform thickness of the silicon layer of a SOI substrate may be difficult to achieve. 
     More complex techniques for fabricating filly-depleted SOI devices include the selective epitaxial growth (SEG) and epitaxial lateral overgrowth (ELO) technique as described in “ SOI - MOSFET Structures Using Silicon Selective  ( SEG )  and Epitaxial Lateral Overgrowth  ( ELO )” by Gerold W. Neudeck, submitted to Semiconductor Research Corporation, May 1997, and the combination of BESOI and Smart-Cut® as described in “Ultra-Cut: A Simple Technique For The Fabrication Of SOI Substrates With Ultra-Thin (&lt;5 nm) Silicon Films” by K. D. Hobart et al., Naval Research Laboratory, published by the IEEE International SOI Conference Proceedings, October 1998. However, none of these techniques appears to be simple, cost-effective, and efficient for fabricating fully-depleted SOI devices. Variation in the uniform thickness of the silicon layer may still be unacceptable and can still impact the threshold voltage of each transistor (memory cell) on the SOI substrate. Accordingly, there is a need for a simple approach to producing highly controlled thicknesses for the silicon layer of SOI substrates for fully-depleted applications. 
     SUMMARY 
     Accordingly, various embodiments of the present invention are directed to a process of fabricating a silicon-on-insulator (SOI) substrate. Such a process comprises forming a dielectric layer on a surface of a semiconductor wafer; forming a barrier layer having a hardness substantially greater than the dielectric layer, on the dielectric layer; forming a first trench through a portion of the barrier layer; forming a second trench through the barrier layer and the dielectric layer to expose a portion of the semiconductor wafer; growing a silicon layer from the exposed portion of the semiconductor wafer to fill the second trench and the first trench; and planarizing the silicon layer using the barrier layer as a polish-stop layer to isolate the silicon within the first trench from the silicon in the second trench. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of exemplary embodiments of the present invention, and many of the attendant advantages of the present invention, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 illustrates an example silicon-on-insulator (SOI) substrate; 
     FIG. 2 illustrates a separation by implantation of oxygen (SIMOX) substrate manufacturing process of fabricating a SOI substrate; 
     FIGS. 3A and 3B illustrate a bond-and-etch-back SOI (BESOI) substrate manufacturing process of fabricating a SOI substrate; 
     FIGS. 4A-4C illustrate a hydrogen implantation and separation (Smart-Cut®) substrate manufacturing process of fabricating a SOI substrate; 
     FIGS. 5A-5E illustrate a selective epitaxial growth (SEG) and epitaxial lateral overgrowth (ELO) substrate manufacturing process of fabricating a SOI substrate; 
     FIGS. 6A-6G illustrate a process of producing controllable thickness of silicon-on-insulator for fully-depleted double-gate applications according to the principles of the present invention; and 
     FIG. 7 illustrates variations of a film thickness as a function of deposited nitride thickness according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is applicable for use with all types of semiconductor substrates and silicon-on-insulator (SOI) devices, including, for example, MOS transistors, CMOS devices, dual-gate MOSFETs, and new memory devices which may become available as semiconductor technology develops in the future. However, for the sake of simplicity, discussions will concentrate mainly on exemplary use of a simple silicon-on-insulator (SOI) substrate, although the scope of the present invention is not limited thereto. 
     Attention now is directed to the drawings and particularly to FIG. 1, an example silicon-on-insulator (SOI) substrate  10  is illustrated. As shown in FIG. 1, the SOI substrate  10  may comprise a semiconductor wafer  12 , a dielectric layer  14  (such as SiO 2  and the like) formed on the main surface of the semiconductor substrate  12  to reduce capacitance, a silicon layer  16  (known as a SOI layer) having a desired thickness formed on the dielectric layer  14 . The semiconductor wafer  12  can be silicon, sapphire, or any appropriate material. Formed on the silicon layer  16  is a SOI device comprised of source/drain regions  16 A, a channel region  16 B and a gate electrode  18 . The SOI device may represent, for example, an NMOS transistor or a pMOS transistor in which the dopant impurity regions  16 A may be heavily doped with high concentration of either n-type of impurity or p-type of impurity. High concentration impurity ions may be implanted in the silicon layer  16  using a mask to form the dopant impurity regions  16 A. In either case, the dielectric layer  14  may serve to reduce junction capacitance between the heavily-doped SOI device and the non-doped or lightly-doped silicon wafer  12  in order to reduce power consumption and obtain greater circuit speed. 
