Abstract:
A method for forming a MOSFET transistor using a disposable gate process which has no need for a chemical mechanical polishing step to expose the disposable gate after deposition of the field dielectric. The field dielectric is deposited non-conformally by HDP-CVD over a disposable gate structure so that the disposable gate remains partially exposed. After deposition, the partially exposed disposable gate may then be removed by selective isotropic etch. In the space left by the removal of the disposable gate, the gate dielectric may be formed and the gate electrode may be deposited. Eliminating the need for exposure of the disposable gate by CMP eliminates the problem of polish rate dependence on gate pattern density.

Description:
This application claims priority under 35 USC § 119 (e) (1) of provisional application No. 60/054,299, filed Jul. 31, 1997. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to integrated circuit structures and fabrication methods, and more specifically to forming an integrated circuit structure using a disposable gate process. 
     BACKGROUND: DISPOSABLE GATE PROCESS 
     A disposable gate process has been shown to provide a method by which a CMOS transistor structure can be scaled further into the sub-micron region while maintaining sufficiently low gate sheet resistance, small junction depth, and low junction capacitance. See provisional applications 60/029,215 filed Oct. 28, 1996 and 60/019,643 filed Oct. 28, 1996, which are hereby incorporated by reference. 
     A conventional disposable gate process is illustrated in FIGS. 2A-2C. FIG. 2A shows a field dielectric  114  which was blanket deposited over a disposable gate  120  (e.g. of silicon nitride) and pad oxide  122  which was formed over a semiconductor active area  102 . The field dielectric  114  is then chemically mechanically polished, leaving the surface of disposable gate  120  exposed as shown in FIG.  2 B. In FIG. 2C disposable gate  120  has been removed (e.g. by a hot phosphoric acid). A gate electrode can now be deposited in the space  115  left by the removal of disposable gate  120 . 
     The chemical mechanical polishing step (CMP) in the conventional process discussed above is a polishing technique which provides global planarization. However, CMP can be a problematic step in this conventional process because of the polish rate dependence on gate density. Yota et al.,  Integration of ICP High-Density Plasnia CVD with CMP and Its Effects on Planarity for Sub- 0.5  μm CMOS Technology , 2875 Proceedings of the SPIE 265 (1997), which is hereby incorporated by reference. The CMP process is difficult to control because of the pattern sensitivity of the polish rate. 
     Background: HDP-CVD 
     The basic HDP-CVD (high density plasma-chemical vapor deposition) process involves a simultaneous deposition and etch component and is already well-established in the semiconductor industry. HDP-CVD can provide very non-conformal deposition, in which material buildup occurs almost complete on the flat surfaces of the starting structure, and not on sidewalls. See e.g., A. Chatterjee et al.,  A Shallow Trench Isolation Study for  0.25/0.18  μm CMOS Technologies and Beyond , 156 Symposium on VLSI Technology Digest (1996); S. Nag et al.,  Comparative Evaluation of Gap-Fill Dielectrics in Shallow Trench Isolation for Sub- 0.25  μm Technologies , 841 IEDM (1996), which are hereby incorporated by reference. 
     CMP-Free Disposable Gate Process 
     The present application solves the problem of polish rate dependence on gate pattern density by using a highly non-conformal field dielectric to leave the disposable gate partially exposed, thereby eliminating the need to chemically-mechanically-polish the field dielectric to expose the disposable gate. In a sample embodiment, HDP-CVD oxide is deposited non-conformally as the field oxide over a disposable gate structure. The deposition process uses a sputter component to achieve minimal deposition on the sidewalls of the disposable gate. The oxide deposition is preferably stopped before the oxide filling up the trench and the oxide depositing on top of the disposable gate meet, thus leaving the sidewalls of the disposable gate partially exposed. Optionally, a short oxide etch can be used to selectively remove any oxide which deposited on the sides of the disposable gate, thereby leaving the sidewalls of the gate partially exposed. Because the sidewalls are exposed, the process can proceed directly to selective removal of the disposable gate, rather than going through a CMP step to expose the disposable gate before removal can proceed. 
     Advantages of the disclosed methods and structures include: 
     eliminating the need for CMP after field-oxide deposition; 
     simplifying the overall disposable gate process; 
     eliminating pattern sensitivity to the degree of planarity; 
     high-k gate dielectrics and/or metal gates (e.g. aluminum) are not subjected to high temperatures in a disposable gate process; 
     limits on lateral dimensions of a disposable gate can be avoided; 
     a thicker disposable gate structure can be used to provide a higher margin of error; 
     the field oxide can provide a thicker oxide etch-stop than the gate oxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
     FIG. 1 shows a sample CMP-free process flow for formation of a disposable gate structure. 
