Self-aligned under-gated thin film transistor and method of formation
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor processing, and more particularly, to thin film transistor formation. 
     BACKGROUND OF THE INVENTION 
     The integrated circuit industry has generally accepted the fact that thin film transistors (TFTs) are necessary for use in some applications. A portion of the integrated circuit industry that requires TFTs for future family generations is a market involving memory products such as fast static random access memories (FSRAMs). Research conducted on TFTs has lead to FSRAM circuits that have: (1) less array current leakage and less parasitic stand-by current; (2) less occurrences of leakage-current-induced soft errors; (3) improved resistance to soft errors and increased cell capacitance; and (4) a higher logic &#34;on&#34; voltage. 
     Although over-gated or top-gated thin film transistors have been researched and in some cases designed into FSRAM load transistor applications, under-gated or bottom-gated transistors are more popular due to FSRAM bit cell area packing concerns. An over-gated transistor is a transistor that has a source, a drain, and a channel region that are formed underlying a gate region. An under-gated transistor is a transistor that has a source, a drain, and a channel region that are formed overlying a gate region. 
     The use of a conventional under-gated TFT has several disadvantages: (1) the source and drain implants are not aligned to the TFT gate; therefore, the TFT is subjected to etch and photolithographic variation as well as substantial device performance alteration due to alignment-related variation; (2) drain offsets are lithographically defined and therefore consistent current leakage limiting is not achieved; and (3) many TFTs are performance limited by short channel behavior, such as an increase in charge carrier punch through as a TFT gate length is decreased. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are fulfilled and other advantages achieved with the present invention. In one form, the present invention comprises a process for fabricating a semiconductor device wherein an insulated substrate is provided. The insulated substrate has a surface with a stacked structure. The stacked structure includes a gate electrode overlying the substrate surface, a dielectric layer overlying the gate electrode, and a sacrificial material layer overlying the dielectric layer. A substantially planar layer is formed on the substrate surface that exposes a portion of the sacrificial material layer. The sacrificial material layer is removed to expose the dielectric layer and form an opening in the substantially planar layer. The opening in the substantially planar layer having a sidewall. A conductive layer is formed overlying the dielectric layer, overlying the substantially planar layer, and lying adjacent to the sidewall. A planarizing layer is formed overlying the conductive layer. A portion of the planarizing layer is removed to form a substantially planar surface and expose portions of the conductive layer. The exposed portions of the conductive layer are doped to form source and drain current electrode regions. A portion of the conductive layer is selectively removed to form a semiconductor device. 
     The present invention will be understood by one skilled in the art from the detailed description below in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1I illustrate, in cross-sectional form, a semiconductor device fabrication process for forming a self-aligned, under-gated thin film transistor in accordance with the present invention; 
     FIGS. 2A-2C illustrate, in cross-sectional form, alternative steps of a portion of the semiconductor device fabrication process of FIG. 1; and 
     FIG. 3 illustrates, in cross-sectional form, another alternative embodiment in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrated in FIG. 1 is a semiconductor device fabrication process for forming a self-aligned, under-gated thin film transistor in accordance with the present invention. FIG. 1A illustrates two locations for under-gated thin film transistor (TFT) formations. The two TFTs are referred to as devices 10 and 11 throughout the process flow of FIG. 1. Two devices are presented to illustrate how devices can be positioned close to each other for memory applications. Using the process of FIG. 1, it is possible to form a single isolated TFT or simultaneously form any number of TFTs. Because of this, two devices 10 and 11, are depicted, but only one device may be explained and described in detail. It should be noted that if a process step or formation is mentioned and described for device 10, an analogous step or formation applies to device 11 as well. Also, regions and layers of device 10 and device 11 that serve similar functions are identically labeled. Devices 10 and 11 can have an underlaying layer 12, which is an optional layer. Underlying layer 12 can be an insulated substrate material, a substrate material, an underlying layer of conductive material such as polysilicon, epitaxial silicon and the like, a dielectric material such as silicon dioxide(SiO 2 ), tetra-ethyl-ortho-silicate (TEOS), or boro-phosphate-silicate-glass (BPSG), or any like semiconductive layer of material. Underlying layer 12 is used to illustrate the use of TFT devices 10 and 11 as stacked, substrate-overlying silicon on insulator (SOI) devices or polysilicon on insulator devices. A base layer material, referred to as a base layer 14, is formed overlying the underlying layer 12. If the underlying layer 12 is not present then base layer 14 is most likely an insulated substrate material. The base layer 14, for most applications, is an isolation dielectric layer but can be a substrate or diffused region for some applications. 
     A first conductive layer 16 is deposited overlying the base layer 14. Conductive layer 16 is usually made of polysilicon and forms a gate electrode. A first dielectric layer 18 is deposited overlying the conductive layer 16. A sacrificial material layer, referred to as a layer of sacrificial material 20, is formed overlying the dielectric layer 18. The layer of sacrificial material 20 can be made of any material that can be etched selectively to dielectric layer 18. Because of the selective etch criterion and due to the fact that dielectric layer 18 is in most cases an oxide, the layer of sacrificial material 20 is usually polysilicon, nitride, or a like material. 