     As shown in FIG. 1, the SOI substrate may be fully-deleted or partially-depleted. However, for reliable non-varied operation, the silicon layer  16  must be thinner than the depletion region, and the thickness of the silicon layer  16  on which the channel is formed must be uniformly controlled. Otherwise, the threshold voltage of the SOI device may be varied and as a result, the performance of the SOI device may be degraded. The characteristic of the threshold voltage of the SOI device, such as a MOS transistor or MOSFET, may be described in detail hereinbelow. 
     Generally the threshold voltage (Vth) of the SOI device can be expressed as follows: 
     
       
           V   th   =V   FB   +Q   B   /C   OX ,  
       
     
     where V th  represents a threshold voltage, V FB  represents a flat band voltage, Q B  represents bulk charge, and C OX  represents capacitance of the dielectric layer  14 . 
     Referring to the above equation, the amount of charge in the channel may vary depending on the thickness of the silicon layer  16 . For example, the threshold voltage of the SOI device decreases as the thickness of the silicon layer is reduced. Accordingly, a variation in thickness of the silicon layer directly influences the threshold voltage of the SOI device. Consequently, the thickness of the silicon layer  16  of a SOI substrate  10  must be uniformly and accurately controlled during its formation (e.g., from SOI device to SOI device across a chip) to avoid variations in the threshold voltage of the SOI devices. 
     The SOI substrate  10  of FIG. 1 may be fabricated by several different techniques, including separation by implantation of oxygen (SIMOX), bonding-and-etch-back SOI (BESOI), hydrogen implantation and separation (also called Smart-Cut®), and selective epitaxial growth (SEG) and epitaxial lateral overgrowth (ELO). However, none of these techniques may be simple, cost-effective, and efficient for fabricating fully-depleted SOI devices while uniformly and accurately controlling the thickness of the silicon layer  16 . 
     For example, FIG. 2 illustrates a separation by implantation of oxygen (SIMOX) substrate manufacturing process of fabricating a SOI substrate  10 ′. As shown in FIG. 2, high-dose oxygen ions  20  may be implanted into the single-crystal silicon wafer  12 ′ and a high temperature anneal processing may be used to cause a portion of the silicon atoms within the silicon wafer  12 ′ and the implanted oxygen ions  20  to react, so that a buried oxide layer  14 ′ is formed in the silicon wafer  12 ′ with a silicon overlayer  16 ′. For example, for high-dose oxygen implantation, an implantation energy of 150-200 KeV, an ion dose of approximately 2×10 18 /cm 2  with substrate temperature greater than 600° C. may be used. The high dose oxygen implantation may then be followed by high annealing temperature of greater than 1300° C. for at least 8 hours. For low dose oxygen implantation, lower dose of oxygen of approximately 4×10 17 /cm 2  and annealing atmosphere of inert gas such as argon (Ar) and oxygen (O 2 ) may be used. The temperature and oxidation time period may be increased or decreased in proportion to the thickness of the buried oxide layer. However, the SIMOX approach can lead to thickness non-uniformity, and can also be cost prohibitive. Moreover, the SOI substrate  10 ′ may be exposed to high surface damage and contamination. 
     FIGS. 3A and 3B illustrate a bond-and-etch-back SOI (BESOI) substrate manufacturing process of fabricating a SOI substrate. As shown in FIG. 3A, two separate silicon substrates (wafers) A and B may be used for diffusion bonding and then grinded, polished and etched to the desired silicon layer. For example, the surface of the second silicon substrate B may be oxidized to form an oxide layer  14 ′. Oxide may be formed by thermal oxidation or chemical vapor deposition (CVD). The oxidized silicon substrate B may then be diffusion-bonded to an unoxidized silicon substrate A at the oxidized surface. After the oxidized, second silicon substrate B is bonded to the unoxidized, first silicon substrate A, and the backside of the oxidized substrate B may then be grinded, polished, and etched to the desired silicon layer  16 ′ as shown in FIG.  3 B. BESOI substrates may avoid the implant damage inherent in the SIMOX approach. However, the BESOI approach may be time consuming since a laborous sequence of grinding, polishing, and etching is required. In addition, substantial silicon may be wasted. Moreover, uniform thickness of both the silicon layer  16 ′ and oxide layer  14 ′ may be difficult to achieve. 