     FIGS. 2A-2C illustrate a previously known step in the process. 
     FIGS. 3A-3E are cross-sectional diagrams of a structure formed according to a preferred embodiment of the invention during various stages of fabrication. 
     FIG. 4 is a cross-sectional diagram of a structure formed according to an alternative embodiment of the invention during various stages of fabrication. 
     FIGS. 5A-5E are cross-sectional diagrams of a structure formed according to an alternative embodiment of the invention during various stages of fabrication. 
     Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. 
     Overview of a CMP-Free Disposable Gate Process 
     FIG. 1 shows a sample CMP-free process flow for forming a disposable gate structure. Those of ordinary skill in the art will realize that the process and benefits of the invention are applicable to structures for the deep submicron regime (i.e., tenth-micron and below) as well as other structures. The steps in FIG. 1 will be discussed with reference to FIGS.  3 A, 3 E. Sample details will be discussed below in a discussion of a sample embodiment. 
     At Step  110 , a disposable gate  220  comprising a first disposable gate material  223  is formed over a sacrificial gate dielectric  222  over an area of substrate  202  where the channel region is desired. Disposable gate  220  may comprise more than one material. If more than one material is used, the materials are chosen such that they may be selectively removed with respect to the substrate, with respect to each other, and/or with respect to other subsequently formed layers (such as the optional sidewalls of Step  115 ). The thickness of disposable gate  220  is variable, but will be high enough to block subsequent source/drain (S/D) implants. Further sample details are provided below in the description of the sample embodiment. 
     Source/Drain regions  206  and  207  are formed in Step  120 . Source/Drain regions may be formed in various ways known in the art and from various materials known in the art. Some examples are discussed and illustrated further below. 
     At Step  130 , a layer of dielectric material  214  is deposited non-conformally over the structure to cover source/drain regions  206  and  207 , and the top of disposable gate  220 , but not the sidewalls of disposable gate  220 . The dielectric material  214  is chosen such that it can be selectively removed with respect to disposable gate material  223  (or any additional materials comprising the disposable gate), and/or with respect to sacrificial gate dielectric  222 . 
     At Step  140 , disposable gate  220  is selectively removed. There is no need to planarize back (e.g., CMP or etchback) the layer of dielectric material  214  to expose the disposable gate because the non-conformal deposition of the layer of dielectric material  214  leaves the sidewalls of disposable gate  220  exposed. 
     At Step  150 , channel doping (e.g., by means such as a Vt implant or gas immersion laser doping) is performed after removal of disposable gate  220 , and either before or after the removal of the sacrificial gate dielectric  222 . Because the layer of dielectric material  214  covers source/drain regions, the introduction of channel dopants is self-aligned to the source/drain regions and therefore substantially limited to only the immediate channel area (i.e., the area of the substrate which had been occupied by the disposable gate). Self-aligned implantation causes a reduction in the capacitance of the subsequently formed source/drain junction region. 
     After introduction of channel dopants, a new gate dielectric  210  can be grown in the space  215  left by removal of sacrificial gate dielectric  222  and disposable gate  220 . A gate electrode  212  is then formed. This completes Step  160 . By performing the gate processing after source/drain formation, heat treatments required by source/drain formation do not affect the gate dielectric and gate electrode. Thus, a doped polysilicon gate electrode can be used with an ultra-thin gate dielectric (i.e., 6 nm or even less than 3 nm) without having harmful dopant diffusion from the doped polysilicon through the gate dielectric into the channel region. Alternatively, a gate electrode comprising, in part, a metal can be used because the heat treatments for the source/drain formation have already been performed. 
     After the process flow described above, processing continues with the formation of interconnections as is known in the art. Various modifications to the process described above will be apparent to persons skilled in the art. 
     Sample Embodiment 
     FIGS. 3A-3E are cross-sectional diagrams of a sample embodiment of the invention during various stages of fabrication. Details of steps in the process flow shown in FIG. 1 will now be discussed in conjunction with FIGS. 3A-3E. 