     A gate-defining etch step is illustrated in FIG. 1B. For device 10, portions of the layer of sacrificial material 20, the dielectric layer 18, and the conductive layer 16 are etched away to form a &#34;pillar-like&#34; stacked structure. The &#34;pillar-like&#34; stacked structure is formed by a remaining region of the layer of sacrificial material 20, a remaining region of the dielectric layer 18, and a remaining region of the conductive layer 16. The remaining region of the layer of sacrificial material 20, the remaining region of the dielectric layer 18, and the remaining region of the conductive layer 16 are hereafter referred to respectively as conductive region 16, dielectric region 18 and sacrificial region 20. The conductive region 16 forms a gate or a control electrode for device 10. 
     FIG. 1C illustrates a second dielectric layer 22 that is formed overlying the conductive region 16, the dielectric region 18, and the sacrificial region 20. Unless gate doping alteration, BPSG reflow, or other doped oxide properties are desired, the dielectric layer 22 is an undoped dielectric to ensure that the gate doping of the conductive region 16 is not altered. A planar layer of material, referred to as planar layer 24, is formed overlying the dielectric region 20. Planar layer 24 can be formed from photoresist, a spin on glass (SOG) material, an oxide deposition-reflow step, or the like. A substantially planar layer of material for planar layer 24 is beneficial for purposes of a uniform subsequent etch-back step. 
     FIG. 1D illustrates the uniform subsequent etch-back step mentioned above. The planar layer 24 and the underlying dielectric layer 22 are plasma etched or removed in a chemical and/or mechanical manner. The chemical and/or mechanical etch/removal procedure must ensure that the planar layer 24 and the dielectric layer 22 are removed at a same relative etch or removal rate. The same relative etch rate will result in a remaining substantially planar layer or surface of the second dielectric, referred to as dielectric layer 22, and the entire removal of planar layer 24. This etch step exposes a top portion of the sacrificial region 20. If the etch rate mentioned above is not the same with respect to planar layer 24 and to dielectric layer 22, a substantially planar surface will be difficult to achieve. 
     The planarization and etch described in FIG. 1D can be performed in many ways. For example, an alternative way to forming the substantially planar dielectric layer 22 that exposes a top portion of the sacrificial region 20 is to deposit BPSG and run a reflow step at least once. Any method of planarizing an overlying surface, etching a planar overlying surface, and exposing sacrificial layer 20 will work. It is important to note that this chemical or mechanical etch-back process must expose a top portion of the sacrificial region 20 while not exposing other regions. The exposure of other regions could result in circuit failure due to electrical shorts, processing related device damage, or other complications. 
     FIG. 1E illustrates a sacrificial region 20 removal step. The sacrificial region 20 is removed forming an opening in the substantially planar layer of material formed by the second dielectric layer 22. The opening in the dielectric layer 22 has a sidewall formed by a sidewall surface of the dielectric layer 22. At this point, the dielectric region 18 can function as a gate dielectric region for the device 10. An alternative to the dielectric region 18 functioning as a gate dielectric is illustrated in FIG. 1F. After the etch of the sacrificial region 20, the dielectric region 18 is also etched away. Following the dielectric layer 18 etch, a gate dielectric 26 is deposited or grown overlying the conductive region 16. The gate dielectric 26 is an oxide or oxide-nitride-oxide (ONO) and is usually very thin. 
     A second conductive layer 28 is deposited overlying the substantially planar surface of the dielectric layer 22 and the gate dielectric 26 as illustrated in FIG. 1G. A planarizing layer is formed overlying the conductive layer 28. The planarizing layer consists of at least one layer of material. The planarizing layer can be formed by a combination of dielectric layers, conductive layers, and like layers of material. In FIG. 1G, a dielectric layer 30 is deposited to form a bottom portion of the planarizing layer. The dielectric layer 30 is used as an etch stop for subsequent etch processing. 
     A top portion of the planarizing layer is formed by a layer 32. Layer 32 can be a polysilicon layer, TEOS, BPSG, nitride, an SOG layer, or a like layer. The planarizing layer is planarized via reflow, SOG, photoresist processing, etch technology, or the like. The planarizing layer is then etched back using selective etch technology and etch stops, such as dielectric layer 30, to form a substantially planar surface which includes an exposed portion or implantation accessible portion of the conductive layer 28 and a plug region 32 as illustrated in FIG. 1H. If the plug region 32, which is a portion of the planarizing layer, is made up of polysilicon and is isolated electrically from conductive region 28, then the device 10 can have several added advantages. Polysilicon is easy to work with mechanically, has very selective etch properties, and a polysilicon plug region 32 can be used to double-gate the device 10 to offer improved device 10 performance. It is important to note that several layers can be used to form the planarizing layer. A nitride deposition, followed by a TEOS film, followed by polysilicon deposition could produce a planarizing layer and form a substantially planar surface with a plug 32. 