     FIGS. 4A-4C illustrate a hydrogen implantation and separation (Smart-Cut®) substrate manufacturing process of fabricating a SOI substrate  10 ′. As shown in FIG. 4A, heavy dose of hydrogen ions  22  may be implanted into the silicon wafer  12 ′ with a thermally grown oxide to form embrittlement  24  in the silicon above the oxide layer  14 ′. The implanted wafer may then be bonded with another silicon wafer with an oxide layer  14 ′ as shown in FIG.  4 B. The bonded wafer may be “cut” across the wafer at the peak location of the hydrogen implant by appropriate annealing, as shown in FIG. 4C, to form the silicon layer  16 ′. The Smart-Cut® approach may not be suitable for fabricating fully-depleted SOI substrates, however, since the uniform thickness of the silicon layer  16 ′ of the SOI substrate  10 ′ may still be difficult to obtain. 
     FIGS. 5A-5E illustrate a selective epitaxial growth (SEG) and epitaxial lateral overgrowth (ELO) substrate manufacturing process of fabricating a SOI substrate  10 ′. As shown in FIG. 5A, an oxide layer  14 ′ may first be formed on the surface of a silicon wafer  12 ′ by way of thermal oxidation or chemical vapor deposition (CVD). The oxide layer  14 ′ may be etched using an etch mask for exposing a portion of the silicon wafer  12 ′ corresponding to a seed hole  26  as shown in FIG.  5 B. Next, the oxide layer  14 ′ may be re-etched using another etch mask for forming a recess (oxide trench)  28  in a portion of the silicon wafer  12 ′ corresponding to a SOI device region to be formed as shown in FIG.  5 C. Thereafter, epitaxial lateral overgrowth (ELO) silicon  32  may then be grown out of the seed hole  26  and over and down into the recess  28  as shown in FIG.  5 D. Finally, the excess of silicon ELO may be polished away (i.e., removed), using the oxide layer  14 ′ as a polish stop, to isolate a silicon layer  16 ′ from the silicon residing in the seed hole  26  as shown in FIG.  5 E. The thickness of the silicon layer  16 ′ may depend on the etching of the oxide layer  14 ′. As a result, variation in the depth of etching may still impact the thickness of the silicon layer  16 ′ of a SOI substrate  10 ′. More particularly, since an oxide layer  14 ′ is not a particularly good polish stop material, depth variations may occur between silicon layers  16 ′ on a single SOI substrate  10 ′ and/or between silicon layer  16 ′ on separate SOI substrates  10 ′ (resulting in inconsistent products). 
     A more complex approach to fabricating a SOI substrate  10 ′ may be a combination of the BESOI technique shown in FIGS. 3A-3B and the Smart-Cut® technique shown in FIGS. 4A-4C as described in “ Ultra - Cut: A Simple Technique For The Fabrication Of SOI Substrates With Ultra - Thin  (&lt;5 nm) Silicon Films” by K. D. Hobart et al. Using this complex approach, the thickness of the silicon layer may be controlled by depositing SiGe epitaxial layer on a silicon wafer, growing highly controllable silicon layer on top of the SiGe epitaxial layer, implanting hydrogen into the SiGe epitaxial layer, transferring the delaminated layer of silicon and SiGe onto an oxidized silicon wafer, and etching the exposed SiGe to leave behind the highly controlled silicon layer adhered to the oxide. 
     Turning now to FIGS. 6A-6F, a simplified approach to producing highly controlled thicknesses of SOI for fully-depleted applications according to an embodiment of the present invention is illustrated. In contrast to the SEG-ELO approach of FIGS. 5A-5E which uses oxide as a polish-stop layer, the inventive approach allows for the fabrication of highly controlled SOI substrate  100  by taking advantage of the thickness control of a deposited nitride layer, and the hardness qualities of a barrier material such as nitride as a chemically-mechanically-polishing (CMP) stop layer. As intended by the present invention, a thin and highly controllable layer of nitride (such as silicon nitride layer) may be concomitantly used both to accurately define the thickness of the thin silicon (SOI) layer, as well as to act as a polish-stop layer. This is primarily because nitride has a hardness that is greater than the hardness of the semiconductor (oxide) layer, control of film thicknesses during nitride deposition have been measured and successfully tested for thin films of 300 and 500 Angstroms which exhibit typical 3-σ uniformity better than 1%. 