     In the cross-section of a sample embodiment shown in FIG. 3A, substrate  202  is silicon. Sacrificial gate dielectric  222  is silicon dioxide, 0.1 microns wide and grown 10 nm thick. Disposable gate  220  comprises one disposable gate material  223 . In this sample embodiment, disposable gate material  223  is silicon nitride, 0.1 microns wide and 150 nm thick. Disposable gate material  223  is deposited under a total pressure of 200 mTorr for 1875 seconds at 700-800 (preferably 750) degrees Celsius with a resulting film composition of Si3N4. 
     Non-elevated source/drain regions  207  were implanted into substrate  202 , then doped in-situ, by appropriate chemistry, with an appropriate dopant for forming the desired conductivity type (e.g, antimony for an n-channel MOSFET). Shallow source/drain regions  206  were then formed by an anneal causing diffusion from source/drain regions  207  as is known in the art. 
     Referring to FIG. 3B, HDP-CVD oxide has been deposited non-conformally as field dielectric layer  214 . Deposition of field dielectric layer  214  is preferably stopped (at a thickness of 150 nm), before it completely covers the disposable gate, so that the sidewalls of the disposable gate are left partially exposed. The non-conformal oxide was deposited from silane and oxygen at a temperature between 300 and 450 degrees Celsius (preferably 330 degrees C.). The HDP-CVD deposition used a medium to high sputtering component to achieve minimal deposition on the sidewalls of the disposable gate  220 . 
     Disposable gate material  223  was then removed by hot phosphoric etch. It is preferable to introduce channel dopants after the removal of disposable gate material  223 , but before the removal of sacrificial gate oxide  222 . After introduction of channel dopants, sacrificial gate oxide  222  was removed by a short wet oxide etch of HF concentration and duration necessary for removal. Field dielectric layer  214  was not substantially removed by either etch removing disposable gate material  223  or sacrificial gate oxide  222 . It should also be noted that substrate  202  was not etched into by removal of disposable gate material  223  or sacrificial gate oxide  222 . Removal of disposable gate material  223  and sacrificial gate oxide  222  leaves space  215  in field dielectric layer  214 , as shown in FIG.  3 C. 
     Referring to FIG. 3D, a clean gate oxide  210  was formed on substrate  202  in space  215 . Gate oxide  210  comprises a thermally grown oxide (e.g., 3-3.5 nm for a 1.2V supply). Gate material  226  was then formed over gate dielectric  210  and field dielectric layer  214 . In this sample embodiment, gate material  226  is aluminum and is 100 nm thick. 
     There are several methods that may be used to form a gate electrode  212 . The preferred embodiment is the T-gate structure, shown in FIG. 3E, which is useful for further reduction of gate sheet resistance. The deposited gate material  226  has been patterned and etched to form T-gate electrode  212  extending over a portion of field dielectric layer  214 . 
     Alternative Embodiment: Silicon-Oxynitride Field Dielectric 
     In this class of embodiments, silicon-oxynitride is deposited non-conformally (as in the sample embodiment) to form the field dielectric. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Silicon-Rich Field Dielectric 
     In this class of embodiments, silicon-rich off-stoichiometric silicon dioxide is deposited non-conformally to form the field dielectric. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Silicon-Germanium Substrate 
     A silicon-germanium substrate may be used instead of the silicon substrate of the sample embodiment. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Extension of Sacrificial Gate Dielectric 
     Rather than being the width of the gate as in the sample embodiment, sacrificial gate dielectric  222  may be extended over the entire substrate. If sidewall dielectrics are subsequently formed, any portions of sacrificial gate dielectric  222  not covered by the formation of sidewall dielectrics and the disposable gate may be removed. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Disposable Gate Materials 
     If sacrificial gate dielectric  222  comprises an oxide, then disposable gate material  223  may comprise silicon-germanium or silicon. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: LPCVD Polysilicon Disposable Gate 
     Polysilicon may be deposited under LPCVD conditions as disposable gate material  223 . If a polysilicon is used as disposable gate material  223 , then sidewall spacers  230  are preferably present. A highly selective polysilicon/oxide etch such as choline would be used to remove the polysilicon. For example, a 5% choline solution etches phosphorous-doped polysilicon at a rate of about 25 nm/minute at room temperature, while it etches LPCVD nitride and thermal oxide at rates of &lt;0.1 nm/minute. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Sacrificial Gate Dielectric 
     If substrate  202  is silicon, silicon-germanium may be used as sacrificial gate material  222  because silicon-germanium may be removed with a high selectivity to silicon. This would prevent damage to the substrate by removal of sacrificial gate material  222 . Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Non-T-Gate by a Damascene Process 
     Although it offers some advantages, the T-gate structure shown in FIG. 3E is not required to practice the invention. Alternatively, a self-aligned non-T-gate structure  212 ′, as shown in FIG. 4, can be formed by numerous conventional processes (e.g, damascene). 
     In a conventional damascene process, an interlevel dielectric layer (ILD) is formed over a patterned underlying metal layer (which may be e.g., Metal-1 or Metal-4, and may be e.g., aluminum alloy or another metal). The ILD is then patterned and etched to cut metal pathways. Next a diffusion barrier and adhesion promotion layer is deposited, followed by blanket deposition of a low-resistivity metal (such as copper). A CMP process is then used to form flush gate structure  212 ′. 
     In an alternative damascene process, transistors are beneath a conductor layer (typically an aluminum alloy) which is surrounded by a lower interlevel dielectric. Thereafter, an upper interlevel dielectric (e.g., BPSG over TEOS-deposited SiO2) is deposited and planarized by conventional methods (e.g. chemical-mechanical polishing). 
     Thereafter, the upper interlevel dielectric is patterned and etched to form slots where lines of metallization are desired, and also to form deeper holes where vias are desired (i.e., where an electrical contact to the underlying conductor layer is desired). Thereafter, a diffusion barrier layer (e.g., a conductive nitride of titanium or tungsten) is deposited. A highly conductive metal (e.g., tungsten) is then deposited overall by conventional methods, and etched back and polished (using one of the methods described above) so that the flat surface of the interlevel dielectric is exposed wherever the metal is not present. 
     Other process conditions remain similar to those stated above. 
     Alternative Embodiments: Elevated Source/Drain Structure with Sidewall Spacers 
     FIGS. 5A-5E are cross-sectional diagrams of an alternative embodiment of the invention during various stages of fabrication. 
     As shown in FIG. 5A, disposable gate  220  comprising a nitride layer  223  was formed over sacrificial gate oxide  222 . 
     FIG. 5B shows sidewall dielectrics  230  formed on the sidewalls of disposable gate  220  to complete optional Step  115 . If sidewalls are desired, they are formed from a material such that materials comprising disposable gate  220  may be selectively removed without substantially removing the sidewall. Sidewall dielectric  230  is thin, less than 20 nm for the deep sub-micron device in this alternative sample embodiment. Combinations of materials and thicknesses for forming sidewall dielectrics  230  will be apparent to those of ordinary skill in the art. 
     At Step  120 , elevated source/drain regions  205  as shown in FIG. 5C are formed by selectively forming an epitaxial layer of silicon over substrate  202  adjacent disposable gate  220 . Disposable gate  220  provides a masking layer for the epitaxial process. Thus, elevated source/drain regions  205  are self-aligned to disposable gate  220 . Angled faceting of an epitaxially formed elevated source/drain region  205  at the edge of the disposable gate  220  may occur. The degree of faceting may be partially controlled by adjusting process parameters of the cleaning and deposition processes. 
     Elevated source/drain regions  205  are next doped and shallow source/drain junction regions  204  are formed by diffusion of dopants from the elevated source/drain regions  205 : The preferred step is to anneal the structure at sufficient time and temperature to diffuse dopant from the elevated source/drain regions  205  to form shallow source/drain junction regions  204 . Diffusing the dopant from an elevated source/drain structure allows for shallower source/drain junction regions  204  than are possible with an implant doped source/drain junction region. This completes Step  120 . 
     In FIG. 5D, dielectric material  214  is HDP-CVD oxide non-conformally deposited such that removal of disposable gate  220  can proceed directly, as described in the main embodiment. As seen in FIG. 5E, dielectric layer  214  and sidewall dielectric  230  are not substantially removed by etches which removed disposable gate  220  and sacrificial gate dielectric  222  and left space  215 . 
     A T-gate electrode (e.g., formed by pattern and etch) or non-T-gate electrode (e.g., formed by a damascene process) can then be formed in space  215 . Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Materials for Sidewall Spacers 
     Various materials can be used to form optional sidewall spacers are present. Some materials include silicon-oxynitride, silicon-rich off-stoichiometric silicon dioxide, or an oxide/nitride composite. The materials comprising optional sidewall spacers will be chosen such that selectivity during etching is maintained as in the sample embodiment. For example, if sacrificial gate dielectric  222  is silicon dioxide and disposable gate material  223  is silicon-germanium or silicon, sidewall dielectric  230  is an oxide/nitride composite. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Formation of Sidewall Spacers 
     As shown in the alternative embodiment of FIG. 5B, sidewall spacers may be aligned next to the sidewalls of the disposable gate prior to removal of the disposable gate. Alternatively, sidewall spacers may be formed after removal of the disposable gate. Sidewalls so formed will be aligned next to the sidewalls of the HDP-CVD oxide in the space left by the removal of the disposable gate. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Gate Cap 
     An additional disposable gate material may be formed over first disposable gate material  223  as a cap for disposable gate  220 . If a cap is present and elevated source/drain regions are to be formed, the cap optionally may be stripped prior to selective epitaxial deposition of the elevated source/drain regions. Polysilicon can then form on disposable gate  220  during the epitaxial deposition and provide extra gate height for the subsequent etch removing disposable gate  220 . Providing extra gate height in this manner is particularly advantageous after sidewall spacers (if desired) are formed adjacent to the disposable gate. This is because the extra height exposes more of the disposable gate above the sidewall spacers. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Larger Gates 
     A limit on the lateral dimensions of a disposable gate can be avoided under the disclosed process. In a larger disposable gate structure, a mask is formed over the structure after deposition of the non-conformal oxide. A patterned removal of the non-conformal oxide (e.g., down the center of the oxide layer covering the top of disposable gate  220 ) will cut slots in the oxide. These slots speed up the removal of the disposable gate because the etch can now work from the top as well as the sides of disposable gate  220 . Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Pattern and Etch to Form Elevated Source/Drain Regions 
     Instead of epitaxially forming elevated source/drain regions  205  of FIG. 5C, elevated source/drain regions  205  may be formed by a non-selective deposition of the desired conductive material followed by pattern and etch of this material. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Metallic Elevated Source/Drain Regions 
     Elevated source/drain regions may be formed from metals or combinations thereof. If the elevated source/drain regions are metallic, doping is unnecessary. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Methods for Forming Ultra-Shallow Source/Drain 
     Rather than the anneal performed to diffuse dopant from elevated source/drain  205  to form ultra-shallow source/drain  204 , adequate diffusion to form ultra-shallow source/drain  204  may be obtained during the deposition of the elevated source/drain if the deposition time or temperature is sufficient. The anneal may also be performed earlier in the process if desired or it may be part of another process such as the formation of a gate dielectric. Other process conditions remain similar to those stated above. 
     It should also be noted that ultra-shallow source/drain regions  204  may be formed in other ways not requiring diffusion from elevated source/drain regions  205 . For example, regions  204  may be diffused from an overlying layer such a polysilicon-germanium or doped oxide, they may implant doped using a variety of techniques to keep the regions  204  shallow, or they may be formed using gas-immersion laser doping. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Cladding of Source/Drain Regions 
     Cladding forms a low resistance material over the desired structure and may be accomplished in a number of ways including salicidation. Salicidation reduces values of contact resistance, sheet resistivity of the shallow junctions of the source/drain regions, and interconnect resistance of the gate lines. A metal, preferably 100 nm of titanium, is deposited over the structure and reacted with the exposed silicon areas of the source/drain regions to form a silicide layer. The unreacted titanium is then selectively removed, leaving the silicide where formed on the structure. Cladding may also be accomplished by a metal deposition, followed by pattern and etch. 
     A selected portion of source/drain regions may be cladded after their formation. Cladding prior to the formation of the gate dielectric and gate electrode reduces the heat treatments seen by the gate dielectric and gate electrode. Cladding of the elevated source/drain regions  204  should not, however, occur over the entire region  205 . Cladding at the ends of the faceted regions is preferably avoided (e.g. by use of a sidewall spacer that is thick enough to cover any facets) since a gate dielectric is desired at that region and a thermally grown gate dielectric would not form over a cladded region. Other process conditions remain similar to those stated above. 
     If the final gate is polysilicon, (whether T-gate or non-T-gate), then it too can be advantageously cladded. The T-gate structure may be cladded after the final gate material is deposited, but before it is etched. After deposition of the cladding material, the cladding material and gate material are patterned and etched. In this case, both the gate material and cladding material extend over the dielectric layer. In a non-T-gate structure, only the cladding layer extends over dielectric layer. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Isotropic Oxide Etch 
     Because dielectric material  214  is chosen such that it may be etched selectively with respect to disposable gate material  223 , a short wet-oxide-etch step may be performed to remove any oxide that deposits on the sides of disposable gate  220 . Disposable gate  220  would then be partially exposed, as it appears in FIG.  3 B. The concentration and duration of the etch will vary with the amount oxide needing removal from the sidewalls (e.g., 2-10 rum of oxide may need to be removed to expose the sidewalls of the disposable gate; 1% HF removes 2 nm of oxide/minute.) This completes optional Step  135 . Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Post-Deposition Doping of Source/Drain 
     Rather than doping elevated source/drain in-situ, as in the sample embodiment, elevated source/drain regions may alternatively be doped after deposition by implantation. Other process conditions remain similar to those stated above. 
     Alternative Embodiment: Removal of Sidewalls and Subsequent Gate Extension 
     After channel doping, but before forming of the gate electrode, sidewall dielectrics  230  (if present) may be selectively removed. If sidewall dielectrics  230  are removed, then the final gate material and/or the final gate dielectric can extend over the tips of elevated source/drain regions (if present). When gate dielectric/gate material extend over the facets of elevated source/drain regions, the gate dielectric in part separates the gate material from the source/drain regions. If sidewall dielectrics  230  are removed, then the gate electrode can be separated from elevated source/drain regions by the final gate dielectric only. Extension of the final gate material over source/drain regions may be beneficial in reducing series resistance of the subsequently formed MOSFET although an increase in gate-to-drain capacitance will additionally result. The existence and/or optimization of the extension of the final gate material depends on the application and trade-offs between such issues as the before mentioned series resistance and overlap capacitance. This completes optional Step  155 . Other process conditions remain similar to those stated above. 
     If sidewall dielectrics  230  are not removed prior to forming of the gate dielectric and gate material, then the final gate material and the final gate dielectric do not extend over the tips of elevated source/drain regions within the space left by the removal of the disposable gate. The presence of sidewall dielectric  230  prevents the subsequently formed gate electrode from being separated from elevated source/drain regions by only the final gate dielectric. Forming the HDP-CVD field oxide such that elevated source/drain regions are separated from the overlying portions of a final T-gate structure results in a low gate-to-drain capacitance between elevated source/drain regions and the overlying portions of the T-gate structure. Other process remain similar to those stated above. 
     Modifications and Variations 
     As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given, but is only defined by the issued claims. 
     While the inventions have been described with primary reference to a single-poly process, it will be readily recognized that these inventions can also be applied to process with two, three, or more layers of polysilicon or polycide. 
     Similarly, it will be readily recognized that the described process steps can also be embedded into hybrid process flows, such as BiCMOS or smart-power processes. 
     Other gate dielectric materials may be used, such as a grown and/or deposited oxide, oxynitride, or any other suitable gate dielectric material including materials with higher dielectric constant than silicon dioxide may be used. 
     Other gate materials may be used such as a non-crystalline material substantially containing silicon or silicon-germanium, a doped polysilicon layer, a doped amorphous silicon layer, a metal layer, a composite material comprised of different metals or a combination of metal and semiconductor material, or other appropriate conductive materials (e.g., materials including tungsten, titanium nitride, or copper). The final gate can also be stacked (e.g., aluminum or tungsten over titanium nitride, or tungsten over tungsten nitride over polysilicon). It is noted that if a semiconductor material is utilized in part for the gate material, this semiconductor material can be doped (with the desired n or p type dopants) in-situ or doped after deposition by means such as implantation and anneal. 
     Instead of the pattern and etch formation, the T-gate structure in the sample embodiment may be formed by means such as a selective epitaxy of semiconductor or metal with the epitaxial overgrowth resulting in a T-gate structure. 
     The thickness of non-conformal dielectric layer may be tailored to allow for a low gate-to-drain capacitance between elevated source/drain regions (if used) and the overlying portion of a T-gate structure, provided the sidewalls of the disposable gate are left partially exposed. 
     Source/Drain regions may be formed using other methods known in the art (e.g., by solid source diffusion into the substrate.) 
     An n-channel MOSFET may be formed using alternative source/drain dopants, such as phosphorous and/or arsenic. Similarly, if a p-channel MOSFET is desired, a p-type dopant, such as boron, may be used to dope source/drain regions.