     FIG. 1H also illustrates a diffusion, implant, knock-in, autodoping, or like step that forms a doped first current region or source region 33 of device 10 and a doped second current electrode or drain region 34 of device 10. In a simultaneous similar manner, a source region 36 and a drain region 38 is formed for device 11. These source and drain regions are formed self-aligned to the gate conductive region 16. An undoped region of conductive layer 28 is formed by the doping of source regions 33 and 36 and drain regions 34 and 38. The undoped region that lies directly above the gate conductive region 16 is a channel region of the device 10. Regions along the sidewall of the dielectric layer 22 between the dielectric layer 22 and the plug region 32 on both sides of the channel region are considered as offset regions 35 and 37 in FIG. 1I. Offset regions are used to help reduce TFT off-state leakage currents and in theory can improve upon or reduce known short channel effects. 
     After formation of the doped and undoped regions described above, portions of the conductive layer 28, and other layer portions are selectively etched away to electrically isolate the source region 33, the drain region 34, and the channel region (not labeled but defined above) of device 10 from device 11, as illustrated in FIG. 1I. 
     FIGS. 2A-2C illustrate a device 10&#39; and a device 11&#39;. Both device 10&#39; and 11&#39; illustrate, in more detail, the formation of a planarizing layer having multiple layers of material. Due to the fact that the process flow of FIGS. 2A-2C is similar to the process flow of FIGS. 1G-1I, the description of FIG. 2 will be brief, and some formations and layers will be identically or similarly labeled. In FIG. 2A a dielectric layer 30 is formed, which in most cases is nitride. A dielectric layer 40 is deposited overlying the dielectric layer 30. Dielectric layer 40 is a TEOS formed layer for example. In FIG. 2B, a final layer 42 is formed, and the same processing used to planarize the planarizing layer for FIG. 1 is used here to form a plug region 42. Plug region 42 is made up of polysilicon or BPSG in most cases. An implant is performed which forms source regions 33 and 36 and drain regions 34 and 38 in a manner similar to FIG. 1H. Isolation etching is performed as illustrated in FIG. 2C to isolate device 10 from device 11. 
     In FIG. 3 an alternative embodiment of FIG. 1I is presented to further illustrate two additional features. The first distinct feature is that a spacer 44 could be formed on the sidewall of the opening in the conductive layer 22, which in this example is made up of a doped region 22&#39; and an undoped region 22&#34;. The opening in the conductive layer 22 was formed in FIG. 1E when the sacrificial region was removed. The formation of the spacer 44 allows the devices 13 and 15, which respectfully are similar to devices 11 and 10, to be more planar, to be easier to etch using conventional etch technology, to be more robust by reducing the known increase in electric fields at sharp edges that cause electrical breakdown and other known electric field related problems. 
     A second feature presented in FIG. 3 is the use of the doped region 22&#39; and the undoped region 22&#34; to form dielectric region 22. The doped region 22&#39; can be made of boro-silicate-glass (BSG) and the undoped oxide 22&#34; can be TEOS formed. The undoped oxide 22&#34; can be a layer of material as illustrated in FIG. 3 or a spacer adjacent the conductive region 16. The function of the undoped oxide 22&#34; is to protect the gate conductive region 16 from the dopant impurities of the doped region 22&#39;. Once undoped region 22&#34; is formed, the doped region 22&#39; is formed and etched similar to the processing of FIGS. 1C-1D. The formation of the source regions 33 and 36 and the drain regions 34 and 38 can performed in a manner different to the preferred method of ion implantation, which was introduced for FIG. 1H and FIG. 2B. The difference is that instead of implanting, which is the desired method for many applications, the source regions 33 and 36 and the drain regions 34 and 38 can be created by heating devices 13 and 15 to thermally drive dopant impurities from the doped region 22&#39; into adjacent portions of the conductive layer 28, therefore forming the source regions 33 and 36 and the drain regions 34 and 38 of FIG. 3. 
     In conclusion, devices 10 and 11, devices 10&#39; and 11&#39;, and devices 13 and 15 are TFT devices that allow the following TFT circuit and design advantages: (1) Source and drain regions that are self-aligned to a gate region for improved circuit density, improved device performance, and reduced lithographic and etch alignment and resolution dependance; (2) improved short channel behavior, such as reducing charge carrier punch through; (3) consistent drain and source offsets to provide consistent current leakage limiting; and (4) a more controllable device series resistance. The inventive devices presented herein, while having the above added advantages, also have several conventional TFT advantages. The conventional TFT advantages are: (1) less array current leakage and less parasitic stand-by current; (2) less occurrences of leakage-current-induced soft errors; (3) increased cell capacitance; and (4) a higher logic &#34;on&#34; voltage. 
     While the present invention has been illustrated and described with reference to specific embodiments, further modifications and improvements will occur to those skilled in the art. For example, there are many ways in which a planarizing layer or a plug region can be formed. There are also many different layers of material that could be added or changed in the above descriptions and still serve the same basic function. Technology like multiple spacers, dual-gated concepts, and other known processing steps can be added to an under-gated self-aligned TFT without substantially altering the functionality or design of the inventive device presented herein. It is to be understood, therefore, that this invention is not limited to the particular forms illustrated and that it is intended in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.