     As shown in FIG. 6A, an oxide layer  140  may be formed on the surface of a silicon wafer  120 . The oxide layer  140  may have a thickness of, for example, between 500 and 10,000 Angstroms, and may be formed by thermal oxidation, or chemical vapor deposition (CVD). CVD may be used to form an oxide layer  140  from borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), or borophospho tetraethylorthosilicate (BPTEOS). A nitride layer  160  may then be formed on the oxide layer  140  by, for example, chemical vapor deposition (CVD), to a thickness of between about 100 to 1000 Angstroms, and preferably between 300 to 500 Angstroms. The selectivity of nitride to oxide may be 9:1. As intended by the present invention, the nitride layer  160  may act as a polish-stop layer and determine the eventual thickness of the silicon layer (SOI island) to be formed. 
     Photo-resist (not shown) may then deposited over the nitride layer  160  and may be patterned by conventional lithography and etching using an etch mask for exposing portions of the silicon wafer  120  corresponding S 01  device regions. The nitride layer  160  may be selectively etched, using the underlying oxide layer  140  as an etch stop, to form a series of first (SOI island) trenches  150  as shown in FIG.  6 B. These SOI island trenches  150  may be used for epitaxial lateral overgrowth (ELO) of silicon (Si) and determine the eventual thickness of the SOI device region. The thickness of these trenches may be controlled directly by the controlling the thickness of the nitride layer  140 . For example, if the nitride layer  140  exhibits a thickness of 500 Angstroms, then the depth of the SOI island trenches  150  defined by the etch mask also exhibits the same thickness of 500 Angstroms. Selected portions of the nitride layer  140  may be removed by wet or dry etching. In particular, the nitride layer  140  may be dry etched by gases of either chlorine (Cl 2 ) or helium (He) in a chemical vapor deposition (CVD) machine. However, other etching techniques may also be used for etching the nitride layer  140 . For example, nitride layer  140  may also be removed by etching with phosphoric acid (H 3 PO 4 ). Alternatively, the nitride layer  140  may be removed by reactive ion etching (RIE) process employing a fluoride etching gas. 
     Next, the nitride layer  140  may further be etched, using the underlying silicon layer  120  as an etch stop, to form second (seed) trenches  170 , as shown in FIG.  6 C. These seed trenches  170  serve as the seed layer for epitaxial lateral overgrowth (ELO) of silicon Si. The seed trenches  170  may be wet etched or etched by the RIE process. The RIE process may consist of a 200 W, 80 mTorr, CHF 3  plasma etch. 
     Referring to FIGS. 6D-6E, the epitaxial lateral overgrowth (ELO) of silicon  155  may be deposited into the seed trenches  170 , and grown over the SOI island trenches  150  using a standard commercial RF heated low-pressure chemical vapor deposition (LPCVD) reactor at, for example, 970° C. at 40 Torr with dichlorosilane, hydrogen, and HCl. 
     The ELO-grown Si layer may now be removed, preferably be chemically/mechanically polished back, using the nitride layer  160  as an effective polish-stop layer, to form the SOI island regions  180  and provide device isolation between the SOI island regions  180 . The chemicalmechanical polishing (CMP) machine may be a dual platen (polish platen and buff platen) machine. The epitaxial overgrowth required to be polished may be approximately 0.3 microns. A normal polish rate of silicon of 1 micron/minute may be used. Since nitride is much harder than silicon, the thickness (depth) control of the SOI island regions  180  may be completely determined by the thickness control of the originally-deposited nitride layer  160 . As a result, the thickness of the SOI island regions  180  may be uniformly and accurately controlled by using the nitride layer  160  as a CMP polish-stop layer. 
     After the SOI island regions  180  may be formed, SOI devices may be now fabricated in the SOI island regions  180  as shown in FIG.  6 G. 
     FIG. 7 illustrates variations of a film thickness as a function of deposited nitride thickness according to the principles of the present invention. Control of film thicknesses during nitride deposition have been measured, and successfully tested as shown in FIG. 7 for thin films of nitride having a thickness of approximately 300 to 500 Angstroms which exhibit typical 3-Sigma (σ) uniformity better than 1%. 
     As described from the foregoing, the present invention provides a simplified, cost-effective approach for fabricating fully-depleted SOI devices while uniformly and accurately controlling the thickness of the silicon (SOI) layer by using a thin and highly controllable layer of nitride both to accurately define the thickness of the silicon (SOI) layer, as well as to act as a polish-stop layer. 
     While there have been illustrated and described what are considered to be exemplary embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. For example, other types of semiconductor materials may be used in lieu of nitride to define the thickness of the silicon (SOI) layer, and to act as a polish-stop layer as long as such semiconductor materials exhibit a hardness that is substantially greater than the hardness of the semiconductor (oxide) layer. Many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. Therefore, it is intended that the present invention not be limited to the various exemplary embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims.